Low viscosity aspartate-terminated prepolymers for adhesive applications

Abstract

In some embodiments, an aspartate-terminated prepolymer may be a reaction product of a reaction mixture comprising a prepolymer having an NCO content of 0.5% to 15% and a lt; an aliphatic NCO-terminated prepolymer having a number average NCO functionality of 1, 2, and a di-or multi-aspartic ester resulting from the reaction of a primary diamine or polyamine with a Michael addition acceptor. The aliphatic NCO-terminated prepolymer and the di-or multi-aspartic ester may be combined in an NCO: NH equivalence ratio of from 1: 1.5 to 1: 5. The adhesive comprises the reaction product of an aliphatic polyisocyanate and an aspartate-terminated prepolymer combined in an NCO: NH equivalence ratio of from 0.9: 1 to 1.75: 1.

Description

Field of the Invention

[0001] The present invention generally relates to low viscosity aspartate-terminated prepolymers. Background of the Invention

[0003] In conventional multilayer flexible packaging, two-component (2K) polyurethane adhesive compositions are often used to laminate the layers. These compositions are usually based on polyurethane systems employing aromatic polyisocyanates. After the adhesive is applied and the film is laminated, the film is wound onto a large roll and stored at elevated temperatures for several days to allow any unreacted polyisocyanate monomers to cure. If this curing step is eliminated or the curing step is not long enough, the laminate may suffer from two problems: first, the mechanical strength of the adhesive bond may be insufficient for further handling and use of the laminated packaging film; and second, the unreacted monomeric aromatic polyisocyanates react with moisture in the product to be packaged to generate aromatic primary amines (PAAs) that may migrate into the contents of the package. This is particularly problematic in high-performance packaging systems, as these packaging systems are subject to elevated temperatures (e.g., 116°C-130°C) during the sterilization (retorting process) of the packaging material and contents.

[0004] For health and safety reasons, the packaging industry has been trying to find adhesive solutions to eliminate this aromatic primary amine migration problem while maintaining the cure speed of the traditional two-component (2K) aromatic polyisocyanate-based adhesives to which they are accustomed. One approach is to replace the aromatic polyisocyanate component in the two-component (2K) polyurethane adhesive composition with an aliphatic polyisocyanate. While this approach eliminates the potential for aromatic amine formation, the lower reactivity observed when using aliphatic isocyanates results in a much slower cure time. As a result, the roll film must be stored at elevated temperatures and humidity for much longer periods of time (potentially up to two weeks) compared to the two to three days of cure time for aromatic isocyanate-based adhesives.

[0005] Thus, there continues to be a need in the art for an adhesive that: 1) is able to meet the necessary mechanical requirements of a film lamination adhesive; 2) does not suffer from PAA migration; and 3) has sufficient cure speed so that extended cure times are not required to meet these requirements. SUMMARY OF THE INVENTION

[0007] The aspartate-terminated prepolymer may be the reaction product of a first reaction mixture comprising an aliphatic NCO-terminated prepolymer having an NCO content of 0.5 wt % to 15 wt % and a number average NCO functionality of less than 2 and a compound according to formula (I):

[0008]

[0009] in,

[0010] X represents a linear, branched and / or cyclic aliphatic group obtained by removing an amino group from a linear, branched and / or cyclic aliphatic polyamine,

[0011] R1 and R2 are the same or different and represent an organic group that is inert to isocyanate groups at 100° C. or lower,

[0012] R3 and R4 are the same or different and represent hydrogen or an organic group which is inert to isocyanate groups at 100°C or lower, and

[0013] n represents an integer having a value of at least 2.

[0014] The aliphatic NCO-terminated prepolymer and the compound according to formula (I) may be combined in an NCO:NH equivalent ratio of 1:1.5 to 1:5.

[0015] The adhesive can be the reaction product of an adhesive reaction mixture comprising an aliphatic polyisocyanate and an aspartate-terminated prepolymer as described herein. The aliphatic polyisocyanate and the aspartate-terminated prepolymer can be combined in an NCO:NH equivalent ratio of 0.9:1 to 1.75:1.

[0016] The multilayer laminate film may include an adhesive as described herein applied to one or more substrate layers.

[0017] A method of minimizing PAA migration in a packaging material may include applying an adhesive as described herein to one or more substrates of the packaging material and curing the adhesive.

[0018] These and other advantages and benefits of the present invention will be apparent from the following detailed description of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Although the following detailed description contains many specific contents for illustration, it will be understood by those skilled in the art that many changes and modifications may be made to the following details, and these changes and modifications are deemed to be included herein. Therefore, the following embodiments are proposed without loss of generality and without imposing limitations on any proposed claims. It should also be understood that the terms used herein are only used to describe specific embodiments and are not intended to be limiting. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present disclosure belongs.

[0021] As used in this written specification, the singular forms "a," "an," and "the" include explicit support for plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a polymer" or "the polymer" may include a plurality of such polymers.

[0022] In this application, "including", "comprising", "containing", "having", etc. may have the meanings assigned to them by U.S. patent law, and may mean "including", "comprising", etc., and are generally interpreted as open terms. The term "consisting of" or "consisting of" is a closed term, and only includes the components, structures, steps, etc. specifically listed with these terms, as well as the content that complies with U.S. patent law. "Substantially consisting of" or "substantially consisting of" have the meanings generally assigned to them by U.S. patent law. Specifically, these terms are generally closed terms, except that additional items, materials, components, steps or elements that do not substantially affect the basic characteristics and novel characteristics or functions of the (one or more) items used in connection therewith are allowed to be included. For example, if present under the wording "substantially consisting of", trace elements that do not affect the properties or characteristics of the composition may be allowed to be present in the composition even if they are not explicitly listed in the list of items following these terms. When open-ended terms such as "comprising" or "including" are used in this written specification, it should be understood that it should also directly support the phrase "consisting essentially of" and the phrase "consisting of", just as if explicitly stated, and vice versa.

[0023] The terms "first", "second", "third", "fourth", etc., if any, in the specification and claims are used to distinguish similar elements and are not necessarily used to describe a particular order or chronological sequence. It should be understood that any terms so used are interchangeable where appropriate, such that the embodiments described herein are, for example, capable of operating in an order different from that shown herein or otherwise described herein. Similarly, if the method described herein includes a series of steps, the order of these steps described herein is not necessarily the only order to perform these steps, and some of the steps may be omitted and / or some other steps not described herein may be added to the method.

[0024] As used herein, the term "substantially" refers to a complete or nearly complete degree or degree of an action, feature, property, state, structure, item, or result. For example, an object that is "substantially" closed means that the object is completely closed, or almost completely closed. In some cases, the degree of allowable deviation from absolute completeness may depend on the specific circumstances. Nevertheless, in general, close completion will have the same overall result as obtaining absolute and complete completion. When used in a negative sense, the use of "substantially" is also applicable to referring to a complete or almost complete lack of an action, feature, property, state, structure, item, or result. For example, a composition that is "substantially free of" particles is completely free of particles, or almost completely free of particles (its effect is the same as completely free of particles). In other words, a composition that is "substantially free of" an ingredient or element may actually still contain this item as long as there is no measurable effect of it.

