WO2022269193A1

WO2022269193A1 - Hot-melt self-adhesives for reclosable packaging

Info

Publication number: WO2022269193A1
Application number: PCT/FR2022/051216
Authority: WO
Inventors: Christophe Robert; Barbara WIEGERT
Original assignee: Bostik Sa
Priority date: 2021-06-24
Filing date: 2022-06-22
Publication date: 2022-12-29
Also published as: FR3124521A1; FR3124521B1

Abstract

The present invention relates to a hot-melt self-adhesive composition comprising: - i) from 5% to 60% by weight of a mixture M1 of styrenic block copolymers, said mixture, based on its total weight, consisting of: - from 30% to 70% by weight of Styrene-Isoprene (SI) diblock copolymers and - from 30% to 70% by weight of Styrene-Isoprene-Styrene (SIS) linear triblock copolymers, the total amount of styrenic units in said mixture M1 being less than 25% and strictly more than 0%, based on the total weight of the mixture M1, and - ii) from 20% to 70% by weight of a mixture M2 of styrenic block copolymers, said mixture, based on its total weight, consisting of: - from 1% to 30% by weight of Styrene-Isoprene (SI) diblock copolymers and - from 70% to 99% by weight of Styrene-Isoprene-Styrene (SIS) linear triblock copolymers, the total amount of styrenic units in said mixture M2 being strictly more than 25%, based on the total weight of the mixture M2, and - iii) at least one non-hydrogenated tackifying resin obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having around 5 carbon atoms that are obtained from petroleum fractions.

Description

HOT-MELT SELF-ADHESIVE FOR RESEALABLE PACKAGING

FIELD OF THE INVENTION

The subject of the present invention is a new hot-melt self-adhesive composition, as well as a multilayer film intended for the manufacture of resealable packaging (or trays), which comprises an adhesive layer constituted by said composition.

TECHNOLOGICAL BACKGROUND

Resealable trays are used in the food industry and mass distribution to package foodstuffs, especially fresh products. Such packages are described by US patent 4673601 and patent application EP 1053952.

After opening the packaging for the first time and consuming part of the food product it contains, the user can manually close the packaging hermetically and therefore ensure, if necessary after placing in the refrigerator, the conservation of the remaining portion of the product. A succession of reopenings and closings is also possible.

These packages generally comprise a container (or receptacle) and a cover forming a lid, which are fixed to each other hermetically by welding.

The receptacle, more or less deep and relatively rigid, consists of a multilayer sheet (also called complex or composite) having a minimum thickness of 200 μm, generally between 200 and 1000 μm. This sheet is thermoformed so as to have a flat bottom on which the food product rests and a periphery in the form of a flat strip. This periphery, generally parallel to the bottom, is bonded by welding to the lid, flexible and flat, which consists of a multilayer film (also called complex or composite) with a thickness generally between 40 and 150 μm, and which is sometimes referred to as lidding film.

When opening the packaging, the lidding film is manually separated from the receptacle at the level of the flat band around the perimeter. This operation reveals an adhesive layer at the level of this flat strip, both on the cover strip and on the receptacle strip which were previously in contact. These 2 "daughter" adhesive layers (continuous or discontinuous) result from the rupture of an initial or "mother" adhesive layer or, possibly, from its separation (or detachment) from one of the 2 layers of the multilayer complex film which are adjacent to it. The initial adhesive layer is therefore one of the layers of said multilayer complex film which is itself an element comprised either in the composite sheet which constitutes the receptacle or, optionally, in the lidding film.

The 2 daughter adhesive layers which are present, after opening the packaging, on the strips located on the respective perimeter of the receptacle and the lid are therefore facing each other. Thus, it suffices to reposition the lid on the receptacle, in accordance with their position in the packaging before opening, in order to put the 2 strips of daughter adhesive layers back into contact. A simple manual pressure then makes it possible to obtain the re-closing of the packaging.

The adhesive composition which constitutes the parent and daughter adhesive layers is therefore necessarily a pressure-sensitive adhesive (also called self-adhesive or even PSA, from the English “Pressure Sensitive Adhesive”).

Hot melt self-adhesive compositions typically comprise tackifying resins and styrenic block copolymers including an elastomeric block. Hot melt adhesive compositions are also commonly referred to as HMPSA corresponding to the initials of the English translation “Hot Melt Pressure Sensitive Adhesive”. They are solid substances at room temperature that do not contain water or solvents. Applied in the molten state, they solidify when they cool, thus forming an adhesive layer which ensures the bond between the 2 thin layers of thermoplastic polymeric material to be assembled, while offering the corresponding packaging the advantageous opening properties. and reclosure.

