FIELD OF THE INVENTION 0001 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.

BACKGROUND OF THE INVENTION 0002 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. 0003 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. 0004 Transparent TPU flat films are normally produced by melt-processing the material in an extruder, wherein it is subsequently pressed one or more times. Among the disadvantages of said process are elevated handling costs, the risk of soiling by dust, pressing defects, formation of deposits and sticking to the polishing rolls. 0005 When thermoplastic polyurethanes are converted into flat films, gel particles, which are disruptive to the optical quality of the product, are often observed. This is particularly disruptive with transparent grades (as, for example, described in EP-A 1 090 940) which, in addition to transparency, are also required to exhibit light stability and flexibility.

SUMMARY OF THE INVENTION 0006 The present invention therefore provides a process for the production of defect-free, transparent TPU flat films, in which it is possible to dispense with labor-intensive pressing operations and in which the problematic issues of soiling, pressing defects, formation of deposits and sticking to polishing rolls are greatly reduced or eliminated.

DETAILED DESCRIPTION OF THE INVENTION 0007 The present invention will now be described for purposes of illustration and not limitation. 0008 The process according to the present invention produces a transparent, smooth, defect-free TPU flat film in a single step, i.e., without additional pressing of the flat films. 0009 The present invention provides a process for the production of a transparent flat film from thermoplastic polyurethane (TPU flat film), wherein 0010 a) a molten thermoplastic polyurethane (TPU) with a temperature of 150 C. to 220 C. is introduced between two sheets of a higher-melting thermoplastic material with a temperature of 0 C. to 80 C., 0011 b) the product from a) is passed between two contrarotating rolls with a temperature of 10 C. to 70 C., 0012 c) cooling is then performed, 0013 d) cutting is optionally performed, and 0014 e) the sheets of the higher-melting plastic are removed mechanically. 0015 Polycarbonate may preferably be used as the higher-melting material by virtue of its good dimensional stability at low wall thickness, its elevated melting point and optical clarity (optionally required for assessing defects in the flat film). In addition to polycarbonate, other polymers, such as for example polyamide, polyesters, polyolefins, mixtures thereof and polycarbonate blends, may also be used. A review of the production, properties and applications of polycarbonates may be found, for example, in Kunststoff-Handbuch (G. Becker, D. Braun, volume 3/1 Polycarbonate, Polyacetale, Polyester, Celluloseester, Munich, Vienna, Carl Hanser Verlag, 1992 or in Handbook of Polycarbonate Science and Technology (D. Legrand, J. Bendler), New York, Basel, Marcel Dekker, 2000. 0016 The sheets of the higher-melting plastic used may be of a thickness of 0.1 mm to 5 mm, preferably of 0.3 mm to 3 mm and particularly preferably of 0.5 mm to 2 mm. 0017 The molten TPU in the present invention may preferably be an aliphatic, transparent TPU, such as for example that described in EP-A 1 090 940. 0018 The TPU flat film produced using the process according to the present invention may have a thickness of 0.1 mm to 5 mm, preferably of 0.3 mm to 3 mm and particularly preferably of 0.5 mm to 2.5 mm. 0019 The thermoplastic polyurethane (TPU) may preferably be melted by means of a extruder before being introduced between the sheets of the higher-melting plastic. 0020 Once removed from the TPU flat film, the sheets of higher-melting plastic may preferably be reused in step a) of the process. 0021 The TPU flat films produced using the process according to the present invention may be particularly useful for optical applications. 0022 The following Examples are intended to illustrate the invention in greater detail.

EXAMPLES

Comparative Example 1 0023 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 m) 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.

Example 2 0024 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. 0025 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.