Interesting information. I knew from personal experience that they were made of some thermoplastic, but not what type. The article said they were made of a plastic "similar to" Tenite. If true, Tenite was one of the earilest plastics in commercial use, and is a cellulose ester. As such it is not petroleum based as the majority of plastics available today are. One of the largest markets for Tenite was in plastic eyeglass frames.