What Are Abrasives?
Abrasives are hard materials used to shape, clean, or prepare a surface by grinding, blasting, or polishing. The choice of abrasive grit significantly affects the cleaning speed and the level of surface roughness achieved.
In blasting applications, a wide variety of abrasives are used—such as walnut shells, glass beads, slags, metallic shot, and grits. However, only a select few are suitable for surface preparation before painting or coating.
Common Abrasives for Coating Preparation
- Chilled Iron Grit or Shot
- Crushed Slag
- Mineral Grits
- Ceramic Grits
Note: Despite the term sandblasting, sand is no longer recognized as a standard abrasive due to the health hazards of free silica, which can cause silicosis.
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Types of Abrasives
1. Metallic Abrasives
Examples include cast steel, steel grit, steel shot, cast iron, and malleable iron.
- Steel Grit & Shot: Effective for scale removal, surface cleaning, and preparation in recycling systems.
- Cast Iron: Extremely hard, used for tough deposits but not suitable in corrosive conditions.
- Applications: Best for heavy-duty cleaning, etching, and controlled surface profiles.
2. Hard Nonmetallic Abrasives (Oxides)
These include aluminum oxide, garnet, and silicon carbide.
- Garnet: Sharp and durable, suitable for blasting rooms.
- Aluminum Oxide: Long-lasting, precise cutting, but more expensive.
- Silicon Carbide: Very sharp and fast-cutting, often used in grinding.
3. Slag Abrasives
Derived from coal, copper, and nickel refining, slag abrasives are low-cost and efficient but tend to break down quickly, making them less recyclable.
4. Free-Silica Containing Abrasives
- Sand, Quartz, Flint/Chert
Although inexpensive and widely available, these abrasives pose severe health risks due to free silica dust and are banned in many regions.
5. Agricultural Abrasives
Eco-friendly alternatives such as crushed walnut shells are used for delicate machinery, aerospace, and stainless steel applications to avoid contamination.
6. Specialty Abrasives
- Dry Ice & Ice Blasting: Effective for removing organic coatings without altering the substrate.
- Plastic Beads: Used for aerospace coatings and delicate surfaces.
- Baking Soda & Sponge Media: Gentle cleaning with minimal surface alteration.
Abrasive Selection Criteria
When selecting an abrasive, consider:
- Type of surface to be cleaned
- Size and shape of the object
- Location of blasting (indoor/outdoor/chamber)
- Desired surface profile
- Recyclability of abrasive
- Coating requirements
Abrasive Recycling Methods
Recycling reduces cost and waste, especially for expensive abrasives. Common methods include:
- Screen Method: Separates usable abrasive from dust and debris.
- Air-Screen Wash: Uses airflow to separate particles by weight.
- Water Wash Method: Removes fines and dust, suitable for non-ferrous abrasives.
- Cyclone Method: Uses suction and airflow to separate fines from reusable material.
Abrasive Cleanliness Testing
Before use, abrasives should be tested to ensure cleanliness and suitability:
- Vial Test: Detects contamination, oil, or dust.
- pH Testing: Determines acidity or alkalinity.
- Sieve Analysis (ASTM C 136): Measures particle size distribution.
- Hardness Testing: Determines durability (Rockwell or Mohs scale).
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