For many years, people could safely assume the clear disposable containers that held their soda or iced coffee were made of polystyrene or polyethylene terephthalate (PET). But thanks to a new generation of clarifying agents, it’s becoming increasingly likely that they are drinking out of polypropylene cups.
Versatile and relatively inexpensive, polypropylene has been poaching applications from other polymers for decades. On the commodity end of the plastics industry, it tends to compete with polyethylene in packaging. On the higher end, it often substitutes for engineering polymers in automotive parts. But polypropylene’s haziness has kept it from encroaching on more transparent polymers in markets such as cups and clamshell deli containers, where high clarity is essential.
That is, until recently. A new generation of polypropylene clarifiers makes the polymer clear enough that it can now compete with the industry’s two standard bearers. Moreover, with the playing field leveled in the clarity realm, proponents say polypropylene can bring to bear other advantages, notably its low density, which can cut costs and provide environmental benefits.
Polypropylene clarifiers are a class of nucleating agents, which induce crystal formation in molten polymer as it solidifies. Traditional nucleating agents include carboxylic acid salts such as sodium benzoate.
Brian Burkhart, global marketing manager for polypropylene clarifiers at Milliken Chemical, likens nucleating agents to the string that’s submerged in a sugar solution to make rock candy. By itself, polypropylene forms large crystals in a disorganized fashion. “When there is a nucleating agent, what happens is that all the crystals grow at the same time, so you end up with a lot more, smaller crystals,” he explains.
The quick formation of smaller, more regular crystals stiffens polypropylene and allows it to set faster in the mold, boosting productivity. Clarifiers disperse through the resin better than standard nucleating agents, allowing growth of even smaller crystals and, thus, improved clarity.
Sorbitol-based clarifiers, which Milliken makes, have been around for a little more than a decade. The industry has seen three generations from the South Carolina-based company: dibenzylidene sorbitol (DBS), di-para-methylbenzylidene sorbitol (MDBS), and di-meta, para-methylbenzylidene sorbitol. Milliken’s latest clarifier, also a sorbitol derivative, was released in 2007 under the trade name Millad NX8000.
“With NX8000, we were able to achieve a level of clarity that we weren’t able to get to with polypropylene in the past,” Burkhart says. The company maintains that Millad NX8000 can reduce haze in polypropylene to half of what previous clarifier generations did.
Mike Musgrave, market manager for molded polyolefin applications at Total Petrochemicals, a major U.S. polymer producer, says the new clarifier “is expanding the properties of polypropylene to encompass the properties of other materials such as styrene copolymers, polycarbonate, and PET.”
Total recently came out with a grade of polypropylene, Lumicene M3382MZ, that incorporates Millad NX8000. The resin is polymerized with Total’s own metallocene catalyst, which boosts clarity compared with a Ziegler-Natta catalyst, Musgrave says, because it yields uniform molecular chain lengths and thus smaller, more uniform crystals.
Although Milliken is the dominant player in the clarifier space, it has a formidable competitor with its own next-generation polypropylene clarifier. In 2006, polyolefin additives giant Ciba unveiled Irgaclear XT 386. The product is now marketed by BASF, which acquired Ciba last year.
BASF won’t disclose the product’s chemistry but does say it is based on trisamide technology. “It has advantages versus the state-of-the-art technology,” says Michael Amone, a manager of new business development for plastics additives at BASF. Irgaclear offers clarity comparable with sorbitol-based clarifiers up to product wall thicknesses of about 50- to 60-thousandths of an inch, he says. It is a natural fit in thin-walled injection-molded articles such as syringes, he adds.
Ralph Maier, applications specialist for plastics additives at BASF, notes that Irgaclear needs to be added at concentrations of only about 150 to 200 ppm, whereas sorbitol-based clarifiers need concentrations that are 15 to 30 times higher. The low concentration, he says, reduces “plate-out,” a phenomenon in which additives form deposits on equipment during processing.
In addition, Maier says, Irgaclear offers higher thermal stability than sorbitol-based clarifiers do, which leads to less discoloration in plastic parts.
Total has worked with the BASF material, and Musgrave says the lower concentration can indeed reduce plate-out and also save money. However, measuring precise doses of Irgaclear for blending into polypropylene can be tricky.
With the help of the new clarifiers, polypropylene is making inroads into thermoformed packaging applications. In thermoforming, a plastic sheet is made pliable by heating and then is stamped with a tool into shapes such as cups and clamshell containers. With clarity no longer a drawback, polypropylene’s other advantages are unlocked, Total’s Musgrave says. For example, the polymer has better heat resistance than PET, a property that might come in handy in packages that are hot-filled or kept under heat lamps.
More important, polypropylene is about 30% less dense than PET, Musgrave notes, and thus less resin is needed to make parts. Proponents claim both financial and environmental benefits.
Cost and sustainability are why two of the biggest wheels in food service, McDonald’s and Starbucks, have transitioned to clarified polypropylene from PET in beverage packaging. Jim Hanna, director of environmental affairs at Starbucks, wrote on a company blog that a company-commissioned life-cycle assessment found that polypropylene cups use 15% less plastic and are responsible for 45% less greenhouse gas emissions during production than PET. “We sell over 1 billion cold cups every year, so any improvement we can make in reducing the greenhouse gas emissions from producing our cups has a pretty big impact,” Hanna wrote.
Similarly, in a best-practices case study, McDonald’s noted that transitioning to clarified polypropylene “realized significant benefits, including 15% cost savings through the reduction in raw material usage and transportation efficiencies,” as well as a resin savings of 2.9 million lb annually.
Polypropylene makers also have been gunning for polycarbonate, which has been under fire over the possible endocrine disruption effects of its raw material bisphenol A. Clarified polypropylene has already made some headway replacing polycarbonate in baby bottles.
Clarity doesn’t provide an entrée to every market that uses a clear plastic. In blow molding, PET’s strong gas-barrier properties give it the upper hand over polypropylene in applications such as beverage containers and food packages that need to stay on supermarket shelves for a long time. But Burkhart points out that blow-molded objects such as squeezable dish detergent bottles are “great targets” for polypropylene.
There are other reasons why companies might be reluctant to switch to clarified polypropylene, according to John Marcantonio, director of the chemicals practice at IAL Consultants in London. “People want a polymer that is cheap, and polypropylene is cheap,” he says. However, switching from one polymer to another also involves changing molding tools and other equipment. “Tools are a major part of the investment because the tools aren’t interchangeable,” he says.
Plastic converters need to be assured of large volumes so the capital costs of buying new tools can be spread out over many units, Marcantonio explains. “Resin costs alone aren’t enough to make the switch,” he says.
Challenges notwithstanding, Milliken’s Burkhart is confident that polypropylene’s newfound clarity will continue to present new opportunities. “Clear polypropylene has got legs under it now to be able to do things that it couldn’t do in the past,” he says.