Glass Fibre Manufacture
Glass in the form of fibres has found wide and varied applications in all kinds of industry. 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 filament glass fibre; manufactured using the continuous filament process, 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.
Continuous Filament Glass Fibre Manufacture
Continuous glass fibre is a continuous strand, made up of a large number of individual filaments of glass. Molten glass is fed from the furnace or "tank" through a channel or "forehearth" to a series of bushings which contain over one thousand six hundred accurately dimensioned holes or "forming tips" in its base.
A constant head of glass is maintained in the tank and forehearth and the temperature of the glass in the bushings is controlled to very fine limits. Fine filaments of glass are drawn mechanically downwards from the bushing tips at a speed of several thousand metres per minute, giving a filament diameter, which may be as small as nine microns, or one tenth the diameter of a human hair. From the bushing the filaments run to a common collecting point where size is applied and they are subsequently brought together as bundles, or "strands", on a high-speed winder.
Glass fibre is produced in a range of filament diameters and strand dimensions to tight tolerances for different end uses. It is used to strengthen and stiffen thermosetting plastics, thermoplastics, nylon and polypropylene as well as inorganic matrices, such as gypsum.
Glass Wool Manufacture
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 fibres by a high velocity blast of hot gas. After being sprayed with a suitable bonding agent, the fibres 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.
In this Section
The oldest examples of glass are Egyptian beads dating from 12,000 BC.