The development story of MR™
Chapter 1Taking on the Challenge of Developing Even Thinner and Lighter Plastic Lenses
From Glass to Plastic Lenses
Forty years ago, in Japan, in the early of 1980s, glass lenses were the mainstream for eyeglasses, and the penetration rate of plastic lenses was only around 10%. On the other hand, in the United States, more than 80% of eyeglass lenses were made of plastic, which is light, break resistant and safe, so it was widely used. The material was made with a resin called allyl diglycol carbonate (ADC). It was developed during World War II as a material for the windshields and fuel pipes of fighter aircrafts. And since mid-1960s, it has been widely used as a material for eyeglass lenses.
Aspiration to Develop Thin Plastic Lenses
Nevertheless, this ADC resin is not sufficient to realize the thin plastic lenses. Since the refractive index is as low as 1.50, a lens with a high prescription power will inevitably become thick and heavy. Consequently, many chemical companies in Japan worked on developing high refractive index lens materials to replace ADC resin, aiming for thinner lenses. Some of those chemical companies successfully developed a high refractive index lens material. However, they have not been widely adopted.
Chapter 2Entrusting Our Last Hope to an Out of the Ordinary Challenge
Research Ran into Difficulties and Faced Suspension
The researchers who had started developing the lens material brainstormed and examined ways to increase the refractive index with reference to the development cases of other companies that had preceded us. However, we could not achieve the desired results. Consequently, the priority level assigned to this research theme was gradually lowered and development was on the verge of being suspended. The number of researchers on this project was also reduced. However, the person who was responsible for business development at that time backed Sakai, who was in charge of development, and Katsuyoshi Sasagawa, who was in charge of research, to continue their research: I will take responsibility for the rest, so continue your research.” If it hadn’t been for a word to continue this research, and without their tenacity, this project would probably have ended. The birth of the MRTM series would never have happened.
Last Hope Entrusted to Thiourethane Resin
The researchers came to think it was necessary to develop their own unique high refractive index lens material looking at this situation in which results were not being produced. They focused on the introduction of sulfur atoms that were known to theoretically increase the refractive index of resins and a urethane resin for which Mitsui Chemicals had accumulated technologies as a material for beds, sheets, paints and adhesives over many years. That was because the isocyanate compound of this urethane raw material and the mercapto compound containing sulfur atoms were known to react well together in theory.However, this mercapto compound was difficult to handle because it had a bad smell characteristic of sulfur. Therefore, this method was not one that was worth pursuing.Moreover, although there were cases in which thiourethane resin (urethane resin containing sulfur) had been considered as an elastic fiber or coating material, it had not been examined and certainly had not been commercialized as an optical resin at that time. The researchers wondered if they could proceed on their research somehow with this approach as their last hope. In this way, research on an original thiourethane resin started.
Application examples of urethane (bed, car seats)
Chapter 3Finally, the Birth of an Ideal Lens Material
Proceeding with the Re-consideration of a Thiourethane-based Material
We tried taking another look at a thiourethane-based material under the belief that there must be a compound we hadn’t tried yet based on the research records compiled by researchers in the past and documents for patent applications. Specifically, we tried reacting various mercapto compounds with a special isocyanate resistant to yellowing
Achievement of the Development Targets: Reward from 5 Years of Hard work
One day, a researcher who was a colleague of Sasagawa said there was such a compound. He then retrieved a compound that had been considered as a material of food packaging from a refrigerator in a laboratory. It had been stored without being discarded even after being considered for that. Neither Sasagawa nor fellow researcher Kanemura thought that this would lead to an important discovery. That is because records of a test on a compound with an extremely similar structure to this compound had been left behind by a researcher stating that the experiment had been a great failure due to intense foaming. It was believed that this compound would also foam violently in the same way. However, when it was mixed together, there was no violent reaction and it hardened transparently. Various optical properties tests were performed on it that day. Something that no one had expected then occurred. Mitsui Chemical had finally cleared the standard of a refractive index of 1.60 and an Abbe number of 35 or more that we had set as our lens material development targets. In addition to that, it was also found that it had a low specific gravity and excellent impact resistance. This was the moment when five years of hard work was rewarded.
Lens inspection process
Improving the Degree of Perfection with Lens Manufacturers
The innovativeness of MR-6™ was highly regarded. Nevertheless, it caused lens manufacturers a lot of confusion. To begin with, MR-6™ was the first urethane-based lens material. The process for lens molding differed greatly to that for ADC. We were asked lots of questions about the material by lens manufacturers. We occasionally received seemingly never-ending complaints from them. We rushed to them and visited their factories to solve these problems each time they arose. We responded to lens manufacturers in this way both in Japan and overseas. The evaluation technologies we accumulated from this response formed the basis of the development of new products subsequently. After developing MR-6™, we were able to develop new lens materials in quick succession: MR-7™ with its improved refractive index of 1.67 and MR-8™ with its improved Abbe number and heat resistance. This was thanks to the technologies we accumulated at that time.
Memento received at the signing ceremony from Silor of America to commemorate the first shipment of MR-6™
The researchers at Mitsui Chemicals are constantly working on the development of lens materials with new possibilities without being complacent with this success.