Properties of Lignin and Poly(hydroxybutyrate) Blends A Thesis by Publication submitted in Partial Fulfilment of the Requirement for the Degree of Doctor of Philosophy Payam Mousavioun M.Sc., B.Sc. (Chemical Engineering) Chemistry Discipline Faculty of Science and Technology Queensland University of Technology Queensland, Australia March 2011
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Properties of Lignin and Poly(hydroxybutyrate) Blends
A Thesis by Publication submitted in
Partial Fulfilment of the Requirement for the
Degree of
Doctor of Philosophy
Payam Mousavioun
M.Sc., B.Sc. (Chemical Engineering)
Chemistry Discipline
Faculty of Science and Technology
Queensland University of Technology
Queensland, Australia
March 2011
II
III
Acknowledgment
It is a pleasure to thank everybody who made this thesis possible including my
supervisory team, my family and Queensland University of Technology (QUT).
First of all, I would like to express my deepest sense of gratitude to my principal
supervisor, Dr William Doherty, who trusted me and quickly discovered my
potential and interest in the research area. I am heartily thankful to Dr Doherty
whose encouragement, supervision, guidance and support from the preliminary
to the concluding level enabled me to develop an understanding of the subject. I
would also like to acknowledge the support of my associate supervisors,
Professor Graeme George and Professor Peter Halley, during my research
programme. Without their brilliant advice and very timely and valid hints
throughout the completion of my PhD programme, this thesis would not have
been possible. It is an honour for me to have worked with such a great and
prestigious supervisory team.
I would also like to convey my thanks to QUT for providing me with such a
pleasant research area and facilities. I gratefully acknowledge QUT and the
Centre for Tropical Crops and Biocommodities (CTCB) for the financial
assistance of this project through the Postgraduate Research Awards (QUTPRA)
Grant. The assistance of QUT Research Portfolio is also highly appreciated.
I am indebted to many of my colleagues and friends at the Australian Institute
for Bioengineering and Nanotechnology (AIBN), Centre High Performance
Polymers (CHPP) in the University of Queensland (UQ) and CTCB for
providing a warm research atmosphere, sharing of knowledge, and
encouragement. I will never forget the pleasant times I had with my friends
during our meetings at CHPP group at UQ and other social events.
I must acknowledge my beloved wife and best friend, Parastoo. Without her
love, encouragement and assistance, I would not have started and finished this
research programme.
Lastly, I offer my regards and blessings to all of those who have supported me in
any respect during the completion of the project.
IV
Table of Contents
Abbreviations and Nomenclature ............................................................. XVII
Abbreviation ............................................................................................ XVII
Table A.2-1 Molecular weight and functional groups of lignins .............. 185
Table A.2-2 Tg of different lignin types (Gargulak and Lebo, 2000) ....... 186
Table A.2-3 Application of lignosulfonate products based on their surface-
active properties .................................................................. 188
Table A.2-4 Lignosulfonate products in speciality markets ..................... 189
XVII
Abbreviations and Nomenclature
Abbrev ia t ion AIBN Australian Institute for Bioengineering and Nanotechnology CHPP Centre High Performance Polymers CTCB Centre for Tropical Crops and Biocommodities DMF N,N’ dimethylformamide DSC Differential scanning calorimetry EL Ether-soluble lignin FT-IR Fourier transform-infrared spectroscopy HPLC high performance liquid chromatography L1 diethyl ether fractionated lignin L2 methanol soluble fractionated lignin L3 Residual solvent fractionated lignin Lignin/PHB Blend of lignin and PHB, the same as PHB/lignin ML Methanol-soluble lignin NMR Nuclear magnetic resonance NREL National Renewable Energy Laboratory PE Polyethylene PEO poly(ethylene oxide) PHA Poly(hydroxyalkanoate) PHB Poly(hydroxybutyrate) PHBV poly(hydroxybutyrate-hydroxyvalerate) PHH Poly(hydroxyhexanoate) PHO Poly(hydroxyoctanoate) PHV Poly(hydroxyvalerate) PLA Poly(lactic acid) PP Polypropylene PVA polyvinyl alcohol QUT Queensland University of Technology QUTPRA QUT Postgraduate Research Award RACI The Royal Australian Chemical Institute RL Residual lignin SEM Scanning electron microscopy TGA Thermogravimetric analysis TnBACl tetra-n-butylammonium chloride UQ University of Queensland XPS X-ray photoelectron spectroscopy analysis
XVIII
Nomencla ture
A s�� the pre-exponential factor � - the degree of conversion β
°C min-1 the linear heating rate ∆Hm J g-1 melting enthalpy η * Pa.s complex viscosity E� kJ mol�� apparent activation energy ΔE� kJ mol�� energy of vaporation to a gas at zero pressure G' Pa storage modulus G" Pa loss modulus G��� J Gibbs free energy K�� - Gordon-Taylor equation adjustable parameter K� - Kwei equation adjustable parameter Mn g mol-1 number average molecular weight Mw g mol-1 weight average molecular weight σ - solubility q - Kwei equation adjustable parameter R JK�� mol�� the ideal gas constant S� JK�� entropy T K absolute temperature t min time
Tan δ
- the ratio of energy dissipated to energy stored Tcc °C cold crystal temperature Tg °C glass transition temperature Tm °C melting temperature T�� °C the equilibrium melting point V cm3 volume v! cm3 mol-1 the molar volume w - weight fraction W g weight x - ratio against the % methoxyl (OCH3) content
x%&' - mass ratio of PHB X! - Bulk crystallinity X)! - PHB crystallinity
XIX
Abstract
The Queensland University of Technology (QUT) allows the presentation of a
thesis for the Degree of Doctor of Philosophy in the format of published or
submitted papers, where such papers have been published, accepted or submitted
during the period of candidature. This thesis is composed of Seven
published/submitted papers and one poster presentation, of which five have been
published and the other two are under review. This project is financially
supported by the QUTPRA Grant.
The twenty-first century started with the resurrection of lignocellulosic biomass
as a potential substitute for petrochemicals. Petrochemicals, which enjoyed the
sustainable economic growth during the past century, have begun to reach or
have reached their peak. The world energy situation is complicated by political
uncertainty and by the environmental impact associated with petrochemical
import and usage. In particular, greenhouse gasses and toxic emissions produced
by petrochemicals have been implicated as a significant cause of climate
changes.
Lignocellulosic biomass (e.g. sugarcane biomass and bagasse), which potentially
enjoys a more abundant, widely distributed, and cost-effective resource base, can
play an indispensible role in the paradigm transition from fossil-based to
carbohydrate-based economy.
Poly(3-hydroxybutyrate), PHB has attracted much commercial interest as a
plastic and biodegradable material because some its physical properties are
similar to those of polypropylene (PP), even though the two polymers have quite
different chemical structures. PHB exhibits a high degree of crystallinity, has a
high melting point of approximately 180°C, and most importantly, unlike PP,
PHB is rapidly biodegradable.
Two major factors which currently inhibit the widespread use of PHB are its
high cost and poor mechanical properties. The production costs of PHB are
significantly higher than for plastics produced from petrochemical resources (e.g.
PP costs $US1 kg-1, whereas PHB costs $US8 kg-1), and its stiff and brittle
nature makes processing difficult and impedes its ability to handle high impact.
XX
Lignin, together with cellulose and hemicellulose, are the three main components
of every lignocellulosic biomass. It is a natural polymer occurring in the plant
cell wall. Lignin, after cellulose, is the most abundant polymer in nature. It is
extracted mainly as a by-product in the pulp and paper industry. Although,
traditionally lignin is burnt in industry for energy, it has a lot of value-add
properties. Lignin, which to date has not been exploited, is an amorphous
polymer with hydrophobic behaviour. These make it a good candidate for
blending with PHB and technically, blending can be a viable solution for price
and reduction and enhance production properties. Theoretically, lignin and PHB
affect the physiochemical properties of each other when they become miscible in
a composite. A comprehensive study on structural, thermal, rheological and
environmental properties of lignin/PHB blends together with neat lignin and
PHB is the targeted scope of this thesis. An introduction to this research,
including a description of the research problem, a literature review and an
account of the research progress linking the research papers is presented in
Chapter 1.
In this research, lignin was obtained from bagasse through extraction with
sodium hydroxide. A novel two-step pH precipitation procedure was used to
recover soda lignin with the purity of 96.3 wt% from the black liquor (i.e. the
spent sodium hydroxide solution). The precipitation process is presented in
Chapter 2. A sequential solvent extraction process was used to fractionate the
soda lignin into three fractions. These fractions, together with the soda lignin,
were characterised to determine elemental composition, purity, carbohydrate
content, molecular weight, and functional group content. The thermal properties
of the lignins were also determined. The results are presented and discussed in
Chapter 2. On the basis of the type and quantity of functional groups, attempts
were made to identify potential applications for each of the individual lignins.
As an addendum to the general section on the development of composite
materials of lignin, which includes Chapters 1 and 2, studies on the kinetics of
bagasse thermal degradation are presented in Appendix 1. The work showed that
distinct stages of mass losses depend on residual sucrose. As the development of
value-added products from lignin will improve the economics of cellulosic
XXI
ethanol, a review on lignin applications, which included lignin/PHB composites,
is presented in Appendix 2.
Chapters 3, 4 and 5 are dedicated to investigations of the properties of soda
lignin/PHB composites. Chapter 3 reports on the thermal stability and
miscibility of the blends. Although the addition of soda lignin shifts the onset of
PHB decomposition to lower temperatures, the lignin/PHB blends are thermally
more stable over a wider temperature range. The results from the thermal study
also indicated that blends containing up to 40 wt% soda lignin were miscible.
The Tg data for these blends fitted nicely to the Gordon-Taylor and Kwei
models. Fourier transform infrared spectroscopy (FT-IR) evaluation showed that
the miscibility of the blends was because of specific hydrogen bonding (and
similar interactions) between reactive phenolic hydroxyl groups of lignin and the
carbonyl group of PHB.
The thermophysical and rheological properties of soda lignin/PHB blends are
presented in Chapter 4. In this chapter, the kinetics of thermal degradation of the
blends is studied using thermogravimetric analysis (TGA). This preliminary
investigation is limited to the processing temperature of blend manufacturing.
Of significance in the study, is the drop in the apparent energy of activation, Ea
from 112 kJmol-1 for pure PHB to half that value for blends. This means that the
addition of lignin to PHB reduces the thermal stability of PHB, and that the
comparative reduced weight loss observed in the TGA data is associated with the
slower rate of lignin degradation in the composite. The Tg of PHB, as well as its
melting temperature, melting enthalpy, crystallinity and melting point decrease
with increase in lignin content. Results from the rheological investigation
showed that at low lignin content (≤30 wt%), lignin acts as a plasticiser for PHB,
while at high lignin content it acts as a filler.
Chapter 5 is dedicated to the environmental study of soda lignin/PHB blends.
The biodegradability of lignin/PHB blends is compared to that of PHB using the
standard soil burial test. To obtain acceptable biodegradation data, samples were
buried for 12 months under controlled conditions. Gravimetric analysis, TGA,
optical microscopy, scanning electron microscopy (SEM), differential scanning
calorimetry (DSC), FT-IR, and X-ray photoelectron spectroscopy (XPS) were
used in the study. The results clearly demonstrated that lignin retards the
XXII
biodegradation of PHB, and that the miscible blends were more resistant to
degradation compared to the immiscible blends.
To obtain an understanding between the structure of lignin and the properties of
the blends, a methanol-soluble lignin, which contains 3× less phenolic hydroxyl
group that its parent soda lignin used in preparing blends for the work reported in
Chapters 3 and 4, was blended with PHB and the properties of the blends
investigated. The results are reported in Chapter 6. At up to 40 wt% methanol-
soluble lignin, the experimental data fitted the Gordon-Taylor and Kwei models,
similar to the results obtained soda lignin-based blends. However, the values
obtained for the interactive parameters for the methanol-soluble lignin blends
were slightly lower than the blends obtained with soda lignin indicating weaker
association between methanol-soluble lignin and PHB. FT-IR data confirmed
that hydrogen bonding is the main interactive force between the reactive
functional groups of lignin and the carbonyl group of PHB. In summary, the
structural differences existing between the two lignins did not manifest itself in
QUT grant -in-aid for attending 10th AIChE meeting, Salt Lake City, UT,
USA, 2010.
XXIX
Statement of Original Authorship
“The work contained in this thesis has not been previously submitted to meet
requirements for an award at this or any other higher education institution. To
the best of my knowledge and belief, the thesis contains no material previously
published or written by another person except where due reference is made.”
Signature Date
1
CHAPTER 1
Introduction
2
1.1. Descr ip t ion of Research Problem It is now widely accepted that in order to help to reduce global warming it is
necessary to use sustainable environmentally friendly plastics instead of the
traditional petroleum-based ones. Many petroleum-based polymers do not
degrade and are usually decomposed by combustion, thereby adding to the
carbon dioxide levels in the atmosphere. Although there are a few commercially
available biodegradable polymers suitable for commodity applications, their cost
is prohibitive. An example is poly(hydroxybutyrate (PHB). However, PHB has
poor mechanical properties and is difficult to process (Khanna and Srivastava,
2005). The main reasons for its poor properties include (a) low Tg, (b)
undergoes secondary crystallisation which occurs during storage at ambient
temperature, and (c) has a low nucleation density which allows large spherulites,
with cracks and splits, to form. Polymer blending is considered to be one of the
most effective methods for lowering the cost of production of these types of
polymers, and in certain cases improves processing and product quality. The
strategy in this project is to blend lignin, an inexpensive biodegradable
amorphous polymer, with high-value biodegradable aliphatic polyester, PHB,
and to investigate the properties of the blends. The project will, therefore
evaluate the physico-chemical properties of lignins, establish suitable processing
conditions for the preparation of the blends, assess the thermal, mechanical and
rheological properties of the blends, and determine the probable environmental
degradation mechanisms of the blends. The overall benefit from the research is
an improved knowledge on the performance and applicability of lignin-based
composite materials. The research activities have been divided into three main
aims:
• Develop composite materials from lignin by investigating the preparation
and characterisation of soda lignins.
• Prepare, characterise and determine the properties of soda lignin/PHB
blends.
• Prepare, characterise and determine the properties of methanol-soluble
lignin/PHB blends. This is to examine whether the structural differences
3
between soda lignin and methanol-soluble lignin will affect the properties
of the corresponding blends derived from them.
Research Aim #1: Develop composite materials of lignin
Sugar cane fibre, bagasse (a lignocellulosic material), is the fibrous residue from
the sugarcane milling process. The Australian sugar industry harvests around 35
million tonnes of sugarcane a year and this is converted into 5 million tonnes of
sugar, 1 million tonnes of molasses and 10 million tonnes of bagasse. There is
now a focus by the industry to increase the income stream by adding value to the
whole sugarcane biomass, including bagasse. Increasing amounts of surplus
bagasse will therefore become available as the Australian sugar industry
continues to move towards increased energy efficiency. Presently, bagasse is
burned for its fuel value to produce steam and electricity for factory operations.
The cellulose component of bagasse (50% of dry matter) has attracted interest as
a potential source of fuel ethanol. The other component of bagasse is lignin
(20% dry matter), a non-toxic amorphous hydrophobic polymer obtained readily
through extraction methods. Its macromolecular structure and low cost makes
lignin and lignin esters a good candidate for blending with aliphatic polyesters
such as PHB.
Lignin is composed of phenylpropane repeat units and possesses aliphatic and
aromatic hydroxyl groups together with vacant para-sites on the aromatic
monomer unit (section 1.2.3.1). This functionality makes lignin amenable to
chemical reactions. However, for lignin to be used as a feedstock to produce
composite materials of consistent quality, it has to be of high purity, susceptible
to chemical reactions, and of narrow molecular weight distribution. Thus, a
process for lignin isolation and purification from bagasse is a sub-objective in
this project. A number of destructive and non-destructive analytical tools were
used for detailed characterisation of lignin, including its molecular weight and
functionality.
