Two-component polyurethane adhesive having substantially temperature-independent mechanical properties and high strength

Abstract

A two-component polyurethane composition containing at least 55% by weight of polybutadiene polyols based on the total amount of all polyols having an average molecular weight of at least 500 g/mol, and at least one latent hardener, where the ratio of the number of reactive groups in the latent hardener to the number of OH groups present is in the range from 0.02 to 0.4. The composition has a long open time, blister-free curing, a very low glass transition temperature, high elasticity and surprisingly high strength which is very constant over a wide temperature range. Furthermore, it has very good adhesion to metallic and nonmetallic substrates, causing barely any stress cracks on glassy thermoplastics.

Claims

1. A composition consisting of a first component comprising at least 55% by weight of polybutadiene polyols having an average molecular weight in the range from 2,000 to 10,000 g/mol and an average OH functionality in the range from 2.1 to 4, based on the total amount of all polyols having an average molecular weight of at least 500 g/mol, and a second component comprising at least one polyisocyanate, where at least one of the two components additionally comprises at least one latent hardener, wherein the ratio of the number of reactive groups in the latent hardener to the number of OH groups present is in the range from 0.02 to 0.4.

2. The composition as claimed in claim 1, wherein the polyisocyanate comprises diphenylmethane 4,4′- or 2,4′- or 2,2′-diisocyanate or any mixture of these isomers.

3. The composition as claimed in claim 1, wherein the latent hardener is a blocked amine having a blocked, hydrolytically activatable amino group and at least one further reactive group selected from the group consisting of hydroxyl group, mercapto group, secondary amino group, primary amino group and blocked, hydrolytically activatable amino group.

4. The composition as claimed in claim 1, wherein the latent hardener is an aldimine of the formula (IV) ##STR00011## where m is 0 or 1 and n is an integer from 1 to 3, where (m+n) is 2 or 3, A is an (m+n)-valent hydrocarbyl radical optionally containing ether oxygen and having 2 to 20 carbon atoms, and Z is an optionally substituted aromatic radical or a radical of the formula ##STR00012## where R.sup.1 and R.sup.2 are each independently a monovalent hydrocarbyl radical having 1 to 12 carbon atoms, or together are a divalent hydrocarbyl radical having 4 to 12 carbon atoms which is part of an optionally substituted carbocyclic ring having 5 to 8 carbon atoms, R.sup.3 is a hydrogen atom or an alkyl or arylalkyl or alkoxycarbonyl radical having 1 to 12 carbon atoms, and ##STR00013## Y is where R.sup.4 is a monovalent hydrocarbyl radical optionally containing ether or aldehyde units and having 6 to 20 carbon atoms, and R.sup.5 and R.sup.6 are each independently a monovalent aliphatic, cycloaliphatic or arylaliphatic hydrocarbyl radical optionally containing heteroatoms in the form of hydroxyl groups or ether oxygen and having 2 to 20 carbon atoms, or together are a divalent aliphatic radical having 4 to 12 carbon atoms which is part of an optionally substituted heterocyclic ring having 5 to 8 ring atoms and as well as the nitrogen atom optionally contains further heteroatoms in the form of ether oxygen, thioether sulfur or tertiary amine nitrogen.

5. The composition as claimed in claim 4, wherein Z is a phenyl radical substituted by a branched alkyl group having 10 to 14 carbon atoms.

6. The composition as claimed in claim 4, wherein Z is a ##STR00014## radical of the formula where R.sup.1 and R.sup.2 are each a methyl radical, R.sup.3 is a hydrogen radical, Y is ##STR00015## and R.sup.4 is a linear alkyl radical having 11 carbon atoms.

7. The composition as claimed in claim 4, wherein Z is a ##STR00016## radical of the formula where R.sup.1 and R.sup.2 are each a methyl radical, R.sup.3 is a hydrogen radical, Y is ##STR00017## and R.sup.5 and R.sup.6 together are a 3-oxa-1,5-pentylene radical which, together with the nitrogen atom, forms a morpholine ring.

8. The composition as claimed in claim 4, wherein Z is a ##STR00018## radical of the formula where R.sup.1 and R.sup.2 are each a methyl radical, R.sup.3 is a hydrogen radical, Y is ##STR00019## and R.sup.5 and R.sup.6 are each 2-hydroxyethyl or 2-hydroxypropyl.

