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15 Biobased Thermosets
Ana Dotan
Pernick Faculty of Engineering, Shenkar College of Engineering and Design, Ramat-Gan, Israel
O U T L I N E
Polymers from Renewable Sources
577
Determination of Bio-Based Content in
Polymers
578
Raw Materials for Renewable Sources
Polymers
579
Polymers from Renewable
Sources
In the past few decades concern about the environment, climate changes, and limited fossil resources
has led to an intensive research of alternatives to
fossil-based polymers. From a historical point of
view, the first polymers used by mankind were from
renewable sources, bio-based polymers, long before
the birth of synthetic polymers. Celluloid, a biobased man-made polymer was invented in the 1860s,
and since then, many other bio-based polymers have
been developed. However, the development of the
crude oil industry in the 20th century transformed the
world of polymers, leading to the use of synthetic
polymers as a replacement for bio-based polymers.
The increasing use of synthetic polymers as a result
of the growing human population and standard of living in the next decades will result in higher demands
on oil production and will contribute to a possible
depletion of crude oil before the end of the 21st century. It is estimated that by 2015 the worldwide
annual production of plastics is very likely to reach
300 million tons (1418% are thermosets), which
will require large amounts of petroleum and will
result in the emissions of hundreds of millions of tons
of CO2 to the atmosphere [1]. A return to bio-based
polymers will reduce the dependency of the polymers
and plastics industry on petroleum, thus creating
more sustainable alternatives.
Thermoset from Renewable Sources
594
References
615
Bio-based polymers are derived from renewable
resources such as plant and animal mass from CO2
recently fixed via photosynthesis [2]. Bio-based polymers can be natural or synthetic. Natural bio-based
polymers are polymers synthesized by living organisms such as animals, plants, algae, and microorganisms. The most abundant bio-based polymers in
nature are polysaccharides [3]. Cellulose and starch
are natural polymers based on polysaccharides, and
are abundant in nature. Proteins and bacterial polyhydroxyalkanoates are also natural bio-based polymers
[4]. Bio-based polymers are not necessarily sustainable; this depends on a variety of issues, including
the source material, production process, and how the
material is managed at the end of its useful life. Sustainable produced bio-based polymers are those
grown without genetically modified organisms
(GMOs), hazardous pesticides, certified as sustainable for the soil and ecosystems, and compostable.
Sustainability also depends on the reduction of
impacts to occupational and public health as well as
the environment throughout their life cycles [2]. Biobased materials are important candidates for sustainable development since they present the potential to
reduce greenhouse emissions by sequestering CO2, to
reduce raw material costs, and to create opportunities
for growth and employment in agriculture [5].
Life cycle assessment (LCA) is the most widely
applied and accepted method to quantitatively assess
the environmental impact of a given material
Handbook of Thermoset Plastics. DOI: http://dx.doi.org/10.1016/B978-1-4557-3107-7.00015-4
© 2014 Elsevier Inc. All rights reserved.
577
578
throughout its life cycle; its principles and framework are described in ISO 14040 [6]. Usually the
impact categories considered in LCA are global
warming, acidification of soil, ozone layer depletion,
aquatic eutrophication, respiratory organics, respiratory inorganics, land occupation, non-renewable
energy, and aquatic ecotoxicity [7]. Additional environmental indicators are resource depletion and
human toxicity [5].
Global warming impact is measured by the
amount of CO2 that is liberated from the material
throughout its life cycle. CO2 is the principal anthropogenic gas that is thought to affect the Earth’s radiative balance. For this reason it is believed that there
is a close correlation between CO2 and the change of
the Earth’s temperature [8]. The CO2 footprint of a
material is assessed by the carbon emissions consequent on the creation of a unit mass of material,
including those associated with transport, generation
of the electric power used by the plant, and that of
feedstocks and hydrocarbon fuels. The CO2 footprint,
which is measured in units of kg of CO2/kg material,
is the sum of all contributions per unit mass of material. Renewable resource polymers have a low CO2
footprint since plants grow by absorbing CO2 from
the atmosphere and thus sequester carbon [9].
Determination of Bio-Based
Content in Polymers
Bio-based polymers can be made totally or partially from renewable source raw materials, produced from photosynthesis and CO2. In order to
determine the bio-based content of the polymer, the
“new” carbon content must be measured. A renewable source is replenished by natural processes at a
rate comparable to its exploitation rate. The carbon
content of such polymers is derived from the socalled short carbon cycle within an expected time
frame between 1 to 10 years. Most industrial polymers and plastics are presently produced from fossil
resources that are non-renewable as they cannot be
replenished at a rate comparable to the exploitation
rate. Fossil resources have a long carbon cycle,
with an expected time frame to convert biomass to
petroleum, gas, and coal of greater than 106 years
[10]. The accepted measure of bio-based content is
the level of 14C isotope in the feedstock (basically,
HANDBOOK
OF
THERMOSET PLASTICS
carbon dating), because ancient petroleum has lost
its 14C through radioactive decay whereas feedstock
derived from recently living organisms have a 14C
content related to the current equilibrium concentration in the atmosphere.
ASTM D6866-11 has developed a protocol to
quantify the bio-based content in materials by comparing the 14C/12C ratio to that of a standard specimen typical of living organisms [11]. CEN/TS
16137 is the equivalent European for the ASTM
standard test [3]. ASTM D6866-11 is the Standard
Test Methods for Determining the Bio-based
Content of Solid, Liquid, and Gaseous Samples
Using Radiocarbon Analysis. It defines bio-based
content as the amount of bio-based carbon in the
material or product as a percent of the weight
(mass) of the total organic carbon in the product.
This standard utilizes two methods to quantify the
bio-based content of a given product: (a) Accelerator Mass Spectrometry (AMS) along with Isotope
Ratio Mass Spectrometry (IRMS); or (b) Liquid
Scintillation Counters (LSC) using sample carbon
that has been converted to benzene. Those methods
directly discriminate between product carbon resulting from new carbon input and that derived from
fossil-based input. A measurement of the 14C/12C
ratio is determined relative to the modern carbonbased oxalic acid radiocarbon Standard Reference
Material (SRM) 4990c (referred to as HOxII), as
the oxalic acid standard is 100% bio-based. The
percent new carbon can be slightly greater than
100% due to the continuing (but diminishing)
effects of the 1950s nuclear testing programs.
Because all sample 14C activities are referenced to
a “pre-bomb” standard, all modern carbon values
must be multiplied by 0.95 to correct for the bomb
carbon and to subsequently obtain the true biobased content of the sample [11].
CEN/TS 16137 specifies the calculation method
for the determination of bio-based carbon content
in monomers, polymers, plastics materials and products using the 14C method based on three test
methods: (a) Proportional scintillation-counter
method (PSM); (b) Beta-ionization (BI); and
(c) Accelerator mass spectrometry (AMS). The analytical test methods specified in this Technical
Specification are compatible with those described
in ASTM D6866-11. The bio-based carbon content
is expressed by a fraction of sample mass, as a fraction of the total carbon content, or as a fraction of
15: BIOBASED THERMOSETS
579
the total organic carbon content. This calculation
method is applicable to any polymers containing
organic carbon, including bio-composites [3].
Calculation of percentage of bio-based content
according to CN/TS 16137:2011 is based on the
calculation of carbon content as a fraction of the
total organic carbon content (TOC) expressed as a
percentage, using Equation (15.1):
XB ;TOC 5 ðXB Þ=ðXTOC Þ
(15.1)
Here, XB 5 is the bio-based carbon content by
mass, expressed as a percentage; and XTOC 5 is the
total organic carbon content, expressed as a percentage, of the sample.
Raw Materials for Renewable
Sources Polymers
Natural Oils
Natural oils, which can be derived from both
plant and animal sources, are considered to be one
of the most important classes of renewable sources
because of the wide variety of possibilities for
chemical transformations, worldwide availability,
and relatively low price [12]. Vegetable oils, fish
oils, and oils from algae can be transformed in raw
materials for polymers. Vegetable oils are inexpensive and offer different degrees of unsaturation
while fish oils have a high degree of unsaturation
[13]. Plant oil is a type of lipid, stored in an organelle in the form of triglycerides, during the oilseed
development. Lipids may be defined as
hydrophobic or amphiphilic (both hydrophilic and
lipophilic) small molecules. Different plant species
contain lipids with different fatty acid compositions
and distributions. Lipids help form a hydrophobic
biological membrane that separates cells from their
surroundings and keeps chloroplasts, mitochondria,
and cytoplasm apart, thus preventing or regulating
diffusion of chemicals [14].
Plant oil is a mixture of various triglycerides
(also called triacylglycerol). One glycerol is
attached to three different fatty acids to form a triglyceride. Glycerolipid and fatty acids are synthesized in the oilseed simultaneously during seed
development, before forming diacylglycerol and
subsequently triacylglycerols. Triglycerides are
composed of three fatty acids joined at a glycerol
juncture, as can be seen in Figure 15.1. Fatty acids
account for approximately 95% of the total weight
of triglycerides and their content and chemistry are
characteristics of each plant oil and geographical
conditions [15]. The most common oils contain
fatty acids that vary from 14 to 22 carbons in
length with 0 to 3 double bonds per fatty acid.
Fatty acids derived from nature have even an number of carbons due to their biosynthesis (acetyl
coenzyme A, two carbon carrier). Some of the fatty
acids may have additional functional groups like
hydroxyl (castor oil), epoxide (vernonia oil), or
ketone (licania oil), as well as triple bonds [14,16].
Although fatty acid pattern varies between crops,
growth conditions, seasons, and purification methods, each of the triglyceride oils has a unique fatty
acid distribution, as can be seen in Table 15.1 [17].
Hydrolysis of vegetable oils provides about 15 different fatty acids (some of them can be seen in
O
1
H2C
O
HC
O
O
5
O
9
H2C
O
2
12
15
ω
α
3
4
6
Figure 15.1 A triglyceride molecule (from top to bottom: saturated palmitic acid and unsaturated oleic acid and
alpha-linolenic acid), glycerol linkage (1), ester group (2), α-position of ester group (3), double bonds (4),
monoallylic position (5), and bisallylic position (6).
Table 15.1 Fatty Acid Composition of Different Plant Oils [14,16]
Fatty Acid Composition (X:Y, where X is the number of carbon atoms and Y is the number of double bonds) %
Oil
Palmitic
16:0
Stearic
18:0
Oleic
18:1
Linoleic
18:2
Linolenic
18:3
Gadoleic
20:1
Erucic
22:1
Ricinoleic
18:1
Linseed
5.5
3.5
19.1
15.3
56.6
Soybean
11
4
23.4
53.2
7.8
Palm
44.4
4.1
39.3
10
0.4
Rapeseed
3
1
13.2
13.2
9
9
49.2
Castor
1.5
0.5
5
4
0.5
87.5
Sunflower
6
4
42
47
1
15: BIOBASED THERMOSETS
Table 15.1). Triglyceride oils have been used
extensively to produce coatings, inks, plasticizers,
lubricants, and agrochemicals.
The stereochemistry of the double bonds, the
degree of unsaturation, and the length of the carbon
chain are important parameters affecting physical
and chemical properties. The degree of unsaturation
can be expressed by the iodine value (the amount of
iodine in grams that can react with double bonds
present in 100 grams of sample). According to iodine
value, oils can be divided into three categories:
(1) drying oils (iodine value above 130, e.g. linseed
oil), (2) semi-drying oils (iodine value between 90
and 130, e.g. sunflower or soybean oil), and (3) nondrying oils (iodine value below 90, e.g. palm kernel
oil) [17]. Triglycerides contain active sites amenable
to chemical reaction: the double bond, the allylic carbons, the ester group, and the carbons alpha to the
ester group. These active sites can be used to introduce polymerizable groups on the triglyceride using
similar techniques applied in the synthesis of
petrochemical-based polymers. The key step is to
reach a higher level of MW and cross-link density, as
well as to incorporate chemical functionalities known
to impart stiffness in a polymer network (e.g. aromatic or cyclic structures) [14].
There are three main routes to obtain polymers
from plant oils: (1) direct polymerization through
the double bonds, or through other reactive functional group present in the fatty acid chain,
(2) chemical modification of the double bonds,
which introduces functional groups to facilitate the
polymerization, and (3) chemical transformation of
plant oils to produce platform chemicals that can be
used to produce monomers for the polymer synthesis, usually through the conversion of the triglycerides into mono/diglycerides or into simple fatty
acids [12].
Direct Polymerization of Natural Oils
Double bonds present in fatty acids can be polymerized through a free radical or cationic mechanism.
Drying oils, such as linseed or tung oil, are
vegetable oils which polymerize through a free-radical
mechanism [18]. The double bonds react with atmospheric oxygen, which leads to the formation of a network in a reaction called oxypolymerization, as can be
seen in Figure 15.2. These oils are film-formers and
are mostly used in paints, coatings, inks, and resins.
The reaction mechanism can be summarized as
581
O2
O
OH
O
Figure 15.2 Oxypolymerization of triglycerides.
follows: an initial hydrogen abstraction on the methylene group between two double bonds that leads to the
formation of conjugated peroxides, and then, radical
recombination that produces cross-linking (alkyl,
ether, or peroxy bridges [1921]. Oils with high
iodine value can also be polymerized directly via cationic polymerization using boron trifluoride diethyl
etherate as initiator [20].
Direct cationic homogenous copolymerization of
some vegetable oils with olefinic copolymers such
as styrene, divinylbenzene, norbornadiene, or dicyclopentadiene can be achieved using BF3OEt2 modified with methyl oleate or Norway fish oil ethyl
ester as initiator [12]. Ronda et al. [12] has shown
that a cationic polymerization of soybean oil with
styrene, divinylbenzene, and trimethylsilylstyrene
can lead to flame retardant bio-based thermosets. In
order to accelerate the reaction that usually proceeds for long periods of time due to low reactivity
of the internal double bonds, microwave irradiation
was used as an alternative to a heat source.
Chemical Modification of the Double
Bonds of Natural Oils
There are two different sites in triglycerides
available to chemical modification: ester groups,
which can be readily hydrolyzed or trans-esterified,
582
HANDBOOK
double bonds along the aliphatic chains, and
hydroxyl groups [22]. Some of the important chemical pathways of functionalization are described in
the following sections.
