Process for the production of a transparent flat film from thermoplastic polyurethane and use thereof

#The present invention relates to a process for the production of a transparent flat film from thermoplastic polyurethane (TPU), wherein the film is obtained by introducing the TPU between sheets of a higher-melting material, passing the material between two contrarotating rolls and subsequently removing the sheets.

#Thermoplastic polyurethanes (TPU) are of great industrial significance due to their good elastomer properties and melt processability. A review of the production, properties and applications of TPU may be found, for example. in G. Becker, D. Braun, Kunststoff-Handbuch, volume 7, “Polyurethane”, Munich, Vienna, Carl Hanser Verlag, 1983 or in M. Szycher, Handbook of Polyurethanes, CRC Press LLC, Boca Raton, Fla. USA, 1999.

#The processing of plastics to yield flat films is a process which has long been known and is described, for example, in “Handbuch der Kunststoffextrusionstechnik”, Hensen, Knappe & Potente, part II, “Extrusionsanlagen”, Hanser Verlag, Munich, Vienna, 1986.

#A homogeneous melt was produced from TPU pellets (Texin® DP7-3007, Bayer Corp.) in an extruder (melt temperature approx. 160° C.). Using a flat film die, the TPU melt was introduced into a chill roll/polishing stack. The flat film was formed, sized and polished from the melt in the polishing stack at roll temperatures of 20° C. After cooling, the flat film was cut. The cut flat films were pressed with matted sheets (roughness height 1.5-2 μm) and then with high gloss polished sheets in platen presses at temperatures of approx. 140° C. In this manner, a transparent, light-stable TPU flat film of a thickness of 2 mm was produced.

#A homogeneous melt was produced from TPU pellets (Texin® DP7-3007, Bayer Corp.) in an extruder (melt temperature approx. 160° C.). Using a flat film die, the TPU melt was introduced into a chill roll/polishing stack. Simultaneously, two PC sheets at room temperature ran into the roll nip, such that the TPU melt was introduced between the PC sheets. The melt was formed, sized and polished between the PC sheets in the polishing stack at roll temperatures of approx. 20° C. After cooling, the multilayer sheet was cut or wound into reels. The two PC sheets remained on the TPU flat film to provide protection until the ultimate use of the film and were then simply peeled off the TPU flat film without leaving any residues. The PC sheets may be reused. In this manner, a transparent, light-stable TPU flat film of a thickness of 2 mm was produced.

#Although the present invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope thereof except as it may be limited by the appended claims.