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Technical Info

Optical Materials Yellowness reduction Casting Yield Mould Adhesion Coatings IPP/ADC

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Yellowness reduction

Optical materials should be water white to enhance their appearance. By far most lenses worldwide are sold in colourless form.

Coloured lenses, can be obtained by tinting the lenses to the desired colour. This is accomplished by adding dyes to the optical monomer, like with RAV 7MC, or by dip dyeing the lens in a dye solution (all other monomers). The latter technique, also provide for gradient coloured lenses.

Intrinsic Yellowness

Most organic materials have some level of yellow colour, or Yellowness Index (YI) even if not always visible to the naked eye. Yellowness can be caused by impurities, or by UV and blue light absorption. Optical monomers of ACOMON, have intrinsically low YI values and are monitored constantly to make sure the yellowness is within a very low specification.

Optical monomers are made into an optical polymer (lens), when an initiator is added and a thermal cure cycle is applied. Both the initiator and the thermal cure cycle may cause some increase of yellowness. During post cure of the final lens, the YI will reduce again to a low level.

Warehouse Yellowing

After the lenses are produced, they are normally stored in a warehouse. Depending on the demand and throughput, they may be stored for several months up to a full year. During the storage, it is sometimes observed that lenses made of ADC (also called CR39™)* are sensitive to so called “warehouse yellowing”. This phenomenon increases costs, as yellow lenses have to be discarded. It may also cause financial claims from down stream customers.

ACOMON has developed special products that have a reduced warehouse yellowing behaviour like RAV 7AT and RAV 7NG. These products are recommended for those lenses that are logistically difficult to process within a short period of time.

UV & Thermal Ageing

Yellowing may also occur on outdoor exposure due to UV (sunlight) radiation as well as thermal ageing. Especially with direct sunshine, UV radiation levels can be high. The organic polymer materials that lenses are made of, can show yellowing under these circumstances. ACOMON’s optical monomers have inherently very good UV stability, the lenses can be protected further by adding small amounts of UV stabilizers. For this purpose UV absorbers are added, which may cause some yellowness, as part of the blue light is also absorbed (blue light is the complementary colour of yellow). In order to shield all UV radiation from our eyes when wearing lenses, UV 400 cut-off lenses were introduced and have become popular. This is achieved by adding even more UV absorber. As a consequence, the yellowness of such lenses tend to be higher as well, but this can be compensated for by adding bluing agents to some extent.

Thermal ageing is mostly seen during thermal curing of hard coatings. As the yellowing occurs at elevated temperatures, it is especially during these hard & AR coat operations that yellowing occurs. Impurities in the material enhance the thermal ageing. Acomon products are developed to have minimal thermal ageing, although most optical monomers show this effect to some degree. The only exception to the rule is RAVolution®, which remains water white during such operations.

Influences of yellowing operations (UV exposure, adding UV-absorbers, high temperature treatment, etc.), can be compensated by adding bluing agents. In case of ADC monomers, special bluing agents have been developed that survive the curing conditions. Such bluing agents may, however, give rise to some haziness. It is therefore paramount that the basic yellowness of the products are low to begin with.

* CR39 is a trademark of PPG Industries

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