ADHESIVE FILM THAT CAN BE WOUND AND STAMPED

Abstract

The invention relates to an adhesive film that can be wound and stamped, having an epoxy-based adhesive compound that can be activated by UV radiation, characterized in that the adhesive compound comprises: a) 2-40 wt % of film former, b) 10-70 wt % of aromatic epoxy resins, c) cycloaliphatic epoxy resins, the cycloaliphatic epoxy resins not exceeding 35 wt %, d) 0.5-7 wt % of cationic initiators, e) 0-50 wt % of epoxy-enhanced polyether compounds, and f) 0-20 wt % of polyol, the proportions adding up to 100%, and the adhesive compound having an open time of 10 seconds to 60 minutes after the UV activation, during which open time the film has a pressure-sensitive adhesive characteristic.

Claims

1. An adhesive film adapted to be wound and stamped, the adhesive film including an epoxy-based adhesive compound that can be activated by UV radiation, wherein the adhesive compound comprises: a) 2-40 wt % of film former, b) 10-70 wt % of aromatic epoxy resins, c) cyclo-aliphatic epoxy resins, the cyclo-aliphatic epoxy resins not exceeding 33 wt %, d) 0.3-7 wt % of cationic initiators, e) 0-50 wt % of epoxy-enhanced polyether compounds, and f) 0-20 wt % of polyol, the proportions adding up to 100%, and the adhesive compound has an open time of 10 seconds to 60 minutes after the UV activation, during which open time the film has a pressure-sensitive adhesive characteristic.

2. The adhesive film of claim 1, wherein the adhesive film has a time until handling strength that does not exceed triple the open time.

3. The adhesive film of claim 1, wherein a share of an epoxy equivalent of the epoxy-enhanced polyether compounds in the epoxy equivalent of all the epoxy resins is between 0 and 40%.

4. The adhesive film of claim 1, wherein a share of an epoxy equivalent of the cyclo-aliphatic epoxy resins in the epoxy equivalent of all epoxy resins is between 0 and 33%.

5. The adhesive film of claim 1, wherein a share of an epoxy equivalent of the aromatic epoxy resins in the epoxy equivalent of all epoxy resins is above 60%.

6. The adhesive film of claim 1, wherein a film former comprises polymers of ethylene vinyl acetate (EVA), polyamide (PA), polyurethane (PU), acrylate or a phenoxy resin.

7. The adhesive film of claim 1, wherein the cationic initiator includes a cross-link initiating cationic initiators selected from arvlsulfonium salts, iodonium salts, thioxanthenium salts, and triarylsulfonium salts.

8. The adhesive film of claim 1, wherein the cationic initiator is activated by UV light.

9. The adhesive film of claim 1, wherein the film is adapted for structural bonding of metals, glass, ceramics, GFRP, CFRP, and high-energy surfaces.

10. The adhesive of claim 1, wherein the adhesive film has an adhesive strength of from about 6 MPA to about 20 MPA.

11. The adhesive film of claim 8, wherein the cationic initiator is activated by UVA light or UVC light

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] FIG. 1 shows the curing curve for two different formulations.

DETAILED DESCRIPTION OF THE EMBODIMENTS

2. Materials Used

[0060]

