with epichlorohydrin (adding a base at room temperature in order to perform the ring closure of intermediate halohydrin species). For example, with a NaOH/OH ratio equal to 10, the tetraglycidyl compound is mainly obtained with about 90% yield. In contrast, the diglycidyl derivative is mainly created (80% yield) at lower NaOH/OH ratios. Separated vanillin‐based epoxy compounds cross‐linked with isophorone diamine, characterized with the glass transition temperature in terms of 138–198 °C, exhibit similar Young modulus and thermal stability values to the bisphenol A‐based epoxy thermoset, but lower elongation at break.
The other possibility of the vanillin dimerization is the electrochemical synthesis of meso‐hydrovanilloin (Figure 1.27) [101]. The symmetrical compound divanillin (two molecules of vanillin coupled by aromatic rings) can be easily prepared by the oxidative dimerization of vanillin catalyzed by FeCl3 or heme iron enzymes [102]. However, vanillin can also be reductively dimerized at the aldehyde function using low‐valent titanium generated via TiCl4‐Mn or by the electrochemical method [103]. The electrochemical coupling is a highly stereoselective reaction giving meso‐hydrovanilloin. The vanillin electrolytic reduction in alkaline solution at a metallic cathode gives interesting bisphenol compound, which can be used as a direct substitute for bisphenol A [104].
The obtained meso‐hydrovanilloin‐based epoxy resin cured using long‐chain aliphatic diamine (1,6‐diaminohexane) and cycloaliphatic amine (isophorone diamine) showed the glass transition temperature and Shore hardness (D‐type) values comparable with commercial diamine‐cured bisphenol A‐based epoxy resins.
Two molecules of vanillin can be coupled by the crossed aldol condensation (Figure 1.28) with cyclopentanone [105].
Figure 1.25 Esterification of vanillic acid, followed by the O‐alkylation and subsequently by the epoxidation of the allylic double bonds.
Figure 1.26 Synthesis of glycidyl derivatives (b) based on the product of vanillin dimerization (a).
Figure 1.27 Synthesis of hydrovanilloin and the epoxy resin based on this vanillin dimer.
Figure 1.28 Synthesis of 2,5‐bis(4‐hydroxy‐3‐methoxybenzylidene)cyclopentanone and its diglycidyl derivative.
The thermal and mechanical properties of the synthesized resin cured with bio‐based (quercetin and guaiacol novolac) hardeners and a petroleum‐based hardener (phenol novolac) are comparable with those of the bisphenol A‐based resins cross‐linked with the same hardeners.
A diamine can also be used to couple two vanillin molecules [106]. Vanillin coupled with aromatic diamines and diethyl phosphite, followed by the reaction with epichlorohydrin, yields high‐performance biorenewable and environment‐friendly flame‐retardant epoxy resins (Figure 1.29).
The coupling product with 4,4‐diaminodiphenylmethane (DDM) or p‐phenylenediamine (PDA) is synthesized (Figure 1.29a) through Schiff base condensation, and the generated Schiff base is further reacted with diethyl phosphite by the phosphorus–hydrogen addition reaction to yield phosphorus‐containing vanillin‐based bisphenols. The resulted bisphenol can be converted into diglycidyl derivative via the above‐described reaction with an excess of epichlorohydrin, preferable under PTC conditions. The reactivity of the epoxy resins synthesized in this way is similar to the bisphenol A‐based epoxy resin. After curing with a stoichiometric amount of 4,4‐diaminodiphenylmethane, both resins showed excellent flame retardancy with UL‐94 V0 rating and high LOI value of 31.4% (coupling with DDM) and 32.8% (coupling with PDA), due to their outstanding intumescent and dense char formation ability. They also exhibit high glass transition temperature value of 183 °C (DDM) and 214 °C (PDA), the tensile strength of 80.3 MPa (DDM) and 60.6 MPa (PDA), and the tensile modulus of 2114 MPa (DDM) and 2709 MPa (PDA), much higher than the cured bisphenol A‐based epoxy resin with a Tg of 166 °C, a tensile strength of 76.4 MPa, and a tensile modulus of 1893 MPa, respectively.
Figure 1.29 Synthesis of the vanillin coupling product (a) and the flame‐retardant epoxy resin based on it (b).
Figure 1.30 The coupling of vanillin with pentaerythritol and synthesis of the epoxy resins containing spiro‐ring structure.
Two molecules of vanillin can also be coupled through the dehydration condensation with pentaerythritol, leading to obtain the bisphenol with the specific spiro‐ring structure (Figure 1.30) [66], which can be further reacted with epichlorohydrin to give the epoxy resin.
This vanillin‐based resin exhibits very interesting properties [107]. This solid resin with an epoxy value of 0.355 mol/100 g, cross‐linked with diamine hardeners, DDM or 3,9‐bis(3‐aminopropyl)‐2,4,8,10‐tetroxaspiro(5,5)undecane, has several relaxations. The first is the β‐relaxation, caused by the micro‐Brownian motion of the aromatic methoxy group, observed from 50 to 100 °C for the spiro‐ring‐type resin systems in both mechanical and dielectric measurements. The peak height and the activation energy of this relaxation are independent of the degree of curing. The second one is the relaxation caused by the hydrogen bonding between the methoxy and the hydroxyl groups at around 0 °C [108]. This relaxation behavior is expected to have a positive effect on the damping characteristics. Moreover, the fracture toughness of the spiro‐ring‐type epoxide resin with methoxy branches is considerably greater above the temperature region of the β‐relaxation than that of the bisphenol A type resin [109].
1.3.3 Cardanol
Cardanol is extracted from the shell of the cashew nut. Cashew nut comes from the cashew tree, Anacardium occidentale, mostly grown in India, East Africa, and Brazil [110]. The nut has a shell of about 1/8 in. thickness inside, which is characterized by a soft honey comb structure containing a dark brown viscous liquid, called cashew