Moreover, these hot-melt self-adhesive compositions, which are prepared by hot mixing of their ingredients, have the additional advantage of being able to be put into the form of granules by means of an extrusion step carried out directly hot after the mixing step, for example by means of a twin-screw extruder fitted with a tool for cutting the extruded product.

Thanks to the granules thus obtained, the film, for example three-layered, which consists of the layer of hot-melt self-adhesive composition and the 2 thin layers of thermoplastic polymeric material to be assembled, can be conveniently manufactured by co-extrusion, in particular by feeding a sheath blowing device by the constituent materials of the 3 layers in the form of granules.

However, the granules may tend to agglomerate over time (self-agglomeration). This is problematic for the customer when he wants to use the hot-melt self-adhesive composition in the form of granules for the preparation of multilayer films. Indeed, the agglomerated granules will not be correctly swallowed by the single-screw extruder used for the preparation of the films. This poor quality swallowing will have consequences on the flow rate during the extrusion process, and impact the quality of the adhesive layer (such as uneven thickness along the film) and its adhesive performance.

The self-agglomeration of granules is particularly linked to the creep behavior of the product, i.e. its ability to deform under stress. If the product flows, it will be likely to release its tack and self-agglomerate.

There is therefore a need for a hot-melt self-adhesive composition making it possible to at least partially remedy at least one of the aforementioned drawbacks.

There is in particular a need for a hot-melt self-adhesive composition having a good compromise between adhesive properties and limited creep. In particular, there is a need for a hot-melt self-adhesive composition having a good compromise between a sufficient first opening force, a maximum reclosing force, and limited creep.

DESCRIPTION OF THE INVENTION

COMPOSITION

The present invention relates to a hot-melt self-adhesive composition comprising:

- i) from 5% to 60% by weight of a mixture M1 of styrenic block copolymers, said mixture consisting of, on the basis of its total weight:

- from 30% to 70% by weight of diblock copolymers of the Styrene-lsoprene (SI) type and of;

- from 30% to 70% by weight of linear Styrene-Isoprene-Styrene triblock copolymers

(SIS); the content of total styrenic units of said mixture M1 being less than 25% and strictly greater than 0%, based on the total weight of mixture M1; and

- ii) from 20% to 70% by weight of a mixture M2 of styrenic block copolymers, said mixture consisting of, on the basis of its total weight:

- from 1% to 30% by weight of diblock copolymers of the Styrene-lsoprene (SI) type and of;

- from 70% to 99% by weight of linear Styrene-Isoprene-Styrene triblock copolymers

(SIS); the content of total styrenic units of said mixture M2 being strictly greater than 25%, based on the total weight of mixture M2; and

- iii) at least one non-hydrogenated tackifying resin obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5 carbon atoms derived from petroleum cuts. The percentages by weight of the mixture M1 are relative to the total weight of said hot-melt self-adhesive composition. The same applies to the percentages by weight of the mixture M2.

Mixture M1\)

The composition according to the invention comprises from 5% to 60% by weight, preferably from 10% to 50% by weight, preferably from 10% to 40%, and even more preferably from 15% to 30% by weight of the mixture M1 relative to the total weight of said composition.

Mixture M1 consists of, based on its total weight:

(SIS); the content of total styrenic units of said mixture M1 being less than 25% and strictly greater than 0%, based on the total weight of mixture M1.

The content of total styrenic units of the mixture M1 can vary from 5% to less than 25%, preferably from 10% to 22%, and even more preferably from 12% to 20%.

The SI and SIS copolymers included in the mixture M1 can have a weight-average molar mass M _w of between 60 kDa and 400 kDa. Unless otherwise indicated, the weight-average molar masses M _w which are given in this text are expressed in daltons (Da) and are determined by Gel Permeation Chromatography, the column being calibrated with polystyrene standards .

Preferably, the content of SI diblocks in the mixture M1 varies from 35% to 65%, more preferably from 40% to 65%, and even more preferably from 45% to 60%.

