Nanoclay reinforced HDPE as a matrix for wood-plastic composites
Introduction
During the last decade, wood-plastic composites (WPCs) have emerged as an important family of engineering materials. They have become prevalent in many building applications, such as decking, docks, landscaping timbers, fencing etc., partially due to the need to replace pressure-treated solid lumber [1]. Although WPCs are commercialized, they are mainly used in semi structural applications because they do not possess the bending strength and modulus required for long spans and load-bearing structural applications. Indeed, most of WPCs have lower bending stiffness and strength compared to solid wood [2].
Enhancing the bending properties of WPCs could expand their acceptance in load bearing structural applications. Two different approaches have recently been reported in the literature to substantially enhance the flexural properties WPCs.
The first approach dealt with the use of engineered and high performance thermoplastic such as poly(phenylene ether) (PPE) as a matrix due to its greater strength and stiffness compared to commodity plastics [3], [4]. However, processing wood fibers with PPE seemed difficult due to the high processing temperature of PPE (in the range of 280–320 °C), which is far above the degradation temperature of wood. Plasticization of the matrix with a low molecular weight epoxy was required to soften the matrix and reduce the processing temperature of PPE, which eased the manufacture of WPC made with PPE. However, epoxy acted as a plasticizer by softening the polymer, leading to lower strength and stiffness for the final products.
The second approach dealt with two different aspects in developing hybrid nanoclay PVC/wood-flour composites. The first aspect involved the development of a novel method for achieving a better dispersion of nanoclay in rigid PVC, which was accomplished by using a melt blending process based on fusion characteristics of PVC [5]. The second aspect involved the use of nanoclay reinforced PVC as a matrix in the manufacture of hybrid PVC/wood-flour composites [2]. These two-step process lead to hybrid PVC/wood-flour nanocomposites having bending properties similar to or better than those of various solid wood lumbers.
Although successful results were achieved in nanoclay–PVC/wood-flour composites, the majority of WPCs are currently made using polyethylene (PE) and polypropylene (PP) matrices. Consequently, the main goal of this study was to examine whether or not the addition of nanoclay could also enhance the bending properties of HDPE-based WPCs. Particular emphasis was placed on examining the effect of nanoclay types and as well as its addition sequence on the properties of the composites.
Section snippets
Materials
High density polyethylene (HDPE) was provided as granules by Nova Chemicals. Its melt flow index was 7.0 g/10 min at 190 °C and density at room temperature was 0.962 g/cm3. Wood flour from hardwood maple species (40-mesh size) supplied by American Wood Fibers was used as filler. The nano particles (montmorillonite clay) were purchased from Southern Clay Products. Five different types of natural montmorillonites modified with a quaternary ammonium salt denoted as Cloisite 10A, Cloisite 15A, Cloisite
Structure of nanoclay/HDPE nanocomposites
The use of small amount of nanoclay (less than 10 wt%) has been an attractive approach of enhancing the mechanical, thermal and barrier properties of the plastics. However, a homogeneous dispersion of nanoparticles (so-called intercalation and exfoliation) in a polymer matrix must be fully accomplished first to achieve those improvements.
Melt blending method was used to evaluate the effect of nanoclay types on the degree of intercalation and mechanical properties of nanoclay/HDPE composites to
Conclusions
Two different methods of introducing nanoclays into HDPE-based WPCs were examined. The first method involved the reinforcement of HDPE matrix with nanoclay, which was then used as a matrix in the manufacture of the composites (melt blending process). The second method consisted of a direct addition of nanoclay into HDPE/wood-flour composites during conventional dry compounding (direct dry blending process). The melt blending process, in which nanoclay/HDPE nanocomposite was used as matrix,
Acknowledgements
The authors gratefully acknowledge the USDA-CSREES Grant-Advanced Technology Applications to Eastern Hardwood Utilization (Grant No. 2006-34158-17581) for the financial support for this research work. The authors are grateful to the manufacturers for the donation of the materials.
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