General Discussion Abrasive Blasting

Abrasive blasting uses mechanical energy to hurl particles at high speeds against metallic and non-metallic surfaces, removing paints and other organic coatings.

Air Pressure Equipment used for abrasive blasting
The common methods for the abrasive blasting process include air pressure and water pressure. These two methods simply use the force of compressed water or air to expel the media at the surface.

Water Pressure Equipment used for Abrasive blasting
The centrifugal wheel method is a less common technique. With this method centrifugal and inertial forces are used to give the media speed and energy.

Abrasive selection is key to a successful coating job. The surface effects produced with various abrasives can range from deep cutting to gentle scouring of the surface. Important factors to consider in selecting an abrasive include

• Type of surface to be cleaned
• Shape of the structure
• Type of material to be removed
• Profile, breakdown rate of the abrasive
• Hazards associated with the use of the abrasive
• Potential damage to equipment located in the repair area

Abrasives commonly used for stripping include steel grit, aluminum oxide, garnet, and glass beads. Steel grit creates a rough surface profile on the substrate that aids coating adhesion. Because it is so hard and durable, steel grit can be reused, and it generates the least amount of waste per unit of surface area stripped. To maximize the reuse of steel grit, companies must keep the blast media dry to avoid rusting. Aluminum oxides are considered to be a multipurpose material that is less aggressive and less durable than steel grit, and it results in a smoother surface profile and less removal of substrate material. Garnet and glass beads are the least aggressive abrasive and often are used in a single-pass operation (i.e., the abrasive is not recycled). Use of garnet and glass beads is most suitable for preparation of soft materials that are easily damaged, and for maintenance of the dimensional tolerance of the part.

Companies can use abrasive blasting to remove paint and corrosion products from larger metal structures in the field (field stripping) or from smaller metal structures in a hanger, booth, or blasting cabinet.

Outdoor blasting can be performed in an open area. Operators must wear self-contained breathing equipment in order to be protected from the stripping dust. After blasting, the used abrasive can be shoveled or vacuumed from the area and processed through the reclaimer. Some systems combine dust control and abrasive recovery by including a vacuum collection pickup device with the blasting nozzle. Abrasive blasting in cabinets is often performed using manual blast cabinets and automated blasting chambers to remove paint from parts. The abrasive is fed into the cabinet or chamber and directed against the part being stripped. Used abrasive and removed paint are then pneumatically conveyed to a reclaimer. Reusable abrasive is separated from the waste and fines (broken-down abrasives and paint chips) are collected in a dust collector.

Plastic Media Blasting
Plastic media blasting (PMB) is an abrasive blasting process designed to replace chemical paint-stripping operations and conventional sand blasting. This process uses soft, angular plastic particles as the blasting medium. PMB is performed in ventilated enclosures such as small cabinets (a glove box), a walk-in booth, a large room, or airplane hangers. The PMB process blasts the plastic media at a much lower pressure (less than 40 psi) than conventional blasting. PMB is well suited for stripping paints, because the low pressure and relatively soft plastic medium have a minimal effect on the surfaces beneath the paint.

Plastic media are manufactured in 6 types and a variety of sizes and hardness. Military specifications (MIL-P-85891) have been developed for plastic media. The specifications provide general information on the types and characteristics of plastic media. The plastic media types are

• Type I Polyester (Thermoset)
• Type II Urea formaldehyde (Thermoset)
• Type III Melamine formaldehyde (Thermoset)
• Type IV Phenol formaldehyde/Clear Cut (Thermoset)
• Type V Acrylic (Thermoplastic)
• Type VI Polyallyl diglycol carbonate (Thermoset)
  

Facilities typically use a single type of plastic media for all of their PMB work. The majority of DOD PMB facilities use either Type II or Type V media. Type V media is not as hard as Type II media and is gentler on substrates. Type V media is more commonly used on aircraft.

After blasting, the PMB media is passed through a reclamation system that consists of a cyclone centrifuge, a dual adjustable air wash, multiple vibrating classifier screen decks, and a magnetic separator. In addition, some manufacturers provide dense particle separators as a reclamation system. The denser particles, such as paint chips, are separated from the reusable blast media, and the reusable media is returned to the blast pot. Typically, media can be recycled 10 to 12 times before becoming too small to remove paint effectively. Waste material consists of blasting media and paint chips. The waste material may be classified as a RCRA hazardous waste because of the presence of certain metals (primarily lead and chrome from paint pigments). An alternative solution to handling the potential hazardous waste is to recycle the media to recapture the metals. Reusing the plastic blasting media greatly reduces the volume of spent media generated as compared to that generated in sand blasting. When compared to chemical paint stripping, this technology eliminates the generation of waste solvent. PMB is also cheaper and quicker than chemical stripping. The U.S. Air Force and airlines have found PMB effective for field stripping of aircrafts, but PMB could also be used to strip vehicles, ships, and engine parts.

