Solvent based paints are widely used, even though there are many water based options/equivalents available. Generally, this is due to their low cost, ease of application and excellent film-forming properties. Many coatings businesses are turning to water-based alternatives as the industry looks to reduce its environmental impact. When properly designed and applied, water-based chemistries can provide products with the same end-use properties as their solvent-based counterparts—UV protection, stain resistance, abrasion resistance, and moisture resistance.
VOC subsidy
VOC measurements for coating chemicals are calculated in pounds per gallon or grams per liter. The same base paint chemical formulation can have different solvent families depending on the regulations of the destination state, region or country.
Just because the coating may contain VOC-free solvents and may be considered EPA compliant in the US, it may not be recognized as exempt in other countries. Some water-based paints may contain co-solvents. Also, water-based paints and 100% polymer chemicals can still be dangerous. A paint safety data sheet will provide information on its environmental and physical effects as well as advice on proper PPE.
Achieve good film properties
The coating should have a lower surface tension than the substrate to achieve adequate wetting of the substrate. Solvent based formulations have inherently low surface tension and will wet on most substrates without the use of additives. This allows the coating to flow into a smooth, continuous film. Water has a very high surface tension compared to solvents. Therefore, water-based coatings require the addition of wetting and defoaming additives to allow continuous film formation, which limits foaming during application. The importance of proper selection of wetting and antifoam additives to optimize the aesthetics of the finished film cannot be overemphasized.
These foaming and wetting issues are part of the reason some manufacturers are hesitant to switch from solvent-based to water-based coatings, despite the positive environmental reasons for implementing the technology.
production considerations
Coating operations can be easily converted from solvent-based to water-based. However, transitioning requires consulting with a coatings supplier who will tailor the chemistry to meet production and end-use performance requirements.
While the drying times of solvent-based paints can be formulated to dry very quickly, the drying times of water-based formulations are limited by the slower rate of water evaporation. Therefore, additional heat and air movement may be required to achieve the desired cure or dry time compared to solvent based equivalents. For many plastic substrates, this additional heat can lead to thermal degradation of the substrate, which can generate toxic vapors. The use of 100% solids polymer coatings as an alternative to VOC heavy solvent coatings also creates production issues. Additional time and temperature may be required to crosslink the polymer to the surface, which may result in damage to some substrates.
Coatings operators should work with their coating suppliers and equipment engineers to understand the equipment modifications required to successfully convert from solventborne to waterborne coatings. They need to take into account any limitations in curing and line speed (i.e. dwell time, oven temperature, air movement, humidity levels) of the coating line and how to adjust them for proper curing when co-solvents and water are used.
