Robust adhesives for laminating flexible packaging material

Abstract

A robust, two component adhesive for laminating flexible packaging material comprising a mixture of Component A and Component B. Component A comprises an isocyanate-functionalized compound. Component B comprises a mixture of a high functionality polyol containing at least four hydroxyl groups per molecule with at least two primary OH groups on the molecule and two secondary OH groups on the molecule and a trifunctional polyol containing three OH groups on the molecule. Component B can contain a difunctional polyol containing two OH groups on the molecule. The adhesive is robust and retains desirable properties when used at varying A:B mix ratios better than conventional laminating adhesives used at the same mix ratios.

Claims

1. A two component adhesive suitable for use in transfer roll lamination equipment to make flexible packaging material comprising Component A and Component B; Component A comprising an isocyanate-functionalized polyurethane prepolymer; and Component B comprising a mixture of a high functionality polyol containing at least four hydroxyl groups per molecule with at least two primary OH groups on the molecule and two secondary OH groups on the molecule and a trifunctional polyol containing three OH groups on the molecule; wherein a mixture of Component A and Component B at a predetermined weight ratio has a pot-life based on viscosity doubling of at least about 25 minutes at a temperature of 40 C., a bond strength at 70 C. of at least 200 grams per inch after 24 hours of cure and has a BfR migration after 3 days of cure of less than 2 parts per billion, and each of these properties remains present throughout the range of Component A:Component B weight ratios of 25% less component A than the predetermined weight ratio, to the predetermined weight ratio, to 25 wt % more Component A than the predetermined weight ratio.

2. The laminating adhesive of claim 1 wherein Component B comprises a difunctional polyol containing two OH groups on the molecule.

3. The laminating adhesive of claim 1 wherein the high functionality polyol is a polyester polyol.

4. The laminating adhesive of claim 1 wherein the trifunctional polyol is a polyether polyol.

5. The two component laminating adhesive of claim 1 comprising about 5 wt % to about 20 wt % of a difunctional polyol.

6. The laminating adhesive of claim 1, wherein the high functionality polyol has the following structure:
(HO).sub.m(R.sup.1)OC(O)R.sup.2C(O)O(R.sup.3)(OH).sub.n wherein m and n are integers which are the same or different and which each have a value of at least 1; m+n=at least 4; and R.sup.1, R.sup.2 and R.sup.3 are hydrocarbon radicals.

7. The laminating adhesive of claim 1, wherein the high functionality polyol has the following structure:
(HO).sub.m(R.sup.1)OC(O)R.sup.2C(O)O(R.sup.3)(OH).sub.n wherein m and n are integers which are the same or different and which each have a value of at least 1 and m+n=at least 4; R.sup.1 is a hydrocarbon radical having 2 to 20 carbon atoms and a valency of m+1 with m OH groups being attached thereto; R.sup.2 is a hydrocarbon radical having 2 to 20 carbon atoms and a valency of 2; R.sup.3 is a hydrocarbon radical having 2 to 20 carbon atoms and a valency of n+1 with n OH groups being attached thereto.

8. The laminating adhesive of claim 1, wherein the high functionality polyol has the following structure:
(HO).sub.m(R.sup.1)OC(O)R.sup.2C(O)O(R.sup.3)(OH).sub.n wherein m and n are integers which are the same or different and which each have a value of at least 1 and m+n=at least 4; R.sup.1 is a CH.sub.2CHCH.sub.2 group with a valency of m+1 with m OH groups being attached thereto; R.sup.2 is a (CH.sub.2).sub.o moiety, where o is an integer of from 2 to 18; and R.sup.3 is a CH.sub.2CHCH.sub.2 group with a valency of n+1 with n OH groups being attached thereto.

9. The laminating adhesive of claim 1, wherein the high functionality polyol has the following structure:
(HO).sub.q(R.sup.4)OC(O)R.sup.5C(O)OR.sup.6OC(O)R.sup.7C(O)O(R.sup.8)(OH).sub.r wherein q and r are integers which are the same or different and which each have a value of at least 1; q+r=at least 4; R.sup.4, R.sup.5, R.sup.7 and R.sup.8 are hydrocarbon radicals; and R.sup.6 is a divalent radical selected from hydrocarbon radicals and polyoxyalkylene radicals.

10. A flexible packaging material comprising two films bonded together by cured reaction products of the adhesive of claim 1.

11. A flexible packaging material comprising a first layer comprised of a first polyolefin or first polyester; a second layer comprised of a second polyolefin, which may be the same or different from the first polyolefin, a second polyester, which may be the same as or different from the first polyester, or a metal foil; and cured reaction products of the adhesive of claim 1 bonding the first layer to the second layer.

12. A method of making a flexible film laminate, comprising: combining Component A and Component B of the laminating adhesive of claim 1 at a predetermined weight ratio to form an adhesive mixture; disposing the adhesive mixture on at least a portion of one surface of a first flexible film, joining the first flexible film and a second flexible film wherein the adhesive mixture is interposed between the first flexible film and the second flexible film; and curing the adhesive mixture.

13. Cured reaction products of the two component adhesive of claim 1.

14. A method of making a flexible film laminate, comprising: combining Component A and Component B of the laminating adhesive of claim 1 at a predetermined weight ratio to form an adhesive mixture; disposing the adhesive mixture on at least a portion of one surface of a first flexible film; joining the first flexible film and a second flexible film wherein the adhesive mixture is interposed between the first flexible film and the second flexible film; and curing the adhesive mixture.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a graph showing improved pot-life of the disclosed robust adhesive compared to a conventional adhesive.

(2) FIG. 2 is a graph showing room temperature cure speed of the disclosed robust adhesive compared to a conventional adhesive.

(3) FIG. 3 is a graph showing room temperature cure speed and bond strength for the disclosed robust adhesive and a conventional adhesive at varying mix ratios.

(4) FIG. 4 is a graph showing elevated temperature cure speed and bond strength for the disclosed robust adhesive and a conventional adhesive at varying mix ratios.

DETAILED DESCRIPTION

(5) The disclosed robust flexible packaging adhesives comprise a substantially homogeneous mixture of Component A, an isocyanate-functionalized compound, and Component B, a mixture of polyol compounds. Components A and B are stored separately and mixed in a predetermined ratio just before use. Flexible packaging adhesives prepared using Components A and B are robust flexible packaging adhesives.

(6) Component A

(7) Component A contains at least one compound having two or more isocyanate groups per molecule. The isocyanate groups may be free NCO groups, but can also be blocked or masked NCO groups. One particular embodiment employs one or more isocyanate-functionalized polyurethane prepolymers in Component A. In the context of this disclosure a polyurethane prepolymer is a compound such as results, for example, from the reaction of a polyol component (or other active hydrogen-functionalized compound) with at least one isocyanate having a functionality of at least two. This reaction can take place without solvent or in a solvent. The term polyurethane prepolymer embraces not only compounds having a relatively low molecular weight, such as are formed, for example, from the reaction of a polyol with an excess of polyisocyanate, but also oligomeric or polymeric compounds. Perfect polyurethane prepolymers, containing a single polyol moiety capped at each end or terminus with a polyisocyanate moiety and very little, if any, free polyisocyanate monomer or oligomeric or polymeric compounds (containing two or more polyol moieties per molecule) may also be utilized.

(8) Molecular weight figures used in this document, unless otherwise indicated, refer to number average molecular weight (M.sub.n). The polyurethane prepolymers used in the context of the present disclosure generally may have a molecular weight of from 500 to 27,000, alternatively from 700 to 15,000, or alternatively from 700 to 8,000 g/mol.

(9) Likewise embraced by the term polyurethane prepolymers are compounds formed, for example, from the reaction of a trivalent or tetravalent polyol with a molar excess of diisocyanate, relative to the polyol. In this case one molecule of the resultant compound bears two or more isocyanate groups.

(10) Polyurethane prepolymers having isocyanate end groups are well known in the art. They can be crosslinked or chain-extended with suitable curing agentsusually polyfunctional alcoholsin a simple way to form substances of higher molecular weight.

