Compressible UV-activatable or thermally activatable (semi-) structural adhesive film that changes color after activation and after curing

Abstract

An adhesive film that can be wound and punched, comprising an epoxy-based adhesive compound that can be activated by UV-radiation or thermally and an expandable filler admixed to the adhesive compound to produce an adhesive film that is compressible when not yet cured.

Claims

1. An adhesive film that can be wound and punched, comprising an epoxy-based adhesive compound that can be activated by UV-radiation and/or thermally, wherein the adhesive compound comprises an admixed dye or an admixed pigment to produce a first color change after activation of the adhesive compound and a second color change after curing of the adhesive compound, and an expandable filler admixed to the adhesive compound to produce an adhesive film that is compressible when not yet cured.

2. The adhesive film that can be wound and punched of claim 1, wherein the adhesive compound further comprises: a. 2-50 percent by weight of film former, b. 10-70 percent by weight of aromatic epoxy resins, c. 0.5 to 7 percent by weight of a cationic initiator, d. 0.001-0.2 percent by weight of dye or pigment, e. cyclo-aliphatic epoxy resins, the cyclo-aliphatic epoxy resins not exceeding 35 percent by weight f. 0.1 to 70 percent by weight of at least one filler agent responsible for the expansion during drying g. 0-50 percent by weight of epoxy-enhanced polyether compounds, and h. 0-20 percent by weight of polyol, the shares adding up to 100 percent.

3. The adhesive film that can be wound and punched of claim 1, wherein the expansive filler agent exhibits an activation temperature between 30° C. and 150° C. and reaches a maximum degree of expansion between 40° C. and 150° C.

4. The adhesive film that can be wound and punched of claim 1, wherein the not yet cured adhesive film exhibits compression between 5 and 80% depending on filler agent concentration and expansion.

5. The adhesive film that can be wound and punched of claim 1, wherein the adhesive film is a UV-activatable transfer adhesive tape without backing.

6. The adhesive film that can be wound and punched of claim 1, wherein the adhesive film comprises a UV-transparent or UV-intransparent backing.

7. The adhesive film that can be wound and punched of claim 1, wherein the adhesive film comprises at least one UV-activatable or thermally activatable adhesive compound.

8. The adhesive film that can be wound and punched of claim 1, wherein 0.001 to 0.2 percent by weight of the dye or the pigment, respectively, are admixed to the adhesive compound.

9. The adhesive film that can be wound and punched of claim 8, wherein the dye or the pigment is an azo dye or an azo pigment.

10. The adhesive film that can be wound and punched claim 9, wherein the azo dye or the azo pigment belong to the azo substances that exhibit a color change subject to the effect of acid.

11. The adhesive film that can be wound and punched of claim 2, wherein the expansive filler agent exhibits an activation temperature between 30° C. and 150° C. and reaches a maximum degree of expansion between 40° C. and 150° C.

12. The adhesive film that can be wound and punched of claim 11, wherein the not yet cured adhesive film exhibits compression between 5 and 80% depending on filler agent concentration and expansion

13. The adhesive film that can be wound and punched of claim 11, wherein the expansive filler agent reaches a maximum degree of expansion between 60° C. and 130° C.

14. The adhesive film that can be wound and punched of claim 2, wherein the adhesive film is a UV-activatable transfer adhesive tape without backing.

15. The adhesive film that can be wound and punched of claim 14, wherein the adhesive film comprises at least one UV-activatable or thermally activatable adhesive compound.

16. The adhesive film that can be wound and punched of claim 2, wherein the adhesive film comprises a UV-transparent or UV-intransparent backing.

17. The adhesive film that can be wound and punched of claim 16, wherein the adhesive film comprises at least one UV-activatable or thermally activatable adhesive compound.

18. The adhesive film that can be wound and punched of claim 3, wherein the expansive filler agent reaches a maximum degree of expansion between 60° C. and 130° C.

19. The adhesive film that can be wound and punched of claim 8, wherein 0.01 to 0.07 percent by weight the dye or the pigment, respectively, are admixed to the adhesive compound.

20. The adhesive film that can be wound and punched of claim 19, wherein 0.015 to 0.04 percent by weight, respectively, of the dye or the pigment are admixed to the adhesive compound

Description

DETAILED DESCRIPTION OF EMBODIMENTS

[0066] Described hereinafter is the manufacture of an adhesive film and its UV-activation. Samples were produced using the manufacturing method described herein. The samples were subjected to various examinations to test their characteristics. The results of the tests are described in detail below.

