Figure 1.59 Preparation of partially depolymerized lignin (PDL) and lignin polycarboxylic acid LPCA) from Kraft lignin.
Another interesting utilization of lignin‐based compounds for the curing purposes of epoxy resin is application of partially depolymerized Kraft lignin [146]. In order to increase its solubility in organic solvents, lignin is subjected to the base‐catalyzed depolymerization in supercritical methanol. The resulting partially depolymerized lignin (PDL) is then converted to lignin‐based polycarboxylic acid (LPCA) by reacting with succinic anhydride (Figure 1.59).
LPCA might be applied as a curing or co‐curing agent for epoxy resins. The curing of a commercial epoxy (DER353, epoxy value: 0.500–0.526 mol/100 g) using LPCA is conducted in the presence of 1 wt% of ethyl‐4‐methyl‐imidazole as a catalyst and at the similar temperature range to the commercial hexahydrophthalic anhydride (HHPA). The obtained cured material exhibits a moderate Tg and comparable storage modulus to that cross‐linked with a commercial anhydride curing agent. Additionally, linear succinate monoester, used in the synthesis of LPCA, enhances the flexibility of the lignin molecules. Therefore, increasing the content of bio‐curing agent in the formulation tends to reduce the Tg of cured resins. For composition of DER353/LPCA with equivalent ratio 1/0.6, 1/0.8, to 1/1, the Tg of the cured resins decreases from 78.5 and 69.4 to 62.3 °C, while the storage moduli at room temperature is comparable (2.4–2.7 GPa). Based on the studies on the application of the solid LPCA together with other liquid curing agents, such as glycerol tris(succinate monoester) and commercial hexahydrophthalic anhydride to cure epoxies, it was observed that using a mixture of LPCA and a liquid curing agent not only adjusts the viscosity of the resin system but also significantly regulates the dynamic mechanical properties and thermal stability of the obtained epoxy materials.
Moreover, interesting two different types of novel cross‐linked epoxy resins from lignin and glycerol are reported [147]. The first one is obtained by mixing the product of sodium lignosulfonate (LS) and glycerol (LSGLYPA, where the content of LSGLYPA varies at 0, 20, 40, 60, 80, and 100%) with polyacid of sodium lignosulfonate and ethylene glycol (LSEGPA) and ethylene glycol diglycidyl ether (EGDGE) at 80 °C (Figure 1.60).
The second one is by mixing LSGLYPA with a mixture of EGDGE/GLYDGE (the content of EGDGE/GLYDGE mixture was 0, 20, 40, 60, 80, and 100%) under similar reaction conditions (Figure 1.61).
The glass transition temperature of the cross‐linked epoxy resins increases with increasing LSGLYPA and GLYDGE contents. The increase of Tg for the product of the first synthesis is due to the increase in cross‐linking density. In turn, for the second sample, an increase in the GLYDGE content increases Tg of the cured epoxy resins (hydrogen bonding became the dominant factor).
Vanillin can be converted into the diamine derivatives (Figure 1.62) [148].
Starting from diglycidyl ethers of 2‐methoxyhydroquinone and vanillyl alcohol through an epoxy ring opening with ammonia, it is possible to obtain two primary amines, which could be applied as epoxy resin cross‐linking agents. Because of β‐hydroxyl groups present in the molecules of diamines, they exhibit the autocatalytic effect on the epoxy–amine reactions. These new β‐hydroxylamines can be used for cross‐linking of diglycidyl ether of methoxyhydroquinone and diglycidyl ether of vanillyl alcohol giving fully bio‐based epoxy systems with good thermomechanical properties and high thermal stability.
Cardanol derivatives are classified as the phenolic curing agents that are cross‐linked with epoxy groups via the phenolic hydroxyl group. Novolac resins (Nov‐I and Nov‐II), containing an amount of unreacted cardanol of 35 and 20 wt%, respectively, are synthesized by the condensation reaction of cardanol and paraformaldehyde using oxalic acid as a catalyst (Figure 1.63) [149].
The cardanol‐based novolacs might be used as curing agents of commercial (the diglycidyl ether of bisphenol A) epoxy resin in the presence of 2‐ethyl‐4‐methyl‐imidazole as a catalyst. The higher cross‐linking density is observed with higher amounts of epoxy resin. Moreover, the resin cured with Nov‐II is characterized by higher Tg and better mechanical properties than Nov‐I‐resin. On the other hand, because of the higher molecular weight and lower unreacted cardanol content, the similar thermal degradation properties are observed for both tested materials.
Figure 1.60 Scheme of preparation cross‐linked epoxy resin.
Figure 1.61 Scheme of epoxy resin by cross‐linking LSGLYPA with a mixture of EGDGE/GLYDGE.
Figure 1.62 Synthesis of dihydroxyaminopropane of 2‐methoxyhydroquinone (a) and (b) vanillyl alcohol.
Figure 1.63 Synthesis of the cardanol‐based novolacs.
The