A coating chemistry’s VOC measurement is calculated in pounds per gallon or grams per liter. The same basic coating chemistry formulations may have different solvent line-ups depending on the regulations of the destination state, region or country.
Just because a coating may contain VOC-exempt solvents and may be considered EPA-compliant in the United States, it might not be recognized as exempt by other countries. Some water-based coatings may contain co-solvents. Plus, water-based coatings and 100 percent polymer chemistries could still be hazardous. The coatings Safety Data Sheet will provide information on its environmental and physical effects as well as proper PPE recommendations.
Achieving good film properties
Coatings should have a lower surface tension than the substrate to achieve adequate wetting of the substrate. Solvent based formulations have an inherently low surface tension and will wet out 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 de-foaming additives to allow for continuous film forming that limits foaming during application. The proper choice of wetting and defoaming additives is crucial to optimizing the finished film’s aesthetics.
These foaming and wetting problems are part of the reason some manufacturers are hesitant to switch from solvent-based coatings to water-based coatings, despite the positive environmental reasons to implement this technology.
Coating operations can easily make the switch from solvent-based to water-based. However, the transition requires consultation with a coatings supplier that will custom-formulate a chemistry to meet production and end-use performance requirements.
While the dry times of the solvent-based coatings can be formulated to dry very quickly, dry times of water based formulas are limited by the slower evaporation rate of water. Therefore, additional heat and air movement may be required to achieve the desired cure or dry times to compare to the solvent-based equivalent. For many plastic substrates, this additional heat may cause thermal degradation of the substrate, which could produce toxic vapors. Using a 100% solids polymer coating as an alternative to VOC-heavy solvent-based coating also creates production concerns. Additional time and temperature may be required to cross-link the polymer to the surface, which can cause damage to some substrates.
Coating operators should work collaboratively with their coatings supplier and equipment engineer to understand the equipment modifications needed to successfully convert from solvent-based to water-based coatings. They must take into account any limitations around the curing and line speeds (i.e., dwell time, oven temperature, air movement, humidity levels) of their coating line and how they may need to be adjusted for proper curing when using co-solvents and water.
Sourcing a high-performance, low-emitting coating can be a challenge. Chances for success improve with a high-tech coatings manufacturer and equipment supplier that have deep engineering expertise and process guidance.
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