The present invention relates to a film based on biaxially oriented polypropylene usable for the production of adhesive tapes, and to adhesive tapes thus obtained.

For such an application one of the faces of the polypropylene film must be highly reactive in order to allow spreading of a layer of adhesion promoter to which a layer of adhesive is subsequently anchored so as to obtain the finished tape. It is known to make this face of the film reactive by raising its surface tension to a value of at least 40/42 mN/m following a flame treatment effected at the end of the extrusion process.

The unrolling of this type of tape is silent and fairly easy as long as the adhesive power of the adhesive used is rather low. However, if it is necessary to utilise an adhesive with a high adhesive power, this results in the loss of the ease of unrolling of the tape.

To overcome this difficulty and to be able to increase the adhesive force of the adhesive even further, a lacquer with an anti-adhesive capability is spread on the face of the film opposite that carrying the adhesive, although this causes an increase in the noise of the tape during unrolling.

On the other hand, in the common case in which the tape must be personalised by printing, the face opposite that carrying the adhesive must be spread with a lacquer which makes the film suitable to receive the appropriate printing inks utilised for this application. The presence of this latter lacquer however induces a greater difficulty in unwinding the roll of tape because of the high affinity and therefore the high adhesion, between the lacquer itself and the adhesive. To counteract this phenomenon chemical agents with anti-adhesive properties are added to the lacquer, which reduce the adhesion between the lacquer and the adhesive, making the unwinding of the tape easier, with benefit to both during the production phase (greater speed of production) and in the final use. However, the application of this layer of lacquer constitutes an additional phase in the production process of the adhesive tape, increasing the costs and complexity.

For the purpose of overcoming the disadvantages of the prior art mentioned above it is an object of the present invention to provide a film based on biaxially oriented polypropylene and an adhesive tape incorporating it, which have the characteristics indicated in the following claims.

Thanks to its composition and to its naturally high surface tension, the auxiliary layer of the film and the adhesive tape of the invention has an intrinsic affinity to conventionally utilised polyamide printing inks and is therefore printable without having to be subjected to further treatments. At the same time, the auxiliary layer has a significant anti-adhesive property in relation to the adhesive, so as to make unrolling of the adhesive tape much easier.

The auxiliary layer may be co-extruded together with the main layer. In this way it is possible to avoid the additional phase of application of anti-adhesive lacquer, with a reduction in costs and a simplification of the adhesive tape production process.

The main layer (A) of the film may have a thickness lying between 20 and 50 μm. It can be composed of polypropylene or a mixture of polypropylene homopolymers with a melting temperature lying between 150 and 170°C and "melt flow index" (MFI) lying between 1 and 5 g/10min at a temperature of 230°C and 2.16kg. The face of the main layer, on which the adhesion promoters and then the adhesive will be subsequently spread, is preferably subjected to a flame or corona treatment in such a way as to form a surface layer with a surface tension of at least 44mN/m and preferably 46mN/m.

In a preferred embodiment of the invention the film comprises a further layer (B) adjacent the main layer on the opposite side from the auxiliary layer.

The further layer (B) may have a thickness lying between 0.5 and 3 μm and may be co-extruded together with the other layers. It may be composed of polypropylene or a mixture of polypropylene homopolymers having characteristics similar to those of the main layer.

Alternatively, the further layer (B) may be composed of polystyrene and a copolymer of ethylene and butyl acrylate. The polystyrene may have an MFI lying between 0.5 and 5 g/lθmin at 200"C and 5kg and a Vicat softening point lying between 80 and 120°C (120°C/h, lkg), such as, for example, Styron 648 (a product of Dow Plastics) . The copolymer of ethylene and butyl acrylate may have a butyl acrylate content lying between 15 and 40%, such as, for example, Elvaloy 3427 AC (a product of DuPont) . Preferably, the further layer (B) comprises polystyrene in quantities lying between 20 and 70%, in particular between 30 and 60%, by weight of the layer, and the copolymer of ethylene and butyl acrylate in quantities complementary to 100% by weight. The further layer (B) is preferably subjected to a flame or corona treatment so as to have a surface tension of at least 44mN/m and preferably 46mN/m.

