EFFECTS OF CHEMICAL FOAMING AGENTS ON THE PHYSICO-MECHANICAL PROPERTIES AND RHEOLOGICAL BEHAVIOR OF BAMBOO POWDER-POLYPROPYLENE FOAMED COMPOSITES
To make full use of bamboo resources in China and explore the foaming mechanism of bamboo powder-polypropylene (PP) foamed composites, a foamed composite of 54 wt% PP and 13 wt% HMSPP containing 33 wt% bamboo powder blends was prepared by injection moulding. Effects of chemical foaming agents (CFA) on the mechanical properties and rheological behavior of foamed composites were investigated. The mechanical measurements and ESEM test results indicated that the composite with 1% modified
... modified exothermic FA had smaller cell size and better cell distribution compared with endothermic FA. It also had better physico-mechanical properties, with a decrease of 14.2% in density and an increase of 16.8% to 40.2% in the specific tensile, bending, and notched impact strength compared with the non-foamed composite. The frequency sweep results indicated that all composites had a shearthinning behavior, and both the modulus and complex viscosity of composite with 1% exothermic FA decreased compared with those of the non-foamed composite. The shear rate scans revealed that the non-Newtonian fluid index increased with the increase of exothermic FA content. The viscous activation energy of the modified composite with 1% exothermic FA was 46.41KJ·mol -1 . This was an increase of 8.9% compared with that of the non-foamed analogue. tion and automotive industries, in place of wood, plastic, glass fiber, metal, and other composites. The first matrix resin used for foamed WPC was polystyrene (PS) (Rizvi et al. 2000) , and current research has focused on the study of polyvinyl chloride (PVC)-based and polyolefin-based foamed WPCs (Matuana et al. 1997 (Matuana et al. , 1998 Nawadon et al. 2011 ). However, PS-based foamed WPC has already been prohibited because of its "white pollution" to the environment. In addition, PVC-based foamed composites always release dioxin, chlorine hydride, and other toxic gases during processing, use, and disposal. In contrast, PP is environmentally friendly, easily recycled and degraded, and also has better resistance to heat and chemical corrosion. Therefore, much attention has been paid to the PP-based foamed WPC. The research interest is primarily focused on the preparation technology, the fiber pretreatment, the cell morphology of the foamed composite, and the effects of the type and concentration of foaming agents on the mechanical and physical properties of foamed composites made with high-density polyethylene ( HDPE)/PP/wood fiber (Rachtanapun 2003), microcellular PP/wood fiber composites (Zhang and Rodrigue 2005; Bledzki and Omar 2006a,b), and PP/flax fiber composite foams (Bledzki et al. 2005) . The major plant materials that have been used as reinforcements for production of PP-based foamed composite are wood fiber, wood flour, and hemp fiber. China has been called the "bamboo kingdom" in recognition of its abundant bamboo resources, the longest history of the cultivation of bamboo, the largest acreage of bamboo forest, and the highest yield of bamboo production in the world. Bamboo possesses numerous advantages, such as low cost, acceptable specific strength, and high wear resistance (Lakkad and Patel 1980) . Moreover, bamboo has demonstrated an ability to be a renewable and sustainable substitute for synthetic fibers. Unfortunately, presently the yield from the physical processing of bamboo is below 40% by weight. More than 60% of the bamboo timber becomes the processing residues in China (Zheng et al. 2005). The present work is based on an assumption that these processing residues may be suitable as the reinforcement in PP-based foamed composites. It is difficult for PP to form a fine cell structure because of its narrow foaming temperature range. Moreover, the difficulty of foaming increases with the increase of bamboo powder content. The physical properties of the foamed composite will also be degraded if the cells are too large or are unevenly distributed. Therefore, it is important to explore foaming mechanisms and preparation technology of bamboo powder/PP foamed composites. In addition, the rheological behavior of composites affects not only the processing performance, but also the mechanical properties of the final product. However, it can be seen from the current information that the effects of foaming agents on the rheological behavior and viscoelastic properties of foamed WPC have been little reported. In this study, 54 wt% PP and 13 wt% HMSPP were blended and mixed with 33 wt% bamboo powder to prepare the foamed composite by injection moulding. The effects of the different types and contents of CFAs on the physico-mechanical properties, cell morphology, and rheological behavior of the foamed composite were investigated to obtain the bamboo powder/PP foamed composite with the highest strength-to-weight ratio and optimal rheological behavior.