Polyester is made from the repeat unit called polyethylene terephthalate (composed from the monomers ethane-1,2-diol and benzene-1,4-dicarboxylic acid) with a degree of polymerisation of 115 to 140.

The polyester polymer is highly crystaline (65-85% crystallinity) which imparts good tensile strength to polyester fabrics. This also has the effect of making polyester hydrophobic (water hating) and thereby enabling it to retain its tensile strength when exposed to water. Polyester will only absorb about 0.4% water compared with 4% for nylon and 7% for cotton when exposed to normal atmospheric conditions.

The high crystallinity coupled with a low degree of polymerisation leaves polyester with some highly desirable properties. The stiffness of the polymer prevents the fabric yielding to distortion whilst the short polymer length ensures that polymer fracture is unlikely to occure. This gives rise to the crease resistance nature of polyester. Polyester can be heat set to take up pleats and fashion creases in garments which coupled with its wrinkle resistance properties has made this fibre very important in the clothing industry.

Polyester fibres usually have significant quantities of finishing oils associated with them. Examples would be Spinfinish oil, antistaic oil or lubricating oil for coning/texturising. This can lead to all sorts of difficulties both to their finishing (in terms of smoke emmission during drying as the oils are driven off) and flame retardancy. The oil residues can have unpredicatble effects during flammability testing.

Polyester is resistant to acid hydolysis but alkali condition encountered in laundering may cause some hyrolysis at the polymer ester groups. This is minimised by virtue of the crystaline nature of the polymer. The benzene rings of the polymer help stabilise the textile when exposed to sunlight. The high crystallinity of polyester makes it difficult for dye molecules to penetrate the fibres. Only the relatively small molecules of the disperse class of dye are used with polyester.