In order to handle the fine filamentary fibres that are necessary for structural composites the fibres are usually in the form of bundles. The bundles are drawn continuously from platinum-rhodium bushings, each producing several hundred filaments. The fibres are pulled away at speeds approaching 1000 to 2000 rn/mm. as molten glass and coated with size which lubricates the surface to prevent abrasion before the filaments (100- 1200) are brought together into a tow and wound onto a mandrel.
The Glass fibres are available in several forms. The main varieties are:
1. Chopped Strands - Short lengths of fibre in bundles of ‘400flbres. Length 3 to 40 mm used in automated pressing and mouldingwith both thermosetting and thermoplastic matrices as well ascement.
2. Chopped Strand Mat - chopped strands in the size 30 to 40mmare distributed over the area of a conveyor belt in randomorientations an a small amount of an organic binder (usually polyvinyl acetate) added to form a loosely bound open mat which isreadily impregnated by resin. Binder must be compatible with theresin - normally used with polyester or epoxy resins.
3. Rovings - A number of strands are grouped together and woundwithout twisting onto a cylindrical package to give a longcontinuous rope or large tow that may be used for filament windingor for chopping and spraying.
4. Yarn - twisted strands used in weaving cloth.
Glass fibre drawing
Copper is no longer the first choice for modern information transmission. Glass fibre cables (optical fibres) have assumed this task over longer distances, as they are vastly superior to the previous metal solution. Only 125 tim thick, a single glass fibre cable is theoretically sufficient to transmit 100,000 million telephone conversations. In contrast to copper, no skin effects occur (forcing of the current with increasing frequency from the middle of the cable to the surface by eddy currents), so that considerably higher frequencies can also be utilised and high transmission rates achieved. They also do not require suppression measures against electromagnetic radiation and can thus be combined with high voltage cables to form inexpensive solutions. High-purity quartz glass forms the core of glass fibre cables, being coated with glass of lower refraction (doped quartz glass). Total reflection thus occurs in the area between the core and the jacket, so information to be transmitted is conveyed in the core in the form of (IR) light. The outer jacket forms a polymer coating that contributes to mechanical stability.
Glass fibre cables are manufactured in a drop tower at approx. 2000 °C through simultaneous drawing (and collapsing) of the core and jacket from the respective preform (tubes). Graphite and CFC resistance heating elements are suitable for heating these furnaces, as these can be produced in ultra-pure forms. However, even the smallest of impurities in glass fibres considerably increase the level of evaporation. They can even resist temperatures up to 2,800 °C, remaining absolutely free of distortion. NTC behaviour enables these materials to realise a high heating efficiency in a vacuum with minimum power.
Glass-reinforced plastic (GRP)
Glass-reinforced plastic (GRP)is a composite material or fiber- reinforced plastic made of a plastic reinforced by fine fibers made of glass. Like graphite-reinforced plastic, the composite material is commonly referred to by the name of its reinforcing fibers (fiberglass). The plastic is chemosetting, most often polyester or vinylester, but other plastics, like epoxy (GRE), are also used. The glass is mostly in the form of chopped strand mat (CSM), but woven fabrics are also used.An individual structural glass fiber is both stiff and strong in tension and compression -- that is, along its axis. (Although one might intuitively imagine the fiber to be weak in compression, it is actually only the long aspect ratio of the fiber which makes it seem so; i.e., because a typical fiber is long and narrow, it buckles easily.) Oh the other hand, the glass fiber is relatively unstiff and unstrong in shear -- that is, across its axis. In other words, the fiber is stiff and strong in a preferred direction, namely, along its length. Therefore if a collection of fibers can be arranged permanently in a preferred direction within a material, and if the fibers can be prevented from buckling in compression, then that material will become preferentially strong in that direction. Furthermore, by laying multiple layers of fiber on top of one another, with each layer oriented in various preferred directions, the stiffness and strength properties of the overall material can be controlled in an efficient manner. In the case of glass- reinforced plastic, it is the plastic matrix which permanently constrains the structural glass fibers to directions chosen by the designer. With chopped strand mat, this directionality is essentially an entire two dimensional plane; with woven fabrics or unidirectional layers, directionality of stiffness and strength can be more precisely controlled within the plane. A glass- reinforced plastic component is typically of a thin “shell” construction, sometimes filled on the inside with structural foam, as in the case of surfboards. The component may be of nearly arbitrary shape, limited only by the complexity and tolerances of the mold used for manufacturing the shell.