Claude Nootens – Luerchem LLC
Stringent VOC regulations for architectural decorative, protective and industrial maintenance coatings/paints are still the biggest driver for switching coating formulations to high solids, water-based, UV or powder systems. Nowadays, although the architectural decorative market segment is predominantly waterborne (mainly vinyl or acrylic emulsion paints), there are still developments ongoing in the area of high solids and waterborne protective and industrial paints, especially for metal coatings, for which specific higher performance is required. Other technologies such as UV and powder are also investigated but suffer from the disadvantage of additional cost associated with the required equipment application.
Nowadays, with sustainability being an important additional element that needs to be taken into consideration, interest in oil-based resins, such as alkyd resins for example, could be reconsidered.
A resinous polyol such as styrene allyl alcohol copolymer (SAA) is a low molecular weight polymer polyol which can be used as part of the backbone to design several binder types(1), including those useful for more environmentally friendly coating formulations.
Firstly, fast cure high solids solvent-borne alkyd resins can be prepared with the resinous polyol SAA as part replacement of pentaerythritol(2,3,4). The resulting coatings have excellent adhesion on various substrates and chemical resistance. Moreover, when applied on metallic substrates, excellent corrosion resistance is observed.
Secondly, in addition to the development of high solids alkyd resins, waterborne (water-dissipatible, water-dispersible, or water-reducible) alkyd resins have also been developed (Fig. 1).
Fig. 1 – Technology evolution for meeting VOC requirements
Alkyd resins have several benefits, including displaying excellent wetting of the substrate, consequently, their coating films offer high gloss and corrosion resistance for metallic substrates. Moreover, during air drying, alkyd resins cross-link via oxidation leading to very high molecular weight networks that further improves their coating resistance properties.
However, it has been reported that there are some drawbacks associated with alkyd resins compared to acrylic resins, in particular the hydrolysis resistance of the resins and the water and alkali resistance of the resulting coatings. To prevent these, especially important in waterborne systems, a novel approach consisted of using a styrene allyl alcohol as the backbone on which the vegetable oil fatty acid is connected (Fig. 2).
Due to limiting the number of ester groups to those resulting from the reaction between the hydroxyl and the vegetable oil fatty acid group, the alkali resistance is significantly improved. In addition, the presence of the hydrophobic aromatic groups gives the water resistance required.
There are several main routes used for making waterborne alkyd resins, and these routes can be used alone or in combination.
-Emulsification of the alkyd resins by external conventional or reactive non-ionic or/and anionic surfactants, or reactive non-ionic or/and anionic resin emulsifiers.
-Incorporation of non-ionic hydrophilic groups (e.g. as polyethylene glycol) or anionic hydrophilic groups (carboxylate or sulphate groups) in the backbone of the alkyd resin.
-Polymerisation and grafting of acrylic copolymer, containing (or not containing) carboxylic acid groups, on alkyd resins to form acrylic-alkyd hybrid polymers.
The waterborne fatty acid ester of SAA can be prepared in similar ways to waterborne alkyd resins.
EMULSIFICATION
The waterborne fatty acid ester of SAA can be obtained via emulsification of the fatty acid ester of SAA by a sodium polyacrylate/aqueous ammonia solution, followed by addition of octyl alcohol as co-solvent (U.S. Pat. 3,293,201).
INCORPORATION OF CARBOXYLIC ACID GROUPS
The waterborne fatty acid ester of SAA can also be prepared via the incorporation of carboxylic acid groups which can subsequently be neutralised by ammonia or a hydroxylamine compound to give a water-dispersible resin.
The incorporation of carboxylic acid groups can be realised via two routes. The first route involves a reaction with anhydride such as maleic anhydride or trimellitic anhydride. The second route is carried out via acrylic grafting. In some cases, the two processes are applied consecutively.
The most representative binders from the first route are maleinised unsaturated fatty acid esters of SAA resins. Various unsaturated vegetable oil fatty acids have been used: linoleic acid (U.S. Pat. 4,143,012), soya fatty acid (U.S. Pat. 3,528,939 and 4,933,380), linseed fatty acid (U.S. Pat. 4,933,380 and 4,107,144), tall oil fatty acid (U.S. Pat. 3,998,716, 3,894,993, 3,558,536 and 3,945,961), tall oil acid/adipic acid (U.S. Pat. 3,663,405 and 3,932,191).
Examples of trimellitic adduct of unsaturated fatty acid ester of SAA binders are based on soya fatty acid or tall oil acid (U.S. Pat. 3,709,846, 3,558,536 and 3,666,649), on dehydrated castor oil fatty acid (U.S. Pat. 3,676,312), on castor oil fatty acid (U.S. Pat. 3,650,998) or on silicone modified soya fatty acid (European Pat. 0 967 235).
Trimellitic adduct of SAA binder based on saturated fatty acid such as stearic acid has been also reported (U.S. Pat. 3,471,388).
Interestingly, a water dispersible phenolic (resole) modified SAA-based alkyd resin containing trimethylolpropane and neopentyl glycol in addition to SAA can also be prepared using isophthalic acid and trimellitic anhydride and after neutralisation, can be formulated in air-drying enamel formulations (U.S. Pat. 4,649,173 and 4,740,567).
The second route involves the grafting of a polyacrylic chain via peroxide-initiated copolymerisation of vinyl monomers containing in some cases (meth)acrylic acid monomer.
This process can be done either on fatty acid ester of SAA (U.S. Pat. 4,107,144; 4,221,647; 4,263,194 and 4,107,114), or on maleinised fatty acid ester of SAA (U.S. Pat. 4,257,933 and 4,436,849), or on trimellitic adduct of fatty acid/crotonic acid ester of SAA (U.S. Pat. 4,735,995), or on mercapto terminated urethane linoleic acid ester of SAA (U.S. Pat. 4,255,541).
The waterborne fatty acid esters of SAA resins are subsequently formulated either in waterborne air-drying primer formulation or baked primer formulation containing a resin hardener (e.g. aminoplast or polyisocyanate). The primer formulation can be applied by conventional application methods (brush, spray, roller) or anionic electrodeposition (AED). Further reaction of the maleinised fatty acid ester of SAA with ethylene diamine and subsequently neutralised with acetic acid gives a water dispersible resin which can be applied by cationic electrodeposition called CED (U.S. Pat. 3,799,854).
In all cases, the resulting films have excellent adhesion on various substrates, and provide good resistance to corrosion for metal primer coatings. The waterborne fatty acid ester of SAA resins therefore provides a class of binders having superior performance than the ‘traditional’ alkyd waterborne resins.
CONCLUSION
Coating films with outstanding performance in terms of adhesion and chemical resistance can be achieved from aqueous medium waterborne SAA-based resins having high hydrolytic stability. These carboxylic acid functional oil-based resins are particularly efficient to protect metallic substrates from corrosion and can be applied by various application methods.