(11) To obtain polyurethane prepolymers having terminal isocyanate groups it is customary to react polyfunctional alcohols with an excess of polyisocyanates, generally at least predominantly diisocyanates. In this case the molecular weight can be controlled at least approximately by way of the ratio of OH groups to isocyanate groups. While a ratio of OH groups to isocyanate groups of 1:1 or near to 1:1 often leads to substances with high molecular weights, it is the case with a ratio of approximately 1:2, for example, when using diisocyanates, that one diisocyanate molecule is attached on average to each OH group, so that in the course of the reaction, in the ideal case, there is no oligomerization or chain extension.

(12) Excess unreacted polyisocyanate monomer may optionally be removed from the polyurethane prepolymer reaction product initially obtained by any known method such as, for example, distillation to provide a prepolymer having a desirably low level of polyisocyanate monomer (e.g., less than 1 weight %).

(13) Polyurethane prepolymers are customarily prepared by reacting at least one polyisocyanate, preferably a diisocyanate, and at least one component having functional groups which are reactive toward isocyanate groups, generally a polyol component, which is preferably composed of diols. The polyol component may contain only one polyol, although it is also possible to use a mixture of two or more polyols as the polyol component. By a polyol is meant a polyfunctional alcohol, i.e., a compound having more than one OH group in the molecule.

(14) By functional groups which are reactive toward isocyanate groups are meant, in the context of the present text, functional groups which can react with isocyanate groups to form at least one covalent bond.

(15) Suitable reactive functional groups containing active hydrogen may be monofunctional in the sense of a reaction with isocyanates: OH groups or mercapto groups, for example. Alternatively, they may also be difunctional with respect to isocyanates (primary amino groups, for example). A molecule containing a primary amino group, accordingly, also has two functional groups which are reactive toward isocyanate groups. In this context it is unnecessary for a single molecule to have two separate functional groups that are reactive toward isocyanate groups. What is critical is that the molecule is able to connect with two isocyanate groups with the formation in each case of one covalent bond.

(16) As the polyol component of Component A it is possible to use a multiplicity of polyols. These are, for example, aliphatic alcohols having from 2 to 4 OH groups per molecule. The OH groups may be primary or secondary. Examples of suitable aliphatic alcohols include ethylene glycol, propylene glycol, butane-1,4-diol, pentane-1,5-diol, hexane-1,6-diol, heptane-1,7-diol, octane-1,8-diol and their higher homologs or isomers such as result in a formal sense from a stepwise extension of the hydrocarbon chain by one CH.sub.2 group in each case or with the introduction of branches into the carbon chain. Likewise suitable are higher polyfunctional alcohols such as, for example, glycerol, trimethylolpropane, pentaerythritol and also oligomeric ethers of said substances with themselves or in a mixture of two or more of said ethers with one another.

(17) As the polyol component it is additionally possible to use reaction products of low molecular weight polyfunctional alcohols with alkylene oxides, referred to as polyether polyols. The alkylene oxides have preferably 2 to 4 carbon atoms. Suitable examples are the reaction products of ethylene glycol, propylene glycol, the isomeric butanediols, hexanediols or 4,4-dihydroxy-diphenylpropane with ethylene oxide, propylene oxide or butylene oxide, or with mixtures of two or more thereof. Also suitable, furthermore, are the reaction products of polyfunctional alcohols, such as glycerol, trimethylolethane or trimethylolpropane, pentaerythritol, sugars or sugar alcohols, or mixtures of two or more thereof, with the stated alkylene oxides to form polyether polyols. Particularly suitable polyether polyols are those having a molecular weight from about 100 to about 10,000, preferably from about 200 to about 5,000. Likewise suitable as the polyol component are polyether polyols such as are formed, for example, from the polymerization of tetrahydrofuran.

(18) The polyether polyols may be synthesized using methods known to the skilled worker, by reaction of the starting compound having a reactive hydrogen atom with alkylene oxides: for example, ethylene oxide, propylene oxide, butylene oxide, styrene oxide, tetrahydrofuran or epichlorohydrin or mixtures of two or more thereof. Examples of suitable starting compounds are water, ethylene glycol, propylene 1,2-glycol or 1,3-glycol, butylene 1,4-glycol or 1,3-glycol, hexane-1,6-diol, octane-1,8-diol, neopentylglycol, 1,4-hydroxymethylcyclohexane, 2-methyl-1,3-propanediol, glycerol, trimethylolpropane, hexane-1,2,6-triol, butane-1,2,4-triol, trimethylolethane, pentaerythritol, mannitol, sorbitol, methylglycosides, sugars, phenol, isononylphenol, resorcinol, hydroquinone, 1,2,2- or 1,1,2-tris(hydroxyphenyl)ethane, ammonia, methylamine, ethylenediamine, tetra- or hexamethyleneamine, triethanolamine, aniline, phenylenediamine, 2,4- and 2,6-diaminotoluene and polyphenylpolymethylene-polyamines (such as are obtainable by aniline-formaldehyde condensation), or mixtures of two or more thereof.

(19) Likewise suitable for use as the polyol component are polyether polyols which have been modified by vinyl polymers. Products of this kind are available, for example, by polymerizing styrene or acrylonitrile, or a mixture thereof, in the presence of polyether polyols.

(20) Polyester polyols having a molecular weight of from about 200 to about 10,000 are likewise suitable as the polyol component. Thus, for example, it is possible to use polyester polyols formed by reacting low molecular weight alcohols, especially ethylene glycol, diethylene glycol, neopentyl glycol, hexanediol, butanediol, propylene glycol, glycerol or trimethylolpropane, with caprolactone. Likewise suitable as polyfunctional alcohols for preparing polyester polyols are 1,4-hydroxymethylcyclohexane, 2-methyl-1,3-propanediol, butane-1,2,4-triol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol, dibutylene glycol and polybutylene glycol.

(21) Further suitable polyester polyols are preparable by polycondensation. For instance, difunctional and/or trifunctional alcohols can be condensed with a substoichiometric amount of dicarboxylic acids and/or tricarboxylic acids, or their reactive derivatives, to form polyester polyols. Examples of suitable dicarboxylic acids are adipic acid or succinic acid and their higher homologs having up to 16 carbon atoms, unsaturated dicarboxylic acids such as maleic acid or fumaric acid, and also aromatic dicarboxylic acids, particularly the isomeric phthalic acids, such as phthalic acid, isophthalic acid or terephthalic acid. Examples of suitable tricarboxylic acids are citric acid or trimellitic acid. These acids may be used individually or as mixtures of two or more thereof. Particularly suitable in the context of this disclosure are polyester polyols formed from at least one of said dicarboxylic acids and glycerol which have a residual OH group content. Particularly suitable alcohols are hexanediol, ethylene glycol, diethylene glycol or neopentyl glycol or mixtures of two or more thereof. Particularly suitable acids are isophthalic acid or adipic acid or their mixtures.

(22) Polyester polyols of high molecular weight include, for example, the reaction products of polyfunctional alcohols, preferably difunctional alcohols (together where appropriate with small amounts of trifunctional alcohols) and polyfunctional carboxylic acids, preferably difunctional carboxylic acids. Instead of free polycarboxylic acids use may also be made (if possible) of the corresponding polycarboxylic anhydrides or corresponding polycarboxylic esters with alcohols having preferably 1 to 3 carbon atoms. The polycarboxylic acids may be aliphatic, cycloaliphatic, aromatic or heterocyclic or both. They may where appropriate be substituted, by alkyl groups, alkenyl groups, ether groups or halogens, for example. Examples of suitable polycarboxylic acids include succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, tetra-chlorophthalic anhydride, endomethylenetetrahydrophthalic anhydride, glutaric anhydride, maleic acid, maleic anhydride, fumaric acid, dimer fatty acid or trimer fatty acid or mixtures of two or more thereof. Where appropriate, minor amounts of monofunctional fatty acids may be present in the reaction mixture.

(23) The polyester polyols may, where appropriate, contain a small fraction of carboxyl end groups. Polyester polyols obtainable from lactones, -caprolactone for example, or hydroxycarboxylic acids, o-hydroxycaproic acid for example, may likewise be used.

(24) Polyacetals and polyester ether polyols are likewise suitable as the polyol component. By polyacetals are meant compounds obtainable from glycols reacted with aldehydes, for example, diethylene glycol or hexanediol or a mixture thereof condensed with formaldehyde. Polyacetals which can be used in the context of the disclosure may likewise be obtained by the polymerization of cyclic acetals.

