-
eg
y a
re, U
D. Mechanical testing
leneeffthevedineatn. I
nt natt for cmechatons oer pulpfor lig
linkages present in lignin are phenolic AOH, aliphatic
hydroxyl,carbonyl, alkyl aryl ether, biphenyl, diaryl ether,
phenylpropane,guaiacyl, syringyl, etc. [1,2]. The details of the
chemical functional-ities and inter unit linkages are reported in
literature [1,2]. Tracesof carbohydrates also remain with lignin.
Lignin nds applicationsin adhesives, asphalts, polyurethanes, and
phenolformaldehyde
creased the modulus but decreased the tensile strength
andelongation of the blends [9]. However, coupling agents have
beenused to improve the mechanical properties of lignin
composites[10]. Lignin itself has also been used as a
compatibilizer in naturalbre composites [11]. It has been reported
that lignin acts as bnucleating agent, re retardant and toughening
agent for neat PP.However, very limited studies have been done on
lignin blendedwith biodegradable biopolymers such as starch,
polyhydroxyalk-anoates and polylactic acid. Lignin acts as a
plasticizing agent forstarch [12], nucleating agent for
polyhydroxybutyrate (PHB) poly-mers [13] and adhesion promoter in
cotton brePLA composites
Corresponding author at: Bioproducts Discovery and Development
Center(BDDC), Department of Plant Agriculture, University of
Guelph, Ontario, CanadaN1G 2W1. Tel.: +1 519 8244120x56664; fax: +1
519 763 8933.
Composites: Part A 42 (2011) 17101718
Contents lists available at
Composite
evE-mail address: [email protected] (A.K. Mohanty).in the
United States in the near future, about 225million tons of lig-nin
generation is expected from the cellulosic bioethanol industry[1].
Only about 2% of the generated lignin is being used for valueadded
applicationswhile the rest is used as burning fuel in the
samegenerating industries. Sustainability of these industries
greatly de-pends upon the value added applications of this
co-product.
Lignin is an amorphous substance that has potential for
mate-rial applications. It is a complex polyfunctional
macromoleculewhich is composed of a large number of polar
functional groups[2]. Important functional groups, chemical units
and inter unit
polyvinyl alcohol (PVA) [4]. LigninPP, ligninPET,
ligninPVC(polyvinyl chloride) and ligninPS (polystyrene) composites
havealso been reported in literature [57]. Lignin is compatible
withpolystyrene (PS) [7] and its compatibility increases with
increasinglignin content. In case of ligninPVC blends, lignin is
more compat-ible to unplasticized PVC than plasticized PVC [7].
Ligninpolyeth-ylene composites compatibilized with ethylene vinyl
alcoholcopolymer was studied by Samal et al. [8]. Incorporation of
ligninin low density polyethylene (LDPE), linear low density
polyethyl-ene (LLDPE), and high density polyethylene (HDPE)
slightly in-E. Extrusion
1. Introduction
Lignin, the second most abundaworld, serves as a matrix
componenlose in plant cell walls and providesbres. Currently, about
70 millionannually as a co-product in the papmore, in order to
fulll the demand1359-835X/$ - see front matter 2011 Elsevier
Ltd.doi:10.1016/j.compositesa.2011.07.025ural biopolymer in
theellulose and hemicellu-nical strength to bio-f lignin are
generatedindustry [1]. Further-
nocellulosic bioethanol
resin formulations [1,3]. Lignin has interesting grafting and
cross-linking abilities that makes it an interesting material for
its usein polyurethanes and other polymeric systems [3]. However,
verylimited studies have been conducted on lignin-based
polymercomposites or polymer blends. Ligninpolymer blends and
com-posites have been reviewed recently [1]. Lignin forms
miscibleblends with polyethylene terephthalate (PET) and
polyethyleneoxide (PEO) and immiscible blends with polypropylene
(PP) andA. Thermoplastic resinB. Strength compatibilized
composites.
2011 Elsevier Ltd. All rights reserved.Enhanced properties of
lignin-based biodusing injection moulding process
Saswata Sahoo a, Manjusri Misra a,b, Amar K. MohantaBioproducts
Discovery and Development Center (BDDC), Department of Plant
Agricultub School of Engineering, University of Guelph, Ontario,
Canada N1G 2W1
a r t i c l e i n f o
Article history:Received 2 December 2010Received in revised form
21 July 2011Accepted 21 July 2011Available online 28 July 2011
Keywords:
a b s t r a c t
Composites from polybutymelt mixing process. The(PMDI)
compatibilizer onmaterial into PBS was achieIncorporation of 1%
PMDIstrength simultaneously. Hnin and PMDI incorporatio
journal homepage: www.elsAll rights reserved.radable polymer
composites
,b,niversity of Guelph, Ontario, Canada N1G 2W1
succinate (PBS) and lignin-based natural material were
fabricated using aects of lignin material and polymeric methylene
diphenyl diisocyanateproperties of composites were investigated.