[0025] As used herein, the term "about" is used to provide flexibility to numerical range endpoints by providing that a given value may be "slightly above" or "slightly below" this endpoint. Unless otherwise indicated, the use of the term "about" in reference to a particular number or numerical range should also be understood to provide support for these numerical terms or ranges without the term "about". For example, for convenience and brevity, a numerical range of "about 50 mg to about 80 mg" should also be understood to provide support for the range of "50 mg to 80 mg".

[0026] In addition, it should be understood that even when the term "about" is used with it, support for actual numerical values ​​is provided in this specification. For example, the expression "about" 30 should be understood to provide support not only for values ​​slightly above and slightly below 30, but also for the actual numerical value of 30. Unless otherwise specified, all numerical parameters should be understood to be preceded and modified by the term "about" in all instances, wherein the numerical parameters have the inherent variability characteristic of the actual measurement technology used to determine the parameter value.

[0027] As used herein, for convenience, multiple items, structural elements, compositional elements, and / or materials may be presented in a common list. However, these lists should be understood as if each member of the list were individually identified as a separate and unique member. Therefore, in the absence of contrary indications, a single member of such a list should not be interpreted as a de facto equivalent of any other member of the same list solely based on its presentation in a common group.

[0028] Concentration, amount and other numerical data can be expressed or presented in the format of range herein.It should be understood that the format of this range is only for convenience and simplicity, and therefore should be flexibly interpreted as not only including the numerical value of the range limit clearly listed, but also including all single numerical values ​​or sub-ranges contained in the range, just as each numerical value and sub-range are clearly listed.As an example, the numerical range of "1 to 5" should be interpreted as not only including the value of 1 to 5 clearly listed, but also including single values ​​and sub-ranges in the indicated range.Therefore, the numerical range includes single values, such as 2,3 and 4, and sub-ranges, such as 1-3,2-4 and 3-5, etc., and single 1,2,3,4 and 5.

[0029] This same principle applies to ranges that list only one numerical value as a minimum or maximum. Furthermore, this interpretation should apply regardless of the breadth of the range or feature being described.

[0030] References to "examples" in this specification mean that a particular feature, structure, or characteristic described in connection with the example is included in at least one embodiment. Therefore, the phrase "in an example" that appears in various places in this specification does not necessarily refer to the same embodiment.

[0031] Unless otherwise stated, any patent, publication or other disclosure described herein is incorporated by reference in its entirety into this specification, but only to the extent that the incorporated material does not conflict with existing definitions, statements or other disclosures explicitly set forth in this specification. Therefore, and to the extent necessary, the explicit disclosures set forth in this specification are above any conflicting materials incorporated by reference herein. Any material or portion thereof, if allegedly incorporated by reference into this specification, but conflicts with existing definitions, statements or other disclosures set forth herein, is incorporated only to the extent that there is no conflict between the incorporated material and the existing disclosure material. Applicants reserve the right to amend this specification to expressly state any subject matter or portion thereof incorporated by reference herein.

[0032] As mentioned above, traditional 2K adhesive formulations for flexible packaging applications can use aromatic polyisocyanates and polyols. However, the residual polyisocyanate reacts with water to form aromatic primary amines (PAA) as unwanted byproducts, which migrate into the contents of the flexible packaging. These traditional 2K adhesive formulations have the advantages of short reaction time and a wide selection of polyols to help adjust the initial viscosity of the resulting adhesive to a reasonable range. Therefore, reaction time and initial viscosity are key considerations for developing adhesive systems that do not produce aromatic primary amines.

[0033] The present disclosure describes flexible polyaspartic esters that can overcome the rigidity issues of conventional polyaspartic ester-based polyureas while maintaining the desired fast, adjustable cure speeds to provide adhesives with minimal or, in some cases, no aromatic primary amines. One of the attractive features of polyaspartic esters is their relatively fast and adjustable cure speeds with aliphatic isocyanates compared to polyurethane systems typically used for 2K laminating films. This reactivity can be adjusted by selecting polyamines with varying degrees of steric hindrance around the amine as the basis for the polyaspartic ester.

[0034] Various embodiments of the present disclosure are directed to aspartate-terminated prepolymers and related adhesives. The aspartate-terminated prepolymer can be the reaction product of a first reaction mixture comprising an NCO-functional prepolymer and a polyaspartate combined in an NCO:NH equivalent ratio of 1:1.5 to 1:5. The NCO-functional prepolymer used to prepare the polyaspartate-terminated prepolymer can have an NCO content of 0.5 wt % to 15 wt % based on ISO 11909:2007, and a number average functionality of less than 2 calculated using the method described in paragraph 0039. The polyaspartate can be a compound according to formula (I)

[0035]

[0036] in,

[0037] X represents a linear, branched and / or cyclic aliphatic group obtained by removing an amino group from a linear, branched and / or cyclic aliphatic polyamine,

[0038] R1 and R2 are the same or different and represent an organic group that is inert to isocyanate groups at 100° C. or lower,

[0039] R3 and R4 are the same or different and represent hydrogen or an organic group which is inert to isocyanate groups at 100°C or lower, and

[0040] n represents an integer having a value of at least 2.

[0041] In more detail, the NCO-terminated prepolymer can be an aliphatic NCO-terminated prepolymer. In some instances, the NCO-terminated prepolymer can be the reaction product of a second reaction mixture, which includes an aliphatic polyisocyanate and an isocyanate-reactive component. In some additional instances, the NCO-terminated prepolymer can be the reaction product of a second reaction mixture comprising an aliphatic polyisocyanate, a polyfunctional isocyanate-reactive component, and a monofunctional component. As used herein, "polyfunctional isocyanate-reactive component" refers to an isocyanate-reactive component having a number average functionality greater than or equal to 2 (e.g., -OH, -NH, -SH, or a combination thereof). As used herein, "monofunctional component" refers to a functional component having a number average functionality less than 2 (e.g., -NCO, -OH, -NH, -SH, or a combination thereof).

[0042] Various aliphatic polyisocyanates may be included in the second reaction mixture. Non-limiting examples may include 1,4-tetramethylene diisocyanate, 1,5-pentamethylene diisocyanate (PDI), 1,6-hexamethylene diisocyanate (HDI), 2,2,4-trimethyl-hexamethylene diisocyanate, dodecamethylene diisocyanate, 2-methyl-1,5-diisocyanatopentane, 1,4-diisocyanatocyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methylcyclohexane (IPDI), 2,4 -diisocyanato-dicyclohexyl-methane, 4,4'-diisocyanato-dicyclohexyl-methane, 1,3-bis(isocyanatomethyl)-cyclohexane, 1,4-bis(isocyanatomethyl)-cyclohexane, 1-isocyanato-1-methyl-3(4)-isocyanatomethyl-cyclohexane (IMCI), bis(4-isocyanato-3-methyl-cyclohexyl)-methane, 1,4-cyclohexane diisocyanate (CHDI), or any mixture thereof. In some specific examples, the aliphatic polyisocyanate may include pentamethylene diisocyanate (PDI), 1,6-hexamethylene diisocyanate (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methylcyclohexane (IPDI), or a combination thereof.