(25) Further suitable polyols include polycarbonates. Polycarbonates can be obtained, for example, by reacting diols, such as propylene glycol, butane-1,4-diol or hexan-1,6-diol, diethylene glycol, triethylene glycol or tetraethylene glycol, or mixtures of two or more thereof, with diaryl carbonates, for example, diphenyl carbonate, or phosgene.

(26) Likewise suitable as the polyol component are polyacrylates which carry OH groups. These polyacrylates are obtainable, for example, by polymerizing ethylenically unsaturated monomers which carry an OH group. Monomers of this kind are obtainable, for example, by esterifying ethylenically unsaturated carboxylic acids and difunctional alcohols, the alcohol generally being present in a slight excess. Examples of ethylenically unsaturated carboxylic acids suitable for this purpose are acrylic acid, methacrylic acid, crotonic acid or maleic acid. Corresponding esters carrying OH groups are, for example, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate or 3-hydroxypropylmethacrylate or mixtures of two or more thereof.

(27) In addition to the aforedescribed polyol compounds, polyisocyanates are important building blocks of the polyurethane prepolymers which can be used in Component A. These include compounds of the general structure 0=CNXNCO, where X is an aliphatic, alicyclic or aromatic radical, such as an aliphatic or alicyclic radical having from 4 to 18 carbon atoms.

(28) As suitable polyisocyanates mention may be made, for example, of 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate (MDI), hydrogenated MDI (H.sub.12MDI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), 4,4-diphenyldimethylmethane diisocyanate, di- and tetraalkylenediphenylmethane diisocyanate, 4,4-dibenzyl diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, the isomers of tolylene diisocyanate (TDI), 1-methyl-2,4-diisocyanatocyclohexane, 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,4,4-trimethylhexane, 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethylcyclohexane (IPDI), chlorinated and brominated diisocyanates, phosphorus-containing diisocyanates, 4,4-diisocyanatophenylperfluoroethane, tetramethoxybutane 1,4-diisocyanate, butane 1,4-diisocyanate, hexane 1,6-diisocyanate (HDI), dicyclohexylmethane diisocyanate, cyclohexane 1,4-diisocyanate, ethylene diisocyanate, bisisocyanatoethyl phthalate and also diisocyanates having reactive halogen atoms, such as 1-chloromethylphenyl 2,4-diisocyanate, 1-bromomethylphenyl 2,6-diisocyanate, 3,3-bischloromethyl ether 4,4-diphenyl diisocyanate.

(29) Sulfur-containing polyisocyanates are obtained, for example, by reacting 2 mol of hexamethylene diisocyanate with 1 mol of thiodiglycol or dihydroxydihexyl sulfide. Further diisocyanates which can be used are, for example, trimethylhexamethylene diisocyanate, 1,4-diisocyanatobutane, 1,12-diiso-cyanatododecane and dimer fatty acid diisocyanate. Particularly suitable are the following: tetramethylene, hexamethylene, undecane, dodecamethylene, 2,2,4-trimethylhexane, 1,3-cyclohexane, 1,4-cyclohexane, 1,3- or 1,4-tetramethylxylene, isophorone, 4,4-dicyclohexylmethane and lysine ester diisocyanates. In one embodiment, tetramethylxylylene diisocyanate (TMXDI) is utilized as the polyisocyanate.

(30) Examples of suitable isocyanates having a functionality of at least three are the trimerization and oligomerization products of the polyisocyanates already mentioned above, such as are obtainable, with the formation of isocyanurate rings, by appropriate reaction of polyisocyanates, preferably of diisocyanates. Where oligomerization products are used, those particularly suitable have a degree of oligomerization of on average from about 3 to about 5.

(31) Isocyanates suitable for the preparation of trimers are the diisocyanates already mentioned above, particular preference being given to the trimerization products of the isocyanates HDI, MDI or IPDI.

(32) Likewise suitable for use are the polymeric isocyanates, such as are obtained, for example, as a residue in the distillation bottoms from the distillation of diisocyanates. Particularly suitable in this context is the polymeric MDI as is obtainable as, a distillation residue from the distillation of MDI.

(33) Component A preferably is formulated to have an isocyanate functionality of greater than 2. Use of a Component A having an isocyanate functionality of 2 or less is not likely to provide a robust laminating adhesive.

(34) Component A preferably is formulated to have a viscosity of not greater than about 10,000 cps (more preferably, not greater than about 5000 cps; most preferably, not greater than about 3500 cps) at 25 degrees C. and a viscosity of not greater than about 2500 cps (more preferably, not greater than about 2000 cps) at 60 degrees C.

(35) Component B

(36) Component B comprises a mixture of a high functionality polyester polyol containing at least four hydroxyl groups per molecule with at least two primary OH groups on the molecule and two secondary OH groups on the molecule; a trifunctional polyol containing three OH groups on the molecule; and a difunctional polyol containing two OH groups on the molecule.

(37) In one embodiment, the high functionality polyol contains two pairs of hydroxyl groups per molecule, wherein the hydroxyl groups within each pair are separated by two or three carbon atoms and the two pairs of hydroxyl groups are separated by at least eight atoms. The hydroxyl groups preferably are primary and/or secondary hydroxyl groups. In one embodiment, the high functionality polyol contains both primary and secondary hydroxyl groups. In another embodiment, the hydroxyl groups are attached to aliphatic carbon atoms.

(38) Limited testing indicates that use of high functionality polyether polyols alone and polyester ether polyols (sometimes also referred to as polyether ester polyols) alone as the Component B mixture does not generally result in the formation of a robust flexible packaging adhesive.

(39) Illustrative high functionality polyester polyols suitable for use may correspond to the following general structure (I):
(HO).sub.m(R.sup.1)OC(O)R.sup.2C(O)O(R.sup.3)(OH).sub.n
wherein m and n are integers which are the same or different and which each have a value of at least 1, m+n=at least 4, and R.sup.1, R.sup.2 and R.sup.3 are hydrocarbon radicals (preferably containing from 2 to 20 carbon atoms). R.sup.1 has a valency of m+1 (with m OH groups being attached thereto), R.sup.2 has a valency of 2, and R.sup.3 has a valency of n+1 (with n OH groups being attached thereto). The hydrocarbon radicals may be linear or branched, aliphatic cycloaliphatic, aromatic or aralkyl, saturated or unsaturated. For example, R.sup.1 and R.sup.3 may each be a CH.sub.2CHCH.sub.2 group. R.sup.2 may, for example, be a (CH.sub.2).sub.o moiety, where o is an integer of from 2 to 18.

(40) High functionality polyester polyols corresponding to the above-mentioned general structure (I) may be prepared by reacting a molar excess of one or more polyols bearing two (preferably three) or more hydroxyl groups per molecule with a dicarboxylic acid or dicarboxylic acid diester, for example. Suitable polyols for such purpose include glycerin, trimethylolpropane, trimethylolethane, pentaerythritol, sugar alcohols, sugars, glycosides and also oligomeric ethers of said substances with themselves or in a mixture of two or more of said ethers with one another. As the polyol component it is additionally possible to use reaction products of such polyols with alkylene oxides, referred to as polyether polyols. The alkylene oxides have preferably 2 to 4 carbon atoms. Suitable examples are the reaction products of polyols such as glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, sugars, glycosides or sugar alcohols, or mixtures of two or more thereof with ethylene oxide, propylene oxide or butylene oxide, or with mixtures of two or more such alkylene oxides. In some embodiments the high functionality polyether polyols have a molecular weight from about 100 to about 5,000, preferably from about 100 to about 1,000.

(41) The dicarboxylic acid reacted with the aforementioned polyol to form the high functionality polyester polyol may be any linear or branched, aliphatic, aromatic, alicyclic, saturated or unsaturated organic compound containing two carboxylic acid groups per molecule. In one embodiment, a linear aliphatic saturated dicarboxylic acid is employed such as, for example, succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, and the like. The dicarboxylic acid may, for example, correspond to the structure HOC(O)(CH.sub.2).sub.mC(O)OH, where n=2-18. Preferably, the polyol and the dicarboxylic acid are reacted at a molar ratio of about 2:1. The reaction may be carried out under conditions effective to remove the water formed as a result of the condensation between the hydroxyl groups of the polyol and the acid groups of the dicarboxylic acid. Catalysts may be employed to accelerate the rate of condensation.

