8th Class Science Synthetic Fibers and Plastics Fibres

        Fibres

Fibres are fine hairlike structure of animal, vegetable, mineral or synthetic origin. Commercially available fibres have diameters ranging from less than 0.004 mm (0.00015 in) to 0.2 mm (0.008 in) and they come in several different forms: short fibres (known as staple or chopped), continuous single fibres (monofilament), untwisted bundles of continuous filaments (tow), and twisted bundles of continuous filaments (yarn). Fibres are classified according to their origin, chemical structure or both.

              Animal Fibres

All animal fibres are complex proteins. They are resistant to most organic acids and to certain powerful mineral acids such as sulphuric acid (H₂SO₄). However, protein fibres are damaged by mild alkalies (basic substances) and may be dissolved in strong alkalies such as sodium hydroxide (NaOH).

             Vegetable Fibres

Vegetable fibres are predominantly cellulose, which, unlike the protein of animal fibres, resists alkalies. Vegetable fibres resist most organic acids but are destroyed by strong mineral acids. Improper use of most bleaches can also weaken or destroy these fibres.

There are four major types of vegetable fibres: seed fibres, which are the soft hairs that surround the seeds of certain plants; bast fibres, the tough fibres that grow between the bark and stem of many dicotyledonous plants; vascular fibres, the tough fibres found in the leaves and stems of mono-cotyledons; and grass-stem fibres. Other fibre types, of limited utility, include strips of leaf skins, such as raffia; the fibre of fruit cases, such as coir; and palm fibres.

           Mineral Fibres

Glass, which is made from silica sand, is the only inorganic (mineral) fibre widely used in commercial applications. There are two main forms of glass fibres: continuous and staple. Continuous glass fibre, which is made by drawing molten glass into threads, is used in textile materials. The use of air, steam, or gas to disrupt the flow of the molten glass stream produces staple fibres. These fibres can be fabricated into mats or into bulk-molding and sheet-molding compounds with the use of resins, or organic binders. Quartz mineral is high silica, high-purity glass that is good for long-term use at temperatures as high as 1400° C (2552° F).

           Synthetic Fibres

Synthetic fibres derived from natural cellulose were first developed at the end of the 19th century and became known as rayons. In a typical rayon-making process/natural cellulose made from wood pulp is treated with chemicals to form a thick liquid. This liquid is then extruded as filaments into a weak acid bath that converts the filaments back into pure cellulose. Rayons are not, therefore, completely synthetic but are actually regenerated fibres.