PET is one of the most important raw materials today when it comes to man-made fibers. Depending on processing and thermal history it may exist both as transparent (amorphous), and white or opaque (semi-crystalline). One of the underlying reasons behind the success of PET containers is that they are virtually unbreakable. This is due to the molecular structure being set in a web. PET molecules are regular and straight, so their inter-chain forces are strong, but not strong enough to prevent melting. Therefore, PET is a "thermoplastic" material; a material that can be melted and then molded and solidified to form specific shapes and sizes. Since it has a high melting point, it does not melt at temperatures that are normally found in drying or washing processes. Another important part of PET is its glass transition temperature (Tg).
When a polymer is above its glass transition temperature, it is easy to change its shape. Below its Tg, the material is dimensionally stable and it resists change to its shape. This is very useful to the textile applications of PET because it allows some fibers, and the fabrics made from them, to be texturized or heat-set into a given shape. It is this property that provides the wrinkle resistance to the fabric that it will produce. These heat-set shapes remain permanent as long as the polymer is not heated above its Tg. Another important feature of PET is that it is fully 100% recyclable. In 1997 the first PET bottle was recycled and turned into a base cup. Very quickly the fiber industry discovered this valuable resource and began using it to make various textiles, carpets and other non-woven material. Today the fiber market is the largest outlet for recovered PET plastic, however, the "bottle to bottle" recycling process is steadily growing.