(42) Other exemplary high functionality polyester polyols suitable for use in the disclosed robust adhesives have the following general structure (II):
(HO).sub.q(R.sup.4)OC(O)R.sup.5C(O)OR.sup.6OC(O)R.sup.7C(O)O(R.sup.5)(OH).sub.r
wherein q and r are integers which are the same or different and which each have a value of at least 1, q+r=at least 4, R.sup.4, R.sup.5, R.sup.7 and R.sup.8 are hydrocarbon radicals which may be the same or different (preferably containing from 2 to 20 carbon atoms), and R.sup.6 is a divalent radical selected from the group consisting of hydrocarbon radicals and polyoxyalkylene radicals. R.sup.4 has a valency of q+1 (with q OH groups being attached thereto), R.sup.5 and R.sup.7 each are divalent, and R.sup.8 has a valency of r+1 (with r OH groups being attached thereto). The hydrocarbon radicals may be linear or branched, aliphatic cycloaliphatic, aromatic or aralkyl, saturated or unsaturated. For example, R.sup.4 and R.sup.8 may each be a CH.sub.2CHCH.sub.2 group. R.sup.5, R.sup.6 and R.sup.7 may, for example, each be a (CH.sub.2).sub.o moiety, where o is an integer of from 2 to 18. R.sup.6 may alternatively be a polyoxyalkylene radical such as, for example, a radical corresponding to the structure [(CH.sub.2).sub.sCHR.sup.9O].sub.t(CH.sub.2).sub.uCHR.sup.10, wherein s and u are integers of 1 to 3, t is at least 1, and R.sup.9 and R.sup.10 are independently selected from the group consisting of H, methyl or ethyl (where R.sup.9 may be the same or different in each (CH.sub.2).sub.sCHR.sup.9O moiety when t is greater than 1). For instance, the polyoxyalkylene radical may be selected from the group consisting of polyoxyethylene radicals, polyoxypropylene radicals and polyoxytetramethylene radicals. High functionality polyester polyols of general structure (II) may generally be prepared by reacting a difunctional alcohol with a dicarboxylic acid so as to react each hydroxyl group of the alcohol with one molecule of the dicarboxylic acid. The remaining unreacted acid groups derived from the dicarboxylic acid are then reacted with one or more polyols containing three or more hydroxyl groups per molecule. The dicarboxylic acid and polyol may, for example, be any of the exemplary compounds discussed hereinabove in connection with the high functionality polyester polyols of general structure (I). The difunctional alcohol may be any monomeric, oligomeric, or polymeric compound containing two hydroxyl groups per molecule such as, for example, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, neopentyl glycol, 2-methyl-1,3-propanediol, 1,4-butanediol, polytetrahydrofuran diol, bis-phenol A, bis-phenol F, and the like. In one embodiment, a polyether glycol (in particular, a polypropylene glycol) having a molecular weight of from about 200 to about 3000 is utilized.

(43) Suitable high functionality polyols may also be prepared by esterification of a compound containing four or more carboxylic acid groups per molecule with a compound containing two hydroxyl groups per molecule (such as, for example, ethylene glycol, propylene glycol, diethylene glycol, butylene glycol, neopentyl glycol, 2-methyl-1,3-propanediol, 1,6-hexanediol and mixtures thereof) under conditions such that each of the carboxylic acid groups is reacted with a molecule of the compound containing two hydroxyl groups per molecule. Polycaprolactone polyols containing four or more hydroxyl groups per molecule, such as the tetrafunctional polycaprolactones sold under the trademark CAPA 4101 by Solvay S.A. may also be used as the high functionality polyol component.

(44) Component B can contain about 50 wt % to about 90 wt % high functionality polyol. Use of lower or higher amounts of high functionality polyol in component B may not provide the advantageous robust performance. In some embodiments the high functionality polyol can have a number average molecular weight of from about 400 to about 2,000.

(45) Component B can contain about 5 wt % to about 30 wt %, more preferably 10 wt % to about 19 wt % trifunctional polyol. Use of lower or higher amounts of trifunctional polyol in component B may not provide the advantageous robust performance. The trifunctional polyol or polyols can be trifunctional polyether polyols, for example based on polypropylene glycol. In some embodiments the trifunctional polyol can have a number average molecular weight of from about 90 to about 2,000, more preferably 90 to 1,000.

(46) Component B can contain about 0 wt % to about 20 wt %, for example about 5% to about 20% and advantageously about 13 wt % to about 17 wt % difunctional polyol. Use of lower or higher amounts of difunctional polyol in component B may not provide the advantageous robust performance. The difunctional polyol or polyols can be difunctional polyester polyols. Difunctional polyester polyols can be obtained by reacting dibasic acids or diester or anhydrides thereof such as adipic acid with glycols such as neopentyl glycol. Polycaprolactone polyols may also be used. In some embodiments the difunctional polyol can have a number average molecular weight of from about 20 to about 2000.

(47) The high functionality polyol(s), trifunctional polyol(s) and difunctional polyol(s) should be selected so as to be compatible with each other. That is, the mixture of the different polyols should be homogeneous in appearance and should not exhibit any tendency to phase separate at normal storage and use temperatures (e.g., about 15 C. to about 100 C.).

(48) The amounts of Component A and Component B used in the robust laminating adhesive systems of this invention will generally be adjusted so as to provide an NCO/active hydrogen equivalent ratio in the range of from about 1:1 to 10:1 in one embodiment of the invention, from about 1.05:1 to about 5:1 in another embodiment, and from about 1.1:1 to about 2:1 in yet another embodiment. Typically, the free isocyanate content (prior to any reaction between Component A and Component B) will be from about 1% to about 25% by weight based on the total weight of the two components adhesive. The weight ratio of Component A to Component B may vary within wide limits, with the optimum ratio being dependent upon the composition of each of Component A and Component B.

(49) The mixed adhesive will have a predetermined or on ratio weight ratio of Component A to Component B. The on ratio weight ratio of A:B in the mixed adhesive is from about 4:1 to about 1:5 (or, in one embodiment, about 1.6:1). Surprisingly, the adhesive is robust, e.g. the amount of Component A and Component B in the mixed adhesive can be varied by 25% from the on ratio and the adhesive will retain its properties.

(50) The mixture of Component A and Component B when first combined will have a viscosity of about 700 cps to about 5000 cps (more preferably, about 900 to about 2500 cps at application temperature. Mixed adhesive viscosities above 5,000 cps at application temperature are difficult or impossible to run on conventional laminating equipment. Typical application temperatures for flexible packaging lamination are about 35 C., although higher or lower application temperatures may be useful in some applications.

(51) Typically, the mixed adhesive will have a pot-life of at least about 25 minutes and more preferably at least about 30 minutes. The viscosity of the mixed adhesive does not increase above about two times the initial viscosity during the pot life after Component A and Component B are mixed and held at a temperature of 40 C.

(52) Where appropriate, in addition to Component A and Component B, the robust, two component laminating adhesive may comprise one or more further additives that are conventionally used in flexible packaging laminating adhesives. The additives may, for example, account for up to about 10% by weight of the overall two component adhesive. The additives may be in either of Components A and B. The optional additives which can be used in the context of the present disclosure include solvents, water, catalysts, curing agents, accelerators, plasticizers, stabilizers, antioxidants, light stabilizers, fillers, dyes, pigments, fragrances, preservatives or mixtures thereof.

(53) In one embodiment, Component B additionally contains up to about 15% by weight (in another embodiment, up to about 10% by weight) of one or more monomeric polyols containing two or three hydroxyl groups per molecule. Exemplary monomeric polyols include glycerol and trimethylolpropane.

(54) The film or films to be coated and adhered to each other using the robust, two component adhesive formulations may be comprised of any of the materials known in the art to be suitable for use in flexible packaging, including both polymeric and metallic materials as well as paper (including treated or coated paper). Thermoplastics are particularly preferred for use as at least one of the layers. The materials chosen for individual layers in a laminate are selected to achieve specific desired combinations of properties, e.g., mechanical strength, tear resistance, elongation, puncture resistance, flexibility/stiffness, gas and water vapor permeability, oil and grease permeability, heat sealability, adhesiveness, optical properties (e.g., clear, translucent, opaque), formability, merchantability and relative cost. Individual layers may be pure polymers or blends of different polymers. The polymeric layers are often formulated with colorants, anti-slip, anti-block, and anti-static processing aids, plasticizers, lubricants, fillers, stabilizers and the like to enhance certain layer characteristics.

