Other factors may be relevant to the formulation decision, such as the interaction of the solvent with other materials in the product or the advantages and disadvantages of various solvents for the application. There will always be tradeoffs. For example, a particular solvent may dissolve a resin very effectively, but dry too slowly for the application. Or a certain solvent may release little or no VOCs, as in a low-VOC paint, but painting may need to be done more often because the paint is less durable and washable.

With knowledge of how the solvent - and final formulation - will be used, a formulator can begin thinking about other factors that will affect the selection of solvents for the formulation.

Solvent Selection

Solvent Type

Solvent type is one of the first things to consider when choosing a solvent for a formulation. Solvents can be organic (contain carbon, such as rubbing alcohol) or inorganic (do not contain carbon, such as water). Organic solvents are further classified by chemical structure. Most organic solvents used in the United States are hydrocarbon or oxygenated solvents, and those are the main focus of this article. Chlorinated solvents also are organic solvents, but are not included in this article because of their limited use in these applications.

Since solvents are used to dissolve other substances - and since a basic principle of how solvents work is "like dissolves like" - the solvent of choice will almost always be one that is chemically similar to the material being dissolved. For example, hydrocarbon resins should have good solubility in hydrocarbon solvents.

Health, Safety and Environment (HSE)

Health, Safety and Environment (HSE) issues, including compliance with state and federal regulations, are important considerations to keep in mind before making a formulation decision. Solvents should be handled carefully to minimize risks of fire or explosion, and they should be used in adequately ventilated areas. HSE issues include:

• VOC content;
  
• flammability;
  
• toxicology profile;
  
• threshold limit values;
  
• photochemical reactivity;
  
• biodegradability;
  
• hazardous air pollutant profile; and
  
• odor.

VOC considerations are especially important. For example, with regulatory mass-based VOC limits for coatings, VOC content can be reduced by (1) using VOC-exempt solvent(s), (2) choosing the most efficient solvent or solvent blend that provides other necessary coating properties, or (3) reducing the molecular weight of the resin (this often requires less solvent, but can result in inferior coating performance). Thus, when choosing the right solvent, it is not always possible to find a non-VOC replacement for a VOC solvent. The interactions of all the materials in the final product are complex, and VOC solvents may be needed to achieve the required formulation performance characteristics.

The formation and accumulation of ozone (primary component of smog) is a complex process involving heat, sunlight, NOx (such as from auto exhaust) and VOCs. A VOC's potential contribution to ozone accumulation depends largely on its "photochemical" reactivity, and VOCs vary greatly in that regard. By using products with low photochemical reactivity, a formulator sometimes can meet performance requirements and contribute to improved air quality. Use of VOC solvents with lower photochemical reactivity lowers the "ozone-creation potential" of a formulated product.

Solvent Performance

Solvent performance is one of the key factors in choosing a solvent for a coating. Solvent selection is system-dependent - the solvent must meet both the performance criteria of the product and be suitable for the desired method of application. Solvent performance is characterized by the physical properties of the solvent itself, as well as by the resulting physical properties of the final coating (e.g., dry time). This article reviews formulation considerations for coatings in terms of the key physical properties that impact product performance.

Solvency

Solvency is one of the most important physical properties, since it defines a solvent's ability to dissolve a resin in a coating and decrease the solution viscosity. The key concept is "like dissolves like." Thus, aliphatic solvents will tend to dissolve "hydrocarbon-like" resins, while polar (e.g., oxygenated) solvents may be required to dissolve other resins, such as polyesters, acrylics and polyurethanes. One of the key measures of solvency is coating viscosity. Viscosity is a measure of a fluid's resistance to flow. The most efficient solvent provides the lowest viscosity at a particular total solids level. Or at a particular viscosity, the desired solvent will provide the highest total solids or require the least amount of solvent.

Solubility Parameters

Solubility parameters are used to characterize solvents and predict which solvents will dissolve which resins. They help narrow the choices from the myriad of potential solvents and blends available. Since "like dissolves like," solubility parameters take into account such things as solvent polarity and hydrogen-bonding to determine which solvents are more like the material that needs to be dissolved.

