On the processing and properties of cellulose-containing polymeric materials

Doctoral thesis, 2013

Cellulose fibres have properties such as renewability, biodegradability, very good availability and low cost that make them very attractive as a reinforcement for thermoplastic polymers. 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, opening 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. The thermal properties of the gels depended only on the type of cellulose whereas the absorption of CA during gelling depended on the types of IL as well as the type of cellulose. The gels showed linear viscoelastic behaviour in terms of the storage modulus G’ but not of the loss modulus G”. A higher concentration of cellulose gave stiffer gels whereas the gel network strength depended on the types of IL and CA. The difficulty of feeding the cellulose fibres into the processing equipment is a serious disadvantage, since continuous feeding is desired for industrial applications. In this work, three different continuous feeding techniques have been studied, more specifically the methods of pelletized cellulose fibres with polymer, the feeding of cellulosic tissue into the compounder and the feeding of wet cellulose masterbatch. Composites containing 20 to 35 wt% of cellulose fibres in ethylene-acrylic acid copolymer were obtained with the different processing techniques. In all cases, injection moulding was performed as a last step. It was shown that if the dispersion of cellulose fibres was performed solely by melt mixing, i.e. without compatibilisers, shorter fibres led to less fibre aggregates. Contrary to what was expected, shorter fibres resulted in composites with better mechanical performance showing the great importance of fibre dispersion together with a good adhesion fibre-matrix. The final fibre length depended on the processing technique and on the fibre concentration, but it was also observed that longer fibres were more affected by the melt processing. Finally, one of the cellulose composites studied was used for injection moulding of bottle caps, demonstrating an application of this type of composite material.