The auxiliary layer (C) , having for example a thickness lying between 0.5 and 4 μm and preferably between 1 and 3 μm, adheres to the face of the main layer (A) opposite that adjacent the further layer (B) and comprises a mixture of copolymers of ethylene and methyl acrylate (for example with a percentage by weight of methyl acrylate lying between 5 and 50% and MFI lying between 1 and 5 g/lθmin at a temperature of 230"C and 2.16kg) and silicon oxide in quantities advantageously lying between 1 and 30% by weight of the mixture. The mixture may be present in quantities lying between 10 and 70%, preferably between 20 and 60% by weight of the layer. The mixture of copolymers of ethylene and methyl acrylate and silicon oxide usable for the present invention may, for example, be the Conpol AC-B product of DuPont.

The auxiliary layer (C) further includes polystyrene - for example having MFI lying between 0.5 and 5 g/lθmin at a temperature of 200°C and 5kg and VICAT softening point lying between 80 and 12O0C (at 120°C/h, lkg) - in quantities complementary to 100% with respect to that of the above- mentioned mixture still with reference to the weight of the layer. The polystyrene usable for the present invention may, for example, be the Styron 648 product of Dow Plastic. The auxiliary layer (C) , whilst being composed of polymers which are heterogeneous among themselves, as well as with respect to the polypropylene of which the main layer is constituted, surprisingly possesses, in addition to a high affinity to polyamide based inks, a high adhesion to the main layer. Without wishing to be tied to a specific theory, it can be hypothesised that the phenomenon arises by the effect of the presence in the copolymer of chains having functional acrylate groups within them.

Whenever it is desired to increase the adhesive power of the adhesive tape by applying more tenacious adhesives to the film of the invention, it is advantageous to formulate the further layer (B) with polymers which are heterogeneous with one another and with respect to the propylene, for example with the mixture of polystyrene and copolymers of ethylene and butyl acrylate described above. Such a formulation has a greater adhesive force in comparison both with the main layer (A) and the adhesion promoter layer on which the adhesive will subsequently be spread, and thus makes it possible to produce adhesive tapes of greater adhesive power.

Further advantages and characteristics of the present invention will become apparent from the following examples of film compositions provided purely by way of non-limitative example, in which all the percentages are to be considered as percentages by weight if not otherwise indicated, and the values in μm represent the thicknesses of the various layers.

EXAMPLE 1 Layer A Polypropylene homopolymer MFI 2 100% 32 μm Layer B Polypropylene homopolymer MFI 3 100% 1 μm Layer C Conpol AC-B (DuPont) 50% 2 μm Styron 648 (Dow Plastics) 50%

EXAMPLE 2 Layer A Polypropylene homopolymer MFI 2 100% 31 μm Layer B Elvaloy 3427 AC (DuPont) 45% 2 μm Styron 648 (Dow Plastics) 55% Layer C Conpol AC-B (DuPont) 50% 2 μm Styron 648 (Dow Plastics) 50%

The auxiliary layers of the films in the examples above have an anti-adhesive power equal to or less than 190 g/cm, which thus is distinctly less than the values generally measured on a surface lacquered in a conventional manner for the purpose of printing, which are about 220-230 g/cm.

The above-indicated values of anti-adhesive power are measured with the VIBAC BVOl method in which a small test strip with dimensions of 60 x 200 mm is taken from the film under examination and anchored onto a rigid support. A PVC tape having a standard value of adhesion on steel of 250 g/cm and width of 25mm is adhered to the auxiliary layer (C) of the film in such a way that it entirely covers the length of the test strip and projects by 50 mm. The test strip is pressed by means of a roller weighing 5kg which is made to roll twice over the entire length first in one direction and then in the other.

The test strip thus prepared is then inserted between the terminals of a dynamometer with which its two ends are pulled apart thus causing separation of the test strip from the PVC tape, with a separation speed of 300πun/min, which latter adopts a configuration inverted by 180° with respect to the original . The value of the force which causes separation between the film under examination and the PVC tape is assumed as an indication of the anti-adhesive power.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments can be widely varied with respect to what has been described purely by way of example, without by this departing from its ambit.