Epoxidation of Triglycerides
Epoxidized triglycerides can be found in natural
oils, such as vernonia (see vernolic acid,
Figure 15.3), or can be obtained from unsaturated
oils by a standard epoxidation reaction. Different
pathways of epoxidation can be found in the literature. Epoxides of fat and oils and their ester derivatives are usually used as plasticizes, toughening
agents, and stabilizers in plastics industry, especially as alternative plasticizers for the polyvinyl
chloride (PVC) industry. Linseed oil is the most
epoxidized oil due to the high content of double
bonds (linolenic acid), and is commercially available. Epoxidized soybean oil is also commercially
available, usually with a functionality of 4.14.6
epoxy rings per trygliceride [14]. The oxirane ring
is useful for further chemical modifications and for
the synthesis of thermosetting resins by crosslinking with anhydride or amine compounds or by
homopolymerization of the oxirane rings initiated
by catalysts [23]. Epoxidation can be done using
organic and inorganic peroxides together with a
metal catalyst; it can also be obtained using halohydrins (haloalcohols), molecular oxygen, and by in
situ epoxidation with percarboxylic acid.
A suitable technique for clean and efficient epoxidation of vegetable oils is the epoxidation in situ
CO2H
O
Figure 15.3 Vernolic acid.
THERMOSET PLASTICS
with percarboxylic acid using Acidic Ion Exchange
Resin (AIER) as the catalyst [24]. It is possible to
use either acetic acid or formic acid as the carboxylic acid in the epoxidation process. Peroxy-acid is
created by the reaction between hydrogen peroxide
and the carboxylic acid. The process can be controlled by the quantitative analysis of oxirane rings
and iodine number [25]. Different catalysts have
been studied, including ion-exchange resins, phosphotungstic acids, rhenium catalysts, titanium catalysts, and enzyme catalysts [26,27]. A schematic
example of a triglyceride epoxidation process can
be seen in Figure 15.4. Chemo-enzymatic epoxidation is a relatively new process and has the advantage of suppressing undesirable ring opening of the
epoxide. In this method, unsaturated fatty acid or
ester is initially converted into unsaturated percarboxylic acid by a lipase-catalyzed reaction with
hydrogen peroxide and is then self-epoxidized via
an intermolecular reaction [28,29]. The epoxidation
of triglycerides makes them capable of reacting via
ring opening [30]. Arkema is already commercializing a line of epoxy plasticizers from renewable
sources under the trade name of Vokoflex® based
on epoxidized linseed oil, soybeans, and tall oil of
fatty acids [31]. The preparation of bio-based epoxy
resins from epoxidized vegetable oils will be discussed later.
Acrylation of Epoxidized Oils
Acrylation of epoxidized oil can be achieved
from the synthesis reaction of acrylic acid with
epoxidized triglycerides [14,32]. Acrylated triglycerides can react via additional polymerization.
Acrylated epoxidized soybean oil is commercially
available for the surface coatings industry. The
reaction of acrylic acid with epoxidized soybean oil
occurs through a standard substitution, as can be
seen schematically in Figure 15.5. Although the
reaction of epoxidized soybean oil with acrylic acid
is partially catalyzed by the acrylic acid, the use of
additional catalysts such as tertiary amines or
O
O
OF
O
Formic/acetic acid
O
O
O
O
O
O
O
O
O
Catalyst
O
O
Peroxide
Figure 15.4 Epoxidation process.
O
O
15: BIOBASED THERMOSETS
583
Figure 15.5 Acrylation of epoxidized triglyceride oil (soybean oil).
organometallic catalysts is common. The resulting
polymer properties can be controlled by changing
the molecular weight of the monomer or the functionality of the acrylated triglyceride, residual
amounts of unreacted epoxy rings, as well as newly
formed hydroxyl groups, both of which can be used
to further modify the triglyceride by reaction with a
number of chemical species (such as diacids, diamines, anhydrides, and isocyanates). The polymer
can be stiffened by the introduction of cyclic or
aromatic groups [14].
Maleinization of Acrylated Epoxidized Oils
According to Wool [14], oligomers of maleinizated acrylated epoxidized oils can be obtained
by reacting the acrylated epoxidized oil with maleic
anhydride (Figure 15.6). The maleinization reaction
introduces more double bonds in the triglycerides.
The reaction, if not controlled, can lead to an
increase in the viscosity, which leads to gelation of
the oligomer. After maleinization the oligomer can
be cured with styrene.
Hydroxylation of Triglycerides
The introduction of hydroxyl groups at the position of double bonds creates polyols that can be
used in the polyurethane and polyester industry, as
well as for creating new pathways for triglyceride
functionalization [33]. Natural occurring hydroxyl
groups can be found in ricinoleic acid (castor oil).
The hydroxylation can be done by reacting an
epoxidized triglyceride with an acid, or alternatively by the hydroxylation of the double bonds.
Ring opening of epoxidized triglycerides can be
obtained by reacting with hydrochloric or hydrobromic acid or by an acid-catalyzed ring-opening reaction with methanol (yielding methoxylated polyol),
or reacting with water forming vicinal hydroxyl
groups, or even through a catalytic hydrogenation
[30]. The cross-linking density of thermoset polymers obtained from hydroxylated oils depends on
the number of hydroxyl groups in the oil and the
position in the fatty acid (end or middle of the fatty
acid chain) [33]. The hydroxylation process can be
controlled, measuring the hydroxyl values according to the ASTM titration method (D 1957-86). The
reactivity of the polyol obtained is relatively low
due to the nature of the secondary alcohol.
Furthermore, multiple numbers of hydroxyl groups
with varied reactivity are also obtained, which leads
to premature gelation [34].
Hydroformylation is another method for preparing polyols; hydrogenation of the formyl group produces hydroxyl, using cobalt or rhodium as
catalysts, at high temperatures [35]. In this process
aldehydes are formed and an extra carbon is introduced per double bond. Hydroformylation creates
primary hydroxyl groups which are more reactive
than the secondary polyols created via epoxidation
(Figure 15.7) [13].
Ozonolysis is another alternative chemical process utilized to synthesize polyols with terminal
hydroxyl groups from vegetable oils. Ozone, a very
powerful oxidation agent, is used to cleave and oxidize the double bonds in the vegetable oil and then
the ozonides formed are reduced to alcohols using
NABH4 or similar catalysts. Tran et al. [34] developed a method to synthesize vegetable oil-based
primary polyol (soybean oil) in a single-step ozonolysis procedure. When the oil is exposed to ozone
in the presence of ethylene glycol and an alkaline
catalyst, double bonds are cleaved, yielding a mixture of polyols that is dependent on the relative
concentration of the unsaturated fatty acids present
(Figure 15.8). In this type of ozonolysis, the ozonides produced react with the hydroxyl group of
the glycol to form an ester linkage with a terminal
hydroxyl group.
584
HANDBOOK
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THERMOSET PLASTICS
H2COH
CH2
H2COH
H2COH
CH2
OCH3
CH
CH2OH
HC
HC
OH
O
H2COH
OCH3
OCH3
CH3O
O
O
HC
OCH3
HC
HC
O
CH
CH
O
H2COH
O
CH
CH
CH
HOC
CH2OH
CH
HC
O
CH
CH3O
HC
HC
CH3O
HC
HO
CH2
O
CH2
CH
O
HCOH
CH3O
O
H2CO
OCH3
OH
Figure 15.6 Maleinization of acrylated epoxidized triglyceride oil (soybean oil).
H
CH O
Catalyst
C C
O
OCH3
HC
H2OCH
HCOH
HCOH
OCH3
HC
CO
CH
O
HC
HCOH2
OH
HOCH2
CH
O
H2OCH
H2COH
HOCH
CO
CHO
CH
O
OCH3
CH
OCH3
CH3O
HOCH2
CH3O
O
O
CHO
O
CH3O
CH3O
HC
O
HCOH
H2COH
HOCH
CH3O
O
HC
H2COH
CH3O
OCH3
HOCH
H2C
H2COH
CH
O
OCH3
O
CH
O
OH
C2H
HC
OH H2C
HOCH2
HC
CH
O
HC
H2COH
(Carbohydrate)
HC
CH3O
HCO
CH
O
H2COH
O
CH3O
CH
CH3O
CH3O
CH
CH3O
HCOH
HO
CH
CH3
HOC
H2COH
CH2
CH2
OH
C
+ CO + H2
Pressure
Figure 15.7 Hydroformylation process [13].
C
H2
Catalyst
H
CH2 OH
C
C
15: BIOBASED THERMOSETS
585
(a)
O
O
O
O3
O
O
O
HO
O
OH−
OH
+
O
O
O
OH
O
HO R
(b)
O
O
O
O3
O
OH−
O
O
O
HO
OH
+
O
O
OH
O
O
O
HO
OH
O
O
+
O
HO
O
(c)
O
O
O
O3
O
O
O
HO
O
O
OH−
OH
O
OH
+
O
O
HO
O
O
+
HO
O
OH
O
O
Figure 15.8 Alkaline-catalyzed ozonolysis of soybean oil with ethylene glycol using triglyceride containing
different fatty acids: (a) oleic acid, (b) linoleic acid, and (c) linolenic acid [34].
Maleinization of Hydroxylated Oils
The synthesis of maleinized hydroxylated oil is
similar to that used to obtain acrylated epoxidized
oil. After the hydroxylation, the oil is reacted with
maleic anhydride in order to functionalize the triglyceride with maleate half-ester, as can be seen in
Figure 15.9. The reaction can be catalyzed with
N,N-dimethylbenzylamine [14].
Enone-Containing Triglycerides
According to Ronda et al. [12], allylic hydroperoxides can be readily prepared by reacting
alkenes with photochemically generated singlet
oxygen, which results in a mixture of enones
(Figure 15.10). The researchers utilized this process
to obtain enone-containing triglycerides from high
oleic sunflower oil. Enone-containing triglycerides
are interesting alternatives to epoxidized oils for
the production of thermosets by cross-linking with
amines via aza-Michael addition [12].
Conversion of Triglycerides into Mono/
Diglycerides/Simple Fatty Acids
The conversion of the triglycerides into mono/
diglycerides or into simple fatty acids can produce
platform chemicals for producing monomers toward
polymer synthesis. Mono or diglycerides can be
obtained using a transesterification reaction with
glycerol (glycerolysis). Standard glycerolysis
586
HANDBOOK
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THERMOSET PLASTICS
Figure 15.9 Maleinization of hydroxylated triglycerides.
O
O
O
O
O
High oleic sulflower oil (85% oleic acid)
O
O2, TPP, hν, CH2CI2, 6h
O
OOH
O
O
OOH
O
O
O
O
O
OOH
Ac2O/Et3N, 4h
O
O
O
O
O
O
O
H2N
Mixture of isomers. 2.6 enone
groups per triglyceride molecule
NH2
90°C to 120°C, 12h
O
O
O
O
O
O
HN
O
Aza Michael cross-linked triglycerides
Soft rubbers Tg (DMTA) = 16°C
O HN
O
O
O
O
O
O
120°C to 160°C, 12h
Tough thermosets Tg (DMTA) = 64°C
Figure 15.10 Synthesis of enone-containing triglycerides from high oleic sunflower oil, cross-linked with 4,40 diaminodiphenylmethane [12].
15: BIOBASED THERMOSETS
587
O
O C
O
O C
1
2
+
OH
230-240°C
OH
%1 Ca(OH)2
O
OH
O C
z
Glycerol
Soybean oil
O
O
O C
O
O C
O C
OH
+
OH
OH
Diglyceride
Monoglyceride
O
CH2
C
55°C
Pyridine,
Hydroquinone
C
O
O
C
C
CH3
CH2
CH3
Methacrylic anhydride
O
O
O C
O C
O
O
O
O
CH3
C
C
O
CH3
C
C
CH2
CH2
O
O
C
+
+
CH2
C
C
OH
CH3
O
O
CH3
C
C
Methacrylic acid
CH2
Figure 15.11 Glycerolysis reaction of soybean oil followed by methacrylation.
requires high temperatures and the use of an inorganic homogeneous catalyst such as sodium, potassium, or calcium hydroxide. Although this process is
energy consuming it has high conversion rates and
relatively short reaction times [36]. Enzymatic glycerolysis of fats and oils at atmospheric pressure at
nearly ambient temperatures has been investigated in
the last decade and has been found to be an attractive
and advantageous route compared to the chemical
process [37,38]. Usually the product of glycerolysis
is a mixture of monoglycerides, diglycerides, and
glycerol. According to Can et al. [39] maleinization
of soybean oil and castor oil monoglycerides can
produce maleate half-esters which in turn can react
via addition polymerization. Maleates are relatively
unreactive with each other and the addition of styrene increases the polymerization conversion rate
and the stiffness of the resultant polymer.
Methacrylation of Monoglycerides
Methacrylation of monoglycerides can be performed after the glycerolysis of the triglyceride (soybean oil). The glycerolysis product can be reacted
with methacrylic anhydride to form the methacrylic
ester of the glycerides and methacrylic acid using
pyridine as catalyst and hydroquinone as inhibitor of
the radical polymerization of the methacrylic esters
(Figure 15.11).
588
Glycerol
Glycerol is a non-toxic, edible, biodegradable
compound. It is an extremely versatile building
block. Glycerol is usually found in pharmaceuticals,
cosmetics and personal care products, alkyd resins,
etc. Glycerol is produced in two forms: natural
glycerol, as a by-product of the oleochemical and
biodiesel industries, and as synthetic glycerol, from
propylene. Glycerol forms the backbone of triglycerides and is mainly produced by saponification of
oils as a by-product of the soap industry. Around
75% of the glycerol produced in the United States
is derived from natural sources [40]. Glycerol can
be used as oligomers or co-monomers in copolymers. Catalyzed self-condensation of glycerol
yields a mixture of linear and branched oligomers.
Linear growth involves only the primary hydroxyl
groups while the secondary ones can have consequences. The preparation of hydroxyesters or
hydroxyacids from glycerol and their polycondensation by transesterification can lead to exquisite
polymers such as hyperbranched polycarbonates
and polyesters [41]. In recent years the amount of
natural glycerol-based monomers increased considerably, and includes diols, diacids, hydroxyacids,
oxiranes, acrolein, and acrylic acid, among others.
Ethylene and propylene glycol are particularly relevant for the polyester market (polyethylene terephthalate and polytrimethylene terephthalate).
Saccharides
Carbohydrates are considered a very important
renewable source of monomers for the preparation
of a variety of polymers since they are the most
abundant class of organic compounds found in living organisms. Sugars and starches are carbohydrates used as sources of metabolic energy in plants
and animals, while cellulose, also a carbohydrate,
serves as structural material. Carbohydrates are also
called saccharides, and small molecules of saccharides are called sugars. They can be divided into
three categories: (1) monosaccharides (glucose,
galactose, and fructose); (2) oligosaccharides, a
combination of two to ten monosaccharides; and
(3) polysaccharides such as cellulose, starch, glycogen, and hemicellulose [42]. The most abundant
oligosaccharide is sucrose, a disaccharide of glucose and fructose. The vast majority of carbohydrates present in nature are polysaccharides, with
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starch and cellulose being the most abundant.
