Its composition depends on the intended use. For building insulation and glass wool the type of glass used is normally soda lime. For textiles, an aluminoborosilicate glass with very low sodium oxide content (E glass) is preferred because of its good chemical durability and high softening point. This is also the type of composition employed for the fibres used in the reinforcement of plastics, familiar for their application in protective helmets, boats, piping, car chassis and many other articles.
In recent years, great progress has been made in making optical fibres which can guide light and thus transmit images round corners. These fibres are applicable to endoscopes for examination of internal human organs, changeable traffic message signs now in common use on motorways for speed restriction warnings, and communications technology for transmitting telephone conversations much more efficiently than copper cable.
There are two broad groups of glass fibre products: continuous glass fibre which is used for the reinforcement of plastics, rubber and cement; and glass wool, which is used for thermal insulation and which is produced by the Crown process.
Glass wool is made in the Crown process. From the forehearth of the “tank” a thick stream of glass flows by gravity from the bushing into a rapidly rotating alloy steel dish “Crown” which has several hundred fine holes round its periphery.
The molten glass is thrown out through the holes by centrifugal force to form filaments, which are further extended into fine fibers by a high velocity blast of hot gas. After being sprayed with a suitable bonding agent, the fibers are drawn by suction onto a horizontally moving conveyor positioned below the rotating dish.
The mat of tangled fibres formed on the conveyor is carried through an oven which cures the bonding agent, then to trimmers and guillotines which cut the product to size. The mat may be further processed into rigid sections for pipe insulation. The mats are made into many products for heat and sound insulation in buildings, transport vehicles and domestic appliances.