15 biobased thermosets - elsevier · 15 biobased thermosets ana dotan ... thermoset from renewable...

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

Thermoset from Renewable Sources 594

References 615

Polymers from RenewableSources

In the past few decades concern about the environ-

ment, 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 bio-

based 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 liv-

ing 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 cen-

tury. It is estimated that by 2015 the worldwide

annual production of plastics is very likely to reach

300 million tons (14�18% 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.

Bio-based polymers are derived from renewable

resources such as plant and animal mass from CO2

recently fixed via photosynthesis [2]. Bio-based poly-

mers can be natural or synthetic. Natural bio-based

polymers are polymers synthesized by living organ-

isms such as animals, plants, algae, and microorgan-

isms. 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 polyhy-

droxyalkanoates are also natural bio-based polymers

[4]. Bio-based polymers are not necessarily sustain-

able; 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. Sus-

tainable produced bio-based polymers are those

grown without genetically modified organisms

(GMOs), hazardous pesticides, certified as sustain-

able 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]. Bio-

based materials are important candidates for sustain-

able 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

577Handbook of Thermoset Plastics. DOI: http://dx.doi.org/10.1016/B978-1-4557-3107-7.00015-4

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throughout its life cycle; its principles and frame-

work 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, respira-

tory inorganics, land occupation, non-renewable

energy, and aquatic ecotoxicity [7]. Additional envi-

ronmental 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 anthro-

pogenic gas that is thought to affect the Earth’s radia-

tive 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 conse-

quent 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 mate-

rial. 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-BasedContent in Polymers

Bio-based polymers can be made totally or par-

tially from renewable source raw materials, pro-

duced from photosynthesis and CO2. In order to

determine the bio-based content of the polymer, the

“new” carbon content must be measured. A renew-

able source is replenished by natural processes at a

rate comparable to its exploitation rate. The carbon

content of such polymers is derived from the so-

called short carbon cycle within an expected time

frame between 1 to 10 years. Most industrial poly-

mers 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,

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 concen-

tration in the atmosphere.

ASTM D6866-11 has developed a protocol to

quantify the bio-based content in materials by com-

paring the 14C/12C ratio to that of a standard speci-

men 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) Accelera-

tor 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 result-

ing from new carbon input and that derived from

fossil-based input. A measurement of the 14C/12C

ratio is determined relative to the modern carbon-

based 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 bio-

based 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 pro-

ducts 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 ana-

lytical 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 frac-

tion of the total carbon content, or as a fraction of

578 HANDBOOK OF THERMOSET PLASTICS

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;TOC5 ðXBÞ=ðXTOCÞ (15.1)

Here, XB5 is the bio-based carbon content by

mass, expressed as a percentage; and XTOC5 is the

total organic carbon content, expressed as a per-

centage, of the sample.

Raw Materials for RenewableSources 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 inexpen-

sive 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 organ-

elle 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 tri-

glyceride. Glycerolipid and fatty acids are synthe-

sized 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 num-

ber 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 meth-

ods, 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 dif-

ferent fatty acids (some of them can be seen in

OO

OO

9 12 15

2

O αω

H2C

O

5

3 64

1H2C

HC

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).

57915: BIOBASED THERMOSETS

Table 15.1 Fatty Acid Composition of Different Plant Oils [14,16]

Oil

Fatty Acid Composition (X:Y, where X is the number of carbon atoms and Y is the number of double bonds) %

Palmitic16:0

Stearic18:0

Oleic18:1

Linoleic18:2

Linolenic18:3

Gadoleic20:1

Erucic22:1

Ricinoleic18:1

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 � � �

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) non-

drying oils (iodine value below 90, e.g. palm kernel

oil) [17]. Triglycerides contain active sites amenable

to chemical reaction: the double bond, the allylic car-

bons, the ester group, and the carbons alpha to the

ester group. These active sites can be used to intro-

duce 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. aro-

matic 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 func-

tional 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 synthe-

sis, usually through the conversion of the triglycer-

ides into mono/diglycerides or into simple fatty

acids [12].

Direct Polymerization of Natural Oils

Double bonds present in fatty acids can be poly-

merized 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 atmo-

spheric oxygen, which leads to the formation of a net-

work 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

follows: an initial hydrogen abstraction on the methy-

lene 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 [19�21]. Oils with high

iodine value can also be polymerized directly via cat-

ionic 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 dicy-

clopentadiene can be achieved using BF3OEt2 mod-

ified 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 pro-

ceeds 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 DoubleBonds of Natural Oils

There are two different sites in triglycerides

available to chemical modification: ester groups,

which can be readily hydrolyzed or trans-esterified,

O2

O OH

O

Figure 15.2 Oxypolymerization of triglycerides.


double bonds along the aliphatic chains, and

hydroxyl groups [22]. Some of the important chem-

ical 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 litera-

ture. Epoxides of fat and oils and their ester deriva-

tives are usually used as plasticizes, toughening

agents, and stabilizers in plastics industry, espe-

cially 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 avail-

able. Epoxidized soybean oil is also commercially

available, usually with a functionality of 4.1�4.6

epoxy rings per trygliceride [14]. The oxirane ring

is useful for further chemical modifications and for

the synthesis of thermosetting resins by cross-

linking 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 halohy-

drins (haloalcohols), molecular oxygen, and by in

situ epoxidation with percarboxylic acid.

A suitable technique for clean and efficient epoxi-

dation of vegetable oils is the epoxidation in situ

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 carbox-

ylic acid in the epoxidation process. Peroxy-acid is

created by the reaction between hydrogen peroxide

and the carboxylic acid. The process can be con-

trolled by the quantitative analysis of oxirane rings

and iodine number [25]. Different catalysts have

been studied, including ion-exchange resins, phos-

photungstic acids, rhenium catalysts, titanium cata-

lysts, and enzyme catalysts [26,27]. A schematic

example of a triglyceride epoxidation process can

be seen in Figure 15.4. Chemo-enzymatic epoxida-

tion is a relatively new process and has the advan-

tage of suppressing undesirable ring opening of the

epoxide. In this method, unsaturated fatty acid or

ester is initially converted into unsaturated percar-

boxylic 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 commercializ-

ing 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 dis-

cussed 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 trigly-

cerides 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

Formic/acetic acid

Catalyst

Peroxide

OO O

O

O

O

O

OO

OO

O

OO

O

O

Figure 15.4 Epoxidation process.

