Two-part polyurethane adhesives made using isocyanate-terminated quasi-prepolymers based on poly(butylene oxide)

Abstract

Two-component polyurethane adhesives include a polyol component and a polyisocyanate component. The polyol component includes a polyether polyol and an aliphatic diol chain extender. The polyisocyanate component includes an isocyanate prepolymer made by reacting an aromatic polyisocyanate with a poly(butylene oxide) polyol. When cured, the adhesive is highly resistant to heat and humidity, and exhibits an unexpectedly high modulus and tensile strength.

Claims

1. A two-component polyurethane adhesive composition having a polyol component and an isocyanate component, wherein: the polyol component includes: a) at least 35 weight percent, based on the weight of the polyol component, of one or more polyether polyols having a hydroxyl equivalent weight of 400 to 2000 and a nominal hydroxyl functionality of 2 to 4, which polyether polyols(s) are selected from homopolymers of propylene oxide and copolymers of 70 to 99% by weight propylene oxide and correspondingly 1 to 30% by weight ethylene oxide; b) 5 to 20 parts by weight, per 100 parts by weight of ingredient a) of the polyol component, of one or more diol chain extenders; c) 0.1 to 3 parts by weight, per 100 parts by weight of ingredient a) of the polyol component, of at least one compound having at least two primary and/or secondary aliphatic amine groups; d) a catalytically effective amount of at least one urethane catalyst; and e) up to 60 weight percent, based on the weight of the polyol component, of at least one particulate filler; and the polyisocyanate component includes: 15 to 60 weight percent, based on the weight of the polyisocyanate component, of at least one isocyanate-terminated poly(butylene oxide) prepolymer having at least 2 isocyanate groups per molecule and an isocyanate equivalent weight of 700 to 3500, a poly(butylene oxide) segment having a weight of 1000 to 6000 g/mol; 20 to 50 weight percent, based on the weight of the polyisocyanate component, of at least one polyisocyanate compound having an isocyanate equivalent weight of up to 350 and 2 to 4 isocyanate groups per molecule; and up to 50% by weight of at least one particulate filler; wherein the isocyanate equivalent weight of the polyisocyanate component and the equivalent weight per isocyanate-reactive group of the polyol component are such that when the polyisocyanate component and the polyol component are mixed at a 1:1 ratio by volume the isocyanate index is 1.1 to 1.8.

2. The adhesive composition of claim 1, wherein the prepolymer is a reaction product of an aromatic polyisocyanate and a poly(butylene oxide) polyol.

3. The adhesive composition of claim 2, wherein the polyisocyanate component contains aliphatic and aromatic isocyanate groups.

4. The adhesive composition of claim 3 wherein the polyisocyanate compound having an isocyanate equivalent weight of up to 350 is a mixture of at least one aromatic polyisocyanate and at least one aliphatic polyisocyanate.

5. The adhesive composition of claim 4 wherein the poly(butylene oxide) segment has a weight of 1800 to 4200 g/mol.

6. A cured adhesive formed by curing the two-component polyurethane adhesive composition of claim 1.

7. A method of bonding two substrates, comprising forming a layer of the two-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.

Description

EXAMPLES 1-2 AND COMPARATIVE SAMPLES A-E

(1) Polyol Components used in Examples 1 and 2 and Comparative Samples A-D are made by mixing ingredients as indicated in Table 1:

(2) TABLE-US-00001 TABLE 1 Parts by Weight Ex. 1, 2, Comp. Ingredient Samples A, C and D Comp. Sample B Polyol A 53.51 53.51 Aminated Polyether A 0 0.6 Aminated Polyether B 0.6 0 1,2-propanediol 0 1 Monoethylene glycol 5.2 4.2 Catalyst Mixture 0.52 0.52 Calcined China Clay 38.18 37.33 Hydrophobically Modified 1.5 1.5 Fumed Silica Dispersing aid 0.5 0.5 Fluorinated surfactant 0.05 0.05

(3) The polyol components are prepared by mixing all the ingredients except the clay and fumed silica in a planetary mixing unit until homogeneous. The clay and fumed silica are then added slowly with mixing, and the resulting composition mixed an additional 40 to 50 minutes under a pressure of 100 atmospheres. The compounded polyol components are then transferred into cartridges.

(4) Polyisocyanate Components are made from the ingredients listed in Table 2.

