January 2003 Q&A Columns

Controlling Block Resistance

How can block resistance be controlled?

Block resistance is the tendency of a coating not to adhere to another surface. For example, when a part is painted and wrapped with paper, the paper may stick to the "dry paint." This is an example of poor block resistance. If the paint is fully dry and not able to flow, block resistance is usually good. Crosslink density improves block resistance. In lacquer systems, the solvent evaporation is the key to block resistance. This solvent includes coalescent aids and many volatile additives. Usually paints will have an increase in glass transition temperature or hardness with time. Use of more volatile, faster evaporating, solvents may help. In addition, additives that reduce adhesion, such as silicones and fluorocarbon additives, may help. Other additives, such as some extender pigments and polyethylene or polypropylene additives, may reduce block adhesion problems. Experimentation will be needed to find what works best in your case. Remember, most additives have multiple effects. Be careful that an additive does not create a bigger problem.

Dual-Cure Technologies for UV

What dual-cure technologies are used in UV- cure systems?

In radical-cure UV systems, peroxides can be used in some systems to thermally cure after passing through the UV system. Moisture-cure urethane chemistry can also be used with the radically cured systems. In cationic systems, thermally induced cationic chemistry can be used in conjunction to cure epoxy systems. The dual cure technology is generally used where line of site is not possible. For example, in heavily pigmented coatings, thick coatings or where the porosity of the substrate will not allow line of site.

DTM Coating Adhesion Factors

What factors will affect adhesion of DTM (direct to metal) coatings?

In direct-to-metal coatings, the adhesion can be changed by additives such as surfactants, defoamers, wetting aids, and dispersants in usually a negative sense. The cleanliness of the surface is very critical today with the use of high solids DTM coatings. Without the solvent to dissolve oils and dirt on the surface, DTM will usually have much poorer performance on these surfaces than the old low solids versions. Additives put into DTM coatings such as silane coupling agents will also help on clean substrates, but they are very ineffective on contaminated surfaces. In the laboratory we do an excellent job of surface preparation, but in the real world there is much more oil and other contaminants on the surface. All evaluations should be done on real-world samples and not just on pristine laboratory coupons.

Acrylates Vs. Methacrylates in UV Curing

Why are acrylates and not methacrylates used in most UV-cure coatings?

Both are used and can be used to make excellent coatings. Both undergo radical cure but the radical chain growth head of the methacrylates is more stable and thus has more time to react with oxygen and have its molecular weight shortened. Thus, methacrylates require a blanket of nitrogen over the paint to reduce the oxygen concentration. When this is done, the methacrylates work well. Acrylates do not require a blanket of nitrogen. However, acrylates will produce better films if they are cured under a nitrogen blanket also. In both cases, the surface of the coating will have more reaction with oxygen and result in different surface properties than the coating deeper in the film. In some cases, this poor surface cure can result in an easily marred coating or a surface that appears to be oily.

Operator Drawdown Deviation

How much deviation should we typically see between operators for drawdowns?

Any methodology in which humans are involved can produce errors. We have found that for a given drawdown bar a film-thickness variation of 1 to 3% is typical if they are all doing it in a similar manner. However, if the methodology is not standardized the error can be larger. There can also be error due to the use of different drawdown bars. Typically this is only 1 to 10%, depending upon the thickness. If multiple drawdown bars are to be used, several can be checked to find the most similar and their use specified. Old worn bars should be retooled or replaced.