ALKYDS

The Uses of ALKYDS

Alkyd resins represent the single largest quantity of solventsoluble resin produced for use in the surface coating industry. They are relatively low molecular weight oil-modified polyesters prepared by reacting together polyols, dibasic acids, and oil (linseed or soya fatty acids). Two of the most common polyols used are glycerol and pentaerythritol. The most common dibasic acids used are phthalic acid and isophthalic acid. According to the oil or fatty acid content, the alkyds are divided into three broad categories:
? Short oil (to 40 percent)
? Medium oil (40-60 percent)
? Long oil (more than 60 percent) alkyd resins

Preparation

Alkyds are formulated from polyester resins, cross-linking monomers, and fillers of mineral or glass. The unsaturated polyester resins used for thermosetting alkyds are the reaction products of polyfunctional organic alcohols (glycols) and dibasic organic acids. Key properties of alkyds are dimensional stability, colorability, and arc track resistance. Chemical resistance, however, is generally poor.

Industrial uses

Several types of alkyds exist.
Alkyd coatings are used for such diverseapplications as air-drying water emulsion wallpaints and baked enamels for automobiles andappliances. The properties of oil-modifiedalkyd coatings depend on the specific oil usedas well as the percentage of oil in the composition.In general, they are comparatively lowin cost and have excellent color retention, durability,and flexibility, but only fair drying speed,chemical resistance, heat resistance, and saltspray resistance. The oil-modified alkyds can be further modified with other resins to produceresin-modified alkyds.
Alkyd resins are a group of thermosettingsynthetic resins known chemically as hydroxycarboxylicresins, of which the one producedfrom phthalic anhydride and glycerol is representative.They are made by the esterificationof a polybasic acid with a polyhydric alcohol,and have the characteristics of homogeneity andsolubility that make them especially suitable forcoatings and finishes, plastic molding compounds,calking compounds, adhesives, andplasticizers for other resins. The resins havehigh adhesion to metals; are transparent, easilycolored, tough, flexible, and heat and chemicalresistant; and have good dielectric strength.
Alkyd plastic molding compounds arecomposed of a polyester resin and usually adiallyl phthalate monomer plus various inorganicfillers, depending on the desired properties.The raw material is produced in threeforms — granular, putty, and glass-fiber-reinforced.As a class, the alkyds have excellentheat resistance up to about 150°C, high stiffness,and moderate tensile and impact strength.Their low moisture absorption combined withgood dielectric strength makes them particularlysuitable for electronic and electrical hardware,such as switch-gear, insulators, and partsfor motor controllers and automotive ignitionsystems. They are easily molded at low pressuresand cure rapidly.