Multilayer film for flexible wrapping

Abstract

A subject of the present invention relates to a multilayer (or laminate) film, that can be used in the field of flexible wrapping, comprising at least 2 thin layers of material bonded together by a layer of an adhesive composition based on a styrene block copolymer. It also relates to a lamination process suitable for the manufacture of said film.

Claims

1. A multilayer film comprising at least 2 thin layers of material having a thickness of 5 to 150 m bonded together by a layer having a thickness 3-5 m of an adhesive composition comprising: a) from 30 to 60% of a mixture of styrene block copolymers comprising at least one elastomer block, comprising: 5 to 75% of at least one triblock copolymer that is SIS, SIBS, SBS, SEBS or SEPS, and from 25 to 95% of at least one diblock copolymer that is SI, SBI, SIB, SB, SEB, or SEP, the overall content of styrene units of said mixture being comprised between 10 and 40%; b) from 10 to 60% of one or more tackifying resins S having a softening temperature greater than or equal to 80 C.; and c) from 5 to 35% of one or more tackifying resins L having a softening temperature less than or equal to 50 C., by weight with respect to the total weight of the adhesive composition, wherein the material(s) forming the multilayer film is(are) not a silicone-coated material; and wherein the said multilayer film is transparent, suitable for flexible wrapping, and has a cohesive force greater than or equal to 1 N/15mm, and wherein the quantity of adhesive composition is approximately 3-5 g/m.sup.2.

2. The multilayer film of claim 1, wherein the triblock and diblock copolymers present in the mixture of styrene block copolymers have the same elastomer block.

3. The multilayer film of claim 1, wherein the triblock copolymers comprised in the adhesive composition have a linear structure.

4. The multilayer film of claim 1, wherein the mixture of styrene block copolymers, at least one triblock copolymer SIS having a linear structure is used, in a mixture with at least one diblock copolymer SI.

5. The multilayer film of claim 1, wherein the tackifying resin(s) S has (have) a softening temperature greater than or equal to 90 C.

6. The multilayer film of claim 1, wherein the tackifying resin(s) L has (have) a softening temperature less than or equal to 30 C.

7. The multilayer film of claim 1, wherein the tackifying resin(s) S, having a softening temperature greater than or equal to 80 C., is: i) rosins of natural origin or modified; ii) resins obtained by hydrogenation, polymerization or copolymerization (with an aromatic hydrocarbon) from mixtures of unsaturated aliphatic hydrocarbons having approximately 5, 9 or 10 carbon atoms, and originating from petroleum cuts; iii) terpene resins resulting from the polymerization of terpene hydrocarbons in the presence of Friedel-Crafts catalysts, optionally modified by the action of phenols; or iv) copolymers based on natural terpenes.

8. The multilayer film of claim 7, wherein the tackifying resin S, having a softening temperature greater than or equal to 80 C., belongs to categories ii) and iii).

9. The multilayer film of claim 1, wherein the tackifying resin L, having a softening temperature less than or equal to 50 C., is selected from: i) rosins of natural origin or modified; ii) resins obtained by hydrogenation, polymerization or copolymerization (with an aromatic hydrocarbon) from mixtures of unsaturated aliphatic hydrocarbons having approximately 5, 9 or 10 carbon atoms, and originating from petroleum cuts; iii) terpene resins resulting from the polymerization of terpene hydrocarbons.

10. The multilayer film of claim 1, wherein the adhesive composition comprises: a) from 35 to 55% of the mixture of styrene block copolymers, b) from 20 to 55% of tackifying resin(s) S and c) from 10 to 35% of tackifying resin(s) L by weight with respect to the total weight of the adhesive composition.

11. The multilayer film according to claim 1, in which the thin layers bonded together by the adhesive composition are made from thermoplastic polymers.

12. The multilayer film of claim 1, wherein one of the 2 thin layers bears a deposit of inks on at least a portion of the face which is in contact with the layer of adhesive composition.