TABLE-US-00001 Identification Type Reference Desmomelt 530 film former, polyester polyurethane Covestro Levamelt 900 film former, ethyl vinyl acetate Lanxess Levamelt 456 film former, ethyl vinyl acetate Lanxess Phenoxy Resin PKHM-301 film former, phenoxy resin Gabrielchem Araldite GT 7072 N aromatic epoxy resin, solid, Huntsman melting point 89 C., epoxy equivalent 570-585 g/eq D.E.R 331 aromatic epoxy resin, liquid resin, Olin highly viscous, epoxy equivalent 182-192 g/eq Uvacure 1534 cyclo-aliphatic epoxy resin, Allnex epoxy equivalent 190-210 g/eq D.E.R. 736 P epoxy-enhanced polyether Olin compound (propylenglycol-epichlorhydrine- copolymer), epoxy equivalent 150-205 g/eq Araldite DY 3601 epoxy-enhanced polyether Huntsman compound (polypropylene glycoldiglycidyl- ether, epoxy equivalent 385-405 g/eq Heloxy Modifier WF epoxy-enhanced polyether Hexion compound (butylene glycol diglycidyl ether, epoxy equivalent 380-420 g/eq PEG 400 polyethylene glycol BASF Chivacure 1176 cationic UV initiator, based on Chitec triarylsulfonium salt Chivacure 1190 Cationic UV initiator, based on Chitec triarylsulfonium salt Omicat 432 Cationic UV initiator, based on IMG resins triarylsulfonium salt Omicat 550 BL Cationic UV initiator, based on IMG resins thioxhanthenium UV 387 C Cationic UV initiator, based on Deuteron iodonium salt Chivacure 2-ITX UV sensitiser for increasing light Chitec yield

2. Manufacture of the Adhesive Films and their UV Activation

[0061] All percentage statements relate to weight percent.

[0062] Preparation of the Adhesive Film:

[0063] Adhesive Compound:

[0064] The film former is pre-dissolved in a suitable solvent mixture, if applicable subject to slight heating of the mixture. Then the remaining components are dissolved separately in a suitable solvent mixture (epoxy resins, epoxy-enhanced polyether compound, polyol and LTV initiator). Finally, the two solutions are mixed together while stirring. The solvents or solvent mixtures are selected from among the solvents known to the skilled person such that the components are easy to solve or disperse, or result in a suitable mixture exhibiting viscous properties such that the mixture can be coated upon a film or a backing. The ratio of the solvent mixture in relation to the remaining components is selected such that coatable viscosity is obtained and that the adhesive compound including solvents is sufficiently stable between its production and the coating process. For the sample formulations, the following solvents are used: Film former: Desmomelt 530 and phenoxy resin PKHM-301: methyl ethyl ketone, Levamelt types: ethyl acetate, remaining components: Ethyl acetate. The solids content of the finished adhesive compound solutions is 50-70%.

[0065] Coating:

[0066] The adhesive compound containing solvents is applied to silicon-enhanced polyester film (thickness: 50 m). Then it first is dried at room temperature for 10 minutes and then at 80 C. in a convection oven for 10 minutes. The amount to be applied is adjusted such that after drying (removal of the solvent mixture) a layer thickness of 50 m is obtained.

[0067] A pressure-sensitive adhesive (tacky) film is obtained of a thickness of ca. 50 m.

[0068] No protective measures against UV light are necessary during handling of the raw materials, the adhesive and for the coating. It is sufficient to work under regular laboratory conditions away from the UV lamp. No further shielding is required.

[0069] UV Irradiation of the Pressure-Sensitive Adhesive Film:

[0070] The pressure-sensitive adhesive film is glued on to the first substrate (plate made of glass fibre-reinforced plastics GFRP, length: 25 mm, width. 25 mm, thickness: 2 mm) at a size of ca. 312 mm.sup.2 (width: 25 mm, length: 12.5 mm). Finally the silicon-enhanced polyester film is removed. This substrate is then irradiated with UV light (either UVC light from a discharging lamp or UVA light from a UVA-LED source). After irradiation, during the open time and outside of the irradiation zone, the second substrate is pressed onto the open adhesive film (the adhesive film is still pressure-sensitive adhesive also after the irradiation) so that the two substrates overlap and the adhesive surface amounts to 25 mm12.5 mm. The two substrates are fixed with braces and stored at room temperature. Tensile shear strength is then measured for that sample after 24 hours, unless specified otherwise.

[0071] UV irradiation, unless specified otherwise, is effected using a UV lab device by Beltron with a conveyor belt and a UVC radiator with a radiation maximum at 256 nm. The conveyor belt is operated at 2 m/min. The radiation dosage in the UVC range, measured using a UV Power Puck II by EIT Intstrument Market Group, amounts to 197 mJ/cm.sup.2.

