CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of International Application No. PCT/FR2017/051902, filed on Jul. 11, 2017, which claims the benefit of French Patent Application No. 16 56653, filed Jul. 12, 2016.

FIELD OF THE INVENTION

The present invention relates to a two-component polyurethane-based adhesive composition. The invention also relates to a multilayer structure (or laminate) comprising at least two layers of material bonded together by a layer of the adhesive composition according to the invention. It also relates to a lamination process suitable for the manufacture of said multilayer structure and also to the use of a multilayer structure according to the invention in the field of flexible packaging, in particular for the manufacture of flexible packagings intended for the packaging of foodstuffs, cosmetic products, pharmaceutical products or body hygiene products.

TECHNOLOGICAL BACKGROUND

The flexible packagings intended for the packaging of the most diverse products, such as those manufactured by the food processing, cosmetics or detergents industries, generally consist of several layers (in the form of sheets or films), the thickness of which is between 5 and 150 μm and which consist of different materials, such as paper, a metal (for example aluminum) or also thermoplastic polymers. The corresponding multilayer structure, the thickness of which can vary from 20 to 400 μm, makes it possible to combine the properties of the different individual layers of material and to thus provide the consumer with a combination of characteristics suitable for the final flexible packaging, such as, for example:its visual appearance (in particular that of the printed elements presenting the information relating to the packaged product and intended for the consumer),a barrier effect to moisture, to gases (O2, N2, CO2and their mixtures) and/or to light and to ultraviolet (UV) rays,contact with food without risk of toxicity or of modification to the organoleptic properties of the packaged foodstuffs,chemical resistance for certain products, such as ketchup or liquid soap,good behavior at high temperature, for example in the case of pasteurization or sterilization. In particular, the adhesive seal formed by crosslinking of the adhesive layer connecting the individual layers of the packaging has to retain a sufficient level of cohesion after heat treatment, in order to avoid any phenomenon of delamination.

In order to form the final packaging, the laminate is generally shaped by heat sealing at a temperature varying from approximately 120 to 250° C., the latter technique also being used for closing the packaging around the product intended for the consumer.

The various layers of material of which the laminate is composed are combined or assembled by laminating during industrial lamination processes.

These processes employ adhesives and devices (or machines) designed for this purpose. The adhesive composition employed for the purpose of producing the laminate is often described by the term of “lamination adhesive”.

These processes first of all comprise a stage of coating the adhesive composition over a first layer of material, which consists of the deposition of a continuous layer of adhesive with a controlled thickness generally of less than 10 μm, corresponding to an amount of adhesive (or grammage) which is also controlled, generally not exceeding 10 g/m2. This coating stage is followed by stage of laminating a second layer of material, identical to or different from the first, consisting of the application, under pressure, of this second layer of material to the first layer of material covered with the layer of adhesive.

Two-component polyurethane-based adhesive compositions are commonly used for this type of application.

These compositions are supplied to the laminator in the form of 2 compositions (or components):one (known as —NCO component) containing chemical entities carrying isocyanate end groups, andthe other (known as —OH component) containing chemical entities carrying hydroxyl end groups.

The mixing of these 2 components is carried out under hot conditions at a temperature of between 35 and 80° C. by the operator of the lamination machine, prior to starting it up, and makes possible, by virtue of an appropriate viscosity, the correct operation of the machine.

On conclusion of the coating of the mixture thus obtained and of the laminating operation, the isocyanate groups of the —NCO component react with the hydroxyl groups of the —OH component, according to a reaction referred to as crosslinking, to form a polyurethane which exists in the form of a three-dimensional network comprising urethane groups, providing the cohesion of the adhesive seal between the 2 laminated layers of material.

The multilayer structures obtained after lamination are generally wound off in the form of reels, known as mother reels, which are subsequently generally stored at ambient temperature or in an oven in order to provide sufficient crosslinking of the adhesive layer.

The lamination operation is very often followed by an operation of cutting the composite multilayer structure, which is also carried out by the laminator, so to generate, from a mother reel of said structure, several daughter reels of lower width, for example of between 10 cm and 1 m.

