Better Foam Structure During PP Extrusion
Foams extruded from linear PP have the advantage of lower material costs compared to the commonly encountered PU foams. Moreover, PP foams are thermoplastic and thus more easily recyclable. Production of closed-cell foams from linear PP, however, is difficult, because the material has no molecular branching or cross-linking. As a consequence, the melt does not strengthen when subjected to elongational flow. This leads to thinning of the cell walls during the foaming process, the foam bubbles combine, creating an open-cell structure.
A research team at the Department of Chemical Engineering and Materials Science of Michigan State University has now succeeded in considerably improving the behavior of PP during foaming through use of a special additive system. The additive is a nanocomposite based on organoclays. Maleic acid anhydride-grafted PP serves as the compatibilizer.
During processing, materials prepared in this way exhibit considerably better foaming, with formation of a closed-cell structure characterized by very tiny bubbles. It was possible to noticeably increase the yield strength of the foam with as little as 3 % organoclay. The researchers observed that the material exhibits a behavior similar to that of blends to which a small amount of cross-linking material has been added. They attribute this to a molecular effect that they call trapped entanglements. Under certain circumstances, the silicates and the grafted PP for a system of physical "cross-linking" that encloses the entanglements and behaves like a chemically cross-lined polymer system. Intentions are to investigate this molecular mechanism in further experiments.