[0072] Tests with a UVA lamp are performed using the LED Spot 100 by Honle. It consists in a UV LED (wavelength 365 nm) and a radiation chamber. The samples are irradiated for 15 seconds in the radiation chamber. The radiation dosage, measured using a UV Power Puck II by EIT Intstrument Market Group, amounts to 5000 mJ/cm.sup.2.

[0073] Note:

[0074] Despite the substantially higher wavelength, the adhesive compounds can also be activated using the UV LED device. Similar radiation times as in the UVC device are feasible and the results regarding open time and adhesive strength are in the same range. Presumably, the significantly higher intensity of the UVA radiation compensates for the smaller energy quantums so that the photo initiators are capable of starting the curing reaction. For example, the intense UVA radiation could have its effect as a consequence of a prohibited transition of the photo initiator.

[0075] Open Time:

[0076] Open time is considered the maximum feasible duration between the removal from the radiation belt (UVC) or removal from the radiation chamber (UVA), respectively, and the point in time when the joining with the second substrate takes place. During this period, the join parts can be joined. It is defined such that the adhesive layer is still pressure-sensitive adhesive (tacky). The open time is determined by finger-checking the tack of the surface of the adhesive films after irradiation. Directly after irradiation, the adhesive film is still tacky. After a certain time, the degree of conceivable tack decreases and reduces further until eventually the surface is non-tacky. The open time is determined as the point in time when tack decreases conceivably. It turns out that as long as the surfaces are still tacky, joining is possible and the subsequent curing results in a homogeneous adhesive bond. As soon as the surfaces lose tack, the curing process has progressed so much already that no joining is possible any longer. This is reflected in the significantly reduced strength values determined based on the quasi-static tensile shear strength. Joining of the adhesive films takes place at the end of the open time.

[0077] Time Until Handling Strength:

[0078] This means the period that elapses after the joining step until the strength of the bond is so hard that glued parts can already be transported and processed further. Here, we presume that handling strength has been reached once the quasi-static tensile shear strength has reached two Megapascal. This is a degree of strength that will tolerate the loads exerted during an industrial manufacturing process.

[0079] Curing Time:

[0080] The curing time is the period between the joining and the final strength of the bond. All sample formulations are fully cured after a maximum of 24 hours. Therefore, for the most part the waiting time was 24 hours before testing for the quasi-static tensile shear strength. When a value in excess of 6 MPa is achieved, structural strength or structural bonding is considered to have been obtained.

[0081] For the use case, sufficient open time is desired. Swift achievement of handling strength is advantageous in case the bond has to withstand a first load soon after joining (e.g. during transport of the parts) or in order to forego fixing the parts, respectively. For full curing, in turn, 24 hours are sufficient because according to experience only after that amount of time the bond tolerates the final load (permanent load or shock loads).

[0082] Open time and curing time are consequences of the reaction speed of the curing reaction. This reaction starts with UV activation and ends upon full curing of the adhesive film. Curing is complete once the final strength of the adhesive bond has been achieved. During the open time and curing time, different phases exhibiting different reaction speeds may pass, delays and accelerations may occur, resulting in a certain overall open time and curing time. The open time and curing time can be controlled by adjusting the formulation, the radiation type and intensity and duration as well as thermal management (temperatures) during the gluing process.

3. Test Methods

[0083] a) Quasi-Static Tensile Shear Test

[0084] After curing, the samples are subjected to a quasi-static tensile shear test. The samples are produced and cured as described above (substrate: GFRP). After a resting time of 24 h until full curing has been achieved, the samples are suspended in a tensile shear testing device and torn apart at room temperature and at a speed of 2 mm/min. The maximum force of the force path curve in relation to the adhesion surface (312 mm.sup.2) is specified as the tensile shear value in N/mm.sup.2 or MPa. The values identified are the mean value of 3 measurements.

