pressure, the yellow dye powder dissolved in the CO2 which transferred it into the plastic. |
CO2
is more than just a waste product. In fact, it has a variety of uses:
the chemical industry makes use of this colorless gas to produce urea,
methanol and salicylic acid. Urea is a fertilizer, methanol is a fuel
additive, and salicylic acid is an ingredient in aspirin.
Researchers
at the Fraunhofer Institute for Environmental, Safety and Energy
Technology UMSICHT in Oberhausen are pursuing a new idea by testing how
carbon dioxide can be used to impregnate plastics. At a temperature of
30.1 degrees Celsius 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. “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,” explains
Dipl.-Ing. Manfred Renner, a scientist at Fraunhofer UMSICHT.
In
tests, 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 the institute’s own
high-pressure laboratory, were killed off completely – a useful function
that could be applied to door handles impregnated with the same
nanoparticles. 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,”
states Renner, “but it cannot be applied to crystalline polymers.”
The
process holds enormous potential, as carbon dioxide is non-flammable,
non-toxic and inexpensive. Whilst it shows solvent-like properties, it
does not have the same harmful effects on health and on the environment
as the solvents that are used in paints, for example. Painted surfaces
are also easily damaged and are not scratch-resistant. Conventional
processes for impregnating plastics and giving them new functions have
numerous drawbacks. Injection molding, for instance, does not permit the
introduction of heat-sensitive substances such as fire retardants or UV
stabilizers. Many dyes change color; purple turns black.
“Our
method allows us to customize high-value plastic components and
lifestyle products such as mobile phone shells. The best about it is
that the color, additive or active ingredient is introduced into layers
near the surface at temperatures far below the material’s melting point,
in an environ mentally friendly manner that does away with the need for
aggressive solvents ,” says Renner.
The
process could, for example, 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. According to the scientist, this new impregnation method is
suitable for a broad range of new applications.
