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What fibre materials make good reinforcing candidates? Glass, of course, is the most common reinforcing material utilized in general moulding applications, and there are high performing but expensive materials such as carbon fibre and aramid. But lately, natural fibres have been making inroads in applications where glass fibres were typically used.
(Published on December 2005 – JEC Magazine #21)
The growing list of natural fibre candidates includes, but is not limited to wood, kenaf, hemp, flax, jute and sisal. Even chicken or turkey feathers have been tried as composite reinforcement materials. In the first quarter of 2005, Composite Products Incorporated (CPI) located in Winona, Minnesota (USA), ran trials with corn hulls, a highly renewable resource, for a customer-specific application.
CPI is always seeking out new applications and reinforcing materials for its Advantage® process, which consists of direct in-line compounding of long fibre thermoplastic composites or D-LFT. In the patented Advantage® process, fibre lengths up to 25.4mm are mixed with a thermoplastic resin to create a molten preform. The preform is placed directly in the mould or transfer cylinder, creating a structural composite with maximum fibre length retention.
Material cost reduction
Besides the domestic availability of natural fibres, their lower price compared to many synthetic fibres offers an overall material cost reduction for the final composite material. With recent resin cost increases, natural fibres offer an excellent way to reduce material costs, especially in applications where structural strength demands are not as stringent.
Natural fibres can be utilised as a filler and reinforcement to reduce material cost.
Natural fibres can be combined with synthetic fibres to offer lower material cost and improved material physical properties. CPI has developed a composite material consisting of 30% flax and 10% glass fibre compounded in a polypropylene resin that offers improved physical properties over a 40%-talc-filled polypropylene composite (see chart above). The unique compounding capability of the Advantage® process allows multiple reinforcing materials, along with colorants and material additives for a specific application.
Temperature and supply issues
One problem commonly associated with the use of natural fibres is temperature sensitivity. Natural fibres tend to burn or caramelise when exposed to the high melt temperatures and shear associated in the melt phase of the resin material. The resulting natural fibre turns dark, producing a “burnt-sugar” odour.
The Advantage® process overcomes this problem by melting the resin in one extruder while introducing the fibres into another. When the two materials are mixed together, CPI’s gentle dispersing mixing action with minimal residence time produces a preform ready for moulding, without the burnt look or smell often associated with natural fibres.
Another concern with natural fibres is the supply side. While natural fibres are abundant and easily renewable, obtaining a regular supply to meet scheduled production demands remains an uncertain proposition. Due to the increased efforts of many European countries to implement recycling requirements in the finished product, Europe may be ahead of North America in the development of an infrastructure for the supply of natural fibres, however.
As development and implementation of natural fibres continue, the supply side should also become more established. Moulders utilising natural fibres need to develop good relations with the suppliers of such fibres to insure uninterrupted supply.
A growing range of applications
The number and range of applications that could benefit from natural fibres continues to grow. Automotive applications are numerous and include seats, door liners, trunk liners, package trays, speaker shelves, under-body panels and acoustical and sound-deadening uses. Agriculture, furniture, trucks recreational vehicles and construction (decking, fencing, etc.) are just a few examples of other industries utilising natural fibres in moulded or extruded products.
As more companies, industries and even countries move towards recycling, environmental conservation continues to play a larger role in material selection. Besides being a good environmental citizen, natural fibres offer additional benefits of lower material cost.
They also lessen dependence on synthetically produced materials and are a naturally renewable resource.