9. The composition as claimed in claim 1, wherein it contains at least one nitrogen-containing compound as catalyst for curing.

10. The composition as claimed in claim 1, wherein its glass transition temperature is below −45° C.

11. A method comprising applying the composition as claimed in claim 1 as elastic adhesive or sealant.

12. A product from the method as claimed in claim 11, which is obtained after the curing of the composition.

13. A method of bonding a first substrate to a second substrate, comprising the steps of: mixing the first and second components of the composition as claimed in claim 1, applying the mixed composition to at least one of the substrate surfaces to be bonded, and joining the two substrates within the open time of the mixed composition.

14. A method of sealing, comprising the steps of: mixing the first and second components of the composition as claimed in claim 1, and applying the mixed composition to a substrate or between two substrates within the open time of the mixed composition.

15. The method as claimed in claim 13, wherein at least one of the substrates is a glassy thermoplastic selected from the group consisting of polycarbonate, polymethylmethacrylate and polystyrene.

Description

EXAMPLES

(1) Working examples are adduced hereinafter, which are intended to elucidate the invention described in detail. It will be appreciated that the invention is not restricted to these described working examples.

(2) “Standard climatic conditions” refer to a temperature of 23±1° C. and a relative air humidity of 50±5%.

(3) 1. Commercial Substances Used:

(4) TABLE-US-00001 Poly bd ® R45 Polybutadiene polyol, OH functionality about 2.5, average molecular weight about 2800 g/mol, OH number 47.1 mg KOH/g (Poly bd ® R-45HTLO from Cray Valley) Kuraray P2010 Polyester diol, average molecular weight about 2000 g/mol, OH number 56 mg KOH/g (Kuraray P-2010 from Kuraray) Voranol ® CP 4755 EO-endcapped polyoxypropylenetriol, OH number 35 mg KOH/g (from Dow) Filler Mineral filler based on calcium carbonate (Winnofil ® SPT from Solvay) Carbon black Monarch ® 120 (from Cabot) DABCO 1,4-diazabicyclo[2.2.2]octane, 33% by weight in dipropylene glycol (DABCO 33 LV ® from Air Products) Zr catalyst Zirconium(IV) chelate complex in reactive diluent and tert- butyl acetate, zirconium content 3.5% by weight (K-Kat ® A- 209 from King Industries) Salicylic acid 5% 5% by weight of salicylic acid in dioctyl adipate Polyisocyanate Modified diphenylmethane diisocyanate containing MDI- carbodiimide adducts, liquid at room temperature, NCO content 29.4% by weight (Isonate ® M 143 from Dow)
2. Preparation of Substances:

(5) The amine content (total content of free and blocked amino groups including aldimino groups) was determined by means of titration (with 0.1N HClO.sub.4 in acetic acid against crystal violet) and is reported in mmol N/g.

Aldimine-1: N,N′-bis(2,2-dimethyl-3-lauroyloxypropylidene)hexamethylene-1,6-diamine

(6) 622 g of 2,2-dimethyl-3-lauroyloxypropanal were initially charged in a round-bottom flask under a nitrogen atmosphere. While stirring, 166.0 g of hexamethylene-1,6-diamine solution (70% by weight in water) were added and then the volatile constituents were removed at 80° C. and a reduced pressure at 10 mbar. 702 g of an almost colorless liquid having an amine content of 2.85 mmol N/g were obtained, corresponding to a calculated aldimine equivalent weight of about 350 g/eq.

Aldimine-2: N,N′-bis(2,2-dimethyl-3-(N-morpholino)propylidene)hexamethylene-1,6-diamine

(7) 359.5 g of 2,2-dimethyl-3-(N-morpholino)propanal were initially charged in a round-bottom flask under a nitrogen atmosphere. While stirring, 166.0 g of hexamethylene-1,6-diamine solution (70% by weight in water) were added and then the volatile constituents were removed at 80° C. and a reduced pressure at 10 mbar. 439.1 g of an almost colorless liquid having an amine content of 9.27 mmol N/g were obtained, corresponding to a calculated aldimine equivalent weight of about 220 g/eq.