Incorporation of 65% ligninwith an improvement in the tensile and
exural properties of composites.50% lignin lled composites enhanced
the tensile, exural and impactdeection temperature (HDT) of the
virgin plastic also increased with lig-mproved interfacial adhesion
was observed from SEM micrographs of the
SciVerse ScienceDirect
s: Part A
ier .com/locate /composi tesa
-
[14]. It has been reported that tensile strength and elongation
at
PBS composites have been studied by many authors [1619].
Nat-
: Parural bres are not compatible with hydrophobic polymers
andhence result in inferior material properties. In order to
improvethe properties of natural bre composites, surface treatments
of -bres have been reported in literature [17,18].
Compatibilizers/cou-pling agents have also been used to improve the
performance ofcomposites. Various types of compatibilizers have
been testedare silanes, titanates, maleic anhydride grafted
polymers and isoc-yantes. Use of polymeric methylene diphenyl
diisocyanate (PMDI),one of the isocyanate compatibilizers, has
improved the tensilestrength and elongation of natural ller based
composites [20,21].
Lignin-based PBS composites are very rarely reported in
litera-ture. In the present study, a lignin-based natural material
with thetrade name Arboform F 45 has been used as reinforcing ller
in aPBS matrix. Arboform F 45 is a melt processable
thermoplasticmaterial available in pellet form. It contains modied
alkali lignin(obtained from paper pulp industry), natural
additives, and 45%ground plant bres like hemp, ax and wood
particles [22,23].Arboform F 45 pellets (described as lignin in
this manuscript)were used as received in this study. It is believed
that lignin ismore compatible with polar polymers due to the
presence of polarfunctionality in its chemical structure. Poor
compatibility of ligninwith conventional polyolen polymers can be
understood from theinferior mechanical properties of lignin lled
polyolen compos-ites. The tensile strength of polyethylene
drastically decreasedwith 27% lignin incorporation [10]. Similarly,
incorporation of lig-nin to polypropylene gradually decreased the
tensile strength ofcomposites [6,24]. Incorporation of lignin to
PLA also reduced theproperties of the polymer [15]. PBS was
selected as the polymermatrix for our study because of its
toughness, polar nature, nearlysimilar solubility parameter with
lignin, biodegradability, and pos-sible renewability. In this
research, the effect of lignin (Arboform
F 45) content and the effect of polymeric dimethylene
diphenyldiisocyanate (PMDI) compatibilizer were tested with respect
tothe mechanical, thermal and thermo-mechanical properties ofthe
generated composite materials.
2. Experimental
2.1. Materials and method
2.1.1. MaterialsLignin Arboform F 45 was received from Tecnaro
GMBH, Ger-
many. PBS (Bionolle 1020), a product of Showa Highpolymers
Co.Ltd., Japan, was received from Toyo Plastics Co. Ltd., Osaka,
Japan.Polymeric methylene diphenyl diisocyanate (PMDI) under
thetrade name Rubinate Mwas used in this study. Rubinate M usedwas
a product of Huntsman polyurethanes, NJ, USA.break of polylactic
acid (PLA) decreased with lignin incorporation.As well, thermal
degradation was accelerated when lignin contentreached 20%
[15].
PLA and polyhydroxyalkanoates (PHAs) are the most widelyused
biopolymers but they are facing challenges due to their infe-rior
impact performance. Polybutylene succinate (PBS) is a
com-paratively tough polymer and is capable of incorporating a
highvolume of biomass. PBS can be made from both fossil and
renew-able resources which offer a great future for the composite
indus-tries regarding availability of raw material. The growing
interestfor PBS manufacturing predicts a future cost reduction
which issignicantly lower than the current price. Natural bre
reinforcedS. Sahoo et al. / Composites2.1.2. Composites
fabricationLignin (Arboform F 45) was not melt processable alone in
a
15 cc microextruder (DSM Xplore, Netherland). Hence, it was
meltmixed with PBS to develop composites from lignin. Before
compos-ite processing, PBS and lignin pellets were dried at 80 C
for 3 and4 h respectively using a convection oven. Lignin of
varying weightpercentages (30%, 50% and 65%) with calculated
quantities of PBSwere melt mixed in a 15 cc microextruder at 160 C
barrel temper-ature, 150 rpm screw rotation (co-rotation
conguration) andcompounded for 6 min. The hot melt was collected
and compositeswere fabricated using a 12 cc microinjection moulder
(DSMXplore) at 160 C melt temperature and 30 C mould
temperature.Composites with compatibilizer were made by adding 12
wt.%PMDI to 50% lignin lled PBS composites. All the composites
weremade at similar processing conditions. All the composite
speci-mens acquired a lignin like dark reddish brown colour.
2.2. Characterisation
The effect of lignin (Arboform F 45) content (30%, 50% and
65%)on the PBS matrix was studied and the properties of the
resultingcomposites were compared with neat PBS polymer (control).
Com-posites with 50% lignin content were selected for PMDI
incorpora-tion. Composites were prepared with the incorporation of
1 and2 wt.% PMDI compatibilizer. The properties of the
compatibilizedcomposites were compared with the properties of 50%
lignin lledcomposites and neat PBS. The types of characterisations
carried outare discussed below. All results presented are the
average values ofve replications for mechanical properties and
three replicationsfor thermal and physical properties.