Research Aim #2: Prepare and characterise soda lignin/PHB blends
The incorporation of an amorphous polymer such as lignin or lignin ester should,
in principle, improve the overall properties of PHB by lowering the melting
point, reducing secondary crystallisation, improving processability and reducing
4
brittleness. To optimise processing conditions for the preparation of lignin/PHB
blends, the thermal properties of lignin, the thermal properties of PHB, and the
kinetics of PHB degradation were investigated. The lignin/PHB blends were
assessed by investigating thermal and miscibility properties, as well as
mechanical and rheological properties. Biodegradation studies of the blends
were based on a standard burial soil test.
Research Aim #3: Prepare and characterise methanol-soluble lignin/PHB
blends
The aim of this phase of the project is to study methanol-soluble lignin/PHB
blends in order to establish whether the differences in lignin structure would
affect the properties of lignin/PHB blends. The experimental protocol used to
prepare methanol-soluble lignin/PHB blends was similar to those of soda lignin-
based PHB blends.
A summary of the research plan is outlined as follows: Phase 1 –
Characterisation of lignins; Phase 2 – Preparation, characterisation and
properties of lignin/PHB blends; Phase 3 – Environmental degradation of soda
lignin/PHB blends; Phase 4 – Preparation, characterisation and properties of
methanol-soluble lignin/PHB blends.
5
1.2. Theor ies and L i terature Review
1.2.1. Miscibility theories
Many researchers have studied polymer blending for the development of new
materials and to tailor properties of the blends by exploiting the physical,
chemical, mechanical and thermal properties of the individual components
(Lipatov and Nesterov, 1997).
There are several theories that have been developed to describe compatability
and miscibility of polymers. One of the most famous is the Flory-Huggins
treatment of polymer/solvent interactions in binary polymer systems (Lipatov
and Nesterov, 1997). This theory devised a general scheme which enables one to
deal with the mixing properties of a pair of polymers. It provides a basic
understanding of the occurrence of different types of phase diagrams
independent of temperature and molecular weight. Figure 1-1 illustrates the
process of mixing two polymers, A and B; where nA and nB are moles of the
polymers A and B, and VA and VB are their respective volumes, with V being the
total volume.
Figure 1-1 The description of the process for mixing two polymers
In order to find out whether true mixing would indeed occur, the change in the
Gibbs free energy has to be considered. This change, called the ‘Gibbs free
energy of mixing’ and denoted by ∆-./0, is given by:
nnnnAAAA
VVVVAAAA
nnnnBBBB
VVVVBBBB
nnnnA A A A , , , , nnnnBBBB
V=VV=VV=VV=VAAAA + V+ V+ V+ VBBBB
GGGGAAAA GGGGBBBB GGGGABABABAB
6
∆-./0 2 -45 6 7-4 3 -58 (1-1)
where -4, -5 and -45 denote the Gibbs free energies of the polymers A and B in
separate states and the mixed state, respectively.
The Flory-Huggins treatment represents ∆-./0 as a sum of two contributions:
∆-./0 = 69∆:; + ∆-<=> (1-2)
where the first component 9∆:; is the product of the temperature (T) and the
translational entropy (∆:;), and the second component is the interactions and
motions of the polymers represented by ∆-<=>.
According to equation (1-2), a decrease in ∆-<=> in association with an increase
in ∆:; will lead to a decrease in ∆-./0, which favours miscibility.
Now ∆:; and ∆-<=> can be represented by:
∆:; 2 ?7@4 A BBCD 3 @5 A B
BED8 (1-3)
∆-<=> 2 ?9F7BCBEBGH
8 (1-4)
where ? is the ideal gas constant and I> is the molar volume of a reference unit
(i.e. solvent) common to both polymers. Principally I> can be chosen arbitrarily,
but usually it is identified as the volume occupied by one of the polymer
components in the polymer solution. The decisive factor that describes the
extent of miscibility is the ‘Flory-Huggins interaction parameter’ χ. χ describes
the thermodynamic ‘quality’ of one component to act as a solvent towards
another. Flory-Huggins interaction parameter χ can be estimated from solubility
parameters using the following equation:
χ 2 7KL�KM8MGHNO (1-5)
where σ� σR are the solubility parameters of polymer 1 and polymer 2.
The ability of a polymer to influence the properties of another depends primarily
on its ability to associate and interact with that polymer. Methods for measuring
the association or compatibility on the nano-level (apart from measuring χ)
include electron microscopic techniques and thermal analysis. The presence of
single glass transition temperature (Tg), and the depression of the equilibrium
7
melting point 9.� , and Tm are useful parameters that can also be used to
demonstrate miscibility. The Tg can be determined using differential scanning
calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). In this
study, all measurements of Tg have been undertaken using only DSC instrument.
It was found that the speciments were too brittle to effectively use the DMTA for
the measurement of Tg.
1.2.2. Kinetics of thermal degradation
It is of practical significance to understand and predict the thermal
decomposition process of polymer blends, since this knowledge will help to
better design the engineering process and to estimate the influence on blend
properties by thermal events. It is necessary to consider the kinetics of
decomposition over a wide range of decomposition temperatures. This limits the
use of the conventional isothermal approach. The non-isothermal approach has
the advantage that the decomposition process can be examined at elevated
temperatures and over a wide temperature range. At these temperatures the
degradation process may follow different mechanisms and so provide useful
practical information for the design engineer.
Yao et al. (2008) describes various methods that are used to calculate kinetic
parameters for the thermal decomposition of compounds based on weight loss.
These include first-order decomposition kinetics with different reaction schemes
involving single or multiple constant heating rate methods (i.e. non-isothermal).
For this work, the Friedman’s method (1964) has been used since the method
was applied for the thermal degradation of polymers.
The general rate equation for a decomposition or degradation process can be
described as:
S�S; ~ USVSO 2 W798X7�8 (1-6)
where � is the degree of conversion, U the linear heating rate (°C min-1), W798 is the rate constant and X7�8 is the reaction rate model, a function which
depends on the actual reaction mechanism. The rate constant, W798 can be
calculated by assuming that the temperature and the degree of conversion, � are
non-dependent functions.
8
In this work,
�2 YZ�YYZ�Y[
(1-7)
where \� is the initial weight, \ is the weight during experiment, and \] is the
final weight of the investigation determinate from the TG thermograms.
The rate constant W798can be represented by the Arrhenius equation as:
W798 2 ^_7�`abc8 (1-8)
where de is apparent activation energy (Wf ghi��8, ? is the ideal gas constant
(8.314 fj�� ghi��), ^ is the pre-exponential factor (gk@��) and 9 is absolute
temperature (j).
For a dynamic TGA process, introducing U, into (1-9) results
S�SO 2 74l8_
7�`abc8X7�8 (1-9)
Equations (1-8) and (1-9) are the fundamental expressions of analytical methods
to calculate kinetic parameters on the basis of TGA data.
The Friedman method, which is a linear differential method of equation 1-8, is:
mU S�SOn 2 6 oa
NO 3 7^X7�88 (1-10)
Then for a given value of � the plots of i@ S�S; vs
�O directly leads to 6 oa
N from
the slope.
The main advantage of using Friedman’s approach or any other iso-conversion
method is that de can be calculated for the main degradation process without
any knowledge of the form of the kinetic equation.
1.2.3. Literature review
Introduction
Polymer blending, a process which involves the mixing of two or more
components by solvent casting or melting, is a cost effective technique to tailor-
make materials with improved physical, chemical, mechanical and thermal
properties.
9
Nowadays, with the high price of crude oil and the associated negative impact of
synthetic polymers, increasing attention is being paid to lignocellulosic biomass
as a provider of chemicals and polymers. Lignin is a component of biomass and
its properties can be exploited in the manufacture of polymer blends.
PHB which is generally obtained via fermentation (and in recent times in plants),
is biodegradable. It is envisaged that the incorporation of lignin/lignin-
derivatives into PHB will produce useful polymers for a wide range of
applications. The review presented here is on lignin, PHB and their polymer
blends.
1.2.3.1. Lignin
Introduction
Lignocellulosic materials refer to plants that are composed of cellulose,
hemicellulose and lignin. Sugarcane bagasse, which is comprised of
lignocellulosic compounds, is one of the most promising industrial residues
obtained from the sugar industries (Pandey, et al., 2000). The lignin extracted
from this source is used in the present research investigation.
The wall of a typical lignocellulosic cell is composed of several layers (Figure 1-
2), which are formed as new cells and created at the cambium layer. The middle
lamella is composed mainly of lignin, and serves as the glue bonding adjacent
cells together. The wall itself is made up of a primary wall and a three-layered
secondary wall, each of which has distinct alignments of microfibrils.
Microfibrils are rope like bundles of cellulose molecules, interspersed with and
surrounded by hemicellulose molecules and lignin (Smook, 1934).
10
Figure 1-2 Cell wall organisation of typical wood presented by Smook (1934)
Cellulose (Figure 1-3), which is a polysaccharide and is the main building
material of all plant cells including sugarcane, makes up about 50% of the dry
weight of bagasse (Doherty and Halley, 2004). Since bonding between and
within glucose molecules is so strong, cellulose molecules are very strong.
Lateral hydrogen bonding between cellulose chains is also quite strong, causing
them to group together to form strands that, in turn, form the thicker, rope like
structures called microfibrils (Milton, 1995).
Figure 1-3 The molecular repeating unit of cellulose
Hemicellulose, the second chemical component of bagasse, makes up 30% of its
dry weight (Glasser, et al., 1999). Unlike cellulose, which is made only from
11
glucose, hemicellulose consists of glucose and several other water-soluble
sugars, such as xylose and arabinose (Figure 1-4), produced during
photosynthesis. The degree of polymerisation (that is, the number of sugar
molecules connected together) is lower for hemicellulose than for cellulose and
branched chains rather than straight chains are formed. Hemicellulose surrounds
strands of cellulose and helps in the formation of microfibrils (Milton, 1995).
Figure 1-4 Structure of hemicellulose monomeric sugar units (a) xylose and (b) arabinose
Lignin is the second most abundant organic substance on earth after cellulose,
and plays several important roles in nature. The word lignin was introduced by
de Candolle in 1819 and is derived from the Latin word lignum, meaning wood
(Sjöström, 1993). Lignin stiffens the plant stem to withstand the forces of
gravity and wind, and makes the wood resistant to vermin. Although lignin
provides plants with a protective barrier against being attacked by
microorganisms, it also plays another important role, since it is recycled in the
natural ecology. When it degrades, it serves the soil as a complexing agent for
minerals and as a moisture-retention aid. Lignin also plays a role in the water
conducting system of plants by sealing the water conducting system against the
hydraulic pressure drop produced by the transport of water from the soil to the
leaves (Glasser, et al., 1999). Lignin makes up around 20% of the dry weight of
bagasse.
(a)
(b)
12
Extraction methods of lignin
For lignin to be used to make new products, it must be removed from the plant.
In addition to the diversity of repeat units and bonding patterns which
characterise natural lignin, is the chemical alteration introduced by each method
of removing lignin from the plant. The recovery process to extract lignin from
woody plants changes the chemical and functional group composition of lignin
(Lora and Glasser, 2002) and makes this material extremely heterogeneous.
Methods for recovering lignin are:
• Alkali (soda) process,
• Sulfite process,
• Kraft process,
• Ball milling,
• Enzymatic process,
• Acid digestion and
• Organosolv process.
Different types of lignin have been described depending on the means of
isolation. These include soda lignin, kraft lignin, organosolv lignin,
lignosulfonate, hydrolytic lignin and Klasson lignin. Ball milled lignin is the
best lignin sample among the many isolated lignins that can be used to study the
chemical structure and reactivity of native lignin. However, there have been no
quantitative relationships found between the structural changes in lignin and the
degree of milling. In this project, lignin will be extracted from bagasse using the
soda process, as this is the process of choice in the bagasse biorefinery project
undertaken at Queensland University of Technology, Brisbane, Australia for the
production of bioethanol. Soda lignin is easily recovered by lowering the pH,
filtering and drying. The purity of extracted lignin, as shown in Table 2-4, was
96.3 wt%. The lignin obtained is hydrophobic and contains no sulfur. Its
solubility properties are different from conventional lignosulfonates obtained
through sulfite pulping.
13
The majority of delignification lignin processes (apart from ball milling and
enzymatic process) involve either acid or alkali mechanisms. The
phenylpropane C9 units in lignin are joined by ether linkages, which readily
undergo both acid and base-induced hydrolysis under specific conditions. Side-
chains may be cleaved depending on the type of substructures, particularly under
alkaline conditions (Doherty and Halley, 2004). In the acid delignification
process α-aryl ether substructures are the most readily broken, but it is likely that
β-aryl ether bonds are also broken under strongly acidic conditions (Figure 1-5).
During delignification, components with the functionalisation of the carbonium
ion intermediates are reactions with aromatic structures (weak nucleophiles)
which form carbon-carbon inter-unit linkages and result in condensation
products. The frequency of such condensation reactions increases with the
acidity of the pulping liquor, and decreases with the concentration of the anion
(e.g., bisulfate anions) (Doherty and Halley, 2004).
Figure 1-5 Structure of the H-type monomer unit of lignin. Labelled are the α ,
β and γ positions of the aryl ether bonds
Lignin structure
Lignin is a large, cross-linked, macromolecule with molecular masses in excess
of 10,000 g mol-1. The degree of polymerisation of natural lignin is difficult to
measure, since it is fragmented during extraction, and since the molecule consists
of various types of substructures, which appear to be repeated in a haphazard
manner (Figure 1-6).
14
Figure 1-6 The structure of a possible lignin macromolecule (Glasser, et al., 1999)
There are three monolignol monomers, methoxylated to various degrees: p-
coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Quideau and Ralph,
1992) (Figure 1-7). These are incorporated into lignin in the form of the
phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringal (S)
respectively (Boerjan, et al., 2003).
(a) (b) (c)
Figure 1-7 The structure of the C9 monomer units of lignin. (a) p-coumaryl alcohol (4-hydroxyl phenyl, H), (b) coniferyl alcohol (guaiacyl, G), (c) sinapyl alcohol (syringyl, S).
OH
OH
OH
OH
OCH3 OH
OH
OCH3
OCH3
15
The polymerisation of lignin can produce a number of bond structures by the
delocalization of and reaction at the free radical sites. The lignin produced by
plants depends not only on the species of plant, but the part of the plant as well.
Therefore, lignin of varying composition exists within a single plant. This
means that the lignin recovered from a lignocellulosic plant will be a mixture of
structure and repeat unit composition that will vary with the source of the wood.
Each class of plants, grasses, softwoods, and hardwoods produces a lignin rich in
one or two types of C9 monolignol repeat unit (Doherty and Halley, 2004).
Hardwoods have a lignin that consists almost entirely of G and S type
monomers. Softwoods also have both G and S types, however the major
component is the S type (Boerjan, et al., 2003). The G predominates in grasses,
but also contains some H monomer units, which enables them to be more
flexible in making combinations with other groups.
Sugarcane bagasse lignin is a grass lignin and has a higher proportion of H
groups and hence a lower methoxyl content (i.e. more monomer units with
vacant ortho- and para-sites), than softwood and hardwood. Based on these
chemical structures, lignin is soluble in polar solvents and insoluble in
hydrocarbons, and hence forms immiscible multi-component systems with non-
polar compounds such as polyethylene (PE) and PP (Doherty and Halley, 2004).
The structural heterogeneity of lignin has also been studied by various methods
in a number of investigations. In several of those studies, lignin was subjected to
fractionation prior to analysis. Robert et al. (1984) fractionated kraft lignin by
successive acidification of kraft black liquor, while Moerck et al. (1986) used
organic solvent, Vanderlaan and Thring (1998) fractionated Alcell® lignin with
an organic solvent and Wallberg et al. (2003) used ultrafiltration. These
fractionations were analysed for functional groups, elemental composition and
molecular weight. The results of these investigations showed that the
fractionation process separated the lignin into distinct molecular weights and that
there were differences in the carboxylic acids, phenolic hydroxyl and methoxyl
contents. The properties of the materials produced were dependent on these
structural properties.
