Biobased Thermosetting Epoxy: Present and Future - ACS Publications

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Biobased Thermosetting Epoxy: Present and Future Rémi Auvergne,† Sylvain Caillol,† Ghislain David,*,† Bernard Boutevin,† and Jean-Pierre Pascault‡,§ †

Institut Charles Gerhardt UMR CNRS 5253 Laboratoire Ingénierie et Architecture Macromoléculaire, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France ‡ INSA-Lyon, IMP, UMR5223, F-69621, Villeurbanne, France § Université de Lyon, F-69622, Lyon, France 3.3.2. Comonomers/Hardeners from Starch/ Sugar 3.3.3. Copolymers Bearing Epoxy Groups 3.4. From Vegetable Oils 3.4.1. Epoxy Monomers from Vegetable Oils 3.4.2. Comonomers/Hardeners from Vegetable Oils 3.5. From Terpenes, Terpenoids, and Resin Acids 3.5.1. Epoxy Monomers from Terpenes 3.5.2. Comonomers/Hardeners from Terpenes 4. Biobased Formulations and Networks 4.1. Some Words about Epoxy Formulations 4.2. Epoxy Formulations with Initiators (i.e., without Hardeners) 4.3. Epoxy Formulations with Hardeners 4.3.1. Formulation Based on Epoxidized Vegetable Oils. How To Increase the Properties? 4.3.2. Sugar-Based Epoxy Networks and Water Uptake 4.3.3. Advanced Biobased Epoxy Networks 4.3.4. Biobased Additives for Toughness Increase 4.3.5. New Properties from Polyesters Based on Epoxy Chemistries 5. Conclusions and Future Trends Author Information Corresponding Author Notes Biographies References

CONTENTS 1. Introduction 2. Background 2.1. Synthesis of Epoxy Monomers 2.1.1. From Epichlorohydrin ECH 2.1.2. From Double Bond Oxidation 2.1.3. From Glycidyl (Meth)acrylate 2.2. Epoxy Monomer Characteristics 2.2.1. Epoxide Content 2.2.2. Chlorine Content 2.3. Polymerization 2.3.1. Step Growth or Polyaddition/Polycondensation 2.3.2. Chain-Growth Polymerization 2.3.3. Dual Cure: Combination of Step-Growth and Chain-Growth Polymerizations 3. Biobased Building Blocks 3.1. From Polyphenols, Tannins, and Cardanol 3.1.1. Epoxy Monomers from Tannins and Derivatives 3.1.2. Epoxy Monomers from Other Natural Polyphenols 3.1.3. Epoxy Monomers from Cardanol 3.1.4. Comonomers/Hardeners from Polyphenols 3.2. From Woody Biomass and Lignin 3.2.1. Some Words about Biomass and Lignin Extraction 3.2.2.. Epoxy Monomers from Lignin 3.2.3. Hardeners from Lignin 3.2.4. Biomacrolecules Extracted Woody Biomass from Lignin 3.3. From Starch and Sugar 3.3.1. Epoxy Monomers

© 2013 American Chemical Society

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1. INTRODUCTION Thermosets (also called thermosetting polymers) are a family of plastics characterized by the fact that they are formed starting from a liquid solution that irreversibly leads to a solid material during the curing step. The initial liquid solution is usually composed of several ingredients, the most important ones being a mixture of comonomers that can react among themselves by an external action such as heating or UV irradiation. The necessary condition to generate a thermosetting polymer is that one or more of the monomers has three or more reactive groups per molecule. This produces a tridimensional cross-linked structure that occupies all the reaction

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Received: March 28, 2012 Published: October 14, 2013 1082

dx.doi.org/10.1021/cr3001274 | Chem. Rev. 2014, 114, 1082−1115

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Scheme 1. Mechanism of Phenolic Compound O-Glycidylation

Scheme 2. Main Commercial Epoxy Monomers/Synthons from Phenolics

by the incorporation of an engineering/high performance property or combination of properties into the final product.1 The global epoxy thermosetting polymers production is estimated to be 2 million tons in 2010 and is projected to reach 3 million tons by the year 2017. Their global market was estimated at roughly US$18 billion in 2012 and is forecasted to reach US$21.5 billion by 2015.2 More than 60% of the global production is used in the coatings industry.3−5 Furthermore, epoxies are probably the most versatile family of engineering/ structural adhesives because they are compatible with many substrates, and can be easily modified to achieve widely varying properties. As explained previously, control of properties and also processing is usually based on the selection of the appropriate epoxy precursors or combination of monomers, on the selection of curing agents and associated reaction mechanism, and on the addition of organic or inorganic fillers and components. Epoxy adhesives are nowadays mainly used in automobile assembly, concrete, sandwich panels, aircraft and aerospace, “do it yourself” (e.g., hobbies, basic home repairs), etc. New markets for structural composites in automotive parts or wind turbine blades are new challenges for epoxies. Due to the need to reduce the weight of transportation vehicles (planes, trains, automobiles, etc.) to reduce fuel consumption, structural composites and adhesives will be the key market drivers for epoxy resins over the next few years. Although epoxies are mainly classified as thermosets, it is also possible to produce linear epoxy polymers by using comonomers with only two reactive sites per molecule. These linear polymers behave as thermoplastics and can be amorphous or semicrystalline. They have interesting optical and barrier properties, but they represent a niche market.6

volume. Also, once the chemical reaction has been completed, it is not possible to reshape the final material. Thermoset materials represent less than 20% of plastic production. A large variety of families of thermosetting polymers is used in industry. Typical examples are phenolic and urea formaldehyde resins, unsaturated polyesters, and epoxy resins. Globally, epoxy resins account for approximately 70% of the market of thermosetting polymers (polyurethanes not included). Epoxy resins are usually low molar mass prepolymers which normally contain at least two epoxide groups. The epoxide group is also referred to as a glycidyl or oxirane group. Epoxy monomers may be reacted either with themselves through anionic or cationic homopolymerization, or with a wide range of co-reactants including polyfunctional amines, acids, anhydrides, phenols, alcohols, and thiols. These co-reactants are often referred to as hardeners. Epoxy formulations contain also some additives and fillers. The aim of blending is to achieve the desired processing and/or final properties, or just to reduce cost. Since the beginning of their commercial production in the late 1940s, epoxy polymers have evolved dramatically to now find adoption in diverse industrial applications requiring superior strength, excellent adhesion, good chemical resistance, and excellent performance at elevated temperatures. Due to that, they are used in coatings, electrical/electronic laminates, adhesives, flooring and paving applications, and high performance composites. Compared to others, epoxy cross-linked polymers are not the lowest-cost resins, potentially available for most applications. Thus, they must provide added value to justify their additional cost. This added value is usually realized 1083


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resources such as tannins, and more generally, no review reports the synthesis of biobased curing agent or material properties to discuss the envisaged applications. Therefore our review is essential and complementary to previous reviews. Biorefinery processes and retreatment of biomass will be out of our scope; we will be limited to the use of natural polymers and molecular biomass for preparing epoxy formulations and networks. In section 2, we recall some background on the epoxy chemistry, including the structures and reactivity of epoxy monomers and hardeners and the description of reactions leading to epoxy networks. Section 3 will focus on the synthesis of biobased building blocks, including epoxy precursors and cross-linkers, and from all sorts of biomass, from carbohydrate, from tannins, or from oleochemical sources. In section 4, because they need optimization and technological developments, we will try to discuss some examples of biobased networks. Finally, in section 5 we will give our opinion on what could be the future of these new materials in different applications.

Uncertainty in terms of price and availability of petroleum, in addition to global political and institutional tendencies toward the principles of sustainable development, is urging the chemical industry toward a sustainable chemistry and particularly the use of renewable resources in order to synthesize biobased chemicals and products. A biobased product is a product synthesized from renewable resources (vegetal, animal, or fungal), but it does not mean that it is a biodegradable material. Biodegradability is a special functionality conferred to a material, biobased or not, and biobased sourcing does not entail biodegradability. It can be said that the tendency of the market is toward the opposite: there is an increasing demand for biobased, sustainable performance materials, where the emphasis is on performance and endurance, and not on biodegradability. Thus, partially or full biobased epoxy cross-linked polymers are nowadays a real target and also a real challenge from both academic and industrial points of view. The use of renewable resources for epoxy monomer synthesis results in reduction of environmental impacts such as nonrenewable resource consumption. However, hazards of chemicals should also be taken into account in order to reduce human toxicity and ecotoxicity impacts. The most popular epoxy monomers are those derived from the reaction of bis(4-hydroxyphenylene)-2,2-propane (called bisphenol A, BPA) and 1-chloroprene 2-oxide (called epichlorohydrin), in the presence of sodium hydroxide. The structure of the major product, bisphenol A diglycidyl ether (DGEBA or BADGE) and its condensed forms, is dependent upon the stoichiometry of the reactants (Schemes 1 and 2a). The aromatic ring of BPA is particularly suitable since it confers a good thermal resistance to epoxy networks, and more than 90% of epoxy cross-linked polymers are provided from BPA as a reactant. Unfortunately, BPA is not only classified as a reprotoxic R2 substance; it is also a compound that was initially synthesized as a chemical estrogen.7 Indeed, this endocrine disruptor can mimic the body’s own hormones and may lead to several negative health effects8−10 including alterations in both the immune and reproductive systems along with a modification in brain chemistry.11 The negative impact of BPA on human health and the environment necessarily implies focusing research on the substitution of BPA, especially since some countries have recently banned the use of BPA in food contact materials. Recently, BPA was avoided in epoxy crosslinked polymers used for bottle feeds or printing inks and also in can coatings and pipe linings for drinking water. Several works have been carried out so far from both the academic and industrial sides to produce epoxy cross-linked polymers by using less toxic or even nontoxic reagents.12 Therefore, there is an increasing interest in the chemical industry for nonharmful aromatic compounds allowing the synthesis of epoxy thermosets without BPA. Renewable molecules and derivatives of biomass can be sources of alternatives to epoxies from BPA. The present review proposes to gather the works undertaken to obtain either partially or fully biobased epoxide materials. In the recent literature there are many papers and reviews on the use of biomass for making polymers and materials. Most of them give an overview on biobased plastics13 or focus on thermoplastic polymers.14 Some interesting reviews have already been published concerning the synthesis of biobased epoxy materials such as the contribution of Gandini.15 However, some authors16−20 only review epoxides from vegetable oils or did not take into account some interesting natural aromatic

2. BACKGROUND A full description of both the chemistry and the properties of conventional epoxy cross-linked polymers can be found in several books and review papers.1,2,21 2.1. Synthesis of Epoxy Monomers

2.1.1. From Epichlorohydrin ECH. The mechanism of the condensation reaction between phenols and 1-chloroprene 2oxide (called epichlorohydrin, ECH) can be described briefly (Scheme 1). In all described procedures, ECH is used in significant excess and reaction is carried out in an aqueous solution of sodium hydroxide (NaOH). The literature22,23 clearly shows that the reaction between the phenate ion (ArO−) 1′ and ECH 2 reveals two competitive mechanisms: one-step nucleophilic substitution (mechanism SN2) with cleavage of the C−Cl bond and a two-step mechanism based on ring opening of ECH (2) with ArO− (1′) followed by intramolecular cyclization (SNi) of the corresponding alcoholate 4, containing one atom of chlorine in the β-position, formed in situ. Depending on the substituent position or nature in the phenol 1, it takes 6−20 h at reflux or 24−26 h at room temperature to complete the reaction. It is important to note that glycidylation of phenols needs first the synthesis of the phenate group with NaOH, which can entail some inconvenience such as moderate isolated yields, moderate purity of products, and poor reaction selectivity (mechanism B vs mechanism A giving directly 3). Indeed, 1-chloro-3-aryloxypropan-2-ols 5 were formed in a nonnegligible amount as nonsuitable byproducts. Another byproduct is the abnormal addition of the phenolic hydroxyl group with ECH. The most popular epoxy monomers are those derived from the reaction of bis(4-hydroxyphenylene)-2,2-propane (called bisphenol A) and ECH in the presence of NaOH. The structure of the major product, bisphenol A diglycidyl ether (DGEBA), and its condensed forms (Scheme 2), is dependent upon the stoichiometry of the reactants. Typically monomers are marketed with n in the range 0.03−10. At room temperature these monomers are crystalline solids for n values close to 0, liquids up to n ∼ 0.5, and vitreous (Tg ∼ 40−80 °C) for higher n. 1084


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Another major group of epoxy monomers derived from ECH is that comprising monomers synthesized with an aromatic amine, such as methylene dianiline (TGMDA) or p-aminophenol (TGpAP) (Scheme 3). For these monomers side reactions are intramolecular cyclization between two adjacent oxiranes.

Scheme 5. Formula of Glycidyl Methacrylate, GMA

These precursors are often used for powder coatings. Whereas the epoxy functional monomer GMA is essential to introducing the primary functional group into the polymer, the choice of the other comonomers has influence on the technological properties of the polymer as well as on the final properties.29

Scheme 3. Main Commercial Epoxy Monomers/Synthons from Amines

2.2. Epoxy Monomer Characteristics

2.2.1. Epoxide Content. As explained, a wide range of epoxy monomers are produced industrially, mainly from petroleum-based raw materials. An important criterion for epoxy precursors is the epoxide content. This is commonly expressed as the epoxide number, which is the number of epoxide equivalents in 1 kg of compound (equiv/kg), or as the equivalent weight, which is the mass in grams of compound containing 1 mole equivalent of epoxide (g/mol). Clearly, one measure may be simply converted to another as equivalent weight (g/mol) = 1000/epoxide number (equiv/kg). The equivalent weight or epoxide number is used to calculate the amount of co-reactant (hardener) to use when curing epoxy precursors. The stoichiometric ratio of functional groups determines very often the physical properties. 2.2.2. Chlorine Content. This characterization is only for commercial monomers such as DGEBA obtained from ECH. In practice, commercial DGEBA may have up to about 1% chlorine. Chlorine may be either hydrolyzable (arising from dehydrochlorination, and usually less than 1%) or inactive (arising from abnormal addition of the phenolic hydroxyl group or reaction of ECH with the secondary alcohol, it can be up to 30% by weight). This presence of chlorine will adversely affect for instance the electrical properties of a printed circuit board’s applications.

