Solvent Usage in Modern Coatings
Formulators are constantly developing coatings based on new and improved polymers. These developments place a greater emphasis on using suitable solvents and solvent combinations. New federal, state and local environmental regulations, however, are making this task more difficult.
No single solvent is capable of dissolving all of the film-forming ingredients used in modern coatings. The ideal reducers and thinners for coating materials will usually consist of blends of various solvents, each capable of dissolving one or more ingredients in a given coating formula.
The question of the thinners and solvents used may not seem too important to some users, but it has a profound effect on the behavior of the coating and the environmental compliance of the product. Selecting and using the proper reducing agent is often the difference between a satisfactory and an unsatisfactory finish.
Thinners and reducersA thinner, or reducer, is a volatile material added to a paint, enamel, lacquer or varnish, usually immediately before application, to give proper body consistency or viscosity for application. It serves merely as a carrying agent, which permits the film-forming components to be applied uniformly. The thinners used in the coatings can affect durability, appearance, gloss, flexibility, adhesion and other properties.
There are four major properties of all solvents that must be considered in the formulation of a liquid coating and in the later use for thinning purposes: flash point, evaporation rate, solvent strength and conductivity. These properties have a major effect on the performance of the coating.
The current federal EPA regulations exempt the following solvents from federal VOC regulations:
- Carbon dioxide
- Methylene chloride
- Oxsol 100
- Trichloroethane and other chromated/fluorinated ethanes and methanes
They will be treated just like water on the Health, Environmental and Regulatory Services (HEARS) program and will be reported as VOC-free exempt solvents.
Some states may not exempt these solvents in their state regulations. For example, Acetone R6K9 is an extremely volatile ketone with a flash point of 1°F. It represents a significant safety hazard because of its volatility and flash point. Products with flashpoints under 21°F have much stricter storage and packaging requirements.
Oxsol 100 is a para chlorobenzene trifluoride (PCBTF) manufactured by Occidental Chemical. It has an evaporation rate somewhat similar to xylene, a flash point of 109°F and is claimed to have solubility with a wide range of resin chemistries. It weighs 11.2 pounds per gallon and is very expensive-much more than the coating itself.
Exempt solvents can be useful to formulators and users to provide solvency without adding VOCs.
Evaporation rateThe evaporation rate of an organic solvent is important for several reasons.
- Flow. A slower evaporating solvent permits the coating to remain in a fluid or wet state longer, thus increasing the leveling of the coating.
- Gloss. The better the flow or leveling, the higher the gloss.
- Blush resistance. A fast-evaporating solvent lowers the surface temperature of the coating very rapidly, which causes condensation of moisture on the wet coating. The moisture can combine with certain film formers, resulting in a precipitation of these film formers that can cause a milky appearance or loss of gloss.
- Sag resistance. A slow-evaporating solvent will cause the coating to remain fluid, which could result in sagging of the coating.
- Bloom, or haze, resistance. This is somewhat similar to blush resistance and results from insufficient solvency to keep the film formers in solution until all the nonsolvents leave the wet film.
These are only some of the factors that can be affected by the evaporation rate of the solvents used in formulation. It is important that a solvent, or solvents, with the correct evaporation rate be selected for each type of coating and application method employed. Weather can also affect the evaporation rate and the behavior of the coating. Humidity and temperature will definitely affect the evaporation rate of a solvent.
Electrostatic spray application is commonly used to provide higher transfer efficiency of the coating onto the part. Some solvents are called polar solvents because they have low ohmic resistance and can conduct electricity. This property enables the formulator of coatings and blended reducers to balance polar and nonpolar solvents so that the conductivity is within a range permitting electrostatic spray application.
Electrostatic spray permits wrap around and greatly reduces application losses compared to other spray techniques. Some solvents are very good conductors (polar) and others are very poor conductors (nonpolar).
Solvents and regulationsThe pollution of air from combustion and industrial activities has created a need to control emissions of solvents. Because of their molecular structure, certain solvents are highly reactive and contribute to smog and related problems, including eye irritation, irritation of the respiratory tract, vegetation damage and reduced visibility.
Legislation has been created to reduce the impact of solvents on the atmosphere. The West Coast, with Los Angeles (Rule 66k) as the forerunner, started the trend of enacting state and local air-quality regulations. It has spread across the country and exists in every major metropolitan area. Originally, organic solvents were classified as photochemically reactive or nonphotochemically reactive.