13. A method for the continuous preparation of the multilayer film as defined in claim 1, comprising: i) coating by the adhesive composition of a first thin layer of material, in which said composition, rendered pourable by heating to an appropriate temperature, is extruded by a coating device without contact with said thin layer, in the form of a substantially continuous layer, which is then placed in contact with the surface of said thin layer, then ii) laminating of a second thin layer, on the first thin layer coated according to step i).

14. The method according to claim 13, in which in the coating i), the substantially continuous layer (6) of adhesive composition extruded by the coating device (5): a) passes across the space between said device and the film (1), in suspension in air, the pressure of which is reduced locally to a value comprised between 500 and 975 mbars, then b) is placed in contact with the film (1) in the vicinity of the edge (10c) formed by the 2 guide surfaces (10a) and (10b) of a deflection element (10) through which a compressed air line passes, opening out via at least one outlet opening in the area of the edge (10c), such that the discharged compressed air forms an air cushion around said edge.

15. A method according to claim 13, in which, in coating i), the substantially continuous layer (6) of adhesive composition extruded by the coating device (5), after it has been placed in contact with the film (1), is pressed onto the latter by a roll (7) the outer surface of which is provided with a non-stick coating.

16. A method of manufacturing flexible wrapping comprising using the multilayer film of claim 1 as wrapping.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 diagrammatically represents an embodiment of the laminating process according to the invention.

(2) FIG. 2 diagrammatically represents another embodiment of the laminating process according to the invention;

DETAILED DESCRIPTION OF THE INVENTION

(3) A subject of the present invention relates to a multilayer (or laminate) film comprising at least 2 thin layers of material bonded together by a layer having a thickness strictly less than 10 m of an adhesive composition comprising: a) from 30 to 60% of a mixture of styrene block copolymers comprising at least one elastomer block, this mixture consisting of: 5 to 75% of at least one triblock copolymer chosen from the group comprising SIS, SIBS, SBS, SEBS and SEPS, and from 25 to 95% of at least one diblock copolymer chosen from the group comprising SI, SBI, SIB, SB, SEB, SEP,
the overall content of styrene units of said mixture being comprised between 10 and 40%; b) from 10 to 60% of one or more tackifying resins S having a softening temperature greater than or equal to 80 C.; and c) from 5 to 35% of one or more tackifying resins L having a softening temperature less than or equal to 50 C.,
by weight/total weight of the adhesive composition.

(4) According to an embodiment, the adhesive composition consists essentially of: a) from 30 to 60% of a mixture of styrene block copolymers comprising at least one elastomer block, this mixture consisting of: 5 to 75% of at least one triblock copolymer chosen from the group comprising SIS, SIBS, SBS, SEBS and SEPS, and from 25 to 95% of at least one diblock copolymer chosen from the group comprising SI, SBI, SIB, SB, SEB, SEP,
the overall content of styrene units of said mixture being comprised between 10 and 40%; b) from 10 to 60% of one or more tackifying resins S having a softening temperature greater than or equal to 80 C.; and c) from 5 to 35% of one or more tackifying resins L having a softening temperature less than or equal to 50 C.,
by weight/total weight of the adhesive composition.

(5) Unless stated otherwise, the percentages used in the present text to express quantities correspond to weight/weight percentages.

(6) The softening temperature (or point) is determined according to standard test ASTM E 28, the principle of which is as follows. A brass ring having a diameter of approximately 2 cm is filled with the resin to be tested in the molten state. After cooling to ambient temperature, the ring and the solid resin are placed horizontally in a temperature-controlled glycerine bath, the temperature of which can vary by 5 C. per minute. A steel ball having a diameter of approximately 9.5 mm is centred on the disk of solid resin. The softening temperature, during the phase of temperature increase of the bath at the rate of 5 C. per minute, is the temperature at which the resin disk is forced downward a distance of 25.4 mm under the weight of the ball.

(7) The styrene block copolymers are constituted by different polymerized monomers, including at least one polystyrene block, and are prepared by the free radical polymerization technique. The triblock copolymers include 2 polystyrene blocks and an elastomer block. They can embody various structures: linear, star (also called radial), branched or also comb. The diblock copolymers include 1 polystyrene block and 1 elastomer block.

