CURABLE ADHESIVE WITH IMPROVED DIECUTTABILITY AND IMPROVED SHOCK PROPERTIES

Abstract

The invention relates to a curable adhesive comprising, based on the mass of the curable adhesive: a) one or more (meth)acrylate block copolymers of the structure A-B-A in a combined mass fraction of 25% or more, where the A blocks independently of one another stand for a poly(meth)acrylate having a glass transition temperature Tg of 50° C. or more which is preparable by polymerization of an A monomer composition composed of A monomers, where the B block stands for a poly(meth)acrylate having a glass transition temperature Tg of less than 50° C. which is preparable by polymerization of a B monomer composition composed of B monomers, b) one or more polymerizable first epoxide compounds E1 in a combined mass fraction of 5% or more, where the first epoxide compound E1 at 25° C. is a solid or a high-viscosity substance, and c) one or more polymerizable second epoxide compounds E2 in a combined mass fraction of 5% or more, where the second epoxide compound E2 at 25° C. is a liquid, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from A monomers in the A blocks, <δ.sub.p>(A), is in the range from 9.0 to 11.0 MPa.sup.0.5, and where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from B monomers in the B blocks, <δ.sub.p>(B) is less than 9 MPa.sup.0.5.

Claims

1. Curable adhesive comprising, based on the mass of the curable adhesive: a) one or more (meth)acrylate block copolymers of the structure A-B-A in a combined mass fraction of 25% or more, where the A blocks independently of one another stand for a poly(meth)acrylate having a glass transition temperature Tg of 50° C. or more which is preparable by polymerization of an A monomer composition composed of A monomers, where the B block stands for a poly(meth)acrylate having a glass transition temperature Tg of less than 50° C. which is preparable by polymerization of a B monomer composition composed of B monomers, b) one or more polymerizable first epoxide compounds E1 in a combined mass fraction of 5% or more, where the first epoxide compound E1 at 25° C. is a solid or a high-viscosity substance having a dynamic viscosity of 50 Pa s or more, and c) one or more polymerizable second epoxide compounds E2 in a combined mass fraction of 5% or more, where the second epoxide compound E2 at 25° C. is a liquid having a dynamic viscosity of 40 Pa s or less, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from A monomers in the A blocks, <δ.sub.p>(A), is in the range from 9.0 to 11.0 MPa.sup.0.5, and where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from B monomers in the B blocks, <δ.sub.p>(B) is less than 9 MPa.sup.0.5.

2. Curable adhesive according to claim 1, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from A monomers in the A blocks, <δ.sub.p>(A), is in the range from 9.1 to 10.0 MPa.sup.0.5.

3. Curable adhesive according to claim 1, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from B monomers in the B blocks, <δ.sub.p>(B), is in the range from 6.0 to 8.9 MPa.sup.0..

4. Curable adhesive according to claim 1, where the A monomer composition comprises one or more A monomers which are selected from the group consisting of methyl methacrylate, ethyl acrylate, methyl acrylate, 2-phenoxydiethylene glycol acrylate and tert-butyl acrylate.

5. Curable adhesive according to claim 1, where the B monomer composition comprises one or more B monomers which are selected from the group consisting of n-butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, isobornyl acrylate, 2-phenoxyethyl acrylate, propylheptyl acrylate and acrylic acid.

6. Curable adhesive according to claim 1, where the combined mass fraction of the (meth)acrylate block copolymers in the curable adhesive is 28% or more.

7. Curable adhesive according to claim 1, where the combined mass fraction of the first epoxide compounds E1 in the curable adhesive is 10% or more.

8. Curable adhesive according to claim 1, where the combined mass fraction of the second epoxide compound E2 in the curable adhesive is 10% or more.

9. Curable adhesive according to claim 1, where the one or the two or more first epoxide compounds E1 and/or the one or the two or more second epoxide compounds E2 are selected from epoxide compounds having at least one cycloaliphatic group.

10. Adhesive tape comprising as adhesive layer a curable adhesive according to claim 1.

11. Curable adhesive according to claim 2, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from A monomers in the A blocks, <δ.sub.p>(A), is in the range from 9.2 to 9.5 MPa.sup.0.5.

