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Emulsion prepared with EE-P20 (“lipophilic–hydrophilic–hydrophobic” structure), as the emulsifier can improve the interface performance and moisture resistance of CF.

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The development of new bio-based epoxy resins and the search for new application areas have become important research directions for bio-based epoxy resins. Using protocatechuic acid (PA) as a bio-based raw material, brominated alkenes with different chain length were introduced into PA through substitution reaction, and then oxidized to obtain epoxy resins (PA-EP) with different epoxy values. After that, it was introduced into epoxy resin (diglycidyl ether bisphenol A [DGEBA]) sizing agent to replace part of DGEBA to prepare new sizing agents. Sizing amount test and sizing CF surface morphology show that the introduction of PA-EP with high-epoxy value made it easier be adhered to carbon fiber (CF) surface during sizing process. Interfacial shear strength and interlaminar shear strength of composite made of sizing CF (20 wt% PA-EP-3) were 56.86% and 30.29% improvement against the bare CF, which was attributed to the increase in surface energy and epoxy groups after CF sizing. Therefore, PA-EP with high-epoxy value was applied as sizing agent for improving interface performance of CF for the first time, and this application was less affected by its low-output problem compared to its application as a matrix resin in the field of composite manufacturing.