Starch is a branched polymer while cellulose is a
long, rigid molecule. Sugar-based monomers can be
introduced into polymer architecture as follows:
(1) Addition reactions involving vinyl-type saccharides; (2) functionalization based on appending the
carbohydrate to a reactive backbone; and (3) polycondensation reactions of sugar-based monomers
[41]. Sugars are basically polyols with a high number of hydroxyl groups along the chain. In order to
control the reactivity of the many different
hydroxyl groups on carbohydrates, simple molecules with two hydroxyl groups are desired.
Dianhydrohexytols, such as isosorbide, isomannide,
and isoidide (Figure 15.12), are a result of intramolecular dehydratation; having two reactive hydroxyl
groups, they can be used as raw material for polycondensation, which leads to polyester, polyether,
or polyurethane chiral polymers. Isosorbide is prepared from starch, isomannide from D-mannose,
and isoidide from isosorbide [41]. Isosorbide, or
1,2,3,6-dianhydrosorbitol, is the product of a multistep process, starting with starch, and passing
through D-glucose and sorbitol. It is produced from
biomass in a combination of enzymatic and chemical technologies [44]. Due to steric effects and
hydrogen bonding, isomannide with two endo
hydroxyl groups is the least reactive compound
compared with isoidide, which has two exo
hydroxyl groups. However, isoidide is rare in
nature while isosorbide is widely available [45].
Isosorbide, which is water soluble and non-toxic,
can be a substitute for bisphenol A in different
polymers, especially in epoxy polymers. It can be
attached to glycidyl ether or allyl ether to make
cross-linkable epoxy monomer with similar properties to bisphenol A-diglycidyl ether [46]. According
to Feng et al. [45] the bisisosorbide diglycidyl ether
can be prepared by heating isosorbide with sodium
hydroxide solution with a large excess of epichlorohydrin. Two equivalents isosorbide are linked to
Figure 15.12 Structures of isosorbide, isomannide,
and isoidide [43]. (Reprinted with permission from
the National Academy of Engineering.)
15: BIOBASED THERMOSETS
589
three molecules of epichlorohydrin to form the
epoxide dimer (Figure 15.13).
Xylitol, sorbitol, mannitol, and maltitol
(Figure 15.14) are additional naturally occurring
sugar alcohols; polyols that can be polycondensated
with dicarboxylic acids such as sebacic acid, citric
acid, glutaric acid, and others [47]. The polymer
obtained can be made photo-cross-linkable by adding methacrylate units to the polymer.
Polyphenols
centers accommodate the inter-flavonoid bonds. Arings in tannins are highly reactive to aldehydes
(formaldehyde) in wood adhesive compositions and
the reaction kinetics can be controlled by the addition of alcohols to the system. Self-condensation
reactions of polyflavonoid tannins are used to prepare adhesives in the absence of aldehydes. The
reaction is based on the opening under alkaline or
acid conditions of the flavonoid repeating unit and
the subsequent condensation of the reactive center
with free sites of a flavonoid unit on another tannin
chain [48].
Naturally occurring polyphenols are characterized by the presence of multiple phenol structural
units. Tannins are a naturally occurring broad class
of polyphenols present in trees and shrubs. Tannins
can appear in nature as condensed tannins (polyflavonoids) or hydrolyzable tannins [41]. Condensed
tannins are oligomeric in nature while hydrolyzable
tannins are non-polymeric. Tannin has been used
for centuries in leather treatment. Tanning leather
involves a process which permanently alters the
protein structure of leather, thus increasing its durability. Condensed tannin constitutes more that 90%
of the total world production of commercial tannins
(Figure 15.15). Hydrolyzable tannins are a mixture
of phenols based on complex substances built with
simple molecules such as gallic acid, ellagic acid,
flavogallonic acid, valoneic acid, etc. Hydrolyzable
tannins have relatively low reactivity. In condensed
tannins the main polyphenolic pattern is represented
by flavonoid moieties based on resorcinol A-rings
and pyrogallol B-rings. A-rings contain one highly
reactive nucleophilic center while the other reactive
Figure 15.14 Naturally occurring polyols.
HO
O
CI
3
NaOH
H2O
+
O
O
OH
Epichlorhydrin
Isosorbide
O
O
O
O
O
O
O
O
OH
Bisisosorbide diglycidyl ether
Figure 15.13 Bisisosorbide diglycidyl ether preparation [45].
O
O
590
Figure 15.15 Condensed tannin.
Lignin is the second most abundant organic polymer on Earth, after cellulose, and the most abundant polyphenol in nature. It is most commonly
derived from wood. Lignin is an integral part of the
secondary cell wall of plants and some algae and
has the function of bonding cells together in the
woody stems, giving the stem rigidity and impact
resistance [14]. Lignin accounts for 24 to 33% of
the dry matter of softwood and 16 to 24% of the
hardwood [49]. Lignin chemical structure is based
on syrigyl, guaiacyl, and p-hydroxyphenol, bonded
together by a set of linkages to form a complex
matrix [50], as can be seen by the schematic
structure in Figure 15.16. Lignin is a complex,
three-dimensional, amorphous, cross-linked phenolic-based polymer. It is a by-product of paper pulping
and biorefineries, and is considered a waste product.
Lignin is separated from wood during pulping and
papermaking operations where it serves as fuel for the
process. Major differences exist between lignin
derived from different pulping processes [51]. Sulfite
processes generate water-soluble sulfonate lignin
while kraft processes generate alkaline soluble lignin.
Hydrolysis lignin is produced by strong hydrolysis,
organosolv lignins are produced from different
organic solvent-based systems, and steam-explosion
lignin is obtained through high temperature/pressure
treatment with steam [42]. Recent pulping trends
involving organic solvents (organosolv) produce lessmodified, sulfur-free lignins, which simplifies the
pyrolysis process for obtaining low-molecular-weight
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lignin sub-products [52]. All chemical pulping processes are associated with the chemical splicing of
natural lignin producing fragments with high molecular weights [41].
Polymeric lignins can be used in thermosetting
resins and as additives in thermoplastic polymers.
Lignin can be chemically modified to improve
polymer-lignin compatibility and to introduce reactive sites. The phenyl propane units in lignin
are generally bonded through carbon and ether
bonds, where only ether bonds can be easily disrupted. The methoxy group substituted at the ortho
positions also influences the reactivity and solubility
of the lignin [49]. Lignin and its degradation products can originate various polymers such as
phenolformaldehyde, poly(azophenylenes), polyimides, polyurethanes, polyesters, polyamides, polyphenylene oxides, polyphenylene sulfide, and others.
Due to the high number of reactive hydroxyl groups
present, esterification and etherification are the most
common reactions used for chemical modification of
the lignin. According to Lora et al. [51], etherification with alkylene oxides (ethylene oxide, propylene
oxide, and butylene oxide [53,54]) results in hydroxyalkyl lignin derivatives with aliphatic hydroxyl
groups. Nonphenolic hydroxypropyl lignins are
already commercially available in the market.
Extended alkyl ether chain lignins can be also prepared, although propylation through heterogeneous
reaction of lignin with propylene oxide can lead to
unpredictable and even dangerous reactions [51].
Etherification and acetylation of lignin provide solubility in a variety of organic solvents such as acetone, tetrahydrofurane, and chloroform. Lignins can
also be sulfonated, sulfomethylated, aminated, halogenated, and nitrated.
According to Wool et al. [14], grafting of lignin
is another possible chemical modification, despite
the inhibiting effect of the phenolic hydroxyl
groups, which reduces the efficiency of the process.
The addition of polar solvents, such as alcohols,
can increase the efficiency though swelling the lignin molecules improves the accessibility of the
reagents. Lignin has reportedly been grafted with
methyl methacrylate, vinyl acetate, styrene, acrylonitrile, acrylic acid, acrylamide, maleic anhydride,
and others. Methacrylated lignin can form crosslinked networks when copolymerized with methyl
methacrylate. Other grafting techniques such as
ionic chain polymerization and chemo-enzymatic
grafting have also been reported [14]. A great deal
15: BIOBASED THERMOSETS
591
H2COH
CH2
H2COH
H2COH
CH2
OCH3
CH
CH3
CH
CH2OH
HC
HC
OH
O
H2COH
OCH3
O
O
HC
O
HC
OCH3
HC
OCH3
CH3O
CH
O
OH H2C
HOCH2
HC
H2C
CH
H2COH
O
O
O
CH
HOCH
O
HOC
CH
CH2OH
CH
HC
HC
HC
CH2
CH3O
HC
OCH3
HCOH2
CH3O
HO
HC
CH
H2OCH
HCOH
HCOH
CH
HC
O
CO
CH
O
CH
H2COH
HOCH
O
H2OCH
CH
CHO
CH
OH
HOCH2
H2CO
O
O
OCH3
O
OCH3
CH3O
HOCH2
CH3O
CO
O
CHO
O
CH3O
CH3O
HC
O
HCOH
H2COH
H2COH
CH
CH
HC
H2COH
CH3O
CH
CH3O
OCH3
O
CH
O
OH
OCH3
HOCH
HC
C2H
HC
HCO
H2COH
(Carbohydrate)
HC
CH3O
CH
CH
O
H2COH
O
CH3O
HO
CH3O
CH3O
CH
CH3O
HCOH
HOC
H2COH
CH2
CH2
OH
O
CH2
HC
OCH3
O
HCOH
CH3O
O
OCH3
OH
Figure 15.16 Generic structure of lignin.
of effort and research has been done to determine
the possible effects of lignin addition to polymers,
introducing functional groups to increase chemical
interactions. Lignin can be used as a co-monomer
in phenolic thermosetting polymers in wood adhesive applications. Lignin can also be co-reacted
with epoxy resins and polyurethane precursors [14].
Star-like macromers from lignin were obtained by
Oliveira et al. [55] through a combination of chain
extension and etherification reaction, synthesizing
lignin with propylene oxide following capping of
OH functional groups with aliphatic ethers.
Polyurethane films were prepared by Kelley et al.
[56] based on hydroxypropyl lignin. In order to
reduce rigidity of the network polyethylene glycol
and polybutadiene glycol extended lignin-based
polyurethanes were prepared mixing two polyol
components prior to cross-linking. Feldman et al.
[50] modified a bisphenol A-based epoxy adhesive
poly-blending with Kraft lignin.
592
Cardanol is the main constituent (97%) of thermally treated cashew nut shell liquid and is based
on phenolic compounds (Figure 15.17). In the last
few decades several new methodologies were
developed for the preparation of polymers using
cardanol [16]. Cardanol is a phenol derivative with
a meta-substituent of a C15 unsaturated hydrocarbon chain with one to three double bonds; it has
potential applications in surface coatings and resins.
Cardanol is used to industrially produce phenolic
resins with formaldehyde (novolac) for coatings
applications, generating high-gloss films for indoor
use [57]. Ikeda et al. [57,58] studied oxidative polymerization of cashew nut shell liquid using iron-N,
N0 -ethylenebis(salicylideneamine) (Fe-salen) as catalyst, in bulk and at room temperature. A soluble
cross-linkable polymer was obtained. The polymer
was cross-linked using heat and a hard, glossy
film was obtained. Gopalakrishnan et al. [59] developed a novel polyurethane based on cardanol, condensing cardanol with formaldehyde using sebacic
acid as catalyst. The resulting resin (novolac
phenolformaldehyde resin) was epoxidized, followed by hydrolysis in order to obtain a hydroxyl
alkylated derivative, a polyol. The resulting polyol
was reacted with hexamethylene diisocyanates,
generating rigid polyurethanes. Benedetti et al. [60]
developed cardanol-based polyurethanes by reacting cardanol derivatives with diisocyanates or polyisocyanates and blowing agents in order to obtain
foamed polyurethanes. Cardanol was derivatized by
condensing cardanol with alkylic aldehydes or
acrylic aldehydes with an acid catalyst. The product
was epoxidized and hydrolyzed to obtain polyols,
which in turn were reacted with isocyanates using
blowing agents. Cardanyl acrylate prepolymers
with controlled molecular weight and polydispersity
can be used in the formulations of inks, coatings,
and adhesives with good adhesion and gloss [16].
Campaner et al. [61] investigated the use of
cardanol-based novolac resins as curing agents of
commercial epoxy resins. The novolacs were prepared by the condensation reaction of cardanol and
paraformaldehyde using oxalic acid as catalyst.
Proteins
Proteins are readily available raw materials from
plants (wheat gluten, soy, sunflower, and zein) and
animals (collagen, keratin, casein, and whey) [62].
Proteins are macromolecules consisting of one or
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Figure 15.17 Structure of cardanol.
more polypeptides, based on amino acids (up to 19
different amino acids) bonded together by peptide
bonds. Polypeptides are usually folded into globular
or fibrous form according to their biological function [63]. Proteins hold together, protect, and provide structure to the body of living organisms in
the form of skin, hair, calluses, cartilage, muscles,
tendons, and ligaments. Proteins can catalyze, regulate, and protect the body chemistry as enzymes,
hormones, antibodies, and globulins [64]. Most proteins fold into 3-dimensional structures. The primary structure of proteins is defined by the
sequence of amino acids in the chain; the folding
pattern defines the secondary structure, resulting
from the rigidity of the amide bond and from intramolecular hydrogen bonding. Coiling and aggregation of polypeptides creates the tertiary structure.
The quaternary structure refers to the spatial
arrangement of a protein, as an oligomeric structure
containing several subunits [42]. Covalent disulfide
cross-linking bonds stabilize the tertiary and quaternary structures. The shape into which a protein naturally folds is known as its native conformation.
Denaturation is the breakdown of the tertiary
structure of a protein, and involves structural or
conformational changes to native structure, such as
unfolding of the protein molecule [14]. These
changes can be induced by pH, detergents, heat,
etc., and are accompanied by irreversible enthalpy
changes [63]. Plasticization of proteins can be
obtained by the interaction between molecules of
plasticizer and protein macromolecules via polar
interactions (with hydroxyl groups) that increase, as
a result, the free volume. Through plasticization,
proteins can be transformed using various
15: BIOBASED THERMOSETS
processing methods [42]. According to Verbeek
[62], melt extrusion of proteins can occur through
denaturation, dissociation, unraveling, and alignment of polymer chains. The presence of disulfide
cross-links in this case is unfavorable, and
decreases chain mobility, increases viscosity, and
prevents homogenization. Proteins usually decompose at temperatures below the softening temperatures, and the introduction of plasticizers helps to
avoid degradation. Animal and vegetable proteins
can be cross-linked by the reaction of tannic acid,
chromic acid, or formaldehyde. The use of proteins
as raw materials for polymers is not new. Some of
the earliest plastics were based on casein, phosphoproteins found in abundance in cow milk. Caseinbased paints were used until the late 1960s, when
they were replaced by acrylic-based paints.
