What Is Polyester? Types, Production and Uses

Polyester is a family of polymers carrying an ester bond in the main chain. It is not a single product but a class with three very different major branches: thermoplastic polyester (PET, PBT), unsaturated polyester resin (composites) and saturated polyester (used as a raw material for coatings and polyurethane). These three branches, discussed under the same name, are almost separate sectors in terms of both production route and application area.

What is polyester?

The ester bond forms when an acid and an alcohol combine with the release of water. When this reaction is repeated, a long chain — polyester — is formed. The rigidity, melting temperature and solubility of the chain depend on the structure of the acid and alcohol used: aromatic acids (terephthalic acid) yield a rigid product with a high melting point, while aliphatic acids (adipic acid) yield a flexible product.

The most significant weakness of the ester bond is hydrolysis: the chain can break down in hot and humid environments. This behavior is a determining factor when selecting the application area for polyester.

How is it produced?

• PET (polyethylene terephthalate): esterification of terephthalic acid with ethylene glycol, followed by polycondensation in the molten phase. For bottle-grade quality, molecular weight is increased through solid-state polycondensation. Ethylene glycol is derived from ethylene oxide.
• Unsaturated polyester resin (UPR): esterification of maleic anhydride and phthalic anhydride with a glycol; the product is dissolved in styrene. The double bond in styrene cross-links with the double bonds in the chain during curing.
• Saturated polyester polyol: made from diacids and glycols without double bonds; used as a hydroxyl carrier in polyurethane and powder coating systems (see polyol).
• Recycled (rPET): reduction to monomers through mechanical recycling (washing, crushing, re-extrusion) or chemical recycling (glycolysis, methanolysis).

Types and varieties

Which sectors use it?

• Textiles: the most widely used synthetic fiber in the world; apparel, home textiles, technical textiles.
• Packaging: PET bottles and film — one of the plastics with the most developed recycling chain.
• Composites: glass fiber reinforced polyester (FRP) — water tanks, boats, panels, pipes.
• Paints and coatings: as powder coating resin; standard for exterior aluminum profiles.
• Plastics: PBT and PET engineering plastics.
• Automotive and electrical: insulation film, connectors, cable tape.

Turkish market and production

Türkiye has an established industry in polyester fiber and yarn production and is a regional player as part of the textile supply chain. Domestic production is also strong on the unsaturated polyester resin side; demand from the FRP sector is largely met domestically. On the other hand, there is import dependency in basic intermediate products such as terephthalic acid.

Key issues on the sector's agenda: rPET capacity and food-contact approvals, the impact of recycled content mandates on exports, reduction of styrene emissions (low-styrene and styrene-free UPR systems), and recycling of composite waste. You can find current news on these topics below.

Technical specifications

On the PET side, the determining figure is intrinsic viscosity: fiber grade, bottle grade and engineering applications each require different IV ranges, and this value decreases slightly with each recycling cycle. On the UPR side, gel time and peak exotherm temperature are fundamental to production planning; high exotherm in thick laminates causes internal stress and cracking.

Points to consider in selection and procurement

• Clarify which "polyester" is meant. PET, UPR and saturated polyester polyol serve different sectors; the application must always be specified when requesting a quote.
• Measure UPR gel time at your own ambient temperature. The value on the datasheet applies to 25°C; if the workshop temperature differs, the production schedule will shift.
• Manage exotherm in thick laminates. Thick sections cast in a single pour produce internal cracking and discoloration; a layer-by-layer approach is required.
• Request IV and color tolerance in writing for rPET. Batch-to-batch variation is more pronounced in recycled content than in virgin resin.
• Follow styrene emission regulations. Closed-mold methods and low-styrene resins are a direct result of this pressure.

Frequently asked questions

Are PET and unsaturated polyester resin the same material?

No. Both carry an ester bond, but PET is a thermoplastic — it melts when heated and can be reshaped. Unsaturated polyester resin, on the other hand, is a thermoset; once cured with peroxide, it cannot be melted. Their applications and recycling routes are entirely different.

Why is polyester fiber so widely used?

It is low-cost, dries quickly, is dimensionally stable and resistant to abrasion. When blended with cotton, it reduces wrinkling. Its disadvantages include low moisture absorption and the ongoing debate over microplastic release; the latter is increasingly on the agenda in technical textiles and apparel.

What is FRP?

"Fiberglass-reinforced polyester" — a composite obtained by curing unsaturated polyester resin together with glass fiber. Because it does not corrode and offers freedom of shaping, it is an alternative to metal in applications such as water tanks, treatment equipment, boat hulls and pipes.

How is polyester protected against hydrolysis?

By selecting the chain structure (isophthalic and vinyl ester systems are more resistant), by adding hydrolysis stabilizers such as carbodiimide, and by avoiding continuous hot-humid contact in the design. In applications involving prolonged contact with hot water, material selection is made accordingly from the outset.

Does rPET deliver the same performance as virgin PET?

In mechanical recycling, molecular weight decreases slightly with each cycle and color/clarity are affected; for this reason, rPET is often blended with virgin resin. Chemical recycling, which breaks the material down to the monomer, yields a product very close to virgin quality, but at a higher cost and energy consumption.