Formulating VOC-Compliant Coatings with Exempt Solvents
Since many common solvents are both HAPs and VOCs, these rules have forced many producers to switch to low-solvent technologies such as high-solids, waterborne, powder and low-energy-curable systems. These technologies have their own inherent limitations that can make them more costly and less convenient to use than the traditional lower-solids formulations.
An alternative approach to these technologies is to use non-HAP, VOC-exempt solvents such as acetone and p-chlorobenzotrifluoride (PCBTF). Unfortunately, both have performance features that make them less-than-ideal solvents for coatings. In addition, the EPA is reviewing its VOC policy5 and, at least in the interim, is proposing to tighten the criteria for exempting VOCs from regulation.6 Therefore, it is unlikely that many new practical coating solvents will be added to the list of VOC-exempt compounds.
However, the EPA has recently proposed7 to add Lyondell Chemical’s tertiary-butyl acetate (TBAc) to the list of VOC-exempt materials based on its negligible photochemical reactivity. This would make TBAc one of only a handful of HAP- and VOC-compliance tools available to coatings formulators. TBAc is an effective viscosity reducer with an intermediate flash point and evaporation. It has been formulated in a variety of low-VOC coatings, ink, adhesives and cleaners, including the following.
- Nitrocellulose wood coatings
- Urethane automotive refinish coatings
- Air-drying and baking alkyd enamels for metal
- Aerosol coatings
- Flexible packaging ink
- Pressure-sensitive adhesives
- Industrial degreasers
- Paint strippers
Ultimately, the EPA’s VOC policy will probably evolve to a weighted reactivity scale similar to what is being proposed by the California Air Resources Board (CARB) for aerosol coating products.8 Another possibility is that VOCs would be classified in reactivity “bins,” again using some reactivity scale. Regardless of the outcome of this policy reevaluation, TBAc and other low-reactivity solvents will likely continue to be less regulated than the more reactive solvents used today.
Formulating Coatings for Future ComplianceAssuming the EPA will move to a reactivity-based VOC policy, reformulating today with low-reactivity solvents and additives could mean fewer compliance issues in the future. This could impact not only the type of solvents used, but also the choice between waterborne and high-solids technologies. For example, waterborne coatings are generally considered more environmentally friendly than solventborne coatings because of their lower VOC content. That’s because the current VOC policy regulates VOCs on a mass (lb/gal) basis and ignores reactivity differences between VOCs. However, the following three factors may become important in the future and should be considered in assessing the environmental impact of coatings.
- The photochemical reactivity of the contained VOCs
- The solids content of the coating
- The durability of the coating
Many of the coalescents, additives and amine neutralizers used in waterborne coatings are VOCs, and some are quite photochemically reactive (see Figure 1).
For example, glycol ethers are commonly used to coalesce acrylic latexes and urethane dispersions. They have maximum incremental reactivities (MIRs)9 in the 3–4.5 g ozone/g VOC range. Glycols, used in architectural paint as freeze-thaw stabilizers, have MIRs in the 3–6 g ozone/g VOC range. Alkylamines are also commonly used to neutralize water-dispersible resins and have MIRs above 10 g ozone/g.
On the other hand, the aliphatic hydrocarbons, esters, alcohols and ketones commonly used to formulate medium- and high-solids coatings have relatively low reactivities. Aromatic solvents, mostly xylene and toluene, are the only solvents used in solventborne coatings that, on average, have higher reactivities than those used in waterborne coatings. By replacing high reactivity solvents with low reactivity ones, it is possible to formulate solventborne coatings with a lower ozone impact than that of their waterborne counterparts. It is also possible to formulate waterborne coatings with lower ozone impacts by selecting less reactive coalescents and additives.
The solids content of the coating may also be considered. When comparing coatings, the most appropriate measure of VOC content is pounds VOC per pound of solids applied, not pounds per gallon of paint. Waterborne coatings typically have solids content in the 30–50% range, whereas high-solids coatings have solids content in the 55–100% range. At best, it takes the same amount of waterborne coating to get the same film build as a high-solids coating. However, in most cases, it takes 30–50% more waterborne coating. This affects the actual amount of VOCs emitted during the coating operation.