The reaction of ECH with an alcohol is more difficult, with many side reactions, since this reaction generates new alcohol groups with similar pKa values which are able to react with the epoxy group of ECH, thus leading to its homopolymerization, and the final product is often a mixture of chlorohydrins and epoxy monomers.24 This particular point will be discussed in section 3.3.1.1. Liquid monomers based on butanediol, neopentylglycol, trimethylolpropane (TMP), and OH terminated polypropylene oxide oligomers are the most common. Due to their low viscosity compared to DGEBA, they are often used as reactive diluents. 2.1.2. From Double Bond Oxidation. Another approach to the formation of the oxirane groups is the peroxidation of a carbon−carbon double bond. The reaction conditions differ according to the type of double bonds. When the double bonds belong to the aliphatic chains, their oxidation is rather simple and only requires the use of hydrogen peroxide (H2O2). However, when the double bonds are chemically incorporated, they are usually glycidyl type and the oxygen in the β-position may alter the double bond reactivity. Stronger oxidative reagents are then required such as m-chloroperbenzoic acid. Purification of the resulting epoxy synthons becomes rather complex, which limits their industrial development.25−28 Various types of oligomers fall into these two categories. A typical one is based on biscyclohexane (Scheme 4). Others such as epoxidized oils and epoxidized rubbers will be discussed later.

2.3. Polymerization

The epoxy/oxirane group is characterized by its reactivity toward both nucleophilic and electrophilic species, and it is thus receptive to a wide range of reagents. Epoxy monomers polymerize through step-growth and chain-growth processes. Linear or cross-linked epoxy polymers are obtained by reaction of the epoxy monomers with comonomers (“hardeners”) and/ or initiators. 2.3.1. Step Growth or Polyaddition/Polycondensation. 2.3.1.1. Reactions with Amines. (a). Epoxy + Amino Hydrogen Reactions. The most typical example of a stepgrowth polymerization of epoxy monomers is the reaction with amines which are the most common curing agents/hardeners used to build up epoxy networks. One epoxy ring reacts with each amino proton (Scheme 6). The reactivity of the amine increases with its nucleophilic character:30 aliphatic > cycloaliphatic > aromatic. While for

Scheme 4. Formula of 3,4-Epoxycyclohexyl Methyl 3′,4′Epoxy Cyclohexane Carboxylate

2.1.3. From Glycidyl (Meth)acrylate. Thermosetting polymers may be formed in two ways: by polymerizing monomers and also by reacting a polymer containing functional groups in its backbone. Acrylic copolymers carrying epoxy groups on the chain is a typical example. They are synthesized in a free radical polymerization reaction of glycidyl methacrylate (GMA; Scheme 5) with other monomers carrying a vinyl or (meth)acrylic double bond.

Scheme 6. Reactions between Epoxy Groups and Primary and Secondary Amines

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polymerization and final shaping are performed in the same process; shaping of the part has to be performed before the gel point where the material is still in the liquid state. (b). Modified Amine Adducts. From a general point of view, amine hardeners lead to epoxy networks with good electrical properties and excellent chemical resistance, particularly against alkalis. Aliphatic amines are used in low temperature curing formulations such as adhesives and coatings, while aromatic amines are used to achieve greater heat resistance (heat deflection temperature of 150−190 °C) and chemical resistance than those from aliphatic amine. They need high temperature cure cycles such as those used for the manufacture of composite materials. Nevertheless, the toxicity of some of these aromatic amines remains a drawback. For instance, m-phenylene diamine (mPDA) and methylene dianiline (MDA) are classified as CMR (carcinogenic, mutagenic, and reprotoxic). Aliphatic amines which rapidly react with epoxy monomers are representative room-temperature curing agents. The aim of their modifications is to improve the workability: to extend the pot life; to control (increase or decrease) the curing speed; to improve compatibility with epoxy monomers; to reduce toxicity and irritation to the skin; to decrease weighing error because the loading amount is increased. Another point is to reduce the capacity of aliphatic amines to react with atmospheric carbon dioxide and water to form carbamates. During reactions at low temperature, carbamates can exude to the surface and produce blush, which has a detrimental effect on coating performance. For all these reasons, curing agent manufacturers have developed a wide variety of modified amine adducts. Their great diversity makes them very suitable for many epoxy applications. (i). Ketimine. Ketimine, which is attracting attention as a curing agent for high solid paints, is formed by the reaction between an aliphatic polyamine and a ketone such as methyl ethyl ketone or isobutyl ketone. Ketimine reacts very slowly when mixed with epoxy monomer, but it works as a kind of latent curing agent for coating applications by absorbing moisture in the air and regenerating amines to cure at room temperature. (ii). Amine Adducts. Amine adducts are prepared by reacting excess primary amines with epoxy monomers. A wide variety of aliphatic and cycloaliphatic amines (mainly diethylene triamine, DETA, triethylene tetramine, TETA, isophorone diamine, IPDA, bis-p-aminocyclohexylmethane, PACM, and 1,2-diaminocyclohexane, 1,2-DACH) are utilized for amine adduct reactions. (iii). Polyamide Amines. Polyamide amines (also called amido-amines) are formed by the condensation reaction between diacid and polyamine, and contain reactive primary and secondary amines in their molecules. They are used to react with DGEBA monomer at room temperature (or even below) with moderate heat generation and a long pot life. They are used in primers or tie coats for steel and concrete. Very often the aliphatic amine is reacted with a tall oil fatty acid which increases the hydrophobicity of the final network.

aliphatic and cycloaliphatic amines, primary and secondary amine hydrogens exhibit similar reactivities, for aromatic amines the reactivity of the secondary amine is 2−5 times less than the reactivity of the primary amine hydrogen. This means that, once the primary amine reacts, the generated secondary amine exhibits a lower reactivity, a fact that is called a “substitution effect”. Hydroxyl groups catalyze the reaction through the formation of a trimolecular complex that favors the nucleophilic attack of the amino group. Apart from species containing OH groups that may be added as catalysts, the epoxy−amine reaction generates OH groups. Therefore, the reaction is self-catalyzed by reaction products. In most cases, when stoichiometric amounts of epoxy and amine comonomers are used, no side reaction takes place. When there is an excess of epoxy groups, the reaction shown in Scheme 7 can take place after most of the amine hydrogens have reacted. Scheme 7. Reaction between Epoxy and Hydroxyl Groups

The epoxy−hydroxy reaction (also called etherification) modifies the initial stoichiometric ratio based on epoxy to amino hydrogen groups. Other factors may also influence the path of the curing reaction, such as the presence of a catalyst or of an initiator (see section 2.3.3). It is interesting to say a few words about the synthesis of amines which is not trivial since they generally show a strong reactivity, which may lead to byproducts. It is widely known that amines are obtained from hydrogenation of nitriles. The nitrile compounds can also be obtained from two pathways: either from carboxylic acids or from alcohols (addition reaction on acetonitrile). Nitrile compounds can be thus hydrogenated, according to different methods such as hydrogenation based on LiAlH4 or hydrogenation with Raney nickel. Poly(oxyalkylene) amines showing terminal −CH2−CH(CH3)−NH2 or −CH2− CH(C2H5)−NH2, are obtained by reaction of ammonia with corresponding secondary alcohol compounds. Nevertheless, alkyl groups situated in the α-position of the amine strongly decrease the amine reactivity. The polyaddition reaction between the 2-functional A2 DGEBA (Scheme 1) and a 4-functional B4 diamine (containing four active amine hydrogens in the structure) leads to a polymer network. Gelation occurs at a conversion where percolation of a giant molecule takes place throughout the system. At this critical conversion the system consists of a large number of finite molecules (the sol fraction) and one giant molecule (the gel fraction). After gelation, the mass fraction of the insoluble giant structure increases continuously and so does the elastic modulus of the sample. As stated in the Introduction, the main characteristic of thermosetting polymers is that Scheme 8. Synthesis of Mannich Base

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(iv). Phenalkamide or Mannich Base. These hardeners are prepared by reacting amines with phenol and formaldehyde to form oligomers (Scheme 8). Usually, Mannich type hardeners also include residual free phenol and diamine. They are designed for low temperature, high humidity applications. Although the condensation decreases the functionality of the amine, the presence of the phenolic hydroxyl functionality on the aromatic ring produces a substantial accelerating effect on the epoxy/amine reaction. Mannich bases rapidly react at low temperatures (0−30 °C). They have better compatibility with epoxy monomers than unmodified alkylene amines and are more resistant to blush and water spotting. 2.3.1.2. Other Polyaddition Reactions of Epoxy Groups. Mercaptans (RSH) are able to react with epoxy groups at room temperature. The basic thiol−epoxy reaction mechanism is reputedly a simple nucleophilic ring-opening reaction by the thiolate anion followed by protonation of the alcoholate anion via the quaternary ammonium, originally formed via reaction of the base catalyst and thiol to generate the initial thiolate. The ring-opening reaction involving thiols and epoxides is important industrially and is involved in the formation of adhesives, high performance coatings and composites, and accounts for one of the two major industrial uses of multifunctional thiols.31 Reactions with phenols (Ar−OH), isocyanates (R−NCO), or acids (RCOOH) are not as clear as expected because catalysts that can also initiate a chain polymerization (tertiary amines, triphenylphosphine, imidazoles, chromates, etc.; see section 2.3.2) are practically always used. With acids, other reactions such as esterification and transesterification are also possible. 2.3.2. Chain-Growth Polymerization. 2.3.2.1. Homopolymerization. In this case it is necessary to generate an active species that attacks a functional group and makes it active for a new reaction with another functional group. This produces a primary chain of reacted functional groups until a termination reaction occurs and the primary chain becomes deactivated. The gel conversion for the ideal chainwise polymerization of an A2 + Af system depends mainly on the ratio q between the propagation rate, Rp, and the sum of the reaction rates: propagation, termination, Rt, and transfer, Rtr, reactions: q = Rp/(Rp + Rt + Rtr). As usually the probability of chain propagation is very much larger than the probability of termination, it means that q → 1 and xgel → 0.30 In practice, the polymerization is far from being ideal and macrogelation occurs, typically at conversions in the range of 5−30%. As stated, epoxy groups can react with both nucleophilic and electrophilic species. Epoxy monomers undergo a chain homopolymerization in the presence of both Lewis acids such as boron trifluoride complexes (cationic homopolymerization) and bases such as tertiary amines, imidazoles, and ammonium salts (anionic homopolymerization). These acids and bases are called initiators of the chain polymerization. The energy released by ring-opening polymerization (ROP) is strongly exothermic and therefore difficult to control. Propagation (Scheme 9) proceeds through an alkoxide (anionic polymerization) or an ozonium (cationic polymerization). In this reaction, each epoxy group acts as a bifunctional reactant in the propagation step and therefore, a diepoxy monomer such as DGEBA (Scheme 2a) becomes a 4-functional monomer, A4, leading to a polymer network. Chain transfer and complex termination steps stop the chain propagation leading to q < 1 and conversion at the gel point, xgel ∼ 20−30%.

Scheme 9. Ionic Propagation Steps of Epoxy Homopolymerization

A usual way to generate a strong acid as initiator of cationic polymerizations is by thermal or UV decomposition (Scheme 10) of a complex aromatic salt of a Lewis acid. Cycloaliphatic Scheme 10. Decomposition Reactions of Some Cationic Photoinitiators

epoxy monomers are used in this reaction because they exhibit higher reactivities than those of glycidyl ether epoxies such as DGEBA. The cationic polymerization proceeds through two classical different mechanisms: the activated monomer (AM) and the activated chain end (ACE) mechanisms.32,33 These formulations are used in photopolymerization processes whose main advantage apart from the fast reaction rate is the insensitivity to oxygen (contrary to free-radical polymerizations).34 2.3.2.2. Chain Copolymerization. While the epoxy−acid reaction follows a stepwise mechanism, the reaction of epoxides with cyclic anhydrides initiated by Lewis bases takes place through a chainwise copolymerization. Initiation (Scheme 11) involves the reaction of the Lewis base with an epoxy group, giving rise to a zwitterion that Scheme 11. Initiation of Epoxy−Anhydride Co-reaction

contains a quaternary nitrogen atom (when the base used is a tertiary amine) and an alkoxide anion. The alkoxide reacts at a very fast rate with an anhydride group, leading to a species containing a carboxylate anion as the active center. Thus the ammonium salt can be considered as the initiator of the chainwise copolymerization. Propagation occurs through the reaction of the carboxylate anion with an epoxy group, regenerating the alkoxide ion which reacts rapidly with an anhydride group, regenerating the carboxylate anion (Scheme 12). As the reaction of an alkoxide group with a cyclic anhydride is much faster than the reaction of a carboxylate group with an epoxy ring (kp2 ≫ kp1), it results in an alternating chainwise copolymerization of epoxide and anhydride groups. 1087


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Scheme 12. Propagation during Epoxy−Anhydride Chain polymerization

It is also the case with cyanoguanidine, or dicyanodiamide (Dicy), a very versatile hardener widely used in one-pack epoxy formulations for prepregs, laminates, powder coatings, etc. Usually these formulations contain an epoxy excess over the other groups.