(8) The styrene block copolymers capable of use in the adhesive composition comprised in the laminate film according to the invention have a weight-average molecular weight Mw generally comprised between 50 kDa and 500 kDa and are constituted by blocks of different polymerized monomers.

(9) The triblock copolymers have the following general formula:
ABA(I)
in which: A represents a styrene (or polystyrene) non-elastomer block, and B represents an elastomer block which can be: polyisoprene. The structure of the block copolymer is then: polystyrene-polyisoprene-polystyrene, known as: SIS; polyisoprene followed by a polybutadiene block. The structure of the block copolymer is then: polystyrene-polyisoprene-polybutadiene-polystyrene, known as: SIBS polybutadiene. The structure of the block copolymer is then: polystyrene-polybutadiene-polystyrene, known as: SBS; totally or partially hydrogenated polybutadiene. The structure of the block copolymer is then: polystyrene-poly(ethylenebutylene)-polystyrene known as: SEBS; totally or partially hydrogenated polyisoprene. The structure of the block copolymer is then: polystyrene-poly(ethylenepropylene)-polystyrene known as: SEPS.

(10) The diblock copolymers have the following general formula:
A-B(II)
in which A and B are as defined previously.

(11) When the adhesive composition comprises several triblock styrene copolymers, the latter being chosen from the group comprising SIS, SBS, SEPS, SIBS, SEBS, it is understood that said triblocks can belong to a single one or to several of these 5 families of copolymers.

(12) It is preferable to use a mixture of triblock copolymers and diblock copolymers having the same elastomeric block, in particular because such mixtures are commercially available.

(13) According to another preferred variant, the diblock copolymer content in the mixture of styrene block copolymers is comprised between 50 and 95%.

(14) The triblock copolymers comprised in the adhesive composition according to the invention preferably have a linear structure.

(15) According to an embodiment of the invention, as styrene block copolymer, a triblock copolymer SIS having a linear structure is used, in a mixture with at least one diblock copolymer SI.

(16) As examples of commercial products having a linear structure there can be mentioned: SIS: Kraton D1113 BT from Kraton (mixture comprising 55% diblock SI of M.sub.w of approximately 100 kDa, 45% linear triblock SIS of M.sub.w of approximately 250 kDa and having an overall content of styrene units of 16%); SIBS: Kraton MD 6465 (56% diblock and 16% styrene) SBS: Kraton D1118 (78% diblock and 33% styrene); SEBS: Kraton G1726 (70% diblock and 30% styrene).

(17) As examples of commercial products having a radial structure there can be mentioned: SIS: Vector 4230 from ExxonMobil (30% diblock and 20% styrene).

(18) The tackifying resin(s) S having a softening temperature greater than or equal to 80 C. represent from 10 to 60% of the adhesive composition, preferably from 20 to 55% of the adhesive composition.

(19) According to an embodiment, the tackifying resin S has a softening temperature greater than or equal to 90 C., preferably ranging from 95 C. to 150 C., more preferably from 100 to 130 C.

(20) According to an embodiment, the tackifying resin S is chosen from: i) rosins of natural origin or modified, such as for example the rosin extracted from pine gum, wood rosin extracted from tree roots and their derivatives, hydrogenated, dehydrogenated, dimerized, polymerized or esterified by monoalcohols or polyols such as glycerol; ii) resins obtained by hydrogenation, polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5, 9 or 10 carbon atoms, and originating from petroleum cuts; iii) terpene resins resulting generally from the polymerization of terpene hydrocarbons such as for example mono-terpene (or pinene) in the presence of Friedel-Crafts catalysts, optionally modified by the action of phenols; iv) copolymers based on natural terpenes, for example styrene/terpene, alpha-methyl styrene/terpene and vinyl toluene/terpene.