Finally, the durability of the coating should be considered. The durability of many waterborne coatings, although significantly improved, still lags behind that of solventborne versions. This means that waterborne coatings need to be applied more frequently, which can increase the actual amount of VOCs emitted. Coating durability is the most difficult performance characteristic to quantify, but gloss or hardness retention data from accelerated UV or chemical resistance testing could be appropriate measures, depending on the application.
Table 1 shows how these concepts may be used to calculate the actual ozone impact of different coatings. For the purpose of illustration, the durability of these coatings was assumed to be equal but may need to be quantified for a more accurate comparison.
In this example, the 2K waterborne urethane had the lowest ozone impact of the five systems considered, despite having a slightly higher VOC content than the acrylic latex. The high-solids system had the same ozone impact as the acrylic latex, despite having a VOC content three times higher (4.5 lbs/gal vs. 1.5 lbs/gal). This demonstrates that a high-solids solventborne system can generate less ozone than a lower VOC waterborne coating.
The urethane dispersion selected here had the highest ozone impact despite a VOC content of 1.8 lbs/gal, mainly because triethylamine (MIR ~10 g ozone/g) was used to neutralize the resin. This does not mean that urethane dispersions cannot be formulated to have low ozone impact. In fact, the 2K-urethane dispersion formulated with TBAc had one of the lowest ozone impacts (1.0 lb. ozone/lb. solids) despite containing a glycol ether coalescent and a relatively high VOC content (3.4 lbs/gal).
This discrepancy between the actual ozone impact of a coating and its VOC content is illustrated in Figure 2. It underscores the limitation of using pounds VOC per gallon of paint for regulatory purposes and the need to refine coatings regulations.
Environmental ConsiderationsDespite these limitations, the current policy does encourage the use of negligibly reactive exempt solvents. Unfortunately, most of these are either halogenated or extremely volatile. Halogenated solvents are seldom used in the coatings industry because of their corrosiveness, poor solvency, toxicity, cost and odor. There are only two nonhalogenated exempt solvents, acetone and methyl acetate. They are so volatile and flammable that they have found only limited use, despite their VOC-exempt status.
TBAc is one of the few nonhalogenated solvents with negligible photochemical reactivity. Based on MIR and other reactivity data, TBAc produces 40–57% less ozone than ethane on a per gram basis, whereas ethane produces 40–57% less ozone than TBAc on a per-mole basis.10 This makes it one of the least photochemically reactive solvents available to date, including currently exempt solvents (see Figure 3).
Because of its limited atmospheric lifetime and low molecular weight, TBAc also does not contribute significantly to global warming, ozone depletion, acid rain formation or fine particulate (PM2.5) formation. It is also biodegradable, does not bioaccumulate and has low toxicity.11 Like reactivity, these factors may become more important in future regulations, and will affect the marketability of solvents and the formulations that contain them.
Using TBAc instead of common coating solvents is environmentally sound, especially when considered on a comprehensive basis. Also, because it is a pound-for-pound replacement for these solvents in most applications, substitution for TBAc should not increase overall emissions. In other words, the risk that increased TBAc emissions could outweigh the benefits of these substitutions is essentially nil.
Over the past three years, since we petitioned the EPA to add TBAc to the list of VOC-exempt compounds, it has been tested in a number of coating, ink, adhesive and cleaner formulations. The following paragraphs compare TBAc’s key properties to other coating solvents and illustrate how it can be a useful tool for formulating compliant two-component urethane coatings, nitrocellulose lacquers, and alkyds.
Key Solvent PropertiesViscosity Reduction. TBAc has solvency properties similar to other esters. It is an efficient viscosity reducer for a range of commercial resins, including nitrocellulose, alkyds, epoxies, polyesters, acrylics, polyamides, urethanes and amino resins. Table 2 lists representative resin viscosity data in TBAc and other common coating solvents.
TBAc is miscible in all proportions with common organic solvents but is virtually insoluble in water (~0.3 wt % at 20°C). Its solubility properties are similar to those of other esters, such as n-Butyl acetate and PM Acetate.
Evaporation Rate. After solvency, evaporation rate is probably the most important property for a coating solvent. Unlike methyl acetate or acetone, TBAc evaporates in the same range as toluene and MEK, making it an especially good substitute for these two HAP-listed VOCs (see Figure 4). Unlike the faster-evaporating solvents, TBAc has good blush resistance in humid conditions.
