Fact Sheet, Minnesota Technical Assistance Program, University of Minnesota
Many industries in Minnesota use paints, varnishes, lacquers and other applied coatings for enhancing the appearance, protection or function of product surfaces. Preparing or restoring old surfaces for these coatings often requires paint stripping to insure a good bond between the new coating and surface.
Currently, many paint strippers are using chemical strippers containing methylene chloride (MeCl), a suspect human carcinogen which has been shown to cause cancer in laboratory animals.
New regulations to safeguard against these hazards are requiring businesses to examine their paint stripping operations closely. For example, the Occupational Safety and Health Administration (OSHA) is further limiting exposure to MeCl. The new standard cuts exposure to MeCl from 500 parts-per-million to 25 parts-per-million in an 8-hour period. The new standard requires employers to monitor employee exposure and may require employees to use supplied-air respirators. OSHA believes this regulation will prevent 34 cancer deaths a year.
In Minnesota, rinsewater from MeCl stripping is a listed hazardous waste. This wastewater may only be discharged to a sanitary sewer system after proper notification and approval is received. In the Twin Cities area no wastewater containing methylene chloride may be discharged unless Metropolitan Council Environmental Services (MCES) guidelines are met.Benefits of Waste Reduction and Using Less Hazardous Materials
Reducing waste and finding less hazardous stripping alternatives is smart business. Just look at the benefits:
This factsheet helps evaluate paint stripping methods, hazardous material alternatives and waste reduction options.Assessing Paint Stripping Needs
Some operations require stripping in order to be effective (like stripping overspray off paint hooks and racking assemblies to ensure metal to metal electrical contact). However, unless your operation is in the business of restoring products to bare surfaces, you may be making mistakes which require unnecessary paint stripping. Identify ALL the reasons why paint stripping is needed in your operation and assess if they are necessary. Is paint stripping done because:
- Parts are improperly cleaned or dried prior to painting/coating;
- Poor coating techniqueeither in the application of the coating or in the curing processresults in rejects;
- Faulty equipment is being used or coating equipment maintenance/cleaning is poor;
- Improper handling, assembly, storage or distribution is resulting in product damage?
The need for paint stripping can signal a problem with your process. Ask yourself, "What did we do that makes us backup and start over with these products? Why is paint stripping necessary? How can it be reduced?"
Paint Stripping Alternatives
Stripping techniques vary with the surface being refinished and the complexity of the stripping technique desired. Combining some techniques may work the best. When selecting a stripping method, determine if the part can handle the stress of the application, whetherheat, cold, pressure, abrasion or chemical resistance. Mechanical removal procedures eliminate the use of hazardous chemicals; however, the part may be damaged during the removal process. Keep in mind parts made of wood, plastics and some metals have limitations.
Equipment layout, capital costs, energy costs, operator training, worker safety, reuse/recycling potential and disposal methods and costs should also be considered. Removed paint should be evaluated for its hazardous characteristics before disposal. Paint which contained heavy metals, such as chromium, cadmium, lead and mercury, will often need to be disposed of as hazardous waste.
Scraping, wire brushing and sanding - These methods are good for small areas of any non-detailed surface. They can be labor and time intensive, however.
Tumbling - Parts are put into a mixer and tumbled with stones or another abrasive material. One advantage to tumbling is it's hands-off and can be labor saving. Tumbling is also used for polishing, deburring and metal finishing. One drawback is that the parts must be separated from the abrasive material, which can be time consuming if the parts are small.
Abrasive blasting - Sand, glass or plastic bead, shells, metal shot or grit, sodium bicarbonate or frozen carbon dioxide is used with air pressure or water pressure to remove paint. The blast media type should be evaluated for the appropriate aggressiveness on the coating/part. Using the most durable abrasive will maximize the repeat use of the blasting material and generate the least amount of waste per part stripped.
Freezing/Cryogenic - A liquid nitrogen immersion at approximately -200 degrees Fahrenheit causes paint to crack and break the adhesive bond.
Burn-off ovens/Pyrolysis - Paint is burned off the substrate at high temperatures. This method is effective with some substrates, but time is needed to bring parts up to the correct temperature and to cool them down. Ash residue will also need to be removed from the part.
