Volatile organic compounds (VOCs) are defined by the EPA as “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photo-chemical reactions.” The most common source of VOCs in the paint and coatings industry are “solvents and thinners” used in the formulation itself or during product application (Figure 1) to reduce viscosity.
Figure 1
Solvents & Thinners
The terms “solvents or thinners” usually refer to organic solvents, which contain carbon. Organic solvents can be classified into three main types outlined below:
Oxygenated Solvents: Contain oxygen and are synthesized from other chemicals.
- Examples: Alcohol, esters, glycol ethers, and ketones, including methyl ethyl ketone.
Hydrocarbon Solvents: Contain hydrogen and are derived mainly from petroleum.
- Examples: Acetone, acetonitrile, benzene, dioxane, gasoline, hexane, kerosene, methanol, phenol, styrene, toluene, turpentine, xylene, and aliphatic and aromatic hydrocarbons.
Halogenated Solvents: Contain one or more of the halogen elements: chlorine, bromine, fluorine, or iodine. Most halogenated solvents contain chlorine and are then called chlorinated solvents.
- Examples: Ethylene dichloride, methylene chloride, perchloroethylene, 1,1,1-trichloroethane and trichloroethylene.
A Growing Demand for Low- and Zero-VOC Coating Products
Over the last few years there has been a growing demand for low and zero-VOC coating products. Driving the market for these types of products is an increased consumer awareness of the negative impact of VOCs in the workplace and in the greater environmental arena, and the subsequent desire to buy “greener” and more environmentally responsible products. These VOC compounds have historically been found useful as an aid in in reducing viscosity to ease application, and in promoting film formation and final film quality.
In recent years the majority of interest and activity related to the level of VOCs in the paint and coatings industry has been related to architectural paints (due to the sheer size and volume of the market). Nonetheless, producers of industrial paints, especially anti-corrosion and heavy duty protective coatings, have been feeling under pressure to reduce the VOC emissions from their products.
These coatings manufacturers are also being motivated by other legislation not specific to VOCs. They are responding to other factors such as REACH in Europe, new guidance from standards organizations such as NACE International, by the plant policies of their downstream customers, or by their own environmentally-friendly strategies to provide the least hazardous option possible without undue sacrifice to performance. As a result they are formulating away from elevated VOC content and focusing instead on systems with low to no emissions.
Assessing VOC Levels in Paint/Coatings
VOC levels in paint systems are categorized as follows:
- >250gm/L is a very high
- 100 – 249 gm/L is high
- 50 – 99 gm/L is moderate
- 5 – 49 gm/L low
- <5gm/L is very low
Advantages of Low- and Zero-VOC Coatings
Within the industrial protective coatings industry, manufacturers are focusing on increasing the solids content (lowering VOCs) within the applied film to promote environmental compliance and to improve performance.
Additional benefits of the lower VOC and the increasingly popular 100% solids coatings are:
- Improved performance = longer re-coat cycles
- Better edge retention during drying = reduced rust zone penetration at edges
- Higher applied film thicknesses = faster application rate at lower labor cost
- Non-flammable allows for lower freight, storage and safety when used concerns
- Improved air quality in work area
As an example, a traditional high VOC coating with a VOC level of 200gm/L (1.6 lbs/G) may lose as much as 15-20% of the applied film thickness during application and curing. Therefore, to achieve a specified dry film thickness for performance requirements multiple coats may need to be applied. During the drying process—as the solvents leave the film—there is some shrinkage (as much as 15-20%), which creates increased stress at the interface region between the film and the substrate. This stress can result in stress relief cracking (Figure 2) and subsequent lower adhesion values that reduce the expected long term performance of the film.
Figure 2
Additionally, solvents may become trapped within the film if insufficient ventilation is used or excessive film thicknesses are applied. This outcome can lead to a plasticized (uncured) film with lower cured mechanical, chemical and thermal properties.
In contrast, a very low VOC coating with a VOC level of 3gm/L (.02 lbs/G) would lose as little as .2% of the applied film thickness during application resulting in lower shrinkage stresses and higher tensile adhesion values.
The very low VOC and 100% solids coatings will also yield:
- Higher applied film thicknesses per coat
- Lower application costs
- Better edge coverage
- Extended performance due to the lack of entrapped solvent (VOC) in the film
Summary
Paints, coatings, and adhesives represent a significant amount of VOC emissions in residential, commercial and industrial markets. Due to the nature of VOCs, their use in paint systems offers specific benefits such as ease of application, surface tolerance, and rapid over-coating as they assist in the film coalescence process during the “drying” stage.
The offsetting negatives are increased emissions of VOCs into the environment, reduced applied film thickness increasing the number of coats required to meet specified total film thickness, shrinkage stresses on cure reducing adhesion and other critical performance attributes, explosive and or flammable work environments due to solvent evaporation impacting work site safety, and increased hazardous waste and freight transport and storage costs.
The protective coatings industry is well aware of the pros and cons of VOCs in their paint systems and is reacting by taking steps to manage to the allowable limits, while seeking alternatives that meet cost, handling and performance requirements. High-solids and 100% solids formulations will continue to gain market share over solvent-borne products in virtually all world markets, as paint makers continue to reduce VOC emissions to comply with government regulations and consumer preferences.
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