16
The Tg is influenced by such factors as the free volume between polymer chains;
the existence and abundance of attractive forces between molecules (which
obviously relates to solubility); the freedom of molecular side groups, branches
and segments to rotate around intermonomer bonds; chain stiffness; and chain
length. The Tg values of some different types of lignin are shown in Table 1-1.
Tab le 1 - 1 Tg va lues o f so me d i f fe ren t t ypes o f l i gn in (G la sse r, e t a l . , 1999)
Types of lignin Tg (ºC)
Lignin in Wood
- Hardwood
- Softwood
65-85
90-105
Milled wood lignin
- Softwood
- Hardwood
138-160
110-130
Periodate lignin 193
Kraft lignin 124-174
Organosolv lignin 91-97
Steam explosion lignin 113-139
1.2.3.2. Poly(hydroxybutyrate)
Introduction
PHB is a polyhydroxyalkanoate (PHA), which belongs to the group of
polyesters. It was first isolated and characterised in 1926 by the French
microbiologist Maurice Lemoigne (1926). PHB is produced by micro-organisms
(such as Alcaligenes eutrophus or Bacillus megaterium), apparently in response
to conditions of physiological stress. The polymer is primarily a product of
carbon assimilation (from glucose or starch) and is utilised by micro-organisms
as a form of energy storage molecule to be metabolized when other common
17
energy sources are not available. Three enzymes (and others) are needed for
production of the PHB polymer. These enzymes include the
3-ketothiolase (PHBA), acetoacetyl-CoA reductase (PHBB), and
PHB-producing bacteria require substrates such as ethanol, sucrose, or glucose,
which are costly. In bacteria, PHB is produced in a diluted aqueous solution.
Therefore, the recovery of PHB from diluted fermentation systems adds to the
cost of fermentation as a means of producing PHB. Recently, significant
attempts have been undertaken to produce PHB from plants (Sticklen, 2008).
Plants produce carbon sources via photosynthesis in concentrated products.
Therefore, the costs of production of PHB in plants may become lower than the
costs of its production in bacteria (Sticklen, 2008).
Production of PHB
The manufacturing process of PHB begins with sunlight (Figure 1-8). Through
photosynthesis, atmospheric carbon dioxide is converted to carbohydrates in
either sugar beets or sugarcane. These carbohydrates are the raw material for the
manufacture of PHB. PHB can be produced from glucose as a raw material, or
from agricultural wastes, such as molasses or material refined from the
processing of sugar beets and lactose, or from a wide variety of sources e.g.
volatile fatty acid fermentation products. The sugar is broken down during
metabolism into C2 building blocks, which are converted, over several steps, to
C4 monomers. Finally, the PHB is polymerised.
18
Figure 1-8 Flow scheme of (a) life cycle of PHB, and (b) PHB manufacturing process (Ghaffar, 2002)
The poly-3-hydroxybutyrate (PHB) form of poly(hydroxyalkanoate) (PHA) is
the polymer used in this project. PHB is probably the most common type of
PHA, but many isolation of this class are produced by a variety of organisms:
these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV),
polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO), and their
copolymers (Figure 1-9).
(a)
(b)
Sugar Beet Sugarcane
Pre-fermentation
19
Figure 1-9 Monomer units of PHB, PHV and their copolymer PHBV
In the future, research using genetic technology, among others, may prove
successful in producing a bacteria-based plastic that has more desirable
properties and is cheaper to produce than PHB. Also, PHB production may
become cheaper if researchers can find a way to make bacteria produce larger
amounts of polymer within shorter time spans or from waste materials using
cheaper production methods. If PHB becomes as cheap as plastics produced
from petrochemicals, then it will probably become widely used, since it has the
potential to be employed for packaging products such as bottles, bags, wrapping
film and disposable nappies (Sykes, 2001).
PHB is also being evaluated as a material for tissue engineering scaffolds and for
controlled drug-release carriers, owing to its biodegradability, limited
cytotoxicity, optical activity and isotacticity (Hasirci, 2003).
There are manyisolation processes that can be used to obtainPHB. Two typical
ones are:
• (1) The extraction method. Mechanical loads are used to destroy the cell
walls and then the polymer is dissolved in chloroform or another solvent
20
such as methyl chloride, 1,2-dichloroethane, pyridine or propylene
carbonate. The remains of the cell must then be separated by
centrifugation and filtration of the solvent.
• (2) Enzymatic method. Enzymes at 37°C destroy the cell wall. The
PHB is then isolated using the same method as that described in previous
section.
Physical and chemical properties of PHB
Some of the main PHB properties are listed below:
• Water insoluble and relatively resistant to hydrolytic degradation. This
differentiates PHB from most other currently available biodegradable
plastics, which are either water soluble or moisture sensitive.
• Good oxygen permeability.
• Good UV resistance but poor resistance to acids and bases.
• Soluble in chloroform and other chlorinated hydrocarbons.
• Biocompatible and hence suitable for medical applications.
• Melting point = 175-177ºC, and Tg = 4ºC.
• Tensile strength of 40 MPa, which is close to that of PP.
• Sinks in water (while PP floats), facilitating its anaerobic biodegradation
in sediments.
• Non-toxic.
Chemistry behind the brittleness of PHB
Crystal structure and crystallisation conditions are responsible for some of the
properties of many PHB products. A sound knowledge and understanding of
crystallisation mechanisms is necessary for designing materials with better
mechanical properties. In practice, crystals formed by polymer molecules are
imperfect. The crystallinity of most melt-crystallised polymers lies in the range
of 30% - 70%. The lamellae thickness can be measured by small angle X-ray
diffraction and directly by electron microscopy. PHB is stiff and brittle with its
brittleness dependent on its Tg, degree of crystallinity and on its microstructure.
21
PHB poses a low nucleation density (Mahendrasingam, et al., 1995, Withey and
Hay, 1999) resulting in the formation of large spherulites. Spherulites contain
crazes, and splitting occurs around the centre of these crazes, hence producing a
significant structural weak point (Barham and Keller, 1986). If PHB is annealed
at high temperatures, stress and brittleness also increases. Another factor which
contributes to the brittle nature of PHB is the fact that it undergoes secondary
crystallisation at room temperature. This process involves the conversion of
amorphous to crystalline material over time, and occurs in PHB because its Tg of
approximately 4°C is close to that of ambient temperature.
22
1.2.3.3. Lignin blends
Introduction
Polymer blending, which involves the mixing of two or more polymeric
components, has been shown to provide the ability to control or tailor properties
to specific desired goals. In many instances, polymer blending results in the
formation of high performance composite materials, this being a consequence of
synergistic interactions. However, many polymer combinations are not miscible
and exist in two different phases in the polymer matrix. The separation into
phases in the polymer matrix results in high interfacial tension and poor
polymer-polymer interactions. This results in materials with poor mechanical
properties, due to poor stress transfer between the phases.
Feldman (2002) reviewed lignin and its polyblends. The review includes phenol
formaldehyde resin-lignin adhesives, epoxy-lignin adhesives, other adhesives
and sealants with lignin, polyolefin lignin blends, polyvinyl chloride/lignin
blends and rubber/lignin blends. Some of these blends are presented here
together with PHB blends and lignin/PHB blends. Since lignin possesses
attractive properties it has been considered by many researchers to provide
compatibility for different polymer types.
Study on miscibility of lignin blends
A very interesting study reported by Pouteau et al. (2004) investigated the
compatibility of lignin-polymer blends by image analysis using visible
spectroscopy. The study looked at the development of lignin-based blends
lignin. It investigated semi-polar polymers (e.g. lignins), very polar polymers
(e.g. starch) and apolar polymers (e.g. PP). The morphology of the blends
obtained from semi polar polymers was very sensitive to the variation of the
solubility parameters. Over a low range of polymer solubility parameters, both
heterogeneous and homogeneous systems were obtained. The properties of the
blends were improved by a careful choice of polymer type. Furthermore, it was
also considered possible to take advantage of lignin variability to improve the
compatibility of the blend. Only low molecular weight lignins were compatible
with a polar and very polar matrix.
23
Kadla et al. (2004) and Kubo et al. (2002) studied the intermolecular interactions
between lignin and synthetic polymers. Their investigations revealed the
immiscible nature of lignin in polyvinyl alcohol (PVA) and PP. However, the
study demonstrated the misciblity behaviour in poly(ethylene oxide) (PEO) and
polyethylene terephthalate (PET), in which the Tg showed a negative deviation
from the linear mixing rule which indicated specific intermolecular interactions.
Furthermore, from these studies Fourier transformed infra red (FT-IR) analysis
revealed strong intermolecular hydrogen bonding between lignin and PET.
Further work by Kadla et al. (2003) investigated the miscibility behaviour over
the entire blend ratio of lignin with PEO. The results of the FT-IR analyses
revealed a strong hydrogen bonding between the aromatic hydroxyl proton of
lignin and the ether oxygen in PEO.
Tinnemans et al. (1984) investigated the mechanical properties of water-
swellable lignin blends. They specifically worked on acylated kraft lignins with
maleic anhydride-styrene copolymers. The resulting blends exhibited a good
tensile strength and demonstrated a high strain at break, owing to favourable
miscibility of the components.
Lignin/PE and lignin/PP blends
Previous attempts at blending PE with lignin in concentrations > 20 wt% yielded
blends with relatively poor mechanical properties. A new method, based on
blending PE with ethylene-vinylacetate (EVA) copolymer has been developed by
Pavol et al. (2004). On the basis of their study, Pavol and co-workers (2004)
found that the addition of 10 wt% EVA caused 200 wt% increase in tensile
strength, and a 1300 wt% increase in elongation at break, compared to those of
the corresponding unmodified samples. Moreover, a composite material
prepared containing 33.6 wt% lignin displayed acceptable processing and
mechanical properties, and was used successfully in preparing blown-films.
Alexy et al. (2000) used lignin as a natural filler in a low-density PE and PP at
concentrations up to 30 wt%. Their study described the influence of lignin
blending on processing stability, mechanical properties and light and long-term
heat degradation, for both polymer blend types. They also showed that different
degradation behaviors between PE-lignin and PP-lignin blends existed. It was
24
determined that lignin concentration influenced both tensile strength and melt
flow index.
The influence of lignin on the oxidative stability of PP and recycled PP has been
examined by Gregorova et al. (2005) using DSC under non-isothermal
conditions. The results showed that lignin exerts a stabilising effect in both
virgin and recycled PP. The protection factor increases with lignin content in the
PP matrix. Moreover, for the evaluation of heat resistance, the influence of the
lignin content on Vicat softening temperature (VST) was determined. VST
showed that the presence of lignin improves the heat resistance of PP and
recycled PP plaques.
The orientation and property correlations of biaxially oriented PE blown films
have been studied by Chen et al. (2006). Correlations between orientation in
both the machine and transverse directions were found with dart impact and
Elmendorf tear strength. These correlations were linked to underlying
morphology and micro-deformation mechanisms.
Košíková et al. (1993) investigated sulfur-free lignins as composites of PP films.
The results showed that PP films containing 2 wt% - 10 wt% spruce organosolv
lignin and/or beech wood prehydrolysis lignin, had good compatibility and
sufficient tensile strength. Also, the physicochemical properties of the lignin-
containing films indicated compatibility between lignin and PP, and
demonstrated that the film acted as a good UV absorber.
Methods for preparing PE blends with organosolv lignin and methods of making
them has been patented by Bono et al. (1995). Another earlier patent by Bono et
al. (1994) involved producing degradable plastic films with ethylene copolymers
and lignin. The lignin was incorporated in the form of very fine powder with a
grain diameter of about 1 µm - 5 µm. The films were homogeneous and
possessed a thickness of about 15 µm - 25 µm. Improved degradation was
achieved with photoactive and oxidizing agents.
Košíková et al. (2001) have reported on the ability of lignin-degrading
microorganisms, phanerochaete chrysosporium, to attack PE in lignin/PE blends.
The isolation of the oligomer fraction from biodegraded polymer blends
25
indicated that the biotransformation of lignin during the cultivation process was
accompanied by the degradation of the PE matrix.
Lignin-polyurethane blends
The morphology of lignin-polyurethane blends has been studied by Feldman et
al. (1989). In this study, although SEM revealed an even distribution of lignin
particles in the polyurethane matrix, it clearly showed the different morphologies
of the constituent phases. The results were confirmed by DSC analysis which
showed immiscibility.
Ciobanu et al. (2004) studied a polyurethane elastomer blended with flax soda
lignin to form dimethylformamide-cast films containing between 4.2 wt% and
23.2 wt% of lignin. The spectral, mechanical and thermal properties of this new
type of blend were investigated in an attempt to establish their potential
applications. Based on that investigation, films containing more than 9.3 wt%
lignin were found to be heterogeneous. The thermal degradation range of
polyurethane and the blends were quite similar. However the presence of lignin
accelerated decomposition at lower temperatures. The tensile strength increased
by up to 370 %, toughness up to 470 % and the elongation at break up to 160 %
for the blends compared to the pure polyurethane film.
Lignin-epoxy blends
Feldman et al. (1991a, 1991b) studied a bisphenol A-polyamine hardener-based
epoxy adhesive modified by kraft lignin. They investigated the curing of these
blends with up to 40 wt% kraft lignin. The curing process was performed either
at room temperature or above the Tg of the components. However, the result was
an enhanced degree of bonding between components, and the reason for the
improvement was thought to be an association between lignin and the unreacted
amine groups of the hardener. In another study, Feldman et al. (1988) observed
that epoxy blends with 10 wt% and 20 wt% lignin improved the adhesion tensile
strength of the epoxy polymer system. However, blending with 5 wt% and 20
wt% lignin had little effect on the adhesive shear strength (by tension loading),
or on the weatherability of the epoxy system. However, after a post curing
process (4 h at 75ºC), a significant improvement of the adhesive strength in shear
of the epoxy-lignin blends was detected.
26
Lignin-based carbon fibres
One of the most interesting applications of lignin is to use it to make carbon-
fibres because of its low cost, high volume and ability to produce fibres, through
melt-spinning. Griffith et al. (2003) studied the use of high-lignin content blends
which could be melt-spun to produce small rows of 10 m - 20 m non-sticking,
drawable filaments. The study was successful and commercial carbon fibres can
now be produced with kraft lignin.
Kadla et al. (2002) reported producing a fusible lignin with excellent spinnability
to form a fine filament following thermal pretreatment under vacuum. Blending
kraft lignin with PEO further facilitated fibre spinning, but at PEO levels
>5 wt%, the blends could not be stabilised without the individual fibres fusing
together. The carbon fibres produced had an overall yield of 45 wt%. The
tensile strength and modulus increased with decreasing fibre diameter, and were
comparable to those of the much smaller diameter carbon fibres produced from
phenolated exploded lignins. In view of its mechanical properties, the tensile
strength of 400 MPa - 550 MPa and the elastic modulus of 30 GPa - 60 GPa,
kraft lignin should be further investigated as a precursor for general grade carbon
fibres.
Effect of UV irradiation on the thermal stability of lignin blends
On the basis of the study by Bittencourt et al. (2005), different films containing
two types of extracted lignin (i.e. kraft lignin and the acetone solvent fraction of
kraft lignin) with different proportions of polyvinyl alcohol (PVA) were
prepared via solvent-casting. The films, with concentrations up to 25 wt%
lignin, were irradiated with UV light for different time intervals. The results of
this analysis indicated better thermal stability and miscibility for the films
prepared with lignin extracted with acetone. This shows that the composition of
the functional group of lignin has a strong bearing on its miscibility behaviour.
A thermal and FT-IR study of polyvinylpyrrolidone (PVP) and bagasse lignin
blends has been undertaken by Silva et al. (2005). The bagasse lignin was
extracted with formic acid and the blends were cast with dimethyl sulfoxide and
formic acid. Blends were also irradiated with UV light. The results showed
miscibility in PVP-lignin blends with 5 wt% lignin content cast from dimethyl
27
sulfoxide, and miscibility in blends containing 5 wt% and 10 wt% lignin cast
from formic acid. Irradiation with UV light resulted in improved thermal
stability.