(55) Particularly preferred polymers for use include, but not limited to, polyethylene (including low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HPDE), high molecular weight, high density polyethylene (HMW-HDPE), linear low density polyethylene (LLDPE), linear medium density polyethylene (LMPE)), polypropylene (PP), oriented polypropylene, polyesters such as poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT), ethylene-vinyl acetate copolymers (EVA), ethylene-acrylic acid copolymers (EAA), ethylene-methyl methacrylate copolymers (EMA), ethylene-methacrylic acid salts (ionomers), hydrolyzed ethylene-vinyl acetate copolymers (EVOH), polyamides (nylon), polyvinyl chloride (PVC), poly(vinylidene chloride) copolymers (PVDC), polybutylene, ethylene-propylene copolymers, polycarbonates (PC), polystyrene (PS), styrene copolymers, high impact polystyrene (HIPS), acrylonitrile-butadiene-styrene polymers (ABS), acrylonitrile copolymers (AN), polyamide (Nylon), polylactic acid (PLA), regenerated cellulose films (Cellophane).

(56) The polymer surface may be treated or coated, if so desired. For example, a film of polymer may be metallized by depositing a thin metal vapor such as aluminum onto the film's surface. Metallization may enhance the barrier properties of the finished laminate. The polymer film surface may also be coated with anti-fog additive or the like or subjected to a pretreatment with electrical or corona discharges, or ozone or other chemical agents to increase its adhesive receptivity. A coating of an inorganic oxide such as SiOx or AlOx may also be present on the polymer surface (for example, an SiOx- or AlOx-coated PET film).

(57) One or more layers of the laminate may also comprise a metal film or foil, such as aluminum foil, or the like. The metal foil will preferably have a thickness of about 5 to 100 m.

(58) The individual films comprising the laminates can be prepared in widely varying thicknesses, for example, from about 5 to about 200 microns. The films, foils, and laminating adhesive formulation can be assembled into the laminate by using any one or more of the several conventional procedures known in the art for such purpose. For instance, the adhesive formulation may be applied to the surface of one or both of two films/foils by means of extrusion, brushes, rollers, blades, spraying or the like and the film/foil surfaces bearing the adhesive composition brought together and passed through a set of rollers (often referred to as nip rollers) which press together the film/foils having the adhesive composition between the films/foils. The resulting laminate may be rolled or wound onto a reel for ageing. The adhesive may be applied by conventional techniques; e.g., by a multi-roll application station.

(59) One way of applying the robust adhesive composition to a substrate such as a film or foil is through the use of a series of smooth surface rubber and steel transfer rollers on a solventless adhesive laminator. The components of the adhesive are mixed using Meter/Mix/Dispense (MMD) equipment capable of automatically measuring and mixing the correct amounts of the components and delivering the resulting mixture to the laminator. The mixed adhesive is deposited on the first two rollers and metered by the remaining rollers in the application station (typically, 3 to 5 rollers). The flow characteristics of the adhesive composition may be improved by heating the first two rollers to a temperature of from about 35 to about 60 degrees C. Typically, the final application roller is heated to a temperature of from about 40 to about 60 degrees C. Modifications of these temperatures may be required depending upon line speed, substrates and roller size.

(60) The coating weight at which the adhesive formulation can be applied to the surface of a film layer is in the range of about 0.12 to about 3.1 lbs/3000 sq. ft, and more typically about 0.8 to about 1.4 lbs/3000 sq. ft.

(61) A second film or foil substrate is pressed against the substrate having the adhesive applied thereon by means of one or more nip rollers. Nip temperatures may be adjusted as needed depending upon line speed, thickness of the laminate, reactivity and other characteristics of the adhesive, and the substrates being laminated, but temperatures of from about 45 to about 90 C. are typically suitable.

(62) It may be desirable to heat the laminate at an elevated temperature (e.g., about 40 C. to about 100 C.) so as to accelerate full curing of the adhesive composition. Alternatively, the adhesive composition can be curable at approximately room temperature (e.g., about 20 C. to about 25 C.) or higher over a period of from about 1 to about 14 days.

(63) Generally speaking, the robust adhesive compositions are believed to be largely chemically cured through the reaction of the formulation constituents containing isocyanate groups and the constituents containing hydroxyl or other active hydrogen groups. However, curing can also be accomplished at least in part through moisture curing. Although sufficient moisture may be inherently present on the film or foil surfaces for this purpose, water may also be deliberately introduced through conventional methods if so desired.

(64) Laminates prepared in accordance with the present disclosure may be used for packaging purposes in the same manner as conventional or known flexible laminated packaging films. The laminates are particularly suitable for forming into flexible pouch-shaped container vessels capable of being filed with a foodstuff and sealed. For example, two rectangular or square sheets of the laminate may be piled in the desired configuration or arrangement; preferably, the two layers of the two sheets which face each other are capable of being heat-sealed to each other. Three peripheral portions of the piled assembly are then heat-sealed to form the pouch. Heat-sealing can easily be accomplished by means of a heating bar, heating knife, heating wire, impulse sealer, ultrasonic sealer, or induction heating sealer.

(65) The foodstuff is thereafter packed in the so-formed pouch. If necessary, gasses injurious to the foodstuff such as air are removed by known means such as vacuum degasification, hot packing, boiling degasification, or steam jetting or vessel deformation. The pouch opening is then sealed using heat. The packed pouch may be heated at a later time.

Example 1

(66) Component A-1: A blend containing 70% by weight of a TDI-based prepolymer obtained from Air Products (7.45% NCO) and 30% by weight of an MDI-based prepolymer obtained from Bayer Chemical Co. (22.9% NCO). Component A-1 has an NCO content of 12% by weight.

(67) Component A-2: A blend containing 92% by weight of an MDI-based prepolymer (16% NCO) obtained from Bayer Chemical Co. and 8% by weight of an aliphatic prepolymer obtained from Bayer Chemical Co. (22% NCO). Component A-2 has NCO content of 17% by weight.

(68) Component B-1: A blend containing 31.8% by weight of a tetrafunctional polyester polyol (TFPP-1), 51.8% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110) and 16.4% by weight of a difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=210). TFPP-1 has a hydroxyl number of 740 and is made by reacting two moles of glycerin with one mole of adipic acid.

(69) Component B-2: A blend containing 31.8% by weight TFPP-1, 51.8% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110), and 16.4% by weight of a difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=55).

(70) Component B-3: A blend containing 50% by weight TFFP-1, 20% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110), and 30% by weight of a difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=55).

(71) Component B-4: A blend of 50% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110) and 50% by weight of a difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=55).

(72) Component B-5: A blend of 20% by weight TFPP-1, 40% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110) and 40% by weight of a difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=55).

(73) Component B-6: A blend of 96% by weight difunctional polyester polyol obtained from Bayer Chemical Company having a hydroxyl number of 210 and 4% trimethylolpropane (a trifunctional polyol).

(74) Component B-7: A blend of 33% by weight of a tetrafunctional polyester polyol (TFPP-2), 61.3% by weight of a difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110), and 5.7% by weight of trimethylolpropane. TFPP-2 has a hydroxyl number of 158 and is prepared by reacting 2 moles of glycerin, 1 mole of polypropylene glycol having a number average molecular weight of 1025 and 2 moles of adipic acid.

(75) Component B-8: A blend of 5 parts by weight pentaerythritol, 6 parts by weight trimethylolpropane and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110). The pentaerythritol did not dissolve in the other components of the blend even after heating 1 hour at 150 degrees C.; Component B-8 was not further evaluated.

(76) Component B-9: A blend of 5 parts by weight pentaerythritol, 6 parts by weight glycerol, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110). The pentaerythritol did not dissolve in the other components of the blend even after heating 1 hour at 150 degrees C.; Component B-9 was not further evaluated.

(77) Component B-10: A blend of 35 parts by weight pentaerythritol ethoxylate (3 moles EO/mole pentaerythritol, obtained from Aldrich), 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110). The components of the blend separated; Component B-10 was not further evaluated.

(78) Component B-11: A blend of 35 parts by weight pentaerythritol ethoxylate (15 moles EO/mole pentaerythritol, obtained from Aldrich), 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110). The components of the blend separated; Component B-11 was not further evaluated.