CO2H

O

Figure 15.3 Vernolic acid.


organometallic catalysts is common. The resulting

polymer properties can be controlled by changing

the molecular weight of the monomer or the func-

tionality 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, dia-

mines, 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 malei-

nizated 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 posi-

tion 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 alterna-

tively by the hydroxylation of the double bonds.

Ring opening of epoxidized triglycerides can be

obtained by reacting with hydrochloric or hydrobro-

mic acid or by an acid-catalyzed ring-opening reac-

tion 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 poly-

mers 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 accord-

ing 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 prepar-

ing polyols; hydrogenation of the formyl group pro-

duces hydroxyl, using cobalt or rhodium as

catalysts, at high temperatures [35]. In this process

aldehydes are formed and an extra carbon is intro-

duced 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 pro-

cess utilized to synthesize polyols with terminal

hydroxyl groups from vegetable oils. Ozone, a very

powerful oxidation agent, is used to cleave and oxi-

dize 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] devel-

oped a method to synthesize vegetable oil-based

primary polyol (soybean oil) in a single-step ozono-

lysis procedure. When the oil is exposed to ozone

in the presence of ethylene glycol and an alkaline

catalyst, double bonds are cleaved, yielding a mix-

ture of polyols that is dependent on the relative

concentration of the unsaturated fatty acids present

(Figure 15.8). In this type of ozonolysis, the ozo-

nides produced react with the hydroxyl group of

the glycol to form an ester linkage with a terminal

hydroxyl group.

Figure 15.5 Acrylation of epoxidized triglyceride oil (soybean oil).


C CCatalyst

CatalystPressure+ CO + H2

CH CH2O

H2

H H

C C

OH

C C

Figure 15.7 Hydroformylation process [13].

OH

HCOHOH

OH

CH

HCHC

HC

HC

O

O

O

O

O

OO

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

H2COH

H2COH

HCOH2

H2COH

H2COHHOCH

H2COH

H2COH

H2COH

H2COH

H2COH

H2COH

H2COH

HCO

HO

H2COH

HCOH

HC

HOCH

HC

CHCH

CH

CH

HC

HC

CO

CH

OH

OH

OH

HC

HO

HC

HOCH2

HOCH2

H2CO

HCOH

HCOH

H2OCH

H2OCH HCOH

HCHOCH2

CH2

CH

CO

OCH3

OCH3

CH2

CH2

CH2

CH2

CH3

(Carbohydrate)

CH CH

CH

CH

CH

OCH3

OCH3

OCH3

OCH3

OCH3

OCH3

H2C

H2C

HOCH

HOC

HC

HC

HC

HC

HOC CH

OCH3

CH2OH

C2H

CH

CHCH

CH CH

CH

CHO

CH2OH

CH3O

CH3O

CH3O

CH3O

CH3O

OCH3

OCH3

CH3O

CH3O

CHO

HC

HC

HC

CH3O

CH3O

CH3O

CH3O

CH3O

CH3O

CH3OCH2

CH3O

CH

Figure 15.6 Maleinization of acrylated epoxidized triglyceride oil (soybean oil).


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 tri-

glyceride 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 hydroper-

oxides 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

HO OH

OH−O3

O

O

O

O

O

O

O

O

O

O

O

O

HO OH

OH−O3

HO

HO

HO

HO

HO

O

+

+

+

+

+

O

O

O

O

O

O

O

O

O

OO

O

O

HO R

O

O

O

O

O

O

O O

O

O

O

O

O

O

OOH

OH

O O

OH

OH−O3

OH

OH

OH

(a)

(b)

(c)

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].


Figure 15.9 Maleinization of hydroxylated triglycerides.

O O

O

High oleic sulflower oil (85% oleic acid)

OO

O

O

OOO

O

O O

O

OO

O

O

O

O O

O O

O

Aza Michael cross-linked triglycerides

Soft rubbers Tg (DMTA) = 16°C

OHN

HNO

O O

O

O O

O

O O

OOOH OOH

Mixture of isomers. 2.6 enonegroups per triglyceride molecule

90°C to 120°C, 12h

OOH

Ac2O/Et3N, 4h

O2, TPP, hν, CH2CI2, 6h

H2N NH2

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].


requires high temperatures and the use of an inor-

ganic homogeneous catalyst such as sodium, potas-

sium, or calcium hydroxide. Although this process is

energy consuming it has high conversion rates and

relatively short reaction times [36]. Enzymatic gly-

cerolysis 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 sty-

rene increases the polymerization conversion rate

and the stiffness of the resultant polymer.

Methacrylation of Monoglycerides

Methacrylation of monoglycerides can be per-

formed after the glycerolysis of the triglyceride (soy-

bean 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).

O

OH

OH

230-240°C

%1 Ca(OH)2

OH

O

O

OH

CH2

CH2

CH3

CH2

CH2

CH3

CH3

CH3

C

C+

+C

O

OO

O

C CO

O

C C

CH3

CH2

CH2

CH3

O

CO

O

O

O

C

C C

O

O C

OH

C O

O O

C C

55°CPyridine,Hydroquinone

O

O

C

C

O

1

CO

O

O

CO

OH

OH

+

+

Soybean oil

Monoglyceride

Methacrylic anhydride

Methacrylic acid

Diglyceride

Glycerol

C

2

zO

O C

Figure 15.11 Glycerolysis reaction of soybean oil followed by methacrylation.


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 trigly-

cerides 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 copoly-

mers. 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 conse-

quences. The preparation of hydroxyesters or

hydroxyacids from glycerol and their polyconden-

sation 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 consid-

erably, and includes diols, diacids, hydroxyacids,

oxiranes, acrolein, and acrylic acid, among others.

Ethylene and propylene glycol are particularly rele-

vant for the polyester market (polyethylene tere-

phthalate 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 liv-

ing organisms. Sugars and starches are carbohy-

drates 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 sacchar-

ides 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, glyco-

gen, and hemicellulose [42]. The most abundant

oligosaccharide is sucrose, a disaccharide of glu-

cose and fructose. The vast majority of carbohy-

drates present in nature are polysaccharides, with

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 sacchar-

ides; (2) functionalization based on appending the

carbohydrate to a reactive backbone; and (3) poly-

condensation reactions of sugar-based monomers

[41]. Sugars are basically polyols with a high num-

ber of hydroxyl groups along the chain. In order to

control the reactivity of the many different

hydroxyl groups on carbohydrates, simple mole-

cules with two hydroxyl groups are desired.