[0085] b) Determination of the Open Time

[0086] The pressure-sensitive adhesive film is glued onto the first substrate (plate made of glass fibre-reinforced plastic GFRP, length: 25 mm, width. 25 mm, thickness: 2 mm) in a size of ca. 312 mm.sup.2 (width: 25 mm, depth: 12.5 mm). Finally, the silicon-enhanced polyester film is removed. This substrate is then irradiated with UV light. Directly after irradiation, the timer is started. The substrate is stored at room temperature in the lab away from the UV light area. In reasonable intervals, the film is finger-checked for tack. Initially it is still tacky, but after a certain time, tack decreases conceivably until it dissipates entirely. The finger-check is always performed on a fresh part of the adhesive film. As soon as tack diminishes, the timer is stopped. The open time is then defined as the period of time where no loss in tack can yet be perceived. It is determined as a mean value from three individual samples.

[0087] For the joining for measuring bond strength, additional samples of the same type are irradiated and then joined before the end of the previously determined open time.

[0088] c) Determination of the Time Until Handling Strength

[0089] For determining the time until handling strength, adhesive strength is monitored for several samples during the curing process. Depending on the curing speed, adhesion strength is built up swiftly or slowly. Based on measuring the quasi-static tensile shear strength, a handling time is determined as the time that elapses until a value of 2 MPa has been reached. FIG. 1 shows the curing curve for two different formulations, whereas the one formulation cures fast (upper curve) and the other cures slowly (bottom curve). For the formulations, the time until handling strength is less than triple the open time.

[0090] d) Calculating the share of a group of epoxy resins in the epoxy equivalent.

[0091] The epoxy equivalent EP.sub.tot of a formulation is calculated based on the epoxy equivalents EP.sub.i of the individual epoxy resins and their respective weight share a.sub.i in the formulation according to the following formula:

EPtot=.sub.i=1.sup.n(a.sub.iEP.sub.i) with n=number of epoxy resins and .sub.i=1.sup.n(a.sub.1)=1

[0092] The share of the cyclo-aliphatic resins (cyc) in the epoxy equivalent is calculated as follows:

A.sub.cyca.sub.cycEP.sub.cyc/EP.sub.tot

[0093] Accordingly, the share of aromatic (arom) epoxy resins is calculated as follows:

A.sub.arom=a.sub.aromEP.sub.arom/EP.sub.tot

[0094] Accordingly, the share of epoxy-enhanced epoxy resins is calculated as follows:

A.sub.ether=a.sub.etherEP.sub.ether/EP.sub.tot

4. Examples

[0095] Hereinafter, the sample and comparative sample formulations are outlined. Only the fixed weight ratios of the adhesive compounds are provided in percent, without taking into account the solvents. The manufacture and coating by way of a suitable solvent are performed as described above and known to the skilled person. Similarly, the manufacture and coating could be performed without solvents as a so-called hotmelt. Glas fibre reinforced plastic (GFRP) is used as the substrate used, as described above. Radiation is performed using a UVC lamp.

[0096] a) Share of Cyclo-Aliphatic Epoxy Resin

[0097] Uvacure 1534 is used as an example of cyclo-aliphatic epoxy resin. Desmomelt 530 is used as a film former, Araldite GT 7072 and D.E.R. 331 are the aromatic epoxy resins used. Moreover, the formulations include the epoxy-enhanced polyether compound D.E.R. 736P and the polyol PEG 400. The Chivacure 1176 is used as UV initiator.

[0098] Examples B1 through B7 show different contents of cyclo-aliphatic epoxy resin.

[0099] The epoxy equivalent EP.sub.tot is calculated using the above formula and in a mean value amounts to 200 g/eq. The share of the cyclo-aliphatic epoxy resin (here: Uvacure 1534) A.sub.cyc is then calculated based on the above formula. The share of the aromatic epoxy resin A.sub.arom is calculated accordingly via the share of the aromatic resins Araldite GT 7072 and D.E.R. 331. The same applies to the share of epoxy-enhanced polyether compound (here: D.E.R. 736 P) A.sub.ether.

