Bio-Based Aromatic Epoxy Monomers for Thermoset Materials

molecules

Review Bio-Based Aromatic Epoxy Monomers for Thermoset Materials

Feifei Ng, Guillaume Couture, Coralie Philippe, Bernard Boutevin and Sylvain Caillol *

Institut Charles Gerhardt—UMR 5253, CNRS, Université de Montpellier, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier, France; [email protected] (F.N.); [email protected] (G.C.); [email protected] (C.P.); [email protected] (B.B.) * Correspondence: [email protected]; Tel./Fax: +330-467-144-327

Academic Editors: Thomas Farmer and James H. Clark Received: 24 November 2016; Accepted: 10 January 2017; Published: 18 January 2017

Abstract: The synthesis of polymers from renewable resources is a burning issue that is actively investigated. Polyepoxide networks constitute a major class of thermosetting polymers and are extensively used as coatings, electronic materials, adhesives. Owing to their outstanding mechanical and electrical properties, chemical resistance, adhesion, and minimal shrinkage after curing, they are used in structural applications as well. Most of these thermosets are industrially manufactured from bisphenol A (BPA), a substance that was initially synthesized as a chemical estrogen. The awareness on BPA toxicity combined with the limited availability and volatile cost of fossil resources and the non-recyclability of thermosets implies necessary changes in the field of epoxy networks. Thus, substitution of BPA has witnessed an increasing number of studies both from the academic and industrial sides. This review proposes to give an overview of the reported aromatic multifunctional epoxide building blocks synthesized from biomass or from molecules that could be obtained from transformed biomass. After a reminder of the main glycidylation routes and mechanisms and the recent knowledge on BPA toxicity and legal issues, this review will provide a brief description of the main natural sources of aromatic molecules. The different epoxy prepolymers will then be organized from simple, mono-aromatic di-epoxy, to mono-aromatic poly-epoxy, to di-aromatic di-epoxy compounds, and finally to derivatives possessing numerous aromatic rings and epoxy groups.

Keywords: epoxidation; aromatic; epichlorohydrin; tannin; lignin; cardanol

1. Introduction Amidst materials widely used in plastic industry nowadays, thermosets (or thermosetting polymers) represent about 20% of plastic production [1]. They are formed from a liquid or solid mixture of various ingredients including at least one or more monomers. One of these monomers at least exhibits a functionality equal or higher than three, thus enabling the creation of a solid three-dimensional non-fusible network via an external action such as heating or UV irradiation [2]. Thermosets include a wide range of reactive systems such as phenolic and urea formaldehyde resins, unsaturated polyesters, and polyepoxides, the latter accounting for nearly 70% of the market. Polyepoxides are one of the most versatile class of compounds with diverse applications, especially coatings, which dominate the market, but also water containers, automotive primer, printed circuit boards, semiconductor capsules, adhesives, and aerospace composites. The global production of epoxy prepolymers is estimated to reach 3 million tons by 2017 for a market of US$ 20 billion in 2015. This success arises from the excellent mechanical strength and toughness, outstanding chemical, moisture, and corrosion resistance of the epoxy thermosets [3,4]. This list does not include various interesting process-related characteristics such as: the absence of volatile products emitted during the

Molecules 2017, 22, 149; doi:10.3390/molecules22010149 www.mdpi.com/journal/molecules Molecules 2017, 22, 149 2 of 48 polymerization reaction, the large choice of monomers available or the high adhesion properties to a variety of surfaces. Over 90% of these epoxy materials are based on bis(4-hydroxyphenylene)-2,2-propane, known as bisphenol A (BPA), a petrol-based molecule first synthesized in the 1890s and used as a synthetic oestrogen [5]. Aromatic compounds are widely used in organic materials for their stability, their toughness, and above all their ability to structure the matter by π-stacking, thus allowing BPA to confer good thermal and mechanical properties to the epoxy thermosets. Commercialized for more than 50 years, BPA, mainly through its epoxy form DiGlycidylEther of Bisphenol A (DGEBA), is nowadays spread in many coatings, adhesives, laminates and composites, but also many domestic or even health related-products such as plastic bags, food containers and metal cans, dental sealants, soaps and lotions [6,7]. However, epoxy thermosets are sensitive to hydrolysis, which may cause BPA to leach, leading to widespread human exposure [6–9]. Unfortunately, it has been classified as carcinogen mutagen and reprotoxic (CMR), and is recognized as an endocrine disruptor [10]. Consequently, many governments have recently hardened the legislation regarding the production and use of BPA, especially in baby’s bottles, food containers and medical supplies [11,12]. Moreover, BPA is synthesized from oil-based phenol and the 3D chemically crosslinked networks of thermosets prevent them from being recycled by heating. The awareness on BPA toxicity combined with the limited availability and volatile cost of fossil resources, and the non-recyclability of thermosets implies necessary changes in the field of epoxy networks. Thus, substitution of BPA has witnessed an increasing number of studies both from the academic and industrial sides. Over the past decades, many bio-based resources have been tested as potential candidates for replacing BPA in epoxy resins, but very few of them have reached the commercialization step. Among these, epoxidized natural oils [13–15] and modified cardanol [16] are the only two types of epoxy resins based on natural and non-toxic precursors, commercially available in the market. However, the low reactivity of the epoxy groups along their aliphatic backbone [1] and the low glass transition temperature caused by the alkyl chain prevent them from competing with BPA-based materials with their high Tg values and glassy moduli [17]. For this reason, many researches have been devoted to using glycidylated aromatic bio-based materials as substitutes for BPA. Very interesting and exhaustive reviews have recently been published on bio-based precursors for thermosets and their hardeners [1,18–21], but to our knowledge, none of them focuses especially on aromatic epoxy monomers based on biomass resources. In fact, only aromatic poly-epoxides seem to be able to compete with DGEBA in terms of thermo-mechanical properties, making them of primary interest for renewability. Thus, the present review proposes to give an overview of the reported aromatic multifunctional epoxide building blocks synthesized from biomass or from molecules that could be obtained from transformed biomass. After a reminder of the main glycidylation routes and mechanisms and the recent knowledge on BPA toxicity and legal issues, this review will provide a brief description of the main natural sources of aromatic molecules. The different epoxy prepolymers will then be organized from simple, mono-aromatic di-epoxy, to mono-aromatic poly-epoxy, to di-aromatic di-epoxy compounds, and finally to derivatives possessing numerous aromatic rings and epoxy groups. For each one, the curing agent used and the thermal properties (especially Tg and thermal degradation) of the crosslinked material will be given along with their DGEBA-based counterparts if available. The potential toxicity of the epoxy precursors will also be mentioned. Tables gathering all these results will be given at the end of each part for comparative purposes.

2. Epoxidation Methods and Processes Poly-functional epoxy compounds are very reactive building blocks and can lead to materials by chain-growth polymerization or crosslinking with anhydrides, phenols and amines. They are generally prepared by direct glycidylation, as showed in Scheme1[ 1,22,23]. It consists in reacting an alcohol or amine derivative with epichlorohydrin (ECH) in the presence of an alkylammonium Molecules 2017, 22, 149 3 of 48 halide as a phase transfer catalyst, such as benzyltriethylammonium chloride, tetrabutylammonium bromide or cetyltrimethyl ammonium chloride. Sometimes, epibromohydrin is used instead of its chlorideMolecules 2017 analogue, 22, 149 [24]. A sodium or potassium hydroxide post-treatment is usually applied in same3 of 47 pot to increase the number of epoxy rings. In fact, the phenolic oxygen may displace the chlorine Molecules 2017, 22, 149 3 of 47 atomdesired to product directly via yield a S theN2 desiredmechanism, product or open via athe SN 2epoxy mechanism, ring causing or open the theformation epoxy ringof a causingchlorinated the formationderivative that of a can chlorinated be closed derivativeby a strong that base can through be closed a SNi bymechanism a strong base(Scheme through 2) [25,26]. a SN iS mechanismNi stands for desired product via a SN2 mechanism, or open the epoxy ring causing the formation of a chlorinated (SchemeSubstitution2)[ 25Nucleophilic,26]. SNi stands internal for Substitutionand is a nucleophil Nucleophilicic substitution internal mechanism and is a nucleophilic that implies substitution a retention derivative that can be closed by a strong base through a SNi mechanism (Scheme 2) [25,26]. SNi stands for mechanismof configuration. that implies a retention of configuration. Substitution Nucleophilic internal and is a nucleophilic substitution mechanism that implies a retention of configuration.

SchemeScheme 1. Synthesis 1. Synthesis of epoxy of derivatives epoxy derivatives from phenol from or aniline phenol by direct or aniline glycidylation by direct with epichlorohydrin. glycidylation Schemewith epichlorohydrin. 1. Synthesis of epoxy derivatives from phenol or aniline by direct glycidylation with epichlorohydrin.

Scheme 2. Mechanism of coupling between phenolic compounds and epichlorohydrin (ECH) in the presence of a phase transfer catalyst (QX) [1,27]. Scheme 2.2. MechanismMechanism ofof couplingcoupling betweenbetween phenolicphenolic compoundscompounds andand epichlorohydrinepichlorohydrin (ECH) in thethe presence of a phase transfer catalyst (QX) [1,27]. presenceThe direct of aglycidylation phase transfer of catalyst phenol (QX) has [1, 27proven]. to yield several possible side products [28–32] including chlorinated and diol derivatives (Figure 1). Furthermore, the higher reactivity of benzoic acid The direct glycidylation of phenol has proven to yield several possible side products [28–32] may Thelead directto an opening glycidylation of the ofepoxy phenol via both has provencarbon atoms, to yield leading several to possible new chlorinated side products (B2), diol [28 –(B332]) including chlorinated and diol derivatives (Figure 1). Furthermore, the higher reactivity of benzoic acid includingand oxetane chlorinated ring (B1) andside-products diol derivatives [33–37]. (Figure Another1). Furthermore, important side-product the higher reactivityobserved ofduring benzoic the may lead to an opening of the epoxy via both carbon atoms, leading to new chlorinated (B2), diol (B3) acidglycidylation may lead step to an with opening epichlorohydrin of the epoxy is via a bothbenzod carbonioxanatoms, derivative leading obtained to new by chlorinated an intra cyclization (B2), diol and oxetane ring (B1) side-products [33–37]. Another important side-product observed during the (occurringB3) and oxetane with two ring phenolic (B1) side-products groups in ortho [33 position–37]. Another (Scheme important 3). side-product observed during the glycidylation step with epichlorohydrin is a benzodioxan derivative obtained by an intra cyclization glycidylation step with epichlorohydrin is a benzodioxan derivative obtained by an intra cyclization occurring with two phenolic groups in ortho position (Scheme 3). occurring with two phenolic groups in ortho position (Scheme3).

Figure 1. Common side-products of the direct glycidylation of phenol (A1–A3) and benzoic acid (B1 –B3). Figure 1. Common side-products of the direct glycidylation of phenol (A1–A3) and benzoic acid (B1–B3).

Molecules 2017, 22, 149 3 of 47 desired product via a SN2 mechanism, or open the epoxy ring causing the formation of a chlorinated derivative that can be closed by a strong base through a SNi mechanism (Scheme 2) [25,26]. SNi stands for Substitution Nucleophilic internal and is a nucleophilic substitution mechanism that implies a retention of configuration.

Scheme 1. Synthesis of epoxy derivatives from phenol or aniline by direct glycidylation with epichlorohydrin.

Scheme 2. Mechanism of coupling between phenolic compounds and epichlorohydrin (ECH) in the presence of a phase transfer catalyst (QX) [1,27].

The direct glycidylation of phenol has proven to yield several possible side products [28–32] including chlorinated and diol derivatives (Figure 1). Furthermore, the higher reactivity of benzoic acid may lead to an opening of the epoxy via both carbon atoms, leading to new chlorinated (B2), diol (B3) and oxetane ring (B1) side-products [33–37]. Another important side-product observed during the

Moleculesglycidylation2017, 22 step, 149 with epichlorohydrin is a benzodioxan derivative obtained by an intra cyclization4 of 48 occurring with two phenolic groups in ortho position (Scheme 3).

Figure 1. 1. CommonCommon side-products side-products of the of thedirect direct glycidylation glycidylation of phenol of phenol (A1–A3 ()A1 and–A3 benzoic) and acid benzoic (B1– acidB3). Molecules(B1 –2017B3)., 22, 149 4 of 47

Scheme 3. ProductsProducts obtained obtained during during the the glycidylation glycidylation of of protocatechuic protocatechuic acid acid with with epichlorohydrin epichlorohydrin [38]. [38 ].

To avoid these drawbacks, Meurs et al. [39] developed a process to obtain glycidyl derivatives from To avoid these drawbacks, Meurs et al. [39] developed a process to obtain glycidyl derivatives phenol, glycidol and propylene carbonate but it requires the use of high temperatures in autoclave and from phenol, glycidol and propylene carbonate but it requires the use of high temperatures in autoclave relatively harsh conditions. A two-step synthesis can also be used to form epoxy compounds: it involves and relatively harsh conditions. A two-step synthesis can also be used to form epoxy compounds: the O- or N-allylation of the corresponding alcohol or amine derivatives using an allyl halide, followed it involves the O- or N-allylation of the corresponding alcohol or amine derivatives using an allyl by the oxidation of the resulting double bond (Scheme 4). Unfortunately, allyl chloride and allyl bromide halide, followed by the oxidation of the resulting double bond (Scheme4). Unfortunately, allyl chloride are both toxic derivatives. Moreover, hydrogen peroxide exhibitis low reactivity toward allyl ether and allyl bromide are both toxic derivatives. Moreover, hydrogen peroxide exhibitis low reactivity oxidation except at high concentrations or in the presence of metal transition catalysts [40]. The use of toward allyl ether oxidation except at high concentrations or in the presence of metal transition stronger, more toxic peracids such as m-chloroperbenzoic acid (mCPBA) is sometimes considered, but catalysts [40]. The use of stronger, more toxic peracids such as m-chloroperbenzoic acid (mCPBA) Aouf et al. [22] found that an excess of peracid is also required and the m-chlorobenzoic acid formed is sometimes considered, but Aouf et al. [22] found that an excess of peracid is also required and during the oxidation of allylated gallic acid is difficult to eliminate. The epoxidation of allyl groups by the m-chlorobenzoic acid formed during the oxidation of allylated gallic acid is difficult to eliminate. potassium peroxymonosulfate (also known as Oxone) can be considered a sustainable pathway. It is The epoxidation of allyl groups by potassium peroxymonosulfate (also known as Oxone) can be based on the Shi epoxidation, which uses a fructose-derived organocatalyst with Oxone and ketones to considered a sustainable pathway. It is based on the Shi epoxidation, which uses a fructose-derived generate in situ dioxiranes [41], which are strong epoxidation agents. However, the epoxidation of organocatalyst with Oxone and ketones to generate in situ dioxiranes [41], which are strong epoxidation electron-deficient alkenes such as allyl groups by dioxiranes can be very slow [42–44]. The reaction may agents. However, the epoxidation of electron-deficient alkenes such as allyl groups by dioxiranes can require the use of ketones bearing highly electroattractive groups such as 1,1,1-trifluoroacetone to be very slow [42–44]. The reaction may require the use of ketones bearing highly electroattractive increase the overall yield [22,45]. Enzymatic catalysts have also been developed as greener alternatives groups such as 1,1,1-trifluoroacetone to increase the overall yield [22,45]. Enzymatic catalysts have for the oxidation of carbon-carbon double-bonds. First developed to replace the Prileshajev reaction also been developed as greener alternatives for the oxidation of carbon-carbon double-bonds. First applied at an industrial scale to produce epoxy vegetable oils [46], one of these catalyst (immobilized developed to replace the Prileshajev reaction applied at an industrial scale to produce epoxy vegetable lipase B from Candida antarctica (Novozym 435)) has also been used by Aouf et al. [40] to obtain epoxy gallic acid and vanillic acid from their allylated precursors in high yields.

Scheme 4. Synthetic pathway to obtain epoxy from phenol or aniline using allyl halide and oxidation.

Overall, the direct glycidylation remains the main synthetic pathway used for the industrial synthesis of Diglycidyl Ether of Bisphenol A, as it allows the recovery of both monomers and oligomers for tunable properties [23,47]. By reacting bisphenol A with a controlled excess of epichlorohydrin, the “taffy” process yields either monomers or short oligomers of DGEBA (Scheme 5). To increase the chain length of the oligomers, “advancement” (with solvent) or “fusion” (without solvent) processes can be

Molecules 2017, 22, 149 4 of 47

Scheme 3. Products obtained during the glycidylation of protocatechuic acid with epichlorohydrin [38].

To avoid these drawbacks, Meurs et al. [39] developed a process to obtain glycidyl derivatives from phenol, glycidol and propylene carbonate but it requires the use of high temperatures in autoclave and relatively harsh conditions. A two-step synthesis can also be used to form epoxy compounds: it involves the O- or N-allylation of the corresponding alcohol or amine derivatives using an allyl halide, followed by the oxidation of the resulting double bond (Scheme 4). Unfortunately, allyl chloride and allyl bromide are both toxic derivatives. Moreover, hydrogen peroxide exhibitis low reactivity toward allyl ether oxidation except at high concentrations or in the presence of metal transition catalysts [40]. The use of stronger, more toxic peracids such as m-chloroperbenzoic acid (mCPBA) is sometimes considered, but Aouf et al. [22] found that an excess of peracid is also required and the m-chlorobenzoic acid formed during the oxidation of allylated gallic acid is difficult to eliminate. The epoxidation of allyl groups by potassium peroxymonosulfate (also known as Oxone) can be considered a sustainable pathway. It is based on the Shi epoxidation, which uses a fructose-derived organocatalyst with Oxone and ketones to generate in situ dioxiranes [41], which are strong epoxidation agents. However, the epoxidation of electron-deficient alkenes such as allyl groups by dioxiranes can be very slow [42–44]. The reaction may require the use of ketones bearing highly electroattractive groups such as 1,1,1-trifluoroacetone to Molecules 2017, 22, 149 5 of 48 increase the overall yield [22,45]. Enzymatic catalysts have also been developed as greener alternatives for the oxidation of carbon-carbon double-bonds. First developed to replace the Prileshajev reaction oilsapplied [46], at one an ofindustrial these catalyst scale to (immobilized produce epoxy lipase vege B fromtable Candidaoils [46], antarcticaone of these(Novozym catalyst 435))(immobilized has also beenlipase used B from by AoufCandida et al.antarctica [40] to obtain(Novozym epoxy 435)) gallic has acid also and been vanillic used acidby Aouf from et their al. [40] allylated to obtain precursors epoxy ingallic high acid yields. and vanillic acid from their allylated precursors in high yields.

Scheme 4. SyntheticSynthetic pathway pathway to to obtain obtain epoxy epoxy from from phenol phenol or oraniline aniline using using allyl allyl halide halide and and oxidation. oxidation.

Overall, the direct glycidylation remains the main synthetic pathway used for the industrial Overall, the direct glycidylation remains the main synthetic pathway used for the industrial synthesis of Diglycidyl Ether of Bisphenol A, as it allows the recovery of both monomers and oligomers synthesis of Diglycidyl Ether of Bisphenol A, as it allows the recovery of both monomers and oligomers for tunable properties [23,47]. By reacting bisphenol A with a controlled excess of epichlorohydrin, the for tunable properties [23,47]. By reacting bisphenol A with a controlled excess of epichlorohydrin, the “taffy” process yields either monomers or short oligomers of DGEBA (Scheme 5). To increase the chain “taffy” process yields either monomers or short oligomers of DGEBA (Scheme5). To increase the chain lengthMolecules of 2017the, 22oligomers,, 149 “advancement” (with solvent) or “fusion” (without solvent) processes can5 of 47be length of the oligomers, “advancement” (with solvent) or “fusion” (without solvent) processes can be chosen. chosen. They They both both consist consist in inreacting reacting BPA BPA with with an anexcess excess of ofa pre-synthesized a pre-synthesized DGEBA DGEBA monomer monomer to toextend extend the the chain. chain. “Fusion” “Fusion” process process is is generally generally preferred preferred for for the the industrial industrial production production of of DGEBA DGEBA oligomersoligomers as as thethe purificationpurification steps are are easier easier and and the the chlorine chlorine content content of ofthe the final final product product is lower is lower than than in inthe the case case of ofthe the “taffy” “taffy” process, process, which which requires requires an excess an excess of epichlorohydrin. of epichlorohydrin.

SchemeScheme 5.5.Two Two main main industrial industrial processesprocesses forfor thethe synthesissynthesis ofof monomersmonomers and oligomers of diglycidyl etherether of of vanillyl vanillyl alcoholalcohol (DGEBA): (i) (i) the the taffy taffy process process with with a controlled a controlled excess excess of epichlorohydrin of epichlorohydrin and and(ii) the advancement/fusion process using an excess of a pre-synthesized DGEBA monomer. (ii) the advancement/fusion process using an excess of a pre-synthesized DGEBA monomer.

In terms of sustainability, both bisphenol A and epichlorohydrin used for the synthesis of DGEBA In terms of sustainability, both bisphenol A and epichlorohydrin used for the synthesis of DGEBA are mostly oil-based. BPA is obtained from reaction of acetone and phenol and epichlorohydrin is are mostly oil-based. BPA is obtained from reaction of acetone and phenol and epichlorohydrin synthesized in two steps by reacting hypochlorous acid on allyl chloride and then treating the alcohol is synthesized in two steps by reacting hypochlorous acid on allyl chloride and then treating the mixture obtained with a strong base [48]. In 2007, Solvay designed the EPICEROLTM process to produce TM alcoholepichlorohydrin mixture obtained from bio-based with a glycerol, strong base thus [ 48allowing]. In 2007, to reduce Solvay the designed content of the fossil EPICEROL resources usedprocess for toDGEBA produce production epichlorohydrin [49–53]. However, from bio-based the percentage glycerol, of thus carbon allowing atoms toin reducethe oligomers the content coming of from fossil resourcesECH is low, used thus for making DGEBA the production impact limited. [49–53 Furthermore,]. However, whatever the percentage synthetic of pathway carbon atomsor process in theis oligomerschosen, reagents coming (e.g., from ep ECHichlorohydrin is low, thus and making allyl halides) the impact are all limited. carcinogen Furthermore, agents (H350). whatever Allyl syntheticbromide pathwayis also very or toxic process for the is chosen, environment. reagents The (e.g.,toxicity epichlorohydrin of these reactions and is a allyl true halides)issue, and are alternatives all carcinogen have agentsto be (H350).found over Allyl time. bromide Although is also the very production toxic for theof epoxides environment. is generally The toxicity well-controlled of these reactions by the ismanufacturer a true issue, and alternativesthe resulting haveresins to and be foundmaterials over do time. not exhibit Although the toxicity the production of these ofreactants, epoxides the is intrinsic toxicity of BPA remains.

3. Toxicity of Bisphenol A and Regulations Bisphenols (BPs) are part of the common class of endocrine disruptors because of their significant hormonal activity, with Bisphenol A being the most famous of them. In fact, BPA exhibits one of the highest production volume of chemicals worldwide [7], with a manufacture of approximately 3.8 million tons per year in 2006 [54]. About 80% of the global production of BPA is used for the synthesis of polycarbonate, 18% for epoxy resins and the remainder for other applications, such as food containers, paper products (e.g., thermal receipts), water pipes, toys, medical equipment, and electronics [6,7]. As a consequence, human beings and environment are constantly exposed to BPA: it has been detected in 95% of human urine samples, which indicates that this compound may leach into food or water [7–9,55]. Furthermore, several studies found that BPA is present in high prevalence in fetuses and infants [7,8,56] and has undeniably an impact on human health leading to precocious puberty, cancer, diabetes, obesity, neurological disorders, and so on. Theoretical and experimental studies can be carried out to forecast the potential toxicity of a compound or to determine the different types of interactions between this compound and estrogen receptors. For example, the “read-across” method, based on analogies between substances is currently developing. It first consists in gathering data on the physical and biological properties of chemicals exhibiting a structure similar to the target molecule. Then, by taking into account previously observed trends, it is considered possible to extrapolate on the target molecule’s behavior [57]. QSAR (Quantitative Structure Activity Relationship) models enable a qualitative and quantitative determination of the endocrine activity in terms of affinity, and give some information about the

Molecules 2017, 22, 149 6 of 48 generally well-controlled by the manufacturer and the resulting resins and materials do not exhibit the toxicity of these reactants, the intrinsic toxicity of BPA remains.

3. Toxicity of Bisphenol A and Regulations Bisphenols (BPs) are part of the common class of endocrine disruptors because of their significant hormonal activity, with Bisphenol A being the most famous of them. In fact, BPA exhibits one of the highest production volume of chemicals worldwide [7], with a manufacture of approximately 3.8 million tons per year in 2006 [54]. About 80% of the global production of BPA is used for the synthesis of polycarbonate, 18% for epoxy resins and the remainder for other applications, such as food containers, paper products (e.g., thermal receipts), water pipes, toys, medical equipment, and electronics [6,7]. As a consequence, human beings and environment are constantly exposed to BPA: it has been detected in 95% of human urine samples, which indicates that this compound may leach into food or water [7–9,55]. Furthermore, several studies found that BPA is present in high prevalence in fetuses and infants [7,8,56] and has undeniably an impact on human health leading to precocious puberty, cancer, diabetes, obesity, neurological disorders, and so on. Theoretical and experimental studies can be carried out to forecast the potential toxicity of a compound or to determine the different types of interactions between this compound and estrogen receptors. For example, the “read-across” method, based on analogies between substances is currently developing. It first consists in gathering data on the physical and biological properties of chemicals exhibiting a structure similar to the target molecule. Then, by taking into account previously observed trends, it is considered possible to extrapolate on the target molecule’s behavior [57]. QSAR (Quantitative Structure Activity Relationship) models enable a qualitative and quantitative determination of the endocrine activity in terms of affinity, and give some information about the underlying mechanism [58]. Recently, Delfosse et al. [59] described for the first time the mode of action of BPA at the molecular scale and developed a bio-informatics tool to predict the interactions between bisphenols and the target receptors (estrogen receptors (ERs) or other members of the nuclear hormone receptor family). Numerous studies demonstrated that bisphenol A has two major modes of actions: steroid related mode and epigenetic mode. Concerning the latter, Dolinoy et al. [60] have shown the ability of BPA to alter DNA methylation. Regarding the steroid related mode, BPA can act as an estrogen agonist when it binds to nuclear estrogen receptors (ERα and ERβ)[61–64]. More recently, Takayanagi et al. [65] demonstrated that BPA also binds to the estrogen-related receptor-γ (ERRγ) with high constitutive activity. Furthermore, BPA also behaves as an androgen receptor antagonist (AR) which affects the activation and function of the AR [66,67]. According to many lines of evidence, BPA acts as an endocrine disruptor even at low doses. A review, based on hundreds of studies, concluded that there is sufficient evidence for low dose effects of BPA. Indeed, for these studies, authors used doses below those used for traditional toxicological studies, and found nonmonotonic dose-response curves [68]. Bisphenols are composed of two phenols linked by a central carbon atom, which in the case of BPA bears two additional methyl groups (Figure2B). These structural features make BPA able to mimic the natural estrogen 17β-estradiol (Figure3), in terms of binding ability to estrogen receptors [ 69,70]. In fact, the main characteristics of the natural ligands required for the steroid activity are the presence of phenol groups on a hydrophobic backbone [71,72]. A recent paper has demonstrated that all the structural elements of BPA are prerequisite for binding the estrogen-related receptor-γ (ERRγ), especially the two phenolic and methyl groups [10]. Firstly, the authors demonstrated that the phenol structure of BPA is an essential element to bind ERRγ and only one of the two phenolic hydroxy groups is required for the full binding. Nevertheless, the presence of a second oxygen-based group increases the steroid activity [73]. Molecules 2017, 22, 149 6 of 47 underlying mechanism [58]. Recently, Delfosse et al. [59] described for the first time the mode of action of BPA at the molecular scale and developed a bio-informatics tool to predict the interactions between bisphenols and the target receptors (estrogen receptors (ERs) or other members of the nuclear hormone receptor family). Numerous studies demonstrated that bisphenol A has two major modes of actions: steroid related mode and epigenetic mode. Concerning the latter, Dolinoy et al. [60] have shown the ability of BPA to alter DNA methylation. Regarding the steroid related mode, BPA can act as an estrogen agonist when it binds to nuclear estrogen receptors (ERα and ERβ) [61–64]. More recently, Takayanagi et al. [65] demonstrated that BPA also binds to the estrogen-related receptor-γ (ERRγ) with high constitutiveMolecules 2017 , activity.22, 149 Furthermore, BPA also behaves as an androgen receptor antagonist (AR) which6 of 47 affects the activation and function of the AR [66,67]. According to many lines of evidence, BPA acts as an underlying mechanism [58]. Recently, Delfosse et al. [59] described for the first time the mode of action of endocrine disruptor even at low doses. A review, based on hundreds of studies, concluded that there is BPA at the molecular scale and developed a bio-informatics tool to predict the interactions between sufficient evidence for low dose effects of BPA. Indeed, for these studies, authors used doses below those bisphenols and the target receptors (estrogen receptors (ERs) or other members of the nuclear hormone used for traditional toxicological studies, and found nonmonotonic dose-response curves [68]. receptor family). Numerous studies demonstrated that bisphenol A has two major modes of actions: steroid related mode and epigenetic mode. Concerning the latter, Dolinoy et al. [60] have shown the ability of BPA to alter DNA methylation. Regarding the steroid related mode, BPA can act as an estrogen agonist when it binds to nuclear estrogen receptors (ERα and ERβ) [61–64]. More recently, Takayanagi et al. [65] demonstrated that BPA also binds to the estrogen-related receptor-γ (ERRγ) with high constitutive activity. Furthermore, BPA also behaves as an androgen receptor antagonist (AR) which affects the activation and function of the AR [66,67]. According to many lines of evidence, BPA acts as an endocrine disruptor even at low doses. A review, based on hundreds of studies, concluded that there is

Moleculessufficient2017 evidence, 22, 149 for low dose effects of BPA. Indeed, for these studies, authors used doses below 7those of 48 used for traditional toxicological studies, and found nonmonotonic dose-response curves [68].