(79) Component B-12: A blend of 35 parts by weight pentaerythritol propoxylate (5 moles PO/mole pentaerythritol, obtained from Aldrich), 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110).

(80) Component B-13: A blend of 35 parts by weight pentaerythritol propoxylate (17 moles EO/mole pentaerythritol, obtained from Aldrich), 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110).

(81) Component B-14: A blend of 35 parts by weight pentaerythritol ethoxylate/propoxylate (obtained from Aldrich), 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and ortho-phthalic acid (hydroxyl number=110). The components of the blend separated; Component B-14 was not further evaluated.

(82) Component B-15: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, and 65 parts by weight polypropylene glycol having a molecular weight of about 2000 (hydroxyl no.=55.4).

(83) Component B-16: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol derived from diethylene glycol and adipic acid (hydroxyl number=55).

(84) Component B-17: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, and 65 parts by weight trifunctional polyether polyol prepared from propylene oxide and a trifunctional starter molecule.

(85) Component B-18: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, and 65 parts by weight polypropylene glycol (hydroxyl no.=264).

(86) Component B-19: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, and 65 parts by weight difunctional polyester polyol obtained from diethylene glycol and adipic acid (hydroxyl no.=210).

(87) Component B-20: A blend of 35 parts by weight TFPP-2, 6 parts by weight trimethylolpropane, 32.5 parts by weight difunctional polyester polyol obtained from diethylene glycol and adipic acid (hydroxyl no.=210), and 32.5 parts by weight polyester polyol obtained from diethylene glycol and adipic acid (hydroxyl no.=55).

(88) Component B-21: A blend of 35 parts by weight TFPP-2, 6 parts by weight glycerol, and 65 parts by weight difunctional polyester polyol obtained from diethylene glycol and adipic acid (hydroxyl no.=210).

(89) Component A-3: A blend containing 97% by weight of MDI-based prepolymer (16% NCO) and 3% by weight of an aliphatic prepolymer (22% NCO) available from Henkel Corporation as Tycel 7660.

(90) Component B-22: A blend containing 95% by weight of difunctional polyester polyol obtained from Bayer Chemical Company having a hydroxyl number of 210 and 5% by weight of trimethylolpropane (TMP).

(91) Component B-23: A blend containing 72% by, weight of a tetrafunctional polyester polyol (TFPP-2); 15% by weight difunctional polyol based on neopentyl glycol and adipic acid; 11% by weight trifunctional polypropyleneglycol; and 2% by weight adhesion promoter (Silquest A-187).

(92) Pot-Life

(93) Components A-3 and B-22 were mixed at about a 1.7:1 ratio by weight to form a laminating adhesive. Components A-3 and B-23 were mixed at about a 1.6:1 ratio by weight to form a robust laminating adhesive. The viscosity of each mixed adhesives was checked initially and at 5 minute intervals. The results are shown in the following Table below and in FIG. 1.

(94) TABLE-US-00001 TABLE 1 A-3 and B-22 comparative A-3 and B-23 minutes adhesive robust adhesive 0 1000 1375 5 1150 1375 10 1500 1563 15 2100 1813 20 2800 2188 25 3650 2500 30 4650 2813 35 5700 3188 40 7000 3563 45 8500 4000 50 10500 4438 55 12500 4938 60 14700 5500

(95) As shown in this Table and FIG. 1 the pot-life of the robust system is almost 30 minutes. The non-robust comparative adhesive even at the predetermined on-ratio mixture is less than 15 minutes for the. As shown by FIG. 1 the comparative adhesive appears to cure at a much faster rate than the robust adhesive. The comparative adhesive reaches a viscosity of 5000 cps in less than 35 minutes. It takes almost 60 minutes for the robust adhesive to reach a viscosity of 5000 cps, which makes the robust adhesive easy to handle in laminating operations and easy to clean from laminating equipment.

(96) Cure Speed and Bond Strength

(97) Given the long pot-life of the robust adhesive a skilled person would expect that it would also cure more slowly and consequently develop strength more slowly, than the conventional adhesive. One way to measure the cure speed is to measure the bond strength of a laminate over time.

(98) Components A-3 and B-22 were mixed at a 1.7:1 ratio by weight to form a conventional laminating adhesive. Components A-3 and B-23 were mixed at a 1.6:1 ratio by weight to form a robust laminating adhesive.

(99) A plurality of 1 inch strips of 48 gauge polyethylene terephthalate were bonded to a 1 inch strips of polyethylene using each adhesive. The samples were cured at room temperature. Samples cured for 24, 48 and 72 hours were placed in a tensile tester in a T peel test configuration. In some cases the tensile test was done at 70 C. in a temperature controlled environment.

(100) As shown by FIG. 2, the robust adhesive surprisingly develops bond strength as fast as the comparative adhesive. In both cases testing showed failure of the strips and not of the adhesive at all times.

(101) Room Temperature Cure Speed and Bond Strength at Varying Mix Ratios

(102) Components A-3 and B-22 were mixed at 1.2:1; 1.7:1 and 2:1 ratios by weight to form a series of comparative laminating adhesives. Components A-3 and B-23 were mixed at 1.2:1; 1.6:1 and 2:1 ratios by weight to form a series of robust laminating adhesives.

(103) A plurality of 1 inch strips of 48 gauge polyethylene terephthalate were bonded to a 1 inch strips of polyethylene using each adhesive. The samples were cured at room temperature (about 20-25 C.) and the force in grams required to pull the sample apart was measured at 24, 48 and 72 hours. In all cases testing showed failure of the strips and not of the adhesive.

(104) In FIG. 3 the results for each cure time are arranged left to right in order of A-3/B22 (not robust) at 1.2:1; 1.7:1 and 2:1 ratios then A-3/B-23 (robust) at 1.2:1; 1.6:1 and 2:1 ratios. As shown by FIG. 3 all samples had acceptable room temperature bond strength at 24 hours cure. it appears that a mix ratio variation of 25% does not affect the bond strength of either conventional or robust adhesives at room temperature.

(105) Elevated Temperature Cure Speed and Bond Strength at Varying Mix Ratios

(106) Components A-3 and B-22 were mixed at 1.2:1; 1.7:1 and 2:1 ratios by weight to form a series of comparative laminating adhesives. Components A-3 and B-23 were mixed at 1.2:1; 1.6:1 and 2:1 ratios by weight to form a series of robust laminating adhesives.

(107) A plurality of 1 inch strips of 48 gauge polyethylene terephthalate were bonded to a 1 inch strips of polyethylene using each adhesive. The samples were cured and the force in g/in required to pull the sample apart at 70 C. (158 F.) was measured at 24, 48 and 72 hours. In some cases testing showed failure of the strips and in other cases testing showed failure of the adhesive.

(108) A skilled person would expect that an excess of polyol will lower heat resistance and elevated temperature bond strength of polyurethane adhesives. In FIG. 4 the results for each cure time are arranged left to right in order of A-3/B22 (not robust) at 1.2:1; 1.7:1 and 2:1 ratios then A-3/B-23 (robust) at 1.2:1; 1.6:1 and 2:1 ratios. FIG. 4 illustrates this expectation, showing the much lower elevated temperature bond strength for the comparative adhesive having excess polyol (mix ratio of 1.2:1) in the adhesive. Very surprisingly, FIG. 4 shows that laminates made with robust adhesive were not affected by shifting the mix ratio by adding an additional 25 wt % polyol.

(109) Boil Test Bond Strength at Varying Mix Ratios

(110) An even more severe heat resistance test for polyurethane laminating adhesives is the so-called boil test.

(111) Components A-3 and B-22 were mixed at 1.2:1; 1.7:1 and 2:1 ratios by weight to form a series of comparative laminating adhesives. Components A-3 and B-23 were mixed at 1.2:1; 1.6:1 and 2:1 ratios by weight to form a series of robust laminating adhesives.

(112) Flexible packaging material is made by laminating multiple layers of film with a test adhesive. Pouches are made from the flexible packaging material. After a predetermined cure period the pouches are filled with water, sealed and boiled for 30 minutes. This test subjects the laminating adhesive to both elevated temperatures and elevated pressures to simulate conditions a flexible package might encounter during use. Test results are shown in the following Table.

