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.
Two-Component Urethane CoatingsTwo-component (2K) urethane coatings are high-performance systems that are finding increased use in automotive and industrial-maintenance applications because of their durability, chemical resistance, and ease of application. Unlike one-component melamine coatings that must be baked for crosslinking to occur, 2K urethane systems cure under ambient conditions when the acrylic (or polyester) polyol and isocyanate parts are mixed just before application. This is particularly useful for coating large parts that cannot be baked or thermally sensitive substrates such as plastics and wood.
High-solids acrylic polyols are typically supplied as 80%-solids solutions in solvents such as MAK or n-BuAc. The isocyanate part, typically HDI or IPDI trimers, may also be supplied with up to 25% solvent. After mixing the two components, additional solvent must be added to achieve sprayable viscosity. Typical solids contents at spray viscosity are in the 55–65% range.
Recently, the EPA published a federal rule2 restricting the VOC content of coating formulations supplied to the automotive refinish industry. The new VOC content limits are listed in Table 3. The compliance date was January 11, 1999.
These new VOC content limits are difficult to achieve without exempt solvents. Waterborne coatings still do not have the dry times and ease of use required by the refinish market. High-solids resins are more difficult to spray and have shorter pot lives than their lower-solids counterparts. Acetone and PCBTF are currently being used to lower the VOC content of some refinish systems, especially for the California market. Their use is limited, however, by their physical properties and cost.
Acetone, for example, evaporates very rapidly and can cause “dry-spray edge” and blushing, a haze caused by moisture condensation in the coating due to excessive evaporative cooling. Its flammability also makes it difficult to use in pigment grinding applications. PCBTF, on the other hand, is relatively expensive and is not as efficient at reducing viscosity as many of the solvents it replaces.
TBAc is an active solvent for acrylic polyols and aliphatic isocyanates, the two main components of thermosetting urethane coatings. It is a slightly less effective viscosity reducer for these polyols than n-BuAc, but still yields low viscosity solutions. These slight viscosity differences in the pure solvents are undetectable in fully formulated systems. For example, TBAc was used to reformulate the automotive refinish clearcoat formulations in Tables 4 and 5.
In both formulations, n-BuAc was replaced with TBAc with no effect on any of the formulation or coating properties. With TBAc being VOC-exempt, the VOC content of these formulations would decrease from 4.4 to 3.3 lbs/gal for the low-solids formulation and from 4.0 to 2.9 lbs/gal for the high-solids formulation. These clearcoats could be applied over conventional 6.6 lb. VOC/gal basecoats and still easily meet the 5.0 lb. VOC/gal limit for 2-stage topcoats based on the standard calculation.
Nitrocellulose LacquersNitrocellulose lacquers are still used extensively in the wood coating industry as clearcoats because of their unmatched clarity, depth and ease of use. They are also commonly used as fast-drying primers in the automotive refinish industry. Because of their high solvent content, these coatings have also come under increasing regulatory pressure.
Wood furniture manufacturers are subject to both Control Technique Guidelines (CTG) that limit VOC content and a National Emission Standard for Hazardous Air Pollutants (NESHAP) that limits the HAP content of the products they use.3 These limits can have a significant economic impact on small to medium-size manufacturers, who either have to install emission control devices or switch to more expensive technologies such as waterborne or UV-curable coatings.
Any switch involves conversion costs and a new set of environmental impacts. For example, switching to waterborne coatings requires that waste products be segregated and can create additional issues in areas such as product application and performance. Even controlling emissions can have a negative environmental impact — incineration of VOC emissions generates NOx, another precursor to tropospheric ozone. In NOx-limited areas, incinerating VOCs could actually make the ozone problem worse than emitting them.
TBAc is an active solvent for nitrocellulose. Figure 6 illustrates how it efficiently reduces the viscosity of both RS and SS-type resins for spray application. Low-VOC and low-HAP wood lacquer formulations were also developed. Replacing xylene, MEK, and n-BuAc with TBAc and TBAc/PMAc blends reduced the VOC content from 4.0 lbs/lb. solids to 1.5 lbs/lb. solids and reduced VHAP (volatile hazardous air pollutant) content from 2.3 lbs/lb. solids to 0.23 lbs/lb. solids.
