Alkyds preparation

Polybasic Acids and Anhydrides. The principal polybasic acids used ki alkyd preparation are Hsted ki Table 2. 

Phtha/k anhydride is the most important type of dibasic acid derivative ki alkyd preparation because of its low cost and the excellent overall properties it imparts to the reski. The anhydride stmcture allows a fast esterification to form half-esters at relatively low reaction temperatures without hberatkig water, thereby avoiding the danger of excessive foaming ki the reactor. However, skice the two carboxyl groups of phthaUc anhydride are ki the ortho position to each other on the benzene ring, cycHc stmctures may and do occur ki the reski molecules. 

Fatty Acid Process. When free fatty acids are used instead of oil as the starting component, the alcoholysis step is avoided. AH of the ingredients can therefore be charged into the reactor to start a batch. The reactants are heated together, under agitation and an inert gas blanket, until the desired endpoint is reached. Alkyds prepared by the fatty acid process have narrower molecular weight distribution and give films with better dynamic mechanical properties (34). 

Polybasic Acids and Anhydrides. The major types of polybasic acids used in alkyd preparation are as follows. 

Phthalic anhydride is by far the most important dibasic acid used in alkyd preparation, because of its low cost and the excellent overall properties it imparts to the... 

The products from the above polyesterifications are brittle materials. They are therefore modified with oils, either drying or nondrying. Such oil-modified resins bear the names of alkyds. While glycerol is widely used, other polyhydroxy compounds (polyols) are also utilized. These may be trimethylolpropane, pentaerythritol, sorbitol, or others. Phthalic anhydride is usually used in alkyd preparations. Other dicarboxylic compounds, however, may also be included for modification of properties. Common modifiers might be isophthalic, adipic, or sebacic acids, or maleic anhydride. In addition, many other acid modifiers are described in the patent literature. 

Epoxy resins may be used as polyols in alkyd preparation. Since the lowest functionality possible - even with liquid epoxies - is four (two epoxide... 

The term modified alkyd, which formerly was used to describe these products, now is associated with chemical modifications that are carried out during alkyd preparation, and that incorporate chemical agents of types other than those included in the definition. For example, the term rosin-based alkyd resins or rosin-modified alkyd refers to alkyd resins in which all or a portion of the monobasic fatty acid is replaced by rosin. Unmodified alkyd resins are polyester products composed of polyhydric alcohol, polybasic acid, and monobasic fatty acid. When no fatty acids are used, or when they are completely replaced by other types of acid, the products can be considered as oil-free alkyds. 

Solomon and Hopwood [24] find that, in the case of alkyd prepared from fatty acid, glycerol, and phthalic anhydride in the molar ratio 1.0 1.26 1.11, the measured hydroxyl values are considerably less than the calculated figures and the difference between these values increases with the molecular weight and the complexity of the polymer molecule. The difficulties encountered in the kinetic analysis of the formation of alkyd resins could be related, at least in part, to the availability of the hydroxyl group. 

In most commercial alkyd preparations, sublimed litharge has been the preferred catalyst, in quantities ranging from 0.02 to 0.05% based on the oil. With recent restrictions on the use of lead compounds, the use of calcium and lithium hydroxides and their soaps has increased. With these catalysts, about 0.008—0.01% as metal, based on the oil, is normally used. The oil is heated to 230-250°C with good agitation and inert gas blanket, the catalyst and the polyol are added, and the mixture is reheated to the reaction temperature of 230-250°C. Since the oil is insoluble and the monoesters are soluble in anhydrous methanol, the course of the alcoholysis reaction is followed when 1 volume of reaction mixture gives a clear solution in 2—3 volumes of anhydrous methanol, the dibasic acid is added, and the polyesterification is conducted at 210—260°C to the desired alkyd specifications. 

Castor oil fatty acids have one double bond per molecule, whereas hydrogenated castor oil fatty acids are saturated consequently, the oils, or alkyds derived from them, are not film forming under autooxidation conditions. However, alkyds prepared from these oils are widely used as polymeric plasticizers for other filmforming resins the two most important types are cellulose nitrate and melamine-formaldehyde condensates. 

Pigmented industrial enamels prepared from the castor oil alkyds were different, particularly for gloss and humidity resistance. The alkyds prepared directly from oil had better properties. 

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