Cookable solventless laminating adhesive

Abstract

The present invention relates to a solvent-free two-component adhesive kit comprising an isocyanate-functionalized aliphatic polyurethane prepolymer and a polyol crosslinker. The present invention further relates to a lamination adhesive comprising the components of said adhesive kit of the invention and to retort pouches comprising said lamination adhesive.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to EP application 22209746.1, which is incorporated herein by reference. Technical Field

[0003] This invention relates to solvent-free two-component adhesives and their use in providing laminated materials for cooking applications. Background Technology

[0004] Cooking is a well-known process in the packaging industry, in which sealed bags containing wet food are processed at high temperatures, typically exceeding 100°C, to cook and sterilize the food.

[0005] Solvent-based adhesives are used in retort applications because, under the conditions of the retort process, they provide bond strength between the layers of the packaging laminate, which is essential to ensure the integrity of the laminate without delamination failure. In particular, solvent-based adhesives exhibit high resistance to thermal degradation, partly because they allow the use of higher molecular weight starting materials, which build up a stronger network that is less prone to forming migratory small molecules during pyrolysis. Solvents maintain the appropriate viscosity of the composition used for lamination, which would otherwise be increased by the incorporation of high molecular weight materials.

[0006] Therefore, it is desirable to provide an alternative solvent-free adhesive for cooking applications, eliminating the need for removal and treatment of volatile solvents such as ethyl acetate and methyl ethyl ketone (MEK). Furthermore, the use of solvent-free adhesives allows for higher lamination press speeds.

[0007] Solvent-free adhesives are known in the art. However, in the absence of solvents, the molecular weight of the components must be limited in order to provide a composition with a viscosity suitable for lamination, and although low molecular weight substances can provide compositions with lower viscosity, they can also migrate through the composition and contaminate the food that comes into contact with it.

[0008] One approach to provide lower viscosity adhesive compositions and also to reduce the amount of migratable substances present in the adhesive is for example the removal of residual isocyanate monomers using evaporation techniques. EP 3176196 relates to a one-component laminating adhesive comprising an aromatic isocyanate polyurethane prepolymer, wherein the amount of free diisocyanate monomers in the polyurethane prepolymer is reduced to less than 0.1 % (w / w) using a thin film evaporator. The residual diisocyanate monomers are removed at 140 °C at a pressure of 0.1 mbar or less. However, EP 3176196 does not disclose two-component adhesives, which have different requirements than one-component adhesives, such as those comprising aliphatic polyurethane prepolymers according to the present application. Furthermore, EP 3176196 does not disclose that these adhesive compositions are suitable for the preparation of flexible packaging laminates for the manufacture of retortable pouches.

[0009] US 5202001 discloses the removal of aromatic diisocyanate (TDI - toluene diisocyanate) from isocyanate functionalized polyurethane prepolymers using a thin film evaporator. The use of aromatic isocyanates in the preparation of polyurethane prepolymers is undesirable because of the risk associated with the generation of potentially harmful aromatic primary amines, which can be generated under retort conditions. Furthermore, US 5202001 does not relate to adhesive compositions and does not disclose adhesives comprising aliphatic polyurethane prepolymers according to the present application. Furthermore, US 5202001 does not disclose the use of adhesives in the preparation of retortable laminates.

[0010] Many prior art documents relate to the reduction of the amount of residual diisocyanate monomers present in polyurethane prepolymers by distillation in the presence of so-called “inert solvents”. In this regard, US 20030065124 discloses the removal of free methylene diphenyl diisocyanate (MDI) via vacuum distillation using a wiped film evaporator. This method uses dimethyl phthalate as “inert solvent”, which has a boiling point lower than the boiling point of MDI. Similarly, WO 2018013688 relates to polyurethane prepolymers prepared from aromatic p-phenylene diisocyanate (PPDI), from which free monomers are also removed using a wiped film evaporator. In this case, dimethyl adipate is used as “inert solvent”.

[0011] US 20030065124 and WO 2018013688 do not relate to adhesive compositions and thus do not disclose two-component adhesives comprising aliphatic polyurethane prepolymers according to the present application. These documents also do not disclose the use of such adhesives in the preparation of laminates suitable for retort applications. Furthermore, the polyurethane prepolymers of US 20030065124 and WO 2018013688 are prepared using solvents, i.e. they are not solvent-free, in contrast to the aliphatic polyurethane isocyanate functional prepolymers of the present application.

[0012] CN110922929 relates to a one-component isocyanate functional polyurethane prepolymer based adhesive that can be used to manufacture flexible laminates. CN110922929 does not relate to a two-component adhesive and does not disclose reducing the amount of free isocyanate monomer to less than 0.1% (w / w) of the polyurethane prepolymer component. Furthermore, there is no teaching on how to maintain the viscosity of the disclosed composition at a level suitable for use in a laminator while also removing residual isocyanate monomer. CN110922929 does not disclose the use of a wiped film evaporator. Furthermore, CN110922929 does not teach the use of an aliphatic polyurethane precursor to provide a laminating adhesive that has improved bond strength under boiling conditions. Furthermore, the one-component CN110922929 composition contains a large amount of catalyst in order to function, which results in a significant reduction in pot life, which is not suitable for use in a two-component adhesive kit.

[0013] CN102604583B relates to a solvent-free two-component adhesive that can be used to manufacture flexible laminates that are resistant to boiling. However, CN102604583B does not disclose reducing the amount of free isocyanate monomer to less than 0.1% (w / w) of the polyurethane prepolymer component nor does it teach how to maintain the viscosity of the disclosed composition at a level suitable for use in a laminator while also removing residual isocyanate monomer. Furthermore, CN102604583B does not disclose the use of a wiped film evaporator. Furthermore, a reactive silane is an essential component of the CN102604583B adhesive. Finally, CN102604583B does not evaluate the disclosed adhesive under boiling conditions.

[0014] To the best of the present inventor’s knowledge, the successful use of a solvent-free two-component laminating adhesive comprising an aliphatic polyurethane prepolymer comprising free isocyanate monomer in an amount less than 0.1% (w / w) and suitable for boiling applications has not been disclosed. Accordingly, the solvent-free laminating adhesive kit of the present invention is advantageous at least in this regard. Other advantages associated with the solvent-free laminating adhesive kit of the present invention are described herein. SUMMARY

[0015] The present invention provides a solvent-free laminating adhesive kit comprising an isocyanate-functionalized aliphatic polyurethane prepolymer and a polyol crosslinker, the isocyanate-functionalized aliphatic polyurethane prepolymer comprising monomeric units derived from an aliphatic isocyanate monomer. The amount of free isocyanate monomer present in the polyurethane prepolymer is less than or equal to 0.1% (w / w) of the prepolymer. The aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, methylene dicyclohexyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, pentamethylene diisocyanate, and combinations thereof.

[0016] The adhesive kit of the present invention is a two-component adhesive kit, i.e. it comprises, in addition to the polyol crosslinker, an isocyanate functional prepolymer.

[0017] The present invention also provides a laminating adhesive comprising the components of the solventless laminating adhesive kit of the present invention mixed together in a single composition. The present invention also provides a retort pouch comprising a laminating adhesive formed from the components of the kit of the present invention.

[0018] The present invention also provides a method of providing a multi-laminated structure, the method comprising forming a laminating adhesive from the components of the kit of the present invention and applying the composition to a flexible film.

[0019] The present invention also provides a method of making the kit of the present invention comprising the steps of: a) reacting an aliphatic isocyanate with a polyester and / or polyether polyol to obtain an isocyanate functional aliphatic polyurethane prepolymer, the isocyanate functional aliphatic polyurethane prepolymer comprising monomeric units derived from the aliphatic isocyanate monomer; b) reducing the residual isocyanate monomer present in the prepolymer to less than 0.1% (w / w) of the prepolymer; and c) introducing a polyol crosslinker to the kit.

[0020] The present invention also provides the use of the kit of the present invention for forming a laminating adhesive. The present invention also provides the use of the kit of the present invention for improving the bond strength under retort conditions of a multi-laminated structure formed from a flexible film and a laminating adhesive formed from the kit of the present invention. These improvements are relative to a multi-laminated structure formed from the same flexible film and a laminating adhesive not formed from the kit of the present invention. The bond strength is the strength required to separate at least two layers of a multi-laminated structure joined together with the adhesive and is measured at room temperature (25°C) and 100 mm / min. DETAILED DESCRIPTION

[0021] DEFINITIONS

[0022] Aromatic monomer = a monomer in which the polymerisable functional group is directly attached to an aromatic group.

[0023] Aliphatic monomer = a monomer in which the polymerisable functional group is not directly attached to an aromatic group.

[0024] Aromatic isocyanate = an isocyanate comprising at least one isocyanate (-NCO) group directly attached to an aromatic ring.

[0025] Aliphatic isocyanate = an isocyanate in which the isocyanate (-NCO) group is not directly attached to an aromatic ring. Aliphatic isocyanates include alicyclic isocyanates. Thus, aliphatic isocyanates can include aromatic rings, provided that the isocyanate group is not directly attached to the aromatic ring, for example, xylene diisocyanate.

[0026] Aromatic polyurethane = a polyurethane formed from aromatic isocyanate. Those skilled in the art will appreciate that it is the nature of the isocyanate itself (aliphatic or aromatic) that determines whether the polyurethane is aliphatic or aromatic. For the purposes of the present invention, an aliphatic polyurethane prepolymer is derived from 95 mol% or more of aliphatic isocyanate monomers relative to the total number of moles of isocyanate monomers incorporated into the prepolymer.