Amidine Catalyst: 1-(3-dimethylaminopropyl)-2-methyl-1,4,5,6-tetrahydropyrimidine

(8) A round-bottom flask was initially charged with 131.63 g of ethyl acetoacetate in 50 mL of toluene, and 161.09 g of N.sup.1-((3-dimethylamino)propyl)-1,3-diaminopropane (from BASF) were slowly added dropwise while stirring and cooling, keeping the temperature at 20 to 30° C. Thereafter, the azeotrope of toluene and water was removed from the reaction mixture by means of distillation at 40° C. and 10 mbar, then residual toluene and ethyl acetate was removed by means of distillation under reduced pressure and the residue was distilled under reduced pressure. This gave 168.74 g of a yellowish oil having a boiling temperature at 95-105° C. at 0.6 mbar.

(9) 3. Production of Polyurethane Adhesives:

Examples K-1 to K-12 and Ref. 1 to Ref. 4

(10) For each example, the ingredients specified in tables 1 to 3 were mixed in the specified amounts (in parts by weight) of the first component (“component-1”) by means of a centrifugal mixer (SpeedMixer™ DAC 150, FlackTek Inc.) with exclusion of moisture to give a homogeneous paste and stored. The second component (“component-2”) used was the amount (in parts by weight) of polyisocyanate specified in tables 1 and 2. The two components were processed by means of the centrifugal mixer with exclusion of moisture to give a homogeneous paste and the paste was immediately tested as follows:

(11) As a measure of the open time, the Tack-free time was determined. For this purpose, a few grams of the adhesive were applied to cardboard in a layer thickness of about 2 mm and, under standard climatic conditions, the time until, when the surface of the adhesive was gently tapped by means of an LDPE pipette, there were for the first time no residues remaining any longer on the pipette was determined.

(12) For determination of the mechanical properties, the adhesive was converted to dumbbell form according to ISO 527, Part 2, 1B, and stored/cured at 23° C. for 24 h and then at 80° C. for 3 h. After a conditioning time of 24 h, Tensile strength, Elongation at break and Modulus of elasticity at 0.5-5% extension of the test specimens thus produced were measured according to ISO 527 on a Zwick Z020 tensile tester at a testing speed of 200 mm/min. The modulus of elasticity values serve here as a measure for the strength of the adhesive. Lap shear strength (LSS for short) was measured by producing various test specimens, by applying the adhesive 1 minute after conclusion of the mixing time in each case between two heptane-degreased cathodically electrocoated steel sheets (“LSS e-coat”) or isopropanol-degreased polycarbonate sheets (Makrolon®) (“LSS polycarb.”) in a layer thickness of 1.6 mm and over an overlapping bonding area of 15×45 mm. The test specimens were stored/cured under standard climatic conditions for 24 h and then at 80° C. for 3 h. After a conditioning period of 24 h under standard climatic conditions, the tensile shear strength was determined according to DIN EN 1465 at a strain rate of 10 mm/min.

(13) To determine whether bonded polycarbonate test specimens have a tendency to environmental stress cracking (ESC for short), further test specimens were produced. To this end, an isopropanol-degreased polycarbonate sheet with dimensions of 150×30 mm was covered with a film having an area of 30×30 mm and a thickness of 2 mm that had been cured under standard climatic conditions for 7 days such that the cured film came to rest in the middle of the polycarbonate sheet and was pressed on thoroughly by hand. Several test specimens of this kind were then clamped across a round piece of timber having a diameter of 35 mm that had been mounted on a board and secured at the ends such that each test specimen was fixed in a curved position. This arrangement was stored at 80° C. in an air circulation oven for 24 h, and then a visual assessment was made as to whether cracks were visible in the polycarbonate. If no cracks occurred in the polycarbonate, “ESC?” was answered “no”, otherwise “yes”.

