What is Polyester?

from Wikipedia

Polyester resins are unsaturated resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Among other uses, it is the basic component of sheet moulding compound and bulk moulding compound.

Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols, organic compounds with multiple alcohol or hydroxy functional groups) and polycarboxylic that contain double bonds. Typical polyols used are glycols such as ethylene glycol. The usual polycarboxylic acids used are phthalic acid and maleic acid. Water, which is a by-product of this esterification reaction, is removed from the reaction mass as soon as it is formed to drive the reaction to completion.

Unsaturated polyesters differ from saturated polyesters such as polyethylene terephthalate which constitutes the polyester films and fibers of commerce in that acids or glycols having double bond unsaturation are included in the formula to provide reactive olefinic unsaturation in the unsaturated polyester alkyd.

Polyester resins are thermosetting; "thermosetting" means the plastic softens when initially heated, but sets permanently rigid once it has cooled (as opposed to "thermoplastics", which re-soften with heat). Polyester resin is often purchased in liquid form for the production of glass-reinforced plastic. In this case, a catalyst (typically methyl ethyl ketone peroxide (MEKP) (also known as butanone peroxide) is used to initiate the polymerization reaction; benzoyl peroxide is a somewhat less hazardous alternative suitable for some purposes. The process of curing polyester resins using a catalyst is an exothermic process. The use of excessive catalyst can cause an excess of heat during the cure process and thus damage the resin by charring or even cause ignition.

Uses of Polyesters

Polyester resin is used for casting, auto body repair, wood filling, and as an adhesive. It has good wear and adhesive properties, and can be used to repair and bond together many different types of materials. Polyester resin has good longevity, fair UV resistance, and good resistance to water. It is important to recognize that all polyester resin products are not created equal; their chemical makeup is complex and can have a wide range of properties. As a filler in auto repair, for example, this material is formulated for superior adhesion to paints and metals, but cures very hard to resist surface trauma; it is therefore only marginally sandable. As a filler for millwork, however, polyester must be softer than the wood substrate so it can be sanded without leaving fills standing proud above surfaces.

Polyester resins adhesion to some materials can be excellent such as in fiberglass composites but others such as timber the adhesion is much weaker.