The SIS and SI type copolymers which are capable of being used as mixture M1 are commercially available often in the form of triblock/diblock compositions. Quintac ^® 3433 N is a composition which consists, respectively, of

44% and 56% of linear SIS triblock (M _w approximately 220 kDa) and of SI diblock (M _w approximately 110 kDa), and of which the total content of styrenic units is 16%. Kraton ^® D1113BT is a composition whose total content of styrenic units is 16%, and which consists of

45% of linear SIS triblock copolymer of M _w approximately 250 kDa, and 55% of SI diblock copolymer of M _w approximately 100 kDa.

Mixture M2 ii)

The composition according to the invention comprises from 20% to 70% by weight, preferably from 20% to 60% by weight, preferably from 25% to 55%, and even more preferably from 30% to 50% by weight of the mixture M2 relative to the total weight of said composition. Mixture M2 consists of, based on its total weight:

(SIS); the content of total styrenic units of said mixture M2 being strictly greater than 25%, based on the total weight of mixture M2.

The content of total styrenic units of the mixture M2 can vary from strictly greater than 25% to less than 50%, preferably from 26% to 45%, and even more preferably from 26% to 40%.

The SI and SIS copolymers included in the mixture M2 can have a weight-average molar mass M _w of between 60 kDa and 400 kDa.

Preferably, the content of SI diblocks in the mixture M2 varies from 5% to 28%, more preferably from 8% to 25%, and even more preferably from 10% to 25%.

The SIS and SI type copolymers which are likely to be used as mixture M2 are commercially available often in the form of triblock/diblock compositions. Kraton® D1165 PT is a composition which consists, respectively, of 80% and 20% of linear SIS triblock and SI diblock, and whose total content of styrenic units is 30%. Europrene® SOL T 9326 from Versalis is a composition which consists, respectively, of 85% and 15% of linear SIS triblock and SI diblock, and whose total content of styrenic units is 30%.

Tackifying resin \\\)

The hot-melt self-adhesive composition according to the invention comprises at least one non-hydrogenated tackifying resin iii) obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5 carbon atoms derived from petroleum cuts.

The tackifying resin iii) may have a softening temperature (or point) ranging from 25° to 150° C., preferably from 30° to 130° C., more preferably from 50° C. to 120° C., and even more preferably from 90° to 120°C.

The softening temperature is determined in accordance with the standardized ASTM E 28 test, the principle of which is as follows. A brass ring with a diameter of approximately 2 cm is filled with the resin to be tested in the molten state. After cooling to room temperature, the ring and the solid resin are placed horizontally in a thermostatically controlled glycerine bath, the temperature of which can vary by 5° C. per minute. A steel ball approximately 9.5 mm in diameter is centered on the solid resin disc. The softening temperature is—during the phase of heating up the bath at a rate of 5° C. per minute—the temperature at which the resin disk flows from a height of 25.4 mm under the weight of the ball. The tackifying resin iii) can have a weight-average molar mass M _w generally between 300 and 5000 Da, preferably between 1000 and 2000 Da.

Among the tackifying resins iii), mention may be made, for example, of Escorez® 1310LC marketed by Exxon Chemical (softening temperature of 94° C. and a Mw of approximately 1800 Da) or even Wingtack 98 marketed by CRAY VALLEY (temperature of softening of 98°C and a Mw of about 2000 Da).

The composition according to the invention may comprise a total content of tackifying resin(s) iii) greater than or equal to 20% by weight, preferably greater than or equal to 25% by weight, even more preferably greater than or equal to 29 % by weight, relative to the total weight of said composition.

The hot-melt self-adhesive composition may comprise a tackifying resin iii) as defined above, or a mixture of tackifying resins iii) as defined above.

Optional tackifying resin ίnΐ

The hot-melt self-adhesive composition may also further comprise one or more tackifying resin(s) iv) different from resin iii) as defined above.

The tackifying resin iv) can be chosen from those having a weight-average molar mass M _w of between 300 and 5000 Da.

The tackifying resin(s) iv) may be chosen in particular from:

- iv-a) rosins of natural or modified origin, such as for example rosin extracted from pine gum, wood rosin extracted from tree roots and their hydrogenated, dehydrogenated, dimerized, polymerized or esterified derivatives by monoalcohols or polyols such as glycerol;

- iv-b) resins obtained by hydrogenation, polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 9 or 10 carbon atoms derived from petroleum cuts.

Among the tackifying resins (iv-a), mention may be made, for example, of Sylvalite® RE 100S from the company Arizona Chemical (ester of rosin and pentaerythritol having a softening temperature of approximately 100° C.).

