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Paint Stripping: Reducing Waste and
Hazardous Materials
Many industries in Minnesota use paints,
varnishes, lacquers and other applied coatings to enhance
function Sand appearance and to protect product surfaces.
Preparing or restoring old surfaces for coatings often
requires stripping existing paint to insure a good bond
between the surface and new coating.
Currently, many paint stripping operations
use chemical strippers that contain methylene chloride
(MeCl)—a suspected human carcinogen which has been
shown to cause cancer in laboratory animals. To safeguard
against these hazards, regulations, such as lead abatement,
require businesses to examine paint stripping operations
closely. For example, the Occupational Safety and Health
Administration (OSHA) limits exposure to MeCl. The standard
cuts exposure to MeCl from the previous limit of 500
parts-per-million to 25 parts-per-million in an 8-hour
period. The standard requires employers to monitor staff
exposure and may require workers to use supplied-air
respirators. OSHA estimates this regulation will prevent
34 cancer deaths a year.
In Minnesota, rinsewater from MeCl
stripping is a listed hazardous waste. This wastewater
can only be discharged to a sanitary sewer system after
proper notification and approval is received. In the
Twin Cities area, no wastewater containing MeCl can
be discharged unless Metropolitan Council Environmental
Services (MCES) guidelines are met. For more information
about MCES, visit the MCES
Web site.
Benefits of Reducing Waste and Hazardous
Materials
Reducing waste and finding alternatives to paint stripping
is smart business. The following list highlights the
benefits of cutting waste.
- Improved working environment, health
and safety for employees
- Reduced regulatory compliance burden
- Savings on materials and disposal
costs
- Marketing advantage of "environmentally
friendly" label
This fact sheet helps businesses evaluate
paint stripping methods and identifies hazardous material
alternatives and waste reduction options.
Assessing Paint
Stripping Needs
In some operations paint stripping is needed, such as
when stripping overspray off paint hooks and racking
assemblies to ensure metal to metal electrical contact.
But, unless your operation is in the business of restoring
products to bare surfaces, you may be requiring unnecessary
paint stripping. Identify why paint stripping is done
in your operation. Is it done because:
- Parts are improperly cleaned or
dried prior to painting/coating;
- Poor coating technique—either
in the application of the coating or in the curing
process—results in rejects;
- Faulty equipment is 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 be a sign of one of these problems in your process. Ask yourself why paint stripping is necessary; and how you can fix the problems that make you have to strip.
Paint Stripping
Alternatives
Stripping techniques vary based on the surface being
refinished and the complexity of the stripping technique
used. Combining techniques may work best. Be sure the
stripping method used is compatible with the part, especially
for parts made from plastic, wood and some metals. Determine
if the part can handle the stress of the techniques
used. Mechanical removal procedures eliminate the use
of hazardous chemicals, but the part can be damaged
during the removal process.
Equipment layout, capital costs, energy costs, operator training, worker safety, reuse and recycling potential, and disposal methods and costs must also be considered. Removed paint should be evaluated for hazardous characteristics prior to disposal. Paint containing heavy metals such as cadmium, chromium, lead and mercury often need to be disposed of as hazardous waste.
Scraping, sanding and wire brushing. These methods are good for small areas of non-detailed
surfaces. They can be labor and time intensive.
Tumbling. Parts are placed in a mixer and tumbled with stones or other abrasive material. Tumbling is hands-off and can save labor. A drawback is that parts must be separated from the abrasive material, which can be time consuming if parts are small. Tumbling is also used for deburring, metal finishing and polishing.
Abrasive blasting. Sand, glass or plastic bead, shell, metal shot or grit, sodium bicarbonate or frozen carbon dioxide is used with air pressure or water pressure to remove paint. Evaluate the type of blast media for the appropriate aggressiveness on the coating and on the part. Use the most durable abrasive to maximize its repeat use 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 extra time is needed to bring parts to the correct temperature and cool them. Ash residue also needs to be removed from the part.
Molten salt baths. Parts are immersed in a molten salt bath at a temperature of 550 to 900 degrees Fahrenheit. This method typically removes paint in several seconds, leaving an ash residue which can be removed by rinsing. Salt stripping is corrosive and can potentially damage equipment and buildings. Air emissions, wastewater and sludges may require additional environmental controls and/or treatment.
Fluidized sand beds. Heated sand or other granulated material vaporizes the coating while the mechanical action gently removes ash from the part. This is a fast, gentle way to remove paint. The configuration of parts within a fluidized bed of sand or salt must be given special consideration to achieve maximum efficiency.
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
solvents—particularly MeCl. These alternatives
are usually heated aqueous solutions or unheated organic
solvents. MnTAP's reference list Safer
Stripping and Cleaning Chemicals for Coatings and Polymers
[#55] provides information about alternatives and includes
a list of suppliers.
In some cases, these alternative chemicals
require more time or stronger physical action to remove
paint. Effectiveness may vary from paint to paint. Heat
may be needed to achieve adequate paint removal. The
chemicals may not be compatible with the composition
of the part—some may attack plastic components
just like they attack paint. Special formulas of chemical
strippers are versatile enough to remove difficult coatings
without damaging the part. Some alternatives are considered
volatile organic compounds (VOCs) and can be flammable
or combustible. Potential health hazards and environmental
problems associated with using corrosive, chlorinated
and flammable materials should be considered.
Aqueous stripping solutions. Aqueous stripping solutions can be acidic, but the most common solutions are alkaline with a pH of 13 to 14. Heat is commonly added to these solutions to improve stripping. Because heat is added these strippers are called hot strippers. Aqueous strippers destroy the bond that holds paint resins together. This reaction loosens paint from the surface. Potassium hydroxide or sodium hydroxide (lye) are often used in these stripping solutions. Although these strippers are not flammable, they can cause severe skin burns making protective clothing very important. Metals such as aluminum and zinc, will react vigorously in alkaline solutions—dissolving parts and endangering employees.
Solvent strippers. 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 U.S. Environmental Protection Agency’s (EPA) Toxic Release Inventory (TRI) regulations.
Biochemical-based stripping agents. Biochemical-based stripping agents are an alternative to petroleum-based chemical strippers. Biochemicals are derived from natural, renewable resources such as fruits, vegetables, trees and other crops. Although they are considered VOCs, the chemicals are not listed on EPA’s TRI list or considered HAPs. Biochemical-based strippers may include:
- terpenes, from pine or citrus
- lactic acids, from corn sugars
- dimethylsulfoxide, from wood pulp
and paper by-products
- soy oil, from soy beans
Additional
Resources
For information about regulations for paint and paint
stripping waste disposal, contact the Minnesota Pollution
Control Agency (MPCA) at 651/297-8363 or 800/657-3724.
Or, contact your Twin Cities county hazardous waste
program.
For a complete list of coating-related
resources request the checklist, Resources
for Minnesota’s Coating Industry [#35]. Resources
can also be found on the Painting and Wood Finishing
page on MnTAP’s Web site. Or, use the search option
to search the site for topics of concern to you and
your business. The following is a sample of the additional
coating resources available from MnTAP.
For More Information
For more information on efficiencies and preventing
painting and stripping waste in manufacturing, contact
Paul Pagel, MnTAP
coatings specialist, at 612.624.4638.
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, and reduce costs.Our information resources are available online. Or,
call MnTAP at 612.624.1300 or 800.247.0015
from greater Minnesota for personal assistance.
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