Issue Date: September 19, 2011
A New Life for Soda Bottles
For anyone who has bought polyester carpeting recently, chances are good that the polyethylene terephthalate, or PET, that makes up the fibers squishing beneath their feet had a previous life as a soda bottle. The same goes for some pillow and ski jacket fillings, cotton-polyester blend sheets, and the plastic strapping wrapped around packages. And of course, some soda bottles find new life as another soda bottle.
Turning garbage into goods may have seemed like a strange idea at one time. Not anymore. In 2009, some 937 million lb of PET soda and water bottles were recycled in the U.S., all bearing the resin identification symbol number one on their bottoms.
But the process is not without its challenges. At the recent American Chemical Society national meeting in Denver, the Division of Polymeric Materials: Science & Engineering hosted a symposium to discuss the benefits of recycling PET and the challenges still facing the industry.
The idea behind the symposium was to educate folks about why they should toss soda and water bottles into the recycling bin, rather than into the trash, said Scott Sutton, a senior principal scientist at Pfizer’s La Jolla, Calif., campus. He coorganized the event with David Schiraldi, a chemistry professor at Case Western Reserve University, and Sara Risch of the food packaging consulting firm Science By Design.
“As long as you recycle your bottle, PET is one of the poster children of sustainability,” Sutton said. “It’s ubiquitous, allowing recyclers to leverage economies of scale, and its unique properties make it both highly recyclable, which saves energy and raw materials, and lightweight for energy efficiency in material transportation.”
Although intuitively it seems like using recycled PET, rather than virgin material, to make new products should be a good thing, there are now hard data that show just how much better it is. Beverly Sauer, a project manager and senior chemical engineer with the life-cycle consulting firm Franklin Associates, a division of ERG, presented a life-cycle analysis of recycled PET and compared it with a similar analysis previously done on virgin PET.
“The purpose of this particular project was to quantify all of the environmental burdens associated with collecting, sorting, and reprocessing postconsumer resin,” she says. That means taking into account things such as the fuel burned to transport bottles from a recycling bin at the curb to a recycling facility and how much energy is actually used in the recycling process.
A lot of complex number crunching went into Sauer’s analysis, but her conclusion is simple. Collecting and recycling PET requires only about 24% of the energy it takes to produce virgin PET. And the steps to collect and recycle PET produce less than half as much greenhouse gas compared with making PET from raw materials. “No matter which way you look at it, there truly are benefits to recycling plastic,” she said.
Forrest Bayer, director of packaging, scientific, and regulatory affairs at Coca-Cola, walked the audience through the PET recycling process. Although there are variations, he said, most bottles go through the same scenario: A bale containing 3,000 to 5,000 bottles arrives at a recycling facility. It gets washed, steam rinsed, and is then mechanically or manually sorted to remove any surprises that might have wound up inside. “Bales have a lot of things in there: bowling balls, toilets, you name it,” Bayer said.
The remaining material is ground up into pieces about half the size of cornflakes and dried. Those pieces include not just bottles but also labels and caps. The flakes are flushed with air to remove the lightweight label materials. What remains is then placed in water, where the PET flakes sink and the polyolefin flakes, which come from caps and other closures, float. At this point, the PET is ready to go into fiber applications.
“If you want to go into food applications you have to go through additional processing,” Bayer said. And there are a few options as to how to proceed. The PET can be chemically depolymerized, purified, and repolymerized. Alternatively, recycled PET can be sandwiched between two layers of virgin PET so that it doesn’t have direct contact with food. Or it can go through a “super cleaning” step to remove anything that might have absorbed into the polymer through physical or chemical means.
“This is very sophisticated technology,” Bayer said of the supercleaning process. “It’s not something that’s just done in a tub.” The PET flakes are coated with a caustic agent and put into a high-temperature reactor so that the surface of the flakes undergoes a saponification reaction. They’re then dried, baked, and rinsed to remove the caustic agent. Then they’re sorted by color. “Anything that has gone through that process that is not PET tends to change color and is very easily sorted out,” Bayer explained. Now the material is good enough to make new food packaging.
Bayer said that when he talks to governments around the world about using recycled material as food packaging, he often meets resistance. “They say, ‘You’re talking about taking garbage and putting it back with food content. We don’t think so.’ That’s a big mind-set hurdle for a lot of people,” he said.