Figure 2. Chemical structures of bisphenol analogues (A); bisphenol A (B); bisphenol F (C) and bisphenol S (D).

Bisphenols are composed of two phenols linked by a central carbon atom, which in the case of BPA bears two additional methyl groups (Figure 2B). These structural features make BPA able to mimic the natural estrogen 17β-estradiol (Figure 3), in terms of binding ability to estrogen receptors [69,70]. In fact, the main characteristics of the natural ligands required for the steroid activity are the presence of phenol groups on a hydrophobic backbone [71,72]. A recent paper has demonstrated that all the structural elements of BPA are prerequisite for binding the estrogen-related receptor-γ (ERRγ), especially the two phenolic and methyl groups [10]. Firstly, the authors demonstrated that the phenol structure of BPA is an essential element to bind ERRγ and only one of the two phenolic hydroxy groups is required for the fullFigure binding.Figure 2. Chemical 2. Nevertheless,Chemical structures structures the of bisphenolpresence of bisphenol analoguesof a second analogues (A); oxygen-based bisphenol (A); bisphenol A (B );group bisphenol A increases (B); F bisphenol (C) and the bisphenol steroid F (C) and Sactivity (D). [73]. bisphenol S (D). Bisphenols are composed of two phenols linked by a central carbon atom, which in the case of BPA bears two additional methyl groups (Figure 2B). These structural features make BPA able to mimic the natural estrogen 17β-estradiol (Figure 3), in terms of binding ability to estrogen receptors [69,70]. In fact, the main characteristics of the natural ligands required for the steroid activity are the presence of phenol groups on a hydrophobic backbone [71,72]. A recent paper has demonstrated that all the structural elements of BPA are prerequisite for binding the estrogen-related receptor-γ (ERRγ), especially the two phenolic and methyl groups [10]. Firstly, the authors demonstrated that the phenol structure of BPA is an essential element to bind ERRγ and only one of the two phenolic hydroxy groups is required for the full binding. Nevertheless, the presence of a second oxygen-based group increases the steroid activity [73].

FigureFigure 3.3. StructureStructure of 17 β-estradiol.-estradiol.

By analogy with 17β-estradiol, these two hydroxyl groups are essential to establish interactions By analogy with 17β-estradiol, these two hydroxyl groups are essential to establish interactions with the hydrophobic pocket created by the receptor [74,75]. This pocket consists of several combining with the hydrophobic pocket created by the receptor [74,75]. This pocket consists of several combining sites where estrogen or other ligands can bind. The size of the pocket is 440 Å [74–76], which is bigger sites where estrogen or other ligands can bind. The size of the pocket is 440 Å [74–76], which is bigger than the natural estrogen molecule size (245 Å) allowing hydrogen binding interactions (Figure 4). Liu et than the natural estrogen molecule size (245 Å) allowing hydrogen binding interactions (Figure4). Liu et al. [77] also showed that the substitution of one of the two aromatic rings by a methyl or ethyl group led to a decrease of the interaction with the hydrophobic part of the receptor, whereas the presence of chlorine substituents on the 3–5 positions of the first aromatic ring strengthens affinity with the receptor. Figure 3. Structure of 17β-estradiol. The presence and the distance between the two hydroxyl groups are not the only critical factors. OkadaBy etanalogy al. [10] with clearly 17 demonstratedβ-estradiol, these that two the hydroxyl alkyl groups groups on theare centralessential carbon to establish atom of interactions bisphenol playwith athe key hydrophobic role in selection pocket of created the human by the estrogen receptor receptors: [74,75]. This ERR pocketγ or ER consisα. ERts αofprefers several thecombining bulkier andsites morewhere electrophilic estrogen or alkylother groups,ligands whereascan bind. ERR Theγ sizeprefers of the the pocket less bulky is 440 and Å [74–76], less electrophilic which is bigger alkyl groups.than the Anatural bulky estrogen group on molecule the central size carbon(245 Å) atomallowing is obviously hydrogen disadvantageous binding interactions in terms (Figure of 4). binding Liu et BPA to ERRγ’s binding pocket and reduces its activity. Furthermore, it was shown that one of the two methyl groups on the central carbon atom of BPA is involved in the hydrophobic intermolecular interaction with the receptor residue, such as CH3-alkyl and CH/π interactions. The increasing concerns about the detrimental effects of BPA has led governments to enforce regulations mostly in the European Union and North America in order to limit the exposition of their citizens to this substance. For example, in 2014, in line with the opinion adopted by the RAC (Risk Assessment Committee), Bisphenol A has been classified in the hazard class reproductive toxicity category 1B “may damage fertility”. In the frame of the REACH regulation, the French proposition of the restriction of BPA in thermal papers was approved on the 6 July 2016 by the REACH Committee. The BPA European legislation involves different elements such as the restriction of the contact of infants and young people with this substance, through toys, feeding bottles, etc. [11,78–80]. BPA is authorized as additive or monomer in the manufacture of plastic materials and articles in contact with Molecules 2017, 22, 149 8 of 48 food and water but a specific migration limit value of 0.6 mg/kg of food has been set [12]. Regarding cosmeticMolecules 2017 products,, 22, 149 Bisphenol A is recorded in the list of the prohibited substances [81]. All the products7 of 47 made of BPA can’t be eligible for a positive Eco-Label [82] and the indicative limit of occupational exposureal. [77] also [83 showed] to BPA that particles the substitution is 10 mg/m of one3. Someof the countriestwo aromatic such rings as Franceby a methyl have or established ethyl group even led moreto a decrease strict legislations of the interaction towards with BPA the [84 ],hydropho and thisbic country part of proposed the receptor, BPA aswhereas a REACH the Regulationpresence of candidatechlorine substituents substance ofon very the 3–5 high positi concernons of (SVHC) the first [ 85aromatic] and confirmed ring strengthens its advert affinity on the with 30 Augustthe receptor. 2016.

Figure 4.4. X-rayX-ray structurestructure ofof thethe hydrophobichydrophobic pocketpocket ofof estrogenestrogen receptorreceptor (ER)(ER)α [[76],76], accordingaccording toto thethe worksworks of Brzozowski et et al. al. [74] [74 ]and and Tanenbaum Tanenbaum et etal. al. [75]. [75 ].

The presence and the distance between the two hydroxyl groups are not the only critical factors. Following the public concern and the stringent regulations on the production and use of BPA, Okada et al. [10] clearly demonstrated that the alkyl groups on the central carbon atom of bisphenol play several bisphenol analogues have been produced as alternative substances. These following analogues, a key role in selection of the human estrogen receptors: ERRγ or ERα. ERα prefers the bulkier and more BPF (4,40-methylenediphenol) and BPS (4-hydroxyphenyl sulfone) (Figure2C,D), are frequently electrophilic alkyl groups, whereas ERRγ prefers the less bulky and less electrophilic alkyl groups. A found in most scientific and environmental studies because they are among the main substitutes bulky group on the central carbon atom is obviously disadvantageous in terms of binding BPA to of BPA in polycarbonate-based plastics and epoxy resins. Available studies have reported a variety of ERRγ’s binding pocket and reduces its activity. Furthermore, it was shown that one of the two methyl detrimental effects of these bisphenol analogues [86] and showed that the toxicity of these analogs groups on the central carbon atom of BPA is involved in the hydrophobic intermolecular interaction is similar to or even greater than that of BPA [87,88]. The toxic effects include endocrine disruption, with the receptor residue, such as CH3-alkyl and CH/π interactions. cytotoxicity, genotoxicity, reproductive toxicity, neurotoxicity, etc. A recent article by Rochester The increasing concerns about the detrimental effects of BPA has led governments to enforce and Bolden [89], focusing on the hormonal activities of BPF and BPS demonstrated that these two regulations mostly in the European Union and North America in order to limit the exposition of their analogues have a hormonal action similar to BPA in vitro and in vivo. Hexafluorobisphenol A (BPAF) citizens to this substance. For example, in 2014, in line with the opinion adopted by the RAC (Risk and 4,40-(1-methylpropylidene) bisphenol (BPB), two others substituents of BPA, have also shown Assessment Committee), Bisphenol A has been classified in the hazard class reproductive toxicity estrogenic and anti-androgenic activities [88]. Therefore, a harmless let alone renewable epoxy building category 1B “may damage fertility”. In the frame of the REACH regulation, the French proposition of the block is still required [90]. For this reason, bio-mass has been considered a cheap source of potentially restriction of BPA in thermal papers was approved on the 6 July 2016 by the REACH Committee. The functionalizable monomers or oligomers. As previously stated, the already commercialized bio-based BPA European legislation involves different elements such as the restriction of the contact of infants and epoxy monomers cannot compete with DGEBA-based materials in terms of thermo-mechanical young people with this substance, through toys, feeding bottles, etc. [11,78–80]. BPA is authorized as properties, because of their characteristic structure and low aromatic content. Thus, the next part will additive or monomer in the manufacture of plastic materials and articles in contact with food and water briefly present the natural sources of renewable aromatic compounds potentially capable of standing but a specific migration limit value of 0.6 mg/kg of food has been set [12]. Regarding cosmetic products, up for BPA replacement. Bisphenol A is recorded in the list of the prohibited substances [81]. All the products made of BPA can’t 4.be Maineligible Natural for a positive Sources Eco-Label of Aromatic [82] Moieties and the indicative limit of occupational exposure [83] to BPA particles is 10 mg/m3. Some countries such as France have established even more strict legislations towardsThe BPA present [84], part andwill this brieflycountry summarize proposed BPA the mainas a REACH sources Regulation of aromatic candidate moieties substance bearing reactive of very groupshigh concern suitable (SVHC) for the[85] and introduction confirmed of its epoxyadvert moieties.on the 30 August This summary2016. will include resources Following the public concern and the stringent regulations on the production and use of BPA, several bisphenol analogues have been produced as alternative substances. These following analogues, BPF (4,4′-methylenediphenol) and BPS (4-hydroxyphenyl sulfone) (Figure 2C,D), are frequently found in most scientific and environmental studies because they are among the main substitutes of BPA in

Molecules 2017, 22, 149 8 of 47

polycarbonate-based plastics and epoxy resins. Available studies have reported a variety of detrimental effects of these bisphenol analogues [86] and showed that the toxicity of these analogs is similar to or even greater than that of BPA [87,88]. The toxic effects include endocrine disruption, cytotoxicity, genotoxicity, reproductive toxicity, neurotoxicity, etc. A recent article by Rochester and Bolden [89], focusing on the hormonal activities of BPF and BPS demonstrated that these two analogues have a hormonal action similar to BPA in vitro and in vivo. Hexafluorobisphenol A (BPAF) and 4,4′-(1-methylpropylidene) bisphenol (BPB), two others substituents of BPA, have also shown estrogenic and anti-androgenic activities [88]. Therefore, a harmless let alone renewable epoxy building block is still required [90]. For this reason, bio-mass has been considered a cheap source of potentially functionalizable monomers or oligomers. As previously stated, the already commercialized bio-based epoxy monomers cannot compete with DGEBA-based materials in terms of thermo-mechanical properties, because of their characteristic structure and low aromatic content. Thus, the next part will briefly present the natural sources of renewable aromatic compounds potentially capable of standing up for BPA replacement.

4. Main Natural Sources of Aromatic Moieties Molecules 2017, 22, 149 9 of 48 The present part will briefly summarize the main sources of aromatic moieties bearing reactive groups suitable for the introduction of epoxy moieties. This summary will include resources naturally naturallycontaining containing small, phenolic small, phenoliccompound compound such as sucheugenol as eugenol extractible extractible from fromplant plant natural natural oils, oils, or orpolyphenolic polyphenolic crosslin crosslinkedked polymers polymers such such as tannins as tannins and and lignin lignin that that can can be bedirectly directly functionalized functionalized or ordepolymerized depolymerized into intosmaller smaller molecules molecules prior to prior epoxidation to epoxidation steps. So steps.me information Some information on their availability on their availabilityor worldwide or production worldwide will production be given willas well be given as the as main well known as the maincharacteristics known characteristics of their structure. of their The structure.reader may The refer reader to the mayvastly refer documented to the vastly review documented from Lochab review et al. from[91] and Lochab book etfrom al. [Belgacem91] and book and fromGandini Belgacem [92] on and renewable Gandini [resources92] on renewable and naturally resources occurring and naturally phenolic occurring derivatives phenolic to obtain derivatives more todetailed obtain data. more detailed data.

4.1. Lignin Lignin Lignin is thethe widestwidest distributeddistributed aromaticaromatic biopolymerbiopolymer and thethe secondsecond mostmost abundantabundant naturallynaturally occurring macromolecule after after cellulose, cellulose, constitu constitutingting from from 1% 1% to to 43% 43% by by weight of the drydry lignocellulosic biomass, biomass, with with a potential a potential availability availability exceeding exceeding 300 billion 300 billion tons [17,93–95]. tons [17, 93It –is95 a]. cell-wall It is a cell-wallcomponent component bonding bondingcells together cells togetherin the woody in the stem woodys, providing stems, providing them with them their with well-known their well-known rigidity rigidityand impact and resistance. impact resistance. Although Although its absolute its absolute structure structure remains remains unknown unknown and varies and variesaccording according to the toplant the plantit originates it originates from fromand andits itsenvironment, environment, lignin lignin is isan an amorphous amorphous th three-dimensionalree-dimensional polymerpolymer network of threethree mainmain methoxylatedmethoxylated phenylphenyl propanepropane unitsunits (Figure(Figure5 5)) with with seven seven major major linkages: linkages: ββ--O-4 (aryl(aryl ether),ether), αα--OO-4,-4, ββ--ββ (pinoresinol),(pinoresinol), ββ-5-5 (phenylcoumaran),(phenylcoumaran), β-1 (diphenylmethane),(diphenylmethane), 5,5 5,5 and 4-4-OO-5-5 (diphenyl(diphenyl ether) linkages. It exhibits various functional groups groups such as aliphatic and phenolic hydroxyl, carboxylic, carbonyl carbonyl and and methoxy methoxy moieties. moieties.

FigureFigure 5. 5.Main Main aromaticaromatic subunits found found in in lignin. lignin.

Usually, lignin is viewed as a waste material derived from the wood pulp in the paper industry and available in large quantity (50–70 million of tons estimated) [96]. Unfortunately, only 1% to 2% of overall lignin is used for more specific applications, the remaining primarily serving as a (bio)fuel for the cellulose extraction [97]. Lignin is extracted from lignocellulosic biomass by two main categories of processes: (i) sulfur processes yielding lignosulfate and Kraft lignin and (ii) sulfur-free processes yielding organosolv and soda lignin. These extraction methods greatly influence the already complex structure of the polymer, sometimes making it difficult to directly use it as a chemical precursor. For this reason, works have been carried out to depolymerize lignin into smaller, simpler aromatic molecules suitable for chemical modification and/or polymerization. For example, when lignin is depolymerized, compounds such as vanillin, phenols derivatives, cresols, ferulic and coumaric acids are released (Figure6). All these molecules are already oil-based but this pathway offers an interesting solution for their renewability, thus increasing thermosets renewability. Molecules 2017, 22, 149 9 of 47

Usually, lignin is viewed as a waste material derived from the wood pulp in the paper industry and available in large quantity (50–70 million of tons estimated) [96]. Unfortunately, only 1% to 2% of overall lignin is used for more specific applications, the remaining primarily serving as a (bio)fuel for the cellulose extraction [97]. Lignin is extracted from lignocellulosic biomass by two main categories of processes: (i) sulfur processes yielding lignosulfate and Kraft lignin and (ii) sulfur-free processes yielding organosolv and soda lignin. These extraction methods greatly influence the already complex structure of the polymer, sometimes making it difficult to directly use it as a chemical precursor. For this reason, works have been carried out to depolymerize lignin into smaller, simpler aromatic molecules suitable for chemical modification and/or polymerization. For example, when lignin is depolymerized, compounds such as vanillin, phenols derivatives, cresols, ferulic and coumaric acids are released (Figure 6). All these Moleculesmolecules2017 are, 22 ,already 149 oil-based but this pathway offers an interesting solution for their renewability,10 of 48 thus increasing thermosets renewability.

Figure 6. Various aromatic moieties obtained from lignin depolymerization [93,94]. Figure 6. Various aromatic moieties obtained from lignin depolymerization [93,94].

4.2. Tannins 4.2. Tannins Tannins are the second bio-based source of natural phenolic moieties and the third most abundant Tannins are the second bio-based source of natural phenolic moieties and the third most abundant compounds extracted from wood biomass, with 160,000 tons bio-synthesized each year [98,99]. They can compounds extracted from wood biomass, with 160,000 tons bio-synthesized each year [98,99]. They be found mainly in the soft tissues such as wood, bark, leaves or needles of all vascular and some can be found mainly in the soft tissues such as wood, bark, leaves or needles of all vascular and some non-vascular plants, in which they play a protective role against outside aggressions and in plant growth non-vascular plants, in which they play a protective role against outside aggressions and in plant regulation [100]. Tannins are polyphenol derivatives with low molecular weights and can be divided growth regulation [100]. Tannins are polyphenol derivatives with low molecular weights and can be into three main categories: hydrolysable tannins, condensed tannins and complex tannins, the latter divided into three main categories: hydrolysable tannins, condensed tannins and complex tannins, the being a combination of the first two. latter being a combination of the first two. Hydrolysable tannins are a mixture of phenolic esters of sugars (Figure 7A) readily hydrolyzed by Hydrolysable tannins are a mixture of phenolic esters of sugars (Figure7A) readily hydrolyzed acids, alkalis or enzymes, and present a low availability (less than 10% of the world’s commercial by acids, alkalis or enzymes, and present a low availability (less than 10% of the world’s commercial production) [101]. They are mainly used in the tanning industry. The condensed tannins may represent production) [101]. They are mainly used in the tanning industry. The condensed tannins may represent the most interesting derivatives with 90% of global production. They are based on four types of the most interesting derivatives with 90% of global production. They are based on four types of repeating flavonoid units: profisetinidin, procyanidin, prorobinetidin, and prodelphinidin linked by C4– repeating flavonoid units: profisetinidin, procyanidin, prorobinetidin, and prodelphinidin linked C6 or C4–C8 bonds (Figure 7B). Thanks to their availability, aromatic moieties and rigid structure, their by C4–C6 or C4–C8 bonds (Figure7B). Thanks to their availability, aromatic moieties and rigid numerous functionalizable hydroxyl functions and nucleophilic sites, condensed tannins may represent structure, their numerous functionalizable hydroxyl functions and nucleophilic sites, condensed an interesting candidate for BPA substitution. Similarly to lignin, some studies are conducted on the tannins may represent an interesting candidate for BPA substitution. Similarly to lignin, some studies depolymerization of tannins to obtain phenolic monomers as building units for thermosets [98,102,103]. are conducted on the depolymerization of tannins to obtain phenolic monomers as building units For example, Roumeas et al. [102] successively used thiol and furan derivatives as nucleophiles for the for thermosets [98,102,103]. For example, Roumeas et al. [102] successively used thiol and furan derivatives as nucleophiles for the acid-assisted depolymerization of condensed tannins into catechin and thioether or furan derivatives of catechin. Finally, a last class of tannins, phlorotannins, can be found in non-vascular plants such as algae and are based on polymerized phloroglucinol (1,3,5-trihydroxybenzene) with a large range of molecular weights [98]. They play a role similar to condensed tannins in vascular plant and can as well be divided into categories according to the link between phloroglucinol units e.g., ether, phenyl bonds, a combination of both, or a dibenzo-p-dioxin bond (Figure8). Molecules 2017, 22, 149 10 of 47

acid-assisted depolymerization of condensed tannins into catechin and thioether or furan derivatives of catechin. Molecules 2017, 22, 149 10 of 47

Moleculesacid-assisted2017, 22 depolymerization, 149 of condensed tannins into catechin and thioether or furan derivatives11 of 48of catechin.

Figure 7. Main structures of tannins from vascular plants: hydrolysable tannins (A) and condensed tannins (B).

Finally, a last class of tannins, phlorotannins, can be found in non-vascular plants such as algae and are based on polymerized phloroglucinol (1,3,5-trihydroxybenzene) with a large range of molecular weights [98]. They play a role similar to condensed tannins in vascular plant and can as well be divided Figure 7. Main structures of tannins from vascular plants: hydrolysable tannins (A) and condensed intoFigure categories 7. Main according structures to of tanninsthe link from between vascular phloroglucinol plants: hydrolysable units e.g., tannins ether, (A) andphenyl condensed bonds, a tannins (B). combinationtannins (B). of both, or a dibenzo-p-dioxin bond (Figure 8). Finally, a last class of tannins, phlorotannins, can be found in non-vascular plants such as algae and are based on polymerized phloroglucinol (1,3,5-trihydroxybenzene) with a large range of molecular weights [98]. They play a role similar to condensed tannins in vascular plant and can as well be divided into categories according to the link between phloroglucinol units e.g., ether, phenyl bonds, a combination of both, or a dibenzo-p-dioxin bond (Figure 8).

FigureFigure 8. Example8. Example of of phlorotannins phlorotannins linkageslinkages with phenyl bonds bonds (in (in red), red), ether ether bonds bonds (in (in green) green) and and dibenzo-dibenzo-p-dioxinp-dioxin bond bond (in(in blue).blue).

4.3. Cardanol 4.3. Cardanol Cashew nut shell liquid (CNSL) is a reddish-brown liquid that can be extracted from the soft honeycombCashew nutstructure shell liquidlocated (CNSL)inside the is acashew reddish-brown nut shell and liquid constitutes that can from be extracted 30 to 35 fromwt % the of softit honeycomb[16,91,104].Figure 8. structure WithExample approximately of located phlorotannins inside 2.1 millions linkages the cashew of with tons nutphenylof cashew shell bonds andnuts (in constitutesproduced red), ether every bonds from year, 30(in green) tomainly 35 wtand from % of it [countries16dibenzo-,91,104 ].inp-dioxin WithAsia approximately(India,bond (in Vietnam) blue). 2.1and millions Africa (Nig of tonseria, ofIvory cashew Coast), nuts CNSL produced represents every an year, abundant mainly fromnon-edible countries by-product in Asia (India, that has Vietnam) already and found Africa various (Nigeria, applications Ivory Coast),in coatings, CNSL laminates represents and an adhesives abundant non-edible4.3.to Cardanol name a by-product few. However, that it has also already represents found an variousinteresting applications source of aromatic in coatings, chemical laminates building and blocks. adhesives In to namefact, CNSL a few. is However,mainly composed it also representsof four major an pheno interestinglic derivatives: source of anacardic aromatic acid, chemical cardanol, building cardol blocks.and Cashew nut shell liquid (CNSL) is a reddish-brown liquid that can be extracted from the soft In2-methyl fact, CNSL cardol is mainly(Figure 9), composed the percentage of four of majorwhich depends phenolic on derivatives: the extraction anacardic method. Among acid, cardanol, these, honeycomb structure located inside the cashew nut shell and constitutes from 30 to 35 wt % of it cardolcardanol and is 2-methyl often regarded cardol as (Figure the most9), theinteresting percentage compound, of which as its depends percentage on can the reach extraction 60% in method. some [16,91,104]. With approximately 2.1 millions of tons of cashew nuts produced every year, mainly from Among these, cardanol is often regarded as the most interesting compound, as its percentage can countries in Asia (India, Vietnam) and Africa (Nigeria, Ivory Coast), CNSL represents an abundant reach 60% in some CNSL grades. Its structure is defined as a mono-aromatic phenol substituted in non-edible by-product that has already found various applications in coatings, laminates and adhesives meta-position by a C15 alkyl chain, on which none to three unsaturations in C8, C11 and C14 are to name a few. However, it also represents an interesting source of aromatic chemical building blocks. In possible [105–107]. Thanks to its hydroxyl function and carbon-carbon double bond(s), cardanol can fact, CNSL is mainly composed of four major phenolic derivatives: anacardic acid, cardanol, cardol and offer various functionalization opportunities, although its long aliphatic chain may severely impact 2-methyl cardol (Figure 9), the percentage of which depends on the extraction method. Among these, materials in terms of thermo-mechanical properties. cardanol is often regarded as the most interesting compound, as its percentage can reach 60% in some

Molecules 2017, 22, 149 11 of 47

CNSL grades. Its structure is defined as a mono-aromatic phenol substituted in meta-position by a C15 alkyl chain, on which none to three unsaturations in C8, C11 and C14 are possible [105–107]. Thanks to its hydroxyl function and carbon-carbon double bond(s), cardanol can offer various functionalization Moleculesopportunities,2017, 22, 149 although its long aliphatic chain may severely impact materials in terms of 12 of 48 thermo-mechanical properties.

Figure 9. Main phenolic constituents of cashew nut shell liquid (from left to right): cardanol, anacardic Figure 9. Main phenolic constituents of cashew nut shell liquid (from left to right): cardanol, anacardic acid, cardol and 2-methyl cardol [104]. acid, cardol and 2-methyl cardol [104]. 4.4. Cellulose and Hemi-Cellulose 4.4. CelluloseAs and previously Hemi-Cellulose stated, cellulose is the most abundant naturally occurring polymer and accounts for Asapproximately previously stated,40–45 wt cellulose % of lignocellulosic is the most biomas abundants depending naturally of wood occurring species [108–111]. polymer With and an accounts annual production estimated around 100 billion tons, it is a remarkable feedstock for the synthesis of for approximately 40–45 wt % of lignocellulosic biomass depending of wood species [108–111]. With renewable chemical building blocks. Cellulose is a crystalline high molecular weight polysaccharide an annual(7000 production to 15,000 monomeric estimated units) around formed 100 of billionD-glucose tons, linked it isby aβ-1,4-glycosidic remarkable feedstockbonds. The forhigh the chain synthesis of renewableorganization chemical through building hydrogen blocks. bonding Cellulose provides is cell a crystallineulose with interesting high molecular mechanical weight properties polysaccharide as (7000 towell 15,000 as poor monomeric solubility units)in water formed and various of D-glucose organic solvents. linked byHowever,β-1,4-glycosidic the several bonds.hydroxyl The groups high chain organizationalong its through backbone hydrogen make cellulose bonding highly provides hydrophilic. cellulose Although with it doesn’t interesting exhibit an mechanical aromatic structure, properties as well as pooraromatic solubility building inblocks water can and be obtained various from organic cellulose solvents. via depolymerization However, the (Scheme several 6) hydroxyl [112–114]. groups Through acid hydrolysis under harsh conditions or bacterial degradation [115] cellulose yields glucose along itsthat backbone can further make isomerize cellulose into fruc highlytose and hydrophilic. then be dehydrated Although into it 5-hydroxymethyl-2-furfural doesn’t exhibit an aromatic (HMF), structure, aromatica buildingfuranic aldehyde. blocks canHMF be can obtained also be fromoxidized cellulose into 2,5-furandicarboxylic via depolymerization acid or (Scheme reduced6 )[into112 –114]. Through2,5-furandimethanol, acid hydrolysisunder two symmetric harsh conditions di-functional or aromatic bacterial moieties. degradation [115] cellulose yields glucose that can furtherThe other isomerize main component into fructose of lignocellulosic and then biomass, be dehydrated namely hemi-cellulose, into 5-hydroxymethyl-2-furfural is an amorphous (HMF),polysaccharide a furanic aldehyde. with short HMF chains can of 500 also to be3000 oxidized monomer into units 2,5-furandicarboxylic with acidic groups. Similarly acid to orcellulose, reduced into it can be depolymerized into pentose that can further be transformed into 2-furfural, 2-furanmethanol 2,5-furandimethanol, two symmetric di-functional aromatic moieties. Moleculesand 2-furancarboxylic 2017, 22, 149 acid. However, these derivatives are mono-functional, thus making them12 of 47 unsuitable for the synthesis of di-epoxy monomers, only fit for reactive diluents. Finally, it is also worth noting that cellulose can also be degraded into other interesting, although non aromatic, building blocks: levulinic and lactic acid.

Scheme 6. Synthesis of furan derivatives from cellulose and hemi-cellulose [112–114]. Scheme 6. Synthesis of furan derivatives from cellulose and hemi-cellulose [112–114]. 4.5. Other Natural Sources of Aromatic Moieties Some other abundant biomasses are potential candidate to extract or synthesize phenolic derivatives. For example, terpenes and terpenoids can be obtained from various plants’ essential oils or as by-products of industrial processes and are largely available at reasonable prices [116–118]. For example, 700 millions of kg of limonene are produced annually as a side-product of orange juice production [117]. Terpenes form a large and diverse class of molecules based on repeating isoprene units that can be linked head to tail or form cycloaliphatic or aromatic rings such as in α-pinene found in pines, limonene from citrus fruits or p-cymene extracted from thyme, only to name a few. Some of them may represent interesting candidates for the synthesis of epoxy monomers such as carvacrol, which exhibits a phenol moiety and a carbon-carbon double bond (Scheme 7) and can be isolated from oregano and thyme essential oils. However, not all terpenoids contain aromatic and/or phenolic moieties, but these requirements can be reached via different synthesis steps. For example, carvacrol can be obtained from other turpentine components such as limonene via an oxidation followed by an isomerization with sulfated zirconia or from p-cymene by the action of concentrated sulfuric acid and sodium hydroxide [117]. Other phenolic compounds can be extracted from various plant essential oils such as 4-allyl-2-methoxyphenol, commonly known as eugenol (Figure 10), obtained from clove oil where it accounts for 80% [119–122], Jamaican chili, cinnamon and bay leaves from California. It is a renewable resource, but it is also considered safe, non-carcinogenic and non-mutagenic by the U.S. Food and Drug Administration and used as a food flavoring agent. Furthermore, it exhibits several pharmacological properties such as anesthetic, antioxidant, and antimicrobial activities. Apart from extraction, eugenol can be synthesized by allylation of guaiacol, another bio-based phenolic derivative. The main advantage of eugenol is that it exhibits a carbon-carbon double bond with fair reactivity and a phenolic group, thus allowing various functionalizations.

