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in Industry, Aachen, Germany and the Skilled Crafts at RWTH Aachen University have recently developed process technology that uses CO₂ in the solid state--so-called dry ice --to produce medium-heavy foamed extrusions. The newly developed process combines the advantages of physical foaming with those of chemical foaming, researchers say.

Like the masterbatch in chemical foaming, the dry ice is fed into the extruder in pellet form. No extruder modifications are needed. Moreover, being a physical blowing agent, dry ice does not leave behind any reaction residues such as water in the extrudate, making the process suitable for hydrolysis-sensitive materials, IKV states. 

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The new metering technique, developed with motan-colortronic GmbH, Isny, Germany, feeds dry ice pellets via a metering screw and gravity mixer directly into the feed zone of the extruder. This avoids premature sublimation of the dry ice and cooling of the plastics granules, researcher says. Through rapid melting of the plastic and fast pressure build-up, the dry ice becomes dissolved in the melt. IKV has since widely tested the new metering technique and has developed relevant process parameters.

With the new process technology, foam densities of a minimum of 350 kg/m³ with LDPE can be achieved, which are comparable with those of chemical foaming, IKV says. And by adjusting the process temperatures, the technique can also reportedly be applied to mateials such as PP.

Despite the comparatively high sublimation losses in the metering of the blowing agent, IKV maintains, the process can compete with chemical blowing agents due to the lower cost of CO_2.The process allows an inexpensive entry into foam extrusion because of the fact that any retrofitting is confined to the metering technology, say the researchers.