The anionic copolymerization of DGEBA = A4 with an anhydride of a diacid = B2 leads to a polymer network. After the amines, acid anhydrides constitute the next most commonly used reactants for curing epoxy monomers (Scheme 13). Scheme 13. Phthalic Anhydride and Its Saturated Derivatives, Trimellitic Anhydride and Pyromellitic Anhydride

3. BIOBASED BUILDING BLOCKS Biobased compounds recently gained strong interest and allowed new developments for epoxy cross-linked polymers especially from both an academic point of view and an industrial point of view. As already mentioned, ECH is conventionally used to obtain epoxy monomers by reaction with hydroxyl compounds. Nevertheless, ECH is classified as CMR, a carcinogen of category 1B. Usually, ECH was obtained from chlorohydrination of allyl chloride, which was synthesized by reaction of chlorine with propylene. 36 Interestingly, ECH is now industrially produced from biobased glycerol (Scheme 14),37 Scheme 14. Synthesis of Biobased Epichlorohydrin

The alternating copolymerization is certainly the reason why in the past this reaction has been erroneously regarded as a stepwise process. The presence of a chainwise mechanism was confirmed by characterizing the linear copolymer formed in the reaction of phenyl glycidyl ether (A2) with phthalic anhydride (B2), using matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectrometry.35 Molar masses were in the range 4−80 kg mol−1, depending on the initiator used and on the purity of the starting materials; impurities can be water, acid from anhydride partial hydrolysis, etc. 2.3.3. Dual Cure: Combination of Step-Growth and Chain-Growth Polymerizations. As explained, the chemistry of epoxies is very versatile. The oxirane ring of the most popular epoxy monomer, DGEBA, can form only a single covalent bond with an amine group but two covalent bonds with an anhydride group. Therefore, DGEBA acts as a bifunctional monomer, A2, in the step-growth polymerization (polyaddition) with a diamine but as a tetrafunctional monomer, A4, in the anionic chain-growth polymerization with an anhydride. In some formulations of commercial use the reality is more complex because both mechanisms can take place in a competitive way that depends on temperature and concentrations of reactants. This is the case of epoxy-amine or -phenol formulations containing an “accelerator” such as a Lewis acid (e.g., a BF3−amine complex) or a Lewis base (e.g., a tertiary amine or an imidazole). The accelerator initiates the chaingrowth polymerization of epoxy groups that occurs in parallel with the step-growth epoxy-amine or -phenol additions.

and this process affords about 100 kt per year.38 It has been found that conversion of glycerol to the chemical ECH is economically attractive.39 The biobased weight content of DGEBA is about 25% if the synthesis is made from this biobased ECH. As mentioned in section 2, epoxy monomers can be also commonly obtained from double bond oxidation. Nevertheless, two different strategies are generally employed, according to the type of double bond in the natural products: when the natural products already contain double bonds in the aliphatic chains, e.g., vegetable oils, epoxidation reaction is carried out in the presence of H2O2. Hydrogen peroxide is probably the best oxidative agent with O2 from both economic and environmental considerations. Lane and Burgess40 reviewed the use of hydrogen peroxide as an oxidative agent and especially focused on the use of metal catalysts. They also showed that oxidation with H2O2 is highly efficient only with electron-rich alkenes. In some cases, the double bonds can be also allyl type and are chemically grafted into the natural product. Oxygen atoms are often in the β-position of the double bond, i.e., glycidyl type, and may deactivate the double bond; thus stronger oxidative reagents than H2O2 must be used to provide the epoxy groups. Below are given some examples of natural product oxidation from the two distinct strategies to provide biobased epoxy monomers. 1088


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occurs on both the carboxylic acid group and at least one phenol group (Scheme 17) in the presence of an ammonium-

We show here the recent trends concerning both the epoxy monomers and the cross-linking agents. 3.1. From Polyphenols, Tannins, and Cardanol

Scheme 17. Epoxy Reactants Obtained from Gallic Acid

3.1.1. Epoxy Monomers from Tannins and Derivatives. Tannins are nonhazardous compounds with antiallergenic, antiatherogenic, and antimicrobial properties.41−45 These positive effects toward human health can be associated with antioxidative properties for these compounds.46−48 Tannins are natural poly(phenol) compounds49 and can be found as hydrolyzed tannins (ellagitannins or gallotannins), condensed tannins, complex tannins, and phlorotannins.50 Both hydrolyzed and condensed tannins are employed for the preparation of epoxy monomers. Gallotannins result from the combination between gallic acid and polyol (i.e., D-glucose), whereas ellagitannins are provided from the oxidative coupling of at least two gallic units of gallotannins. Condensed tannins come from flavan-3-ol polymerization (Scheme 15). Depending on the substituents R1−R4, different flavan-3-ols exist and can be polymerized owing to the bond in parentheses.51

type phase transfer catalyst in anhydrous medium. Thus, Tomita reports epoxy equivalent weight (EEW) values ranging from 137 to 160, corresponding to epoxy functionality ranging from 1 to 4. The epoxidized compound was then cross-linked with a conventional cross-linking agent (i.e., polyamine or acid anhydride).54 However, theoretical calculations show that epoxy functionalities of 1, 2, 3, and 4 correspond respectively to EEW values of 226, 141, 113, and 99. This demonstrates that Tomita obtained an average epoxy functionality of 2, despite a ratio of 4 epihalohydrin per phenol group. In a recent publication, Aouf et al.55 reported conditions allowing the synthesis of tetraepoxygallic acid. Previous studies only reported functionalization with two to three epoxy groups. Moreover, they investigated the mechanism of O-glycidylation for several model phenolic compounds in order to establish a relationship between the chemical structures of phenolic compounds and their behavior toward ECH. Indeed, an understanding at the molecular level of the reactivity of phenolic monomers toward glycidylation represents a crucial step in the development of biobased epoxy monomers based on polymeric tannins as phenolic sources.55 Nouailhas et al.53 also recently proposed another synthetic pathway leading to epoxy prepolymers from gallic acid allylation by reaction of gallic acid with allyl bromide, followed from double bond oxidation by using m-chloroperbenzoic acid. This method allows obtaining epoxy prepolymers with epoxy functionality up to 3. 3.1.1.3. Other Tannins. Following a former study based on model molecules of tannins, beyond the feasibility, Benyahya et al.56 proposed the direct use of condensed tannins naturally rich in polyphenols for the synthesis of aromatic biobased epoxy oligomers. These natural and inexpensive green tea tannins are directly used after extraction from renewable resources and open a route to a new aromatic renewable resource usable for polymer synthesis. 3.1.2. Epoxy Monomers from Other Natural Polyphenols. Some other polyphenols were investigated to provide new biobased epoxy monomers, such as curcumin and resveratrol (Scheme 18). The double bonds can be epoxidized by reaction with m-chlorobenzoic acid, resulting in glycidyl

Scheme 15. Structures of Flavan-3-ol Monomers: (+)-Afzelechin (R1 = R2 = R3 = H; R4 = OH); (−)-Epiafzelechin (R1 = R2 = R4 = H; R3 = OH); (+)-Catechin (R1 = R3 = H; R2 = R4 = OH); (−)-Epicatechin (R1 = R4 = H; R2 = R3 = OH); (+)-Gallocatechin (R3 = H; R1 = R2 = R4 = OH); (−)-Epigallocatechin (R4 = H; R1 = R2 = R3 = OH)

Notably, catechin and gallic acid are among the most studied tannin derivatives. The epoxy monomers from such compounds are given in sections 3.1.1.1 and 3.1.1.2. 3.1.1.1. Catechin. Catechin was epoxidized either by reaction with ECH or by alkylation with an unsaturated halogenated compound followed by its oxidation.52 A full characterization of the obtained compounds shows the presence of byproducts with benzodioxane groups, which then decreases the average epoxy functionality.53 These byproducts result from an internal cyclization reaction between phenolic alcohol in an ortho position and ECH after addition by an SN2 mechanism (Scheme 16). 3.1.1.2. Gallic Acid. Gallic acid is found in most astringent vegetables, and especially in gallic nuts. Gallic acid may be combined with a tannin structure. Gallic acid epoxidation with epihalohydrin was first reported by Tomita.54 The addition

Scheme 16. Epoxy Monomer and Byproduct from Catechin Epoxidation53

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(8Z,11Z,14Z-pentadecatrienyl)phenol, with mainly cis conformation67,68 (Scheme 20).

Scheme 18. Structures of Curcumin and Resveratrol

Scheme 20. Structures of Cardanol and Compounds Extracted from Cashew Nut Shell Liquid

ether compounds, thus enabling cross-linking to provide epoxy cross-linked polymers.57 Resorcinol is produced by mixing resins (galbanum, asafetida, ...) with potassium hydroxide and shows an aromatic structure.58 Prior to its use for the synthesis of epoxy monomer, resorcinol is reacted with the condensed form (mesityl oxide) of acetone (Scheme 19).59 Then the polyphenol compound is able to react with ECH in the presence of (CH3)4NBr (Scheme 19). Scheme 19. Products Obtained by Reaction of Acetone with Resorcinol (left) and Epoxy Monomer from Resorcinol (right)

Due to both its aromatic and aliphatic structures, cardanol seems to be a promising candidate for the substitution of petroleum-based phenol derivatives. Furthermore, cardanol enhances coating properties such as both chemical and mechanical resistances, anticorrosion, and flexibility.69 This will be discussed more deeply in section 4. A patent claims the use of commercial epoxidized cardanol from Cardolite Co.70 PPG Industries also denoted the use of cardanol to provide biobased epoxy cross-linked polymers. Cardanol was in fact mixed with bisphenol A.71 Tan72 reported the epoxidation of phenolated lignin by cardanol. Cardanol was added to lignin between 0.3 and 0.9 mol/lignin unit. Moreover, a study73 was performed on the enzymatic epoxidation of cardanol. More interestingly, epoxy monomers were synthesized from cardanol74−77 by double bond epoxidation in the presence of enzymes (lipase) and acetic acid in toluene. A continuous addition of H2O2 is required during 6 h to quantitatively epoxidize the reactive double bonds. The epoxidized compound is then polymerized with H2O2 in the presence of 2-propanol at ambient temperature (Scheme 21). After 24 h, the crude product is concentrated under vacuum. The cross-linking reaction is allowed with phenylamine at 150 °C.73 The synthesis of monoepoxidized cardanol is actually obtained from direct reaction with ECH, which is used as reactive diluting compound. The synthesis of diepoxy cardanol is obtained via a two-step reaction: cardanol phenolation followed by reaction with ECH of the resulting diphenol. Notably, the phenolation mechanism of unsaturated aliphatic chains is well-known and occurs on the allyl carbons in the presence of strong acids such as HBF4.78 The idealized structure of diepoxy cardanol is given in Scheme 22. It is also possible to obtain a cardanol−novolac prepolymer79 from a conventional reaction of cardanol with formaldehyde catalyzed with p-toluenesulfonic acid at 120 °C during 7 h. Epoxidation is performed with ECH in basic medium. 3.1.4. Comonomers/Hardeners from Polyphenols. Polyphenols are the easiest route to biobased cross-linking agents. For instance, Shibata et al.80,81 carried out the crosslinking reaction of sorbitol polyglycidyl ether (SPE) in the presence of quercetin (QC or 3,5,7,30,40-pentahydroxyflavone). Phenalkamide and Mannich bases obtained from the monophenol and diphenol of cardanol are commercial products. More details about formulations and properties using these hardeners will be given in section 4.

Different patents show the synthesis of epoxy cross-linked polymers from epoxy monomers of resorcinol or phloroglucinol (three hydroxyl groups), previously epoxidized by reaction with ECH.60−62 (Notably, resorcinol can be also obtained from phloroglucinol which is synthesized from glucose through an enzymatic pathway63). 3.1.3. Epoxy Monomers from Cardanol. Cashew nut shell liquid (CNSL) is a renewable natural resource obtained from the cashew (Anacardium occidentale) nut as a byproduct during the process of removing the cashew kernel from the nut. The total production of CNSL approaches 1 million tons annually,64 and CNSL is one of the few major and economic sources of naturally occurring phenols. CNSL can be regarded as a versatile and valuable raw material for polymer production and represents a good natural alternative to petrochemically derived phenols.65 CNSL constitutes nearly 25% of the total mass of the nut and is composed of anacardic acid (3-npentadecylsalicylic acid) and smaller amounts of cardanol (3-npentadecylphenol), cardol (5-n-pentadecylresorcinol), and methylcardol (2-methyl-5-n-pentadecylresorcinol), with the long aliphatic side chain being saturated, monoolefinic (C8), diolefinic (C8, C11), and triolefinic (C8, C11, C14) with an average value of two double bonds per molecule. The thermal treatment of cashew nuts and CNSL induces the partial decarboxylation of anacardic acid, which is completed by the subsequent purifying distillation. The result is an industrial grade cardanol, in the form of a yellow oil containing mainly cardanol (about 90%), with smaller percentages of cardol and methylcardol.66 Cardanol itself is a mixture of four malkylphenols differing by unsaturation degree of aliphatic chain: 3% 3-(pentadecyl)phenol, 42% 3-(8Z-pentadecenyl)phenol, 17% 3-(8Z,11Z-pentadecadienyl)phenol, and 38% 31090


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Scheme 21. Cardanol Epoxidation and Polymerization

Scheme 22. Structure of Diepoxy Cardanol

Scheme 23. Structure of Softwood Lignin

3.2. From Woody Biomass and Lignin

3.2.1. Some Words about Biomass and Lignin Extraction. Natural polymers, including cellulose, hemicellulose, polysaccharide, lignin, chitin, and chitosan, are considered as abundant and low-cost feedstocks. The first step of the chemical modification of wood20 consists of wood destructuring in order to gain access to a range of valorizable synthons. Such destructuring can be carried out through either physical or chemical processes, i.e., high temperature vapor, acids, bases, phenols, or ozone, followed by water or methanol extraction. Lignocellulose comprises lignin, cellulose, and hemicellulose, which are covalently linked.82,83 Lignocellulosic biomass represents a significant, abundant sustainable feedstock. Cellulose is primarily built by C6 and C5 sugars (by polymerization of glucose), while hemicellulose is a polymer of glucose and xylose. Lignin is a highly cross-linked complex polymer built of substituted phenols. The major functional groups of lignin consist of hydroxyl, methoxyl, and phenylpropane units allowing copolymer grafting. Its aromatic structure confers relatively good thermostability84 and good mechanical properties. The structure of lignin is based on three phenylpropanoids: coumaryl, synapyl, and coniferyl alcohols.85 These units are linked by aryl−alkyl−ether bonds to form a tridimensional network (Scheme 23). The heterogeneous structure of lignin may vary with the vegetable resource as well as with the mode of extraction.86−89 For instance, lignin provided from softwood is mainly constituted of coniferyl alcohols. Lignin can be extracted from different processes, aiming at biomass fractionation to lower molar mass oligomers.90 The main processes are the following: • sulfite process using SO2, sulfites, and sulfuric acid • kraft process leading to lignins with aliphatic thiols • cellulose process mainly used for preparation of paper coating