[0043] Therefore, a variety of aliphatic polyisocyanates can be used to prepare NCO-functional prepolymers. As used herein, the term "polyisocyanate" refers to a compound comprising at least two unreacted isocyanate groups. Polyisocyanates include diisocyanates, and diisocyanate reaction products including, for example, biuret, isocyanurate, uretdione, carbamate, urea, iminooxadiazinedione, oxadiazinedione, carbodiimide, acylurea, allophanate groups or prepolymers, or a combination thereof. The term "aliphatic polyisocyanate" also includes alicyclic polyisocyanates. A variety of polyfunctional isocyanate reactive components can also be used in the second reaction mixture. Non-limiting examples can include polyols or polyamines with a number average molecular weight of 300 to 6000, which are based on polyethers, polyesters, polycarbonates, polycarbonate esters (polycarbonate ester), polycaprolactone, polybutadiene, etc. or one of their combinations.

[0044] Various embodiments include polyether polyols formed by the alkoxylation of various polyols, including diols such as ethylene glycol, 1,2-butanediol, 1,3-butanediol or 1,4-butanediol, 1,6-hexanediol, etc. or higher polyols such as trimethylolpropane, pentaerythritol, etc. One useful alkoxylation method is by reacting the polyol with an alkylene oxide such as ethylene oxide or propylene oxide in the presence of a basic catalyst or a coordination catalyst such as double metal cyanide (DMC).

[0045] Suitable polyester polyols can be prepared by polyesterification of organic polycarboxylic acids, their anhydrides or their esters with organic polyols. Preferably, the polycarboxylic acids and polyols are aliphatic or aromatic dibasic acids and diols.

[0046] The diols that can be used to prepare the polyesters include alkylene glycols such as ethylene glycol, 1,2-butanediol, 1,3-butanediol or 1,4-butanediol, neopentyl glycol and other diols such as cyclohexanedimethanol, polycaprolactone diols (e.g., the reaction product of caprolactone and ethylene glycol), polyether diols such as poly(oxytetramethylene) glycol, etc. However, in various embodiments of the present invention, other diols of various types and higher functionality polyols as indicated may also be used. Such higher polyols may include, for example, trimethylolpropane, trimethylolethane, pentaerythritol, etc., as well as higher molecular weight polyols such as those produced by alkoxylating low molecular weight polyols.

[0047] The acid component of polyester is mainly composed of monomeric carboxylic acids or their anhydrides or esters having 2 to 18 carbon atoms per compound. Among them, the acids that can be used are phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, adipic acid, succinic acid, azelaic acid, sebacic acid, maleic acid, glutaric acid, chlorendic acid, tetrachlorophthalic acid and different types of other dicarboxylic acids. In addition, higher polycarboxylic acids such as trimellitic acid and trisaccharic acid can also be used.

[0048] In addition to polyester polyols formed from polyacids and polyols, polycaprolactone-type polyesters can also be used. These products are formed by reacting cyclic lactones (e.g., ε-caprolactone) with polyols containing primary hydroxyl groups (e.g., those mentioned above). For example, such products are described in U.S. Pat. No. 3,169,949.

[0049] Suitable hydroxy-functional polycarbonate polyols may be those prepared by reacting monomeric diols (e.g., 1,4-butanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol or tetraethylene glycol, dipropylene glycol, tripropylene glycol or tetrapropylene glycol, 3-methyl-1,5-pentanediol, 4,4'-4,4'-dimethylolcyclohexane and mixtures thereof) with diaryl carbonates (e.g., diphenyl carbonate), dialkyl carbonates (e.g., dimethyl carbonate and diethyl carbonate), alkylene carbonates (e.g., ethylene carbonate or propylene carbonate) or phosgene. Optionally, small amounts of higher-functionality monomeric polyols, such as trimethylolpropane, glycerol or pentaerythritol, may be used.

[0050] In various embodiments, low molecular weight diols, triols and higher alcohols may also be included in the NCO-functional prepolymers. In many embodiments, they are monomers and have a hydroxyl value of 375 to 1810. Such materials may include aliphatic polyols, particularly alkylene polyols containing 2 to 18 carbon atoms. Examples include ethylene glycol, 1,4-butanediol, 1,6-hexanediol and alicyclic polyols, such as cyclohexanedimethanol. Examples of triols and higher alcohols include trimethylolpropane and pentaerythritol. Polyols containing ether bonds, such as diethylene glycol and triethylene glycol are also available.

[0051] The second reaction mixture may also include various monofunctional components. In some examples, the monofunctional component may be or include a monofunctional isocyanate reactive component. In this case, the monofunctional isocyanate reactive component may be or include methanol, ethanol, propanol, butanol, 2-ethylhexanol, other C5-C 38 Alkanol, C3-C 25 enols (containing one or more double bonds) or mixtures of any of these. It is worth noting that equivalent amines or thiols may also be used.

[0052] In some additional examples, the monofunctional component can be or include a monofunctional isocyanate. In this case, the monofunctional isocyanate can be or include methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, hexyl isocyanate, octyl isocyanate, isopropyl isocyanate, phenyl isocyanate, (methyl) acryloyl isocyanate, 2-(methyl) acryloyloxyethyl isocyanate, 3-(methyl) acryloyloxypropyl isocyanate, 2-(methyl) acryloyloxy-1-methylethyl isocyanate, 2-(methyl) acryloyloxy-2-methylethyl isocyanate, naphthyl isocyanate or any mixture of these.

[0053] In some examples, the monofunctional component can include a monofunctional isocyanate reactive component and a monofunctional isocyanate. In other examples, the monofunctional component can include a monofunctional isocyanate reactive component or a monofunctional isocyanate component. A small amount of a monofunctional component (e.g., a monofunctional alcohol or a monofunctional isocyanate) can be incorporated into the NCO-terminated prepolymer backbone to "chain terminate" the NCO-terminated prepolymer, thereby helping to maintain a fairly low viscosity.

[0054] Typically, the second reaction mixture may include a ratio of polyfunctional isocyanate reactive component equivalents to monofunctional component equivalents of 1: 1 to 4: 1. In other examples, the second reaction mixture may include a ratio of polyfunctional isocyanate reactive component equivalents to monofunctional component equivalents of 1: 1 to 3: 1, or 2: 1 to 4: 1.

