BGAS Painting Study Material Chapter 2 continuation

Other properties of an abrasive have an effect on the resulting substrate also, these being.

A. Size of the particles

B. Hardness of the material

C. Density of the material

D. Shape of the particle

E. Velocity of abrasive

F. Angle of impingement

G. Time & distance

For example steel has a density of approximately 7.6 gm/cc and copper slag, approximately 4.2 gm/ccIf one particle of each material, of identical size, hit a steel substrate, then it would be logical to say that the steel would impinge further into the substrate, resulting in a deeper trough.

A spherical particle would not impinge as deeply because the large smooth surface area would use its energy up in preening or work hardening the surface rather than cutting into it. So a shot blasted surface is different in appearance and texture to that of grit blasted surface.

Sizing of abrasives

G Prefix = Grit (amorphous, points and cutting edges, irregular profile)

S Prefix = Shot (spherical, smoother profile)

The G or S notation is followed by a number, which denotes the particle size.

G-24 or S-330.  BS 2451 the 24 means nominally 24 thousandths of an inch.

SAE (society automotive engineer) USING THE J 444 SIEVE SYSTEM.it represents ¹/₂₄^" = approximately 40 thou.

 

New BS ref. 7079 pt PARTICLE SIZE DISTRIBUTION Uses a different method again, in metric units. G140 would mean a nominal particle size of 1.4mm

Adhesion and Profile

A commonly used definition of Adhesion is: - The force required separating two surfaces in touch.

A newly rolled plate, perfectly smooth, 1m x 1m has an apparent surface area of 1m² and an actual area of 1m².  Abrasive blasting roughens the surface and increases the actual area, (the apparent area is still 1m²), thus increasing the adhesion.

Two theories of adhesion are:

1. Molecular Interference

Because the surface is rough and uneven the paint wets, and locks into the profile, Analogy Velcro Physical.

2. Molecular Attraction

Negatively charged particles attracted to positive areas, and vice versa Analogy Magnet (sometimes called Ionic Bonding) Chemical.

continue............

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