Flash Point. Flammability is another important solvent property, especially as it affects worker safety. Very flammable solvents like acetone are difficult to use safely, especially in pigment-grinding applications. TBAc has an intermediate flash point, similar to that of many solvents used in coatings today (see Figure 5).
Density. The density, in pounds per gallon, of a solvent can also have an impact on VOC content and the cost of a formulation. For most coatings, VOC content is calculated by subtracting the pounds of exempt solvent from the numerator and the gallons of exempt solvent from the denominator. Theoretically, high-density solvents such as PCBTF (11.2 lbs/gal) should reduce the VOC content more than low-density solvents like acetone (6.6 lbs/gal) or TBAc (7.2 lbs/gal), provided they require the same amount of solvent to reduce viscosity. However, more PCBTF is usually required to achieve the same viscosity reduction per gallon of paint. Also, the formulated cost of the solvent increases with increasing density since solvents are bought by the pound but coatings are sold by the gallon.
Sidebar: VOC Policy RetrospectiveThe first step in the EPA’s VOC policy review was a Reactivity Workshop, held in March of 1998 in Research Triangle Park, NC. One outcome of this meeting was the creation of a Reactivity Research Work Group whose function is to make recommendations about what additional reactivity research is needed. However, the EPA’s goal is much broader: Laying the foundation for a new reactivity-based policy, which may consider not only the ozone-forming potential of VOCs but also their impact on fine particulates, global warming, acid rain, stratospheric ozone depletion, and human health. Many wonder where this will take us, how much it will cost and how long it will take. Others find it difficult to comply today and want to know what, if any, regulatory relief will be available in the interim. Studying the evolution of the EPA’s VOC-policy provides some insights into these questions.
In 1977, the EPA published its first policy regulating VOCs in an attempt to control the formation of toxic ozone in polluted urban environments. VOCs were known to react with nitrogen oxides (NOx) in the presence of sunlight to give ozone. However, it was already clear that all VOCs did not contribute equally to the ozone problem. Some VOCs reacted rapidly to generate ozone while others were relatively inert. Consequently, the first rule stated that VOCs that were less photochemically reactive than ethane would be considered to have negligible impact on ozone formation and therefore could be exempted from VOC regulations and emission controls. The rule also stated that for VOCs with reactivities close to ethane, other environmental benefits could be considered for the purpose of granting VOC exemptions. The rule did not specify how photochemical reactivity would be measured or define the petition process in detail.
In 1990, the Clean Air Act (CAA) amendments were passed, providing a definition of VOC and a list of exempt compounds. These exempt compounds were said to have “negligible photochemical reactivity” because they reacted with atmospheric hydroxyl radicals much more slowly than ethane. These radicals abstract hydrogens from VOCs to form water and organic radicals. The organic radicals then undergo further decomposition and reactions with NOx to yield ozone.
The current VOC policy considers only two classes of VOCs — exempt and nonexempt — with ethane as the boundary between the two classes. This policy has the following two limitations.
- VOCs spanning a wide range of reactivities are regulated the same way. Hence, there is no incentive to use less reactive solvents unless they are less reactive than ethane. In reality, formulations with the same amount of VOC can have vastly different ozone impacts. In fact, some low-VOC waterborne formulations actually produce more ozone than high-solids systems formulated with low-reactivity solvents.
- The use of ethane as the boundary severely limits the number of useful solvents that are VOC exempt. If the EPA changes the cutoff from maximum incremental reactivities (MIRs) expressed on a weight basis to a stricter mole-based standard, as it has recently proposed, this limitation will become even more severe.
The end result is that paint companies have little incentive to reformulate to less reactive VOCs. Since the new ethane cutoff virtually eliminates all oxygenates and hydrocarbons from contention as exempt solvents, solvent suppliers also are more limited in their ability to develop better solvents. The only solvents likely to meet the new ethane standard are halogenated solvents, which are typically not used in coatings.
Most parties recognize that there are better ways to reduce ozone formation than by continually reducing VOC content limits. The EPA and scientific community agree that all VOCs do not contribute equally to ozone formation. Most of the current discussion centers around which models best describe the extent to which VOCs’ reactivity differs. There are several methods used to estimate the ozone impact of VOCs, each with different implications for a reactivity-based VOC policy.