Saturated Polyester Resins

General

The saturated polyester resins are the precursors of the alkyd and unsaturated polyester resins.These resins are formed from the reaction of organic acids with organic bases to form ester group, via condensation reactions, to produce high molecular mass polymers. The chemicals used in the production of saturated polyesters are mostly always of synthetic origin. The solvents used depend on the constituent monomers within the resin polymer itself but might be none, where the product is to be utilised as a hot melt adhesive, or medium to strong solvents when the final application is a coating. These resins have no oxidative potential and are otherwise thermoplastic in nature. Other than those resins designed for application at 100% solids, the saturated polyester reins are generally co-reacted with a second polymer or chemical.

The saturated polyesters fit into two groups:

Thermoplastic Saturated Polyesters

These resins are designed to be solvent free, of high molecular mass and associated high melting points, and utilised as 'Hot Melt Adhesives'. In use, they are heated until molten and then applied by glue guns, or alternative mechanical application methods, to one, or both, surfaces to be joined. After application of the resins, the two part must be immediately joined before the adhesive starts to harden ('set'). Mostly the items to be joined are of dissimilar materials, e.g rubber to metal, paper to glass etc. Depending on the bond strength the adhesive has with the substrates, the joint strength may be anything from fair to good. Cooling of the adhesive results in rapid hardening and the finished articles can be handled in a short period of time. The downside of these adhesives is that they will again melt if subjected to temperatures above their softening point, thereby reducing the strength of the bond. They may also be subjected to dissolving in the presence of strong solvents.

Thermoset Saturated Polyester

Thermoset saturated polyesters are invariably dissolved in medium to strong solvents, the composition of the solvent blends used is normally dictated by the end application.

Oven cured

In applications where the coated article is to subjected to temperatures well above ambient to effect cure, a suitable co-reactant, normally a urea or melamine formaldehyde resin, is added to the coating during the manufacture process. The coating is either applied by spray, or other mechanical means (roller or 'curtain' coating), to the article. Subsequent to the application of the coating to the substrate, and after a brief 'flash-off' period to allow much of the solvent to evaporate, the article passes through a hot oven (≥ 120°C) in order to react the saturated polyester with the co-reactant. The reaction forms additional cross-links between the saturated polyester polymer chains resulting in a polymer with a higher molecular mass that is impervious to most solvents, and has improved mechanical and environmental durability. Applications for coatings based on these saturated polyesters are home appliance finishes, can coatings, and industrial metal side/roof cladding/sheeting coatings.

Room Temperature Cured

In applications where the saturated polyester is required to react, and cross link, at room temperature, a reactive polymer, such as a polyisocyanate, is the co-reactant of choice. The polyisocyanate reacts with free hydroxyls of the saturated polyester to form urethane linkages, giving rise to these coating systems being known better as 'polyurethanes' rather than saturated polyesters or even polyesters.There are two types of polyisocyanates that are used, one based on an aromatic backbone and one on an aliphatic backbone. The aromatic polyisocyanates provide coatings with faster drying, are cheaper, but provide coatings with inferior properties such as gloss and colour retention so are most often used in non-critical applications. The more expensive aliphatic polyisocyanates are specifically used where maximum performance, particularly in terms of exterior durability, is required. The coating containing the saturated polyester is admixed with the polyisocyanate immediately prior to use. As the mixed coating reacts at room temperature, there is a limited period before the coating thickens and becomes unusable. These '2 pack' resins are generally applied by spray application to also aid solvent evaporation and touch dry times. Applications of these coatings are plentiful, from wood and plastic coatings through to the coating of transportation vehicles and commercial aircraft.

Chemistry

To be completed at a later date