[0027] Aliphatic polyurethane = a polyurethane formed from aliphatic isocyanate. Those skilled in the art will appreciate that it is the nature of the isocyanate itself (aliphatic or aromatic) that determines whether the polyurethane is aliphatic or aromatic. For the purposes of the present invention, an aliphatic polyurethane prepolymer is derived from 95 mol% or more of aliphatic isocyanate monomers relative to the total number of moles of isocyanate monomers incorporated into the prepolymer.

[0028] Aromatic-aliphatic polyurethane = a polyurethane formed from aromatic and aliphatic isocyanate. For the purposes of the present invention, an aromatic-aliphatic polyurethane prepolymer is derived from greater than 5 mol% to less than 95 mol% of aromatic isocyanate monomers and greater than 5 mol% to less than 95 mol% of aliphatic isocyanate monomers relative to the total number of moles of isocyanate monomers incorporated into the prepolymer.

[0029] Aromatic carboxylic acid = a carboxylic acid comprising at least one acid (-COOH) group directly attached to an aromatic ring.

[0030] Aliphatic carboxylic acid = a carboxylic acid in which the carboxylic acid group (-COOH) is not directly attached to an aromatic ring. Aliphatic carboxylic acids include alicyclic carboxylic acids. Thus, aliphatic carboxylic acids can include aromatic rings, provided that the carboxylic acid group is not directly attached to an aromatic ring.

[0031] Free isocyanate monomer = monomeric isocyanate species that has not undergone reaction with a co-monomer (e.g. a polyol). Unreacted starting material from a polymerisation reaction, for example starting material used to form a polyurethane prepolymer.

[0032] Solvent-free = comprises 5 wt% or less of any solvent.

[0033] Retort pouch = a type of food packaging made from a laminate of flexible plastics

[0034] Retort conditions = a method of cooking and sterilising food in a retort package, typically involving heating at high temperature (e.g. 115°C to 125°C) for a period of time, for example 20 to 60 minutes.

[0035] Two-component adhesive = an adhesive composition comprising two components, where one component induces crosslinking of the other component.

[0036] Single component adhesive = adhesive composition comprising one component which typically undergoes crosslinking by reaction with residual moisture.

[0037] PET = polyethylene terephthalate

[0038] PE = polyethylene

[0039] ALU = aluminum

[0040] CPP = cast polypropylene

[0041] OPA = nylon

[0042] AlOx = aluminum oxide

[0043] SiOx = silicon oxide

[0044] OPP = oriented polypropylene

[0045] OPE = oriented polyethylene

[0046] LDPE = low density polyethylene

[0047] LLDPE = linear low density polyethylene

[0048] VM-PET = vacuum metallized polyethylene terephthalate

[0049] Methylene dicyclohexyl diisocyanate includes isomers: 1 -isocyanato-1 - [(1 -isocyanatocyclohexyl)methyl] cyclohexane and 4,4-diisocyanatodicyclohexylmethane.

[0050] Toluene diisocyanate includes its regioisomers.

[0051] Unless otherwise indicated, all ranges include the corresponding endpoints. For example, a range of 3 to 9 includes the endpoints 3 and 9. However, when the endpoints are defined as “greater than” one value and / or “less than” another, the range does not include the corresponding endpoint.

[0052] Unless otherwise indicated, wt% (w / w) means the mass of the component in question relative to the mass of all components present in the composition.

[0053] The wt% (w / w) of free isocyanate monomer is the mass of the substance relative to the total mass of the polyurethane prepolymer derived from the monomer.

[0054] The present invention

[0055] The present invention provides an improved two-component solventless laminating adhesive kit comprising an aliphatic polyurethane prepolymer. The adhesive kit of the present invention is suitable for the preparation of flexible packaging laminates useful in retort applications.

[0056] To the best of the inventors' knowledge, this is the first reported example of using a solvent-free aliphatic two-component lamination adhesive in the preparation of flexible multilayer laminates for retort applications, where the laminates withstand heat treatment temperatures of 100°C or higher.

[0057] The two-component solvent-free lamination adhesive kit of the present invention comprises an aliphatic isocyanate functional polyurethane prepolymer prepared from aliphatic isocyanate monomers such as diisocyanates. The amount of residual monomer in the polyurethane prepolymer used in the present invention is less than 0.1% (w / w) of the prepolymer. The amount of residual monomer can be reduced by using an evaporator (e.g., wiped film evaporator). The adhesive of the present invention is particularly suitable for the preparation of flexible packaging laminates for the manufacture of retortable, pasteurizable, and boilable pouches.

[0058] Thus, the kit of the present invention can be used to improve the bond strength of a multilayer laminate structure formed from a flexible film and a lamination adhesive formed from the components of the kit of the present invention under retort conditions. The improved bond strength involves an increase in the resistance to delamination of the laminate structure compared to a comparative laminate structure formed from the same flexible film and a lamination adhesive not formed from the components of the kit of the present invention.

[0059] Retort conditions include heating at a temperature of 100°C or higher for a period of 20 minutes, such as heating at 120°C for 30 minutes or heating at 135°C for 20 minutes. Retort conditions result in a pressure differential between the pressure inside the retort package and atmospheric pressure, which puts the adhesive used to seal the retort package under strain. For example, a low retort test can involve heating a retort pouch at about 120°C for at least 30 minutes in an autoclave, where the autoclave is under a pressure of about 1.5 bar. A high retort test can involve heating a retort pouch at about 135°C for at least 20 minutes in an autoclave, where the autoclave is under a pressure of about 2.6 bar.

[0060] Advantages associated with the present invention

[0061] The inventors have found that incorporating an aliphatic polyurethane prepolymer comprising monomeric units derived from aliphatic isocyanate monomers into a solvent-free lamination adhesive for the present invention can provide an adhesive with improved bonding under retort conditions. The improvement can be observed relative to a comparative adhesive comprising an aromatic polyurethane prepolymer comprising monomeric units derived from aromatic isocyanate monomers.

[0062] The adhesive kit of the present invention is also improved relative to a comparative adhesive kit comprising a polyurethane prepolymer comprising a residual isocyanate monomer content higher than required by the present invention.

[0063] The aliphatic two-component solventless laminating adhesive kit of the present invention is suitable for the preparation of flexible packaging laminates that are resistant to boiling conditions at > 100°C for a period of > 10 minutes (i.e. form seals that do not fail under said conditions). To date, two-component solventless laminating adhesives have been widely used for lower demanding applications, but not for boiling applications.

[0064] Furthermore, the result of using aliphatic isocyanate monomers in the preparation of the adhesives for the present invention is that the risk associated with the generation of harmful aromatic primary amines is eliminated. Such aromatic amines can be generated when adhesives comprising polyurethane prepolymers formed from aromatic diisocyanates are placed under boiling conditions. The use of aliphatic isocyanate based adhesives can reduce the risk of the formation of aromatic primary amines under boiling conditions.

[0065] Another advantage is that the amount of free diisocyanate monomer in the polyurethane prepolymer component of the two-component adhesive kit of the present invention is less than 0.1% (w / w) based on the weight of the polyurethane prepolymer. This is important in relation to hazards associated with prepolymers. In particular, reducing the free diisocyanate monomer content in the polyurethane prepolymer to less than 0.1% (w / w) can ensure that the hazards associated with monomeric isocyanates (e.g. diisocyanates such as hexamethylene diisocyanate) can be mitigated to such an extent that the prepolymer does not have any labelling related to hazards associated with free diisocyanate monomers in accordance with the current Classification, Labelling and Packaging (CLP) guidelines. This is advantageous as it allows for easier handling of the adhesive kit of the present invention compared to conventional two-component laminating adhesive kits / compositions which contain larger amounts of residual isocyanate. For example, the use of a wiped film evaporator to reduce the amount of free monomeric isocyanate (such as xylene diisocyanate) present in the adhesive has not been previously reported.

[0066] Furthermore, even with the use of aliphatic isocyanate monomers, which generate aliphatic primary amines when subjected to boiling conditions, reducing the amount of free isocyanate monomer to less than 0.1% (w / w) can still minimise the risk associated with the presence of migratable substances in the adhesive composition.

[0067] Solventless laminating adhesives do not contain organic solvents or water in order to reduce the viscosity to a level such that the material can be applied with a laminator at temperatures of < 90°C, for example < 80°C. However, due to the above advantages, the present inventors have found a method of producing solventless adhesives in which the viscosity required for use in a laminator at said temperatures can be achieved by controlling the molecular weight of the components and without the problems normally associated with the presence of migratable low molecular weight / small molecule components.

[0068] The solvent-free laminating adhesive kit of the present invention comprises 5 wt% or less of any solvent. Preferably, the solvent-free laminating adhesive kit comprises 3 wt% or less of any solvent, and more preferably 1 wt% or less of any solvent. Even more preferably, the solvent-free laminating adhesive kit of the present invention is essentially free of solvent.

[0069] Furthermore, the inventors have also found that the incorporation of a polyester polyol comprising monomeric units derived from a mixture of aromatic and aliphatic monomers into the polyurethane prepolymer for the present invention improves its mechanical properties.

[0070] When compared to similar one-component adhesive compositions, the two-component adhesive kit of the present invention can provide the adhesive with the additional advantages of stronger bonding, better chemical resistance and faster curing. One-component adhesives cure using only moisture present in / on the substrates and in the air. Typically, one-component adhesives work well with paper substrates laminated to paper or film. However, for example, for film-film lamination or film-aluminium lamination, the reaction and bond strength development of one-component adhesives can be very slow as there is not so much moisture available and / or it is difficult for the moisture to reach the isocyanate.

