03-07-2013, 02:35 PM
DEVELOPMENT OF WOOD FLOURRECYCLED POLYMER COMPOSITE
PANELS AS BUILDING MATERIALS
DEVELOPMENT OF WOOD.pdf (Size: 6.97 MB / Downloads: 198)
ABSTRACT
Wood plastic composites (WPCs) were made using matrices of recycled high-density
polyethylene (rHDPE) and polypropylene (rPP) with sawdust (Pinus radiata) as filler.
Corresponding WPCs were also made using virgin plastics (HDPE and PP) for
comparison with the recycled plastic based composites. WPCs were made through melt
compounding and hot-press moulding with varying formulations based on the plastic
type (HDPE and PP), plastic form (recycled and virgin), wood flour content and
addition of coupling agent. The dimensional stability and mechanical properties of
WPCs were investigated. Durability performances of these WPCs were studied
separately, by exposing to accelerated freeze-thaw (FT) cycles and ultraviolet (UV)
radiation. The property degradation and colour changes of the weathered composites
were also examined. Dimensional stability and flexural properties of WPCs were further
investigated by incorporation of nanoclays in the composite formulation. To understand
the changes in WPCs stability and durability performance, microstructure and thermal
properties of the composites were examined. Two mathematical models were developed
in this work, one model to simulate the moisture movement through the composites in
long-term water immersion and the other model to predict the temperature profile in the
composites during hot-press moulding.
Introduction
Wood-plastic composites (WPCs) are emerging as one of the dynamic growth materials
in the building industry. WPC is manufactured by dispersing wood particles into molten
plastic with coupling agent or additives to form composite material through various
techniques of processing such as extrusion, compression or injection moulding. It was
first made commercially from phenol-formaldehyde and wood flour that was used for
Rolls-Royce gearshift knob in 1916, and it was reborn as a modern concept in Italy in
the 1970s, and popularized in North America in early 1990s [1]. Wood-thermoset
composites date back to early 1900s; however, thermoplastic polymers in WPC is a
relatively new innovation. In 1983, an American Woodstock company (Lear
Corporation in Sheboygan, WI) began producing automotive interior substrates by using
extrusion technology from the mixture of polypropylene (PP) and wood flour [2]. Since
then production and markets demand for the WPCs have been growing rapidly
worldwide. The share in North American decking market has grown considerably, from
2% in 1997 to an estimated 18 % in 2005, with total sales of US$3.9 billion in 2005 for
residential, industrial deck-boards and railing market in USA [3]. It is expected to
increase to about 500 million to 1 billion pounds over the next five years in the UK and
other European markets [4]. Currently WPCs are mainly used for building products like
decking, fencing, siding, garden furniture, exterior windows and doors [3, 4], although
other applications can also be found in marine structures, railroad crossties, automobile
parts and highway structures such as highway signs, guardrail posts, and fence posts [5].
WPCs possess many advantages over the raw materials of polymers and wood filler.
WPC had better dimensional stability and durability against bio-deterioration as
compared to wood. In addition, WPC also reduce the machine wear and tear of
processing equipment, and lower the product cost against inorganic fillers when waste
streams such as sawdust are used [4].
Wood filler in WPCs
Wood has been used as reinforcing filler in thermoset polymer for decades, however, its
use in thermoplastics is a relatively new spurred by improvement in processing
technology and development of coupling agents. Use of wood as the filler in WPCs has
advantages such as low-cost, renewable, biodegradability, low specific gravity, and low
abrasion to equipment as compared with inorganic fillers (e.g. glass fibres and clay).
Commonly used wood species for WPCs manufacturing are pine, maple and oak,
although other species can also be used. As the physical, chemical and micro-structural
properties of wood species depends on the type of species such as for softwood and
hardwood. Hence selection of wood species for the use in WPCs could have a
significant influence on the microstructure and properties of WPCs [25].
Waste wood for reinforcing filler
The large amount of wood waste is generated at different stages in the wood processing
and a proportion of this waste is mainly destined for landfill although major part of the
wood processing waste used for energy. About 1,038,996 m3 round wood equivalents of
wood chip residuals (wood off-cuts, slab wood, planer shavings and sawdust) was
produced by New Zealand forest industry in 2002, which are mostly from radiata pine
forest [60]. The use of waste wood in WPCs helps to offset these disposal costs. The
waste wood in the form of sawdust, fibres or pulp are suitable filler for polyolefin’s
matrix composites [13, 14]. The Pinus radiata fibre possesses physical and mechanical
properties suited to the reinforcement of plastics [15]. According to Lightsey et al. [61]
there was little difference in tensile modulus of composites made either from wood flour
or pulp mill wood residue with HDPE matrix. Wood particles for the use in WPCs need
to be dried to 0-2% moisture content to process adequately with thermoplastic
polymers. Due to the thermal stability of wood under temperature of 200ºC, most
common thermoplastics (PE, PP, PS, and PVC) are easily processed with wood below
this thermal decomposition temperature [5].