Abrasives General Information Abrasive cleaning, as it is used in SAGE, refers primarily to blasting operations, such as sand-blasting. This process uses a media propelled by compressed gases or a liquid to impinge on a surface. Blasting media can vary widely and include the following: Metal needles or shot "Glass" particles Ground walnut or peanut shells Ground corn cob Sodium bicarbonate Wheat starch CO2 pellets Ice Plastic particles (7 types are described in MIL-P-85891). Abrasive cleaning is usually preferred for removal of heavy scale and paint on large surfaces. Metals subject to hydrogen embrittlement are frequently cleaned using a blasting process. The corrosion resistance of stainless steels, and other substrates, can be affected if a corrosion-prone material is used as the abrasive media. Abrasive cleaning is primarily a surface material removal process. It is not well suited to removal of fluid residues. Abrasive cleaning will affect surface finish. It may also cause dimensional changes in thin parts. Some materials may be subject to work hardening of the surface. Wet blasting is most commonly used for more delicate or finer operations than air blasting. For example, wet blasting is most commonly used for removing: Minute burrs on precision parts Fine tool marks Light mill scale Surface oxide (electronics) Welding scale. Wet blasting is not recommended for cleaning heat-resistant alloys with a high titanium or aluminum content. Plastic abrasives can be used to remove primers and topcoats from substrates such as honeycomb panels, fiberglass, composites, and plastics without damaging them. The use of wheat starch has attracted attention because it increases in efficiency as it is used, is inexpensive, and it is biodegradable. Successful stripping requires multiple passes rather than a continued attack on one area. Continuous blasting in one area may cause overheating and damage the substrate. Factors affecting the viability of abrasive cleaning for any given application are: Blasting pressure Media flow rate Blasting standoff distance Type of coating being removed Substrate materials and thickness Selection of blast media type and size Determination of blast nozzle size Masking requirements Angle of blast nozzle Plastic particles may be pneumatically applied at low pressures of 20 to 40 psi. The plastic particles vary in hardness from 3.0 to 4.0 Mohs compared with hard abrasives in the 7-Mohs range. The plastic should be softer than the substrate but harder than the coating to be removed so that the substrate will not be damaged. Plastic particles are good for sensitive substrates such as aluminum, brass, copper, magnesium, thin steel, titanium, and composites including fiberglass and for surfaces where chemical strippers can't be used. They have been used for more than 6 years principally for the stripping of aircraft and aerospace components. If you select the appropriate media hardness, plastic particles can remove surface coatings while leaving harder undercoatings intact. Most abrasives are not sensitive to changes in temperature and humidity. Use of baking soda as the abrasive usually requires copious amounts of water for disposal to a sewer. Baking soda is not recyclable as a blasting media. WHEAT STARCH BLASTING New media is less aggressive than used media. However, new media gives the operator more control than used media when stripping Kevlar. Wheat starch can strip a wide range of coatings from graphite, fiberglass, and Kevlar without risk of substrate damage. Large media works better on elastomeric coatings, while small media works better on more brittle coatings (i.e. epoxy primers and topcoats). The media works best by breaking through the paint surface and peeling the paint up from the edges of the stripped area. Some of the harder coatings will break the media down more quickly than others. Wheat starch blasting uses 100 percent crystallized wheat starch. This process is used as a replacement media for sand blasting type operations. Wheat starch blasting is used for paint stripping on aircraft. This process does not damage metal or composite material substrates. Wheat starch media does not remove alodine. The alodine that remains after depainting passes 7-day salt-spray exposure, but it is dehydrated so paint adhesion suffers. This can be remedied by a quick dip in an alodine tank. State Information | Glossary | Conversion Checklist | Comments Home | Process Advisor | Alternatives | Links | Search All SAGE material, Copyright© 1992, Research Triangle Institute Last Update: 10 March 1995 sage@rti.org http://clean.rti.org/alt.cfm?id=ab&cat=gi