Polyester resins: types, properties, applications / Articles

Polyester resins are among the most common and versatile classes of thermosetting synthetic resins, occupying a critically important position in modern industry. They are synthetic heterochain oligomers and polymers containing ester groups and carbon-carbon double bonds, forming insoluble and infusible products upon curing. By production volume, polyester resins rank third among all plastics globally, behind only polyethylene and polypropylene.

Chemical Structure and Synthesis

The most common polyester resins are based on polyalkylene glycol maleates and polyalkylene glycol fumarates — products of polycondensation of unsaturated dibasic acids (maleic or fumaric) with aliphatic glycols. The resulting linear unsaturated polyester is dissolved in a copolymerisable monomer (usually styrene, less commonly methyl methacrylate, diallyl phthalate). This solution is the commercial polyester resin.

A key feature of the chemical structure of unsaturated polyester resins is the presence of double bonds in their macromolecules, capable of radical copolymerisation with vinyl monomers. The curing process proceeds via a free-radical polymerisation mechanism, initiated by organic peroxides (e.g., methyl ethyl ketone peroxide, MEKP, or dibenzoyl peroxide).

Classification of Polyester Resins

By composition and application, polyester resins are divided into several main types:

Orthophthalic resins — standard resins with good mechanical strength at short curing times (pot life about 7–40 minutes), but with limited chemical resistance. Used in boatbuilding, tank construction and polymer concrete.
Isophthalic resins — high-quality resins with good strength parameters, hydrolysis and chemical resistance. Recommended for applications requiring corrosion resistance (acids, water).
Isophthalic/Neopentyl glycol resins (ISO/NPG) — feature very high mechanical stability and resistance to hydrolysis, chemicals and UV. Used for polymer concrete and CIPP.
Terephthalic resins — offer the highest resistance to chemicals and hydrolysis, high heat resistance and very good mechanical stability.
Dicyclopentadiene resins (DCPD) — characterised by very low styrene emissions and low shrinkage, making them preferred for low-styrene-emission production.
Bisphenol A resins — feature reduced ester bond density, providing enhanced corrosion resistance.
Vinyl ester resins — represent a hybrid of epoxy and polyester resins, combining high chemical resistance with good mechanical properties.

By properties and application, resins are also classified as: general purpose; increased heat resistance; reduced flammability; for translucent fibreglass; chemically resistant; containing no volatile monomers; increased elasticity.

Physical, Mechanical and Chemical Properties

Unsaturated polyester resins possess a wide range of valuable properties:

Density. Relative density is 1.11–1.20 kg/m³.

Mechanical Properties. Cured polyesters are characterised by high tensile, flexural and compressive strength, good hardness and stiffness. Elastic strength is 80–100 MPa, elastic modulus — 3.2–3.9 GPa.

Thermal Resistance. Most unsaturated polyesters can be used at temperatures up to 250 °C, but their properties deteriorate significantly over time, especially at elevated temperatures. Heat resistance of most grades is 50–60 °C (heat deflection temperature), with some grades withstanding up to 120 °C. Some special grades (e.g., naphthalene-based resins) withstand temperatures up to 150–160 °C.

Chemical Resistance. Cured polyesters are resistant to mineral and organic acids, salt solutions, gasoline and oils. However, they are not resistant to alkalis, hot acids and chlorinated hydrocarbons. Bisphenol A-based resins offer enhanced corrosion resistance due to reduced ester bond density.

Optical Properties. Light transmission of transparent polyester-based materials can reach 92%.

Electrical Properties. Polyester resins have good electrical insulation properties.

Shrinkage. Polyester resins are characterised by significant volume shrinkage during curing, which must be considered in product design.

Curing of Polyester Resins

Curing of unsaturated polyester resins proceeds via a free-radical mechanism. Organic peroxides are used to initiate the process, decomposing to form free radicals. Curing can occur at room temperature (cold curing) or with heating (hot curing). At room temperature, methyl ethyl ketone peroxide (MEKP) is typically used in combination with accelerators (e.g., cobalt salts). The curing process can also be initiated by ultraviolet radiation.

An important feature of polyester resins is the ability to cure at room temperature and atmospheric pressure, making them particularly convenient for large-scale products and field applications.

Modification of Polyester Resins

To improve properties, polyester resins undergo various types of modification:

Improving chemical resistance is achieved by introducing hydrophobic cyclic radicals (isophthalic, endomethylenetetrahydrophthalic anhydrides), rosin, anthracene, as well as branched glycols.
Increasing elasticity is achieved by using aliphatic acids (adipic, sebacic) and long-chain glycols.
Reducing porosity and improving working conditions is achieved by replacing styrene and methyl methacrylate with less volatile monomers — diallyl phthalate, triethylene glycol dimethacrylate, diallyl isophthalate or oligoester acrylates.
Improving UV resistance is achieved by introducing light stabilisers.
Bio-based modifications — use of vegetable oils, cellulose and other renewable sources to reduce volatile organic compound emissions.

Applications

Polyester resins find wide application in various industries:

Fibreglass and Composites. This is the primary use of polyester resins. They serve as binders for fibreglass, carbon fibre and other reinforced composites. Products include boat and yacht hulls, automotive components, sporting goods, pipelines and tanks.

Construction. Polyester resins are used for polymer concrete, pipe rehabilitation (CIPP), floor coatings, roofing materials and cladding panels.

Automotive Industry. Used for body panels, bumpers, interior parts and structural components.

Marine Industry. Shipbuilding and yacht building are among the largest consumption segments due to corrosion resistance and light weight.

Electrical Engineering. Used for potting and sealing electronic components, as insulation materials.

Wind Energy. Wind turbine blades are made from fibreglass composites based on polyester resins.

Coatings and Adhesives. Widely used in the paint and coatings industry and adhesive production.

Household Appliances and Consumer Goods. Used for various products by casting and moulding.

Global Polyester Resin Market

The global unsaturated polyester resin market shows steady growth. In 2025, the market was valued at USD 14.6–14.92 billion. It is projected to reach USD 21.8–31.18 billion by 2034 at a CAGR of 4.43–8.54%.

The Asia-Pacific region dominates the market with a share of over 58% in 2025. China is the world's largest producer and consumer of polyester resins. The polyester resin segment held the largest revenue share in the composites market in 2025, driven by low cost, good mechanical properties and ease of processing.

Key market growth drivers: rising demand in construction, automotive and marine industries due to lightweight, high-strength and corrosion-resistant properties; increasing infrastructure projects; expanding wind energy applications; growth of electric vehicles; introduction of bio-based and low-emission resins.

Conclusion

Polyester resins represent one of the most versatile and sought-after classes of thermosetting polymers in modern industry. The unique combination of affordable cost, ease of processing, good mechanical and electrical insulation properties, corrosion resistance and room-temperature curing makes them indispensable in fibreglass production, construction, automotive, shipbuilding and wind energy. Continuous improvement of formulations and the development of bio-based and environmentally friendly technologies open new prospects for this critically important class of materials.