Dry times, gloss, and whiteness were slightly improved over the high-VOC formulation. Propylene glycol methyl ether acetate (PMAc), a non-HAP glycol ether commercially available from Lyondell, reduced the viscosity of this nitrocellulose-based lacquer formulation better than n-butyl acetate and gave equivalent dry times and coating performance.
Alkyd CoatingsAlkyds are one of the leading coating technologies because of their low cost, ambient curing capability and film properties.12 They are popular coatings in the architectural, industrial-maintenance and OEM market sectors. However, they also have come under regulatory pressure because of their relatively high VOC content.
Alkyd resins are commonly supplied as 50–60% solids solutions in xylene or mineral spirits and further reduced to 35–50% solids before application. Resin producers and coating formulators have developed high-solids, air-drying alkyds but drying rates and performance are typically inferior to their low-solids counterparts. The wood furniture, aerosol, general metal, light transportation, and machinery and equipment markets would benefit from low-VOC, low-solids alkyd systems that performance as well as conventional formulations.
Table 6 lists solution viscosities for three alkyd resins from Reichhold Chemical in a range of solvents and blends. Solvents and blends are listed in order of decreasing evaporation rate. The 70:30 TBAc:MAK blend is a good match for n-BuAc, giving similar solution viscosities for all three resins and a similar evaporation rate. The properties of xylene can be approximated with blends of TBAc and slower solvents such as MAK, PM Acetate, EEP, or Exxates. Low-VOC formulations with these base resins have been developed and new TBAc-based resins are now available from Reichhold. Figure 7 illustrates how using TBAc significantly reduces the VOC content and ozone-forming potential of these formulations.
First, xylene was replaced with a TBAc/MAK/DPM acetate blend in a white enamel formulation based on the commercial chain-stopped alkyd, Beckosol 12-102. This reduced the VOC content of the formulation from 4.8 to 4.0 lbs/gal. More importantly, it reduced its ozone-forming potential from 5.4 to 4.0 lbs ozone per pound of solids applied.
A more significant reduction in both VOC content and ozone impact was achieved when the resin itself was supplied in 95/5 TBAc/xylene. The VOC content was reduced to 1.7 lbs/gal and the ozone yield was 0.8 lbs ozone per pound of solids applied. These numbers are identical to the 2K waterborne urethane system described earlier. In addition, the ozone impact is lower than either the commercial acrylic latex or urethane dispersions shown in Table 6.
This example demonstrates that medium-solids alkyd systems, when properly formulated with exempt solvents, can not only rival the VOC content of water-based and high-solids systems but can also have a lower ozone impact.
ConclusionThe EPA is proposing to exempt TBAc from VOC-regulations because of its negligible potential to form ozone in polluted urban environments. Although TBAc would become the latest VOC-exempt solvent, it also would be one of the first practical solvents for coatings, ink, adhesives and cleaners to be added to the exempt list. In addition, its low reactivity, low toxicity and negligible environmental impact make it likely that TBAc will continue to enjoy a preferred regulatory status even after the EPA completes its VOC policy revision.
Solventborne coatings, when properly formulated with exempt solvents, can be as environmentally friendly as their waterborne versions despite having higher solvent contents. TBAc gives the formulator an additional tool for achieving VOC and HAP compliance without compromising performance, switching to a different technology, negatively impacting the environment, or significantly increasing costs.
AcknowledgementsThe authors wish to thank Pat Peterson of Zeneca Resins, Carol Fox of Hercules Inc., Roy and Carol Williams of Reichhold Chemical, and Henri Barnowski of Lyondell Chemical for their technical contributions.
For more information on solvents, contact Lyondell Chemical, 3801 West Chester Pike, Newtown Square, PA 19073; phone 888/777.0232; visit www.lyondell.com.
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