• CIMV (Compagnie Industrielle de la Matière Végétale) process that selectively separates cellulose, hemicellulose, and lignin The sulfite process allows hydrolysis of both ether and ester bonds in acid medium.91 This technique, applied at high temperature and under high pressure, leads to fragmentation of lignin, which becomes degraded at the end of the process thus ending into lignosulfate. The kraft process corresponds to hydrolysis in basic medium92 in the presence of Na2S. Another process is based on the use of anthraquinone as catalyst in the presence of NaOH.93 Both processes afforded lignin of modified structure. The cellulose process consists of an extraction from solvolysis; this method is based on lignin solubilization in organic solvents, leading to “organosolve lignins”. This process affords lignins with a high degree of purity. Several organic solvents were used, such as methanol, phenol 1,4-dioxane, ethanol, formic acid, and acetic acid.94−103 The last process, i.e., the CIMV process, allows the destructuring of the vegetable matter at atmospheric pressure by a catalyst−solvent system of formic acid/acetic acid/water to produce a white lignin.104,105 Some other chemical (such as solvent extraction) or physical processes are also developed but rarely at an industrial scale. 1091


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3.2.2.. Epoxy Monomers from Lignin. Only a few publications deal with the modification of lignin to be used as an epoxy monomer.82,106−109 Mainly three processes have been described in the literature20 enabling the synthesis of epoxidized lignin. The first process principally aims at increasing the content of phenolic groups in lignin.110−114 The phenols used are basically phenol and bisphenol A, but acetone is also used to in situ create interphenol bridges from lignin. The phenolated lignin is then reacted with ECH, leading to epoxidized lignin, which can be cross-linked with polyamine curing agent. It was especially shown that the lignin−epoxy precursor provided better waterproof adhesion strength after curing at 140 °C with triethylene tetramine (TETA). Recently, Hitachi Co. claimed the use of lignin to provide epoxy cross-linked polymers based on this process. Hitachi also claimed through two patents that their performances as thermal insulating materials are similar to those of conventional epoxy cross-linked polymers (based on DGEBA). Further, Hitachi enhanced their good flame retardant performances added to a good solubility in organic solvents.115 It is also mentioned that high molar mass lignins were removed to reach biobased lignins with number-average molar masses, Mn, ranging from 300 to 10 000 g/mol. Below 300 g/mol, the prepolymer will not possess enough reactive groups (hydroxyl, carbonyl, or acid), and above 10 000 g/mol the material will lose its solubility in organic solvents. This compound is primarily epoxidized with ECH in basic medium. Then, the epoxy monomer, based on lignin, is cross-linked with a very low molar mass lignin compound. The glass transition temperature, Tg, of the resulting epoxy network, based on lignin, is above 180 °C.116 Araco Corp., a Japanese company, also claimed the use of lignin as epoxy monomer.117 They actually use a phenolated lignin mixed with a phenol compound to increase the amount of aromatic groups and thus to increase both the heat resistance and the hydrophobicity of the final material. In another process, lignin reacts with an aliphatic diepoxide compound in DMF at 80 °C to afford epoxide-containing lignin.118,119 Hirose et al. also proposed the synthesis of epoxy monomer from reaction between kraft lignin and diglycidyl ether of polyethylene glycol. The cross-linking occurred by reaction with poly(azealic anhydride), leading to an epoxy network of very low Tg, i.e., below 0 °C, whatever the epoxide/ acid ratio used for cross-linking reaction.120 The third process is based on the oxypropylation of lignin to afford a lignin “decorated” with hydroxyl groups. These hydroxyl groups are then converted into epoxide groups by reaction with ECH, and cross-linking occurs in the presence of diamines. The authors mentioned that the cross-linking rate is rather low, compared to a conventional system, due to the poor mobility of epoxide groups. With aromatic diamines, the Tg ranges from 80 to 200 °C, depending on both the type of lignin and the degree of oxypropylation.121 3.2.3. Hardeners from Lignin. Much work has been carried out on the synthesis of lignin-based cross-linking agents for epoxy cross-linked polymers. In one process, lignin reacts with ozone in the presence of NaOH.122−124 The aromatic rings of lignins are cut and unsaturated diacids are then formed and used as curing agents for conventional aqueous epoxy monomers. Usually, crosslinking occurs at 150 °C and the reactive systems are used as wood adhesives.

Acid groups have also been introduced by another way: sulfuric acid modified lignin is dissolved in a polyol in order to increase the hydroxyl content and then is able to react with acid anhydride leading to a carboxylic acid (Scheme 24).125 Scheme 24. Synthesis of Carboxylic Acid from Lignin

In another example, organosolve lignin and ethylene glycol (Scheme 25) react with succinic anhydride to form a carboxylic acid derivative (A) and a polyacid (B). Compounds A and B are mixed, and both are able to cross-link ethylene glycol diglycidyl ether in the presence of a tertiary amine. Another carboxylic acid may also be added during the cross-linking reaction at 130 °C/5 h. The epoxy cross-linked polymers show Tg values below 0 °C whatever the ratios of reactants.126,127 Ismail et al. performed a similar synthesis of lignin-based cross-linking agent. Compound A is mixed either with compound B or with compound C, i.e., obtained from reaction between glycerol and succinic anhydride. The cross-linking agent is in fact a mixture of A + B and A + C and enables crosslinking reaction with ethylene glycol diglycidyl ether. The increase of lignin content in the formulation leads to an increase of the grafting density, which of course increases the Tg of the network. When the glycerol content increases, the Tg value also increases despite a higher chain length as compared to ethylene glycol. This behavior was attributed to the higher content of hydroxyl groups which enhance hydrogen bonding.128 As a hardener of their epoxy-based lignin, Hitachi proposed the synthesis of a biobased anhydride cross-linking agent obtained by esterification reaction between a low molar mass lignin and trimellitic anhydride chloride129 (Scheme 26). It is interesting to note that, prior to its use as curing agent, lignin was introduced into epoxy cross-linked polymers before cross-linking to increase the mechanical properties or to form a polyblend. Xie et al.130 showed that lignin itself could not crosslink the epoxy monomer; thus a cross-linking agent such as an amine type needs be added to the polyblend.122,131 3.2.4. Biomacrolecules Extracted Woody Biomass from Lignin. One route for biobased building blocks involves chemical degradation and transformation of natural polymers. Considering their aromatic and polymeric characteristics, their renewability, and their vast supply, lignin sources of lignocellulosic biomass undoubtedly represent a significant sustainable feedstock for chemicals.132 Lignin is challenging to break down into chemically useful fragments. For example, different processes used in biorefineries lead to primary synthons such as vanillin, 2-pyrone, 4,6-dicarboxylic acid, pcoumarylic acid, coniferic acid, sinapylic acid, and muconic acid.39 Some research teams have already reported works on vanillin for epoxy synthesis and various applications. A Schiff base synthesized was used for metal ion chelation applications (Scheme 27).133 Moreover, vanillin was used to synthesize biobased epoxy monomer by a chemoenzymatic process with Candida antartica lipase134 (Scheme 28). In all cases, epoxy networks exhibited interesting thermal properties. 1092


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Scheme 25. Synthesis of Lignin-Based Cross-Linking Agent and Cross-Linking Reaction with Ethylene Glycol Diglycidyl Ether

that interactions between methoxy and hydroxyl groups improved the impact strength as well as tensile strength and elongation. Some other original structures were also described in the review article of Koike20 such as 2-pyrone-4,6-dicarboxylic acid diepoxide, which is obtained from vanillin (Scheme 30). Many technologies proposed to convert in the same reactor and in a one-step process the lignocellulosic feedstocks into intermediates. As an example, the Biofine Process136,137 developed a high-temperature, dilute acid hydrolysis process that converts cellulosic biomass into soluble sugars which are then transformed to a mixture, the major component of which (50%) is levulinic acid (LA). LA can react as both a carboxylic acid and a ketone. A catalyzed hydroxyalkylation gives a biobased diphenol (Scheme 31). DPA was once used commercially in various epoxy formulations before it was replaced by the petrochemically derived BPA which could be supplied at a lower price. An expected reduced cost of LA production may allow DPA to recapture some of the market share. However, strangely, no one has recently used DPA to prepare the diglycidyl ether of DPA.

Scheme 26. Structure of Lignin Modified with Acid Anhydride

In the review paper of Koike,20 an interesting process was described allowing the synthesis of an epoxy monomer from vanillin (Scheme 29). The intermediate bisphenol showed a melting point of 175 °C. The diepoxy was further cross-linked with methylene dianiline (MDA). Ochi et al.135 demonstrated Scheme 27. Synthesis of Schiff Base from Vanillin

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Scheme 28. Epoxidation of Allyl-Containing Vanillin by Chemoenzymatic Process

are commercially available138,139 and are mainly aliphatic glycidyl ether types. Some studies report the synthesis of poly(epoxide) prepolymers from glycerin140,141 (Scheme 32).

Scheme 29. Synthetic Route for a Difunctional Epoxy Monomer Based on Vanillin

Scheme 32. Epoxy Monomers Based on Glycerin and Sorbitol

As stated in section 2, the reaction of ECH with natural aliphatic polyols, bearing both primary alcohol groups at the chain end and secondary alcohol groups in the chain, is not trivial. Indeed, this reaction generates new alcohol groups, which are also reactive toward ECH.24 Unlike the reaction between phenol groups and ECH, the hydroxyl groups resulting from the reaction between the polyols and ECH show reactivity fairly similar to that of the starting polyols. This thus implies multiple addition of ECH onto the same alcohol, which is then not able to undergo intracyclization. This can explain the chlorine content since these chloride atoms are not situated in the α-position and so no cyclization into epoxide is then possible. Furthermore, sorbitol is not soluble in water, i.e., reaction medium, and an excess of ECH is then required. This in fact reveals that the above-mentioned structures provided from the chemical companies are idealized structures since other byproducts containing chloride atoms are also obtained. It is important to mention that most of these commercial products contain about 10−20 wt % Cl. This actually has two consequences: first, the epoxy formulations become harder and, second, the resulting epoxy networks may undergo HCl formation. Sorbitol and maltitol have also been converted in multifunctional epoxy monomers through oxidation of allyl142 or crotonic143 double bonds, and in this case there was no chlorine group. Interest in the production of 1,4:3,6-dianhydrohexitols, especially isosorbide, has been generated by potential industrial

Scheme 30. Structure of 2-Pyrone-4,6-dicarboxylic Acid Diepoxide

3.3. From Starch and Sugar

As glucose is produced from starch, but also from cellulose, or from woody biomass by chemical transformation, biomass carbohydrates are the most abundant renewable resources. 3.3.1. Epoxy Monomers. 3.3.1.1. Epoxy Monomers from (Cyclo)aliphatic Polyols. Many poly(epoxide) prepolymers obtained from biobased polyols such as glycerol and sorbitol

Scheme 31. Synthesis of 4,4-Bis(4′-hydroxyphenyl)pentanoic Acid or Diphenolic Acid (DPA)

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Scheme 33. Synthesis of Diglycidyl Ether of Isosorbide and Structure of Diisosorbide

Scheme 34. Synthesis of Diglycidyl Ether of Isosorbide by Oxidation Reaction

Scheme 35. Synthesis of Epoxy Monomer from Lactic Acid

behavior can be ascribed to a better solubility of isosorbide in the reaction medium. The second method is a two-step reaction in anhydrous medium between ECH and isosorbide. The first step consists of generating the alcoholate of isosorbide in the presence of NaH for instance, which leads to chlorohydrin ECH ring opening. The second step corresponds to the formation of the oxirane ring in the presence of a strong base (Scheme 33).152 This method is selective since it only leads to diglycidyl ether of isosorbide. Epoxidized isosorbide was also obtained by applying a method based on the use of allyl bromide via a two-step reaction (Scheme 34).149−151 First, diallyl of isosorbide was prepared from reaction between isosorbide and allyl bromide in the presence of potassium hydroxide. Then, modified

applications such as the synthesis of polymers. Isosorbide is obtained from dehydration (−2 mol) of sorbitol. As already shown by Fenouillot et al.,144 isosorbide is a nontoxic and chiral molecule which also brings stiffness to polymer chains such as polyesters, polycarbonates, and polyurethanes. Different processes145−150 allow synthesis of diglycidyl ethers of isosorbide as well as oligoglycidyl ethers of isosorbide. Furthermore, two methods lead to either monomer or oligomers of epoxidized isosorbide by reaction with ECH. The first one, fairly similar to the synthesis of DGEBA, is based on the reaction between diol, ECH, and a strong base in aqueous medium (Scheme 33).143,150,151 This method often generates epoxy oligomers of isosorbide as well. In this case, the chlorine content remains slightly higher than that of polyphenols but much lower than that of sorbitol. This 1095


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isosorbide was oxidized by using m-chloroperbenzoic acid (MCPBA) in dichloromethane at room temperature in order to epoxidize the double bonds. It is nevertheless important to mention that this synthesis was not industrially developed due to the high required content of MCPBA. 3.3.1.2. Epoxy Monomers from Natural Polyacids. Common acids including lactic acid, succinic acid, itaconic acid, and levulinic acid can be extracted from starch and sugar. Lactic acid, used as lactide and glycidol, may provide either linear or hyperbranched epoxide prepolymer, depending on the experimental conditions (Scheme 35).153 This synthetic process does represent a real novelty for the synthesis of epoxy monomers since so far neither conventional nor biobased epoxy monomers were provided from this synthetic pathway; i.e., see section 2. 3.3.1.3. Epoxy Monomers from Sugars. Epoxy monomers from sugars are usually obtained by double bond oxidation, which requires the use of strong oxidative agents such as MCPBA in large excess (about 4−12 excess of oxidative agent to double bond). Sachinvala et al.143 performed the oxidation of sucrose bearing both allyl and crotonic groups. The use of aqueous peracetic acid (32% solution in the presence of sodium acetate) allows performing an efficient epoxidation reaction. The authors were able to evaluate the content of epoxide groups from both FAB mass spectroscopy and 13C NMR and showed that from octaallyl sucrose 3.7 epoxide groups are obtained whereas 7.2 epoxide groups could be reached from crotonyl sucrose due to the electron-donor character of the crotonyl. They also specified that either magnesium peroxyphthalate, or m-chloroperoxybenzoic acid, or oligomers of phosphotungstic acid with H2O2 or molybdenum hexacarbonyl with tert-butyl hydroperoxide were unable to develop satisfactory conditions. 3.3.2. Comonomers/Hardeners from Starch/Sugar. 3.3.2.1. Chitosan. Chitosan (Scheme 36), soluble in water and bearing primary amines, was used as epoxy hardener, particularly to react with DGEBA in emulsion154 or with PEG diglycidyl ether155 to synthesize epoxy networks.

