What a VOC is
In US air-quality regulation, a volatile organic compound is broadly any carbon compound that takes part in atmospheric photochemical reactions, apart from a list of specific exclusions. VOCs evaporating from coatings react in sunlight with nitrogen oxides to form ground-level ozone, which is why coatings are regulated.
How limits are expressed
Coating VOC content is usually stated in grams per liter of coating, calculated less water and exempt compounds. That convention means water and exempt solvents do not count toward the limit, and also do not dilute the VOC figure. Limits vary by coating category, for example floor coatings versus industrial maintenance coatings.
The rules in the US
- Federal: the National VOC Emission Standards for Architectural Coatings (40 CFR Part 59, Subpart D) apply to architectural coatings manufactured since 1999.
- State and local: many states and air districts set tighter limits, often based on model rules. California air districts are among the strictest.
- Industrial facilities can face separate rules covering shop coating operations.
VOC-exempt solvents
US EPA lists compounds with negligible photochemical reactivity as exempt in 40 CFR 51.100(s). Those most relevant to coatings include:
| Solvent | Exempted | Notes |
|---|---|---|
| PCBTF | 1994 | Xylene-like evaporation |
| Acetone | 1995 | Very fast, very flammable |
| Methyl acetate | 1998 | Fast ester |
| tert-Butyl acetate | 2004 | Former recordkeeping requirements removed in 2016 |
| Dimethyl carbonate | 2009 | Low-toxicity, fast |
| Propylene carbonate | 2009 | Very slow, high boiling |
Low-VOC strategies
- 100%-solids systems: spray polyurea, many epoxies and polyaspartics
- Waterborne binders such as acrylic latex and polyurethane dispersions
- High-solids resins and reactive diluents
- Exempt solvents to adjust application properties