Carbon Fibre

 

Carbon fibre is produced in three types, simplistically speaking, these are; low, medium and high stiffness – but more accurately, extremely high, incredibly high and phenomenally high. These are termed high strength (HS,) intermediate modulus (IM) and high modulus (HM) with the supply cost reflecting the speed of manufacture – the latter two are heated higher and stretched further in the production furnace – but all use the same type of feedstock (termed precursor), a polymer fibre, polyacrlonitrile (PAN.) A further complication comes from the width of tow produced. Tow is the term for the size of fibre string. Tows of 6K and 12K are most common having approximately 6000 and 12000 individual carbon fibres held as a bundle. The thicker the tow, the thicker the composite lamina (layer) and the lower the cost per kilogramme since a furnace can only hold a fixed number of tows to be tensioned, unrolled, rerolled.

 

Glass Fibre

 

There are three main types, standard E type, S and R types which have higher strength and better chemical resistance respectively. Only E type are used in standard structures owing to its very low cost, S and R being relatively high cost fibres.

 

Natural Fibres

 

There is growing interest in these resulting from the Green agenda and that being grown with solar energy rather than being manufactured by CO2 intensive processes, their embodied energy for manufacture is much (up to 80%) lower than for glass fibre. Of these fibre types, flax is favoured owing to its relatively high stiffness and strength. The main structural benefit of flax is its low density of 1.5g/cc c.f. glass at 2.6 g/cc and the densities of typical polymer matric composite laminates being 1.3 and 1.9 respectively. With most structural sizing of panels being flexural stiffness dominated (related to thickness cubed), this is of great benefit since part thicknesses using flax are higher for equivalent mass panels. This higher panel thickness also provides enhanced impact resistance and these two attributes result in their practicality for semi-structural ‘cladding’ type applications such as car body panels. All production applications are for car interiors however, as a replacement to injection moulded thermoplastics or cardboard. The three awkward downsides for flax laminates are: susceptibility to moisture and fluid damage, very low compressive strength and stiffness and an immature supply chain.

 

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