Polyurethane Hot Melt Adhesive for Low Temperature Application

Abstract

Disclosed is a moisture reactive polyurethane hot melt adhesive composition that can be applied to substrates at a low coating weight even at application temperatures as low as 60 to 95° C. while retaining the advantageous high tack, green strength, cured bond strength and toughness of conventional high melting point polyurethane hot melt adhesives.

Claims

1. A moisture curable, polyurethane, hot melt adhesive comprising: an isocyanate functional prepolymer that is the reaction product of a mixture comprising: 3 to 30 wt. % of an amorphous polyester polyol having a Tg greater than 0° C.; 10 to 40 wt. % of at least one crystalline polyester polyol, wherein the crystalline polyester polyol has a melting point in the range of 20° C. to 100° C.; wherein the weight ratio of crystalline polyester polyol:amorphous polyester polyol is in the range of 2:1 to 1:2; 10 to 30 wt. % of a diisocyanate; 5 to 50 wt. % of at least one polyether polyol; catalyst; either 0.1 to 3 wt. % of a polyisocyanate having a functionality greater than 2 or 0.1 to 10 wt. % of a thermoplastic polyurethane having hydroxyl functionality; optionally one or more additives wherein the hot melt adhesive has a green strength of at least 0.4 psi or more 5 minutes after application of the hot melt adhesive to a substrate at a temperature of 60 to 95° C. and a coating weight of 10 gsf or less.

2. The moisture curable, polyurethane, hot melt adhesive of claim 1, further comprising a second amorphous polyester polyol having a Tg in the range of −60° C. to 0° C.

3. The moisture curable, polyurethane, hot melt adhesive of claim 1, further comprising a second crystalline polyester polyol different from the first crystalline polyester polyol and having a melting point in the range of 20° C. to 100° C.

4. The moisture curable, polyurethane, hot melt adhesive of claim 1, further comprising a second polyether polyol different than the at least one polyether polyol.

5. The moisture curable, polyurethane, hot melt adhesive of claim 1, comprising 10 to 30 wt. % of the additives.

6. The moisture curable, polyurethane, hot melt adhesive of claim 1, being free of organometallic catalysts.

7. The moisture curable, polyurethane, hot melt adhesive of claim 1, wherein the weight ratio of crystalline polyester polyol:amorphous polyester polyol is in the range of 2:1 to 1:2.

8. The moisture curable, polyurethane, hot melt adhesive of claim 1, wherein the hot melt adhesive has a Lauan board to vinyl foil green strength of 1.0 psi or more.

9. The moisture curable, polyurethane, hot melt adhesive of claim 1, wherein the hot melt adhesive has an Azdel board to vinyl foil green strength of 0.4 psi or more.

10. The moisture curable, polyurethane, hot melt adhesive of claim 1, comprising 0.1 to 3 wt. % of the polyisocyanate having a functionality greater than 2 and 0.1 to 10 wt. % of the thermoplastic polyurethane having hydroxyl functionality; wherein the hot melt adhesive has an Azdel board to vinyl foil green strength of 0.8 psi or more.

11. The moisture curable, polyurethane, hot melt adhesive of claim 1, comprising 0.1 to 3 wt. % of the polyisocyanate having a functionality greater than 2 and 0.1 to 10 wt. % of the thermoplastic polyurethane having hydroxyl functionality.

12. Cured reaction products of the moisture curable, polyurethane, hot melt adhesive of claim 1.

13. An article comprising the moisture curable, polyurethane, hot melt adhesive of claim 1.

14. An article comprising cured reaction products of the moisture curable, polyurethane, hot melt adhesive of claim 1.

15. (canceled)

16. The moisture curable, polyurethane, hot melt adhesive of claim 1, having a maximum viscosity of 12,000 cps at 95° C.

17. The moisture curable, polyurethane, hot melt adhesive of claim 1, having a maximum viscosity of 5,000 cps at 95° C.

18. The moisture curable, polyurethane, hot melt adhesive of claim 1, having a coating weight of 10 grams per square foot (gsf) or less when heated to 60 to 95° C.