1.2.3.4. PHB blends
Most studies reported in the literature on PHB blends deal with miscibility,
thermal and mechanical properties. Little has been reported on their
processability and on their rheological properties. This project will investigate
the processability of lignin/PHB blends by studying detailed thermal events,
viscoelastic behavior and storage and loss modulus.
Avella et al. (2000) in a comprehensive review summarizes the properties of
blends of PHB and poly(hydroxybutyrate-hydroxyvalerate) (PHBV). The
mechanical, morphological, and miscibility properties of blends with polyesters,
polyethers, polyvinylacrylates and polysaccharides were studied, as well as the
biodegradation of the blends. The results from the study showed that the
microstructure of the blends controlled the mechanical and biodegradation
behavior of the blends.
Antunes and Felisberti (2005) studied blends of PHB and poly(ε-caprolactone)
(PCL), which is a semi-crystalline polymer that is used as a biomaterial. PHB
and PCL were blended by melting mixtures in an internal mixer. The blends
compositions varied from 0 wt% to 30 wt% PCL. DMTA, DSC and SEM were
used to characterise the blends. The blends were found to be immiscible with no
indication of interaction either in the amorphous or crystalline state. The
morphology of the blends revealed PHB as the matrix and PCL as the dispersed
phase.
El-Taweel et al. (2004) studied the stress-strain behaviour of blends of PHB (of
molar mass 30,000 g mol-1), with different miscible amorphous polymers (of
molar mass 600 g mol-1 to 200,000 g mol-1). They found that a high extension
ratio was obtained only if the PHB content was less than 60 wt%.
Liu et al. (2004) studied the crystallisation of poly(vinylidene fluoride) (PVDF)
and PHB blends using DSC. They found that solid PVDF possibly acts
heterogeneously, nucleating and accelerating PHB crystallisation.
28
An investigation of PHB blends containing starch or starch derivatives has been
reported by Innocentini-Mei et al. (2003). Their work showed a significant
decrease of both the Tg and the melting point (Tm) for all formulations. Best
results in terms of modulus and Tg were obtained with grafted starch-urethane
blends.
The melting and crystallisation behaviour and phase morphology of PHB blends
with poly(DL-lactide)-co-poly(ethylene glycol) (PELA) have been studied by
Zhang et al. (1997). Compared to pure PHB, the cold crystallisation peak
temperatures (Tcc) of PHB blends shifted to higher temperatures. The growth of
spherulites of PHB in the blends was affected significantly by a 60 wt% PELA
content. Similar results were also obtained by Deng et al. (1993).
An investigation on the thermal properties of PHB blends with cellulose esters
containing acetate, propionate, or butyrate substituents has been reported by
Scandola et al. (1993). They observed a good PHB miscibility with the cellulose
esters. The morphology of blends of PHB with cellulose acetate butyrate (CAB)
by compression molding followed by different thermal treatments has been
carried by Tomasi et al. (1995) The results also showed good miscibility
between CAB and PHB.
Yoshie et al. (1995) using high-resolution solid-state carbon-13 nuclear magnetic
resorance (13C-NMR) and proton (1H NMR) spectroscopy observed hydrogen-
bonding interaction in the amorphous phases of PHB and PVA blends. The DSC
measurement confirmed the compatibility of the blends and showed that the
blends have lower crystallinity than the individual polymers.
1.2.3.5 Studies on the biodegradation of PHB blends
Biodegradation of polymer blends is determined both by the degradability of
blend components themselves and by the blend composition. Ikejima et al.
(1999) studied the environmental biodegradability and crystallisation behaviour
of blend films of PHB with chitin and chitosan. The crystallisation behaviour
was similar between blends and with PHB alone. However, several of the blends
showed faster biodegradation than either of the polymer components.
Zhao et al. (2005) studied the effect of aging on the fractional crystallisation of
PEO component in the PEO-PHB blends. Their investigation confirmed that
29
nearly all the PEO component that had remained trapped within the interlamellar
regions of PHB affected aging.
Nagahama et al. (2005) wrote a review on manufactured biodegradable plastics
through forming PVA-PHB blends, fibre-polymer composites and aliphatic
polyester blends. Their work showed effective biodegradation in the composites
made with PHB. Teryshnaya and Shibryaeva (2006) also studied oxidative
degradation of biomicrobial PHB-low density PE and ethylene-PP rubber-PHB
blends. Their results showed improved degradation of the olefins components.
Kikkawa et al. (2006) investigated the enzymatic hydrolysis of poly(L-lactide)
and atactic PHB blends showed that either poly(L-lactide) or atactic PHB
domains were attacked depending on the kind of enzyme used. The larger
number of enzyme molecules was found on poly(L-lactide) domains suggesting
a higher affinity of the enzyme for poly(L-lactide).
Gonvaleves et al. (2009) investigated the biodegradation of PHBV, PP and their
blends in soil. They found the PHBV degraded faster than PP, and that in the
blends, PP only showed changes in the amorphous region.
The type of environment in which the biodegradation is performed has a
significant effect on the rate of degradation. El-Hadi et al. (2002) found that for
blends of PHB and nucleating agents (e.g. tributyrin), aerobic biodegradation
was easier in river water and compost, than in the soil. Imam et al. (1998)
reported that in a natural composite environment, the weight loss correlated with
the amount of starch present in the blends. Imam et al. (1998) also found that
there was no significant difference in molecular weight decrease between neat
PHBV compared to PHBV/starch blends. Imam et al. (1995), on the other hand,
reported that in an activated sludge environment, the rate of weight loss were
quite similar with neat PHBV and PHBV/starch blends.
Recently, Woolnough et al. (2010) studied the biodegradation of PHB and some
other “green plastics” in mature soil detecting mass loss, topographical changes
and biofilm attachments and found that PHB itself has a better degradability
among polyhydroxyoctanoate and poly(DL-lactide) and polystyrene and ethyl
cellulose.
30
1.2.3.6. Lignin/PHB blends
Based on the published information available, there are five articles specifically
relating to lignin and PHB blends. (Camargo, et al., 2002, Ghosh, et al., 2000,
Mihaela, et al., 2010, Naegele, et al., 2000, Weihua, et al., 2004).
Ghosh et al. (2000) investigated the thermoplastic blends of several
biodegradable polymers with lignin and lignin esters, based on both solvent
casting and melt processing. The biodegradable polymer they used contained
cellulose acetate butyrate (CAB), a starch-caprolactone copolymer blend and
PHB. In addition to organosolv lignin, they investigated organosolvo lignin
esters of acetate, butyrate, hexanoate and laurate. They detected a high level of
compatibility between blends of lignin acetate, lignin butyrate and CAB. They
observed a significant amount of retarded crystallisation of PHB with the
addition of lignin, which result in lower melting points of the blends. The
addition of lignin also increased the modulus of the blends significantly at room
temperature, probably because it increased the crystallinity of PHB.
Weihua et al. (2004) investigated the effect of lignin fine powder on the
nucleation of PHB by studying the kinetics under both isothermal and
nonisothermal crystallisation processes. The DSC results showed that lignin not
only acted as a nucleating agent and decreased the activation energy of the
crystallisation process, but it also increased the number of the spherulites formed
(Figures 1-10 and 1-11). However the size of spherulites had decreased.
31
Figure 1-10 DSC cooling and heating curves of pure PHB and PHB/lignin blend samples showing the melt nonisothermal and cold crystallisation temperature, Tmc and Tcc: (A) cooling, and (B) heating (Weihua, et al., 2004)
32
Figure 1-11 Spherulitic growth rate at various crystallisation temperatures for both a pure PHB and a PHB/lignin blend (Weihua, et al., 2004)
Understanding the mechanical and rheological properties of polymer blends is
necessary for understanding changes in the viscoelastic response and
processability conditions.
None of the studies reported to date on lignin/PHB blends have examined in
detail the macroscopic and microscopic associations between lignin and PHB
that would help explain observed thermal, rheological and biodegradation
properties of blends.
33
1.3. Account of Research Progress L inking the
Research Papers This project commenced with a comprehensive literature review of lignin
chemistry, properties and applications. It was evident from the review that
limited work has been carried out with lignin/PHB blends and no biodegradation
evaluation studies for these blends. The following sections link the various
research papers covering the research program.
1.3.1. Chemical and thermal properties of soda lignin
The first step in making lignocellulosics (such as bagasse) amenable to
enzymatic hydrolysis for the production of sugars and subsequently ethanol is to
pretreat it either by mechanical or chemical means. Sodium hydroxide is one of
the pretreatment options used to fractionate lignocellulosics. The advantage of
using sodium hydroxide is that the lignin component of lignocellulosics can
readily be recovered. The lignin recovered by this process has high ash content
and hence is of low purity (Lora and Glasser, 2002). A purification step is
necessary if the soda lignin is to be used in chemical reactions, such as in resin
synthesis. In the present study, a two-stage process was developed which
improved the purity of soda lignin derived from bagasse. Soda lignin produced
by this process was characterised by physical, thermal and chemical means. The
soda lignin was fractionated into three parts using two solvents, diethyl ether and
methanol, which have different polarities. Figure 1-12 shows the flow diagram
for the fractionation process.
Figure 1-12 Fractionation process of soda lignin.
34
Based on this process, only a very small portion of the lignin sample, ~8 wt%,
was recovered using diethyl ether (EL). The major proportion, ~ 68 wt%, was
methanol soluble (ML), and the residue (RL) makes up the remaining 24 wt%.
The results clearly demonstrated the heterogeneity of soda lignin. The two-stage
lignin precipitation process, and the chemical and thermal properties of soda
lignin and its fraction was published in Industrial Crops and Products
(Mousavioun and Doherty, 2010) titled: “Chemical and thermal properties of
fractionated bagasse soda lignin”.
This study on lignin chemistry showed that the lignin with the highest proportion
of phenolic hydroxyl functional group (i.e. ether-soluble lignin) has the highest
potential to interact with PHB to form miscible blends. However, the proportion
of this lignin type in soda lignin was too small to merit investigation.
1.3.2. Addendum: Kinetics of bagasse decomposition, Lignin applications
During the course of the study on the thermal properties of lignin and PHB, it
was established that to obtain optimised conditions for production of lignin/PHB
blends with minimum PHB degradation, the kinetics of PHB degradation should
be studied. Both isothermal and non-isothermal conditions were used in the
study. Similar Ea values for the decomposition process were obtained using both
approaches. As the non-isothermal approach is rapid, it was decided, as an add-
on to the project, to investigate the kinetics of the thermal degradation of bagasse
from which lignin originates. The results of this work were published in
Industrial Engineering & Chemistry Research (Maliger et al., 2011). The title of
the article is, “Thermal decomposition of bagasse. Effect of different sugarcane
cultivars”. The article is presented in Appendix 1. The contribution by the
author of this thesis for this piece of work was 30 wt%.
Having reviewed hundreds of articles on lignin chemistry, properties and uses, a
review paper was deemed necessary to illustrate the potential of lignin-based
polymers for improving the economics of producing cellulosic ethanol from
lignocellulosics. This review paper was published in Industrial Crops and
Products (Doherty, et al., 2011), with the title: “Value-adding to cellulosic
ethanol: Lignin polymers”. The paper is in Appendix 2. The author of this
thesis contributed 20 wt% towards writing the paper.
35
1.3.3. Thermal stability and miscibility of PHB and soda lignin blends
Literature review showed that there were five articles that specifically relate to
lignin and PHB blends (Camargo, et al., 2002, Ghosh, et al., 2000, Mihaela, et
al., 2010, Naegele, et al., 2000, Weihua, et al., 2004). None of these studies
examined the association and interactions between the functional groups of
lignin and those of PHB, which may lead to a better understanding of observed
thermal stability and miscibility properties of lignin/PHB blends.
As mentioned in section 1.3.2, there was concern about the thermal stability of
PHB during processing. So in the 2nd phase of the project, isothermal
degradation tests with PHB were performed at temperatures from 165°C to
190°C. Results showed 175°C was the optimum processing temperature that
will result in minimum PHB degradation.
To produce blends, lignin and PHB were dried at 100°C for 12 h and then stored
in desiccators under vacuum prior to use. Lignin/PHB blends with lignin
contents from 10 wt% to 90 wt% were mixed in a Haake mini lab twin screw
extruder (Figure 1-13) using the procedure reported by Ghaffar (2002). To
minimise PHB degradation, the temperature of the extruder was maintained at
175°C for 2 min. The polymer blends were extruded as strands then cooled and
pelletised. The pellets were stored in a desiccator to avoid moisture absorption,
prior to use.
Figure 1-13 Haake mini lab twin extruder
Feeder
Screw
Heater
Controller
36
In this study, the thermal properties and miscibilities of PHB and soda lignin
blends were investigated by TGA, DSC, SEM and FT-IR over the entire range of
composition. The most important outcome of the study was that lignin reduced
the initial temperature of decomposition of PHB, but stabilised PHB over a wider
temperature range at higher temperatures. This may be because the carbohydrate
components in lignin start to decompose at an earlier temperature than PHB. A
single Tg, which depicts miscibility, was obtained for blends containing up to 40
wt% lignin. The Tg results correlated well with the SEM and FT-IR data. The
FT-IR data showed that the miscibility of the blends is probably associated with
specific hydrogen bonding interactions between the reactive functional groups in
lignin and the carbonyl groups of PHB. This result showed the anticipation of
improvement in properties of PHB by lignin (outcome of phase 1 of the project)
was valid. Results of this work were published in a paper in Industrial Crops and
Products (Mousavioun, et al., 2010), titled “Thermal stability and miscibility of
poly(hydroxybutyrate) and soda lignin blends”.
At this stage of the project it was concluded that lignin, to a certain extent,
improves the thermal properties of PHB. However, the key question still
remained of whether lignin could enhance the rheological properties of PHB and
hence its processibility.
1.3.4. Combination of thermal stability and rheological properties
A comprehension of the rheological properties of polymer blends is required to
determine changes in the viscoelastic responses and determine blend
compositions suitable for easy processing. The miscibility between components
is a significant parameter that dictates viscoelastic responses. Thus, the next
phase of the project was to study the effect of lignin loading on the viscosity of
PHB and its thermophysical properties. The results have been submitted to
Polymer International, titled “Thermophysical properties and rheology of
PHB/lignin blends”. The conclusion drawn from this work is that lignin not only
affects the thermal stability of PHB (based on Ea values, confirming the mass
loss data to some degree), it also affects PHB crystallisation. The rheological
study showed that lignin contents of 10 wt% and 30 wt% plasticise PHB,
resulting in blends having lower viscosities than PHB alone. For blends
37
containing 60 wt% and 90 wt% lignin respectively, lignin acts as a filler, and
blends have viscosities higher than PHB.
1.3.5. Environmental degradation of lignin/PHB blends
In this phase of study, for various compositions of lignin/PHB blends, four
samples were prepared and buried in the soil. The standard burial test method
was used (Woolnough, et al., 2010). Samples were removed every 4 months for
analysis during the 12 months of the trials. The samples were analyzed before
and after exposure, using gravimetric analysis, TGA, DSC, optical microscopy,
SEM, X-ray Photoelectron Spectroscopy (XPS) and FT-IR. The gravimetric
analysis results showed that lignin significantly protects PHB against
degradation, while the DSC results showed that hydrogen bonding of lignin with
PHB plays a significant role to protect PHB against degradation. XPS data
revealed an accumulation of biofilms on the surface of buried film samples.
XPS and FT-IR confirmed that PHB is the most susceptible component against
degradation. FT-IR analysis showed that low lignin contents (<30 wt%)
accelerate PHB degradation, while high lignin contents retard the process. The
results have been explained using the miscibility concept. Results of this work
have been submitted to Polymer Degradation and Stability, titled
“Environmental degradation of lignin/PHB blends”.
