Weathered Wood-Plastic Composites Produced from Modified Wood Flour

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Abstract

The effects of weathering on the surface properties of wood-plastic composites(WPC)were examined. High-density polyethylene(HDPE)based WPCs made from modified wood flour(untreated,extractives free, and holocellulose(delignified)fibers)were subjected to accelerated(xenon-arc)weathering.Colorimetery and Fourier-transform infrared spectroscopy were employed to monitor the color change and surface chemistries of the weathered WPC surface.The compositional change that occurred on the WPC surface was determined using pyrolysis GC-MS analysis.The study showed that longer exposure time caused higher oxidation,lower wood lignin and higher plastic content of the weathered WPC surface.From this study, the use of holocellulose fibers improved the weathering performance(color)of WPCs.

Introduction

Fading color and properties change of wood-plastic composites(WPC)during weathering is becoming a great concern both to the manufacturers and consumers.It must be noted that several works have been conducted on weathering and durability issues of WPCs (Stark and Matuana 2004,Fabiyi et al.2006).Weathering has been reported to cause an increase in surface oxidation(C=O),unsaturation(C=C),while wood lignin content decreased in WPCs(Stark and Matuana 2004,Fabiyi et al.2006).Lignin has been suggested to be a great contributor to WPC color change during weathering(Fabiyi et al.2006).There is little or inadequate information,however,about the effects of using extractive-free wood and holocellulose(lignin free)fiber as filler/reinforcement on the weathering of highdensity polyethylene(HDPE)-based WPCs.

The use of cellulose or holocellulose fiber for WPC production may be a good alternative to minimize the problem of color change during weathering.The use of pure cellulose fibers over wood fiber in WPCs offers the benefit of higher thermal stability(up to 270°C)in that thermoplastics(e.g.,nylon)can be used to obtain materials for structural applications(Sears et al.2001). Therefore,cellulose fibers would be a suitable reinforcement for HDPE-based WPCs since they are commonly produced at temperatures below 200°C during pulping.

Background information on the chemistry of wood and plastic behavior during weathering may likely foster research toward production of WPCs with improved weathering performance(color).The determination of crystallinity could provide useful information about the plastic behavior during WPC weathering. Chemical changes that occur during weathering affect the overall properties of the polymer including melt flow/viscosity,molecular weight,and mechanical strength(Philip et al.2004).Another reason for studying crystallinity is that it has been reported to be an indicator of polymer chain scission during photodegradation(Stark 2003).The goal of this study was to examine the effect of modifying wood fibers for use in WPCs with enhanced weathering performance.The accelerated weathered WPCs were assessed by several spectroscopic techniques and thermal analysis.

Materials and Methods

Extractive-free wood fiber was prepared from commercial wood flour(pine,100 mesh,American Wood Fibers)by batch extraction using acetone.Holocellulose(lignin-free wood fiber)was prepared using an acidic-sodium chlorite solution at 70°C according to the method of Wise et al.(1946).The bleached(delignified)wood fiber was recovered by filtration through a polypropylene(PP)screen(100 mesh),then exhaustively washed with deionised water until a conductivity of 1.5 to 2.5μS was reached(VWR conductivity meter)and then finally rinsed with acetone to aid in drying.The resultant holocellulose fiber(1.5%lignin) was then dried to achieve a yield of 67 percent.

WPCs irrespective of the wood flour(untreated,extractive-free wood,and holocellulose)were composed of 40 percent HDPE(Equistar petrothene,LB 0100-00) and 60 percent wood flour.The materials were weighed and premixed,then compounded and extruded on a 35-mm conical counter-rotating twin-screw extruder (Cincinnati-Milacron)to a profiled dimension of 9.5 by 38 mm.The temperatures of the barrel and die ranged from 150°to 190°C and were held constant throughout each extrusion run.The extruded profiles were machined to give equal exposure of wood and plastic at the WPC surface.Samples for xenon-arc weathering tests (Q-Sun)were cut into 5-by 38-by 102-mm specimens and subjected to accelerated weathering with water spray according to ASTM D6662.

Surface color of weathered WPC specimens(at three locations)was determined in accordance with ASTM 2244 using a StellarNet EPP2000 UV-Vis spectrometer(190 to 850 nm)using a krypton light source (SL1,Stellar Net)and a diffuse reflection fiber optic probe.The spectrometer SpectraWiz software transforms spectral data into CIE LAB color coordinates on the basis of a D65 light source(L*,a*,and b*)(CIE 1986).Color coordinates for each sample,before and after exposure,were measured at three measurements per sample.