Dianhydrohexytols, such as isosorbide, isomannide,

and isoidide (Figure 15.12), are a result of intramo-

lecular dehydratation; having two reactive hydroxyl

groups, they can be used as raw material for poly-

condensation, which leads to polyester, polyether,

or polyurethane chiral polymers. Isosorbide is pre-

pared from starch, isomannide from D-mannose,

and isoidide from isosorbide [41]. Isosorbide, or

1,2,3,6-dianhydrosorbitol, is the product of a multi-

step process, starting with starch, and passing

through D-glucose and sorbitol. It is produced from

biomass in a combination of enzymatic and chemi-

cal 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 proper-

ties 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 epichloro-

hydrin. 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.)


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 add-

ing methacrylate units to the polymer.

Polyphenols

Naturally occurring polyphenols are character-

ized 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 (polyfla-

vonoids) 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 dura-

bility. 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

centers accommodate the inter-flavonoid bonds. A-

rings in tannins are highly reactive to aldehydes

(formaldehyde) in wood adhesive compositions and

the reaction kinetics can be controlled by the addi-

tion of alcohols to the system. Self-condensation

reactions of polyflavonoid tannins are used to pre-

pare 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].

3O

O O

O O

O

O OO

OHO

O

CI

HO

OH

O NaOHH2O

O

Epichlorhydrin

Bisisosorbide diglycidyl ether

Isosorbide

+

Figure 15.13 Bisisosorbide diglycidyl ether preparation [45].

Figure 15.14 Naturally occurring polyols.


Lignin is the second most abundant organic poly-

mer on Earth, after cellulose, and the most abun-

dant 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 pheno-

lic-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 less-

modified, sulfur-free lignins, which simplifies the

pyrolysis process for obtaining low-molecular-weight

lignin sub-products [52]. All chemical pulping pro-

cesses are associated with the chemical splicing of

natural lignin producing fragments with high molecu-

lar 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 reac-

tive sites. The phenyl propane units in lignin

are generally bonded through carbon and ether

bonds, where only ether bonds can be easily dis-

rupted. The methoxy group substituted at the ortho

positions also influences the reactivity and solubility

of the lignin [49]. Lignin and its degradation pro-

ducts can originate various polymers such as

phenol�formaldehyde, poly(azophenylenes), polyi-

mides, polyurethanes, polyesters, polyamides, poly-

phenylene 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], etherifica-

tion with alkylene oxides (ethylene oxide, propylene

oxide, and butylene oxide [53,54]) results in hydro-

xyalkyl lignin derivatives with aliphatic hydroxyl

groups. Nonphenolic hydroxypropyl lignins are

already commercially available in the market.

Extended alkyl ether chain lignins can be also pre-

pared, 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 solu-

bility in a variety of organic solvents such as ace-

tone, tetrahydrofurane, and chloroform. Lignins can

also be sulfonated, sulfomethylated, aminated, halo-

genated, 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 lig-

nin molecules improves the accessibility of the

reagents. Lignin has reportedly been grafted with

methyl methacrylate, vinyl acetate, styrene, acrylo-

nitrile, acrylic acid, acrylamide, maleic anhydride,

and others. Methacrylated lignin can form cross-

linked 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

Figure 15.15 Condensed tannin.


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 adhe-

sive 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.

OH

HCOHOH

OH

CH

HCHC

HC

HC

O

O

O

O

O

OO

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

H2COH

H2COH

HCOH2

H2COH

H2COHHOCH

H2COH

H2COH

H2COH

H2COH

H2COH

H2COH

H2COH

HCO

HO

H2COH

HCOH

HC

HOCH

HC

CHCH

CH

CH

HC

HC

CO

CH

OH

OH

OH

HC

HO

HC

HOCH2

HOCH2

H2CO

HCOH

HCOH

H2OCH

H2OCH HCOH

HCHOCH2

CH2

CH

CO

OCH3

OCH3

CH2

CH2

CH2

CH2

CH3

(Carbohydrate)

CH CH

CH

CH

CH

OCH3

OCH3

OCH3

OCH3

OCH3

OCH3

H2C

H2C

HOCH

HOC

HC

HC

HC

HC

HOC CH

OCH3

CH2OH

C2H

CH

CHCH

CH CH

CH

CHO

CH2OH

CH3O

CH3O

CH3O

CH3O

CH3O

OCH3

OCH3

CH3O

CH3O

CHO

HC

HC

HC

CH3O

CH3O

CH3O

CH3O

CH3O

CH3O

CH3OCH2

CH3O

CH

Figure 15.16 Generic structure of lignin.


Cardanol is the main constituent (97%) of ther-

mally 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 hydrocar-

bon 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 poly-

merization of cashew nut shell liquid using iron-N,

N0-ethylenebis(salicylideneamine) (Fe-salen) as cat-

alyst, 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] devel-

oped a novel polyurethane based on cardanol, con-

densing cardanol with formaldehyde using sebacic

acid as catalyst. The resulting resin (novolac

phenol�formaldehyde resin) was epoxidized, fol-

lowed 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 react-

ing cardanol derivatives with diisocyanates or poly-

isocyanates 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 pre-

pared 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

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 func-

tion [63]. Proteins hold together, protect, and pro-

vide structure to the body of living organisms in

the form of skin, hair, calluses, cartilage, muscles,

tendons, and ligaments. Proteins can catalyze, regu-

late, and protect the body chemistry as enzymes,

hormones, antibodies, and globulins [64]. Most pro-

teins fold into 3-dimensional structures. The pri-

mary 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 intra-

molecular hydrogen bonding. Coiling and aggrega-

tion 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 quater-

nary structures. The shape into which a protein nat-

urally 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

Figure 15.17 Structure of cardanol.


processing methods [42]. According to Verbeek

[62], melt extrusion of proteins can occur through

denaturation, dissociation, unraveling, and align-

ment 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 decom-

pose at temperatures below the softening tempera-

tures, 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, phospho-

proteins found in abundance in cow milk. Casein-

based 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 oxy-

gen 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 prop-

erties improved cross-linking of the resulting poly-

mer with methylene bisphenyl diisocyanate [67].

