One-component hot melt polyurethane adhesive

Abstract

One-component, moisture-curable polyurethane adhesives exhibit excellent handling strength before being fully cured, and excellent creep properties after curing. The adhesives include an isocyanate-terminated prepolymer made from polyols that include a crystalline polyester and hydroquinone bis (2-hydroxyethyl) ether, and a polyoxazolidine compound.

Claims

1. A one-component, moisture-curable, hot melt polyurethane adhesive composition which is a heat-softenable room temperature solid comprising: A) 50 to 98% by weight, based on the total weight of the adhesive composition, of a polyurethane prepolymer containing free isocyanate groups, the prepolymer being a reaction product of: 1) A mixture of polyols that comprises i) at least 35 weight percent, based on the weight of the mixture of polyols, of at least one hydroxyl-terminated polyester that is liquid at 22° C.; ii) at least 10 weight percent, based on the weight of the mixture of polyols, of at least one hydroxyl-terminated polyester that is a solid at 22° C. and exhibits a crystalline melting temperature of 70 to 130° C.; and iii) 5 to 20 weight percent, based on the weight of the mixture of polyols, of hydroquinone bis-(2-hydroxyethyl) ether; with 2) An excess of at least one organic polyisocyanate; B) 1 to 10% by weight, based on the total weight of the adhesive composition, of at least one polyoxazolidine compound having a molecular weight of up to 2000; and C) a catalytically effective amount of a catalyst for a reaction of water with an isocyanate group.

2. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 1 wherein component ii) has a crystalline melting temperature of 75 to 100° C.

3. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 1 wherein component ii) has a hydroxyl equivalent weight of 700 to 2500 and a hydroxyl functionality of 1.8 to 3.

4. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 1 wherein component i) has a hydroxyl equivalent weight of 700 to 1500 and a hydroxyl functionality of 1.8 to 3.

5. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 1 wherein the polyoxazolidine compound is represented by structure I or structure II: ##STR00004## wherein R is hydrogen, alkyl, aryl-substituted alkyl, phenyl or alkyl-substituted phenyl, x is from 1 to 20, and each y is independently 1 to 20.

6. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 5 wherein the polyoxazolidine compound has the structure: ##STR00005##

7. The one-component, moisture-curable, hot melt polyurethane adhesive composition of claim 1 wherein the mixture of polyols contains 12 to 25% by weight of component ii), based on the weight of the mixture of polyols.

8. A cured adhesive formed by curing the one-component polyurethane adhesive composition of claim 1.

9. A cured adhesive formed by contacting the one-component polyurethane adhesive composition of claim 1 with H.sub.2O.

10. A method of bonding two substrates, comprising forming a layer of the one-component polyurethane adhesive of claim 1 at a bondline between two substrates, and curing the layer at the bondline to form a cured adhesive bonded to each of the substrates.

11. A method of bonding two substrates, comprising forming a layer of the one-component polyurethane adhesive of claim 1 at a bondline between two substrates, and curing the layer at the bondline by contacting the layer with H.sub.2O to form a cured adhesive bonded to each of the substrates.

Description

EXAMPLE 1 AND COMPARATIVE SAMPLES A-D

(1) Talc-filled NCO polyurethane prepolymers are made from the formulations indicated in Table 1.

(2) TABLE-US-00001 TABLE 1 Parts by Weight Ingredient Comp. A* Comp. B* Comp. C* Comp. D* Ex. 1 HQEE 2.65 3.05 2.65 2.30 2.95 Crystalline 15.00 0 0 15.00 9.00 Polyester A Crystalline 0 0 15.00 15.00 0 Polyester B Liquid 36.52 48.62 35.42 22.00 40.72 Polyester Crosslinker 0.23 0.23 0.23 0.23 0.23 Liquid MDI 20.50 23.00 21.60 20.37 22.00 Tin Catalyst 0.002 0.002 0.002 0.002 0.002 Talc 20.00 20.00 20.00 20.00 20.00

(3) In each case, the polyesters, crosslinker and HQEE are dried at 105° C. under vacuum for 24 hours. The tale is dried at 200° C. for 24 hours. The dried materials are kept under nitrogen until used.