19. A method of coating a substrate, comprising: providing a first substrate having a coating surface; providing the moisture curable, polyurethane, hot melt adhesive of claim 1; heating the moisture curable, polyurethane, hot melt adhesive to a temperature in the range of 60 to 95° C.; and coating the heated moisture curable, polyurethane, hot melt adhesive onto the first substrate coating surface at a coating weight of 10 grams per square foot (gsf) or less.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0021] Unless specifically noted, throughout the present specification and claims the term molecular weight when referring to a polymer refers to the polymer's number average molecular weight (Mn). The number average molecular weight Mn can be calculated based on end group analysis (OH numbers according to DIN EN ISO 4629, free NCO content according to EN ISO 11909) or can be determined by gel permeation chromatography according to DIN 55672 with THF as the eluent. If not stated otherwise, all given molecular weights are those determined by gel permeation chromatography.

[0022] Unless otherwise specified weight % or wt. % is based on weight of the moisture reactive polyurethane hot melt adhesive composition.

[0023] An adhesive's open time refers to the time during which an adhesive can bond to a material.

[0024] An adhesive's green strength refers to the initial holding strength prior to full chemical cure. In one variation green strength more particularly refers to the strength developed within 5 minutes after bonding a lauan board to a vinyl foil using the disclosed hot melt adhesive composition. In another variation green strength more particularly refers to the strength developed within 5 minutes after bonding an Azdel board to a vinyl foil using the disclosed hot melt adhesive composition. Below this level of initial strength, the green strength is generally insufficient for the bonded layers to withstand in-line finishing operations without exhibiting some debonding or delamination. Azdel is a product of Azdel Onboard in the U.S.

[0025] The present disclosure is directed toward providing reactive polyurethane hot melt adhesives that have some or all of the following properties. An application temperature in the range of 60° C. to 95° C. A maximum viscosity of 20,000 cps at 95° C. An open time of 10 minutes or less. A green strength of 0.4 or more pounds per square inch (psi), and preferably greater than 1 pound per square inch, measured 5 minutes after application from the operating temperature. A final cured mechanical strength of at least 3 psi and preferably greater than 6 psi.

[0026] In one embodiment the moisture reactive polyurethane hot melt adhesive composition is prepared from a combination comprising an amorphous polyester polyol, a crystalline polyester polyol, a polyether polyol, a diisocyanate, one of a polyisocyanate having a functionality greater than 2 or a thermoplastic polyurethane having hydroxyl functionality, a catalyst and optionally one or more additives.

[0027] Amorphous polyester polyols are polyester polyols that exhibit only a glass transition (Tg) and no substantial melting endotherm or crystallization exotherm in a DSC scan. Preferably at least one amorphous polyester polyol will have a Tg greater than 0° C. In one variation the moisture reactive polyurethane hot melt adhesive composition will comprise a mixture of at least one amorphous polyester polyol having a Tg greater than 0° C. and at least one amorphous polyester polyol having a Tg of less than 0° C.

[0028] Crystalline and/or semi-crystalline polyester polyols are polyester polyols that exhibit both a glass transition and melting and crystallization peaks in a DSC scan. In some embodiments useful crystalline and semi-crystalline polyester polyols have a melting point™ of greater than 25° C. Useful crystalline polyester polyol polymers include the Dynacoll materials available from Evonik Industries, for example Dynacoll 7490.

[0029] Preferably, the polyester polyols are used at an amorphous polyester polyol:crystalline polyester polyol ratio of 2:1 to 1:2. Below a 2:1 ratio the initial strength of the resulting adhesive will be too low to be acceptable. Above a 1:2 ratio the viscosity at 95° C. will be too high to be acceptable and the final bond strength will suffer.

