terpenes. It is worth highlighting here that the linear prenyl diphosphates: geranyl diphosphate (GPP, C10), farnesyl diphosphate (FPP, C15), and geranylgeranyl diphosphate (GGPP, C20) are precursors of terpenes obtained via the biosynthesis in the presence of prenyltransferases (Figure 1.46). Due to the fact that terpenes consist of multiple isoprene units (C5H8, 2‐methyl‐1,4‐butadiene), they are categorized based on the number of units into hemiterpene (one isoprene unit, C5), monoterpene (two units, C10), sesquiterpene (two units, C15), diterpene (four units, C20), and so on [131] (Figure 1.46) [132].
Terpenes are built from a wide range of cycloaliphatic and hydrocarbon chain structures with repeating isoprenyl double bonds. Terpenoids can be viewed as modified terpenes with added, missing, or shifted methyl and oxygenated functional groups (Figure 1.47).
The global turpentine production is more than 300 000 tons per year, and it includes α‐pinene (45–97%) and β‐pinene (0.5–28%), with smaller amounts of other monoterpenes [130]. The pinenes and limonenes are natural chemical precursors to a wide variety of compounds used in the pharmaceutical, fragrance, and flavor industry. Pinenes, additionally, are a source of other less common terpenes (Figure 1.48).
Figure 1.47 Outline of the biosynthetic pathway leading to the major monoterpenes from a single precursor.
Figure 1.48 Isomerization and oxidation processes for converting pinenes into other terpenes and a terpenoid.
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].