Molecules 2017, 22, 149 13 of 48

The other main component of lignocellulosic biomass, namely hemi-cellulose, is an amorphous polysaccharide with short chains of 500 to 3000 monomer units with acidic groups. Similarly to cellulose, it can be depolymerized into pentose that can further be transformed into 2-furfural, 2-furanmethanol and 2-furancarboxylic acid. However, these derivatives are mono-functional, thus making them unsuitable for the synthesis of di-epoxy monomers, only fit for reactive diluents. Finally, it is also worth noting that cellulose can also be degraded into other interesting, although non aromatic, building blocks: levulinic and lactic acid.

4.5. Other Natural Sources of Aromatic Moieties Some other abundant biomasses are potential candidate to extract or synthesize phenolic derivatives. For example, terpenes and terpenoids can be obtained from various plants’ essential oils or as by-products of industrial processes and are largely available at reasonable prices [116–118]. For example, 700 millions of kg of limonene are produced annually as a side-product of orange juice production [117]. Terpenes form a large and diverse class of molecules based on repeating isoprene units that can be linked head to tail or form cycloaliphatic or aromatic rings such as in α-pinene found in pines, limonene from citrus fruits or p-cymene extracted from thyme, only to name a few. Some of them may represent interesting candidates for the synthesis of epoxy monomers such as carvacrol, which exhibits a phenol moiety and a carbon-carbon double bond (Scheme7) and can be isolated from oregano and thyme essential oils. However, not all terpenoids contain aromatic and/or phenolic moieties, but these requirements can be reached via different synthesis steps. For example, carvacrol can be obtained from other turpentine components such as limonene via an oxidation followed by an isomerization with sulfated zirconia or from p-cymene by the action of concentrated sulfuric acid and sodiumMolecules hydroxide 2017, 22, 149 [ 117]. 13 of 47

Scheme 7. Synthesis of carvacrol from limonene or p-cymene according to Harvey et al. [117]. Scheme 7. Synthesis of carvacrol from limonene or p-cymene according to Harvey et al. [117].

Other phenolic compounds can be extracted from various plant essential oils such as 4-allyl-2-methoxyphenol, commonly known as eugenol (Figure 10), obtained from clove oil where it accounts for 80% [119–122], Jamaican chili, cinnamon and bay leaves from California. It is a renewable resource, but it is also considered safe, non-carcinogenic and non-mutagenic by the U.S. Food and Drug Administration and used as a food flavoring agent. Furthermore, it exhibits several pharmacological properties such as anesthetic, antioxidant, and antimicrobial activities. Apart from extraction, eugenol can be synthesized by allylation of guaiacol, another bio-based phenolic derivative.

The main advantage of eugenol is that it exhibits a carbon-carbon double bond with fair reactivity and a phenolic group, thus allowingFigure various 10. Structures functionalizations. of eugenol and ferulic acid.

Lignocellulosic biomass may also contain p-coumaryl, coniferyl and sinapyl acids, depending on the plant species, that act as crosslinkers between lignin and polysaccharides (cellulose and hemi-cellulose) to increase the rigidity of the materials. Among these, 3-methoxy-4-hydroxycinnamic acid (Figure 10), also known as ferulic acid, is an abundant derivative that can be extracted in good yields from various non-food resources such as bagasse, rice, wheat and sugar beet roots [123–126] and has both a hydroxyl group and an aliphatic carboxylic acid, thus enabling functionalization. It also exhibits antioxidative, anti-tumor, photoprotective and anti-hypertensive activities.

5. Bio-Based Aromatic Epoxy Compounds

5.1. Mono-Aromatic Epoxy Compounds Mono-aromatic glycidyl compounds are defined by having strictly one aromatic ring per molecule and by having two or more epoxy groups per molecule. As previously stated, the characteristic structure of BPA, e.g., its length, its two aromatic rings and alcohols functions greatly influence its endocrine disruptor activity. Thus, mono-aromatic molecules with reduced hydrophobic skeleton may potentially be less likely to bind with estrogen receptor, making their epoxy monomers interesting candidates for BPA substitution, however, specific studies would be necessary before these compounds can be proposed as BPA substitutes.

5.1.1. Phenyl-Based Di-Functional Epoxy Monomers Various mono-aromatic phenyl-based epoxy monomers have been considered as potential candidates for the replacement of DGEBA (Figure 11). One of the simplest, diglycidyl ether of resorcinol (1), is obtained from resorcinol, a meta-substituted di-phenol that can be obtained from biomass by

Molecules 2017, 22, 149 13 of 47

Molecules 2017, 22, 149 14 of 48 Scheme 7. Synthesis of carvacrol from limonene or p-cymene according to Harvey et al. [117].

FigureFigure 10. 10.Structures Structures ofof eugenol and and ferulic ferulic acid. acid.

Lignocellulosic biomass may also contain p-coumaryl, coniferyl and sinapyl acids, depending on Lignocellulosic biomass may also contain p-coumaryl, coniferyl and sinapyl acids, depending the plant species, that act as crosslinkers between lignin and polysaccharides (cellulose and on the plant species, that act as crosslinkers between lignin and polysaccharides (cellulose and hemi-cellulose) to increase the rigidity of the materials. Among these, 3-methoxy-4-hydroxycinnamic hemi-cellulose) to increase the rigidity of the materials. Among these, 3-methoxy-4-hydroxycinnamic acid (Figure 10), also known as ferulic acid, is an abundant derivative that can be extracted in good yields acid (Figure 10), also known as ferulic acid, is an abundant derivative that can be extracted in good from various non-food resources such as bagasse, rice, wheat and sugar beet roots [123–126] and has yields from various non-food resources such as bagasse, rice, wheat and sugar beet roots [123–126] and both a hydroxyl group and an aliphatic carboxylic acid, thus enabling functionalization. It also exhibits has both a hydroxyl group and an aliphatic carboxylic acid, thus enabling functionalization. It also antioxidative, anti-tumor, photoprotective and anti-hypertensive activities. exhibits antioxidative, anti-tumor, photoprotective and anti-hypertensive activities. 5. Bio-Based Aromatic Epoxy Compounds 5. Bio-Based Aromatic Epoxy Compounds

5.1. Mono-Aromatic Epoxy Epoxy Compounds Compounds Mono-aromatic glycidyl compounds are defineddefined by having strictly one aromatic ring perper moleculemolecule and by by having having two two or or more more epoxy epoxy groups groups per molecule. per molecule. As previously As previously stated, the stated, characteristic the characteristic structure structureof BPA, e.g., of BPA,its length, e.g., its its length, two aromatic its two aromaticrings and ringsalcohols and functions alcohols functionsgreatly influence greatly its influence endocrine its endocrinedisruptor activity. disruptor Thus, activity. mono-aromatic Thus, mono-aromatic molecules with molecules reduced with hydrophobic reduced hydrophobic skeleton may skeleton potentially may potentiallybe less likely be to less bind likely with to estrogen bind with receptor, estrogen making receptor, their makingepoxy monomers their epoxy interesting monomers candidates interesting for candidatesBPA substitution, for BPA however, substitution, spec however,ific studies specific would studies be wouldnecessary be necessarybefore these before compounds these compounds can be canproposed be proposed as BPA assubstitutes. BPA substitutes.

5.1.1. Phenyl-Based Phenyl-Based Di-F Di-Functionalunctional Epoxy Epoxy Monomers Monomers Various mono-aromaticmono-aromatic phenyl-based epoxy monomers have been consideredconsidered asas potentialpotential candidates for for the the replacement replacement of DGEBA of DGEBA (Figure (Figure 11). One11). of One the simplest, of the simplest, diglycidyl diglycidyl ether of resorcinol ether of resorcinol(1), is obtained (1), is from obtained resorcinol, from resorcinol,a meta-substituted a meta-substituted di-phenol that di-phenol can be thatobtained can befrom obtained biomass from by biomass by fermentation [127] of catechins or by fermentation of glucose into inositol, chemical conversion of the latter into 1,3,5-benzenetriol (or phloroglucinol) and finally reduction. As a part of catechin’s skeleton structure, it has been used as model molecule for catechin characterization [128]. It is commercially available or can be prepared by direct O-glycidylation of resorcinol with good yield (87%) [38]. Because of the resonance and inductive effects of the meta-substituted aromatic ring, diglycidyl ether of resorcinol should be more reactive than its para-substituted analogue, diglycidyl ether of hydroquinone (3)[129]. Its high toxicity may explain why this epoxy has not been much used as material component. Similarly, a substituted resorcinol, methyl-2,4-dihydroxybenzoate has been glycidylated with epibromohydrin (2) with a 78% yield but no materials have been developed from this compound [24]. On the contrary, diglycidyl ether of hydroquinone, the para isomer or resorcinol can be obtained via the microbial synthesis of phloroglucinol or quinic acid from glucose, converted into hydroquinone (or resorcinol) [130]. This epoxy was cured with diethyl toluene diamine (EPIKURE W) and the obtained materials exhibited slightly higher glass transition temperature (Tg) than those based on DGEBA (up to 8 ◦C) [131], being the greatest data obtained with a di-functional epoxy. However, contrary to resorcinol, hydroquinone is carcinogen (H351) and mutagen (H341). Molecules 2017, 22, 149 14 of 47

fermentation [127] of catechins or by fermentation of glucose into inositol, chemical conversion of the latter into 1,3,5-benzenetriol (or phloroglucinol) and finally reduction. As a part of catechin’s skeleton structure, it has been used as model molecule for catechin characterization [128]. It is commercially available or can be prepared by direct O-glycidylation of resorcinol with good yield (87%) [38]. Because of the resonance and inductive effects of the meta-substituted aromatic ring, diglycidyl ether of resorcinol should be more reactive than its para-substituted analogue, diglycidyl ether of hydroquinone (3) [129]. Its high toxicity may explain why this epoxy has not been much used as material component. Similarly, a substituted resorcinol, methyl-2,4-dihydroxybenzoate has been glycidylated with epibromohydrin (2) with a 78% yield but no materials have been developed from this compound [24]. On the contrary, diglycidyl ether of hydroquinone, the para isomer or resorcinol can be obtained via the microbial synthesis of phloroglucinol or quinic acid from glucose, converted into hydroquinone (or resorcinol) [130]. This epoxy was cured with diethyl toluene diamine (EPIKURE W) and the obtained materials exhibited slightly higher glass transition temperature (Tg) than those based on DGEBA (up to 8 °C) [131],

Moleculesbeing 2017the , greatest22, 149 data obtained with a di-functional epoxy. However, contrary to resorcinol,15 of 48 hydroquinone is carcinogen (H351) and mutagen (H341).

FigureFigure 11.11. Mono-aromaticMono-aromatic phenyl-basedphenyl-based di-functional epoxy compounds. compounds.

The same research team prepared the diglycidyl ether of p-xylene alcohol (5) by direct glycidylation The same research team prepared the diglycidyl ether of p-xylene alcohol (5) by direct of the non-toxic 1,4-benzenedimethanol with an overall yield of 90% and a purity of 99% assessed by 1H glycidylationNMR [131]. ofIt thehas non-toxic been cured 1,4-benzenedimethanol with cycloaliphatic with and an aromatic overall yielddiamines, of 90% 4,4 and′-methylene a purity 1 ofbiscyclohexylamine 99% assessed by (PACM)H NMR and [131 diethyl]. It hastoluene been diam curedine with(EPIKURE cycloaliphatic W), respectively and aromatic yielding diamines,materials 4,40-methylene biscyclohexylamine (PACM) and diethyl toluene diamine (EPIKURE W), respectively with Tg values up to 67 °C lower than those of DGEBA-based equivalents, because of the methylene ◦ yieldinglinkages. materials with Tg values up to 67 C lower than those of DGEBA-based equivalents, because of theThe methylene ester form linkages. of the latter compound was prepared by direct glycidylation of the non-toxic terephthalicThe ester acid form [132], of theto form latter digl compoundycidyl ester was of terephthalic prepared by acid direct (6) with glycidylation a yield of 80%. of the This non-toxic epoxy terephthalicwas cured acidwith [132methylhexahydrophthalic], to form diglycidyl ester anhydride of terephthalic and poly(propylene acid (6) with aglycol) yield ofbis(2-aminopropyl 80%. This epoxy wasether). cured The with materials methylhexahydrophthalic showed similar Tg values anhydride as those and based poly(propylene on DGEBAglycol) with both bis(2-aminopropyl curing agents. ether).Furthermore, The materials terephthalic showed acid similar is synthesizedTg values from as thosep-xylene based that on can DGEBA be obtained with bothfrom curing bio-mass agents. by Furthermore,various processes terephthalic [133]. Du acide to isthe synthesized large amount from of pby-products-xylene that using can be both obtained direct glycidylation from bio-mass and by variousallylation processes methods, [133 You]. Dueet al. to [134] the largeprepared amount diglycid of by-productsyl ester of terephthalic using both directacid by glycidylation esterification and of allylationdiacyl chloride methods, by glycidol, You et al. with [134 a] 50% prepared yield. diglycidylDiglycidyl esterester ofof terephthalicterephthalic acid bywas esterification cured with ofcarboxylic diacyl chloride acid derivatives, by glycidol, and the with corresponding a 50% yield. materials Diglycidyl were esterused ofas biocompatible, terephthalic acid biodegradable was cured withand functional carboxylic polymers acid derivatives, for biomedical and theapplications corresponding. No thermal materials properties were were used reported. as biocompatible, However, biodegradablethe low hydrolytic and functionalstability of polymersaromatic ester for biomedical groups is expected applications. to negative No thermally impact properties the potential were reported.materials However,[134]. the low hydrolytic stability of aromatic ester groups is expected to negatively impactThe the double potential functionalization materials [134 of]. eugenol (7) requires a three-step route: (i) protection of the hydroxy groupThe via double an acetylation functionalization using acetic ofanhydr eugenolide; (ii) (7) oxidation requires of a three-stepthe double bond route: using (i) protection mCPBA and of (iii) the hydroxy group via an acetylation using acetic anhydride; (ii) oxidation of the double bond using mCPBA and (iii) glycidylation by deacetylation using epichlorohydrin. The resulting solid was obtained with an overall yield of 53% [119]. The epoxy was cured with anhydrides and the obtained materials showed similar thermal and mechanical properties than those of DGEBA equivalents. For example, when diglycidyl ether of eugenol was cured with hexahydrophtalic anhydride, the ◦ ◦ obtained Tg reached 114 C, which was slightly higher than that of DGEBA counterparts (106 C). Eugenol can be used as a precursor for vanillin synthesis. The latter is an important bio-based phenolic aldehyde, which is the main component of vanilla bean extract, widely used as flavouring in food, beverages and pharmaceuticals [135]. It is also a bio-sourced chemical derived from lignin currently produced by Borregaard [123] that easily allows to lead to diepoxy compounds, the structures of which are showed in Figure 12. All diglycidyl ethers deriving from vanillin can be prepared by direct O-glycidylation of the corresponding alcohol derivatives, with good yield (85%–89%) [27]. It is interesting to note that the alcohol derivatives, e.g., 4-hydroxy-3-methoxybenzyl alcohol, 2-methoxyhydroquinone and 4-hydroxy-3-methoxybenzoic acid are all commercially available, Molecules 2017, 22, 149 15 of 47

glycidylation by deacetylation using epichlorohydrin. The resulting solid was obtained with an overall yield of 53% [119]. The epoxy was cured with anhydrides and the obtained materials showed similar thermal and mechanical properties than those of DGEBA equivalents. For example, when diglycidyl ether of eugenol was cured with hexahydrophtalic anhydride, the obtained Tg reached 114 °C, which was slightly higher than that of DGEBA counterparts (106 °C). Eugenol can be used as a precursor for vanillin synthesis. The latter is an important bio-based phenolic aldehyde, which is the main component of vanilla bean extract, widely used as flavouring in food, beverages and pharmaceuticals [135]. It is also a bio-sourced chemical derived from lignin currently produced by Borregaard [123] that easily allows to lead to diepoxy compounds, the structures of which are showed in Figure 12. All diglycidyl ethers deriving from vanillin can be prepared by direct

MoleculesO-glycidylation2017, 22, 149 of the corresponding alcohol derivatives, with good yield (85%–89%) [27].16 It of 48is interesting to note that the alcohol derivatives, e.g., 4-hydroxy-3-methoxybenzyl alcohol, 2-methoxyhydroquinone and 4-hydroxy-3-methoxybenzoic acid are all commercially available, non-toxic and also used forfor foodfood flavouring.flavouring. The threethree diglycidyldiglycidyl ether compoundscompounds were cured withwith isophorone diamine diamine (IPDA) (IPDA) and and showed showed Tg Tvaluesg values lower lower than than that thatof DGEBA of DGEBA by 69 by (9), 69 34 (9 (),8) 34 and (8 )14 and °C ◦ ◦ 14(10).C( The10 )authors. The authors considered considered that the that presence the presence of a methylene of a methylene spacer spacer decreased decreased Tg by T35g °Cby 35and Cwhen and ◦ whencarbonyl carbonyl group groupwas used was usedas spacer, as spacer, Tg increasedTg increased by 20 by °C. 20 AdditionaC. Additionally,lly, when when diglycidyl diglycidyl ether ether of ofhydroquinone hydroquinone and and diglycidyl diglycidyl ether ether of of methoxyhydroquinone methoxyhydroquinone were were compared, compared, the the presence presence of thethe ◦ methoxy group involved a a decrease decrease of of TTgg byby 25 25 °C.C. Similarly, Similarly, when when comparing comparing (8 () 8and) and diglycidyl diglycidyl ether ether of ofeugenol eugenol (7), ( 7it), seems it seems that that the theloss loss of the of the(CH (CH2-O-)2- Ogroup-) group between between the oxirane the oxirane ring ringand andthe aromatic the aromatic ring ringallows allows to reach to reach higher higher glass tran glasssition transition temperatures, temperatures, closer to closer those to of those DGEBA-based of DGEBA-based materials. materials.

Figure 12. Diglycidyl ether compounds obtained from vanillin: ( 8)) diglycidyl ether of vanillylvanillyl alcohol, ((9) diglycidyl ether ether of of methoxyhydroquinone methoxyhydroquinone and and (10 (10) diglycidyl) diglycidyl ether ether of vanillic of vanillic acid acid [135]. [135 ].

Recently, Hernandez et al. [17] continued the researches to determine the influence of the methoxy Recently, Hernandez et al. [17] continued the researches to determine the influence of the methoxy group of diglycidyl ether of vanillyl alcohol (DGEVA) (9) by comparing it with diglycidyl ether of group of diglycidyl ether of vanillyl alcohol (DGEVA) (9) by comparing it with diglycidyl ether of gastrodigenin (DGEGD) (4). When cured with 4,4′-methylbiscyclohexylamine, the DGEVA-based gastrodigenin (DGEGD) (4). When cured with 4,40-methylbiscyclohexylamine, the DGEVA-based materials exhibited a lower Tg than the DGEGD ones due to the methoxy moiety, but it appears that it materials exhibited a lower T than the DGEGD ones due to the methoxy moiety, but it appears that it enables to obtain a higher rigidityg in the glassy state, as observed via the higher storage modulus. enables to obtain a higher rigidity in the glassy state, as observed via the higher storage modulus. Following its previous work on lignin depolymerisation using hydrogenation in the presence of Following its previous work on lignin depolymerisation using hydrogenation in the presence of Zn/Pd catalysts [136,137] the team of Abu-Omar [138] synthesized di-epoxides from propylcatechol (11) Zn/Pd catalysts [136,137] the team of Abu-Omar [138] synthesized di-epoxides from propylcatechol (Scheme 8). As previously explained, the presence of hydroxyl groups in ortho position will yield (11) (Scheme8). As previously explained, the presence of hydroxyl groups in ortho position will yield benzodioxane derivatives during the glycidylation step, but the authors claimed to have improved the benzodioxane derivatives during the glycidylation step, but the authors claimed to have improved synthesis conditions in terms of epoxy equivalent weight and mass ratio. The epoxy monomer was then the synthesis conditions in terms of epoxy equivalent weight and mass ratio. The epoxy monomer mixed with octadecylamine-modified nano-montmorillonite and cured with diethyl triamine. The was then mixed with octadecylamine-modified nano-montmorillonite and cured with diethyl triamine. bio-based polymers were characterized in terms of thermal stability and mechanical properties but no The bio-based polymers were characterized in terms of thermal stability and mechanical properties but possible leaching of the benzodioxane derivatives non-integrated in the network was discussed. no possible leaching of the benzodioxane derivatives non-integrated in the network was discussed. Another abundant and natural product is rosin. With a production of approximately 1.2 million of tons per year [19], this compound is obtained by heating fresh tree resin to remove the volatile liquid terpenes. It thus contains a mixture of isomerized acid with large hydrogenated phenanthrene ring structures providing them with high rigidity. Most of them do not contain aromatic structures, but Liu et Zhang [139] recently synthesized a mono-aromatic di-glycidyl ester based on rosin acid (12) (Figure 13), cured it with 1,2-cyclohexanedicarboxylic anhydride and compared the results with DER332, an epoxy resin from Dow Chemical Company cured in the same conditions. The resulting ◦ material exhibited a Tg of 154 C slightly higher than its counterpart, and a good thermal stability with a temperature of 5% weight loss of 311 ◦C. Molecules 2017, 22, 149 16 of 47

Molecules 2017, 22, 149 17 of 48 Molecules 2017, 22, 149 16 of 47 Scheme 8. Synthetic pathway to glycidylated propylcatechol as described by Zhao and Abu-Omar [138].

Another abundant and natural product is rosin. With a production of approximately 1.2 million of tons per year [19], this compound is obtained by heating fresh tree resin to remove the volatile liquid terpenes. It thus contains a mixture of isomerized acid with large hydrogenated phenanthrene ring structures providing them with high rigidity. Most of them do not contain aromatic structures, but Liu et Zhang [139] recently synthesized a mono-aromatic di-glycidyl ester based on rosin acid (12) (Figure 13), cured it with 1,2-cyclohexanedicarboxylic anhydride and compared the results with DER332, an epoxy resin from Dow Chemical Company cured in the same conditions. The resulting material exhibited a Tg Scheme 8. Synthetic pathway to glycidylated propylcatechol as described by Zhao and Abu-Omar [138]. of 154Scheme °C slightly 8. higherSynthetic than pathway its counterpart, to glycidylated and a good propylcatechol thermal stability as described with a temperature by Zhao and of 5% weightAbu-Omar loss of 311 [138 °C.]. Another abundant and natural product is rosin. With a production of approximately 1.2 million of tons per year [19], this compound is obtained by heating fresh tree resin to remove the volatile liquid terpenes. It thus contains a mixture of isomerized acid with large hydrogenated phenanthrene ring structures providing them with high rigidity. Most of them do not contain aromatic structures, but Liu et Zhang [139] recently synthesized a mono-aromatic di-glycidyl ester based on rosin acid (12) (Figure 13), cured it with 1,2-cyclohexanedicarboxylic anhydride and compared the results with DER332, an epoxy resin from Dow Chemical Company cured in the same conditions. The resulting material exhibited a Tg of 154 °C slightly higher than its counterpart, and a good thermal stability with a temperature of 5% weight loss of 311 °C.

FigureFigure 13. 13.Chemical Chemical structurestructure of a rosin-based diepoxy diepoxy compound compound [139]. [139 ].

5.1.2. Phenyl-Based Poly-Functional Epoxy Monomers 5.1.2. Phenyl-Based Poly-Functional Epoxy Monomers Phloroglucinol is a bio-based tri-phenol and is used as an active ingredient in medicines Phloroglucinol is a bio-based tri-phenol and is used as an active ingredient in medicines (SPASFON). The glycidylated compound (13) (Figure 14) was prepared by direct glycidylation with a (SPASFON). The glycidylated compound (13) (Figure 14) was prepared by direct glycidylation with yield of 68%, leading to a mixture mainly containing triglycidyl ether of phloroglucinol [140]. Its a yield of 68%, leading to a mixture mainly containing triglycidyl ether of phloroglucinol [140]. symmetric tridimensional and tri-functional meta-substituted structure makes it the most reactive epoxy Its symmetric tridimensional and tri-functional meta-substituted structure makes it the most reactive [129]. The reached materials exhibited much higher Tg than those of DGEBA, up to 60 °C for ◦linear epoxy [129]. The reached materials exhibited much higher Tg than those of DGEBA, up to 60 C for aliphatic [141] and 20 °C for cycloaliphatic diamines [142]. linear aliphatic [141Figure] and 2013. ◦ChemicalC for cycloaliphatic structure of a rosin-based diamines [diepoxy142]. compound [139]. Another simple tri-functional glycidyl ether is based on pyrogallol (14), which is also a bio-based Another simple tri-functional glycidyl ether is based on pyrogallol (14), which is also a molecule5.1.2. Phenyl-Based obtained Poly-Ffromunctional decarboxylation Epoxy Monomers of gallic acid extracted from hydrolysable tannins. bio-based molecule obtained from decarboxylation of gallic acid extracted from hydrolysable tannins. Unfortunately, it is mutagen (H341) and its structure produces more bulkiness than its meta-substituted Unfortunately,Phloroglucinol it is mutagen is a bio-based (H341) and tri-phenol its structure and producesis used moreas an bulkiness active ingredient than its meta in-substituted medicines analogue. Moreover, as previously described, the ortho position of the phenol groups yields a 50:50 analogue.(SPASFON). Moreover, The glycidylated as previously compound described, (13) (Figure the ortho 14)position was prepared of the phenolby direct groups glycidylation yields awith 50:50 a mixture of triglycidyl ether of pyrogallol and benzodioxane derivatives, with an overall yield of 66% [38]. mixtureyield of of68%, triglycidyl leading etherto a mixture of pyrogallol mainly and containi benzodioxaneng triglycidyl derivatives, ether of withphloroglucinol an overall [140]. yield Its of The low yield and by-products synthesis were likely the reason why no material has been prepared from 66%symmetric [38]. The tridimensional low yield and and by-products tri-functional synthesis meta-substituted were likely structure the reason makes why it the no most material reactive has epoxy been this epoxy. prepared[129]. The from reached this epoxy.materials exhibited much higher Tg than those of DGEBA, up to 60 °C for linear Similarly, protocatechuic acid, which is a relatively toxic (H315, H319, H335) major metabolite of aliphaticSimilarly, [141] and protocatechuic 20 °C for cycloaliphatic acid, which diamines is a relatively [142]. toxic (H315, H319, H335) major metabolite of antioxidant and anti-inflammatory bio-based polyol found in green tea, yields a mixture of 60% of the antioxidantAnother and simple anti-inflammatory tri-functional glycid bio-basedyl ether polyol is based found on in pyrogallol green tea, ( yields14), which a mixture is also of a 60% bio-based of the triglycidyl ether (15) and 9% of the benzodioxane derivative [38]. The presence of the carboxyl group in triglycidylmolecule obtained ether (15 ) andfrom 9% decarboxylation of the benzodioxane of gal derivativelic acid [38extracted]. The presence from hydrolysable of the carboxyl tannins. group meta-position apparently limited the cyclization compared to pyrogallol. Despite the good yield, no inUnfortunately, meta-position it is apparently mutagen (H341) limited and the its cyclization structure produces compared more to pyrogallol. bulkiness than Despite its meta the-substituted good yield, material has been reported. noanalogue. material Moreover, has been reported.as previously described, the ortho position of the phenol groups yields a 50:50 mixtureAnother of triglycidyl triglycidylated ether of compoundpyrogallol and was benzodio synthesizedxane from derivatives, trimellitic with acid an ( 16overall), which yield is of a relatively66% [38]. toxic The low triacid. yield The and epoxy by-products form was synthesis found were at 25% likely in a the mixture reason with why 75% no material of diglycidyl has been terephthalate prepared from acid ester,this epoxy. supplied by Vantico-Switzerland. This mixture was used as polyester curing agent for coating applications.Similarly, No protocatechuic thermal properties acid, which of the is intrinsic a relatively materials toxic have(H315, been H319, reported H335) [major143,144 metabolite], but a low of hydrolyticantioxidant stability and anti-inflammatory is expected similarly bio-based to the polyol diglycidyl found esterin green of terephthalic tea, yields a acid mixture (6). of 60% of the triglycidyl ether (15) and 9% of the benzodioxane derivative [38]. The presence of the carboxyl group in meta-position apparently limited the cyclization compared to pyrogallol. Despite the good yield, no material has been reported.

Molecules 2017, 22, 149 17 of 47

Another triglycidylated compound was synthesized from trimellitic acid (16), which is a relatively toxic triacid. The epoxy form was found at 25% in a mixture with 75% of diglycidyl terephthalate acid ester, supplied by Vantico-Switzerland. This mixture was used as polyester curing agent for coating applications.Molecules 2017, 22No, 149 thermal properties of the intrinsic materials have been reported [143,144], but 18a oflow 48 hydrolytic stability is expected similarly to the diglycidyl ester of terephthalic acid (6).

Figure 14. Mono-aromaticMono-aromatic phenyl-based poly-functional epoxy compounds.