Another important cow milk protein is the whey
protein, isolated from the whey, a liquid by-product
of cheese production. The protein in cow’s milk is
20% whey protein and 80% casein protein. Whey
protein films and coatings have been the focus of
recent research as potential materials for high oxygen barrier packaging [65,66]. Whey protein can be
plasticized using glycerol and sorbitol [66]. Gao
et al. developed whey protein-based aqueous
polymer-isocyanate adhesives. The adhesive properties improved cross-linking of the resulting polymer with methylene bisphenyl diisocyanate [67].
Zein, a class of prolamine protein found in corn,
was historically used as polymeric resins and coatings in a variety of applications such as fibers and
tough, glossy, and grease-resistant coatings. Zein
has also antimicrobial resistance and can be cured
with formaldehyde. Zein has regained interest in
the last few years, and the water resistance was
improved by chemical modification where hydrophylic groups such as aNH2, aOH, aCOOH, and
aSH were modified by hydrophobic groups or
polymer segments. Plasticizers can be introduced in
order to flexibilize zein films. Wu et al. [68] introduced dibutyl L-tartarate (DBT), which contains
ester and hydroxyl moieties to form potential
hydrogen bonds with zein. Sessa et al. [69] introduced isocyantes and diisocyanates to reduce the
hydrophilicity of zein and, as a consequence,
decrease the water moisture uptake. Cross-linking
through isocyanate moieties should improve dimensional stability. Lai et al. [70] used oleic acid to
plasticize zein, thus creating more flexible and
tough films.
593
Collagen, a triple helical, self-organizing protein,
is the main component of connective tissue and is
the most abundant protein in mammals. Mano et al.
[71] has shown that collagen can be effectively
cross-linked with glutaraldehyde in order to overcome the drawbacks of fast biodegradation and
lack of mechanical properties.
Albumin usually refers to any protein that is
water soluble and denatures by heat. Oss-Ronen
et al. [72] conjugated serum albumin to polyethylene glycol (PEG) and cross-linked it to form
PEGylated albumin hydrogels. The main application is as scaffolds for controlled drug release.
Gluten is a protein composite based on gliadin and
glutenin, forming, together with starch, the endosperm of grass-related grains. The term gluten comes
from Latin, meaning “glue,” due to its adhesive properties. Gliadin and glutenin comprise around 80% of
the overall protein in wheat grains. Wheat gluten is
an important resource for bio-based polymers due to
its viscoelastic properties, mechanical strength, excellent gas barrier properties, low price, and large-scale
availability [73]. Gliadin and glutenin form a crosslinked network that contributes to the extensibility of
mixed dough. In order to reduce the strong intramolecular interactions, plasticizers are usually required,
which leads to an undesired reduction in mechanical
properties. Gluten has found applications in the
pressure-sensitive adhesives field [74]. A glutenbased PSA, with internal plasticizer, was developed
by Aranyi et al. [75]. An acetic acid soluble hydrolysate was obtained using a hydrochloric and acetic
acid mixture. The gluten hydrolysate was then epoxidized using ethylene oxide, and generated a water
soluble, epoxidized gluten hydrolysate. This hydrolysate was then reacted with hydroxyethyl methacrylate
giving rise to an acrylic copolymer of gluten with
PSA properties. A reduction in water vapor transmission of gluten coatings was obtained by Cho et al.
[76] using glycerol as plasticizer and oleic acid as a
hydrophobic component. Kim et al. [77] developed a
gluten/zein composite with flexural properties similar
to those of polypropylene.
Soy protein is the major co-product of soybean
oil extraction comprising around 50% of the
defatted soy flour [42]. Soy protein is a storage protein held in discrete particles called protein bodies
which provide amino acids during soybean seed
germination. Soy protein comprises a high percentage of water soluble albumins and salt solution soluble globulins. Water is considered a major
594
plasticizer in the processing of soy proteins. High
quantities of water content reduce the denaturation
temperature of soy proteins. Soybean protein plastics can be prepared using a variety of different
processes: compression molding, injection molding,
and lamination [78]. According to Sun [14], smooth
surface soy protein plastics can be prepared by hot
pressing soy protein at 150°C, the temperature at
which soy protein molecules melt and unfold, leading to a homogeneous distribution. A posterior
interaction between the molecules leads to entanglement upon curing.
Plastics based exclusively on soy protein are
rigid and brittle and plasticizers are usually introduced in order to increase the flexibility and toughness. Polyols have often been used as plasticizers.
Plasticizers disrupt molecular interactions, decreasing the forces holding the chains. According to Sun
[14], polypropylene glycol and glycerol appear to
be more compatible with soybean protein and easier to introduce between protein chains. Mo et al.
[78] studied the effect on soy protein properties of
different plasticizers, such as glycerol, polyethylene
glycol, and butanediols (1,2- and 1,3-substituted).
Ethylene glycerol, propylene glycerol, sorghum
wax, and sorbitol have also been studied for this
purpose [42,79]. Addition of cross-linking agents
such as formaldehyde, glutaraldehyde, and adipic/
acetic anhydride improve mechanical properties as
well as the water resistance of the resulting soy
protein plastic [80]. Gonzalez et al. [81] used the
naturally occurring cross-linker genipin, a chemical
compound found in gardenia fruit extract, in order
to tailor mechanical properties and biodegradability. Denaturants such as sodium dodecyl sulfate and
urea also act as plasticizers, reducing the glass transition temperature and increasing tensile strength,
elongation, and water resistance. A high degree of
entanglements and cross-links is expected when a
high degree of denaturation is obtained, leading to
an increase in stiffness and strength. Su et al. [82]
investigated the moisture barrier properties of
blends of soy protein with polyvinyl alcohol using
glycerol as plasticizer. Water vapor transmission
was significantly affected by the PVA content.
Studies on chemical modification of soy proteins
with monomers or oligomers have been done based
on functional groups able to react with hydroxyl or
amino groups in the protein. Acetylation and esterification are known pathways for chemical modification [80].
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Soy protein can be modified using polycaprolactone/hexamethylene diisocyanate prepolymer
[68]. It was found that HDI-modified PCL forms
ureaurethane linkages with the amino acids in the
protein, increasing substantially the water resistance
and the elongation. Grafting can be obtained with
acrylate-based polymers via free radical reaction,
although branching is created, disrupting the strong
inter and intramolecular interactions [80]. Grafting
with polyurethane pre-polymer has been proved to
enhance toughness and water resistance [42].
Soy can be used as an adhesive and the performance can be controlled depending on the particle
size and structure of the protein, viscosity, and pH
[83]. In order to enhance the adhesion strength of
soy protein, a chemical modification is needed to
break the internal bonds and uncoil or disperse the
polar molecules. Dispersion and unfolding of protein is obtained by hydrolysis or by an alkali
environment.
Thermoset from Renewable
Sources
Epoxy
Approximately 90% of all non-bio-based epoxy
in the market is based on diglycidyl ether of
bisphenol A (DGEBA) derived from epichlorohydrin and bisphenol A. Epichlorohydrin is an epoxide. The conventional, petrochemical process of
producing epichlorohydrin is the chlorohydrination
of allyl chloride, which in turn is made by chlorination of propylene. Until recently, epichlorhydrin
has also been used to produce glycerol (glycerine).
The large availability of bio-based glycerol,
obtained as a by-product of biodiesel production,
has made the production of glycerol using epichlorohydrin superfluous [4]. Epichlorohydrin can be
produced using glycerol from renewable feedstock.
The glycerin-to-epichlorohydrin (GTE) is a process
based on two chemical steps: (1) hydrochlorination
of glycerin with hydrogen chloride gas at elevated
temperature and pressure using a carboxylic acid as
catalyst, and (2) conversion of the dichlorohydrin
formed in the first step to epichlorohydrin with a
base [84]. This new glycerin-to-epichlorohydrin
process reduces energy consumption by about onethird, generates less than one-tenth the waste water,
and produces less chlorinated organics, when
15: BIOBASED THERMOSETS
compared to conventional processes [85]. Solvay
Chemicals produces epichlorohydrin from biobased glycerol, a by-product of biodiesel production using rapeseed, according to EPICEROL®
technology [86]. The Dow Chemical Company,
currently the world’s largest producer of epichlorohydrin, also announced its intent to manufacture
epichlorohydrin via a novel, acid-catalyzed hydrochlorination process using bio-based glycerin [4].
Krafft et al. [87] developed a process from glycerol via 1,3-dichloropropanol. Bio-based DGEBA is
chemically identical to the fossil one, and the biobased epichlorohydrin accounts for approximately
20% of the molecular weight of DEGBA. Glycerolbased epoxy resins such as glycerol polyglycidyl
ether and polyglycerol polyglycidyl ether are industrially available, and have been used in the textile
and paper industry as processing agents, tackifiers,
coatings, etc. Shibata et al. [88] developed a biobased epoxy system based on a mixture of commercially available sorbitol polyglycidyl ether (SPE,
DENACOL EX-614B) [89] and glycerol polyglycidyl ether (GPE,DENACOL EX-313) [89] cured
with e-poly(L-lysine) as the bio-based curing agent.
Sorbitol polyglycidyl ether is a multifunctional
epoxy resin obtained by the reaction of epichlorohydrin and sorbitol, which in turn is obtained by
the reduction of glucose. Tannic acid was used as
the bio-based curing agent.
Bio-based epoxy can also be obtained using
plant oils and fatty acids. Unsaturated vegetable
oils can be converted into epoxidized oils using
performic acid and peroxide (see the section on natural oils) or through enzymatic processes [90].
Natural epoxidized oil can be found in vernolic
acid, present in Vernonia species. Epoxidized UVcurable resins can be synthesized via transesterification of vernonia fatty acids with hyperbranched
hydroxyl functional polyether. The resin can then
be cationically polymerized in the presence of vernolic acid and methyl ester as diluents [17].
Epoxidized vegetable oils (castor oil, soybean
oil, linseed oil, etc.) are currently used in epoxy
compositions [91]. Epoxidized oils can be polymerized in the presence of latent catalysts or in the
presence of curing agents, such as anhydrides. The
low reactivity of the epoxy groups together with a
tendency for intramolecular bonding lead to a low
degree of cross-linking. As a result, poor mechanical and thermal properties are obtained with higher
water uptakes, compared to fossil epoxy systems.
595
On the other hand, the polymer obtained is potentially biodegradable via hydrolytic cleavage of
glycerol ester bonds present in the triglyceride oils.
Blends of epoxidized oils and fossil epoxy resins
have also been studied. Epoxidized soybean oil
(EBSO) is considered to be the second largest
epoxide following epichlorohydrin, and it is prepared commercially by epoxidation with percarboxylic acids. Epoxidized linseed oil (ELSO) can
be produced by epoxidation with formic acid and
hydrogen peroxide [4].
Tan et al. [92] added epoxidized palm oil to a
fossil-based epoxy blend of diglycidyl ether of
bisphenol A/cycloaliphatic epoxide resin/epoxy
novolac resin in order to obtain a thermal curable
partially bio-based epoxy system. The epoxidized
palm oil acts as plasticizer, thus reducing the glass
transition temperature of the system. In order to
overcome those limitations, Tan et al. [93] synthesized a thermally curable epoxidized soybean oil in
the presence of tetraethylammonium bromide catalyst. Increasing catalyst concentrations led to a
reduction of curing cycles and temperatures, and to
higher degrees of conversion and cross-linking density. As a result, higher storage modulus and glass
transition temperatures were obtained.
Chemical modification of the epoxidized oils is
another method used to improve final properties.
Transesterification of soybean oil with allyl alcohol
followed by epoxidation in the presence of benzoyl
peroxide, and cured with anhydride, resulted in
highly cross-linked polymers with better mechanical
properties. cis-9,10-Epoxy-18-hydroxyoctadecanoic
acid is a naturally occurring epoxidized monomer
present in birch tree. Lipase-catalyzed condensation
polymerization of this naturally occurring epoxidized acid creates epoxidized oligomers [91].
Lu et al. [94] synthesized high stiffness polymers
from epoxidized linseed oils as a result of the
high degree of unsaturation of the linseed oil.
Chandrashekhara et al. [95] developed a soy-based
epoxy resin consisting of mixtures of epoxidized
fatty acid esters, specifically, epoxidized allyl
soyate. Epon 9500 and Epicure 9550 from Shell
Chemical Company were used as base epoxy resin.
Epoxidized allyl soyate (EAS) was synthesized
through a lab-scale process from food grade soybean
oil. The epoxidized allyl soyate was prepared by
transesterification of triglycerides, and yielded fatty
acid methyl ester and allyl ester using methyl alcohol and allyl alcohol, respectively. The fatty acid
596
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THERMOSET PLASTICS
80
10% EAS
60
Stress (MPa)
esters were then epoxidized and yielded soyate
epoxy resins. The resin formulations were prepared
by directly mixing the epoxidized soyate resins into
base Epon resin and curing in one step. The following ratios of Epon/epoxidized soyate resins were
used: 100% Epon resin, 90/10%, 80/20%, and 70/
30%. Tensile tests results can be seen in
Figure 15.18. An increase in the ductility could be
noticed with the increase of the epoxidized soyate
resin content, accompanied by a corresponding
decrease in the ultimate strength and modulus. The
increase in the ductility was related to the higher
molecular weight of the soybean oil. It could be concluded that epoxidized allyl soyate provided better
intermolecular cross-linking, yielding tougher materials as compared to commercially available epoxidized soybean oil. The addition of epoxidized allyl
soyate to a commercial epoxy resin led to a viable
low-cost, high-performance thermoset product with
improved properties when used with glass fibers in
the pultrusion process. The pultruded composites,
based on soy-based resin systems, have shown comparable mechanical properties compared to neat
resin. The presence of soybean oil increased the
lubricity, thus reducing the pull force during the pultrusion process, an additional bonus.
Espana et al. [96] cured epoxidized soybean oil
of commercial grade (Traquisa S.A.Barcelona,
Spain, EEW of 238 g/equivalent) with maleic anhydride (AEW index of 98.06 g/equivalent) with the
aid of a mixture of catalysts (1,3-butanediol anhydrous and benzyldimethylamine). The mixture was
cured at various temperatures for 5 hours using different epoxidized soybean oil (EBSO):anhydride
(AEW) ratios. The best-balanced mechanical and
thermal properties were obtained for a ratio of 1:1,
representing a high level of cross-linking.