[0030] Polyisocyanates, which may be used to prepare the present adhesive include f=2 diisocyanates such as alkylene diisocyanates, cycloalkylene diisocyanates, aromatic diisocyanates and aliphatic-aromatic diisocyanates and f>2 polyisocyanates. Examples of suitable polyisocyanates for use in the present disclosure include, by way of example and not limitation: methylenebisphenyldiisocyanate (MDI), isophoronediisocyanate (IPDI), hydrogenated methylenebisphenyldiisocyanate (HMDI), toluene diisocyanate (TDI), ethylene diisocyanate, ethylidene diisocyanate, propylene diisocyanate, butylene diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, cyclopentylene-1,3-diisocyanate, cyclo-hexylene-1,4-diisocyanate, cyclohexylene-1,2-diisocyanate, 4,4′-diphenylmethane diisocyanate, 2,2-diphenylpropane-4,4′-diisocyanate, xylylene diisocyanate, 1,4-naphthylene diisocyanate, 1,5-naphthylene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, diphenyl-4,4′-diisocyanate, azobenzene-4,4′-diisocyanate, diphenylsulphone-4,4′-diisocyanate, 2,4-tolylene diisocyanate, dichlorohexa-methylene diisocyanate, furfurylidene diisocyanate, 1-chlorobenzene-2,4-diisocyanate, 4,4′,4″-triisocyanatotriphenylmethane, 1,3,5-triisocyanato-benzene, 2,4,6-triisocyanato-toluene, 4,4′-dimethyldiphenyl-methane-2,2′,5,5-tetratetraisocyanate, and the like. Preferred polyisocyanates include methylenebisphenyl diisocyanate (MDI).

[0031] Aromatic polyisocyanates having a functionality greater than 2 (f>2) can also be optionally used to prepare the adhesive in some embodiments. Preferably, these aromatic polyisocyanates have an average functionality of 2.5 to 3.0. Useful aromatic polyisocyanates having a functionality greater than 2 include polymeric MDI, such as Rubinate M.

[0032] The moisture reactive polyurethane hot melt adhesive composition combination comprises polyether polyols including linear and branched polyethers having hydroxyl groups. Examples of the polyether polyol may include a polyoxyalkylene polyol such as polyethylene glycol, polypropylene glycol, polybutylene glycol and the like. Further, a homopolymer and a copolymer of the polyoxyalkylene polyols may also be employed. Particularly preferable copolymers of the polyoxyalkylene polyols may include an adduct of at least one compound selected from the group ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, 2-ethylhexanediol-1,3, glycerin, 1,2,6-hexane triol, trimethylol propane, trimethylol ethane, tris(hydroxyphenyl)propane, triethanolamine, triisopropanolamine, ethylenediamine and ethanolamine. Most preferably the polyether polyol comprises a polypropylene glycol. Typically, the polyether polyol has a number average molecular weight of greater than 300 Daltons with a preferred range of 400 to 6000 Daltons. The polyether polyol may comprise a mixture of different polyether polyols.

[0033] The adhesive can optionally include a catalyst. Some useful catalysts include, for example 2,2′-dimorpholinodiethylether, triethylenediamine, dibutyltin dilaurate and stannous octoate. In some variations the adhesive free of organometallic catalysts. A preferred catalyst is 2,2′-dimorpholinodiethylether.

[0034] The adhesive formulation can optionally include one or more of a variety of known hot melt adhesive additives such as filler; additional resins such as reactive acrylic polymer, nonreactive acrylic polymer, EVA, amorphous polyalphaolefin; plasticizer; colorant; rheology modifier; flame retardant; UV pigment; nanofiber; defoamer; tackifier; anti-oxidant; adhesion promoter; stabilizer; a thixotropic agent such as fumed silica; and the like. Conventional additives that are compatible with a composition according to this invention may simply be determined by combining a potential additive with the composition and determining if they are compatible. An additive is compatible if it is homogenous within the product at room temperature and at the use temperature.

[0035] Examples of useful fillers for use in the present disclosure include inorganic materials such as calcium carbonate, kaolin and dolomite. Calcium carbonate has been referred to as a non-fossil fuel based, sustainable, renewable material. Other examples of suitable fillers can be found in Handbook of Fillers, by George Wypych 3.sup.rd Edition 2009 and Handbook of Fillers and Reinforcements for Plastics, by Harry Katz and John Milewski 1978.

