Growing concern over volatile organic compounds (VOCs) and other emissions is motivating adhesive and coating formulators to capture and recycle solvents, reduce solvent use, or switch to solvents with better environmental profiles. Although environmentally suitable, “drop-in” replacements for conventional solvents are still a distant future vision, and a strong evolutionary path is developing to complete this quest.
Even in “water-based” adhesives and coatings, solvents are sometimes used in conjunction with water to dissolve or suspend components or to provide a more efficient film forming function. For example, many water-reducible coatings contain small amounts of a “co-solvent” such as a glycol ether or alcohol to aid in coating and handling.
This article reviews new solvents that have been developed for the purposes of: (1) formulating and processing adhesives, (2) cleaning equipment, and (3) removing contaminants from substrates prior to adhesive bonding or coating. A leading technology in this pursuit is the development of bio-based or “green” organic solvents.
| Performance Properties | Non-Performance Properties |
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Industrial Solvents Industrial solvents are vital In the adhesives and coatings industries. They are used as component ingredients in formulated products or as processing aids in manufacturing (e.g., cleaning fluids). Different applications require specific solvating or other properties, and different solvents can be often blended to achieve the specific properties required for an application.
Solvents operate on the principle of “like dissolves like.” Therefore, for a solvent to work, it needs to have similar chemical characteristics to the substance that it is trying to dissolve. Other factors that are important in selecting a solvent or solvent blend are listed in Table 1.
Two of the primary applications for solvents are for cleaning and formulating. As one might expect, the evolution of environmentally safe solvents has been faster in the cleaning area than in the formulating area. This is primarily due to the greater technical requirements on the solvent with regard to formulating. But another contributing factor is the shear volume of cleaning solvents that are used. For an article focusing on greener cleaning compounds, the reader is directed to a previous Metal Finishing article,1 which concentrates on the use of newer solvents in adhesive and coating formulations.
Solvent replacement technology is a diverse field that affects many industries. It is a field driven by regulations implemented to protect our environment and health. However, these regulations often translate to economic incentives and have given rise to many exciting and innovative technologies that otherwise might never have developed.
| Property | Ethyl Lactate | D-Limonene | Methyl Soyate |
| Boiling point, °C | 154 | 179 | 333 |
| Flash point, °C | 47 | 51 | 140 |
| Density | 1.03 | 0.844 | 0.882 |
| Solubility in water, % | 100 | insoluble | N/A |
| Surface tension, dynes/cm | 28.9 | 26.7 | 31.6 |
| Dielectric constant | 15.4 | 2.36 | 42.2 |
| Exposure limit, ppm | 4 | 30 | Not recommended |
Minimizing Conventional Solvents One of the most straightforward methods of minimizing solvent in formulations is to assure that the efficiency of the conventional solvent system is optimal. To obtain low viscosity, or alternatively high solids content at a given viscosity, solvents with good solvency power must be employed. The solvent selection for high solids formulations is not simple.
Ketones, such as methyl isobutyl ketone (MIBK) and methyl ethyl ketone (MEK), constitute solvents that combine good solvency and effectiveness in reducing viscosity. Moreover, the relative low density of these solvents permits the most volume possible per unit weight of solvent—an important factor in complying with VOC regulations, generally expressed as grams/liter.
High solids formulations can only be achieved with the proper choice of solvent for the specific resin in question. Fortunately, several tools have been developed to make this task easier, including solubility parameters. These are widely used to construct solubility maps, which can be used to select optimal solvent systems for a specific resin base.
Shell Chemical Company has developed an even more efficient tool. This is a software program called BlendPro. The program uses a database of more than 180 solvents, including water, and contains a database of more than 30 solubility maps of commercial resins. Physical properties of a theoretical formulation can easily be calculated via the program. BlendPro is claimed to calculate which of the blended products best meets the technical requirements at the lowest costs.
There is also an excellent website, http://clean.rti.org (Solvent Alternatives Guide - SAGE), which provides a comprehensive tool for pollution prevention information. This is a computerized expert system that allows evaluation of specific processes and generates a ranked list of alternative solvents. Alternative solvents are presented in the form of case studies, economic and environmental information, references, and so forth. The U.S. EPA Air Pollution Prevention and Control Division developed SAGE for assistance on parts cleaning and degreasing.
| Solvent | Relative Price |
| Conventional solvents: | |
| Methyl ethyl ketone | 1.0 |
| Trichloroethylene | 1.4 |
| Perchloroethylene | 0.8 |
| Methylene chloride | 0.7 |
| N-Methyl pyrrolidone | 3.3-4.0 |
| Bio-based solvents: | |
| Ethyl lactate | 1.9 |
| D-Limonene | >0.9 |
| Methyl Soyate | 0.7-1.0 |
Bio-Based Solvents In recent years a new classification (“bio-based”) can be added to industrial solvents. These green solvents can provide alternatives for conventional solvents when regulatory, environmental, or safety and health pressures are exerted. They are based on agricultural or bio-based feedstock. As such, they not only provide environmental benefit from their use, but they also avoid the more costly petroleum-based route to production. It is hoped that bio-based solvents will avoid the high cost (economic and environmental) and significant price fluctuation associated with petroleum-derived solvents.