12. Curable adhesive according to claim 2, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from A monomers in the A blocks, <δ.sub.p>(A), is in the range from 9.3 to 9.4 MPa.sup.0.5.

13. Curable adhesive according to claim 3, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from B monomers in the B blocks, <δ.sub.p>(B), is in the range from 6.5 to 8.8 MPa.sup.0.5.

14. Curable adhesive according to claim 3, where the amount-of-substance-weighted polar component of the Hansen solubility parameters <δ.sub.p> of the monomer units derived from B monomers in the B blocks, <δ.sub.p>(B), is in the range from 7.0 to 8.7 MPa.sup.0.5.

15. Curable adhesive according to claim 6, where the combined mass fraction of the (meth)acrylate block copolymers in the curable adhesive is 30% or more.

16. Curable adhesive according to claim 6, where the combined mass fraction of the (meth)acrylate block copolymers in the curable adhesive is 33% or more.

17. Curable adhesive according to claim 7, where the combined mass fraction of the first epoxide compounds E1 in the curable adhesive is 15% or more.

18. Curable adhesive according to claim 7, where the combined mass fraction of the first epoxide compounds E1 in the curable adhesive is 20% or more.

19. Curable adhesive according to claim 8, where the combined mass fraction of the second epoxide compound E2 in the curable adhesive is 20% or more or 30% or more.

20. Curable adhesive according to claim 9, where epoxide compounds having at least one cycloaliphatic group are selected from epoxide compounds having at least one a cyclohexyl group and/or dicyclopentadienyl group.

Description

[0121] A. Production of the Curable Adhesives:

[0122] Synthesis of a Random Comparative Copolymer (VP1):

[0123] The reactions were carried out under nitrogen atmosphere at room temperature (25° C.) in a screw-top EPA bottle with a volume of 60 ml. The radiation source used comprised two Skymore 110W UV LED nail-dryer lamps having a power each of 110 W and an emitted wavelength of 365 nm, the lamps being placed in such a way as to allow the reaction vessel to be positioned at a distance of 2 cm from the LEDs.

[0124] A mixture of 320 mg of S,S-dibenzyl trithiocarbonate (DBTTC), 16 g of methyl methacrylate (MMA, δ.sub.p=9.31 MPa.sup.0.5), 24 g of n-butyl acrylate (BA, δ.sub.p=8.60 MPa.sup.0.5) and 9 g of toluene was homogenized and then flushed with nitrogen for 10 min. The polymerization was initiated by irradiation of the reaction mixture. To end the reaction, the irradiation was paused and the high-viscosity reaction mixture was dissolved in THF, precipitated dropwise from an excess of cold methanol, and filtered to recover the precipitate. The molecular weight of the random comparative copolymer VP1 was 120 000 g/mol.

[0125] Production of the Curable Adhesives:

[0126] From the random comparative copolymer VP1 and a commercially available A-B-A (meth)acrylate block copolymer with the same fundamental monomer composition (P1, Kurarity LA2250 (M.sub.n: around 60 000 g/mol and M.sub.w: around 66 000 g/mol) or P2, Kurarity LA3320 (M.sub.n: around 108 000 g/mol and M.sub.w: around 119 000 g/mol)), with A blocks consisting of polymethyl methacrylate (δ.sub.p=9.31 MPa.sup.0.5) and a B block consisting of poly-n-butyl acrylate (δ.sub.p=8.60 MPa.sup.0.5), curable adhesives were obtained in a customary way by mixing with the further components.

[0127] As first epoxide compounds E1, use was made of a commercially available solid bisphenol A diglycidyl ether (E1a, D.E.R. 662E or E1b, Araldite GT 7072). As second epoxide compounds E2, use was made of a commercially available liquid cycloaliphatic epoxide (E2a, epoxycyclohexylmethyl 3′,4′-epoxycyclohexancarboxylate; Uvacure 1500) or a commercially available liquid bisphenol A diglycidyl ether (E2b, Epikote 828 or E2d, Araldite GY 250) or a commercially available liquid hydrogenated bisphenol A diglycidyl ether (E2c, HBE-100).

[0128] As open time additive, use was made of polyethylene glycol 400 (PEG 400) or polyethylene glycol dimethyl ether 500 (CAS: 24991-55-7), and as initiator, use was made of triarylsulfonium hexafluoroantimonate (CAS: 109037-75-4).

