A new technology to manufacture polypropylene foam sheet and biaxially oriented foam film: Dissertation

Foamed polymers are future materials with their wide application spheres. They can be used to enhance appearance as insulating structures or to save material costs. This study addresses all of these and concerns a new technology to produce foamed tubes and films. An entire blown film extrusion line was built up to fulfil the final target, biaxially oriented foam film with closed cell, microcellular foam structure. Extrusion conditions, which in general means temperature, pressure and flow control of viscoelastic material, is discussed and examined experimentally to define the dominating characteristics. Screw and die design together with foam stabilisation is taken under stringent examination, resulting a three layer blown film die focused for foamed extrudates. The effects of chemical foam nucleating agents and physical and chemical foaming are examined during the study concluding in formation of cell embryos by chemical foaming agents and expanding them into low density foam with physically compressed nitrogen gas. The effect of processing parameters in foam structure is examined and adapted according to the polypropylene. The main characteristics of polypropylene were defined by rheological measurements to promote foaming studies in extrusion. Sharp melting point and linear structure of polypropylene caused troubles in keeping the foam structure closed during expansion, but was also partly resolved with new bimodal type polypropylene owing to high melt strength properties. Some applications, leading to new commercialised innovations in foam films, will also be discussed in greater detail.