-
Deakin Research Online
This is the published version:
Kong, Lingxue, Chen, Ying, Peng, Zhen and Li, Puwang 2008,
Latex-based nanocomposites, in Progress in polymer nanocomposite
research, Nova Science Publishers, Hauppauge, N.Y., pp.83-104.
Available from Deakin Research Online:
http://hdl.handle.net/10536/DRO/DU:30016979
Reproduced with the kind permissions of the copyright owner.
Copyright : 2008, Nova Science Publishers
-
In: Progress in Polymer Nanocomposite Research Editors: S.
Thomas, G. E. Zaikov
ISBN: 978-1-60456-484-6 © 2008 Nova Science Publishers, Inc.
Chapter 5
LATEX-BASED NANOCOMPOSITES
Ling Xue Kong 1' 2'a, Ying Chen 2, Zhen Peng 1' 2 and Puwang Li
1' 2 1Centre for Materials and Fiber Innovation,
Deakin University, Geelong Vic 3217, Australia 2Agriculture
Ministry Key Laboratory of Natural Rubber Processing,
South China Tropical Agricultural Product Processing Research
Institute,
P.O. Box 318, Zhanjiang 524001, P.R. China
ABSTRACT
Nanoparticles have been widely used as filler in polymer because
of their unique reinforcing effect. There are many compounding
methods for nanocomposites. The recent development on latex
nanocomposites, a group of special nanocomposites, is reviewed in
this chapter. They include carbon black/latex nanocomposite,
silica/latex nanocomposite, layered silicate/latex nanocomposite,
ZnO/latex nanocomposite, carbon nanotube/latex nanocomposite,
lignin/latex nanocomposite, starch/latex nanocomposite,
nano-fiber/latex nanocomposite, and Chitin whiskers/latex
nanocomposite. Advanced compounding techniques and the latest
advance on these latex nanocomposites are described. The
nano-reinforcing theories of latex nanocomposites are also
studied.
1. INTRODUCTION
As far as the mechanical properties (tensile strength,
stiffness, abrasion resistance and fatigue resistance) are
concerned, most polymers do not possess practical value if they are
not reinforced by fillers. For example, the pure rubber product is
difficult to be made due to its high elasticity and thus has no
practical value. Therefore, the exploration and development of
highly effective, simple and economic reinforcing methods and
agents becomes one of the
'Corresponding author: [email protected].
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84 Ling Xue Kong, Ying Chen, Zhen Peng et al.
major challenges for polymer industry. Consequently, the study
on polymer reinforcement has been an important topic in polymer
industry.
As well known carbon black and silica particles, particularly of
nano dimension, as fillers can significantly reinforce polymer
materials, including rubber [1-9]. How could carbon black and
silica of nano dimension offer outstanding reinforcement in
comparison to general micrometer fillers? Extensive research in
recent years has significantly improved the understanding of these
phenomena which are widely accepted now by researchers in polymer
science. One of the most important breakthroughs is the
identification ofthree most impmiant factors, particle size,
particle structure and surface activity influencing the reinforcing
effect of the fillers. Detailed studies have been made on the
influence of particle size [ 1 0], structure [11, 12], and surface
[13] on the reinforcement effect. It has particularly been
demonstrated that it is the particle size of nano dimension that
contributed most significantly to the excellent reinforcing effect
of carbon black and silica [1]. In other words, the effect of
particle size is the most important among these three factors.
With the rapid advancement in science and technology, the
production of all kinds of nano-particles is no longer a problem,
but how to uniformly disperse the nanoparticles into polymer matrix
is still a major challenge. The most common method of dispersing
nanoparticles into polymer is mechanical blending [14-17]. Due to
high viscosity of most polymer matrices, strong shear and tensile,
and compressive forces are exerted on the agglomerates, therefore
the nano-particles can be easily dispersed into polymer matrix
[18]. However, there are many obvious shortcomings for this
technique, such as fly dust, long mixing time, high energy
consumption, and the creation of nano-aggregates or agglomerates
(nano-particles directly in contact) in the nanocomposites [1],
which will weaken the reinforcement effect of nano-particles and
the properties of nanocomposites.
