Cross-linking alkyd

Only a limited amount of resin can be added to white paints due to its intrinsic color. Addition of these hard resins increases the hardness and the gloss of the paint films, accelerates and improves the drying of oxidatively cross-linking alkyd resins, and optimises sanding properties and corrosion protection in putties. Modified phenolic resins have lost much of their importance because they have been replaced by more efficient binder systems (e.g., thermoplastic and cross-linkable acrylic resins, polyurethane systems). Rosin-modified phenolic resins are, however, still extremely important in the production of resins for printing inks. 

This process is therefore used commercially only for relatively low-molecular-weight, linear, aliphatic polyesters, or for branched or cross-linked alkyd resins. 

Urea, melamine, phenol or substituted phenols can be modified with formaldehyde to produce corresponding resins. An excess of free formaldehyde must be removed in order to prevent interference with the film-forming properties of the paint. These resins can also be used to cross-link alkyd resins. The curing takes place by heating. Phenol-formaldehyde resins are stable to variations of temperature and have a good resistance to moisture, acids and solvents (Piper 1965 Mathias 1984 Fischer and Adams 1990). 

Dissolving cross-linked alkyd resins for effective paint and film... 

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. 

The most important industrial use of pentaerythritol is in a wide variety of paints, coatings, and varnishes, where the cross-linking capabiUty of the four hydroxy groups is critical. Alkyd resins (qv) are produced by reaction of pentaerythritol with organic acids such as phthaUc acid or maleic acid and natural oil species. 

Substituted nonheat-reactive resins do not form a film and are not reactive by themselves, but are exceUent modifier resins for oleoresinous varnishes and alkyds. Thein high glass-transition temperature and molecular weight provide initial hardness and reduce tack oxygen-initiated cross-linking reactions take place with the unsaturated oils. 

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. 

Paint Driers and Polymer Additives. Paints based on alkyd resins (qv) dry by the oxidation and cross-linking of unsaturated side chains. Metal catalysts are included in paint formulations to promote this drying. Cerium carboxylates, eg, the naphthenate, are used as through driers, ie, to promote drying in the body of the paint film rather than at the film s surface (44). 

The other broad class of refinish coatings is cross-linking coatings, caHed enamels in the trade. In the United States, most shops cure the enamels at room temperature in Europe, cure is commonly carried out at temperatures of 60—75°C. TraditionaHy, the vehicle was an oxidi2ing medium oH alkyd. 

Alkyds. Alkyd resins (qv) are polyesters formed by the reaction of polybasic acids, unsaturated fatty acids, and polyhydric alcohols (see Alcohols, POLYHYDRic). Modified alkyds are made when epoxy, sUicone, urethane, or vinyl resins take part in this reaction. The resins cross-link by reaction with oxygen in the air, and carboxylate salts of cobalt, chromium, manganese, zinc, or zirconium are included in the formulation to catalyze drying. 

In modem manufacturing methods the oil is sometimes reacted directly with the glycerol to form a monoglyceride and this is then reacted with the acid to form the alkyd resin. When the resulting surface coating is applied to the substrate the molecules are substantially linear. However, in the presence of certain driers such as lead soaps there is oxidative cross-linking via the unsaturated group in the side chain and the resin hardens. 

Epoxy-DICY systems, 10 454 Epoxy equivalent weight (EEW), 10 399 Epoxy ester alkyd resins, 2 165-166 Epoxy esters, 10 380-384, 443 Epoxy formulations, performance of, 10 428 Epoxy groups, 10 567 hydrolysis of, 10 358 acid-catalyzed cross-linking of, 15 171 Epoxy molding compounds (EMC), 10 373, 430 34, 458... 

Smith condensed phthalic anhydride with glycerol in 1901 to prepare a glassy brittle material. Kienle in 1924 started investigating these resins and called them alkyds, He modified the polyesters with drying oils. By this modification they become soluble, on exposure to oxygen rapid cross-linking occurs and... 

The most important alkyds are obtained from glycerol and phthalic anhydride. A linear polymer can be obtained by heating them at 180°C in ratio 2 3 (i.e., excess of acid). This polymer when heated forms cross-links and yields a thermosetting insoluble resin. 

Commercial alkyd resins contain catalysts which speed up cross-linking reaction. These catalysts is called Driers. 

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