Technical PaperAir Drying Waterborne Alkyd Resin Emulsions

byMichael Gobec - Vianova Resins AG - Austria

Dr. Walter Weger - Vianova Resins AG - AustriaMario Pschaidt - Vianova Resins - U.S.A.

As a result of some fairly recent developments, the great versatility of synthetic resin chemistry is now reflected in the area of eco-friendly binders. The benefits are apparent in both the binders' chemical composition and range of applications.

Since their launch in the late 1950s, waterborne synthetic resins have grown in popularity to the point where they are now used in virtually all types of coatings. Water reducible binders were originally used mainly for dip coatings because of their low flammability, as in the case of electrodeposition coatings for the automotive industry. But increasingly strict environmental legislation has led to a further expansion in the use of these binders. Even today, because of the versatility permitted by a wide range of raw materials and synthesis, the development of waterborne binders has by no means reached its limit. Among the many aqueous systems, alkyd-resin emulsions have steadily gained in popularity; they already cover a wide field of forced and air dried coatings. But before we discuss the current products that make up this binder class, a brief look at the chemistry of alkyd-resin emulsions is necessary.

Structure of Alkyd-Resin EmulsionsThe molecules of modern resin emulsions consist of a hydrophobic alkyd-resin segment onto which high-acid polymer segments have been grafted and then neutralized to make them hydrophilic. During the emulsification process, these molecules associate in a particular way to form emulsion droplets with a core shell structure 

(Figure 1).

The hydrophobic alkyd-resin segments form the core, and the hydrophilic but unsaponifiable polymer segments form the shell. This structure gives these self-emulsifying alkyd-resins a significantly longer storage life. Previously, alkyd-resins with high acid numbers and large proportions of organic cosolvent had only limited stability, since the ester bonds were soon hydrolyzed in water as a result of the anchimeric effect (Figure 2).

Self-emulsifying, alkyd-resin emulsions are more complicated to produce. First, methacrylic acid and its esters are grafted onto drying fatty acids (stage 1). Then, a low molecular weight alkyd-resin with a high hydroxyl value and a low acid number is produced (stage 2). Finally, the end product is manufactured (stage 3) by a partial esterification of the two intermediates (stages I and 2). Through suitable control of the reaction, all the primary carboxyl groups of the fatty acids are esterified, while the majority of the tertiary, sterically hindered carboxyl groups of methacrylic acid remain free and are then neutralized with ammonia and diluted with water (see Figure 3).

If polyethylene glycol constituents are incorporated into alkyd-resin emulsions, then hydrolytic splitting must be prevented. The following are advantages of alkyd-resin emulsions produced in this way.

• Low acid numbers.

• Better stability with certain basicpigments.

• Improved drier compatibility.

The cross section of an emulsion droplet in a core-shell emulsion is shown in Figure 4. On the inside, the alkyd-resin core is surrounded by an intermediate layer of alkyl chains of the fatty acid links; on the outside, grafted onto these chains, are the hydrophilic-polymer segments and the polyethylene-glycol chains forming a swollen shell.The hydrophilic polymer segments and the polyethylene glycol

portions are responsible for rheology, interaction with pigments and driers, and substrate wetting; the nature of the alkyd-resin core is cru-cial to film formation, wetting and stability. Water dilutability is achieved by neutralizing the carboxyl groups with ammonia.

Self-Emulsifying Alkyd ResinsVianova Kunstharz AG (a subsidiary of Hoechst AG) supplies self-emulsifying alkyd-resins in the form of 35% to 45% aqueous emulsions. The content of organic cosolvents is low; in some cases, there is no organic cosolvent. When the emulsions are diluted, no water mountain occurs since that step has already been accomplished. Furthermore, the alkyd-resin emulsions are not irreversibly damaged by freezing. If the emulsions or paint produced from them have been frozen, they can be thawed and restored to a usable state by slowly stirring at temperatures up to a maximum of 120°F (50°C). The shear stability of these alkyd-resins provides a number of additional benefits, including:

• good pigmentation properties;• high gloss;

• rapid drying;• good corrosion resistance with certain anti-corrosive pigments; and• good storage stability.

These alkyd-resin emulsions also exhibit marked pseudoplasticity, including viscosity decreases with a rise in shear rate and increases with a rise in pH level. Cosolvents, as well as the nonvolatile portion, have an adverse effect on viscos-ity values. This group is represented by Resydrol AY 586w and its thixotropic version, 548w TIX. The development of these binders was guided by the following three objec-tives.

• They must permit the manufacture of coatings with a solvent content of less than 5%.• They should be suitable for a variety of applications, ranging from decorative paint to anticorrosion and industrial coatings.• The coatings produced from these emulsions must have good flow, high gloss and good weathering resistance.

In Figure 5, paint based on 586w is compared with paint based on a conventional, long-oil alkyd. Even after outdoor weathering, paint based on 586w shows good gloss retention see (Figure 6) and thus compares well with conventional, solvent-based systems.

Externally Emulsified Alkyd ResinsSelf-emulsifying alkyd resins have a relatively low solids content; hence, the paint body is slightly poorer than that obtained with conventional alkyd-resins. In addition, ammonia is required as a neutralizing agent. These two factors became the starting point for developing externally emulsified alkyd resins. The creation of externally emulsified resins was guided by the desire to achieve ecological benefits and to produce a

high-solids content, which is customary with conventional alkyd resins for decorative paint. The quality of the binder and, therefore, the quality of the paint is significantly influenced by the type and amount of emulsifier used, as well as by the emulsification process itself. Our Vianova research team has developed its own emulsification process. The sole emulsifiers used are nonionic, nonylphenol ethoxylate-free products with sufficient double bonds for crosslinking/ This prevents the emulsifier from causing flooding and floating, and leads to high initial gloss, good gloss retention and good repaintability.Nevertheless, drying both to the touch and complete is impaired by the emulsifier. For this reason, medium-oil alkyd-resins of this type with an oil content of about 50% are highly suitable for decorative paint, unlike conventional types that dry too hard and brittle. Since no neutralization is required for the emulsification process, there is no need to use agents such as amines or ammonia. The same applies to auxiliary solvents and coalescents.Paint production is carried out with a pigment paste. Additol® VXW 6208 has been developed for dispersion of white pigments in finishes and primers. This dispersing agent is a nonionic, modified, acrylic copolymer in aqueous solution. Colored paint can be produced by tinting a low pigmented white paint with color pastes based on Colanyl®. The emulsification of solvent-free alkyd resins with special emulsifiers is an ecologically sensible method of converting alkyd-resins into an aqueous supply form without changing their structure.