[0067] Adhesive compound 1 comprises the following composition: [0068] a. 2-50 percent by weight of film formers, [0069] b. 10-70 percent by weight of aromatic epoxy resins, [0070] c. 0.5-7 percent by weight of a cationic initiator, [0071] d. 0.001-0.2 percent by weight of dye or pigment, [0072] e. cyclo-aliphatic epoxy resins, the cyclo-aliphatic epoxy resins not exceeding 35 percent by weight, [0073] f. 0.1 to 70 percent by weight of at least one additive responsible for the expansion during drying (expandable additive), [0074] g. 0-50 percent by weight of epoxy-enhanced polyether compounds, and [0075] h. 0-20 percent by weight of polyol,
the shares adding up to 100 percent.

[0076] Following UV-activation, the adhesive compound has an open time of 10 seconds to 60 minutes, during which the film is tacky.

[0077] The dye and the pigment, respectively, can be an azo dye or an azo pigment and in particular such azo substances that exhibit color change after exposure to acid. The table below lists a few exemplary azo dyes:

TABLE-US-00001 TABLE 1 Fett-Blau B 01 Blue anthraquinone dye by the company Clariant [Oil Blue B 01]. Produkte (Deutschland) GmbH. Orasol Yellow 081. Yellow metal complex dye by the company BASF Colors & Effects GmbH. Heliogen Green Green halogenated copper phthalocyanine pigment L 8730. by the company BASF Colors & Effects GmbH.

[0078] Color change following UV-activation or thermal activation occurs at additive volumes between 0.001 and 0.2 percent by weight of the dye and the pigment, respectively. In concentrations less than 0.001 percent by weight, the coloration of the adhesive compound is no longer visually detects able to ensure a sufficient extent of process reliability, in concentrations higher than 0.2 percent by weight the dye or the pigment and their amine groups or nitrogen compounds create an alkaline milieu, preventing a reaction of the superacid with the epoxy groups and an azo group.

[0079] The experiments indicated a range of 0.01 to 0.07 percent by weight, for example, 0.015 to 0.04 percent by weight of the dye and the pigment, respectively.

[0080] Subject to increased atmospheric humidity, the color change is less pronounced, because the resulting acid particles can bond to the OH ions of the water and thus to a lesser extent to the dye or the pigment, respectively. Simultaneously, in these cases the completely cured adhesive tape is less densely cross-linked, which manifests itself in decreased strength values in tensile testing and associated increased elongation at break values.

[0081] The two filler agents responsible for expansion are thermo-expandable micro spheres, which are produced by encapsulating liquid low evaporation hydrocarbons in thermoplastic polymer sleeves. Those typically exhibit particle sizes of 5 μm to 50 μm and their activation temperatures range from 30° C. to 300° C.

[0082] It derives from experiments that the temperature range for a maximum degree of expansion is between 40° C. and 150° C., for example, between 60° C. and 130° C. This results in an adhesive film that is compressible prior to final curing.

[0083] The adhesive compound containing solvents is applied on a silicon-enhanced polyester film (thickness: 50 μm) using a blade. Then it first is dried at room temperature for 10 minutes and then at 80° C. in a convection oven for 10 minutes.

[0084] The applied amount is set such that after drying (removal of the solvent mixture) a pressure-sensitive adhesive (tacky) film with a thickness of 150 μm is obtained.

[0085] No protective measures against UV-light are necessary regarding the handling of the raw materials, the adhesive and the coating. It is sufficient to work under regular laboratory conditions away from the UV-lamp. No further shielding is required.

[0086] The UV-source required for UV-activating the UV-activatable adhesive compound may for example be UV-C light from a discharging lamp or UV-A light from a UV-A-LED source.

[0087] Tests with a UV-C lamp are carried out in a UV-lab device by the company Beltron with a conveyor belt and a UV-C radiator with a radiation maximum at 256 nm. The conveyor belt is operated at 2 m/min. The radiation dosage in the UV-C range, measured using a UV Power Puck II by the company EIT Instrument Market Group, amounts to 197 mJ/cm.sup.2.

[0088] In the alternative, regardless of a substantially higher wavelength, the adhesive compounds can also be activated with a UV-LED device. Similar irradiation times as in the UV-C device are feasible, and the results regarding open time and adhesion strength are in the same range.

[0089] Tests with a UV-LED device are carried out with a LED spot lamp 100 by the company Hönle, comprising a UV-LED (wavelength 365 nm) and a radiation chamber. The samples are irradiated in the radiation chamber for 15 seconds. The radiation dosage, measured using a UV Power Puck II by the company EIT Instrument Market Group, amounts to 5000 mJ/cm.sup.2.