(113) TABLE-US-00002 TABLE 2 Boil Test results 24 hr cure 48 hr cure 72 hr cure A-3 and B-22 comparative (1.2:1) fail fail fail A-3 and B-22 comparative (1.7:1) pass ND.sup.1 ND.sup.1 A-3 and B-22 comparative (2:1) pass ND.sup.1 ND.sup.1 A-3 and B-23 robust (1.2:1) pass ND.sup.1 ND.sup.1 A-3 and B-23 robust (1.6:1) pass ND.sup.1 ND.sup.1 A-3 and B-23 robust (2:1) pass ND.sup.1 ND.sup.1 ND.sup.1 Not tested but expected to pass.

(114) As discussed above it is expected that an excess of polyol will lower heat resistance and elevated temperature bond strength of polyurethane adhesives. The boil test results show that this is true for non-robust comparative adhesives having excess polyol (mix ratio of 1.2:1). Very surprisingly, the boil test results show that pouches made from flexible packaging material bonded with robust adhesive have sufficient hot strength to pass the boil test at all mix ratios, even when the adhesive had excess polyol component (mix ratio of 1.2:1). Even more surprising is the ability of the robust adhesive at this mix ratio to pass the boil test after only 24 hours of curing.

(115) Migratory Primary Aromatic Amines

(116) FDA compliance requires that the concentration of primary aromatic amines in a flexible packaging material used for food contact be below the detection limit (2 parts per billion (ppb) when tested by the migration test, also referred to as the BfR test method). Excess isocyanates in a laminating adhesive can react with moisture in packaged products to form primary aromatic amines. Thus, while the use of excess isocyanates may increase bond strength it can also lead to excessive primary aromatic amines and prolonged curing time for the finished packaging material to avoid failure under the photometric migration test.

(117) Components A-3 and B-22 were mixed at 1.2:1; 1.7:1 and 2:1 ratios by weight to form a series of non-robust comparative laminating adhesives. Components A-3 and B-23 were mixed at 1.2:1; 1.6:1 and 2:1 ratios by weight to form a series of robust laminating adhesives.

(118) Flexible packaging material is made by laminating multiple layers of film with a test adhesive. Pouches are made from the flexible packaging material. After a predetermined cure period the pouches were tested using the BfR test method. Test results are shown in the following Table.

(119) TABLE-US-00003 TABLE 3 BfR test method results (<2 ppb to pass) 24 hr cure 48 hr cure 72 hr cure A-3 and B-22 comparative (1.2:1) ND.sup.1 A-3 and B-22 comparative (1.7:1) ND.sup.1 fail pass A-3 and B-22 comparative (2:1) ND.sup.1 ND.sup.1 ND.sup.1 A-3 and B-23 robust (1.2:1) pass pass A-3 and B-23 robust (1.6:1) pass pass A-3 and B-23 robust (2:1) ND.sup.1 fail pass ND.sup.1 Not tested but expected to fail.

(120) The results confirm that an excess of isocyanate leads to the presence of primary aromatic amines for longer periods of time. The comparative laminating adhesive produced by mixing A-3 and B-22 at standard (1.7:1) mix ratios required curing for 72 hours to fully react isocyanates and provide acceptable test results. Other comparative polyurethane laminating adhesives typically require curing for more than 72 hours to fully react isocyanates and provide acceptable test results. The robust laminating adhesive cures more quickly and at standard on-ratio (1.6:1) mix ratio passes the photometric migration test in only 48 hours, a 24 hour reduction compared to the comparative laminating adhesive. Even with an excess of isocyanate (2:1 mix ratio or 25 wt % excess) the robust adhesive produces laminates which pass the BfR migration test in only 72 hours, a result which is not possible with the comparative adhesive having an excess of isocyanate (2:1 mix ratio or 25 wt % excess).

(121) Resistance Against Solvents

(122) As mentioned earlier to avoid loss of bond strength the recommended limit for mono-alcohols in a flexible packaging material application is very low (<3,900 mg/ream or less). The inventors added different amounts of Dowanol PM in a range of 4,000 mg/ream up to 16,000 mg/ream to adhesive mixtures of Components A-3 and B-23. Surprisingly, there were no negative effects on heat resistance of the robust adhesive even at the higher amounts. All pouches made with the robust adhesive provided acceptable bond strength using the boil test after curing for 24 hours.

Example 2

(123) The following compositions were prepared.

(124) Component A-4: MDI-based prepolymer (NCO=12%) prepared by reacting 4,4-MDI with blend of a polyester polyol based on adipic acid, isophthalic acid, and diethylene glycol, and a polypropyleneglycol with an average molecular weight of 1025.

(125) Component B-24: A tetrafunctional polyester polyol with a hydroxyl number of 158, which is prepared by reacting 2 moles of glycerin, 1 mole of polypropylene glycol having an average molecular weight of 1025 and 2 moles of adipic acid.

(126) Component B-25: A blend containing 82.7% by weight of TFPP-2 and 17.3% by weight of a difunctional polypropyleneglycol with an average molecular weight of 1025. The blend has a hydroxyl number of 160.

(127) Component B-26: A blend containing 86.3% by weight of TFPP-2 and 13.7% by weight of a trifunctional polypropyleneglycol with an average molecular weight of 260. The blend has a hydroxyl number of 216.

(128) Component B-27: A blend containing 82.7% by weight of TFPP-2 and 17.3% by weight of a difunctional polyol based on neopentyl glycol and adipic acid. The blend has a hydroxyl number of 150.

(129) Component B-28: A blend of 43.1% of a trifunctional polypropyleneglycol with an average molecular weight of 260 and 56.9% of a bisphenol A ethoxylate. The mixture has a hydroxyl number of 440.

(130) Selected components were mixed to form a laminating adhesive. The mixtures were tested for pot-life. The mixtures were used as previously described to form a flexible packaging material. The prepared flexible packaging material was tested as previously described for high temperature bond strength after 24 hours and 96 hours of cure; and for migration, e.g. BfR test, after 3 days of cure. Results are summarized in the following Tables.

(131) TABLE-US-00004 TABLE 4 non-robust laminating adhesives Isocyanate material 2 2 2 3 3 3 Polyol material 7 7 7 22 22 22 mol ratio n ratio n ratio 25% 1.6:1 25% 25% 1.6:1 25% Pot - Life 0 minutes 250 100 000 119 70 60 5 489 269 269 089 69 69 10 969 639 629 319 279 249 15 539 999 019 779 709 609 20 189 489 479 389 219 049 25 879 049 989 109 839 549 30 659 639 539 969 549 3119 35 518 269 099 958 359 3779 40 508 968 718 128 288 4539 45 588 748 428 478 6368 5418 50 858 648 198 058 7618 6408 55 0000 628 018 9038 7508 60 648 918 8788 pot-life 5 7 5 8 15 16 (doubling) pot-life 2 0 2 35 38 42 (>5000 cP) Bond strength 70 C. (g) 24 hr cure 225 910 815 390 860 815 4 day cure 525 970 835 615 865 865 Primary aromatic amines detected after 3 days No No No No No No

(132) All of the above laminating adhesives unacceptably doubled in viscosity in 15 to 18 minutes at all ratios. Each of the above laminating adhesives also had a very short pot-life to 5,000 cP at all ratios. None of the above laminating adhesives are robust.

(133) TABLE-US-00005 TABLE 5 non-robust and robust laminating adhesives Isocyanate material A-4 A-4 A-4 A-1 A-1 A-1 Polyol material B-23 B-23 B-23 B-23 B-23 B-23 mol ratio non-robust robust on ratio on ratio 25% 1.6:1 +25% 25% 1.6:1 +25% Pot - Life 0 minutes 1500 1625 1750 1039 1189 1209 5 1250 1125 1313 889 1059 1019 10 1500 1250 1438 999 1119 1039 15 1938 1500 1750 1199 1309 1129 20 2625 1938 2188 1449 1519 1299 25 3250 2375 2688 1669 1739 1469 30 4063 2875 3188 1859 1969 1649 35 4938 3438 3875 2069 2139 1839 40 5938 4063 4500 2299 2389 2059 45 7125 4813 5250 2539 2679 2289 50 8375 5625 6000 2839 2969 2539 55 9750 6563 7000 3109 3309 2819 60 11380 7625 8000 3469 3659 3109 pot-life 25 35 33 25 30 35 (doubling) pot-life 36 46 43 78 77 87 (>5000 cP) Bond strength 70 C. (g) 24 hr cure 140 820 975 550 730 785 4 day cure 95 740 870 710 720 700 Primary aromatic amines detected after 3 days No No No No No No

(134) The A-1/B-23 system maintained properties over the entire low, on ratio and high mix ratio range illustrating that the A-1/B-23 laminating adhesive is robust.