Maximum values of flexural modulus (432 MPa)
and Shore D (70) were obtained for an EBSO:AEW
ratio of 1:1, while maximum glass transition temperature (42.6°C) was obtained for a ratio of 1:0.9.
Lignin can be used as the hard segment in epoxy
resin networks, increasing the glass transition of the
resultant polymer. Hirose et al. [97] investigated
the properties of ester-type epoxy resins derived
from lignin. An ester-carboxylic acid derivative of
lignin was previously obtained from the alcoholysis
of lignin with succinic acid anhydride. The estercarboxylic acid obtained can be reacted with ethylene glycol diglycidyl ether to form epoxy resins.
Glass transition temperature increased with
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Pure epon
40
20% EAS
20
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Strain (m/m)
Figure 15.18 Mechanical properties of soy-based
resin systems [95].
increasing ester-carboxylic acid derivative content,
suggesting that lignin indeed acts as the hard segment (reaching a maximum value of 211°C).
Sugar-based epoxy systems were also developed.
East et al. [98] developed substitutes for bisphenol
A based on bisglycidyl ethers of anhydro-sugars,
such as isosorbide, isomannide, and isoidide
(Figure 15.12). The curing agent used could be
either bio-based polyamines or polycarboxylic
acids. The novel sugar-based epoxy system developed can be synthesized to be water soluble.
Isosorbide epoxy with equivalent weight 230 g/eq
was cured with methylenedianiline (MDA, with
equivalent weight of 49.6 g/eq) at 80°C for 2 hours
and 16 hours at 120°C. The glass transition temperature obtained for the cured epoxy was 89°C.
Boutevin et al. [99] developed epoxy prepolymers from phenol compounds based on flavonoids, condensed tannin, or hydrolyzable tannin,
and derivatives of catechin (Figure 15.19), epicatechin, gallocatechin, epigaloctechin, etc. Those flavonoids can be epoxidized with epichlorohydrin
according to the schematic reaction shown in
Figure 15.19. The resulting epoxidized flavonoid
can be cross-linked with amine or anhydride, or
mixed with resorcinol diglycidyl ether for posterior
cure. Epoxidized catechin was added to DGEBA at
different ratios: 25:75% and 40:60%. The mixtures
were cured using cycloaliphatic Epamine PC 19
(PO.INT.ER SRL, Italy). Glass transition temperatures obtained for the cured resin containing 25%
epoxidized catechin mixed with 75% DGEBA
(Epikote 828 Resolution Performance Products)
was 48°C, while the commercial DGEBA/Epamine
15: BIOBASED THERMOSETS
597
Figure 15.19 Reaction of epoxidation of catechin using epichlorohydrin [99].
O
O
NH2
S
R
HO
O
N
N
O
O
OH
O
O
S
O
O
O
S
S
H2N
O
NH2
HO
O
N
NH2
S
O
AGSO
O
+
O
O
O
O
O
O
O
S
O
N
R
HO
N N
O
O
OH
O
O
O
S
O
R
O
O
O
O
O
O
O
ELO
N
OH
O
Figure 15.20 Cross-linking reaction between epoxidized linseed oil (ELO) and amine grapeseed oil
(AGSO) [90].
PC 19 system Tg is 47°C. For the ratio of 40:60%,
a Tg of 32°C was obtained.
Stemmelen et al. [90] developed cross-linking
materials from renewable resources and created
fully bio-based epoxy systems. Amines are considered the most popular cross-linking agent due to
their nucleophilicity, which makes possible reactivity at room temperature. Functionalization of natural oils, such as grapeseed oil, with amine groups
involving the reaction of cysteamine chloride and
UV initiated thiol-ene chemistry created bio-based
cross-linking agents. The curing reaction with a
commercial epoxidized linseed oil can be seen in
Figure 15.20. An additional bio-based curing agent
was developed by Takahashi et al. [100].
A terpene-derived acid anhydride (TPAn) was synthesized by Diels-Alder reaction of maleic anhydride and allo-ocimene obtained by the
isomerization of α-pinene, a terpene found in coniferous trees.
The authors compared the thermal and mechanical properties of epoxidized soybean oil (ESO)
cured with three different curing agents: terpenebased acid anhydride (TPAn), hexahydrophthalic
anhydride (HPAn), and maleinated linseed oil
(LOAn). ESO-TPAn showed a higher glass transition temperature (67.2°C) and a higher tensile modulus and strength.
Wang et al. [101] used rosin as a bio-based curing agent. Rosin is a natural and abundant product
598
Figure 15.21 Structure of abietic acid, one of the
main components of rosin.
obtained from coniferous trees. The acidic component of rosin is mainly a mixture of isomeric
abietic-type acids (Figure 15.21). Rosin and its
derivatives have been used as tackifiers for adhesives, varnishes, paints, etc.
The researchers have studied a rosin-based,
anhydride-type curing agent and rosin-based glycidyl ether-type epoxies. Glycidyl ether of abietic
alcohol was synthesized, reducing first the carboxyl
group of rosin acid to a hydroxyl group, and then
reacting with epihalohydrin to obtain glycidyl
ether.
Cimteclab [102] developed a series of products
based on the phenolic structure currently derived
from cashew shell oil, a by-product of the cashew
nut industry. Novocardt is a curing agent developed for epoxy systems based on the reaction of
cardanol and paraformaldehyde using oxalic acid as
catalyst [61]. Using Novocardt (18%) as a curing
agent for a composite matrix based on DGEBA
epoxy (EC01, Camattini, Spa, Italy), a Tg of 110°C
was obtained, compared to the 122°C obtained using
a regular amine hardener. Mechanical properties
obtained were similar to the conventional system
except for the impact strength. The bio-based hardener increased the impact strength from 9.2 kJ/m2 to
24.3 kJ/m2 [103].
Wang et al. [104] synthesized rosin-based flexible anhydride-type curing agents based on
maleopimarate-terminated polycaprolactone (MPAterminated PCL). Commercial epoxy resin (DER
332 Epoxy resin, DOW) was cured with (MPAterminated PCL) in different stoichiometric ratios,
HANDBOOK
OF
THERMOSET PLASTICS
using 2-ethyl 4-methylimidzole as accelerator.
Epoxide/anhydride ratio affects the mechanical
properties. At a ratio of 3:2, the cured resin exhibited clear stress yield. As the ratio increased to 5:2,
the cured resin became less ductile, stronger, and
stiffer. These changes can be attributed to an
increase in the amounts of rigid polyether segments
formed. The length of the soft segment in the curing agent affected the cross-linking density. As the
molecular weight of the PCL segment increased,
the corresponding cured epoxy became more ductile and less strong.
It can be concluded that the obstacles to replace
petroleum-based epoxy resins with bio-based polymers in commercial applications are mainly due to
inferior mechanical and thermo-physical properties.
In the meantime, bio-based epoxy systems can be
used to partially replace conventional epoxy
systems.
Bio-Based Unsaturated Polyester
Unsaturated polyester resins are widely used as
the matrix in commodity composite materials, usually reinforced with fiber glass. Unsaturated polyesters are produced by polycondensation of
unsaturated and saturated dicarboxylic acids with
diols. Curing reactions are usually performed
through radical or thermal processes in the presence
of vinyl monomers, such as styrene. Polyester resins
are usually classified as: (1) Ortho resins, (2) Isoresins, (3) Bisphenol A-Fumarates, (4) Chlorendics,
and (5) Vinyl ester [105]. The most widely used diol
for standard unsaturated polyester is propylene glycol
(1,2-propanediol). Additional polyols are dipentaerythritol, glycerol, ethylene glycol, trimethylpropane,
and neopentylglycol [91]. Maleic anhydride and
fumaric acid are among the most common unsaturated
acid monomers used. Phthalic acid (iso or ortho acid)
is the saturated dicarboxylic acid used in all standard
unsaturated polyester resins [4]. Ortho resin is the
least expensive among all polyester resins. Monomers
of styrene are usually used as cross-linking agents,
and solutions of unsaturated polyesters and styrene
vinyl monomers (reactive diluents) are known as
unsaturated polyester resins (UPR). The curing reaction of UPR is a free-radical chain growth polymerization between reactive diluent (styrene) and the
resin. Curing procedures are considered versatile,
from room temperature to elevated temperature,
according to the catalyst used [105].
15: BIOBASED THERMOSETS
These building blocks can be substituted by biobased analogs, either partially or even totally, in
some cases. Bio-based propylene glycol (1,2-propanediol) and 1,3-propanediol (PDO) are currently
being commercially produced from glycerol. In the
presence of metallic catalysts and hydrogen, glycerol can be hydrogenated to propylene glycol (1,2propanediol), 1,3-propanediol, or ethylene glycol.
1,3-Propanediol (PDO) is also being used for production of bio-based unsaturated polyesters [86].
Several entities are working to develop and/or commercialize glycerin-to-propylene glycol technology:
Senergy/Suppes (University of Missouri), Cargill/
Ashland, Archer Daniels Midland (ADM), UOP/
Pacific Northwest National Laboratory (PNNL),
Virent Technologies (University of Wisconsin),
Huntsman, and Dow Chemical. The cost of production for propylene glycol made from crude
biodiesel-based glycerin is compared to conventional propylene oxide-based propylene glycol
[106]. Page et al. [107] detailed the polymerization
of unsaturated polyester resins based on biologically derived 1,3-propanediol. PDO monomers can
be obtained via a fermentation process of corn feed
stocks, using bacterial strains able to convert glycerol into 1,3-propanediol (Susterrat, DuPont).
Maleic anhydride was chosen as the unsaturated
diacid, and anhydride was added in order to
increase the solubility in styrene since aromatic diacids increase the solubility of unsaturated polyesters
in vinyl monomers (Figure 15.22). Solubility in styrene enables storage, handling, and processing of
the unsaturated polyester/vinyl monomer solution.
The solubility is dependent on the ratio between the
saturated and unsaturated diacids, e.g. the ratio
between phthalic anhydride (PA) and maleic anhydride (MA). Compositions containing orthophthalic acid, maleic anhydride, and Susterrat
1,3-PDO were compared to compositions containing
ortho-phthalic acid, maleic anhydride, and 1,2-propylene glycol (instead of 1,3-PDO) using the same
ratio between the components. The curing was
obtained using styrene (60%). The bio-based composition has shown higher tensile strength (70 MPa
compared to 44 MPa), higher flexural strength
(112 MPa compared to 67 MPa), but slightly lower
HDT (67°C compared to 75°C) [107].
Szkudlarek et al. [108] synthesized low-viscosity
unsaturated polyester resins based on bio-based 1,3propanediol and C5-C10 unsaturated dicarboxylic
building blocks, such as bio-based itaconic acid or
599
O
O
O
O
O
O
+
+
Maleic
anhydride
(MA)
Phthalic
anhydride
(PA)
OH
HO
1,3-propanediol
(1,3-PDO)
O
O
O
O
O
O
O
O
O
O
O
O
Fumaric
fragment
(FA)
Figure 15.22 Unsaturated polyesters based on biobased 1,3-propanediol [107].
anhydride. Maleic anhydride was added as the
unsaturated diacid and ortho/isoanhydride was
added as the aromatic saturated diacid building
block. A styrene or methacrylate-containing compound was used as reactive diluent. Radical inhibitors based on phenolic groups such as
hydroquinones, catechols, or phenothiazines may
be added. A tertiary aromatic amine was used as
co-initiator for the free radical polymerization. The
composition containing bio-based itaconic acid and
1,3-propanediol were compared to a fossil-based
composition containing maleic anhydride and 1,2propylene glycol (with similar ratio between the
components). The compositions were cured using
the same amount of styrene (65%). The bio-based
unsaturated polyester compositions showed comparable mechanical properties to similar fossil compositions with higher elongation and slightly lower
HDT (105°C compared to 109°C). Fatty acids or
oils can be used as polyacids, while rigid carbohydrates, such as isosorbide, can be used as polyols.
600
Szkudlarek et al. [109] also synthesized unsaturated
polyester resins using corn-based isosorbide and
1,3-propanediol, corn-based itaconic acid/anhydride, and maleic anhydride in styrene solution.
Isosorbide provides the aromatic building block
needed to obtain the solubility in styrene. Low thermal stability obtained by Szkudlarek et al. was
improved by introducing bio-based itaconic, citraconic, and mesaconic ester units [110]. The unsaturated polyester described by the researchers can be
obtained by polycondensation of a polyol (biobased 1,3-propanediol or 1,2-propanediol) and
itaconic, citraconic, and/or mesaconic acid or anhydride as unsaturated dicarboxylic acids. The heat
deflection temperature of this new composition was
considerably improved (from 70 to 105°C). Citric
acid can also be converted into bio-based polyol.
Kraft lignin, esterified with anhydrides, is soluble
in styrene, and can be used as an additive in unsaturated polyesters, which improves toughness and
connectivity in the polymer network [91].
Epoxidized vegetable oils can be used as a replacement for polyester resins. Robert et al. [111]
reported a new strategy, based on tandem (serial)
catalysis, to obtain alternating polyesters from
renewable sources, using available complexes to
catalyze the cyclization of dicarboxylic acids followed by alternating copolymerization of the resulting anhydrides with epoxides. Rosh et al. [112]
prepared cross-linked partially bio-based polyesters
by curing epoxidized soybean oil with various
dicarboxylic acid anhydrides in the presence of
cure catalysts such as tertiary amines, imidazoles,
or aluminum acetylacetonate. The anhydride dictates the final thermal and mechanical properties.
Anhydrides of hexahydrophthalic acid, succinic
acid, and norbornene dicarboxylic acid have led to
high flexibity with glass transition temperatures
below room temperature. Maleic anhydrides gave
rise to more rigid, stiff polyesters with higher glass
transition temperatures between 43 and 73°C.
Haq et al. [113] replaced partially petroleumbased unsaturated polyester with functionalized
vegetable oils such as epoxidized methyl soyate.
Ortho-unsaturated polyester resin (UPE, Polylite
32570, Reichhold Inc., USA) containing 33.5 wt.%
styrene was used in the research. The bio-resin
based on epoxidized methyl soyate (EMS) was
obtained from Arkema Inc., USA (Vikotex 7010).
The amount of bio-based portion in the resin varied
from 0 to 20%. A reduction in the tensile modulus
HANDBOOK
OF
THERMOSET PLASTICS
of 28% was observed for polyester resins containing 10% EMS in the blend, and 42% for 20%
EMS, relative to neat UPE. The failure strain
increased, while the tensile strength decreased,
which led to increased toughness with increasing
EMS content. The blend containing 10% EMS
showed 44% higher toughness relative to neat UPE.
