ABSTRACT:A one-way analysis of variance and thermal analysis were performed in this study to examine the influences of the contents, types (exothermic vs. endothermic), and forms (pure vs. masterbatch) of chemical foaming agents (CFAs), as well as the use of coupling agents, on the density reduction (or void fraction) and cell morphology of extrusion-foamed neat high density polyethylene (HDPE) and HDPE/wood-flour composites. The CFA types and forms did not affect the void fractions of both the neat HDPE and HDPE/wood-flour composites. However, a gas containment limit was observed for neat HDPE foams whereas the average cell size achieved in the HDPE/wood-flour composite foams remained insensitive to the CFA contents, irrespective of the foaming agent types. The experimental results indicated that the use of coupling agent in the formulation was required to achieve HDPE/wood-flour composite foams with high void fraction.
ABSTRACT:The interfacial adhesion between a wood fiber and a plastic matrix strongly influences the performance of wood-fiber-reinforced thermoplastic composites. Fiber surface modification with coupling agents is generally needed to induce bond formation between the fiber and polymer matrix. This study investigated the chemical reactions between cellulosic materials and functionalized polyethylene coupling agents. Both wood flour and cotton cellulose powder were treated with acrylic acid-functionalized polyethylene and maleic anhydride-functionalized polyethylene (maleated polyethylene) for surface modifications, and chemical changes resulting from these treatments were followed by a study of the Fourier transform infrared and X-ray photoelectron spectroscopy spectra. Variations in the band intensities, oxygen-to-carbon ratios, and concentrations of unoxidized carbon atoms were related to changes that occurred on the surfaces of modified cellulosic materials. The experimental results indicated that chemical bonds between the hydroxyl groups of the cellulosic materials and the functional groups of the coupling agents occurred through esterification reactions.
The performance of fiber reinforced thermoplastic composites strongly depends on solid fiber-matrix adhesion to allow stress transfer between the phases. Fiber surface modification with coupling agents is generally needed to induce bond formation between the fiber and the polymer. This study investigated the effects of coupling agent's functional monomer (acrylic acid vs. maleic anhydride) and base resin (polyethylene (PE) vs. polypropylene (PP)) types on the tensile and flexural properties of high-density polyethylene (HDPE)-wood-flour composites. The interfacial adhesion between wood flour and HDPE matrix was examined using environmental scanning electron microscope. The experimental results indicate that the types of functional monomer and base resin are important factors determining the effectiveness of functionalized coupling agents for HDPEwood-flour composites. Maleic anhydride-functionalized polyolefins perform better than acrylic acid counterparts whereas PE-based maleated coupling agents are more effective than PP-based counterparts in improving the strength properties of HDPE-wood-flour composites.
A factorial design was performed to determine the statistical effects of material compositions and extrusion processing variables on the foamability of polypropylene (PP)/wood-flour composites. Two levels with centrepoint values of wood flour content, chemical foaming agent (CFA) content, extruder's die temperature and screw speed were employed. The isolated main and interaction effects of these variables on the void fraction of foamed composite samples were analysed using Design Expert software. Statistical analysis of data revealed that the void fraction data was best fit with a linear model. The extruder's screw speed showed no discernible effect within the narrow range studied (20 to 50 rpm) whereas the other three main factors showed significant effects (values of “Prob > F” less than 0.0001) on the void fraction. Wood flour content/CFA content and wood flour content/die temperature constitute the important interaction effects. The experimental results indicate that void fraction of extrusion foamed composites is a strong function of the extruder's die temperature. A large amount of gas molecules available for the cell growth is not the only requirement for the production of composite foams with a high void fraction. Processing at a high die temperature is also very important for the development of proper viscoelastic properties of the matrix suitable for cell growth.
In this paper, a regression model was constructed to optimize the relationships between the void fraction of foamed HDPE/wood-flour composites, the processing conditions (extruder’s die temperature and screw speed rate), and the formulation compositions [chemical foaming agent (CFA) concentration and the moisture content of wood flour] by applying a four-factor central composite design (CCD) statistical approach. Design Expert software was employed to carry out the experimental design, statistical analysis, and numerical optimization. The analysis of variance (ANOVA) of the model showed that the void fraction of HDPE/wood-flour composites was a strong function of the extruder’s die temperature, the screw speed rate, and the moisture content of wood flour, while independent of the CFA content within the studied range (0 1%). In addition, the moisture content of wood flour and the extruder’s screw speed rate exhibited significant interaction effect on the void fraction of the foamed composites. The response surface optimization generated using the regression model suggests that HDPE/wood-flour composite foams with the highest void fraction can be achieved at the highest levels of extruder’s screw speed rate (120 rpm) and moisture content of wood flour (12%), together with the lowest levels of extruder’s die temperature (170 BC) and no chemical foaming agent.
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