Gallic acid is a bio-based, trihydroxybenzoic acid compound encountered in the plant hydrolysable Gallic acid is a bio-based, trihydroxybenzoic acid compound encountered in the plant tannins, as gallic acid derivatives (esters, glycosides) or as the acyl group of some polyols (glucose, quinic hydrolysable tannins, as gallic acid derivatives (esters, glycosides) or as the acyl group of some acid). It is as toxic as protocatechuic acid and is the only mono-aromatic compound with four functions polyols (glucose, quinic acid). It is as toxic as protocatechuic acid and is the only mono-aromatic that can be glycidylated. Tetraglycidyl ether of gallic acid (17) can be prepared by direct glycidylation or compound with four functions that can be glycidylated. Tetraglycidyl ether of gallic acid (17) can be allylation [22,145]. The direct glycidylation of gallic acid allowed to reach the targeted product with a prepared by direct glycidylation or allylation [22,145]. The direct glycidylation of gallic acid allowed yield of 68% [22,146,147], whereas, the allylation route led to a better control of the functionalization, but to reach the targeted product with a yield of 68% [22,146,147], whereas, the allylation route led to a a lower overall yield of tetraglycidyl derivative of 48%. The materials cured using isophorone diamine better control of the functionalization, but a lower overall yield of tetraglycidyl derivative of 48%. reached Tg values of 73 °C higher than that of DGEBA, which is the highest value obtained for The materials cured using isophorone diamine reached T values of 73 ◦C higher than that of DGEBA, mono-aromatic epoxy compounds [22]. g which is the highest value obtained for mono-aromatic epoxy compounds [22]. Cardanol has also been considered as a BPA substitute for epoxy monomer synthesis. The Cardanol has also been considered as a BPA substitute for epoxy monomer synthesis. conversion of the C=C double bonds to epoxide groups can be almost complete by reaction with The conversion of the C=C double bonds to epoxide groups can be almost complete by reaction hydrogen peroxide in the presence of formic acid and p-toluenesulfonic acid, with a yield of 82%. The with hydrogen peroxide in the presence of formic acid and p-toluenesulfonic acid, with a yield of resulting product is only glycidylated along the alkyl chain and is used for its antioxidative activity in 82%. The resulting product is only glycidylated along the alkyl chain and is used for its antioxidative soybean oil [106]. activity in soybean oil [106]. Fully epoxidized cardanol (18) (Figure 15) was prepared by direct O-glycidylation of the phenol, Fully epoxidized cardanol (18) (Figure 15) was prepared by direct O-glycidylation of the phenol, followed by double bonds oxidation using mCPBA (78%) or perbenzoic acid and used as a natural followed by double bonds oxidation using mCPBA (78%) or perbenzoic acid and used as a natural plasticizer for PVC films [105] or as diluent to improve the mechanical properties of DGEBA cured by plasticizer for PVC films [105] or as diluent to improve the mechanical properties of DGEBA cured by phthalic anhydride for glass-fiber reinforcement applications. For this latter application, Tg increased phthalic anhydride for glass-fiber reinforcement applications. For this latter application, T increased with the amount of epoxidized cardanol, from 145 to 180 °C using 40% of diepoxy cardanolg [107]. No with the amount of epoxidized cardanol, from 145 to 180 ◦C using 40% of diepoxy cardanol [107]. epoxy/amine-based material has been reported, likely because the epoxy groups along the alkyl chain No epoxy/amine-based material has been reported, likely because the epoxy groups along the alkyl exhibit a lower reactivity toward amines. In fact, glycidyl ether compounds are found to be the most chain exhibit a lower reactivity toward amines. In fact, glycidyl ether compounds are found to be the reactive species towards amine compounds because of the inductive effect of the oxygen on the most reactive species towards amine compounds because of the inductive effect of the oxygen on the epoxy-ring and the fact that the ether oxygen is able to form hydrogen bonds with the amine [129]. An interesting approach is thus to combine both characteristics in one molecule. Molecules 2017, 22, 149 18 of 47

Moleculesepoxy-ring2017, 22, 149 and the fact that the ether oxygen is able to form hydrogen bonds with the amine [129]. An 19 of 48 interestingMolecules 2017 approach, 22, 149 is thus to combine both characteristics in one molecule. 18 of 47

epoxy-ring and the fact that the ether oxygen is able to form hydrogen bonds with the amine [129]. An interesting approach is thus to combine both characteristics in one molecule.

Figure 15. Theoretical structure of epoxidized cardanol. Figure 15. Theoretical structure of epoxidized cardanol.

Triglycidyl ether of vanillylamine (19) (Figure 16) is the only tri-functional mono-phenolic Figure 15. Theoretical structure of epoxidized cardanol. Triglycidylglycidylated ether derivative of vanillylamine synthesized from ( 19the) bio-based (Figure and16) non-toxic is the onlyvanillin tri-functional reported in the mono-phenolicliterature [148]. It was prepared in a four-step synthesis from hydroxylamine hydrochloride: (i) aldoximation of glycidylatedTriglycidyl derivative ether synthesized of vanillylamine from (19 the) (Figure bio-based 16) is andthe only non-toxic tri-functional vanillin mono-phenolic reported in the vanillin; (ii) reduction of the aldoxime using dihydrogen in the presence of Pd/C; (iii) neutralization in glycidylated derivative synthesized from the bio-based and non-toxic vanillin reported in the literature literaturebasic [148 medium]. It leading was preparedto a potential in alkanolamine a four-step hardener; synthesis (iv) direct from N-glycidylation hydroxylamine of the hydrochloride:resulting [148]. It was prepared in a four-step synthesis from hydroxylamine hydrochloride: (i) aldoximation of (i) aldoximationvanillylamine, of with vanillin; an overall (ii) reductionyield of 61%. of The the materials aldoxime cured using with dihydrogenisophorone diamine in the showed presence of vanillin; (ii) reduction of the aldoxime using dihydrogen in the presence of Pd/C; (iii) neutralization in Pd/C; (iii)similar neutralization Tg than that of triglycidyl in basic mediumether of phloroglucinol. leading to a potential alkanolamine hardener; (iv) direct basic medium leading to a potential alkanolamine hardener; (iv) direct N-glycidylation of the resulting N-glycidylation of the resulting vanillylamine, with an overall yield of 61%. The materials cured with vanillylamine, with an overall yield of 61%. The materials cured with isophorone diamine showed isophoronesimilar diamine Tg than that showed of triglycidyl similar etherTg ofthan phloroglucinol. that of triglycidyl ether of phloroglucinol.

Figure 16. Aromatic derivatives with epoxy groups grafted on nitrogen atoms.

5.1.3. Epoxy Monomers Based on Other Aromatic Rings Figure 16. Aromatic derivatives with epoxy groups grafted on nitrogen atoms. FurfurylFigure diglycidyl 16. Aromatic ether derivatives (20) (Figure with 17) epoxy was groupsprepared grafted by ondirect nitrogen glycidylation atoms. of the 5-hydroxymethyl-2-furfural,5.1.3. Epoxy Monomers Base dwith on Other a yield Aromatic of 60% Ringsand a purity of 99%—however 5-hydroxymethyl-2- 5.1.3. Epoxyfurfural Monomers is a toxic substance. Based on Hu Other et al. [149] Aromatic found Ringsthat the overall reactivity of furan derivatives was Furfuryl diglycidyl ether (20) (Figure 17) was prepared by direct glycidylation of the significantly greater than that of the phenyl analogue and led to materials with a glass transition 5-hydroxymethyl-2-furfural, with a yield of 60% and a purity of 99%—however 5-hydroxymethyl-2- Furfuryltemperature diglycidyl higher than ether their (20benzene) (Figure analogs. 17 )Ho waswever, prepared in a recently by corrected direct glycidylationarticle [150], the of the furfural is a toxic substance. Hu et al. [149] found that the overall reactivity of furan derivatives was 5-hydroxymethyl-2-furfural,research group changed their with observations a yield of and 60% conclusions and a purity as the benzyl-based of 99%—however materials showed 5-hydroxymethyl-2 higher significantly greater than that of the phenyl analogue and led to materials with a glass transition Tg values than the furan-based ones, due to a higher freedom degree of the furan ring and the hydrogen -furfuraltemperature is a toxic substance.higher than Hutheir et benzene al. [149 analogs.] found Ho thatwever, the overallin a recently reactivity corrected of furanarticle derivatives[150], the was bonding between atoms in the furan rings and hydroxyl groups created by the epoxy opening. significantlyresearch greater group changed than that their of observations the phenyl and analogueconclusions andas the led benzyl-based to materials materials with showed a glass higher transition In another article, their further investigation on furfuryl diglycidyl ether cured with temperatureTg values higher than the than furan-based their benzene ones, due to analogs. a higher freedom However, degree in of athe recently furan ring corrected and the hydrogen article [150], difurfurylamine led them to conclude that each methylene spacer per ring reduced the Tg by 32–34 °C bonding between atoms in the furan rings and hydroxyl groups created by the epoxy opening. the researchand thatgroup furanyl changed structure their led to observations higher freedom and degree, conclusions which decreased as the Tg benzyl-based [151]. When compared materials with showed In another article, their further investigation on furfuryl diglycidyl ether cured with higher thoseTg values obtained than from theDGEBA, furan-based the mono-a ones,romaticdue furan-based to a higher materials freedom showed adegree Tg in between of the 66 furanand ring difurfurylamine led them to conclude that each methylene spacer per ring reduced the Tg by 32–34 °C and the97 hydrogen °C lower. bonding between atoms in the furan rings and hydroxyl groups created by the and that furanyl structure led to higher freedom degree, which decreased Tg [151]. When compared with epoxy opening. those obtained from DGEBA, the mono-aromatic furan-based materials showed a Tg in between 66 and In97 another °C lower. article, their further investigation on furfuryl diglycidyl ether cured with ◦ difurfurylamine led them to conclude that each methylene spacer per ring reduced the Tg by 32–34 C and that furanyl structure led to higher freedom degree, which decreased Tg [151]. When compared with those obtained from DGEBA, the mono-aromatic furan-based materials showed a T in between g 66 and 97 ◦C lower. Furfuryl diglycidyl ester (21) was prepared by allylation and epoxidation of the bio-based but irritant 2,5-furandicarboxylic acid, in an overall yield of 59% [132]. The authors reported that the furandicarboxylic-based materials displayed higher curing reactivity, higher Tg, similar mechanical properties and thermal stability than those based on terephtalic derivatives. The freedom degree of Molecules 2017, 22, 149 20 of 48 furandicarboxylic structure is lower than with a methylene spacer, which is consistent with previous observations [135,151], but lower stability toward hydrolysis is expected. Molecules 2017, 22, 149 19 of 47

Figure 17. Furan-based diglycidylated compounds. Figure 17. Furan-based diglycidylated compounds. Furfuryl diglycidyl ester (21) was prepared by allylation and epoxidation of the bio-based but N,N0-Diglycidylirritant 2,5-furandicarboxylic furfurylamine acid, (22 in) an was overall synthesized yield of 59% by double[132]. TheN authors-glycidylation reported that of thethe relatively furandicarboxylic-based materials displayed higher curing reactivity, higher Tg, similar mechanical toxic 2-aminomethylfurfuran,properties and thermal stability with an than overall those based yield on of terephtalic 50% [152 derivatives.]. When theThe resultingfreedom degree multi-functional of epoxy wasfurandicarboxylic cured with maleimide structure is lower and than anhydride with a me species,thylene spacer, stable which thermally is consistent reversible with previous linkages were formed, leadingobservations to self-healing [135,151], but lower thermosetting stability toward materials. hydrolysis is expected. RecentlyN Liu,N′-Diglycidyl et al. [153 furfurylamine] developed (22) awas six-armed synthesized epoxyby double resin N-glycidylation (23) based of onthe linoleicrelatively acid and toxic 2-aminomethylfurfuran, with an overall yield of 50% [152]. When the resulting multi-functional hexamethylolepoxy melamine was cured inwith a two-stepmaleimide processand anhydride of esterification species, stable and thermally epoxidation reversible with linkages H2 Owere2 (Figure 18). Their goalformed, was to leading reduce to theself-healing high flexibility thermosetting induced materials. by the long aliphatic chain of the vegetable oils by introducing aRecently rigid aromatic Liu et al. group. [153] developed Resins werea six-armed cured epoxy using resin 4-methyl (23) based hexahydrophthalic on linoleic acid and anhydride hexamethylol melamine in a two-step process of esterification and epoxidation with H2O2 (Figure 18). and 1,8-diazabicyclo[5.4.0]undec-7-ene as a catalyst and exhibited high degrees of cure, Tg values Their goal was◦ to reduce the high flexibility induced by the long aliphatic chain of the vegetable oils by between 28introducing and 42 Ca rigid higher aromatic than group. those Resins observed were whencured using epoxidized 4-methylsucrose hexahydrophthalic soyate was anhydride used. However, ◦ their temperatureand 1,8-diazabicyclo[5.4.0]undec-7-ene at 5% weight loss turned as a out catalyst to be and from exhibited 20–90 highC degrees lower, of thus cure, indicating Tg values a poorer thermo-oxidativebetween 28 stability. and 42 °C Withhigher similarthan those hardener observed when content, epoxidized the resinsucrose presented soyate was used. a bio-based However, content of 70% vs. 80%their for temperature the sucrose at 5% soyate-based weight loss turned one. out to be from 20–90 °C lower, thus indicating a poorer thermo-oxidativeMolecules 2017, 22, stability.149 With similar hardener content, the resin presented a bio-based content20 ofof 47 70% vs. 80% for the sucrose soyate-based one.

5.1.4. Conclusions

Bio-based mono-aromatic epoxy materials show interesting properties in terms of Tg and thermal stability. While cardanol-based materials exhibit flexibility and low Tg values, tri- and tetraglycidyl ether, led to materials with higher Tg than DGEBA-based materials. Among the di-functional mono-aromatic substances, only the materials based on epoxy with no spacer and separated by a carbonyl group showed higher Tg. Unfortunately, since most of these epoxides are in development, their toxicity is not known and at that moment, little information is available on the potential endocrine disruption of their precursors. Tables 1 and 2 gather the known data on materials based-on mono-aromatic di- and poly-epoxy monomers from renewable resources.

Figure 18. Structure of the glycidylated hexa(linoleoyl hydroxymethyl) melamine [153]. Figure 18. Structure of the glycidylated hexa(linoleoyl hydroxymethyl) melamine [153]. Table 1. Mono-aromatic, di-epoxy monomers and thermal properties of the cured materials. Tg

values are indicated in plain text, Tα are in italics. 5.1.4. Conclusions Tg (°C) or Tα (°C) Epoxy Curing Agent DGEBA Td,5% (°C) Reference Bio-based mono-aromatic epoxy materials show interestingMaterials properties in terms of T and thermal Comparison g stability. While cardanol-based materials exhibit flexibility and low Tg values, tri- and tetraglycidyl

2,5-difluoroterephthalic acid 69 66 242 [154]

2,5-difluoroterephthalic acid 74 66 305 Diethyl toluene diamine 193 a 185 a/198 b - EPIKURE W

4,4′-methylene 100 a/111 b 167 a/176 b - [150] biscyclohexylamine PACM Diethyl toluene diamine 132 a/140 b 185 a/198 b - EPIKURE W Methyhexahydrophthalic 129 b 125 284 anhydride MHHPA [132] Poly(propylene glycol) 92 b 97 266 bis(2-aminopropyl ether) D230

4,4′-methylenebiscyclohexylamine 100 a/107 b 149 a/158 b 341 [17]

Isophorone Diamine 97/106 b 166/182 b 361 c [27,135]

Molecules 2017, 22, 149 21 of 48

ether, led to materials with higher Tg than DGEBA-based materials. Among the di-functional mono-aromatic substances, only the materials based on epoxy with no spacer and separated by TableTable 1.1. Mono-aromatic,Mono-aromatic, di-epoxydi-epoxy monomersmonomers andand therthermalmal propertiesproperties ofof thethe curedcured materials.materials. TTgg a carbonylvaluesvalues areare group indicatedindicated showed inin plainplain higher text,text, TTαα areareT g inin. italics. Unfortunately,italics. since most of these epoxides are in development, Table 1. Mono-aromatic,Mono-aromatic, di-epoxydi-epoxy monomersmonomers andand therthermal properties of the cured materials. Tgg theirTable toxicity 1. Mono-aromatic, is not known di-epoxy and monomers at that and moment, thermal properties little information of the cured materials. is available Tg on the potential values are indicated in plain text, Tα areare inin italics.italics. Tg (°C)Tg (°C) or T orα (°C) Tα (°C) valuesTable 1.are Mono-aromatic, indicated in plain di-epoxy text, T αmonomers are in italics. and thermal properties of the cured materials. Tg endocrine disruption of their precursors. Tables1 and2 gather the knownT datad,5% on materials based-on values areEpoxy indicated in plain text, Tα areCuring Curingin italics. Agent Agent Tgg (°C)(°C) oror Tα (°C)(°C) Td,5% (°C) Reference Reference Table 1. Mono-aromatic, di-epoxy monomers and thermal propertiesTg (°C) or of T DGEBAαthe (°C)DGEBA cured materials.(°C) Tg mono-aromatic di- and poly-epoxy monomers fromMaterials renewableMaterials resources. values areEpoxy indicated in plain text, Tα areCuring in italics. Agent g Comparisonα Tdd,5%5% (°C) Reference Epoxy Curing Agent T (°C) or T (°C)Comparison Td,5% (°C) Reference Table 1. Mono-aromatic, di-epoxy monomers and thermal properties of DGEBAthe cured materials. Tg Table 1. Mono-aromatic, di-epoxy monomers and thermalMaterials properties ofDGEBA the cured materials. Tg Epoxy Curing Agent MaterialsTg (°C) or TComparisonα (°C) Td,5% (°C) Reference values are indicated in plain text, Tα are in italics. ComparisonDGEBA Tablevalues are 1. Mono-aromatic, indicated in plain text, di-epoxy Tα are in monomers italics. andMaterials thermal properties of the cured materials. Tg values Epoxy Curing Agent Comparison Td,5% (°C) Reference T Tg (°C) or TDGEBAα (°C) are indicated in plain text, 2,5-difluoroterephthalic2,5-difluoroterephthalicα are in italics. acid acid Materials 69Tg 69(°C) or Tα (°C) 66 66 242 242 Comparison Epoxy Curing Agent Td,5% (°C) Reference[154][154] Epoxy 2,5-difluoroterephthalicCuring Agent acid 69 DGEBA 66 Td,5% 242 (°C) Reference 2,5-difluoroterephthalic acid Materials 69 DGEBA◦ 66 ◦ 242 TComparisong ( C) or T α ( C) Materials [154][154] ◦ Epoxy 2,5-difluoroterephthalicCuring Agent acid 69 Comparison 66 242 [154]Td,5% ( C) Reference

2,5-difluoroterephthalic2,5-difluoroterephthalic acid acid Materials 74 74 DGEBA66 66 Comparison 305 305 [154] 2,5-difluoroterephthalic acid 69 66 242

2,5-difluoroterephthalic acid 74 66 305 [154] 2,5-difluoroterephthalicDiethyl toluene diamine acid 74 66 305 Diethyl2,5-difluoroterephthalic toluene diamine EPIKURE acid W 69193 193 a a 185 66185 a /198 a/198 b b 242 - - 2,5-difluoroterephthalicEPIKURE W acid 74 66 305 2,5-difluoroterephthalic2,5-difluoroterephthalic acid acid 69 69 66 66 242 [154] 242 Diethyl toluene diamine [154] Diethyl toluene diamine aa aa bb 193 a 185185 a/198/198 b -- [154] 4,42,5-difluoroterephthalic′-methyleneEPIKURE biscyclohexylamine W acid 74193 185 66 /198 305 - Diethyl4,4EPIKURE toluene′-methylene diamine W a b a b [150] 100193100 a /111a /111 b 185167 a/198167 a/176 b /176 b - - - [150] biscyclohexylamineEPIKUREPACM W PACM 2,5-difluoroterephthalic2,5-difluoroterephthalicDiethyl toluene diamine acid acid 74 74 66 66 305 305 4,4′′-methylene-methylene a a b 2,5-difluoroterephthalic4,4′-methylene acid 193 74aa bb 185 /198 66aa bb - 305 100 a/111/111 b 167 a/176/176 b -- [150][150] EPIKURE W 100 /111a b 167 /176a b - [150] Diethylbiscyclohexylamine Diethyltoluene4,4′-methylene diamine toluene PACM EPIKURE W 132 /140a 185 /198 a b- biscyclohexylamineDiethylDiethyl toluene toluene diamine diamine PACM a 193b a b 185 /198 - diamine EPIKURE W 100132193 /111 a /140a b 167185185 a/176/198 a/198 b b -- - [150] biscyclohexylamineDiethylEPIKURE toluene diamineW PACM 4,4EPIKURE′-methylene W a a b Diethyl0 toluene diamine 193a b 185a /198b - MethyhexahydrophthalicDiethyl4,4EPIKURE-methylene toluene Wdiamine anhydride 100 /111aa bb 167 /176aa bb - [150] biscyclohexylamine PACM 132100 a/140/140129a/111 bb b 185 a/198/198125 b167 a/176--284 b - [150] biscyclohexylamineDiethylEPIKURE tolueneMHHPA diamine W PACM Methyhexahydrophthalic4,4EPIKURE′-methylene W a b a b 132100 a129/140/111 b b 185167 a/198/176125 b -- 284 [150] biscyclohexylamineanhydride4,4EPIKURE′-methylene MHHPA W PACM [132] DiethylDiethyl toluene toluene diamine a a b b a b a b Methyhexahydrophthalic 132100132 a /140/111/140 b 185167 a/198 /176 b 185 /198- - [150] - biscyclohexylamineMethyhexahydrophthalicPoly(propylene glycol)PACM bb diamineEPIKURE EPIKURE W W 129 92b b 125125 97 284266 [132] bis(2-aminopropylMethyhexahydrophthalicanhydride MHHPA ether) D230 129 125 284 DiethylPoly(propyleneanhydride toluene MHHPA diamine glycol) 129 b 125 284 Methyhexahydrophthalic 132 a/14092 b b b 185 a/19897 b - 266 bis(2-aminopropylDiethylanhydrideEPIKURE toluene MHHPA W diamineether) D230 129 125 [132][132] 284 Methyhexahydrophthalicanhydride MHHPA a b a b Poly(propylene glycol) 132129 /140 b 185125 /198 284- [132] Poly(propyleneEPIKURE W glycol) bb [132] anhydride MHHPA 92 b 97 266 bis(2-aminopropylPoly(propylenePoly(propylene glycol)ether) glycol) D230 92 97 266 bis(2-aminopropylMethyhexahydrophthalic ether) D230 92 b 92 b 97 97266 [132] 266 bis(2-aminopropylbis(2-aminopropyl ether) ether) D230 D230 129 b 125 284 4,4Methyhexahydrophthalic′-methylenebiscyclohexylaminePoly(propyleneanhydride MHHPA glycol) 100 a/107 b 149 a/158 b 341 [17] 92129 b b 97125 266284 4,4bis(2-aminopropyl′-methylenebiscyclohexylamineanhydride MHHPA ether) D230 100 a/107 b 149 a/158 b 341 [132][17] Poly(propylene glycol) [132] 92 b 97 266 aa bb aa bb 4,4bis(2-aminopropyl0 ′′-methylenebiscyclohexylamine-methylenebiscyclohexylaminePoly(propylene ether) glycol) D230 100 a/107/107a b b 149 a/158/158 b a 341b [17] 4,44,4-methylenebiscyclohexylamine′-methylenebiscyclohexylamine 100100 /107b /107 149 /158 149 /158341 [17] 341 [17] a92 b a 97 b 266 4,4bis(2-aminopropyl′-methylenebiscyclohexylamine ether) D230 100 /107 149 /158 341 [17]

a b a b 4,4′-methylenebiscyclohexylamine 100 /107 149 /158 341 [17] Isophorone Diamine 97/106 b 166/182 b 361 c 4,4′-methylenebiscyclohexylamine 100 a/107 b 149 a/158 b 341 [17] Isophorone Diamine 97/106 b 166/182 b 361 c 4,4′-methylenebiscyclohexylamine 100 a/107 b 149 a/158 b 341 [17]

bb b bb b cc c IsophoroneIsophoroneIsophorone Diamine DiamineDiamine 97/10697/106 b 166/182166/182 b 166/182361 c 361 Isophorone Diamine 97/106 b 166/182 b 361 c [27,135] Isophorone Diamine 97/106 b 166/182 b 361 c [27,135]

Isophorone Diamine 152/166 b 166/182 b 315 c b b c Isophorone Diamine 97/106 166/182 361 [27,135][27,135] [27,135] Isophorone Diamine 152/166 b 166/182 b 315 c [27,135][27,135] Isophorone Diamine 97/106 b 166/182 b 361 c

bb b bb bcc c IsophoroneIsophoroneIsophorone Diamine DiamineDiamine 152/ 152/152/166166 b 166/166/182 b 166/182315315 c [27,135] 315 Isophorone Diamine 152/166b 166/182b 315c Isophorone Diamine 152/166 166/182 315 Isophorone Diamine 132/154 b 166/182 b 328 [27,135] Isophorone Diamine 152/166 b 166/182 b 315 c b b Isophorone Diamine 132/154 166/182 328 Bisfurfurylamine A 85 111 312 [27,135] Isophorone Diamine 152/166 b b 166/182 b 315 c b Isophorone Diamine 132/154bb bb 166/182 328 IsophoroneIsophorone DiamineDiamine 132/ 132/154 b 166/166/182 b 328328 IsophoroneIsophoroneBisfurfurylamine Diamine Diamine A 132/ 132/154 85154 b 166/166/182 111182 b 328 312328 BisfurfurylamineIsophoroneVanillylamine Diamine A 152/166 8564 b 166/18267 b 111315262 c [148] 312 IsophoroneBisfurfurylamine Diamine A 132/154 85 b 166/182 111 b 328 312 BisfurfurylamineBisfurfurylamineVanillylamineVanillylamine A A 8564 85 64 111 11167 67 312262 312 [148] 262 [148] 1,10-diaminodecane 74 98 328 Isophorone Diamine 132/154 b 166/182 b 328 1,10-diaminodecaneBisfurfurylamineVanillylamine A 8564 74 11167 98 312262 [148] 328 1,10-diaminodecaneVanillylamineVanillylamine 647464 679867 262328262 [148][148] b b HexahydrophthalicIsophoroneBisfurfurylamine Diamine anhydrideA 132/ 85 154 114 166/ 111182 106 312328 321 Hexahydrophthalic1,10-diaminodecaneVanillylamine anhydride 6474 114 6798 106262328 [148] 321 Hexahydrophthalic1,10-diaminodecane1,10-diaminodecane anhydride 74 11474 98 10698 328 321328 [119] [119] [119] Molecules 2017, 22, 149 Bisfurfurylamine A 85 111 312 2 of 7 1,10-diaminodecaneVanillylamineMaleopimaric acid 6474 155 6798 - 262328 317 [148] HexahydrophthalicMaleopimaric anhydride acid 114 155 106 - 321 [119] 317 HexahydrophthalicHexahydrophthalic anhydride anhydride 114 114 106 106 321 321 [119] [119] 1,10-diaminodecaneVanillylamine 7464 9867 328262 [148] Hexahydrophthalic anhydride 114 106 321 [119] MoleculesMolecules 20172017,, 2222,, 149;149; doi:10.3390/molecules22010149doi:10.3390/molecules22010149 www.mdpi.com/journal/moleculeswww.mdpi.com/journal/molecules 1,2-cyclohexanedicarboxylicHexahydrophthalic1,2-cyclohexanedicarboxylic1,10-diaminodecane anhydride 11474 10698 321328 [119] 154 154144 311 144 [139] 311 [139] Molecules 2017,, 2222,, 149;149;149; doi:10.3390/molecules22010149doi:10.3390/molecules22010149doi:10.3390/molecules22010149anhydrideanhydride www.mdpi.com/journal/moleculeswww.mdpi.com/journal/molecules Hexahydrophthalic anhydride 114 106 321 [119] Molecules 2017, 22, 149; doi:10.3390/molecules22010149 www.mdpi.com/journal/molecules

a b Moleculesa T T2017αα measuredmeasured, 22, 149; doi:10.3390/molecules22010149by byDMA DMA at the at peak the peakposition position of loss ofmodulus loss modulus curve; b T curve;α measuredwww.mdpi.com/journal/moleculesT αbymeasured DMA at the by DMA at the maximum c maximumof tan δ; ofT tand,50% δ; c .Td,50%. MoleculesTable 2017 ,2. 22 Mono-aromatic,, 149; doi:10.3390/molecules22010149 poly-epoxy resins and thermal properties of the curedwww.mdpi.com/journal/molecules materials. Tg values

are indicated in plain text, Tα are in italics.