2.2.1. C, N, S analysisTotal carbon, nitrogen and sulphur
content of lignin pellets
were evaluated through elemental analysis at Laboratory
Services,University of Guelph.
2.2.2. Mechanical testingTensile and exural properties of the
composites were mea-
sured by a Universal testing machine, Instron 3382, according
tostandards ASTM D 638 and ASTM D 790 respectively. System con-trol
and the data analysis were done using Blue Hill software.
Thenotched Izod impact strength was measured with a TMI
MonitorImpact tester (model No. 4302-01) according to ASTM D
256using a pendulum of 5 ft-lb.
2.2.3. Fourier transform infrared spectroscopy (FTIR)Thermo
Scientic Nicolet 6700 FTIR spectrometer in attenu-
ated total reection infrared (ATR-IR) mode with a resolution of4
cm1 and a number of 32 scans per sample was used to obtainthe
spectra.
2.2.4. Differential scanning calorimeter (DSC)Heat ow as a
function of temperature was studied by a Differ-
ential scanning calorimeter (DSC Q 200, TA Instruments Inc.)
usingheatcoolheat mode. Nitrogen was used as purge gas during
theexperiment. The data was collected by heating the specimen
from50 to 200 C at a constant heating and cooling rate of 10 C
perminute. The data was analysed through TA instrumentss
Universalanalysis software.
2.2.5. Dynamic mechanical analysis (DMA)The storage modulus,
loss modulus and tan delta of the com-
posite specimens as a function of temperature were measuredusing
a Dynamic mechanical analyser (TA Instrument Inc. DMA Q800).
Experiments were carried out by heating the specimens from50 to 110
C at a constant heating rate of 3 C/min, 20 lm oscil-
t A 42 (2011) 17101718 1711lating amplitude, and 1HZ frequency.
Heat deection temperature(HDT) measurements were done at 0.455 MPa
load (ASTM D 648).Specimens were heated from room temperature to
110 C using a
-
heating rate of 2 C /min. A three point bending clamp was used
forall the tests.
2.2.6. Thermogravimetric analysis (TGA)Thermogravimetric
analysis was carried out by a Thermogravi-
metric analyser (TA Instrument Inc. Q500).The samples
werescanned from room temperature to 600 C at a heating rate of20
C/min in a nitrogen atmosphere.
2.2.7. Density measurementThe density of polymer and composites
was measured by an
Electronic densimeter MD-300S (Alfa Mirage) that takes
measure-ments according to Archimedes principle.
material. Peaks due to a CAH stretching vibration appear at
2920
ites gradually increased with increasing lignin content in
thecomposites. The improvement in the properties of composites
indi-cates an interaction, possibly polarpolar interaction between
lig-nin and the polyester matrix. Improvement of the tensile
strengthof the hydroxypropyl lignin and polyethylene blend with
the
nset Maximumdegradationtemperature (C)
Weight loss at 400 C(%)
Charred residues at 600 C(%)
402.9 57.1 0.2392.3 67.8 12.0387.9 343.2 63.8 20.4383.7 341.3
57.5 27.3
388.1 344.2h 61.7 22.1357.8 353.1 62.6 21.8
341 380480b 53.4 31.6
1712 S. Sahoo et al. / Composites: Part A 42 (2011) 171017182845
cm1. Characteristic peaks in the spectra appear at1685 cm1, 1590
cm1, 1511 cm1 are due to aromatic C@Cstretching, and peaks at 1040
cm1 and 1260 cm1 appear due toCAOAC stretching from ether groups
(broader peak than CAOstretching of polyester). Weak peaks appear
at 1220 cm1 and1370 cm1 (not very clearly distinguished due to the
overlappingof spectra) also indicate phenolic CAO and phenolic OH
in ligninrespectively. A chemical and structural analysis of this
materialwas also reported by Haensel et al. [26].
Table 1Thermo gravimetric analysis of composites.
Specimens Degradation onset(C)
Weight loss at degradation o(%)
PBS 306.4 1.030% LigninPBS 260.3 2.450% LigninPBS 237.6 2.665%
LigninPBS 236 2.7
50% LigninPBS-1% PMDI 244.5 2.450% LigninPBS-2% PMDI 236.7
2.6
Lignin 179 3.22.2.8. Scanning electron microscopy (SEM)The
morphology of tensile fractured surfaces of the composites
was observed through a Hitachi S-570 scanning electron
micro-scope (Hitachi High Technologies, Tokyo, Japan) at room
tempera-ture. The samples were gold sputtered up to a thickness of
21 nmby means of a Emitech K-550 sputter coater (Ashford Kent,
UK).An accelerating voltage of 10 kV was used to collect
themicrographs.