Zein, a class of prolamine protein found in corn,

was historically used as polymeric resins and coat-

ings 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 hydro-

phylic 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] intro-

duced dibutyl L-tartarate (DBT), which contains

ester and hydroxyl moieties to form potential

hydrogen bonds with zein. Sessa et al. [69] intro-

duced 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 dimen-

sional stability. Lai et al. [70] used oleic acid to

plasticize zein, thus creating more flexible and

tough films.

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 over-

come 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 polyethyl-

ene glycol (PEG) and cross-linked it to form

PEGylated albumin hydrogels. The main applica-

tion is as scaffolds for controlled drug release.

Gluten is a protein composite based on gliadin and

glutenin, forming, together with starch, the endo-

sperm of grass-related grains. The term gluten comes

from Latin, meaning “glue,” due to its adhesive prop-

erties. 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, excel-

lent gas barrier properties, low price, and large-scale

availability [73]. Gliadin and glutenin form a cross-

linked network that contributes to the extensibility of

mixed dough. In order to reduce the strong intramo-

lecular 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 gluten-

based PSA, with internal plasticizer, was developed

by Aranyi et al. [75]. An acetic acid soluble hydroly-

sate was obtained using a hydrochloric and acetic

acid mixture. The gluten hydrolysate was then epoxi-

dized using ethylene oxide, and generated a water

soluble, epoxidized gluten hydrolysate. This hydroly-

sate was then reacted with hydroxyethyl methacrylate

giving rise to an acrylic copolymer of gluten with

PSA properties. A reduction in water vapor transmis-

sion 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 pro-

tein held in discrete particles called protein bodies

which provide amino acids during soybean seed

germination. Soy protein comprises a high percent-

age of water soluble albumins and salt solution sol-

uble globulins. Water is considered a major


plasticizer in the processing of soy proteins. High

quantities of water content reduce the denaturation

temperature of soy proteins. Soybean protein plas-

tics 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, lead-

ing to a homogeneous distribution. A posterior

interaction between the molecules leads to entan-

glement upon curing.

Plastics based exclusively on soy protein are

rigid and brittle and plasticizers are usually intro-

duced in order to increase the flexibility and tough-

ness. Polyols have often been used as plasticizers.

Plasticizers disrupt molecular interactions, decreas-

ing the forces holding the chains. According to Sun

[14], polypropylene glycol and glycerol appear to

be more compatible with soybean protein and eas-

ier 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 biodegradabil-

ity. Denaturants such as sodium dodecyl sulfate and

urea also act as plasticizers, reducing the glass tran-

sition 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 esteri-

fication are known pathways for chemical modifi-

cation [80].

Soy protein can be modified using polycapro-

lactone/hexamethylene diisocyanate prepolymer

[68]. It was found that HDI-modified PCL forms

urea�urethane 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 perfor-

mance 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 pro-

tein is obtained by hydrolysis or by an alkali

environment.

Thermoset from RenewableSources

Epoxy

Approximately 90% of all non-bio-based epoxy

in the market is based on diglycidyl ether of

bisphenol A (DGEBA) derived from epichlorohy-

drin and bisphenol A. Epichlorohydrin is an epox-

ide. The conventional, petrochemical process of

producing epichlorohydrin is the chlorohydrination

of allyl chloride, which in turn is made by chlorina-

tion 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 epichlo-

rohydrin 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 one-

third, generates less than one-tenth the waste water,

and produces less chlorinated organics, when


compared to conventional processes [85]. Solvay

Chemicals produces epichlorohydrin from bio-

based glycerol, a by-product of biodiesel produc-

tion using rapeseed, according to EPICEROL®

technology [86]. The Dow Chemical Company,

currently the world’s largest producer of epichloro-

hydrin, also announced its intent to manufacture

epichlorohydrin via a novel, acid-catalyzed hydro-

chlorination process using bio-based glycerin [4].

Krafft et al. [87] developed a process from glyc-

erol via 1,3-dichloropropanol. Bio-based DGEBA is

chemically identical to the fossil one, and the bio-

based epichlorohydrin accounts for approximately

20% of the molecular weight of DEGBA. Glycerol-

based epoxy resins such as glycerol polyglycidyl

ether and polyglycerol polyglycidyl ether are indus-

trially available, and have been used in the textile

and paper industry as processing agents, tackifiers,

coatings, etc. Shibata et al. [88] developed a bio-

based epoxy system based on a mixture of commer-

cially available sorbitol polyglycidyl ether (SPE,

DENACOL EX-614B) [89] and glycerol polyglyci-

dyl 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 epichloro-

hydrin 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 nat-

ural oils) or through enzymatic processes [90].

Natural epoxidized oil can be found in vernolic

acid, present in Vernonia species. Epoxidized UV-

curable resins can be synthesized via transesterifi-

cation of vernonia fatty acids with hyperbranched

hydroxyl functional polyether. The resin can then

be cationically polymerized in the presence of ver-

nolic 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 polymer-

ized 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 mechani-

cal and thermal properties are obtained with higher

water uptakes, compared to fossil epoxy systems.

On the other hand, the polymer obtained is poten-

tially 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 pre-

pared commercially by epoxidation with percar-

boxylic 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] synthe-

sized a thermally curable epoxidized soybean oil in

the presence of tetraethylammonium bromide cata-

lyst. Increasing catalyst concentrations led to a

reduction of curing cycles and temperatures, and to

higher degrees of conversion and cross-linking den-

sity. 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 epoxi-

dized 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 alco-

hol and allyl alcohol, respectively. The fatty acid


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 follow-

ing 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 con-

cluded that epoxidized allyl soyate provided better

intermolecular cross-linking, yielding tougher mate-

rials as compared to commercially available epoxi-

dized 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 com-

parable mechanical properties compared to neat

resin. The presence of soybean oil increased the

lubricity, thus reducing the pull force during the pul-

trusion 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 anhy-

dride (AEW index of 98.06 g/equivalent) with the

aid of a mixture of catalysts (1,3-butanediol anhy-

drous and benzyldimethylamine). The mixture was

cured at various temperatures for 5 hours using dif-

ferent 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 tem-

perature (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 ester-

carboxylic acid obtained can be reacted with ethyl-

ene glycol diglycidyl ether to form epoxy resins.