Among the tackifying resins (iv-b), mention may be made, for example, of Escorez ^® 5400 marketed by the company 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 has a softening temperature of 100° C. and a Mw of approximately 570 Da; or even Régalite™ R1125 available from Eastman, which is a fully hydrogenated resin having a softening temperature of 125° C., a molar mass Mw of 1200 Da, obtained by polymerization of a composition of unsaturated hydrocarbons having 9 carbon atoms.

Preferably, the hot-melt self-adhesive composition of the invention does not comprise a tackifying resin chosen from terpene resins (terpene resins).

Terpene resins cover in particular unmodified terpene resins, terpene resins modified by the action of phenols (terpene-phenol resins) as well as terpene resins resulting from copolymerization (for example styrene/terpene). Terpene resins can result from the polymerization of terpene hydrocarbons such as, for example, mono-terpene (or pinene), in the presence in particular of Friedel-Crafts catalysts. Among the terpene resins are in particular known Dercolyte® 115 available from the company DRT (terpene resin having a softening temperature of 115° C. and a Mw of approximately 2300 Da, or else Sylvares® TR7115 from Arizona Chemicals.

According to a preferred embodiment, the composition according to the invention does not comprise any tackifying resin other than the tackifying resin(s) iii) mentioned above.

Additional/optional components

According to one embodiment, the aforementioned hot-melt self-adhesive composition consists of:

- i) from 5% to 60% by weight of a mixture M1 as defined above;

- ii) from 20% to 70% by weight of a mixture M2 as defined above;

- iii) at least one non-hydrogenated tackifying resin obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5 carbon atoms derived from petroleum cuts;

- from 0% to 5% by weight of additional component chosen from the group consisting of stabilizers (antioxidants), plasticizers, anti-caking agents, pigments, dyes, organic fillers, mineral fillers, and mixtures thereof .

The total amount of additional component(s) can range from 0.01% to 2% by weight, preferably from 0.1% to 2% by weight relative to the total weight of said composition.

Antioxidants can typically be introduced to protect the composition from degradation resulting from reaction with oxygen which is liable to form by the action of heat, light or residual catalysts on certain raw materials such as tackifying resins. These compounds can include primary antioxidants that scavenge free radicals and 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 such as ^Irgafos® 168 also from CIBA, or even with UV stabilizers such as amines. The anti-caking agents can be chosen from talc, calcium carbonate, calcium stearate, silica (natural or synthetic) or mixtures thereof.

It is possible to use as plasticizer a paraffinic and naphthenic oil (such as ^Primol® 352 from the company ESSO) optionally comprising aromatic compounds (such as Nyflex 222B).

Composition

The hot-melt self-adhesive composition according to the invention may have a flow index (or MFI) ranging from 2 to 70 g/10 min, and preferably from 10 to 50 g/10 min.

The flow index (or Melt Flow Index MFI) of the hot-melt self-adhesive composition is measured at 190° C. and for a total weight of 2.16 kg, in accordance with condition d) of standard ISO 1133. The MFI is the mass of composition (previously placed in a vertical cylinder) which flows in 10 minutes through a die of fixed diameter, under the effect of pressure exerted by a loaded piston having the total weight of 2 .16 kg. Unless otherwise stated, the MFI values given in this text were measured under these same conditions.

The hot-melt self-adhesive composition according to the invention may have a total content of styrenic units strictly greater than 20%, preferably strictly greater than 22%, and even more advantageously from 23% to 35%.

The total content of SI diblocks in the hot-melt self-adhesive composition can be strictly less than 45%, preferably can range from 20% to 42%, even more preferably from 25% to 40%.

The hot-melt self-adhesive composition according to the invention may be in the form of granules of ellipsoidal shape, the average dimensions of the major axis and of the minor axis of which are preferably the following: average dimension of the major axis of between 1 and 10 mm, preferably between 2 and 6 mm; average dimension of the minor axis between 1 and 10 mm, preferably between 2 and 6 mm.

Average dimensions are typically calculated by averaging ten measurements.

The measurement of the dimensions of the major axis and the minor axis of the granules can be done using a vernier caliper.

The hot melt adhesive composition can be prepared in this form by a process which comprises: - a step of mixing the ingredients hot, between 150°C and 200°C, preferably at about 160°C, by means of a twin-screw extruder equipped with a tool for cutting the extruded product of the cutting type under water at the outlet of the die, then

- a drying and cooling step, for example at ambient temperature (23° C.).

MULTILAYER FILMS

The present invention also relates to a multilayer film comprising 2 thin layers B and C of thermoplastic material bonded together by an adhesive layer A, characterized in that said layer A consists of the hot-melt self-adhesive composition as defined previously.

