Sealable, biaxially oriented polyester films are known. These known films either have good sealing performance or good optical properties, or acceptable processing performance.
GB-A 1 465 973 describes a coextruded, two-layer polyester film in which one layer is composed of isophthalic acid-containing and terephthalic acid-containing copolyesters and in which the other layer is composed of polyethylene terephthalate. The specification gives no useful information concerning the sealing performance of the film. Lack of pigmentation means that the process for producing the film is not reliable (the film cannot be wound) and that there are severe limitations on further processing of the film. The film can certainly not be further processed on systems which operate using a vacuum (metalization, coating with ceramic materials, etc.).
EP-A-0 035 835 describes a coextruded sealable polyester film which has particles admixed in the sealable layer to improve winding and processing performance, the median size of these particles exceeding the thickness of the sealable layer. The particulate additives form surface protrusions which prevent undesired blocking and sticking to rollers or guides. No further detail is given concerning incorporation of antiblocking agents in the other, non-sealable layer of the film. If no antiblocking agents are present in this layer, the film has restricted further processability, and especially has restricted capability for processing in a vacuum. The preferred use of bimodal particle systems makes the distribution of particle diameter very broad. This, together with the concentrations stated in the examples of the specification, impairs the sealing performance of the film. The specification gives no information on the sealing temperature range, in particular on the minimum sealing temperature (MST) of the film. Seal seam strength is measured at 140° C. and is in the range from 85 to 120 N/m (corresponding to from 1.275 to 1.8 N/15 m of film width). In addition, the film has unsatisfactory optical properties. The haze of the film is in the range from 5 to 15%, and this is markedly too high for most applications.
EP-0 379 190 discloses a coextruded polyester film which has a sealable layer whose sealing energy is greater than 400 g·cm/15 mm. The sealable layer comprises inorganic, or else additionally organic particles, added at a concentration of from 0.01 to 5% to the sealable layer. The diameter of these particles is smaller than the thickness of the sealable layer, but no definitive information is given on the absolute size of the particles. In the examples, the diameter varies from 2 to 3.5 μm. In the preferred embodiment, the particles are monodispersed and almost spherical. The base layer may likewise comprise particulate additives of this type. However, that specification gives no teaching concerning, for example, how the base is to be pigmented in order to make the sealable film reliably processable on systems in a vacuum. The capability of the film for, for example, vacuum-coating is therefore non-existent or only very restricted.
EP-A-0 432 886 describes a coextruded multilayer polyester film which has a first surface on which a sealable layer has been arranged and a second surface on which an acrylate layer has been arranged. Here, too, the sealable outer layer may be composed of isophthalic acid-containing and terephthalic acid-containing copolyesters. The reverse-side coating gives the film improved processing performance. The specification gives no information concerning the sealing range of the film. Seal seam strength is measured at 140° C. For a sealing layer of thickness 11 μm the seal seam strength given is 761.5 N/m (corresponding to 11.4 N/15 mm of film width). A disadvantage of the reverse-side acrylate coating is that this side does not seal with respect to the sealable outer layer. This gives the film a limited field of application.
EP-A-0 515 096 describes a coextruded, multilayer sealable polyester film which comprises an additional additive on the sealable layer, and which has appropriate topography (number of elevations and range for the average height of the elevations). The additive may comprise inorganic particles, for example, and is preferably applied in an aqueous layer to the film during its production. Furthermore, the additive (=antiblocking agent) may also be directly incorporated into the sealable layer. The result is said to be that the film retains good sealing properties and processes well. The reverse side of the film preferably comprises only very few, relatively small, particles, which pass into this layer mainly via the regrind. The specification gives no information concerning the sealing temperature range of the film. Seal seam strength is measured at 140° C. and is more than 200 N/m (corresponding to 3 N/15 mm of film width). For a sealing layer of thickness 3 μm the seal seam strength given is 275 N/m (corresponding to 4.125 N/15 mm of film width). The film has only limited suitability for further processing on systems which operate in a vacuum (physical vapor deposition (PVD) processes, such as metalization and coating with ceramic substances). The reasons for this are the low roughness of the hot-sealable layer, the low average height of the elevations of the particles present in this layer, and the provision of only a low concentration (e.g. via the regrind) of antiblocking agents in the other, non-sealable layer of the film. In addition, the optical properties of the film are unsatisfactory. The haze of the film is greater than 3%, and this is too high for many applications.
WO 98/06575 describes a coextruded multilayer polyester film which comprises a sealable outer layer and a non-sealable base layer. The base layer here may be composed of one or more layers, one of the layers being in contact with the sealable layer. The other (exterior) layer then forms the second non-sealable outer layer. Here, too, the sealable outer layer may be composed of isophthalic acid-containing and terephthalic acid-containing copolyesters, but no antiblocking particles are present in these. In addition, the film also comprises at least one UV absorber, which is added to the base layer in a weight ratio of from 0.1 to 10%. The base layer of this film has conventional antiblocking agents. The film has good sealability, but is relatively unsuitable for metalization. In addition, it has shortcomings in optical properties (gloss and haze).
EP-A 1 138 480 describes a coextruded, biaxially oriented, sealable polyester film with at least one base layer (B), with a sealable outer layer (A), and with another outer layer (C). The sealable outer layer (A) has a minimum sealing temperature of not more than 110° C., and a seal seam strength of at least 1.3 N/15 mm of film width, and is also characterized by appropriate features with regard to the topography of the two outer layers (A) and (C). The relatively smooth sealable layer, which in comparison comprises only relatively few antiblocking particles, has excellent sealing properties and is easy to produce, and has good processability (printing, cutting, laminating, etc.) It has no, or only very restricted, capability for use on systems which operate in a vacuum.
It was therefore an object of the present invention to provide a transparent, coextruded, sealable, biaxially oriented polyester film which does not have the disadvantages of the prior-art films mentioned, and which in particular has very good processability on systems which operate in a vacuum (metalization, coating with ceramic substances, and in general the use of PVD processes and chemical vapor deposition (CVD) processes), and which has very good optical properties. For example, the film is intended not to block during coating with ceramic substances (e.g. SiOx, Al2O3) in a vacuum, and is intended to give good winding performance in those processes. This applies both to the unwinding of the film prior to the coating process and also to the winding-up of the film after the coating process. Blocking of the two surfaces of the film after the coating process can cause the (sealable) reverse side of the film to “stick” to the coated (non-sealable) side, and this is undesirable. On rewinding of the blocked roll (where the blocking sites mostly have random distribution), the material can be completely torn away from the film surface and transferred to the other, sealable film surface, and this is likewise undesirable. In addition, the blocking of the two surfaces of the film during the subsequent steps of processing can lead to break-off of the film web. A further object of the present invention is to improve the processability of the film in processes in which at least one of the two surfaces is coated in such a way that this side becomes duller after coating. This means that the coefficient of friction of that side increases, giving that side of the film a susceptibility to blocking. This may occur, for example, when scratch-resistant coatings (acrylates, epoxy resins) are applied. The film is also intended to have good sealability, the intention here being that the sealable layer (A) be sealable with respect to itself (fin sealing) and also with respect to the reverse side (non-sealable side (C)) (lap sealing). A further intention is to ensure that cut material arising during production of the film can be returned as regrind to the production process in amounts of up to 60% by weight, based on the total weight of the film, without any significant resultant adverse effect on the physical and optical properties of the film. In summary, the properties which were in particular to be improved over prior-art films were the following:    the properties needed for good processing of the film in a vacuum    winding, in particular during processing of the film in a vacuum    blocking performance of the two surfaces of the film,    the optical properties of the film, in particular haze and gloss.