drate and obtained dendrimers of PL (Scheme 37). The pendant α-amino groups are able to react with epoxy groups. Polylysine was also employed to cross-link polyglycidyl ether of glycerin (GPE) or even polyglycidyl ether of polyglycerol. Pioneer Surgical Technology especially patented a formulation for dental cement comprising epoxy monomers mixed with lysine and polylysine.161 Details are given in section 4.3.1.1. 3.3.2.3. Amines from Isosorbide. Recently, van Es et al.162,163 developed a semicatalytic route starting from isosorbide by subsequent transformation into the corresponding bistosylates, nucleophilic substitution by benzylamine, and finally catalytic hydrogenolysis to obtain the pure stereoisomers (Scheme 38). The authors showed that this three-step strategy gave the desired product dideoxy-diamino isosorbide with absolute stereocontrol in >80% overall isolated yield and high purity (>99% based on NMR and GC). Another strategy to enhance the reactivity of isohexides is based on chain extension at C2 and C5 of the isohexide skeleton. This strategy is based on the activation of isosorbide by reaction with triflic anhydride, followed by cyanation, yields the exo−exo configured dinitrile, which serves as a platform chemical for the syntheses of the corresponding diamine, diacid, and diol (Scheme 38).164 Gillet et al. also proposed the synthesis of di(aminopropyl) isosorbide (Scheme 39). Isosorbide reacts first with acrylonitrile, followed by hydrogenation in the presence of Raney nickel.165 3.3.2.4. Acids and Anhydrides. All recent biobased diacids such as succinic acid can be used as biobased hardeners. By a process similar to that for lignin, Hirose et al. developed a carboxylic acid based cross-linking agent from saccharide and succinic anhydride and used it with ethylene glycol diglycidyl ether.166 The Tg values remain below 0 °C and increase with the saccharide content. 3.3.2.5. Monomers and Hardeners from Furan Derivatives. Furfural (F) and 5-hydroxymethylfurfural (HMF), two widely developed furan-derived monomers, can be prepared from C5 and C6 carbohydrate resources, respectively. Both compounds seem to be promising as platform organic molecules. From F it is possible to access the monoamine compound, whereas the diepoxide one can be obtained by reaction with ECH.167 While F has been an industrial commodity for making polymers, the production of HMF has been slowed down by difficulties in terms of isolating it in good yields and purity. However, it is clear that this process will soon become a reality. Notably, if epoxy hardeners have been synthesized from HMF (Scheme 40),168 no epoxy network has been produced from HMF so far. 3.3.3. Copolymers Bearing Epoxy Groups. 3.3.3.1. From Itaconic Acid. Itaconic acid (IA) is an unsaturated dicarboxylic acid produced via fermentation from carbohydrates. It was used to synthesize copolymers from radical copolymerization with vinyl monomers such as styrene followed by functionalization with ECH169 (Scheme 41). Epoxy monomers can be also synthesized by the esterification reaction between an acid and ECH. Recently an itaconic acid (IA) based epoxy monomer with curable double bonds (EIA) was prepared. Different characterization techniques indicate the presence of oligomers, which could be explained by the competition during the reaction. If ECH always reacted with the carboxy groups of IA, the linear oligomers would be formed (a in Scheme 42). When the reaction between ECH and the secondary hydroxyl groups, which were formed during the esterification reaction between

Scheme 36. Structure of Chitosan

3.3.2.2. Lysine. Lysine or 2,6-diaminohexanoic acid has two amine groups, thus enabling cross-linking of epoxy monomers. Li et al. used lysine as a cross-linking agent of cycloaliphatic epoxy monomers.156 Lysine can condense itself from both α and ε sites, but, according to pKa NH2 ε is a more reactive site. The general structure of the polymer polylysine is represented in Scheme 37 with a higher proportion of ε structure. ε-Poly(Llysine) (PL) is produced by aerobic bacterial fermentation using Streptomyces albulus in a culture medium containing glucose, citric acid, and ammonium sulfate.157,158 PL differs from usual proteins since the amide linkage is not between the α-amino and carboxylic groups as is typical of peptide bonds, but is between the ε-amino and carboxyl groups. Scholl et al.159,160 studied the homopolymerization of lysine chlorohy1096


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Scheme 37. Synthesis and Structure of Polylysine (PL)

Scheme 38. Synthetic Route To Provide Dideoxy-diaminoisosorbide, 2,5-Diaminoisosorbide, and Other Derivatives

IA and ECH, was carried out, the branched oligomers would be found (b in Scheme 42).170 3.3.3.2. Glycidyloxystyrene from Coumaric Acid. Glycidyloxystyrene (Scheme 43) is obtained from the reaction of phydroxystyrene (p-HS or 4-vinylphenol) with ECH. Microbial production of p-HS from glucose is carried out in three steps: (i) glucose is first converted to the aromatic amino acid Ltyrosine, (ii) which is deaminated by an enzyme to yield phydroxycinnamic acid, p-HCA (or coumaric acid), and (iii) and subsequent decarboxylation of p-HCA which gives p-HS.171−178 Glycidyloxystyrene can be copolymerized with vinyl monomers by radical polymerization.

Scheme 39. Synthesis of Di(aminopropyl) Isosorbide

3.4. From Vegetable Oils

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Scheme 40. Epoxy Hardeners from HMF

Scheme 41. Copolymerization of Itaconic Acid with Styrene Followed by Its Epoxidation169

Scheme 43. Formula of Glycidyloxystyrene

Different vegetable oils have various compositions in fatty acids, but most of the fatty acids have natural functional groups such as double bonds, hydroxyl groups, and also sometimes epoxy groups. Natural carbon−carbon double bonds with low reactivity can be replaced with new functional groups, which are then readily polymerized. 3.4.1. Epoxy Monomers from Vegetable Oils. Vegetable epoxidized oils are generally used as either plasticizers or stabilizers for PVC synthesis or for paints and coatings.180 They are also used to replace conventional poly(epoxide)s such as DGEBA to synthesize epoxy cross-linked polymers. Arkema is one of the main industrial producers of epoxidized vegetable oils, with the development of VIKOFLEX based on epoxidized soybean oil. Interestingly, vernonia oil is a natural vegetable oil having an epoxide group in the aliphatic chain (Scheme 44).181

Scheme 42. Direct Epoxidation of Itaconic Acid with ECH170

Scheme 44. Chemical Structure of Vernonia Oil

Another alternative consists of modifying the vegetable oil in order to chemically incorporate the epoxide group at the chain ends of the triglycerides (Scheme 45).182 Indeed, vegetable oils having the epoxide group in the ω-position show a reactivity similar to that of conventional epoxy monomers, especially when anhydride acid is used as cross-linker. Then, Tg values are increased up to 50 °C until, e.g., 133 °C. By comparison, the epoxy network based on DGEBA shows a Tg value of 184 °C whereas the Tg value drops to 54 °C with epoxidized triolein (when using the same cross-linker). 3.4.2. Comonomers/Hardeners from Vegetable Oils. Dimers of fatty acids (Scheme 46) are commercially available with different proportions of I, II, and III. These hardeners have been reacted with DGEBA to prepare hybrid epoxy networks

oils are liquid materials at room temperature, and they are currently the most employed renewable resources in the chemical industry.18,179 The main components of this important class of abundant natural resources are triglycerides. Triglyceride usually has three ester bonds, which can be hydrolyzed to form glycerol and three fatty acids. The fatty acids are about 95% of the total weight of triglycerides. 1098


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Scheme 45. Epoxide Group Incorporated at the Triglyceride Chain End182

Scheme 46. Formulas of a Fatty Acid 9,11-Linoleic Acid I, Diacid II, and Triacid III That Can Be Obtained by a Diels− Alder Reaction183

Scheme 47. Reaction Pathway for the Synthesis of AmineFunctionalized Vegetable Oil

combining chemical and supramolecular hydrogen-bonding cross-links.183 Several polyamines are directly obtained from dimers of fatty acids or by amidification reaction with polyamines such as NH2−CH2−CH2−NH−CH2−CH2−NH2. If a vegetable oil is functionalized with at least one amine group, it can be used as a cross-linking agent of epoxy cross-linked polymers. To this aim, Zhao et al. developed a complex five-step synthesis leading to amine-functionalized vegetable oil.184 Oleic acid is first modified into a triglyceride by reaction with oxalyl chloride, followed by esterification in the presence of glycerin and pyridine. Then the triglyceride is epoxidized in the presence of m-chloroperbenzoic acid in dichloromethane, which is reduced to a triol by use of sodium cyanoborohydride and boron trifluoride in THF. The obtained triol undergoes bromination by reaction with tetrabromomethane in the presence of triphenylphosphine, and finally is reduced under hydrogen atmosphere in the presence of palladium/carbon (Scheme 47). This synthesis nevertheless requires a multistep reaction, based on the use of hazardous reagents. Another reaction consists of the modification of ester groups. For instance, Dubois and Gillet185 suggest first the triglyceride methanolysis, leading to three fatty acids, which are then able to react with ammonia to provide fatty nitriles. These nitriles are reduced in the presence of hydrogen to get fatty amines (Scheme 48). Finally, in a recent paper, Stemmelen et al. suggested a more simple method to functionalize vegetable oils186 by thiol−ene chemistry. The reaction occurs between grapeseed oil and cysteamine chlorohydrate (Scheme 49). The conversion rate of unsaturated functions is 87%, which corresponds to an average amine functionality of 4.13 per triglyceride. This compound was used to cross-link the epoxidized prepolymer of linseed oil, leading to an epoxy network with a low Tg = −38 °C. We can note that this last amine functionalization pathway is more in accordance with the green chemistry principles.

trees (particularly, pine and conifer trees) produce this class of molecular biomass. 3.5.1. Epoxy Monomers from Terpenes. 3.5.1.1. Epoxidation of Terpenes/Limonene. Limonene is a six-memberedring terpene,187 present in agricultural wastes derived from citrus peels. Its world production is estimated to be between 110 million and 165 million lbs/year. Limonene is a relatively stable terpene which has two double bonds: one vinylene group allocated in the ring and one vinylidene side group. It can be distilled without decomposition. It is used as a fragrance and as a solvent for coatings. Limonene oxidation allows obtaining limonene monoepoxide and diepoxide which are commercially used as reactive diluents in epoxy applications (Scheme 50). Another study188 shows the possibility of using limonene to build steric epoxy monomers by reaction with napthol. Sellers189 describes another synthetic pathway, based on the reaction of ECH onto a polyhydric phenol, previously obtained from reaction between terpene and phenol. Finally, Fenn et al.190 performed the radical copolymerization of terpene with monomers carrying glycidyl groups. 3.5.1.2. Epoxidation of Rosin and Adducts. Rosin is the solid nonvolatile fraction of resin. Depending on its specific

3.5. From Terpenes, Terpenoids, and Resin Acids

Terpenes, terpenoids, and resin acids are also a class of hydrocarbon-rich natural molecular biomass. Many plants and 1099


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Scheme 48. Synthesis of Fatty Amines from Triglycerides185

Scheme 50. Limonene and Epoxy Derivatives

Scheme 51. Diterpene Carbon Skeletons Found in the Most Common Resin Acids192

Scheme 52. Epoxide Oligomers from Epoxidized Imidodicarboxylic Resinic of Maleic Anhydride origin (gum, wood, and tall oil), Rosin consists primarily of abietic- and pimaric-type resin acids (or rosin acids) with characteristic hydrophenanthrene structures.191 The intrinsic acidity, rigidity, and renowned hydrophobicity, coupled with other chemical properties, enable rosin acids to be converted to a large number of derivatives. The most common resin acids found in pine rosin are derived from the three basic tricyclic carbon skeletons abietane, pimarane, and isopimarane and the less common bicyclic labdane skeleton (Scheme 51). Each skeleton has several derivatives depending on the double bond positions.192 According to their structures, resinic acids behave like aromatic or cycloaliphatic compounds in terms of rigidity. Thus, they are solid compounds and solvents have to be used for chemical modifications or processing. They can be good candidates for the preparation of rigid epoxy cross-linked polymers.193 Some works194 were especially conducted to provide the first imidodicarboxylic resinic of maleic anhydride. This compound was then epoxidized with ECH yielding the corresponding glycidyl ether (Scheme 52). Wang et al. also suggested the synthesis of epoxide synthon from abietic acid. The reaction with epibromohydrin leads to monoepoxy monomer (Scheme 53).195 Liu et al. performed the synthesis of rosin-based epoxy monomer by modification of rosin acid leading first to maleopimaric acid. The corresponding triglycidyl ester of maleopimaric was obtained by reaction with ECH and aqueous

sodium hydroxide with tetrabutylammonium bromide as the phase transfer catalyst.196 The Diels−Alder reaction of maleic anhydride with levopimaric acid leads to maleopimaric anhydride and its corresponding diacid, from which Atta197 synthesized a diepoxide by reaction with ECH (Scheme 54). Resin dimer adducts with maleic anhydride and acrylic acid were also used to prepare epoxy networks.198−200 The epoxy precursors were prepared by the reaction of the Diels−Alder adducts with diethanolamine, followed by treatment with ECH under alkaline conditions, as shown in Scheme 55 for acrylic acid adduct. The real structure is nevertheless probably more complex than the one given in Scheme 55. We have nevertheless to mention that the extraction of such acid compounds is very complex and not environmentally friendly, so the availability of these renewable resources is rather poor. Thus the development of epoxy cross-linked polymers from resinic acid remains low so far.

