Polyester Resins

Polyester Resins

Table of Contents


Applications of Polyester Resins

What to look for when buying Polyester Resins

Frequently Asked Questions about Polyester Resins



Polyester resins are synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Polyester resins are transparent, high-density polymers with excellent strength and water resistance with high usage in textiles and packaging. Compared with other polymers, Polyester resins are moderately expensive, shatter-resistant, food resistant, and withstands repeated recycling.

Applications of Polyester Resins

Polyester has been a staple of the textile industry since not long after its invention in the 1940s. When combined with cotton, the material produces a fabric that does not shrink or wrinkle. The strength and flexibility of polyester makes it more resistant to tearing. In addition to fabrics, the fibers are used to make ropes, carpeting, and other consumer products. Polyester textiles are used extensively in automotive interiors.

Polyester’s non-reactance with many foods makes it an ideal packaging material. Available as a film for packaging and in preforms (parisons) for bottle blow-molding, the lightweight material long ago replaced glass in many bottling applications as it is shatterproof and resists bacteriological attack. It has not caught on in beer and wine bottling – partly by tradition – but also because it can oxidize to some extent, imparting off-tastes to these long shelf-life products. A special form of the material is available for foods intended for both microwave and conventional ovens and turns up in so-called dual-ovenable trays. While the material’s permeability to gas is low, it can pass moisture; these characteristics are often improved through metalizing.

Polyester has a low shrink rate making it well suited to molding. It flows easily in its melted state. It is used as a filament for 3D printing although oftentimes suppliers will print prototypes in ABS as this material is especially prevalent in the industry. Polyester is readily machined and available in common forms. Often, the material is reinforced with glass fiber for increased strength. The material has many applications in the automotive sector, particularly in electrics and electronics (connectors, breakers, etc.) with other mechanical products used under-the-hood and on exteriors (wiper arm assemblies) as well.

The material is transparent making it especially suited for packages where product visibility is desirable, such as egg cartons.

What to Look for When Buying Polyester Resins

There are several things to consider when buying it.

The glass fibre mats used in laminating and mould making are available in many different thicknesses. The choice here can be very confusing. The same applies to the polyester resin itself. There are many products on the open market and not all of them are suitable for your planned project. If this is your first try with polyester resin, we recommend that you inform yourself thoroughly about the products before you buy.

Pay attention with the purchase to considerable manufacturers. Do not buy a product that is offered cheaply, but may then prove to be unsuitable. With products from good manufacturers, you are on the safe side. Otherwise, you may have to put a lot of effort into mould making, only to discover that the material does not harden completely.

Laminating with Polyester Resin

The prepared glass fiber mat is impregnated by applying the polyester resin to the substrate using a roller or brush and then laying the glass fibre mat on top. This works either cut or torn, with frayed edges giving you an advantage when working with overlaps.

After you have applied another coat of resin, the fiberglass mat becomes soft and can be shaped.

Epoxy vs Polyester Resin

When applying, make sure that neither white nor glittering spots remain, because there is too little or too much resin.

After the entire mat has been painted, remove the air bubbles by brushing them out with a roller or brush. There are special ventilation rollers for this purpose, which can be used for larger areas.

When the material is dry, it can still be painted with a topcoat of polyester resin to make it weatherproof. For this it must be lightly sanded. If you want to paint your object in a specific color, you can add color pigments or color paste to the polyester resin.

After work, clean the tools used, to which liquid resin still adheres, promptly with acetone and let it air out well. Once the resin has hardened, you can only sand it down.

The Curing Phase

How long the finished workpiece takes to harden completely depends on the material, among other things. How long this takes for your used resin brand can be found in the enclosed data sheet of the manufacturer. A factor that significantly influences the duration of the curing process is, besides the material, the size and shape of the workpiece, the temperature. Synthetic resins are available on the open market that cure completely at a temperature of 20 to 25° Celsius, others require a high temperature. These are tempered.

The Tempering

The curing of synthetic resins is a chemical process that can be partially controlled by heat input. The viscosity of the material increases until it reaches the gelling point. Then the resin hardens completely; it changes into the solid state. At higher temperatures, the processing time is reduced because the resin is more fluid and therefore less viscous. The lower the temperature, the longer both processing and curing time will last. In the latter case, it can take up to several days.

Filler materials are used extensively with polyester resins for a variety of reasons including:

  • To reduce the cost of the moulding
  • To facilitate the moulding process
  • To impart specific properties to the moulding

Fillers are often added in quantities up to 50% of the resin weight although such additional levels will affect the flexural and tensile strength of the laminate.

Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a monomer which is usually styrene. The addition of styrene in amounts of up to 50% helps to make the resin easier to handle by reducing its viscosity. The styrene also performs the vital function of enabling the resin to cure from a liquid to a solid by ‘cross-linking’ the molecular chains of the polyester, without the evolution of any by-products. These resins can therefore be moulded without the use of pressure and are called ‘contact’ or ‘low pressure’ resins. Polyester resins have a limited storage life as they will set or ‘gel’ on their own over a long period of time. Often small quantities of inhibitor are added during the resin manufacture to slow this gelling action.

Frequently Asked Questions About Polyester Resins

Is Polyester Resin Harmful?

Polyester resins are being increasingly used in industry. These resins require the addition of catalysts and accelerators. The handling of polyester resin system materials may give rise to skin irritations, allergic reactions, and burns. The burns are probably due to styrene and organic peroxides. Atmospheric pollution from styrene and explosion and fire risks from organic peroxides must be prevented. Where dimethylaniline is used, scrupulous cleanliness and no-touch technique must be enforced. Handling precautions are suggested.

What is the difference between Polyester Resin and Epoxy Resin?

Polyester resin is produced by free-radical polymerization. Epoxy resin is produced by condensation polymerization.
Polyester resins do not have adhesive properties. Epoxy resins have adhesive properties.
The shrinkage is high.The shrinkage is low.
Environmental durability is low. Environmental durability is high.
Polyester resins are less likely to be used in high thermal applications.Epoxy resins are more likely to be used in high thermal applications.
Glass transition temperature is 40 to 110 °C. Glass transition temperature is 40-300 °C.
Polyester resin is not expensive. Epoxy resin is expensive.
Polyester resin is highly toxic.Epoxy resin is less toxic.


Polyester resin is widely applied in manufacturing fiberglass-reinforced plastics (FRP) profiles, which are used for structural engineering applications and making FRP rebars. Polyester resins can be used as a strengthening material and as a corrosion resistant polymer composite. Unsaturated polyester resin is the most widely used type of polyester resin which contains double-covalent bonds in its polymer chains.

Both polyester resin and epoxy resin are two polymer matrix materials widely used in the manufacturing of fiber composites for structural engineering applications. Polyester resin is produced by free radical polymerization between dibasic organic acids and polyhydric alcohols in the presence of catalysts, whereas epoxy resins are produced by the condensation polymerization of bisphenol A and epichlorohydrin. Polyester resins provide strength and corrosion resistance, whereas epoxy resins provide adhesive properties, strength, and high environmental stability. This is the difference between polyester resin and epoxy resin.

The use of fillers can be beneficial in the laminating or casting of thick components where otherwise considerable exothermic heating can occur. Addition of certain fillers can also contribute to increasing the fire-resistance of the laminate.

Contact GZ Industrial Supplies Nigeria for more information about Polyester Resins. 

May 17, 2021 Anthony Ubong

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