3. Results and discussion
3.1. Characterisation of lignin
Lignin pellets were analysed for density, elemental
composi-tion, functional groups, and thermal properties. The
density of lig-nin was 1.34 g/cm3. The total carbon, nitrogen and
sulphur contentin the material were 57.2%, 0.27% and 0.6%
respectively. This ligninmaterial showed thermal degradation onset
at about 179 C (Ta-ble 1). Lignin degrades at a very broad range of
temperatures, be-tween 150 and 800 C. Lignin decomposition occurs
by severalcompeting reactions during which various bonds cleave at
widerange of temperatures releasing gases like CO, CO2, H2O and
CH4[25]. The FTIR spectrum of lignin (Fig. 1) shows a strong and
broadpeak at 3342 cm1 which depicts the presence of OH group in
theb Broad.h Hump.3.2. Mechanical properties
3.2.1. Tensile propertiesTensile properties of composites are
shown in Table 2. It is ob-
served that the tensile strength of the composites rst
decreasedwith 30% lignin incorporation and then increased gradually
byincreasing lignin content to 65% (Table 2). Tensile strength of
com-posites also decreased with increasing ller (agro bre) content
inbiodegradable polymers like PLA and PBS [27,28]. This outcomewas
attributed to the weak interfacial adhesion between thehydrophilic
ller and the hydrophobic polymer matrix. However,improvement in the
tensile properties of composites at higher lig-nin content (65%)
was observed in this investigation. At 65% lignincontent, the
tensile strength increased by a factor of 10 over the50% lignin
lled composites, and the strength was about 13% high-er than the
neat polymer. The increase in the tensile strength athigher lignin
content indicates the reinforcing effect of lignin inPBS polymer
that may be attributed to the similarity in the solubil-ity
parameter of lignin and PBS, crosslinking ability and
adhesivenature of lignin. A highly viscous appearance of the
composite meltwas observed during the processing of 65% lignin lled
compos-ites; however, the composite melt at 30% and 50% lignin
contentwas comparatively less viscous. This result may be
attributed tothe crosslinking ability of lignin that increased with
increasing lig-nin content in the composites. The tensile modulus
of the compos-
Fig. 1. FTIR spectra of lignin, PMDI and composites.
-
addition of vinyl acetate, a polar component in non polar
polyolenmatrix, also supports this interaction concept [29].
Authors haveinterpreted that the interaction was due to the
presence of polarcarbonyl group [29]. Therefore, a hydrogen bond
formation couldbe possible between the carbonyl group of the
polyester matrixand the hydroxyl group of lignin.
Based upon the processing suitability and properties
combina-tion, 50% ller based composites were selected for
incorporation
failure of composites can be observed from the gure. Strain
atbreak of materials decreased with ller incorporation and
slightlyimproved by PMDI addition. Reduction of elongation at break
withthe ller incorporation and slight improvement by PMDI
additioncan also be observed from Table 2. It is reported that PBS
is a quiteductile polymer and the percentage elongation reduces
signi-cantly even with 10% bioller incorporation [17]. Similar
trendsin properties by the addition of PMDI to sugar beet pulp
basedpolylactide composites have also been reported [21].
The rule of mixtures as presented in Eq. (1) was used to
predictthe modulus of composites. The rule is generally applied to
randomoriented short bre composites [27].
Ec VmEm kVf Ef 1Ec, Em, Ef are the elastic modulus of the
composite, polymer matrixand ller (lignin) respectively. A modulus
of lignin of 6.27 GPa [22]was considered for calculating the
modulus of composites. Vm andVf are the volume fraction of polymer
matrix and ller respectively.
Table 2Tensile, exural, HDT and impact properties of
composites.
Specimen label Tensilestrength (MPa)
Tensilemodulus (GPa)
Elongationbreak (%)
PBS 35 1.5 0.6 0.01 122 2130% LigninPBS 26 1.8 1.1 0.03 4.6
0.350% LigninPBS 29 3.4 2.3 0.35 2.0 0.865% LigninPBS 39 1.1 3.3
0.04 1.5 0.1
50% LigninPBS-1% PMDI 37 6.1 2.0 0.03 3.1 1.350% LigninPBS-2%
PMDI 42 4.7 1.9 0.19 4.3 0.7
S. Sahoo et al. / Composites: Parof PMDI compatibilizer.
Incorporation of 1% and 2% PMDI increasedthe tensile strength by 27
and 44% over the uncompatibilised com-posites, and was about 7.5%
and 22% higher than the neat polymerrespectively (Table 2). From
these results it was concluded thatPMDI improved the interfacial
adhesion in the composites[20,21]. Addition of PMDI expects the
formation of a urethane(AHNACOOA) linkage due to the reaction
between the ANCOgroup of PMDI and the AOH group of lignin [30].
Furthermore,the urethane linkage leads to possible secondary
intermolecularbonding (i.e. the hydrogen bonding between NAH group
of ure-thane linkage and carbonyl group of polyester [30]) which
couldbe the cause of improved interfacial adhesion of PMDI
compatibi-lized composites. The schematics of the possible
interactions areshown in Fig. 2.
