MOISTURE-CURABLE POLYURETHANE HOTMELT ADHESIVE HAVING HIGH INITIAL STRENGTH

Abstract

Described herein is a moisture-curing polyurethane hot-melt adhesive including at least 80% by weight, based on a total weight of the moisture-curing polyurethane hot-melt adhesive, of isocyanate-terminated prepolymer obtainable by mixing diisocyanate (a) with compounds having at least two isocyanate-reactive groups (b) and reacting the mixture to form the isocyanate-terminated prepolymer, where the compounds having at least two isocyanate-reactive groups (b) include at least one polylactide (b1) obtainable by reacting lactide with a linear difunctional starter molecule having 2 to 20 carbon atoms and the isocyanate content of the isocyanate-terminated prepolymer is 1 to 5% by weight. Also described herein is a process for producing such a moisture-curing polyurethane hot-melt adhesive and a method of using the moisture-curing polyurethane hot-melt adhesive in bonding of substrates.

Claims

1. A moisture-curing polyurethane hot-melt adhesive comprising at least 80% by weight, based on a total weight of the moisture-curing polyurethane hot-melt adhesive, of isocyanate-terminated prepolymer obtainable by mixing diisocyanate (a) with compounds having at least two isocyanate-reactive groups (b) and reacting the mixture to form the isocyanate-terminated prepolymer, wherein the compounds having at least two isocyanate-reactive groups (b) comprise at least one polylactide (b1) obtainable by reacting lactide with a linear difunctional starter molecule having 2 to 20 carbon atoms and an isocyanate content of the isocyanate-terminated prepolymer is 1 to 5% by weight, wherein the starter molecule is selected from the group consisting of diethylene glycol, propanediol, neopentyl glycol, dipropylene glycol, butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, decanediol, dodecanediol, and mixtures of two or more of these compounds.

2. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the starter molecule is 1,6-hexanediol.

3. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the polylactide (b1) is a copolymer and a proportion of lactide groups based on a total weight of the groups in the polylactide (b1) added to the starter molecule is 50 to less than 100% by weight.

4. The moisture-curing polyurethane hot-melt adhesive according to claim 3, wherein the polylactide (b1) is obtainable by reacting the starter molecule in a first step with lactide and in a second step with alkylene oxide.

5. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the polylactide (b1) is a copolymer and wherein a proportion of lactide groups based on a total weight of the groups in the polylactide (b1) added to the starter molecule is 100% by weight.

6. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein a hydroxyl value of the polylactide (b1) is 35 to 230 mg KOH/g.

7. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein a DMC catalyst is used in the production of the polylactide (b1).

8. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the diisocyanate (a) is selected from the group consisting of 2,4-MDI, 4,4′-MDI, and mixtures thereof.

9. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the compounds having at least two isocyanate-reactive groups (b) further comprise polyols (b2) that are different from the polylactide (b1).

10. The moisture-curing polyurethane hot-melt adhesive according to claim 9, wherein a proportion of polylactide (b1) based on a total weight of compounds having at least two isocyanate-reactive groups (b) is 5 to 90% by weight.

11. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein, in addition to the isocyanate-terminated prepolymer, further comprises thermoplastic polymer that has no isocyanate-reactive groups.

12. The moisture-curing polyurethane hot-melt adhesive according to claim 1, wherein the moisture-curing polyurethane hot-melt adhesive further comprises auxiliaries and additives.

13. A process for producing a moisture-curing polyurethane hot-melt adhesive comprising at least 80% by weight, based on a total weight of the moisture-curing polyurethane hot-melt adhesive, of isocyanate-terminated prepolymer, wherein diisocyanate (a) is mixed with compounds having at least two isocyanate-reactive groups (b) and the mixture is reacted to give the isocyanate-terminated prepolymer, wherein the compounds having at least two isocyanate-reactive groups (b) comprise at least one polylactide (b1) obtainable by reacting lactide with a linear difunctional starter molecule having 2 to 20 carbon atoms and wherein quantitative proportions of the diisocyanate (a) and the compounds having at least two isocyanate-reactive groups (b) are adjusted so that the resulting isocyanate-terminated prepolymer has an isocyanate content of 1 to 5% by weight and optionally mixing the isocyanate-terminated prepolymer obtained with a thermoplastic polymer and/or auxiliaries and additives, wherein the starter molecule is selected from the group consisting of diethylene glycol, propanediol, neopentyl glycol, dipropylene glycol, butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, decanediol, dodecanediol, and mixtures of two or more of these compounds.

14. A method of using the moisture-curing polyurethane hot-melt adhesive according to claim 1 for bonding substrates by applying the moisture-curing polyurethane hot-melt adhesive to a substrate at temperatures of greater than 80° C., applying a second substrate to the moisture-curing polyurethane hot-melt adhesive, and allowing the moisture-curing polyurethane hot-melt adhesive to cure.

15. (canceled)

Description

[0031] The invention is elucidated hereinbelow with reference to examples.