(21) Among the tackifying resins S capable of use in the adhesive composition comprised in the laminate film according to the invention, there can be mentioned as examples of commercially available resin: i) Sylvalite RE 100S available from Arizona Chemical which is a rosin resin having a softening temperature of approximately 100 C. ii) Escorez 5600 (respectively 5615) available from Exxon Chemicals which is a hydrogenated dicyclopentadiene resin modified by an aromatic compound having a softening temperature of approximately 100 C. (respectively 115 C.) and a molecular weight M.sub.w of approximately 800 g/mol; Escorez 5400 also from Exxon Chemicals which is a resin obtained by polymerization, then hydrogenation of a mixture of unsaturated aliphatic hydrocarbons having approximately 9 or 10 carbon atoms and which have a softening temperature of approximately 100 C. and a M.sub.w of approximately 670 g/mol; Escorez 1310 LC available from Exxon Chemicals which is a resin obtained by polymerization of a mixture of unsaturated aliphatic hydrocarbons having approximately 5 carbon atoms, and which has a softening temperature of approximately 94 C. and a M.sub.w of approximately 1900 g/mol; Escorez 2203 LC available from Exxon Chemicals which is an aliphatic hydrocarbon-containing resin modified by an aromatic compound having a softening temperature of approximately 92 C. and a M.sub.w of approximately 2200 g/mol. iii) Dercolyte S115 available from DRT (Drivs Rsiniques et Terpniques) which is a terpene resin obtained by polymerization of beta-pinene having a softening temperature of approximately 114 C. and a M.sub.w of approximately 2300 g/mol; Dertophene T available from DRT which is a terpene resin modified by phenols having a softening temperature of approximately 95 C. and a Mw of approximately 700 g/mol; Sylvarez TR7115 available from Arizona Chemicals which is a terpene resin having a softening temperature of approximately 115 C. and a M.sub.w of approximately 1040 g/mol. iv) Sylvarez ZT 105 LT available from Arizona Chemicals which is a styrene/terpene copolymer having a softening point of approximately 105 C.

(22) The tackifying resin(s) L having a softening temperature less than or equal to 50 C. represent from 5 to 35% of the adhesive composition, preferably from 10 to 35% of the adhesive composition.

(23) According to an embodiment, the tackifying resin L has a softening temperature less than or equal to 30 C., preferably ranging from 10 C. to 30 C., more preferably from 5 C. to 20 C.

(24) According to an embodiment of the invention, the tackifying resin L is chosen from: i) rosins of natural origin or modified, such as for example the rosin extracted from pine gum, wood rosin extracted from tree roots and their derivatives, hydrogenated, dehydrogenated, dimerized, polymerized or esterified by monoalcohols or polyols such as glycerol; ii) resins obtained by hydrogenation, polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5, 9 or 10 carbon atoms, and originating from petroleum cuts; iii) terpene resins resulting generally from the polymerization of terpene hydrocarbons such as for example mono-terpene (or pinene) in the presence of Friedel-Crafts catalysts, optionally modified by the action of phenols.

(25) Among the tackifying resins L capable of use in the adhesive composition comprised in the laminate film according to the invention, there can be mentioned as examples of commercially available resin: i) Sylvatac RE12 available from Arizona Chemicals which is a rosin ester resin having a softening temperature of 12 C. ii) Regalite R1010 available from Eastman which is a hydrogenated hydrocarbon-containing resin having a softening temperature of 10 C.

(26) The adhesive composition can also comprise from 0.1 to 2% of one or more stabilizers (or antioxidants). These compounds are introduced in order to protect the composition from degradation resulting from a reaction with oxygen which is likely to be caused by the action of heat, light or residual catalysts on certain raw materials such as the tackifying resins. These compounds can include primary antioxidants which trap the free radicals and which are generally substituted phenols such as Irganox 1010 from CIBA. The primary antioxidants can be used alone or in combination with other antioxidants such as phosphites like Irgafos 168 also from CIBA, or also with UV stabilizers such as amines.

(27) The adhesive composition can also comprise a plasticizer, but in a quantity not exceeding 5%. It is possible to use as a plasticizer a paraffinic and naphthenic oil (such as Primol 352 from ESSO) optionally comprising aromatic compounds (such as Nyflex 222B).

(28) According to a preferred variant, the adhesive composition does not comprise a plasticizer.