These daughter reels are intended to be transported and delivered to the various client manufacturers of the laminators, who employ them directly on their packaging lines in order to carry out the packaging of their own products, for example processed foodstuffs, cosmetics or detergents, which are intended in particular for the consumer.

The chemical entities present in the —NCO component are generally polymeric chemical compounds carrying isocyanate end groups (often known as prepolymers as they are precursors of the constituent final crosslinked polyurethane of the adhesive seal), which are themselves generally polyurethanes produced by the reaction of a stoichiometric excess of at least one diisocyanate with at least one polyether polyol and/or polyester polyol.

The chemical entities present in the —OH component are generally polymeric or non-polymeric compounds, sometimes of natural origin (such as castor oil), which generally include polymers of polyether polyol and/or polyester polyol type, with a number-average molecular weight Mn (or molar mass) ranging from 400 to 4000 g/mol.

However, these two-component polyurethane-based adhesive compositions generally exhibit the disadvantage of employing an —NCO component comprising high residual contents of diisocyanate monomers originating from the reaction for the synthesis of the polyurethane prepolymer carrying NCO groups (or having NCO endings). This is because these residual diisocyanate monomers are capable of resulting in a number of undesirable effects.

“Diisocyanate monomer” is understood to mean a hydrocarbon compound of low molar mass (of less than 700 g/mol) having two isocyanate groups. In particular, “aromatic diisocyanate monomer” is understood to mean a diisocyanate monomer as defined above, in which one of the NCO groups is connected via a covalent bond to a carbon atom forming part of an aromatic hydrocarbon ring, such as a phenyl group.

The disadvantage of aromatic diisocyanate monomers, such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI), is that they are capable of migrating through the packaging and of forming, by hydrolysis on contact with water or with moisture present in the foods or other packaged products, primary aromatic amines regarded as toxic.

In order to take into account the undesirable effects related to the presence of these aromatic diisocyanate monomers, regulations require, for some types of products, in particular in the food field, a specific labelling of the product, if the concentration of aromatic diisocyanate monomers exceeds 0.1% by weight of the weight of the product.

One of the solutions envisaged to overcome these disadvantages and meet the abovementioned health requirements was to reduce the residual content of aromatic diisocyanate monomers. However, the processes employed for this purpose are restrictive.

Another solution was to prepare the polyurethane prepolymer having NCO endings from aliphatic diisocyanate monomer, that is to say from nonaromatic diisocyanate monomer and in particular from diisocyanate monomer in which neither of the NCO groups is connected by a covalent bond to a carbon atom forming part of an aromatic hydrocarbon ring, such as a phenyl group. This is because aliphatic diisocyanate monomers do not exhibit the abovementioned disadvantages. However, it is desirable to also limit their content as some of them, such as IPDI, can also present health problems, being classified as sensitizing.

Patent application WO2006/026670 discloses two-component polyurethane-based adhesive compositions comprising, in a first component denoted A, a composition comprising, in solvent, a polyurethane prepolymer obtained by reaction of a mixture of IPDI and of an IPDI triisocyanate isocyanurate with a mixture of a polyester diol 1 and 2 and of bisphenol A propoxylate, with an aminosilane, and, in a second component denoted B, a composition comprising, in solvent, at least one branched polyester polyol and an epoxy resin resulting from bisphenol A and from epichlorohydrin.

However, the two-component adhesive compositions described in this document are not entirely satisfactory and remain to be improved in a number of respects, in particular from the viewpoint of their adhesive performance, of their behavior toward sterilization, of their harmlessness and of their rate of crosslinking.

This is because these compositions crosslink slowly and do not make it possible to rapidly achieve the desired adhesive performance qualities, in particular in the case of the adhesive bonding of metal materials, which requires the operator to place the multilayer structures in an oven at 60° C., in order to reduce the duration of crosslinking to an acceptable level.

In particular, the two-component adhesive compositions described do not make it possible to rapidly manufacture multilayer structures exhibiting a high level of cohesion, as desired in some types of applications or processes.

In particular, when it is desired to transform or treat the multilayer structures obtained, for example by a cutting or heat-sealing operation, followed by an optional pasteurization or sterilization treatment, the operator has to wait for the multilayer structure to exhibit a sufficient level of cohesion before being able to operate, if an imperfect cutting and/or a nonconforming packaging is not to be obtained.

