Plastic pollution is a major environmental and sustainability issue, with millions of tons of plastic produced from petroleum products entering landfills, waterways and oceans each year. Despite decades of research, plastic recycling is still very limited; only about 9% of plastic is recycled globally, with much of it downcycled into less valuable products.<\/p>\n\n\n\n
New technologies, however, may help close the loop on recycling, producing high-quality recycled plastic just as good as fresh, \u201cvirgin\u201d plastic. Since 2020, researchers at UW-Madison have made great strides in chemical recycling through a pioneering process called solvent-targeted recovery and precipitation (STRAPTM).<\/p>\n\n\n\n
STRAP is particularly good at recycling colored multilayer, flexible plastics, which include food packaging like bags, pouches, wrappers and films. These types of plastic often incorporate multiple specialized layers that prevent moisture, seal out oxygen and improve strength. STRAP uses a series of solvent washes to dissolve each layer of plastic, which is then recovered and processed into near-virgin plastic. These films also contain a variety of color bodies that are put in by brand owners to market their products.<\/p>\n\n\n\n
In recent years, Huber\u2019s team has refined STRAP. However, the researchers found that the final plastic films they produced often had a yellowish hue to them. That tinge makes the recycled end product much less appealing to manufacturers, reducing the value of the plastic by more than half.<\/p>\n\n\n\n
\u201cTo consumers, yellow might be a sign of age or degradation,\u201d says Granger. \u201cIn these recycled plastics, that\u2019s not the case. It\u2019s just from pigments. But either way, it looks gross.\u201d<\/p>\n\n\n\n
That\u2019s why Granger and Yan set out to discover why recycled plastic film produced via STRAP looked yellow, and what they could do about it. They first tested dozens of pigments, adding them individually to polyethylene, the plastic most used in flexible packaging, running them through the STRAP process to see if they caused the yellowing. Soon, they narrowed the culprit down to Yellow 12, a common organic pigment used to print packaging.<\/p>\n\n\n\n
Most other pigments break down during STRAP processing and are removed by solvents or filtering. But elements of Yellow 12 survive the process, remaining in the solvents used to dissolve the plastic. In the final processing step, in which the recycled plastic is dried, the researchers found that evaporating solvents left behind the pigment in the plastic, causing a yellow sheen in the final product.<\/p>\n\n\n\n Armed with that knowledge, the team was able to come up with a method to get rid of the color. \u201cThe yellow pigment has a higher solubility in STRAP solvents than other types of plastic pigments due to its chemical structure,\u201d says Huber. \u201cSo the first thing was to pick a solvent that has a lower solubility of that pigment. Then there are a couple of extra steps that really make that plastic come out crystal clear.\u201d<\/p>\n\n\n\n![\"George](\"https:\/\/engineering.wisc.edu\/wp-content\/uploads\/2024\/01\/huber_george-gwhuber-5x7-1-731x1024.webp\")