Although the properties of polymeric materials can be
dramatically improved through the introduction of nanoparticles,
uniformly distributing nanoparticles into polymer matrices without
aggregation is still one of the most challenging issues in
preparing polymer nanocomposites because the prerequisites of
marked reinforcement are the uniform dispersion of nano-patticles,
and a certain interfacial interaction between fillers and polymer
[19]. It is difficult for inorganic nano-particles to be evenly
dispersed in polymer, such as rubber matrix, and attain the strong
interface strength due to their surface characteristics (which
generally cause the strong filler-filler interaction and weak
filler-rubber interaction). Therefore, it is very important and
challenging to find an economic and effective technique to improve
the dispersion of nano-particles in polymer matrix and strengthen
the interfacial interaction between inorganic nano-particles and
polymer which enhance the structure effect of compound, reduce the
hysteresis loss and improve the physical properties of
composites.
As one of the most important materials, latex nanocomposites
have attracted more and more attention because they usually exhibit
much better performance properties when homogeneous dispersion of
nanoparticles in latex matrix is achieved. Different synthesis
methods are now available for the preparation of latex
nanocomposite. In order to disperse particles evenly in the matrix,
various nanocomposites compounding methods have been developed,
such as sol-gel [20, 21], in situ polymerization [22-24],
intercalation polymerization [25-27], blending as well as other
methods [28]. In this chapter, recent development on latex
nanocomposites is reviewed, including synthesis methods and their
compounding techniques.
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Latex-Based Nanocomposites 85
2 . SYNTHESIS METHODS
2.1. Blending Method
Blending is one of the widely used synthesis methods, which
involves mixing polymers with water slurry ofnanoparticles or
precursors under high shear stirring directly [28-30]. The
advantage of this method is that the process is very simple and
controllable while the disadvantage is that without surface
treatment, some nanoparticles are hydrophobic which
makes it difficult to disperse nanoparticles homogeneously into
latex matrix.
2.2. Polymerization Method
There are two popular polymerization methods. The first
synthetic approach mixes nanoparticle precursors with monomers,
followed by simultaneous polymerization of the precursors and
monomers. For example, silica composites based on polystyrene,
poly(2-hydroxyethyl methacrylate) or poly(furfuryl alcohol) have
been prepared via this method. The second synthetic approach is
called in situ polymerization, which is carried out in the presence
of nano-silica particles and either using emulsifier-free
conditions, emulsions or dispersions [31, 32).
2.3. Self-Assembling Compounding Method [19, 33]
Recently a novel synthesis method called self-assembly
compounding method has been developed [25]. The self-assembly is
based on the electrostatic interaction of oppositely charged
polyelectrolytes and is a promising method to produce latex
nanocomposites. The electrostatic adsorptive interactions between
latex particles and the nanoparticles are used as driving forces to
ensure an effective inter-assembly at latex state. Different from
other conventional methods such as intercalation and blending
process, this novel process combines the self-assembly and latex
compounding technique and offers a new approach to synthesize
nanocomposites. The main advantage of the approach is the
constraint of free movement of nanoparticles in latex matrix during
the synthesis which significantly reduces the probability of
particle aggregation.
3. LATEX NANOCOMPOSITES FILLED WITH INORGANIC PARTICLES AND
THEIR COMPOUNDING TECHNIQUES
3.1. Carbon Black/Latex Nanocomposites
Like many other nano-materials, nano carbon black exhibits
characteristics that are significantly different from those of
micro-patticles, such as strong absorption effect, electron tunnel
effect, and unsaturated valence effect [34]. Carbon black is widely
used in rubber
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86 Ling Xue Kong, Ying Chen, Zhen Peng et al.
industry as a reinforcing agent and in the plastics industry for
its antistatic [35, 36], conductive [37, 38], and UV-protective
[39, 40] capacities. For example, the tensile strength of
vulcanized rubber filled with fine carbon black is several or even
tens times higher than that of non reinforced rubber [1]. The
general method of dispersing carbon particles into rubber is
mechanical blending. Due to the high viscosity of most rubber
matrix, the strong shear, tensile and compressive forces are
exerted on the agglomerates; therefore carbon black nano-particles
can be dispersed in rubber matrix. At the same time, the diffusion
resistance from high melts viscosity can prevent dispersed
nano-particles from re-agglomeration during mixing. However, there
are obvious shortcomings with the blending process as described
previously. This will influence the reinforcement effect of
nano-particles and thus result in obvious drawback of ultimate
products.
Carbon blacks are structurally complex particles containing
90-99% of carbon. The smallest dispersible units of carbon black
are irregularly shaped aggregates ranging in size from 50 to 500
nm. The aggregates are composed of chemically coalesced and
spherically shaped primary particles with diameter of 10-75 nm.