In addition, the multilayer structures described, obtained with the compositions of the prior art, exhibit, after sterilization, signs of degradation of the adhesive seal which are reflected in particular by a significant loss in cohesion of the adhesive seal, above 30%, with respect to its initial value measured before sterilization.

Furthermore, the use of a significant amount of solvent in the adhesive compositions is not very desirable as this results in a low-rate lamination process which is not very compact and which is potentially more dangerous in the case of a flammable and volatile solvent.

Thus, there exists a need to develop a polyurethane-based adhesive composition which does not exhibit the disadvantages of the abovementioned prior art.

The aim of the present invention is to provide a polyurethane-based adhesive composition, substantially, indeed even completely, devoid of aromatic and/or aliphatic diisocyanate monomers, which is suitable for the high-output production of multilayer structures which can be used in the manufacture of flexible packagings.

In particular, the aim is to develop a polyurethane-based adhesive composition which exhibits a satisfactory rate of crosslinking at ambient temperature (23° C.), making it possible to rapidly produce (in less than 5 days) laminates with an excellent degree of cohesion.

In particular, the aim is to develop a polyurethane-based adhesive composition which exhibits a better thermal resistance, in particular which exhibits a better behavior with regard to the pasteurization and/or sterilization test.

In particular, the aim is to develop a polyurethane-based adhesive composition which is suitable for the manufacture of a broad range of multilayer structures, especially those comprising at least one layer of metal material, which structures can be used in the manufacture of flexible packagings.

In particular, the aim is to develop a polyurethane-based adhesive composition which can be employed with a reduced content of solvent (preferably less than 45% by weight of the total weight of the adhesive composition).

In particular, the aim is to develop a polyurethane-based adhesive composition which is simple to prepare by mixing of its components at ambient temperature (23° C.) and easy to employ by a conventional industrial lamination process, without it being necessary to accelerate the crosslinking reaction of the adhesive layer by heating.

It has now been found that the adhesive composition which is a subject matter of the present patent application makes it possible to meet its needs.

In particular, the adhesive composition according to the invention makes it possible to manufacture, with a satisfactory level of cohesion, a multilayer structure suitable for the manufacture of flexible packaging and capable of withstanding a high-temperature heat treatment ranging from 60° C. to 135° C., such as a pasteurization (70° C. to 99° C.) and better still a sterilization (100° C. to 135° C.).

In particular, the adhesive composition according to the invention makes it possible to manufacture, with an excellent level of cohesion, different types of laminates which can be used in the manufacture of flexible packagings, including those based on metal materials.

In particular, the adhesive composition according to the invention can be employed with a content of solvent of less than or equal to 45% by weight of the weight of the adhesive composition. In particular, the —NCO component used according to the invention can comprise a reduced content of solvent while exhibiting a viscosity satisfactory for being mixed at ambient temperature.

In particular, the adhesive composition according to the invention rapidly crosslinks at ambient temperature after mixing of the —NCO and —OH components and thus makes it possible to produce, with high line speeds, multilayer structures with an excellent level of cohesion.

In particular, the adhesive composition according to the invention satisfies the health requirements required in order to be used to manufacture multilayer structures intended for the packaging of foodstuffs, cosmetic products, pharmaceutical products or body hygiene products.

In particular, the adhesive composition according to the invention exhibits a suitable viscosity at ambient temperature and is sufficiently stable to be employed in conventional industrial lamination processes.

The level of cohesion and the thermal resistance of the adhesive compositions can be evaluated in a way well known to a person skilled in the art, as illustrated in the examples.

SUMMARY OF THE INVENTION

A first subject matter of the invention relates to a two-component polyurethane-based adhesive composition comprising an —NCO component and an —OH component, such that:the —NCO component is a composition comprising:A) at least one polyurethane prepolymer comprising at least two NCO end groups obtained by a polyaddition reaction:of at least one aliphatic polyisocyanate chosen from aliphatic diisocyanate monomers and mixtures of at least one aliphatic diisocyanate monomer with at least one triisocyanate based on XDI, andof at least one polyester diol,B) at least one triisocyanate based on XDI, andthe —OH component is a composition comprising at least one polyester polyol.