(4) The dried polyesters, crosslinker and HQEE are charged to a reactor and heated with agitation to 95° C. under nitrogen until the HQEE has dissolved. The tale is added and the mixture is maintained at the same temperature with agitation until the tale has dispersed. The polyisocyanate is added and the resulting reaction stirred under nitrogen until the exothermic temperature rise is completed. The tin catalyst is then added. And the mixture is heated under nitrogen to 120° C. to complete the prepolymer-forming reaction. The resulting product is an isocyanate-terminated polyurethane prepolymer. The prepolymer (exclusive of talc) in each case contains approximately 4% isocyanate groups. The filled prepolymer contains about 3.3% isocyanate groups.

(5) To make the adhesive, the hot talc-filled prepolymer in each case is combined under nitrogen with 0.10 parts 2,2-dimorpholinodiethyl ether and 5 parts of a bis-oxazolidine having the structure:

(6) ##STR00003##
The bis-oxazolidine has a molecular weight of about 518. After thorough mixing, the resulting adhesive is poured into cartridges and sealed.

(7) When cooled, the adhesives each are room temperature solids that have viscosities of over 1,000,000 cps at 25° C.

(8) Each of Comparative Samples A-D and Example 1 are evaluated for bonding a 40%-talc-filled polypropylene sheet to polycarbonate. The dimensions of the test sheets are 3.8 mm thick, 2.54 cm wide and 110.6 cm in length. The polypropylene sheet is plasma treated approximately an hour before the adhesive is applied to produce a surface energy of at least 60 dynes/cm. 0.76 mm glass spacer beads are applied to a surface of the polypropylene substrate. The adhesive in each case is melted by heating to 120° C. and applied onto the polypropylene substrate. The polycarbonate sheet is applied over the adhesive to produce a 25.4 mm overlap. The assembled specimens in each case are bound together with binder clips and cured for 3 days at 25° C. and 80% relative humidity for creep testing. Duplicate specimens are cured for 10 minutes at ambient temperature and humidity to evaluate handling strength.

(9) Handling strength is performed on the 10-minute-cured samples by measuring lap shear strength according to ISO standard ISO 4587. The apparatus is an Instron® 5500R Materials Testing System equipped with mechanical grips. The distance between grips is 17.8 cm and the crosshead speed is 12.7 mm/minute. On this test, a handling strength of at least 300 N is considered adequate.

(10) Creep testing is performed by punching holes at each end of the fully cured specimens. The specimen is hung from one end by a hook, and from the other end a 4.5 kg weight is attached via another hook. After 24 hours at 85° C., the specimens are visually inspected. A “pass” rating indicates that no visible movement of the adhesive joint is seen. A “fail” rating indicates movement or failure of the adhesive joint.

(11) Results are as indicated in Table 2.

(12) TABLE-US-00002 TABLE 2 Creep at Handling 85° C./ Designation Strength, N 24 hours Comp. A* 561 Fail Comp. B*  33 Pass Comp. C*  25 Fail Comp. D* 534 Fail Ex. 1 304 Pass

(13) Comparative Sample B shows the effect of including about 6% of the HQEE in the mixture of polyols without the presence of a polyester having a crystalline melting temperature of 70 to 130° C. Handling strength is poor, although adequate creep performance is obtained.

(14) Adding a crystalline polyester that has a melting temperature of only about 53° C. to a polyol mixture that contains about 4.8% HQEE (Comp. Sample C) worsens handling strength and creep performance.

(15) Comp. Sample D contains both the higher-melting and lower-melting crystalline polyesters, but the polyol mixture contains only about 4.2% of the HQEE. This combination allows one to obtain very good handling strength, but creep performance is poor.

(16) Comp. A contains the high-melting crystalline polyester and the polyol mixture used to make the prepolymer contains about 4.8% of the HQEE. Handling strength is good, but creep performance is again poor.

(17) Example 1 achieves a good balance of handling strength and creep performance. Handling strength is about 300 N, which is satisfactory. The polyol mixture in Example 1 contains 5.6% HQEE and 17% of the high-melting polyester.

(18) This data set suggests that handling strength is strongly dependent on the presence of the crystalline polyester having a melting temperature of 70 to 130° C. during prepolymer preparation; Example 1, which is made using less of that material than Comparative Samples A and B, it is believed to have somewhat lower handling strength for that reason. Increasing the amount of that material would be expected, on the basis of this data set, to further increase handling strength to be commensurate with those of Comp. Samples A and D.

(19) On the other hand, the data for Comp. Samples A-D suggests that the presence of that polyester in the prepolymer preparation has an adverse effect on creep. Compare, for example, the creep results for Comp. Sample B with those of Comp. Samples A, C and D. When coupled with the requisite amount of HQEE, however, good creep performance is obtained despite the presence of that polyester during the prepolymer preparation.