Stock material or miscellaneous articles – Composite – Of polyester
Patent
1991-08-23
1993-09-07
Thibodeau, Paul J.
Stock material or miscellaneous articles
Composite
Of polyester
15624411, 156247, 156344, 427171, 428483, 428910, B32B 2706
Patent
active
052427579
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to novel polyester films which can be used, in particular, for the preparation, by extrusion coating, of composite films containing a layer of an olefin polymer on at least one of the sides of the polyester film and to the composite films resulting therefrom.
The composite films containing a layer of oriented semi-crystalline polyester and, on at least one side, a layer of an olefin polymer, and in particular of polyethylene, are used in the packaging sector as sealing tape to ensure the leaktightness of numerous recipients intended to contain liquids (milk, fruit juice, wines). The process for the production of these composite films which industrially is the most advantageous consists in carrying out the extrusion of a molten olefin polymer on at least one side of an orientated semi-crystalline polyester film, separating close to the extrusion die for the olefin polymer; this process is generally denoted by the term "extrusion-coating process". The composite film which results from the deposition of the olefin polymer on the polyester support is then cooled, for example by placing the polyolefin side against the surface of a cooling drum. In order to obtain composite films carrying a polyolefin coating on each side of the polyester film support, hereinafter termed "double-sided layered composite films", the complex film resulting from the first step of extrusion-coating is subjected, after cooling, to a repeat identical operation on the non-coated side of the polyester.
In spite of the advantage which it shows, the extrusion-coating process suffers from the drawback of leading to composite films having various anomalies with respect to the olefin polymer/polyester support bond. Thus, it has been found that the delaminating force (that is to say the force necessary to break the bond between the polyolefin coating and the support film) varies, during delaminating, between extreme values on either side of a mean value. This fluctuation in the delaminating force makes its determination uncertain and imprecise and is prejudicial to the industrial and commercial advantage of composite films. It has been found that it can affect the two layers of polyolefin coating when the support film has been the subject of a double extrusion-coating and that it can manifest itself whatever the mean value of the delaminating force.
The studies carried out by the Applicant have also enabled it to be shown that in the case of polyester support films subjected to a double extrusion-coating the delaminating force of the layer resulting from the second extrusion-coating is lower than the delaminating force of the polyolefin layer provided on the other side of the polyester film du ring the first extrusion-coating stage and can assume values less than the minimum value demanded for certain applications in the packaging sector, that is to say a value of less than 200 g/10 mm.
Without the present invention being limited by the explanation of the cause of these anomalies, it would appear that they are caused by the thermal shock to which the polyester support is subjected during the extrusion-coating. In fact, during the production of the polyolefin coating the polyester support film is brought into contact with the olefin polymer brought to a temperature of the order of 280.degree. to 330.degree. C., as a function of the nature of the latter and/or the extrusion-coating conditions, and is then cooled to ambient temperature. Where relevant, the polyester support film is subjected to a second thermal shock during the production of another polyolefin coating on its second side. The sudden rise in the temperature of the film can cause modifications in its structure which could be the origin of the anomalies found. In particular, it would appear that the rapid and more or less significant fluctuations in the delaminating force are due to a variation in the dimensions of the support film at a temperature greater than 200.degree. C. and in particular to a too substantial transverse shrinkage at this temperature
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Buisine Abel
Corsi Philippe
Eyraud Marcel
Le Hoa T.
Rhone-Poulenc Films
Thibodeau Paul J.
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