On the processing and properties of cellulose-containing polymeric materials
Licentiate thesis, 2011
Cellulose fibres have properties that make them very attractive as a reinforcement for thermoplastic polymers. Renewability, biodegradability, very good availability and low cost are some general advantages. Other more specific favourable properties are low density, high specific stiffness, low abrasive nature and their potential for modification. There are however, some drawbacks or limitations when processing cellulose-fibre-reinforced composites that need to be resolved in order to improve the mechanical properties of the composite. This work is focused on how to adapt the processing towards this goal and on a study of the mechanical properties of polymer composites with a high content of fibre reinforcement.
Cellulose is not soluble in water or conventional organic solvents. Recently it was found that ionic liquids (IL) are able to dissolve cellulose, and this opens many new possibilities. In this work, microcrystalline cellulose (MCC) was dissolved in two different imidazolium-based ionic liquids and when a coagulation agent (CA) was added, a gel structure was obtained. In this case, MCC was also modified to lauric acid cellulose ester (LACE) in order to study the effect of surface hydrophobisation. It was concluded that the thermal transition range of the cellulose gels was affected only by the type of cellulose and that the absorption of CA during gelling depended on the types of IL and cellulose. The rheological measurements with a stress-controlled parallel-plate rheometer showed linear vicoelastic behaviour in terms of the storage modulus G’ but not in the loss modulus G”. Gels having a high concentration of cellulose exhibited higher plateau values for G’ and the critical stress value for linearity depended on the type of IL and the type of CA. It was concluded that the G’ and G” of the gels were independent of the frequency.
The difficulty of feeding the cellulose fibres into the extruder or injection moulding machine is a serious disadvantage, since continuous feeding is desired when processing this type of composite. Pelletizing the cellulose fibres made it possible to achieve continuous feeding, but this process resulted in a significant shortening of the fibre lengths and a consequent reduction in the mechanical properties of the composite. Cellulose pellets or agglomerates of cellulose in ethylene-acrylic acid copolymer (EAA) were processed with different techniques. Pellets with 30 and 70 wt% cellulose content were used. The 70 wt% cellulose content pellets were blended with additional EAA to a final cellulose content of 30 wt%. It was concluded that the melt processing after the initial agglomeration had a small or insignificant effect on the fibre length. Elongation dispersive mixing improved both stiffness and strength, probably because the number and size of the fibre aggregates were reduced.
Keywords: cellulose fibres, ethylene-acrylic acid copolymer, compounding, dispersion, ionic liquids, gels, rheological properties
ethylene-acrylic acid copolymer