Unsaturated Polyester Resin

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Unsaturated polyester resins are the most commonly used thermoset resins in the world. More than 2 million tonnes of unsaturated polyester resins are utilised globally for the manufacture of a wide assortment of products, including sanitary-ware, pipes, tanks, gratings and high performance components for the marine and automotive industry.

Unsaturated polyester resins are produced by chemical reaction of saturated and unsaturated di-carboxylic acids with alcohols. Unsaturated polyester resins form highly durable structures and coatings when they are cross-linked with a vinyl reactive monomer, most commonly styrene. The properties of the cross-linked unsaturated polyester resins depend on the types of acids and glycols used and their relative proportions.

Unsaturated polyester resins are further classified into the following categories:

  • Ortho-phthalic polyesters – resins made from ortho-phthalic anhydride are generally cheaper than the other two classes of unsaturated polyester resins. They are usually used to manufacture general purpose composite laminates where only moderate structural properties are required.
  • Iso-phthalic polyesters – resins made from Iso-phthalic acid. These resins are much more structurally competent than the ortho-phthalic resins. They also have superior corrosion resistance and are used for more demanding applications
  • Tere-phthalic polyesters – Tere-phthalate resins are made from tere-phthalic acid. These resins are currently made in small volumes and are considered a specialty resin. Although they tend to have better thermal and chemical resistance than iso-phthalic resins they are difficult to manufacture. 
Propylene glycol is the predominant alcohol used in producing the various types of unsaturated polyester resins. Other alcohols like neo-pentyl glycol (NPG), di-ethylene glycol (DEG) and ethylene glycol (EG) are also used used in the production of unsaturated polyester resins. Each of these alcohols contributes to the final polymer characteristics, which includes heat distortion temperature (HDT), physical strength, water uptake and weather resistance.

Their versatility in use allows unsaturated polyester resins to be used in a myriad of composite applications. Composite parts can be made at temperatures as low as 15°C to as high as 150°C depending on the processing requirement of the application.

Unsaturated polyester resins also have excellent service temperatures. They have good freeze-thaw resistance and can be designed for use in many low to moderate temperature applications ranging from refrigerated enclosures to hot water geysers.

When it comes to weight for cost comparisons, unsaturated polyester resins are much favoured over their metallic counterparts. With the current fuel and processing costs, the increasing prices of steel and aluminium are pushing more fabricators to use unsaturated polyester resin composites instead. Another major advantage is the increased productivity potential. While metals involve the use of specific smelters, expensive tooling and processing requirements, unsaturated polyester resins are far cheaper and afford the use of low cost tooling. An unsaturated polyester resin can be moulded at ambient temperature whereas metals need to be heated to well over 2000°C before they are melted and poured into mould cavities. Although the perception is that metals are generally structurally superior, there has been much advancement in the development of technologies for producing higher strength composites made from unsaturated polyesters resins.

Adding beauty to structure is easily attained with unsaturated polyester resins. Easily pigmented resin systems can be employed to produce parts with aesthetic appeal without compromising structural properties. Natural stone can be simulated with granite like effects in solid surface features made with special NPG based unsaturated polyester resins. These products are advantageous over their natural stone counterparts in that they offer non porous surfaces which allow them to be used in hygienic environments like kitchens, hospitals and biological laboratories. High gloss blemish free parts can also be produced in-mould using pigmented gel-coat technologies. In the case of glass-fibre boats a smooth and attractive high gloss surface is achieved that need not be sprayed or finished with a paint.

Recently there has been a global focus on greener processes and technologies. This is where unsaturated polyesters have the most potential over their metallic counterparts. Production of metals continues to deplete our natural resources and consumes most of our fossil fuels. Unsaturated polyesters although previously synthesised from fossil fuels can now be manufactured from biological resources instead. Starches, plant oils and other naturally derived building blocks have been discovered that can be used to prepare unsaturated polyester resins.

Collectively there is an ever increasing potential for unsaturated polyester resins. Their low cost, ease of use and weight advantages make them prime candidates for a wide variety of structural and decorative applications. Imagine a world where all metal components are replaced with thermoset composite components manufactured from unsaturated polyester resins –these are the possibilities of “pure science.”

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