TABLE-US-00002 TABLE 1 Formulations B1 to B7 B1 B2 B3 B4 B5 B6 B7 Desmomelt 530 11% 11% 11% 11% 11% 11% 11% Araldite GT 7072 38% 38% 38% 38% 38% 38% 38% D.E.R. 736 P 12% 12% 12% 12% 12% 12% 12% D.E.R. 331 30% 25% 20% 15% 10% 0% 0% Uvacure 1534 0% 5.0% 10% 15% 20% 30% 40% PEG 400 7.0% 7.0% 7.0% 7.0% 7.0% 7.0% 7.0% Chivacure 1176 2.0% 2.0% 2.0% 2.0% 2.0% 2.0% 2.0% Epoxy equivalent 371 372 373 374 375 377 357 in g/eq A.sub.arom 93% 90% 86% 83% 80% 73% 69% A.sub.cyc 0% 3% 7% 10% 13% 20% 25% A.sub.ether 7% 7% 7% 7% 7% 7% 6%

[0100] The share of the cyclo-aliphatic epoxy resin A.sub.cyc amounts from 0 to 25%. Accordingly, the share of the aromatic epoxy resin A.sub.arom is calculated as 93% to 69%. The share of the epoxy-enhanced polyether compound A.sub.ether is 7%.

TABLE-US-00003 TABLE 2 Measurement results B1 to B7 B1 B2 B3 B4 B5 B6 B7 Open time 30 min 20 min 15 min 10 min 5 min 4 min 2 min Time until 50 min 45 min 30 min 20 min 12 min 10 min 5 min handling strength: Quasi-static 18.7 MPa 15.3 MPa 15.5 MPa 17.1 MPa 18.0 MPa 14.0 MPa 13.1 MPa tensile shear strength

[0101] The results in table 1 and 2 show that the composition of the epoxy resins determines the open time. In formulations without cyclo-aliphatic epoxy resin (B1) the open time is 30 minutes. If the share of the cyclo-aliphatic resin Uvacure 1534 is increased, the open time reduces until it only amounts to 2 minutes any longer when the share is 2%. Times until handling strength are slightly longer accordingly. As a general rule, however, a large share of aromatic epoxy resins helps keeping the open time long. It has turned out that a large share of aromatic epoxy resin in the epoxy equivalent is advantageous, preferably a share of higher than 60%, in order to achieve a long open time.

[0102] As derives from the table, in B1 to B7 a structural bond is achieved (value higher than 6 MPa). It is not necessary to irradiate the already joined substrates the whole time. In fact, it is sufficient to irradiate the adhesive surface for a few seconds prior to joining. Afterwards, the adhesive film can still be joined. It is still adhesive and wets the second substrate to a sufficient extent. The joining also does not have to occur within seconds. The formulation can be adjusted such that an open time in a range of minutes is feasible. Joining takes place after the open time and without additional irradiation or heating a structural adhesive bond is obtained. By way of this open time it is possible to join substrates with the adhesive film regardless of their permeability for UV light.

[0103] b) Content of Epoxy-Enhanced Polyether Compound and Polyol Compound

[0104] Examples B8 to B11 show the significance of the epoxy-enhanced polyether compound in the formulation. D.E.R. 736 P was used as epoxy-enhanced polyether compound.

TABLE-US-00004 TABLE 3 Formulations B8 to B11 B8 B9 B10 B11 Desmomelt 530 11% 11% 11% 11% Araldite GT 7072 45% 45% 45% 45% D.E.R. 331 0% 0% 36% 45% D.E.R. 736 P 42% 36% 0% 0% PEG 400 0% 6.0% 6.0% 0% Chivacure 1176 2% 2% 2% 2% A.sub.arom 78% 81% 100% 100% A.sub.cyc 0% 0% 0% 0% A.sub.ether 22% 19% 0% 0%

TABLE-US-00005 TABLE 4 Measurement results B8 to B11 B8 B9 B10 B11 Open time 35 min 35 min 10 min <10 seconds Quasi-static tensile 16.5 MPa 14.1 MPa 4.2 MPa 1.9 MPa shear strength

[0105] Due to the D.E.R. 736, longer open times can be achieved, with or without polyethylene glycol. In the absence of D.E.R. 736, the polyol may extend the open time, but overall, the times are shorter and no structural adhesion strength is achieved (in B11). In this respect, the epoxy-enhanced polyether compound is advantageous for the properties of the adhesive tape.

