Unsaturated polyesters are thermosetting resins widely used in the composites industry.
These resins are obtained by polycondensation of one or more glycols with one or more diacids, at least one of which contains an ethylene double bond.
The polycondensate obtained is soluble in some vinyl, acrylic or allyl monomers that can subsequently react with the polycondensate's double bonds.
This solution can copolymerise under the action of heat and/or free radicals. In practice, hardening is obtained by adding of a catalyst (an organic peroxide) leading to three-dimensional cross-linking generally accompanied by quite significant shrinkage and an exothermic reaction.
There are 12 types of polyester resin:
• orthophthalic resins (the commonest);
• tetrahydrophthalic resins for use in the food industry;
• isophthalic resins for gel-coats and the chemical industry.
These resins have a very good capacity to impregnate glass fibre, their price is low and they harden quickly without releasing any by-products.