1.3.6. Methanol-soluble lignin/PHB blends
This phase of the project examined the impact the composition of the functional
groups of lignin influenced the properties of lignin/PHB blends. Soda lignin
contains 3× the proportion of xylan and phenolic hydroxyl group than ML. It
however, has 1.5× less carboxylic acid groups. ML/PHB blends with lignin
contents from 10 wt% to 90 wt% were assessed in a similar fashion as soda
lignin/PHB blends. The result of the study was presented and published as a full
paper in the Proceedings (CD-ROM) at the 10th AIChE Annual meeting, Salt
Lake City, UT, USA. The title of the paper is: “Thermal stability and miscibility
of poly(hydroxybutyrate) and methanol-soluble soda lignin blends”.
38
The T0 values of ML/PHB blends were higher than the T0 values of soda
lignin/PHB blends. This may be because of the proportion of xylan in the
composite. Xylans are known to decompose at lower temperatures than
cellulose and lignin.
Tg results of ML/PHB blends indicated that blends containing up to 40 wt% ML
are miscible with PHB. The similar results were obtained for soda lignin/PHB
blends (section 1.3.3). FT-IR spectra showed that for blends up to 50 wt% ML,
there was a small but definitive shift to lower wavenumbers, indicating hydrogen
bonding interactions. The similarities between the results and those of
lignin/PHB blends indicated that the differences observed in the composition of
lignin functional groups were not significant to influence the glass transition
temperature of lignin/PHB blends derived from the two lignin types.
39
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Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001
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Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception,
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2. they take public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;
3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the
editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and
5. they agree to the use of the publication in the student’s thesis and its publication on
the Australasian Digital Thesis database consistent with any limitations set by publisher requirements.
In the case of this chapter: Publication title and date of publication or status: “Thermal stability and miscibility of poly(hydroxybutyrate) and soda lignin blends”, Industrial Crops and Products, Vol 32, 656-661, 2010
Contributor Statement of contribution* Payam
Mousavioun Experimental design, Conducted experiments, Data analysis. Signature
Date William O.S.
Doherty Wrote the manuscript, Data analysis.
Graeme A. George Data analysis.
Principal Supervisor Confirmation I have sighted email or other correspondence from all Co-authors confirming their certifying authorship. _______________________ ____________________ ______________________ Name Signature Date
Antunes, M.C.M., Felisberti, M.I., 2005. Blends of poly(hydroxybutyrate) and poly(ecaprolactone) obtained from melting mixture. Polym. Sci. Technol. 15, 134–138.
Barham, P.J., Keller, A., 1986. The relationship between microstructure and mode of fracture in polyhydroxybutyrate. J. Polym. Sci. Part B: Polym. Phys. 24, 69–77.
Barsbay, M., Güner, A., 2007. Miscibility of dextran and poly(ethylene glycol) in solid state: Effect of the solvent choice. Carbohyd. Polym. 69, 214–223.
Chiu, H.J., Chen, H.L., Lin, J.S., 2001. Crystallization induced microstructure of crystalline/crystalline poly(vinylidenefluoride)/poly(3-hydroxybutyrate) blends probed by small angle X-ray scattering. Polymer 42, 5749–5754.
Dence, C.W., 1992. Determination of carboxyl groups by non-aqueous potentiometric titration. In: Lin, S.Y., Dence, C.W. (Eds.), Methods in Lignin Chemistry. Springer, Berlin, Heidelberg, pp. 458–464.
Dong, J., Ozaki, Y., 1997. FT-IR and FT-Raman studies of partially miscible poly(methyl methacrylate)/poly(4-vinylphenol) blends in solid states. Macromolecules 30, 286–292.
ElMiloudi, K., Djadoun, S., Sbirrazzuoli, N., Geribaldi, S., 2009. Miscibility and phase behaviour of binary and ternary homoblends of poly(styrene-coacrylic acid), poly(styrene-co-N,N-dimethylacrylamide) and poly(styrene-co-4- vinylpyridine). Thermochim. Acta 483, 49–54.
Fox, T.G., 1956. Influence of diluent and of copolymer composition on the glass temperature of a polymer system. Bull. Am. Phys. Soc. 2, 123.
Ghaffar, A.M.E.A., 2002. Development of a biodegradable material based on Poly(3- hydroxybutyrate) PHB. Ph.D. Thesis, Martin-Luther University, Wittenberg, Germany.
Ghosh, I., 1998. Blends of biodegradable thermoplastics with lignin esters. M.Sc. Thesis, Virginia Polytechnic Institute and State University, VA, USA.
Ghosh, I., Jain, R.K., Glasser, W.G., 1999. Multiphase materials with lignin. XV. Blends of cellulose acetate butyrate with lignin esters. J. Appl. Polym. Sci. 74, 448–457.
Ghosh, I., Jain, R.K., Glasser, W.G., 2000a. Blends of biodegradable thermoplastics with lignin esters. In: Glasser, W.G., Northey, R.A., Schultz, T.P. (Eds.), Lignin: Historical, Biological, and Materials Perspectives. American Chemical Society, Washington, DC, pp. 331–350.
Ghosh, I., Jain, R.K., Glasser, W.G., 2000b. Multiphase materials with lignin. Part 16. Blends of biodegradable thermoplastics with lignin esters. ACS Symp. Ser. 742, 331–350.
85
Gosselink, R.J.A., Abächerli, A., Semke, H., Malherbe, R., Käuper, P., Nadif, A., van Dam, J.E.G., 2004. Analytical protocols for characterisation of sulphur-free lignin. Ind. Crops Prod. 19, 271–281.
Guo, L., Sato, H., Hashimoto, T., Ozaki, Y., 2010. FT-IR study on hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) and pol(4-vinylphenol). Macromolecules 43, 3897–3907.
Kuo, S.W., Chang, F.C., 2001. Effects of copolymer composition and free volume change on the miscibility of poly(styrene-co-vinylphenol) with poly(ɛ- caprolactone). Macromolecules 34, 7737–7743.
Kuo, S.W., Chan, S.C., Chang, F.C., 2002. Miscibility enhancement on the immiscible binary blend of poly(vinyl acetate) and poly(vinyl pyrrolidone) with bisphenol A. Polymer 43, 3653–3660.
Lizymol, P.P., Thomas, S., 1993. Thermal behaviour of polymer blends: a comparison of the thermal properties of miscible and immiscible systems. Polym. Degrad. Stab. 41, 59–64.
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Mousavioun, P., Doherty, W.O.S., 2010. Chemical and thermal properties of fractionated bagasse soda lignin. Ind. Crops Prod. 31, 52–58.
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Viswanathan, S., Dadmun, M.D., 2002. Guidelines to creating a true molecular composite: inducing miscibility in blends by optimizing intermolecular hydrogen bonding. Macromolecules 35, 5049–5060.
Yong, H., Naoki, A., Yoshio, I., 2001. Blend of poly(ɛ-caprolactone) and 4,4’-thiodiphenol: hydrogen bond formation and some solid properties. Macromolecules Chem. Phys. 202, 1035–1043.
Yoshie, N., Azuma, Y., Sakurai, M., Inoue, Y., 1995. Crystallization and compatibility of poly(vinyl alcohol)/poly(3-hydroxybutyrate) blends:
86
influence of blend composition and tacticity of poly(vinyl alcohol). J. Appl. Polym. Sci. 56, 17–24.
Zheng, S., Mi, Y., 2003. Miscibility and intermolecular specific interactions in blends of poly(hydroxyether of bisphenol A) and poly(4-vinyl pyridine). Polymer 44, 1067–1074.
87
CHAPTER 4
Thermophysical properties and
rheology of PHB/lignin blends
Payam Mousaviouna, Peter Halleyb and William O.S. Dohertya a Sugar Research and Innovation, Centre for Tropical Crops and
Biocommodities, Queensland University of Technology, GPO Box 2434,
Brisbane, Australia. b Centre High Performance Polymers (CHPP), School of Chemical Engineering
and AIBN, St Lucia, The University of Queensland, QLD 4072, Brisbane,
Australia
Submitted to the Polymer International, 2011
Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001
Suggested Statement of Contribution of Co-Authors for
Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception,
execution, or interpretation, of at least that part of the publication in their field of expertise;
2. they take public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;
3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the
editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and
5. they agree to the use of the publication in the student’s thesis and its publication on
the Australasian Digital Thesis database consistent with any limitations set by publisher requirements.
In the case of this chapter: Publication title and date of publication or status: “Value-adding to cellulosic ethanol: Lignin polymers”, published in Industrial Crops and Products, Vol 33, 259-276, 2011.
Contributor Statement of contribution* Payam
Mousavioun Collating of literature. Signature
Date William
O.S.Doherty Wrote the manuscript.
Christopher M.Fellows Edited and wrote some sections of the manuscript.
Principal Supervisor Confirmation I have sighted email or other correspondence from all Co-authors confirming their certifying authorship. _______________________ ____________________ ______________________ Name Signature Date
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Antunes, M.C.M., Felisberti, M.I., 2005. Blends of poly(hydroxybutyrate) and poly(ε-caprolactone) obtained from melting mixture. Polym. Sci. Technol. 15, 134-138.
Aoyagi, Y., Yamashita, K., Doi, Y., 2002. Thermal degradation of poly[(R)-3-hydroxybutyrate], poly[ε-caprolactone], and poly[(S)-lactide]. Polym. Degrad. Stab. 76, 53-59.
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Chiu, H.J., Chen, H.L., Lin, J.S., 2001. Crystallization induced microstructure of crystalline/crystalline poly(vinylidenefluoride)/poly(3-hydroxybutyrate) blends probed by small angle X-ray scattering. Polymer 42, 5749-5754.
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Pizzoli, M., Scandola, M., Ceccorulli, G., 1994. Crystallization kinetics and morphology of poly(3-hydroxybutyrate)/cellulose ester blends. Macromolecules 27, 4755-4761.
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106
Yoshie, N., Azuma, Y., Sakurai, M., Inoue, Y., 1995. Crystallization and compatibility of poly(vinyl alcohol)/poly(3-hydroxybutyrate) blends: influence of blend composition and tacticity of poly(vinyl alcohol). J. Appl. Polym. Sci. 56, 17-24.
Zhang, L., Deng, X., Zhao, S., Huang, Z., 1997. Biodegradable polymer blends of poly(3-hydroxybutyrate) and poly(DL-lactide)-co-polyethylene glycol. J. Appl. Polym. Sci. 65, 1849-1856.
107
CHAPTER 5
Environmental degradation of soda
l ignin/ poly(hydroxybutyrate) blends
Payam Mousaviouna, Graeme A. Georgeb and William O.S. Dohertya
a Sugar Research and Innovation, Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. b Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. Submitted to the Journal of Polymer Degradation and Stability, 2011
Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001
Suggested Statement of Contribution of Co-Authors for
Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception,
execution, or interpretation, of at least that part of the publication in their field of expertise;
2. they take public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;
3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the
editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and
5. they agree to the use of the publication in the student’s thesis and its publication on
the Australasian Digital Thesis database consistent with any limitations set by publisher requirements.
In the case of this chapter: Publication title and date of publication or status: Thermal Decomposition of Bagasse: Effect of Different Sugar Cane Cultivars, published in Industrial & Engineering Chemistry Research, Vol 50, 791-798, 2011
Contributor Statement of contribution* Payam
Mousavioun
Data analysis. Signature
Date Vanita R. Maliger
Experimental design and conducted experiments.
William O. S. Doherty Wrote the manuscript,
Data analysis.
Ray L. Frost Edited manuscript.
Principal Supervisor Confirmation I have sighted email or other correspondence from all Co-authors confirming their certifying authorship. _______________________ ____________________ ______________________ Name Signature Date
kinetics and morphology of poly(β-hydroxybutyrate) and poly(vinyl acetate) blends. Eur. Polym. J. 33, 1449-1452.
Antunes, M.C.M., Felisberti, M.I., 2005. Blends of poly(hydroxybutyrate) and poly(ε-caprolactone) obtained from melting mixture. Polym. Sci. Technol. 15, 134-138.
Avella, M., Rota, G.L., Martuscelli, E., Raimo, M., Sadocco, P., Elegir, G., Riva, R., 2000. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fibre composites: thermal, mechanical properties and biodegradation behaviour. J. Mater. Sci. 35, 829-836.
Barham, P.J., Keller, A., Otun, E.L., Holmes, P.A., 1984. Crystallization and morphology of a bacterial thermoplastic: poly-3-hydroxybutyrate J. Mater. Sci. - Mater. Med. 19(9), 2781-2794.
Chiu, H.J., Chen, H.L., Lin, J.S., 2001. Crystallization induced microstructure of crystalline/crystalline poly(vinylidenefluoride)/poly(3-hydroxybutyrate) blends probed by small angle X-ray scattering. Polymer 42, 5749-5754.
Cronin, D.J., 2008, Formation of multi-component films using anionic liquid, Honours Chemistry Thesis In, Queensland University of Technology, Brisbane.
Dizhbite, T., Telysheva, G., Jurkjane, V., Viesturs, U., 2004. Characterization of the radical scavenging activity of lignins--natural antioxidants. Bioresource Technol. 95, 309-317.
ElMiloudi, K., Djadoun, S., Sbirrazzuoli, N., Geribaldi, S., 2009. Miscibility and phase behaviour of binary and ternary homoblends of poly(styrene-co-acrylic acid), poly(styrene-co-N,N-dimethylacrylamide) and poly(styrene-co-4-vinylpyridine). Thermochim. Acta 483, 49-54.
Farmer, V.C., 1974. The infrared spectra of minerals, Mineralogical society, London.
Ghaffar, A.M.E.A., 2002, Development of a biodegradable material based on Poly(3-hydroxybutyrate) PHB, In, Martin-Luther University, Wittenberg, pp. 115.
Ghosh, I., Jain, R.K., Glasser, W.G., 1999. Multiphase materials with lignin. XV. Blends of cellulose acetate butyrate with lignin esters. J. Appl. Polym. Sci. 74, 448-457.
Grassie, N., Murray, E.J., Holmes, P.A., 1984. The thermal degradation of poly(β-hydroxybutyric acid): Part 2--Changes in molecular weight. Polym. Degrad. Stab. 6, 95-102.
Grassie, N., Murray, E.J., Holmes, P.A., 1984. The thermal degradation of poly(β-hydroxybutyric acid): Part 3--The reaction mechanism. Polym. Degrad. Stab. 6, 127-134.
Guo, L., Sato, H., Hashimoto, T., Ozaki, Y., 2010. FT-IR study on hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) and pol(4-vinylphenol). Macromolecules 43, 3897-3907.
Hablot, E., Bordes, P., Pollet, E., Avérous, L., 2008. Thermal and thermo-mechanical degradation of poly(3-hydroxybutyrate)-based multiphase systems. Polym. Degrad. Stab. 93, 413-421.
129
Ikejima, T., Cao, A., Yoshie, N., Inoue, Y., 1998. Surface composition and biodegradability of poly(3-hydroxybutyric acid)/poly(vinyl alcohol) blend films. Polym. Degrad. Stab. 62, 463-469.
Kumagai, Y., Doi, Y., 1992. Enzymatic degradation and morphologies of binary blends of microbial poly(3-hydroxy butyrate) with poly(ε-caprolactone), poly(1,4-butylene adipate and poly(vinyl acetate). Polym. Degrad. Stab. 36, 241-248.
Li, S.D., He, J.D., Yu, P.H., Cheung, M.K., 2003. Thermal degradation of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as studied by TG, TG–FTIR, and Py–GC/MS. J. Appl. Polym. Sci. 89, 1530-1536.
Lim, S.P., Gan, S.N., Tan, I., 2005. Degradation of medium-chain-length polyhydroxyalkanoates in tropical forest and mangrove soils. Appl. Biochem. Biotech. 126, 23-33.
Mousavioun, P., Doherty, W.O.S., George, G., 2010. Thermal stability and miscibility of poly(hydroxybutyrate) and soda lignin blends. Ind. Crops Prod. 32, 656-661.
Pandey, K.K., 1999. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J. Appl. Polym. Sci. 71, 1969–1975.
