Non-drying alkyds

Nitrocellulose based lacquers often contain short or medium oil alkyds to improve flexibiUty and adhesion. The most commonly used are short oil non drying alkyds. Amino resins or urethane resins with residual isocyanate functional groups may be added to cross-link the coating film for improved solvent and chemical resistance. The principal appHcations are furniture coatings, top lacquer for printed paper, and automotive refinishing primers. 

Compatible resins used with N/C include non-drying alkyds (see Chapter 12), acrylic resins and many natural resins and derivatives, such as dammar and ester gum. Natural resins are mixtures of relatively low molecular weight chemicals, often of uncertain chemistry (see rosin. Chapter 12). Ester gums are produced by reacting polyhydric alcohols, e.g. glycerol, with rosin. An increase in molecular weight occurs, since more than one molecule of monobasic rosin acid will react with one molecule of the alcohol. Replacement of... 

Rapid drying is one of the most important characteristics of nitrocellulose-based coatings. Nitrocellulose alone produces hard and brittle films, and therefore, nitrocellulose-based coatings generally are formulated with plasticizers to improve flexibility and adhesion as well as compatibility with other resins. They are also blended with other polymers such as hard resins and non-drying alkyd resins, to name a couple. On the limiting side, nitrocellulose-based lacquers have generally poor resistance to chemicals and poor durability. 

Drying, semi-drying and non-drying oils are used for modifying alkyd resins. Alkyd resins can also classified according to the percentage of oil present in them. Short oil resins contain less than 40 per cent oil. Medium oil resins contain 50-70 per cent oil, while long oil resins contain more than 70 per cent oil. 

Non-drying oil resins are soluble only in Aromatic hydrocarbons. They are used with amino resins for stoving finishes for appliances. Medium resins are used as plasticisers for cellulose nitrate. Along with natural oils several natural occurring and synthetic acid like resin (abiotic acid) pelargonic acid and isooctanoic acid are added to modify alkye resins. The alkyd resins are obtained by two processes, i.e., (1) Fatty Acid Process and (2) Alcoholysis process. 

Urethane alkyds are basically similar to ordinary alkyds and may be drying or non-drying, long oil or medium oil. Short oil urethane alkyds are theoretically possible, but are difficult to make. Irreversible gelation tends to occur in the reaction vessel. 

A non-oxidizing alkyd contains non-drying (saturated) oils or fatty acids and therefore the alkyd is not capable of forming coherent film by air oxidation. Hence, the non-oxidizing alkyd is used as polymeric plasticizer or as hydroxyl-functional resin, which is cross-linked by melamine-formaldehyde or irrea-form-aldehyde resins or by isocyanatecross-linkers. The oil length for non-oxidizing alkyds is usually formulated below than 45% (Wicks et al., 1999). 

Drying or non-drying oils can be used, according to the type of alkyd required. The non-drying oils, such as castor oil, give the plasticizing... 

Soybean oil is widely used to produce alkyd resins because it produces non-yellowing products. When discoloration is less important, linseed oil or tung oil are used for faster drying. Sometimes a non-drying oil, such as cottonseed or coconut is used when the alkyd resin is to serve as a plasticizer rather than a film former. 

Modifications of the above epoxy types are used as well. For example, phthalic anhydride is sometimes used to partially replace fatty acids in producing epoxy resin alkyds. Also short oil length epoxy esters (sometimes based on a non-drying oil), are occasionally modified with an amino resin to yield a coating binder capable of forming a very hard, chemically resistant film. There are also epoxy systems based on modifications of the epoxy resins with acrylic resins or polyisocyanate ciuing agents. 

Fig. 82 Scanning electron micrographs of long-oil alkyd resin systems containing P.R.3, (a) non-hazing, (b) hazing. Drying time 8 days.

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