[0129] The composition of the adhesives is summarized in table 4. From the adhesives, by coating out and evaporation of the solvent, adhesive tapes having a thickness of about 100 μm were produced.

TABLE-US-00004 TABLE 4 Composition of adhesives together with solvents; all figures in parts by weight B1 B2 B3 B4 B5 B6 B7 P1 (Kurarity LA2250) 30 40 60 40 P2 (Kurarity LA3320) 35 35 35 E1a (D.E.R. 662E) 40 30 20 30 E1b (Araldite GT 7072) 20 40 40 E2a (Uvacure 1500) 30 30 25 E2b (Epikote 828) 30 E2c (HBE-100) 20 25 25 E2d (Araldite GY 250) 20 PEG 400 5 5 5 5 Polyethylene glycol 5 5 5 dimethyl ether 500 Triarylsulfonium 1 1 1 1 0.5 0.5 0.5 hexafluoroantimonate Acetone 100 100 100 100 100 100 100 V1 V2 V3 P1 (Kurarity LA2250) 15 VP1 30 50 E1a (D.E.R. 662E) 40 40 E2a (Uvacure 1500) 30 50 E2b (Epikote 828) 40 PEG 400 5 5 5 Triarylsulfonium 1 1 1 hexafluoroantimonate Acetone 100 100 100

[0130] B. Bonding Experiments:

[0131] The peel adhesions were determined in analogy to ISO 29862 (method 3) at 23° C. and 50% relative humidity, with a removal velocity of 300 mm/min and a removal angle of 180°. The thickness of the layer of adhesive in each case here was 100 μm. The reinforcing film used was an etched PET film having a thickness of 50 μm, as is available from Coveme (Italy). The substrate used comprised steel plates in accordance with the standard. The uncured measuring strip was bonded here by means of a roll-on machine with 4 kg at a temperature of 23° C. The adhesive tapes were removed immediately after application. The measured value (in N/cm) was obtained as the mean value from three individual measurements, and the failure mode was documented as follows: adhesive failure (A) or cohesive failure (C).

[0132] The lap-shear strength was also determined on the cured adhesives. The bond strength was determined in a dynamic lap-shear experiment in accordance with DIN-EN 1465 at 23° C. and 50% relative humidity for a test velocity of 1 mm/min, the determination being quantitative in each case (results in N/mm2=MPa). The test bars employed were steel bars which had been cleaned with acetone prior to bonding. The layer thicknesses of the adhesive tapes corresponded in each case to the details above. In this case the adhesive tapes, prior to the assembly of the test bars but after the removal of the second liner, were irradiated using appropriate light and the test specimens were assembled immediately thereafter. The measurement took place after seven days of storage at 23° C. and 50% relative humidity. The result reported is the mean value from three measurements.

[0133] Moreover, the shock resistance of the cured adhesives was investigated. The shock test employed for this purpose provides information about the bond strength of an adhesive product in the direction normal to the adhesive layer. Provided for this test are a circular first substrate (1) (polycarbonate, Makrolon 099, thickness 3 mm) having a diameter of 21 mm, a second substrate (2) (polycarbonate, Makrolon 099, thickness 3 mm), which is implemented in a square shape with a side length of 40 mm and which has a circular opening (drilled hole) arranged centrally, 9 mm in diameter, and the adhesive film samples for investigation, which were likewise produced circularly with a diameter of 21 mm (cut to size or diecut).

[0134] From the corresponding three components, a test element is produced by first bonding the adhesive film sample by the free surface exactly onto the substrate (1). The temporary protective film (siliconized PET liner) is then removed and the curable adhesive is activated by irradiation with at least 1000 mJ/cm.sup.2 from a 365 nm UV-LED. The assembly thus produced is then applied, by the now exposed side of the adhesive product, concentrically onto the substrate 2 within two minutes, concentrically meaning that the circular cut-out in the substrate 2 is positioned precisely centrally above the circular first substrate 1 (with a resulting bond area of 282 mm.sup.2) and is compressed with a force of at least 280 N for at least 10 s, to produce the test element.

[0135] After having been pressed, the test elements are conditioned for 72 hours at 23° C. and 50% relative humidity.