The adhesive layer A preferably ensures the connection between a thin complexable layer B and a thin sealable and divisible layer C.

The thickness of the adhesive layer A can be between 7 and 50 μm, preferably between 7 and 35 μm, and even more preferably between 7 and 25 μm.

The complexable layer B can be complexed (or laminated) with other layers for producing the packaging, for example with a rigid layer for producing the receptacle.

The sealable and breakable layer C makes it possible to ensure, at the level of the periphery along which the receptacle is bonded by welding to the lid, the first opening of the packaging, by means of a breakable weakened zone. After opening, the weakened area reveals:

- the parent adhesive layer on the seal strip and/or on the receptacle strip which were in contact in the closed packaging, and/or

- 2 daughter adhesive layers resulting from the rupture of the parent adhesive layer and located on the cover band and/or the receptacle band.

The material that can be used to form the 2 layers B and C can be a thermoplastic polymer (identical or different for the 2 layers) such as:

- polyethylene (PE),

- polypropylene (PP),

- a copolymer based on ethylene and propylene,

- polyamide (PA),

- polyethylene terephthalate (PET), or even

- a copolymer based on ethylene such as for example a maleic anhydride graft copolymer, a copolymer of ethylene and vinyl acetate (EVA), a copolymer of ethylene and vinyl alcohol (EVOH), a copolymer of ethylene and an alkyl acrylate such as methyl acrylate (EMA) or butyl acrylate (EBA),

- polystyrene (PS), - polyvinyl chloride (PVC),

- polyvinylidene fluoride (PVDF),

- a polymer of lactic acid (PLA), or

- a polyhydroxyalkanoate (PHA).

It is preferred to use to form the 2 layers B and C a polyolefinic material, and most particularly PE, and even more preferably a low density PE (also designated by the English name of Low Density PolyEthylene or LDPE).

The thickness of layers B and C can vary within a wide range from 5 to 150 μm.

The multilayer film according to the invention may comprise, in addition to layers A, B and C, 2 bonding layers D and E such that the adhesive layer A is:

- connected to layer B via layer D, and

- connected to layer C via layer E.

The connecting layers D and E are also designated by the name of “intermediate layers” or by the English term of “tie layers” and generally have a thickness of between 1 and 10 μm, preferably between 2 and 8 μm. The use of such bonding layers advantageously makes it possible to improve the quality of the reclosure of the multilayer film, and therefore of the corresponding reclosable packaging, thus ensuring the consumer more effectively the preservation of the remaining portion of the perishable food product contained in the package, after first opening it.

The tie layers D and E are respectively advantageously made up of compositions d and e, which are identical or different, having a melting point of approximately 80 to 120° C. and essentially comprising polymers chosen from ethylene homopolymers or copolymers, propylene homopolymers or copolymers, copolymers of ethylene with a polar comonomer, and grafted olefin homopolymers or copolymers. Reference is made for further details on these compositions d and e to patent application US2013/0029553.

The multilayer film according to the invention may comprise, in addition to the 3 essential layers A, B and C, and the 2 optional layers D and E, other thin layers necessary for producing the packaging, such as for example:

- a rigid layer necessary for the mechanical strength of the receptacle, or

- a printable layer, or

- a barrier effect layer against oxygen, water vapor or carbon monoxide.

The materials that can be used to form said layers can be identical or different and generally include thermoplastic polymers which can be chosen from the polymers mentioned above for layers B and C. According to a variant embodiment, the multilayer film according to the invention is a 3-layer film consisting of the adhesive layer A and the 2 layers B and C, according to the sequence B/A/C/ in which the sign "/" means that the faces of the layers concerned are in direct contact.

According to another variant embodiment, the multilayer film according to the invention is a 5-layer film consisting of the adhesive layer A, the 2 intermediate layers D and E and the 2 outer layers B and C, according to the sequence B/D /A/E/C in which the sign "/" means that the faces of the layers concerned are in direct contact.

The present invention also relates to a process for manufacturing the multilayer film as defined above, characterized in that it comprises a step of co-extrusion of the hot-melt self-adhesive composition and of the constituent materials of the layers B and C and, where appropriate , layers D and E.