[0106] Apart from the D.E.R. P, also other epoxy-enhanced polyether compounds are capable of fulfilling the same task, such as Araldite DY 3601 (company: Huntsman) and Heloxy Modifier WF (company: Hexion).

[0107] c) Polyol Content

[0108] In examples B12 to B19 the polyol share (here: PEG 400) varies whilst the median cyclo-aliphatic resin share remains constant and D.E.R 736 P only accounts for 12% in the formulation.

TABLE-US-00006 TABLE 5 Formulations B12 to B19 B12 B13 B14 B15 B16 B17 B18 B19 Desmomelt 530 11% 11% 11% 11% 11% 11% 11% 11% Araldite 38% 38% 38% 38% 38% 38% 38% 38% GT 7072 D.E.R. 736 P 12% 12% 12% 12% 12% 12% 12% 12% D.E.R. 331 22% 20% 18% 16% 15% 14% 12% 7% Uvacure 1534 15% 15% 15% 15% 15% 15% 15% 15% PEG 400 0% 2.0% 4.0% 6.0% 7.0% 8.0% 10% 15% Chivacure 1176 2% 2% 2% 2% 2% 2% 2% 2% A.sub.arom 84% 83% 83% 83% 83% 83% 83% 83% A.sub.cyc 10% 10% 10% 10% 10% 10% 10% 10% A.sub.ether 6% 7% 7% 7% 7% 7% 7% 7%

TABLE-US-00007 TABLE 6 Measurement results B12 to B19 B12 B13 B14 B15 B16 B17 B18 B19 Open time 30 sec 3 min 8 min 15 min 10 min 20 min 25 min 45 min Quasi-static 2.2 MPa 8.3 MPa 10.4 MPa 10.3 MPa 17.1 MPa 17.1 MPa 16.7 MPa 15.5 MPa tensile shear strength

[0109] It turns out that increasing the polyol compound share results in an increased open time. Reducing the polyol share to zero results in shorter open times. They are too short to still allow for the formation of a structural bond (see B12).

[0110] Examples B1 to B19 show that varying the shares of aromatic epoxy resin, cyclo-aliphatic epoxy resin, epoxy-enhanced polyether compound and polyol within the preferred limits allows for a variation of the open time between 10 seconds and 60 minutes.

[0111] d) Film Formers and UV Initiators

[0112] Polymers of ethylene vinyl acetate (EVA), polyamide (PA), polyurethane (PU), acrylate or a phenoxy resin can be used as film formers. Examples B20 to B24 show formulations with different film formers. The share of the epoxy-enhanced polyether compound is high at 33 to 35% so that also without polyol, sufficiently long open times (10 seconds to 60 minutes) and structural adhesive strength are generated.

TABLE-US-00008 TABLE 7 Formulations of examples B20 to B24 B20 B21 B22 B23 B24 Film former 13.6% 11.4% 13.6% 20.5% 27.8% Levamelt Desmomelt Phenoxy Levamelt Levamelt 900 530N Resin 900 456 PKHM-301 Araldite GT 7072 48.1% 52.6% 48.1% 41.2% 37% D.E.R. 736 35.2% 33% 35.2% 35.2% 32.2% Chivacure 1176 3.1% 3.1% 3.1% 3.1% 3.0% Epoxy equivalent in g/eq 409 424 409 393 391 A.sub.arom 82% 84% 82% 79% 79% A.sub.cyc 0% 0% 0% 0% 0% A.sub.ether 18% 16% 18% 21% 21%

TABLE-US-00009 TABLE 8 Measurement results B20 to B24 B20 B21 B22 B23 B24 Open time 30 minutes 25 minutes 30 minutes 30 minutes 20 minutes Quasi-static tensile shear 13.4 MPa 14.9 MPa 12.9 MPa 11.2 MPa 13.3 MPa strength after 24 h

[0113] Different UV initiators were used, on different chemical bases, with and without photo-sensitizer. Here it turns out (see also tables 9 and 10) that the type of UV initiator has an impact on the open time. However, the preferred open times of 10 seconds to 60 seconds can be achieved with different types of UV initiators.

