STRUCTURAL POLYURETHANE ADHESIVE HAVING GOOD ADHESION AFTER SHORT-DURATION HEATING

Abstract

A two-component polyurethane adhesive including a polyol component K1 and a polyisocyanate component K2; wherein the polyol component K1 includes at least one diol A1 having two primary hydroxyl groups and a molecular weight in the range from 60 to 150 g/mol, at least one triol A2 having an average molecular weight in the range from 1,000 to 10,000 g/mol, at least one polyester polyol A3 based on dimer fatty acids and/or dimer fatty alcohols, especially based on dimer fatty acids; and the polyisocyanate component K2 includes at least one polyisocyanate B1 and/or at least one polyurethane polymer B2 having isocyanate groups.

Claims

1. A polyurethane adhesive consisting of a polyol component K1 and a polyisocyanate component K2; wherein the polyol component K1 comprises at least one diol A1 having two primary hydroxyl groups and a molecular weight in the range from 60 to 150 g/mol, at least one triol A2 having an average molecular weight in the range from 1,000 to 10,000 g/mol, at least one polyester polyol A3 based on dimer fatty acids and/or dimer fatty alcohols; and the polyisocyanate component K2 comprises at least one polyisocyanate B1 and/or at least one polyurethane polymer B2 having isocyanate groups.

2. The polyurethane adhesive as claimed in claim 1, wherein the triol A2 is a polyether triol.

3. The polyurethane adhesive as claimed in claim 1, wherein the triol A2 has exclusively primary hydroxyl groups.

4. The polyurethane adhesive as claimed in claim 1, wherein the at least one polyester polyol A3 is a polyester polyol A3 based on dimer fatty acids formed from C10-C30 fatty acids.

5. The polyurethane adhesive as claimed in claim 1, wherein the polyol component K1 additionally includes at least one aliphatic polyol A4 which is a hydroxylation product of a triglyceride based on fatty acids.

6. The polyurethane adhesive as claimed in claim 5, wherein the aliphatic polyol A4 has an average OH functionality in the range from 2 to 4.

7. The polyurethane adhesive as claimed in claim 5, wherein the aliphatic polyol A4 is a hydroxylation product based on soya oil or a hydroxylation product based on castor.

8. The polyurethane adhesive as claimed in claim 1, wherein the mixing ratio in volume between the polyol component K1 and the polyisocyanate component K2 is in the range from 1:3 to 3:1.

9. The polyurethane adhesive as claimed in claim 1, wherein the diol A1, the triol A2, the polyester polyol A3 and optionally the aliphatic polyol A4 are present in the adhesive in such an amount that the weight ratio (A1+A2)/(A3+A4) is in the range from 0.3 to 1.3.

10. The polyurethane adhesive as claimed in claim 1, wherein the diol A1 and the triol A2 are present in the adhesive in such an amount that the weight ratio A1/A2 is in the range from 0.1 to 0.5.

11. The polyurethane adhesive as claimed in claim 1, wherein the polyester polyol A3 and the optionally present aliphatic polyol A4 are present in the adhesive in such an amount that the weight ratio A3/A4 is in the range from 1.0 to 3.0.

12. A method of adhesive bonding of a first substrate to a second substrate, comprising the steps of: a) mixing the polyol component K1 and the polyisocyanate component K2 of a polyurethane adhesive as claimed in claim 1, b) applying the mixed polyurethane adhesive to at least one of the substrate surfaces to be bonded, c) joining the substrates to be bonded within the open time, d) heating the polyurethane adhesive to a temperature of 80-170° C., e) curing the polyurethane adhesive.

13. The method as claimed in claim 12, wherein one or both substrates is a metal or an alloy.

14. The method as claimed in claim 12, wherein the polyurethane adhesive is applied in step b) in the form of an adhesive bead and, in step d), 30%, of the length of the adhesive bead applied is heated in step d).

