Unsaturated Polyester Resins form the bulk of all resins used in FRP moulding. Unsaturated Polyester Resins are formed by the reaction of Dibasic Organic acids and Polyhydric alcohols. Polyester resins are Thermosetting which means the plastic softens when initially heated but, sets permanently rigid once it has cooled. Polyester Resins are the third largest class of thermoset molding resins. The Polyesters are low molecular weight viscous liquids dissolved in vinyl monomers like styrene to facilitate shaping of the resin into a desired form before curing into rigid solids. The remarkable versatility of fibre reinforced polyesters is largely due to the almost unlimited possibilities to modify the basic polymer by varying parameters like type of glycols and unsaturated acids. Proportion of these ingredients and processing, enabling unsaturated polyesters to be preferred choice of processors. It has good wear and adhesive properties, and can be used to bond/repair many different types of materials. Polyester Resin has good longevity, fair UV resistance and good resistance to water. The chemical makeup of Polyester Resins are complex and can have a wide range of properties.

When Polyester is used as a casting medium, Fibre Glass is added for strength. Fibre can either be chopped up to give the final product internal strength or laid-up in polyester-saturated strips to create high strength external forms.

Curing Mechanism : The curing is carried out either at elevated temperature of about 100^oC where press moulding are being carried out and at room temperature in the case of Hand lay-up. More frequently used curing agent is Methyl Ethyl Ketone Peroxide in conjunction with Cobalt Octoate for room temperature cure. Benzoyl peroxide is most commonly used for elevated temperature curing. Generally, these peroxides are called Catalysts and cobalts are called Accelerators. When resin is to be used, first accelerator to be mixed well with resin and then catalyst.

Types of Unsaturated Polyester Resins

Orthophthalates : Orthophthalic Resins(General Purpose) resins are made of propylene glycol, phallic anhydride  and maleic anhydride and styrene is used as a co-monomer. The variation in molar ratios of dibasic acids could alter the characteristics of resin (from rigid to soft). Dipropylene glycol enhances the flexibility without affecting the weatherability of the cured resin. Use of adipic acid in place of phthalic anhydride results in a flexible resin with improved impact resistance.

Major Applications : Boats, Doors, Automobile Parts, Cooling Towers, Electrical Insulators, Water Proofing, Coating, Roofing Sheets etc..

Variants  : Marine Grade Resins, Rooflite Resin, Fire Retardant Resin, Spray-Up Resin

Isophthalic Resins : Isophthalic Acid is used in place of phthalic anhydride to make Isophthalic Resin. It has enhanced water resistance, acid and alkali resistance and mechanical properties. The resin can be made more resilient by partial substitution of adipic acid by isophthalic acid which gives very high impact resistance to the resin.

Major Applications : Chemical, Marine, Automobile, Electrical, Wind Energy, Piping, Roofing etc.,

Variants  : Fire Retardant Resin, UV Stabilized Resin, Spray-Up Resin

Bisphenol Resins :  Bisphenol-A dipropoxy ether and hydrogenated Bisphenol-A are used in place of propylene glycol. The resin has more chemical resistance to acids and alkalies. But mechanical properties suffer due to the presence of bulky molecules of Bisphenol-A dipropoxyethers.

Gelcoat Resins  : Gelcoat Resins containing optimum amount of thixotropic agents are available in spary-up and hand lay-up grades. They offer superior abrasion and chemical resistance. But for critical applications, resins without thixotropic agents should be used. Wherever superior water resistance is desired, NPG based gelcoats offer special advantages.

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