TABLE-US-00010 TABLE 9 Formulations of examples B25 to B29 B25 B26 B27 B28 B29 Desmomelt 530 11.5% 11.5% 11.5% 11.5% 11.5% Araldite GT 53.0% 53.1% 53.1% 53.1% 53.1% 7072 D.E.R. 736 P 33.5% 33.0% 33.4% 31.5% 33.1% UV initiator 2.0% 2.4% 2.0% 3.9% 2% Chivacure Chivacure Omicat Omicat UV 387C + 1176 1190 432 550 0.3% Chivacure I-TX A.sub.arom 84% 84% 84% 85% 84% A.sub.cyc 0% 0% 0% 0% 0% A.sub.ether 16% 16% 16% 15% 16%

TABLE-US-00011 TABLE 10 Measurement results B25 to B29 B25 B26 B27 B28 B29 Open time 35 minutes 30 minutes 35 minutes 60 minutes 45 minutes Quasi-static tensile shear 13.6 MPa 14.7 MPa 11.8 MPa 12.1 MPa 17.5 MPa strength after 24 h

[0114] As expected, also the amount of the UV initiator plays a role. As to be expected, when the initiator concentration increases, the open time decreases (see tables 11 and 12), whereas to a large extent structural adhesive strength is still achieved. In turn, long open times are achieved due to a large share of D.E.R. 736 P. Adjusting the open time so as to achieve shorter and thus faster curing formulations could still be implemented by adding cyclo-aliphatic epoxy resin in accordance with examples B1 to B7.

TABLE-US-00012 TABLE 11 Formulations of examples B22 to B26 B22 B23 B24 B25 B26 Desmomelt 530 11.6% 11.5% 11.5% 11.4% 11.2% Araldite GT 7072 53.7% 53.3% 53.0% 52.5% 51.8% D.E.R. 736 P 33.6% 33.4% 33.2% 33.0% 32.5% Chivacure 1176 1.2% 1.7% 2.3% 3.1% 4.5%

TABLE-US-00013 TABLE 12 Measurement results B22 to B26 B22 B23 B24 B25 B26 Open time 60 minutes 45 minutes 35 minutes 25 minutes 15 minutes Quasi-static tensile shear 13.9 MPa 13.8 MPa 13.6 MPa 14.9 MPa 15.4 MPa strength after 24 h

[0115] e) UV Dosage

[0116] The open time can also be adjusted by regulating the UV dosage. A formulation (B27, consisting in 11.5% Desmomelt 530, 53.1% Araldite GT 7072, 16.1% D.E.R. 736 P, 17.1% Uvacure 1534, and 2.3% Chivacure 1176) is irradiated with different UV dosages (see table 13).

TABLE-US-00014 TABLE 13 Measurement results B27 UV dosage (UVC lamp) in 270 197 125 93 76 47 mJ/cm.sup.2 Open time in min 0.5 1 1.5 3 5 10

[0117] f) Shelf Life

[0118] The shelf life of the adhesive tapes (prior to joining) is determined based on a sample according to example 22, which is packed light-tight in an aluminium bag. It is stored at room temperature. Also after 6 months of storage, sufficient open time and structural adhesive strength have been maintained.

TABLE-US-00015 TABLE 14 Measurement results storage of B22 Storage time Fresh 2 months 6 months Open time 60 minutes 60 minutes 50 minutes Quasi-static tensile 13.9 MPa 13.8 MPa 12.0 MPa shear strength after 24 h