Glass transition temperature increased with

increasing ester-carboxylic acid derivative content,

suggesting that lignin indeed acts as the hard seg-

ment (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 devel-

oped 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 temper-

ature obtained for the cured epoxy was 89°C.Boutevin et al. [99] developed epoxy pre-

polymers from phenol compounds based on flavo-

noids, condensed tannin, or hydrolyzable tannin,

and derivatives of catechin (Figure 15.19), epicate-

chin, gallocatechin, epigaloctechin, etc. Those fla-

vonoids 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 tempera-

tures 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

80

60

40

Str

ess

(MP

a)

20

00 0.01 0.02 0.03 0.04

20% EAS

10% EAS

Pure epon

Strain (m/m)

0.05 0.06 0.07

Figure 15.18 Mechanical properties of soy-based

resin systems [95].


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 consid-

ered the most popular cross-linking agent due to

their nucleophilicity, which makes possible reactiv-

ity at room temperature. Functionalization of natu-

ral 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 syn-

thesized by Diels-Alder reaction of maleic anhy-

dride and allo-ocimene obtained by the

isomerization of α-pinene, a terpene found in conif-

erous trees.

The authors compared the thermal and mechani-

cal properties of epoxidized soybean oil (ESO)

cured with three different curing agents: terpene-

based acid anhydride (TPAn), hexahydrophthalic

anhydride (HPAn), and maleinated linseed oil

(LOAn). ESO-TPAn showed a higher glass transi-

tion temperature (67.2°C) and a higher tensile mod-

ulus and strength.

Wang et al. [101] used rosin as a bio-based cur-

ing agent. Rosin is a natural and abundant product

Figure 15.19 Reaction of epoxidation of catechin using epichlorohydrin [99].

O

O

O

SNH2

H2N

O

S S

SNH2

NH2

AGSO

ELO

+

O

O

O

O

O

O

O

O O

O

O

O

O

O O

R

O

O

O S

HO OH

OH

S S

HO

OH O

O

O

O

O

OR

N

N

N

N N

N

R

O

O O

O

O

O

NHO

O O

Figure 15.20 Cross-linking reaction between epoxidized linseed oil (ELO) and amine grapeseed oil

(AGSO) [90].


obtained from coniferous trees. The acidic compo-

nent of rosin is mainly a mixture of isomeric

abietic-type acids (Figure 15.21). Rosin and its

derivatives have been used as tackifiers for adhe-

sives, varnishes, paints, etc.

The researchers have studied a rosin-based,

anhydride-type curing agent and rosin-based glyci-

dyl 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 devel-

oped 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°Cwas 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 hard-

ener increased the impact strength from 9.2 kJ/m2 to

24.3 kJ/m2 [103].

Wang et al. [104] synthesized rosin-based flexi-

ble anhydride-type curing agents based on

maleopimarate-terminated polycaprolactone (MPA-

terminated PCL). Commercial epoxy resin (DER

332 Epoxy resin, DOW) was cured with (MPA-

terminated PCL) in different stoichiometric ratios,

using 2-ethyl 4-methylimidzole as accelerator.

Epoxide/anhydride ratio affects the mechanical

properties. At a ratio of 3:2, the cured resin exhib-

ited 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 cur-

ing agent affected the cross-linking density. As the

molecular weight of the PCL segment increased,

the corresponding cured epoxy became more duc-

tile and less strong.

It can be concluded that the obstacles to replace

petroleum-based epoxy resins with bio-based poly-

mers 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, usu-

ally reinforced with fiber glass. Unsaturated polye-

sters 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) Iso-

resins, (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 dipentaery-

thritol, 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 reac-

tion of UPR is a free-radical chain growth polymeri-

zation between reactive diluent (styrene) and the

resin. Curing procedures are considered versatile,

from room temperature to elevated temperature,

according to the catalyst used [105].

Figure 15.21 Structure of abietic acid, one of the

main components of rosin.


These building blocks can be substituted by bio-

based analogs, either partially or even totally, in

some cases. Bio-based propylene glycol (1,2-propa-

nediol) and 1,3-propanediol (PDO) are currently

being commercially produced from glycerol. In the

presence of metallic catalysts and hydrogen, glyc-

erol can be hydrogenated to propylene glycol (1,2-

propanediol), 1,3-propanediol, or ethylene glycol.

1,3-Propanediol (PDO) is also being used for pro-

duction of bio-based unsaturated polyesters [86].

Several entities are working to develop and/or com-

mercialize 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 produc-

tion for propylene glycol made from crude

biodiesel-based glycerin is compared to conven-

tional propylene oxide-based propylene glycol

[106]. Page et al. [107] detailed the polymerization

of unsaturated polyester resins based on biologi-

cally derived 1,3-propanediol. PDO monomers can

be obtained via a fermentation process of corn feed

stocks, using bacterial strains able to convert glyc-

erol 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 dia-

cids increase the solubility of unsaturated polyesters

in vinyl monomers (Figure 15.22). Solubility in sty-

rene 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 anhy-

dride (MA). Compositions containing ortho-

phthalic acid, maleic anhydride, and Susterrat

1,3-PDO were compared to compositions containing

ortho-phthalic acid, maleic anhydride, and 1,2-pro-

pylene glycol (instead of 1,3-PDO) using the same

ratio between the components. The curing was

obtained using styrene (60%). The bio-based com-

position 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 1,3-

propanediol and C5-C10 unsaturated dicarboxylic

building blocks, such as bio-based itaconic acid or

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 com-

pound was used as reactive diluent. Radical inhibi-

tors 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 1,2-

propylene 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 compa-

rable mechanical properties to similar fossil compo-

sitions with higher elongation and slightly lower

HDT (105°C compared to 109°C). Fatty acids or

oils can be used as polyacids, while rigid carbohy-

drates, such as isosorbide, can be used as polyols.

O O

HO OH

OOO

OO

O O O

O

O

O

O

Fumaricfragment

(FA)

O

Maleicanhydride

(MA)

Phthalicanhydride

(PA)

1,3-propanediol(1,3-PDO)

O O O

+ +

Figure 15.22 Unsaturated polyesters based on bio-

based 1,3-propanediol [107].


Szkudlarek et al. [109] also synthesized unsaturated

polyester resins using corn-based isosorbide and

1,3-propanediol, corn-based itaconic acid/anhy-

dride, and maleic anhydride in styrene solution.

