Surface functionalization of cellulose fibres and their incorporation in renewable polymeric matrices

The surface of model cellulose fibres (Avicell) as well as kraft softwood pulps (ksp) was chemically modified with different coupling agents, namely: two difunctional anhydrides, two diisocyanates, and two alkoxysilanes. These grafting agents were pyromellitic dianhydride (PMDA), benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BPDA), 1,4-phenylene diisocyanate (PPDI), methylene-bis-diphenyl diisocyanate (MDI), and γ-mercaptopropyltriethoxysilane (MRPS), and γ-methacrylopropyltriethoxysilane (MPS). In all cases stiff monomers were used with the aim of ensuring the reaction of only one of the functionalities with the cellulose surface, leaving the remaining moiety to react with the polymer matrix during composite processing to provide a covalent linkage between the matrix and the reinforcing elements, thus enabling perfect stress transfer. Thus, the rigidity of the used molecules and their quite small size exclude bridging two fibres or reacting both functionalities at the surface of the same fibres. After modification, the ensuing fibres were submitted to soxhlet extraction, in order to remove all the unbounded and physically adsorbed molecules. The occurrence of the grafting was confirmed by elemental analysis, infrared, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and contact angle measurements. The modified fibres were then incorporated into cellulose acetate-butyrate and natural rubber matrices and the mechanical properties of the ensuing composites determined. PMDA- and BPDA-treated Avicell were found to reinforce efficiently the mechanical properties of CAB, whereas MPS-treated cellulose enhanced those made with natural rubber matrix.