[0036] Reactive acrylic polymers comprise moieties that are reactive with other materials. Examples of useful reactive acrylic polymers include Elvacite 4014 and Elvacite 2978.

[0037] Nonreactive acrylic polymers are free of moieties that are reactive with other materials. Examples of useful nonreactive acrylic polymers include Elvacite 2013 and Elvacite 2016.

[0038] Tackifiers useful in the adhesive composition are not limited. Examples of useful materials for the optional tackifier include aliphatically modified C.sub.9-C.sub.10 hydrocarbons such as Novares TK 100H, Norsolene A-90, Henghe HH2-1004; alpha methyl styrene based tackifiers such Kristalex 3100; rosin ester tackifiers such as Sylvalite RE 100L; terpene phenolic tackifiers such as Sylvarez TP2040 and mixtures thereof.

[0039] Examples of useful adhesion promoters include silane based adhesion promoters such as Silquest A-link, Dynasylan AMEO and Dynasylan AMMO.

[0040] In the Table below the components of some embodiments of the presently disclosed adhesive composition are presented. The amounts are the percentage by weight of that component based on the total adhesive weight.

TABLE-US-00001 preferred range range component wt. % wt. % amorphous polyester polyol with Tg > 0° C. 3-30  8-10 total amorphous polyester polyols 3-40 18-30 crystalline polyester polyol 10-40  18-30 polyether polyol 5-50 10-20 diisocyanate 5-30 10-25 polyisocyanate having a functionality greater 0.1-3   0.3-1.sup.  than 2 thermoplastic polyurethane having hydroxyl 0-10 0.5-3.sup.  functionality catalyst 0-1  0.01-0.1  one or more additives. 0-70  0-60 amorphous polyester polyol to crystalline 2:1 to 1:2 1:1 polyester polyol ratio

[0041] The hot melt adhesives according to the present disclosure can be prepared by as follows. Add functional polyols, other polymers or resins (for example, thermoplastic urethane polymer, acrylic resin) into a reactor. Optionally add defoamer, filler, plasticizer or other non-reactive additives into the reactor. Mix the components at an elevated temperature until homogeneous. Dehydrate the mixture, for example by maintaining at an elevated temperature and under vacuum. To the dehydrated mixture add the diisocyanate (f=2) with mixing, under vacuum and at a temperature of about 120 to 140° C. and allow the reaction to proceed until for 1 hour or until a desired viscosity and NCO content are reached. Subsequently, add the polyisocyanate (f>2), if desired, catalyst, pigment, colorant, stabilizer, adhesion promoter and mix under vacuum for about 30 min to 1 hour. Pack the final adhesive product into a container under an inert atmosphere to exclude moisture.

[0042] The hot melt adhesives according to the present disclosure can be applied in a variety of manners including by spraying, roller coating, via a slot die, extruding and as a bead. The disclosed hot melt adhesive is stable during storage provided moisture is excluded. The disclosed hot melt adhesive has a commercially acceptable pot life in the molten state during application. It can be applied to a range of substrates including metals, wood, plastics, glass and textiles.

[0043] The invention also provides a method for bonding articles together which comprises applying the reactive hot melt adhesive composition of the disclosure in a liquid melt form to a first article, bringing a second article in contact with the composition applied to the first article, allowing the adhesive to cool and solidify and subjecting the applied composition to conditions which will allow the composition to fully cure to a composition having an irreversible solid form, said conditions comprising moisture. The composition is typically distributed and stored in its solid form and stored in the absence of moisture to prevent curing during storage. The composition is heated to a molten form prior to application and applied in the molten form. Typical application temperatures are in the range of from about 60° C. to about 145° C. and more typically about 60° C. to about 95° C. Thus, this disclosure includes reactive polyurethane hot melt adhesive compositions in both its uncured, room temperature solid form, as it is typically stored and distributed and its molten form after it has been melted just prior to its application and in its irreversibly solid form after curing.

