Eastman Chemical may have hit upon the right polymer at the right time. The company had been upgrading its polyester copolymers to give them an advantage over polycarbonate. No sooner had it launched Tritan, the result of those efforts, when the controversy over polycarbonate’s raw material, bisphenol A (BPA), moved into consumer revolt. Baby product makers scrambled to find replacements, and Eastman had one.
Tritan is the latest step in the evolution of Eastman’s specialty polymers business, which is focused on polyester copolymers. Business began more than 30 years ago, when it looked to improve upon the properties of polyethylene terephthalate (PET), which is made by polymerizing ethylene glycol and either terephthalic acid or dimethyl terephthalate (DMT).
PET is fine for bottle applications, but it lacks the toughness necessary for some durable goods and higher-end packaging. Thus, the company developed a range of polyesters that substitute 1,4-cyclohexanedimethanol (CHDM) for some of the ethylene glycol. “It is one of the few monomers we know of that toughens PET,” says David S. Porter, a group leader at Eastman.
Depending on the proportion of CHDM to ethylene glycol, the copolymers have various degrees of chemical resistance, toughness, and stiffness, Porter says. But their poor temperature resistance kept them out of dishwasher-safe housewares.
There was another diol with a long name that Eastman scientists had known since the 1960s could boost the temperature performance of polyesters: 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD). This monomer, CHDM, and DMT form the polymer backbone of Tritan.
But starting the business wasn’t simply a matter of copolymerizing the monomers and marketing the stuff, Porter says. The company had to make sure there was a market for the new polymer and a route to make TMCD economically.
Porter won’t disclose how Eastman makes the monomer—only that it is made of raw materials “in Eastman’s streams.” In the course of researching an upcoming report on plastics for baby bottles, Jeffrey S. Plotkin, director of Nextant’s process evaluation and research planning program, and Marisabel Dolan, senior analyst at the consulting firm Nexant, put together a possible reaction scheme based on patents Eastman and others have filed.
Plotkin and Dolan say the probable route to TMCD is a six-step process beginning with propylene. The first three intermediates are isobutyraldehyde, isobutyric acid, and isobutyric anhydride, molecules that Eastman already sells as specialty chemicals. Dimethyl ketene, 2,2,4,4-tetramethylcyclobutanedione, and finally TMCD, are made in subsequent steps.
As Eastman was working on the monomer technology, it was also sampling the polymer with customers that use polycarbonate and styrene acrylonitrile resins to make housewares such as cutlery, containers, and blenders. Porter found it striking that all of the companies working with polycarbonate that Eastman contacted wanted improved dishwasher durability. That was odd, because the glass transition temperature of polycarbonate is 150 °C, well above the temperature in dishwashers.
Porter then recalled that polycarbonate locks in residual stresses in the molding process because of its rapid vitrification. In the hot, wet, and alkaline dishwashing environment, these stresses start to crack. This is why old polycarbonate cups develop crazing on the bottom. Tritan, he says, doesn’t have those residual stresses or sensitivity to the dishwashing environment.
Frederick L. Colhoun, market development manager for durables at Eastman, says the company started capturing housewares market share soon after it launched the polymer toward the end of 2007. Then, in early 2008, concerns started mounting over the effects of BPA on infant and child development. Retailers such as Wal-Mart banned BPA in infant care products, and manufacturers of baby bottles sought replacements.
According to Nexant, materials being substituted for polycarbonate in baby bottles include Tritan, polyethylene, clarified polypropylene, polyethylene naphthalate, and polyethersulfone. Tritan is found in bottles made by Evenflo, Playtex, and Weil Baby. Plotkin says they have a good, clear appearance and a nice feel. “If you look at baby bottles made with Tritan, they really are very attractive,” he says.
Tritan costs about the same to manufacture as polycarbonate, Plotkin estimates. Its priciest component is the novel monomer, which costs about $1.30 per lb to make, he figures. However, Tritan is cheaper than polyethersulfone, he notes.
Colhoun says Eastman is agnostic on the health effects of BPA, but he notes that if companies are looking for alternatives to polycarbonate, his company sells one.
Eastman will complete an expansion from market development to commercial-scale manufacturing of Tritan later this year. But Colhoun is reluctant to acknowledge the BPA issue as a boon. “It certainly helped to have people looking for alternative materials,” he says. “I would characterize it as a tail wind that aided the adoption of Tritan. But if there was no BPA issue, we could still stand on the strengths of the material itself.”