Isosorbide provides the aromatic building block

needed to obtain the solubility in styrene. Low ther-

mal stability obtained by Szkudlarek et al. was

improved by introducing bio-based itaconic, citra-

conic, and mesaconic ester units [110]. The unsatu-

rated polyester described by the researchers can be

obtained by polycondensation of a polyol (bio-

based 1,3-propanediol or 1,2-propanediol) and

itaconic, citraconic, and/or mesaconic acid or anhy-

dride as unsaturated dicarboxylic acids. The heat

deflection temperature of this new composition was

considerably improved (from 70 to 105°C). Citricacid 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 unsatu-

rated polyesters, which improves toughness and

connectivity in the polymer network [91].

Epoxidized vegetable oils can be used as a replace-

ment 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 fol-

lowed by alternating copolymerization of the result-

ing 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 dic-

tates 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 petroleum-

based 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

of 28% was observed for polyester resins contain-

ing 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 acry-

lated 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 combina-

tions containing AESO. Two Tg were detected for

the polymerized AESO (at 250 and 15°C), proba-bly due to the multi-functionality and high unsa-

turation 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 tough-

ness of the system. The resistance to thermal

decomposition of the hybrid resin system was

improved due to the interpenetrating network struc-

ture created.

Liu et al. [115] obtained unsaturated polyester-

like resins from functionalized tung oil. Tung oil is

extracted from the seeds of tung trees. The princi-

pal compound of this oil is a glyceride based on

alpha-elaeostearic acid (cis-9, trans-11, trans-13-

octadecatrienoic acid). This compound is a highly

unsaturated conjugated system that is used as a dry-

ing 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


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 commer-

cially available bio-based unsaturated polyester

resin (Envirezt) based on soybean oil triglycerides.

Different grades of Envirezt are available that con-

tain from 8 to 22% bio-based content [116].

Envirezt is obtained by a process where a carbox-

ylic acid or corresponding anhydride containing an

ethylenic unsaturation is first reacted with a satu-

rated, 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 func-

tionality of the polyol determines the properties of

the final polyurethane. Diols lead to linear thermo-

plastic polyurethane whereas polyols with three or

1

O

2

O

O

Tung oil (TO)

Free radical copolymerization

Rigid thermoset polymer

(3) Styrene unt (St)

Malinated products (TOPERMA)

Pentaerythritol alcoholysis products (TOPER)

+

+

+

C O

C

O

HOOC CH CH CH2 CH2

H2C

CH

CH CH

CH O

O

O

O O

N,N-Di methyl benzylemine

Pentaerythritol (PER)

(1) 200–210°C

Hydroquionone

T = 95°C

O

O

(2)

C

O

O

O

O

C

C

C

CH

CH

CH

CH

COOH

COOH

COOH H2C

CH2OH

CH2OH

CH2OH

CH2OHCa(OH)2

HOH2C

CH2OH

CH2OH

CH2 O

O

CHOH2C

CH2OH

H2CCOOH

H2C

O

C

C

C

C

O

O

O

O

O

O

O

O C

Figure 15.23 Synthesis of tung oil-based unsaturated polyester-like polymer [115].


more hydroxyl groups are required to prepare ther-

moset, polyurethane networks.

The terminology “bio-based polyurethanes” usu-

ally 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 poly-

ols 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 poly-

urethane production are divided into three groups:

polyether polyol, polyester polyol, and oleochem-

ical polyols from vegetable oils.

a. Bio-based polyether polyols: Sucrose and sor-

bitol 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]. Bio-

based 1,3-propanediol can be used to produce

polytrimethylene ether glycol as the soft seg-

ment 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 oxida-

tion, compared to polyether-based. Polyester-

based polyurethanes are more sensitive to

hydrolysis and microorganism attack [119].

Longer and hydrophobic chain polyols can

result in greater flexibility and hydrolytic sta-

bility of the resulting polyurethane.

c. Vegetable oil-based polyols: These can be pre-

pared using distinct methods such as

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 subse-

quent 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 natu-

rally occurring polyols, however, the number

of hydroxyl groups in castor oil are substan-

tially 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 distri-

bution 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 seg-

ments resulting from the saturated portions) in

polyurethane networks obtained from vegetable oil-

based polyols [33]. The functionality of the polyol

determines the properties of the final polyurethane

polymers. Diols lead to linear thermoplastic polyur-

ethanes, 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.


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 dur-

ing gas expansion might weaken the three-

dimensional network of the polyurethane foam,

thus creating more open cells with lower carbon

dioxide content and reduced thermal isolation prop-

erties [121].

Castor oil can be used in the synthesis of cross-

linked 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 polyure-

thane 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 poly-

urethanes are obtained when the ratio is high [122].

Most vegetable oil-based polyols are of relatively

low molar mass (around 1000 Da) with a functional-

ity distribution that ranges from 1 to 8 hydroxyl

groups per molecule. As a result, the average

hydroxyl equivalent (molecular weight divided by

the hydroxyl functionality) varies from 200 to 300,

making them suitable for rigid and semi-rigid appli-

cations, rigid foams, cast resins, coatings, and adhe-

sives [13]. Rigid polyurethane foams can be obtained

from 100% vegetable oil-based polyols, while flexi-

ble foams are obtained by mixing vegetable oil-

based 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 elasto-

mer with a glass transition temperature around 7°C.According to Petrovic [13], in order to obtain flexi-

ble foams and elastomers, polyols with molecular

weight greater than 3000 Da and hydroxyl equiva-

lent weight of 1000 and higher are required. These

polyols were synthesized by Petrovic from ricino-

leic acid via transesterification, which produced

polyricinoleic acids (Figure 15.24). The dangling

chains act as plasticizers, and inhibit crystallization.

Elastomers with glass transition temperatures rang-

ing from 233°C to 258°C were obtained when

polyricinoleic acids were polymerized with diphe-

nylmethane diisocyanate. According to Petrovic,

lower glass transition temperatures and higher elon-

gations 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 func-

tionality of the polyols ranged from 3.0 for the mid-

oleic sunflower polyol to 5.2 for the linseed oil.

The conversion of the double bonds to epoxy

groups during the epoxidation reaction was rela-

tively 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].


followed by ring opening. Polyurethanes were

obtained by reacting the polyols with MDI. Linseed

oil-based polyurethanes presented higher cross-

linking 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

15�23 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 oil-

based 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 differ-

ent hydroxyl contents using the synthesis of poly-

ether 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 function-

ality of the polyol, the higher the degree of cross-

linking 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 tai-

lored to generate substances quickly and reliably,

mimicking nature, and was designed to generate sub-

stances by joining small units together.