The modulus of the composites decreased slightly with 1%
PMDIaddition (Table 2) and was reduced further by increasing
PMDIconcentration from 1% to 2%. Lowering of the modulus may
beattributed to possible plasticisation of the composite
materialsdue to the PMDI addition. It is reported that the moisture
presentin the bioller reacts with PMDI producing amine or urea
com-pounds. These compounds plasticize the composites [21]
resultingin a lowering of modulus and an increase in the
elongation. Thestressstrain curves of the composites are shown in
Fig. 3. Brittle
-O-(CH2)4-O-C-(CH2)2-C-
OH
O
Lignin
O
n
(a)a
O
O-C
OHLignin
OCN
CH2-n
+
N-C=O
CH2-n
H
O-Lignin
(b)Lignin
Polyester Polyurethane linkage
PMDI
Fig. 2. Schematics of reaction between lignin, PBS and PMDI. (a)
Interactionbetween lignin and PBS through hydrogen bonding, (b)
polyurethane linkageformation.at Flexuralstrength (MPa)
Flexuralmodulus (GPa)
Impactstrength (J/M)
HDT (C)
28 0.4 0.6 0.01 40 8.4 78 1.940 0.5 1.1 0.01 29 1.0 83 3.046 0.3
2.2 0.03 15 0.9 86 3.152 1.1 3.8 0.15 11 0.9 85 0.6
68 1.8 2.3 0.07 29 2.3 90 1.966 0.7 2.1 0.03 24 3.7 94 1.6
i (at full strain)
i (within 6 % strain)
Fig. 3. Stressstrain curve of the composites. (i) Neat PBS
specimen, (ii) 30% ligninPBS composite, (iii) 50% ligninPBS
composite, (iv) 65% ligninPBS composite, (v)50% ligninPBS composite
with 1% PMDI, (vi) 50% ligninPBS composite with 2%PMDI.
t A 42 (2011) 17101718 1713A factor k (contribution of bre
length and orientation) was used tot the data. The volume fraction
of the bre and matrix was calcu-lated using the density of PBS
(1.26 g/cm3) and lignin (1.34 g/cm3).Correlation between
theoretical modulus and experimental modu-lus of composites is
shown in Table 2. An increase in the moduluswith increasing lignin
content can be observed from the Fig. 4.The theoretical modulus
calculated from the rule of mixture washigher than the experimental
modulus of composites. With a ttingparameter k = 0.67, the
theoretical modulus exactly matches theexperimental modulus of 50%
lignin lled composites. However,modulus values obtained from 30%
and 65% lignin lled compositesshowed slightly lower and higher
values than the respective calcu-lated modulus. Considering the
random orientation of wheat strawbres, a bre efciency factor of k =
0.9 was reported by authors[27]. However, in our research, lignin
pellets (having 45% nely
-
: Parground biobre, lignin particles and other additives
[22,23]) wereused as ller instead of biobre alone, hence, a k value
of 0.67 isquite reasonable. The lower modulus values for 30% lignin
lledcomposites and the higher modulus values for 65% lignin
lledcomposites as compared to the theoretical modulus values maybe
attributed to the ligninpolymer interaction which is not takeninto
account in the equation.
Fig. 4. Correlation analysis of experimental tensile modulus and
rule of mixture(ROM).
Table 3Thermal properties of composites from DSC.
Types of specimen Tg (C) Tm (C)
PBS 31.1 113.230% LigninPBS 26.4 112.050% LigninPBS 20.5
112.065% LigninPBS 12.1 110.450% LigninPBS-1% PMDI 15.8 111.650%
LigninPBS-2% PMDI 21.1 110.9
1714 S. Sahoo et al. / Composites3.2.2. Flexural propertiesIt
can be observed from Table 2 that the exural strength and
exural modulus of composites increased gradually with
increas-ing lignin content. Flexural strength and modulus increased
by41 to 84% and 81 to 503% respectively with increasing lignin
con-tent from 30 to 65 wt.%. It is believed that, the polar
biollers aremore compatible with the polar polymers that improves
the misci-bility of the two phases and promotes a good interfacial
morphol-ogy. However, the difference in the trend of the tensile
and exuralproperties was not clearly understood. The possible cause
may bethe behaviour of the material towards the stretching and
bendingforces. The addition of 1% PMDI improved exural strength
by48% compared to the uncompatibilised material and by 143%
com-pared to the neat polymer (Table 2). Greater stress transfer
fromthe matrix to the ller through a compatibilizer modied
interfaceis believed to be the cause of this signicant improvement.
Theexural modulus of composites with 1% compatibilizer
remainedalmost the same as that of uncompatibilised materials
(Table 2).Increasing PMDI content to 2% slightly decreased the
exuralstrength and modulus of the composites. This result may be
attrib-uted to the resultant effect of various competing reactions
such asplasticisation, urethane linkage formation, and secondary
hydro-gen bonding.
3.2.3. Impact strengthImpact strength measures the ability of
the material to resist
fracture under high rate stress applied at a high speed. Fibres
playa key role towards the impact resistance or toughness of a
material.In this research, impact strength of composites decreased
drasti-cally with ller incorporation (Table 2). A similar trend
wasobserved in the unnotched Izod impact strength of ligninPP
com-posites [24]. It is reported that the incorporation of lignin,
a brittlematerial, decreases the impact strength of composites.
