Issue Date: June 6, 2011
BPA Is Indispensible For Making Plastics
Polymer producers are in a pickle. Bisphenol A (BPA) is a versatile and valuable chemical building block that makers of high-performance plastics don't want to give up. But BPA is under intense scientific scrutiny for being an endocrine disrupter, and consumers are pressuring companies to find alternatives.
"In terms of viable alternatives, there have certainly been attempts to displace BPA-based plastics such as polycarbonate and epoxy resins, but until recently, none were successful," says polymeric materials chemist Daniel F. Schmidt of the University of Massachusetts, Lowell.
As Schmidt notes, chemists have been active in seeking out BPA substitutes, especially for food and drink packaging applications, starting with compounds that are easy surrogates. One idea has been to turn to chemicals that already have Food & Drug Administration approval for food-contact uses and are considered safe. Products made from such FDA-approved components generally do not need to undergo any additional safety testing in the U.S., at least for now. Congress, however, may address this gap in U.S. environmental regulations if it makes revisions to the Toxic Substances Control Act.
Some examples of compounds that are approved for food-contact uses include isosorbide and isoidide, which are bicyclic sugars derived from sorbitol. These compounds are used as diuretics, and nitrated versions are used to treat angina. But as with other potential alternatives for making polymers tough and clear, they don't yet match BPA's attributes.
"From the standpoint of plastics engineering, it's material costs and properties that matter, not simply replacing one specific molecule with another," Schmidt points out.
The only large-scale BPA-free success so far is Eastman Chemical's Tritan copolyester, which was originally designed as a polycarbonate substitute for plastic dinnerware that is less prone to developing microcracks and becoming hazy (C&EN, Aug. 31, 2009, page 20). Introduced in October 2007 just before the BPA safety alarm sounded, it unexpectedly became a hit for making reusable baby bottles and water bottles.
Thought to be assembled from a combination of dimethyl terephthalate, 1,4-cyclohexanedimethanol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol, Tritan's properties in combination with its comparable production costs and consumer demands for BPA-free materials are driving its success, Schmidt says.
Because all of Tritan's components were previously approved for food-contact use in the U.S., Eastman received an exemption from FDA and thus no additional safety data were required. But Eastman was required to obtain additional approval for Tritan from European authorities. And to reassure consumers, the company reported in May 2010 that independent lab tests it commissioned showed Tritan to be BPA-free and free from estrogenic activity in standard in vitro assays.
It turns out that BPA-free alternatives, including some grades of Tritan, can still display estrogenic activity, according to Stuart I. Yaniger, vice president of research and product development at PlastiPure, a polymer technology company based in Austin, Texas.
Yaniger and his colleagues use standard human breast cancer cell proliferation assays to test polymer formulations to see if they stimulate estrogenic activity and then try to design new plastics to avoid it (C&EN, March 14, page 48). Estrogenic activity—the ability to simulate the effects of the female sex hormones—is a common kind of endocrine disruption. But it doesn't necessarily signal toxicity.
The estrogenic activity from BPA-free plastics might arise from the polymer or from additives such as clarifiers that modify the crystal morphology of the plastic to keep it clear enough to compete with polycarbonate, Yaniger says. These clarifiers include traditional compounds such as sodium benzoate and newer versions based on sorbitol derivatives and benzene trisamides.
"The key is aromatics—if there's anything that can possibly hydrolyze to form a phenolic species or contains a phenol structure right out of the jar, it's likely to be an issue," Yaniger explains. "But our research shows that the estrogenic activity can be avoided through the choice of appropriate base polymers and, just as important, choosing additives carefully."
Among BPA-free alternatives being explored, glycol-modified polyethylene terephthalate, a type of copolyester, has clarity and processability comparable with polycarbonate, Yaniger says. Glasslike transparency is desirable for aesthetics and functionality—clear containers look better and allow users to see what's inside or whether the container appears clean. But the copolyester is still not quite as tough or heat-stable as polycarbonate and has a slightly higher cost, he notes. In bioassays, it has about the same estrogenic activity as polycarbonate, he adds.
Another BPA-free replacement plastic is polyether sulfone, which is typically made by condensation of chlorinated diphenyl sulfone with disodium bisphenolate salts. Diphenyl sulfone is a chemical cousin to BPA, and the two compounds are sometimes copolymerized. Polyether sulfone is very stable and holds its properties over a wide temperature range, Yaniger says, but the polymer is significantly more expensive and still triggers a positive response in estrogenic activity assays.
Cyclic olefin copolymers, often based on norbornene, are a less well-known class of plastics, but they can have outstanding clarity and chemical resistance, Yaniger adds. These polymers are somewhat more expensive than polycarbonate, but they can readily be made estrogenic-activity-free and have target applications that include baby bottles. "Its Achilles' heel is brittleness, but we think that's a problem that can be solved," Yaniger says.
The other popular polycarbonate alternative is clarified polypropylene (C&EN, Sept. 6, 2010, page 34). It doesn't produce quite as clear a plastic as polycarbonate, but it's inexpensive, tough, highly processable, and chemically inert, Yaniger notes. "Although most commercial examples are estrogenic, our research shows that not all are. For the moment, this is the overall best choice for our baby bottle customers," he says.
Replacing BPA in epoxy can liners is a more intractable problem, Yaniger says. "But given the intense interest and research activity in industry and academia, it's one that will certainly eventually be solved."
Without any lining, a typical aluminum or steel can creates a strong air and light barrier by itself. But over time contact with the food corrodes the metal and can lead to microbial contamination.
That's where BPA-based epoxy resin liners come in. They give canned goods improved shelf life and their good safety record. But residual amounts of BPA can leach into can contents, becoming an unwanted source of human exposure to the chemical (C&EN, July 20, 2009, page 28).
There are a couple of approaches to avoiding BPA in epoxies, Yaniger says. Manufacturers could substitute polyesters or oleoresins for BPA-based coatings. In the case of polyesters, that requires avoiding potentially toxic metal catalyst residues. As for the oleoresins, they often are plant-derived compounds, which could be endocrine disrupters that cause adverse health effects. The other way is to create new types of epoxies using isosorbide or other sugar-based materials. Both approaches cost more than BPA-based resins, he adds.
Epoxy replacement polymers have started to gain some traction, especially polyethylene terephthalate in Japan, Yaniger says. But they remain limited in their use. It's also unknown whether these alternatives are BPA-free or show any reduction in estrogenic activity, he notes.
Another alternative is to not use cans, Yaniger says. Aside from glass, there are plastic packaging options that have the favorable barrier properties of metal cans but don't require BPA-containing epoxy liners, he explains. "The materials that are used for food contact in these technologies are usually polyolefins, which probably aren't estrogenic-activity-free, but they could be made that way." These options provide challenges in heating to sterilize food and can create off-flavors, however.
Although extensive research continues on one-for-one replacements for BPA in polycarbonate and in epoxy resins, Yaniger says, indications are that the implementation of most of those solutions "is still years away."
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