Coatings R&D Notebook
Waterbornes Without SolventsWhat problems are expected with zero-VOC waterborne technologies?
Waterborne coatings usually have two VOCs: the antifreeze, usually a glycol, and the coalescent aid. The glycol aids in both wet edge retention and freeze/thaw stability. Wet edge retention is more important for architectural coatings than OEM, but it can affect early re-coat. The glycol helps water leave but is slow to leave the film itself. Some OEM systems use a more volatile alcohol, methanol, as the antifreeze. The antifreeze, as well as most coalescent aids, is a VOC. Removal of these solvents will make the coating more susceptible to freeze damage. Very few paints are stable to freeze/thaw cycling without the glycol or methanol. Without the glycol, mud cracking and other problems when thicker coatings are applied can arise. Mud cracking can occur when water is not lost fast enough as the paint begins to form a film. The volume then slowly changes as the remaining water is lost, stressing the coating and resulting in cracks. The lack of anti-freeze also usually means that the coating must be kept above the freezing point to avoid in-can gelling.
The coalescent aid allows the Tg to be temporarily lowered but recovers when the volatile solvent leaves. The coalescent aid can be removed more readily than the antifreeze without harming the properties of the coating. Reactive diluents or reactive groups on the resin can allow the resin to self-coalesce at ambient temperatures but react to produce a harder film with time or through reaction with oxygen from the air. If the coating remains soft, poor block resistance will result. If the coating does not coalesce fully, the coating will have lower tensile strength, lower gloss, lower abrasion resistance and will be more permeable. Many coatings require forced hot air to accelerate the drying and coalescence process and give the system enough energy to cross-link. It should be noted that if forced hot air is used, fuel is used to heat the air. The result of this type of coating is not as environmentally friendly. In general, the dry time, wet edge retention, tensile strength and block resistance can all be affected by the lack of key solvents.
UV Vs. Visible-Light CuringComparing UV and visible-light curing systems, does one have an advantage over the other?
UV cure requires a mercury vapor lamp activated by a high-voltage discharge or microwave irradiation, or a xenon or similar arc system. The UV light can react with pigments, resin or additives to degrade or alter the color of the component. This damage can produce the equivalent of photo bleaching, yellowing of the resin, or a color change due to photo-oxidation of the additives. UV light also has the potential to damage workers' eyes, cause skin cancer and produce ozone, which is both environmentally harmful and potentially hazardous to workers' health.
Visible light is of lower energy and can be generated by a large variety of bulb technologies. The blue and violet portions of the spectra are potentially useful for initiation of polymerization. The difficulty with working with visible-light cure coatings is that normal room light could cause the coating to cure in an open can. The use of incandescent lighting, yellow or red lights would reduce this problem. The current availability of visible-light curing agents is quite limited. The cost of the more efficient agents can be over $1,300 per pound. Luckily, very little photoinitiator is used. With this high price, it would cost approximately $2.50 per gallon of coating for the initiator.
As the need for reduction in VOCs increases, the technology around photo-initiated coatings will increase. This segment of the coatings industry is growing at a very high rate. The low cost of visible light will only serve to increase the use of photo-cured systems. The few problems surrounding the limiting of the paint's exposure to light can readily be avoided. This technology could be employed in OEM and maintenance coatings as well as architectural coatings.