Incorpora-tion of agrobres also decreases impact strength of
composites[27,28]. Incorporation of 1% PMDI compatibilizer to
lignin compos-ites improved the impact strength by 92% as compared
to theuncompatibilized counterpart. This improvement may be
attrib-uted to the possible plasticisation as discussed earlier. On
increas-ing the PMDI content from 1% to 2%, the impact strength
ofcomposites decreased by a small extent. Decrease of
impactstrength with increasing PMDI concentration was also
reportedby the authors [20]. They observed a detrimental effect at
1% PMDIwhile using bamboo pulp in composites however, the same
wasobserved at 2% for lignin lled composites in our study.
3.3. FTIR analysis
FTIR spectra of neat PBS, lignin, and 50% ligninPBS
compositeswith and without PMDI are shown in Fig. 1. Characteristic
carbonyl(C@O) stretching at 17351750 cm1, CAO stretching at
11451155 cm1, and CAH stretching at 28502950 cm1 are presentin the
spectra of PBS and all its composites. Broad peaks for hydro-gen
bonded AOH groups at 34003100 cm1 appear in the spectraof lignin
and all the composites. Characteristic peaks of lignin havebeen
discussed in the characterisation of lignin. It can be observedthat
all the characteristic peaks of PBS and lignin appear in
thecomposites. The peak at 1370 cm1 in lignin spectra (due to
pheno-lic OH group) shifted to 1388 cm1 in the spectra of
compositeswhich is possibly caused due to the interaction between
OH groupsof lignin and C@O groups of PBS matrix. No characteristic
peak forNCO (isocyanate) group at 2270 cm1 appears in the spectra
ofcompatibilized composites which indicates a complete reactionof
isocyanate in the composite system. As shown in Fig. 2, NAHand C@O
bond formation are expected during the reaction. AsC@O is already
present in the matrix and NAH stretching appearsat 33503180 cm1
(overlapping with hydrogen bonded OHstretching), disappearance of
peak at 2270 cm1 and increasedintensity of the peak at 33503180 cm1
can be considered as aconrmation of urethane formation in the
composites.
3.4. Dynamic mechanical analysis (DMA and HDT analysis)
DHm (J/g) Tc (C) DHc (J/g) v (%)
64.8 78.4 62.2 30.957.3 71.6 43.7 39.049.1 64.3 31.6 46.831.6
63.9 21.6 42.925.7 77.2 25.5 24.726.3 76.9 27.03 25.6
t A 42 (2011) 17101718Dynamic mechanical analysis (DMA) is
widely used for theinvestigation of the viscoelastic behaviour and
structure of com-posite materials. Damping measurement (Tand) gives
informationabout the glass transition temperature (Tg) and the
storage modu-lus gives information about the stiffness. The storage
modulus ac-counts for the elastic component of the complex modulus
ofmaterial. Storage modulus, loss modulus and tan delta of
compos-ites as a function of temperature are shown in Fig. 5.
Storage mod-ulus of the polymer and composites decreased with
increasingtemperature (Fig. 5a). The reduction of storage modulus
with tem-perature can be attributed to the softening of the polymer
due tothe increase in the chain mobility of the polymer matrix at
hightemperatures. As compared to neat PBS, storage modulus of
the
-
: ParS. Sahoo et al. / Compositescomposites at room temperature
(25 C) increased by 96495% at3065 wt.% lignin loading. Similar
results were observed in agroour lled biodegradable composites
[31]. The storage modulusof composites remained almost constant at
1% PMDI addition.However, the storage modulus decreased on
increasing the PMDIcontent to 2%. This observation may be
attributed to various com-petitive reactions caused by the addition
of PMDI.
The loss modulus accounts for the contribution of the
viscouscomponent in the complex modulus of the material. At room
tem-perature, the loss modulus of composites increased with
increasingller content (Fig. 5b). However, very little difference
in the lossmodulus was observed near the glass transition
temperature. Theglass transition temperature (Tg) obtained from the
loss moduluspeak (Fig. 5b) increased with increasing lignin
content. Two peaksare observed in the thermogram of 65% lignin lled
composites
Fig. 5. Dynamic mechanical analysis. (a) Storage modulus, (b)
loss modulus, (c) tandelta of composites. (i) neat PBS specimen,
(ii) 30% ligninPBS composite, (iii) 50%ligninPBS composite, (iv)
65% ligninPBS composite, (v) 50% ligninPBS compositewith 1% PMDI,
(vi) 50% ligninPBS composite with 2% PMDI.where the rst peak
corresponds to the Tg of the polymeric phaseand the second possibly
represents the ller (because of very highcontent of ller). Addition
of compatibilizer to 50% lignin lled PBScomposites decreased the
glass transition temperature slightly.
The damping behaviour of the material is measured by
themagnitude of tan d since it is the ratio of loss modulus to
stor-age modulus or energy dissipated to energy stored during
adynamic loading cycle [16]. Tand decreased with ller
incorpora-tion (Fig. 5c). The result indicates that addition of
ller de-creased the molecular mobility of the composite materials
andthe mechanical loss occurred to overcome the inter-friction
be-tween molecular chains was also reduced. Similar observationwas
reported for biobre reinforced PBS [16] and PLA [27] com-posites.