Scheme 49. Amine Functionalization of Grapeseed Oil

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Scheme 53. Synthesis of Epoxy Monomer from Abietic Acid195

Scheme 54. Synthesis of Epoxy Monomer from Levopimaric Acid

Scheme 55. Synthesis of Tetraepoxy Monomer from Levopimaric Acid

Scheme 57. Formula of Terpene Diphenol (TPD)

biobased polycarbonate but up to now never as a hardener for epoxy networks. Notably, a novolac-type terpene phenol was synthesized from terpene, phenol, and formaldehyde, resulting in a polyphenolbased terpene cross-linker.206 3.5.2.2. Acids from Rosin. Resinic acids were recently investigated as cross-linking agents of epoxy monomers. Interestingly, resinic acids allow an increase of both the Tg value and the storage modulus for the resulting epoxy crosslinked polymers. Two cross-linking agents were synthesized based on abietic acid: resinic maleic anhydride imidocarboxylic (RMID; Scheme 58) and diresinic maleic anhydride imidocarboxylic (D-RMID; Scheme 58). Wang et al. also modified abietic acid by reaction with polycaprolactone; the cross-linking reaction was undergone from the maleopimarate end groups (Scheme 59).193

3.5.2. Comonomers/Hardeners from Terpenes. 3.5.2.1. Diamine and Diphenol from Limonene. Terpenes can be also found as hardeners for epoxy materials, carrying either amine or phenol groups. For instance, menthane diamine (Scheme 56) was obtained from limonene201 and was used to

4. BIOBASED FORMULATIONS AND NETWORKS

Scheme 56. Formula of Menthane Diamine

4.1. Some Words about Epoxy Formulations

In section 3 we have tried to list most of the biobased molecules found in the literature and able to be used in epoxy formulations. If monomers, oligomers, and hardeners are the main components that define a formulation, initiators, catalysts, rubbers, fillers, short or continuous fibers, pigments, etc. can be also very important. This means that, for all these reasons, synthesis of biobased monomers is just the first step before preparation of fully or partially biobased materials for welldefined applications. As explained, owing to their superior mechanical properties, ease of processing, and excellent chemical resistance, epoxybased polymers have been widely used as structural, coating, and adhesive materials in many demanding application fields: automotive, aeronautics, electronics, and electrical engineering.

decrease the exothermicity during the cross-linking reaction of epoxy.202 Furthermore, the ring opening of dicyclocarbonate obtained from a terpene derivative, by using a diamine compound (in excess compared to dicyclocarbonate), allows obtaining diamine telechelic oligomers.203 Terpene diphenol (TPD) synthesized from monoterpene and phenol is used as precursor of functional phenolic resins (Scheme 57).204,205 TPD has a rigid molecular structure of aromatic and cyclohexane rings; thus TPD is expected to be a monomer of engineering bioplastics, which possess high Tg’s and high dimensional stability. It has been used to prepare a 1101


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Scheme 58. Synthesis of (top) RMID from Abietic Acid and (bottom) D-RMID from Abietic Acida

a

BMI = 1,10-(methylene di-4,1-phenylene)bismaleimide.

goal is a one-component system with a long pot life at room temperature and a high reactivity in the mold at a temperature as low as possible, which is clearly in contradiction to an Arrhenius law which can, however, be bypassed by the use of latent/blocked reactants or by reactions activated by radiations. Another example of the complexity of epoxy formulations is the introduction of a reactive diluent which is used primarily to reduce viscosity; it also permits higher filler loading and gives better wetting and impregnation of fibers (for composite applications) but it also decreases very often the thermal properties of the final epoxy network. In some cases, additives (or modifiers) are also used to improve mechanical and thermal shock resistance, increase elongation, and obtain higher impact strength and flexibility, but the problem is that they also increase the viscosity very often. All these examples mean that it is not easy to optimize the different requirements for a given formulation and that development of fully biobased epoxy formulations is a long way off. Most of the authors from the literature, after having prepared biobased monomers, are just giving some information concerning reactivity and thermal properties. Of course, these parameters are important to know, but from our point of view it is still mainly the first step. Therefore, instead of giving a list of Tg and thermogravimetric analysis results, our aim is to focus on some potential formulated systems which illustrate some important points for the future developments of epoxy formulations based on renewable resources. Lignin and rosin based epoxy and hardeners have been largely discussed in section 3 and will not been discussed again in this section.

Scheme 59. Cross-Linking Agent Based on Polyester of Abietic Acid with Maleopimarate End Groups

Each application has its unique performance requirements, so formulations are a complex compromise between performance and processing, and also cost aspects. However, some special considerations are common to many applications, and knowledge of them is essential in determining practical solutions for biobased formulations. Formulations have to be adapted to the process conditions. An overview of the processing techniques for epoxy thermosets is out of our scope. However, four different steps that depend on the temperature, pressure, reaction rate, and evolution of viscoelastic properties can be taken into account: (i) the pot life of the reactive system at the storage temperature, (ii) flow inside the processing machine and into the mold, (iii) reactions in the mold or on a substrate (for coatings and adhesives), and (iv) demolding and possible postreaction.207 While some epoxy monomer/hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150 °C being common and up to 200 °C for some specialist systems. On the other hand, the reactive system can be a one-pack or two-pack system. The end-user

Scheme 60. Structures of N-Benzylpyrazinium Hexafluoroantimonate (BPH) and N-Benzylquinoxalinium Hexafluoroantimonate (BQH)

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4.2. Epoxy Formulations with Initiators (i.e., without Hardeners)

gelatinous materials when cooled to room temperature. Analyses demonstrate that HSA did not react with ESO during the heating at 100 °C, and through UV irradiation only photocationic polymerization of ESO proceeded. At the end, supramolecular nanofibers were successfully formed in the biobased epoxy matrix. Various interesting properties such as mechanical properties and also self-repairing are expected for these biocomposites compared with conventional polymer/ fiber composites.220

As an epoxy precursor may be reacted with itself in the presence of few weight percent of an anionic or a cationic initiator, it is certainly the easiest way to have a cured biobased network with a high content of renewable resources. Some cationic initiators such as N-benzylpyrazinium and Nbenzylquinolinium salts (antimonate salts, Scheme 60) dissolve readily in epoxy monomers and have been shown to be excellent latent initiators. They are inert under normal conditions, i.e., at ambient temperature and light, but they show activity with only certain external stimulation, such as heating or photoirradiation. They are widely used for adhesives, paintings, inks, and photoresist applications. Cationic photopolymerizations are often used for coating applications.208 As an example, thin film materials containing biorenewable epoxidized-cardanol (ECD) have been investigated.209,210 The initiator was a solution of triaryl sulfonium hexafluoroantimonate salt dissolved in propylene carbonate. Monomer conversion during cationic photopolymerization and as a function of relative humidity was studied for thin film containing 10 wt % ECD and 5 wt % hydroxy-functional reactive diluents. Interesting is the fact that ECD-containing films exhibited higher surface hydrophobicity due to the enrichment of the long, hydrophobic alkyl chain of the ECD on the material−air interface. Therefore, hydrophobic, biorenewable ECD showed great potential for use as a reactive ingredient in cationic UV curable materials, especially as a “humidity blocker”. More difficult is the synthesis of composites.211,212 BPH and BQH (Scheme 60) have been used for the homopolymerization of epoxidized vegetable oils such as epoxidized castor oil, ECO, or epoxidized soybean oil, ESO.213−216 Their thermal decomposition into HSbF6 at 50−60 °C is required to start the polymerization. ECOs have been found to have higher Tg’s (but always less than 50 °C!) and lower coefficients of thermal expansion than analogous networks prepared from ESO. BPH usually leads to epoxy cross-linked polymers of higher Tg values, whereas BQH affords better mechanical properties: tearing energy, tensile strength, and elongation at break. It was attributed to a higher cross-linking density. However, as these epoxy cross-linked polymers show low Tg values, authors have tested some blends of ECO or ESO with classical DGEBA. Epoxidized vegetable oils were used like a toughening agent for DGEBA networks. As expected, compared to the neat DGEBA network, the presence of epoxidized vegetable oils decreases the Tg (from 200 to 62 °C with 80% ECO) and improves significantly the Izod impact strength. It is mostly a flexibilizer rather than a toughener additive. In order to improve their properties, biobased epoxy networks have been reinforced with naturally occurring layered silicates such as montmorillonite (MMT). For example, nanocomposites prepared by the curing of ESO using a latent cationic initiator at 150 °C in the presence of octadecylammonium-modified MMT have been reported by some groups.217,218 Biocomposites have also been made with 12-hydroxystearic acid (HAS), which is derived from castor oil. HAS is known to form thermoreversible gels with supramolecular nanofibrous networks in organic solvents and also in DGEBA.219 This property was also applied to biobased monomers. Mixtures of ESO/HSA (10/1 and 20/1) containing a cationic photoinitiator, which were homogeneous liquids at 100 °C, became

4.3. Epoxy Formulations with Hardeners

4.3.1. Formulation Based on Epoxidized Vegetable Oils. How To Increase the Properties? 4.3.1.1. Biobased Hardeners. There has been little literature on the combination of both biobased epoxy monomer and hardener. For example, the thermal and mechanical properties of the ESO cured with maleinated soybean oil221 or terpene derived acid anhydride222 were reported. Recently, networks from epoxidized linseed oil cross-linked with a vegetable oil polyamine cross-linker (prepared by thiol−ene chemistry),186 with elastomeric properties and low Tg (Tα ∼ −40 °C), have been prepared. Shibata and co-workers140,141 have focused their works on commercial glycerol polyglycidyl ether (GPE; epoxy functionality = 2) and polyglycerol polyglycidyl ether (PGPE; epoxy functionality = 4.1) reacted with two biobased hardeners. First, biobased nanocomposites have been prepared by the curing of GPE or PGPE with ε-poly(L-lysine) (PL) at 110 °C in the presence of nonmodified montmorillonite (MMT).141 Interesting mechanical properties (modulus, tensile properties, etc.) have been published but always with a Tg less than 60 °C. The aerobic biodegradability of the PGPE−PL networks in an aqueous medium was about 4% after 90 days and decreased with the MMT content. The second hardener used was a tannic acid (TA) and, in the presence of microfibrillated cellulose,140 cured at 160 °C. With this hardener Tg could reach 100 °C and again with interesting mechanical properties. In both cases, the use of water as a mixing solvent of the monomers and hydrophilic nanofillers is interesting. However, it can be said that, as explained in section 3, the main drawback of these commercial epoxy monomers is that they contain many nonhydrolyzable chlorine groups which can also explain the water resistance of the final networks. Pressure sensitive adhesives (PSA) are also a different topic. The synthesis of renewable PSA via cationic UV initiated polymerization of ESO or other epoxidized vegetable oils has been disclosed in a patent application.223 Vendamme and Eevers224 have synthesized functional PSA derived from carboxylic acid terminated polyesters. Polyesters were prepared from bulk polycondensation of dimerized fatty acids with several diols such as dimer fatty diol, butanediol, or isosorbide. The resulting polymers were then cured with two commercially available epoxidized plant oils with differing oxirane functionality (ELO and ESO) to form viscoelastic bioelastomers with tunable stickiness degrees. The authors highlighted how the viscoelastic and adhesion properties of the glues can be tailored by incorporation of isosorbide. Interestingly, these fully renewable coatings combine the intrinsic flexibility of lipids with the polarity of sugars and demonstrate interesting performances. 4.3.1.2. Nonbiobased Anhydride Hardeners. As there are few biobased hardeners, most of the literature is focused on partially biobased epoxies, and mainly on how to increase the initial poor properties (low Tg) of networks just based on 1103


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Scheme 61. Epoxidized Sucrose Ester of Fatty Acids (ESEFA)230

Very often the modified natural oils are used as reactive diluents and/or flexibilizers.231 As an example, blends of DGEBA plus increasing amounts of ESO and with methyl tetrahydrophthalic anhydride (MTHPA) as hardener and 1-methylimidazole (1MI) as an initiator have been studied. As expected, the Tg values of the resulting epoxy cross-linked polymers decrease with the amount of epoxidized vegetable oil from 108 to 57 °C (without DGEBA).227 For a composition, i.e., DGEBA/ epoxidized oil 40/60 (wt %), a good compromise of mechanical properties was obtained, but the biobased content is still rather low. Similar results have been obtained with mixtures of 50% DGEBF with both soybean and linseed oils218,243 and reacted also with MTHPA (+1-MI) or a triamine, poly(oxypropylene) triamine. 4.3.1.3. Miscellaneous Chemistry. As stated, triglyceridebased materials made up of aliphatic chains do not have the rigidity and strength required for most applications. In some cases, for an increase of properties of the final material, complex chemistries were used.214,232−235 A prereaction for instance was made to graft phenol units on the epoxidized vegetable oil backbone. One example is the grafting of bisphenol A at 160 °C in the presence of lithium chloride in order to form a new epoxy precursor (Scheme 62). Then, cross-linking was performed in the presence of different hardeners.233 Perhaps more interesting because it introduces flame retardant properties to the final material is the synthesis of a novel epoxy monomer containing phosphorus moieties obtained by first reacting 10-undecenoyl chloride with a diphenol (resulting from the reaction of 9,10-dihydro-9-oxa10-phosphaphenanthrene-10-oxide and benzoquinone) and then oxidizing the double bonds (Scheme 63). The resulting diepoxy monomer was cross-linked with methylene dianiline (MDA, which is not a good choice because MDA is known to be carcinogenic) or bis(m-aminophenyl)methylphosphine oxide (BAMPO). The thermal, thermomechanical, and flame retardant properties of the cured materials were measured. Tg

vegetables oils. As the epoxidized oils react more rapidly and with better properties with anhydrides than with amine hardeners, it explains why the use of aromatic or cycloaliphatic anhydride has been preferred.225−229 The hardener concentration in the range 1/1 with anhydride (i.e., high percent by weight of hardener) is certainly also one explanation for the better properties. As an example, epoxidized linseed oil (ELO) has been crosslinked by using different cyclic acid anhydride hardeners and tertiary amine or imidazole as initiators.228 Both the Tg and flexural modulus of the resulting networks depend on the anhydride content. A result comparable to classical epoxies, Tg ∼ 110 °C and flexural modulus at 30 °C is ∼2100 MPa, was obtained with tetrahydrophthalic anhydride (THPA), ratio 1/1 and 0.9% 2-methylimidazole (2-MI). One way to increase properties of networks based on epoxidized vegetable oils is to increase the functionality of the epoxy precursor. This has been done by preparing epoxidized sucrose esters of fatty acids (ESEFAs)230 (Scheme 61). These new monomers were cross-linked with a liquid cycloaliphatic anhydride, 4-methyl-1,2-cyclohexanedicarboxylic anhydride (MHHPA), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) used as initiator. The properties of coatings on steel substrates were studied to determine coating hardness, adhesion, solvent resistance, and mechanical durability. Compared with a commercial ESO, the anhydride-cured ESEFAs have high modulus values and are hard and ductile, high-performance thermoset materials while maintaining a high biobased content (∼75%). The exceptional performances are attributed to the unique structure of these precursors: well-defined compact structures with high epoxide functionality. These biobased thermosets may have potential uses in applications such as composites, adhesives, and coatings. By contrast, many thermosetting polymers were synthesized just by the partial replacement of the classical epoxy prepolymers such as DGEBA with epoxidized vegetable oils. 1104


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abundant flavonoids found in plants such as onion, capers, and tea.

