January/February 2011

An upside to carbon dioxide

Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen, Germany, are pursuing a new idea by testing how carbon dioxide can be used to impregnate plastics, a process that could lead to new applications ranging from colored contact lenses to bacteria-resistant door handles. At a temperature of 30.1°C and a pressure of 73.8 bar, CO2 goes into a supercritical state that gives the gas solvent-like properties. In this state, it can be introduced into polymers, or act as a "carrier" in which dyes, additives, medical compounds, and other substances can be dissolved.

"We pump liquid carbon dioxide into a high-pressure container with the plastic components that are to be impregnated, then steadily increase the temperature and the pressure until the gas reaches the supercritical state. When that state is reached, we increase the pressure further. At 170 bar, pigment in powder form dissolves completely in the CO2 and then diffuses with the gas into the plastic. The whole process only takes a few minutes. When the container is opened, the gas escapes through the surface of the polymer but the pigment stays behind and cannot subsequently be wiped off," said Dipl.-Ing. Manfred Renner, a scientist at Fraunhofer UMSICHT.

In tests, the researchers have even managed to impregnate polycarbonate with nanoparticles that give it antibacterial properties. E-coli bacteria, placed on the plastic's surface in the institute's own high-pressure laboratory, were killed off completely-a useful function that could be applied to door handles impregnated with the same nanoparticles. Tests conducted with silica and with the anti-inflammatory active pharmaceutical ingredient flurbiprofen were also successful. "Our process is suitable for impregnating partially crystalline and amorphous polymers such as nylon, TPE, TPU, PP and polycarbonate," said Renner, "but it cannot be applied to crystalline polymers."

The process holds enormous potential, as carbon dioxide is non-flammable, non-toxic and inexpensive.

Whilst it shows solvent-like properties, it does not have the same harmful effects on health and on the environment as the solvents that are used in paints, for example. Conventional processes for impregnating plastics and giving them new functions have numerous drawbacks. Injection molding, for instance, does not permit the introduction of heat-sensitive substances such as fire retardants or UV stabilizers. Many dyes change color; purple turns black. "Our method allows us to customize high-value plastic components and lifestyle products such as mobile phone shells. The best about it is that the color, additive, or active ingredient is introduced into layers near the surface at temperatures far below the material's melting point, in an environmentally friendly manner that does away with the need for aggressive solvents," said Renner. The process could, for example, be used to dye contact lenses and lenses could even be enriched with pharmaceutical compounds that would then be slowly released to the eye throughout the day, representing an alternative to repeated applications of eye drops for the treatment of glaucoma.

Cleaning up pollution with banana peels

carbon1Just when you thought banana peels were destined merely to be the tormentor of cartoon characters and video-game go-kart racers, science has found a use for them. Turns out peels can clean dirty water.

According to Brazilian media outlet Folha.com, Milena Boniolo, a chemist from the Federal University of São Carlos, near São Paulo, made a discovery that could potentially save thousands of banana peels from the garbage heap. When dried and ground into a powder, said Boniolo, peels have the ability to clean up polluted water.

Boniolo estimates her city's restaurants alone discard around four tons of the stuff each week. If her technique is implemented, it could be a low-cost alternative to the expensive methods used by industries currently-which often use things like magnetic nanoparticles to clean water.

Banana peels, evidently, are rich in negatively charged molecules, so they attract the heavy and positively charged metal pollutants in water. For every treatment with the peel powder, around 65% of the water was decontaminated. The process can also be repeated purify the water almost completely.

"I started doing it at home. It's really easy," said Boniolo.

The discovery was made as part of her Ph.D. dissertation, but Boniolo said she is hoping it will actually be used by industries as a way to keep contaminants out of the water and find a new use for organic waste.