Stripping Powder Coatings

Before and After photos of paint stripper in action.

2. Correlation between acid concentration and titration points.

Before and After photos of paint stripper in action.

1. Performance of stripper vs. acid concentration

The need for an environmentally safe, user friendly paint stripper that works as well as cold solvent strippers containing methylene chloride has been a pressing issue for finishers. Since 1997, OSHA and EPA have placed serious restrictions on the use of methylene chloride.

A goal in developing a safe alternative stripper was to meet regulatory restrictions, improve performance and meet customer needs for a user-friendly product.

The alternative stripper is a single-phase stripping technology that combines an organic solvent and a moderately strong acid in a homogenous solution. Bath control involves titration to determine acid points. Replenishment is necessary if titration points fall below a certain number, in which case a replenisher is added to the bath to bring the titration back to working parameters. Water, a replenisher or more of the initial make-up product is added to make up volume lost during processing when the titration is incorrect.

The alternative metal stripper was compared to similar low-temperature alkaline strippers on steel substrates. For comparison to stripping powder coatings from aluminum, methylene chloride and hot alkaline strippers were used.

Several Q panels of different substrates were powder coated with four types of powder at three dry-film thicknesses. The panels were pretreated with a molybdate-accelerated iron phosphate before painting. Black polyester, black epoxy-polyester hybrid, black polyurethane and black epoxy powder coatings were used. All powder coatings were cured according to manufacturer specifications.

The time to 100% strip 1, 2 and 3 mils of powder at 140 and 160F was recorded. Results showed that the alternative stripper was comparable, if not better, in performance to the hot alkaline stripper, removing epoxy and hybrid coatings faster.

The alternative stripper was compared to a methylene-chloride-based stripper and a hot alkaline stripper on removing powder coating from aluminum panels. The same powder coatings were used at a dry-film thickness of 3 mils. The hot alkaline stripper was used at 70% by weight with mild agitation at 140 and 160F. The methylene-chloride-based stripper was used at room temperature. Results indicated that the alternative stripper performed better at removing epoxy powder coatings. It performed similarly to the stripper tested on hybrid and polyurethane paints and stripped polyester paint at a slower rate.

One component of the alternative stripper is an acid that is low in corrosivity, odor, toxicity and is biodegradable. This dual chemistry functionality places both alcohol and a moderately strong acid functional group on the same molecule. The acid component contributes to the unusual solvency properties. The performance of the product was examined by varying the percent by weight of the acid. The test was conducted on aluminum coupons coated with approximately 3 mils of epoxy paint. The stripper was used at 100% with slight agitation at a 160F.

The alternative stripper effectively stripped tenacious coatings from virtually all substrates, including aluminum, at lower temperatures and faster when compared to traditional stripping systems. This contributes to energy and water conservation. It is user friendly, strips in sheets, is easy to control and offers minimal safety, environmental and health risks compared to methylene chloride and hot alkaline strippers. On-going field tests have verified that, due to its stripping mechanism, removing stripped paint requires just a mesh screen most of the time.

TABLE I—Features and Benefits
FEATURE:	BENEFITS:
Operating temperatures of 100-180F	Lower energy costs
No reportable SARA 313 ingredients	Cost savings related to handling and disposal
Inhibition, resulting in safe use on virtually all metals, including most aluminums	One product is used for multi-metal processing
Inhibition, resulting in safe use on virtually all metals, including most aluminums	One product is used for multi-metal processing
Most paint sheets off in entire layers, similar to stripping mechanism of methylene chloride	Allows recycling, lower waste volume and longer bath life
Easy to use and control	Lower processing costs

TABLE II—Time in Minutes to 100% Strip One Mil of Powder Coating
Hot Alkaline Stripper			New Stripper
Paint Epoxy Hybrid Polyester Polyurethane	140F 5 30 45 20	160F 2 20 30 14	140F 2 12 50 18	160F 45 seconds 10 30 12

TABLE III—Time in Minutes to 100% Strip Three Mils of Powder Coating
Stripper Hot Alkaline @ 140F Hot Alkaline @ 160F Methylene Chloride @ RT New Stripper @ 140F New Stripper @ 160F	Epoxy 15 6 240 10 3	Hybrid 10 5 60 12 8	Polyester 15 6 90 25 15	Polyurethane 5 3 180 5 3

TABLE IV—Performance of Stripper Based on Acid Concentration
Percent Acid 5.0 6.0 7.5 9.0 10.0 12.5 15.0 17.5 20.0	Stripping Time (In Minutes) 3 3 3 2 2 2 2 6 9

Methylene Cloride

Methylene chloride has been a popular paint stripper because of its aggressive solvency, and the fact that it does not contribute significantly to atmospheric pollution through the formation of smog. Its greatest use is in paint stripping followed by adhesives and aerosols.

Although it is popular, it is painful and irritating if splashed into the eyes or held in contact with the skin. Overexposure to the vapor can cause central nervous system depression and irritation to the skin, eyes, mucous membranes and respiratory tract. However, these effects are not observed below exposures of 200 ppm.

The International Agency for Research on Cancer considers methylene chloride a "possible carcinogenic to humans," and OSHA considers it a potential human carcinogen. Methylene chloride is carcinogenic in laboratory animals at high doses, but these doses are not comparable to those workers are exposed to. Available documentation indicates that it is not likely to cause cancer in humans except under unusual methods or levels of exposure.

A white paper published by the Halogenated Solvents Industry Alliance, Inc., states that "on the basis of extensive evidence, it can be concluded that methylene chloride does not pose a significant health risk to humans under normal conditions of occupational exposure and when products are used in accordance with the manufacturer's instructions."

The white paper reviews studies on genotoxicity, reproductive toxicity, neurotoxicity, carcinogenicity and epidemiology, including general health effects related to methylene chloride. The paper can be accessed at www.hsia.org.

To learn more visit Chemetall US, Inc.