Pizzoli, M., Scandola, M., Ceccorulli, G., 1994. Crystallization kinetics and morphology of poly(3-hydroxybutyrate)/cellulose ester blends. Macromolecules 27, 4755-4761.
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Weihua, K., He, Y., Asakawa, N., Inoue, Y., 2004. Effect of lignin particles as a nucleating agent on crystallization of poly(3-hydroxybutyrate). J. Appl. Polym. Sci. 94, 2466-2474.
Woolnough, C.A., Yee, L.H., Charlton, T., Foster, L.J.R., 2010. Environmental degradation and biofouling of ‘green’ plastics including short and medium chain length polyhydroxyalkanoates. Polym. Int. 59, 658-667.
Wu, C.S., 2006. Assessing biodegradability and mechanical, thermal, and morphological properties of an acrylic acid-modified poly(3-hydroxybutyric acid)/wood flours biocomposite. Journal of Applied Polymer Science 102, 3565-3574.
Xing, P., Dong, L., An, Y., Feng, Z., Avella, M., Martuscelli, E., 1997. Miscibility and crystallization of poly(β-hydroxybutyrate) and poly(p-vinylphenol) blends. Macromolecules 30, 2726-2733.
Yoshie, N., Azuma, Y., Sakurai, M., Inoue, Y., 1995. Crystallization and compatibility of poly(vinyl alcohol)/poly(3-hydroxybutyrate) blends: influence of blend composition and tacticity of poly(vinyl alcohol). J. Appl. Polym. Sci. 56, 17-24.
Zhang, L., Deng, X., Zhao, S., Huang, Z., 1997. Biodegradable polymer blends of poly(3-hydroxybutyrate) and poly(DL-lactide)-co-polyethylene glycol. J. Appl. Polym. Sci. 65, 1849-1856.
130
Zhang, L., Xiong, C., Deng, X., 1996. Miscibility, crystallization and morphology of poly(β-hydroxybutyrate)/poly(d,l-lactide) blends. Polymer 37, 235-241.
131
CHAPTER 6
Thermal stabil i ty and miscibil i ty of
poly(hydroxybutyrate) and methanol-
soluble soda l ignin blends
Payam Mousaviouna, William O.S. Dohertya, Graeme A. Georgeb and Peter
Halleyc a Sugar Research and Innovation, Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. b School of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane, Australia. c Centre High Performance Polymers (CHPP), AIBN, St Lucia, The University of Queensland, QLD 4072, Brisbane, Australia
Published in 10th AIChE meeting, Salt Lake City, UT, USA, November 2010
Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001 Ph: +61 7 3138 4475 or 3138 5306 e-mail [email protected]
http://www.rsc.qut.edu.au/studentsstaff/ Correct as at: 7-6-10
Suggested Statement of Contribution of Co-Authors for
Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception, execution, or
interpretation, of at least that part of the publication in their field of expertise; 2. they take public responsibility for their part of the publication, except for the responsible author
who accepts overall responsibility for the publication; 3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or
publisher of journals or other publications, and (c) the head of the responsible academic unit, and 5. they agree to the use of the publication in the student’s thesis and its publication on the
Australasian Digital Thesis database consistent with any limitations set by publisher requirements.
In the case of this chapter: Publication title and date of publication or status: Environmental degradation of soda lignin/PHB blends”, Polymer Degradation and Stability, 2011.
Contributor Statement of contribution* Payam
Mousavioun Wrote the manuscript, Experimental design, Conducted experiments, Data analysis.
Signature
Date
Graeme A. George
Aided experimental design, Data analysis.
William O.S. Doherty
Data analysis.
Principal Supervisor Confirmation I have sighted email or other correspondence from all Co-authors confirming their certifying authorship. _______________________ ____________________ ______________________ Name Signature Date
The shift to a lower wavenumber is indicative of hydrogen bonding interactions
(Barsbay and Güner, 2007). As shown in Figure 6-9 (and Table 6-2), for blends
containing 10 wt%, 30 wt%, 40 wt% and 50 wt% ML, there is a small but
definitive shift (2 cm-1to 4 cm-1) to a lower wavenumber relative to the PHB
peak of 1722 cm-1. This implies that the reactive functional groups of ML are
engaged in hydrogen bonding interactions with the carbonyl oxygen in PHB as
has been reported in a previous study (Mousavioun et al., 2010). This explains
143
the compatibility obtained between PHB and lignin for the blends containing up
to 50 wt% lignin. The reason why there were no differences in some of the
wavenumber shifts between these blends containing different proportions of
lignin is not known.
Figure 6-9 also shows that for the PHB band at 1733 cm-1, there is probably a
slight shift to a lower wavenumber (~ 4 cm-1) for the 10 wt% and 30 wt% blends.
Although this band is of less intensity compared the main band at 1722 cm-1 it
shows some favourable interactions between the amorphous part of PHB and
lignin. The slight shift to a higher wavenumber for the other blends may also be
linked to some sort of association between ML and PHB.
6.4. Conclus ion The addition of lignin to PHB has been found to improve the overall thermal
stability of PHB. For blends containing up to 50 wt% lignin, the addition of
lignin raised the initial decomposition temperature i.e. T0 of PHB by a few
degrees. Glass transition temperature and microscopy studies indicated
miscibility with blends containing 10 wt% - 40 wt%. At up to 30 wt% lignin, the
experimental data fitted the Gordon-Taylor and Kwei models. The
intermolecular interactions between the two polymer components were found to
be due to hydrogen bonding formation between their functional groups.
144
6.5. References Antunes, M.C.M., Felisberti, M.I., 2005. Blends of poly(hydroxybutyrate) and
poly(ε-caprolactone) obtained from melting mixture. Polym. Sci. Technol. 15, 134-138.
Barham, P.J., Keller, A., 1986. The relationship between microstructure and mode of fracture in polyhydroxybutyrate. J. Polym. Sci. Part B: Polym. Phys. 24, 69-77.
Barsbay, M., Güner, A., 2007. Miscibility of dextran and poly(ethylene glycol) in solid state: Effect of the solvent choice. Carbohyd. Polym. 69, 214-223.
Chiu, H.J., Chen, H.L., Lin, J.S., 2001. Crystallization induced microstructure of crystalline/crystalline poly(vinylidenefluoride)/poly(3-hydroxybutyrate) blends probed by small angle X-ray scattering. Polymer 42, 5749-5754.
ElMiloudi, K., Djadoun, S., Sbirrazzuoli, N., Geribaldi, S., 2009. Miscibility and phase behaviour of binary and ternary homoblends of poly(styrene-co-acrylic acid), poly(styrene-co-N,N-dimethylacrylamide) and poly(styrene-co-4-vinylpyridine). Thermochim. Acta 483, 49-54.
Fox, T.G., 1956. Influence of diluent and of copolymer composition on the glass temperature of a polymer system. Bull. Am. Phys. Soc. 2, 123.
Garcìa-Pèrez, M., Chaala, A., Yang, J., Roy, C., 2001. Co-pyrolysis of sugarcane bagasse with petroleum residue. Part I: thermogravimetric analysis. Fuel 80, 1245-1258.
Ghaffar, A.M.E.A., 2002, Development of a biodegradable material based on Poly(3-hydroxybutyrate) PHB, In, Martin-Luther University, Wittenberg, pp. 115.
Ghosh, I., 1998, Blends of biodegradable thermoplastics with lignin esters, In, Virginia Polytechnic Institute and State University, VA, pp. 139.
Ghosh, I., Jain, R.K., Glasser, W.G., 1999. Multiphase materials with lignin. XV. Blends of cellulose acetate butyrate with lignin esters. J. Appl. Polym. Sci. 74, 448-457.
Ghosh, I., Jain, R.K., Glasser, W.G., 2000. Multiphase materials with lignin. Part 16. Blends of biodegradable thermoplastics with lignin esters. ACS Symp. Ser. 742, 331-350.
Gosselink, R.J.A., Abächerli, A., Semke, H., Malherbe, R., Käuper, P., Nadif, A., van Dam, J.E.G., 2004. Analytical protocols for characterisation of sulphur-free lignin. Ind. Crops Prod. 19, 271-281.
Guo, L., Sato, H., Hashimoto, T., Ozaki, Y., 2010. FTIR study on hydrogen-bonding interactions in biodegradable polymer blends of poly(3-hydroxybutyrate) and pol(4-vinylphenol). Macromolecules 43, 3897-3907.
Kuo, S.W., Chan, S.C., Chang, F.C., 2002. Miscibility enhancement on the immiscible binary blend of poly(vinyl acetate) and poly(vinyl pyrrolidone) with bisphenol A. Polymer 43, 3653-3660.
Kuo, S.W., Chang, F.C., 2001. Effects of Copolymer Composition and Free Volume Change on the Miscibility of Poly(styrene-co-vinylphenol) with Poly(ε-caprolactone). Macromolecules 34, 7737-7743.
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146
CHAPTER 7
Conclusions and Further Research
147
7.1. Conclus ions
In this thesis, the advantages and disadvantages of blending lignin with PHB
have been studied. The properties of the lignin/PHB blends that were studied
are:
� Thermal properties and miscibility
� Thermophysical and rheological properties
� Environmental degradation
� Thermal properties of PHB blends with different types of lignin
7.1.1. Thermal properties and miscibility study.
In this study, lignin was found to increase the overall thermal stability of the
PHB/lignin blend, although it reduces the initial onset temperature of PHB
degradation. Thermal analyses (TGA and DSC) indicate that it is the
intermolecular interaction between PHB and lignin which causes miscibility
within a range of blends. One of the fundamental outcomes of these
investigations is the evaluation of the range of miscibility of these two polymers
and thermal analyses revealed that 40 wt% lignin is the highest amount of lignin
which gives a miscible blend. There was a significant difference between the
properties of miscible and immiscible blends reflected in the thermal, rheological
and environmental degradation properties. One of the factors which control
miscibility is believed to be hydrogen bonding between carbonyl groups of PHB
and hydroxyl groups of lignin.
7.1.2. Thermophysical and rheological properties of lignin/PHB
blends
The mechanical properties of PHB are affected by the high degree of crystallinity
(62 %) and the Tg of 4°C. PHB has a low concentration of nucleation sites so it
has relatively big crystals which make it brittle and susceptible to secondary
crystallisation. Lignin has been shown to reduce the bulk crystallinity of PHB
up to 65% (see Table 4-4). Another benefit which lignin provides to PHB during
blending is a reduction in the melting temperature and so enables PHB to be
148
processed at a lower temperature. Decreasing the processing temperature not
only reduces the risk of thermal degradation of PHB, but also saves energy.
A low concentration of lignin is also found to plasticise PHB. In the miscible
region, lignin lowers by up to 10 times the lignin/PHB blend melt viscosity
which facilitates the processing of PHB and saves energy.
7.1.3. Environmental investigation of lignin/PHB blends
Investigations of lignin/PHB blends on soil burial for up to 12 months showed
lignin does not improve the biodegradation properties of PHB. Lignin not only,
even in low concentration, inhibits the biodegradation of PHB, it decreases the
rate of degradation as burial time is increased. Surface composition analysis
using XPS show a presence of both PHB and lignin on the surface of film
samples. The analysis also revealed the presence of biofilms on the buried films.
The presence of biofilms is evidence of biodegradation. In a future study it is
worth to investigate the antimicrobial effect of lignin on lignin/PHB blends.
7.1.4. Thermal properties of PHB blends with different types of lignin
According to chemical analyses in this study, soda lignin (SL) has a far higher
concentration of phenolic hydroxyl group with lower content of carboxylic acid
and methoxyl groups compared to ML (see Table 2-3). Also, modelling results
based on the Gordon-Taylor equation showed stronger hydrogen bonding in
SL/PHB blends 7K�� 2 22.08 compared with ML/PHB 7K�� 2 3.348. Therefore, the conclusion is that the phenolic hydroxyl group could make a
stronger contribution in hydrogen bonding. The T0 values of ML/PHB blends
were higher than the T0 values of soda lignin/PHB blends. This may be because
of the proportion of xylan in the composite (see Table 2-4). Xylans are known to
decompose at lower temperatures than cellulose and lignin.
Tg results of ML/PHB blends indicated that blends containing up to 40 wt% ML
are miscible with PHB. The similar results were obtained for soda lignin/PHB
blends (section 1.3.3). FT-IR spectra showed that for blends up to 50 wt% ML,
there was a small but definitive shift to lower wavenumbers, indicating hydrogen
149
bonding interactions. The similarities between the results and those of
lignin/PHB blends indicated that the differences observed in the composition of
lignin functional groups were not significant to influence the glass transition
temperature of lignin/PHB blends derived from the two lignin types.
7.2. Future Research
The opportunities for future related research can be classified in the following
areas:
� Study of molecular structure of PHB during processing with lignin
� Modelling the viscoelasticity of lignin/PHB blends
� Study of antimicrobial effect of lignin
� Study of mechanical properties of lignin/PHB blends
7.2.1. Study of molecular structure of PHB during blend processing
In the processing of lignin/PHB blends, PHB is sensitive to thermal degradation.
The degradation temperature of PHB is close to its melting point. An
investigation for monitoring the molecular structure of lignin/PHB blend while
processing could be linked with the proposed methods to evaluate the kinetics of
the degradation reaction of PHB. The FT-IR monitoring of the extruder chamber
for the lignin/PHB blend could be a good approach to detect changes in
molecular structure of PHB and lignin/PHB blends. A near infrared spectroscopy
(NIRS) method is available (Siesler, et al., 2007) for real-time monitoring of the
processing in the Haake Minilab extruder and this could be adapted for these
studies.
7.2.2. Modeling the viscoelasticity of lignin/PHB blends
Data on the complex viscosity of lignin/PHB blends at different temperatures,
lignin content and frequencies could be used to develop a model that can be
used to provide processing conditions for different blend compositions. For
creating such a model, it is essential to trial a frequency sweep of the complex
150
viscosity of lignin/PHB blends at different temperatures. However, the sensitivity
of PHB to degradation in such close range to its melting point makes this
correlation hard. Using FT-IR detection during the rheological study could be
useful to detect when degradation is occurring.
7.2.3. Study of antimicrobial effect of lignin
An investigation of the radical scavenging activity of lignin could clarify why
lignin protected PHB from biodegradation. Lignin is known to be a natural
antioxidant (Dizhbite, et al., 2004) with a variety of functional groups containing
oxygen (for example hydroxyl and carboxylic acid) which could play a
considerable role in antibacterial and antifungal activity (Nada, et al., 1989).
Based on the outcomes of this thesis, lignin resisted the degradation of PHB even
on the surface of buried films which were exposed to the soil. A more detailed
laboratory-based microbiological study could help to understand this.
7.2.4. Study of mechanical properties of lignin/PHB blends
Utilizing lignin in low concentration leads to miscible blends with PHB. Also,
lignin in higher amounts acts as a filler in lignin/PHB blends and moreover,
lignin affects the bulk crystallinity of lignin/PHB blends both of which alter the
mechanical properties of PHB. Both miscible and immiscible blends of
lignin/PHB need to be investigated in terms of mechanical properties such as
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interpretation, of at least that part of the publication in their field of expertise; 2. they take public responsibility for their part of the publication, except for the responsible author
who accepts overall responsibility for the publication; 3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or
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In the case of this chapter: Publication title and date of publication or status: Environmental degradation of soda lignin/PHB blends”, Polymer Degradation and Stability, 2011.
Contributor Statement of contribution* Payam
Mousavioun Wrote the manuscript, Experimental design, Conducted experiments, Data analysis.
Signature
Date
Graeme A. George
Aided experimental design, Data analysis.
William O.S. Doherty
Data analysis.
