Why Photochemical Reactivity is Important

Regulation based on the photochemical reactivity of individual solvents offers a more effective and efficient means of reducing the impact of VOCs on the formation of ground-level ozone. Another attractive feature of photochemical reactivity-based approaches to VOC control is that they provide formulators with greater flexibility than traditional mass-based approaches. The result is that the required coating performance attributes can be maintained as the contribution to ozone levels is reduced.

EPA Interim Guidance Policy

Direction for Future VOC Control

The EPA recognizes that reactivity-based approaches should be more efficient and effective than traditional approaches that do not distinguish among VOCs. As a result, the EPA has updated its VOC policies by endorsing photochemical reactivity as a sound, science-based approach for VOC control. The EPA Interim Guidance policy¹ was published in the Federal Register in September 2005 as guidance for states to pursue reactivity-based approaches in their State Implementation Plans (SIPs). The guidance encourages states “to consider how they may incorporate VOC reactivity information to make their future VOC control measures more effective and efficient.” In effect, the guidance represents a new paradigm for VOC control for coatings – moving from the old paradigm of “low-VOC paints” to the new paradigm of “low-ozone paints.”

CARB Aerosol Coatings Reactivity Regulation

Difficult reformulation challenges led the California Air Resources Board (CARB) to conclude that it may not be feasible to achieve additional VOC reductions from a traditional mass-based program for aerosol coatings. As a result, CARB worked with industry experts and scientists to develop a reactivity-based aerosol coatings rule, which was approved by EPA² in September 2005. The CARB rule encourages reduction in the use of higher reactivity VOCs in aerosol coatings, to achieve more ozone reduction than would have been achieved by traditional mass-based regulations. CARB estimates this new rule will achieve the equivalent of an additional 3.1 tons/day of VOC reductions in California. The EPA is now developing a national reactivity-based aerosol coatings rule, using the CARB rule as its starting point for rule development. CARB is now considering a reactivity-based rule for AIM coatings, which is due in 2007.

Ozone Formation

Ozone in the upper atmosphere (stratospheric ozone) absorbs UV light and protects the earth from harmful ultraviolet radiation. On the other hand, ground-level ozone (tropospheric ozone) is the main component of smog, and thus can have adverse effects on human health.

Ozone is formed in the atmosphere when UV light from the sun reacts with oxides of nitrogen (NOx) from automotive or other emissions. This happens even when VOCs are not present. When VOCs are present, the overall equilibrium between ozone and NOx is shifted (that is, ozone breaks down more slowly because of competing reactions in the atmosphere) and more ground-level ozone may accumulate.

There are many contributors to VOC emissions in the atmosphere including natural or “biogenic” sources, such as trees and vegetation. Man-made sources such as vehicle emissions, petroleum refining, manufacturing plants and power generation facilities also contribute to VOC levels. VOC emissions come from the use of organic solvents if they evaporate into the air. In most rural areas of the country, biogenic VOCs predominate over man-made VOCs. These rural areas also often have limited NOx concentrations, due to fewer urban man-made emissions sources as discussed above. Thus, reducing man-made VOCs in rural areas is not expected to provide the same benefit as in urban areas.

Some areas of the country do not meet national standards for ground-level ozone and are referred to as “ozone non-attainment areas.” Under the Clean Air Act, these areas generally are required to reduce VOC emissions (not including vehicle emissions) by 3 percent each year until the national standard is met. To make these reductions and reach ozone attainment, virtually all sources of VOC emissions in these areas, including solvent uses, may be regulated.

VOCs – Not All Are Alike

Traditionally, VOCs have been regulated using a mass-based control technology approach, by limiting the mass or amount of VOCs in various products or formulations, such as paint. Under this approach, VOCs are either considered reactive, and therefore subject to VOC regulation, or negligibly reactive, and thus exempt from VOC regulation. This mass-based approach treats all non-exempt VOCs alike in their ability to contribute to ozone levels. However, scientists now concur that VOCs vary significantly in their potential to impact ozone levels (i.e., the higher the reactivity, the greater the potential contribution to ozone levels). For example, alkenes (olefins) are more photochemically reactive than aromatics, which in turn are more reactive than both aliphatic hydrocarbon and oxygenated solvents.

Mass-Based VOC Limits vs. Photochemical Reactivity-Based VOC Approaches

Mass-based VOC limits provide no differentiation with regard to the photochemical reactivities of different solvents used in coating formulations. Therefore, two formulations that meet mass-based limits can have very different potentials to contribute to ozone levels, depending on the solvents used.

Photochemical reactivity, on the other hand, is a science-based approach to reducing air pollution and smog, which can provide greater reduction of ozone levels compared to mass-based approaches. In other words, distinguishing between more reactive and less reactive VOCs allows ozone concentrations to be reduced further and more efficiently than by controlling all VOCs equally.

Using a reactivity-based approach to reducing ozone levels provides an incentive for formulators to move from high- to medium-reactivity solvents, or from medium- to low-reactivity solvents. This approach also helps increase formulation flexibility by not imposing mass-based limits that might lead to performance decrements. Additionally, reactivity-based approaches result in further benefits to the overall environment. For example, substrates may need to be repainted less often since the paint can be formulated to last longer, saving the materials and energy associated with reapplications.

