Figure 1.49 Mechanism of the cationic polymerization of β‐pinene.
A methyl group or other electron‐donating groups on the double bond present within the structure of compounds obtained via the isomerization and oxidation of pinenes makes them susceptible for cationic polymerization. However, because of the presence of highly reactive exo‐methylene double bond within the β‐pinene structure, most of the polymerization reactions involve β‐pinene, not α‐pinene (Figure 1.49).
Some cationic polymerization of terpenes leads to oligomers or low molecular weight polymers, which along with other terpene monomers might be used for the synthesis of epoxy resins. One of the interesting applications of terpenes is the synthesis of hydrogenated terpinene‐maleic ester type epoxy resin [133]. A bio‐based epoxy resin (denoted as TME) and a waterborne dispersion of TME (denoted as WTME) obtained from the turpentine (Figure 1.50) are nontoxic alicyclic structure epoxy resins without BPA.
The resulting film with good thermal stability and antifouling properties is transparent and flexible. However, because the obtained cured terpene‐based products are characterized by worse mechanical properties than those of BPA‐based epoxy resin, two different approaches had also been studied: (i) incorporation of cellulose nanowhiskers (CNWs) suspension, hydrolyzed from microcrystalline cellulose [134], and (ii) in combination with polyurethane [133]. In the first modification of the synthesis, the incorporation of CNWs in the WTME matrix (0.5–8 wt%) results in the increase of the storage modulus at 150 °C (from 0.8834 to 4.756 MPa), Young's modulus (from 295.6 to 800.1 MPa), and tensile strength (from 7.08 to 15.2 MPa) compared to unmodified WTME. Noted improved properties might be attributed to the formation and increase of interfacial interaction by hydrogen bonds between CNWs nanofiller and the WTME matrix. On the other hand, in the second approach (Figure 1.51), an anionic polyol (T‐PABA) dispersion is prepared by modifying terpene‐based epoxy resin with para‐aminobenzoic acid and then cross‐linked with a hexamethylene diisocyanate (HDI) tripolymer to prepare waterborne polyurethane/epoxy resin composite coating. These new cross‐linked products combine the rigidity and weatherability of the saturated terpinene alicyclic epoxy resin with the flexibility and tenacity of the polyurethane.
Figure 1.50 Chemical structures of TME, TME‐based emulsifier, and aliphatic amine.
Figure 1.51 Preparation of T‐PABA and T‐PABA dispersion.
On the other hand, the oxidation of limonene, the terpene derivative, which within the structure contains two double bonds, (i) vinylene group allocated in the ring and (ii) vinylidene side group, results in limonene monoepoxide and diepoxide – compounds commercially applied as reactive diluents in epoxy applications [135]. Additionally, D‐limonene might also be used in the synthesis of epoxy resins as a bio‐based replacement for conventional DGEBA resins [136]. Hybrid epoxy resin, including naphthalene and limonene moieties, is obtained on the course of the three‐step reaction (Figure 1.52) consisting of (i) alkylation of naphthol with limonene in the presence of Friedel–Crafts catalyst, (ii) introduction of methylene linkage between naphthalene rings to obtain product 2 with higher molecular weight, and subsequently (iii) epoxidation of 2 with epichlorohydrin in the presence of sodium hydroxide and polyethylene glycol to give epoxy resin 3.
The obtained epoxy resin is mixed with dicyanodiamide and a bisphenol A formaldehyde novolac resin used as curing agents in a molar stoichiometric ratio of 1 : 1 and in the presence of 2‐methylimidazole as an accelerator. Compared to DGEBA resins, the cured D‐limonene/naphthol‐based epoxy resins are characterized by higher Tg (by 75 °C) and thermal stability with higher temperatures of maximum rate of weight loss in air by about 40 °C.
Epoxy resins might also be synthesized from rosin, produced by heating fresh tree resin to remove the volatile liquid terpenes [137]. Because of the presence of rigid hydrogenated phenanthrene ring in the molecular structure, rosin is suitable as an alternative to DGEBA [138, 139]. The interesting examples are the flame‐retardant rosin‐based epoxy thermosets, such as a rosin‐based siloxane epoxy monomer (AESE), which is prepared by the reaction of ethylene glycol diglycidyl ether modified acrylpimaric acid (AP‐EGDE) with polymethylphenylsiloxane (PMPS) (Figure 1.53) [140].
Figure 1.52 Synthesis of epoxy resins starting from naphthol and limonene.
Figure 1.53 Synthetic route to a rosin‐based siloxane epoxy monomer (AESE).
The incorporation of PMPS results in the improvement in the LOI value compared to the AP‐EGDE/MHHPA thermosets. The highest LOI value of 30.2% is noted for the cured product containing 30 wt% of AESE (AESE30/MHHPA). Because of the presence of the flexible chains of the PMPS, all the AESE/MHHPA thermosets display a relatively low tensile strength (<15 MPa) but on the other hand much larger elongation at break (>50%).
Terpenes are an interesting group of raw materials that might be used in the synthesis of epoxy resins. Even though not all terpenoids contain aromatic and/or phenolic moieties, these requirements can be reached via different synthesis steps (e.g. carvacrol, for instance, might be obtained from other turpentine components, limonene, for instance, throughout an oxidation, followed by an isomerization process).
1.4 Bio‐Based Epoxy Curing Agents
Cross‐linking of epoxy resins takes place with the participation of oxirane rings present in them. The strained structure of these rings is the reason for their high reactivity and facilitates their opening under the influence of very different factors. Epoxy resin curing generally takes place under the influence and with the participation of multifunctional chemical compounds with active protons. These are mainly polyamines (aliphatic, aromatic, and cycloaliphatic) and carboxylic compounds (acids and anhydrides). Cross‐linking of the epoxy resins can also be accomplished by a mechanism of ring‐opening polymerization using suitable catalysts. This is an important method of cross‐linking, especially coating materials. However, the search for compounds of natural origin, which could replace petrochemical hardeners, is the most important in the case of polyamines and carboxyl compounds that are used for cross‐linking in stoichiometric