Plastic ophthalmic lenses are now being used for safety reason. Plenty of plastic lenses (e.g. CR-39) are now available in Indian market and abroad. Among the various plastic lenses available in India, CR39 ophthalmic lenses are dominating the market presently. This CR-39 plastic is optically transparent and can withstand high impact than glass. However this is not scratch resistant enough and as a result, its optical property is deteriorated quickly due to surface damage. 

The available global technologies in this particular area are mainly based on CVD, PVD, plasma CVD etc. These methods are not only expensive but also the coatings are not durable enough. The inorganic-organic hybrid sol-gel method seems to be very appropriate to fabricate hard and scratch resistant coatings on plastic substrates. Present published literature shows that some sol-gel hybrid materials exhibit abrasion resistance in addition to optical clarity. However, hitherto known prior art processes based on inorganic-organic hybrids have many drawbacks e.g. high curing temperature not very suitable for plastics, the inorganic and organic polymerization reactions are not well controlled and as a result of this shelf life of sol is less, use of silica or alumina nanoparticles in the hybrid sol from outside could not produce high hardness, UV curing, which causes yellowing effects etc. 

A process has been developed for deposition of ‘Anti Scratch Coating’ (ASC) on CR39 grade plastic ophthalmic lenses based on sol-gel processing. This is a thermally curable coating and no yellow coloration is observed after curing of the  coating. This optically transparent coating is found to be very suitable for the surface of CR39 grade ophthalmic lenses and showed the hardness of the order of 6H after curing at 90-100^oC.  All essential tests (ASTM and DIN standard) e.g. adhesion, abrasion, chemical and thermal tests are also passed which are required for practical application of the coatings. The high hardness of the coatings is due to the generation of dense oxide nanoparticles in situ in the sol which remain covalently bonded with the hybrid network. This anti-scratch coating can also found to be suitable for the previously tinted Cr39 lenses. 

Market Demand :

Plastic lenses demand in India is about three million pairs of lenses per annum. This is about 6% of the total market size of 50 million pairs per annum (data 2002-03). The market share of plastic lenses is growing at about 20% per year. Already, in the metro cities, plastic lenses have nearly 25% share of the market. The trend is expected to grow, as in the more economically advanced countries, plastic lenses have occupied about 90% market shares. Worldwide lens sales quantities are not known, but it works out to as business of the order of a few Billion US dollars per year. Transport hard coatings improve the surface and scratch resistance property of plastic lenses and therefore, it has a potential demand in the market.

Process:

This process involves dip-coating of ophthalmic lenses in to a sol of controlled chemical composition, drying the deposited layer at 60^oC followed by curing at 90^oC. The sol can also be used for spin-coating purposes. 

Product:

Anti-scratch Coatings on CR-39 grade ophthalmic lenses

Plant Parameters:

           Capacity                                  :            1,20,000 pairs of lenses per year

           No. of shifts/day                       :            One

           Working days/annum                  :            300

           Cost of coating per pair of lens: Rs.10/-

Manpower                                   :            5 Nos. 

Land                                          :            2000 sq.ft.

Covered area (Building & Shed)       :            1000 sq.ft. 

NRDC Terms & Conditions for Release – Negotiable.