Introduction

The estimated polyester fibre production, in 2007, was 34 million tons (approx)-39.7% of the whole textile fibre production-while, cotton production was 28 million tons (approx) and other cellulosic fibres, 2.8 million tons (approx). On a positive note, polyester fibres are estimated to grow by 5% annually, even as other textile fibres stagnate or decrease.

What is PET?

The development of fibres from polyester have come a full circle since scientists at ICI, UK, first produced dimethyl terephthalate from ethylene glycol and terephthalic acid. Subsequent years saw the arrival of this new fibre in the market, chemically known as polyethylene terephthalate (PET).

Independent of this research, DuPont (US) and Hoechst (Germany) also developed this synthetic fibre. By today's standards, it is the most versatile synthetic fibre ever invented. It lends itself beautifully alone, as a textile fibre, or as blends with cotton, viscose, polynosic, linen, silk, wool or hemp. The chief characteristics of this fibre include:

• Good crease recovery
• Good shape retention
• High bulking capacity
• Good abrasion
• Tear resistance
• Good dye ability
• Acceptable moisture absorption
• No strength loss during wetting and other chemical treatments
• Resistance to microorganism, insects, mildew, etc.

Structure and modifications

Mainly manufacture by the direct reaction of terephthalic acid and ethylene glycol, polyester is devoid of functional groups.

Polyester modifications are made to alter the inner structure of the fibre. These modifications are physical, using special methods, or they contain copolymers or different chemical components, giving the desired end results like low pilling, dyeing behavior to anionic and cationic dyes, etc. An interesting modification is the process known as texturising, where the molten fibre is first stretched and then cooled. It gives the fabric a luxurious bulkiness, softness and extra shine.

Various types of additions and modification also affect the performance of the fibre/ fabric. For example, delustering agents (usually titanium dioxide) results in dull polyester, while adding optical whitener to the fibre before spinning produces white fibre.

Physical Properties

• Density: 1.38 - 1.5
• Melting point: 252 - 256^oC
• Softening point: 230 - 250^oC
• Glass transition temperature: 68^oC

Drawbacks

The major drawback of the low moisture regain polyester fibres is its wearing comfort. Hence, 67/ 33% blends with cotton result in excellent fabrics having the best properties of both the fibres. Another disadvantage is that oils, fats, grease, etc. can easily soil polyester fabrics because of the electrostatic charge build up and the fibres tend to pill. Hence, adding a singeing process takes care of this problem by burning off the surface fibre.