Current EPA regulations address air quality by defining allowable emission levels for various industries and finishing operations. In current EPA regulations, VOCs are measured in pounds per gallon of coating and tons of emissions in a unit of time. In addition, hazardous air pollutants (HAPs) are listed solvents that face additional restrictions.
The EPA has established National Emission Standards for Hazardous Air Pollutants (NESHAPs) as part of the Clean Air Act. There are many volatile HAPs listed in the regulation; the major solvents impacting the coatings industry are listed in Table 1.
Though wood finishing operations have been the only industrial sector affected thus far, NESHAPs are likely to affect all market areas in the near future. In the miscellaneous metals category, HAPs regulations are anticipated over the next few years. Like wood finishers, metal and plastic finishers are also affected by Title V, which requires operating permits for all major sources.
While the initial rules are developed at the federal level, the final regulations are created at the state and local level. Contact your state air pollution agency to determine the compliance schedule for your area.
The HAPS regulation for wood finishing is applicable to all major sources, which are wood finishers with a potential to emit 10 tons per year of a single HAP or 25 tons of combined HAPS, based on 24 hours per day, 365 days per year production. For major-source wood finishers, the regulation requires less than 1.0 pound of HAPs emission per pound of solids for existing sources. For new sources, the requirement is less than 0.8 pound HAPs per pound of solids.
There are several programs to reduce air pollution that impact solvent users. The federal Clean Air Act Amendments (CAAA) of 1990 was the most far-reaching of these programs. Solvent users are also often subject to the Emergency Planning and Community Right-to-Know Act (EPCRA).
Understanding these programs and their effects on commonly used solvents is very important to the coatings formulators and users. Understanding the federal perspective on regulations is helpful, but many solvent users are also affected by additional state or local air-quality requirements. It is important to communicate with the appropriate state agencies.
The Clean Air ActThe Clean Air Act creates dozens of regulatory programs to deal with a variety of environmental concerns, including acid rain, hazardous air pollutants, visibility in national parks, depletion of the stratospheric ozone layer and formation of ground-level ozone. Programs that are concerned with ground-level ozone and HAPs can have a significant impact on solvent users.
Ground-level ozone is formed by the photochemical reaction of VOCs and nitrogen oxides (NOx) in the atmosphere. It is the main component of urban smog. In order to reduce ozone levels, the CAA regulates manmade emissions of VOCs and NOx. VOC emissions can come from "biogenic" sources such as trees and vegetation, vehicle emissions, petroleum refining and combustion. The critical source that affects coaters is emission from organic solvents used in paint formulation.
Many areas of the country do not meet the standards set by the CAA for ground-level ozone. These "nonattainment" areas are generally required to reduce VOC emissions by 3% 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 are regulated.
The EPA recently issued a more stringent ozone standard that will take effect in the next few years. To meet this new standard, existing nonattainment areas will likely need to make more far-reaching VOC reductions. In addition, many areas that meet the current standard may not be able to meet the new standard.
For industries that use solvents in coatings, the regulation limits the amount of VOC content per gallon. One significant new development in the regulations of VOCs involves the use of "relative reactivity" to rate the potential of each particular chemical to contribute to the formation of ground-level ozone. Relative reactivity recognizes that chemicals have different levels of photochemical reactivity and that the potential to contribute to ozone formation varies from one solvent to another.
In recognizing this difference, the California Air Resources Board has developed a compliance option for aerosol coating products that allows them to contain a higher total volume of VOCs if the overall photochemical reactivity of the product does not exceed a specified level. An organization of state and local air administrators has offered a generic version of the California rule to reduce VOCs from paints and architectural coatings. EPA is evaluating whether a similar approach should be adopted at the federal level.
- Currently, HAPs regulations apply only to wood furniture manufacturing operations.
- The compliance date for the largest users (more than 50 tons per year emissions based on 1996 emissions) was November 21, 1997. The compliance date for users emitting 10 to 50 tons per year was December 7, 1998.
- The term is HAPs compliant, not HAPs free.
- The regulations do not require HAPs free.
- The regulations also restrict manual, conventional-spray equipment.
A component of the regulation governing HAPs, the National Emission Standards for Hazardous Air Pollutants, requires that reducing solvents (reducers) contain less than 10% HAPs. Many lacquer thinners, POLANE reducers and blended solvents are not HAPs-compliant because xylene, toluene, MEK and MIBK are typically used in these types of solvents.
Nearly all waterborne coatings contain some HAPs, such as Butyl Cellosolve, Butyl Carbitol, other glycol ethers or amine solubilizers, but these coatings generally are still HAPs-compliant.