Beyond the basic structures, aggregates of carbon black can easily
form agglomerates which are physically held together [41].
Despite its hydrophobic nature, carbon black can be dispersed in
water by using ionic or nonionic surfactants. In these systems, the
hydrophobic parts of the surfactants adsorb onto carbon black while
the hydrophilic parts interact with water, providing steric/static
repulsions between carbon black aggregates. The colloidal stability
of these systems depends on the amount of adsorbed surfactants and
the hydrodynamic thickness of the adsorbed layer [ 41]. The
stability of dispersed carbon black decreases upon addition of some
solvents or prolonged storage due to desorption of the surfactant
molecules from the carbon black surface. The surfactants or
polymers that chemically linked to carbon black should provide high
dispersion stability.
Since carbon black can easily be dispersed into water to produce
colloidal dispersion, the production of water master batches (WMB)
by directly blending rubber latex and carbon black slurry seems
feasible. During solid mixing process of rubber and carbon black,
productivity is improved, energy consumption is reduced, and most
importantly, environmental pollution is greatly alleviated.
However, the mechanical properties of vulcanized rubbers made from
WMB are not ideal.
Based on this, He et al [ 42] recently introduced a novel way to
modify carbon black. Vinyl monomers with hydrotropic group or NaSS
were in-situ grafted onto the surface of carbon black by
thermo-mechanical means in non-solvent environment. Under strong
shear force and higher temperature vinyl monomers were easily
grafted into the surface of carbon black. The carbon black modified
by thermo-mechanical means can disperse in water uniformly with
higher dispersion stability. In the subsequent process, NaSS
grafted hydrophilic carbon black (NaSS-g-CB) was obtained.
NaSS-g-CB was dispersed into natural rubber latex (NRL), the
mixture was then coagulated. After that the process was similar to
the traditional NR processing method and WMB was obtained. SEM
micro-morphology (Figure 1.) shows that hydrotropic carbon black
particles were more uniformly dispersed in the NRL matrix than that
of non-modified carbon black particles. And the ultimate
vulcanization NR filled with these hydrophilic carbon black
particles have excellent mechanical properties (Table 1 ).
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Latex-Based Nanocomposites 87
Figure 1. SEM micrograph of vulcanized NR filled by (a) 35phr CB
and (b) NaSS-g-CB [42].
Table 1. Tear Strength of Vulcanized NR filled by CB and
NaSS-g-CB [42]
Sample CB (phr) Tear Strength (KN/m) 10 41.8
NRL/ NaSS-g-CB 20 85.8 30 98.1 40 133.8
NRL/CB 40 52.8 NR/CB 40 52.2
3.2. Silica/Latex Nanocomposite
Silica is one of another widely used reinforcing fillers.
Composites filled with silica exhibit better mechanical, thermal,
optical properties compared with the corresponding pure composites
[19, 43, 44]. The silica particles act as reinforcing agents,
making the polymers harder, delivering higher strength, improving
their heat distortion temperature, and lowering the coefficient of
thermal expansion [45].
However, due to the strong hydrogen bonding interaction and high
surface free energy, the fumed Si02 nanoparticles usually exhibit a
strong tendency to form large aggregates, which lead to separation
in the composites. Numerous methods have been used to solve the
dispersing problem. For example, Li et al. [19] introduced a novel
self assembly synthetic method by evenly dispersing Si02
nanoparticles into the NR matrix. The process of self-assembling
Si02/latex nanocomposites involves two assembly steps (Figure 2).
At the first stage, Si02 particles are negatively charged at a pH
of 10, and act as templates to assemble the
poly(diallyldimethylammoniumchloride) (PDDA) molecular chains that
are positively
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88 Ling Xue Kong, Ying Chen, Zhen Peng et a!.
charged at the same pH value of I 0. The driving force is the
electrostatic adsorptive interaction. In the second step, the
natural rubber latex particles are negatively charged. As the
protein particles adsorbed on the surface of the natural rubber
latex particles contain carboxyl and amino functional groups,
acidic ionization of the proteins can be generated. These natural
rubber latex particles with negative charges are then assembled
onto the surface of the Si02 particles that are covered with PDDA
as treated in the first stage. Finally, the Si02 nanoparticles are
uniformly dispersed in NR matrix to form nanocomposite.