(29) According to another preferred variant, the adhesive composition comprises from 35 to 55% of the mixture of triblock and diblock copolymers, from 20 to 55% tackifying resin(s) S and from 10 to 35% tackifying resin(s) L.

(30) According to an embodiment, the adhesive composition consists essentially of 35 to 55% of the mixture of triblock and diblock copolymers, 20 to 55% tackifying resin(s) S and 10 to 35% tackifying resin(s) L.

(31) Finally, it is preferred to use in the multilayer (or laminate) film according to the invention an adhesive composition having a Brookfield viscosity measured at 190 C. greater than 5000 cP (5 Pa.Math.s), preferably greater than 8000 cP (8 Pa.Math.s).

(32) The adhesive composition is prepared by simple mixture of the hot ingredients, between 150 and 200 C., by means for example of a blade mixer or a twin screw extruder.

(33) The laminate film according to the invention comprises the adhesive composition as described previously in the form of a continuous layer having a thickness strictly less than 10 m, preferably less than or equal to 9 m, preferably less than or equal to 8 m, preferably less than or equal to 7 m, preferably less than or equal to 6 m, and also more preferentially ranging from 1 to 5 m.

(34) According to an embodiment, the quantity of adhesive composition comprised in the multilayer film is strictly less than 10 g/m.sup.2, preferably less than or equal to 9 g/m.sup.2, preferably less than or equal to 8 g/m.sup.2, preferably less than or equal to 7 g/m.sup.2, preferably less than or equal to 6 g/m.sup.2 and even more preferentially ranges from 0.5 to 5 g/m.sup.2.

(35) According to an embodiment, the layer of adhesive composition has a thickness ranging from 0.5 to 10 m, preferably from 1 to 9 m, preferably from 1 to 7 m, more preferably from 2 to 5 m.

(36) The layer of adhesive composition makes it possible to ensure the cohesion between two thin layers of material the thickness of which can vary from 5 to 150 m.

(37) The cohesion of the multilayer film can be measured by a peel test at 180 C. as described in the French standard NF T 54-122. The cohesion of the multilayer film of the invention is preferably greater than or equal to 1 N/15 mm, preferably greater than or equal to 1.2 N/15 mm, more preferably greater than or equal to 1.8 N/15 mm.

(38) The materials of which the thin layers surrounding the adhesive layer are constituted are generally chosen from the paper, aluminium or thermoplastic polymers such as: polyethylene (or PE), polypropylene (or PP), oriented or not, copolymers of ethylene and of vinyl acetate (or EVA), polyester, polyamide.

(39) According to an embodiment, the materials of which the thin layers surrounding the adhesive layer are constituted are chosen from the heat-sealable materials.

(40) Preferably, the material forming the multilayer film is not a silicone-coated material.

(41) The laminate film according to the invention can comprise several layers of the aforementioned materials, and also several adhesives layers of the composition as previously defined. Its total thickness is capable of varying within a wide range from 20 to 400 m.

(42) According to a preferred embodiment, one of the 2 thin layers comprised in the multilayer film according to the invention is constituted over at least a portion of its surface by a transparent material and bears on at least a portion of the face which is in contact with the layer of adhesive composition an ink deposit, for example for the purposes of information relating to the wrapped product. This deposit of inks is carried out by a suitable printing process on the thin transparent layer, before it is laminated. It is advantageously protected from the outside by the thin transparent layer. The transparent material is for example polyester or polypropylene. The inks used are generally dispersions in a continuous phase, organic or mineral, of insoluble, organic or mineral pigments, very finely divided.

(43) According to an embodiment, the multilayer film according to the invention comprises a thin layer based on a metallized material and a thin layer constituted by a transparent material over at least a portion of its surface area. Thus, thanks to the uniform and transparent appearance of the adhesive layer, the user is able to read for example printed information, without being inconvenienced by any visible defects by light reflection on the metallized portion of the thin layer.

(44) As well as very good cohesion making it possible to prevent delamination of the films, the multilayer film according to the invention has an excellent visual appearance. Said film has over at least a portion of its surface area, a transparent and uniform visual appearance, making it possible in particular to view the food item or the object wrapped in the multilayer film.

