Polymers sorting and recycling

Sorting has become a major challenge for plastic recycling mainly due to the complexity of plastic waste stream and the drawbacks of the available separation technologies based on near-infrared (NIR) spectroscopy, which fails in some cases to separate different polymers grades. For instance, black and dark-colored plastics form a serious problem for the recycling industry since the standard carbon black pigment absorbs nearly all NIR irradiation. As a result, insufficient radiation is reflected to the detector to enable classification of the polymer. It is essential to find an appropriate and cost-effective method for the sorting of polymers, since it would ease the following recycling processes and allow high purity polymers to be re-used, avoiding plastic waste to be incinerated or end in landfills. Some recent methods have focused on changes in the polymer production process, like the development of alternative black pigments and dyes that can be detected by NIR. However, these additives are expensive and do not present the same properties as carbon black, which can also act as stabilizer and reinforcing filler. Additionally, luminescent tracers that emit light under UV irradiation (to overcome the issue of NIR absorption) have been developed. Nevertheless, these tracers are not very efficient as they are dispersed uniformly throughout the resin and therefore require relatively high concentrations (as opposed to only tagging the plastic surface). Another potential approach focusing on “digital watermarks” included during label printing or molding, would require an immense collaboration between different stakeholders involved in plastics processing, and it would be limited only to plastic packaging. Here, all the above challenges will be overcome focusing on the use of the “tag approach”. This approach does not require changes to manufacturing processes and can be implemented using standard sorting equipment. It is possible to add a tag molecule to the plastic waste during a standard washing step at a recycling facility. These tags molecules will have a well-selected spectral signature and will be able to differentiate between different plastics based on their affinity for the polymer surface and how fast the tag diffuses into the polymer.