Preferably, the compositions and constituent materials of layers A, B, C and, where applicable, D and E are fed into the co-extrusion device in the form of granules of the dimensions mentioned above. Thus, the hot-melt self-adhesive composition used in the multilayer film according to the invention makes it possible, in a particularly advantageous manner, to ensure both the properties required for said film and the possibility of presenting said adhesive composition under the shape of the aforementioned granules. The other layers possibly included in the multilayer film can be obtained either by incorporating into the co-extrusion device the corresponding constituent materials in the form of granules of the same size (as for example for the bonding layers D and E), or by a process of complexing the film directly resulting from the co-extrusion, implementing for example a polyurethane-based adhesive.

The co-extrusion device used in the manufacturing process is preferably a bubble-blowing co-extrusion device (also known as “sheath blowing co-extrusion”). As known to those skilled in the art, this method comprises:

- the melting, in separate extruders, of the compositions and constituent materials of layers A, B and C, as well as, where appropriate, D and E, then

- the passage of the corresponding flows through a set of annular and concentric dies, so as to form a tubular bubble with several layers, in the order corresponding to that desired for the final structure, then

- the radial expansion (relative to the annular die) and the stretching (in the axial direction) of the bubble, then

- the cooling of the bubble. The geometric characteristics of the dies, as well as the parameters of the process such as the rate of radial expansion and the speed of stretching are fixed so as to obtain the desired thickness for the various layers constituting the multilayer film. Reference is made in particular for a fuller description of the co-extrusion process by bubble blowing to patent application US2013/0029553.

The hot-melt self-adhesive composition according to the invention advantageously exhibits a good compromise between adhesive properties and limited creep. In particular, the hot-melt self-adhesive composition advantageously presents a good compromise between:

- a sufficient first opening force (preferably less than or equal to 6N/cm, even more preferably less than or equal to 5 N/cm),

- a maximum reclosing force, in particular a maximum 5th opening force (for example greater than 0.7 N/cm), and

- limited creep.

USES

The present invention also relates to the use of the multilayer film as described above for the manufacture of resealable packaging.

The use for the manufacture of resealable trays is particularly advantageous, and according to a particularly preferred embodiment for the manufacture of the lidding film of these trays.

In the context of the invention, by “between x and y”, or “ranging from x to y”, is meant an interval in which the limits x and y are included. For example, the range "between 1 and 10" includes the values 1 and 10 in particular.

The following examples are given for purely illustrative purposes of the invention, and can in no way be interpreted to limit its scope.

EXAMPLES

Ingredients used:

Quintac® 3433N (Zeon supplier): Blend of styrenic block copolymer comprising 56% by weight of diblock copolymer of Styrene-lsoprene (SI) type and 44% of linear triblock Styrene-lsoprene-Styrene (SIS) copolymer. The styrenic unit content of this mixture is 16% by weight. It has an MFI of 12g/10min at a temperature of 190°C and under a weight of 5kg Kraton® D1165 PT (Kraton supplier): Blend of styrenic block copolymer comprising 20% by weight of diblock copolymer of the Styrene-lsoprene (SI) type and 80% of linear triblock Styrene-lsoprene-Styrene (SIS) copolymer. The content of styrenic units of this mixture is 30% by weight. It has an MFI of 7g/10min at a temperature of 190°C and under a weight of 5kg

Kraton® D1111 KT (Kraton supplier): Blend of styrenic block copolymer comprising 18% by weight of diblock copolymer of the Styrene-lsoprene (SI) type and 82% of linear triblock Styrene-lsoprene-Styrene (SIS) copolymer. The content of styrenic units of this mixture is 22% by weight. It has an MFI of 4g/10min at a temperature of 190°C and under a weight of 3kg.

Escorez® 1310 LC: Tackifying resin obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of a mixture of unsaturated aliphatic hydrocarbons containing approximately 5 carbon atoms derived from petroleum fractions. It has a softening temperature of 94°C.

SONGNOX® 1010 (SONGWON supplier): Primary antioxidant

LDPE grade LD0304: low density polyethylene manufactured in a high pressure autoclave (MFR at 190°C, 2.16 kg = 4 g/10 min) (TOTAL supplier)

Example 1: Preparation of the adhesive compositions

The adhesive compositions (see Table 1) are first prepared in the form of granules of ellipsoidal shape, the major axis and the minor axis of which respectively have an average dimension of between 1 and 10 mm, preferably between 2 and 6 mm, by mixing of their ingredients at around 160°C using a 2-screw extruder, extrusion through a die then cutting the product using a granulation tool such as an underwater cutter, then drying and cooling to room temperature ( 23°C). Table 1 shows percentages by weight.

