As a result of the general increase in environmental awareness, the use of natural fiber reinforced polymer composites has risen across sectors such as the automotive, construction, and packaging industries. These composites are employed as a replacement for inorganic fiber reinforced polymer composites. Currently, glass fiber remains one of the most widely used fibers to reinforce polymers; composites with glass fiber reinforcements typically exhibit superior mechanical properties, but are not as environmentally friendly as composites with natural fiber reinforcements.
Cellulose fiber has emerged as a possible replacement for glass fiber (1). Its enhanced mechanical and thermal properties, low density, high aspect ratio, and nonabrasiveness are attractive features. Moreover, unlike other natural fillers/fibers, cellulose fiber contains little extractives, which means that the emission of volatiles is reduced during processing; it is thus a suitable material for extrusion or injection processing. However, cellulose fiber is polar (hydrophilic) in nature while polyolefin, such as polyethylene and polypropylene, is nonpolar (hydrophobic). The poor compatibility between cellulose fiber and polyolefin causes poor dispersion of cellulose fiber in the polyolefin matrix and a weak interface between cellulose fiber and polyolefin, thus resulting in poor mechanical properties of the composites (2-7).
To improve the mechanical properties of cellulose fiber reinforced polyolefin composites, efforts have been made to modify the interface between the cellulose fiber and polyolefin (e.g. (2), (3). for cellulose/PE composites and (4-7) for cellulose/PP composites). In fact, past modifications were found to increase the tensile or flexural strength. However, notched impact strength needed to be further improved. It was reported that maleated thermoplastic elastomers, such as maleated styrene/ethylene/butylene/styrene copolymer and maleated ethylene/propylene/nonconjugated diene elastomer, were useful in improving the notched impact strength of wood/polyolefin composites (8-12) and rice hull/polyolefin composites (13). However, these have not yet been applied to the cellulose fiber reinforced polyolefin composites.
Microcellular foaming (14) or fine-celled foaming is another way to improve the impact strength of natural filler/fiber composites while reducing material weight and material use. To achieve such improvements, it is essential to control the cell morphology of the composites. However, there are technical obstacles that arise when one attempts to control the foam structure of natural filler/fiber composites. Although natural fillers/fibers can act as nucleating agents (15), the inherent moisture of natural fillers/fibers, as well as the volatiles that are released during processing due to the presence of extractives, cause significant deterioration of the foam cell structure, that is, nonuniform cell distribution and a large average cell size (16-21). Furthermore, the large amount of natural filler/fiber causes irregular cell nucleation and cell growth, generating foams with low cell density and a large, nonuniform cell size (22), (23).
It has been reported that the modification of the interface between natural filler/fiber and polymer affected the foamability of the composites. However, only limited information is available in the literature. Matuana et al. (24), (25) investigated the effect of surface modification with silane on the cell morphology of polyvinyl chloride (PVC)/cellulosic-fiber composites in a batch process. They suggested that the surface modification of fibers increased the void fraction of the composites. Li and Matuana (26) and Zhang et al. (27), using high-density polyethylene/wood flour composites and polypropylene/wood flour composites, respectively, examined the foaming behavior in extrusion with chemical blowing agents. They indicated that the addition of the coupling agents increased the void fraction and cell size of the composites.
In general, the end users will decide if natural fiber reinforced polymer composites are foamed or not based on their solid-state properties, especially their mechanical properties. In this context, the mechanical properties of solid cellulose fiber reinforced high-density polyethylene (HDPE) composites were investigated in this article. Four types of maleated polymers were applied to improve mechanical properties, such as the flexural and impact properties of the composites. As the manufacturability is also important, the foaming behaviors of the composites were also studied in the extrusion foaming process. [N.sub.2] was applied to the composites as a physical blowing agent.
EXPERIMENTAL
Materials
HDPE (SCLAIR2710, Nova Chemicals) was used as a polymer matrix. HDPE has a melt flow index of 17 g/10 min (190[degrees]C/2.16 kg) and a density of 0.951 g/[cm.sup.3]. Two types of cellulose fibers were employed as reinforcement materials. Both fibers were similar in thickness (1-2 /[micro]m) and width (20 [micro]m), but differed in length. One had a length of 60 /[micro]m (TC90, CreaFill Fibers Corp.) and the other was 700 [micro]m (TC750, CreaFill Fibers Corp.). SEM micrographs of the short and long fibers are shown in Fig. la and b, respectively. The fibers were coated with a thin layer of platinum, and then observed using a scanning electron microscope, SEM (JSM-6060, JEOL). The figures clearly show that there is a large difference between the short and long fibers in terms of the length and aspect ratio.
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