SCIENCE NEWS ONLINE
Food for Thought
August 24, 1996
If it sits on the shelf long enough, even an unopened 2-liter bottle of Coca-Cola or Diet Sprite will lose its zesty effervescence. What happens is the pent up carbon dioxide slowly leaks through microscopic holes in the molecular structure of the container’s plastic. Fortunately, not all plastics are as permeable as the inexpensive polyethylene terephthalate (PET) used to make large soft-drink bottles. One novel class of more rigid polyesters appears to offer particular promise for bottled drinks. It develops extraordinary barrier properties after it’s been transformed into a liquid crystal, for example, by heating.
Benny D. Freeman of North Carolina State University began working with these experimental materials 5 years ago under a grant issued jointly by the National Science Foundation and Electric Power Research Institute (EPRI). Initially, his mission was one of basic research: to understand how heat alters the structure and barrier properties of these polyesters.
At room temperature, they’re frozen glasses. On a molecular level, they resemble microscopic pick-up sticks that had been dropped onto a flat surface — splaying into a disordered pile with ends sticking every which way. Between individual sticks are big gaps, ones large enough for a soft drink’s carbonation to slowly sneak through.
Under heating, Freeman has found, this polyester begins to align and order itself into tightly packed rows of parallel sticks — like boxed toothpicks. This structure possesses far smaller holes for carbon dioxide or any other molecules to slip through. In fact, heat treating can improve the barrier properties of the starting polyester 10 to 100 fold (depending on what’s trying to escape). That increase is substantial, Freeman notes, since the plastic had started out about as leaky as the PET used in today’s soft drink bottles.
Based on Freeman’s findings and EPRI’s financing, a small company in Waltham, Mass., is already developing one such experimental liquid crystal polymer into a superior moisture guard for underground electric cables (above, right). Though these designer plastics are expensive — typically about $10 to $15 per pound, so little is needed that they’re expected to add no more than perhaps a penny per foot to the cost of cable that now runs about $1.25 per foot.
If the new cable sheathing becomes commercially successful, Freeman says, “that single application would double the worldwide market for liquid crystal polymers to about 20 million pounds per year.” Such a dramatic increase in demand for this plastic should also bring down its cost, making it more attractive to bottlers of carbonated beverages, including many who eschew plastics today.
For instance, commercial plastics are so permeable to oxygen, which can destroy the taste of beer, that brewers have generally stuck to glass and metal. Liquid-crystal polyester bottles should preserve the flavor of your ale or lager far longer — though still not as long as glass.
Or consider juice purveyors. Limonene and many other trace flavorants in fruit juices and soft drinks can migrate into PET and other conventional packaging plastics. Not only can this change a drink’s taste, but if the plastic were later reused for some other application, it could shed those trace contaminants into other foods or materials where they might not be appreciated. Liquid crystal polymers appear to make such good barriers, Freeman says, that they could seal in or out any possible adulterants far better than today’s commercial plastics — though, again, not quite as well as glass.
In fact, where bottlers want to sterilize and reuse containers, these experimental polyesters might well stand in for glass, offering the convenience of no breakage and lighter weight.
While it might be fun to imagine these bottles changing colors, like mood rings of yore (photos, above), bottlers will probably opt for a more prosaic clear or milky opaque form. Indeed, Freeman points out, the liquid crystalline materials in watch faces and some toys change their hue only after they have been sandwiched between two sheets of polarized film and then subjected to a force that temporarily imposes order onto their normally amorphous rod-like structure.
References:
McDowell, C.C., H.C. Shen, and B.D. Freeman. 1966. Thermal transitions and structure evolution in PICT, a soluble nematic LCP exhibiting a kinetically trapped, disordered structure. American Chemical Society annual meeting (polymer division), New Orleans.
Benny D. Freeman, Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905.
This week’s Food for Thought is prepared by Janet Raloff, senior editor of Science News.