There are many reasons why recycled PET can turn yellow when remelted into pellets or new products.
Exposure to ultraviolet light, contamination from residuals and even additives all can help discolor the recycled plastics.
Chemists around the world make good money figuring out formulas that allow companies to use recycled PET while minimizing yellowing in new products.
But for all of the talk over the years about the impact of things like coatings, labels, adhesives and light, organic chemist Frank Schloss does not remember actually hearing about somebody studying the real-world impact of light exposure on recycled PET containers before they are reprocessed.
So the vice president at Plastic Technologies Inc. of Holland, Ohio, set out to do just that.
“We sit in meetings and we debate all of the time how do we make recycled material better and what are the reasons why we are seeing these sorts of problems today,” Schloss said. “We've debated these things for years and still don't have a really good answer as to what the primary contributor is to yellowing.
“This is just a study that gives you just a little more information around what might be causing some of the yellowing of PET,” he said.
Brand owners have to balance the use of recycled content with the visual impact that plastic could have on their products.
Just how big of an issue is yellowing?
“Well, if you like yellow bottles, it's not an issue at all,” Schloss joked. “It's a matter of everybody in the industry wishes to buy the best quality recycled PET possible.”
“You don't want the consumer to feel they are getting an inferior product because it has recycled material in it,” Schloss said. “The consumer can't really determine the quality of the bottle other than how it looks.”
PTI exposed containers full of loosely placed bottles to a year's worth of sunlight on its roof, agitating them from time to time to ensure equal distribution of UV light exposure. Other bottles were stored inside under fluorescent lighting, and a third set was stored away from light as a control group.
The research project showed that while the bottles really don't look any different after a year in the sun, the problem of yellowing occurs when you create a “melt history” — including when the material is recycled into resin pellets and when those pellets are then used to create new products.
“When that pellet gets melted once again, it just adds more yellowness to it,” he said.
While Schloss said his study shows UV light plays a role in recycled PET yellowing, it's still unclear how much of an impact it has compared to other factors such as additives and residuals.
“For MRFs and reclaimers who buy the bales from the MRFs, they would want to ensure that they have their bales that they may have stockpiled not stored in an open sunlight areas and possibly have them in warehouses,” he said.
Recyclers and reclaimers, at least in the United States, tend not to store their bales outside and exposed to sunlight, “not for very long at least,” the chemist said. “There might be some that do that,” he added, and it “might be a more prevalent condition” in other countries.
“I hate to think it's one more thing to worry about, but it's one more thing to consider,” Schloss said.
“The only point I would stress so we keep it in perspective: I'm not trying to say this is the primary cause of yellowing. This is simply another factor that we must consider. So if you have a choice, don't leave your bales in the sunlight, which I think most of the reclaimers here in the U.S. probably follow today, but they may not know the impact,” he said.
The study is available at www.plastictechnologies.com/solve/white-papers-and-articles/how-light-impacts-rpet.aspx.
