Aramid is a contraction of aromatic and polyamide. BISFA defines these fibres as "fibre composed of linear macromolecules made up of aromatic groups joined by amide or imide linkages". There are two types of aramid: the meta-aramids and the para-aramids.
The polymer poly-metaphenylene isophthalamide is used to make meta- aramids and the polymer p-phenylene terephthalamide to make para-aramids. Because the aramids decompose before they melt they are produced by wet and dry spinning methods. Sulphuric acid is the normal solvent used in the spinning processes. In wet spinning a strong solution of the polymer, which also contains inorganic salts, is spun through a spinneret into weak acid or water. In this bath the salts leach out. In the dry spinning process the salts are more difficult to remove and this process is only used to produce the weaker meta-aramid fibres. In both processes post treatment of the fibres by additional drawing is used to optimise fibre properties. Aramid products are available as filament yarn, staple fibre or pulp.
Properties and End-Uses
The p-aramid fibres have a very high strength, 5 times stronger than steel, little loss of strength during repeated abrasion, flexing and stretching. It has an excellent dimensional stability. The m-aramid fibres are used for their excellent heat resistance.
Some of the main end-uses for meta-aramids are protective clothing, hot gas filtration and electrical insulation. Para-aramids are used to replace asbestos in brake and clutch linings, as tyre reinforcement, and in composites such as materials for aircraft, boats, high-performance cars and sports equipment. Members of police forces and armed forces wear anti-ballistic aramid apparel.