15. An article formed from the method of adhesive bonding as claimed in claim 12.

Description

EXAMPLES

Substances Used:

[0140]

TABLE-US-00001 A1, diol butane-1,4-diol from LyondellBasell A2, triol EO-endcapped polyoxypropylene triol, OH number 35.0 mg KOH/g (Voranol ® CP 4755 from Dow) A3, Room temperature liquid polyester diol based on dimer polyester fatty acids of C14-C22 fatty acids, dimer acid content of polyol more than 95%, average molecular weight about 2000 g/mol A4, Room temperature liquid hydroxylated castor oil, OH aliphatic number about 120 mg KOH/g, average OH functionality polyol about 2.2. Polyether EO-endcapped polyoxypropylene diol, OH number about diol 28 mg KOH/g (Voranol ® EP 1900 from Dow) Cat. Acid-blocked amine catalyst, tertiary amine based on 1,8- Diazabicyclo[5.4.0]undec-7-ene (DBU) Carbon black Zeolite Sylosiv ® A3 from Grace

[0141] Polymer-1 was prepared by reacting 1300 g of polyoxypropylene diol (Acclaim® 4200 N, from Bayer; OH number 28.5 mg KOH/g), 2600 g of polyoxypropylene-polyoxyethylene trial (Caradol® MD34-02, from Shell; OH number 35.0 mg KOH/g), 600 g of 4,4′-methylene diphenyl diisocyanate (Desmodur 44 MC L, from Bayer) and 500 g of diisodecyl phthalate by a known method at 80° C. to give an NCO-terminated polyurethane polymer having a content of free isocyanate groups of 2.1% by weight.

[0142] Production of Polyurethane Adhesives

[0143] For each adhesive, the ingredients specified in table 1 were processed in the specified amounts (in parts by weight) of the polyol component K1 by means of a vacuum dissolver with exclusion of moisture to give a homogeneous paste, and stored. The ingredients of the polyisocyanate component K2 specified in table 1 were likewise processed and stored. Subsequently, the two components were processed in the specified mixing ratio K2/K1 (in parts by weight, w/w) by means of a SpeedMixer® (DAC 150 FV, Hauschild) for 30 seconds to give a homogeneous paste, and the latter was tested immediately as follows:

[0144] For determination of the mechanical properties, the adhesive was fashioned into dumbbell shape according to ISO 527, Part 2, 1B, and stored/cured at 25° C. for 24 h and then at 80° C. for 3 h.

[0145] After a conditioning period of 24 h at 23° C., modulus of elasticity in the range from 0.05 to 0.25% elongation, tensile strength and elongation at break of the test specimens thus produced were measured to ISO 527 on a Zwick Z020 tensile tester at the respective temperature specified in the table and a testing speed of 50 mm/min.

[0146] Lap shear strength was measured by producing various test specimens, in each case by applying the adhesive 1 minute after the end of the mixing time between two isopropanol-degreased cathodically-electrocoated steel sheets in a layer thickness of 2 mm and over an overlapping bonding area of 15×45 mm. These test specimens were used to determine lap shear strength to DIN EN 1465, with different storage of the test specimens prior to the measurement: either, as a measure for early strength, at 23° C. for 1 h (“ZSF 1 h RT”) or at 23° C. for 24 h (“ZSF 24 h RT”), and measured at 23° C., or, in order to measure lap shear strength in the fully cured state, at 23° C. for 7 days, followed by measurement at 23° C. (“ZSF 7d RT”), or at 23° C. for 7 days and then at 80° C. for 3 h, and then measurement at 80° C. (heat resistance, “ZSF 7d RT (80° C.)”). For measurement of lap shear strength after storage at high temperature and humidity (“ZSF 3d HTH”), the test specimens were stored at 80° C. and 100% air humidity for 3 days, then stored at 23° C. for 3 h, and then measured at 23° C.

[0147] Lap shear strength immediately after preliminary curing (“ZSF precuring”) was measured by producing various test specimens, in each case by applying the adhesive 1 minute after the end of the mixing time between two isopropanol-degreased cathodically-electrocoated steel sheets in a layer thickness of 2 mm and over an overlapping bonding area of 15×45 mm. These test specimens were heated to a temperature of 150° C. by means of an IR lamp and kept at that temperature for 40 seconds. 15 seconds later, lap shear strength was determined to DIN EN 1465 on the still-hot test specimens (no active cooling of the test specimens).

[0148] The attributes (af) and (cf) each denote the fracture profile, where “af” represents an adhesive fracture profile and “cf” a cohesive fracture profile.

[0149] Pot life was measured by introducing about 20 g of adhesive (comp. K1 and K2) into a beaker. The clock was started and the two components were mixed thoroughly with a spatula for about 30 seconds. After 30 seconds, the mixture was applied to the lower plate of a rheometer. The plate has been equilibrated to 25° C. Then the measurement was started: plate/plate rheometer from Anton Paar, spindle diameter 25 mm, gap 0.2 mm, temperature 25° C., shear rate 10 1/s. The pot life is the time elapsed since commencement of mixing before the mixture attains a viscosity of 500 Pas.