Principal Supervisor Confirmation I have sighted email or other correspondence from all Co-authors confirming their certifying authorship. _______________________ ____________________ ______________________ Name Signature Date
Vanita, R. Maligera, William O.S. Dohertya, Ray L. Frostb and Payam
Mousaviouna
a Sugar Research and Innovation, Centre for Tropical Crops and
Biocommodities, Queensland University of Technology, GPO Box 2434,
Brisbane, Australia. b Inorganic Materials Research Program, School of Physical & Chemical
Sciences, Queensland University of Technology, GPO Box 2434, Brisbane,
Australia
Published in the Journal of Industrial & Engineering Chemistry Research, Vol
50, Page 791, 2011
halla
Due to copyright restrictions, this article is not available here. Please consult the hardcopy thesis available from QUT Library or view the published version online at: http://dx.doi.org/10.1021/ie101559n
Research Students Centre, Level 4, 88 Musk Avenue, Kelvin Grove Campus, GPO Box 2434. Brisbane QLD 4001 Ph: +61 7 3138 4475 or 3138 5306 e-mail [email protected]
http://www.rsc.qut.edu.au/studentsstaff/ Correct as at: 7-6-10
Suggested Statement of Contribution of Co-Authors for
Thesis by Published Paper The authors listed below have certified* that: 1. they meet the criteria for authorship in that they have participated in the conception, execution, or
interpretation, of at least that part of the publication in their field of expertise; 2. they take public responsibility for their part of the publication, except for the responsible author
who accepts overall responsibility for the publication; 3. there are no other authors of the publication according to these criteria; 4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or
publisher of journals or other publications, and (c) the head of the responsible academic unit, and 5. they agree to the use of the publication in the student’s thesis and its publication on the
Australasian Digital Thesis database consistent with any limitations set by publisher requirements.
In the case of this chapter: Publication title and date of publication or status: Thermal stability and miscibility of poly(hydroxybutyrate) and methanol-soluble soda lignin blends”, presentation in 10th AIChE meeting, Salt Lake City, UT, USA, November 2010. CD Rom.
Contributor Statement of contribution* Payam
Mousavioun Wrote part of the manuscript, Experimental design, Conducted experiments, Data analysis.
Signature
Date William O.S.
Doherty
Wrote the manuscript, Data analysis.
Graeme A. George Data analysis.
Peter Halley Data analysis
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William O.S. DohertyA, Payam MousaviounA and Christopher M. FellowsB
A Centre for Tropical Crops and Biocommodities, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4000, Australia B Chemistry, School of Science and Technology, The University of New England, Armidale, NSW 2351, Australia
Published in Industrial Crops and Products, Vol 32, Page 259, 2011
175
Abstract- Lignocellulosic waste materials are the most promising feedstock for
generation of a renewable, carbon-neutral substitute for existing liquid fuels.
The development of value-added products from lignin will greatly improve the
economics of producing liquid fuels from biomass. This review gives an outline
of lignin chemistry, describes the current processes of lignocellulosic biomass
fractionation and the lignin products obtained through these processes and finally
outlines the current and potential value-added applications of these products, in
particular as components of polymer blends.
A.2.1. I ntroduc t ion Concern about the depletion of fossil fuel resources and climate change
attributed to anthropogenic carbon dioxide emissions is driving a strong global
interest in renewable, carbon-neutral energy sources and chemical feedstocks
derived from plant sources. Commercial products, which are capturing an
increasing share of the liquid fuel market, are esters of long-chain fatty acids
from plant oils (biodiesel) and ethanol from the enzymatic digestion and
fermentation of starch or sucrose. As an example of the use of biomass as a
chemical feedstock, a consortium led by Dupont is working to convert maize
starch to the monomer, 1,3-propandiol, using genetically modified Escherichia
coli (Caimi, 2004, Wehner et al., 2007). This monomer can then be used to
prepare poly(trimethylene terephthalate), a polyester which is traditionally
synthesised by the polycondensation of trimethylene glycol with either
terephthalic acid or dimethyl terephthalate (Kurian, 2005, Kurian and Liang,
2008).
Industrial production of fuels and feedstocks from plant sources has concentrated
on those sources that can be most readily and economically processed, such as
oil palm, sugarcane, and corn. However, these compete for arable land with
crops intended for human or animal consumption, putting upward pressure on
food prices and accelerating environmental degradation. For this reason, current
research efforts have concentrated on lignocellulosic biomass from sources that
do not compete with food crops: e. g., agricultural waste products, such as sugar
cane bagasse, wheat straw, rice stalk, cotton linters, and forest thinnings, or
novel crops that can be grown in environments too marginal for food production,
such as switchgrass and eucalypts (Sierra et al., 2008). In order for biomass to
176
be a sustainable source of liquid fuel, technologies are required to enable the
economic production of suitable compounds from these sources, the dry mass of
which consists primarily of a matrix of cellulose, hemicellulose, and lignin
intimately mixed on a microscopic scale.
Current research is focussed on increasing the effectiveness and reducing the
cost of cellulase and xylanase enzymes for cellulose and hemicellulose
saccharification, (Maki et al., 2009, Oehgren et al., 2007, Rattanachomsri et al.,
2009), developing enzymes capable of converting the range of sugars produced
by the digestion of hemicelluloses to ethanol, (Bettiga et al., 2009, Yano et al.,
2009) and improving the pre-treatment process for the fractionation of cellulose,
hemicellulose and lignin from biomass (Fox et al., 1987, Kim, 2009, Moxley,
2008). Whatever the means, for producing ethanol from lignocellulosic biomass,
large volumes of lignin will be produced. Current pilot plants producing ethanol
from lignocellulosic material use the residual lignin for energy generation,
sequester it as ‘biochar’ as a carbon sink, or must dispose of it as waste. The
viability of biofuel production would clearly be greatly enhanced by the
development of markets for lignin-derived products. Any value-added lignin
derived product will improve the economics of biomass conversion, while high-
volume bulk commodity applications will also address the problem of waste
lignin disposal. There are a number of physicochemical factors which suggest a
bright future for lignin-based products: (a) compatibility with a wide range of
industrial chemicals; (b) presence of aromatic rings providing stability, good
mechanical properties, and the possibility of a broad range of chemical
transformations; (c) presence of other reactive functional groups allowing facile
preparation of graft copolymers; (d) good rheological and viscoelastic properties
for a structural material; (e) good film-forming ability; (f) small particle size; and
(g) hydrophilic or hydrophobic character depending on origin, allowing a wide
range of blends to be produced (Mousavioun and Doherty, 2010).
The focus of this review is the preparation of possible value-added polymers
derived from the varieties of lignin likely to be generated in significant amounts
from the production of cellulosic ethanol.
177
A.2.2. L ignin Struc ture Lignocellulose materials refer to plants that are composed of cellulose,
hemicellulose and lignin. The cellulose microfibrials (formed by ordered
polymer chains that contain tightly packed, crystalline regions) are embedded
within a matrix of hemicellulose and lignin (Figure 3-1). Covalent bonds
between lignin and the carbohydrates have been suggested to consist of benzyl
esters, benzyl ethers and phenyl glycosides (Smook, 2002).
Figure A.2-1 Cellulose strands surrounded by hemicellulose and lignin (Department of energys genomic, http://genomics.energy.gov, 1986)
Lignin is primarily a structural material to add strength and rigidity to cell walls
and constitutes between 15 wt% and 40 wt% of the dry matter of woody plants.
Lignin is more resistant to most forms of biological attack than cellulose and
other structural polysaccharides, (Akin and Benner, 1988, Baurhoo et al., 2008,
Kirk, 1971) and plants with a higher lignin content have been reported to be
more resistant to direct sunlight and frost (Miidla, 1980). In vitro, lignin and
lignin extracts have been shown to have antimicrobial and antifungal activity,
(Cruz et al., 2001) act as antioxidants, (Krizkova et al., 2000, Pan et al., 2006,
Ugartondo et al., 2008) absorb UV radiation, (Toh et al., 2005, Zschiegner,
1999) and exhibit flame-retardant properties (Reti et al., 2008).
178
Lignin is a cross-linked macromolecular material based on a phenylpropanoid
monomer structure (Figure A.2-2). Typical molecular masses of isolated lignin
are in the range 1,000 g mol-1 to 20,000 g mol-1, but the degree of polymerisation
in nature is difficult to measure, since lignin is invariably fragmented during
extraction and consists of several types of substructures which repeat in an
apparently haphazard manner. In this review the term ‘lignin’ will be used both
for the in vivo material and the various fractions isolated from living matter,
which invariably undergo some degree of chemical and physical change.
The monomer structures in lignin consist of the same phenylpropenoid skeleton,
but differ in the degree of oxygen substitution on the phenyl ring. The H-
structure (4-hydroxy phenyl) has a single hydroxy or methoxy group, the G-
structure (guaiacyl) has two such groups, and the S-structure (syringyl) has three
(Figure A.2-2). The polymerisation of the phenylpropanoid monomers is
initiated by oxidases or peroxidases. While the precise mechanism is obscure, it
is postulated that radical-radical combination of free radicals produced by
enzymatic dehydrogenation is the key reaction, either under enzymatic control
(Davin et al., 2008) or in a random ‘combinatorial’ manner (Ralph et al., 2004).
Another important parameter is the glass transition temperature, Tg, which is an
indirect measure of crystallinity and a degree of cross-linking and directly
indicates the rubbery region of the material (Table A.2-2) (Gargulak and Lebo,
2000).
Tab le A.2-2 Tg o f d i f f e ren t l i gn in t ypes (Ga rgu la k and Lebo, 20 00)
Types of lignin Tg (°C)
Milled wood lignin
-Hardwood
-Softwood
110-130
138-160
Kraft lignin 124-174
Organosolv lignin 91-97
Steam explosion lignin 113-139
Lignin Tg will depend on the amount of water and polysaccharides, as well as
molecular weight and chemical functionalisation, but in general the Tg will be
lower the greater the mobility of the lignin molecules. While Tg generally
increases with increasing molecular weight, the impact of structural variation
based on the degree of polymerisation has only recently been established.
Baumberger and co-workers (2002) showed using a series of transgenic poplars
that the variations in Tg were closely related to the degree of polymerisation of
lignin as determined by thioacidolysis. This is illustrated in Figure A.2-4, where
the Tg increases with the degree of condensation, expressed as the fraction of
phenylpropanoid units involved in C-C linkages.
187
Figure A.2-4 Correlation between the glass transition temperature (Tg) and
the degree of condensation (% phenylpropanoid units involved in C-C linkages) of milled wood and enzyme lignins isolated from control and transgenic poplars (Baumberger et al., 2002). Data for control plants are shown as open symbols, and data for transgenic plants derived from those controls are shown as closed symbols. Figure redrawn with permission from Baumberger et al. (2002)
The reactivity and physicochemical properties of lignins are dependent to certain
extent, on their molecular weight distribution. Recently, Baumberger et al.
(2007) developed the use of size-exclusion chromatography to measure the
molecular weight distribution of lignin.
More potential applications of lignin can be realised if the miscibility of lignin
with other polymeric materials can be improved. This may be done through the
chemical modification of lignin with appropriate hydrophobic groups (e.g.
butyrate, hydroxypropyl, ethyl) (Ghosh et al., 2000, Uraki et al., 1997) or
through the formation of lignin copolymers (Wang et al., 1992). Pouteau and his
coworkers (2004) examined the compatibility of lignin-polymer blends by image
analysis. A correlation (Figure A.2-5) between the solubility parameter of kraft
lignin (20.5-22.5 (MPa)1/2) and the solubility parameters of different polymers
was obtained. The data shown does not discriminate between the molecular
170
175
180
185
190
45 50 55 60 65 70 75 80
Tg
(C
)
% units involved in C-C bonds
(°C
)
188
weight of lignin fractions, but only low molecular weight lignins are compatible
with apolar and very polar matrices.
Figure A.2-5 Correlation between total aggregate surface area observed per photo and the solubility parameter of the polymer matrix (Pouteau et al., 2004). Figure redrawn with permission from Pouteau et al. (2004).
A.2.5. App l icat ions There are many commercial applications of low value where lignins
(predominantly lignosulfonates) are used because of their surface-active
properties (Gargulak and Lebo, 2000, Stewart, 2008). Table A.2-3 gives the
variety of these lignosulfonate products.
Tab le A.2-3 App l i ca t i on o f l i gno su l fon ate p rodu c ts based o n the i r su r fa ce-a c t i v e p rope r t ies
Products Reference
Concrete additives (Sestauber et al., 1988, Shperber et al.,
2004)
0
10000
20000
30000
40000
15 17 19 21 23 25 27
Ave
rag
e s
urf
ace
per p
ho
to
Solubility Parameter (MPa)1/2
189
Animal feed pelleting aid (Winowiski and Zajakowski, 1998)
Metallic ore processing (Clough, 1996)
Oil well drilling muds (Detroit and Sanford, 1989, Kelly, 1983)
Dust control (Buchholz and Quinn, 1994, Fiske, 1992)
Phenol-formaldehyde resins (Raskin et al., 2002)
Lignosulfonates are also used to produce a number of value-added products for
specialty markets (Gargulak and Lebo, 2000). Table A.2-4 gives the variety of
these lignosulfonate products.
Tab le A.2-4 L igno su l fon ate p rod uct s in spec ia l i t y ma rket s
Products Reference
Vanillin (Bjorsvik and Minisci, 1999, Gogotov, 2000)
Pesticides (Lebo, 1996)
Dispersant for carbon black (Goncharov et al., 2001)
Dyes and pigments (Hale and Xu, 1997)
Gypsum board (Northey, 2002)
Water treatments (Jones, 2004, Zhuang and Walsh, 2003)
Scale inhibitors (Ouyang et al., 2006)
Industrial cleaners (Jones, 2008)
Emulsifiers (Gundersen et al., 2001, Sjoblom et al., 2000)
Matrix for micronutrient (Docquier et al., 2007, Meier et al., 1993,
190
fertilisers Niemi et al., 2005)
Wood preservatives (Dumitrescu et al., 2002, Lin and Bushar,
1991)
Battery expanders (Pavlov et al., 2000)
Specialty chelants (Khabarov et al., 2001)
Bricks, refractories and
ceramics
(Pivinskii et al., 2006)
Retention aids in papermaking (Vaughan et al., 1998)
Blending of two or more polymers provides the ability to tailor material
properties to achieve specific goals with higher value. While a particular
homopolymer will have properties that can be tailored by controlling molecular
weight and the degree of branching and crosslinking, blending of polymers
makes a vastly greater range of potential materials properties available. As well
as making simple additive properties accessible, in many instances polymer
blending results in high-performance composite materials as a result of
synergistic interactions between the components. However, many polymer
combinations are immiscible and so exist in two different phases in the polymer
matrix. This separation into phases can result in poor stress transfer between the
phases, thereby lowering the mechanical properties of the blend to that at least of
one of the individual components. When incorporated in blends with natural and
synthetic polymers, lignin generally increases the modulus and cold
crystallisation temperature but decreases the melt temperature. The addition of
plasticisers to such systems have been found to improve the mechanical
properties by reducing the degree of self-association between lignin molecules,
improving lignin-polymer miscibility (Feldman et al., 2001). Because lignin
possesses easily-functionalisable hydroxyl and carboxylic acid groups, its
compatibility with different polymer types has been extensively examined. The
191
following section presents some examples of lignin blends with natural and
synthetic polymers.
Natural polymers are synthesised by living organisms or by enzymes isolated
from living organisms, through sophisticated biosynthetic pathways requiring
carbon dioxide consumption. These ‘environmentally friendly’ polymers include
cellulose, hemicellulose, lignin, starch, proteins, nucleic acids and linear
aliphatic polyesters. The ability to control the hydrophilicity of lignin means that
it could in principle form composite materials with any of these polymers, while
the physicochemical qualities of lignin means that it can in many cases improve
the tensile strength and bulk modulus of these biopolymers, and protect the
composite against oxidative degradation under UV light or elevated temperature.
Feldman (2002) and more recently Stewart (2008) have reviewed lignin blending
with synthetic polymers. The present review will discuss protein-lignin blends,
starch-lignin blends, epoxy-lignin composites and phenol-formaldehyde resins
where all or part of the phenol is derived from lignin, polyolefin-lignin blends,
lignin blends with vinyl polymers, lignin-polyester blends, lignin as a component
of polyurethanes, synthetic rubber-lignin blends, graft copolymers of lignin and
the prospects of lignin incorporation into further polymer systems. Most of these
copolymers and polymer blends are currently in the research phase with the
intent of commercial applications.