[0071] Furthermore, a catalyst (such as ethylmorpholine, DMDEE = 2,2-dimorpholinodiethylether or another tertiary amine containing compound or metal salt) is typically added to one-component adhesives in order to compensate for the slower curing speed by making the isocyanate groups more reactive / sensitive to moisture. Two-component adhesives do not require a catalyst, which means that the amount of migrant substances can be further reduced relative to comparative one-component adhesives. Instead, the isocyanate groups in the two-component adhesive react with the polyol which is intimately mixed into the polyurethane prepolymer and provide readily available reaction sites. However, the isocyanate groups of the polyurethane prepolymer of the two-component adhesive can also react with residual moisture.

[0072] Aliphatic polyurethane prepolymer

[0073] Polyurethanes with NCO end groups are typically obtained by reacting a polyfunctional alcohol with an excess of polyisocyanate monomer. Typically, diisocyanates are used in order to obtain a molecular weight and viscosity which is compatible with the use of the prepolymer in a laminator at temperatures of 80 to 90 °C. This is because diisocyanates (as opposed to higher functionality isocyanates) are only able to react with two polyol species, thus not forming crosslinks.

[0074] The polyurethane prepolymer for the present invention is characterized by having a viscosity of 800 mPas to 20,000 mPas, or preferably 1,000 mPas to 10,000 mPas, or more preferably 2,000 mPas to 7,000 mPas at 80 °C.

[0075] The content of reactive isocyanate groups (%NCO) of the polyurethane prepolymer used in the present application is preferably from 3% to 16%, or more preferably from 5% to 10%.

[0076] Those skilled in the art will appreciate that while the advantages associated with the present application arise from the use of aliphatic polyurethane precursors comprising monomeric units derived from aliphatic isocyanates, this does not preclude the presence of small amounts of non-aliphatic isocyanates in the prepolymer, so long as such amounts do not affect these advantageous properties. Thus, for the purposes of the present application, an "aliphatic polyurethane prepolymer" is derived from 95 mol% or more aliphatic isocyanate monomers relative to the total number of moles of isocyanate monomers incorporated into the prepolymer. Preferably, the aliphatic polyurethane prepolymer used in the present application is derived from greater than 98 mol%, more preferably greater than 99 mol% aliphatic isocyanate monomers relative to the total number of moles of isocyanate monomers incorporated into the prepolymer. Most preferably, the only isocyanate monomers incorporated into the aliphatic polyurethane prepolymer used in the present application are aliphatic isocyanate monomers.

[0077] An amount of residual diisocyanate monomers (due to the use of a stoichiometric excess) will still be present in the reaction mixture at the end of the reaction, regardless of the reaction time. The aliphatic polyurethane prepolymer used in the present application is isocyanate-functionalized, such that reactive NCO groups are attached to the prepolymer. The aliphatic polyurethane prepolymer used in the present application comprises at least two isocyanate groups per prepolymer. The aliphatic polyurethane prepolymer used in the present application is preferably difunctionalized with isocyanate groups, such as where the aliphatic polyurethane prepolymer used in the present application is linear and comprises isocyanate groups at either end.

[0078] The polyurethane prepolymer used in the present application can comprise three or more isocyanate groups per prepolymer, such as four or more, or five or more isocyanate groups per prepolymer.

[0079] The polyurethane prepolymer is preferably linear. For the purposes of the present application, a "linear" polyurethane prepolymer is generally derived from a difunctional isocyanate and a difunctional polyol, such that each isocyanate-derived moiety is covalently linked to at most two polyols, and each polyol-derived moiety is covalently linked to at most two isocyanates. Linear polyurethane prepolymers can comprise monomeric components that are branched in their own right, such as branched diols.

[0080] The aliphatic polyurethane prepolymer used in the present application is preferably derived from a polyester polyol, and more preferably from a polyester polyol comprising monomeric units derived from aromatic monomers, such as aromatic dicarboxylic acids.

[0081] It is preferred that at least 50 wt% of the polyol component used to make the polyurethane prepolymer for use in the present application is a polyester polyol, such as at least 60 wt%, at least 70 wt%, more preferably at least 80 wt% or at least 90 wt% is a polyester polyol. It is more preferred that all of the polyols used to make the polyurethane prepolymer for use in the present application are polyester polyols. The present inventors have found that polyester polyols can provide the polyurethane prepolymer for use in the present application with the desired high temperature mechanical properties.

[0082] The molar ratio of NCO groups of the aliphatic isocyanate monomer used to make the polyurethane prepolymer for use in the present application to HO groups of the polyol used to make the polyurethane prepolymer for use in the present application can be 5: 1 to 2: 1. Preferably, the molar ratio of NCO groups of the aliphatic isocyanate monomer used to make the polyurethane prepolymer for use in the present application to HO groups of the polyol used to make the polyurethane prepolymer for use in the present application is 4: 1 to 2: 1, such as 3: 1 to 2: 1.

[0083] The ratio of NCO:OH groups of the isocyanate and polyol used to make the prepolymer of the two-component adhesive kit for use in the present application can be lower than that required for one-component adhesives which rely on residual moisture content to cure. Without wishing to be bound by theory, when making polyurethane prepolymers for two-component adhesives, a lower NCO:OH ratio can be used; the lower ratio results in a polyurethane prepolymer of higher molecular weight and viscosity which can effectively function when combined with the polyol crosslinker. By contrast, when making polyurethane prepolymers for one-component adhesives, a higher NCO:OH ratio must be used; the excess NCO provides a polyurethane prepolymer of lower molecular weight and therefore of sufficiently low viscosity for one-component adhesive applications. The excess isocyanate monomer must then be removed from the one-component adhesive after use.

[0084] Reduction of free isocyanate monomer content

[0085] Even at room temperature, diisocyanates such as hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI) have a significant vapor pressure. This can cause serious health risks at the application stage, as these substances are toxic due to their sensitizing and irritating effects. Legislation requires users to take specific measures to protect personnel during use, involving significant additional equipment such as devices designed to keep the air breathable and / or to keep these substances below the maximum concentrations allowed in the workplace. As a result, users are required to install expensive protection devices for personnel exposed to such product vapors and / or aerosols generated by dynamic application conditions on rotating machines.

[0086] This type of agent is regulated by hazardous substances legislation and requires labelling with a hazardous substances label. The labelling obligation is accompanied by the requirement to employ special packaging and transport measures.

[0087] Many polyurethane adhesives in the art contain more than 0.1 % (w / w) of residual isocyanate monomers (typically volatile diisocyanates such as free TDI, MDI or other isocyanate monomers). In particular, when using a molar excess of isocyanate functionality relative to the hydroxyl functionality of the polyol, after formation of the polyurethane prepolymer, an excess of free isocyanate monomers will remain in the reaction mixture. Unless an additional processing step is employed to remove this excess of free monomers, more than 0.1 % (w / w) of free isocyanate monomers will remain in the prepolymer, necessitating the safety precautions described above. Furthermore, if a prepolymer containing more than 0.1 % (w / w) of free isocyanate monomers is used as an adhesive for food packaging, it can also lead to contamination of the food with migratable isocyanate monomers.

[0088] The solventless laminating adhesive kit of the present invention comprises an isocyanate functional aliphatic polyurethane component having an amount of free isocyanate monomers of less than or equal to 0.1 % (w / w) relative to the prepolymer. For example, the isocyanate functional polyurethane component preferably comprises less than 0.1 % (w / w) or 0.08 % (w / w) or less of free isocyanate monomers. The polyurethane prepolymer is subjected to an additional processing step outlined herein to reduce the amount of free isocyanate monomers to the amount required by the present invention.

[0089] The low amount of free monomer isocyanate monomers required by the present invention can be achieved by stripping the unreacted isocyanate monomers from the polyurethane prepolymer using evaporation (e.g. wiped film evaporator). The evaporator can be used at a temperature of 100 °C to 250 °C, preferably 120 °C to 200 °C, more preferably 140 °C to 180 °C. The evaporator can be operated at a pressure of less than 5 mbar, preferably less than 0.5 mbar, more preferably less than 0.1 mbar. The evaporator can be run for a total contact time of less than 30 minutes, preferably less than 15 minutes, more preferably less than 5 minutes. The evaporator can be run at a pressure of 0.1 mbar or less and a temperature of 140 °C to 180 °C.

[0090] Preparation of polyurethane prepolymers for use in the present invention

[0091] The polyurethane prepolymer for use in the present invention can be obtained by a process comprising the following scheme:

[0092] a) reacting an aliphatic diisocyanate with a polyester polyol and / or a polyether polyol, each having a number average molecular weight (Mn) of < 1000 g / mol, preferably < 800 g / mol, more preferably < 600 g / mol, and a polyol functionality of 2 to 6, preferably 2 to 4, more preferably 2 to 3, and n), the NCO:OH molar ratio is greater than 2.0:1.0;

[0093] i. optionally using a catalyst; and

[0094] ii. optionally using an acidic compound,

[0095] to obtain an NCO-terminated prepolymer; and

[0096] b) subjecting the NCO-terminated prepolymer obtained in step a) to one or more stripping stages using a series of one or more wiped film evaporators and / or short path evaporators.

[0097] Steps a) and b) are carried out according to known procedures and operating conditions.

[0098] The final polyurethane prepolymer thus obtained, after removal of free isocyanate monomers, has the following characteristics:

[0099] - NCO% > 7%

[0100] - viscosity at 50°C < 30,000 mPas

[0101] - wt% of free diisocyanate monomers < 0.1%

[0102] The process for preparing the polyurethane prepolymer for use in the present application preferably does not require solvents. In other words, the polyurethane prepolymer for use in the present application can be formed by using only the reagents which are themselves incorporated into the prepolymer.

