At the Ludwig-Maximilians-Universitaet (LMU) in Munich, researchers have developed a faster and easier way to identify and separate plastics (e.g., polyethylene terephthalate PET/PETE and polyvinyl chloride PVC) for recycling. The method uses a flash of light to cause the material to fluoresce.
LMU chemistry professor, Dr. Heinz Langhals had this to say:
Plastics emit fluorescent light when exposed to a brief flash of light, and the emission decays with time in a distinctive pattern. Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers and can serve as an identifying fingerprint.
After photoexcitation, the plastic passes through a photoelectric sensor that measures the amount of illumination. This process determines the progress of the plastic’s decay. Polymers used in plastics show individual fluorescence lifetimes, therefore, the shape of the decay curve can detect it.
“Polymers represent an interesting basis for the sustainable cycling of technological materials. The crucial requirement is that the recycled material should be chemically pure,” Langhals said.
Purity Prevents Downcycling Waste
Polymers are processed as thermoplastics by melting at high-heat and injection molding to create the finished product. When recycling, the plastic is again exposed to high heat, which could change its properties if the sorted material is contaminated by another polymer. The material is then downcycled. Downcycling happens when a material loses quality or functionality in the process of recycling, but the end product can be used. Polymers don’t mix well with other polymers. Even a 5% contamination can drastically decrease the quality of the final product. This is why new high-quality plastics are always made from virgin materials, without the use of recycled materials at all.
The way for recycling plants to avoid downcycling is to try to achieve the highest purity when sorting the plastics.
“The waste problem can only be solved by chemical means, and our process can make a significant contribution to environmental protection because it makes automated sorting feasible,” Langhals said.
The novel method developed by the LMU team using fluorescence allows the sorting of up to 1.5 tons of plastic per hour. As a result, it already meets the mandatory requirements for industrial use and the creators will be applying for a patent on the new technology.
The specifics can be found in the journal Green and Sustainable Chemistry, where the group tested the technical polymers Luran® (styrene, polyacrylonitrile copolymer from BASF), Delrin® (polyoxymethylene from DuPont) and Ultramid® (polyamide with glass fiber from BASF).
The Here and Now
Current technology for sorting plastics for recycling include near-infrared spectroscopy that scans the plastics on a high-speed conveyor belt under the powerful light source. Some of the wavelengths are absorbed, and some reflected and captured by lenses. The lenses transmit a signal identifying the material to the shutter valves, and the material is blown into the appropriate chute. This video below gives a behind-the-scenes glimpse of a plant using NIR technology:
Sorting by hand is still very much in use today to separate plastics. Of course, this method allows for human error in the sorting, leading to downcycling.
The only way to save our “plastic planet” is through recycling. The more fast and efficient techniques we find to sort the materials, the better. Amazing things are being created with recycled plastics, like filaments for 3D printing, polyester fleece, and carpet. So, look out Pacific Ocean garbage patch we’re coming for you next. Who knows? There may be enough recyclable plastic out there to build the first recycled amusement park.
Image by belchonock/123RF.
Source: “Breakthrough Trial for Recycling Based on Fluorescent Light of Technical Polymers,” by Jenny Eagle, www.foodproductiondaily.com, August 26, 2014.
Source: “Novel Recycling Methods: Fluorescent Fingerprint of Plastics,” Science Daily, www.sciencedaily.com, August 21, 2014.
Source: “High Performance Recycling of Polymers by Means of Their Fluorescence Lifetimes,” by Heinz Langhals, et al., Green and Sustainable Chemistry, August 2014, DOI: 10.4236/gsc.2014.43019.