Example 2: preparation of B/A/C three-layer films comprising a layer A of the composition of example 1

This three-layer film is manufactured using a continuously operating bubble blowing co-extrusion pilot device, in which 3 extruders are fed:

- for one, by the composition of example 1 (composition n°1, n°2 or n°3), and

- for the other 2, by LDPE.

The process parameters are adjusted so as to manufacture a three-layer film consisting of:

- as layer A, a layer 15 μm thick consisting of the composition of example A, - as a complexable thin layer B, a 30 μm thick layer consisting of LDPE;

- as a thin sealable and breakable layer C, with a layer 15 μm thick also consisting of LDPE.

Several usual parameters were set, namely a radial expansion rate of the bubble at 3, a stretching speed of 7 m/minute and an overall flow rate of 11 kg/hour.

The three-layer film thus obtained has a total thickness of 60 μm, a length of 50 m and is packaged in the form of a roll 250 mm wide.

The different materials were introduced into each extruder, in order to fill the screws and form the polymer bubble at the exit of the extrusion die. After purging for 30 minutes, the thickness and stability of the bubble being checked, 50m of film was wound on mandrels.

Measurement of the force of first opening by peeling in T at 23°C:

Rectangular specimens E1 25 cm long and 3 cm wide are cut from the B/A/C three-layer film obtained above.

A PET/PE complex film is used, moreover, consisting of a PET layer 23 μm thick which is laminated with polyurethane (2 μm layer) on a PE layer 50 μm thick. Rectangular specimens E2 25 cm long and 3 cm wide are cut from this complex film.

A test piece E1 is then placed opposite between 2 test pieces E2, the 3 test pieces being superimposed, so that the layers B and C of E1 are in contact with the PE layer of E2.

Partial sealing is then carried out, by means of two jaws heated to 130°C, applied under a pressure of 4.6 bar for 1 second, so as to obtain a sealed zone of rectangular shape (15 cm long and 1 cm of width). Said sealed zone is arranged lengthwise and is located in the center of the assembly of the 3 superimposed test specimens, so that 5cm of unsealed films protrude on either side in the lengthwise direction, and 1cm unsealed films protrude on both sides in the direction of the width.

The strips of free films located on the same side of the assembly of the 3 test specimens, with respect to the sealed zone, are fixed:

- to a first attachment device (called a jaw) which is connected to the fixed part of a traction device, as regards that of the 2 bands E2 which is facing the layer C of E1, and

- A second jaw connected to the movable part of said traction device, as regards the other band E2 and the band E1. The fixed and mobile parts of the traction device, which is a dynamometer, are located on the same vertical axis.

The 3 bands thus fixed, allow, by moving the jaws of the dynamometer, to stress (or peel) the interface between the layer C of the B/A/C film constituting E1 and the PE of E2 which faces it.

While a drive mechanism communicates to the mobile part a uniform speed of 300 mm/minute leading to the peeling of the 3 sealed specimens E1 and E2, the ends move progressively along a vertical axis forming an angle of 180°. A force sensor connected to said movable part measures the force supported by the specimen thus held. The measurement is carried out in a climatic room maintained at a temperature of 23°C.

The force obtained is the first opening force (in N/cm) and is indicated in the following Table 1.

Measurement of the fifth opening force by T-peel at 23°C:

The 2 parts of the previous specimen are, after peeling, repositioned facing each other and brought into contact manually. They are then subjected to pressure exerted by means of a roller of mass 2 kg with which one proceeds to a back and forth movement in a direction parallel to the length of the specimen.

A tensile specimen of identical shape to that prepared for the previous peeling test is thus obtained, which is then repeated.

The two steps are performed 4 times until the fifth opening force is obtained. This is shown in the following Table 1.

Example 3 creep test at 20°C

A single-layer film with a thickness of approximately 1.5 mm is manufactured by means of a hydraulic press by hot pressing of the adhesive composition granules obtained in Example 1 at a temperature of approximately 120° C. and under a pressure around 50 bar. A circular disc with a diameter of 25mm is then cut out of the film obtained.

The sample is placed hot at approximately 120° C. between the circular plates with a diameter of 25 mm of an MCR 301 type rheometer (supplier Anton Paar) in order to bring the sample into good contact with the geometries. Then the temperature dropped to 20°C. An air gap of 1.5 mm is maintained during the test. A stress of 5000Pa is first applied for 10 min and then released. The deformation (expressed in %) of the sample is measured over time. The acquisition time is variable with a logarithmic ramp from 0.05 to 10s during the stress application time. The value of the strain at the time of stress release is recorded and shown in Table 1.

