Effect of fibre orientation on the mechanical properties of polypropylene–lyocell composites

Abstract

Nature uses the concept of optimised reinforcing fibre orientation to produce plant structures to withstand natural forces, such as the bending of plants by wind power. This concept was the inspiration to design similar man made composites and to study the effect of fibre orientation in a systematic experimental investigation. In general, fibre-reinforced plastics show enhanced properties such as low weight, corrosion resistance, and high tensile strength and modulus. In technical applications the use of carbon, glass and aramid-fibres as reinforcement in matrix-polymers is common (Spoljaric et al. 2009 ). But these fibres have disadvantages such as high production costs, difficulties in their recycling and their end of life (Shanks et al. 2004 ). Therefore the use of natural and regenerated cellulose fibres to reinforce matrix polymers has generated much interest in recent years. Especially cellulose type polymers, for example rayon and lyocell (CLY) are able to substitute conventional inorganic reinforcing materials. The advantages of natural and regenerated fibres over conventional inorganic fibres are for example economic viability, low tool wear, reduced dust formation, reduced dermal irritation and biodegradability (Bledzki and Gassan 1999 ; Wambua et al. 2003 ). Natural and regenerated fibre reinforced polymer composites also have some disadvantages such as the low adhesion between the hydrophilic natural fibres and the hydrophobic polymer matrix (Bledzki and Gassan 1999 ). In the literature, physical and chemical surface treatments of the reinforcing natural fibres are described to improve the adhesion between fibres and matrix (Belgacem et al. 2003 ; Gray 2008 ; Joly et al. 1996 ; Qiu et al. 2003 ; Quan et al. 2005 ; Quillin et al. 1992 , 1993 ). For example the hydrophilicity of polypropylene (PP) can be increased by modification with maleic anhydride, resulting in an improved adhesion between the matrix polymer and natural fibres (Bataille et al. 1989 ; Joseph et al. 2003 ; Khalid et al. 2006 ; Shubhra et al. 2011 ; Spoljaric et al. 2009 ; Qiu et al. 2005 , 2006 ). The preparation and characterization of composites, consisting of PP matrix and cellulose (Amash and Zugenmaier 1998 , 2000 ; Bengtsson et al. 2007 ; Ganster et al. 2006 ; Kuboki 2014 ; Zugenmaier 2006 ) or cellulose-based plant fibres like bagasse (Ashori and Nourbakhsh 2009 ), jute (Acha et al. 2007 ; Doan et al. 2006 , 2007 ; Gassan and Bledzki 1997 ; Karmaker and Youngquist 1996 ; Rahman et al. 2010 ), hemp (Niu et al. 2011 ) and sisal (Joseph et al. 2003 ) have already been reported in the literature. These composites suffer from the problem of moisture absorption in the presence of humid air or when immersed in water (Espert et al. 2004 ). The absorbed water accumulates at the interface of fibres and matrix and induces debonding. Thus, the propensity for stress transfer between the matrix and fibre is reduced, which results in a reduction of mechanical properties. The negative influence of water sorption can be reduced by the use of coupling agents and special fibre treatments. Composites prepared from thermoset matrix suffer from the problem of difficult processing after curing. It would consequently be desirable to replace thermosetting by thermoplastic matrix materials. The use of thermoplastic polymers combined with natural or regenerated fibres, like CLY allows the efficient mass production of composites. During the last decade, many efforts are made in order to generate thermoplastic composites reinforced with man-made fibres with special emphasis on fibre orientation in the composites. Starting with a wet-lay process which is similar to papermaking (Johnson et al. 2008 ), reported on the mechanical properties of PP/cellulose fibre composites with random oriented fibres. By employing typical thermoplastic compounding technology such as extrusion process, PP/CLY compounds were prepared (Mirza et al. 2009 , 2010 ; Borja et al. 2005 ; Ganster et al. 2008 ; Huque et al. 2012 ; Paunikallio et al. 2003a , b ; Abdennadher et al. 2016 ; Zarges et al. 2017 ) in order to get good distribution of the cellulose fibres in the PP matrix. However, only chopped fibres were suitable for compounding technologies and the fibre orientation cannot be influenced, especially when conversion techniques such as compression moulding (Mirza et al. 2009 , 2010 ; Huque et al. 2012 ) or injection moulding (Paunikallio et al. 2003a , b , Abdennadher et al. 2016 ; Zarges et al. 2017 ) were used to produce sample specimens after the compounding step. In case of compression moulding the chopped fibres are oriented randomly in the press mould. When injection moulding technique was used, the chopped fibres will be forced to align predominantly along the flow channel during the mould filling process. In both cases, there was no control on the fibre orientation possible. Other research groups have developed pultrusion technique to produce continuous coated cellulose tows (Ganster and Fink 2006 ; Ganster et al. 2006 ; Feldmann 2016 ). The coated tows, however, were pelletised for further conversion step such as injection moulding. The developed pultrusion techniques allowed a good fibre wetting and distribution of the matrix polymers. However, during the pelletising step, the fibres were cut and no orientation control of the chopped fibres could be done during the part shaping process.