Can wetting and dispersing additives improve the durability of coatings?

Transcription

1 Can wetting and dispersing additives improve the durability of coatings? János Hajas BYK-Chemie, Wesel, Germany Topics: WETTING AND DISPERSING ADDITIVE, DURABILITY, CORROSION RESISTANCE 1. Abstracts There are many coating additives known, which are designed to improve durability of coatings (e.g. UV absorbers, antioxidants, radical scavengers, corrosion inhibitors ). Not very often we can hear from the possible negative or positive impact of wetting and dispersing additives on the coatings durability. The lecture gives an overview of studies, showing the possible negative and positive impact on water, humidity, salt spray resistance of various metal primers and on light fastness/chalking behavior of topcoat formulations. It will be shown that there is a close correlation between the additive structure and the influence on durability. 2. Possible impacts if wetting and dispersing additives on durability There are many positive or negative impacts on the coating durability possible, when using wetting and dispersing additives in a coating formulation. Fig. 1. gives an overview on the possible impacts. Fig.1. We can see in many cases negative effects (e.g. by using too hydrophilic structures, which then decrease the corrosion resistance), but in several cases also significant improvements have been observed, especially in non-aqueous systems.

2 3. Improvement of chalking resistance of white topcoats The chalking resistance of white glossy topcoats is a very important feature e.g. for automotive and industrial coatings. Titanium dioxide, as a photoactive pigment, even when treated with inorganic oxides (Fig.2.), may cause some gloss drop during the natural or artificial UV exposure. Fig.2. During our studies we have observed that by using certain additive structures the gloss retention of coatings can be significantly improved. Fig.3 is showing exterior durability results of a polyester/melamine type baking topcoat on Okinawa Island (tropical, humid environment). Fig.3.

3 Similar tendency of improvements can be achieved in highly durable acrylic/melamine systems, but the improvement is less pronounced, than in polyester/melamine coatings (Fig.4.). Fig.4. These positive experiences have been obtained by using specially designed organic phosphate type structures (Fig.5.). Such additives are used since the mid 90s very widely in the coating industry, with many confirmed positive customer results in white coatings. Fig.5.

4 Fig.6. suggests an explanation, why just these additive structures result in remarkable improvements of the gloss retention. The first point (better deflocculation of TiO2) is a result of a very strong and almost complete adsorption of the acidic organic phosphate on the TiO2 surface, together with a good steric hindrance of the pigment particles against flocculation. The organic phosphate chemistry must have obviously some capabilities to capture or desactivate free radicals, preventing the oxidation of the organic binder which results in chalking and gloss reduction. Fig Improvement of corrosion resistance of primers Since many years, coating formulators avoid to use very hydrophilic structures as a wetting and dispersing additive in anti-corrosive primers, because they used to cause a significant negative impact of the corrosion resistance. Typical examples for this group of additives are lecithine based products, or non-ionic surfactants (alkylphenol ethoxylates, alkyl ethoxylates). After carrying out numerous studies, we found that ~ 70 % of commercially available additives for non-aqueous systems have a visible negative impact on the corrosion resistance of the film. The negative impact can be observed as an increased rate of blistering in accelerated corrosion tests such as humidity test, water immersion test and salt spray test, and the so called undercut corrosion values are also getting worse when using these structures. However, a smaller group of additives has been found very supportive for improved corrosion properties. Chemistry and typical fields of application of these additives are highlighted in Fig.7. Four of the five products are controlled flocculating additives, just one belongs to the deflocculating category.

5 Fig.7. Fig.8 shows a negative and a positive result for a hydrophobic, and hydrophilic additive structure in an air-drying alkyd primer with Ca modified silica anticorrosive pigment, obtained in a one-week salt spray test. Fig.8 Fig.9. highlights salt spray results with a short oil alkyd primer with phosphosilicate anticorrosive pigment, using two different polycarboxylic acid based controlled flocculation additives, compared to the control.

6 Fig.9. Fig.10 displays results with an inorganic Zn rich primer, where the usage of a phosphate based non-polar additive structure resulted in visible improvement of corrosion resistance. Fig.10 According to Fig.11, and 12., the improvement is mainly due to a more homogenous Zn dust distribution in the cross-cut section of the film, also resulting in less porosity and more compact film structure.

7 Fig.11 Fig.12. Very similar results have been obtained in epoxy based zinc rich primer formulations as well.

8 5. Results in aqueous formulations The general experience in aqueous systems is the same like in solvent-borne or solvent-free systems: too hydrophilic structures may reduce the corrosion resistance of the coating film, while hydrophobic structures are neutral or have a slightly positive effect in this respect. Another important experience is that ammonium or amine salt type structures behave better, than comparable sodium or potassium salts of the same acidic polymeric dispersants. Ammonia or the amines evaporate during the film formation, but the sodium or other metal salts remain in the film, causing a more hydrophilic film structure. Fig. 13. is showing salt spray results in an aqueous epoxy system, using a hydrophilic non-ionic wetting agent and a hydrophobic anionic type wetting and dispersing additive. Fig.13 According to our general experience, almost 90 % of commercially used and recommended wetting and dispersing additives, dispersants, wetting agents show a visible negative impact on the corrosion resistance! Fig. 14 and 15 display undercut corrosion and blistering resistance results in a styrene-acrylic air drying anticorrosive primer formulation by using eight different additive structures. By using only one (hydrophobic) structure we got acceptable results for both properties.

9 Fig.14 Fig.15 As a summarizing experience of numerous test series, Fig. 16 is showing the impact of commercially used dispersants, wetting agents and wetting and dispersing additives of various chemical structures on the corrosion resistance in styrene-acrylic latex based primer formulations. The ranking of additive structures may change, when a different resin type is utilized.

10 Fig Summary It has been shown, that certain wetting and dispersing additive structures may contribute significantly to a better exterior durability. In white pigmented topcoats the gloss retention and chalking resistance is improved by using acidic organic polyester-polyether phosphate type products. In non-aqueous primer formulations especially polycarboxylic acid based hydrophobic wetting and dispersing additives improve the corrosion resistance. In aqueous primer formulations the wetting and dispersing additive selection is the most difficult task, since many commercially used products have a pronounced negative impact on the anti-corrosive properties. Best performance can be achieved by using relatively hydrophobic molecules.