Tg (°C) or Tα (°C)

Epoxy Curing Agent Td,5% (°C) Reference DGEBA Materials Comparison

PE-C9-NH2 102 a 57 266 c [141] PE-C18-NH2 112 a 52 283 c

Isophorone Diamine 177 157 306 (10%)

Decane-1,10-diamine 137 - 300 (10%) [142]

Difurfuryl amine 134 - 290 (10%)

Isophorone Diamine 233 a 160 300 [22]

Isophorone Diamine 176/194 b 166/182 b 308 [148]

4,4′-metylene 71 a/80 b 167 a/176 b 303 biscyclohexylamine PACM [[150]] Diethyl toluene diamine 88 a/94 b 185 a/198 b - EPIKURE W

5,5′-methylene 56 a/62 b 121 a/128 b 272 difurfurylamine [151] 5,5′-ethylidene 56 a/69 b 128 a/142 b 272 difurfurylamine

methyhexahydrophthalic 152 b 125 293 anhydride MHHPA [132] poly(propylene glycol) 101 b 97 267 bis(2-aminopropyl ether) D230

Molecules 2017, 22, 149 2 of 7 MoleculesMolecules 2017 2017, ,22 22,, 149149 2 of 7 2 of 7 Molecules 2017, 22, 149 2 of 7 Molecules 2017, 22, 149 2 of 7

1,2-cyclohexanedicarboxylic1,2-cyclohexanedicarboxylic 154 154 144 144 311 311 [139] [139] 1,2-cyclohexanedicarboxylicanhydrideanhydride 154 144 311 [139] 1,2-cyclohexanedicarboxylicanhydride 154 144 311 [139] anhydride

a b a aTα αmeasured by DMA at the peak position of loss modulus curve;b αb Tα measured by DMA at the T Tα measuredmeasured by DMA atat thethe peakpeak position position of ofloss loss modulus modulus curve; curve; T measuredTα measured by DMA by DMA at the at the a b Tα measured by DMAc at the peak position of loss modulus curve; Tα measured by DMA at the maximummaximum of of tantan δ; c Td,50%.. maximum of tan δ; Td,50%. a maximum of tan δ; c Td,50%. b Table TableTα measured 2. 2. Mono-aromatic, Mono-aromatic, by DMA at poly-epoxypoly-epoxy the peak resins positionresins and and of ther lossthermal malmodulus properties properties curve; of theof thecuredTα measuredcured materials. materials. by T gDMA values Tg valuesat the Table 2. Mono-aromatic, poly-epoxy resins and thermal properties of the cured materials. Tg values MoleculesTable2017 2., Mono-aromatic,22, 149 c poly-epoxy resins and thermal properties of the cured materials. Tg values 22 of 48 maximumareare indicated indicated of tan inin plainδ; Td,50% text,. TTαα are are in in italics. italics. are indicated in plain text, Tα are in italics. are indicated in plain text, Tα are in italics. Table 2. Mono-aromatic, poly-epoxy resins and thermal properties of the cured materials. Tg values Tg (°C) or Tα (°C) Tg (°C) or Tα (°C) Tg (°C) or Tα (°C) are indicated in plain text, Tα are in italics. Tg (°C) or Tα (°C) Epoxy Curing Agent Td,5% (°C) Reference Epoxy Curing Agent Td,5% (°C) Reference Table 2.EpoxyMono-aromatic, poly-epoxyCuring Agent resins and thermalDGEBA properties ofT thed,5% (°C) cured Reference materials. Tg values Epoxy Curing Agent MaterialsTg (°C) or TDGEBAα (°C) Td,5% (°C) Reference MaterialsComparison DGEBA are indicated in plain text, Tα are in italics. Materials Comparison Epoxy Curing Agent Comparison Td,5% (°C) Reference DGEBA PE-C9-NH2 102Materials a 57 266 c PE-C9-NH2 102 a 57◦ ◦ 266 c PE-C9-NH2PE-C9-NH2 102 102a a Comparison57T (57C) or T266( c C)266 c [141] g α ◦ Epoxy PE-C18-NH2Curing Agent 112 a 52 283 c [141] [141] Td,5% ( C) Reference Materialsaa DGEBA Comparisoncc PE-C18-NH2PE-C18-NH2PE-C9-NH2 112102 112a a 52 5752 283 c 266283 c IsophoronePE-C9-NH2 Diamine 177 102 a 157 306 (10%)57 [141] 266 c IsophoroneIsophoronePE-C18-NH2 Diamine Diamine 177112 177 a 157 15752 306 (10%) 306283 (10%) c [141] IsophoronePE-C18-NH2 Diamine 177 112 a 157 30652 (10%) 283 c Decane-1,10-diamine 137 - 300 (10%) [142] Decane-1,10-diamineIsophorone Diamine 137 177 - 300 (10%) 157 [142] 306 (10%) Decane-1,10-diamineIsophorone Diamine 177137 157 - 306300 (10%)(10%) [142] Difurfuryl amine 134 - 290 (10%) Decane-1,10-diamine 137 - 300 (10%) [142] Difurfuryl amine 134 - 290 (10%) Decane-1,10-diamineDifurfuryl amine 137 134 - - 300 290 (10%)(10%) [142] Difurfuryl amine 134 - 290 (10%)

Difurfuryl amine 134 - 290 (10%)

Isophorone Diamine 233 a 160 300 [22] Isophorone Diamine 233 a 160 300 [22] a IsophoroneIsophorone Diamine Diamine 233 a233 a 160 160300 [22] 300 [22]

a Isophorone Diamine 233 160 300 [22]

Isophorone Diamine 176/194 b 166/182 b 308 [148] Isophorone Diamine 176/194 b 166/182 b 308 [148]

b b b b IsophoroneIsophorone Diamine Diamine 176/176/194 b 194 166/182 b 166/308182 [148] 308 [148] Isophorone Diamine 176/194 166/182 308 [148]

b b Isophorone4,4′-metylene Diamine 176/194 166/182 308 [148] 71 a/80 b 167 a/176 b 303 biscyclohexylamine4,4′-metylene PACM 71 a/80 b 167 a/176 b 303 biscyclohexylamine0 PACM [[150]] 4,44,4′-metylene-metylene a b a b 4,4′-metylene a 71 b /80 a b 167 /176 303 biscyclohexylamineDiethyl toluene diamine PACM 71 a/80 b 167 a/176 b 303 [[150]] biscyclohexylamine PACM 88 a71/94 b/80 185 a167/198 b/176 - 303 biscyclohexylamineDiethylEPIKURE toluene diamineW PACM [150] 88 a/94 b 185 a/198 b - Diethyl4,4 toluene′-metylene diamine a b a b [[150]] EPIKURE W 71 a/8088 b /94 167 a/176 b 185 303/198 [[150]] - DiethylEPIKURE toluene diamine W biscyclohexylamineDiethyl5,5′-methylene toluene diamine PACM a b a b 56 a88/62 ab/94/94 b 121 a185/128 a /198b/198 b 272 - difurfurylamine5,55,5EPIKURE′-methylene0-methylene W [[150]] 56 a/62 b 56 a/62121b a/128 b 121272 a/128 b 272 Diethyldifurfurylaminedifurfurylamine toluene diamine [151] 88 a/94 b 185 a/198 b - 5,5′-methylene [151] [151] 5,55,5EPIKURE′-ethylidene0′-methylene W a b a b 5,5 -ethylidene 56 a56/69 ab/62 b 128 a121/142 a /128b b 272 272 difurfurylamine5,5difurfurylamine′-ethylidene 56 /6256 a/69 b 121 /128 128 a272/142 b 272 difurfurylaminedifurfurylamine 56 a/69 b 128 a/142 b 272 difurfurylamine5,5′-methylene [151] 56 a/62 b 121 a/128 b 272 [151] methyhexahydrophthalicdifurfurylamine5,5′-ethylidene b 5,5′-ethylidene 15256 b a/69152 b 125128 a/142 b 293 125272 293 methyhexahydrophthalicanhydrideanhydride MHHPA MHHPA 56 a/69 b 128 a/142 b 272 difurfurylamine 152 b 125 293 [151] anhydride MHHPA poly(propylene5,5′-ethylidene glycol) [132] [132] poly(propylene glycol) 56 a/69 b b 128 a/142 b 272 [132] Molecules 2017, 22, 149 bis(2-aminopropylmethyhexahydrophthalic ether) b 101 973 of 7 267 methyhexahydrophthalicdifurfurylamine 101 b 97 267 bis(2-aminopropylpoly(propylene ether) glycol) D230 152 b 125 293 anhydrideD230 MHHPA 101 152b 97 125 267 293 bis(2-aminopropylanhydride MHHPA ether) D230 methyhexahydrophthalic [132] 152 b 125 293 [132] poly(propylene glycol) poly(propyleneanhydride MHHPA glycol) b 101 b 9797 267 267 bis(2-aminopropyl ether) D230 [132] poly(propylene glycol) b 101 97 267 4-methylbis(2-aminopropyl4-methyl hexahydrophthalichexahydrophthalic ether) D230 78–9278–92 - 246–316 - [153] 246–316 [153] anhydrideanhydride

a b a TTα αmeasuredmeasured by byDMA DMA at atthe the peak peak position position of loss of lossmodulus modulus curve; curve; b Tα measuredTα measured by DMA by at DMA the at the maximum of c maximumtan δ; T ofd tanunder δ; c T aird under flow. air flow. Table 3. Di-epoxy resins with two aromatic rings separated by one atom, and thermal properties of the 5.2. Di-Aromaticcured materials. Epoxy Compounds

Tg (°C) or Tα (°C) Its bisphenolic structure provides BPA with high thermo-mechanical properties but also toxicity. Epoxy Curing Agent Td,5% (°C) Reference DGEBA Materials In an attempt to mimic but alter the BPA structure to match itsComparison qualities but not its defaults, researchers have developed various di-aromatic derivatives based on bio-resources. The diaromatic glycidyl ether compounds are defined as having strictly two benzene rings bearing two or more epoxy groups per 4,4′-methylene-biscyclohexylamine 104 a/111 b 149 a/158 b 363 [17] molecule. As the spacer between these two aromatic rings plays an important role in the estrogenic disruption of BPA, the epoxy monomers will be presented in the following sections according to the number of atoms between the two aromatic moieties: (i) two benzene rings without spacer; (ii) separated by one atom; (iii)Isophorone separated Diamine by two atoms 158 and (iv) separated361 by long spacers.

Isophorone Diamine 136 363

165 [157]

Isophorone Diamine 96 363

Isophorone Diamine 86 362

4,4′-diaminodiphenyl methane 161 179 367/368 c

[116] 4,4′-methylene 143 154 363/360 c bis(5-isopropyl-2-methylaniline)

4,4′-diaminodiphenyl methane 184/183 b - 346/355 c [159]

a Tα measured by DMA at the peak position of loss modulus curve; b Tα measured by DMA at the maximum of tan δ; c Td under air flow. Table 4. Di-epoxy monomers with two aromatic rings separated by more than one atom exhibiting ester or amide bonds, and thermal properties of the cured materials.

Molecules 2017, 22, 149 22 of 47

4-methyl hexahydrophthalic 78–92 - 246–316 [153] anhydride

a Tα measured by DMA at the peak position of loss modulus curve; b Tα measured by DMA at the

maximum of tan δ; c Td under air flow.

5.2. Di-Aromatic Epoxy Compounds Its bisphenolic structure provides BPA with high thermo-mechanical properties but also toxicity. In an attempt to mimic but alter the BPA structure to match its qualities but not its defaults, researchers have developed various di-aromatic derivatives based on bio-resources. The diaromatic glycidyl ether compounds are defined as having strictly two benzene rings bearing two or more epoxy groups per molecule. As the spacer between these two aromatic rings plays an important role in the estrogenic disruption of BPA, the epoxy monomers will be presented in the following sections according to the Molecules 2017 22 number of, atoms, 149 between the two aromatic moieties: (i) two benzene rings without spacer; (ii) separated23 of 48 by one atom; (iii) separated by two atoms and (iv) separated by long spacers. 5.2.1. Two Aromatic Rings without Spacer 5.2.1. Two Aromatic Rings without Spacer Grelier et al. [155] synthesized a diglycidyl ether derivative (24) from the bio-based but highly Grelier et al. [155] synthesized a diglycidyl ether derivative (24) from the bio-based but highly toxic toxic for the environment (H411) 2,6-dimethoxyphenol using a three-steps pathway: (i) enzymatic for the environment (H411) 2,6-dimethoxyphenol using a three-steps pathway: (i) enzymatic coupling coupling using laccase from Tramates versicolor; (ii) reduction of the carbon-oxygen double bonds into using laccase from Tramates versicolor; (ii) reduction of the carbon-oxygen double bonds into alcohols and alcohols(iii) O-glycidylation and (iii) O-glycidylation with epichlorohydrin with epichlorohydrin (Scheme 9). The (Scheme di-epoxy9 ).monomer The di-epoxy obtained monomer was crosslinked obtained T ◦ wasusing crosslinked isophorone using diamine isophorone yielding diamine a material yielding that exhibits a material a Tg thatof 126 exhibits °C and a a goodg of 126thermalC and stability a good ◦ thermalwith a stabilityTd,5% of 312 with °C. aUnfortunately,Td,5% of 312 noC. DGEBA Unfortunately, comparison no DGEBAwas provided. comparison was provided.

SchemeScheme 9. 9.Synthetic Synthetic pathway pathway developed developed byby GrelierGrelier et al. [[155]155] to obtain diphenolic diphenolic derivatives derivatives from from 2,6-dimethoxyphenol.2,6-dimethoxyphenol.

Following this method, the same team synthesized di-phenols from vanillin [156] (Scheme 10). A methylationFollowing step this using method, methyl the sameiodide team can synthesizedbe applied prior di-phenols to the fromreduction vanillin in [order156] (Schemeto obtain 10 ). A methylationdi-functional stepderivatives. using methylNo epoxides iodide were can besynthesized applied prior from to these the reductioncompounds in orderbut a tosimple obtain di-functionalglycidylation derivatives. step could easily No lead epoxides to potential were bio-based synthesized monomers. from these compounds but a simple Moleculesglycidylation 2017, 22 step, 149 could easily lead to potential bio-based monomers. 23 of 47 5.2.2. Two Aromatic Rings Separated by One Atom Hernandez et al. [17] recently synthesized epoxy resins by a two-steps pathway involving the electrophilic condensation of vanillyl alcohol and guaiacol, a mono phenolic derivative that can be obtained in large quantities when carrying out the pyrolysis of lignin. This original method is an interesting alternative to the use of formaldehyde, especially for the synthesis of an analogue to bisphenol F. The obtain bis-guaiacol was then glycidylated (25) (Figure 19) and cured with 4,4′-methylene-biscyclohexylamine (Amicure PACM), yielding a material with a Tg value of 111 °C compared to 158 °C for the DGEBA-based material and a slightly lower thermal stability.

Scheme 10. Synthetic pathway developed by Grelier et al. [156] to obtain diphenolic derivatives from vanillin. Scheme 10. Synthetic pathway developed by Grelier et al. [156] to obtain diphenolic derivatives from vanillin. N-alkyl diphenolate diglycidyl ethers (26) were synthesized by Maiorana et al. [157] in yields ranging from 85% to 97% in a two-step pathway using diphenolic acid. First, it was esterified with 5.2.2. Two Aromatic Rings Separated by One Atom various alcohols and then glycidylated using epichlorohydrin. The advantage of these epoxides is their liquidHernandez state at room et al.temperature [17] recently and synthesized the possibility epoxy to have resins fully by bio-based a two-steps materials. pathway In involvingfact, levulinic the acidelectrophilic is produced condensation from cellulose of by vanillyl the acid alcohol hydrolysis and of guaiacol, C6 sugars a at mono the pilot phenolic plant scale derivative by Biofine that [110] can andbe obtained Segetis [158]. in large The quantitiesmaterials cured when with carrying isophorone out the diamine pyrolysis showed of lignin. glass tran Thissition original temperatures method in is betweenan interesting 86 and alternative 158 °C, depending to the use of of the formaldehyde, alkyl chain, especiallywhich is lower for the than synthesis that of ofDGEBA-based an analogue equivalentsto bisphenol (165 F. °C). The T obtaing decreases bis-guaiacol with increasing was then alkyl glycidylated chain length. (No25) trend (Figure was 19 observed) and cured regarding with 0 ◦ storage4,4 -methylene-biscyclohexylamine modulus or tensile strength (Amicure but materials PACM), yieldingexhibited a materialmechanical with properties a Tg value similarof 111 toC ◦ DGEBA-basedcompared to 158 materialsC for theand DGEBA-based slightly lower thermal material stability. and a slightly lower thermal stability.

Figure 19. Bio-based di-aromatic epoxy derivatives with a single carbon atom spacer.

Harvey et al. [117] synthesized di-epoxides (27) through methylene bridging between two carvacrol molecules with 1,3,5-trioxane in dilute HCl at elevated temperature (Figure 19) and O-glycidylation using epichlorohydrin, with an overall yield of 42%. The resulting monomer was cured with 4,4′-diaminodiphenyl methane and 4,4′-methylene bis(5-isopropyl-2-methylaniline), also prepared from

Molecules 2017, 22, 149 23 of 47

Scheme 10. Synthetic pathway developed by Grelier et al. [156] to obtain diphenolic derivatives from vanillin.

N-alkyl diphenolate diglycidyl ethers (26) were synthesized by Maiorana et al. [157] in yields ranging from 85% to 97% in a two-step pathway using diphenolic acid. First, it was esterified with various alcohols and then glycidylated using epichlorohydrin. The advantage of these epoxides is their liquid state at room temperature and the possibility to have fully bio-based materials. In fact, levulinic acid is produced from cellulose by the acid hydrolysis of C6 sugars at the pilot plant scale by Biofine [110] and Segetis [158]. The materials cured with isophorone diamine showed glass transition temperatures in between 86 and 158 °C, depending of the alkyl chain, which is lower than that of DGEBA-based equivalents (165 °C). Tg decreases with increasing alkyl chain length. No trend was observed regarding Moleculesstorage 2017modulus, 22, 149 or tensile strength but materials exhibited mechanical properties similar24 of to 48 DGEBA-based materials and slightly lower thermal stability.

Figure 19. Bio-basedBio-based di-aromatic epoxy derivatives with a single carbon atom spacer.

Harvey et al. [117] synthesized di-epoxides (27) through methylene bridging between two carvacrol N-alkyl diphenolate diglycidyl ethers (26) were synthesized by Maiorana et al. [157] in yields molecules with 1,3,5-trioxane in dilute HCl at elevated temperature (Figure 19) and O-glycidylation ranging from 85% to 97% in a two-step pathway using diphenolic acid. First, it was esterified with using epichlorohydrin, with an overall yield of 42%. The resulting monomer was cured with various alcohols and then glycidylated using epichlorohydrin. The advantage of these epoxides is 4,4′-diaminodiphenyl methane and 4,4′-methylene bis(5-isopropyl-2-methylaniline), also prepared from their liquid state at room temperature and the possibility to have fully bio-based materials. In fact, levulinic acid is produced from cellulose by the acid hydrolysis of C6 sugars at the pilot plant scale by Biofine [110] and Segetis [158]. The materials cured with isophorone diamine showed glass transition temperatures in between 86 and 158 ◦C, depending of the alkyl chain, which is lower than that of ◦ DGEBA-based equivalents (165 C). Tg decreases with increasing alkyl chain length. No trend was observed regarding storage modulus or tensile strength but materials exhibited mechanical properties similar to DGEBA-based materials and slightly lower thermal stability. Harvey et al. [117] synthesized di-epoxides (27) through methylene bridging between two carvacrol molecules with 1,3,5-trioxane in dilute HCl at elevated temperature (Figure 19) and O-glycidylation using epichlorohydrin, with an overall yield of 42%. The resulting monomer was cured with 4,40-diaminodiphenyl methane and 4,40-methylene bis(5-isopropyl-2-methylaniline), also prepared from p-cymene [116]. The ortho-methylene substituents led to lower degree of cure and higher moisture resistance, and likely lower hydrolysis. The obtained materials showed glass transition temperatures ranging from 143 to 161 ◦C, which is slightly lower than DGEBA-based materials. When this di-epoxide was cured with 4,40-diaminodiphenyl methane and compared with diglycidyl ether of ◦ tetramethylbisphenol F, the material based on the former showed lower Tg than the latter, up to 23 C. Diglycidyl ether of tetramethylbisphenol F (28) was prepared in a two-step synthesis route from 2,6-dimethylphenol and the mutagenic and carcinogenic (H341-350) formaldehyde: (i) aldol condensation reaction between formaldehyde and two molecules of 2,6-dimethylphenol followed by (ii) the direct O-glycidylation using epichlorohydrin, with an overall yield of 79% [159]; The Tg of the 0 ◦ material cured with 4,4 -diaminodiphenyl methane showed a high Tg value, up to 184 C. When this epoxide was compared with its non-spacer analogue, 3,30,5,50-tetramethyl-4,40-biphenol, the additional methylene spacer led to a slight decrease of the glass transition temperature, up to 15 ◦C lower. Diglycidyl ether of bisfuran (29) was prepared in a four-step synthesis from the bio-based 2-furoic acid and acetone: (i) protection of the carboxylic groups using methanol; (ii) coupling of two resulting molecules in the presence of acetone and concentrated sulfuric acid; (iii) reduction of the ester groups Molecules 2017, 22, 149 24 of 47

p-cymene [116]. The ortho-methylene substituents led to lower degree of cure and higher moisture resistance, and likely lower hydrolysis. The obtained materials showed glass transition temperatures ranging from 143 to 161 °C, which is slightly lower than DGEBA-based materials. When this di-epoxide was cured with 4,4′-diaminodiphenyl methane and compared with diglycidyl ether of tetramethylbisphenol F, the material based on the former showed lower Tg than the latter, up to 23 °C. Diglycidyl ether of tetramethylbisphenol F (28) was prepared in a two-step synthesis route from 2,6-dimethylphenol and the mutagenic and carcinogenic (H341-350) formaldehyde: (i) aldol condensation reaction between formaldehyde and two molecules of 2,6-dimethylphenol followed by (ii) the direct O-glycidylation using epichlorohydrin, with an overall yield of 79% [159]; The Tg of the material cured with 4,4′-diaminodiphenyl methane showed a high Tg value, up to 184 °C. When this epoxide was compared with its non-spacer analogue, 3,3′,5,5′-tetramethyl-4,4′-biphenol, the additional methylene spacer led to a slight decrease of the glass transition temperature, up to 15 °C lower.

MoleculesDiglycidyl2017, 22, 149ether of bisfuran (29) was prepared in a four-step synthesis from the bio-based 2-furoic25 of 48 acid and acetone: (i) protection of the carboxylic groups using methanol; (ii) coupling of two resulting molecules in the presence of acetone and concentrated sulfuric acid; (iii) reduction of the ester groups using lithium aluminum hydride and then;then; (iv)(iv) directdirect OO-glycidylation-glycidylation ofof thethe di-alcohol,di-alcohol, withwith anan overalloverall yieldyield of 48% [113]. [113]. However, However, no no materials materials ha haveve been been prepared prepared from from this this epoxy epoxy resin. resin. Meylemans et al. [160] [160] proposed an an interesting stra strategytegy to to synthesize synthesize di-p di-phenolshenols from from creosol, creosol, a aphenolic phenolic derivative derivative coming coming from from the the reduct reductionion of ofvanillin vanillin (Figure (Figure 20). 20 Through). Through Zn(AcO) Zn(AcO)2 or 2acidor acidcatalyzed catalyzed coupling coupling with aldehydes, with aldehydes, they obtained they obtained two different two different di-phenol di-phenol suitable for suitable epoxide for synthesis. epoxide synthesis.Unfortunately, Unfortunately, no glycidylation no glycidylation step was carried step was out carried on these out original on these compounds, original compounds, although the although use of thebio-based use of bio-basedaldehydes aldehydessuch as benzaldehyde such as benzaldehyde may produce may producehighly bio-sourced highly bio-sourced monomers monomers as well asas wellgreatly as greatlyinfluence influence the mechanical the mechanical and thermal and thermalproperties. properties. However, However, despite the despite presence the presenceof methoxy of methoxymoieties and moieties a meta and substitution, a meta substitution, the di-phenol the derivatives di-phenol derivativesexhibit a structure exhibit very a structure close to very bisphenol close toE bisphenoland F, according E and F,to accordingthe aldehyde to the chosen, aldehyde thus requir chosen,ing thus toxicological requiring studies toxicological to determine studies their to determine potential theirdangerousness potential dangerousnessor estrogen disruptor or estrogen behavior. disruptor behavior.

FigureFigure 20. 20.Creosol-based Creosol-based di-phenolic di-phenolic derivativederivative synthesized by by Meylemans Meylemans et et al. al. [160]. [160 ].

5.2.3. Two Aromatic Ring Ringss Separated Separated by by Two Two Atoms Atoms Duann et al. [161] [161] prepared prepared a abis-phenolic bis-phenolic glycidylated glycidylated derivative derivative by byreacting reacting p-aminophenolp-aminophenol and andp-hydroxybenzaldehydep-hydroxybenzaldehyde prior priorto a to direct a direct epoxidation epoxidation with with epichlorohydrin epichlorohydrin (30 (30) )(Figure (Figure 2121).). p-hydroxybenzaldehyde can can be be obtained obtained from from biom biomassass by depolymerizing lignin lignin [162] [162] but to ourour knowledge no bio-basedbio-based commercial p-aminophenol is is currently availa available.ble. However, However, Ng Ng et al. [[163]163] claimed in aa patentpatent toto havehave developeddeveloped aa processprocess forfor producingproducing thisthis compoundcompound usingusing geneticallygenetically engineered microorganisms and and biomass-based suga sugars,rs, possibly possibly providing providing this this resin resin with a highhigh renewabilityrenewability percentage. After curing with 4,44,40′-diamino diphenyl methane, they obtainedobtained aa materialmaterial ◦ ◦ withwith aa TTgg ofof 200200 °CC andand aa temperaturetemperature ofof 5%5% weightweight lossloss underunder airair ofof 342342 °C.C. Unfortunately,Unfortunately, nono comparison with a DGEBA-basedDGEBA-based material was providedprovided to determinedetermine the rolerole ofof thethe carbon-nitrogencarbon-nitrogen double-bondMolecules 2017, 22 on on, 149 the the thermal thermal and and mechanical mechanical properties. properties. 25 of 47

FigureFigure 21. Epoxy 21. Epoxy resin based resin on based p-aminophenol on p-aminophenol and p-hydroxybenzaldehyde and p-hydroxybenzaldehyde synthesized by synthesized Duann et al. by[161]. Duann et al. [161]. Using vanillin as a starting material, Harvey et al. [164] synthesized a di-phenol stilbene using titanium chloride and magnesium, and further hydrogenated it on platinum oxide (Scheme 11). These Using vanillin as a starting material, Harvey et al. [164] synthesized a di-phenol stilbene using compounds were turned into cyanates and used to obtain polycyanurates, but no glycidylation step has titanium chloride and magnesium, and further hydrogenated it on platinum oxide (Scheme 11). been considered by the authors despite the interesting potential of these molecules for di-epoxy These compounds were turned into cyanates and used to obtain polycyanurates, but no glycidylation monomers. step has been considered by the authors despite the interesting potential of these molecules for di-epoxy monomers.

Scheme 11. Synthesis of di-phenolic compounds from vanillin according to Harvey et al. [164]

5.2.4. Two Aromatic Rings Separated by More Than Two Atoms The small size of BPA allows it to mimic the natural estrogen 17β-estradiol and enter the estrogen receptor pocket. Thus, by increasing the length of the spacer, it may be possible to decrease the activity of the epoxy monomer precursors. In this part, all monomers with long aliphatic or cycloaliphatic spacer will be presented, including both di- and poly-epoxy monomers. Di-Epoxy Monomers Recently, Zou et al. [165] synthesized bio-based epoxy monomers by coupling glycidylated eugenol via photo-initiated thiol-ene reaction using three different aliphatic di-thiols (31) (Figure 22). The resins were cured by 4,4′-diaminodiphenylmethane and the resulting materials exhibited Tg and Tα values ranging from 39 to 60 °C by DSC and 54 to 70 °C by DMA, respectively. As expected, the glass transition decreased when increasing the di-thiol chain length.

Figure 22. Eugenol-based epoxy prepolymers synthesized by Zou et al. [165].

Following the same synthetic pathway, Maiorana et al. [123] and Menard et al. [166] synthesized bio-based bis-ferulate epoxy monomers (32) (33) and (34) (Scheme 12) via a three step synthetic pathway: (i) synthesis of ethyl ferulate by reacting with HCl and hydrogenating over Pd/C; (ii) enzyme-catalyzed transesterification with various bio-based diols including n-alkyl diols and isosorbide or transamidification with 1,4-diaminobutane; and (iii) O-glycidylation with epichlorohydrin in high yields.

Molecules 2017, 22, 149 25 of 47 Molecules 2017, 22, 149 25 of 47

Figure 21. Epoxy resin based on p-aminophenol and p-hydroxybenzaldehyde synthesized by Duann et al. [161]. Figure 21. Epoxy resin based on p-aminophenol and p-hydroxybenzaldehyde synthesized by Duann et al. [161]. Using vanillin as a starting material, Harvey et al. [164] synthesized a di-phenol stilbene using titaniumUsing chloride vanillin and as magnesium,a starting material, and further Harvey hydrog et al.enated [164] itsynthesized on platinum a oxidedi-phenol (Scheme stilbene 11). usingThese titaniumcompounds chloride were andturned magnesium, into cyanates and and further used hydrog to obtainenated polycyanurates, it on platinum but oxide no glycidylation (Scheme 11). step These has Moleculescompoundsbeen considered2017, 22were, 149 turnedby the intoauthors cyanates despite and usedthe interesting to obtain polycyanurates, potential of these but nomolecules glycidylation for di-epoxystep26 has of 48 beenmonomers. considered by the authors despite the interesting potential of these molecules for di-epoxy monomers.

SchemeScheme 11. 11.Synthesis Synthesis of of di-phenolic di-phenolic compounds compounds fromfrom vanillin according according to to Harvey Harvey et etal.al. [164] [164 ]. Scheme 11. Synthesis of di-phenolic compounds from vanillin according to Harvey et al. [164] 5.2.4. Two Aromatic Rings Separated by More Than Two Atoms 5.2.4. Two Aromatic Rings Separated by More Than Two Atoms 5.2.4. TheTwo small Aromatic size ofRings BPA Sepa allowsrated it byto mimicMore Than the natural Two Atoms estrogen 17β-estradiol and enter the estrogen The small size of BPA allows it to mimic the natural estrogen 17β-estradiol and enter the estrogen receptorThe pocket.small size Thus, of BPAby incr allowseasing it theto mimiclength theof the natural spacer, estrogen it may be17β possible-estradiol to anddecrease enter the the activity estrogen of receptor pocket. Thus, by increasing the length of the spacer, it may be possible to decrease the activity receptorthe epoxy pocket. monomer Thus, precursors. by increasing In thisthe lengthpart, all of monomersthe spacer, itwith may long be possible aliphatic to or decrease cycloaliphatic the activity spacer of of the epoxy monomer precursors. In this part, all monomers with long aliphatic or cycloaliphatic thewill epoxy be presented, monomer including precursors. both Indi- this and part, poly-e allpoxy monomers monomers. with long aliphatic or cycloaliphatic spacer spacer will be presented, including both di- and poly-epoxy monomers. willDi-Epoxy be presented, Monomers including both di- and poly-epoxy monomers. Di-Epoxy Monomers Di-EpoxyRecently, Monomers Zou et al. [165] synthesized bio-based epoxy monomers by coupling glycidylated eugenol via photo-initiatedRecently,Recently, Zou etthiol-ene et al. al. [165] [165 reaction synthesized] synthesized using bio-based three bio-based different epoxy epoxy aliphaticmonomers monomers di-thiols by coupling ( by31) coupling(Figure glycidylated 22). glycidylated The eugenol resins eugenolviawere photo-initiated cured via photo-initiatedby 4,4 ′thiol-ene-diaminodiphenylmethane thiol-enereaction using reaction three and using different the three resulting aliphatic different materials di-thiols aliphatic exhibited(31 di-thiols) (Figure Tg (22).31and) (FigureTheTα valuesresins 22 ). 0 Thewereranging resins cured from were by 39 cured4,4 to ′-diaminodiphenylmethane60 °C by by 4,4 DSC-diaminodiphenylmethane and 54 to 70 °C and by theDM resultingA, andrespectively. the materials resulting As expected,exhibited materials theT exhibitedg glassand Ttransitionα valuesTg and ◦ ◦ Trangingdecreasedα values from rangingwhen 39 toincreasing 60 from °C 39by the toDSC 60di-thiol andC by54 chain DSCto 70 length. °C and by 54 DM toA, 70 respectively.C by DMA, As respectively. expected, the As glass expected, transition the glassdecreased transition when decreased increasing whenthe di-thiol increasing chain thelength. di-thiol chain length.