(45) The invention also relates to a process for the continuous preparation of the multilayer film according to the invention, comprising: (i) a coating step by the adhesive composition as previously defined of a first thin layer of material, in which said composition, rendered pourable by heating to an appropriate temperature, is extruded by a coating device without contact with said thin layer, in the form of a substantially continuous layer, which is then placed in contact with the surface of said thin layer, then (ii) a laminating step of a second thin layer, on the first thin layer coated according to step (i).

(46) FIGS. 1 and 2, to which reference is made below, describe diagrammatically 2 embodiments of the lamination process according to the invention.

(47) The thin layer 1 is presented in the form of a film which is packed by winding on a reel (not shown in the figure) the width of which corresponds to the standard width of said film and which is rotated by drive means (also not shown) so as to give said film a certain speed of movement in the direction indicated by the arrow, which may be up to 450 m/minute or even more.

(48) The film (or thin layer) 1 is conveyed by the guide rolls 3 and/or 4 in the vicinity of the coating device 5 from which it remains separated by a distance comprised between 0.5 and 20 mm, preferably between 0.5 and 2 mm. The coating device is advantageously of the slot nozzle (also called lip) type, the slot of which is rectangular in shape with the long side (or width) corresponding to the standard width of the film 1 to be coated (which may range up to approximately 2 m), and the short side of which (or height) can measure from 100 to 1000 m.

(49) The temperature at which the adhesive composition is rendered pourable can vary from 140 to 210 C., and is obtained by means of heating said composition placed according to circumstances in a melting tank or extruder.

(50) The substantially continuous layer 6 of adhesive composition has a thickness at the outlet of the nozzle slot that corresponds substantially to the height of the slot, i.e. a thickness that can vary from 100 to 1000 m. Under continuous operation, said layer is moved suspended in the air over the distance between its placing in contact with the film 1 to be coated and the coating device 5.

(51) Under continuous operation the substantially continuous layer of adhesive composition 6 is stretched under the effect of the force resulting from the movement of the film, so that its thickness, comprised between 100 and 1000 m on leaving the nozzle, is brought to a thickness less than 10 m in the vicinity of its contact with the film 1, or less than or equal to 7 m, or even less than or equal to 5 m. This difference in thickness is not shown in FIGS. 1 and 2.

(52) According to a first embodiment, shown in FIG. 1, of the coating step (i) of the process according to the invention, the substantially continuous layer 6 of adhesive composition extruded by the coating device 5: (a) passes through the space between said device and the film (1), suspended in the air, the pressure of which is reduced locally to a value comprised between 500 and 975 mbars, preferably between 750 and 900 mbars, then (b) is placed in contact with the film (1) in the vicinity of the edge (10c) formed by the 2 guide surfaces (10a) and (10b) of a deflection element (10) through which a compressed air line (not shown in the figure) opening out via at least one outlet opening into the area of the edge 10c, such that the discharged compressed air forms an air cushion around said edge. The air cushion advantageously reduces the friction between the edge 10c and the film 1 during the movement at high speed of the latter.

(53) Lowering the atmospheric pressure in the vicinity of the layer 6 suspended in the air is advantageously obtained by a vacuum box 11 having suitable geometry, which is connected to a vacuum source (not shown in FIG. 1).

(54) The 2 guide surfaces 10a and 10b of the deflection element 10 are adapted to the guidance of the film 1 and to the modification of its direction of transport, as indicated in FIG. 1. The film 1 passes along the inward guidance surface 10a of the deflection element 10, its direction of transport being radically modified by the edge 10c, and then passes along the outward guidance surface 10b, in the direction of the arrow. An air cushion is present between the film 1 and the deflection element 10, the 2 surfaces 10a and 10b being configured as air-cushioned surfaces.

(55) The combination of lowering the atmospheric pressure and the deflection element 10 is particularly advantageous, because it avoids the inclusion of air between the layer 6 of adhesive composition and the film 1, inclusion which can become particularly troublesome for high movement speeds of the film, in particular greater than 450 m/minute.

