Materials for engineering, 3rd Edition - (Malestrom)

Composite materials 187
(b) Alternatively, fibres may be produced by melt-spinning molten pitch.
During the spinning process, the orifice causes the planar molecules to
become aligned. It is then treated, while held under tension in order to
maintain its preferred orientation, at temperatures up to 2000°C to form
the requisite grains of graphite.
(c) It is also possible to stretch either of the fibre types described above
during the graphitization stage, giving further orientation of the layers
parallel to the fibre axis.
6.2.2 Glass fibres
Glass fibres are commonly produced in ‘E-glass’ (E is for electrical), because
it draws well and has good strength and stiffness. A typical composition
(wt %) would be 52 SiO 2 , 17 CaO, 14 Al 2 O 3 , 10 Ba 2 O 3 with some oxides of
Mg, Na and K, and molten glass is gravity-fed into a series of platinum
bushings each of which has several hundred holes in its base. Fine glass
filaments are drawn mechanically as the glass exudes from the holes, then
wound on to drums at speeds of several thousand metres per minute.
The strength of the glass fibres is dependent upon the surface damage
arising when they rub against each other during processing. The application
of a size coating at an early stage during manufacture minimizes this degradation
in properties, by reducing the propensity for forming these ‘Griffith’ cracks.
The size consists of an emulsified polymer in water and also has the effect
of binding the fibres together for ease of further processing.
6.2.3 Organic fibres
Organic fibres of high strength and stiffness may be manufactured and one
of the most successful commercial organic fibres has been developed by the
DuPont Company with the trade name of Kevlar. It is an aromatic polyamide
and the aromatic rings result in the polymer molecules having the properties
of a fairly rigid chain. The fibres are produced by extrusion and spinning
processes, which align the stiff polymer molecules parallel to the fibre axis.
We will now consider how various types of composite materials may be
fabricated.
6.2.4 Polymer matrix composites
Fillers for plastics
Apart from fibres, a wide range of other solids may be added to plastics in
order to modify their properties or to reduce their cost. These additives are
known as fillers, and many of them are based on ground minerals such as