Agriculture Reference
In-Depth Information
PE is probably the most commonly used plastic polymer for WPC. HDPE is used in
WPC applications because of its better physical properties than LDPE or linear-LDPE.
HDPE with a melt flow index (MFI) of 0.3-1.0 g/10 min (known as 'fractional melt
grades') are better suited for WPC applications such as decking and other outdoor
uses but with limitations. HDPE with very low melt flows, such as 0.2 g/10 min,
require higher specific energy to process in the extruder, which means higher melt
temperatures. Because the wood content in a WPC mix will degrade at >190 °C,
to maximise throughput a WPC producer will have to choose a plastic resin with a
high MFI that will give a consistent physical properties as required by the targeted
end product.
6.2.1 Variations in Polymer Matrices
Plastics are chemically simple but complex with respect to chain lengths, chain
configuration, molecular-weight distribution and crystallinity. These properties are
seldom measured at WPC extrusion plants but can have a dramatic impact on WPC
production. The producers generally depend on the manufacturers of polymer resins
for accurate information.
Most of the plastics used in WPC (or in part) may consist of recycled material or
part virgin material. WPC producers may also use off-spec virgin material quite
successfully. Because the properties of these streams are not tightly controlled, there
can be significant variation in the parameters of the extrusion process over time. These
variations can affect the maximum throughput volume and scrap rates. Variations in
process throughput of ≈15% over a few days are not uncommon. These variations
can occur as the same frequency as plastic delivery to the WPC production plant.
Measuring the melt index of the incoming plastic polymers gives an incomplete (but
still useful) indication of the processing characteristics of a particular plastic resin to
be used. However, even this relatively simple measurement is often lacking in WPC
producing facilities.
WPC production makes significant use of lubricants as a process aid. Lubricants
in WPC have two broad functions. The first is minimising the heat of the exrudate due
to shear forces and to limit the melt temperature. The second function is to overcome
the no-slip boundary condition on the surface of the profile die so that the profile has
a smooth 'skin'. General lubricant levels in PE-based WPC formulae are ≈3-5% by
weight. PP-based WPC formulae generally contain 1-3% lubricant.
In the largest single market for WPC, which probably is outdoor applications such
as decking, mechanical properties are important to the installer and the end user of
the product. The resistance to bending under a load or the modulus of elasticity of the
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