Engineering Plastics from Lignin. 22. Cure of Lignin Based Epoxy Resins

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Abstract
The kinetic cure parameters-activation energy, pre-Arrhenius factor, and reaction order-for three lignin copolymer epoxides were determined by multiple heating rate dynamic DSC experiments in accordance with the procedure of Ozawa and Doyle. The three epoxides differed in degree of polyether chain extension (and thereby in lignin content) and in molecular weight. Exothermic cure reactions were observed over a temperature range of between 100 and 240°C. Activation energy, pre-Arrhenius constant, and reaction order ranged between 53 and 66 kJ/mol, 10,000 and 440,000, and 1.21 and 1.28, respectively. Lignin content and molecular weight differences had surprisingly little effect on cure performance. This suggests that there are no steric or mobility obstacles to cure. Whereas the activation energy of the cure reaction is similar to that of commercial epoxy resins (i.e., approximately 60 kJ/mol), the reaction order of the n-th order reaction ranges from 1.21 to 1.28 instead of the expected value of 2. This may be explained by a similar rate of the activation and ring-opening step of the cure reaction.