Table 1

The composition according to the invention advantageously leads to a good compromise between limited creep (7.4%), ^1st opening peel strength of 4.7 N/cm and ^satisfactory 5th opening strength (0.8 N/cm ). On the other hand, comparative composition n°1 leads to a high creep ^deformation (15.2%), while comparative composition no. 3 according to the invention.

Claims

1. Self-adhesive hot-melt composition comprising:

- from 30% to 70% by weight of linear Styrene-Isoprene-Styrene (SIS) triblock copolymers; the content of total styrenic units of said mixture M1 being less than 25% and strictly greater than 0%, based on the total weight of mixture M1; and

- from 70% to 99% by weight of linear Styrene-Isoprene-Styrene (SIS) triblock copolymers; the content of total styrenic units of said mixture M2 being strictly greater than 25%, based on the total weight of mixture M2; and

- iii) at least one non-hydrogenated tackifying resin obtained by polymerization or copolymerization (with an aromatic hydrocarbon) of mixtures of unsaturated aliphatic hydrocarbons having approximately 5 carbon atoms derived from petroleum cuts.

2. Composition according to claim 1, characterized in that it comprises from 10% to 50% by weight, preferably from 10% to 40%, and even more preferably from 15% to 30% by weight of the mixture M1 relative to the total weight of said composition.

3. Composition according to any one of claims 1 or 2, characterized in that the content of total styrenic units of the mixture M1 varies from 5% to less than 25%, preferably from 10% to 22%, and even more preferably from 12% to 20%.

4. Composition according to any one of claims 1 to 3, characterized in that the content of SI diblocks in the mixture M1 varies from 35% to 65%, more preferably from 40% to 65%, and even more preferably from 45% to 60%.

5. Composition according to any one of claims 1 to 4, characterized in that it comprises from 20% to 60% by weight, preferably from 25% to 55%, and even more preferably from 30% to 50% by weight of the mixture M2 relative to the total weight of said composition.

6. Composition according to any one of claims 1 to 5, characterized in that the content of total styrenic units of the mixture M2 varies from strictly greater than 25% to less than 50%, preferably from 26% to 45%, and even more preferably from 26% to 40%.

7. Composition according to any one of claims 1 to 6, characterized in that the content of SI diblocks in the mixture M2 varies from 5% to 28%, more preferably from 8% to 25%, and even more preferably from 10% to 25%.

8. Composition according to any one of claims 1 to 7, characterized in that the total content of tackifying resin(s) iii) is greater than or equal to 20% by weight, preferably greater than or equal to 25% by weight, even more preferably greater than or equal to 29% by weight, relative to the total weight of said composition.

9. Composition according to any one of claims 1 to 8, characterized in that it does not comprise a tackifying resin chosen from terpene resins.

10. Composition according to any one of claims 1 to 9, characterized in that it does not comprise any tackifying resin other than the tackifying resin(s) iii).

11. Composition according to any one of claims 1 to 10, characterized in that it has a flow index (or MFI) ranging from 2 to 70 g/10 min, and preferably from 10 to 50 g/10 min. .

12. Composition according to any one of claims 1 to 11, characterized in that it has a total content of styrenic units strictly greater than 20%, preferably strictly greater than 22%, and even more advantageously ranging from 23% to 35%.

13. Composition according to any one of claims 1 to 12, characterized in that it has a total content of SI diblocks strictly less than 45%, preferably ranging from 20% to 42%, even more preferably from 25% to 40%.

14. Composition according to any one of claims 1 to 13, characterized in that it consists of:

- i) from 5% to 60% by weight of the mixture M1;

- ii) from 20% to 70% by weight of the mixture M2;

15. Composition according to any one of claims 1 to 14, characterized in that it is in the form of granules of ellipsoidal shape whose average dimensions of the major axis and of the minor axis are preferably the following: average dimension of the major axis between 1 and 10 mm, preferably between 2 and 6 mm; average dimension of the minor axis between 1 and 10 mm, preferably between 2 and 6 mm.

16. Multilayer film comprising 2 thin layers B and C of thermoplastic material bonded together by an adhesive layer A, characterized in that said layer A consists of the hot-melt self-adhesive composition as defined in one of claims 1 to 15.

17. Use of the multilayer film as defined in claim 16, for the manufacture of resealable packaging.