Within the simplest terms, polyester resins are unsaturated polymers formed by the reaction of poly functional acids and poly functional alcohols. They are classified as Saturated and Unsaturated Polyester Resins based on their chemical structure. PET (Polyethyleneterephthalate) is a common example for saturated Polyester Resin. This material exhibits plastic behavior therefore can be heat formed.
Unsaturated polyester resins contain carbon-carbon double bond (C=C) in their structure hence, once cured, they form a cross-linked structure, which is not reversible. Such materials that form a cross-linked hard and tough structure are called “Thermoset” materials. Polyester resins are normally hard and brittle at room temperature if they are not dissolved in styrene. To make the resin usable and cross-linkable, it is dissolved in styrene monomer and then cured by adding peroxide. After the addition of peroxide, double bonds of styrene monomer react with the double bonds present in unsaturated polyester resin to form a strong, durable thermoset material.
There are many kinds of raw materials for polyester resin industry and the selection of poly functional alcohols and acids provide different variety of products. Some of the raw materials are very well known in the sector and they give their names to standart resins used in the composites industry.
Orthophthalic Polyester : Most of the polyester resins in the market are of orthophthalic base. They have average specifications and price/performance value. They can be used for general-purpose applications unless is specified.
Isophthalic Polyester : These types of resins contain isophthalic acid in their back bone. They are used generally for chemical resistance purposes. Isophthalic acid yields resins with high hydrolytic stability and chemical resistance. Isophthalic polyesters have higher mechanical and heat resistance compared to orthophthalic resins. Although there is no established rule, sometimes resins containing a small amount of isophthalic acid are also named and marketed as isophthalic resins. Boytek`s isophthalic polyesters are always 100% isophthalic and never blended with orthophthalic.
NPG Polyester : These resins contain Neopentyl Glycol as the glycol component and mostly combined with isophthalic acid to produce durable resins for gel coat production. NPG improves weatherability, UV and heat resistance properties of resins.
DCPD Polyester : DCPD is an interesting molecule that can be incorporated into the polyester resin backbone through different reaction mechanisms. When properly formulated and reacted, DCPD resins are used to produce low viscosity and low styrene content resins.