A.2.5.1. Protein-lignin blends
Proteins have long been used for the production of plastics and resins (Huang et
al., 2004, Nagele et al., 2000). The main drawbacks of protein-based materials
are high water absorption and the difficulty of separating the proteins from
naturally occurring colourants without denaturation, however these obstacles are
gradually being overcome (John and Bhattacharya, 1999, Otaigbe and Adams,
1997, Zhong and Sun, 2001). As a crosslinked material with a largely aromatic
structure, lignin has the capacity to increase the tensile strength, Young’s
modulus, thermal stability and elongation at break of protein materials.
The addition of soda lignin to soy protein plastics has been shown to reduce
water absorption, as well as improving the mechanical properties of soy
protein/glycerol blends. Blends containing 50 wt% soda lignin have a tensile
192
strength twice that of unblended soy protein (Huang et al., 2003). Thermoplastic
materials comprising lignin and protein blended with natural rubber, have been
patented. These materials have been shown to have improved impact resistance
compared to lignin-free formulations (Nagele et al., 2000).
Hydrogen-bonding interactions are often insufficient to ensure adequate mixing
of lignin with protein. Huang et al. (2004) blended kraft lignin with soy protein
using methylene diphenyl diisocyanate (MDI) as a compatibiliser. MDI will
form urethane links between hydroxy groups on lignin molecules and in the
protein. Only a slight reduction in water absorption was observed, but the
addition of 2 wt% MDI caused a simultaneous enhancement of modulus,
strength, and elongation at break of the polymer blends, which was attributed to
graft copolymerisation and crosslinking (Huang et al., 2004).
An alternative strategy for enhancing the compatibility of lignin with protein,
rather than adding a compatibiliser, is chemical or enzymatic modifications of
the lignin. Blending soy protein with hydroxypropylated soda lignin resulted in a
200 % increase in the tensile strength of the blended material, (Chen et al., 2006,
Wei et al., 2006) without reducing the elongation at break (Huang et al., 2006).
Wei et al. (2006) suggested that improved mechanical properties of protein
blended with hydroxylpropyl lignin molecules were due to: (a) the formation of
supramolecular domains by hydroxylpropyl lignin, (b) the strong adhesion
between the hydroxylpropyl group and soy protein and, (c) the interpenetration
of the soy protein molecules into the supramolecular hydroxylpropyl domain.
Protein has also been incorporated in more complex composite materials, e.g., an
adhesive composition of low molecular weight polyaminopolyamide-
epichlorohydrin resin and protein has been patented (Spraul et al., 2008).
While most processing of gluten protein increases the degree of cross-linking,
incorporation of kraft lignin in gluten reduced protein/protein interactions,
prevented loss of solubility (Kunanopparat et al., 2009). This has obvious
implications for processibility of gluten-based materials, suggesting kraft lignin
is a promising additive for such materials. It was suggested that kraft lignin had a
radical scavenging activity, reacting with the sulfur-centred radicals responsible
for gluten crosslinking.
193
A.2.5.2. Starch-lignin blends
The use of starch-based films for packaging materials has increased recently as
they degrade readily in the environment in comparison to conventional synthetic
materials. However, a significant disadvantage of starch films is that they have
very poor water resistance. Blending with hydrophobic polymers can clearly
improve the water resistance of starch, and lignin has a high compatibility with
starch making it an obvious candidate for blending. Baumberger (2002) has
reviewed studies involving starch-lignin films, giving an overview of methods of
preparation, thermomechanical properties, mechanisms of starch-lignin
interactions and potential target applications of starch-lignin blends.
Lepifre et al. (2004) compared the reactivity of films of three soda lignins (one
derived from sugarcane bagasse and the other two from wheat straw) with starch
on exposure to radiation doses of 200 kGy and 400 kGy, using spectroscopic and
chromatographic techniques. Infrared analysis of the bagasse lignin-starch film,
in contrast to the wheat straw lignin, showed evidence of condensation probably
related to the presence of reactive ferulic acid, and that irradiation improved
compatibility of the two polymers.
Lepifre et al. (2004) found that grafting of starch films with lignin gave
significant improvements in water resistance. The higher water resistance of
lignin/starch blends is attributable to the partial compatibility of lignin with the
amylose component of starch, the presence of hydrophobic lignin at the surface
of the material due to surface activity of phenolic groups, and cross-linking
between the starch-rich phase and the lignin-rich phase (Baumberger et al.,
2000). The work by Baumberger et al. (1998) established that reduced water
content and water solubility starch-kraft lignin blends was due to the amount of
water soluble phenolics present in lignin as these hydrophilic compounds are
likely to interact with the starch matrix, through hydrogen bonding, and lead to
increased bonding to lignin. Figure A.2-6 shows the bonding between β-1
stilbene (a component) found in lignin and the amylose portion of starch. The
increase in elongation at break for the starch-kraft lignin blend compared to
starch was attributed to the increased plasticity of the starch matrix due to the
presence of low molecular phenolics and amphiphilic fatty acids.
194
Figure A.2-6 Hydrogen bonding between β-1 stilbene and amylose. Composite films of lignin, starch, and cellulose have been cast from ionic liquid
at room temperature, with the product showing good mechanical properties,
thermal stability, and resistance to gas permeation (Wu et al., 2009).
Ke et al. (2003) studied the effect of amylose content on the mechanical
properties and moisture uptake of starch films. Three dry corn starches with
lignin blends have also been obtained by treating steam explosion lignin from
straw with a range of isocyanates. The presence of ethylene glycol reduced the
yields, and the best results were obtained using an isocyanate terminated
poly(butylene terephthalate) (Bonini and D'Auria, 2007). Ciobanu et al. (2004)
used a polyurethane elastomer blended with flax soda lignin to form
homogeneous solvent-cast films containing between 4.2 wt% and 23.2 wt%
lignin. While the thermal degradation ranges of unmodified polyurethane and the
blends were similar, the presence of lignin accelerated decomposition at lower
temperatures. The tensile strength increased up to 370 wt%, toughness up to 470
wt% and elongation at break up to 160 wt%, for the blends compared to the
unmodified polyurethane film.
A.2.5.11. Rubber-lignin blends
Lignin has attracted most attention as a filler in natural and synthetic rubbers -
that is, as a component of a multiphase mixture, not in a homogeneous blend. It
has been applied as a filler in butadiene-styrene-butadiene and isoprene-styrene-
butadiene rubbers for shoe soles, (Savel'eva et al., 1983) in styrene-butadiene
elastomer, (Kosikova et al., 2003, Kramarova et al., 2007) and in natural rubber
(Kramarova et al., 2007). Soda lignin and calcium lignosulfonate were compared
as fillers in natural rubber, and though neither had properties entirely comparable
to carbon black, soda lignin had better filler properties than calcium
lignosulfonate and showed potential as a low-cost substitute for carbon black
(Lazic et al., 1986). Low molecular weight lignins have been shown to be more
effective in improving the tensile strength of natural rubber than of styrene-
butadiene rubber, being significantly more effective than starch or protein as a
filler for natural rubber but not for styrene-butadiene rubber (Kramarova et al.,
2007).
Lignin-based phenol-formaldehyde resin has demonstrated good mechanical
properties, oil resistance, and resistance to environmental oxidation when used as
a filler in nitrile rubber (Wang et al., 1992).
212
Lignin has also been applied in combination with an oligomeric polyester as a
modifier of isoprene rubber and methylstyrene-butadiene rubber (Savel'eva et al.,
1988). The vulcanisation rate of the rubbers increased and optimum
vulcanisation time decreased, and improvements were obtained in the
mechanical properties suitable for applications as tyre rubber (fatigue strength,
adhesion to reinforcing cord). Improved adhesion to textiles in blends with lignin
has also been observed in blends of lignosulfate with natural rubber (Piaskiewicz
et al., 1998) and styrene-butadiene rubber (Lora et al., 1991). While in these
applications lignin incorporation increases the adhesiveness of the material, a
hydrophobically modified lignin has been applied to pre-vulcanised natural
rubber latex in order to decrease the stickiness of natural rubber latex as a
paperboard coating material (Wang et al., 2008).
A.2.5.12. Lignin-graft-copolymers
Apart from the uses of lignin as a filler in thermoplastics and as a copolymer in
thermosetting polymers, there is the potential for lignin to be used in free-radical
copolymerisation with unsaturated polymers. This potential is limited by the
ability of the phenolic hydroxyl groups in lignin to act as radical scavengers,
initiating the formation of quinonic structures (Barclay et al., 1997, Lu et al.,
1998).
The residual double bonds in lignin are 1,2-disubstituted and hence not reactive
towards free-radical attack, but lignin has a high concentration of benzylic sites
that should be susceptible to hydrogen abstraction and hence afford grafting sites
(Figure A.2-9). The chief limitation on achieving grafted copolymers based on
free-radical monomers and lignin is hence not normally the intrinsic reactivity of
the ligand, but that the high polarity of the hydroxyl groups leads to a molecule
insoluble in non-polar comonomers such as styrene and methyl methacrylate.
213
(a)
(b)
Figure A.2-9 Potential sites for hydrogen abstraction for free-radical grafting from lignin ; (a) benzylic hydrogen, (b) allylic hydrogen from double bond from dehydrozylation
Lignin has been shown to retard the polymerisation of styrene and methyl
methacrylate, (Rizk et al., 1984) but good yields of PMMA-grafted lignin have
been prepared, (Meister and Zhao, 1992) and successful grafting using
conventional radical initiation has also been achieved with acrylamide, (Ibrahim
et al., 2006, Meister et al., 1991) vinyl acetate, (Corti et al., 2003) cationic vinyl
monomers, (Meister and Li, 1990) acrylic acid, (Maelkki et al., 2002),
acrylonitrile (Chen et al., 1996) and sodium acrylate (Potapov et al., 1990).
Interest in grafting polyelectrolytes to lignin arises from the possibility of
incorporating the thermal and mechanical resistance of lignin into polyelectrolyte
applications for extreme environments, such as additives for drilling muds
(Ibrahim et al., 2006). Chemical grafting of PMMA or polystyrene to lignin
produces surface-active materials which have possible applications as wood
coatings (Chen et al., 1995, Gardner et al., 1993). Contact angle on wood
surfaces coated with lignin-PMMA graft copolymer, a measure of
OCH3
O
OH
OCH3
OH
H
OCH3
O
OH
OCH3
OH
-H
O
OCH3
OCH3
OH
H
O
OCH3
OCH3
OH-H
214
hydrophobicity, increased with lignin content, and copolymers of relatively low
molecular weight gave larger contact angles than copolymers of low molecular
weight (Gardner et al., 1993). Sailaja (2005) has reported that lignin grafted with
PMMA using manganese pyrophosphate initiator gave much improved
mechanical properties in blends at up to 50 wt% with PE, in comparison to
blends of PE with unmodified lignin.
A promising means for producing graft copolymers of lignin and free-radical
monomers appears to be initial derivatisation of lignin with more readily
polymerisable moieties, e.g., with isocyanatomethacrylate to give pendant
methacrylate groups readily polymerisable with methyl methacrylate or styrene,
(Glasser and Wang, 1989) or with chloromethylstyrene and methacryloyl
chloride (Da Cunha et al., 1993). Feldman et al. (1991c) carried out free-radical
grafting of maleic anhydride onto lignin in order to facilitate incorporation of the
modified lignin into a polyurethane. They reported both free-radical grafting to
the lignin backbone and a degree of esterification of the phenol hydroxy groups
on treatment with maleic anhydride and a persulfate radical source.
Grafting of methyl methacrylate to lignin using radiation was first reported by
Koshijima and Muraki (1964). Alkoxylation of the phenol groups improved the
effectiveness of radiation grafting, and radiation-curable coatings have been
produced using acrylic acid and propoxylated lignin (Reich et al., 1996).
Radiation-induced grafting of styrene to lignin was facilitated in the presence of
an organic solvent, with better efficiency as the proportion of methanol in the
reactants was increased (Phillips et al., 1972). Increasing moisture content in
wood was correlated with increasing radiation-induced grafting of PMMA to
lignin, presumably a phenomenon related to monomer diffusion within the
matrix (Sutyagina et al., 1987).
Grafting to lignin has also been accomplished through anionic and cationic chain
polymerisation, and chemical (De Oliveira and Glasser, 1994a) or enzymatic
(Huttermann et al., 2000) grafting of complete polymer chains. Oliveira and
Glasser prepared star-like graft copolymers of lignin and poly(caprolactone)
using anionic polymerisation (De Oliveira and Glasser, 1994b) and
heterogeneous composites of these copolymers with poly(vinyl chloride) (De
Oliveira and Glasser, 1994c). While these lignin-PCL copolymers were brittle
215
and had poor mechanical strength on their own (De Oliveira and Glasser, 1990),
they were found to exhibit good plasticisation properties with PVC. Anionic
polymerisation has been used to graft well-characterised polystyrene chains onto
mesylated lignin, producing copolymers suitable for use as compatibilisers for
blends of kraft lignin and polystyrene (Narayan et al., 1989).
Another route to lignin-PCL graft copolymers is by enzymatic polymerisation of
ε–caprolactone (Enoki and Aida, 2007). A similar chemo-enzymatic
polymerisation pathway has also been reported as a means of grafting acrylamide
(Mai et al., 2000a) and acrylic acid (Mai et al., 2001) onto lignin, in a process
where the role of the laccase enzyme appears to be primarily to catalyse the
production of peroxide-derived radicals (Mai et al., 2002). Although grafting of
acrylic acid to calcium lignosulfonates could be successfully carried out with a
hydroperoxide initiator alone, the process was much more effective when the
initiator was used in combination with laccase (Mai et al., 2000b).
A.2.6. Conc lus ions Lignin is a very abundant naturally occurring polymer with good properties for
the applications, of many materials which can play a role in replacing or partly
replacing petroleum-based components in a broad range of composite materials.
Lignin can be isolated in fractions of varying molecular weight and may readily
be functionalised to play a role in a broad range of composite materials. In
addition, lignin can serve as a feedstock for the production of both liquid fuel
and a broad range of commodity chemicals. The importance of lignin in these
applications is likely to increase, as society becomes less tolerant of product
streams that dispose of lignin by landfill or burning and as the exploitation of
lignocellulosic sources for biofuels increase the amount of lignin generated.
Widespread exploitation of these lignocellulosic sources would also dramatically
change the nature of the lignin isolated: today most lignin is hydrophilic sulfated
material produced as a by-product of the pulp and paper industry, but the
thermal, chemical, and biological methods employed in digesting lignocellulosic
material are all likely to give rise to unfunctionalised lignin. For many
applications, this material will be processed in order to improve its quality and
hence lead to the emergence of a viable lignocellulosic biofuels industry. Lignin
of superior quality will afford a significant opportunity to apply it to a much
216
greater extent in polymer composites, controlled-release formulations, and as a
feedstock for fuels and commodity chemicals. Conversely, the development of
these applications on a commercially viable scale will exert a ‘pull’ effect on
lignocellulosic biofuel development, making the industry economically viable at
an earlier stage of fossil fuel resource depletion. Despite hundreds of years of
experience in the pulping of biomass, technically feasible processes for
separation of biomass into its main components still lie mostly below the
threshold of economic viability. The present treatment strategies, whether
thermal, thermochemical or thermomechanical, still require considerably energy
input. Thus an important future research direction is the cost-effective
fractionation of lignocellulosic biomass. Specifically, the processes involved in
lignin recovery from black liquor (such as acid precipitation and membrane
filtration) need to be improved so that better separation, decreased losses during
washing of the precipitated lignin, and improved purity can be achieved.
Research into the use of flocculants, surfactants and ions for effective lignin
isolation from black liquor produced from various fractionation strategies would
also be worthwhile.
217
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