Polymeric Composites with Different Constituents - Introduction to Polymeric Composites with Rice Hulls - page 74

technologies have been adopted in the automotive sector. Furniture components offer
considerable potential for WPC and several products have been marketed. Various
studies have forecasted that the dilution of expensive polymers with wood flour to
form polymeric composites will become of increasing economic benefit. Furthermore,
the end products are environmentally friendly, largely free from rot and do not require
painting because aesthetically pleasing finishes can be easily achieved. They are also
machinable like natural wood and have practical advantages over traditional products.
Another interesting end application is the production of 'green' computer parts,
especially computer keyboards. A German moulding company, Amper Plastik R.
Dittrich, is using a biodegradable grade of cellulose ester compound to produce the
base of the world's first ecological computer keyboard. This product also contains
Arboform , a colourable and biodegradable material based on lignin from waste
wood. The lignin is combined with other natural fibres such as flax or hemp and
some polymer resin, and this compound can be injection moulded. Arboform has
a 'PS-like' feel and properties and compares well with traditionally produced ones.
PCRH will have distinct advantages over WPC with wood flour due to the high content
of silica in the rice hulls flour. Thus, one could expect additional and a wider range of
end applications due to superior physical and mechanical properties. PCRH technology
and products are virtually new but, given time, the demand for them would expand.
6.2 Role of Polymers
Common understanding may be that plastic is plastic and that wood is wood. There
are a wealth of differences between each type and grade, and a designer of a WPC
composite and the producer must have a good understanding of both for successful
production. If a combination of polymers is used, the designer must ensure complete
compatibility between the polymeric matrices and a good knowledge of their properties
that will be needed.
PVC, PE and PP are the most commonly used polymers in WPC extrusion. PVC is
suited for counter-rotating extrusion with low screw speeds because it degrades the
material if subjected to high shear or temperatures higher than normal standard
temperatures. PE and PP are suited for co-rotating extruders because these polymers
can withstand high shear and temperatures better than PVC. These features allow
co-rotating extruders to achieve throughputs >3,000 lb/h with a screw diameter of
≈100 mm. Because WPC is a 'temperature-limited' extrusion process, the rule of
thumb for scale-up is the square of the screw diameter of the extruder. For example,
given a throughput of 1,600 lb/h on a 73-mm extruder, the expected throughput of
a 103-mm extruder is (103/73) 2 × 1,600, which is = ≈3185 lb/h.