Figure 22. Eugenol-based epoxy prepolymers synthesized by Zou et al. [165]. Figure 22. Eugenol-based epoxy prepolymers synthesized by Zou et al. [165]. Figure 22. Eugenol-based epoxy prepolymers synthesized by Zou et al. [165]. Following the same synthetic pathway, Maiorana et al. [123] and Menard et al. [166] synthesized bio-basedFollowing bis-ferulate the same epoxy synthetic monomers pathway, (32) ( 33Maiorana) and (34 )et (Scheme al. [123] 12) and via Menard a three stepet al. synthetic [166] synthesized pathway: bio-based(i) synthesisFollowing bis-ferulate of ethyl the ferulate sameepoxy syntheticmonomers by reacting pathway,(32 with) (33 HC) andl Maioranaand (34 )hydrogenating (Scheme et 12) al. via [ 123over a ]three Pd and/C; step Menard(ii) synthetic enzyme-catalyzed et pathway: al. [166 ] synthesized(i)transesterification synthesis of bio-based ethyl withferulate bis-ferulate various by reacting bio-based epoxy with monomers HC diolsl and hydrogenatingincluding (32)(33 )n and-alkyl over (34 Pddiols) (Scheme/C; (ii)and enzyme-catalyzed 12isosorbide) via a three or steptransesterificationtransamidification synthetic pathway: withwith 1,4-diaminobutane; (i)various synthesis bio-based of ethyland (iii)diols ferulate O -glycidylationincluding by reacting n with-alkylwith epichl diols HClorohydrin and and hydrogenatingisosorbide in high yields. or overtransamidification Pd/C; (ii) enzyme-catalyzed with 1,4-diaminobutane; transesterification and (iii) O-glycidylation with various with bio-based epichlorohydrin diols including in high yields.n-alkyl diols and isosorbide or transamidification with 1,4-diaminobutane; and (iii) O-glycidylation with epichlorohydrin in high yields. The epoxy monomers were cured by isophorone diamine, 1,10-diaminodecane and difurfurylamine. Regarding the thermosets based on the n-alkyl bis-ferulates ◦ and IPDA, the Tα values ranged from 50 to 65 C and decreased while increasing the diol chain length, compared to 170 ◦C for DGEBA-based material. The change of ester to amide bonds lead to higher Tg values thanks to the hydrogen bonding induced by the nitrogen atoms. These values were roughly similar to those obtained with the isosorbide derivative, the cycloaliphatic structure of which brought more rigidity to the polymeric network. However, none of these materials could compete with DGEBA-based resources in terms of Tg/Tα. An interesting point to mention nonetheless is that Maiorana et al. [123] also focused on the degradability of the obtained materials for thermosets recyclability issues, and studied the estrogenic activity of bisferulates compared to bisphenol A and 17β-estradiol. The n-alkyl bis-ferulates derivatives showed no significant estrogenic activity for ER α Molecules 2017, 22, 149 26 of 47 Molecules 2017, 22, 149 26 of 47 The epoxy monomers were cured by isophorone diamine, 1,10-diaminodecane and difurfurylamine. RegardingThe epoxy the monomersthermosets werebased cured on the by n -alkylisophorone bis-ferulates diamine, and 1,10-diaminodecane IPDA, the Tα values andranged difurfurylamine. from 50 to 65 Regarding°C and decreased the thermosets while increasingbased on the the n -alkyldiol ch bis-ferulatesain length, andcompared IPDA, theto 170 Tα values°C for rangedDGEBA-based from 50 to material.65 °C andThe changedecreased of esterwhile to increasing amide bonds the leaddiol toch higherain length, Tg values compared thanks to to 170 the °Chydrogen for DGEBA-based bonding inducedmaterial. by theThe nitrogen change ofatoms. ester Theseto amide values bonds were lead roughly to higher similar Tg valuesto those thanks obtained to the with hydrogen the isosorbide bonding induced by the nitrogen atoms. These values were roughly similar to those obtained with the isosorbide Moleculesderivative,2017, 22, 149the cycloaliphatic structure of which brought more rigidity to the polymeric network.27 of 48 However,derivative, none the of cycloaliphaticthese materials stru couldcture comp of whichete with brought DGEBA-based more rigidity resources to thein termspolymeric of Tg /Tnetwork.α. An interestingHowever, point none to ofmention these materialsnonetheless could is that comp Maioraete withna et DGEBA-based al. [123] also focused resources on thein termsdegradability of Tg/Tα of. An at concentrationstheinteresting obtained pointmaterials where to mention bisphenol for thermosets nonetheless A does, recyclability is highlighting that Maiora issues,na theet al.and potential [123] studied also offocused the these estrogenic on compounds the degradability activity as of a saferof the obtained materials for thermosets recyclability issues, and studied the estrogenic activity of substitutebisferulates for BPA. compared to bisphenol A and 17β-estradiol. The n-alkyl bis-ferulates derivatives showed no bisferulates compared to bisphenol A and 17β-estradiol. The n-alkyl bis-ferulates derivatives showed no Insignificant a similar estrogenic way, Aoufactivity et for al. ER [40 α] at synthesized concentrations glycidylated where bisphenol bio-based A does, bis-vanillic highlighting acid the (35) significant estrogenic activity for ER α at concentrations where bisphenol A does, highlighting the in a three-stepspotential of these pathway: compounds (i) synthesisas a safer substitute of bis-vanillic for BPA. acid by reaction with 1,5-dibromopentane; potentialIn a similar of these way, compounds Aouf et asal. a [40] safer synthesized substitute for glycidylated BPA. bio-based bis-vanillic acid (35) in a (ii) allylation of the hydroxyl functions with allyl bromide and (iii) epoxidation of the allyl bond three-stepsIn a pathway: similar way, (i) synthesis Aouf et of al. bis-vanillic [40] synthesized acid by reactionglycidylated with 1,5-dibromopentane;bio-based bis-vanillic (ii) acid allylation (35) in a withof an three-stepsthe enzymatic hydroxyl pathway: functions catalyst (i) withsynthesis (Figure allyl 23ofbromide ).bis-vanillic Unfortunately, and acid(iii) epoxidationby reaction no materials withof the 1,5-dibromopentane; allyl were bond synthesized with an enzymatic(ii) allylation with these di-epoxycatalystof monomers.the (Figure hydroxyl 23). functions Unfortunately, with allyl no materials bromide wereand (iisynthesizedi) epoxidation with of these the allyldi-epoxy bond monomers. with an enzymatic catalyst (Figure 23). Unfortunately, no materials were synthesized with these di-epoxy monomers.

Scheme 12. Synthetic pathway to obtain bio-based di-epoxy monomers from ferulic acid proposed Scheme 12. Synthetic pathway to obtain bio-based di-epoxy monomers from ferulic acid proposed by by SchemeMaiorana 12. et Synthetical. [123] and pathway Ménard to etobtain al. [166]. bio-based di-epoxy monomers from ferulic acid proposed Maioranaby etMaiorana al. [123 et] and al. [123] Ménard and Ménard et al. [166 et al.]. [166].

Figure 23. Di-epoxy monomers with longer ester spacers between the aromatic rings. Figure 23. Di-epoxy monomers with longer ester spacers between the aromatic rings. Figure 23. Di-epoxy monomers with longer ester spacers between the aromatic rings.

Fourcade et al. [167] used 1,6-hexanediol to synthesize an epoxy resin using a two-steps synthetic route: (i) phenolization of the 1,6-hexanediol using ethyl-4-hydroxybenzoate with 92% of yield; and (ii) direct O-glycidylation using epichlorohydrin in the presence of sodium carbonate (36) (Figure 23). The resulting epoxide was used as a DGEBA diluent and blended with 50%–90% of DGEBA and then cured with dicyandiamide. The glass transition temperature of the blend materials ranged from 77 to 109 ◦C, decreasing when increasing the amount of aliphatic epoxide. Materials exhibited high thermal stabilities under air, but unfortunately no sample without DGEBA was mentioned. While the 1,6-hexanediol used by the authors was oil-based, it can be obtained by hydrogenation of adipic acid coming from different biomass such as glucose, lignin and fatty acids [168]. It is considered lowly Molecules 2017, 22, 149 27 of 47

Fourcade et al. [167] used 1,6-hexanediol to synthesize an epoxy resin using a two-steps synthetic route:Molecules (i) phenolization 2017, 22, 149 of the 1,6-hexanediol using ethyl-4-hydroxybenzoate with 92% of yield;27 of and47 (ii) direct O-glycidylation using epichlorohydrin in the presence of sodium carbonate (36) (Figure 23). The Fourcade et al. [167] used 1,6-hexanediol to synthesize an epoxy resin using a two-steps synthetic resultingroute: epoxide (i) phenolization was used of as the a 1,6-DGEBAhexanediol diluent using and ethyl-4 blended-hydroxybenzoate with 50%–90% with of 92%DGEBA of yield; and and then (ii) cured with directdicyandiamide. O-glycidylation The usingglass epichlorohydrintransition temperatur in the presencee of the ofblend sodium materials carbonate ranged (36) (Figure from 7723). to The 109 °C, decreasingresulting when epoxide increasing was used the as amount a DGEBA of diluent aliphatic and epoxide. blended withMaterials 50%–90% exhibited of DGEBA high and thermal then cured stabilities underwith air, dicyandiamide.but unfortunately The glassno sample transition without temperatur DGEBAe of thewas blend mentioned. materials While ranged the from 1,6-hexanediol 77 to 109 °C, used Molecules 2017, 22, 149 28 of 48 by thedecreasing authors waswhen oil-based, increasing itthe can amount be obtained of aliphatic by hydrogenationepoxide. Materials of exhibited adipic ac highid coming thermal fromstabilities different biomassunder such air, as but glucose, unfortunately lignin no and sample fatty withoutacids [168]. DGEBA It is was considered mentioned. lowly While toxic the [169]1,6-hexanediol and is not used labelled by the authors was oil-based, it can be obtained by hydrogenation of adipic acid coming from different toxicas carcinogenic [169] and or is notmutagen. labelled Ethyl-4-hydroxybenzoate as carcinogenic or mutagen. is obtained Ethyl-4-hydroxybenzoate by esterification of 4-hydroxybenzoic is obtained by biomass such as glucose, lignin and fatty acids [168]. It is considered lowly toxic [169] and is not labelled esterificationacid, a naturally of 4-hydroxybenzoic occurring product acid, synthesized a naturally occurringindustrially product from synthesizedphenol, carbon industrially dioxide fromand as carcinogenic or mutagen. Ethyl-4-hydroxybenzoate is obtained by esterification of 4-hydroxybenzoic ® phenol,bio-ethanol.acid, carbon a Moreover,naturally dioxide occurring the and authors bio-ethanol. prod useduct epichlorohydrinsynthesized Moreover, industrially the authorsfrom glycerolfr usedom phenol, epichlorohydrin(Epicerol carbon process) dioxide from making and glycerol the ® (Epicerolresinsbio-ethanol. possiblyprocess) highly Moreover, making renewable, the the authors resins although used possibly epichlorohydrin no value highly was renewable, given. from glycerol although (Epicerol no® process) value was making given. the Diglycidylresins possibly ether ether highly derivatives derivatives renewable, including although including spiro-ringno value spiro-ring was as given. spacer as spacer can be can prepared be prepared in a two-step in a two-steppathway pathwayfrom non-toxic, fromDiglycidyl non-toxic, bio-based ether derivatives bio-based reactants, including reactants, e.g., spiro-ringva e.g.,nillin vanillin as[27,135,170,171] spacer [27 can,135 be,170 prepared ,and171] andpentaerythritolin a pentaerythritoltwo-step pathway [167]: [ 167 (i)]: (condensationi) condensationfrom non-toxic, of ofboth both bio-based starti startingng substancesreactants, substances e.g., and and va(ii) (ii)nillin direct direct [27,135,170,171] OO-glycidylation-glycidylation and using usingpentaerythritol epichlorohydrin epichlorohydrin [167]: (i) in in thethe presencecondensation of sodium of hydroxid hydroxide.both startie.ng However, However,substances no noand other other (ii) detail,direct detail, O such-glycidylation such as as reaction reaction using yield yield epichlorohydrin could could be befound found in the[172]. [172 In]. In1986 1986 presenceand and 1987, 1987, of Ochisodium Ochi et ethydroxidal. al. [173] [173e. ]investigated investigatedHowever, no aother aseries series detail, of of diglycidyl diglycidylsuch as reaction ether yield containing could be spiro-ring found [172]. buildingbuilding In 1986 and 1987, Ochi et al. [173] investigated a series of diglycidyl ether containing spiro-ring building blocks (37)() (38))( (39), as shown inin FigureFigure 2424.. MaterialsMaterials cured withwith hexahydrophthalichexahydrophthalic anhydride showed blocks (37) (38) (39), as shown in Figure 24. Materials cured with hexahydrophthalic anhydride showed similar or higher glass transition temperatures than that of their DGEBA counterparts. Curing with more similarsimilar or higher or higher glass glass transition transition temperatures than than that that of their of theirDGEBA DGEBA counterparts. counterparts. Curing with Curing more with rigid anhydrides, e.g., phthalic anhydride and nadic anhydride allowed to slightly increase Tg, in morerigid rigid anhydrides, anhydrides, e.g., e.g., phthalic phthalic anhydride anhydride and andnadic nadic anhydride anhydride allowed allowed to slightly to slightly increase increase Tg, in Tg, incomparison comparisoncomparison to hexahydrophthalic to to hexahydrophthalichexahydrophthalic anhydride. anhydride. anhydride.

Figure 24. Diglycidyl ether derivatives containing spiro rings. Figure 24. Figure 24.Diglycidyl Diglycidyl etherether derivatives containing containing spiro spiro rings. rings. Rao and Samui [174] synthesized diepoxy monomers including bisbenzylidene segments by Raoreaction andand between SamuiSamui vanillin [[174]174] synthesizedorsynthesized syringaldehyde diepoxydiepoxy with acetone monomersmonomers (40) and includingincluding cycloaliphatic bisbenzylidenebisbenzylidene ketones (41) segmentsfollowed by reactionby glycidylation between vanillin with epichlorohydrin or syringaldehydesyringaldehyde (Scheme with 13). acetoneacetone These (( 40monomers) andand cycloaliphaticcycloaliphatic were used to ketonesketones synthesize ((41 )linear followedfollowed by glycidylationglycidylationpolyesters for withphotoactive epichlorohydrin liquid crystal (Scheme applications 13).13). These but also monomers reacted with were benzene-1,3,5-tricarboxylic used to synthesizesynthesize linearlinear polyestersacid to for obtain photoactivephotoactive dendrimers, liquidliquid although crystalcrystal their applicationsapplications protocol seems butbut more alsoalso likely reactedreacted to yield withwith crosslinked benzene-1,3,5-tricarboxylicbenzene-1,3,5-tricarboxylic materials. No data on potential thermo-mechanical properties is thus mentioned. acid to obtain dendrimers, dendrimers, although although their their protocol protocol seems seems more more likely likely to yield to yield crosslinked crosslinked materials. materials. No Nodata data on potential on potential thermo-mechanical thermo-mechanical properties properties is thus is mentioned. thus mentioned.

Scheme 13. Synthesis of di-epoxy monomers from vanillin and syringaldehyde with acetone or cycloaliphatic ketones according to Rao and Samui [174].

Scheme 13.13. Synthesis of of di-epoxy monomers from van vanillinillin and syringaldehyde with acetone or cycloaliphatic ketones according toto RaoRao andand SamuiSamui [[174].174].

Poly-Epoxy Monomers Triglycidyl ether of polyphenol (42) (Figure 25) was synthesized in a two-steps pathway from resorcinol and acetone: a coupling reaction followed by direct O-glycidylation using epichlorohydrin [175]. The glass transition temperature of the resulting material cured with diaminodiphenyl sulfone was not observed neither in DSC, nor in DMA, up to 300 ◦C. The authors ◦ concluded that the Tg was above 300 C, thus making it much higher than that of DGEBA-based Molecules 2017, 22, 149 28 of 47

Poly-Epoxy Monomers MoleculesTriglycidyl 2017, 22 ,ether 149 of polyphenol (42) (Figure 25) was synthesized in a two-steps pathway28 of from47 resorcinol and acetone: a coupling reaction followed by direct O-glycidylation using epichlorohydrin MoleculesPoly-Epoxy2017, 22, 149 Monomers 29 of 48 [175]. The glass transition temperature of the resulting material cured with diaminodiphenyl sulfone was not observedTriglycidyl neither ether in DSC,of polyphenol nor in DMA, (42) (Figureup to 300 25) °C.was The synthesized authors concluded in a two-steps that thepathway Tg was from above material.300resorcinol °C, The thus three makingand acetone: epoxy it much groupsa coupling higher and than reaction the that rigidity offollowed DGEBA-based ofby thedirect cyclic material. O-glycidylation backbone The three usin may epoxyg epichlorohydrin in groups fact explain and the such [175]. The glass transition temperature of the resulting material cured with diaminodiphenyl sulfone was a result.rigidity of the cyclic backbone may in fact explain such a result. not observed neither in DSC, nor in DMA, up to 300 °C. The authors concluded that the Tg was above 300 °C, thus making it much higher than that of DGEBA-based material. The three epoxy groups and the rigidity of the cyclic backbone may in fact explain such a result.

FigureFigure 25. Triglycidyl 25. Triglycidyl ether ether derivative derivative based on on resorcinol resorcinol and and acetone. acetone.

Aouf et al. [176] studiedFigure 25. hydrolysab Triglycidyl etherle tara derivative tannins based as potential on resorcinol cand andidates acetone. for a source of phenolic Aoufmoieties. et al. These [176 galloylquinic] studied hydrolysable acid oligomers tara were tannins first depolymerized as potential candidatesby tannase-assisted for a source hydrolysis of phenolic to moieties.produce TheseAouf mainly galloylquinicet al. gallic [176] acidstudied and acid hydrolysab galloylquinic oligomersle tara wereacid. tannins firstThe asepoxidation depolymerized potential cand of thisidates by mixture tannase-assistedfor a source was carried of phenolic out hydrolysis by to producedirectmoieties. O mainly-glycidylation, These gallic galloylquinic acidleading and acidto galloylquinicgalloylquinic oligomers were esters acid. first an depolymerized Thed glycidylated epoxidation by dimerized tannase of this-assisted gallic mixture moieties hydrolysis was (Figure carried to out by direct26).produce TheO-glycidylation, obtained mainly gallicmonomer acid leading andmixture galloylquinic to galloylquinic was cured acid. using Th esterse epoxidationisophorone and glycidylated ofdiamine this mixture and dimerized the was resulting carried gallic out material by moieties direct O-glycidylation, leading to galloylquinic esters and glycidylated dimerized gallic moieties (Figure (Figureshowed 26). Thea Tg obtainedup to 129 monomer °C, which mixtureis 20 °C lower was cured than that using of isophoroneDGEBA-based diamine materials and in the same resulting 26). The obtained monomer mixture◦ was cured ◦using isophorone diamine and the resulting material materialconditions. showed The a T presenceg up to 129of estersC, which bonds is may 20 Chowever lower thanmake thatthese of thermosets DGEBA-based more materialssensitive to in the hydrolysisshowed anda Tg favourup to 129the use°C, ofwhich condensed is 20 °C instead lower ofthan hydrolysable that of DGEBA-based tannins. materials in the same same conditions.conditions. TheThe presence of of esters esters bonds bonds may may however however make make these these thermosets thermosets more sensitive more sensitive to to hydrolysishydrolysis and favour and favour the the use use of of condensed condensed instead instead of ofhydrolysable hydrolysable tannins. tannins.

Figure 26. Main oligomers obtained from the glycidylation of depolymerized tara tannins by Aouf et al. [176]. Figure 26. Main oligomers obtained from the glycidylation of depolymerized tara tannins by Aouf et al. [176]. Figure 26. Main oligomers obtained from the glycidylation of depolymerized tara tannins by For this reason, Nouailhas et al. [128,177] focused their research on catechin, one of the constitutive Aouf etFor al. [this176 ].reason, Nouailhas et al. [128,177] focused their research on catechin, one of the constitutive units of condensed tannins. Although it is not yet bio-based, it can be obtained by acid-depolymerization units of condensed tannins. Although it is not yet bio-based, it can be obtained by acid-depolymerization as previously described (Section 4.2.). The direct O-glycidylation of catechin (Figure 27) led mainly to the Foras thispreviously reason, described Nouailhas (Section 4.2.). et al. The [128 direct,177 O-glycidylation] focused their of catechin research (Figure on 27) catechin,led mainly to one the of the tetraglycidylated catechin (45) (46%) and to two di-glycidylated benzodioxane derivatives (46), with a constitutivetetraglycidylated units of condensedcatechin (45) (46%) tannins. and to Although two di-glycidylated it is not benzodioxane yet bio-based, derivatives it can (46 be), with obtained a by yieldyield of of18% 18% and and 15%, 15%, respectively. respectively. As As thethe mixturemixture was aa solidsolid at at room room temperature, temperature, it wasit was mixed mixed with with acid-depolymerization as previously described (Section 4.2.). The direct O-glycidylation of catechin 25 25and and 50% 50% of ofDGEBA DGEBA as as a areactive reactive diluent diluent andand cured withwith Epamine Epamine PC PC 19, 19, which which is iscomposed composed of benzylof benzyl (Figurealcohol, alcohol,27) led1,3-bis(aminomethyl)benzene, 1,3-bis(aminomethyl)benzene, mainly to the tetraglycidylated 3-amino-3-amino-methyl-3,5,5-trimethylcyclohexylamine catechin (45) (46%) and to two and di-glycidylatedand BPA- BPA- benzodioxane derivatives (46), with a yield of 18% and 15%, respectively. As the mixture was a solid at room temperature, it was mixed with 25 and 50% of DGEBA as a reactive diluent and cured with Epamine PC 19, which is composed of benzyl alcohol, 1,3-bis(aminomethyl)benzene, 3-amino-methyl-3,5,5-trimethylcyclohexylamine and BPA-epichlorohydrin polymer. The materials based on the 50% and 75% blends of glycidylated catechin exhibited slightly lower and 10 ◦C higher Tg than DGEBA-based materials, respectively. These results may seem surprising as the introduction of tetra-epoxy monomers with the catechin rigid structure was expected to increase glass transition temperature. However, the role of the di-epoxy benzodioxane derivative is not elucidated and may explain these results. Molecules 2017, 22, 149 29 of 47

epichlorohydrin polymer. The materials based on the 50% and 75% blends of glycidylated catechin exhibited slightly lower and 10 °C higher Tg than DGEBA-based materials, respectively. These results may seem surprising as the introduction of tetra-epoxy monomers with the catechin rigid structure was Moleculesexpected2017, 22 to, 149 increase glass transition temperature. However, the role of the di-epoxy benzodioxane30 of 48 derivative is not elucidated and may explain these results.

Molecules 2017, 22, 149 3 of 7 Molecules 20172017,, 2222,, 149149 33 ofof 77 Molecules 2017Figure, 22, 149 27. Epoxy monomers obtained by glycidylation of catechin according to Nouailhas3 of 7 et al. [128]. Figure 27. Epoxy monomers obtained by glycidylation of catechin according to Nouailhas et al. [128]. Molecules 2017, 22, 149 3 of 7 5.2.5. Conclusions 5.2.5. Conclusions A wide range of di-aromatic bio-based epoxy monomers has been synthesized and cured over the 4-methyl hexahydrophthalic 4-methyl hexahydrophthalic 78–92 - 246–316 [153] Apast wide years range with ofmost di-aromatic of them4-methyl exhibiting bio-based hexahydrophthalicanhydride interesting epoxy monomers78–92 and varied has - properties been246–316 synthesized in terms[153] of andglass curedtransition over the 4-methylanhydride hexahydrophthalicanhydride 78–92 - 246–316 [153] anhydride 78–92 - 246–316 [153] temperature and thermal stability.anhydride Shrewd strategies have been developed to slightly or largely step past years with most of them4-methyl exhibiting hexahydrophthalic interesting and varied properties in terms of glass transition 78–92 - 246–316 [153] temperatureaway from and the thermal detrimental stability. structureanhydride Shrewd of BPA, strategies but similarly have to mono-aromatic been developed compounds, to slightly very or few largely were step studied for their estrogen activity. The following tables (Tables 3–5) gather the known data on cured away from the detrimental structure of BPA, but similarly to mono-aromatic compounds, very few a b materialsa Tα measured based by on DMA the atdi-aromati the peak positionc di- and of losspoly-epoxy modulus curve;monomers. b Tα measured by DMA at the a Tα measured by DMA at the peak position of loss modulus curve; b Tα measured by DMA at the were studieda Tα measured for by their DMA estrogen at the peak activity. position of Theloss modulus following curve; tables b Tα measured (Tables by3 DMA–5) gatherat the the known data on maximum Tα measured of tan by δ ;DMA c Td under at the air peak flow. position of loss modulus curve; Tα measured by DMA at the amaximumα of tan δ; c Td under air flow. b α maximummaximum T measured of of tan tan by δ δ; ;cDMA T Td under under at the air air flow.peak flow. position of loss modulus curve; T measured by DMA at the curedmaximum materials of tan based δ; c Td under on the air flow. di-aromatic di- and poly-epoxy monomers. aTable Tα Tablemeasured 3. Di-epoxy 3. Di-epoxyby DMA cresins at resinswith the peaktwo with aromaticposition two aromatic ofrings loss separated modulus rings separatedbycurve; one batom, Tα measuredby and one thermal atom, by DMA propertiesand at thermal the of the properties of the TablemaximumTable 3. 3. Di-epoxy Di-epoxy of tan δ ;resins resins Td under with with air two two flow. aromatic aromatic rings rings separated separated by by one one atom, atom, and and thermal thermal properties properties of of the the Table 3. Di-epoxy cresins with two aromatic rings separated by one atom, and thermal properties of the maximumcured materials. of tan δ ; Td under air flow. curedTablecuredcured materials. 3.materials. Di-epoxy materials. resins with two aromatic rings separated by one atom, and thermal properties of the curedTable materials.3. Di-epoxy resins with two aromatic rings separated by one atom, and thermal properties of the Tablecured 3.materials.Di-epoxy resins with two aromatic rings separatedTg (°C) byor Tαone (°C) atom, and thermal properties of the Tg (°C) or Tα (°C) Tg (°C) or Tα (°C)T g (°C) or Tα (°C) cured materials. Tg (°C) or Tα (°C) cured materials.Epoxy Curing Agent Tg (°C) or Tα (°C) Td,5% (°C) Referenced,5% EpoxyEpoxy Curing AgentCuring Agent Td,5%DGEBA (°C) ReferenceT (°C) Reference Epoxy Curing Agent Tg (°C) or Tα (°C)Materials DGEBA Td,5% (°C) Reference Epoxy Curing Agent Materials DGEBADGEBA TComparisond,5% (°C) Reference Materials Comparison d,5% Epoxy Curing Agent Materials ComparisonDGEBA T (°C) Reference Epoxy Curing Agent Materials Comparison◦DGEBA Td,5% (°C)◦ Reference Materials DGEBATgComparison( C) or Tα ( C) Comparison ◦ Epoxy Curing Agent Materials a b a b Td,5% ( C) Reference 4,4′-methylene-biscyclohexylamineMaterials Comparison104 /111 DGEBA Comparison149 /158 363 [17] a b a b 4,4′-methylene-biscyclohexylamine 104 a/111 b 149 a/158 b 363 [17] 4,44,4′-methylene-biscyclohexylamine′-methylene-biscyclohexylamine 104104 a /111/111 b 149149 a /158/158 b 363363 [17][17] 4,4′-methylene-biscyclohexylamine 104 a/111 b 149 a/158 b 363 [17] a b a b 4,40′-methylene-biscyclohexylamine a104 b/111 a 149b /158 363 [17] 4,4′-methylene-biscyclohexylamine-methylene-biscyclohexylamine 104 /111104 a/111149b /158 149363a /158 b [17] 363 [17]

Isophorone Diamine 158 361

Isophorone Diamine 158 361 IsophoroneIsophorone Diamine Diamine 158 158 361361 Isophorone Diamine 158 361 IsophoroneIsophoroneIsophorone Diamine Diamine Diamine 158 158 158 361 361 361

Isophorone Diamine 136 363

Isophorone Diamine 136 363 IsophoroneIsophorone Diamine Diamine 136 136 363 363165 [157] IsophoroneIsophoroneIsophorone Diamine Diamine Diamine 136 136 136 363 363 363 Isophorone Diamine 136 363

165 [157] 165165 [157][157] Isophorone Diamine 165 96165 [157] [157] 363 165 165 [157] [157]

Isophorone Diamine 96 363 IsophoroneIsophorone Diamine Diamine 96 96 363 363 IsophoroneIsophoroneIsophorone Diamine Diamine Diamine 96 96 96 363 363 363 Isophorone Diamine 96 363

Isophorone Diamine 86 362

Isophorone Diamine 86 362 c IsophoroneIsophoroneIsophoroneIsophorone4,4′ -diaminodiphenylDiamine Diamine Diamine Diamine methane 86 86 86 86 161 362 362 362 179 367/368 362 [116] Isophorone Diamine 86 362 Isophorone Diamine 86 362

c c 4,44,4′-diaminodiphenyl′-diaminodiphenyl methane methane 161 161 179 179 367/368 367/368 c 4,44,40 ′-diaminodiphenyl′-diaminodiphenyl methane methane 161 161 179 179 367/368 367/368 c c 4,44,4-diaminodiphenyl′-diaminodiphenyl methane methane 161 161 179 179 367/368 c 367/368 4,4′-diaminodiphenyl methane 161 179 367/368 c [116] [116] [116] 4,4′4,4-methylene′-methylene [116][116] 4,40′-methylene 143 154 363/360 c c 4,4′-methylene 143 154 363/360 c [116] bis(5-isopropyl-2-methylaniline)bis(5-isopropyl-2-methylaniline)4,44,4′-methylene-methylene 143 154 363/360c [116] c bis(5-isopropyl-2-methylaniline) 143 143154 154363/360 c 363/360 bis(5-isopropyl-2-methylaniline)bis(5-isopropyl-2-methylaniline)4,4′-methylene 143 154 363/360 bis(5-isopropyl-2-methylaniline) 143 154 363/360 c bis(5-isopropyl-2-methylaniline)

b c 4,4′0-diaminodiphenyl methane 184/183 b b - 346/355 [159]c c 4,44,4-diaminodiphenyl′-diaminodiphenyl methane methane 184/184/183183 b - -346/355 c [159] 346/355 [159] 4,44,4′-diaminodiphenyl′-diaminodiphenyl methane methane 184/ 184/183183 b - - 346/355346/355 c [159][159] 4,4′-diaminodiphenyl methane 184/183 b - 346/355 c [159] b c 4,4′-diaminodiphenyl methane 184/183 - 346/355 [159] a α b α a T measured by DMA at the peak position of loss modulus curve; T measuredb b by DMA at the a Tα measured by by DMA DMA at at the the peak peak position position of ofloss loss modulus modulus curve; curve; b Tα measuredT measured by DMA byDMA at theat the maximum of a Tαα measured byc DMA at the peak position of loss modulus curve; b Tα measuredα by DMA at the a maximumTα measuredc of tan by δ; DMA Td under at airthe flow. peak position of loss modulus curve; b Tα measured by DMA at the tanmaximum Tα δmeasured; T ofunder tan by δ ;DMA cair Td under flow. at the air peak flow. position of loss modulus curve; Tα measured by DMA at the amaximumα d of tan δ; c Td under air flow. b α maximumTablemaximum T measured 4. Di-epoxy of of tan tan by δ δ;monomers ;cDMA T Td under under at withthe air air flow.peak flow.two aromatic position rings of loss sepa modulusrated by morecurve; than Tone measured atom exhibiting by DMA ester at the c d maximumTable 4. Di-epoxy of tan δ; cmonomers T under air with flow. two aromatic rings separated by more than one atom exhibiting ester TablemaximumorTable amide 4. 4. Di-epoxy bonds,Di-epoxy of tan and δ ;monomers monomers thermalTd under properties airwith with flow. two two of aromatic thearomatic cured rings materials.rings sepa sepa ratedrated by by more more than than one one atom atom exhibiting exhibiting ester ester Tableor amide 4. Di-epoxy bonds, and monomers thermal propertieswith two aromatic of the cured rings materials. separated by more than one atom exhibiting ester Tableor amide 4. Di-epoxy bonds, and monomers thermal propertieswith two aromatic of the cured rings materials. separated by more than one atom exhibiting ester or amide bonds, and thermal properties of the cured materials. or amide bonds, and thermal properties of the cured materials.