Molten salt baths - Material is immersed into a molten salt bath at a temperature of 550 to 900 degrees Fahrenheit. This method usually removes paint within several seconds and leaves an ash residue which can be removed by rinsing. Salt stripping is corrosive, potentially damaging to the equipment and building. Air emissions, wastewater and sludges may require environmental controls and/or treatment.
Fluidized sand beds - Heated sand or another granulated material vaporizes the coating and mechanical action gently removes the ash from the part. This is a fast, gentle way to remove paint. The geometry or configuration of the parts within a fluidized bed of sand or salt must be given special consideration to achieve maximum efficiency of the system.
Laser - A laser beam is used to decompose the coating. This procedure works best on flat materials and can be slow.Chemical Stripping Alternatives
Alternative stripping chemicals for paints and coatings are frequently marketed as alternatives to chlorinated solventsmethylene chloride in particular. These alternatives are commonly heated aqueous solutions or unheated organic solvents. MnTAP has put together a reference list titled "Safer Stripping and Cleaning Chemicals for Coatings and Polymers #55" which you can request by calling 612/627-4646 or 800/247-0015. This list provides information on available alternatives, including their suppliers.
In some cases, these alternative chemicals require more time or stronger physical action to remove the paint. The effectiveness of the alternative may vary from paint to paint. Heat may be required to achieve adequate paint removal. The chemicals may not be compatible with the composition of the partsome may attack plastic components just like they attack the paint. Special formula of chemical strippers are versatile enough to remove difficult coatings without damaging the part. Some alternatives are considered volatile organic compounds (VOCs) and they may be flammable or combustible. Potential health hazards and environmental problems associated with using corrosive, chlorinated and flammable materials should be considered.
Aqueous stripping solutions can be acidic, but the most common solutions are alkaline with a pH of 13 to 14. To improve stripping, heat is commonly added to these solutions, hence the name "hot strippers." Aqueous strippers destroy the bonds that hold paint resins together. This reaction with the paint loosens it from the surface. Potassium hydroxide or sodium hydroxide (lye) is often one of the chemicals used in these stripping solutions. Although these strippers are not flammable they can cause severe burns to the skin making protective clothing very important. Metals such as zinc and aluminum will react vigorously in alkaline solutionsdissolving the parts and endangering employees.
Solvent strippers are often used at room temperature and called "cold strippers." They can be applied by immersion, brushing or flowing. Solvent strippers remove paint by dissolving, softening or both. Solvent strippers can include ketones, glycols, esters, phenols, and other hydrocarbons. Aside from being VOCs, many of the solvent strippers contain hazardous air pollutants (HAPs) and chemicals subject to reporting under the EPAs Toxic Release Inventory (TRI) regulations.
Biochemical-based stripping agents are an alternative to petroleum-based chemical strippers. Biochemicals are derived from natural, renewable resources, such as crops, fruits, vegetables and trees. Although they may be considered VOCs, the chemicals are not listed on EPAs TRI list or considered HAPS. Biochemical-based strippers may include:
Businesses that strip paint need to understand how to best conduct their work to maximize their resources and minimize the waste. New kinds of techniques, equipment and chemistries are continually being developed. New regulations, such as lead abatement, also impact stripping businesses.
Stripping coatings in manufacturing requires a close look to evaluate the process needs. Remember: start at the beginning of your process and ask "Why are we removing coatings?" If you work through the manufacturing process, you may find the key to your paint stripping needsor the clue to your paint stripping problems. If you find problems, use your employees, suppliers, vendors, and trade associations to help you achieve the best possible coating resultsthe first time.
For information on the regulations for paint and paint stripping waste disposal, contact the Minnesota Pollution Control Agency at 612/297-8363 or 800/ 657-3724, or your metropolitan county hazardous waste program.
For further information on efficiencies and waste prevention in painting and stripping in manufacturing, contact MnTAP at 612/627-4646 or in greater Minnesota 800/247-0015.
Related resources available from MnTAP include:
Evaluating Paint and Ink Waste (MPCA), #44 (9/97)
Management Options for Old Paint and Paint-related Materials, #69 (1/95)
Stripping and Cleaning Chemicals for Coatings and Polymer, #55 (2/96)
Suppliers of Low-VOC and Waterborne Wood Finishes, #7 (1/97)
Waste Reduction and Cost Saving Ideas for Wood Finishers, #39 (5/98)
Waste Reduction Alternatives for Spray Painting and Coating, #85 (10/97)