Good interaction between PBS matrix and lignin can beunderstood
from the increased tan d peak temperature (often re-ferred as glass
transition temperature, Tg) and broadening ofTand thermograms due
to lignin incorporation. Two effects aretaken into account as the
cause of increase in the Tg of the com-posites. The rst one may be
the creation of an amorphous com-ponent in the composite structure
where both the polymer andthe ller coexisted in a closely
associated state reducing free vol-ume in the composites and hence
increased Tg. Another cause ofincrease in Tg may be the possibility
of secondary bonds thatacted as quasi-crosslinks and restricted the
Brownian motion oflong chain molecules [29]. The Tg of composites
rst increasedat 1% and then decreased by about 6C at 2% PMDI
addition. Thismay be attributed to be effects such as improved
interaction andplasticisation due to PMDI addition. Improved
interaction be-tween polymer and ller improves the Tg and
plasticisation low-ers the Tg of a material.
Heat deection temperature (HDT) is a measure of the dimen-sional
stability of the material under a particular load and temper-ature.
It is considered as an essential property requirement for awide
range of material applications. HDT values of neat PBS andall
composites are shown in Table 2. HDT of composites increasedwith
increasing lignin content up to 50 wt.% and remained
almostunaltered with further increasing the lignin content to
65%.Improvement in the HDT values was also observed in
compositesfor biobre reinforcement [32,33]. It may be attributed to
the high-er crystallinity of the bio-composites [32] compared to
the neatpolymers. Incorporation of compatibilizer further enhanced
theHDT of composites which is believed to have occurred due to
im-proved interfacial adhesion [32].
3.5. Thermal analysis
3.5.1. Differential scanning calorimetryThe effect of lignin on
the crystallisation and melting behaviour
of composites was studied in non-isothermal DSC experiments
(3).The degree of crystallinity was calculated using Eq. (2), given
be-low [27,34]
v% DHmf DH0m
100 2
where v = degree of crystallinity (%), DHm = enthalpy of fusion
ofmaterial studied, DH0m = enthalpy of fusion of 100% crystalline
PBSi.e. 210 J/g [35], f = weight fraction of polymer in
composite.
Glass transition temperature (Tg) increased by 519 C
withincreasing lignin content in the composites (Table 3) which
indi-cates good interaction between the polymer matrix and lignin.
Lig-nin acts as a nucleating agent for PHB composites and
facilitatescrystallisation [13]. In our current study, the degree
of crystallinity
t A 42 (2011) 17101718 1715increased with ller incorporation up
to 50% and slightly decreasedat 65% ller content (Table 3). It is
believed that the amount of lig-nin content (amorphous in nature)
might have played some role in
-
the nucleation activity of the polymer. The decrease in
crystallinityat 65% lignin lled composites may have been due to
very high l-ler content compared to the polymer matrix. Lignin had
no effecton the melting behaviour of the polymer. The addition of
PMDIcompatibilizer decreased the crystallinity of composites
however;the effect is not consistent for varying PMDI content.
Similarly,the glass transition temperature showed varying trends
with theincreased PMDI content. Although, the effect of PMDI on the
ther-mal properties of natural ller based composites is not very
clear[21], polymer-ller interaction and plasticisation due to the
PMDIaddition could be considered as the possible causes for the
ob-served results in this study.
3.5.2. Thermogravimetric analysisThermal degradation onset,
weight loss (%), major decomposi-
tion temperature and charred residue left after 600 C for
lignin,neat polymer and all composites are shown in Table 1.
Degradationonsets of lignin and PBS polymer are 179 and 306.4 C
respectively.Degradation onset and maximum decomposition
temperature ofcomposites decreased on increasing lignin content in
the compos-ites. Composites with 50 and 65 wt.% lignin content
showed veryclose degradation onset temperatures. The addition of
PMDI in lig-ninPBS composites increased the degradation onset of
compositesby 7 C. Weight loss around 100 C can be attributed to the
loss ofmoisture from the materials and degradation at the onset
temper-
1716 S. Sahoo et al. / Composites: Part A 42 (2011) 17101718Fig.
6. SEM micrograph of composites. (a and c) 50% LigninPBS composite
(lower magni(lower magnication and higher magnication
respectively), (e) 50% ligninPBS with 1%cation and higher
magnication respectively), (b and d) 65% LigninPBS compositePMDI
(lower magnication).
-
at 400 C compared to neat polymer and composites. The
charred
[10] Alexy P, Kokov B, Crkonov G, Gregorov A, Marti P.
Modication of lignin
reinforced poly (lactic acid) (PLA) composites. J Mater Sci
2008;43:52229.
: Parresidue left after 600 C was highest (31.6%) for lignin due
to thepresence of high ratio of highly condensed aromatic
structures.Charred residue increased with the increase in lignin
content inthe composites. The addition of PMDI compatibilizer
slightly in-creased the percentage of charred residues in the
composites,which may be attributed to the presence of aromatic
componentsof PMDI. Char yield is directly related to the ame
retardant poten-tial of the material [17]. Hence, the ame retardant
ability of ligninis reected from this result which is again
synergized by PMDIaddition.