According to one embodiment, the aliphatic diisocyanate monomer(s) is (are) chosen from:xylylene diisocyanate (XDI),aliphatic diisocyanate monomers other than XDI,and their mixtures.

According to one embodiment, the amount of aliphatic diisocyanate monomer(s) with respect to the amount of polyester diol(s) is such that the NCO/OH molar ratio, denoted r1, ranges from 1.0 to 2.2, preferably from 1.2 to 2.0.

This ratio r1corresponds to the ratio of the number of NCO groups (present in the total amount of aliphatic diisocyanate monomer(s) used to synthesize the polyurethane prepolymer having NCO endings) to the number of OH groups (present in the total amount of diol(s) used to synthesize the polyurethane prepolymer having NCO endings).

According to one embodiment, the aliphatic diisocyanate monomer used to prepare the polyurethane prepolymer having NCO endings is a mixture of aliphatic diisocyanate monomers comprising at least XDI, preferably at least 50% by weight of XDI with respect to the weight of said mixture, it being possible for the latter to be in the form of a pure XDI isomer or in the form of a mixture of XDI isomers.

According to a preferred embodiment, the aliphatic diisocyanate monomer used to prepare the polyurethane prepolymer having NCO endings is XDI, it being possible for the latter to be in the form of a pure isomer, such as meta-xylylene diisocyanate (m-XDI), or in the form of a mixture of XDI isomers.

According to one embodiment, the molar ratio of the number of NCO groups present in the total amount of aliphatic diisocyanate monomer(s) used to synthesize the polyurethane prepolymer having NCO endings A) to the number of NCO groups present in the total amount of triisocyanate(s) based on XDI, denoted r2, ranges from 3 to 11, preferably from 5 to 11.

According to one embodiment, the polyaddition reaction takes place in the presence or absence of at least one reaction catalyst, at a reaction temperature T1 of less than 95° C. and preferably ranging from 65° C. to 80° C., under anhydrous conditions.

According to a preferred embodiment, the triisocyanate(s) based on XDI is (are) chosen from XDI isocyanurates, XDI biurets, adducts of XDI and of triols, and their mixtures.

According to one embodiment, the —NCO component comprises a content of aliphatic diisocyanate monomer(s) of less than or equal to 0.5% by weight, with respect to the weight of the —NCO component.

According to one embodiment, the polyester diol(s) and/or polyester polyol(s) has (have) a number-average molar mass ranging from 1000 to 4000 g/mol.

According to one embodiment, the —NCO and —OH components, which are intended to be mixed, are present in the two-component adhesive composition according to the invention in amounts such that the NCO/OH molar ratio, denoted r3, ranges from 2 to 15, more preferably from 6 to 11.

This ratio r3corresponds to the ratio of the number of NCO groups present in the total amount of NCO component intended to be mixed to the number of OH groups present in the total amount of OH component intended to be mixed.

Another subject matter of the invention is a multilayer structure comprising at least two layers of material bonded together by an adhesive layer, characterized in that said adhesive layer consists of the adhesive composition according to the invention in the crosslinked state.

According to one embodiment, the multilayer structure according to the invention comprises at least one layer of aluminum-based material.

Another subject matter of the invention is a process for the manufacture of a multilayer structure according to the invention, comprising the following stages:

(i) the mixing of the —NCO and —OH components of the two-component adhesive composition according to the invention, then

(ii) the coating of said mixture over the surface of a first layer of material, then

(iii) the laminating of the surface of a second layer of material over said coated surface, then

(iv) the crosslinking of said mixture.

According to one embodiment of the manufacturing process, the mixing of the —NCO component and of the —OH component comprises at least one solvent and the process additionally comprises a stage of evaporation of the solvent or solvents.

Another subject matter of the invention is the use of a multilayer structure according to the invention in the manufacture of flexible packagings and in particular of heat-sealable, sterilizable and/or pasteurizable flexible packagings.

Other subject matters and characteristics of the present invention will become more clearly apparent on reading the description and the examples.