Undecylenic acid was obtained by heating rici-

noleic acid under vacuum pyrolysis to create unde-

cylenic acid and heptaldehyde. Using thiol-ene

click chemistry, Lligadas et al. [120] applied photo-

initiated coupling of 2-mercaptoethanol and methyl

esters of oleic and undecylenic acids, followed by

reduction (Figure 15.25).

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 elimi-

nation to alkyne functionality, and then subsequent

reduction of carboxylate groups to obtain primary

hydroxyl groups (Figure 15.26). The resulting poly-

ol was reacted with methylene diphenyl isocyanate

(MDI) using 1,4-butanediol as the chain extender.

Desroches et al. [125] synthesized ester-

containing 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 differ-

ent 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 trans-

esterification with a diol or amidification with

hydroxylamine or through thio-ene radical cou-

pling. 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 iso-

propanolamine (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 poly-

ol 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 poly-

ol), Bayer (BAYDUR, castor oil-based polyol) and

Mitsui Chemicals (castor oil-based polyols) � for

making polyurethane foams for the automotive, fur-

niture, spray insulation, and other industries.

Polyurethane rigid foams are widely used as insula-

tion and structural materials for construction, trans-

portation, decoration, and appliances, which

accounts for approximately one-third of the


polyurethane market. Flexible polyurethane foams

are used in automobile seating, upholstered furni-

ture, 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-

O

(a)

(b)

O

O

O

OH HO

HOSH

OH

O

O

OL

O

OLM

O

4

4

6

6

HOSH

HO O

O

DMPA/hν7

7

4

4

4

4

6

6

6

6

O

O

O O

O

S

S

HO

OH

OH

O

OLA OLA-diol

OLM-diol

HO

OH

SHS O

O

UDA-diol

S

SS

7

7

S

S

OH

OH

OH

HO

OH

SH

OHUDA85%

DMPA/hν

DMPA/hν

95%

70%

DMPA/hν

80%

7UD

7

UDMUDM-diol

OH p-TSA/reflux

80%

CH3OH

p-TSA/reflux

85%

CH3OH

p-TSA/reflux

85%

p-TSA/reflux

80%

LiAIH4/THF

84%

LiAIH4/THF

75%

Figure 15.25 Preparation of undecylenic and oleic acid-derived diols using thiol-ene click chemistry [120].


based polyols, raw materials, and producers can be

seen in Table 15.3.

Polyols from Lignocellulosic Materials

Lignocellulosic materials including wood, agri-

cultural, or forestry wastes are a mixture of natural

polymers based on lignin, cellulose, and hemicellu-

lose, 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 thermochem-

ical treatments at high temperatures and high pres-

sure [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.

Figure 15.26 Preparation of undecylenic aromatic triol (UDT) and oleic aromatic triol (OLT) using

cyclotrimerization process [120].


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 polyur-

ethanes [22,41]. Starches and cellulose can be liq-

uefied in the presence of alcohols in order to obtain

polyols to be used in the preparation of polyur-

ethanes. The process includes the use of liquefac-

tion solvents based on polyethylene glycol,

glycerol, and sulfuric acid. The solution obtained is

then neutralized with caustic soda [122].

Polyols from Natural Polyphenols

As detailed in the section on polyphenols, natu-

rally occurring polyphenols such as tannins and lig-

nins 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 con-

tent. High content (30�35%) of kraft lignin results

in rigid and brittle materials, while lower amounts

provide soft polyurethanes [122]. Kelley et al.

[140] synthesized polyurethanes from kraft hydro-

xypropyl lignin with varied molecular weights and

hexamethylene diisocyanate. The mechanical prop-

erties 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 lig-

nin. 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 copoly-

mer 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 poly-

urethanes. 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 result-

ing mixture can be reacted with aromatic diisocya-

nate 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 polyure-

thane foams [59].

Isocyanate-Free Polyurethanes andBio-Based Isocyanates

Isocyanates are the derivatives of isocyanic acid

(H-NQCQO). The functionality of the isocyanate

(R-NQCQO) group is highly reactive toward

Figure 15.27 Synthesis of bio-based polyurethane

from epoxidized soybean oil and isopropanolamine

[126].


proton-bearing nucleophiles, and the reaction of

isocyanate proceeds with addition to the carbon-

nitrogen bond [142]. Important isocyanates used

in polyurethane manufacturing include 2,4-toluene

diisocyanate, 2,6-toluene diisocyanate, 1,6-hexam-

ethylene diisocyanate, and 1,5-naphthalene diiso-

cyanate, 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 pres-

ence of electron-withdrawing substituents on iso-

cyanates 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 chemi-

cal 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 alterna-

tive routes to create bio-based, non-toxic isocya-

nates. 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 synthe-

sized from any epoxy monomers. The cyclic car-

bonate groups were added to the triglyceride by

reacting epoxidized linseed oil with carbon dioxide

in the presence of a catalyst.

Cayli et al. [144] synthesized isocyanate deriva-

tives from unsaturated plant oil triglycerides. The

triglyceride was first brominated at the allylic posi-

tions 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 tetraa-

mine polyurea showed a glass transition

Table 15.3 Summary of Bio-Based Polyols for Polyurethanes [4]

Source MaterialTradeName Producer Application Reference

Soybean oil BiOH Cargill Flexible foams [128]

Renuva Dow Flexible foams and CASE� [129]

SoyOil Urethane SoySystems

Flexible and rigid foams,spray foams, elastomers

[4]

Baydur BAYER Rigid and flexible foams [130]

Agrol BioBasedTechnologies

CASE�, molded foams [131]

Castor oil LupranolBALANCE

BASF Rigid foams, mattresses [132]

Polycin Vertellus Coatings [133]

MistuiChemicals

Rigid and flexible foams [134]

Renewable andrecycled natural oil

Enviropol IFL Chemicals Rigid foams for insulationand refrigeration

[135]

Rapeseed/Sunflower oil

Metzeler-SchaumGmbH

Flexible foam [136]

�CASE: Coatings, Adhesives, Sealants, and Elastomers.


temperature of 31°C. The polyurethanes synthe-

sized 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

Figure 15.28 Isocyanate free urethane: Reaction of cyclic carbonate with phenylalkylamine [143].