3.6. Density of composites
Density of lignin, 1.34 g/cm3, was obtained from the
compositesby using the rule of mixtures. The density of the neat
polymer andthe composites were measured by a densimeter. The
compositesand neat polymer showed densities of nearly 1.3 g/cm3
and1.26 g/cm3 respectively.
3.7. Surface morphology
The SEM photographs of the composites are shown in Fig. 6.Phase
separation between polymer and ller can be clearly ob-served from a
polymer rich part in 50 wt.% lignin lled PBS com-posites (Fig.
6a).This might have been caused due to loweramounts of crosslinking
components in the ller present in thecomposites. However, more
homogeneous distribution of polymerand lignin ller can be observed
in the micrograph of the 65% lig-nin lled composite (Fig. 6b).
Tensile strength data for these com-posites (50 and 65% lignin
lled) also supports this observation.Micrographs at higher
magnication (Fig. 6c and d) clearly depictthe exact fracture
surface morphology of these two composites.More pulled out bres and
holes in 50% lignin lled compositesindicate poor interfacial
adhesion. The possible cause for the morebres pull-out in 50%
lignin lled composites may be the phaseseparation owing to lower
lignin content compared to 65% ligninlled composites. More
interaction in 65% lignin lled compositespossibly occurred due to
higher lignin content. Pulled out brewith adhered resin matrix and
a more compatibilized phase indi-cate a stronger interface in the
composites having 1% PMDI(Fig. 6e). The improved interface is well
reected in the mechanicalproperties of the compatibilized
composites.
4. Conclusions
Lignin acted as reinforcing ller in PBS matrix that
synergisti-cally improved tensile, exural, some thermal and
thermo-mechanical properties of composites. Incorporation of a
highweight fraction (65%) of lignin was achieved. Impact strength
andthermal degradation onset of the composites gradually
decreasedwith increasing lignin content. The addition of PMDI
compatibiliz-er to 50 wt.% lignin lled composites improved all
mechanicalature may correspond to the scission of weak ether bonds
[36]present in lignin inter units (b-O-4 linkage). Maximum
decomposi-tion temperature of lignin was much lower than the neat
polymer.Unlike the polymer, additional peaks appeared in the
derivativecurves of lignin. Second major decomposition peaks
between340345 C appeared in composites having higher lignin
content(50 and 65 wt.%). As discussed before, lignin decomposition
occursby several competing reactions that release many gaseous
compo-nents. A lowest percentage of weight loss was observed for
lignin
S. Sahoo et al. / Compositesstrength of the composites at 1 wt%
incorporation. Increasing PMDIcontent to 2 wt% further improved
tensile strength of compositeswhile the exural and impact strength
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acid) and lignin. Polym Int 2003;52:94955.
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promoter in cotton bre-The effect of PMDI was not very consistent
for tensile and exuralmoduli of composites however, both the moduli
showed a slightreduction with increasing PMDI content from 1 to 2%.
A higheramount of char content obtained from lignin indicates that
it canact as a ame retardant in the composites. Degree of
crystallinityof composites increased by the incorporation of lignin
up to50 wt.% and slightly decreased with increasing lignin content
to65%. Incorporation of PMDI to the composite blends resulted inan
improvement of HDT but decreased the degree of
crystallinitydrastically compared to the composite with no PMDI. A
strong -brematrix interface was observed from the fractured surface
mor-phology of PMDI compatibilized composites.
Acknowledgements
Authors are thankful to 2009 Ontario Ministry of
Agriculture,Food and Rural Affairs (OMAFRA)/U of G project
Renewable, recy-clable lightweight hybrid green composites from
lignin, switch-grass, miscanthus and bioplastics and Ontario
BioCarInitiative, Ontario Ministry of Research and Innovation for
thenancial support to this research work.
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1718 S. Sahoo et al. / Composites: Part A 42 (2011) 17101718
Enhanced properties of lignin-based biodegradable polymer
composites using injection moulding process1 Introduction2
Experimental2.1 Materials and method2.1.1 Materials2.1.2 Composites
fabrication
2.2 Characterisation2.2.1 C, N, S analysis2.2.2 Mechanical
testing2.2.3 Fourier transform infrared spectroscopy (FTIR)2.2.4
Differential scanning calorimeter (DSC)2.2.5 Dynamic mechanical
analysis (DMA)2.2.6 Thermogravimetric analysis (TGA)2.2.7 Density
measurement2.2.8 Scanning electron microscopy (SEM)
3 Results and discussion3.1 Characterisation of lignin3.2
Mechanical properties3.2.1 Tensile properties3.2.2 Flexural
properties3.2.3 Impact strength
3.3 FTIR analysis3.4 Dynamic mechanical analysis (DMA and HDT
analysis)3.5 Thermal analysis3.5.1 Differential scanning
calorimetry3.5.2 Thermogravimetric analysis
3.6 Density of composites3.7 Surface morphology
4 ConclusionsAcknowledgementsReferences