[0055] The isocyanate component(s) and the isocyanate-reactive component(s) may be combined and allowed to react in a certain ratio to generate an NCO-functional prepolymer having 0.5 wt % NCO to 15 wt % NCO based on ISO 11909:2007. In various embodiments, the NCO-functional prepolymer has 0.5 wt % NCO to 10 wt % NCO, 1 wt % NCO to 5 wt % NCO, or 2 wt % NCO to 4 wt % NCO. In any case, an excess of aliphatic polyisocyanate is used to generate the NCO-functional prepolymer. In addition, in some embodiments, the resulting NCO-functional prepolymer may be stripped to remove residual monomeric polyisocyanate (e.g., by thin film evaporation) prior to termination with a polyaspartic acid ester.

[0056] Since the NCO-functional prepolymer is formed from monofunctional components, the number average NCO functionality of the NCO-functional prepolymer is less than 2. In some examples, the NCO-terminated prepolymer can have a number average NCO functionality of 1.1 to less than 2, or 1.2 to 1.9, or 1.3 to 1.8, or 1.4 to 1.7. The number average NCO functionality of the prepolymer can be calculated as follows:

[0057]

[0058] in

[0059]

[0060] Eq 单体NCO =NCO equivalents of monomer (after reaction) = Eq 多异氰酸酯A -Eq 聚酯A -Eq 单官能组分

[0061]

[0062] It is also noted that various analytical techniques can be used to demonstrate that the NCO-functional prepolymer contained in the first reaction mixture used to prepare the aspartate-terminated prepolymer has a number average NCO functionality of less than 2.

[0063] Surprisingly, as will be discussed in further detail below, the addition of a monofunctional component to an NCO-terminated prepolymer produces an aspartate-terminated prepolymer having a lower number average molecular weight, which, upon combination with the aliphatic polyisocyanate described herein, can have an acceptable initial viscosity at a solids content of 35 wt % to 40 wt % solids without negatively affecting other performance characteristics of the adhesive, such as bond strength development and / or pot life. % solids can be determined according to ASTM D2369:2020.

[0064] As described elsewhere herein, in two-component (2K) adhesive systems, aliphatic polyisocyanates generally have lower reactivity and slower cure times than aromatic polyisocyanates. With this in mind, polyaspartates can serve multiple functions in aspartate-terminated prepolymers, depending on the specific formulation. For example, aspartate-terminated prepolymers can have faster reaction rates with co-reactant polyisocyanate components than polyols commonly used in the art.

[0065] In certain embodiments, various polyaspartic acid ester components can be used to prepare aspartic acid ester-terminated prepolymers. As known to those skilled in the art, polyaspartic acid esters can be produced by the reaction of polyamines with Michael addition acceptors (i.e., olefins substituted with electron withdrawing groups such as cyano, keto or ester groups (electrophiles) on one or both olefinic carbons in a Michael addition reaction). Examples of suitable Michael addition acceptors include, but are not limited to, acrylates and diesters, such as dimethyl maleate, diethyl maleate, dibutyl maleate, dimethyl fumarate, diethyl fumarate, and dibutyl fumarate.

[0066] The polyaspartate may include one or more polyaspartates corresponding to formula (I):

[0067]

[0068] in:

[0069] n is an integer of at least 2;

[0070] X represents an aliphatic residue, for example, a linear, branched and / or cyclic aliphatic group obtained by removing an amino group from a linear, branched and / or cyclic aliphatic polyamine;

[0071] R1 and R2 each independently represent an organic group that is inert to isocyanate groups under the reaction conditions; and

[0072] R3 and R4 each independently represent hydrogen or an organic group which is inert towards isocyanate groups under the reaction conditions.

[0073] As will be described in further detail, the polyaspartic acid esters can be prepared from a variety of polyamines, including low molecular weight polyamines, high molecular weight polyamines, or combinations thereof. In addition, the polyamines can have a wide range of amine functionalities, repeat unit types, distributions, etc. This wide range of molecular weights, amine functionalities, repeat unit types, and distributions can provide versatility for the design of new compounds or mixtures.

[0074] Suitable low molecular weight polyamines have a molecular weight of 60 to 400 in various embodiments, and in selected embodiments have a molecular weight of 60 to 300. Suitable low molecular weight polyamines include, but are not limited to, ethylenediamine, 1,2- and 1,3-diaminopropane, 1,5-diaminopentane, 1,3-, 1,4- and 1,6-diaminohexane, 1,3-diamino-2,2-dimethylpropane, 2-methyl-1,5-pentanediamine, isophoronediamine, 4,4′-diamino-dicyclohexylmethane, 4,4′-diamino-3,3′-dimethyldicyclohexylmethane, 1,4-bis(2-amino-propan-2-yl)-cyclohexane, hydrazine, piperazine, bis(4-aminocyclohexyl)methane, and mixtures of these polyamines. Representative polyaspartic acid esters prepared from these low molecular weight polyamines include DESMOPHEN NH-1220, DESMOPHEN NH-1420, and DESMOPHEN NH-1520, all commercially available from Covestro.

[0075] In some embodiments of the present invention, a single high molecular weight polyamine can be used. A mixture of high molecular weight polyamines can also be used, such as a mixture of difunctional materials and trifunctional materials and / or different molecular weights or different chemical composition materials. The term "high molecular weight" is intended to include polyamines having a molecular weight of at least 400 in various embodiments. In selected embodiments, the polyamine has a molecular weight of 400 to 6,000. Non-limiting examples can include polyethylene glycol bis (amines), polypropylene glycol bis (amines) or polytetramethylene glycol bis (amines) etc., or a combination thereof.

[0076] In certain embodiments, the polyamine may be, for example, one or more of the JEFFAMINE series of amine-terminated polyethers from Huntsman Corporation, such as JEFFAMINE D-230, JEFFAMINE D-400, JEFFAMINE D-2000, JEFFAMINE D-4000, JEFFAMINE T-3000, and JEFFAMINE T-5000.

[0077] In some specific examples, X represents a group obtained by removing an amino group from 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, 1-amino-3,3,5-trimethyl-5-aminomethyl-cyclohexane, 2-methyl-1,5-pentanediamine, 2,4-hexahydrotolylenediamine, 2,6-hexahydrotolylenediamine, 4,4′-diaminodicyclohexylmethane, 3,3-dimethyl-4,4′-diamino-dicyclohexylmethane, 3,3-diethyl-4,4′-diamino-dicyclohexylmethane, or a combination thereof.

[0078] In some embodiments, the obtained aspartate-terminated prepolymer can be further diluted in an organic solvent to form an aspartate-terminated prepolymer composition. In other words, the aspartate-terminated prepolymer composition can further include an organic solvent as a diluent. Non-limiting examples of organic solvents can include ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, methoxypropyl acetate, N-methyl pyrrolidone, petroleum hydrocarbons, etc., or combinations thereof.