MoleculesMolecules2017 2017, 22, ,22 149, 149 4 of 7 31 of 48 Molecules 2017, 22, 149 4 of 7 Molecules 2017, 22, 149 4 of 7

Tg (°C) or Tα (°C) Molecules 2017, 22, 149 Tg (°C) or Tα (°C) 4 of 7 Molecules 2017, 22, 149 Tg (°C) or Tα (°C) T4d,5% of 7 TableMolecules 2017 4. ,Di-epoxy 22, 149 Epoxy monomers with two aromaticCuring Agent rings separated by moreT4d,5% of 7 thanReference one atom exhibiting Molecules 2017, 22, 149 Epoxy Curing Agent DGEBA (°C)T4d,5% of 7 Reference Epoxy Curing Agent Materials DGEBA (°C) Reference ester or amide bonds, and thermal properties of theTg cured(°C)Materials or Tα (°C) materials. ComparisonDGEBA (°C) Tg (°C)Materials or Tα (°C) Comparison Tg (°C) or Tα (°C) ComparisonTd,5% g α Epoxy Curing Agent T (°C) or T (°C) Td,5% Reference Epoxy Curing Agent DGEBA Td,5%(°C) Reference Epoxy Curing Agent Materials DGEBA ◦ T(°C)d,5% Reference◦ Epoxy Curing Agent Materials ComparisonDGEBAT g ( C)(°C) or Tα Reference( C) Materials ComparisonDGEBA (°C) ◦ Epoxy Curing AgentMaterials Comparison T ( C) Isophorone Comparison d,5% Reference Isophorone Materials66 b 170 DGEBA b Comparison300 [123] Isophoronediamine 66 b 170 b 300 [123] diamine 66 b 170 b 300 [123] diamine Isophorone Isophorone b b Isophorone 66b 170b 300 [123] Isophoronediamine 66 b 170 b 300 [123] IsophoronediamineIsophorone 66 b 170 b 300 [123] diamine 66 b 66170b b 300 170[123]b 300 [123] diaminediamineIsophorone b b Isophorone 51/61 150/174 314 Isophoronediamine 51/61 b 150/174 b 314 diamine 51/61 b 150/174 b 314 Isophoronediamine Isophorone 51/61 b 150/174 b 314 [166] Isophorone 51/61 b 150/b 174 b 314b diamine1,10-diaminodecane 51/61 b 33/33150/ 174 b 98/97314 314 [166] Isophoronediamine 51/61 150/b 174 314b diamineIsophorone1,10-diaminodecane 51/61 b 33/33150/ 174b b 98/97314 314 b [166] diamine1,10-diaminodecane 51/33/3361 b 98/97 b 150/314174 314 diamine [166] b b [166] 1,10-diaminodecane 33/33b b98/ 97b 314b [166] 1,10-diaminodecaneDifurfurylamine 33/ 33 b 32/4198/ 97 b 92/110314 [166]306 1,10-diaminodecaneDifurfurylamine 33/ 33 b 32/4198/ b 97 b 92/110314 b [166]306 [166] 1,10-diaminodecane 1,10-diaminodecaneDifurfurylamine 33/ 33 b 33/32/413398/ b 97b b 92/110314 b 98/30697 b 314 b b Difurfurylamine 32/41b 92/110b 306 Difurfurylamine 32/41 b 92/110 b 306 Difurfurylamine 32/41 b 92/110b b 306 b DifurfurylamineDifurfurylamine 32/41 b 32/92/41110 b 306 92/110 306 Isophorone Isophorone 50 b 170 b 331 [123] Isophoronediamine 50 b 170 b 331 [123] Isophoronediamine b b Isophorone b 50 b 170 331 [123] IsophoroneIsophoronediamine 5050 b 170b170 b 331331 [123][123]b Isophoronediaminediamine 50 b 50 170 b 331 170[123] 331 [123] diaminediamine 50 b 170 b 331 [123] diamine

Isophorone Isophorone 85/99 b 150/174 b 295 Isophoronediamine 85/99 b 150/174 b 295 IsophoroneIsophorone b b Isophoronediamine 85/85/9999 b 85/99150/150/ b 174174 b 150/174295295 b 295 Isophoronediaminediaminediamine 85/99 b 150/174 b 295 diamineIsophorone 85/99 b 150/174b b 295 b diamine1,10-diaminodecane 85/54/6899 b 98/97 b 150/292174 295 diamine b b 1,10-diaminodecane b 54/68 b 98/97 292 1,10-diaminodecane1,10-diaminodecane 54/54/6868 b 98/98/b 9797 b 292292b 1,10-diaminodecane1,10-diaminodecane 54/68 b 54/6898/ 97 b 98/97292 292 1,10-diaminodecane 54/68 b 98/97 b 292 Difurfurylamine 63/85 b b 92/110 b 304 b 1,10-diaminodecane b b 68b b b b 97 292 DifurfurylamineDifurfurylamineDifurfurylamine 63/8563/85 b 54/63/8592/11092/110 b 92/110304304 98/304 DifurfurylamineDifurfurylamine 63/85 b 63/8592/110 b b 92/110304 b 304 [166] b b Difurfurylamine 63/85 92/110b 304 [166][166] b [166] [166] Difurfurylamine [166] 304 63/85 92/110 [166] Isophorone [166] IsophoroneIsophoroneIsophorone 75 150 282 Isophorone 7575 150150 282 282 IsophoroneIsophoronediamine 75 75 150 150282 282 Isophoronediaminediamine 75 150 282 diaminediamine 75 7575 150 150282 150282 282 diaminediaminediamine

1,10-diaminodecane1,10-diaminodecane1,10-diaminodecane 6969 69 98 98 98297297 297 1,10-diaminodecane1,10-diaminodecane1,10-diaminodecane 69 69 69 98 98297 98297 297 1,10-diaminodecane 69 98 297 1,10-diaminodecane 69 98 297

DifurfurylamineDifurfurylamineDifurfurylamineDifurfurylamine 74 74 74 74 92 92 92271 271 92271 271 Difurfurylamine 74 92 271 Difurfurylamine 74 92 271 a DifurfurylamineDifurfurylamine 74 74 92 b 92271 271 Ta α b b α T a aαTαmeasured αmeasured byby by DMADMA DMA atat thethe at peakpeak the peakpositionposition position ofof lossloss modulusmodulus of loss curve;curve; modulus b T Tα αmeasured measured curve;b α by by DMA DMAα measured at at the the by DMA at the maximum of a TTα measured measured by by DMA DMA at theat thepeak peak position position of loss of modulus loss modulus curve; curve; Tα measured b T measured by DMA byat theDMA at the a Tα cmeasured byc DMA at the peak position of loss modulus bcurve; Tα measured by DMA at the maximum aTδα measuredT of tan by δ; DMAcc Td under at the air peakflow. position of loss modulus curve; Tα measuredb by DMA at the tanmaximum T;α measuredd ofunder tan δ ;by; c Tair ddcDMA underunderd flow. atairair theflow.flow. peak position of loss modulus curve; Tα measured by DMA at the maximummaximum of of tan tan δ; δ T; dc underT under air flow. air flow. Tablemaximummaximum 5. Di-epoxy of oftan tan δ; monomers cδ T; d underTd under air with flow. air two flow. aromatic rings separated by more than one atom, and thermal TableTablemaximum 5. 5. Di-epoxy Di-epoxy of tan monomers monomersδ ; monomersc Td under withwith air with twotwo flow. twoaromaticaromatic aromatic ringsrings separatedringsseparated separated by by more more by than than more one one than atom, atom, one and and atom, thermal thermal and thermal propertiesTableTable 5. 5.Di-epoxy ofDi-epoxy the cured monomers monomers materials. with with two aromatictwo aromatic rings separatedrings separated by more by than more one than atom, one and atom, thermal and thermal propertiespropertiesTable 5. ofDi-epoxy ofthe the cured cured monomers materials. materials. with two aromatic rings separated by more than one atom, and thermal Tablepropertiesproperties 5. Di-epoxy of ofthe the cured cured monomersmaterials. materials. with two aromatic rings separated by more than one atom, and thermal TTg g(°C) (°C) or or T Tα α(°C) (°C) properties of the cured materials. Tg (°C) or Tα (°C) properties of the cured materials. Tg (°C) orT Tgα (°C) (°C) or Tα (°C) Tg (°C) or Tα (°C) d,5% Epoxy CuringCuring Agent Agent TTd,5%d,5% (°C) (°C) Reference Reference Epoxy Curing Agent TgDGEBA (°C)DGEBA or T α (°C)T d,5% (°C) Reference EpoxyEpoxy CuringCuring Agent AgentMaterials Materials DGEBA Td,5% (°C) Td,5% Reference (°C) Reference Epoxy Curing AgentMaterials ComparisonComparisonDGEBA DGEBA Td,5% (°C) Reference Epoxy Curing AgentMaterials Materials Comparison DGEBA ◦ Td,5% (°C)◦ Reference MaterialsComparison ComparisonDGEBA Tg ( C) or Tα ( C) Epoxy Materials Comparison ◦ Curing Agent Comparison Td,5% ( C) Reference 4,44,4′-Diamino′-Diamino Materials DGEBA Comparison 4,4′-Diamino 60/7060/70 - - - - diphenyldiphenyl4,4′-Diamino methane methane 60/70 - - diphenyl 4,4methane′-Diamino 0 60/70 - - diphenyl 4,4methane′-Diamino4,4 -Diamino 60/70 - - diphenyl4,4′-Diamino methane 60/70 - - diphenyl methanediphenyl 60/70 60/70- - - - diphenyl methane 4,4 ′-Diamino methane 4,44,4′-Diamino′-Diamino 45/58 - - [165] diphenyl4,4′-Diamino methane 45/5845/58 - - - - [165][165] diphenyldiphenyl methane methane 0 45/58 - - [165] diphenyl 4,4methane′-Diamino4,4 -Diamino 4,4′-Diamino 45/58 - - [165] diphenyl4,4′-Diamino methanediphenyl 45/58 45/58- - -[165] - diphenyl methane 45/58 - - [165] diphenyl methanemethane [165] 4,4′-Diamino 4,44,4′-Diamino′-Diamino 39/54 - - diphenyl4,4′-Diamino methane 39/5439/54 - - - - Molecules 2017, 22, 149 diphenyldiphenyl methane methane4,4 0 -Diamino39/54 - - 5 of 7 Molecules 2017, 22, 149 diphenyl 4,4methane′-Diamino 5 of 7 Molecules 2017, 22, 149 4,4′-Diaminodiphenyl 39/54 39/54- - -5 of 7 - Molecules 2017, 22, 149 diphenyl4,4′-Diamino methane 39/54 - - 5 of 7 diphenyl methanemethane 39/54 - - diphenyl methane 4,4′-Diamino 4,4′-Diamino 200 - 341/342 c [16]] diphenyl methane0 200 - 341/342 c [16]] 4,4′-Diamino4,4 -Diamino 4,4diphenyl′-Diamino methane c c diphenyl200 200 200- - 341/342 c 341/342 [16]]- [16]] 341/342 [16] diphenyldiphenyl methane methane methane

Hexahydrophthalic b b Hexahydrophthalic 134 b 132 b - HexahydrophthalicanhydrideHexahydrophthalic 134 132 - Hexahydrophthalicanhydride b b b b 134 134 b 132134 132 b - - 132 - anhydrideanhydrideanhydride Phthalic anhydride 138 b - - Phthalic anhydride 138 b - - Phthalic anhydridePhthalic 138 b - b - b 138 - - Phthalic anhydrideanhydride 138 - - b Nadic anhydride 147 b - - NadicNadic anhydride anhydride 147 b 147 - b - - - NadicNadic anhydride anhydride 147 b 147 - - - - Hexahydrophthalic [173] HexahydrophthalicHexahydrophthalicHexahydrophthalic 170 b 132 b - [173] [173] [173] anhydride 170 b 170 b 132 b b132 b - - b - Hexahydrophthalicanhydrideanhydride 170 132 [173] anhydride 170 b 132 b - anhydride

HexahydrophthalicHexahydrophthalicHexahydrophthalic Hexahydrophthalic 149 b 149 b 132149 b b132 b - - 132 b - anhydrideanhydride 149 b 132 b - Hexahydrophthalicanhydrideanhydride 149 b 132 b - anhydride

a a b b a Tα measured by DMA at the peak position of loss modulus curve; Tα measuredb by DMA at the Tα Tmeasuredα measuredby by DMADMA at the peak peak position position of loss of lossmodulus modulus curve; curve; Tα measuredTα measured by DMA at by the DMA at the maximum of a α b α maximumT c measured of tan by δ; cDMA Td under at airthe flow. peak position of loss modulus curve; T measured by DMA at the aδ T c b tanmaximum T; α measuredd under of tan by δ air; DMA T flow.d under at the air flow.peak position of loss modulus curve; Tα measured by DMA at the maximumTable 6. Poly-aromatic of tan δ; c T epoxyd under monomers air flow. from crude bio-mass and thermal properties of the cured materials. Table 6. Poly-aromaticc epoxy monomers from crude bio-mass and thermal properties of the cured materials. Tablemaximum 6. Poly-aromatic of tan δ; T epoxyd under monomers air flow. from crude bio-mass and thermal properties of the cured materials. Table 6. Poly-aromatic epoxy monomers from crude bio-mass Tandg (°C) thermal or Tα (°C) properties of the cured materials. Tg (°C) or Tα (°C) g α Epoxy Curing Agent T (°C) or T (°C) Td,5% (°C) Reference Tg (°C)DGEBA or Tα (°C) Epoxy Curing Agent Materials Td,5% (°C) Reference Epoxy Curing Agent ComparisonDGEBA Td,5% (°C) Reference Materials DGEBA Epoxy Curing Agent Materials Comparison Td,5% (°C) Reference ComparisonDGEBA Diethylenetriamine - Materials - 252 d Comparison d Epoxidized depolymerized kraft lignin Diethylenetriamine - - 252 d 4,4-diaminodiphenylDiethylenetriamine - - 252 - - 290 d d Epoxidized depolymerized kraft lignin Diethylenetriaminemethane - - 252 Epoxidized depolymerized kraft lignin 4,4-diaminodiphenyl 4,4-diaminodiphenyl - - 290 d [186] Epoxidized depolymerized kraft lignin methane - - 290 d Diethylenetriamine4,4-diaminodiphenylmethane - - 228 d - - 290 d [186] Epoxidized depolymerized methane [186] d organosolv lignin 4,4-diaminodiphenylDiethylenetriamine - - 228 d [186] Diethylenetriamine - - - - 257 d 228 Epoxidized depolymerized d Epoxidized depolymerized Diethylenetriaminemethane - - 228 organosolv lignin 4,4-diaminodiphenyl Epoxidizedorganosolv depolymerized lignin 4,4-diaminodiphenyl - - 257 d methane - - 257 d organosolv lignin 4,4-diaminodiphenylmethane - - 257 d methane 94 -

94 - Phenol Novolac 94 95 - [189] 94 -

Phenol Novolac 95 [189] Phenol Novolac 134 95 - [189] Phenol Novolac 95 [189]

134 - Phenol Novolac (TD2131) - 134 - 293 - Epoxidized lignin (Cedar) 134 -

Lignin (Cedar) - - 296

Phenol Novolac (TD2131) - - 293 PhenolPhenol Novolac Novolac (TD2131) (TD2131) - - - - 275 293 [190] Epoxidized lignin (Cedar) EpoxidizedEpoxidized lignin lignin (Eucalyptus) (Cedar) Phenol Novolac (TD2131) - - 293 Lignin (Cedar) - - 296 Epoxidized lignin (Cedar) LigninLignin (Eucalyptus) (Cedar) - - - - 274 296 Lignin (Cedar) - - 296 Phenol Novolac (TD2131) - - 275 [190] Epoxidized lignin (Bamboo) PhenolPhenol Novolac Novolac (TD2131) (TD2131) - - - - 266 275 [190] Epoxidized lignin (Eucalyptus) Epoxidized lignin (Eucalyptus) Phenol Novolac (TD2131) - - 275 [190] Lignin (Eucalyptus) - - 274 Epoxidized lignin (Eucalyptus) Lignin (Eucalyptus) - - 274 Lignin (Eucalyptus) - - 274 Epoxidized lignin (Bamboo) Phenol Novolac (TD2131) - - 266 Epoxidized lignin (Bamboo) Phenol Novolac (TD2131) - - 266 Epoxidized lignin (Bamboo) Phenol Novolac (TD2131) - - 266

Molecules 2017, 22, 149 32 of 48

5.3. Polyaromatic Epoxy Compounds As observed in the previous parts, it is hard to match up with the properties of DGEBA-based materials, especially in term of glass transition temperature, thus making the use of polyaromatic and possibly poly-epoxy derivatives a viable alternative. Moreover, the direct use of the biomass would avoid the costly and time-consuming depolymerization and purification steps. The next part will present the epoxy prepolymers containing at least three aromatic moieties and two or more epoxy rings.

5.3.1. Glycidylated Lignin Chemical modifications of lignin functional groups were carried out to increase its solubility and chemical reactivity, and thus its range of applications. Among the possible functionalizations, modification of the hydroxyl groups is the most versatile since phenolic hydroxyl groups are the most reactive groups and can significantly affect the chemical reactivity of the materials. Allylation [178], esterification [179,180], phenolation [181] or etherification [172,182–184] of lignin have been widely investigated. However, only few research groups attempted to epoxidize raw depolymerized lignin by direct O-glycidylation. As the isolation of pure building blocks from lignin is both economically and environmentally costly, Fache et al. [185] developed model mixtures of depolymerization products of lignin from both hardwood and softwood, including for example vanillin, syringaldehyde and their acid equivalents. After a Dakin oxidation to increase the number of reactive hydroxyl groups and an O-glycidylation step, the materials cured with IPDA showed interesting thermo-mechanical properties, with Tg values of 99 ◦C and 113 ◦C for hardwood and softwood model mixtures, respectively. These temperatures remain lower than that of DGEBA-based materials synthesized by the same authors in fairly similar conditions [135], probably due to the mono-epoxy derivatives included in the mixtures. Both materials exhibited a good thermal stability with onset degradation temperatures above 250 ◦C under nitrogen atmosphere and high char yields of 20%–23% at 600 ◦C. This study demonstrated that purification steps are not necessarily required to achieve high-performance epoxy thermosets from biomass. Ferdosian et al. [186] carried out the glycidylation of de-polymerized organosolv or Kraft lignin with epichlorohydrin and highlighted the presence of epoxy by normalized FTIR. Then, they prepared materials of the resulting epoxidized lignin by curing with aromatic and aliphatic amines to show the influence of the curing agent on the thermal stability. Despite the good thermal stability and very high char rate at 800 ◦C, no glass transition temperature or mechanical properties were given. Following the work of Zhao et al. [187], Sun et al. [188] prepared a liquid epoxy resin from phenolated lignosulfonate and focused their study on its curing kinetics with maleic anhydride. No data on the material properties were given. Kaiho et al. [189] used a three-steps pathway to synthesize lignin-based epoxy resins (Figure 28): (i) selective β-O-4 bond cleavage of eucalyptus globulus lignin, (ii) transacetalization for the synthesis of spiro rings (47) or annulation to form a phenyl-naphthalene derivative (48) and (iii) O-glycidylation with epichlorohydrin. The obtained resins were cured with phenol novolac yielding materials with glass transition temperatures ranging from 67 to 134 ◦C and of 95 ◦C for an epoxy/bisphenol A system. As expected from the structures, the more rigid structure of the phenyl-naphthalene lignin epoxy prepolymers lead to higher Tg values and higher flexural strength. Similarly, Asada et al. [190] used various low molecular weight lignins extracted with methanol to synthesized bio-based epoxy resins. The glycidylation was assessed by FTIR and 1H NMR spectroscopies and four sets of materials were cured using two epoxy resins: glycidylated lignin and an oil-based commercial Epikote 828 (EP828, DGEBA from Japan Epoxy Resins Co. Ltd., Nagoya, Japan) and two phenolic hardeners: non-glycidylated lignin and TD2131 (a phenol novolac from DIC Corp., Tokyo, Japan). The materials exhibited high thermal stabilities with Td,5% ranging from 259 to 326 ◦C, still lower than the fully oil-based material (361 ◦C) but with higher char rates, up to 41% at 800 ◦C. Unfortunately, no glass transition temperatures or mechanical properties were provided. It is Molecules 2017, 22, 149 32 of 47

Following the work of Zhao et al. [187], Sun et al. [188] prepared a liquid epoxy resin from phenolated lignosulfonate and focused their study on its curing kinetics with maleic anhydride. No data on the material properties were given. Kaiho et al. [189] used a three-steps pathway to synthesize lignin-based epoxy resins (Figure 28): (i) selective β-O-4 bond cleavage of eucalyptus globulus lignin, (ii) transacetalization for the synthesis of spiro

Moleculesrings (472017) or, 22 annulation, 149 to form a phenyl-naphthalene derivative (48) and (iii) O-glycidylation33 with of 48 epichlorohydrin. The obtained resins were cured with phenol novolac yielding materials with glass transition temperatures ranging from 67 to 134 °C and of 95 °C for an epoxy/bisphenol A system. As worthexpected noting from that the the structures, lignin extract theinsoluble more rigid in methanol structure was of enzymaticallythe phenyl-nap transformedhthalene lignin into glucose, epoxy increasingprepolymers the lead biomass to higher promotion. Tg values and higher flexural strength.

FigureFigure 28. 28.Lignin-based Lignin-based epoxyepoxy resinsresins synthesized by by Kaiho Kaiho et et al. al. [189]. [189 ].

Similarly, Asada et al. [190] used various low molecular weight lignins extracted with methanol to Other uses have been found to lignin for the improvement of epoxy resins. Lignin functionalized synthesized bio-based epoxy resins. The glycidylation was assessed by FTIR and 1H NMR spectroscopies by a dicarboxylic acid prepared by dimerizing unsaturated fatty acids was considered as a phenolic and four sets of materials were cured using two epoxy resins: glycidylated lignin and an oil-based aldehyde amine curing agent by Fang et al. [191]. Similarly, Engelmann et Ganster [192] blended a low commercial Epikote 828 (EP828, DGEBA from Japan Epoxy Resins Co. Ltd., Nagoya, Japan) and two molecular weight lignin fraction with the bio-based tri-glycidylated glycerol for the preparation of phenolic hardeners: non-glycidylated lignin and TD2131 (a phenol novolac from DIC Corp., Tokyo, composites reinforced with cellulosic fibers. Japan). The materials exhibited high thermal stabilities with Td,5% ranging from 259 to 326 °C, still lower 5.3.2.than the Glycidylated fully oil-based Tannins material (361 °C) but with higher char rates, up to 41% at 800 °C. Unfortunately, no glass transition temperatures or mechanical properties were provided. It is worth noting that the ligninFew extract research insoluble papers in methanol are devoted was enzymatically to the use of tanninstransformed and tannin-basedinto glucose, increasing molecules the for biomass epoxy materialspromotion. synthesis compared to lignin ones. Maybe the reduced availability of the former is a key factorOther in this uses result, have asbeen the found complexity to lignin and for the impr variabilityovement of of both epoxy these resins. bio-resources Lignin functionalized seem similar. by Asa dicarboxylic previously mentionedacid prepared before, by Aoufdimerizing et al. [unsaturated176] considered fatty the acids hydrolysable was considered tannins as from a phenolic tara as potentialaldehyde precursoramine curing for epoxyagent by prepolymer Fang et al. synthesis,[191]. Similarly, but they Engelmann first carried et Ganster out a depolymerization [192] blended a low to obtainmolecular gallic weight acid mixtureslignin fraction prior with to glycidylation. the bio-based Benyahya tri-glycidylated et al. [ 193glycerol] focused for the their preparation work on the of inexpensivecomposites reinforced green tea with (Camellia cellulosic sinensis fibers.) tannins, which are condensed tannins mainly composed of epicatechin, catechin dimers and their galloylated equivalents. The tannins extracts were glycidylated with5.3.2. epichlorohydrinGlycidylated Tannins and cured with isophorone diamine. The obtained materials exhibited a Tα of 142 ◦C, a value similar to the 140 ◦C of the material from IPDA and DER352 (a mixture of DGEBA and DGEBFFew supplied research by papers Dow (France)). are devoted Swelling to the ratios use andof tannins soluble and fractions tannin-based of these materialsmolecules also for proved epoxy tomaterials be very synthesis low. Jahanshahi compared et to al. lignin [194 ones.,195] recentlyMaybe the published reduced availability two papers of on the the former use ofis a condensed key factor tanninsin this result, for the as synthesis the complexity of bio-based and materials.the variabili Inty the of first both one these [194 ],bio-resources Mimosa (Acacia seem mearnsii similar.) bark As tanninspreviously were mentioned glycidylated before, with Aouf epichlorohydrin et al. [176] considered but the the epoxy hydrolysab rings werele tannins then from reacted tara with as potential acrylic acidprecursor for adhesive for epoxy synthesis. prepolymer On synthesis, the contrary, but inthey their first second carried paper, out a theydepolymerization focused their to efforts obtain on gallic the glycidylationacid mixtures prior optimal to glycidylation. parameters andBenyahya characterization et al. [193] focused of the epoxy their work prepolymers, on the inexpensive thus very green brief resultstea (Camellia on tensile sinensis shear) tannins, tests of which glycidylated are condensed tannins tannins mixed withmainly DGEBA composed were of mentioned. epicatechin, catechin dimers and their galloylated equivalents. The tannins extracts were glycidylated with epichlorohydrin 5.3.3.and cured Glycidylated with isophorone Cardanol diamine. The obtained materials exhibited a Tα of 142 °C, a value similar to the 140 °C of the material from IPDA and DER352 (a mixture of DGEBA and DGEBF supplied by Dow To increase the low glass transition temperatures obtained with materials based on glycidylated (France)). Swelling ratios and soluble fractions of these materials also proved to be very low. Jahanshahi cardanol, a polyaromatic derivative of it can be designed by a two-steps synthesis, beginning by et al. [194,195] recently published two papers on the use of condensed tannins for the synthesis of a phenolation step and followed by direct O-glycidylation. The resulting product is idealized by bio-based materials. In the first one [194], Mimosa (Acacia mearnsii) bark tannins were glycidylated with a diepoxidized diaromatic cardanol structure and marketed with the name NC514 by Cardolite. epichlorohydrin but the epoxy rings were then reacted with acrylic acid for adhesive synthesis. On the However, Jaillet et al. [196] showed that the commercial product is a mixture (49), in which chains contrary, in their second paper, they focused their efforts on the glycidylation optimal parameters and are crosslinked and some epoxy rings are opened, as shown in Figure 29. This product may cause an characterization of the epoxy prepolymers, thus very brief results on tensile shear tests of glycidylated allergic skin reaction (H317), but can be used as food contact material, according to the supplier. tannins mixed with DGEBA were mentioned. Epoxy-amine based materials showed moderate glass transition temperature, from 13 to ◦ 30 C with linear aliphatic diamines [197–199]. The highest Tg was reached when epoxidized cardanol was cured with isophorone diamine, up to 50 ◦C, which is still 100 ◦C lower than that of DGEBA-based material [196]. Nevertheless, the fatty chain of the epoxidized cardanol allows more flexibility, conferring mechanical properties suitable for particular coating applications. Glycidyl ether Molecules 2017, 22, 149 33 of 47

5.3.3. Glycidylated Cardanol To increase the low glass transition temperatures obtained with materials based on glycidylated cardanol, a polyaromatic derivative of it can be designed by a two-steps synthesis, beginning by a Moleculesphenolation2017, 22step, 149 and followed by direct O-glycidylation. The resulting product is idealized34 by of 48a diepoxidized diaromatic cardanol structure and marketed with the name NC514 by Cardolite. However, Jaillet et al. [196] showed that the commercial product is a mixture (49), in which chains are crosslinked groupsand some have epoxy also rings been convertedare opened, to as methacrylate, shown in Figu allowingre 29. This radical product polymerization may cause for an other allergic coating skin applicationsreaction (H317), [200 but,201 can]. be used as food contact material, according to the supplier.