Figure 3 is the SEM micrograph of NR/Si02 nanocomposite. It can
be seen that the Si02 nanoparticles are homogenously distributed
throughout the NR matrix as spherical nano-clusters with an average
size of 75 nm. The initial degradation temperature (To) and final
degradation temperature (Tr ) and the peak degradation temperature
(Tp) of NR/Si02 nanocomposite, are significantly higher than those
of the pure NR (Table 2). And the degradation rates Cp and Cr
(corresponding to Tp and Tr, respectively.) of the nanocomposite
are lower than those of the pure NR, due to the retardant effect of
Si02.
+
Silica POOA
Electrostatic ;if? Adsorption
---+110 ·~
nano-particle Molecular Chain
NRJSi02 Nanocomposne
Dry ~
Figure 2. The schematic of the self-assembly process [19].
Electrostatic 1 Adsorption
e Natural Rubber Latex Particle
Table 2. Thermal degradation temperatures and rates ofNR
andNR/Si02 nanocomposite [19]
NR NR/Si02
382.6 400.5
407.2 424.2
461.2 476.1
50.6 44.6
Cf(%/min) 4.9 10.3
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Latex-Based Nanocomposites 89
Figure 3. SEM micrograph ofNR!Si02 nanocomposite [19].
3.3. Layered Silicate/Latex Nanocomposites
The research on clay as filler in polymer science has a long
history. Since the diameter of over 80% of clay particles is below
20 nm, the reinforcement effect of clay can be achieved, but it can
not be compared with that of carbon and silica [1]. A series of
clay/polymer nanocomposites with good performance were prepared by
making use of special layer structure of clay (i.e. there are
numerous layers with 1 nm thickness and 1 00~ 1 OOOnm width in clay
particles, which tightly stack together one by one by virtue of the
cation presented in inner space). But these clay/polymer composites
prepared by conventional mechanical mixing are microcomposite.
Hence, in order to obtain clay/polymer nanocomposites with
remarkable performance, it must generate strong intercalating force
to drive macromolecules into inner space of clay layers [14]. At
present, the preparation methods of clay/polymer nano compo-sites
are in situ polymerization [24, 46], liquid rubber reaction,
polymer melt intercalation [26], polymer solution intercalation
[47], and polymer latex compounding [48-50]. In this Chapter we
mainly focus on the polymer latex compounding methods.
Because the sheet shaped fillers have large aspect ratio, it
strongly limits the deformation of macromolecules due to a highly
efficient stress transfer. As a result, the composite possesses
high modulus, stiffness, strength [1 0, 51], low elasticity and
large permanent set, and has e~cellent resistance to gas permeation
[52-54], solvent swelling, oil-proof, vibration-absorption, flame
resistance [55], and heat resistance [17].
However, the improvement in mechanical property has not been as
good as expected. There are two reasons for this. One is the clay
exfoliation, which is quite difficult to achieve in rubber unless
using expensive in situ polymerization. The other is the weak
interfacial
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90 Ling Xue Kong, Ying Chen, Zhen Peng eta!.
strength/toughness unless a chemical reaction happens between
the clay and rubber molecules.
There are three main strategies reported to prepare rubber/clay
nanocomposites, that is, melt intercalation, solution
intercalation, and emulsion compounding. Among these, emulsion
compounding has advantages over the other two because of its
environmental friendliness and simple processing procedure.
Ma et al.[54] used emulsion compounding methods to produce an
exfoliated styrene butadiene rubber(SBR)/clay nanocomposite by
mixing organic modified clay suspension with SBR latex. The mixture
was then coagulated by diluted H2S04 solution and washed with water
until neutral. The experimental results showed that there are
significant improvements in mechanical properties. Transmission
electron microscopy (TEM) analysis of the intercalation/exfoliation
phenomena is shown in Figure 4, which proves that both exfoliation
and a strong interface play critical roles in nano
reinforcement.
(a) (b)
Figure 4. TEM image of a SBR!allylamine (AA)-clay nanocomposite,
revealing the disorderly exfoliated silicate layer structure in
SBR: (a) low magnification TEM micrograph for the SBRIAA-clay
nanocomposite, in which exfoliated nanolayers are observed; (b) a
typical aggregate observed at higher magnification with a
disorderly exfoliated structure [54].
3.4. ZnO/ Latex Nanocomposites
Besides carbon black and silica, other inorganic nano-particles
were developed and commercialized, such as ZnO, Ti02, Mg(OH)z,
AL(OH)3, and Fe304. Many researchers investigated the properties of
polymers filled with these novel particles. Experimental results
proved that the reinforcement effect of these nano-particles was
much better than that of conventional argil, talc micro-reinforcing
fillers, even without surface treatment.