(56) For the description of said deflection element 10, reference is made to international application WO 2005/099911.

(57) According to a second embodiment of coating step (i) of the process according to the invention, illustrated by FIG. 2, the substantially continuous layer 6 of adhesive composition extruded by the coating device 5, after it has been placed in contact with the film 1, is pressed onto the latter by a roll (7) the outer surface of which is provided with a non-stick coating, for example based on polytetrafluoroethylene. Said roll 7, with roll 4, exerts pressure on the adhesive layer deposited on the film 1, and thus avoids trapping air between said film and the adhesive layer for high movement speeds of the film, in particular greater than 100 m/minute, or even 450 m/minute.

(58) For the description of this second embodiment, reference is specifically made to the aforementioned application WO 99/28048.

(59) Coating step (i) comprised in the process according to the invention is followed by a step (ii) of laminating a second thin layer 2 of material on the first thin layer 1 covered by the substantially continuous layer of adhesive composition 6. This second thin layer 2 is also presented in the form of a film which is also advantageously packaged like the first in reel form; it is composed of a material identical to or different from that used for the first thin layer 1. The lamination is carried out by application of a pressure exerted by the rolls 8 and 9.

(60) Thus advantageously 2 thin layers of material bonded by a layer of adhesive composition are obtained, the whole being comprised in the laminate film according to the invention, as defined previously. This laminate film is also packaged on a reel.

(61) The laminate films according to the invention are capable of use for the manufacture of the most varied flexible wrappings, which are shaped then closed (after the step of packaging the product intended for the consumer) by the technique of heat sealing (or heat welding).

(62) According to an embodiment, the flexible wrapping obtained using the multilayer film according to the invention has a transparent window allowing the food item or the wrapped object to be viewed.

(63) Peel Test at 180:

(64) The cohesion of the composite film according to the invention is tested by the peel test at 180 as described French standard NF T 54-122. The principle of this test consists of determining the force necessary for the separation (or peeling) of 2 individual layers of films bonded by the adhesive.

(65) A rectangular-shaped test piece of 1.5 cm wide and approximately 10 cm long is cut from the laminate film. From the end of this test piece, and over approximately 2 cm, the 2 individual layers of film comprised in this strip and the 2 free ends obtained are fixed onto two attachment devices linked, respectively, to a fixed portion and a mobile portion of a traction device that are situated on a vertical axis.

(66) When a drive mechanism gives the mobile portion a uniform speed of 100 mm/minute, leading to the detachment of the 2 layers the ends of which move progressively along a vertical axis, forming an angle of 180, the fixed portion, connected to a dynamometer, measures the force which the test piece thus held is able to withstand.

(67) The result is expressed in N/15 mm.

(68) Visual Test

(69) The visual appearance is assessed by one or more skilled persons. In general terms, the eye in fact allows defects (or bubbles, spots, etc) of a size greater than or equal to 40 m to be detected. The observation is carried out under natural light. The sample is assessed over its entire width (more than a meter in industrial use) and at different moments (yardage) of the production. The appearance is scored by referring to a colour chart.

(70) The score from 1 to 5 indicated in Table 1 below is described as follows: 1: PoorNon-uniform, 2: PoorUniform (slightly cloudy (haze), orange peel, etc), 3: Acceptable (average): 4: GoodStandard quality obtained in the trade, 5: Equivalent to that obtained with a polyurethane-based solvent adhesive or in the absence of adhesive.

(71) The score is an integer value, with an assessment that is as close as possible to the reference (colour chart).

(72) The visual aspect is assessed during production, and after 3 days' storage at 23 C. +/2 C. It is the value at 3 days which is recorded within the framework of the invention.

(73) The visual appearance of the multilayer film according to the invention is preferably greater than or equal to 3.

(74) The following examples are given purely by way of illustration of the invention and can in no way be interpreted as limiting its scope.

EXAMPLE 1

(75) The adhesive composition given in Table 1 is prepared by simple hot mixing of the ingredients at 170 C.

(76) For this adhesive composition a Brookfield viscosity at 190 C. of 14000 mPa.Math.s and a softening temperature of 117 C. are measured.

