German researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology are impregnating plastics with compressed CO2 using a process that could lead to new products ranging from coloured 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.
In the process developed by the Fraunhofer researchers, liquid carbon dioxide is first pumped into a high-pressure container with the plastic components that are to be impregnated, then the temperature and the pressure are increased until the gas reaches the supercritical state.
When that state is reached, the pressure is increased further and, at 170 bar, pigment in powder form dissolves completely in the CO2 and then diffuses with the gas into the plastic. When the container is opened, the gas escapes through the surface of the polymer but the pigment stays behind.
In tests, the researchers have used the system 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. 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 Manfred Renner, a scientist at Fraunhofer, ’but it cannot be applied to crystalline polymers.’
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 eliminates the need for aggressive solvents.
Renner said that the process could 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.