[0044] After application, to adhere articles together, the reactive hot melt adhesive composition is subjected to conditions that will allow it to solidify and cure to a composition that has an irreversible solid form. Solidification or setting occurs when the liquid melt begins to cool from its application temperature to room temperature. This provides green strength. Curing, i.e. chain extending, to a composition that has an irreversible solid form, takes place in the presence of ambient moisture. This provides final or cured strength.

[0045] In some embodiment the disclosed reactive hot melt adhesive provides one or more advantages over conventional reactive hot melt adhesives. The disclosed hot melt adhesive can be applied at 60-95° C. while conventional reactive hot melt adhesives are generally applied above 110° C., for example above 135° C. The disclosed hot melt adhesive has a maximum viscosity of 20,000 cps at 95° C. In some embodiments the disclosure provides a moisture reactive polyurethane hot melt adhesive composition has a maximum viscosity of 15,000 cps or 12,000 cps or 10,000 cps or 8,000 cps or 5,000 cps at 95° C. These low viscosities at 95° C. allow use in commercial application equipment at this temperature. Conventional reactive hot melt adhesives have a viscosity of 50,000-60,000 cps or more at 95° C., a viscosity too high for use in commercial application equipment to achieve a desired low coating weight. The disclosed hot melt adhesive when heated to 60 to 95° C. can be applied at a coating weight of 10 grams per square foot (gsf) or less. Conventional reactive hot melt adhesives cannot be applied at a coating weight of about 10 gsf when used at a temperature of 60 to 95° C. The disclosed hot melt adhesive provides significant green strength of at least 0.4 psi, and more preferably at least 1 psi, 5 minutes after application of 10 gsf or less, more preferably application of 7 gsf or less. Other low temperature hot melt adhesives require substantially higher coating weights to increase their green strength. The disclosed hot melt adhesive has similar cured strength to conventional reactive hot melt adhesives.

[0046] The invention is further illustrated by the following non-limiting examples.

EXAMPLES

[0047] The following components, in Table 1, were utilized in the examples that follow.

TABLE-US-00002 TABLE 1 Component amorphous polyester Dynacoll 7130, OH # 32, Tg > 20° C. molecular polyol weight 1750, viscosity at 130° C. 14 Pas. A solid polyester polyol at room temperature. amorphous polyester Dynacoll 7230, OH number 30; Tg −30° C., polyol molecular weight 3500, viscosity at 80° C. 10 Pas. A liquid polyester polyol at room temperature. amorphous polyester Dynacoll 7250, OH number 21, Tg −50° C., polyol molecular weight 5500, viscosity at 80° C. 5 Pas. A liquid polyester polyol at room temperature. crystalline polyester Piothane 50-2000 HAI, OH number 55, Tm < 25° polyol C., crystalline polyester Dynacoll 7360, OH number 30, Tm 55° C., polyol molecular weight 3500, viscosity at 80° C. 2 Pas. A solid polyester polyol at room temperature. crystalline polyester Dynacoll 7380, OH number 30, Tm 70° C., polyol molecular weight 3500, viscosity at 80° C. 2 Pas. A solid polyester polyol at room temperature. diisocyanate f = 2 4,4′ MDI polyisocyanate Rubinate M, equivalent wt. 133 having f > 2 polyether polyol PPG Mw 425 polyether polyol PPG Mw 1,000 polyether polyol PPG Mw 2,000 thermoplastic Pearlbond 521, a thermoplastic polyurethane polyurethane based on polycaprolactone catalyst DMDEE (dimorpholinodiethylether amine) additive Novares TK 100H tackifier additive A-Link 25 adhesion promoter additive Elvacite 2013 acrylic resin based on methyl methacrylate and butyl methacrylate

[0048] Viscosity was measured using a Brookfield viscosimeter at 80° C. using a number 28 spindle and 10 rpm.

