With the popularity of thermosetting plastics such as CR39 and the softer thermoplastic materials like polycarbonate, abrasion resistance has become an important issue when considering the durability and longevity of a spectacle lens. Compared to mineral glasses, plastic lenses are far more susceptible to surface damage and degradation. This is clearly evident when we consider Table 1. The Bayer abrasion test is one of the most widely cited test methods for abrasion resistance, subjecting coated and uncoated lenses to a cycle of abrasive tests using oscillating sand or alumina zirconia. The difference between plastics materials and mineral glasses is quite startling and provides very good justification for the recommendation of a protective coating for most plastics materials. While it may be preferable to coat all spectacle lenses, there are some materials that provide an acceptable level of abrasion resistance without being coated, such as Crown glass, which is generally accepted as a bench mark for abrasion resistance. The hard coating of CR39 lenses is also generally considered as optional when no reflection-free (RF) coating is specified. For all other materials, the provision of either a hardcoat or an RF coating and hardcoat is essential if a reasonable level of surface durability is to be expected. A hard coating should exhibit some or all of the following features:
Increased scratch resistance to help withstand the damage caused when a sharp object comes into contact with the lens surface
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