Process for preparing soft, elastic polyurethane

Details Process for preparing soft, elastic polyurethane Process for preparing soft, elastic polyurethane

2000-07-06

2003-03-04

Lechert, Jr., Stephen J. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming articles by uniting randomly associated particles

Autogenously or by activation of dry coated particles

Reexamination Certificate

active

06527995

ABSTRACT:

CROSS REFERENCE TO RELATED PATENT APPLICATION
The present patent application claims the right of priority under 35 U.S.C. §119 (a)-(d) of German Patent Application No. 199 33 261.4, filed Jul. 15, 1999, and German Patent Application No. 199 42 393.8, filed Sep. 6, 1999.
FIELD OF THE INVENTION
This invention relates to soft, elastic thermoplastic polyurethane films (TPU films), which are produced by sintering pulverulent thermoplastic polyurethanes—also referred to below by the abbreviation TPU—with the aid of a heated mould, a process for their production and their use.
BACKGROUND OF THE INVENTION
The production of PU films (polyurethane films) by casting from liquid PU formulations or by extrusion of TPU is known and is described, for example, in Kunststoff-Handbuch “Polyurethane”, Volume 7, Second Edition, page 469 (edited by Dr. G. Oertel; Carl Hanser Verlag, Munich, Vienna, 1983).
SUMMARY OF THE INVENTION
Starting from the liquid formulations, either blocks are cast and films are cut from them, or the films are produced directly in the centrifugal-moulding process. Films made from TPU in thicknesses of 0.03 to 0.3 mm are conventionally produced in the blowing process; thicker films, for example, up to about 3 mm, are produced by slot-die extrusion. The production of PU films from TPU by sintering is not mentioned in the monograph cited above.
The use of decorative plastics films in the interiors of motor vehicles is also known [R. Pfriender, Kunststoffe, 76 (1986), 10, page 960 ff.]; here the plastics mouldings are coated with films or the films or skins are foamed at the back with foamed plastics, preferably PU foamed plastic.
Where PU is used, the surface layers are in most cases produced from two-component PU systems by the IMC (in-mould coating) process. In this process the mould, heated to about 50° C., is first of all sprayed with a release agent, then the PU two-component coating and subsequently the PU supporting layer are introduced into the open mould. This manufacturing technique for the production of corresponding component parts is arduous and to date has hardly been used by processing technicians (Dr. M. Wachsmann, Kunststoffberater, 10/1987, pages 27 to 28).
DE-A 4 203 307 discloses the use of aliphatic TPU powders for producing sintered films. In DE-A 4 203 307 no reference is made to a specific melt index range (MVR range) which would result in a favourable processing behaviour. On the contrary, a very wide MVR range is covered by the range of the NCO to OH ratio (index) mentioned, extending from 97 to 99.
In prior art, PVC/ABS films are conventionally formed by the thermoforming process and subsequently foamed at the back in a second processing step. PVC films can be produced by the PVC powder slush process. For this, the mould is heated in a furnace to about 250° C., the pulverulent PVC is then distributed evenly in it and the mould is again heated in the furnace in order to gel the PVC skin. After the mould has been cooled, for example, in a water bath, the film can be removed and then foamed at the back. The films produced by the PVC powder slush process are considerably less expensive than ABS/PVC films, PU IMC films and TPU films. A disadvantage of mouldings made from PVC films foamed at the back with PU foamed plastics is the negative interaction of the PVC film and the PU backing foam. Thus constituents such as, for example, catalysts, stabilisers et cetera diffuse out of the PU foamed plastic into the decorative film, and conversely the plasticiser migrates from the PVC film into the PU foamed plastic. As a result of these migration processes, the mouldings are mechanically damaged, for example, by contraction or embrittlement, and their appearance is altered as a result of discoloration and specking (Kunststofftechnik, VDI Verlag GmbH, Düsseldorf, 1987, “Kunststoffe als Problemlöser im Automobilbau”, pages 141 ff.).
EP-A 399 272 describes elastic polyurethane films which are produced from pulverulent TPUs by sintering. A melt index (MVR) [at 190° C. and an applied weight of 212 N] of from 50 to 350 is specified for the TPUs. It is expressly stated in EP-A 399 372 that a TPU which is suitable for the sintering process has to meet the criteria mentioned with regard to melt index. The required high processing temperatures of 220° C. to 280° C., preferably of 230° C. to 270° C., are a disadvantage. At these high processing temperatures TPUs in general, and particularly TPU based on polyethers, tend firstly to undergo a molecular weight decrease as a result of chain splitting (deterioration of the mechanical properties) and secondly, above all in TPUs based on polyethers and/or aromatic diisocyanates, to become yellow (discoloration even of dyed test pieces; see also the Dissertation by Wolfgang Endres in the Faculty of Chemistry at the University of Osnabrück, 1994). A TPU which has an MVR of 50 at 190° C. and 212 N exhibits an MVR of 2 at 21.2 N and 190° C. A TPU which has an MVR of 40 at 190° C. and 21.2 N exhibits an MVR of>500 at 212 N and 190° C., that is, the TPU “runs through” and is no longer measurable. The high processing temperatures of 220° C. to 280° C. described in EP-A 399 272 are also disadvantageous owing to the associated long cycle times as a result of the long cooling and heating times.
The object of the present invention, accordingly, was to provide a material which can be processed into films in the sintering process even at low processing temperatures of 180° C. to 215° C., preferably 190° C. to 215° C.
Surprisingly, this object was achievable by means of TPU films produced from certain TPUs by sintering.
The invention provides soft, elastic thermoplastic polyurethane films obtainable by sintering pulverulent thermoplastic polyurethanes—also referred to below by the abbreviation TPU—having a melt index at 190° C. and at an applied weight of 21.2 N (2.16 kp) of from 20 to 100, preferably 30 to 60, and a relative solution viscosity of from 1.15 to 1.45, preferably of 1.25 to 1.35, as well as a Shore hardness of from 80 to 90, preferably from 85 to 94, with the aid of a heated mould, the pulverulent TPU being obtainable by the reaction of
a) organic diisocyanates,
b) a polyol (polyether diol, polyester diol, polycarbonate diol or mixtures thereof)
c) a chain extender (diol or diol/diamine mixture) and
d) optionally conventional auxiliary substances and additives.
In connection with the production of the TPU films according to the invention, the sintering of TPUs includes the following processes: production by the “powder slush process”, as described, for example, in DE-A 3 932 923; production by the calendering process, as described, for example, in Kunststoff-Handbuch “Polyurethane”, Volume 7, or production by the coextrusion process, as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition.
The pulverulent TPU is obtained preferably by the reaction of
a) aromatic diisocyanates, preferably 4,4′-diphenylmethane diisocyanate,
b) a polyol having an average molecular weight of between 600 and 5000 g/mol,
c) a chain extender having an average molecular weight of between 60 and 500 g/mol and
d) optionally conventional auxiliary substances and additives.
TPUs based on aromatic diisocyanates are not particularly suitable for applications having especially high requirements with regard to light resistance. In EP-A 399 272 it is stated that TPUs based on aliphatic diisocyanates are light-resistant, but that they can be handled only with difficulty after thermoplastic processing. For that reason, mixtures with TPUs based on aromatic diisocyanates are proposed. However, in many cases these mixtures likewise do not meet the high requirements with regard to light resistance. These high requirements are only met, surprisingly, if the TPUs are built up predominantly (more than 90 wt. %) on the basis of aliphatic diisocyanates. It has now been found that a satisfactory handling of these aliphatic TPUs is achieved even after thermoplastic processing, if the above-mentioned pulve

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