Research accelerates on advanced water-treatment technologies as their use in purification grows
MAIRIN B. BRENNAN, C&EN WASHINGTON
Conventional water purification is a tried-and-true process that hasn’t changed much in decades: Coagulation and flocculation, sedimentation, sand or gravel filtration, and chlorine disinfection are the customary steps. Wresting fresh water from seawater is also a long-standing technique, especially in oil-rich, water-starved countries where the cost of the energy-intensive process is not an issue.
RALEIGH, North Carolina (CNN) — Ever wonder why soft drinks sometimes go flat even before you’ve opened the bottle? Ever wonder when somebody’s going to do something about it?
Wonder no more. Researchers at North Carolina State University (NCSU) are experimenting with liquid crystal polymers they say could be used to make plastic soda bottles and other plastic packaging virtually impervious to gas leakage, thus greatly increasing the “shelf life” of hundreds of products.
Leakage is a universal problem in plastic packaging. Every plastic soda bottle that rolls off a production line, every food product packed in plastic, and every plastic container of any kind on any store shelf anywhere will leak to some degree.
It may not be apparent to the casual observer, but slow, invisible leaks can affect the quality of products.
“Anything we have is vulnerable to some degree to air, the oxygen in the air, loss of flavor, gain of outside odors,” said packaging consultant Aaron Brody.
The problem, NCSU researchers say, is that oxygen and other gasses dissolve into the walls of polymer-based plastic containers much like sugar dissolves in coffee.
“The molecules of water actually dissolve into the body of the polymeric film, and then move through the polymeric film itself,” says Dr. Benny Freeman, one of the researchers trying to solve the problem. Dr. Freeman speaking about this issue.
Freeman and others are conducting experiments to compare the ability of various liquid crystal polymers to form gas-tight barriers.
The test involves suspending polymer samples from springs inside gas-filled chambers. The more gas a material absorbs, the heavier it gets, says doctoral candidate Chris McDowell.
Freeman compares the molecules in polymers to logs lined up together to form an oxygen barrier 100 times better than that of today’s soda bottles.
Depending on the outcome of the experiments, the airtight polymers may have other uses. Freeman says the electrical industry is already considering using a sleeve made of liquid crystal polymers to extend the life of underground power cables.
And Brody, co-author of an authoritative book on packaging, says the day may soon come when the public will find even beer packaged in plastic, just like soft drinks.
“We’re converting just about everything else into plastic,” he says, “Why not beer?”
Tomorrow Today examines a new weapon against termites — one that drastically reduces the amount of toxic chemicals needed to control the pests.
Anyone who’s ever opened a bottle of soda knows that sound. But why is it that even unopened plastic soda bottles lose their fizz over time? I’m Jim Metzner, and this is the Pulse of the Planet.
“The carbon dioxide that gives soda what people normally associate with as fizz is soluble and will dissolve into the wall of the plastic and be transported through the plastic and escape. In much the same way that air in your tires will eventually escape and the tire pressure goes down with time.”
Benny Freeman is an Associate Professor of chemical engineering at North Carolina State University. He’s been studying the effectiveness of plastic packaging.
“Any plastic will permit leakage of small molecules through the plastic. The current packaging materials for things like soda bottles have leak rates that are acceptable for large sizes, but become unacceptable for smaller size bottles or for applications like beer packaging which are more sensitive to things like oxygen coming in from the outside of the package.”
And that’s why you don’t currently see beer packaged in plastic containers.
“For an application like beer where beer is very very sensitive to even small amounts of oxygen, the major factor limiting the use of plastic packaging for beer is that oxygen from the surrounding atmosphere gets into the package and causes the beer to get stale or taste flat.”
But Professor Freeman and his colleagues are on the track of using new kinds of plastic which form more effective barriers. We’ll hear more in future programs.
Pulse of the Planet is presented by DuPont, makers of better things for better living.
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.
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.
Raleigh (AP) – Does your soda go flat? Is the gas tank on your car corroding? Are your wine bottles to heavy?
If so, researchers at North Carolina State University may have the solution to your problems.
Chemical engineers at the school are working on a strain of liquid crystalline polymer plastic that could seal in flavor and carbonation while keeping out air. The plastic could also be used to stop corrosion of fuel tanks and underground power lines.
“The main science behind it is these polymer molecules pack very well,” said Chris McDowell, a doctoral student working on the project. “The molecules are long and stiff like a pen. That allows them pack together so there is no room for small molecules like oxygen to pass through.”
Beer and wine are extremely sensitive to oxygen and quickly lose their flavor if air gets into the packaging. But the new variety of plastic, known as PICT, could keep enough oxygen out to make plastic beer and wine containers practical.
The plastic also could be used to extend the shelf life of bottled soda, which now keeps for six to eight weeks, and to make smaller, 12-ounce, soda bottles. Existing plastic bottles, which are made from another kind of plastic called PET, can be used to hold only 16 or more ounces of soda.
“Due to how fast these small molecules migrate through PET polymers, you cannot have small packages, ” McDowell explained. “It has to do with the amount of surface area in the smaller packages.”
Dr. Benny Freeman, associate professor of chemical engineering, presented the team’s findings Monday at the American Chemical Society’s annual meeting in New Orleans.
Researchers are uncertain how long commercial development of their plastics might take, but estimate the new food bottles could appear on store shelves within the next decade. The process is expensive now, but Freeman believes his group’s work will pave the way for cost-effective production of food containers and specially coated underground cables.
“Underground electric power cables with a thin coating of these plastics would have substantially longer lifetimes than non-coated cables because of the prevention of moisture ingress and subsequent corrosion,” Freeman said. “Yet adding the coating only adds about one cent per foot to the total production cost.”
Freeman also hopes the plastic will be used to make lightweight, corrosion resistant fuel tanks for cars running on reformulated gasoline.
The project is currently funded by the National Science Foundation and the chemical company Hoechst Celanese.