(135) The A-4/B-23 system had unacceptable bond strength for a flexible packaging laminating adhesive at the lower ratio but acceptable bond strength on ratio and at the higher ratio. This lack of acceptable bond strength over the entire low to high mix ratio range illustrates a non-robust laminating adhesive system.

(136) TABLE-US-00006 TABLE 6 robust laminating adhesives Isocyanate material A- 3 A-3 A-3 A-2 A-2 A-2 Polyol material B-23 B-23 B-23 B-23 B-23 B-23 mol ratio on ratio on ratio 25% 1.6:1 +25% 25% 1.6:1 +25% Pot - Life 0 minutes 875 1000 938 1059 949 849 5 813 875 750 1039 959 869 10 937 938 875 1089 1029 979 15 1125 1125 1063 1289 1209 1129 20 1250 1438 1250 1509 1419 1299 25 1500 1688 1438 1749 1629 1489 30 1688 2000 1688 1999 1849 1699 35 1938 2313 1938 2259 2089 1899 40 2188 2688 2125 2519 2319 2129 45 3413 3063 2438 2809 2569 2329 50 2750 3500 2750 3119 2839 2579 55 3063 3938 3063 3359 3129 2839 60 3375 4438 3375 3749 3459 3129 pot-life 31 30 33 34 30 30 (doubling) pot-life 80 65 80 75 81 87 (>5000 cP) Bond strength 70 C. (g) 24 hr cure 285 890 840 275 915 905 4 day cure 360 875 830 660 910 710 Primary aromatic amines detected after 3 days No No No No No No

(137) Both the A-3/B-23 and A-2/B-23 laminating adhesive compositions had a pot-life (time to initial viscosity doubling) of 30 minutes or more at low, normal and high mix ratios. Both the A-3/B-23 and A-2/B-23 laminating adhesive compositions had acceptable hot (70 C.) bond strength for flexible packaging lamination use after both 24 and 96 hour cure. Neither of the A-3/B-23 and A-2/B-23 laminating adhesive compositions had any detectable leaching of primary amines. Both the A-3/B-23 and A-2/B-23 laminating adhesive compositions were robust.

(138) TABLE-US-00007 TABLE 7 non-robust laminating adhesives Isocyanate material A-2 A-2 A-2 A-2 A-2 A-2 Polyol material B-24 B-24 B-24 B-25 B-25 B-25 mol ratio on ratio on ratio 25% 1.6:1 +25% 25% 1.6:1 +25% Pot - Life 0 minutes 1500 1250 1250 1019 929 879 5 1188 1063 1000 939 899 839 10 1125 1125 1000 939 919 909 15 1250 1250 1125 1029 1029 1019 20 1375 1438 1250 1139 1159 1129 25 1563 1625 1375 1249 1309 1249 30 1688 1750 1500 1339 1419 1369 35 1875 1938 1625 1449 1519 1509 40 2000 2125 1750 1569 1639 1889 45 2250 2375 1875 1689 1769 2009 50 2375 2563 2000 1809 1909 2149 55 2625 2813 2188 1949 2049 60 2813 3000 2375 2079 2199 pot-life >60 49 >60 57 47 38 (doubling) pot-life >60 >60 >60 >60 >60 >60 (>5000 cP) Bond strength 70 C. (g) 24 hr cure 70 810 750 0 280 860 4 day cure 250 825 840 105 740 810 Primary aromatic amines detected after 3 days No No No No No No

(139) The A-2/B-24 and A-2/B-25 laminating adhesive compositions both had acceptable pot-life at all ratios. Neither of the laminating adhesive compositions had any detectable leaching of primary amines at any ratio. The A-2/B-24 and A-2/B-25 laminating adhesive compositions both had unacceptable hot strength (70 C.) at the low isocyanate ratio. This lack of adhesive bond strength over the entire low to high mix ratio range illustrates a non-robust laminating adhesive system.

(140) TABLE-US-00008 TABLE 8 non-robust laminating adhesives Isocyanate material A-2 A-2 A-2 A-2 A-2 A-2 Polyol material B-27 B-27 B-27 B-28 B-28 B-28 mol ratio on ratio on ratio 25% 1.6:1 +25% 25% 1.6:1 +25% Pot - Life 0 minutes 1312 1125 1125 839 779 709 5 1125 1000 938 979 859 739 10 1250 1125 1063 1539 1159 949 15 1438 1313 1188 2479 1649 1269 20 1688 1500 1375 3789 2319 1699 25 1938 1688 1563 5448 3159 2209 30 2188 1938 1750 7668 4199 2809 35 2438 2125 2000 >10K 5458 3489 40 2688 2375 2188 028 4299 45 3000 2688 2438 8988 5038 50 3313 2938 2688 >10K 6108 55 3625 3188 2938 7378 60 4000 3500 3250 8848 pot-life 39 36 41 11 14 17 (doubling) pot-life 74 64 >60 22 32 44 (>5000 cP) Bond strength 70 C. (g) 24 hr cure 95 890 835 420 1000 760 4 day cure 255 810 745 650 910 750 Primary aromatic amines detected after 3 days No No No Yes No No

(141) The A-2/B-27 laminating adhesive composition had acceptable pot-life over all ranges. The A-2/B-28 laminating adhesive composition had unacceptable pot-life at any ratio. The A-2/B-27 laminating adhesive composition had unacceptable hot (70 C.) adhesive bond strength at the low range but acceptable hot bond strength in the on ratio and high ranges. The A-2/B-28 laminating adhesive composition had acceptable hot (70 C.) adhesive bond strength at all ranges. The A-2/B-27 laminating adhesive composition had no detectable leaching of primary amines at any ratio. The A-2/B-28 laminating adhesive composition had detectable leaching of primary amines at the low ratio. The lack of pot-life at any range is unacceptable for a laminating adhesive. The unacceptable adhesive strength over the entire low to high mix ratio range illustrates that the A-2/B-27 laminating adhesive composition is not robust. Leaching of primary amines at the low ratio illustrates that the A-2/B-28 laminating adhesive composition is not robust.

(142) TABLE-US-00009 TABLE 9 robust laminating adhesive Isocyanate material A-2 A-2 A-2 Polyol material B-26 B-26 B-26 mol ratio on ratio 25% 1.6:1 +25% Pot-Life 0 minutes 1375 1375 1063 5 1000 1000 813 10 1063 938 875 15 1250 1063 1000 20 1500 1250 1188 25 1750 1437 1313 30 2000 1688 1500 35 2313 1938 1688 40 2688 2188 1938 45 3000 2438 2188 50 3375 2750 2438 55 3813 3063 2688 60 4250 3375 2938 pot-life (doubling) 41 50 44 pot-life (>5000 cP) 67 80 89 Bond strength 70 C. (g) 24 hr cure 315 850 700 4 day cure 690 810 820 Primary aromatic amines detected after 3 days No No No

(143) The A-2/B-26 laminating adhesive composition had acceptable pot-life throughout the low, on ratio and high mix ratio ranges. The A-2/B-26 laminating adhesive composition had acceptable hot (70 C.) adhesive strength throughout the low, on ratio and high mix ratio range. The A-2/B-26 laminating adhesive composition did not leach primary amines at any ratio in the low, on ratio and high range. Retention of properties throughout the low, on ratio and high mix ratio range illustrates that the A-2/B-26 laminating adhesive is robust.

(144) Every two part polyurethane adhesive is not suitable for use as a flexible packaging laminating adhesive. Many two part polyurethane adhesives are not capable of application using lamination equipment conventionally used for producing flexible packaging material or do not have cured properties such as pot life or hot adhesive bond strength suitable for use in flexible packaging or migrate undesirable components into the food stuff contained in the flexible packaging.

(145) Further, the results show that every flexible packaging laminating adhesive is not robust, e.g. does not retain properties throughout a range of excess isocyanate to excess polyol mix ratios. The robust laminating adhesives described herein are a selection of components that surprisingly retain properties throughout a range of excess isocyanate to excess polyol mix ratios.