[0103] Aliphatic isocyanate for the preparation of polyurethane prepolymers

[0104] The aliphatic isocyanate for preparing the isocyanate-functionalized aliphatic polyurethane prepolymer for use in the present application is selected from the group consisting of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), methanedicyclohexyl diisocyanate, xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), pentamethylene diisocyanate (PDI), and combinations of any of the foregoing.

[0105] Preferably, the aliphatic isocyanate monomer for use in the present application is a diisocyanate selected from the group consisting of isophorone diisocyanate (IPDI) and isomeric mixtures thereof, 1,6-hexane diisocyanate (HDI), xylylene diisocyanate (XDI), and combinations thereof. More preferably, the aliphatic isocyanate monomer for use in the present application is xylylene diisocyanate (XDI).

[0106] Aliphatic isocyanates for use in the present application comprise isocyanate groups that are not directly attached to an aromatic ring. Thus, aliphatic isocyanates for use in the present application can comprise aromatic groups, provided that the isocyanate groups are not directly attached.

[0107] Polyol for the preparation of polyurethane prepolymers for use in the present invention

[0108] Polyols for use in making isocyanate-functional aliphatic polyurethane prepolymers for use in the present application can be selected from the group consisting of polyester polyols, polyether polyols, and combinations thereof. Polyols for use in the present application are preferably polyester polyols, and more preferably polyester polyols comprising monomer units derived from aromatic monomers.

[0109] Polyester polyols for use in the present application have been selected to provide a desired combination of properties. In particular, the polyols have been selected so that when incorporated into polyurethane prepolymers for use in the present application, the polyurethane prepolymers have desirable mechanical properties at high temperature and a final viscosity suitable for use in a laminator at 80°C to 90°C.

[0110] The inventors have found that particularly preferred polyester polyols for use in the present application are characterized as follows:

[0111] 1. comprise a mixture of aromatic dicarboxylic acids and aliphatic dicarboxylic acids; and / or

[0112] 2. comprise a mixture of linear short chain diols and branched short chain diols; and / or

[0113] 3. have an OH value of 110 mgKOH / g to 400 mgKOH / g; and / or

[0114] 4. have a viscosity of 500 mPas to 10,000 mPas at 23°C.

[0115] While the polyester polyols for use in the present application can comprise other components, such as polyether segments, it is preferred that at least 50 mol% of the monomer units present are polyester monomer units, such as at least 60 mol%, at least 70 mol%, preferably at least 80 mol%, or more preferably at least 90 mol% of the monomer units present are polyester monomer units. The polyester polyols for use in the present application can comprise only polyester monomer units.

[0116] When used in formulating laminating adhesives of the present application, the inventors have found that polyurethane prepolymers made with polyester polyol backbones are advantageous because the resulting polyurethane prepolymers exhibit improved mechanical properties (e.g., tear strength) at the typical temperatures (100°C to 135°C) used in the retort process.

[0117] The polyester and / or polyether polyols used in the present application are preferably linear. In the context of the present application, a "linear" polyester polyol is derived from di-functional (and optionally also mono-functional) monomers, such that each monomer unit can react with up to two other monomers. In other words, the maximum functionality of the monomers is two. For example, dicarboxylic acids and dihydric alcohols produce linear polyesters.

[0118] Aromatic and aliphatic monomers

[0119] When used to formulate polyurethane prepolymers for use in the present application, it is preferred that the polyester polyol comprises at least 10 wt% of monomer units derived from aromatic monomers. More preferably, at least 20 wt% of the monomer units of the polyester polyol are derived from aromatic monomers. This is because, in general, polyurethane prepolymers derived from such polyols exhibit improved mechanical properties at the typical temperatures used in the retort process, particularly improved bond strength under retort conditions. Without wishing to be bound by theory, the present inventors hypothesize that these improvements are related to the higher hydrolysis resistance and higher glass transition temperature (T g ) of these polyester polyols.

[0120] Polyurethanes prepared with polyester polyol backbones are generally more viscous than polyurethanes prepared with polyether backbones; and polyester polyols containing aromatic rings in the backbone are even more viscous than purely aliphatic polyester polyols. The present inventors have found that by incorporating a mixture of aliphatic and aromatic groups into the polyester polyols used in the present application, polyurethane prepolymers having improved mechanical properties and viscosity suitable for use in adhesive lamination applications can be provided.

[0121] The polyester polyols used in the present application can be derived from 10 wt% to 55 wt% aromatic monomers, preferably 15 wt% to 45 wt%, more preferably 15 wt% to 35 wt%, such as 20 wt% to 30 wt% aromatic monomers, relative to the total wt% of the polyester polyol. The aromatic monomers are preferably aromatic dicarboxylic acids, such as isophthalic acid.

[0122] The polyester polyols used in the present application can be derived from 45 wt% to 90 wt% aliphatic monomers, preferably 55 wt% to 85 wt% aliphatic monomers, and more preferably 65 wt% to 85 wt% aliphatic monomers, relative to the total wt% of the polyester polyol. The aliphatic monomers are preferably aliphatic dicarboxylic acids, such as sebacic acid. The aliphatic monomers are also preferably aliphatic polyols, such as aliphatic diols and dihydric alcohols.

[0123] Polyols used to prepare the polyester polyols.

[0124] The polyol used to make the polyester polyol is preferably a diol and / or a diol. The diol and diol used to make the polyester polyol is preferably selected from the group consisting of neopentyl glycol, 1,6-hexanediol, 3-methyl, 1-5-pentanediol, monoethylene glycol, and combinations thereof.

[0125] The polyester polyol used in the present application can comprise 15% to 35% of monomer units derived from aromatic dicarboxylic acids and 65 wt% to 85 wt% of monomer units derived from aliphatic dicarboxylic acids and aliphatic diols and diols.

[0126] The polyester polyol used in the present application can be formed from 10 wt% to 55 wt% of aromatic monomers, preferably 15 wt% to 45 wt%, and more preferably 15 wt% to 35 wt%, such as 20 wt% to 30 wt% of aromatic dicarboxylic acids, relative to the total amount of monomers used to form the polyester polyol. The polyester polyol used in the present application can be formed from 45 wt% to 90 wt% of aliphatic dicarboxylic acids and aliphatic diols and diols, preferably 55 wt% to 85 wt% and more preferably 65 wt% to 85 wt% of aliphatic dicarboxylic acids and aliphatic diols and diols, relative to the total amount of monomers used to form the polyester polyol.

[0127] The aromatic dicarboxylic acid used to make the polyester polyol is preferably selected from the list consisting of terephthalic acid, isophthalic acid, phthalic anhydride, and combinations thereof. The aliphatic dicarboxylic acid used to make the polyester polyol is preferably a linear C2-C 14 dicarboxylic acid, such as a linear C2-C 14 dicarboxylic acid: succinic acid, glutaric acid, adipic acid, sebacic acid, azelaic acid, and combinations thereof.

[0128] The weight ratio of aromatic dicarboxylic acid: aliphatic dicarboxylic acid incorporated into the polyol can be 10: 1 to 1 : 10, preferably 4: 1 to 1 :4, more preferably 2: 1 to 1 :2. The weight ratio of aromatic dicarboxylic acid: aliphatic dicarboxylic acid incorporated into the polyol used in the present application can be 1.5: 1 to 1 : 1.5, such as incorporated in substantially equal wt%.

[0129] Short chain diols

[0130] Short chain diols suitable for making the polyester polyol used in the present application include, but are not limited to, linear C1-C6 diols, such as 1,2 ethylene glycol, 1,3 propanediol, 1,4 butanediol, 1,5 pentanediol, 1,6 hexanediol, and combinations thereof. Short chain diols comprising alkyl substituents are also suitable for making the polyester polyol used in the present application and are preferably selected from C1-C6 diols: 1,2 propanediol, 1,2 butanediol, 1,3 butanediol, neopentyl glycol, 1,3 methyl pentanediol, 2-methyl- 1,3-propanediol, and combinations thereof.

[0131] The diols used in the present application preferably have a molecular weight of less than 500 gmol -1 , such as less than 300 gmol -1 and preferably less than 200 gmol -1 .

[0132] The weight percentage of linear short chain diol: branched short chain diol incorporated into the polyol used in the present application can be 10: 1 to 1 : 10, preferably 4: 1 to 1 :4, and more preferably 2: 1 to 1 :2. The weight ratio of linear short chain diol dicarboxylic acid: branched short chain diol dicarboxylic acid incorporated into the polyol used in the present application can be 1.5: 1 to 1 : 1.5, such as incorporated in substantially equal wt%.

[0133] Polyurethane prepolymer additive

[0134] The polyurethane prepolymer used in the present application can be used "as is" (i.e. in the form provided by the above described process) or after the addition of additives such as adhesion promoters, viscosity and rheology modifiers, water scavengers, anti-skinning agents and anti-foaming agents.

[0135] In particular, when the adhesive system is to be used on a metal or metallised substrate such as aluminium, it is advantageous to use an adhesion promoter, for example a silane containing groups which react with isocyanate groups (such as 3-aminopropyltriethoxysilane). Such adhesion promoters can also be incorporated into the polyol crosslinker. However, such components are not essential to the adhesive kit of the present application. The present inventors have found that the adhesives of the present application perform well under retort conditions even in the absence of silane additives which react with isocyanates. Thus, the polyol functional crosslinker used in the present application can not contain silane compounds which are capable of reacting with isocyanates.