(5) TABLE-US-00002 TABLE 2 Parts By Weight Comp. Comp. Comp. Comp. Ingredient Ex. 1 Ex. 2 Sample A Sample B Sample C Sample D Calcined China Clay 40 39 40 40 37.3 37.3 Hydrophobically 1 1 1 1 1 1 Modified Fumed Silica Polyisocyanate A 8 8 8 8 10.5 10.5 Polyisocyanate B 25.5 24 25.5 25.5 27 27 Polyisocyanate C 6 6 6 6 5 5 Polyol B 0 0 0 0 19.2 0 Polyol C 0 0 19.5 0 0 0 Polyol D 0 0 0 19.5 0 0 Polyol E 0 0 0 0 0 19.5 BO Polyol A 19.5 0 0 0 0 0 BO Polyol B 0 22 0 0 0 0

(6) Each of the Polyisocyanate Components are made by mixing the polyol (if any), the BO Polyol and the aromatic polyisocyanate(s) in a planetary mixture under ambient temperature and pressure until a homogeneous mixture is obtained. The clay and fumed silica are then added slowly. Mixing is continued for 40 to 50 minutes under a pressure of 100 atmospheres and ambient temperature, after which the temperature is increased to 75 C. and mixing is continued for an additional 60 minutes. After cooling the mixture to 40 C., it is filled into cartridges.

(7) To produce cured adhesives for physical property testing, equal volumes of the respective polyol and polyisocyanate components are mixed to produce a mixture in which the isocyanate index is 1.15-1.65. The mixture is formed into a 2-mm thick layer and cured for 7 days at 23 C. and 50% relative humidity. For each experiment, duplicate test specimens are cut for physical property testing. Physical property testing is performed on a portion of the specimens. The remaining specimens are wrapped in cotton. The wrapped samples are saturated with water and then wrapped in aluminum foil and polyethylene film to prevent water from escaping. The wrapped sample is exposed to 70 C. for 7 days, and then cooled to 20 C. for 16 hours. The sample is then brought to ambient temperature and stored at 23 C. for 2 hours before performing physical property testing.

(8) In each case, Young's modulus, tensile strength and elongation are measured on the samples as aged at 23 C./7 days, and again after the humid aging regimen. Results are as indicated in Table 3. In Table 3, Comp. Sample E is a commercially available two-part polyurethane adhesive that represents a baseline case.

(9) TABLE-US-00003 TABLE 3 Sample Designation Comp. Comp. Comp. Comp. Comp. Property Ex. 1 Ex. 2 Sample A Sample B Sample C Sample D Sample E Young's modulus (MPa) Before humid aging 158 156 135 131 73 125 23 After humid aging 147 157 86 107 60 114 15 Change (%) 7 1 36 18 18 9 35 Tensile Strength (MPa) Before humid aging 16.7 17.1 13.5 14.3 17.2 15.3 11 After humid aging 13.7 13.7 11.7 12.2 11.2 9.9 8 Change (%) 18 23 13 15 35 34 27 Elongation (%) Before humid aging 60 64 55 66 70 67 190 After humid aging 50 62 64 73 84 93 318 Change (%) 17 3 +16 +11 +20 +39 +40

(10) Example 1 and Comparative Sample A are direct comparisons, their formulations being identical except for the substitution of BO Polyol A for the poly(propylene oxide) diol in Comparative Sample A. As can be seen from the foregoing data, before humid aging, Example 1 has a much higher Young's modulus than Comp. Sample A (158 MPa vs. 135 MPa), and a much higher tensile strength (16.7 MPa vs. 13.5 MPa). Elongations are similar.

(11) Example 1 and Comparative Sample A show greatly different behavior after humid aging. Comparative Sample A exhibits a large decrease in modulus and an increase in elongation, indicating that significant water absorption occurs during humid aging. In contrast, Example 1 shows little change in modulus and a decrease in elongation after humid aging. The Example 1 humid aging results indicate that little if any water absorption has occurred.

(12) Polymers of propylene oxide such as Polyol C (used in Comparative Sample A) are known to contain significant amounts of monol impurities, whereas poly(butylene oxides) tend to contain much lower monol levels. To compensate for the potential effect of monol content, Comparative Sample B is made using Polyol D, which has a low monol content, similar to that of BO Polyol A. As seen in Table 3, the change from Polyol C to Polyol D (Comp. Sample A vs. Comp. Sample B), results in little change in modulus, tensile strength or elongation before humid aging. Like Comparative Sample A, Comparative Sample B exhibits a large decrease in modulus and a significant increase in elongation after humid aging. These results suggest that the better performance of Example 1 is attributable to the selection of a poly(butylene oxide) polyol, rather than being merely an effect of a reduced monol content.

(13) Comparative Samples C and D show the effect of replacing Polyol C with lower equivalent weight polyols. In each case, the modulus before humid aging is much lower than Example 1. Like Comparative Samples A and B, Comparative Sample C exhibits a large loss in modulus and a significant increase in elongation when humid aged, contrary to Example 1. Comparative Sample D shows a very large increase in elongation and tensile strength when humid aged.

(14) Comparative Sample E is a commercial benchmark. It has a very low modulus and tensile strength, and much higher elongation, before humid aging. Upon humid aging, it exhibits a large decrease in modulus and tensile strength and a large increase in elongation.

(15) Example 2 performs similarly to Example 1, despite the higher equivalent weight of the poly(butylene oxide) polyols used in Example 2. Initial modulus is much higher than any of the comparatives, and is essentially unchanged after humid aging. Elongation also is essentially unaffected by humid aging.