Agriculture Reference
In-Depth Information
With the advent and availability of high-end additives and constant development, WPC
producers have been able to offer improved products, especially for the construction
industry.
The basic benefits of using wood fibre or wood flour with plastics are increases in
the modulus of elasticity and lowering of the coefficient of thermal expansion, which
result in automatic cost reductions. The standard process for producing WPC lumber
is by extrusion, which is technically demanding and requires efficient equipment, with
the design of the screw being the most important. Other important factors are the
moisture content and the particle size of wood flour. For general-purpose applications,
after the particle-reduction stage a final pass through a 120-mesh screen may suffice
but, if smooth finishes are desired, a pass through a 80/100 mesh is required. Moisture
content may vary but should ideally be ≈1% and ≤2%. The choice of additives and
fillers is dependent upon the properties of the end applications, whereas colours can
be obtained by pigments or masterbatches. If recycled polymers are used and they
comprise different colours, then achievement of the final desired colours may be
more difficult.
Ground wood fibre may contain moisture content as high as 8-12% by weight
depending on humidity. Excess residual water in the extrudate leaving the extrusion
die will cause swelling of the WPC profile. Hence, it is essential to remove any excess
moisture content well before the compounded mass enters the extrusion barrel.
Extruders have de-volatisation capacity using atmospheric and vacuum venting but
the amount of water that can be removed from a WPC extrudate is limited. For
counter-rotating extruders, the highest production that can be achieved is if the
moisture content of the wood flour is dried to ≤1% before it is fed into the extruder.
Co-rotating extruders can achieve much better finishes than counter-rotating extruders,
and can also tolerate higher contents of moisture in the feed, with a range of 2-4%
by weight. For co-rotating operations, reduction of the moisture content to <2% may
be counter-productive with lesser throughput because evolution of steam from the
extrudate has a beneficial cooling effect.
Counter-rotating and co-rotating extruders can process wood fibre extrudates at
moisture levels of 1 and 4%, respectively, but at the expense of throughput. Reducing
the throughput increases the time the mass stays in the extruder, which increases the
water percentage that has to be removed from the extrudate. If the amount of water
in the wood that is fed into the extruder is double the recommended amount (e.g., 2%
in counter-rotating extruders and 8% in co-rotating extruders) then, as a general rule,
the maximum throughput will be reduced by ≈50%. Extrudates with excess moisture
at the die develop localised bubbles under the 'skin' of the profile. If the moisture
level is sufficiently high, the skin separates itself entirely from the underlying profile
as water expands and vaporises.
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