Polymeric Composites with Rice Hulls for Extrusion - Introduction to Polymeric Composites with Rice Hulls - page 124

Recommended specifications for the use of fibres to achieve the best material properties
and highest production speeds (output rates) are:
• Fibre size : 0.1-1 mm, ideally ≈350-500 µm, equal to mesh numbers 35, 40 or 45.
• Moisture content : <12% for direct extrusion or <3% for compounded, pelletised
extrusion.
• Fibre types : generally a wide range of natural fibres are possible but softwood
fibres are the easiest to process. In the case of rice hulls, specially designed screws
are used.
Fibres with other specifications can also be used but with limitations regarding the
maximum line-speed (output) and end-product properties. Composite manufacturers
also have the option of having a 1-3-kg sample of the fibres analysed by most
machinery suppliers, free of charge, before usage in production.
Besides the fibre raw material, the polymer matrix to be used also has a significant
effect on the production equipment. In general, the most popular and easiest-to-process
composites are based on polyolefin (PP or PE) as the matrices for the following reasons:
• Higher ibre-illing ratios are possible (≤80% natural ibres).
• Reduced costs for raw materials.
• Final products have a more natural wood-like feel to touch.
• Recyclability of polyolein conforms to the natural and environmentally friendly
image of WPC/PCRH due to use of renewable and sustainable raw materials.
• PVC is (normally) not allowed to be energetically reused (burned) because it
would be harmful to the environment.
Depending on the size and capacity of an extrusion system and thickness of a profile,
it is generally accepted that profile widths ≤1,000 mm can be produced. For general
composite lumber applications, widths ≈200 mm can be produced easily.
10.4 Extrusion Systems for Polymeric Composites with Rice Hulls
The key to an ideal selection of an extrusion system for PCRH is the end products to
be produced. For example, a producer may opt to manufacture only PCRH lumber for
decking, fencing and other outdoor applications, or set himself/herself up to produce
different composite products on the same system using different formulations. Also,