Several of the more common bio-based solvents that have recently been commercialized include ethyl lactate, d-limonene, and methyl soyate. The physical properties of these solvents are described in Table 2. All are produced from renewable resources such as corn, citrus, or soybeans. Relative prices for these bio-based solvents and more traditional solvents are shown in Table 3. Ethyl Lactate. A very common cleaning solvent is ethyl lactate. It is an environmentally benign solvent with properties superior to many conventional petroleum-based solvents. Unlike other solvents, which can damage the ozone layer or pollute groundwater, ethyl lactate is so benign that the U.S. Food and Drug Administration long ago approved its use in food products.
Ethyl lactate is produced by the reaction of lactic acid with ethanol. Both reactants are produced from agricultural materials. Lactic acid is produced by fermentation of lactose via specially developed bacteria, and ethanol is produced from corn.
Ethyl lactate can also be blended with methyl soyate (produced from soybean oil as indicated later in this article) to create custom tailored solvents for various applications. Companies such as Vertec Biosolvents currently produce ethyl lactate in soy oil solvent blends. Applications that are targeted include replacement of methlyene chloride, methyl ethyl ketone, and N-methyl pyrrolidone.
Until recently, the use of ethyl lactate has been limited due to high production costs. However, advances in lactic acid fermentation and separation and conversion technologies have driven down cost. It has been suggested by industry experts that ethyl lactate could replace conventional solvents in more than 80% of the applications. D-Limonene. D-limonene is a solvent that is produced from citrus feedstock. The process involves squeezing the rind or peels of fruit (orange or lemon) in a steam extractor to produce oil. When the steam is condensed, a significant layer of oil (d-limonene) floats to the surface of the water and can be collected. In the past decade, the use of d-limonene has expanded tremendously. The largest growth segment has been the use of d-limonene in cleaning products. Much of the product goes into making paint, imparting orange fragrance, and as a secondary cooling fluid.
As a straight solvent, d-limonene can replace a wide variety of products, including mineral spirits, methyl ethyl ketone, acetone, toluene, glycol ethers, and halogenated solvents. D-limonene has solubility close to that of chlorinated fluorocarbons, indicating that it is a much better solvent than typical mineral spirits. Soy-Based Solvents. As the name implies, soy-based solvents are produced from the oil in soybeans. The most commercial of these is methyl soyate. Methyl soyate has good solvency with many resins and contaminants and is readily biodegradable. It has low toxicity relative to many conventional solvents. Its high flash point and high boiling point also make it safer to store and handle than most commercial solvents.
As with ethyl lactate and d-limonene, the best market opportunities for soy-based solvents involve solvent cleaning applications. This includes equipment cleaning as well as the removal and cleanup of resins. The use of soy-based solvents in printing inks is well known and fully commercial. Potential also exists in adhesives and coatings, but further product and process development work is necessary to ensure commercialization. The best market and application opportunities for methyl soyate appear to include: co-solvents, resin removal, asphalt release agent emulsions, and adhesive/coating additives.
Numerous formulated consumer products using methyl soyate are already being produced and marketed. These range from hand cleaners to auto care products as a replacement for mineral spirits and other solvents. Additional market opportunities are expanding utilizing methyl soyate-based co-solvents with ethyl lactate (Vertec Gold), D-limonene (CITRUSoy), and other organic solvents. Others. The bio-based solvents described previously are commercially available. There are also many products under development that use proprietary technology to manufacture solvents that mimic petroleum-derived products. One example of this activity is Gevo’s renewable isobutanol, which will begin commercial production in 2012. This product is claimed to result in a fully renewable and cost-competitive isobutanol product that is chemically identical to petroleum-derived isobutanol, making it a sustainable “drop-in” replacement for all n-butanol and other solvents.3
| Attribute | Chlorinated | Hydro-carbon | Methyl Ethyl Ketone | Ethyl Lactate | D-Limonene | Methyl Soyate |
|
Good solvency |
Y | N | Y | Y | Y | Y |
| Low VOC | Y | N | N | Y | Y | Y |
| Non HAP | N | N | Y | Y | Y | Y |
| Nonflammable | Y | N | N | N | N | Y |
| Low toxicity | N | N | Y | Y | Y | N |
| Fast evaporation | Y | Y | Y | Y | Y | N |
| No surface residue | Y | Y | Y | Y | Y | Y |
| Biodegradable | N | N | N | Y | Y | Y |
| Low odor | N | N | N | Y | N | Y |
| Material compatibility | Y | Y | N | Y | Y | Y |
| Competitive cost | Y | Y | Y | Y | N | Y |
Summary Across all market applications, bio-based solvents are increasingly becoming important as regulatory pressures mount. However, performance and cost factors must be the first priority to the formulator when seeking to replace conventional solvents that have long been an integral component in successful commercial products.
Table 4 lists solvent properties and performance requirements that are nearly widespread across most sub-segments of the adhesives and sealants market. Conventional solvents are compared to the evolving “green” solvents, and the possibility of a green solvent future can readily be seen. BIO Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary, N.C.–based consulting firm focused on solving problems in the adhesives and sealants industry. He also works as a technical expert for SpecialChem. For more information, visit www.specialchem4adhesives.com. REFERENCES
- Durkee, J., “Cleaners from the Farm”, Cleaning Times Column, Metal Finishing, January 2008.
- BlendPro, Shell Chemical Company, www.shell.com, accessed June 2011.
- “Renewable Isobutanol for Solvents and Coatings”, SpecialChem Renewable Isobutanol Center, www.specialchem4coatings.com, accessed June 2011.
- Wildes, S.G., Solvents:A Market Opportunity Study, Report prepared by OmniTech International, Ltd. for the United Soybean Board, December 2007.