The addition of EMS also increased the moisture
absorption of the resulting bio-based resin.
Acrylated epoxidized vegetable oils can be cured
alone or mixed with unsaturated polyester resins.
Grishchuk et al. [114] developed hybrid thermosets
with interpenetrating network (IPN) structures
based on vinyl ester/acrylated epoxidized soybean
oil hybrids with IPN structure, cross-linked with
styrene, and anhydride as an additional cross-linker.
The styrene diluted (B30 wt%) bisphenol A-type
vinyl ester (VE; Daron-XP-45-A2) was obtained
from DSM Composite, Nederland, and the acrylated epoxidized soybean oil (AESO) containing
monomethyl ether hydroquinone as inhibitor, was
obtained from Sigma-Aldrich Chemie GmbH. The
following vinyl ester (VE)/AESO combinations
were synthesized: 75/25, 50/50, and 25/75 wt.%.
Lower storage moduli were obtained for combinations containing AESO. Two Tg were detected for
the polymerized AESO (at 250 and 15°C), probably due to the multi-functionality and high unsaturation level of the AESO generating hard and soft
segments. The presence of the acrylated epoxidized
soybean oil in the hybrid system led to a reduction
of the flexural modulus (around 50% lower for the
50/50 composition) and to an increase in the toughness of the system. The resistance to thermal
decomposition of the hybrid resin system was
improved due to the interpenetrating network structure created.
Liu et al. [115] obtained unsaturated polyesterlike resins from functionalized tung oil. Tung oil is
extracted from the seeds of tung trees. The principal compound of this oil is a glyceride based on
alpha-elaeostearic acid (cis-9, trans-11, trans-13octadecatrienoic acid). This compound is a highly
unsaturated conjugated system that is used as a drying oil, mostly for coatings, paints, and varnishes,
but cannot compete with the properties of a general
purpose unsaturated polyester. In order to obtain
unsaturated polyester-like resin with enhanced
properties, tung oil was functionalized in two steps:
(a) alcoholysis with pentaerythritol to produce tung
oil pentaeritritol; and (b) maleination to produce
15: BIOBASED THERMOSETS
601
tung oil pentaerithritol (maleinated). The product
was blended with styrene, and cross-linking
took place via a free radical polymerization, as can
be seen in Figure 15.23. Promising mechanical
properties were obtained: tensile strength was
35.9 MPa, tensile modulus was 1.94 GPa, flexural
strength was 46.2 MPa, and flexural modulus was
2.08 GPa.
Ashland Performance Materials, a commercial
unit of Ashland Inc., developed the first commercially available bio-based unsaturated polyester
resin (Envirezt) based on soybean oil triglycerides.
Different grades of Envirezt are available that contain from 8 to 22% bio-based content [116].
Envirezt is obtained by a process where a carboxylic acid or corresponding anhydride containing an
ethylenic unsaturation is first reacted with a saturated, monohydric alcohol to form the half ester of
the acid or anhydride. The half ester is then reacted
with a polyol to form the polyester. Soybean oil is
introduced in the reaction step of the half ester and
the poyol [117]. A comparison between mechanical
properties of one of Envirezt grades and a standard
UPR can be seen in Table 15.2.
Bio-Based Polyurethanes
Polyurethanes are extremely versatile polymers
with a great variety of applications: flexible and
rigid foams, elastomers, coatings, adhesives, and
sealants. Polyurethanes can be thermoplastic or
thermoset. Polyurethanes are synthesized by the
reaction of a polyol and a diisocyanate. The functionality of the polyol determines the properties of
the final polyurethane. Diols lead to linear thermoplastic polyurethane whereas polyols with three or
O
O C
CH2OH
(1) 200–210°C
O
+
C O
1
2
CH2OH
HOH2C
Ca(OH)2
O
CH2OH
O C
Pentaerythritol (PER)
Tung oil (TO)
O
CH2OH
CH2OH
O
O
(2)
O
O
HOH2C
CH2
O
+
C
O
C
N,N-Di methyl benzylemine
Hydroquionone
CH2OH
CH2OH
T = 95°C
Pentaerythritol alcoholysis products (TOPER)
O
O
H2C
O
C
O
HOOC
CH
CH
C
CH
CH
H2C
COOH
O
CH2
CH2
O
+
C
O
C
O
C
O
C
CH
COOH
CH
CH
COOH
O
O
H2C
CH
O
O
O
C
CH
CH
H2C
COOH
Malinated products (TOPERMA)
(3)
Styrene unt (St)
Rigid thermoset polymer
Free radical copolymerization
Figure 15.23 Synthesis of tung oil-based unsaturated polyester-like polymer [115].
602
HANDBOOK
more hydroxyl groups are required to prepare thermoset, polyurethane networks.
The terminology “bio-based polyurethanes” usually refers to polyurethanes based on renewable
source polyols [42]. Bio-based isocyanates have
been introduced only recently and the results have
not yet been conclusive. Bio-based content of polyols can range from 30 to 100%, and as a result,
polyurethane bio-based content varies from 8 to
70%, depending on the building blocks chosen [4].
Bio-based polyols available in the market for polyurethane production are divided into three groups:
polyether polyol, polyester polyol, and oleochemical polyols from vegetable oils.
a. Bio-based polyether polyols: Sucrose and sorbitol are short-chain polyether polyols used for
rigid foams [4]. Polyether polyols can also be
obtained by condensation of 1,3-propanediol
synthesized for bio-based glycerol [118]. Biobased 1,3-propanediol can be used to produce
polytrimethylene ether glycol as the soft segment in elastomers and spandex fibers.
b. Bio-based polyester polyols: These can be
obtained by polycondensation of bio-based
dicarboxylic acids such as adipic or succinic
acid with bio-based polyols (1,3-propanediol)
[118]. Polyester-based polyurethanes have
better mechanical properties and are more
resistant to oil, grease, solvents, and oxidation, compared to polyether-based. Polyesterbased polyurethanes are more sensitive to
hydrolysis and microorganism attack [119].
Longer and hydrophobic chain polyols can
result in greater flexibility and hydrolytic stability of the resulting polyurethane.
c. Vegetable oil-based polyols: These can be prepared using distinct methods such as
OF
THERMOSET PLASTICS
epoxidation of the double bonds with further
oxirane ring opening with alcohols or other
nucleophiles [120], transesterification with
multifunctional alcohols, and the combination
of hydroformylation or ozonolysis with subsequent reduction of carbonyl groups (see the
section on natural oils for more details). Fatty
acids can be easily isolated from triglycerides
and can be used to prepare diols and polyols.
Triglycerides of castor oil and lesquerella oils
are characterized by the presence of ricinoleic
and lesquerolic fatty acids, respectively, both
presenting hydroxyl groups on their backbones
[42]. Both oils are important sources of naturally occurring polyols, however, the number
of hydroxyl groups in castor oil are substantially higher compared to lesquerella oil.
Polyurethanes from Vegetable Oils
Most of the bio-based polyols for polyurethanes
are synthetized from vegetable oils. The hydroxyl
groups present in oils can react with isocyanates to
form branched polyurethanes. Natural oils vary
greatly regarding the type, composition, and distribution of fatty acids in the triglycerides molecules.
The principal variation in fatty acid composition of
the oils results from variations in chain length,
degree of unsaturation, and position of the double
bond in the fatty acid chains. As a result, there is a
great variation in the length of elastically active
network chains (related to the double bonds/
hydroxyl groups) and dangling chains (the soft segments resulting from the saturated portions) in
polyurethane networks obtained from vegetable oilbased polyols [33]. The functionality of the polyol
determines the properties of the final polyurethane
polymers. Diols lead to linear thermoplastic polyurethanes, whereas polyols with three or more
Table 15.2 Mechanical Properties of Envirezt 70302 [116]
Property
Bio-based Envirezt 70302
Aropol S 542 H (Std ref. resin)
Tensile Strength (MPa)
73
75
Tensile Modulus (MPa)
2500
4300
Elongation at Break (%)
4.0
2.9
Heat Deflection Temperature (°C)
90
94
Envirezt 7030 (Ashland) is an isophthalic-based resin with 22% bio-content used for pultrusion.
Aropolt S 542 H (Ashland) is a standard reference unsaturated polyester resin used for pultrusion.
15: BIOBASED THERMOSETS
hydroxyl groups are required to obtain thermoset
polyurethane networks [120]. Petroleum-based
polyols usually present hydroxyl groups as primary
alcohols while the majority of functional groups in
vegetable oils are secondary. The reaction rate of
primary alcohols with isocyanate is about 3.3 times
faster than that of the secondary ones [121].
Additionally, when polyurethane foams are created
from secondary polyols, slower reaction rates during gas expansion might weaken the threedimensional network of the polyurethane foam,
thus creating more open cells with lower carbon
dioxide content and reduced thermal isolation properties [121].
Castor oil can be used in the synthesis of crosslinked polyurethanes and interpenetrating networks.
Soybean oil, palm oil, and rapeseed oil are more
popular and cheaper oils compared to castor and
lesquerella oils (natural polyols). In order to obtain
polyols from these oils, chemical modifications
have to be made (refer to the section on natural oils
for more details). Polyurethanes produced from
vegetable oil-based polyols include elastomers
obtained from oils with low hydroxyl content and
rigid foam and plastics from high hydroxyl content.
The extent of unsaturation conversion to hydroxyl
groups can tailor the final properties of the polyurethane since the hydroxyl content controls the degree
of cross-linking and the resulting stiffness of the
polymer. The polymer structure must be highly
cross-linked when a rigid foam is required, whereas
less cross-linking gives rise to flexible foams [115].
The degree of cross-linking is also dependent on the
NCO/OH ratio. Highly cross-linked and stiffer polyurethanes are obtained when the ratio is high [122].
Most vegetable oil-based polyols are of relatively
low molar mass (around 1000 Da) with a functionality distribution that ranges from 1 to 8 hydroxyl
groups per molecule. As a result, the average
603
hydroxyl equivalent (molecular weight divided by
the hydroxyl functionality) varies from 200 to 300,
making them suitable for rigid and semi-rigid applications, rigid foams, cast resins, coatings, and adhesives [13]. Rigid polyurethane foams can be obtained
from 100% vegetable oil-based polyols, while flexible foams are obtained by mixing vegetable oilbased polyols with petroleum-based polyols.
Castor oil, for example, is a naturally occurring
polyol with a functionality of 2.7 hydroxyls per
molecule. Polyurethane obtained from castor oil
with diphenylmethane diisocyanate is a hard elastomer with a glass transition temperature around 7°C.
According to Petrovic [13], in order to obtain flexible foams and elastomers, polyols with molecular
weight greater than 3000 Da and hydroxyl equivalent weight of 1000 and higher are required. These
polyols were synthesized by Petrovic from ricinoleic acid via transesterification, which produced
polyricinoleic acids (Figure 15.24). The dangling
chains act as plasticizers, and inhibit crystallization.
Elastomers with glass transition temperatures ranging from 233°C to 258°C were obtained when
polyricinoleic acids were polymerized with diphenylmethane diisocyanate. According to Petrovic,
lower glass transition temperatures and higher elongations can be achieved using a triol polyricinoleic
acid obtained by introducing a triol during the
transesterification step.
Zlatanic et al. [33] synthesized polyurethanes
from different vegetable oils midoleic sunflower,
canola, soybean, sunflower, corn, and linseed oil with 4,4-diphenylmethane diisocyanate. The functionality of the polyols ranged from 3.0 for the midoleic sunflower polyol to 5.2 for the linseed oil.
The conversion of the double bonds to epoxy
groups during the epoxidation reaction was relatively high for all the oils, and ranged from 91 to
94%. The polyols were obtained by epoxidation
Figure 15.24 Polyester diol obtained from ricinoleic acid [13].
604
followed by ring opening. Polyurethanes were
obtained by reacting the polyols with MDI. Linseed
oil-based polyurethanes presented higher crosslinking density, better mechanical properties, and
higher glass transition temperatures. The lowest Tg
was obtained for the polyurethane from midoleic
sunflower oil (33°C), and the highest was observed
for the linseed oil-based polyurethane (77°C).
Tensile strengths of all polyurethanes ranged from
1523 MPa, except for the polyurethane based on
linseed oil, which showed tensile strength three
times higher than the others (56 MPa). The tensile
modulus of linseed oil-based polyurethane was near
four times higher (2.0 GPa) compared to other oilbased polyurethanes. The variation in properties
resulted primarily from the different cross-linking
densities and less from the position of the reactive
sites in fatty acids chains.
Dwan’Isa et al. [122] used soy phosphate ester
polyol with hydroxyl content ranging from 122 to
145 mg KOH/g and diphenylmethane diisocianate
to
prepare
highly
cross-linked
bio-based
polyurethanes.
Del Rio et al. [124] obtained polyols with different hydroxyl contents using the synthesis of polyether polyols through the combination of cationic
ring-opening polymerization of epoxidized methyl
oleate and the reduction of carboxylate groups to
hydroxyl groups. Polyurethanes were obtained from
the reaction of the polyols with MDI or L-lysine
diisocyanate (LDI), a non-toxic diisocyanate. It was
observed (as expected) that the higher the functionality of the polyol, the higher the degree of crosslinking that occurred, which in turn led to higher
Tg values. Additionally, the aromatic MDI led to
higher Tg values compared to the aliphatic LDI.
Lligadas et al. [120] synthesized diols and polyols
from oleic acid (C18 fatty acid found mostly in olive
oil) and undecylenic acid (C11 fatty acid derivative
with a terminal double bond obtained from castor
oil) using click chemistry. Click chemistry was tailored to generate substances quickly and reliably,
mimicking nature, and was designed to generate substances by joining small units together.
Undecylenic acid was obtained by heating ricinoleic acid under vacuum pyrolysis to create undecylenic acid and heptaldehyde. Using thiol-ene
click chemistry, Lligadas et al. [120] applied photoinitiated coupling of 2-mercaptoethanol and methyl
esters of oleic and undecylenic acids, followed by
reduction (Figure 15.25).
HANDBOOK
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Bio-based triols were also obtained by transition
metal-catalyzed cyclotrimerization of methyl
10-undecynoated and methyl 9-octadecynoated
compounds that can be synthesized from oleic and
undecylenic acid via bromination and further elimination to alkyne functionality, and then subsequent
reduction of carboxylate groups to obtain primary
hydroxyl groups (Figure 15.26). The resulting polyol was reacted with methylene diphenyl isocyanate
(MDI) using 1,4-butanediol as the chain extender.