CH

CH

Br O

O

O

CH

CH

CH O

O

O

O

O

NCO O

O

O

O

AgNCO in THFR.T.

CH

Figure 15.29 Bio-based isocyanate from vegetable oil [144].


decomposition of acyl azide. Diacids such as azea-

lic 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 fol-

lowed by purification and oxidation of the aldehyde

to obtain the diacid. Hojabri et al. compared physi-

cal 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 avail-

able diisocyanate: 1,6-hexamethylene diisocyanate

(HDI). Canola polyols were synthesized using ozo-

nolysis and hydrogenation. Desmophen 800 (Bayer)

was used as the commercial polyol. The polyur-

ethanes obtained had comparable properties to

fossil-based diisocyanates.

Bio-Based Alkyd Resins

Alkyds are polyesters obtained by polycondensa-

tion of three monomers: polyols, dicarboxylic acids,

or anhydrides, and natural fatty acids or triglycer-

ides. The term alkyd is a modification of the origi-

nal 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 compat-

ible with a vast number of polymers, making them

very versatile to produce coatings, binders, paints,

lacquers, and varnishes, both transparent and semi-

transparent [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 con-

tent of the final polymer [4] and extends up to 70%

of the bio-based content [147]. Fossil-based phtha-

lic acid and maleic acid (and anhydrides) are the

most commonly used organic acids; glutaric and

succinic anhydrides can also be used in alkyd for-

mulations [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

Figure 15.30 Synthesis of bio-based isocyanate from oleic acid [145].


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 neutral-

ized 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 dimethylolpro-

pionic 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 esteri-

fication of dipentaerythritol with fatty acids.

In the 1980s and 1990s, environmentally

friendly, zero VOC (Volatile Organic Contents)

versions of alkyd waterborne resins were devel-

oped. 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 unsatu-

rated fatty acid, styrene, and methyl methacrylate

[17]. Kuhneweg [149] developed a waterborne

acrylic-modified alkyd resin based on the polymeri-

zation 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 con-

tent 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. High-

solid 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 formula-

tions. 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 glyc-

erol in medium oil-length, soybean oil-based, air-

drying alkyd resin formulations [103].

Bio-Based Phenolic Resins

Phenol�formaldehyde is one of the oldest com-

mercial 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.

CH2-OHCH2 CH2CH

O

O

O

O

O C

O

C O O

On

C O

R

CH2-OH

CH-O-CO-R

Figure 15.31 Synthesis of alkyd resin [17].


The ratio between formaldehyde and phenol deter-

mines 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

phenol�formaldehyde resins where the molar ratio

of formaldehyde to phenol is less than one. The poly-

merization 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 phenol�formaldehyde 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. Cross-

linking 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 phenol�formaldehyde 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 compo-

sites). 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 phenol�formaldehyde 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 phenol�formaldehyde

resins; it is a relatively expensive petroleum-based

chemical [154].

Lignin is highly cross-linked; three-dimensional

aromatic polymers with phenyl�propane 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

Figure 15.32 Structure of phenol formaldehyde.


nature, after cellulose, and it is produced as a by-

product 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 compo-

sites [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 para-

substituted by the propyl chain of the propylphen-

4-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 dete-

riorating the mechanical and adhesion properties,

and observed that an additional environmental ben-

efit 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 liq-

uefying wood, bark, and forest and wood industry

residues using a fast pyrolysis process. Monomeric

phenols can be produced by thermochemical con-

version 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, hard-

wood, 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, guaia-

col, syringol and para-substituted derivatives, car-

bohydrate 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

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 mix-

ture 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 pheno-

lic compound (Figure 15.17) that can be used to

substitute phenol in novolac or resole resins.

Cardanol�formaldehyde 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 form-

aldehyde. Tannins are composed mostly of flavan-

3-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

Figure 15.33 A- and B-ring in tannin, where R1

can be resorcinol or phlorogucinol and R2 can be

pyrocathecol or pyrogallol [91].


reactions with hexamine at lower temperatures,

compared to fossil-based phenol formulations.

However, the high reactivity and the large mole-

cules 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 formal-

dehyde. 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 hydroxy-

methylolation can be used to introduce hydroxy-

methyl groups (aCH2OH) to lignin molecules

using formaldehyde in an alkaline medium.

Methylolated lignin can be directly incorporated in

phenol�formaldehyde 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 bio-

based, 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

[162]. Cetin et al. [164,165] utilized phenolated-

lignin (20�30% of overall phenol) instead of

unmodified lignin in phenol�formaldehyde 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 phenol-

formaldehyde 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 methylo-

lated with formaldehyde (Figure 15.34) in the pres-

ence of sodium hydroxide. The methylolated bio-

oil was polymerized with formaldehyde, replacing

up to 75 wt.% of phenol in the resin for the produc-

tion of plywood. The methylolation treatment

improved the thermal stability of the bio-based phe-

nolic resin. Phenolation and methylolation have

shown the best results in terms of number of reac-

tive sites created. The origin of the lignin may

affect the reactivity. Softwoods may yield more

reactive phenolics than hardwoods due to the

CH3O

CH2O

CH3O CH2OH

OH

+NaOH

O+–

NaConiferyl monolignols

Figure 15.34 Methylolation reaction of bio-oil with formaldehyde [167].

Figure 15.35 Guaiacol and syringol chemical

structures (in lignin) [154].


relatively high presence of guaiacol with one meth-

oxy group (Figure 15.35) [154].

Formaldehyde-containing resins (urea�formal-

dehyde, melamine�formaldehyde, and phenol�formaldehyde) for applications as wood adhesives

and mineral fiber binders for thermal insulation are

under intense pressure to reduce or eliminate form-

aldehyde content due to ecological concerns and

toxicity issues [152]. There is a need to develop non-

toxic, bio-based formaldehyde substitutes. Ramires

et al. [169,170] utilized glyoxal (OCHaCHO), a bio-

based 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 glyoxal-

phenol resins were developed for application in com-

posite materials using sisal fibers as reinforcement, and

showed improved results for novolac-type resins in

terms of mechanical and physical properties. The novo-

lac glyoxal-phenol composite showed higher impact

strength (237 J m21) than the resole glyoxal�phenol

composite (118 J m21), indicating a better fiber/matrix

interaction. The novolac glyoxal�phenol composite

exhibited a higher storage modulus (E’), and conse-

quently 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].

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