[0136] After the corresponding storage, the test elements are each clamped into a sample holder so that the assembly is aligned horizontally. The test element with the polycarbonate sheet (substrate 1) is inserted downwardly into the sample holder. The sample holder is subsequently inserted centrically into the provided holder of the apparatus used (“DuPont Impact Tester”, from Cometech, Taiwan, model QC-641). The impact head is inserted such that the circular, rounded striking geometry with the diameter of 5 mm lies centrically and flush on the bonding side of the substrate 1. A weight (carriage) guided on two guide rods and having a mass of 307 g is caused to drop perpendicularly from a height of initially 5 cm onto the above-prepared assembly composed of sample holder, test element and impact head (measuring conditions: 23° C., 50% relative humidity). The height from which the weight is dropped (h) is increased in steps of 5 cm until the impact energy introduced destroys the test elements as a result of the impact load, and the polycarbonate sheet (substrate 1) parts from the baseplate (substrate 2). In order to be able to compare experiments with different test elements, the energy is calculated as follows:

DuPont shock [mJ/cm.sup.2]=(m(carriage)[kg]*9.81[kg/m*s.sup.2]*h[m])/A(bond area)[cm.sup.2]

[0137] Five samples per adhesive are tested, and the mean value for the energy calculated is reported as an index for the impact strength. The results of the experiments are summarized in table 5.

TABLE-US-00005 TABLE 5 Summary of the bonding experiments B1 B2 B3 B4 V1 V2 V3 Fracture mode*/ A A A A C C C peel adhesion (>1 N/cm) (>1 N/cm) (>1 N/cm) (>1 N/cm) Lap-shear test/MPa >4  >4  >5  >4  >4  >4  >4  Shock (PC-PC)/mJ/cm.sup.2 >200 >200 >200 >200 <50 <100 <150 B5 B6 B7 Fracture mode*/ A A A peel adhesion (>1 N/cm) (>1 N/cm) (>1 N/cm) Lap-shear test/MPa >5  >4  >5  Shock (PC-PC)/mJ/cm.sup.2 >250 >250 >250 *A: adhesive failure; C: cohesive failure

[0138] On the basis of the fracture mode in the peel adhesion test it is possible to demonstrate the advantageous cohesion-boosting effect of the (meth)acrylate block copolymers employed in adhesives of the invention, advantageously also when large amounts of reactive resin are used. The experiments show, moreover, that with (meth)acrylate block copolymers in combination with specific reactive resins, reactive adhesives are obtainable that have significantly improved shock resistance. In this context, examples B5 to B7 show that the shock resistance can be improved through the use of relatively high molecular weight (meth)acrylate block copolymers (such as Kurarity LA3320) in the adhesives of the invention. This effect may perhaps be attributable not only to the high molecular weight of LA3320 (Mw: around 119 000 g/mol) in comparison to LA2250 (Mw: around 66 000 g/mol for LA2250). Without being tied to a particular theory, the inventors believe that possibly in LA3320 the lower hard block fraction of just below 20% of MMA, in combination with the increased molecular weight, means that the middle block in the cured epoxide adhesive tape is present with better phase separation and therefore has a stronger positive influence on the shock performance. In further experiments the inventors have determined that this surprising effect occurs even in a comparison of the even more similar LA2330 and LA3320 and leads to an improvement in the shock resistance (more than 50 mJ/cm.sup.2).

[0139] In comparison to the prior art, it has surprisingly been found that in curable adhesives of the invention, significantly greater amounts of (meth)acrylate block copolymer are outstandingly suitable for achieving not only good pressure-sensitive adhesives in the uncured state (adhesive failure in the peel adhesion test) but also strong bonds in the cured state. If the skilled person employs quantities of (meth)acrylate block copolymer known in principle from other adhesive systems, as shown in V3, the resulting adhesives tend to be paste-like, and lack suitability as pressure-sensitive adhesive.

[0140] The lap-shear tests after curing show that the adhesives of the invention cure to enable good bond strengths to be achieved, this being an indicator of sufficient adaptation behaviour. The advantage of the adhesives of the invention is evident in particular in the positive combination of the properties before and after curing.

[0141] Furthermore, examples B3, B5 and B7 show vividly that hydrogenated epoxide compounds in the adhesives of the invention result in higher bond strengths.