Alternative Surface Preparation Methods
There are alternatives to traditional abrasive blast cleaning methods that may reduce surface preparation costs, dust, or fouling of machinery by small abrasive particles. These alternative surface preparation methods, unless otherwise noted, assume an existing surface (anchor) profile on the substrate surface does exist. If a surface profile does not exist, or new material requires a deeper profile, traditional abrasive blast cleaning must be employed. Alternative surface preparation methods include soda and ice blasting,sponge blasting and chemical strippers.
Sponge Abrasive Blasting
Sponge blasting is one of the alternative surface preparation methods that follows the traditional abrasive blast methods, except that the abrasive particles are encapsulated in a sponge material. The sponge material prevents abrasive particles from breaking up and diffusing on impact, thus reducing the amount of dust. Sponge blasting creates about 10 to 20 percent of the dust that would be created by non-capsulated abrasive materials. The method does create a surface profile, and the surface cleanliness can be compared to SSPC-VIS 1.
Soda Bicarbonate Blasting
This method propels large crystals of soda bicarbonate (baking soda) by pressurized air or water. It is used mostly as a stripper for cleaning contaminants and for thin coatings. There is no surface cleanliness standard for these alternative surface preparation methods; however, cleanliness can be specified to meet the requirements of consensus surface preparation.
This method propels ice particles by pressurized air. On impact, the ice exerts a sheer force across the substrate surface, removing contaminants and thin coatings. If dry ice is used, it should not be used in confined spaces because of carbon dioxide build up. There is no surface cleanliness standard for this method; however, cleanliness can be specified to meet the requirements of a consensus surface preparation.
Chemical strippers can be classified into two generic composition types: (1) bond breakers and (2) caustic. Bond breaker strippers work by breaking the paint’s molecular bonds between paint layers and between the paint and the substrate so that paint will crinkle up and be easily removed. Bond breaker strippers can contain toluene, methylene chloride, or methyl ethyl ketone that removes paints in a relatively short time but may be considered hazardous to workers. Less hazardous bond breakers contain N methyl-pryrrolidone (NMP) or dibasic ester (DBE) compounds, but these remove paint less quickly. Bond breaker strippers will remove all coatings except oil-based, inorganic, and metallic coatings. Caustic strippers work by softening the entire paint system rather than breaking molecular bonds and can contain sodium, calcium, and magnesium hydroxide. Caustic strippers are restricted to oil-based paints but will not work on oil-based paints that are pigmented with aluminum flakes. This is because hydrogen gas is generated when caustic compounds come in contact with aluminum, thus preventing the caustic stripper from penetrating the paint system. Chemical strippers are commonly used for small areas where power is not available, abrasive and water jet blasting is not economically feasible, hose distance is too great to achieve necessary air pressure for blasting operations, or where accessibility is limited. They are also used to minimize airborne paint particles for organic paints or heavy-metal based paints (e.g., lead). In general, chemical strippers may be messy, may require repetitive applications to remove all foreign matter from the substrate, and may leave a residue on the substrate that requires solvent cleaning. There is no surface cleanliness standard for these alternative surface preparation methods; however, cleanliness can be specified to the requirements of a consensus surface preparation.
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