Scheme 62. Reaction of Epoxidized Vegetable Oil with Bisphenol A

Scheme 64. Chemical Structures of Quercetin (QC), Pyrogallol (PG), Vanillin (VN), and the Reaction Product of PG + VN in Acidic Conditions: Pyrogallol−Vanillin Calixarene (PGVNC)140

values were in the range 90−100 °C, and as expected, the flame retardancy of these epoxy networks was improved.234,235 4.3.2. Sugar-Based Epoxy Networks and Water Uptake. 4.3.2.1. Biobased Hardeners. In their studies on biobased nanocomposites with microfibrillated cellulose and tannic acid (TA) as hardener, Shibata et al.140 have also used industrially available sorbitol polyglycidyl ether (SPE; 172 g/ equiv for an average number of epoxy groups of 3.6). Results similar to those with glycerol polyglycidyl ether (GPE) have been obtained. Expecting a thermal resistance increase, Shibata et al. have tested other hardeners having a lower hydroxyl value and a higher aromatic content than TA. Quercetin (3,3′,4′,5,7pentahydroxyflavone, QC; Scheme 64) is one of the most

Monomers were first dissolved in tetrahydrofuran to obtain a homogeneous solution. After solvent vaporization at 40 °C for 24 h, the resulting viscous liquid was prepolymerized at 150 °C for 0.5 h, and then compression-molded at 170 °C for 3 h. Thermomechanical measurements show that the SPE−QC network had a higher tan δ peak temperature than SPE−phenol novolac (PN). A similar comparison was obtained with liquid DGEBA. These results indicate that QC is a superior epoxy hardener to generate epoxy networks with high heat resistance.

Scheme 63. Diepoxide Compound Obtained from 10-Undecenoyl Chloride and 9,10-Dihydro-9-oxa-10-phosphaphenanthrene10-oxide

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Even if it is possible to find old patents,145,146 many studied have been devoted the past 5 years to isosorbide-based epoxies as safe and renewable alternatives to petroleum-based BPA epoxies.147,149,151,238−240 Isosorbide is also known to bring stiffness to the polymer chain of linear polyesters or polycarbonates.144 Depending on the chemistry used, different types of diglycidyl ether of isosorbide (DGEDAS) have been prepared in the literature. The cross-linking reaction was performed with aliphatic triamine (Jeffamine T403) and cycloaliphatic diamine (isophorone diamine, IPDA) with different curing schedules but similar to the ones used for DGEBA with the same hardeners. The Tg’s of networks synthesized from DGEDAS were always lower than those of the corresponding DGEBA based networks, about 40 °C with Jeffamine T403. This result was a little disappointing. However, they have higher rubbery moduli due either to the lower molar mass or to the higher functionality of the prepolymer. Storage of these biobased prepolymers must be done in dry conditions because these compounds are hygroscopic. However, the main problem to solve is the water uptake of the networks that can be as high as 50% by weight (with T403 for example), which deteriorates the properties of the networks. To optimize the mechanical properties and to decrease the water uptake (less than 2%) of the isosorbide-derived epoxies for industrial applications such as can coating, some solutions have been proposed. Feng et al.240 have added hydrophobic functional groups (such as 4-allyloxybenzoyl chloride) into the backbone of isosorbide epoxy (leading to isosorbide bisglycidyl benzoate). It was also possible to add in the formulation hydrophobic epoxidized cardanol (ECD).241 However, a more efficient way which is under testing is to adjust the amount and type of cross-linker, such as terpene diphenol (TPD, from terpene compounds and phenol as raw materials, melting point, T m = 85 °C),242 4,4-bis(4′hydroxyphenyl)pentanoic acid, or diphenolic acid (DPA, from levulinic acid and phenol, Tm = 171 °C), the biobased phenols with high aromatic content for a low hydroxyl value (such as the ones used with SPE monomers), or other phenols extracted from lignin.20 4.3.3. Advanced Biobased Epoxy Networks. 4.3.3.1. Coatings. The Mannich reaction of cardanol with formaldehyde and some amines leads to partially biobased phenalkamine. The high hydrophobicity of these commercial phenalkamines provided by the long linear side chain also brings many benefits to coating formulations compared with some other hardeners. Depending on the amine type they may have very good pot life, but also can be used at low temperature cure (even below 0 °C). The final networks have good flexibility (rather low Tg < 100 °C), with good surface appearance, good chemical resistance, and excellent water and salt water resistances. Adhesion to poorly prepared or tough wet surfaces, such as water saturated concrete, is especially good with these hardeners. These unique properties make them an excellent choice for marine and offshore coatings, solventfree industrial floor coatings, construction equipment coatings, portable water coatings, and tank and pipe linings. They are also used for adhesive, automobile, and electrical potting applications. These commercial products are potential hardeners for biobased epoxy monomers, such as epoxidized cardanol or others.209,210 4.3.3.2. Structural Materials. The epoxy networks with high Tg’s are required to retain the dimensional stability and rigidity at a high temperature in electrical and structural materials.

In their research for biobased phenols with high aromatic content for a low hydroxyl value, Shibata et al.236 have also prepared a pyrogallol−vanillin calixarene (PGVNC) (Scheme 64). This phenolic hardener was obtained by reacting pyrogallol (PG, prepared by decarboxylation of gallic acid) and vanillin (VN) in the presence of p-toluenesulfonic acid. PGVNC was well characterized and blended with SPE with the help of a solvent (tetrahydrofuran). After a similar curing schedule, measurements indicate that the tan δ peak temperature of SPE−PGVNC was 148 °C, which was much higher than that of the SPE−PN. Based on these monomers, biocomposites were also made with different percentages of wood flour with the aim of increasing the tensile modulus and strength at room temperature and, also, the storage modulus at the rubbery plateau. These phenols are interesting hardeners, but as the chlorine contain of the SPE monomer used in these three studies was 9.6%, it can be said that they need to be evaluated with a free chlorine epoxy monomer. Another point which is questionable concerns the toxicity and carcinogenicity of the quercetin molecule.237 4.3.2.2. Nonbiobased Hardeners. As stated in section 3.3.1, sorbitol and maltitol were converted in multifunctional epoxy monomers through oxidation of allyl double bonds and were used in combination with diethylene triamine (DETA) to produce biobased networks with Tg values in the range 70−80 °C.142 Epoxy allyl and epoxy crotyl sucroses (EAS and ECS) were synthesized in one step from octa-O-allyl and octa-O-crotyl sucroses (Scheme 65) by epoxidation. The average number of epoxy groups (functionality, f) per sucrose was controlled by changing the concentration of peracetic acid in the reaction mixture.143 Scheme 65. Chemical Structures of Sucrose (1), Allyl Sucrose (2), and Crotyl Sucrose (3)

While both DGEBA and EAS cured with DETA at about 100 °C, ECS cured at about 150 °C. EAS/DETA thermosets exhibited a wide range of Tg values depending on the average functionality of EAS. For stoichiometric ratio and f = 3.7 epoxy groups per sucrose, EAS cured with DETA have Tg in the range of 50 °C, compared to ∼130 °C for DGEBA. Despite incomplete cure, the high functionality ECS (f = 7.3) indicated a highly cross-linked network but with no visible Tg up to 220 °C. A Tg at 134 °C was observed with an epoxy:NH ratio = 1:2. Sucrose based epoxies cured with DETA were found to bind aluminum, glass, and steel. Comparative lap shear tests showed that DETA-cured epoxy allyl sucroses, EAS, with f = 3.2 generated a flexible adhesive comparable in bond strength to DGEBA. However, DETA-cured ECS-7.3 outperformed the bonding characteristics of both DGEBA and EAS-3.2. 1106


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However, we have seen in previous sedctions that Tg’s of the biobased epoxy networks and their composites were still lower than those of cured materials based on the popular DGEBA or other epoxy precursors with higher functionality, such as TGMDA. The development of heat resistant biobased epoxy monomer/hardener having a comparable Tg to that of a conventional epoxy curing system is a real challenge. Strategies using epoxidized vegetable oils and complex chemistries leading to biobased epoxy cross-linked polymers which can be used as comonomers or be directly polymerized215,221,222,243−250 have demonstrated their limits. Compared to vegetable oils, phenols have a higher potential. They can be chosen as hardeners, but also as epoxy precursors (after an epoxidation reaction). Furthermore, gallic acid epoxidation leads to monomers with molar mass values ranging from 137 to 160 g/mol, which corresponds to epoxy functionality from 1 to 3. The epoxidized compounds were then cross-linked with a conventional crosslinking agent (i.e., polyamine type or anhydride acid.52,53 Epoxidized tannins have also been prepared. An epoxy monomer was formed from commercialized hydrolyzed tannin. This last compound was pretreated by alcohol or ether extraction in order to obtain polyphenol oligomers (molar masses from 500 to 5000 g/mol), soluble in organic solvents.251 Then, phenol groups were epoxidized by action of ECH, by using a phase transfer catalyst in order to avoid reaction with ester groups. Interestingly, in this study the cross-linking reaction was performed by using phenolic groups of hydrolyzed tannins. Furthermore, Nouailhas et al tried to enhance the tannin reactivity toward the synthesis of epoxy monomers.53 Epoxy monomer was indeed obtained from glycidylation of tannin units, i.e., (−)-epicatechin and (+)-catechin. This monomer was mixed with DGEBA (25/75 and 50/50 wt %), and the mixture was cross-linked with a commercially available cycloaliphatic amine at 60 °C during 24 h. Table 1 gives a

depending on the epoxy functionality, and with a high elastic modulus at the rubbery plateau (Tg + 30 °C). Another example is the cross-linking reaction of epoxy obtained from resinic acid with 1,2-cyclohexanedicarboxylic anhydride (CHDB) in the presence of 2-ethyl-4-methylimidazole as initiator at 100 °C for 2 h and at 180 °C for 2 h. The epoxy network exhibits a Tg = 153 °C, 10 °C higher than that of the corresponding network based on DGEBA as epoxy monomer.253 Synthons extracted from lignin have certainly a high potential for structural epoxies.20 Nevertheless, they are solid, whereas a liquid allows at room temperature an easier mixing and processing without the help of solvents. As an example, the difunctional epoxy monomer based on vanillin254 is a white solid with a high melting point, Tm ∼ 175 °C (Scheme 27). This epoxy monomer has been reacted with MDA. It was reported that due to several β-relaxations the impact strength, the tensile strength, and the elongation were improved.254−256 4.3.4. Biobased Additives for Toughness Increase. Typically, DGEBA and other di- or multifunctional monomers containing an aromatic ring structure will cure to hard, rigid compositions having rather low impact and elongation characteristics. There are many approaches to improving these properties. Among them are modifications with vegetable oils, polyamide or polysulfide curing agents, or long chain polyglycols which increase network flexibility, but always with a decrease of Tg and modulus. To prevent this drawback, particular modifiers such as a rubber or a thermoplastic that are miscible before the reaction of the epoxy formulation but become phase separated in the course of polymerization (reaction induced phase separation) are employed to produce different appropriate morphologies and toughness increase in the final products.257 As an example, natural rubber, i.e., poly(1,4-isoprene), was first exploited to perform biobased epoxy monomers. Namely, the epoxidation by using peracid compound was performed on the unsaturations leading to epoxidized natural rubber (ENR). Gandini,15 in his review paper, described the process leading to ENRs, which was operated on an industrial scale. ENRs have been cured in the presence of diamines, which also acted as radical scavengers able to delay ENR degradation.258 Further, Hong and Chan259 incorporated ENR in a conventional epoxy formulation, i.e., DGEBA, dicyanodiamide, and 2-methylimidazole as initiator. The authors pointed out the (macro)heterogeneous nature of the resulting material, probably due to the high molar mass of the modifier. Thus, the degradation of ENR, which leads to liquid ENR with low molar mass, might be more suitable for being incorporated in conventional epoxy formulations15,260 and can increase the toughness of the thermosetting polymer. A totally different example is the use of a synthesized cardanol-based epoxy curing agent (MBCBE) from cardanol butyl ether, formaldehyde, and DETA.210 This partially biobased hardener has a butoxy taking the place of phenol’s hydroxyl, which can improve its color stability and decrease its viscosity. Without the phenol hydroxyl, MBCBE is less reactive with DGEBA than common phenalkamine. However, because of a decrease in the solubility of MBCBE during reaction, authors have observed by scanning electron microscopy that the morphology of the cured sample consisted of cavities dispersed within a continuous epoxy matrix. Also, these cavities markedly improved the lap shear strength and impact strength of the network.