Nano-ZnO, as one of the multifunctional inorganic nanoparticles,
has drawn increasing attention in recent years due to its
significant physical and chemical properties, such as high
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Latex-Based Nanocomposites 91
chemical stability, low dielectric constant, large
electromechanical coupling coefficient, high
luminous transmittance, high catalysis activity, intensive
ultraviolet and infrared absorption
[29]. Zinc oxide can react with accessory ingredients and
accelerators to form organic zinc salts, which can further improve
the activity accelerators. Decreasing the particle size of zinc
oxide and increasing the surface area can increase the reaction
activity. The experimental
results demonstrated that when the dosage of nano-ZnO was half
of micro-ZnO, the curability
and mechanical properties of composites with nano-ZnO reached
the same level as those of composites with macro-ZnO, which could
greatly reduce the cost and was beneficial to environmental
protection [ 1 ].
Owing to the functional properties of nano-ZnO, it can
potentially be applied as catalysts [56], gas sensors,
semiconductors, varistors [57, 58], piezoelectric devices,
field-emission
displays and UV-shielding (59, 60] materials. The introduction
of nano-ZnO into polymers could improve the mechanical and optical
properties of the polymers due to a strong
interfacial interaction between the organic polymer and the
inorganic nanoparticles and nanoparticle's small size and large
specific area, and quantum effect, respectively. Consequently,
these nanocomposites could be widely applied in coatings, rubbers,
plastics, sealants, fibers and other fields.
Table 3. Mechanical Properties of Pure, Al, A2 and A3 Samples
[29)
Samples Content of Tensile strength Elongation ZnO(%) (MPa) at
break(%)
Pure 0 wt 2.30 448.4 3 wt 4.63 374.5 5 wt 4.83 390.6 7wt 4.95
389.9
Al
9 wt 5.04 385.8 3 wt 4.96 369.2 5 wt 5.03 367.5 7wt 5.18
371.9
A2
9 wt 5.33 389.9 A3 3 wt 3.54 394.8
5 wt 4.13 387.6 7wt 4.30 388.2
9 wt 4.57 377.2
Xiong et a!. [29] prepared poly(styrene butylacrylate)
latex/nano-ZnO composites by
blending poly(styrene butylacrylate) latex with a water slurry
of nano-ZnO particles which
have a diameter ranging from 60nm, 1 OOnm to micron scale and
defined as samples A 1, A2
and A3, respectively. The tensile strength (Table 3), UV and NIR
shielding properties (Figure 5) of the nanocomposite increase with
an increasing nano-ZnO content.
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92 Ling Xue Kong, Ying Chen, Zhen Peng et al.
10"'1""-------------------. e
c i)
/'r--a , I\
200
4
§ 3 -e 0
300
200 300
400 500
a Owl% b--3wt% c.--Swt% d--7wl% e--9wr%
600
Wavelength/nm
400
a -~··-· tlwt% b-·~--3wt%
o~5wt%
d--· 1wt% e --·-- 9wl%
500 600
Wavelength/run
a--OWl.% b--3wt% c--Swt% d--1wt% $--iw't%
400 500 600
Wavelengthlnm
700
a)
700
b)
700
c)
Figure 5. UV-VIS spectra of polymers obtained form AI series
composites (a) scattering, (b) absorbance and (c) transmittance
[29].
Recently, Gatos et al. [61] produced nanocomposites by mixing
boehmite aluminas with polyurethane (PU) latex. The mechanical
performance, thermal stability, water uptake and dielectric
response of these nanocomposites were investigated, as a function
of the nominal nanoparticle size of the boehmites. The stiffness,
determined by tensile experiment and
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Latex-Based Nanocomposites 93
dynamic mechanical thermal analysis (DMTA), was enhanced with
decreasing particle size at the same filler content. The composite
with 25 nm boehmite alumina has the highest improvement in modulus,
however, at cost of thermal stability compared to these with 90 nm
and 220 nm boehmite alumina. The water uptake increases with the
addition of fillers; however, the effect becomes less significant
when the size of alumina particles increases. The dielectric
response of the nanocomposites was examined by means of broadband
dielectric spectroscopy in the frequency range 10-3~107 Hz, at
ambient temperature. Three distinct relaxation modes were recorded
in the spectra of all systems. They were attributed to interfacial
polarization, glass transition (a-relaxation) and movement of polar
side groups (b-relaxation).