[0049] % NCO was measured by the conventional titration method.

[0050] Bond strength was measured at room temperature using a 90° peel test at a rate of 6 inches per minute. Samples were prepared and aged for either 5 minutes (short term bond strength or green strength) or 24 hours (long term bond strength or cured bond strength) before testing. Samples were tested by pulling at 90° to the bond line.

[0051] Open time was measured by drawing a 5 mil film of adhesive and pressing strips of Kraft paper onto the adhesive in intervals of 5-10 seconds. After 20 min, or when the adhesive is hardened, the paper strips are pulled off the adhesive. Paper strips applied after the open time will have less than 50% fiber tear.

[0052] Samples were prepared by roll coating adhesive to one or both substrates and pressing or nipping the coated substrates together.

[0053] The following Table lists comparative samples. Amounts are in weight percent (wt. %).

TABLE-US-00003 C1 C2 C3 polyether polyol.sup.1 9 9 9 amorphous polyester polyol.sup.2 13 13 23 crystalline polyester polyol.sup.3 24 24 34 crystalline polyester polyol.sup.4 31 30 11 crystalline polyester polyol.sup.5 7 7 2 diisocyanate, f = 2.sup.6 16 17 16 catalyst.sup.7 0.05 0.05 0.05 % NCO 2.7 2.9 2.6 Viscosity at 80° C. (cps) 15,000 13,000 15,000 amorphous polyester polyol to 1:4.8 1:4.7 1:2.6 crystalline polyester polyol ratio .sup.1PPG Mw 2000 .sup.2Dynacol 7250 .sup.3Piothane 50-2000 HAI .sup.4Dynacol 7360 .sup.5Dynacol 7380 .sup.6MDI .sup.7 DMDEE

[0054] Samples were made by coating each of the above samples at about 95° C. on both test substrates, contacting the coated surfaces of the test substrates and holding the contacted surfaces together for 5 minutes or 24 hours. Coating weights were 2-3 grams per square foot (gsf) on the vinyl foil and 4-5 grams per square foot on the board. The vinyl foil is a commercially available 4 to 6 mil thick product. The board is commercially available Lauan plywood or Azdel board. Sample sizes for vinyl foil and board were 1 inch by 5 inches and adhesive was applied to the entire 1 inch by 5 inch surface. Results are shown in the following Table.

TABLE-US-00004 C1 C2 C3 90° peel strength, Lauan board poor no adhesion no adhesion to vinyl foil, 5 min cure 90° peel strength, Lauan board I-SF I-SF I-SF to vinyl foil, 24 hour cure 90° peel strength, Azdel board poor no adhesion no adhesion to vinyl foil, 5 min cure 90° peel strength, Azdel board SF SF SF to vinyl foil, 24 hour cure I-SF immediate substrate failure, immediate fiber tear of the board or the vinyl foil. SF substrate failure - vinyl foil tears after peeling about one inch.

[0055] The following Table lists moisture reactive polyurethane hot melt adhesive composition samples. Amounts are in weight percent (wt. %).

