Composite comprises a polymer material of polyvinyl chloride, lignocellulose or cellulose plant fibers and an agent for promoting the bond between the polymer and the fibers, where the promoting agent is polyphenol and carboxylic acid comprising a phenol group. An independent claim is included for a process to obtain the composite comprising mixing the plastic material and promoting agent with the plant fibers then extruding or injecting a profile or an aggregate of the composite at a temperature of 150-200[deg] C.