In most cases, HAPs-compliant counterparts, either as a coating or a reducer, are more expensive than the listed HAPs that they replace. Xylene, toluene, MEK, MIBK and Methanol are much less costly than their replacements.
Many companies are not be affected by HAPs regulations because they are small and emit less than the threshold level. An exception exists for small companies that use no more than 250 gallons per month of coating, gluing and cleaning, wash-off, diluent and reducing solvents
Regulation of HAPsAs noted previously, in addition to the regulation of VOCs, some common solvents are also regulated as HAPs. There are several important differences between VOC regulations and HAP regulations:
- Most regulations for VOCs are in place or will be soon. HAP regulations that affect solvent use will not be in place for a few years. They are expected to be phased in between now and 2006. Newly constructed facilities (or major changes to existing facilities) will be subject to HAP regulations at the time of construction.
- HAP regulations apply to facilities that have the "potential to emit" 10 tons or more per year of any one HAP or 25 tons or more per year of all HAPs combined. VOC regulations apply to facilities in nonattainment areas or newly constructed facilities.
- Most HAP regulations are set by the EPA. Most VOC regulations are set by state or local regulatory agencies based on federal guidelines.
- Several HAP regulations are focused specifically on solvents. Rules are being prepared for coatings in different industrial groups, including metal parts, wood furniture, automobiles and plastic parts.
Compliance optionsOptions for compliance depend in part on the area and the industry involved. There are three basic approaches to compliance:
- Reformulate the current paint to a lower VOC product.
- Switch to an alternative technology that has fewer VOCs.
- Install control or recovery technology.
In many cases, companies are able to switch to a waterborne or high-solids formula with lower VOCs than the current technology. However, these products still contain some VOCs and HAPs. Other companies are able to convert to powder coating or radiant-cure products to virtually eliminate emissions. In many other cases, neither of these options is practical.
Reformulation may be a good option for many companies, especially if they are located in an attainment area. Unless they make major changes to their system, they will be more affected by HAPs than they will be by VOC limitations. This gives them flexibility to reformulate using solvents that are not listed as HAPs. A careful analysis of the advantages of this strategy should be conducted.
Advantages of reformulation:
- It is often the lowest capital cost approach to compliance.
- It usually does not require major operational changes.
- It may also help reduce emissions that are governed by the Emergency Planning and Community Right-to-Know Act (EPCRA).
- It may reduce HAPs below the threshold levels at which such regulations apply. Disadvantages of reformulation:
- The reformulated product may cost more or compromise product quality.
- The compliance may turn out to be temporary as solvents are continually added to the list of HAPs.
- Safe handling and long-term safety on the alternatives used in the new formula may be difficult to obtain or limited in scope.
Reformulation will be more difficult if a company needs to reduce VOCs as well as HAPs. Higher solids formulas may not be a good fit for the system or product, and there are cases where reformulations for reduction of HAPs can actually increase VOCs.
Converting to products that are very low in VOCs or are 100% solids, which is the case with powder coatings and UV-cure products, certainly helps with compliance. For some companies, this is easily the best compliance option, but it may not fit every product. As with reformulation, it requires an analysis of the pros and cons.
Advantages of alternative technologies:
- They provide substantial reduction of VOCs and HAPs.
- >They reduce record keeping and emission-related paperwork.
- They may provide long-term compliance.
- The reduction of hazardous waste can provide a substantial cost savings.
Disadvantages of alternative technologies:
- Initial investment in capital equipment for the change can be high.
- Cure technologies may have limitations due to part size, shape or substrate material.
- Some liquid nonsolvent formulations have materials that are more toxic than the solvents that they replace.
Some larger users of solvents in coating may choose to install control technology such as absorption systems, incineration or solvent recovery to control VOCs and HAPs. Like other compliance strategies, control technology has advantages and disadvantages.
Advantages of control technology:
- Allows the continued use of proven solvents and existing application equipment.
- Avoids threshold limits on discharge volumes.
Disadvantages of control technology:
- Very expensive capital cost for installation.
- High operating cost with no value added to the customer.
- Requires regulatory approval.
- May be difficult to install equipment now with the HAPs rules still unfolding. For example, some incinerators generate levels of NOx that may someday be unacceptable.
Solvent users need to understand the changing rules and what their compliance options are. Noncompliance may lead to serious consequences for a business. Seek advice from government agencies and coatings suppliers to be sure that your operation is safe and meets current standards. Before making any changes, carefully consider the options and work towards long-term solutions that will be HAPs compliant.