[0150] The viscosity of component K1, or K2 (“Viscosity of component K1/K2”) was measured by determination by means of a rheometer as described above.

[0151] The viscosity of the mixture of components K1 and K2 (“Viscosity of K1+K2 mixture”) was determined by introducing about 20 g of adhesive (comp. K1 and K2) into a beaker and mixing the two components thoroughly with a spatula for about 30 seconds. After 30 seconds, the mixture was measured by means of a rheometer as described above.

[0152] The results are reported in table 2.

[0153] The attributes (A1+A2)/(A3+A4), A1/A2 and A3/A4 in table 1 are based on the weight ratios of the diols A1, triols A2, polyester polyol A3 and optionally aliphatic polyols A4 present in the respective adhesive.

[0154] Rf.1 and Rf.2 are comparative examples; Z-1 and Z-2 are inventive examples.

TABLE-US-00002 TABLE 1 Examples Rf. 1 Z-1 Rf. 2 Z-2 Polyol component K1: A1, butane-1,4-diol 10 9 6 8 A2, polyether triol 24 24 24 24 Polyether diol 49 — — — A3 — 49 — 29 A4 — — 49 20 Zeolite  4 4 4 4 Carbon black 18 16 16 19 Cat.   0.7 0.7 0.7 0.7 (A1 + A2)/(A3 + A4) — 0.67 0.61 0.65 A1/A2  .sup. 0.42 0.38 0.25 0.33 A3/A4 — — — 1.45 Polyisocyanate component K2: Prepolymer-1 85 85 85 85 Carbon black 13 13 13 13 K2/K1 (w/w) 1/1 1/1 1/1 1/1

TABLE-US-00003 TABLE 2 Examples Rf. 1 Z-1 Rf. 2 Z-2 Viscosity of component 73.6/110.5 73.6/65.3 54.6/65.3 126.6/37.4 K1/K2 [Pas] Pot life [min] 12 12.5 12.9 13.3 Viscosity of K1 + K2 40.1 77.3 47.5 67 mixture, [Pas] ZSF, precuring [MPa] 2.4 (AF) 2.4 (CF) 1.8 (CF) 2.1 (CF) ZSF, 1 h RT [MPa] 0.56 CF 0.32 CF 0.53 CF 0.52 CF ZSF, 24 h RT [MPa] 9.9 CF 4.0 CF 8.0 CF 5.5 CF ZSF, 7 d RT [MPa] 11.5 CF 8.5 CF n.d. 8.1 CF ZSF, 7 d RT (80° C.) [MPa] 5.0 CF 5.3 CF 3.4 CF 4.9 CF ZSF, 3 d HTH 10.2 CF 10.2 CF 7.7 CF 9.5 CF Modulus of elasticity [MPa] 40 36 40 40 Tensile strength [MPa] 11.3 11.1 10.4 14.4 Elongation at break [%] 260 230 230 270 n.d. not determined

[0155] Inventive example Z-1, compared to Rf.1, shows better adhesion after IR preliminary curing at 150° C. for 40 seconds on a cathodically electrocoated substrate. Rather than adhesive fracture characteristics of Rf.1, cohesive fracture characteristics are obtained.

[0156] Moreover, comparison of Z-1 with Rf.1 shows an improvement in pot life with otherwise comparable mechanical properties and the adhesion values of final strength (ZSF 7 d RT), and also the adhesion values after storage at high temperature and humidity.

[0157] The addition of A4 to the inventive composition Z-2, compared to Z-1, leads to a faster development of adhesion (ZSF 1 h RT and 24 h RT), an improvement in pot life and higher values of modulus of elasticity, tensile strength and elongation at break. It has further been found that, surprisingly, the addition of A4 to component K1 leads to an improvement in storage stability thereof. In the case of storage of component K1 of Z-1 in a cartridge at 40° C. for 7 days, slight separation of component K1 was detected. In the course of the same storage of component K1 of Z-2, by contrast, no separation was detected.

[0158] It is further apparent from comparative example Rf.2 that, in the case of compositions containing solely A4 and no A3, adhesion values after IR preliminary curing are significantly lower. Adhesion values for storage at high temperature and humidity and for heat resistance are also significantly worse.