[0136] Optional further aliphatic polyisocyanate

[0137] The polyurethane prepolymer used in the present application can also be formulated by the addition of one or more further aliphatic polyisocyanates to further reduce the viscosity of the final system, i.e. the further aliphatic polyisocyanate can be incorporated into part a) of the kit of the present application. The further aliphatic polyisocyanate used in the present application can be low viscosity, i.e. the further aliphatic polyisocyanate used in the present application can have a viscosity of 3,000 mPas or less at 23 °C, preferably 2,000 mPas or less, such as 1,000 or less, or 500 or less.

[0138] The additional aliphatic polyisocyanate is preferably selected from the group consisting of aliphatic polyisocyanate trimers (i.e. isocyanurates), aliphatic polyisocyanate allophanates, aliphatic polyisocyanate oligomers, aliphatic polyisocyanate biurets, aliphatic polyisocyanate uretdiones, and combinations thereof.

[0139] Specific examples of these compounds include hexamethylene diisocyanate (HDI) trimers (e.g., POLURENE MT100, POLURENE MT100LV, POLURENE MT100LLV, Wannate HT100, and Wannatye HT 600 from Wanhua), hexamethylene diisocyanate (HDI) allophanates, hexamethylene diisocyanate biurets, uretdione / allophanate modified HDI oligomers, and HDI oligomers (e.g., Polurgreen MT 100 01, Polurgreen MT100LV 01, Polurgreen MT 100LLV 01, Tolonate HDT, Tolonate HDT LV, Tolonate HDTLV2, Tolonate XFLO 100, Basonat HI, Basonat HI-2000, Basonat HA3000, Desmodur ULTRA N 3300, Desmodur ULTRA N 3600, Desmodur ULTRA N 3900, Desmodur XP2860). Allophanates are isocyanate dimers.

[0140] More preferably, the additional aliphatic polyisocyanate is selected from the group consisting of aliphatic polyisocyanate trimers such as HDI trimers, aliphatic polyisocyanate allophanates such as HDI allophanates, and combinations thereof. Even more preferably, the additional aliphatic polyisocyanate is an aliphatic polyisocyanate trimer such as an HDI trimer.

[0141] The aliphatic polyurethane prepolymer for use in the present application can be incorporated into a composition comprising an additional polyisocyanate for use in the present application. The wt% ratio of the polyurethane prepolymer for use in the present application to the additional polyisocyanate for use in the present application can be 8: 1 to 1 : 1, more preferably 5: 1 to 2: 1, and even more preferably 3: 1 to 2: 1.

[0142] Catalyst

[0143] If desired, the urethane formation reaction can be accelerated by the addition of a suitable catalyst during the preparation stage. Catalysts suitable for urethanization reactions are known and include amines and organometallic compounds. However, such components are not essential to the adhesive kits of the present invention. The inventors have found that the adhesives of the present invention perform well under retort conditions even in the absence of a catalyst. Thus, the polyurethane prepolymer for use in the present invention (and kits of the present invention) can not contain a catalyst.

[0144] Examples of catalysts suitable for use in the present invention include triethylamine, tributylamine, dimethylbenzylamine, dicyclohexylmethylamine, dimethylcyclohexylamine, N,N,N',N-tetramethyl- dimethylamine, bis(dimethylaminopropyl)urea, N-methyl or N-ethyl morpholine, N,N'- dimorpholinodiethylether (DMDEE), N-cyclohexylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethylbutanediamine, N,N,N',N'-tetramethyl-1,6-hexanediamine, pentamethyldiethylene triamine, dimethylpiperazine, N-dimethylaminoethylpiperidine, 1,2- dimethylimidazole, N-hydroxypropylimidazole, 1 -azabicyclo-[2,2,0]-octane, 1,4- diazabicyclo-[2,2,2]octane (DABCO), alkanolamines such as triethanolamine, triisopropanolamine, N-methyl and N-ethyldiethanolamine, dimethylethanolamine, 2-(N,N'- dimethylaminoethoxy)ethanol, N,N',N-tris(dialkylaminoalkyl)-hexahydrothiazines such as N,N',N-tris(dimethylaminopropyl)-S-hexahydrothiazine, tetraalkylammonium hydroxides such as tetramethylammonium hydroxide, basic hydroxides such as sodium hydroxide, basic alkoxides such as sodium methoxide, basic salts of long chain fatty acids, iron (II) chloride, zinc chloride, lead octoate, tin salts such as tin dioctoate, tin diethylhexanoate, dibutyl tin dilaurate, dibutyl dilauryl tin mercaptide, organometallic compounds of titanium such as titanium (IV) butoxide, organometallic compounds of tin, lead, iron, titanium, bismuth and zirconium, tin oxides and sulfides and bismuth carboxylates.

[0145] The polyurethane prepolymer adhesive obtained as described above is characterized by a free monomer of less than 0.1 wt% (“free monomer”), does not require a hazardous label as it is completely safe for the user, and does not contain substances (primary aromatic amines and cyclic esters) that are prone to migrate from the packaging to food, even when the packaged product has a long shelf life.

[0146] Polyol crosslinker

[0147] The polyol crosslinking agent used in the kits of the present invention can be any polyol suitable for curing the isocyanate-functionalized aliphatic polyurethane prepolymer for use in the present invention.

[0148] The polyol crosslinker used in the kits of the present application can be selected from linear or branched polyether and / or linear or branched polyester polyols. The polyol crosslinker used in the present application can be a mixture of polyester polyols and polyether polyols. The polyol crosslinker used in the present application is preferably a polyester polyol.

[0149] The polyol crosslinker used in the present application can be a polyester polyol formed from a diol and a dicarboxylic acid. The diol can be selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, neopentyl glycol, tetramethylene glycol, polyethylene glycol (including the methyl ether thereof), polypropylene glycol (including the methyl ether thereof), and polybutylene glycol (including the methyl ether thereof). The dicarboxylic acid can be selected from the group consisting of the aliphatic dicarboxylic acids malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, methyl succinic acid, aspartic acid, malic acid; and the aromatic dicarboxylic acids terephthalic acid and isophthalic acid. The polyester polyol can also be derived from non-diolic polyols, including trimethylolpropane.

[0150] The polyol crosslinker used in the present application can be a polyester polyol comprising a tertiary amine group in the backbone.

[0151] The polyol crosslinker used in the present application, such as a polyester polyol, can have a number average molecular weight of 200 Da to 10,000 Da, preferably 400 Da to 5,000 Da, and more preferably 500 Da to 2000 Da.

[0152] The polyol crosslinker used in the present application can be incorporated into the crosslinking composition of part b) which also comprises a polyether monool (i.e., comprising a single hydroxyl substituent, such as a polypropylene glycol monoalkyl ether). The polyether monool is preferably selected from the list consisting of polyethylene glycol monoalkyl ether, polypropylene glycol monoalkyl ether, polybutylene glycol monoalkyl ether, and combinations thereof. For example, the polyol crosslinker used as part b) in the present application can be incorporated into a crosslinking composition which also comprises polypropylene glycol monomethyl ether.

[0153] The polyether monool can have a number average molecular weight of 200 Da to 2000 Da, such as 200 Da to 1000 Da or 200 Da to 500 Da.

[0154] When present, the polyether monool (e.g., polypropylene glycol monoalkyl ether) can be incorporated into the polyol crosslinker composition of part b) in an amount of up to 50 wt%, such as up to 40 wt%, relative to the total weight of both the polyol crosslinker and the polyether monool. In other words, the wt% ratio of the polyol crosslinker of part b) to the polyether monool (when present) can be 100:0 to 50:50, such as 80:20 to 50:50, or 70:30 to 60:40, such as about 60:40.

[0155] The kit of the present application comprises an aliphatic polyurethane prepolymer and optionally an additional polyisocyanate as part a), and a polyol crosslinker as part b). The wt% ratio of part a) to part b) can be from 5: 1 to 1 :2, and preferably from 4: 1 to 1 : 1, and more preferably from 3: 1 to 1 : 1.

[0156] Laminate

[0157] There are important markets associated with adhesively formed multi-layer laminates for retort applications. Typical structures include the following substrate combinations: PET / CPP, PET / Al / CPP, PET / Al / OPA / CPP, PET-SiOx / CPP, PET-AlOx / CPP, OPA / CPP, OPA / LDPE, PET / Al / LDPE, VM-PET, AlOx-PET, SiOx-PET, AlOx-OPP, SiOx-OPP, AlOx-OPE, SiOx-OPE, AlOx-OPA, SiOx-OPA, PET / OPA / Al / CPP, OPA / VM-PET / LLDPE, PET / OPA / CPP, PET / OPA-SiOx / CPP, PET / OPA-AlOx / CPP, PET-SiOx / OPA / CPP, PET-AlOx / OPA / CPP, OPP / CPP, OPP-SiOx / OPA / CPP, OPP-AlOx / OPA / CPP, OPE-SiOx / OPA / CPP, OPE-AlOx / OPA / CPP, OPA-SiOx / OPA / CPP, OPA-AlOx / OPA / CPP.

[0158] The adhesive kit of the present application can be used with any of the above substrates to form a laminate.

[0159] Cooking conditions

[0160] The laminating adhesive kit of the present application can provide a laminate having improved bond strength under retort conditions relative to comparative laminates. Retort conditions include subjecting a sealed retort package to a temperature for a period of time. For example, retort conditions can involve heating the package at 115°C to 150°C for 10 to 60 minutes, such as 120°C to 140°C for 20 to 35 minutes. Retort conditions can involve a pressure of 1.5 bar to 4 bar, such as 1.5 bar to 3 bar. Retort conditions can involve heating at 120°C and 1.5 bar for 30 minutes, or at 135°C and 2.6 bar for 20 minutes.

