Transparent, oriented polyester films have been known for a long time. Films produced from an antimony-free polyester are likewise known. Roasting bags and sleeves made of polyester film are likewise known.
Film bags for the baking of bread, and in particular film bags and tubes intended for cooking an entire dish in the oven, have to withstand temperatures above 200° C. for more than one hour without mechanical failure (e.g. due to bursting as a consequence of embrittlement). Industrial practice is to use almost exclusively film bags and tubes made of polyethylene terephthalate (PET), because this material has high heat resistance.
The traditional catalysts for producing polyesters continue to be antimony compounds. Some antimony compounds can be hazardous to health in particular at relatively high concentrations and if exposure is frequent. The EU therefore prescribes a maximum permissible migration of antimony from a film into a food. The high usage temperatures in oven applications increase antimony migration and for this reason alone it is desirable to reduce antimony content. Studies by the Swiss Department of Health show that, in actual foods, the migration limit for antimony is exceeded, sometimes considerably, at oven temperatures as low as 180° C. (see M. Haldimann, A. Blanc and V. Dudler, in Food Additives and Contaminants, 2007; 24(8): 860-868). Indeed, there are discussions in the technical literature where much smaller traces of antimony from PET are considered to be potentially hazardous (e.g. W. Shotyk and M. Krachler in Environ. Sci. Technol., 2007, (5), pp. 1560-1563). For an application in direct contact with food at greatly elevated temperatures it would therefore be important to use an antimony-free film.
However, an analysis carried out for the purposes of this invention, taking more than 10 different roasting film bags and sleeves from various producers revealed the presence of antimony in all of the instances studied, albeit with values reduced (from 50 to 110 ppm) in comparison with standard antimony-containing films (standard film mostly >150 ppm). Antimony is easily detected at above 10 ppm by commonly used methods of digestion and analysis, and migration of antimony out of a film is similarly detectable by selecting suitable conditions (high temperature >100° C. in foods).
The reasons for the use of antimony compounds as polycondensation catalyst may well be found in the low heat resistance of antimony-free films. The oven test (see test methods) was not passed by any of the commercially available films studied that had been produced exclusively with a titanium catalyst. Germanium-catalyzed films were even poorer than the titanium-catalyzed films.
Although experiments with use of phosphorus stabilizers, as described in JP 2007077220, to improve performance in the oven test by deactivating the catalyst led to reduced embrittlement, they did not lead to any reliable passing of the oven test. Nor was it possible to achieve reliable passing of the test by reducing, or entirely eliminating, use of regrind. Again, this achieved only some degree of improvement. This type of reduction moreover considerably impairs the cost-effectiveness of film production. Nor did any specific selection of the transesterification catalyst (elements studied being Zn, Mg, and Mn) lead to roasting film bags and sleeves which reliably pass the oven test.
EP-A-2 164 079 (whose United States equivalent is United States Patent Application Publication No. 2010/209722) describes an electrical insulation film made of a polyester which was produced at least to some extent by using titanium catalyst, where the film comprises a mixture of free-radical scavengers, in order to achieve greater thermal stability. However, the high content of free stabilizer, and a selection of the stabilizers that is possible within the context of the disclosure can lead to films which are unsuitable for the direct food-contact application (e.g. by exceeding limiting migration values). Necessary preconditions for a roasting film bag or sleeve are not only thermal stability and the suitability of all of the starting materials for direct contact with foods but also suitable transparency and color giving good visibility of the roasting contents for the user, and also minimized clouding and yellowing during roasting, in order to provide good-quality appearance, and also compliance with certain mechanical properties which permit reliable handling of the bag or sleeve prior to and after roasting.