Impregnating plastics with carbon dioxide

Compared to carbon dioxide, Jack the Ripper enjoys a pretty good reputation.

The most important and plentiful greenhouse gas produced by human activities (it’s what human beings and other mammals exhale), carbon dioxide – more widely known as CO2 – is the number one culprit fingered for global warming, and as such is a constant target of the environmental movement.      

But the gas also has positive characteristics – the chemical industry makes use of this colorless gas to produce urea, methanol and salicylic acid, for example – and some of these are now being used to bring new qualities to another favorite target of environmentalists: plastics.

Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen, Germany are now impregnating plastics with compressed CO2 in a process that could lead to new applications ranging from colored contact lenses to bacteria-resistant door handles.

Here’s how the science works: At a temperature of 30.1 degrees Celsius and a pressure of 73.8 bar, CO₂ goes into a so-called “supercritical state” that gives the gas solvent-like properties; in this state, according to Fraunhofer UMSICHT’s Dr. Manfred Renner, 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,” Renner explained. “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. 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.”  

To date, 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 our laboratory, were killed off completely – a useful function that could be applied to door handles impregnated with the same nanoparticles, Renner said. “Our process is suitable for impregnating partially crystalline and amorphous polymers such as nylon, TPE, TPU, PP and polycarbonate.”

According to Renner, big benefits of using C02 are that it’s non-flammable, non-toxic and inexpensive (we exhale the stuff, after all). Conventional processes for impregnating plastics and giving them new functions have numerous drawbacks, he continued; injection molding, for example, does not permit the introduction of heat-sensitive substances such as fire retardants or UV stabilizers. Also, many dyes have the bad habit of changing color. “Our method allows us to customize high-value plastic components and lifestyle products such as mobile phone shells,” Renner said. “Best of all, 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 dangerous and harmful solvents.”

The process is suitable for a broad range of new applications, Renner said, some of them fairly exotic – adding dyes to contact lenses, for instance, and also embedding them with time release medicine to treat such maladies as glaucoma. 

Not bad for a widely unpopular trace gas.