Lignocellulosic Thermoplastic Composites

The weathering properties of thermoplastic matrix cellulose fiber composites have been studied more extensively than other matrix types. HDPE-based composites have received most attention, followed by polypropylene (PP) matrix composites. The response of these composites to both natural weathering and accelerated weathering conditions has been studied, and Table 15.8 includes a summary of some of the studies into the effects of accelerated weathering on mechanical properties of cellulose fiber-reinforced thermoplastics. It is clear from the table that weathering has adverse effects on the mechanical properties of almost all the composites studied. 

Matrix Fiber/filler Weathering cycle Duration (h) Degradation in mechanical properties (%) Reference 

Abbreviations T. S. Tensile Strength T. M. Tensile (Yoimg s) Modulus F. S. Flexural Strength F. M. Flexural Modulus I. S. Impact Strength. 

Similar increases in carbonyl index, vinyl index and crystallinity have also been reported for rape straw flom (RSF)/HDPE and nano-SiO /RSF/ HDPE composites exposed to natural weathering for 120 days by Zuo et al. [61]. 

Panthapulakkal et at [65] studied the effects of accelerated LTV radiation exposure on the flexural properties of rice husk/HDPE composites for 745 hours. Although the surface of the composites showed considerable discoloration, their flexural strength and modulus were not significantly different from those of the xmexposed composite samples. It was suggested that the exposure time was not sufficient to cause any significant deterioration in flexural properties. On the other hand, water immersion for 1600 hours had a significant effect of flexural properties, particularly the flexural stiffness. 

---

The use of waste wood and post-consumer thermoplastics will help to solve the severe environmental and recycling problems. The increasing concern about our environment promotes recycling of thermoplastics for lignocellulosic-thermoplastic composites... 

Le Digabel F, Boquillon N, Dole P et al (2004) Properties of thermoplastic composites based on wheat-straw lignocellulosic fillers. J Appl Polym Sci 93 428-436 Lee S -R, Park FI-M, Lim H et al (2002) Microstructuie, tensile properties, and biodegradability of aliphatic polyester/clay nanocomposites. Polymer 43 2495-2500 Lee SFI, Ohkita T, Kitagawa K (20(M) Eco-composite from poly(lactic acid) and bamboo fiber. Flolzforschung 58 529-536... 

The lignocellulosic materials mostly used as fillers in thermoplastic composites include wood flour, starch, rice husk and a wide variety of vegetable fibers available such as jute, sisal, flax, hemp, coir, banana, pineapple, among others. And whenever vegetable fiber reinforced thermoplastic composites with higher properties are needed, possible solutions include improved adhesion, better fiber orientation, and filler hybridization with synthetic fibers or mineral fillers. The latter solution is an intermediate alternative regarding environmental friendliness, cost, weight and performance compared to an all synthetic composite [12,26]. 

Moreover, wood has limited thermoplasticity. Although it can be bent under steam and chemical treatment, wood normally bums before it melts or becomes sufficiently plastic for heat molding or extrusion. These two techniques are important ways of shaping materials in high-speed composite production and are therefore keys to the cost-efficient penetration of lignocellulosic materials into the composites market. Chemical modification of wood offers a means of improving its thermoplasticity. 

The inherent polar and hydrophilic nature of lignocellulosic fibers and the nonpolar characteristics of most thermoplastics results in compounding difficulties leading to non-uniform dispersion of fibers within the matrix, which impairs the efficiency of the composite. This is a major disadvantage of natural fiber-reinforced composites. 

The anhydride functionality in the compatibilization research described before may react with the lignocellulosic, but there is no evidence to support that at this time. A higher level of grafted anhydride on the polypropylene would be required for the alloy reactions, and it would be expected that the reaction between grafted thermoplastic and jute or kenaf would take place both on the matrix polymers (lignin and hemicelluloses) and in the cellulose backbone. Some decrystallization of the cellulose may be desired to give more thermoplastic character to the entire composite. 

Yet another limitation associated with the use of lignocellulosic fillers is the fact that the processing temperature of composites must be restricted to just above 200°C (although higher temperatures can be used for short periods of time), because of their susceptibility to degradation and/or the possibility of volatile emissions that could affect the composite properties. This limits the types of thermoplastics that can be used to polymers like polyethylene, PP, poly-vinyl chloride and polystyrene, which constitute, however, about 70 per cent of all industrial thermoplastics. Nevertheless, technical thermoplastics like polyamides, polyesters and polycarbonates, which are usually processed at temperatures higher than 250°C, cannot be envisaged as matrices for these types of conposite. 

Extrusion and injection-moulding are the economically most attractive processing methods of thermoplastic-based composites. The extrusion press processing (express-processing) has been developed for the production of flax fibre reinforced PP at the research centre of Daimler Benz [7]. In this process, flax fibre non-wovens and PP melt films are alternatively deposited and moulded. A production process for PP semi-products reinforced with lignocellulosic fibres in the form of mats has been developed by BASF AG [7]. Fibre mats are produced by stitching together layers of fibres which have previously been crushed. 

Ihere is a growing trend to use natural fibers as fillers and/or reinforces in plastic composites as thermosetting and thermoplastic materials. Ihe use of lignocellulosic materials as reinforcements has received increasing attention due to the improvements that natural fibers can provide such as low density, biodegradabihty and highly specific stiffness, as well as the fact that these materials are derived from renewable and less expensive sources [11-... 

As mentioned previously, the main bottleneck in the broad use of these fibers in thermoplastics is the poor compatibility between the fibers and the matrix. The inherent high moisture sorption of lignocellulosic fibers certainly has an effect on their dimensional stability [28]. This may lead to the microcracking of the composites and degradation of mechanical properties [28]. Like other natural fibers, kenaf absorbs moisture due to its hydrophilicity. The key issue related to the development and production of natural fiber-reinforced composites is the interfacial adhesion between the fiber and polymer matrix. Because of their inherent dissimilarities, natural fibers/polymer matrix composites are not compatible and interfadal adhesion in these composites tends to be poor. The weak bonding at the interfaces between natural fibers and polymer matrix is surely a critical cause of the reduction of useful properties and performance of the... 

---