TABLE-US-00005 E1 E2 E3 E4 E5 E6 E7 E8 polyether polyol.sup.1 12.5 12.5 12.5 12.5 12.7 12.6 12.8 12.1 polyether polyol.sup.2 3.0 3.0 3.0 3.0 3.0 3.0 3.1 2.9 amorphous polyester polyol.sup.3 6.9 13.8 19.4 10.0 10.2 10.1 10.2 9.6 amorphous polyester polyol.sup.4 9.3 18.6 13.0 13.5 13.7 13.6 13.8 12.9 crystalline polyester polyol.sup.5 28.4 14.2 14.2 22.0 22.3 22.2 22.5 21.2 crystalline polyester polyol.sup.6 4.0 2.0 2.0 3.1 3.2 3.1 3.2 3.0 diisocyanate, F = 2.sup.7 17.7 17.7 17.7 17.7 18.0 17.9 18.1 17.1 polyisocyanate, F > 2.sup.8 0.8 0.8 0.8 0.8 0.8 0 0 0.8 thermoplastic urethane.sup.9 1.5 1.5 1.5 1.5 0 1.5 0 1.5 tackifier.sup.10 15.3 15.3 15.3 15.3 15.5 15.4 15.7 15.3 acrylic resin.sup.11 0 0 0 0 0 0 0 3 catalyst.sup.12 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 additive.sup.13 (colorant, defoamer, 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 adhesion promoter) % NCO 3.9 3.9 3.9 3.9 4.0 3.7 3.7 3.8 viscosity at 80° C. (cps) 8,000 18,000 15,000 13,000 10,000 13,000 11,000 16,000 amorphous polyester polyol to crystalline 1:2 2:1 2:1 1:1 1:1 1:1 1:1 1:1 polyester polyol ratio .sup.1PPG Mw 2000 .sup.2PPG Mw 425 .sup.3Dynacoll 7230 .sup.4Dynacoll 7130 .sup.5Dynacol 7360 .sup.6Dynacoll 7380 .sup.74,4′ MDI .sup.8Rubinate M polymeric MDI .sup.9Pearlbond 521 .sup.10Novares TK 100H tackifier .sup.11Elvacite2013 .sup.12DMDEE .sup.13Thermoplast Blue > 0.1 wt. %; Resiflow LF 0.4 wt. %; A Link 25 < 0.1 wt. %

[0056] Samples were made by coating each of the above samples at about 95° C. on test substrates, contacting the coated surfaces of the test substrates and holding the contacted surfaces together for 5 minutes or 24 hours. Coating weights were 2-3 grams per square foot (gsf) on the vinyl foil and 4-5 grams per square foot on the board. Results are shown in the following Table.

TABLE-US-00006 E1 E2 E3 E4 E5 E6 E7 E8 90° peel strength, Lauan board to 0.7 nm 1   2   1.1 1.1 0.6 2.6 vinyl foil, 5 min cure (psi) 90° peel strength, Lauan board to 3.7 nm I-SF I-SF 9.3 10.6 SF I-SF vinyl foil, 24 hour cure 90° peel strength, Azdel board to 0.2 nm 0.3 0.8 0.5 0.4 0.2 1.2 vinyl foil, 5 min cure 90° peel strength, Azdel board to 1.1 nm 6.2 D-SF 5.9 5.1 SF 6.1 vinyl foil, 24 hour cure nm not measured I-SF immediate substrate failure, immediate fiber tear of the board or the vinyl foil. SF substrate failure - vinyl foil tears after peeling about one inch. D-SF deep substrate failure, Azdel fibers were torn out to about 2 mm depth

[0057] As shown in results composition E7 with only f=2 diisocyanate (no f>2 polyisocyanate or thermoplastic urethane) provided acceptable green and cured strength on Lauan to vinyl foil bonding and acceptable cured strength on Azdel board to vinyl foil bonding. Composition E7 did not have acceptable green strength for Azdel board to vinyl foil bonding. Composition E5 comprised f=2 diisocyanate and f>2 polyisocyanate but no thermoplastic urethane. Composition E6 comprised f=2 diisocyanate and thermoplastic urethane but no f>2 polyisocyanate. Compositions E5 and E6 provided improved green and cured strength on Lauan to foil bonding compared to composition E7 and marginal green and cured strength on Azdel board to vinyl foil bonding for some applications. Composition E4 comprised all of f=2 diisocyanate; f>2 polyisocyanate and thermoplastic urethane. The surprisingly improved strengths of this composition across all materials and cure levels indicate an unexpected synergy from using all of f=2 isocyanate; f>2 polyisocyanate and thermoplastic urethane.

[0058] The present disclosure provides a moisture reactive hot melt adhesive composition that can be applied to substrates at a coating weight of 7 grams per square foot or less even at temperatures as low as 60 to 95° C. while providing high tack and green strength as well as retaining the cured bond strength and toughness of conventional high melting point PURHM.

[0059] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.