Figure 1

Although the concept of photochemical reactivity has gained greater attention in recent years, it is not a new concept. Scientists in fact have known for years that two VOCs may have profound differences in their contributions to ozone formation. Regulations in California, particularly those pertaining to low-emitting vehicles and clean fuels, have made distinctions based on reactivity as far back as 1991. Since 1998, EPA has supported additional study of reactivity through the Reactivity Research Working Group (RRWG), which consists of representatives from academia and industry as well as state and federal regulators. Modeling has demonstrated that reactivity-based limits can be effective in reducing ozone in non-attainment areas such as urban environments. The RRWG and others have also addressed the need for developing scientifically valid “reactivity scales.”  One of the most common scales in use today is the Maximum Incremental Reactivity (MIR) scale³, illustrated in Figure 1.

The MIR scale measures the relative photochemical reactivity of solvents on a common, continuous scale. The MIR values are usually expressed in units of grams of ozone formed per gram of VOC reacted.

Figure 2

To assess the potential impact of reactivity on ozone reduction, analyses of the solvents used in coatings in the United States were conducted using published market data.⁴ The market data consisted of speciated solvent volume data, broken down by coating end-use type. Assumptions were made on the end-uses that were “open” or emissive, versus those that were “closed” or captured, to estimate the quantity of solvent emissions to the air. The overall solvent volume being used in open end-uses was estimated to be 60%, for all coatings end-uses on a weight-average basis. The MIR scale was then used to calculate the ozone creation potential from the evaporated solvents. The solvent volume data and corresponding ozone creation potential were normalized to 100%, for simplicity, as shown in the pair of bars labeled as “Today” in Figure 2.

Regulatory “scenarios” or case studies were then conducted on the open/emitted volume, so that comparisons of ozone creation potentials could be made between a mass-based scenario and two reactivity cases. For the mass-based case, an ozone reduction target of 50% was chosen, wherein the open mass of each solvent type was cut in half in order to force the calculated ozone creation potential to decrease by 50%. The second pair of bars in Figure 2 illustrates this result, where the ozone level is seen to have dropped by 50%. The corresponding solvent mass however only decreased by 30% (70% remaining), because, as noted earlier, the open coating end-uses were estimated to be only 60% of the total market; i.e., removing half of that 60% resulted in the 30% solvent volume reduction. (Said another way, if an ozone reduction target of 100% had been chosen, then the solvent volume reduction would have been 60%, or all open volume.)

Next, a reactivity case (3rd pair of bars, labeled “Reactivity Model #1” in Figure 2) was conducted for comparison. In this simple model, the legend/key under the bars shows the assumed impact of reactivity on open solvent volumes. That is, solvents with MIR < 1 were assumed to grow in use by 20%; solvents with MIR between 1-3 were assumed to decrease in use by 10%; and solvents with MIR > 3 were assumed to no longer be used in open coating end-uses. These growth (decrease) factors were applied to the corresponding open solvent volumes, so that the impact on overall solvent volume and ozone creation potential could be calculated. As shown in Figure 2, the ozone creation level again decreased by about 50%. However, the overall solvent volume only decreased by 13% (87% remaining), meaning that more solvent options are still available for formulating. That is, the same 50% ozone reduction target was achieved, but by eliminating less solvent volume than in the mass-based case, because the reactivity scenario focused on the higher reactivity solvents. Thus, a more efficient approach for ozone reduction that also provides increased formulating flexibility.

However, simply eliminating solvents arbitrarily as in the last example does not address the solvent property requirements that may be needed to meet coating performance criteria. The last pair of bars in Figure 2 (labeled “Replacement Scenario”) is a more refined reactivity case, with some consideration now given to solvent property (e.g., solvency) impact on coating performance. That is, instead of completely eliminating some solvent volumes as in the previous example, the replacement scenario suggests replacing certain solvents with other solvents (or solvent blends), to maintain the solvent properties being replaced but by using lower reactivity solvents. For example, this scenario replaces the aromatic solvents with a 60:40 blend of light dearomatized aliphatic and oxygenated solvents (the oxy solvent in the model assumed a 50:50 mix of ketones and esters). This approach may simulate the solvency requirements needed by a particular resin, for example. In this case, the ozone reduction was still significant at 43%, while none of the solvent volume was lost – maximizing formulation options.

VOC Formulation Considerations

With traditional mass-based limits, formulators may have only limited reformulation options and often may be forced to change coating technology type instead. For example, changing technology from solventborne to waterborne, powder or energy-curable systems. These options are generally expensive, research-intensive tasks, made more expensive by the need to fully re-qualify at the end-user. This can be a multi-year effort.

With reactivity-based limits, regulators can encourage formulators to reduce the amounts of highly reactive VOCs in their formulations via substitution with blends of lower reactivity solvents. This allows the formulator much greater latitude in choosing the best solvents, and is a more economical solution. In most cases, this would not require the extensive effort associated with technology changes that mass-based regulations drive.