Relative Evaporation Rate

Relative Evaporation Rate (RER) is a measure of how fast a solvent evaporates. Solvents work by dissolving resins or polymers to produce a useable liquid, then evaporating after application to leave a coating. n-Butyl acetate is typically used as a reference, with its RER set at 1 or 100. Faster-evaporating solvents are typically used in air-dry applications, while slower-evaporating solvents are used in baking applications.

Coatings formulations often include several solvents that evaporate at different rates. For example, a typical spray application paint formulation may include fast-, medium- and slow-evaporating solvents. The fast-evaporating solvent provides a lower initial paint viscosity for easier application (good atomization of the spray), while allowing a higher viscosity after application (after the fast evaporating solvent quickly disperses) to help prevent dripping and sagging. The medium-evaporating solvent provides a more controlled release to help prevent film defects arising from too rapid solvent evaporation. And the slow-evaporating solvent, the last to leave the system, finalizes flow and leveling, and thus provides a uniform film thickness, which impacts the final appearance and the final mechanical properties (such as adhesion).

Vapor Pressure/Boiling Point

The vapor pressure and boiling point of a solvent are typically not major considerations for coatings (RER is the determining property). Vapor pressure, the pressure at which a vapor is in equilibrium with its liquid, is directly proportional to RER. Boiling point, the temperature at which a liquid changes to a gas at atmospheric pressure, is inversely proportional to RER.

Flash Point

Flash point is the temperature at which a liquid gives off enough vapor to form an ignitable mixture. It is an important safety consideration that affects the application of the solvent, including selection of equipment and facilities for manufacturing and applying the formulation. Flash point is inversely proportional to RER.

Density

Density is the weight of a solvent divided by its volume, typically measured as lb/gal or g/L. The importance of density is its effect on the VOC content of formulations. Solvents with greater densities contribute to higher VOC content, everything else being equal.

Electrical Resistivity

Electrical resistivity is a critical property for solvents used in coatings applied by electrostatic spray, a process that atomizes and charges paint particles, increasing their attraction to metal parts and decreasing paint usage and emissions.

Viscosity

Viscosity is a measure of a fluid's resistance to flow. However, the solvency of the solvent has a greater impact on the formulation viscosity, since solvents with higher solvency decrease formulation viscosity by dissolving the resin.

Surface Tension

Surface tension is the force per unit length in the surface of a liquid. Low surface tension allows for enhanced substrate wetting, improved sprayability, and reduced number and severity of coating defects.

Conclusion

Solvents are important components in coatings, and selecting the optimal solvent or solvent blend for a formulation is complex. Consideration of system, application, solvent type and HSE often narrows the choices considerably, but solvent performance, based on the physical properties of the solvent and the final formulation, is typically used to make the final selection. Thus, no matter which solvent or solvent blend is chosen in the final formulation, there are trade offs between all these important variables. Formulating requires solvent suppliers to work with downstream users to find the best solvent system for a particular formulation. n

Legal Notice

The information contained in this article is believed to be accurate and reliable as of the date of publication. However, neither the American Chemistry Council nor the American Solvents Council assumes any liability resulting from the use of, or reliance upon, the information provided in this article. All persons involved in handling, storing, or using solvents have an independent obligation to ascertain that their actions are in compliance with current federal, state, and local laws and regulations, and should consult with legal counsel concerning such matters.

American Solvents Council (ASC) of the American Chemistry Council

This article was prepared by the American Solvents Council (ASC) of the American Chemistry Council. The ASC includes producers of oxygenated and hydrocarbon solvents. The ASC addresses health, safety and environmental issues that affect producers, distributors, and users of modern hydrocarbon and oxygenated solvents. The ASC supports scientific research, participates in regulatory activities pertaining to solvents, and works to ensure solvents continue to be recognized as important components of a wide range of products that make our lives safer and healthier while meeting the challenges of today's environmentally conscious world. Members of the ASC are: ExxonMobil Chemical Company, The Dow Chemical Company, Shell Chemical LP, Eastman Chemical Company, Sasol North America Incorporated and CITGO Petroleum Corporation. For more information about this article, or the ASC, please visit www.americansolventscouncil.org.