The present invention relates to an adhesive tape, namely to a pressure-sensitive product intended for sealing packages and for generic industrial use, and to the associated production method.

More particularly, such an adhesive tape typically comprises a support layer based on substantially homopolymeric bioriented polypropylene (BOPP) and an adhesive layer. The latter is typically based on natural rubber and hydrocarbon resins dissolved in solvent, such as hexane, or based on thermoplastic rubber and hydrocarbon resins, namely hot melt adhesives.

According to a prior art, an anti-adhesive release layer is applied on the opposite side of the support layer from that facing towards the adhesive layer, which release layer is capable of ensuring unrolling of the finished tape. The anti-adhesive substance, generally based on polyvinyl carbamate, is applied in the form of a solution in xylene and/or toluene. After the deposition thereof, the tape passes through a drying tunnel, in order to remove the solvent, which is then recovered. On the other hand, EP-0 965 625 Al describes an adhesive tape which can be made without a release layer by making use of a BOPP film having a "skin" composed of high density polyethylene (HDPE). The high density polyethylene does not, however, ensure ideal adhesion to the homopolymer polypropylene film due to the physicochemical incompatibility between the two materials. Such a tape thus exhibits an unwind force which is much higher than that of an adhesive tape having a standard release layer of the above-stated kind. Furthermore, using HDPE on equipment normally used for polypropylene leads to a decline in the yield and efficiency of the production process.

Example 1 of prior art document US-4 673 61 1 discloses a tape having an adhesive layer of rubber and a support therefor including a surface layer opposite to the adhesive layer and containing 1.5 % by weight of dimethylpolysiloxane. An object of the present invention is therefore that of providing an adhesive tape of the general type indicated at the beginning of the present description, which exhibits optimum application characteristics, in particular with regard to unwindability while still lacking a release layer applied by means of solvent.

Said object is achieved thanks to an adhesive tape comprising a support layer and an adhesive layer of the hot melt type, as well as a skin layer located on the opposite side of the support layer from that facing towards the adhesive layer, said skin layer being formed of a propylene -butene copolymer, a propylene-ethylene -butene terpolymer, a propylene homopolymer or mixtures thereof, there furthermore being present in said skin layer a quantity of between 32,000 and 100,000 ppm of polyorganosiloxane having a viscosity of

2 2

between 10,000 mm /s and 2,000,000 mm /s, which viscosity is measured at 25°C according to ASTM D445. Preferred embodiments of the tape of the invention are stated in the claims dependent on claim 1.

The tape of the present invention has the advantage that it can be produced in a straightforward and economic manner. This is because dispensing with the release layer permits considerable savings in terms of raw materials and manufacturing energy costs, which are due inter alia to the necessity of recovering the solvent used for spreading the release agent. As a consequence, there are lower emissions of solvent into the atmosphere and the risks associated with storing considerable quantities of flammable and harmful solvents are reduced.

Furthermore, according to the invention, the provision of a skin layer containing fairly high amounts of polyorganosiloxane of high viscosity allows to the user to smoothly draw the desired length of tape without having to exercise high traction forces which would break the tape, even when an "hot melt" adhesive layer is present. The present invention further provides a production method for the above-stated adhesive tape by co-extrusion of the support layer and the skin layer, and subsequent application of the adhesive layer. Said method makes it possible to simplify the adhesive spreading operation and the associated equipment: in particular, hot melt technology, which already does not involve using an adhesive comprising solvent, becomes completely "solventless".

According to the invention, production of an adhesive tape without a release layer provides the use of a film having a support layer and a "skin" (namely an outer layer forming the back of the adhesive tape) obtained by coextrusion, with a thickness which typically varies from 0.5 to 3 microns, and formed from a C3-C4 (propylene, butene) copolymer or from a C3-C2-C4 (propylene, ethylene, butene) terpolymer or from a C3 (propylene) homopolymer, which contains a quantity of polyorganosiloxane (32,000 ppm to

2 2 100,000 ppm) of a viscosity of between 10,000 mm /s and 2,000,000 mm /s , and

2 2

preferably between 50,000 mm /s and 500,000 mm /s. Advantageously, the outer surface of the skin is flame- or corona-treated to increase the anti-adhesive effect.

