Glowing crystals provide bright line in sorting wastestreams

May 6, 2021
German company patents use of fluorescent crystals to separate difficult-to-process materials, such as glass-fiber-reinforced PVC, from plastics to be recycled. Recyclers would need a special light source and cameras to take advantage of the process.

A patent awarded to Freiburg, Germany-based Polysecure GmbH describes a marker that can be blended into a plastic material to make it fluoresce at certain wavelengths, so that recycling plants can efficiently sort material that might otherwise damage their equipment. The process requires a special light source – preferably a laser – that can luminesce a material stream, as well as optical equipment, such as a camera, that can detect the glowing materials. 

The patent notes that recycling mixed plastic waste, for example PVC plastic windows, profiles and production waste, is an established process. However, it can be difficult to detect foreign substances, such as glass-fiber reinforcement or additives, that has been mixed with a stream of a homogenous plastic material. Such substances, which are abrasive, can damage extruders and filter apparatuses. 

The proposed method involves blending Anti-Stokes crystals or pigments into certain plastic material when it’s produced to avoid trouble downstream when it’s recycled. The materials could include glass-fiber-reinforced PVC. The crystals fluoresce when a light source emits a particular wavelength that excites them. According to the patent, …it has been found that the fluorescent materials … exhibit such chemical, thermal and UV stability that they can fulfil their function throughout the entire lifecycle of the material, even if the material is processed in several recycling cycles.” 

The patent noted that some current processes require more-sophisticated cameras than those needed for the described process. 

After materials are reduced to a size range of between 0.1cm and 2.0cm, a specific wavelength of light can be directed at the particles, and only particles containing that specific marker will fluoresce, so they can be removedThe patent states that crystals attuned to different wavelengths can be blended into materials that have different additions or attributes. This would make it possible to separate multiple fractions from the material stream using multiple sorting stages. 

Patent 10,717,113; issued July 21, 2020