Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 259 of 331

stripping CHEMICAL IMMERSION PAINT STRIPPING BY RUBIN M. OPEROWSKY CONSULTANT, HASTINGS-ON-HUDSON, N.Y. The two most common methods of chemical paint stripping are hot and cold stripping (see Table I). Hot strippers are normally associated with the use of hot alkali and the stripping of organic coatings from steel, copper, and magnesium; however, this is not the only application of hot strippers. HOT STRIPPERS There are many hot strippers that are not highly alkaline but are of relatively neu- tral pH, which form an emulsion when brought in contact with water or are solubilized and form a clear solution. These hot strippers can strip coatings from aluminum, zinc, and brass without etching or darkening the base metals. The hot alkaline paint strippers are formulated with sodium hydroxide as the pri- mary element acting together with chelating agents, surface activating agents, and any other solvents that can be added to the mixture to enhance the ability to strip without causing the compounds to cake. Originally, the principal solvents used were pine oil, phenol, and phenol de- rivatives; however, environmental regulations affecting both the air and water have caused the banning of phenols and phenolics from the workplace in many states. Most alkaline strippers have been used for coatings that are easily saponified such as alkyds, nitrocellulose, and ureas. Coatings, such as epoxy and the new poly- mers used in the high solids coatings, waterborne coatings, and powders, show a resistance to strippers composed primarily of alkali. The new school of stripping technology embraces the use of hot diphase strip- pers. The lower phase generally consists of concentrated alkaline solution and the upper phase consists of odorless blends of nonphenolic water or alkaline-insol- uble organic compounds. These compounds have certain characteristics such as (1) high boiling points without being able to steam distill, (2) odorless, and (3) a composition of organic compounds such as glycols, polyglycols, ketones, or amines. Another phase of the new alkaline paint stripping technology solubilizes the normally insoluble solvents and strong alkaline solutions to form a clear single- phase product, which strip normally alkaline-insoluble coatings rapidly when used at elevated temperatures. Since one can never be sure of the type of coating be- ing stripped, the organic coating often dissolves or lifts away from the substrate. This depends upon the resin being stripped and solvents being used. In the past, the hot-emulsion-based paint stripper was used on all metals and was generally composed of a blend of emulsifiers, neutral soap, phenolics, high- boiling chlorinated solvents, and solubilizer. These strippers were generally used on coatings that were unstrippable in the hot alkali. Today, chlorinated solvents and phenolics are prohibited from use in many factories. Currently, solvents are being sought that have high boiling properties, low tox- icity, and good ability to strip paint. At the present time the solvents being test- ed are not as efficient as the chlorinated or phenolic solvents. 258

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