The remainder of the film is generally composed of substantially homopolymeric polypropylene. It should be noted that, in the context of the present description, "substantially homopolymeric polypropylene" should be taken to mean a polypropylene which may contain up to 3% of ethylene.

Advantageously, the opposite side of the film is also flame- or corona-treated to ensure excellent adhesion with the adhesive compound which will be applied thereon.

Furthermore, particulate substances based on synthetic silica, clay, PMMA of a mean size of around 3-4 microns may be present on the surface of both outer sides of the film as antiblocking agents. These substances are typically mixed into the polymer in the form of a masterbatch during the extrusion phase. The above-stated film is advantageously produced using a Brueckner Maschinenbau GmbH & Co KG LISIM ^® double-bubble sequential stretching/simultaneous stretching installation with a longitudinal stretching ratio (MDx) of between 4 and 7 and a transverse stretching ratio (TDx) of between 6 and 11.

The total thickness of the adhesive tape is advantageously between 20 and 60 microns.

The preferred characteristics of the homopolymer PP used for the support layer are stated in Table I below.

TABLE I

Examples of commercially available homopolymer PP which may be used are: Moplen HP522H - LyondellBasell; Sabic® PP 524P - Sabic; Moplen 2152 HOXP - LyondellBasell; Moplen HP422H - LyondellBasell; 100-GD02 INEOS Polyolefms.

The characteristics of the preferred polyolefmic materials used for the skin layer are stated in Table II.

TABLE II

MT Vicat Melt index

(melting temperature) (softening point) (ISO 1133; 230°C, 2.16 kg) ASTM D 3417 A50 (50°C/h 10N)

ISO 306

100-145°C 90-135°C 1.5-11 g/lO min Examples of commercially available copolymers which may be used are: Adsyl 3C30F HP - LyondellBasell; Adsyl 5C39F - LyondellBasell; Eltex® P KS359 - INEOS Polyolefms; Eltex® P KS357 - INEOS Polyolefms.

Masterbatches which may be used for the polysiloxane component of the skin layer, which forms the back of the adhesive tape, advantageously have the characteristics shown in Table III.

TABLE III

Further advantages and features of the present invention will become clear from the following exemplary embodiment which is provided by way of non-limiting example.

Example

A film of a total thickness of 25 microns is produced having the following three-layer structure in which layer A is the skin layer and layer B the support layer:

A ADSYL 5C39F (LyondellBasel) 50% + SAB 06579 PPR (Constab) 50% 1 μιη B MOPLEN HP522H (LyondellBasell)

C ADSYL 5C39F (LyondellBasell) 1 μιη

Layer A contains thus polydimethylsiloxane in a quantity of 40,000 ppm. Both sides of the film are subjected to corona treatment. The film was produced with a sequential stretching installation, with a longitudinal stretching ratio (MDx) of 5 and a transverse stretching ratio (TDx) of 9.

Said film was then used to produce an adhesive tape with "hot melt" technology which involves hot application of an adhesive based on thermoplastic rubber and hydrocarbon resins onto layer C. The adhesive was applied in a quantity, measured when dry, of 16.0 g/m ² .

The composition of the adhesive used is stated in Table IV below.

TABLE IV - Adhesive composition

Total (%) 100.00

A release layer based on polyvinyl carbamate was applied onto one part of the back (or skin layer A) of the resultant adhesive tape, while no release layer was applied onto the remaining part of the back.

Table V below shows the experimental data obtained for the two parts of the adhesive tape, which were or were not provided with a release layer.

TABLE V

Parameter Method (unit of Adhesive tape Adhesive tape measurement) without with release release layer layer

As may be seen, the results obtained on both parts of the tape are substantially similar, confirming that the tape of the invention makes it possible to dispense with the application of a release layer, so avoiding the associated costs and drawbacks, but without suffering substantial performance penalties.

For comparison purposes, a further tape having the same structure and composition of the above-disclosed tape of the invention, except that Layer A contains polydimethylsiloxane in a quantity of 20,000 ppm, exhibits an unwind force measured according to PSTC-8 of 180 g/cm.

Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may be varied widely with respect to those described, which have been given purely by way of example, without thereby departing from the scope thereof. For example, further layers could be interposed between the support layer and the adhesive layer.