Desroches et al. [125] synthesized estercontaining diols of fatty acids via transesterification
with diol compounds followed by thiol-ene radical
coupling. Polyurethanes were prepared from the
synthesized oleochemical pseudo-telechelic diols,
which were reacted with methylene diphenyl-4,4
diisocyanate (MDI). In Desroches’ study, soybean
oil was used as raw material, and contained different fatty acids with 0 to 3 double bonds. The soft
segments of vegetable oils were based on ester
groups or amide groups with various spacer lengths
in between. These were obtained through the transesterification with a diol or amidification with
hydroxylamine or through thio-ene radical coupling. Amide groups containing polyurethanes
exhibited the highest glass transition temperatures
(62°C) due to hydrogen bonding enhancement.
Miao et al. [126] developed a polyol with high
hydroxyl value from epoxidized soybean oil and isopropanolamine (Figure 15.27). Both ester groups and
epoxide groups in epoxidized soybean oil reacted
with amino group generating hydroxyls, leading to a
hydroxyl value of 317 mg KOH/g. The resulting polyol was reacted with 1,6-diisocyanato-hexane to obtain
polyurethane, and additionally used 1,3-propanediol
(PDO) as the chain extender. A single glass transition
was noticed (24.4 to 28.7°C).
Natural oil polyols are produced commercially
by several companies Agribusiness Cargill
(BiOH, soybean-based polyol), Dow Chemical
(Renuva soybean-based polyols), Urethane Soy
Systems Company, and BioBased Technologies
(Agrol) BASF (BALANCE, castor oil-based polyol), Bayer (BAYDUR, castor oil-based polyol) and
Mitsui Chemicals (castor oil-based polyols) for
making polyurethane foams for the automotive, furniture, spray insulation, and other industries.
Polyurethane rigid foams are widely used as insulation and structural materials for construction, transportation, decoration, and appliances, which
accounts for approximately one-third of the
15: BIOBASED THERMOSETS
605
OH
(a)
SH
O
DMPA/hν
7
p-TSA/reflux
7
UD
80%
O
O
O
OH
S
HO
O
HO
85%
UDA
S
7
UDA-diol
OH
CH3OH
p-TSA/reflux
85%
OH
OH
SH
HO
O
O
S
7
DMPA/hν
UDM
95%
LiAIH4/THF
O
S
84%
O
OH
7
7
UDM-diol
(b)
OH
O
O
OH
4
HO
HO
4
DMPA/hν
p-TSA/reflux
O
S
6
6
70%
80%
OL
O
S
O
4
6
SH
OLA
HO
OLA-diol
CH3OH
p-TSA/reflux
85%
OH
O
HO
4
O
OH
SH
O
S
4
DMPA/hν
6
80%
6
OLM
LiAIH4/THF
O
S
4
75%
OH
6
OLM-diol
Figure 15.25 Preparation of undecylenic and oleic acid-derived diols using thiol-ene click chemistry [120].
polyurethane market. Flexible polyurethane foams
are used in automobile seating, upholstered furniture, carpet backing, and bedding (mattresses and
pillows). Bayer recently developed a visco-elastic
polyurethane foam based on castor oil polyol for
application in cushioning and vibration-damping
materials [127]. The visco-elastic effect is obtained
due to small open cells (with little opening, which
delays the air re-entrance after compression and
leads to a slow-down recovery). A summary of bio-
606
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Figure 15.26 Preparation of undecylenic aromatic triol (UDT) and oleic aromatic triol (OLT) using
cyclotrimerization process [120].
based polyols, raw materials, and producers can be
seen in Table 15.3.
Polyols from Lignocellulosic Materials
Lignocellulosic materials including wood, agricultural, or forestry wastes are a mixture of natural
polymers based on lignin, cellulose, and hemicellulose, and tannins with more than two hydroxyl
groups per molecule, and can be used as polyols
for polyurethane preparation [137]. In order to
obtain polyols from lignocellulosic materials, they
must first be liquefied by chemical or thermochemical treatments at high temperatures and high pressure [42,138,139]. In the presence of alcohols such
as ethylene glycol, liquefied wood with hydroxyl
content suitable to reaction with isocyanate is
obtained. Foam with varied densities can be
obtained with properties similar to conventional
rigid polyurethane foams.
15: BIOBASED THERMOSETS
607
Polyols from Natural Polyphenols
Figure 15.27 Synthesis of bio-based polyurethane
from epoxidized soybean oil and isopropanolamine
[126].
Polyols from Carbohydrates
Sugars and starches are carbohydrates that are
considered a very important renewable resource for
polyols (see the saccharides section for more
details). Sugars are basically polyols with a high
number of hydroxyl groups along the chain, and
therefore are potential raw materials for polyurethanes [22,41]. Starches and cellulose can be liquefied in the presence of alcohols in order to obtain
polyols to be used in the preparation of polyurethanes. The process includes the use of liquefaction solvents based on polyethylene glycol,
glycerol, and sulfuric acid. The solution obtained is
then neutralized with caustic soda [122].
As detailed in the section on polyphenols, naturally occurring polyphenols such as tannins and lignins are characterized by the presence of multiple
phenol structural units. Polyurethanes having
mechanical properties ranging from soft to hard
were synthesized by varying the kraft lignin content. High content (3035%) of kraft lignin results
in rigid and brittle materials, while lower amounts
provide soft polyurethanes [122]. Kelley et al.
[140] synthesized polyurethanes from kraft hydroxypropyl lignin with varied molecular weights and
hexamethylene diisocyanate. The mechanical properties of the polyurethane obtained improved as the
molecular weight of the hydroxypropyl lignin
increased. Kelley et al. [56] tried to increase the
elongation and toughness of polyurethanes based
on lignins by introducing chain extenders to the lignin. Polyethylene glycol and polybutadiene glycol
were used as chain extenders. In order to eliminate
phase separation, a polyether soft segment was
attached to the lignin derivative by propylene oxide
chain extension, which created a star-like copolymer with rigid aromatic core and flexible polyether
arms. Saraf et al. [141] compared the properties of
polyurethanes synthesized from two types of lignin:
kraft and steam explosion lignin. Kraft lignin
showed inferior mechanical properties.
Cashew nut-based phenols, especially cardanol,
are considered an attractive raw material for polyurethanes. Cardanol can be recovered from cashew
nut shell liquid by double vacuum distillation and
can be converted into a diol to be reacted with a
diisocyanate [60]. According to Mohanty et al.
[123], cardanol can be reacted with glycol in the
presence of phosphoric acid as catalyst. The resulting mixture can be reacted with aromatic diisocyanate to obtain films of polyurethane. Mixtures of
cardanol, formaldehyde, and diethanolamine,
blended first with polyethylene glycol and then
with methylene diphenyl diisocyanates; MDI and
hexmethylene diisocyanate led to rigid polyurethane foams [59].
Isocyanate-Free Polyurethanes and
Bio-Based Isocyanates
Isocyanates are the derivatives of isocyanic acid
(H-NQCQO). The functionality of the isocyanate
(R-NQCQO) group is highly reactive toward
608
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Table 15.3 Summary of Bio-Based Polyols for Polyurethanes [4]
Source Material
Trade
Name
Producer
Application
Reference
Soybean oil
BiOH
Cargill
Flexible foams
[128]
Renuva
Dow
Flexible foams and CASE
[129]
SoyOil
Urethane Soy
Systems
Flexible and rigid foams,
spray foams, elastomers
[4]
Baydur
BAYER
Rigid and flexible foams
[130]
Castor oil
Renewable and
recycled natural oil
Rapeseed/
Sunflower oil
Agrol
BioBased
Technologies
CASE , molded foams
[131]
Lupranol
BALANCE
BASF
Rigid foams, mattresses
[132]
Polycin
Vertellus
Coatings
[133]
Mistui
Chemicals
Rigid and flexible foams
[134]
IFL Chemicals
Rigid foams for insulation
and refrigeration
[135]
MetzelerSchaum
GmbH
Flexible foam
[136]
Enviropol
CASE: Coatings, Adhesives, Sealants, and Elastomers.
proton-bearing nucleophiles, and the reaction of
isocyanate proceeds with addition to the carbonnitrogen bond [142]. Important isocyanates used
in polyurethane manufacturing include 2,4-toluene
diisocyanate, 2,6-toluene diisocyanate, 1,6-hexamethylene diisocyanate, and 1,5-naphthalene diisocyanate, among others (all petroleum-derived). The
reactivity of isocyanates depends on their chemical
structures. Aromatic isocyanates are usually more
reactive than their aliphatic counterparts. The presence of electron-withdrawing substituents on isocyanates increases the partial positive charge on the
carbon atom and moves the negative charge further
away from the reaction site, which results in a fast
reaction. Isocyanate is obtained from the reaction
between amines and phosgene, a hazardous chemical that requires special precautions, is toxic, and is
an irritant to mucous membranes. Polyurethanes are
currently being produced from bio-based polyols
with toxic isocyanate. Environmental and public
health concerns motivate the research for alternative routes to create bio-based, non-toxic isocyanates. Isocyanate-free, environmentally friendly
polyurethane systems can be obtained from
vegetable oils. Mahendran et al. [143] developed
a new bio-based non-isocyanate urethane by the
reaction of a cyclic carbonate synthesized from a
modified linseed oil and an alkylated phenolic
polyamine from cashew nut shell liquid
(Figure 15.28). Cyclic carbonates can be synthesized from any epoxy monomers. The cyclic carbonate groups were added to the triglyceride by
reacting epoxidized linseed oil with carbon dioxide
in the presence of a catalyst.
Çayli et al. [144] synthesized isocyanate derivatives from unsaturated plant oil triglycerides. The
triglyceride was first brominated at the allylic positions by a reaction with N-bromosuccinimide, and
then, the brominated species were reacted with
AgNCO to convert them to isocyanate-derivative
triglycerides (Figure 15.29).
Polyurethanes and polyureas were synthesized
curing the fatty isocyanates obtained with alcohol
and amines, respectively. Glycerin polyurethane
exhibited a glass transition temperature of 19°C,
castor oil polyurethane showed two glass transition
temperatures at 24 and 36°C, and triethylene tetraamine polyurea showed a glass transition
15: BIOBASED THERMOSETS
609
Figure 15.28 Isocyanate free urethane: Reaction of cyclic carbonate with phenylalkylamine [143].
Br
C
H
C
H
O
O
CH
O
O
O
O
AgNCO in THF
R.T.
NCO
C
H
C
H
O
O
CH
O
O
O
O
Figure 15.29 Bio-based isocyanate from vegetable oil [144].
temperature of 31°C. The polyurethanes synthesized demonstrated similar mechanical properties
(tensile modulus around 50 kPa and tensile strength
around 100 kPa). Despite the low tensile strength
and modulus, castor oil polyurethane and glycerin
polyurethane showed excellent elongation, 410%
and 353%, respectively.
Hojabri et al. [145] described the synthesis of
linear saturated terminal aliphatic diisocyanates
from fatty acids via Curtis rearrangement, a thermal
610
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Figure 15.30 Synthesis of bio-based isocyanate from oleic acid [145].
decomposition of acyl azide. Diacids such as azealic acid, can be produced from oleic acid, as in
Figure 15.30.
Saturated diacids can be prepared by ozonolysis
of oleic acid to the corresponding aldehyde followed by purification and oxidation of the aldehyde
to obtain the diacid. Hojabri et al. compared physical properties of polyurethanes obtained using the
same polyol with fatty acid-derived diisocyanate
(1,7-heptamethylene diisocyanate, HPMDI) and a
similar but petroleum-derived commercially available diisocyanate: 1,6-hexamethylene diisocyanate
(HDI). Canola polyols were synthesized using ozonolysis and hydrogenation. Desmophen 800 (Bayer)
was used as the commercial polyol. The polyurethanes obtained had comparable properties to
fossil-based diisocyanates.
Bio-Based Alkyd Resins
Alkyds are polyesters obtained by polycondensation of three monomers: polyols, dicarboxylic acids,
or anhydrides, and natural fatty acids or triglycerides. The term alkyd is a modification of the original name “alcid” coined by R. H. Kienle, meaning
from alcohol and organic acids [146]. Since alkyd
resins emerged in the market in the late 1920s, they
have always had a substantial bio-based content
[4]. These resins are used in paints and coatings. In
1927 Kienle combined fatty acids with unsaturated
esters while searching for a better insulating resin
for General Electric [146]. Double bonds present in
the fatty acids are capable of oxidative coupling
reactions that lead to “air drying” of plant oils, thus
creating film coatings. This is the chemistry behind
the alkyd resins [14]. When the resin is applied to a
substrate, heat or oxidizing agents are achieved by
changing the oil length or chemical modification,
such as introducing chain stopping agents, phenolic
resins, acrylic monomers, styrene, vinyl toluene,
silicones, isocyanates, etc. Alkyd resins are compatible with a vast number of polymers, making them
very versatile to produce coatings, binders, paints,
lacquers, and varnishes, both transparent and semitransparent [147]. Fatty acids are produced from
vegetable oil. The common polyols are synthetic
glycol or glycerol, although bio-based glycerol can
be used instead, which increases the bio-based content of the final polymer [4] and extends up to 70%
of the bio-based content [147]. Fossil-based phthalic acid and maleic acid (and anhydrides) are the
most commonly used organic acids; glutaric and
succinic anhydrides can also be used in alkyd formulations [17]. Drying times can be controlled by
the type and amount of anhydride, with drying
times increasing with the amount of anhydride. An
illustrative reaction between glycerol and phthalic
anhydride is shown in Figure 15.31.