Then Bayer asked the audience where they thought virgin PET came from. “Crude oil,” he responded. “There’s a lot of toxic stuff in crude oil—carcinogens, teratogens, you name it. It is there. But we don’t have any problem understanding that we can take crude oil and go through physical and chemical processes that generate p-xylene, which can be oxidized into terephthalic acid, and we can react that with ethylene glycol to make PET. And that’s accepted around the world.” Why then, Bayer wondered, is it such an intellectual hurdle to take something that’s far more pure to start out with and clean it enough to get something that’s safe to drink from?
“Everything you can do with virgin PET you can do with recycled PET,” said David Cornell, technical director of the Association of Postconsumer Plastic Recyclers (APR). Besides overcoming the mind-set challenge that Bayer talked about, there are several technical challenges, many involving chemistry, facing PET recyclers, he noted. “Quality is very important, and cost is always an issue. Recycled materials do not sell at a premium for very long. They have to be cost-competitive,” Cornell said.
Part of the problem, Cornell explained, is that PET packages hold a wide variety of products, from soda to soap to maraschino cherries, each requiring different additives and barrier layers. “If they were all 2-L soft-drink bottles, life would be very nice,” he joked.
Such additives include ultraviolet light inhibitors, acetaldehyde scavengers, and colorants. “Some of those make a difference in recycling and some don’t,” Cornell said.
There are also low-melting look-alikes—plastics that look like PET to the naked eye, but aren’t PET and cause trouble when they get into the recycling process, usually because they melt in the dryer. Polystyrene and polyvinyl chloride are some examples.
“We want the bottles to be designed properly in the first place so that we don’t have to deal with mistakes at the end of the process,” Cornell said. To that end, APR puts out guidelines for recyclability, and they notify manufacturers when they find a bottle that doesn’t conform. These guidelines include making sure that certain polymers are never combined. For example, a PET bottle with a PVC bottle-cap liner would likely cause problems in the recycling process.
Cornell said that it’s also important to keep PET transparent, so manufacturers should avoid adding odd colorants or agents that make the PET opaque. Any additives, UV inhibitors, or acetaldehyde scavengers should be nonyellowing, nonhazing, and nondestructive.
Additive makers should also take into consideration the fact that PET is likely to be recycled more than once, Cornell said. Some additives can be recycled once without breaking down, but not twice.
Recently, oxo-degradable additives (C&EN, June 13, page 20), which are supposed to degrade long polymer chains into smaller fragments, have become an issue for PET recyclers. “We have challenged the folks that make these materials to show us that with appropriate aging the service life of the recycled products is not imperiled,” Cornell said. “They have not done that.”
There are also challenges in making labels and adhesives. “We want the labels to be separable and not to bleed their ink colors into the wash water and stain the PET, and we want adhesives to stay with the label and detatch from the PET,” Cornell added.
Finally, both virgin and recycled PET, left untreated, appear slightly yellow. “Manufacturers do all sorts of things to squelch that yellow color,” Cornell said. “But that yellow color will come back if you keep melting the PET carelessly.” He suggested that there’s an opportunity for chemists to come up with longer-lasting ways of neutralizing yellow color in PET.
Perhaps the biggest challenge when it comes to recycling in the U.S. is simply getting people to recycle, Cornell said. Recycling rates have been slowly climbing over the past decade, but they’re still fairly lackluster. According to the National Association for PET Container Resources (NAPCOR), a trade association for the PET packaging industry, just 28% of the PET bottles that went on store shelves in the U.S. in 2009 were collected for recycling.
“There are people who think that all of their bottles that are collected go to the landfill,” Cornell said. “That’s not true. We need more bottles collected.” One easy way consumers can boost the supply of postconsumer PET is to recycle all household PET containers. The recycling rate of PET bottles, such as soap containers and peanut butter jars is just around 10%. “People are pretty good about soft-drink bottles, but they’re not very good about bottles that don’t look like soft-drink bottles,” Cornell pointed out.
“We find ourselves as a country in a very embarrassing position where we have the lowest recycling rates of commodities versus other parts of the world, both developed and undeveloped nations,” said Dennis Sabourin, NAPCOR’s executive director. “We need public policy in order to drive the recycling of all commodities.”
Sabourin pointed to S. Res. 251 as a step in the right direction. The resolution, which was introduced last month, expresses support for improvement in the collection, processing, and consumption of recyclable materials throughout the U.S.
He advocated for making the folks who sell stuff in bottles more responsible for what ultimately happens to those bottles. He noted many companies have already stepped up to the challenge. Coca-Cola, for example, has committed to recycling 50% of the PET bottles it puts out in the U.S. by 2015.
“If the nation is looking at ways of becoming oil independent, of saving energy in the conversion of products, of being more prudent in our carbon footprint of products, and creating green jobs, recycling is the answer,” Sabourin said.
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