(77) This adhesive composition is used for the manufacture of a composite film comprising 2 layers each constituted by an oriented polypropylene film of thickness 20 m and bonded together by a layer of said composition of thickness comprised between 3 and 5 m according to the tests.

(78) In order to produce this film a laminating machine is used the structure of which corresponds diagrammatically to the device shown in FIG. 1. The 2 films of oriented polypropylene are packaged on a reel of 40 cm standard width. The polypropylene reel to be coated is driven by a motor allowing a speed of movement to be given to the substrate ranging up to 600 m/minute. The coating device 5 comprises a lip nozzle marketed by NORDSON under reference BC 70, the slot of which has the shape of a rectangle with length 30 cm and height 600 m. This nozzle is placed at 1 mm from the thin-layer substrate to be coated.

(79) The guide surfaces 10a and 10b of the deflection element 10 form an angle of approximately 60. The pressure created in the vicinity of the continuous film of adhesive composition 6 between the coating device 5 and the film 1 is kept between 750 and 900 mbar by means of the vacuum box 11.

(80) The adhesive composition is extruded by the lip nozzle at the extrusion temperature given in Table 1 and at a pressure comprised between 70 and 90 bars. The lamination of the film 2 onto the film 1 thus coated is carried out by means of the rolls 8 and 9.

(81) The polypropylene film to be coated is given a speed of movement of 100 m/minute. At the output of the nozzle, after establishing steady state, a continuous and cohesive adhesive film 6 is obtained, ensuring the production of the bi-layer film.

(82) The movement speed is then increase to a value of 300 m/minute, then to a value of 500 m/minute, obtaining the same result as for the movement speed of 100 m/minute.

(83) The maximum movement speed of the film is 500 m/minute, corresponding to the maximum speed at which a continuous and cohesive film 6 of adhesive composition is obtained at the outlet of the nozzle. This film 6 leads to depositing on the film 1a layer of thickness comprised between 3 and 5 m, corresponding to a grammage approximately comprised between 3 and 5 g/m.sup.2.

(84) The visual test gives a visual appearance after 3 days of 3.

EXAMPLES 2 to 8

(85) The same experimental protocol is applied with each of the different adhesive compositions the contents of which in constituents are given in Table 1.

(86) The results of the peel test and of the visual test are also given in Table 1.

(87) TABLE-US-00001 TABLE 1 Content in % Composition 1 2 3 4 5 6 7 8 Kraton D1113BT 39.80 39.80 39.80 39.80 39.80 39.80 39.80 Vector 4111 39.80 Dercolyte S115 22.39 Escorez 5400 49.75 34.83 24.88 59.70 14.93 Escorez 5615 13.93 Escorez 1310 LC 37.31 Escorez 2203 LC 49.75 Regalite R1010 9.95 24.88 34.83 44.78 Sylvatac RE12 9.95 45.77 Irganox 1010 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Viscosity at 190 C. 14000 11000 9000 11500 30000 13000 7000 15000 (mPa .Math. s) Softening 117 113 113 113 140 115 111 70 temperature ( C.) Extrusion 190 170 170 160 195 185 170 165 temperature ( C.) Peel test at 23 C. 2.0 1.2 1.1 2.2 2.8 2.2 0.9 0.9 (N/15 mm) Visual test 3 3 5 3 2 2 5 5

(88) Vector 4111 is an SIS copolymer of linear structure having a diblock content of 0% and a styrene content of 19%.

(89) Compositions 1 to 4 correspond to multilayer films according to the invention. In fact, the tests above show, for these compositions, a result in the peel test greater than or equal to 1 N/15 mm and a result in the visual test greater than or equal to 3.

(90) Compositions 5 and 6, although they show good results in the peel test (respectively 2.2 and 2.8 N/15 mm), they do not have satisfactory results in the visual test (less than 3).

(91) Compositions 7 and 8, although they show good results in the visual test (a result of 5), they do not have satisfactory results in the peel test (less than 1 N/15 mm).

(92) Compositions 5 to 8 are comparative examples and do not form part of the present invention.