Enhancing Interactions Between Cellulose Fibers and Synthetic Polymers

Doctoral thesis, 1993

The nature of the interfacial interactions between cellulose fibers and synthetic polymers was studied. The goal was to gain knowledge of and enhance interactions in cellulose- based multicomponent systems. The methodology employed for this purpose involved altering the surface properties of cellulose fibers using chemical or plasma treatments, and then investigating the influence of these alterations on the interface character and performance of the combined material. The fiber properties whose effects on interfacial interactions were studied included surface atomic composition, surface energetics, acid- base characteristics and surface topography. The effect of a transcrystalline interphase morphology on interfacial shear strength was also evaluated. Surface treatment of cellulose fiber with maleated polypropylene (MAPP) considerably improved the adhesion between the fiber and polypropylene (PP). The maleic anhydride groups of the MAPP were found to react with the hydroxyl groups of the cellulose and to covalently bond the MAPP to the fiber. A dependency of fiber/matrix adhesion on the molecular weight of the surface modifying agent was registered, which indicates that the adhesion-promoting effect at the treated fiber/PP matrix interface is mainly caused by interdiffusion and the formation of entanglements. Segmental or co-crystallization of MAPP chains and PP matrix chains was found not to be relevant. Chemical treatments using silanes and plasma treatments using nitrogen, ammonia and methacrylic acid as plasma media were employed to elucidate the importance of acid- base interactions in cellulose-based composites. Mechanical testing of composites of treated fibers and selected acidic and basic matrices showed that acid-base forces, not necessarily dominant, merit consideration. When carefully controlled, plasma treatment was demonstrated to be a useful technique for varying the acidity/basicity of cellulose surfaces. Plasma treatment of fibers using oxygen as the medium led to a dramatic increase in the adhesion between low-density polyethylene (LDPE) and cellulose. A plausible explanation for this increase was the formation of covalent bonds between the phases, as hydroperoxides, created on the fibers during plasma treatment, decompose at compounding temperatures and form reactive radicals. The presence of a transcrystalline interphase in cellulose fiber/PP composites was found to increase interfacial shear strength. This was proposed to be a consequence of interfacial adsorption and/or a stronger mechanical interlock provided by the transcrystalline phase. The knowledge gained of surface modification and interaction effects in cellulose- polymer systems may contribute to an increased number of applications for cellulose fibers in combination with polymers. The development of new bio-based materials which can meet environmental demands will particularly benefit from this knowledge.