3.5. Carbon Nanotube/Latex Nanocomposite
Ever since the first scientific report of carbon nanotubes
(CNTs) in 1991, carbon nanotubes become a world research focus in
recent 15 years [62, 63]. According to its structure it can be
divided into single-walled carbon nanotubes (SWTs) and multi-walled
carbon nanotubes. CNTs have attracted enormous interest owing to
their potential applications in field-emission devices,
electronics, fibers, composites, sensors, detectors, capacitors,
hydrogen storage media, and fuel cell, and so on [37, 64]. Recent
work has demonstrated how nanocomposites of polymers and CNTs offer
the advantages of polymers, such as optical clarity,
viscoelasticity and good barrier properties, combined with the
strength and high thermal and electrical conductivity ofCNTs [65,
66].
Single-walled carbon nanotubes (SWNTs) remain an interesting
filler material for polymers due to their large aspect ratio, small
diameter and relatively large length, high elastic modulus [67],
high intrinsic electrical conductivity [68, 69] and high thermal
conductivity and their surprising high malleability and elastic
deformation. The malleability of nanotubes is 200 times of other
fibers, which makes them easily restore their original shape
without any destruction after removing the exerted high pressure
[1]. SWNT-filled polymers are a unique class of composites due to
their ability to achieve significant enhancements with a very low
filler concentration. Significant improvements in thermal
transport, electrical conductivity, and mechanical properties of
polymers have been achieved with the addition of less than 1 wt%
SWNTs [66].
Vandervorst et a!. [ 64] used latex processing and
functionalized S WNTs to create crack-free, water-resistant, and
optically clear nanocomposite coatings. In this approach, there is
no requirement for adding surfactant or emulsifier. Nanocomposites
with carbon nanotubes exhibit high electrical and thermal
conductivity and enhanced mechanical properties.
4 . LATEX N ANOCOMPOSITES FILLED WITH ORGANIC PARTICLES AND
THEIR COMPOUNDING METHODS
In last few decades, the development of biodegradable polymers,
based on renewable resources, has attracted increasing interest
because of environmental pollution and the petroleum crisis. Many
organic particles, such as starch, fiber, lignin and chitin
whiskers,
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94 Ling Xue Kong, Ying Chen, Zhen Peng et al.
have been used to substitute inorganic particles in polymer
reinforcement. Great efforts have been made in organic-particle
modification and compounding methods in order to achieve
homogeneous dispersion.
4.1. Starch/Latex Nanocomposites
In comparison with inorganic particles, such as carbon black and
silica, starch is a renewable natural product with the following
advantages: low density [70], cost-effectiveness, abundant supply,
and environmental amity. Therefore it is widely used in food,
paper-making [71, 72], fine chemicals, and packing material [73,
74] industry. Given the declining storage of petroleum, much effort
has been devoted to develop starch-based polymers: that is,
chemically modified starch being used as a matrix or filler. This
promising technology has been attracting an increasing attention,
since it provides an environmentally friendly alternative to the
present petroleum-based plastics.
In the past, some researchers used resorcinolformaldehyde(RF) to
modify starch xanthathate/rubber composites to improve interfacial
strength, but the result was not as good as expected. In order to
achieve fine dispersion of starch in rubber matrix, Tang et al.
[75] and Angellier et al. [76] developed a novel dispersion
technique- the latex compounding method. This method was to mix
rubber latex with starch paste and then coagulate the mixture. The
method has two merits:
I. water is an excellent medium to dissociate the hydrogen
bonding of starch, and 2. most rubbers have a latex form.
In comparison with previous direct compounding techniques, this
method can produce smaller size of starch granules and help
homogeneous dispersion.
4.2. Nano-Fiber!Latex Nanocomposites
To take the advantage of the biomass, fibrous cellulose (e.g.,
wood, coir, pineapple, sisal) is gaining ever more attention as a
reinforcing agent in polymer matrices [77, 78]. Short fibers are
particularly interesting because of their great reinforcing
potential and their ability to make recyclable composites with
thermoplastic matrices. However, in accordance with the
reinforcement theory, this interesting potential cannot be fully
exploited without:
I. an adequate interfacial adhesion between fibers and matrix,
2. an optimized fiber aspect ratio length to diameter ratio (LID),
and 3. an acceptable dispersion level of the fibers into the matrix
phase.
