Alkyds structure

Although blending with other coating resins provides a variety of ways to improve the performance of alkyds, or of the other resins, chemically combining the desired modifier into the alkyd structure eliminates compatibility problems and gives a more uniform product. Several such chemical modifications of the alkyd resins have gained commercial importance. They include vinylated alkyds, silicone alkyds, urethane alkyds, phenolic alkyds, and polyamide alkyds. 

Just as diisocyanates can be substituted for acids in the alkyd structure to produce a urethane alkyd or oil, so a simpler structure similar to a polyester can be made. Diols such as those described in the next section as being suitable for use in polyesters, and lower molecular weight polyethers, are reacted with diisocyanates to form linear high molecular weight polyurethanes. These are used as clear lacquers and applied as very low solids solutions on flexible substrates such as plastics and wood. 

Other dibasic acids used in alkyds to impart special properties are adipic acid, azelaic acid, sebacic acid, tetrachlorophthalic anhydride, chlorendic anhydride, dimerized fatty acids, and trimellitic anhydride. Adipic, azelaic, and sebacic acids impart flexibility in the alkyd structure and are primarily used in alkyds designed for application as plasticizers. Tetrachlorophthalic anhydride and chlorendic anhydride are used to impart fire-retardant properties to the resin system. 

These resins are produeed by reacting a polyhydric alcohol, usually glycerol, with a polybasic acid, usually phthalic acid and the fatty acids of various oils such as linseed oil, soya bean oil and tung oil. These oils are triglycerides of the type shown in Figure 25.30. R], R2 and R3 usually contain unsaturated groupings. The alkyd resins would thus have structural units, such as is shown in Figure 25.31. 

Alkyds are the most widely used protective-coatings at 1-million pounds annual world consumption. Brittle and insoluble in common solvents, they are manufactured with a plasiici/m. -Manipulating the molecular structure produces alkyds with the desirable properties. 

Saponification Paints are most commonly used to protect steel from corrosion by seawater in marine applications and soil in the case of buried structures. Additional protection is often supplied by the application of cathodic protection to the steel. Any paint coating used in conjunction with cathodic protection must be resistant to the alkali which is produced on the steel at defect sites in the coating. The amount of alkali generated depends on the potential to which the steel is polarized. Some paint binders such as alkyds and vinyl ester are very susceptible to saponification, and should not be used on cathodically protected structures. Cathodic disbondment testing should be undertaken if the relevant information is not available. 

Modification of alkyd resins with high proportions of silicones considerably reduces rates of attack, but the most spectacular extension of life is shown by fluorinated polymers such as polyvinylidene fluoride where erosion rates can be reduced to 0 -1 /tm/year. If this level of durability can be achieved an initial coating, if firmly adherent and free from any breaks, may often be expected to maintain protection over a metal substrate for the likely life of the structure. The considerably increased first cost, as compared with more conventional coatings, has to be balanced against the probable saving in maintenance costs or consequences of failure. 

From these, prepolymers are prepared where the diisocyanates may be completely reacted as in the case of the urethane oils which resemble the oil-modified alkyds but have urethane (—NHCOO—) links in place of the ester (—COO—) links of the alkyds, or where one only of the isocyanate groups is combined, leaving the other to participate in crosslinking reactions. Such a reactive prepolymer is the biuret that may be prepared from hexamethylene diisocyanate, has the following structure ... 

The chemistry and structure-properties relationships of alkyd resins are well established and discussed in many textbooks.50... 

Another commercially important crosslinking process that involves unsaturated polymer precursors is the so-called drying of alkyd resins in paints. This process is not drying at all, at least not in the sense of mere loss of solvent to leave behind a solid residue. Instead, the main process is the conversion of high relative molar mass molecules to a crosslinked structure via... 

Covalent polymeric networks which are completely disordered. Continuity of structure is provided by an irregular three-dimensional network of covalent links, some of which are crosslinks. The network is uninterrupted and has an infinite molecular weight. Examples are vulcanized rubbers, condensation polymers, vinyl-divinyl copolymers, alkyd and phenolic resins. 

In this chapter, a general overview of the physical and material aspects of PHAs, such as their chemical structure and composition, as well as their thermal and mechanical properties is presented. To conclude, some recent developments in various application areas such as the development of high solid alkyd-... 

J.M. Challinor, Structure determination of alkyd resins by simultaneous pyrolysis methylation,... 

Chain stationary insertion, 16 110 Chain stiffness, of fiber polymers, 11 175 Chain-stopped alkyds, 2 152 Chain structure of PVDC, 25 699... 

Pettersson and Sorensen have described a number of different thermoset resin structures based on hyperbranched aliphatic polyesters [123]. Their results can best be exemplified by a study on hyperbranched alkyd coating resins. A comparative study was performed between an alkyd resin based on a hyperbranched aliphatic polyester and a conventional high solid alkyd, which is a less branched structure. The hyperbranched resin had a substantially lower viscosity than the conventional resin of comparable molecular weight, that is, less solvent was needed to obtain a suitable application viscosity. The hyperbranched resin also exhibited much shorter drying times than the conventional resin, although the oil content was similar. These achievements would not have been possible without a change in architecture of the backbone structure of the resins (Figs. 12,13). 

A combination of enhanced reactivity and reduced viscosity for alkyd resins has been achieved by using hyperbranched polyester structures as discussed in Sect. 4.2.3 [ 123]. This study clearly showed the benefits of using highly branched structures in coating applications to obtain improved properties. 

The structural chemistry of alkyds has already been covered in Chapter 15, Section 7. Although there are over 400-500 varieties of such resins, they are all polyesters with carbon-carbon double bonds that can be cross-linked. They are very versatile in coatings, and their diverse properties can be matched for particular uses. They are the most widely used resins for protective coatings. Their best points can be summarized as follows (1) easy to apply (2) can have flat, semigloss, or high-gloss finish (3) useful for most surfaces except concrete or plaster (alkaline) (4) good color retention and (5) odorless (some of them). 

Another resin application based on the same hyperbranched polyester structure described herein is low-VOC alkyds, which have very low viscosity and high reactivity compared to conventional high-solid alkyds. Other resin structures are unsaturated polyesters, polyurethane dispersions, and epoxides. ... 

A highly concentrated dispersion of carbon black is first prepared with a portion of the binder and solvent. The viscosity of this concentrate is a function of the particle size, structure, and surface chemistry of the black, the type of binder and its interaction with the pigment black, and the proportions of black, binder, and solvent. The final paint is made from the concentrate by adding more binder and solvent, its carbon black concentration is 3-8% referred to the solids content. Wetting agents are sometimes added to improve dispersibility and prevent flocculation. A number of concentrates for paint manufacture e.g., carbon black-nitrocellulose chips or carbon black -alkyd resin pastes, can be obtained from paint producers. 

Alkyd parts are generally very rigid, and the surfaces are hard and stiff. Surface preparation for alkyd parts consists of simple solvent cleaning and mechanical abrasion. Epoxies, urethanes, cyanoacrylates, and thermosetting acrylics are commonly used as structural adhesives. 

---