As with any blasting operations, airborne dust is a safety and health concern with PMB. Proper precautions should be taken to ensure that personnel do not inhale dust and particulate matter. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and other paint compounds are known to be carcinogenic. Inhalation of paint solvents can irritate the lungs and mucous membranes. Prolonged exposure can affect respiration and the central nervous system. Operators must wear continuous-flow airline respirators when blasting operations are in progress in accordance with OSHA requirements. PMB systems can range in cost from $7,000 for a small portable unit to $1,400,000 for a major facility for aircraft stripping.

Black Beauty Abrasives
Black beauty abrasives also known as coal slag, black diamond, black blast and boiler slag is an inexpensive media. It is one of the safer forms of abrasive media, containing less than 1% or no silica. Black beauty media also produces little dust, however, may release hazardous air pollutants (HAP) into the surrounding air.

Black Beauty abrasives are made from crushed liquid coal slag from utility boilers. The abrasive contains iron (Fe), Aluminum (Al), Magnesium (Mg) and Calcium (Ca). The media comes in sharp angular grains ranging in many sizes including coarse, medium, fine and extra fine grained. The coarser grains can be used to remove heavy rust and provide a high degree of profile, good for coating attachment and bonding. The finer grains can be used for cleaning surfaces and for smoothing surfaces. The media is used on many applications including steel, buildings, railroads and bridges. Coal slag is a fast-cutting media with a hardness of 6-7 on the Moh's hardness scale. The media is non-recyclable.

Copper Slag Abrasives
Copper slag abrasives are used with water jetting equipment to produce a fast cutting removal of surface contaminants. These abrasives provide a good surface profile when used with coarser grains. The finer grained copper slags will provide removal of lighter rust and mill scale. The material is used applications including steel, offshore oil rigs, power plants and tanks.

Copper slag abrasive material
These abrasives are formed from the smelting process using the by-product, iron silicate. The media comes in cubical form with sharp edges, ranging 6-7 on the Moh's scale. Copper slag produces low dust and contains less than .1% silica.

Sponge Blasting
Sponge blasting systems are a class of abrasive blasting that uses (1) grit-impregnated foam and (2) nonabrasive blasting media using foam without grit. These systems incorporate various grades of water-based urethane-foam cleaning media. Firms use the nonabrasive media grades to clean delicate substrates. The abrasive media grades are used to remove surface contaminants, paints, protective coatings, and rust from a variety of surfaces. In addition, the abrasive grades can be used to roughen concrete and metallic surfaces. A variety of grit types are used in abrasive media including aluminum oxide, steel, plastic, or garnet.

Sponge Media
The foam-cleaning medium is absorptive and can be used either dry or wet with various cleaning agents and surfactants to capture, absorb, and remove a variety of surface contaminants such as oils and greases. The capability of using the foam-cleaning medium in a wet form provides for dust control without excessive dampening of the surface being cleaned. The equipment consists of three transportable modules, which include the feed unit, the classifier unit, and the wash unit. The feed unit is pneumatically powered for propelling the foam-cleaning medium. The unit is portable and produced in several sizes. A hopper, mounted at the top of the unit, holds the foam medium.

Foam-cleaning unit
The medium is fed into a metering chamber that mixes the foam-cleaning medium with compressed airBy varying the feed-unit air pressure and type of cleaning medium used, sponge blasting can remove a range of coatings from soot on wallpaper to high-performance protective coatings on steel and concrete surfaces.

The classifier unit removes large debris and powdery residues from the foam medium after each use. The used medium is collected and placed into an electrically powered sifter. The vibrating sifter classifies the used medium with a stack of progressively finer screens. Coarse contaminants, such as paint flakes and rust particles, are collected on the coarse screens. The reusable foam medium is collected on the corresponding screen size. The dust and finer particles fall through the sifter and are collected for disposal. After classifying, the reclaimed foam medium can be reused immediately in the feed unit. The abrasive medium can be recycled approximately six times and the nonabrasive medium can be recycled approximately 12 times.

This system removes paint, surface coatings, and surface contaminants from a variety of surfaces. Waste streams produced from this system include: coarse contaminants, such as paint flakes and rust particles; dust and finer particles; and the concentrated residue from the bottom of the wash unit. Sponge blasting systems are compatible in most situations where other types of blasting media have been used.

As with any blasting operations, airborne dust is a safety and health concern. The key advantage to sponge blasting is the low/reduced generation of dust. Proper precautions should be taken to ensure that inhalation of dust and particulate matter is avoided. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and some compounds are known to be carcinogenic. Proper personal protective equipment should be used.

NAVSEA Surface Preparation Point of Contact
Mr. Andrew Seelinger
Naval Sea Systems Command, SEA 05P23
1333 Isaac Hull Ave., SE
Washington Navy Yard
Washington, D.C. 20376
andrew.seelinger@navy.mil
(202) 781-3670