Unfortunately, materials properties such as high viscosity of
the molten polymer and systematic formation of hydrogen bonds
between cellulose fibers usually lead to heterogeneous dispersion
with the formation of cellulose aggregates. Otherwise, a decrease
of the LID ratio is frequently occurred by fiber breakage during
the mixing step. Moreover, these thermoplastic matrices and
hydrophilic cellulose fillers usually result in composites with
poor
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Latex-Based Nanocomposites 95
performance. Many investigations have shown that the use of
various surface treatments applied onto cellulose fibers or
chemical modifications of the matrix (e.g., by addition of maleic
anhydride) can improve the dispersion and the mechanical properties
of the composites (i.e. tensile strength, and impact strength)
[70].
The preparation methods include direct blend, for example
attapulgite/rubber nanocomposites, electric nano-fiber/rubber
nanocomposites and nano-whisker/rubber nanocomposites by
traditional mechanical mixing processing; in-situ polymerization,
in which rigid monomers in-situ condensation polymerize in
styrene-butadiene block copolymers (SBS) to improve the modulus and
heat resistance [79], and solution blend coprecipitation, which
prepares poly(p-phenylene terephthalamide) (PPT A)/ Nitrile rubber
(NBR) molecular composites by blending PPT A and NBR in solution.
Obviously, compared to other preparations, direct solution blending
is a simple and promising method to prepare fibrous
nanocomposites.
Hajji et a! [78] produced cellulose whisker/latex nanocomposite
by adopting a water suspension mixing procedure instead of using
compatibilizing agents. Cellulose whiskers have been used to
reinforce a copolymer matrix prepared from a latex phase (i.e.,
water suspension of polymer spheres with low viscosity) to process
nanocomposites with quasi-isotropic mechanical properties to
improve the dispersion quality and limit the degradation of the
whisker aspect ratio during the compounding step.
4.3. Lignin/Latex Nanocomposites
Lignin is a waste material, and can be largely extracted from
waste solution originating from paper making. It is a kind of
renewable reinforcing filler for polymer. The mechanical properties
of composites that filled with lignin can be significantly improved
due to lignin's multifunction, high-impact strength, thermal
resistance, and biodegradable properties [80, 81 ].
Lignin is the reinforcing part of plant, composed of C, H and 0
elements, as the form of aromatic compound of phenylpropyl units
connected by -0-, -C-C-. It can be dissolved during papermaking,
and its molecular weight is from several hundred to several
million. Lignin can be precipitated out by acid from its lye, but
the diameter of lignin particles is relatively large (2~50)-tm) by
precipitating method, and cannot evidently decrease even by further
grinding.
The preparation of lignin nano-reinforcing (more accurately
fine- reinforcing) rubber is through coprecipitation [1]. It
dissolves the lignin into lye, then mixes the lye with latex, and
lastly coagulates them together by acid. The composite obtained by
this way has light color, low density, and higher physical and
mechanical properties than those of rubber with furnace carbon
black. In addition, the mixing time is greatly shortened, and the
size of dispersed lignin particles in rubber is below 1 OOnm.
Lignin is independent of petroleum resources and the polymer
reinforced with it has good surface properties, low density, the
same strength as polymer filled with carbon black, and a certain
level of flame resistance. Now much of the work related to
polymer-lignin products has focused on increasing the reactivity of
the lignin by modifying specific reactive groups, such as carboxyl
and hydroxypropyl groups. Especially, hydroxyalkylation of lignin
has been recognized as a promising technique for overcoming the
frequently observed adverse effect of
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96 Ling Xue Kong, Ying Chen, Zhen Peng et al.
lignin on mechanical properties of solid polymer materials. This
kind of materials have wide applications in various industry, and
therefore will be of great interest to researchers [70, 80].
4.4. Chitin Whiskers /Latex Nanocomposite
Chitin whiskers are very promising reinforcing materials for
polymer because of their high stiffness and strength [82]. Owing to
their small diameter, whiskers are nearly free of internal defects,
thereby yielding strength close to the maximum theoretical value
predicted by the theory of elasticity. The extent of their
reinforcement depends on many factors, such as the nature of the
matrix, the generation of a strong polymer matrix interface through
physicochemical bonding, the aspect ratio, and dispersion of the
whiskers in the matrix. Compared with inorganic whiskers, whiskers
from renewable resources have advantages such as renewability, low
cost, easy availability, good biocompatibility, and easy
modification by chemical and mechanical methods. Chitin, from
shellfish, insects, and microorganisms, is the second most abundant
structural biopolymer. Chitin whiskers have been used as a new kind
of nanofillers in both synthetic polymeric matrices and natural
ones [82]. Utilizing natural fillers from renewable resources not
only contributes to a healthy ecosystem but also makes them
economically interesting for industrial applications due to the
high performance of the resulting composites.
