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Polycarbonate! |
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When economic times are tough, we all start searching for ways to lower our cost of goods sold. Some of us look for discount lab services, while others look for frame deals. We’ve grown so accustomed to only promoting the latest and greatest technology, that we have forgotten a lens material that was once the “latest and greatest”---polycarbonate. Yup, I said it out loud.
I LOVE polycarbonate and here is my story. |
Sometime during the late 1970’s, one of my labs introduced me to a wonderful new resin lens product. It was more impact resistant than the CR-39 lenses I was currently using and lighter weight as well. It was the best thing since night baseball - for kids, for safety glasses and for high-powered lenses that might be uncomfortably heavy. This new and wonderful lens material was polycarbonate.
I used the heck out of that material. The only real complaint was how easily it scratched. CR-39 scratched, but not as easily. However, most of us - ECPs and patients alike - were more accustomed to the durability of crown glass. All of us had to learn to take better care of our eyewear.
Polycarbonate also had some other very important characteristics.
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It’s UV absorptive, a serious consideration following the study of the link between exposure to UV light and cataract formation published in 1985.
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It’s superior tensile strength allows surfacing to a 1.0mm center thickness, a real break for those high myopes and hyperopes who wanted something thinner and more cosmetically attractive.
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With a specific gravity of 1.22, it is lighter weight than CR-39 and crown glass. This is another plus for those patients who have developed “dents” where their heavy eyeglasses rested on the bridge.
And polycarbonate had some real significant problems.
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The optics were marginal.
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The substrates were poor. Lack of temperature control was responsible for the “black specs” often seen imbedded in the material.
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Front and back-side coatings were poor.
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The material could not be tinted.
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There were very few lens styles available.
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A lack of good processing procedures and equipment created issues with thicknesses.
My friend polycarbonate was not off to a very impressive start.
The next generation of polycarbonate lenses occurred in the mid to late 1980’s. The quality of the resins improved as did the factory applied coatings. Equipment was developed specifically to process polycarbonate lenses, such as more aggressive generator wheels, dry-cut edging systems and back-side coaters. There were more lens styles available and the lenses could be processed to a much thinner center thickness. The first tintable polycarbonate lens was introduced in 1988. True, it didn’t get very dark, but it was tintable none the less.
During this same time, a new recording industry was born and with it the need for a purer form of polycarbonate to deliver the crystal clear sound requirements of the CD industry.
Polycarbonate lenses are manufactured from the same “water-white” material used for CD’s and DVD’s. This new material is free of the contamination seen in earlier lenses.
The tooling used to produce today’s polycarbonate lenses are cut from multi-axis milling machines and electroplated to maintain exacting curve specifications.
The process of transforming raw pellets into a flowing plastic has also improved. The raw material is vacuum fed into a dryer, then drawn into the machinery and melted under strict and optimum temperature controls. The “black specs” seen in the early material were the result of inconsistent temperature control.
The injection-compression process subjects the molten raw material to a high degree of compression after being injected into mold cavity, reducing the internal stress visible in earlier lenses.
Most polycarbonate manufacturers use robotics to remove the lenses from the molds, transport them to a coating station and then place them in a position to be cured, yielding the cleanest quality lenses available.
Today’s improvements in polycarbonate technology are centered around lens treatments including enhanced tintability and anti-reflective coating compatibility. Not only has the chemistry improved, but the application technology has improved for both manufacturer-applied as well as lab-applied hard coats.
On the cutting edge of this new coating technology is a product from Polylens, Inc. Their entry in the tintable polycarbonate category is PolyTintz, a proprietary technology that offers the ability to tint a polycarbonate lens to sunglass density and still maintain scratch-resistance greater than CR-39. This coating can withstand higher temperatures for longer times than conventional coatings and is compatible with current tinting products, such as those manufactured by BPI. In challenging economic times, this may offer ECPs an alternative to higher priced sunglass options.
Once the “red-headed stepchild” of the lens family, polycarbonate has become a full fledged member. These lenses are now available in many forms including polarized and photochromics. They can be A/R coated and mirror coated. Almost every major lens manufacturer offers an array of designs in polycarbonate, from single vision to premium progressive products. Polycarbonate has been approved for use with most premium anti-reflective lens treatments and is still used by NASA for face shields and in the airline industry for aircraft windows.
My friend polycarbonate continues to be a viable and cost-effective lens choice for most of today’s eyeglass wearers. |
