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Safety Lenses 183

Surface Hardness
There have been many efforts to study the problems associated with surface
abrasion. However, there are distinct advantages in coating plastic lenses,
particularly polycarbonate, which is soft thermoplastic. A thinly coated
polycarbonate lens is superior to an uncoated CR lenses.
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Chemical Resistance

Glasses are more resistant to most chemicals. Plastics, however, may show
crazing and surface clouding with some strong chemical solutions. CR 39
has good chemical resistance, and is frequently used for chemical visors
and box goggle windows.

Thermostability
Polycarbonate and PMMA are prone to distortion more readily than glasses.

Flammability
All the plastic materials are flammable. However, as their ignition
temperatures are high, they are considered safe for use.

Resistance to Hot Particles

Eye protectors must be able to withstand hot particles impinging upon
them, as can occur in such processes as grinding or welding. A glass surface
is very easily pitted by these particles, as they fuse with the surface. Whereas,
plastics do not pit easily. This could be due to the elasticity of the surface
when heated by the particle.

Fig. 12.8: Resistance to hot particles by a glass lens (left) and a CR (right) lens.
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After equal exposure to spatter from an arc welder the glass lens is considerably
more pitted than the CR lenses
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