Table 1. Mechanical Properties of Epoxy Cross-Linked Polymers from Tanninsq storage modulus (GPa)

q

epoxy monomer

decompsn T5% (°C)

glassy region

elastic region

DGEBA 75DGEBA/25GEC 50DGEBA/50GEC

209 221 202

2.81 2.46 2.40

0.019 0.016 0.014

GEC = glycidyl ether of catechin.

comparison of mechanical properties for epoxy networks with different contents of epoxy monomer based on tannin. It can be said that tannin brings softness to the epoxy network in both the glassy and elastic regions, compared to the neat network obtained from DGEBA only. The resulting epoxy monomer of resorcinol (Scheme 19) was cross-linked with diamino diphenyl sulfone (DDS, which is known as a nontoxic aromatic amine). The thermal analysis (differential scanning calorimetry, DSC) of the epoxy network did not reveal any glass transition in the 50−250 °C region. Furthermore, the thermal stability of the polymer is clearly improved, compared to conventional DGEBA.252 Recently, Benyahya et al.56 have prepared epoxy monomers based on catechin and green tea extracted phenols. These monomers have been reacted with isophorone diamine, IPDA. DSC results showed a high reactivity for these epoxy monomers. Obtained Tg’s were in the range 140−190 °C 1107


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Scheme 66. Synthetic Strategy for Generation of Epoxy Networks Combining Chemical and Supramolecular Hydrogen Bonding183

4.3.5. New Properties from Polyesters Based on Epoxy Chemistries. 4.3.5.1. Supramolecular Properties and Exchange Reactions. The addition−esterification reaction between a carboxylic acid and an epoxy leads first to a βhydroxyester, but as staed in section 2, the chemistry is more complex and many side reactions could occur depending on the stoichiometry ratio and the curing schedule. By contrast, the reaction of epoxies with cyclic anhydride initiated by Lewis bases takes place through a chainwise polymerization, but due to terminations and transfer reactions there are many hydroxyl groups on the network. This means that in both chemistries there are many hydroxyl groups near or not far from the ester groups. Leibler et al.183,261,262 have used this particularity to develop new concepts. In a first work they combine the supramolecular chemistry of heterocyclic ureas with the chemistry of epoxies to synthesize new cross-linked materials incorporating both chemical and supramolecular hydrogen-bonded links. In a first step a supramolecular prepolymer from a mixture of di- and trifatty acids (Scheme 46) was partially amidified with aminoethylimidazolidone (UDETA) (Scheme 66). During this first step all the UDETA molecules (from 10 to 50%) were grafted. During a second step an epoxy monomer, DGEBA (but it can be also an epoxidized vegetable oil) was reacting with the remaining carboxylic acid functions in the presence of 2-MI (6 mol %) for chemical cross-links. Both “chemical cross-links” and “supramolecular cross-links” exist at low temperatures, but because supramolecular bonds will disassemble at high temperatures, only “chemical cross-links” persist and explain the elastic plateau observed above T = 130 °C. This combination should offer many advantages. For example, adhesive properties inherent to supramolecular Hbonding groups may improve the matrix to filler adhesion, important for both composites and adhesives applications. In a second work Leibler et al. proposed epoxy networks (hardeners can be biobased acids or anhydrides) which are insoluble and processable as they can rearrange their topology

by exchange reactions without depolymerization. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications. 4.3.5.2. Exchange Reactions with Depolymerization: Recycling. European Union directives such as End of Life Vehicles (ELV) and Waste Electrical and Electronic Equipment (WEEE) will put more pressure on solving the problem of waste management through recycling and reuse. As thermosets cannot be dissolved or liquefied, their efficient recycling is a real challenge. In recent years, the microelectronics industry has developed a need for a class of epoxy materials known as “reworkable” epoxy networks. One proposal in the literature is to use an epoxy monomer with ester groups that contain a tertiary ester linkage. It is interesting that one partially biobased epoxy monomer derived from the esterification of cycloaliphatic acid with α-terpineol and subsequent epoxidation of double bonds have been proposed (Scheme 67). Anhydrides of diacids have been proposed as hardeners to increase the chemical and thermal reworkability at moderate temperatures.263−265 The chemical degradation mechanism of the α-terpene polyester network was found to be consistent with tertiary ester linkage cleavages at elevated temperature (250 °C), thereby breaking down the polyester network and forming carboxylic acid and alkene moieties. Kinetic and activation energies of the chemical degradation were measured by the decrease of the Tg.

5. CONCLUSIONS AND FUTURE TRENDS As already mentioned, biorefinery processes and retreatment of biomass were out of our scope. However, it is clear that the future of biobased polymers, including building blocks for 1108


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constituents (VOC), the “Restriction of certain Hazardous Substances” (RoHS), or the “Registration, Evaluation and Authorisation of Chemicals”, (REACh). Biobased compounds will not be the only solution for these policy pressures, but they will be an opportunity. Developing highly efficient, safe, low waste, low toxicity, and atom economy processes are the keywords for green chemistry. Chemistry has to be simple, practical, and operational, and catalysts are expected to play an important role. ECH is the preferred way to prepare epoxy monomers, but even if ECH can be biobased it remains a toxic molecule which has to be manipulated in a safe environment. Epoxidation without the use of ECH is a key challenge. We have seen that allylation or crotonization of alcohols can be a first step for epoxidation, but the second step, i.e., double bond oxidation, requires the use of expansive and hazardous catalysts. Use of glycidol (2-epoxy-1propanol) could be another route for esterification or etherification (i.e., Mitsunobu type) reactions, but it has not been tested very often up to now. Some industrial countries have declared bisphenol A (BPA) to be a toxic substance that causes risks to human health as well as to the environment; thus it has to be banned for all food contact applications in the next coming years, and in the immediate future the pressure is on its replacement. BPA is industrially produced from condensation of acetone with phenols. Bacterial anaerobic fermentation can be used to produce acetone but also 1-butanol and ethanol (ABE) from starch in the respective ratio 3−6−1 (it was the primary process used to make acetone during World War II). 270,271 Furthermore, the wastes produced from the forest industry can be distilled to lead to phenol compounds. The synthesis of biobased BPA is one option, which has been tested to produce biobased epoxy oligomers (EPOBIOX from AMROY Cie). Nevertheless, from petroleum or renewable resources, BPA remains classified as reprotoxic R2 and this route is certainly not the solution. Other biobased phenols discussed in this review can be proposed to replace BPA. The oldest one is diphenol acid (DPA; Scheme 31) synthesized from levulinic acid. Based on the same hydroxyalkylation synthesis, terpene diphenol (Scheme 57) or diphenol based on vanillin (Scheme 29) could be also excellent candidates. Natural phenols such as quercetin (Scheme 64) or phenols extracted from tannins and derivatives (Scheme 15) are also good candidates. Because of its long aliphatic chain, diphenol of cardanol is too flexible (Scheme 22, R = −H) for BPA replacement. However, the main problem remaining with phenols is their toxicity, which is usually unknown or questionable such as for quercetin.237 On the other hand, isosorbide is known to be nontoxic and is also rigid, but the problem to solve in this case is the too-low water resistance of the resulting epoxy networks. Therefore, biobased aromatic/rigid epoxy monomers are still needed in order to fulfill a good compromise between processing and properties, and able to replace BPA. The more important epoxy hardeners are the amines and their derivatives. As noted in section 2.3.1.1, aromatic amines are less reactive but with higher mechanical properties than those of aliphatic amines. However, these aromatic amines can be also toxic and there is a need for biobased and nonharmful amine hardeners. Unfortunately, there are very few biobased amines; some have been prepared such as isosorbide diamine but through complex chemistries. Chitosan has characteristic amino groups around its polymeric framework; however, chitosan is only soluble in a few dilute acid solutions due to

Scheme 67. Formula of Diepoxy and Scheme of Degradation of the Polyester Network263

thermosetting epoxies, is strongly dependent on the future of biorefineries. The goal of a biorefinery is to produce high quality chemicals for fuels, monomers, and finally polymers and materials. New screening techniques such as “green chemistry” and technoeconomic and life cycle assessment (LCA) indicators can help in the chemical-based biorefinery project developments.266 However, efficient, economical, and largescale synthesis of monomers is crucial, and the first key parameter remains a resource pool that should be abundant and easily accessed. Waste is a worldwide problem. The largest waste source for carbohydrates and lignin is from lignocellulosic biomass residues, which are estimated to exceed 2 × 1011 t/year worldwide.267 Kraft lignins represent a major low value byproduct of the pulp and paper industry, with their utilization predominantly limited as a fuel to fire the pulping boilers. It has been estimated that only 1−2% of lignin is isolated from pulping liquors and used for specialty products. However, as previously mentioned, lignin is the only large volume renewable feedstock that comprises aromatics. Up to now, despite extensive research, there have been very few reports of efficient ways for recovering such aromatic products. The only noticeable commercial process has been the production of vanillin (section 3.2.4) from lignosulfonates with a yield less than 10% by weight and levulinic acid which is an interesting chemical platform.268 Food residues and byproducts are being generated in very significant quantities by both the food industry and the agricultural sector. Waste orange peel is one excellent example of a located wasted resource (3 × 106 t/year of orange peel in Brazil) that can be used to produce fuels but also chemicals such as terpenes. Another example is protein waste which is generated from the production of foods and beverages. These proteins could lead to platform chemicals such as amino acids.269 As a first conclusion, there are many opportunities to develop molecular biomass and monomers for preparation of renewable epoxy formulations and materials. However, the biggest changes for thermosetting epoxy formulations in the near future will certainly come from the regulation changes such as the environmental directives for reducing volatile organic 1109


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Biographies

its crystalline nature. More simple is the hydrolysis and decarboxylation of amino acids which can lead to diamines, mostly aliphatic diamines. One example is lysine, which can be an interesting platform chemical and which can be converted into a number of industrial monomers, including 1,5diaminopentane.272−275 Partially biobased phenalkamines (Mannich bases) are available from biobased cardanol with petro-based polyamines or amines from fatty acid dimers but with several reaction steps and also residual formaldehyde. The thiol−ene reaction is also a very powerful synthesis method to obtain, under mild conditions, monomers from fatty acids,186 but also from terpenes such as limonene. An additional remark concerns biodegradability, which is a special functionality conferred to a material. There are few results in the literature,141 which means that, except for some biomedical application, it is not a required property for thermosetting epoxies. More important is certainly recycling, which has been demonstrated to be easier for polyesters. Thermosetting polyesters from epoxy can be obtained with anhydride hardeners, which unfortunately are usually not biobased. One idea to maximize the utilization of natural resources and to improve material performances is to combine natural polymers and natural molecular biomass. There are a few examples in the literature: nanocomposites prepared with microfibrillated cellulose;140 lignin which has been reacted with cardanol72 or with rosin acids.276 This route merits further development. In conclusion, it is clear that we are at the beginning of a new story for thermosetting materials. Important research is needed to obtain fully biobased epoxy networks. This research is not only important for epoxy thermosetting materials, but it will also impact other materials as biobased epoxies are also a platform for other chemistries: (i) The catalytic carbonation of biobased epoxies with CO2 leads to biobased carbonates. These carbonates are able to react with aliphatic amines and are precursors of nonisocyanate poly(hydroxyurethane)s (NIPUs).277,278 (ii) Epoxies are able to react with (meth)acrylic acid to give formulations for coating applications or vinyl ester monomers and networks after radical polymerization. Thus, petroleum refining started at the end of the 19th century and the industrial development of synthetic polymers began in the 1930s. These processes have been continuously optimized since the beginning in terms of economy and environment. Biorefinery of biomass is nowadays just at the beginning of its history, and several processes are still to be developed, according to the type of building blocks (i.e., both hardeners and epoxy monomers). Thus biorefining is particularly a challenge for aromatic molecules since they do not benefit from previous industrial agroindustry and the food industry.

Rémi Auvergne was born in 1975. He received his Ph.D. degree in polymer science in 2006 from the University of Montpellier. In 2008 he was appointed assistant professor in the polymer team of the Charles Gerhardt Institute of Montpellier to develop a new research topic dedicated to biobased polymers. His research interests include thiol−ene reactivity and the use of renewable resources for the synthesis of biobased building blocks and polymers for materials and composites. He is a coauthor of 20 articles and patents.

Sylvain Caillol was born in 1974. He first graduated from the engineering school of Chemistry of Montpellier in 1998. Then he received his Ph.D. degree in polymer science in 2001 from the University of Bordeaux. Subsequently he joined the Rhodia group and headed the polymer department in the research center of Rhodia. In 2007 he joined CNRS in the University of Montpellier, where he

AUTHOR INFORMATION

started a new research topic dedicated to the synthesis of biobased

Corresponding Author

building blocks and polymers. He is cofounder and director of the

*E-mail: [email protected]. Notes

ChemSuD Chair. Coauthor of several articles and patents, he won the

The authors declare no competing financial interest.

Innovative Techniques for Environment award. 1110


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Prof. Jean-Pierre Pascault is emeritus since 2005 at the National Institute of Applied Science (INSA), Lyon, France. He was a professor at the same institute from 1983 to October 2005, director of the Laboratory of Macromolecular Materials (associated with CNRS) from 1982 to 1998, director of a CNRS Polymer Network Group (FR CNRS) from 2000 to 2006, and president of the French Polymer Group, GFP, and of the Polymer Division of the French Chemical Society, SFC, from 2001 to 2004. He has authored over 320 scientific publications including several book chapters and two books (Thermosetting Polymers and Epoxy Polymers: New Materials and Innovations) and 32 patents. His main research themes concern polyaddition/polycondensation reactions and new green polyurethane/epoxy/polyester materials.

Ghislain David was born in Sète (France) in 1975. He obtained his Ph.D. degree in 2002 at the University Montpellier II. In 2003, he obtained a postdoctoral position in the laboratory of Prof. Gilbert (KCPC, Sydney). In 2004, he joined the laboratory of Prof. Boutevin to perform postdoctoral research on CRP of vinyl phosphonates, with the collaboration of Rhodia Chemicals. In 2006 he was awarded as an associate professor at the Institut Charles Gerhardt. His main research projects are in the field of phosphorus-containing polymers as well as in the phosphorus functionalization of biobased monomers and

REFERENCES

polymers. He has coauthored 70 scientific publications including

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several book chapters and six patents.

Bernard Boutevin was born in 1948. He received his Ph.D. degree in 1974 from the University of Montpellier and joined CNRS. Subsequently he created a polymer laboratory in Montpellier in 1985. Then he was director of several research units with more than 100 researchers. He was appointed full professor in 1999. His research interests included the use of fluorine, phosphorous, and silicon in monomers and polymers. Recently, he became interested in the synthesis of new biobased building blocks and polymers. Now professor emeritus, he was codirector of more than 200 Ph.D. theses and coauthor of more than 250 patents and 1000 articles. 1111


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