OPTICAL CORRECTION

  The physical data: What you need to know

Safety Materials

Compared with glass, the two chief advantages of plastics spectacle lenses are their lightness and safety. The weight of a material depends upon its density, which expresses the weight of 1 cubic centimetre of the material in grams. Table 1 shows that the densities of plastics materials are far less than those of glass materials of similar refractive indices. When glass breaks, the broken fragments are notoriously sharp and dangerous and for this reason, glass can hardly be claimed to be an ideal lens material to place before the eyes. CR39 lenses are more difficult to break and the broken edges are less sharp than fractured glass. However, for real safety in spectacle lens materials there is nothing to beat the materials polycarbonate (available from several sources) and Trivex" from PPG Industries.

POLYCARBONATE

Polycarbonate material was first produced some 50 years ago, since when there has been a steady increase in its quality and use. It was used widely to manufacture goods such as drinking glasses, lamp housings for lighting in public areas, and as screens for protecting bank staff. Since 1960 much research and development has taken place for the material and as a result polycarbonate lenses are now readily available, not only as safety lenses, but as high-quality spectacle lenses for dress wear. These can be more reliably coated with both hard and multilayer antireflection coatings. This development is in some part due to the advances that have been made in the production of compact discs, which is probably the widest use for the material today. Polycarbonate material has a resistance to distortion by heat, is quite transparent and has good insulating properties. It is also very tough, and is widely used to make eye protectors.

It is a thermoplastic polymer that melts at a fairly low temperature and can easily be moulded and remoulded by a process of heating and cooling. When the strength of polycarbonate is compared to that of metals, the mechanical strength is fairly low, but it is very light and very strong. Lenses can be produced by both moulding and surfacing techniques. Both methods produce good-quality, impact-resistant lenses. Polycarbonate crazes when hit. The crazed areas surrounding the point of impact can be seen easily by the way in which light is scattered through the lens. The amount of crazing produced is dependent on the time-period of the load and the temperature of the material. When the temperature is low the material crazes more readily and fracture by a small high-velocity particle will occur at a lower velocity. Polycarbonate is a fairly good material for the production of spectacle lenses, as can be seen from the physical data presented in Table 2.

Its only drawbacks are its low Abbe number and the need for some specialized equipment. However, regarding the Abbe number, experience has shown that correctly centred lenses do not cause most patients any visual problems. It is therefore possible to produce lightweight lenses that are of good optical quality and very strong. Regarding equipment, polycarbonate lenses generally need specialized machining methods, both when surfacing the lenses and when glazing. Dedicated machines fitted with special edging wheels need to be employed, as the material is difficult to cut with standard diamond edging wheels.

TRIVEX TM

Recognizing the strength of polycarbonate as a safety material for plastics lenses, PPG Industries (Pittsburgh Plate Glass Industries), the originators of CR39 monomer, has introduced its own lightweight safety plastics material known as Trivex'. It is a normal-index material and is claimed to be even stronger than polycarbonate but more flexible, to enhance even further its safety aspect. It offers 100% UV attenuation and is easily tinted by the usual surface dyeing process. Minus lenses can be surfaced down to just 1.0mm centre thickness without the lens losing its inherent safety features. Two lens manufacturers now produce lenses in Trivex" material. Younger Optics calls its lenses Trilogy" and Hoya calls its lenses Phoenix" (PNX). Each company has modified the monomer to its own needs.

TREATMENTS FOR SAFETY MATERIALS

Both polycarbonate and Trivex" lenses are available in photochromic form (by means of the Transitions treatment) from several sources, and can be coated to improve their resistance to abrasion. Polycarbonate, in particular, is too soft to be used without a hard coating and is always supplied coated with a hard lacquer to protect the lens surfaces. The hard coating is usually applied in the form of a polysiloxane lacquer by a dipping process. An ancillary effect of the coating is to reduce the impact resistance of the lens, but even coated Polycarbonate is still much stronger and, therefore, more difficult to break than CR39 and the mid-index plastics materials. Polycarbonate lenses can also be supplied with multilayer reflection-free surfaces. The transmission of white reflection-free lenses can be as high as 99%, offering improved contrast and freedom from the ghost images often experienced by wearers of uncoated lenses. Most multilayer coatings are finished with a topcoat, which has antistatic and hydrophobic properties, helping to maintain the transparency of the surfaces and making them easier to clean.

COMPARISON OF MATERIALS AND THE TEST

EN Standards 166, 167, and 168 apply to impact-resistant lenses. These have now completely replaced the old BS 2092 Standard which, in general, used a drop-ball test for impact resistance. The EN Standards are somewhat more demanding as the impact velocities recommended for impact-resistant lenses have been increased significantly. All lenses used for safety have to pass a basic level "hat is referred to as "increased robustness", which distinguishes these lenses from those that do not pretend to embody any safety feature. All appliances that have impact-resistance features are classified as able to pass either low, medium or high-energy impact. All these are marked with a symbol to indicate the level of safety that they offer. This is shown in Table 3.

Note that whereas the standards for increased robustness, low-energy impact and medium-energy impact are very similar to those given in the old BS 2392 Standard, the new high-energy impact requirement is a much more severe test. The best materials for the production of impact-resistant safety spectacles are, without doubt, polycarbonate and Trivex'. Spectacle lenses made in these materials easily pass the general robustness and low-energy impact test suggested in EN 166. They need be glazed correctly and ideally in a plastics frame wich will, to some extent, allow for movement of the lens in the rim. Their use will depend on the environment in which they will be employed. If it is felt that a higher level of safety is needed than that provided by safety spectacles, then goggles or shields might be suggested.
20/20 05/03