[0079] In those embodiments where aspartate-terminated prepolymer is diluted in an organic solvent, it can be diluted to varying degrees to maintain a reasonable viscosity for intended application. In these embodiments, aspartate-terminated prepolymer can be diluted to a solid content of 20 % by weight to 90 % by weight solids based on the total weight of aspartate-terminated prepolymer composition. In certain embodiments, aspartate-terminated prepolymer can be diluted to a solid content of 20 % by weight to 60 % by weight or 40 % by weight to 90 % by weight solids based on the total weight of aspartate-terminated prepolymer composition. In various embodiments, aspartate-terminated prepolymer composition can include 20 % by weight to 40 % by weight, 30 % by weight to 50 % by weight, 40 % by weight to 60 % by weight, 50 % by weight to 70 % by weight, 60 % by weight to 80 % by weight or 70 % by weight to 90 % by weight solids based on the total weight of aspartate-terminated prepolymer composition. In some embodiments, the aspartate-terminated prepolymer composition comprises 60 wt% to 90 wt% solids based on the total weight of the aspartate-terminated prepolymer composition. % Solids can be calculated based on ASTM D2369:2020.

[0080] As previously described, the addition of a monofunctional component to an NCO-terminated prepolymer can help reduce the viscosity of the aspartate-terminated prepolymer composition relative to a "comparative control," which is an equivalent aspartate-terminated prepolymer prepared using an otherwise equivalent aliphatic NCO-terminated prepolymer having a number average NCO functionality of ≥2. Thus, the "comparative control" referred to herein is an aspartate-terminated prepolymer prepared with or from an aliphatic NCO-terminated prepolymer having a number average NCO functionality of ≥2, but otherwise prepared at the same solids content, with the same solvent dilution, and otherwise prepared with the same reactants, with the same reaction method, and with the same NCO:OH ratio, etc., wherein the only difference is that the monofunctional component is omitted from the second reaction mixture. In other words, the "comparative control" is the same as the aspartate-terminated prepolymer of the present invention, except that it is prepared with or from an NCO-terminated prepolymer prepared by a reaction mixture that does not contain the monofunctional component content, resulting in a number average NCO functionality of ≥2. With this in mind, in some cases, the viscosity of the aspartic acid ester-terminated prepolymer at 25°C based on ASTM D1084:2016 can be ≤50% of the viscosity of the comparative control group. In some additional examples, the viscosity of the aspartic acid ester-terminated prepolymer at 25°C based on ASTM D1084:2016 can be ≤45%, ≤40%, or ≤30% of the viscosity of the comparative control group.

[0081] In some additional examples, the monofunctional component can provide an aspartate-terminated prepolymer having a lower molecular weight and polydispersity than a comparative control group. For example, in some cases, the weight average molecular weight of the aspartate-terminated prepolymer can be ≤90%, ≤80%, ≤70%, or ≤60% of the weight average molecular weight of the comparative control group. The weight average molecular weight can be determined by gel permeation chromatography.

[0082] In some specific examples, the aspartate-terminated prepolymer may have a weight average molecular weight of 6000 g / mol to 12,000 g / mol based on gel permeation chromatography. In other examples, the aspartate-terminated prepolymer may have a weight average molecular weight of 6000 g / mol to 8000 g / mol, 8000 g / mol to 10,000 g / mol, or 10,000 g / mol to 12,000 g / mol based on gel permeation chromatography.

[0083] In some additional examples, the polydispersity (MW / MN) of the aspartate-terminated prepolymer can be ≤95%, ≤90%, ≤85%, or ≤80% of the polydispersity of the comparative control group.

[0084] In some specific examples, the aspartate-terminated prepolymer can have a polydispersity of ≤ 4 based on gel permeation chromatography. In other examples, the aspartate-terminated prepolymer can have a polydispersity of 3 to 3.5, 3.2 to 3.8, 3.4 to 3.9, or 3.5 to 4 based on gel permeation chromatography.

[0085] The aspartate-terminated prepolymer or aspartate-terminated prepolymer composition can be part of a multi-component (e.g., two-component (2K)) adhesive system, which can include a co-reactant polyisocyanate or polyisocyanate prepolymer component for combining with the aspartate-terminated prepolymer to form an adhesive.

[0086] In some examples, the adhesive can be or include the reaction product of an adhesive reaction mixture including an aliphatic polyisocyanate and an aspartate-terminated prepolymer as described herein. Typically, the aliphatic polyisocyanate and the aspartate-terminated prepolymer can be combined with an NCO:NH ratio of 0.9:1 to 1.75:1. In some specific examples, the aliphatic polyisocyanate and the aspartate-terminated prepolymer can be combined with an NCO:NH ratio of 0.9:1 to 1.3:1, 1.1:1 to 1.5:1, or 1.3:1 to 1.7:1.

[0087] In various embodiments, various aliphatic polyisocyanate components may be included in the adhesive. For example, in certain embodiments, the aliphatic polyisocyanate component may be or include a monomeric polyisocyanate, such as any aliphatic polyisocyanate monomer and aliphatic polyisocyanate adduct described elsewhere herein, or a combination thereof. In certain other embodiments, the polyisocyanate component may be or include an aliphatic polyisocyanate prepolymer, and may optionally be diluted with a solvent to reduce viscosity in a manner similar to that described herein for the aspartic acid ester-terminated prepolymer component of the adhesive.

[0088] When the aliphatic polyisocyanate component includes an aliphatic polyisocyanate prepolymer, the polyisocyanate prepolymer can be the reaction product of the second aliphatic polyisocyanate and the second isocyanate reactive component. The type of polyisocyanate used in the polyisocyanate prepolymer includes the same polyisocyanate and polyisocyanate adducts listed herein about NCO-functional prepolymers. Similarly, the type of isocyanate reactive component used in the polyisocyanate prepolymer can include those listed herein about NCO-functional prepolymers. In some embodiments, the polyisocyanate prepolymer can be the same as the NCO-functional prepolymer. In certain embodiments, the polyisocyanate prepolymer can be different from the NCO-functional prepolymer. In selected embodiments, the polyisocyanate prepolymer can include (one or more) polyisocyanates identical to the NCO-functional prepolymer, and (one or more) identical or different isocyanate reactive components. In embodiments, the polyisocyanate and isocyanate-reactive components of the polyisocyanate prepolymer may be combined in a ratio to produce a polyisocyanate prepolymer having from 0.5% NCO to 35% NCO, e.g., from 2% NCO to 12% NCO, from 10% NCO to 18% NCO, from 12% to 20% NCO, or from 18% NCO to 25% NCO.

[0089] In some examples, the adhesive can be further diluted with an organic solvent, such as ethyl acetate, ethyl butyrate, methyl ethyl ketone, methyl isobutyl ketone, methoxypropyl acetate, N-methylpyrrolidone, petroleum hydrocarbons, etc. or a combination thereof. As mentioned above, since the aspartic acid ester-terminated prepolymer is prepared with an NCO-functional prepolymer having a number average NCO functionality of <2, the initial viscosity of the adhesive can be relatively low, which can reduce the amount of solvent required to achieve a suitable initial viscosity.