(14) Glass transition temperature, abbreviated in the tables to T.sub.g, was determined from DMTA measurements on strip samples (height 2-3 mm, width 2-3 mm, length 8.5 mm) which were stored/cured at 23° C. for 24 h and then at 80° C. for 3 h, with a Mettler DMA/SDTA 861e instrument. The measurement conditions were: measurement in tensile mode, excitation frequency 10 Hz and heating rate 5 K/min. The samples were cooled down to −70° C. and heated to 200° C. with determination of the complex modulus of elasticity M* [MPa], and a maximum in the curve for the loss angle “tan 6” was read off as T.sub.g.

(15) The “Aldimine/OH ratio” in tables 1 to 3 referred to the ratio of the number of aldimine groups to the number of OH groups in the adhesive.

(16) The results are reported in tables 1 to 3.

(17) To determine the storage stability or aging stability or behavior under heat aging, further specimens were produced for some of the examples for determination of the mechanical properties and in some cases of tensile shear strength. These results are reported in table 4.

(18) Storage stability of the first component was determined by storing it in a moisture-tight container at 60° C. for 7 days before using it to produce the test specimens. These values are identified in table 4 by “Comp.-1 aged”. Aging stability was determined by storing further test specimens that had been cured/stored as described above, prior to the testing, additionally at 70° C. and 100% relative air humidity for 7 days and then conditioning them under standard climatic conditions for 24 h. These values are identified in table 4 by “+7 d 70° C./100% RH”.

(19) Behavior under heat aging was determined by producing further test specimens for determination of the mechanical properties which were stored/cured in an air circulation oven at 80/100/130° C. for 3 h and then conditioned under standard climatic conditions for 24 h. These values are identified in table 4 by “Curing 3h 80° C.”, “Curing 3h 100° C.” and “Curing 3h 130° C.” respectively.

(20) All the test specimens produced were visually impeccable, with a nontacky surface and free of blisters.

(21) Examples K-1 to K-12 are adhesives of the invention. Examples Ref. 1 to Ref. 4 are comparative examples.

(22) TABLE-US-00002 TABLE 1 Example Ref. 1 K-1 K-2 K-3 K-4 K-5 Ref. 2 Component-1: Poly bd ® R45 60.00 60.00 60.00 60.00 60.00 60.00 60.00 Aldimine-1 — 1.20 2.00 4.80 — — 9.00 Aldimine-2 — — — — 0.70 3.00 Filler 20.00 20.00 20.00 20.00 20.00 20.00 20.00 Carbon black 10.00 10.00 10.00 10.00 10.00 10.00 10.00 DABCO 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Salicylic acid 5% 0.20 0.20 0.20 0.20 0.20 0.20 0.20 Component-2: Polyisocyanate 8.03 8.65 9.09 10.57 8.52 10.18 12.18 Aldimine/OH ratio — 0.07 0.11 0.27 0.06 0.27 0.51 Tack-free time 18 15 15 13 10 10 18 [min.] LSS e-coat [MPa] 2.9 2.9 3.2 3.3 3.0 3.6 3.5 LSS polycarb. [MPa] 1.8 1.8 1.4 1.3 1.4 0.9 1.2 ESC? no no no no no no no Tensile strength 3.7 3.3 3.4 4.2 3.7 4.3 3.2 [MPa] Elongation at break 233 201 193 185 198 148 97 [%] Modulus of elasticity 3.6 5.0 5.3 5.7 5.0 6.3 5.9 [MPa] T.sub.g [° C.] −54 −53 −54 −54 −55 −54 −52

(23) TABLE-US-00003 TABLE 2 Example Ref. 3 K-6 K-7 K-8 K-9 Component-1: Poly bd ® R45 60.00 60.00 60.00 60.00 60.00 Kuraray P2010 10.0 10.0 10.0 10.0 10.0 Aldimine-1 — 2.00 4.80 — — Aldimine-2 — — — 0.70 3.00 Filler 20.00 20.00 20.00 20.00 20.00 Carbon black 10.00 10.00 10.00 10.00 10.00 DABCO 0.10 0.10 0.10 0.10 0.10 Salicylic acid 5% 0.20 0.20 0.20 0.20 0.20 Component-2: Polyisocyanate 9.20 10.10 11.35 10.20 11.63 Aldimine/OH ratio — 0.09 0.23 0.05 0.23 Tack-free time [min.] 15 23 20 12 7 LSS e-coat [MPa] 2.9 2.7 2.7 2.8 2.7 LSS polycarb. [MPa] 2.5 2.7 2.5 2.8 2.9 ESC? no no no no no Tensile strength 3.7 3.7 3.4 4.0 3.7 [MPa] Elongation at break 275 233 212 207 141 [%] Modulus of elasticity 3.6 4.4 4.2 4.9 4.5 [MPa] T.sub.g [° C.] −55 −54 −52 −54 −52