[0032] Moisture-curing polyurethane adhesives according to the present invention (inventive examples) and comparative examples were produced and the increase in viscosity on curing thereof was investigated. The following starting materials were used for this:

[0033] Polyesterol 1: polyesterol formed from hexanediol and adipic acid having a functionality of 2, a hydroxyl value of 30 mg KOH/g, and a melting point of 55° C., obtainable under the trade name Dynacoll© 7360 from Evonik

[0034] Polyetherol 1: polypropylene glycol having a functionality of 2 and a hydroxyl value of 56 mg KOH/g

[0035] Polyetherol 2: polypropylene glycol having a functionality of 2 and a hydroxyl value of 28 mg KOH/g

[0036] The lactide polyols were produced by reaction of an OH-containing starter with Puralact L ((3S-cis)-3,6-dimethyl-1,4-dioxane-2,5-dione, CAS number 4511-42-6)

[0037] Lactide polyol 1 polylactide having 1,6-hexanediol as starter molecule, a functionality of 2, and a hydroxyl value of 56 mg KOH/g, produced using 100 ppm of tin bis(2-ethylhexanoate) at 175° C. (catalyst amount based on overall mixture).

[0038] Lactide polyol 2 polylactide having 1,6-hexanediol as starter molecule, a functionality of 2, and a hydroxyl value of 56 mg KOH/g, produced using double-metal cyanide catalyst (1000 ppm based on overall mixture) at 200° C. (catalyst amount based on overall mixture).

[0039] Lactide polyol 3 polylactide having neopentyl glycol as starter molecule, a functionality of 2, and a hydroxyl value of 56 mg KOH/g, produced using 100 ppm of tin bis(2-ethylhexanoate) at 175° C. (catalyst amount based on overall mixture).

[0040] Lactide polyol 4 polylactide having neopentyl glycol as starter molecule, a functionality of 2, and a hydroxyl value of 37 mg KOH/g, produced using 100 ppm of tin catalyst at 175° C.

[0041] Acrylate polymer: thermoplastic acrylate polymer having a number-average molecular weight of 34 000 g/mol, obtainable under the trade name Elvacite© 2013 from Lucite International.

[0042] Isocyanate: MDI mixture comprising approx. 99% by weight of 4,4′-MDI and approx. 1% by weight of 2,4′-MDI

[0043] The moisture-curing polyurethane adhesives were produced by reacting the starting materials in the weight ratio shown in table 1 (values are in parts by weight unless otherwise stated).

TABLE-US-00001 TABLE 1 Comparison Example 1 Example 2 Example 3 Example 4 Polyesterol 1 35.0 13.0 13.0 13.0 13.0 Polyetherol 1 14.0 14.0 14.0 14.0 14.0 Polyetherol 2 18.0 18.0 18.0 18.0 18.0 Acrylate polymer 22.0 22.0 22.0 22.0 22.0 Lactide polyol 1 15 Lactide polyol 2 15 Lactide polyol 3 15 Lactide polyol 4 15 Isocyanate 11.0 11.0 11.0 11.0 10.2 Isocyanate 1.89 1.89 1.89 1.89 1.89 content

[0044] The viscosities of the moisture-curing polyurethane adhesives obtained were determined at different temperatures in accordance with ASTM D 3236 in a Brookfield viscometer with single-measurement geometry SC27 spindle. These values are shown in table 2.

TABLE-US-00002 TABLE 2 Comparison Example 1 Example 2 Example 3 Example 4 η at 150° C. 3.2 Pa .Math. s 3.4 Pa .Math. s 4.0 Pa .Math. s 3.4 Pa .Math. s 4.0 Pa .Math. s η at 130° C. 7.5 Pa .Math. s 7.4 Pa .Math. s 9.0 Pa .Math. s 10.1 Pa .Math. s 17.5 Pa .Math. s η at 120° C. 12.6 Pa .Math. s 11.8 Pa .Math. s 15.4 Pa .Math. s 12.6 Pa .Math. s 70.7 Pa .Math. s η at 110° C. 14.8 Pa .Math. s 21.8 Pa .Math. s 43.2 Pa .Math. s 44.8 Pa .Math. s 183 Pa .Math. s η at 100° C. 251 Pa .Math. s 206 Pa .Math. s 1560 Pa .Math. s η at 90° C. 65.8 Pa .Math. s 178 Pa .Math. s 1042 Pa .Math. s 2258 Pa .Math. s η at 80° C. 966 Pa .Math. s η at 70° C. 200 Pa .Math. s

[0045] The viscosity values obtained for the inventive examples show that application at temperatures of greater than 100° C. is possible without difficulty on account of the low viscosity, but, on cooling to temperatures of lower than 100° C., there is a rapid increase in viscosity that results in the adhesive joint attaining high initial strength before curing of the adhesive is complete. The comparative examples likewise demonstrate an increase in viscosity, but this is smaller with decreasing temperature than that of the inventive examples.