[0090] In some examples, the adhesive can be diluted with an organic solvent to a total solid content of 35% to 50% by weight based on the total weight of the adhesive. In some further examples, the adhesive can be diluted with an organic solvent to a total solid content of 35% to 45% or 40% to 50% by weight based on the total weight of the adhesive. In further examples, the adhesive can be diluted with an organic solvent to a total solid content of 35% to 40% or 40% to 45% by weight based on the total weight of the adhesive.

[0091] In some examples, the adhesive reaction mixture may have an initial viscosity of from 15 centipoise (cP) to 30 cP based on ASTM D1084:2016 at 23°C, and a total solids content of 38 wt% solids based on the total weight of the adhesive reaction mixture. In additional examples, the adhesive reaction mixture may have an initial viscosity of 15 cP to 25 cP or 20 cP to 30 cP based on ASTM D1084:2016 at 23°C, and a total solids content of 38 wt% solids based on the total weight of the adhesive reaction mixture. In yet additional examples, the adhesive reaction mixture may have an initial viscosity of 15 cP to 20 cP or 20 cP to 25 cP based on ASTM D1084:2016 at 23°C, and a total solids content of 38 wt% solids based on the total weight of the adhesive reaction mixture. The initial viscosity refers to the viscosity of the adhesive reaction mixture when the aliphatic polyisocyanate and the aspartic acid ester-terminated prepolymer are initially combined and mixed.

[0092] The pot life of the adhesive reaction mixture, measured by doubling the initial viscosity at 23°C according to ASTM D 1084:2016, can generally be greater than or equal to 1 hour. In more detail, the aspartic acid ester-terminated prepolymer and the aliphatic polyisocyanate component can be combined in various ratios to produce an adhesive reaction mixture. In various embodiments, the viscosity of the two-component adhesive reaction mixture at 23°C will require at least 0.5 hours to double, in certain embodiments, the viscosity at 23°C will require at least 2 hours to double, in other embodiments, at least 4 hours to double, and in selected embodiments, the viscosity at 23°C will require at least 8 hours to double the viscosity.

[0093] In additional examples, the adhesive can develop acceptable bond strength to the substrate, defined as having a minimum value of at least 150 g / in. or substrate tear as measured according to ASTM D1876-01 at 23° C., in less than or equal to 5 days after the substrates are laminated with the adhesive. In other examples, the adhesive can develop acceptable bond strength to the substrate, defined as having a minimum value of 150 g / in. or substrate tear as measured according to ASTM D1876-01 at 23° C., in less than or equal to 1 day or less than or equal to 7 days after the substrates are laminated with the adhesive.

[0094] Typical flexible packaging materials may include polyethylene terephthalate, metallized polyethylene terephthalate, polypropylene, aluminum foil, or a combination thereof. Thus, in some examples, the substrate used to determine acceptable bond strength may include polyethylene terephthalate, metallized polyethylene terephthalate, polypropylene, aluminum foil, or a combination thereof.

[0095] The present disclosure also describes a method for minimizing the migration of aromatic amines in a multilayer substrate, such as packaging material. The method includes forming or applying an adhesive described herein to one or more packaging material substrates and curing the adhesive.

[0096] Adhesive of the present invention can be formed on or applied to various substrates, including multilayer laminated films, such as laminated films for packaging materials, etc., particularly flexible packaging materials. Non-limiting examples of substrates include metals (aluminum, copper and steel), plastics, wood, cement, concrete, glass or a combination thereof. Adhesive of the present invention can be applied by coating, rolling, pouring, spraying, dipping, casting, dispensing, etc. or a combination thereof. Adhesive-substrate layers of the present invention can be laminated together by methods known in the art.

[0097] In some specific examples, the multilayer laminate film may include an adhesive as described herein applied to one or more substrate layers. The adhesive may bond one or more substrate layers together. In some examples, one or more substrate layers may be or include polyethylene terephthalate, metallized polyethylene terephthalate, polypropylene, aluminum foil, or a combination thereof. Example

[0098] The following non-limiting and non-exhaustive examples are intended to further describe various non-limiting and non-exhaustive embodiments without limiting the scope of the embodiments described in this specification. Unless otherwise indicated, all amounts given in "parts" and "percents" are to be understood as being by weight.

[0099] Polyester A: an aliphatic and aromatic polyester diol having a hydroxyl value of about 56 mgKOH / g, commercially available from Covestro LLC as BAYCOLL AD 2047;

[0100] Polyaspartic acid ester A: a diaspartic acid ester prepared from 3,3'-dimethyl-4,4'-diamino-dicyclohexylmethane and diethyl maleate, having an amine value of about 193 mgKOH / g and commercially available as DESMOPHEN NH 1520 from Covestro LLC;

[0101] Polyaspartic acid ester B: a diaspartic acid ester prepared from 4,4'-diamino-dicyclohexylmethane and diethyl maleate, having an amine value of about 204 mgKOH / g and commercially available as DESMOPHEN NH 1420 from Covestro LLC;

[0102] Polyaspartic acid ester C: a diaspartic acid ester prepared from 4,4'-diamino-dicyclohexylmethane and diethyl maleate, having an amine value of about 204 mgKOH / g and commercially available as DESMOPHEN NH 1423 from Covestro LLC;

[0103] Polyisocyanate A: an aliphatic diisocyanate (1,6-hexamethylene diisocyanate) having an average isocyanate content of 49.7% by weight and commercially available as DESMODUR H from Covestro LLC;

[0104] Polyisocyanate B: HDI trimer (isocyanurate) having an average isocyanate content of 21.8 wt%, commercially available as DESMODUR N-3300 from Covestro LLC;

[0105] Monofunctional compound A: 2-ethylhexanol, commercially available from Sigma Aldrich;

[0106] Ethyl acetate: commercially available from Sigma Aldrich.

[0107] Overview of Adhesive Preparation and Testing Procedures

[0108] This disclosure describes the use of reduced viscosity aspartic acid ester-terminated prepolymers in combination with aliphatic polyisocyanates to produce solvent-based polyurethane-polyurea laminating adhesives. A general three-step procedure for their preparation and use is outlined herein, with more detailed information for each step provided below:

[0109] Step 1. Preparation of NCO-terminated prepolymer

[0110] The NCO-terminated prepolymers are prepared by reacting polyols, monool chain terminators and polyisocyanates in excess NCO equivalents / OH equivalents. The compositions and properties of the NCO-terminated prepolymers produced in the examples are summarized in Table I.

[0111] Step 2. Preparation of aspartic acid ester-terminated prepolymer

[0112] The NCO-terminated prepolymer prepared in step 1 was reacted with polyaspartic acid ester. The ratio of NH / NCO equivalents was 2.0. The composition and properties of the aspartic acid ester-terminated prepolymers produced in the examples are summarized in Table II.