(24) TABLE-US-00004 TABLE 3 Example K-10 K-11 K-12 Ref. 4.sub.0 Component-1: Poly bd ® R45 60.00 60.00 60.00 30.00 Voranol ® CP 4755 — — — 30.00 Aldimine-1 2.00 3.00 2.00 1.20 Aldimine-2 — — — Aldimine-3 — — — Filler 20.00 20.00 20.00 20.00 Carbon black 10.00 10.00 10.00 10.00 Amidine catalyst 0.20 0.20 — — Zr catalyst — — 0.04 0.04 Salicylic acid 5% 0.20 0.20 0.20 0.20 Component-2: Polyisocyanate 9.09 10.18 9.00 7.44 Aldimine/OH ratio 0.11 0.27 0.11 0.08 Tack-free time [min.] 8 5 n.d. 50 LSS KTL [MPa] 3.7 4.0 2.1 2.7 LSS polycarb. [MPa] 1.4 1.3 n.d. n.d. ESC? no no Tensile strength 4.5 3.9 3.4 2.7 [MPa] Elongation at break 212 118 253 208 [%] Modulus of elasticity 5.3 6.8 3.7 3.3 [MPa] T.sub.g [° C.] −55 −54 −53 −37 “n.d.” stands for “not determined”

(25) TABLE-US-00005 TABLE 4 Example Ref. 1 K-2 K-3 K-4 K-5 Ref. 2 Comp.-1 aged: LSS e-coat [MPa] 3.1 3.4 3.8 3.4 4.2 n.d. LSS polycarb. [MPa] 2.3 1.4 1.9 1.6 1.4 n.d. ESC? no no no no no Tensile strength 3.9 3.6 3.5 2.8 3.7 n.d. [MPa] Elongation at break 203 147 108 115 117 n.d. [%] Modulus of elasticity 4.9 5.9 6.4 5.5 6.6 n.d. [MPa] +7 d 70° C./100% RH: LSS e-coat [MPa] 3.5 3.3 3.8 3.5 4.4 n.d. LSS polycarb. [MPa] 1.0 1.7 1.3 1.1 1.1 n.d. Tensile strength 3.9 3.4 4.1 4.5 4.2 n.d. [MPa] Elongation at break 240 149 135 148 218 n.d. [%] Modulus of elasticity 4.0 5.6 7.1 4.5 4.8 n.d. [MPa] Curing 3 h 80° C.: Tensile strength 3.1 3.1 3.3 3.5 n.d. 2.0 [MPa] Elongation at break 231 157 153 201 n.d. 86 [%] Modulus of elasticity 4.0 5.2 5.2 4.9 n.d. 4.0 [MPa] Curing 3 h 100° C.: Tensile strength 3.2 3.3 3.4 3.5 n.d. 2.8 [MPa] Elongation at break 238 163 112 183 n.d. 65 [%] Modulus of elasticity 4.1 5.4 6.5 5.2 n.d. 6.9 [MPa] Curing 3 h 130° C.: Tensile strength 3.0 3.7 n.d. 3.5 n.d. n.d. [MPa] Elongation at break 183 157 n.d. 186 n.d. n.d. [%] Modulus of elasticity 4.3 5.5 n.d. 4.7 n.d. n.d. [MPa]

(26) FIG. 1 shows the progression of the complex modulus of elasticity M* as a function of temperature for examples Ref. 1, K-2, K-3, K-4 and K-5, determined by means of DMTA as described above. The diagram serves as a measure for the constancy of the mechanical properties against temperature, and a flat curve profile represents high constancy.

(27) It is clear from the curve profile that inventive examples K-2 to K-5 have higher strength and lower temperature dependence of strength across the temperature range shown, especially in the range from −40 to 90° C., than the comparative example Ref. 1.