Carbon dioxide may be used like a solvent to impregnate molded articles with pigments, antimicrobials, or other chemicals. One big advantage is that chemicals are embedded in the polymer structure, not just applied on the surface like a paint. The process may also be less expensive than compounding pigments or other chemicals into an entire molded part.
Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen, Germany are testing how carbon dioxide can be used to impregnate plastics. They say that carbon dioxide goes into a supercritical state that gives the gas solvent-like properties at a temperature of 30.1C and a pressure of 73.8 bar. In this state, carbon dioxide can 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,” says Manfred Renner, a scientist at Fraunhofer UMSICHT. “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 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.”
The propeller shown below was dyed yellow in only five minutes at 90Celsius and 200 bar. (© Fraunhofer UMSICHT)