[0113] Step 3. Preparation and Processing Characteristics and Performance of Multilayer Laminated Structures Using Aspartic Acid Ester-Terminated Prepolymers of the Invention

[0114] The aspartic acid ester-terminated prepolymer prepared in step 2 was combined with a polyisocyanate in ethyl acetate to produce a laminating adhesive. The processable pot life of the mixture was determined by monitoring the change in viscosity over time. In addition, laminates were prepared from these adhesive compositions using pairs of representative films commonly used in packaging laminates. The development of adhesive strength over time was measured by peeling the laminate at different time intervals after lamination.

[0115] Detailed general procedure for the preparation of NCO-terminated prepolymers (step 1)

[0116] Polyester A was melted (60-80°C) and charged to a round bottom flask equipped with an overhead mechanical stirrer, a heating mantle and a nitrogen inlet. In NCO-terminated prepolymers B and C (inventive examples), monofunctional compound A was added and the mixture was cooled to 40°C before adding polyisocyanate A. For NCO-terminated prepolymers A and D (comparative examples), monofunctional compound A was not added before adding polyisocyanate A.

[0117] The reaction mixture was heated to 80°C with stirring and maintained under nitrogen purge for ~3-4 hours. The reaction mixture was cooled to 50°C and ethyl acetate was added to the reaction vessel. The contents of the reactor were removed and stored in a sample jar at 40°C until used in step 2. The viscosity of the reaction product was recorded at 25°C and the sample was titrated using standard n-butylamine titration according to ASTM D2572-19 to determine the NCO content. The detailed compositions and properties of NCO-terminated prepolymers A, B, C and D are summarized in Table I.

[0118] Table I

[0119]

[0120] Detailed general procedure for the preparation of aspartate-terminated prepolymers (step 2)

[0121] The NCO-terminated prepolymer in step 1 is charged into a round bottom flask equipped with an overhead mechanical stirrer, a heating jacket, and a nitrogen inlet. The NCO-terminated prepolymer is maintained at ~20°C before the addition of polyaspartate A, polyaspartate B, or polyaspartate C. After the addition of the polyaspartate, the reaction mixture is stirred and heated to 50°C under a nitrogen atmosphere until the isocyanate band of the reaction mixture is no longer visible in infrared analysis (~4 hours). The reaction product is cooled to room temperature and removed from the reaction flask. The viscosity is recorded at 25°C. The amine value is determined by standard titration according to ASTM D6979. The composition and properties of the aspartate-terminated prepolymers used to prepare the laminating adhesives are summarized in Table II.

[0122] Table II

[0123]

[0124] Determination of the pot life of adhesive compositions

[0125] Polyisocyanate B and each aspartate-terminated prepolymer in Table II were combined at 23°C. The samples were diluted to a target viscosity of 20 + / - 8 centipoise (cP) using dry ethyl acetate. The initial viscosity was recorded and measured sequentially after 1, 2, 4, and 8 hours at 23°C according to ASTM D1084:2016. All formulation data and viscosity measurement results are summarized in Table III.

[0126] Adhesion strength

[0127] In the following examples, polyisocyanate B and one of the aspartate-terminated prepolymers in Table II were combined at 23°C. The samples were diluted to 50% solids using dry ethyl acetate to ensure consistent adhesive application thickness between samples. Each formulation was applied to corona treated polyethylene terephthalate (PET) film using a wire wound rod to give a 2.5-5.0 g / m 2 The samples were dried at 60°C for 60 seconds and laminated to corona treated PET, metallized PET (MPET), corona treated cast polypropylene (cPP), and aluminum foil (Al) using a hot roll laminator at 50 psig, 65°C, and a travel speed of two feet per minute.

[0128] Adhesive strength was measured according to ASTM D 1876-01 using an INSTRON machine at a peeling speed of 12 inches / minute. The measurement results were recorded at intervals of four hours, one day, seven days, and fourteen days after lamination. All results are given in g / in., where the failure mode is specified as follows: ST is "substrate tearing", indicating that one or more substrates are torn during the analysis; P is "peeling", indicating that the sample is peeled off smoothly during the analysis; Z is "zipper", indicating that the adhesive strength of the sample increases and decreases rapidly during the analysis; C is "cohesive", indicating that the adhesive splits during the analysis, and part remains adhered to the two substrates; and AF is "adhesive failure", indicating that the adhesive is completely and cleanly separated from one of the two substrates during the analysis. Table III presents detailed formulations and adhesive strength data with various substrates.

[0129] Acceptable Application Viscosity and Application Solids Limits

[0130] Application viscosity is particularly important to adhesive manufacturers and converters. If the viscosity is not appropriate, the adhesive may not transfer properly to the web, resulting in adhesion problems in the laminate. In addition, application solids are also important. If the adhesive must be diluted too much with solvent to achieve the target application viscosity, the economics of the adhesive will become unsatisfactory. For the purposes of the present invention, the inventors have limited the minimum acceptable solids content in ethyl acetate to 35% by weight solids when the adhesive can achieve a viscosity of 20 (+ / - 8) cP or less.

[0131] Similarly, if the viscosity of the adhesive composition increases too quickly after combining the aspartate-terminated prepolymer with the polyisocyanate component, the processable pot life will be too short for viable commercial production. For the purposes of the present invention, the inventors define an acceptable pot life as 1 hour or longer, wherein the pot life is defined as the time required for the adhesive composition to double in viscosity at 23°C at a solids content that produces an initial viscosity of 20 (+ / -8) cP after mixing the polyisocyanate and polyaspartate in a solvent.

[0132] Limits on acceptable cure speed

[0133] Ideally, the laminate will cure rapidly after the lamination process. The cure speed is determined by measuring the bond strength of the laminate at various time intervals after the lamination process. For the purposes of the present invention, the inventors define an acceptable cure speed as one where the bond strength between all tested substrate pairs reaches a minimum of 150 g / in. at 23°C within a storage time of less than five days when measured according to ASTM D 1876-01 at 23°C. Alternatively, the cure speed is considered acceptable if substrate tearing is observed in a peel test performed within five days of lamination and storage at 23°C.

[0134] Table III

[0135]

[0136]

[0137] Note: ST = substrate tear; P = peel; Z = zipper; C = adhesion; and AF = adhesive failure

[0138] Those skilled in the art will appreciate that the comparative laminates (Examples 1 and 5) include adhesives made from aspartate-terminated prepolymers that have acceptable pot life and cure speed. However, the adhesive must be diluted with ethyl acetate to less than 35% solids to achieve the target initial application viscosity of 20 + / - 8 cP. This example was made from an NCO-terminated prepolymer that did not contain a monofunctional compound to reduce the viscosity of the resulting aspartate-terminated prepolymer solution.

[0139] Furthermore, as will be apparent to one skilled in the art, the laminates according to the present invention (Examples 2, 3 and 4) are formulations in which the monofunctional compound 2-ethylhexanol is contained in an NCO-terminated prepolymer formulation. In each of these examples, the application solids content is greater than 35% at an application viscosity of 20+ / -8 cP. The adhesive compositions of the present invention also exhibit acceptable pot life and cure speed as defined herein.

