Salient: Victoria University Students' Paper. Vol. 29, No. 6. 1966.

Alkyd resins

Alkyd resins

Alkyd resins are the backbone of the surface coating industry. The estimated distribution of alkyd resins in coatings is approximately 10 per cent. These resins are the basic vehicle for such paints as gloss enamels and undercoats for interior and exterior surfaces, for both household and industrial purposes.

The properties of these resins are governed by the ratio of three basic compounds used in their preparation. The three compounds, or building blocks, are polyhydric alcohols such as glycerine dibasic acids usually phthalic anhydride, and oils such as linseed oil. soya bean oil, safflower oil, tung oil or dehydrated Castor oil.

The resins based on these three building blocks can be divided into two groups.

(i) Alkyd resins used in the surface coating industry.

(ii) Polyester resins used in the plastics industry.

Both types are manufactured in New Zealand and used in a wide variety of applications.

(i) Alkyd resins In the surface coating industry: In general alkyds with a higher ratio of oil. about 60 per cent to 70 per cent, are more tlexible and show greater exterior durability and hence are used in synthetic high gloss enamels. The resins with a lower proportion of oil become correspondingly harder and faster drying, and are used more in undercoats and primers.

Alkyd resins arc manufactured in New Zealand in a number of plants throughout the country. It entails a chemical fusion by heat of the compounds listed above, in the presence of suitable catalysts.

The polymer formed by the reaction is essentially an ester, which point must be kept in mind when applying the resin to a particular application.

This versatile resin type is capable of many variations to meet specific problems. The surface coating chemists are constantly faced with demands to meet a particular condition or end requirement. With the knowledge and literature at his disposal this can be predicted, in theory— not always borne out in practice—and modifications to his product can be done.

The range of modifications that can be built into an alkyd resin is quite large. Alkyds are compatible or react with other groups because of:—

(1) Functional hydroxyl groups which can react with acid groups, amide groups, epoxy groups or urethanes.

(2) Functional acid groups that react with hydroxy groups or epoxy groups.

(3) Unsaturated double bonds in the fatty acid chain which can copolymerise with unsaturated monomers such as styrene. vinyl toluene, isocyanates, and various acrylate compounds.

The modifications to the alkyd structure that are carried out by New Zealand technical staff are:—

(a) Phenolic modification to improve hardness, water resistance and resistance to chemicals and abrasion, but here exterior durability suffers.

(b) Alteration in type and oil content to improve hardness, exterior durability, flexibility, speed of dry and yellowing resistance on interior application.

(c) Assessment of residual acidity. Reaction with basic pigments such as zinc oxide, calcium plumbate, white and red lead and aluminium is possible if the acidity is too high.

(d) Establishing the degree of polymerisation. Factors of film build, degree of penetration, and sag resistance are invohed here.

(e) Styrene or vinyl toluene modification to give greatly increased speed of dry but the vehicle when cured becomes sensitive to solvents.

(f) Polyamide modification to induce a "gel structure" to the paint.

(g) Epoxy modification to ungrade water, solvent, acid, alkali and chemical resistance.

All these modifications, and perhaps more, can be built in. and a new resin evolves to meet a nnrticular end requirement. The final selection of the alkyd depends on its ability to meet and nerform satisfactorily under the conditions laid down by the end user.

(ii) Polyester resins: Polyester resins are used in the plastics industry in the moulding or laminating operations usually in conjunction with fibre glass. Applications are in automotive bodies, transparent roofing, surfacing of boats, etc.

The formation of polyester resins forms a close parallel with the formation of alkyd resins. In fact polyester resins are alkyd resins without the oil modification. Whereas alkyd resins are thinned with a solvent that plays no part in the drying mechanism but just evaporates off. the polyester resin is thinned in styrene that does take part in the curing mechanism.

The formation of a polyester resin is, as above, through a reaction of a polyhydric alcohol such as propylene glycol or dipropylene glycol with phthalic anhydride or isophthalic acid.

Esterification is carried out at temperatures ranging from 350deg.F to 425deg.F. The time and temperature vary with the relative proportions of the two components. When ester formation is complete lurther modification is made with maleic or fumaric acid.

The resultant reaction product is then dissolved in styrene.

Variations in hardness and flexibility can be done quite readily to meet a particular end requirement. Modifications at the phthalic anhydride stage by replacing a moderately large percentage of the phthalic by adipic acid produces a flexible resin. On the other hand a high maleic content gives a harder brittle film by oroducing a high rate of reactivity during the film curing processes.

Polyester resins cure through the cross linking of the unsaturated groups catalysed by an organic peroxide such as methyl ethyl ketone peroxide or benzoyl peroxide. A reaction initiator is also necessary which normally is an organic cobalt salt, cobalt naphthenate. The curing reaction proceeds, provided the air temperature is not too low. rapidly, with the evolution of heat. As the film is 100 per cent reactive the product polymerises into a solid mass that is highly resistant to most solvents, acids, bases and salts.

Resin modifications are being developed in New Zealand laboratories continually. There Is not the same scope for modification as in the alkyd resin field, and once a formula has been established the product is fixed and serves a variety of similar applications.