Our recent post, Plastic Offers Better Eyewear Options, talked about some of the varieties of plastics used to make eyeglass frames and lenses. While plastics have some advantages over other materials used to make eyewear, they should still be analyzed for product quality assurance, quality control verification and, as needed, to carry out failure analysis. The cosmetic prominence of eyeglasses makes any visual imperfection unacceptable. An FTIR spectrometer, fitted with an Attenuated Total Reflectance (ATR) sampling accessory based on a single bounce diamond crystal, is an excellent instrument for analyzing many of the problems that may arise during the plastic lens or frame manufacturing process.
Fourier transform infrared (FTIR) spectroscopy is also a useful technique for analyzing the plastic blends which may be used to make eyeglass frames. Plastic blends are often engineered to improve the strength and durability of the product, or to facilitate processing and manufacturability. Mixing dissimilar polymers can result in materials with even more valuable properties than those of the individual component polymers, but the interplay between components in these blends is complex; multiple techniques are required for complete characterization.
For FTIR analysis, thin films can be produced by heating thermoplastics to just below their melting point, then pressing them into films. These films can be used for rapid and accurate quantitative analysis of polymer constituents and additives and for determination of crystallinity and monomeric ratios in copolymers. ATR simplifies the analysis of polymer surfaces even further through a simple contact-and-collect process.
An article in the September issue of Plastics Engineering illustrates an example where polymer blend analysis using FTIR may be useful. It describes the efforts of a company experimenting with making eyeglass frames from a variety of recycled plastics. The company is blending polypropylene with polyethylene to increase the strength of the weld lines in the frames made from recycled plastics recovered from low-density polyethylene milk bottle lids and high-density polyethylene keg caps.
The use of recycled plastic, and even the recycling process itself, presents many challenges, as we explained in the previous post, Strategies for Recycling Polyethylene. Separating different recyclable plastics from each other so that they are processed correctly is also a challenge. In addition to separating polyethylene (PE) items from other plastics, different types (densities) of PE need to be separated, as do items co-polymerized with other types of plastics. This is difficult to do without complex chemical analysis. Read Density and Copolymer Content in Polyethylene Samples by FT-NIR Spectroscopy to learn how Fourier transform near-infrared spectroscopy (FT-NIR) provides a means to identify and analyze various polyethylenes.