[0140] This specification has been written with reference to various non-limiting and non-exhaustive embodiments. However, those of ordinary skill in the art will recognize that any disclosed embodiment (or portion thereof) may be variously replaced, modified, or combined within the scope of this specification. Therefore, it is conceivable and understood that this specification supports additional embodiments not explicitly set forth herein. For example, such embodiments may be obtained by combining, modifying, or reorganizing any steps, components, elements, features, aspects, characteristics, limitations, etc. disclosed in the various non-limiting embodiments described in this specification. In this way, the applicant reserves the right to amend the claims during the review to add the features described in this specification, and such amendments comply with 35 U.S.C. § 112 (a), and 35 U.S.C. § 132 (a).

Claims

1. An aspartic acid ester-terminated prepolymer, comprising: A reaction product of a first reaction mixture, the first reaction mixture comprising: Aliphatic NCO-terminated prepolymers having an NCO content of 0.5 to 15 wt. % based on ISO 11909:2007 and a number average NCO functionality of <2; and Compounds according to formula (I) in, X represents a linear, branched and / or cyclic aliphatic group obtained by removing an amino group from a linear, branched and / or cyclic aliphatic polyamine, R1 and R2 are the same or different and represent an organic group that is inert to isocyanate groups at a temperature of 100° C. or lower, R3 and R4 are the same or different and represent hydrogen or an organic group which is inert to isocyanate groups at 100°C or lower, n represents an integer with a value of at least 2, Therein the aliphatic NCO-terminated prepolymer is combined with the compound according to formula (I) in an NCO:NH equivalent ratio of 1:1.5 to 1:

5.

2. The aspartic acid ester-terminated prepolymer according to claim 1, wherein the first reaction mixture further comprises a solvent.

3. The aspartate-terminated prepolymer according to claim 2, wherein the aspartate-terminated prepolymer has a total solid content of 60 wt% to 90 wt%.

4. The aspartate-terminated prepolymer of claim 1, wherein the viscosity of the aspartate-terminated prepolymer at 25°C based on ASTM D1084:2016 is ≤50% of the viscosity of a comparative control aspartate-terminated prepolymer prepared with an otherwise equivalent aliphatic NCO-terminated prepolymer having a number average NCO functionality ≥2.

5. The aspartic acid ester-terminated prepolymer of claim 1, wherein the aliphatic NCO-terminated prepolymer is the reaction product of a second reaction mixture comprising: Aliphatic polyisocyanates; a polyfunctional isocyanate-reactive component; and A monofunctional component selected from the group consisting of a monofunctional isocyanate-reactive component and a monofunctional isocyanate.

6. The aspartic acid ester-terminated prepolymer of claim 5, wherein the second reaction mixture comprises an equivalent ratio of polyfunctional isocyanate-reactive component equivalents to monofunctional isocyanate-reactive component equivalents of 1:1 to 4:

1.

7. The aspartate-terminated prepolymer of claim 5, wherein the polyfunctional isocyanate-reactive component comprises a polyester polyol.

8. The aspartic acid ester-terminated prepolymer according to claim 5, wherein the monofunctional component comprises methanol, ethanol, propanol, butanol, 2-ethylhexanol, other C5-C 38 Alkanol, C3-C 25 enol, or a mixture of any of these.

9. The aspartic acid ester-terminated prepolymer of claim 5, wherein the monofunctional component comprises methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, hexyl isocyanate, octyl isocyanate, isopropyl isocyanate, phenyl isocyanate, (meth)acryloyl isocyanate, 2-(meth)acryloyloxyethyl isocyanate, 3-(meth)acryloyloxypropyl isocyanate, 2-(meth)acryloyloxy-1-methylethyl isocyanate, 2-(meth)acryloyloxy-2-methylethyl isocyanate, naphthyl isocyanate, or a mixture of any of these.

10. The aspartic acid ester-terminated prepolymer according to claim 1, wherein X represents a group obtained by removing an amino group from 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, 2-methyl-1,5-pentanediamine, 1-amino-3,3,5-trimethyl-5-aminomethyl-cyclohexane, 2,4-hexahydrotolylenediamine, 2,6-hexahydrotolylenediamine, 4,4′-diamino-dicyclohexylmethane, 3,3-dimethyl-4,4′-diamino-dicyclohexylmethane, 3,3-diethyl-4,4′-diamino-dicyclohexylmethane, or a combination thereof.

11. The aspartate-terminated prepolymer of claim 1 , wherein the aspartate-terminated prepolymer has a weight average molecular weight based on gel permeation chromatography that is ≤ 90% of the weight average molecular weight of a comparative control aspartate-terminated prepolymer prepared with an otherwise equivalent aliphatic NCO-terminated prepolymer having a number average NCO functionality of ≥ 2.

12. The aspartate-terminated prepolymer of claim 1, wherein the aspartate-terminated prepolymer has a polydispersity based on gel permeation chromatography that is ≤ 95% of the polydispersity of a comparative control aspartate-terminated prepolymer prepared with an otherwise equivalent aliphatic NCO-terminated prepolymer having a number average NCO functionality of ≥ 2.

13. An adhesive comprising: A reaction product of an adhesive reaction mixture, the adhesive reaction mixture comprising: Aliphatic polyisocyanates; and The aspartic acid ester-terminated prepolymer according to claim 1, The aliphatic polyisocyanate is combined with the aspartic acid ester-terminated prepolymer in an NCO:NH equivalent ratio of 0.9:1 to 1.75:

1.

14. The adhesive of claim 13, wherein the adhesive reaction mixture further comprises a solvent.

15. The adhesive of claim 14, wherein the adhesive reaction mixture has an initial viscosity of 15 to 30 cP based on ASTM D1084:2016 at 23°C and 38% solids.

16. The adhesive of claim 13, wherein the adhesive reaction mixture has a pot life of greater than or equal to 1 hour as measured by doubling of viscosity at 23°C according to ASTM D1084:2016, and Wherein the adhesive develops an acceptable bond strength to the substrate, defined as a minimum of 150 g / in. or substrate tear as measured at 23°C according to ASTM D1876-01, within 5 days or less after the substrates are laminated with the adhesive.

17. The adhesive of claim 16, wherein the substrate comprises polyethylene terephthalate, metallized polyethylene terephthalate, polypropylene, aluminum foil, or a combination thereof.

18. A multi-layer laminate film comprising the adhesive of claim 13 applied to one or more substrate layers.

19. The multilayer laminate film of claim 18, wherein one or more substrate layers comprises polyethylene terephthalate, metallized polyethylene terephthalate, polypropylene, aluminum foil, or combinations thereof.

20. A method for minimizing the migration of aromatic amines in a packaging material, comprising: applying the adhesive of claim 13 to one or more packaging material substrates; and The adhesive is cured.

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