Plastics improved with CO2

The process has been invented by researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen, Germany. At a temperature of 30.1^°C and a pressure of 73.8 bar, CO2 goes into a supercritical state that allows the gas to act as a solvent. In this state, it can be introduced into polymers, or act as a carrier in which additives can be dissolved. Manfred Renner, a scientist at Fraunhofer UMSICHT explained, "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." In tests, the researchers have also managed to impregnate polycarbonate with nanoparticles that give it antibacterial properties. E-coli bacteria, placed on the plastic's surface in the institute's laboratory, were killed off completely. 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," stated Renner, "but it cannot be applied to crystalline polymers." Conventional processes for impregnating plastics and giving them new functions have numerous drawbacks. Injection moulding, for instance, does not permit the introduction of heat-sensitive substances such as fire retardants or UV stabilisers. Many dyes change colour; purple turns black. "Our method allows us to customise high value plastic components and lifestyle products such as mobile phone shells. The best about it is that the colour, 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 ," Renner said. The process could also be used to dye contact lenses as well as enrich them 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.