The simple replacement scenario described in Figure 2 begins to address some of the solvent property requirements that may be needed by a given coating system. Reformulating from one solvent blend to another solvent blend with lower reactivity must also give consideration to the solvent properties that are important to the system. As noted earlier, for example, solvency or solvent strength is important for dissolving the resin and achieving acceptable coating viscosity; solvent evaporation rate is important for controlling coating dry-time. With reactivity-based approaches to VOC management, more solvent options remain available to the formulator, allowing increased flexibility to meet these and other needed coating property requirements.

The coating reformulation examples at the end of this paper will look at specific, actual coating reformulations that take into account all solvent and coating property requirements, while simultaneously reducing ozone creation potentials.

Reactivity-Adjusted VOC Content

The traditional mass-based VOC content of a coating provides a measure of the VOCs present in simple units such as lbs/gallon or grams/liter. However, two different coatings with similar VOC contents may actually have significantly different impacts on the environment, depending on the reactivities of the solvents used. Several “metrics” or parameters have been developed for quantifying the reactivity impact of a given coating. For example, in CARB’s aerosol coatings reactivity rule, the product-weighted MIR in Grams Ozone per Gram Product is used. Another available metric is the reactivity-adjusted VOC content, or RAVOC, which has the same units as traditional VOC content (e.g., g/L or lbs/gal). RAVOC is the traditional VOC content of the coating, multiplied by a simple weighted-average of the relative reactivities of the solvents used. That is,

RAVOC = VOC x

Table 1

Reformulation Examples

To illustrate the practical implementation of reactivity, two reformulation examples were carried out in the laboratory. The first example, illustrated in Table 1, used an alkyd resin-based industrial maintenance coating formulation. Table 1 lists the solvents used in both the “control” or high ozone version of the formulation, and also the alternative solvents used in the low-ozone version of the coating. The MIR values and relative reactivities (i.e., MIRi/MIRBC) of all solvents are also shown.

As may be seen in Table 1, the alkyd coating reformulation involved replacing xylene in the control formulation with a blend of lower reactivity aliphatic hydrocarbon and oxygenated solvents. A software program that employs Hansen/Hildebrand solubility theory, as well as solvent relative evaporation rates, was used to ensure that the low-ozone alternative solvent blend would be effective in dissolving the alkyd resin and would also match the evaporation profile (or dry-time) of the control formulation. With the composition of the low-ozone blend established, the environmental impacts of the two coating formulations could be compared.

In this example, the VOC contents of the two formulations are very similar (i.e., 342 and 351 g/L), but inspection of the RAVOC contents reveals a dramatic difference between the two coatings. The RAVOC content of the control formulation was 529 g/L, while that of the low-ozone formulation was only 89 g/L, a reduction in ozone creation potential of 83%. Thus, despite the very similar traditional mass-based VOC contents, the environmental impacts of the two coatings are quite different.

Table 2

The second reformulation example, illustrated in Table 3, used an epoxy-polyamide resin primer formulation. A software program was again used to replace two higher reactivity solvents with a blend of lower reactivity aliphatic hydrocarbon and oxygenated solvents. The solvent blend compositions of the control and the low-ozone formulation, and their respective environmental impacts, are also shown in Table 3. In this example the traditional VOC contents of both coatings are again similar (i.e., 536 g/L and 531 g/L), but the RAVOC values, which reflect the ozone creation potentials of the formulations, are again dramatically different. The control epoxy-polyamide formulation had a RAVOC content of 842 g/L, while the low-ozone version had a RAVOC value of only 156 g/L, a reduction in ozone creation potential of 81%.

Table 3

A theory for why the low-ozone formulation showed more viscosity rise is that the control formulation used a ketone as one of the solvents, while the low-ozone version didn’t contain any ketone. The amide group in the epoxy-polyamide resin may be prone to hydrogen bonding with the highly polar ketone, inhibiting the polymerization reaction somewhat by keeping the amide tied up longer with solvent. If this had been anticipated beforehand, the low-ozone formulation probably could have been designed to include a ketone. But since the pot life of the low-ozone coating was acceptable in this example, no further analysis was undertaken.

Table 4

Summary

Reactivity-based approaches provide coatings formulators with a multitude of formulation options, enabling the industry to retain important product attributes, while achieving and/or exceeding targeted reductions in ozone creation potential. The use of photochemical reactivity approaches provides formulators with greater flexibility to meet product performance criteria because there are many lower-reactivity solvents to choose from that provide the properties necessary for the particular application. This, in turn, benefits consumers by providing affordable, high-quality, durable coatings with low ozone impact on the environment.

This paper was presented at The Waterborne Symposium, sponsored by the University of So. Mississippi, School of Polymers and High Performance Materials, New Orleans, February, 2007.

The American Solvents Council

Members of the American Solvents Council include: The Dow Chemical Company, ExxonMobil Chemical Company, Eastman Chemical Company, Shell Chemical LP and Sasol North America, Inc.