Nair et al.[82-84] produced nanocomposite materials from a
colloidal suspension of chitin whiskers as the reinforcing phase
and latex of both unvulcanized and prevulcanized natural rubber as
the matrix. The chitin whiskers, prepared by acid hydrolysis of
chitin from crab shell, consisted of slender parallelepiped rods
with an aspect ratio close to 16. After the two aqueous suspensions
were mixed and stirred, solid composite films were obtained either
by freeze-drying and hot-pressing or by casting and evaporating.
The processing and swelling behavior of composite films were
evaluated. The resistance against swelling in an organic solvent
medium is a good evidence that a rigid chitin network exists. The
diffusion coefficient, bound rubber content, and relative weight
loss also supported the presence of a three-dimensional chitin
network within the evaporated samples. The mechanical behavior of
the composites gives additional insight and evidence for this
fact.
5. THE NANO-REINFORCING THEORIES
Why can the filler with small particle size offer the polymer
significant reinforcement effect? Is the reinforcing effect
improved with the decrease of particle size? What role does the
interface play in the reinforcement? Many theories have been
developed in order to explain the phenomenon. Some representing
theories are briefly described.
5.1. The Surface Structure Model
The surface of active fillers is not smooth, and the surface
structure has marked effects on the reinforcement. Three
reinforcing mechanisms originated from different scales are
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Latex-Based N anocomposites 97
involved. In the range of small scale (less than I 00 nm), due
to the multi-dimensions of surface structure of fillers, the shape
and activity of the fillers play an important role in the physical
and chemical interactions between fillers and polymers. At the
intermediate scale, particle agglomerates and gel cause
reinforcement. In a large scale, filler networks exhibit
reinforcement. At the low loading content, the diameter of
dispersion phase is very small, and therefore the reinforcing
mechanism is absorption. At the high loading content, hydrodynamic
reinforcement play a main role, and the deformation and rupture of
branched agglomerate dissipate energy [85].
5.2. Mechanical Model
In this model, the polymer-aggregate of fillers and
aggregate-aggregate interactions are taken into consideration. The
polymer-aggregate interaction can be represented by a linear
spring-dumper system; the force of neighboring aggregates is the
London-vander waals interaction, the magnitude of which decreases
with the seventh power of the distance, which can be represented by
a non-linear elastic spring. When the elastomer is stretched, the
agglomerate-polymer interaction makes the agglomerates move to new
rest positions. The model includes the following main points
[86]:
I. the linear viscoelastic behavior of the polymer matrix and
the nonlinear elastic behavior of agglomerates contribute to the
complex modulus;
2. upon deformation, the polymer pulls agglomerate apart, while
the London-van der waals force pushes them together;
3. due to the London-van der waals force, a pair of aggregates
have two stable equilibrium points, combination or separation
state; and
4. the transition of agglomerates from combination to separation
state is the reason that the complex modulus decreases with the
increase of shear rate.
6. CONCLUSION
As one of the most important materials, latex nanocomposites
have attracted more and more attention because of their simple
compounding methods, uniform nanoparticle dispersion in latex
matrix, unique properties, and wide range of applications. Great
achievements have been made to prepare latex nanocomposites.
However, much more efforts have to be done to modify the surface of
organic or inorganic particles so as to further improve the
nano-dispersion phase in polymer matrix and it is still a long way
to develop a valid reinforcement mechanism in the future.
The fundamental understanding of the interaction between fillers
and latex molecules will play an important role in developing novel
compounding methods. This includes the understanding of the
materials properties at molecular level, the forces existing during
the compounding, the validation of theoretical models. There are
many mature and emerging techniques to facilitate the development
of theoretical models. For example, various forces at molecular
level can be measured with equipment such as atomic force
microscope; the
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98 Ling Xue Kong, Ying Chen, Zhen Peng et al.
assembly process can be monitored with techniques including
small angle X-ray scattering and small angle neutron scattering.
All these advanced technologies will significantly enhance the
understanding of filler-latex interaction, which consequently
contribute significantly to the development of latex-based
nanocomposites with greatly improved properties.
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