Polyester resin

Polyester Resin - Unsaturated Polyester Resin. The term generally used for unsaturated (means containing chemical double bonds) resins formed by the reaction of dibasic organic acids and polyhydric alcohols, basic component of SMC/BMC.

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.

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 of surfaces.

Environmental Issues

The drive to be environment-conscious has led also to the development of water-extendable polyester resins and LSE (Low Styrene Emmision) polyester resins. [ cite web
title =Water-Extendable Resins
url =http://chemrez.com/default.asp?link=FiberComposite]

Taking the place of styrene monomer, water reduces the resin solids and adjust working viscosity. This reduction of styrene fume and odor greatly benefit the workers' health and safety.

LSE resins allow the workers to improve productivity without the unnecessary exposure to styrene emissions.

When polyester is used as a casting medium, fiberglass is often added for strength. Fiberglass 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. A viscous polyester "surface coat" is first laid into the mold to create a crisp, highly-detailed surface. The fiberglass-reinforced polyester is applied into this surface coat to give the cast strength, volume, and rigidity. Casting occurs as follows:

# A mold is made of the desired element
# Mold-release is applied to keep the casting from adhering to the mold
# Surface coat is brushed into the mold to create a detailed surface
# A catalyst such as butanone peroxide is added to the liquid polyester resin to promote polymerisation
# A series of layers of polyester resin and resin-impregnated polyester fiber are laid down
# After resin has cured, the casts are pulled from the mold.

This molding process is used in a variety of applications including: Film set design, auto prototyping, and the creation of lost architectural details such as plaster rosettes, etc.

Types of Unsaturated Polyester Resins

References