[0161] Examples

[0162] The present invention is defined by the following non-limiting examples, which further illustrate the present invention and are not intended to, nor should they be interpreted to, limit the scope of the present invention.

[0163] Test methods

[0164] - Free diisocyanate monomer content (eg % HDI, % XDI): determined by gas chromatography with internal standard according to ASTM D3432. Expressed in wt% relative to the total amount of polyurethane prepolymer.

[0165] - Reactive NCO group content (% NCO): determined by back titration with an excess of n-butylamine acid according to ASTM D2572.

[0166] - Viscosity : determined using a Brookfield rotational viscometer Mod. LVDV II according to ASTM D1084 at the specified temperature. Viscosity is measured at 23 °C unless otherwise stated.

[0167] - Hydroxyl value (OH value) : milligrams of potassium hydroxide required to neutralize one gram of a chemical substance containing free hydroxyl groups when acetylated. The hydroxyl value is determined using the standard procedure defined in ISO 4629-1 :2016(E).

[0168] - Molecular weight:

[0169] a) The molecular weight of non-polymeric or oligomeric compounds (i.e. defined monomeric substances) is defined and calculated by the molecular structure of the compound. Typically, this is provided by the supplier technical data sheet of the monomer, or can be found on the web page of the European Chemicals Agency (ECHA).

[0170] b) Oligomeric and polymeric substances typically include a chain length distribution, and thus a molecular weight distribution. Therefore, unless otherwise stated, the molecular weight of oligomeric and polymeric substances (and components that exist as a mixture of substances with individual molecular weights above 500 Da (and thus have a distribution - e.g. vegetable oils)) is measured by gel permeation chromatography (GPC) equipped with two GPC Ultrastyragel columns 103 and a refractive index detector 104, according to ISO 16293-2:2012(E). were performed on a Hewlett-Packard 1050 series HPLC system with a mixed-mode column (5 pm mixed-mode, 300 mm x 19 mm, Waters Millipore Corporation, Milford, MA, USA) and THF as mobile phase. The column temperature was 40 °C. The molecular weight was calculated by comparison with polystyrene standards. The skilled person will understand that this definition of molecular weight is applicable to polymeric materials which typically have a molecular weight distribution. Unless otherwise stated, the molecular weights reported herein for oligomers and polymers are number average molecular weights.

[0171] - Bond strength is the strength required to separate a multi-layered structure formed by a flexible film and by a lamination adhesive formed from components of the adhesive kit. The multi-layered structure comprises at least two layers which are joined together with a lamination adhesive. The bond strength is measured at room temperature (25 °C) and at 100 mm / min. All other parameters comply with ASTM D3330-F (90° peel test).

[0172] The improved bond strength is related to an increased resistance to delamination of a laminated structure composed of a flexible film and of a lamination adhesive formed from components of the inventive kit compared to a comparative laminated structure formed from the same flexible film but using a lamination adhesive not formed from components of the inventive kit.

[0173] Example 1 : Synthesis of a polyester polyol

[0174] The polyester polyol of Example 1 was prepared by mixing the following diols and dicarboxylic acids and reacting them in a typical polycondensation using an esterification reactor and well-known esterification conditions.

[0175] Table 1 : Reagents used for the preparation of the polyester polyol of Example 1.

[0176] Components % range Isophthalic acid 20-30 Sebacic acid 20-30 Monoethylene glycol 10-20 Neopentyl glycol 5-15 1,6-hexanediol 5-15 3-methyl 1-5 pentanediol 5-15

[0177] Example 1 is a typical polyester polyol obtained from the reagents incorporated in the amounts indicated above and has the following characteristics:

[0178] OH number = 265 mg KOH / g

[0179] Viscosity at 23 °C = 3,000 mPas

[0180] Example 2: Synthesis of an NCO-terminated polyurethane prepolymer

[0181] To a 1 -liter reaction flask equipped with a stirrer and reflux condenser, 550 parts of XDI were added under continuous nitrogen flow. The mixture was heated at 50 °C and 450 parts of the polyester polyol A of Example 1 (having 2 functional groups and a number average Mwof 1,000 g / mol) were added under stirring over 240 minutes.w OH-terminated polyester polyol) was added dropwise to the mixture, the temperature of the reaction mixture was monitored and the polyol addition rate was adjusted to ensure that it did not exceed 60°C at all times.

[0182] At the end of the addition, the mixture was heated to 80°C for 2 hours until the NCO% was about 15.5%.

[0183] The product thus obtained was distilled in a thin layer evaporator at a pressure of about 0.1 mbar and a temperature of 140°C to remove unreacted monomers. 760 parts of a transparent colourless liquid were obtained having the following characteristics:

[0184] NCO% = 7.6%

[0185] Viscosity at 50°C = 18,700 mPas

[0186] XDI% = 0.04 wt%

[0187] Example 3: Preparation of a solvent-free aliphatic isocyanate prepolymer with residual monomer level below 0.1%.

[0188] 700 parts of Example 2 were mixed with 300 parts of a low viscosity HDI trimer (Polurgreen MT100 LV 01) with residual HDI content <0.1% to obtain 1000 parts of a transparent colourless liquid having the following characteristics:

[0189] NCO% = 12.3%

[0190] Viscosity at 40°C = 11,000 mPas

[0191] XDI% = 0.04%

[0192] HDI% = 0.04%

[0193] Example 3 is a solvent-free aliphatic isocyanate prepolymer with residual monomer level below 0.1 wt% prepared according to the method described previously. Its performance has been compared with a solvent-free aromatic adhesive for the evaluation on a PET (12 pm) / aluminium (8 pm) / cast polypropylene (CPP) (60 pm) three-layer structure. Laminates using the adhesives described in Table 2 below were produced on a Labo Combi400 laminator manufactured by Nordmeccanica Group using the following conditions:

[0194] Coating weight: 2.5 gsm / dry;

[0195] Adhesive roll speed: 80;

[0196] Adhesive roll temperature: 50°C;

[0197] Coating roll temperature: 50°C

[0198] Tension unwinder A: 23 N;

[0199] Tension unwinder B: 20 N;

[0200] Tension rewinder (laminate): 28 N;

[0201] Coating head pressure: 3 bar;

[0202] Nip pressure: 3 bar;

[0203] Nip temperature 50°C;

[0204] Laminate cured at 20°C for 20 days.

[0205] Table 2: Comparative adhesive systems and inventive adhesive systems and mixing ratios

[0206]

[0207] Sunlam NS-4158A (polyurethane polyisocyanate) / HA-328 (polyester polyol) is a two-component, solvent-free, partially aromatic laminate adhesive that can be processed at 50°C. The polyurethane prepolymer (NS-4158A) is aromatic, i.e., the polyurethane contains monomeric units derived from aromatic isocyanates.

[0208] Sunlam ZA-1000 (polyurethane polyisocyanate) / ZB-301 (polyester polyol based on a combination of propylene glycol, trimethylolpropane, and adipic acid, containing a small amount of a tetrol with a tertiary amine on the backbone) is a commercial two-component, aromatic, solvent-free, ultra-low isocyanate monomer (<0.1 wt%) laminate adhesive that can be processed at 50°C to 55°C. The polyurethane (ZA-1000) is aromatic, i.e., the polyurethane contains monomeric units derived from aromatic isocyanates.

[0209] MP40 is a 60 / 40 mixture of DIC Dry HA930 (polyester polyol, DIC Graphics Corporation) and Smack MP-40 (Kao Corporation). Smack MP-40 is a polypropylene glycol monomethyl ether with a molecular weight of about 260 Da.

[0210] MP70 is a 60 / 40 mixture of DIC Dry HA930 (polyester polyol, DIC Graphics Corporation) and Smack MP-70 (Kao Corporation). Smack MP-70 is a polypropylene glycol monomethyl ether with a molecular weight of about 450 Da.

[0211] Sunlam HA450B is a commercial solvent-free polyester polyol based on monoethylene glycol, neopentyl glycol, trimethylolpropane in combination with isophthalic and adipic acid, typically used in combination with aromatic isocyanate functionalized prepolymers.

[0212] ZB301 is a solvent-free polyester polyol.

[0213] The bond strength between aluminum and CPP, measured at room temperature, expressed in N / 15mm, after various heat treatments, and measured at 100 mm / min, is reported in the following table:

[0214] Table 3: Bond strength of selected adhesives before and after pasteurization and retort

[0215]

[0216]

[0217] Alu-F = Aluminum failure / Ad-Alu = Adhesive failure.

[0218] Comparative examples (samples 1 and 2) prepared using aromatic-based polyisocyanate prepolymers with standard or ultra-low levels of free monomer did not pass the high retort test and almost did not pass the low retort test. The laminates that survived the low retort process by maintaining their appearance and not showing delamination, ended up having almost no bond strength and therefore were not suitable for retort applications.

[0219] The four samples based on example 3 of the present invention (i.e. samples 3, 4, 5 and 6) were cured with various polyol crosslinking agents and survived the low and high retort processes, maintaining their appearance and integrity while showing sufficient bond strength.

[0220] Having described the application in detail, including various embodiments thereof, it will be apparent to those skilled in the art that modifications and / or improvements can be made to the application which fall within the scope and spirit of the application.

[0221] Numbered embodiments of the present invention

[0222] The present application is defined by the following numbered embodiments, which form part of the specification.

[0223] 1. A solvent-free laminating adhesive kit comprising:

[0224] a) an isocyanate functionalized aliphatic polyurethane prepolymer comprising monomeric units derived from aliphatic isocyanate monomers; and

[0225] b) a polyol crosslinking agent,

[0226] wherein the amount of free isocyanate monomer present in the polyurethane prepolymer is less than or equal to 0.1% (w / w) of the prepolymer.