Alkyd resins can be classified according to oil
length, which refers to the oil percentage in the
alkyd. Short oil alkyds contain below 40% oil;
between 40 and 60% they are called medium, and
above 60%, long oil alkyds [17]. The oil length
affects the properties of the final product. A typical
long oil alkyd is made of 60% soybean fatty acids,
21.5% polyol (pentaerythritol), and 25.4% phthalic
anhydride [4]. Most alkyd-based coatings are used
for industrial goods (vehicles, wood products, etc.)
and infrastructure (traffic control striping, bridges,
etc). Alkyd coatings are inexpensive, durable, and
heat resistant. Durability and abrasion resistance
can be increased by modifying alkyd with rosin
(pine resin) [146]. Phenolic and epoxy resins
improve hardness and resistance to chemicals and
water. Styrene extends flexibility of coatings and
can be used to cross-link alkyd resins (using
15: BIOBASED THERMOSETS
CH2-OH
611
O
CH-O-CO-R
CH2-OH
O
O
O
O C
O
C O CH2 CH CH2 O
n
O
C
O
R
Figure 15.31 Synthesis of alkyd resin [17].
peroxides as initiators). Thermoset alkyds can be
used to produce billiard balls, appliance housings,
motor cases, switches, electronic encapsulations,
etc. More ecological friendly versions of alkyds
were produced based on the synthesis of alkyd
resins with relatively high acid value, and neutralized using amines to form colloidal solutions in a
mixture of water and water miscible solvents such
as glycol ether [147]. Nevertheless, solvents present
in alkyd formulations at an amount of 20 to 30%
present an environmental problem. High-quality
alkyd coatings with low solvent amounts are an
important target in the coating industry. High solid
content alkyd resins can be obtained by reducing
polymer viscosities using lower-molecular-weight
polymers, thus enabling a reduction in the solvent
content [17]. The same effect can be obtained by
synthesizing highly branched and star-structured
resins [148]. Hyperbranched alkyd resins can be
obtained using trimethylpropane and dimethylolpropionic acid. Saturated polyesters with hydroxyl end
groups are first obtained. The alkyd resin is
obtained by the esterification of the polyester
obtained in the first step, with unsaturated fatty
acids. According to Gruner [17], star-like resins
with three or four arms can be formed by the esterification of dipentaerythritol with fatty acids.
In the 1980s and 1990s, environmentally
friendly, zero VOC (Volatile Organic Contents)
versions of alkyd waterborne resins were developed. In order to obtain water-based resins, alkyd
resins with high acid numbers were prepared and
neutralized by amines. Amines were avoided by
using suitable surfactants. According to Hofland
[147], surfactants can also be eliminated by in situ
polymerization of hydrophilic monomers within
solvent-less alkyd resin, followed by inverse-phase
emulsification. Water-soluble alkyd resins were
synthesized by the copolymerization of acrylic
acid, glycidyl methacrylate esterified with unsaturated fatty acid, styrene, and methyl methacrylate
[17]. Kuhneweg [149] developed a waterborne
acrylic-modified alkyd resin based on the polymerization of a sulfonated alkyd resin with an acrylated
fatty acid. After polymerization, the polymer could
be dissolved in water.
Hofland [147] described an ultimate low carbon
footprint alkyd resin that had not only a low content of fossil carbon, but also low energy content
(i.e. low energy bio-based raw materials such as
succinic acid, colophonium (rosin), glycerol, and
isosorbide). Water-based alkyd and high-solid alkyd
resins can be ecological friendly solutions. Highsolid alkyd solvents can be replaced by bio-based
ones like ethanol, methyl ester of soybean fatty
acids, or methyl lactate.
Kemwerke (Philippines) developed eco-friendly
short oil coconut alkyd resins for paint formulations. Coconut alkyds are a range of tough resinous
products formed by reacting polybasic organic
acids or anhydrides with polyhydric alcohols in the
presence of coconut fatty acid or the monoglyceride
state of the refined coconut oil [150]. Soy
Technologies LCC (KY, EUA) produces soy-based
alkyd emulsions for coating formulations (Soyanol)
[151]. Perstop (Sweden) produces bio-based glycerol in medium oil-length, soybean oil-based, airdrying alkyd resin formulations [103].
Bio-Based Phenolic Resins
Phenolformaldehyde is one of the oldest commercial synthetic polymers, first introduced by Leo
Hendrik Bakeland in 1907 [152]. The polymer is
the result of a step-growth polymerization of two
simple chemicals: phenol or a mixture of phenols
and formaldehyde using an acidic or basic catalyst
[153]. Phenol is reactive towards formaldehyde at
the ortho and para sites, allowing up to three units
of formaldehyde to attach to the aromatic ring. The
main product of the reaction between them is the
production of methylene bridges between aromatic
rings, as can be seen in Figure 15.32.
612
HANDBOOK
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Figure 15.32 Structure of phenol formaldehyde.
The ratio between formaldehyde and phenol determines the degree of cross-linking. When the molar
ratio is one, theoretically, every phenol can be
linked together via methylene bridges, enabling total
cross-linking. Novolacs (novolaks originally) are
phenolformaldehyde resins where the molar ratio
of formaldehyde to phenol is less than one. The polymerization is catalyzed by acids such as oxalic acid,
hydrochloric acid, or sulfonate acids; the reaction
is slow and relatively low molecular weights are
obtained [153]. In a second step, a hardener such as
hexamethylenetetramine can be added to cross-link
the resin. The hexamine forms methylene and
dimethylene amino bridges between the aromatic
phenol rings at high temperatures. Resoles are
one-stage phenolformaldehyde resins obtained
with formaldehyde/phenol ratios greater than one
(B1.5) using a basic catalyst. Hydroxymethyl
phenols and benzylic ether groups are formed in the
first step of the reaction, at around 70°C. Crosslinking occurs when the temperature reaches around
120°C to form methylene and methyl ester bridges
through the elimination of water molecules. A
three-dimensional, cross-linked network is formed.
Resole-type phenolformaldehyde resin becomes
hard, and heat and chemically stable, after curing.
Resoles are used as adhesives (plywood, oriented
strand boards, laminated composite lumber, etc.) and
in composite materials (glass and carbon composites). Resole and novolac are soluble and fusible
low-molecular-weight products at A-stage, rubbery
at B-stage, and rigid, hard, and insoluble at C-stage
[153]. One of the most common applications of
novolac phenol resins is as photoresists.
Since phenolformaldehyde is one of the major
adhesive resins in the manufacture of plywood, and
the raw materials are petroleum-based, their
replacement by bio-based, non-toxic substitutes has
become a necessity. In the polyphenols section,
bio-based polyphenols were presented as well as
the chemical pathways used to transform them into
raw materials for polymers. Lignin, a renewable,
non-toxic, widely available, low-cost raw material
was presented as having a high potential to be used
as a phenol substitute in phenolformaldehyde
resins; it is a relatively expensive petroleum-based
chemical [154].
Lignin is highly cross-linked; three-dimensional
aromatic polymers with phenylpropane units are
linked together by carbon-carbon or ether bonds
with phenolic and hydroxyl groups (Figure 15.16).
Lignin is the second most abundant polymer in
15: BIOBASED THERMOSETS
nature, after cellulose, and it is produced as a byproduct of the paper industry. One of the functions
of lignin in plants, together with hemicellulose, is
to bind cellulose fibers, although isolated lignins
have proven to be poor adhesives for wood composites [155]. Lignin is not structurally equivalent to
phenol. Phenol has five free sites on the aromatic
ring and no ortho and para substituents around the
hydroxyl group. In lignin, the aromatic ring is parasubstituted by the propyl chain of the propylphen4-ol(coumaryl) structural unit, which affects lignin
reactivity during cross-linking with formaldehyde
[155]. Lignin presents most ortho and para sites as
blocked by functional groups, which leads to
slower reaction with formaldehyde when compared
to phenol. For this reason, lignin is used to replace
phenol in a limited amount, between 40 to 70% in
adhesives formulations. Brosse et al. [156] added
lignin to phenol-formaldehyde resins without deteriorating the mechanical and adhesion properties,
and observed that an additional environmental benefit was obtained: the presence of lignin reduced
the formaldehyde emissions for both the resin and
the finished products.
Phenolic precursors can also be prepared by liquefying wood, bark, and forest and wood industry
residues using a fast pyrolysis process. Monomeric
phenols can be produced by thermochemical conversion of biomass using fast and vacuum pyrolysis
[51,147]. Liquefation, phenolysis, and fractionation
methods can be also used. Fast pyrolysis is based
on fast heating rates using temperatures between
400 and 600°C. Chum et al. [157] synthesized
novolac and resole resins using phenol derivatives
obtained by fast pyrolysis from softwood, hardwood, and bark residue. Reactive components for
resin synthesis were obtained by fractioning the
pyrolysis oils and isolating phenolic and neutral
fractions, which were directly polymerized
[154,157]. These pyrolytic oils contained a complex
mixture of compounds including phenolics, guaiacol, syringol and para-substituted derivatives, carbohydrate fragments, polyols, organic acids,
formaldehyde, acetaldehyde, furfuraldehyde, and
other oligomeric products. The fraction containing
phenolic and neutral components substituted for not
only the phenol, but also some of the formaldehyde
present in the fraction.
Giroux et al. [158] developed a technology
called Rapid Thermal Processing [159] based on
fast pyrolysis or rapid destructive distillation that
613
Figure 15.33 A- and B-ring in tannin, where R1
can be resorcinol or phlorogucinol and R2 can be
pyrocathecol or pyrogallol [91].
enables one to obtain natural resins from a ligninic
fraction of the liquid pitch produced, all without
having to extract the phenol fraction using solvents.
The total phenolic content obtained ranged from 30
to 80 wt.% and was considered highly reactive and
suitable for use within resin formulations without
requiring any further fractionation procedure.
Resole-type resins were used in the production of
oriented strand board and plywood, with similar
results compared to fossil-based resins [154].
Vergopoulo-Markessini et al. [160] used a mixture of phenol compounds obtained from pyrolysis
and cashew nut shell liquid. A synergistic effect
between phenolic-based pyrolysis products and
cashew nut shell liquid was noticed, which enabled
the substitution of up to 80 wt.% of the phenolic
component of a standard formaldehyde-based resin.
Cashew nut shell liquid contains cardanol, a phenolic compound (Figure 15.17) that can be used to
substitute phenol in novolac or resole resins.
Cardanolformaldehyde resins have shown
improved flexibility due to an internal plasticization
effect of the long chain and better processability.
Additionally, hydrophobic behavior was noticed
due to side chains, making the resin water-repellent
and resistant to weathering [91].
Tannins are also polyphenolic compounds
(Figure 15.15), with good reactivity towards formaldehyde. Tannins are composed mostly of flavan3-ol units, for which two phenolic rings are joined
together by a heterocyclic ring, as can be seen in
Figure 15.33 [91]. The high reactivity of the tannin
towards formaldehyde is due to the A-ring. In
novolac resins, the presence of tannins enable
614
HANDBOOK
+
CH2O
CH3O
OF
THERMOSET PLASTICS
NaOH
CH2OH
CH3O
OH
O
–
Na
+
Coniferyl monolignols
Figure 15.34 Methylolation reaction of bio-oil with formaldehyde [167].
reactions with hexamine at lower temperatures,
compared to fossil-based phenol formulations.
However, the high reactivity and the large molecules of tannin can result in premature gelation,
which leads to brittleness [91]. Pizzi et al. [161]
developed polymers derived from the cross-linking
of condensed tannins by polycondensation reactions
with furfuryl alcohol and small amounts of formaldehyde. The addition of blowing agents created
tannin-based rigid foams with fire resistance similar
to synthetic phenolic foams.
Lignin has fewer reactive sites compared to
petroleum-based phenol. More than half of the
potentially reactive aromatic hydroxyl groups in
kraft lignin, for example, are blocked by methyl
groups, which hinders the aromatic hydroxyl group
[162]. Sulfur-mediated demethylation can be used
to remove methyl groups on the aromatic ring, thus
increasing the reactivity. Methylolation or hydroxymethylolation can be used to introduce hydroxymethyl groups (aCH2OH) to lignin molecules
using formaldehyde in an alkaline medium.
Methylolated lignin can be directly incorporated in
phenolformaldehyde resin, replacing part of the
phenol, in wood adhesives compositions [155,162].
El-Mansouri et al. [163] replaced the formaldehyde
in the methylolation process by glyoxal, a biobased, non-toxic dialdehyde. The process was
called glyoxalation.
Lignin can be treated with phenol in the presence
of organic solvents such as methanol or ethanol in
a process called phenolation or phenolysis [154].
The phenolation process is based on a thermal
treatment of lignin with phenol in an acidic
medium, which leads to the condensation of phenol
with the aromatic ring of the lignin and side chains.
A reduction of the molecular weight of the lignin is
also observed, as a result of ester bond cleavage
Figure 15.35 Guaiacol and syringol chemical
structures (in lignin) [154].
[162]. Cetin et al. [164,165] utilized phenolatedlignin (2030% of overall phenol) instead of
unmodified lignin in phenolformaldehyde resins
to achieve an improvement in mechanical and
physical properties when utilized as wood adhesive
in particleboard. Hu et al. [166] modified a foamed
resole resin using phenolated lignosulfonate (lignin
from the sulfite process) to achieve mechanical and
physical properties similar to fossil-based phenolformaldehyde foams. Perez et al. [167] synthesized
two types of novolac resins based on lignin:
(a) ammonium lignosulfonate, which was used
directly, as filler, and (b) ammonium lignosulfonate
modified by methylolation. Cheng et al. [168]
obtained bio-oil from hydrothermal liquefaction of
pine sawdust. The bio-oil obtained was methylolated with formaldehyde (Figure 15.34) in the presence of sodium hydroxide. The methylolated biooil was polymerized with formaldehyde, replacing
up to 75 wt.% of phenol in the resin for the production of plywood. The methylolation treatment
improved the thermal stability of the bio-based phenolic resin. Phenolation and methylolation have
shown the best results in terms of number of reactive sites created. The origin of the lignin may
affect the reactivity. Softwoods may yield more
reactive phenolics than hardwoods due to the
15: BIOBASED THERMOSETS
relatively high presence of guaiacol with one methoxy group (Figure 15.35) [154].
Formaldehyde-containing resins (ureaformaldehyde, melamineformaldehyde, and phenol
formaldehyde) for applications as wood adhesives
and mineral fiber binders for thermal insulation are
under intense pressure to reduce or eliminate formaldehyde content due to ecological concerns and
toxicity issues [152]. There is a need to develop nontoxic, bio-based formaldehyde substitutes. Ramires
et al. [169,170] utilized glyoxal (OCHaCHO), a biobased chemical obtained by the oxidation of lipids, a
non-toxic and non-volatile dialdehyde that can be used
as a substitute for formaldehyde in phenolic resins.
Both resole and novolac resins based on glyoxalphenol resins were developed for application in composite materials using sisal fibers as reinforcement, and
showed improved results for novolac-type resins in
terms of mechanical and physical properties. The novolac glyoxal-phenol composite showed higher impact
strength (237 J m21) than the resole glyoxalphenol
composite (118 J m21), indicating a better fiber/matrix
interaction. The novolac glyoxalphenol composite
exhibited a higher storage modulus (E’), and consequently was more rigid than the resole composite. In
addition to glyoxal, oxazolidine, solid resole, epoxy,
resorcinol, and tannins are being examined to substitute
formaldehyde in phenol resins [152].
615
[6]
[7]
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[9]
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[11]
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