Figure 29. 29. IdealizedIdealized structure structure of phenolated of phenolated cardanol cardanol (left) and (left) real and structure real structure (49) of NC514 (49) of by NC514 Cardolite. by Cardolite.The numbers The between numbers brackets between indicate brackets the locations indicate theof the locations phenol ofmoieties the phenol along moieties the aliphatic along alkyl the aliphaticchain. alkyl chain.

Epoxy-amine based materials showed moderate glass transition temperature, from 13 to 30 °C with 5.3.4. Other Bio-Based Poly-Aromatic Monomers linear aliphatic diamines [197–199]. The highest Tg was reached when epoxidized cardanol was cured with Fourcadeisophorone et diamine, al. [167] up applied to 50 the°C, samewhich strategy is still 100 as °C previously lower than described that of DGEBA-based (see Section 5.2.2 material.) on various[196]. Nevertheless, aliphatic polyols the fatty (Scheme chain 14of). the Glycerol epoxidiz (50ed) andcardanol sorbitol allows (51 )more are bio-basedflexibility, precursors,conferring pentaerythritolmechanical properties (52), di-pentaerythritol suitable for particular (53) and coating trimethylolpropane applications. Glycidyl (54) are ether obtained groups via have formaldehyde also been andconverted acetaldehyde to methacrylate, from allowing syngas ofradical the polyme Biomass-to-Liquidrization for other (BtL) coating process applications or carbon [200,201]. monoxide hydrogenation. Moreover, according to their corresponding MSDS files, these polyols and ethyl-4-hydroxybenzoate5.3.4. Other Bio-Based Poly-Aromatic show no toxic Monomers risk. Only the dibutyltin dilaurate (DBTDL), which is used as catalyst in the phenolation step and the unavoidable epichlorohydrin present safety risks. MoleculesFourcade 2017, 22 , et149 al. [167] applied the same strategy as previously described (see Section 5.2.2.)34 of on 47 various aliphatic polyols (Scheme 14). Glycerol (50) and sorbitol (51) are bio-based precursors, pentaerythritol (52), di-pentaerythritol (53) and trimethylolpropane (54) are obtained via formaldehyde and acetaldehyde from syngas of the Biomass-to-Liquid (BtL) process or carbon monoxide hydrogenation. Moreover, according to their corresponding MSDS files, these polyols and ethyl-4-hydroxybenzoate show no toxic risk. Only the dibutyltin dilaurate (DBTDL), which is used as catalyst in the phenolation step and the unavoidable epichlorohydrin present safety risks. The phenolation step allowed 75% to 100% of conversion and the glycidylation of the resulting phenolic compounds led to an overall functionality ranging between 2 and 5. The synthesized polyaromatic glycidyl epoxides were blended with DGEBA and cured with dicyandiamide. From a general point of view, the addition of these new resins decreased the glass transition temperature of the blend materials, up to 24% when 25% are used. Glycidyl ether of trimethylolpropane-tris (4-hydroxybenzoate) was cured with dicyandiamide in the absence of DGEBA and the corresponding material showed a Tg of 105 °C, slightly lower than that of DGEBA-based material (119 °C). This result is very promising since materials can be prepared from non-toxic hydroxyl compounds and almost meet the thermal properties of the harmful DGEBA-based materials.

Scheme 14. 14. SyntheticSynthetic pathway pathway to obtain to obtain poly-aromatic poly-aromatic poly-glycidyl poly-glycidyl ethers developed ethers developed by Fourcade by etFourcade al. [167]. et al. [167].

Similarly, Ménard et al. [166] synthesized a tri-aromatic tri-epoxy monomer based on glycerol and ferulic acid (55) (Figure 30), although the esterification step was carried out with an enzymatic catalyst. After curing with IPDA, difurfurylamine and 1,10-diaminodecane, the thermosets showed Tg and Tα values ranging from 54 to 73 °C and 67 to 92 °C, respectively, with lowest values observed for the aliphatic diamine. These values remain significantly lower than those observed for DGEBA-based materials, as the increased functionality does not compensate the effect of the long aliphatic segments.

Figure 30. Tri-epoxy monomer based on ferulic acid and glycerol synthesized by Ménard et al. [166].

Very recently, Wan et al. [121,202] prepared two eugenol-based di-epoxy compounds, using a two-steps synthesis. The first step consisted in a Williamson etherification reaction between the hydroxyl group of eugenol and chloride aromatic derivatives, α,α′-dichloro-p-xylene [202] or cyanuric chloride [121]. The resulting allylated intermediate was achieved with an almost quantitative yield. Then, the carbon-carbon double bonds were oxidized using mCPBA with a moderate yield (44%–70%), yielding di- (56) and tri-glycidylated (57) derivatives (Figure 31). Despite a high net bio-based content (70%) of these new epoxy resins, the chosen synthetic pathway requires the use of mCPBA, which is known to be dangerous, and chlorinated derivatives, which are highly toxic (H302-H314-H330) for cyaniric chloride and detrimental for the environment (H400) for α,α′-dichloro-p-xylene.

Molecules 2017, 22, 149 34 of 47

Molecules 2017, 22, 149 35 of 48

The phenolation step allowed 75% to 100% of conversion and the glycidylation of the resulting phenolic compounds led to an overall functionality ranging between 2 and 5. The synthesized polyaromatic glycidyl epoxides were blended with DGEBA and cured with dicyandiamide. From a general point of view, the addition of these new resins decreased the glass transition temperature of the blend materials, up to 24% when 25% are used. Glycidyl ether of trimethylolpropane-tris (4-hydroxybenzoate) was cured with dicyandiamide in the absence of DGEBA and the corresponding material showed a T of 105 ◦C, slightly lower than that of DGEBA-based material (119 ◦C). This result Scheme 14. Syntheticg pathway to obtain poly-aromatic poly-glycidyl ethers developed by Fourcade is veryet al. promising [167]. since materials can be prepared from non-toxic hydroxyl compounds and almost meet the thermal properties of the harmful DGEBA-based materials. Similarly, Ménard Ménard et et al. al. [166] [166 ]synthesized synthesized a tri-a a tri-aromaticromatic tri-epoxy tri-epoxy monomer monomer based based on glycerol on glycerol and andferulic ferulic acid ( acid55) (Figure (55) (Figure 30), although 30), although the esterification the esterification step was step carried was carriedout with out an withenzymatic an enzymatic catalyst. catalyst.After curing After with curing IPDA, with difurfurylamine IPDA, difurfurylamine and 1,10-diaminodecane, and 1,10-diaminodecane, the thermosets the thermosets showed showed Tg and TTgα ◦ ◦ andvaluesTα rangingvalues rangingfrom 54 from to 73 54 °C to and 73 C67 and to 6792 to°C, 92 respectively,C, respectively, with withlowest lowest values values observed observed for the for thealiphatic aliphatic diamine. diamine. These These values values remain remain signific significantlyantly lower lower than than those those observed observed for DGEBA-basedDGEBA-based materials, as as the the increased increased function functionalityality does does not not compensate compensate the the effect effect of ofthe the long long aliphatic aliphatic segments. segments.

FigureFigure 30. 30.Tri-epoxy Tri-epoxy monomer monomer based basedon on ferulicferulic acid and glycerol synthesized synthesized by by Ménard Ménard etet al. al. [166]. [166 ].

Very recently, Wan et al. [121,202] prepared two eugenol-based di-epoxy compounds, using a Very recently, Wan et al. [121,202] prepared two eugenol-based di-epoxy compounds, using two-steps synthesis. The first step consisted in a Williamson etherification reaction between the hydroxyl a two-steps synthesis. The first step consisted in a Williamson etherification reaction between the group of eugenol and chloride aromatic derivatives, α,α′-dichloro-p-xylene [202] or cyanuric chloride hydroxyl group of eugenol and chloride aromatic derivatives, α,α0-dichloro-p-xylene [202] or cyanuric [121]. The resulting allylated intermediate was achieved with an almost quantitative yield. Then, the chloride [121]. The resulting allylated intermediate was achieved with an almost quantitative yield. carbon-carbon double bonds were oxidized using mCPBA with a moderate yield (44%–70%), yielding Then, the carbon-carbon double bonds were oxidized using mCPBA with a moderate yield (44%–70%), di- (56) and tri-glycidylated (57) derivatives (Figure 31). Despite a high net bio-based content (70%) of yielding di- (56) and tri-glycidylated (57) derivatives (Figure 31). Despite a high net bio-based content these new epoxy resins, the chosen synthetic pathway requires the use of mCPBA, which is known to be (70%) of these new epoxy resins, the chosen synthetic pathway requires the use of mCPBA, which is dangerous, and chlorinated derivatives, which are highly toxic (H302-H314-H330) for cyaniric chloride known to be dangerous, and chlorinated derivatives, which are highly toxic (H302-H314-H330) for and detrimental for the environment (H400) for α,α′-dichloro-p-xylene. cyaniric chloride and detrimental for the environment (H400) for α,α0-dichloro-p-xylene. Both eugenol-based epoxies showed lower reactivity towards amine than DGEBA, due to the lower electron withdrawing effect of the -CH2Ph epoxy-ring, compared to the usual -CH2OPh in BPA, but they avoid the use of epichlorohydrin and still, eugenol-based materials were successfully prepared by curing with aromatic diamines. The material based on the diglycidyl ether compounds 0 ◦ prepared from α,α -dichloro-p-xylene showed a Tg 40 C lower than for DGEBA-based materials, probably due to the methoxy groups on the aromatic rings and the -CH2O linkages between them. For the material obtained from cyanuric chloride, the glass transition temperature proved to be 33 ◦C higher despite the methoxy moieties, probably thanks to the tri-functional nature of the prepolymer. Molecules 2017, 22, 149 35 of 47

Molecules 2017, 22, 149 36 of 48 Molecules 2017, 22, 149 35 of 47

Figure 31. Eugenol-based poly-aromatic epoxy monomers.

Both eugenol-based epoxies showed lower reactivity towards amine than DGEBA, due to the lower electron withdrawing effect of the -CH2Ph epoxy-ring, compared to the usual -CH2OPh in BPA, but they avoid the use of epichlorohydrin and still, eugenol-based materials were successfully prepared by curing with aromatic diamines. The material based on the diglycidyl ether compounds prepared from α,α′-dichloro-p-xylene showed a Tg 40 °C lower than for DGEBA-based materials, probably due to the methoxy groups on the aromatic rings and the -CH2O linkages between them. For the material obtained Figure 31. Eugenol-based poly-aromatic epoxy monomers. from cyanuric chloride, theFigure glass 31. transitionEugenol-based temperature poly-aromatic proved epoxy to be monomers. 33 °C higher despite the methoxy moieties,Both probably eugenol-based thanks epoxies to the tri-fu showednctional lower nature reactivi of thety towards prepolymer. amine than DGEBA, due to the lower Wan etet al.al. [[120]120] alsoalso preparedprepared aa bio-basedbio-based epoxyepoxy buildingbuilding blockblock in aa two-steptwo-step synthesissynthesis from electron withdrawing effect of the -CH2Ph epoxy-ring, compared to the usual -CH2OPh in BPA, but they eugenol and terephthaloyl chloride: (i) a coupling reaction between eugenol and terephthaloyl chloride, eugenolavoid the and use terephthaloyl of epichlorohydrin chloride: and (i) still, a coupling eugenol-base reactiond materials between were eugenol successful and terephthaloylly prepared bychloride, curing followed by (ii) an epoxidation by oxidation of the C=C double bonds (58). The coupling step followedwith aromatic by (ii) diamines.an epoxidation The bymaterial oxidation based of theon C=Cthe doublediglycidyl bonds ether (58 ).compounds The coupling prepared step allowed from 93%allowed of yield 93% and of yieldthe final and product the final was product a white was solid. a white However, solid. the However, high toxicity the highof the toxicity commercially of the α,α′-dichloro-p-xylene showed a Tg 40 °C lower than for DGEBA-based materials, probably due to the availablecommercially terephthaloyl available chloride terephthaloyl has to chloride be taken has in toaccount. be taken The in account.material Theobtained material by obtainedcuring with by methoxy groups on the aromatic rings and the -CH2O linkages between them. For the material obtained curing with 3,30-diaminodiphenyl sulfone showed lower T and thermal stability and slightly higher 3,3from′-diaminodiphenyl cyanuric chloride, sulfone the glass showed transition lower temperature Tg and thermal provedg stability to be 33 and °C higherslightly despite higher the mechanical methoxy mechanical properties than the one based on DGEBA. Furthermore, it exhibited interesting intrinsic propertiesmoieties, probably than the thanks one tobased the tri-fu on nctionalDGEBA. nature Furthermore, of the prepolymer. it exhibited interesting intrinsic flame flame retardancy. retardancy.Wan et al. [120] also prepared a bio-based epoxy building block in a two-step synthesis from Czub [203] considered recycling polyethylene terephthalate (PET) wastes, especially coming from eugenolCzub and [203] terephthaloyl considered chloride: recycling (i) polyethylene a coupling re terephthalateaction between (P ET)eugenol wastes, and especially terephthaloyl coming chloride, from bottles, for the synthesis of epoxy resins (59). The precursor of PET, terephtalic acid, is mostly produced bottles,followed for by the (ii) synthesis an epoxidation of epoxy by resins oxidation (59). Theof the precursor C=C double of PET, bonds tereph (58talic). The acid, coupling is mostly step produced allowed by air oxidation of oil-based p-xylene [204]. However, as previously stated, p-xylene can be obtained by93% air of oxidation yield and of the oil-based final product p-xylene was [204]. a white However, solid. However,as previously the highstated, toxicity p-xylene of thecan commercially be obtained from cellulosic biomass, for example via dehydration of iso-butanol into iso-butene, dimerization into fromavailable cellulosic terephthaloyl biomass, chloridefor example has viato bedehydration taken in account.of iso-butanol The material into iso -butene,obtained dimerization by curing withinto iso-octene and catalytic conversion. It It can can also also be synthesized by dehydrogenation of 3-carene extracted 3,3′-diaminodiphenyl sulfone showed lower Tg and thermal stability and slightly higher mechanical from pine tree or limonene extracted from citrus fruits into p-xylene. Thus, the author degraded PET fromproperties pine treethan or thelimonene one based extracted on fromDGEBA. citrus Furthermore, fruits into p -xylene.it exhibited Thus, interesting the author intrinsicdegraded flame PET waste by glycolysis reaction into small aromatic hydroxy telechelic oligomers. Using epichlorohydrin, wasteretardancy. by glycolysis reaction into small aromatic hydroxy telechelic oligomers. Using epichlorohydrin, these polyhydroxyl derivatives were glycidylated and mixed with DGEBA oligomers leading to an these Czubpolyhydroxyl [203] considered derivatives recycling were polyethyleneglycidylated terephthalateand mixed with (PET) DGEBA wastes, oligomers especially leading coming to from an improved water absorption and resistance to HNO3 ,H2 SO4 and ethyl acetate (Figure 32). The addition improvedbottles, for water the synthesis absorption of epoxyand resistance resins (59 to). HNOThe precursor,3 H SO2 and4 of PET, ethyl tereph acetatetalic (Figure acid, 32).is mostly The addition produced of 5%–20%of 5%–20% of glycidylated of glycidylated PET-wastes PET-wastes in oligo(DGEBA) in oligo(DGEBA) cured cured with with isopho isophoronerone diamine diamine led to led similar to similar and by air oxidation of oil-based p-xylene [204].◦ However,◦ as previously stated, p-xylene can be obtained and slightly higherg T values, from 56 C to 53–63 C measured in DMTA. slightlyfrom cellulosic higher T biomass, values,g fromfor example 56 °C to via53–63 dehydration °C measured of iniso -butanolDMTA. into iso-butene, dimerization into iso-octene and catalytic conversion. It can also be synthesized by dehydrogenation of 3-carene extracted from pine tree or limonene extracted from citrus fruits into p-xylene. Thus, the author degraded PET waste by glycolysis reaction into small aromatic hydroxy telechelic oligomers. Using epichlorohydrin, these polyhydroxyl derivatives were glycidylated and mixed with DGEBA oligomers leading to an improved water absorption and resistance to HNO3, H2SO4 and ethyl acetate (Figure 32). The addition of 5%–20% of glycidylated PET-wastes in oligo(DGEBA) cured with isophorone diamine led to similar and slightly higher Tg values, from 56 °C to 53–63 °C measured in DMTA.

FigureFigure 32. 32.Epoxy Epoxy monomers monomers obtainedobtained from glyc glycolyzedolyzed PET PET waste waste by by Czub Czub [203]. [203 ].

5.3.5. Conclusions Due to their complexity and variability, bio-mass resources such as lignin or tannins have not received as much attention as smaller molecules for BPA substitution. However, as observed in the previous parts, they have been successfully turned into poly-epoxy monomers and used for the synthesis of materials exhibiting interesting properties. Similarly, smaller bio-based molecules such Figure 32. Epoxy monomers obtained from glycolyzed PET waste by Czub [203].

MoleculesMolecules 2017 2017, ,22 22, ,149 149 55 of of 7 7

4,4′-Diamino 4,4′-Diamino 200 - 341/342 c [16]] diphenyl methane 200 - 341/342 c [16]] diphenyl methane

Hexahydrophthalic Hexahydrophthalic 134 b 132 b - anhydride 134 b 132 b - anhydride

b Phthalic anhydride 138 b - - Phthalic anhydride 138 - -

Nadic anhydride 147 b - - Nadic anhydride 147 b - -

Hexahydrophthalic [173] Hexahydrophthalic 170 b 132 b - [173] anhydride 170 b 132 b - anhydride

Hexahydrophthalic Hexahydrophthalic 149 b 132 b - anhydride 149 b 132 b - anhydride Molecules 2017, 22, 149 37 of 48

a Tα measured by DMA at the peak position of loss modulus curve; b Tα measured by DMA at the a Tα measured by DMA at the peak position of loss modulus curve; b Tα measured by DMA at the maximum of tan δ; c Td under air flow. as eugenolmaximum and ferulicof tan δ; cacid Td under were air flow. turned into poly-aromatic epoxy monomers, but still few materials TableTable 6. 6. Poly-aromatic Poly-aromatic epoxy epoxy monomers monomers from from crude crude bio-mass bio-mass and and thermal thermal properties properties of of the the cured cured materials. materials. reached the high Tg values of their DGEBA counterparts. These results are gathered in Tables6 and7. Tg (°C) or Tα (°C) Tg (°C) or Tα (°C)

Epoxy Curing Agent Td,5% (°C) Reference Table 6. Poly-aromaticEpoxy epoxy monomersCuring Agent from crude bio-mass and thermalTd,5% (°C) properties Reference of the DGEBADGEBA MaterialsMaterials cured materials. ComparisonComparison

d Diethylenetriamine - ◦ - ◦ 252 d Diethylenetriamine T- g ( C) or Tα -( C) 252 ◦ Epoxy Curing Agent Td,5% ( C) Reference Epoxidized depolymerized kraft lignin Epoxidized depolymerized kraft lignin 4,4-diaminodiphenyl Materials DGEBA Comparison 4,4-diaminodiphenyl d - - - - 290290 d Diethylenetriaminemethanemethane - - 252 d Epoxidized depolymerized [186] 4,4-diaminodiphenyl [186] kraft lignin d d Diethylenetriamine --- - 228 d 290 Diethylenetriaminemethane - - 228 [186] Epoxidized depolymerized Epoxidized depolymerized Diethylenetriamine - - d organosolv lignin 4,4-diaminodiphenyl 228 Epoxidizedorganosolv depolymerized lignin 4,4-diaminodiphenyl - - 257 d 4,4-diaminodiphenylmethane - - 257 d organosolv lignin methane -- 257 d methane

94 - 94 94 - -

Phenol Novolac 95 [189] PhenolPhenol Novolac Novolac 95 95 [189] [189]

134134134 - - -

Phenol Novolac Phenol Novolac (TD2131) - - - - 293 293 Epoxidized lignin (Cedar) Phenol(TD2131) Novolac (TD2131) - - 293 Epoxidized lignin (Cedar) Epoxidized lignin (Cedar) Lignin (Cedar) - - 296 LigninLignin (Cedar) (Cedar) - - - - 296 296 Phenol Novolac - - 275 Epoxidized lignin (Eucalyptus) Phenol(TD2131) Novolac (TD2131) - - 275 [190] Phenol Novolac (TD2131) - - 275 [190] [190] Epoxidized lignin (Eucalyptus) Lignin Epoxidized lignin (Eucalyptus) - - 274 (Eucalyptus) LigninLignin (Eucalyptus) (Eucalyptus) - - - - 274 274 Phenol Novolac - - 266 Epoxidized lignin (Bamboo) (TD2131) EpoxidizedEpoxidized lignin lignin (Bamboo) (Bamboo) Phenol Phenol Novolac Novolac (TD2131) (TD2131) - - - - 266 266 Lignin (Bamboo) - - 259

Isophorone Epoxidized green tea extract 142 a,b 140 a,b 256/267 c [193] Diamine Isophorone 50 a,b 155 a,b 350 [205] Diamine Jeffamine T403 23 70 352 e [198] 41 121 350 e Isophorone e b b 366 /363 Diamine 50/59 158/158 c,e

e [196] Epoxidized cardanol (NC514) b 362 /361 Jeffamine D400 15/9 - c,e

PE-C9-NH2 13 279 - [141] PE-C18-NH2 14 284 - Phenalkamine 325 f/322 30/38 a,b - NX5454 c,f [206] Cardanol 328 f/311 19/21 a,b - cysteamine c,f a b Tα measured by DMA at the peak position of loss modulus curve; Tα measured by DMA at the maximum of c d e f tan δ; Td under air flow; IDT = initial decomposition temperature; Td,30%; Td,10%. Molecules 2017,, 22,, 149149 6 of 7

Lignin (Bamboo) - - 259

a,b a,b c Epoxidized green tea extract Isophorone Diamine 142 a,ba,b 140140 a,ba,b 256/267 cc [193][193]

a,b a,b IsophoroneIsophorone DiamineDiamine 50 a,ba,b 155155 a,ba,b 350 [205]

e JeffamineJeffamine T403T403 23 23 70 70 352 352 ee

[198][198] e 41 121 350 ee IsophoroneIsophorone DiamineDiamine b b e c,e 50/59 bb 158/158/158 bb 366366 ee/363/363 c,ec,e

[196][196] b e c,e Epoxidized cardanol (NC514) JeffamineJeffamine D400D400 15/ 15/9 bb -- 362362 ee/361/361 c,ec,e

PE-C9-NH2 13 279 -

[141][141] PE-C18-NH2 14 284 -

a,b f c,f Phenalkamine NX5454 30/38 a,ba,b -- 325325 ff/322/322 c,fc,f

[206][206] a,b f c,f Cardanol cysteamine 19/21 a,ba,b -- 328328 ff/311/311 c,fc,f Molecules 2017, 22, 149 38 of 48 a α b α a Tαα measuredmeasured byby DMADMA atat thethe peakpeak positionposition ofof lossloss modulusmodulus curve;curve; b Tαα measuredmeasured byby DMADMA atat thethe c d d e d,30% ff d,10% maximum of tan δ;; c Tdd underunder airair flow;flow; d IDTIDT == initialinitial decompositiondecomposition temperature;temperature; e Td,30%d,30%;; f Td,10%d,10%.. Table 7. Poly-aromaticPoly-aromatic di-di- andand tri-epoxytri-epoxy monomersmonomers andand thermalthermal propertiesproperties ofof thethe curedcured materials.materials. Table 7. Poly-aromatic di- and tri-epoxy monomers and thermal properties of the cured materials. g α Tgg (°C)(°C) oror Tαα (°C)(°C)

Epoxy Curing Agent Td,5% (°C) Reference Epoxy Curing Agent ◦ Td,5%d,5% (°C)(°C)◦ Reference Reference DGEBATg ( C) or Tα ( C) ◦ Epoxy Curing AgentMaterials Td,5% ( C) Reference Comparison Materials DGEBA Comparison

4,4′-diaminodiphenyl0 4,4′′-diaminodiphenyl-diaminodiphenyl4,4 -diaminodiphenyl b b 114 bb 114 b 154154 bb 341154 b [202] 341 [202] methanemethane

b 1,10-diaminodecane 54/70 bb 293 1,10-diaminodecane 54/70 b 293 b IsophoroneIsophorone DiamineDiamine 73/ 73/92 bb 294 b b 150/174 bb 150/174 [166][166] [166]

Isophorone b b 73/92 294 DifurfurylamineDiamine 58/67 bb 284

Difurfurylamine 58/67 b 284

3,3′-diaminodiphenyl0 3,3′′-diaminodiphenyl-diaminodiphenyl3,3 -diaminodiphenyl b b c 168 bb 168 b 174174 bb 338/337174 ccb [120][120] 338/337 c [120] sulfonesulfonesulfone

b b b c b c DicyandiamideDicyandiamide 105 bb 105 119119 bb 305119 cc [167][167] 305 [167] Molecules 2017, 22, 149 7 of 7

3,3′-diaminodiphenyl3,30-diaminodiphenyl 207 b 207 b 174 b 326174 b [121] 326 [121] sulfonesulfone

a a b b Tα Tmeasuredα measuredby by DMADMA atat thethe peak position position of of loss loss modulus modulus curve; curve; Tα measuredTα measured by DMA by DMAat the at the maximum of c c tanmaximumδ; Td under of tan δ air; T flow.d under air flow.

6. General Conclusions Biorefinery processes and retreatment of biomass was out of our scope. However, it is clear that the future of bio-based polymers, including building blocks for thermosetting epoxies is strongly dependent on the future of biorefinery. The goal of a biorefinery is to produce high quality chemicals for fuels, monomers, and finally polymers and materials. New screening techniques like “Green Chemistry” and techno-economic and Life Cycle Assessment (LCA) indicators can help chemical-based biorefinery project developments. But efficient, economical, and large-scale synthesis of monomers is crucial, and the first key parameter remains a resource pool that should be abundant and easily accessed. At first glance, there are a lot of opportunities to develop molecular biomass and monomers for preparation of renewable epoxy formulations and materials. But, the biggest changes for thermosetting epoxy formulations in the near future will certainly come from the regulation changes like the environmental directives for reducing Volatile Organic Constituents (VOC), the “Restriction of certain Hazardous Substances” (RoHS), or the “Registration, Evaluation and Authorisation of Chemicals”, (REACh). Bio-based compounds will not be the only solution for these policy pressures but will be an opportunity. There are many different routes to synthesize bio-based aromatic epoxy monomers and hence to replace DGEBA. However, we have to take into account several parameters such as the epoxy functionality; the different applications—for example coatings are totally different from composites and the adaptation of properties to applications. Hence, even if DGEBA is very interesting since it allows to cover all applications, it seems difficult to imagine to replace DGEBA by the same bio-based epoxy monomer in all applications. Currently, only a handful of patents led to commercialization of products, such as epoxided cardanol or vegetable oils. However, these epoxide monomers led to

Molecules 2017, 22, 149 39 of 48

low Tg polyepoxide networks for low Tg applications. Epoxy aromatic monomers leading to high Tg polyepoxide are desired and correspond to an important challenge. 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. Therefore, bio-based aromatic/rigid epoxy monomers are still needed in order to fulfil good compromise between processing and properties, and able to enable replacement of BPA. Developing highly efficient, safe, low waste, low toxicity and atom economy processes are the key words for green chemistry. Chemistry has to be simple, practical, and operational, and catalysts are expected to play an important role. Epichlorohydrin is the preferred way to prepare epoxy monomers; but epichlorohydrin is a toxic molecule which has to be manipulated in a safe environment. Epoxidation without the use of epichlorohydrin is a key challenge. Even if allylation or crotonization of alcohols can be a first step for epoxydation, the second step, i.e., double bond oxidation, requires the use of hazardous catalysts. Therefore, the oxidation of terminal double bonds is an interesting route but still needs research to propose industrial routes.

Conflicts of Interest: The authors declare no conflict of interest.

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