[0227] 2. The kit of embodiment 1, wherein the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, methylene dicyclohexyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, pentamethylene diisocyanate, and combinations thereof; optionally wherein the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of isophorone diisocyanate, 1,6-hexane diisocyanate, xylylene diisocyanate, and combinations thereof.

[0228] 3. The kit of any preceding embodiment, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is difunctionalized with isocyanate groups, optionally wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is linear and functionalized with isocyanate groups at each end.

[0229] 4. The kit of any preceding embodiment, wherein

[0230] i) the isocyanate-functionalized aliphatic polyurethane prepolymer has a viscosity at 80 °C between 800 mPa and 20,000 mPas, such as between 1,000 mPas and 10,000 mPas, or between 2,000 mPas and 7,000 mPas; wherein the viscosity is measured according to the method described in the specification; and / or

[0231] ii) the isocyanate-functionalized aliphatic polyurethane prepolymer has an isocyanate group content (%NCO) between 3% and 16%, such as between 5% and 10%; and / or

[0232] iii) the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from greater than 80 mol% aliphatic isocyanate relative to the total moles of isocyanate incorporated into the prepolymer, such as greater than 90 mol% or greater than 95 mol% aliphatic isocyanate relative to the total moles of isocyanate incorporated into the prepolymer; and / or

[0233] d) wherein the only isocyanate incorporated into the isocyanate-functionalized aliphatic polyurethane prepolymer is an aliphatic isocyanate.

[0234] 5. The kit of any preceding embodiment, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from a polyester polyol and / or a polyether polyol; optionally

[0235] wherein the isocyanate-functional aliphatic polyurethane prepolymer is derived from a polyester polyol comprising monomeric units derived from aromatic monomers.

[0236] 6. The kit of embodiment 5, wherein

[0237] i) the isocyanate-functional aliphatic polyurethane prepolymer is derived from at least 50 wt% of a polyester polyol relative to the total amount of polyols incorporated into the prepolymer; and / or

[0238] ii) the polyester polyol comprises monomeric units derived from a mixture of aromatic dicarboxylic acids and aliphatic dicarboxylic acids; and / or

[0239] iii) the polyester polyol comprises between 10 wt% and 55 wt% of monomeric units derived from aromatic monomers, such as between 15 wt% and 45 wt%, or between 15 wt% and 35 wt% of monomeric units derived from aromatic monomers.

[0240] 7. The kit of embodiment 5 or 6, wherein

[0241] i) the polyester polyol comprises between 45 wt% and 90 wt% of monomeric units derived from aliphatic monomers, such as between 55 wt% and 85 wt%, or between 65 wt% and 85 wt% of monomeric units derived from aliphatic monomers; and / or wherein the polyester polyol comprises monomeric units derived from a mixture of linear diols and branched diols; optionally wherein the linear diols and the branched diols have a molecular weight of less than 500 gmol -1 , such as less than 200 gmol -1 ; and / or

[0242] ii) the polyester polyol and / or polyether polyol each have a number average molecular weight of less than 1000 g / mol; and / or

[0243] iii) the polyester polyol and / or polyether polyol is linear.

[0244] 8. The kit of any preceding embodiment, wherein

[0245] i) the polyol crosslinker does not comprise a silane compound capable of reacting with isocyanate, such as an aminosilane; and / or

[0246] ii) the polyurethane prepolymer is incorporated into a composition further comprising an additional polyisocyanate; optionally

[0247] wherein the additional polyisocyanate is selected from the group consisting of hexamethylene diisocyanate trimer, hexamethylene diisocyanate allophanate, or a combination thereof.

[0248] 9. A laminating adhesive comprising the components of the solventless laminating adhesive kit of any preceding embodiment mixed together in a single composition.

[0249] 10. A retort pouch comprising the laminating adhesive of embodiment 9.

[0250] 11. A method of providing a multi-layered laminate structure comprising

[0251] i) forming a laminating adhesive from the components of the kit of any one of embodiments 1 to 8 or providing the laminating adhesive of embodiment 9; and

[0252] ii) applying the laminating adhesive to a flexible film.

[0253] 12. The method of embodiment 11, wherein

[0254] i) the flexible film is selected from the group consisting of polyethylene terephthalate, nylon, aluminum, oriented polypropylene, cast polypropylene, low density polyethylene, linear low density polyethylene, vacuum modified polyethylene terephthalate, aluminum oxide-polyethylene terephthalate, silicon oxide-polyethylene terephthalate, aluminum oxide-oriented polypropylene, silicon oxide-coated oriented polypropylene, aluminum oxide-oriented polyethylene, silicon oxide-oriented polyethylene, aluminum oxide-nylon, and silicon oxide-nylon, and any coated films thereof; and / or

[0255] ii) the method further comprises applying the adhesive at a press speed of more than 50 m / min or more than 100 m / min.

[0256] 13. A method of making the kit of any preceding embodiment comprising the steps of

[0257] a) reacting an aliphatic isocyanate with a polyester and / or polyether polyol to obtain an isocyanate-functionalized aliphatic polyurethane prepolymer comprising monomeric units derived from aliphatic isocyanate monomers;

[0258] b) reducing residual isocyanate monomers present in the prepolymer to less than 0.1% (w / w) of the prepolymer;

[0259] c) introducing a polyol crosslinker into the kit.

[0260] 14. The method of embodiment 13, wherein

[0261] i) the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, methylene dicyclohexyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, pentamethylene diisocyanate, and combinations thereof; and / or

[0262] ii) the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of isophorone diisocyanate, 1,6-hexane diisocyanate, xylylene diisocyanate, and combinations thereof; and / or

[0263] iii) the amount of isocyanate monomer is reduced by stripping the prepolymer using a series of one or more thin-film evaporators and / or molecular evaporators.

[0264] 15. The method of either of embodiments 13 or 14, wherein

[0265] i) the isocyanate-functionalized aliphatic polyurethane prepolymer is the isocyanate- functionalized aliphatic polyurethane prepolymer of any one of embodiments 3 to 6; and / or

[0266] ii) step a) comprises reacting the aliphatic isocyanate with a polyester polyol; optionally wherein the polyester polyol is the polyester polyol of either of embodiments 6 or 7; and / or

[0267] iii) prior to step a), the method further comprises a step of reacting a mixture of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid with at least one polyol to form the polyester polymer; optionally

[0268] wherein the at least one polyol is a mixture of a linear diol and a branched diol, optionally wherein the linear diol and the branched diol have a molecular weight of less than 500 gmol -1 , such as less than 200 gmol -1 .

[0269] 16. The method of any one of embodiments 13 to 15, which does not use a solvent.

[0270] 17. Use of the kit according to any one preceding embodiment to form a laminating adhesive; optionally wherein the laminating adhesive forms part of a retort package, such as a pouch.

[0271] 18. Use of a kit according to any preceding embodiment for improving the bond strength under retort conditions of a multi-layered laminate structure formed from a flexible film and a laminate adhesive formed from components of the kit; optionally wherein the retort conditions comprise heating at 100 °C or higher for a period of 20 minutes, such as heating at 120 °C for 30 minutes or heating at 135 °C for 20 minutes.

Claims

1. A solventless laminating adhesive kit comprising: a) a composition comprising: i) an isocyanate-functionalized aliphatic polyurethane prepolymer comprising monomeric units derived from aliphatic isocyanate monomers, wherein the aliphatic polyurethane prepolymer is formed from aliphatic isocyanate monomers, wherein only the nature of the isocyanate determines whether the polyurethane is aliphatic or aromatic, and wherein the aliphatic isocyanate monomers are monomers in which the isocyanate group is not directly attached to an aromatic ring; wherein the aliphatic isocyanate monomers are diisocyanates selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, methylene dicyclohexyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, pentamethylene diisocyanate, and combinations thereof; wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from a polyester polyol and / or a polyether polyol; wherein the molar ratio of NCO groups of the aliphatic isocyanate monomers to HO groups of the polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived is from 5:1 to 2:1; and ii) an additional aliphatic polyisocyanate selected from the group consisting of aliphatic polyisocyanate trimers, aliphatic polyisocyanate allophanates, aliphatic polyisocyanate biurets, aliphatic polyisocyanate oligomers, aliphatic polyisocyanate uretdiones, and combinations thereof; and b) a polyol crosslinker, wherein the polyol crosslinker is a linear or branched polyether polyol and / or a linear or branched polyester polyol; wherein the amount of free isocyanate monomers present in the polyurethane prepolymer is less than or equal to 0.1% w / w of the prepolymer; wherein the amount of free isocyanate monomers is determined according to ASTM D3432 by gas chromatography with internal standard in wt% relative to the total amount of polyurethane prepolymer; and wherein the polyol crosslinker does not comprise a silane additive capable of reacting with isocyanate.

2. The kit of claim 1, wherein the aliphatic isocyanate monomers are diisocyanates selected from the group consisting of isophorone diisocyanate, 1,6-hexane diisocyanate, xylylene diisocyanate, and combinations thereof.

3. The kit of claim 2, wherein the aliphatic isocyanate monomers are xylylene diisocyanate.

4. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is difunctionalized with isocyanate groups, optionally wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is linear and functionalized with isocyanate groups at each end.

5. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer has a viscosity at 80°C of 800 mPa to 20,000 mPas; wherein the viscosity is measured according to ASTM D1084 using a Brookfield rotational viscometer Mod. LV DVII at 23°C.

6. The kit of claim 5, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer has a viscosity at 80 °C of 1,000 mPas to 10,000 mPas.

