Promising trials around the world on using carbon dioxide as a feedstock for plastics production (see Plasteurope.com of 18.08.2010) show that CO₂ could be more useful than its reputation suggests. Now the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT, Oberhausen / Germany; www.umsicht.fraunhofer.com) has come up with another good use for carbon dioxide. Based on the versatility of this colourless gas, which is used in the chemical industry to produce substances such as urea (fertilisers), methanol (fuel additives) and salicylic acid (found in aspirin), scientists at UMSICHT are looking into completely different applications. They are trying to find out whether carbon dioxide can be used to impregnate plastics.

At 30.1 °C and 73.8 bar, the gas takes on a supercritical state in which its behaviour is similar to a solvent. According to UMSICHT, in this state it can be used as a carrier, for example, to dissolve colorants, additives and medical compounds and encase them in polymers. “We pump liquid carbon dioxide into a high-pressure container with the plastic components that are to be impregnated, then steadily raise both temperature and pressure until the gas reaches the supercritical state. Then we raise the pressure. At 170 bar, the powdered colorant dissolves completely in the CO₂ and diffuses with the gas into the plastic,” explains Manfred Renner, a scientist working at UMSICHT. This process only takes a few minutes. When the container is opened, the gas vaporises from the surface but the colorant remains in the polymer and can no longer be wiped off.

The researchers report that they have conducted trials in which they impregnated polycarbonate with nanoparticles that produce antibacterial properties. E-coli bacteria applied to the surface were completely killed off in tests in the institute's high-pressure lab. The scientists anticipate that this method could be used to impregnate doorhandles with nanoparticles. Tests with the anti-inflammatory active ingredient flurbiprofen and silicon dioxide have also been successful. “Our process is suitable for impregnating partially crystalline and amorphous polymers such as nylon, TPE, TPU, PP and polycarbonate, but it cannot be applied to crystalline polymers,” says Renner.

The process has enormous potential because carbon dioxide is non-flammable, non-toxic and, most importantly, inexpensive. Although its behaviour is similar to a solvent, it does not have the adverse health and environmental effects associated with solvents used in paints. Moreover, painted surfaces are easily scratched and damaged. Conventional methods of impregnating plastics to alter their functional properties have a number of drawbacks according to UMSICHT. For example, heat-sensitive substances such as fire retardants and UV stabilisers are not suitable for injection moulding. "The best thing about [our method] is that the colour additive or active ingredient is introduced into layers near the surface at temperatures far below the material’s melting point,” explains Renner.

Other potential applications for the process include the coloration of contact lenses. The lenses could also be enriched with pharmaceutical ingredients that would be released into the eye continuously during the day as an alternative to repeated application of eye drops, for example, for the treatment of glaucoma.