Abstract
Owing to its chemical structure, wide availability and renewable nature, lignin is a promising candidate for the partial replacement of fossil-based raw material in the synthesis of epoxy resins. Its poor compatibility has been reported to be one of the main drawbacks in this domain. On the other hand, a well-established modification method for lignin epoxidation has been used for many years for the improvement of lignin compatibility. However, the extent of the effect of lignin epoxidation on the improvement of bio-based epoxy mechanical properties, applied as adhesives, is still an open question in the literature. In this context, a pristine and industrial grade kraft lignin (AKL) was reacted with epichlorohydrin to yield epoxidized lignin (E-AKL) in this work. Afterwards, AKL or E-AKL were separately blended with petroleum-based epoxy resin at 15 and 30 wt% and cured with a commercial amine. The adhesive curing kinetic was evaluated using a novel technique for thermal transition characterization, Temperature Modulated Optical Refractometry (TMOR); the results showed that the incorporation of AKL reduces the crosslinking rate, and that this effect is overcome by lignin modification. Mechanical tests revealed an improvement of impact and practical adhesion strength for samples containing 15 wt% of E-AKL. These results elucidate the effect of lignin epoxidation on the application of lignin-based epoxy adhesives, and might support the further development and application of these bio-based materials.
Highlights
Good mechanical and thermal properties, chemical resistance, and proper adhesion to many materials are some of the properties that explain why epoxy resins are among the widely most applied thermoset polymers [1,2]
We investigate the effects of lignin epoxidation on thereactivity mechanical properties of epichlorohydrin, this approach has the disadvantage of demanding additional reaction steps, which epoxy adhesives
Kraft lignin was chemically modified by reaction with epichlorohydrin
Summary
Good mechanical and thermal properties, chemical resistance, and proper adhesion to many materials are some of the properties that explain why epoxy resins are among the widely most applied thermoset polymers [1,2]. The third method is based on lignin chemical modification Previous lignin modification might improve lignin reactivity towards epichlorohydrin, this approach has the disadvantage of demanding additional reaction steps, which might involve hazardous/expensive reagents or complex processes (high pressure and/or temperature), increasing safety requirements, costs and hindering its industrial application. We investigate the effects of lignin epoxidation on thereactivity mechanical properties of epichlorohydrin, this approach has the disadvantage of demanding additional reaction steps, which epoxy adhesives. Lignin epoxidation on the temperature), increasing safety requirements, andhand, hindering industrial investigate the effects of lignin epoxidation on properties of mechanical properties of we epoxy adhesives are still understudied inthe themechanical literature.
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