7. The kit of claim 5, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer has a viscosity at 80 °C of 2,000 mPas to 7,000 mPas.

8. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer has an isocyanate group content (%NCO) of 3% to 16%.

9. The kit of claim 8, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer has an isocyanate group content (%NCO) of 5% to 10%.

10. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from equal to or greater than 95 mol% aliphatic isocyanate monomers relative to the total isocyanate monomers incorporated into the prepolymer.

11. The kit of claim 1, wherein the only isocyanate monomers incorporated into the isocyanate-functionalized aliphatic polyurethane prepolymer are aliphatic isocyanate monomers.

12. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from a polyester polyol comprising monomeric units derived from aromatic monomers.

13. The kit of claim 1, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is derived from at least 50 wt% polyester polyol relative to the total amount of polyol incorporated into the prepolymer.

14. The kit of claim 1, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived comprises monomeric units derived from aromatic dicarboxylic acids and monomeric units derived from aliphatic dicarboxylic acids.

15. The kit of claim 1, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 10 wt% to 55 wt% aromatic dicarboxylic acids.

16. The kit of claim 15, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 15 wt% to 45 wt% aromatic dicarboxylic acids.

17. The kit of claim 15, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 15 wt% to 35 wt% aromatic dicarboxylic acids.

18. The kit of claim 1, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 10 wt% to 55 wt% aliphatic dicarboxylic acids.

19. The kit of claim 18, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 15 wt% to 45 wt% aliphatic dicarboxylic acids.

20. The kit of claim 18, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived is itself derived from 15 wt% to 35 wt% of aliphatic dicarboxylic acid.

21. The kit of claim 1, wherein the polyester polyol from which the isocyanate- functionalized aliphatic polyurethane prepolymer is derived comprises monomeric units derived from a mixture of linear diols and branched diols; optionally wherein the linear diols and the branched diols have a molecular weight of less than 500 gmol -1 .

22. The kit of claim 21, wherein the linear diol and the branched diol have a molecular weight of less than 200 gmol -1 .

23. The kit of claim 1, wherein the polyester polyol and / or polyether polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived has a number average molecular weight of less than or equal to 1000 g / mol.

24. The kit of claim 1, wherein the polyester polyol and / or polyether polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived is linear.

25. The kit of claim 1, wherein the molar ratio of NCO groups of the aliphatic isocyanate monomer to HO groups of the polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived is 4: 1 to 2:

1.

26. The kit of claim 25, wherein the molar ratio of NCO groups of the aliphatic isocyanate monomer to HO groups of the polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived is 3: 1 to 2:

1.

27. The kit of claim 1, wherein i) the polyol crosslinker is a linear or branched polyester polyol, or ii) the polyol crosslinker is a mixture of at least one linear or branched polyester polyol and at least one linear or branched polyether polyol.

28. The kit of claim 1, wherein the polyol crosslinker is a polyester polyol formed from one or more diols and one or more dicarboxylic acids.

29. The kit of claim 28, wherein the one or more diols are selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, neopentyl glycol, tetramethylene glycol, polyethylene glycol, polypropylene glycol, polybutylene glycol, methyl ether derivatives of any of the aforementioned diols, and combinations thereof.

30. The kit of claim 28, wherein the dicarboxylic acid is selected from the group consisting of aliphatic dicarboxylic acids: malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, methyl succinic acid, aspartic acid, malic acid; and aromatic dicarboxylic acids: terephthalic acid, isophthalic acid, and combinations thereof.

31. The kit of claim 28, wherein the polyester polyol of the polyol crosslinker is derived from a non-diolic polyol.

32. The kit of claim 31, wherein the polyester polyol of the polyol crosslinker is derived from trimethylolpropane.

33. The kit of claim 1, wherein the polyol crosslinker of the polyol crosslinker is a polyester polyol comprising a tertiary amine group in the backbone.

34. The kit of claim 1, wherein the further aliphatic polyisocyanate has a viscosity of 3,000 mPas or less at 23 °C.

35. The kit of claim 34, wherein the further aliphatic polyisocyanate has a viscosity of 2,000 mPas or less at 23 °C.

36. The kit of claim 34, wherein the additional aliphatic polyisocyanate has a viscosity of 1,000 mPas or less at 23 °C.

37. The kit of claim 34, wherein the additional aliphatic polyisocyanate has a viscosity of 500 mPas or less at 23 °C.

38. The kit of claim 1, wherein the additional aliphatic polyisocyanate is selected from the group consisting of hexamethylene diisocyanate trimer, hexamethylene diisocyanate allophanate, hexamethylene diisocyanate biuret, and combinations thereof.

39. The kit of claim 1, wherein the wt% ratio of the polyurethane prepolymer to the additional polyisocyanate in a composition comprising the additional aliphatic polyisocyanate is from 8:1 to 1:

1.

40. The kit of claim 39, wherein the wt% ratio of the polyurethane prepolymer to the additional polyisocyanate in a composition comprising the additional aliphatic polyisocyanate is from 5:1 to 2:

1.

41. The kit of claim 39, wherein the wt% ratio of the polyurethane prepolymer to the additional polyisocyanate in a composition comprising the additional aliphatic polyisocyanate is from 3:1 to 2:

1.

42. A laminating adhesive comprising the components of the solventless laminating adhesive kit of any one of claims 1-41 mixed together in a single composition.

43. A retort pouch comprising the laminating adhesive of claim 42.

44. A method of providing a multi-layered laminate structure comprising a) forming a laminating adhesive from the components of the kit of any one of claims 1 to 41 or providing the laminating adhesive of claim 42; and b) applying the laminating adhesive to a flexible film.

45. The method of claim 44, wherein the flexible film is selected from the group consisting of polyethylene terephthalate, nylon, aluminum, oriented polypropylene, cast polypropylene, low density polyethylene, linear low density polyethylene, vacuum modified polyethylene terephthalate, aluminum oxide-polyethylene terephthalate, silicon oxide-polyethylene terephthalate, aluminum oxide-oriented polypropylene, silicon oxide-coated oriented polypropylene, aluminum oxide-oriented polyethylene, silicon oxide-oriented polyethylene, aluminum oxide-nylon, and silicon oxide-nylon, and any coated films thereof.

46. The method of claim 44, comprising applying the adhesive at a press speed of more than 50 m / min.

47. The method of claim 46, comprising applying the adhesive at a press speed of more than 100 m / min.

48. A laminating adhesive structure resulting from the method of any one of claims 44 to 47.

49. The structure of claim 48, which is suitable for use in making a retort pouch, wherein delamination failure does not occur at retort conditions of 100 °C or higher.

50. A method of making the kit of any one of claims 1-41, comprising the steps of: a) reacting an aliphatic isocyanate monomer with a polyester polyol and / or a polyether polyol to obtain an isocyanate-functionalized aliphatic polyurethane prepolymer comprising monomer units derived from the aliphatic isocyanate monomer; wherein aliphatic polyurethane prepolymer is formed from aliphatic isocyanate, wherein only the nature of the isocyanate determines whether the polyurethane is aliphatic or aromatic, wherein aliphatic isocyanate is an aliphatic isocyanate in which the isocyanate group is not directly attached to an aromatic ring; wherein the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, methylene dicyclohexyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, pentamethylene diisocyanate, and combinations thereof; wherein the molar ratio of NCO groups of the aliphatic isocyanate monomer to HO groups of the polyol from which the isocyanate-functionalized aliphatic polyurethane prepolymer is derived is from 5:1 to 2:1; b) reducing the residual isocyanate monomer present in the aliphatic polyurethane prepolymer to less than 0.1% w / w of the prepolymer; c) introducing an additional aliphatic polyisocyanate selected from the group consisting of aliphatic polyisocyanate trimer, aliphatic polyisocyanate allophanate, aliphatic polyisocyanate biuret, aliphatic polyisocyanate oligomer, aliphatic polyisocyanate uretdione, and combinations thereof; and d) introducing a polyol crosslinker to the kit; wherein the polyol crosslinker is a linear or branched polyether polyol and / or a linear or branched polyester polyol; wherein the polyol crosslinker does not comprise a silane additive capable of reacting with isocyanate.

51. The method of claim 50, wherein the aliphatic isocyanate monomer is a diisocyanate selected from the group consisting of isophorone diisocyanate, 1,6-hexane diisocyanate, xylylene diisocyanate, and combinations thereof.

52. The method of claim 50, wherein the amount of isocyanate monomer is reduced by stripping the prepolymer using a series of one or more wiped film evaporators and / or short path evaporators.

53. The method of claim 50, wherein the isocyanate-functionalized aliphatic polyurethane prepolymer is the isocyanate-functionalized aliphatic polyurethane prepolymer of any one of claims 4 to 13.

54. The method of claim 50, wherein step a) comprises reacting the aliphatic isocyanate with a polyester polyol.

55. The method of claim 54, wherein the polyester polyol is the polyester polyol of any one of claims 14 to 24.

56. The method of claim 50, wherein prior to step a), the method further comprises a step of reacting a mixture of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid with at least one polyol to form the polyester polyol.

57. The method of claim 56, wherein the at least one polyol is selected from the group consisting of neopentyl glycol, 1,6-hexanediol, 3-methyl, 1-5 pentanediol, monoethylene glycol, and combinations thereof.

58. The method of claim 50, wherein the kit is free of solvent.

59. Use of the kit of any one of claims 1-41 to form a lamination adhesive.

60. The use of claim 59, wherein the lamination adhesive forms a portion of a retort package.

61. The use of claim 60, wherein the retort package is a pouch.

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