[0090] Below, we specified the terms open time, time until handling strength and curing time in further detail as they are to be understood in the context in hand.

[0091] Open time is considered the maximum feasible duration between the removal from the radiation belt (UV-C) or removal from the radiation chamber (UV-A), respectively, and the joining with the second substrate. During this period, the join parts can be joined. The open time is defined such that the adhesive layer is still pressure-sensitive adhesive (tacky). It is determined by finger-checking the tackiness of the surface of the adhesive films after radiation. Directly after radiation, the adhesive film is still tacky. After a certain time, the degree of tangible tack decreases and diminishes further until eventually the surface is non-tacky. The open time is determined as per the point in time when tack tangibly decreases so conceivably that afterwards no tack remains.

[0092] It turns out that as long as the surfaces are still tacky, joining is possible and that the subsequent curing results in a homogeneous adhesive bond. The curing process proceeds more and more to the same extent as the surfaces gradually lose tack so that ultimately no joining is possible any longer. This is reflected in the significantly reduced strength values determined based on the quasi-static tensile shear strength.

[0093] Additionally, by way of the color change, which is novel for such adhesive films, activation can be detected, and the additional color change over time following activation allows for a determination of the open time. The adhesive films are joined directly after UV-activation.

[0094] The curing time is the period between the joining of the adhesive partners 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 the quasi-static tensile shear strength was measured. When a value in excess of 6 MPa is achieved, structural strength or structural bonding is obtained. Due to the added dye and the added pigment, respectively, the color hue allows for the determination whether the formulation has cured completely. The adhesive compound has fully reacted in terms of its cross-linking status in relation to the temperature provided during the cross-linking.

[0095] Sufficient open time is desired for application. Rapid 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 prolonged fixing of the parts, respectively. For full curing, however, 24 hours is sufficient because according to experience only after that amount of time the bond is subjected to its final load (permanent load or shock loads).

[0096] Open time and curing time are consequences of the reaction speed of the curing reaction. This reaction starts with the UV-activation and ends with the complete 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 with different reaction speeds may take place, there may be delays and accelerations, resulting in a specific overall open time and curing time. The open time and the curing time can be controlled via the formulation, the radiation type and intensity and duration as well as thermal management (temperatures) during the gluing process.

[0097] The time until handling strength means the period that elapses after the joining step until the strength of the bond is so high that glued parts can already be transported and processed further. Experience has shown that handling strength is achieved once the quasi-static shear strength has reached 2 MPa. This strength allows for sufficient leeway for the loads that occur during an industrial manufacturing process.

[0098] Table 2 lists the raw materials used:

TABLE-US-00002 TABLE 2 Phenoxy Resin Solid phenoxy resin by the company Gabriel PKHH-25-B Chemie GmbH D.E.R. 331 Epoxy resin based on bisphenol-A-diglycidylether with an epoxy equivalent weight of 182-192 g/eq by the company DOW Chemical Co. D.E.R. 736 Epoxy resin based on propylene glycol diglycidylether with an epoxy equivalent weight of 175-205 g/eq by the company DOW Chemical Co. Struktol Polycavit Epoxy functional polyester polyether co-polymer 3550 with an epoxy equivalent weight of 1800 g/eq by the company Firma Schill + Seilacher “Struktol”. Araldite DY 3601 Di-functional, reactive thinning agent for epoxy resins by the company HUTSMAN HOLLAND BV. Omnicat 432 Hexa-fluor-phosphate based photo-initiator by the company IGM RESINS. Dynasylan Ethoxy silane based gluing agent by the company GLYEO EVONIK Industries. Fett-Blau B 01 Blue anthraquinone dye by the company Clariant [Oil Blue B 01]. Produkte (Deutschland) GmbH. Microsphere F-35D Thermally expandable filler by the company Microsphere F-36D MATSUMOTO YUSHI-SEIYAKU Co., Ltd. Expancel 920 Thermally expandable filler by the company DU 40 Nouryon Akzo Nobel Chemicals GmbH

[0099] Test Methods

[0100] a) Color Change

[0101] The color change is observed visually and documented photographically. The process is documented prior to activation by temperature or UV-radiation, immediately following activation and 24 hours after activation. The indicated color hue reflects to the perception of five different observers involved in the test.

[0102] a) Quasi-Static Tensile Shear Test

[0103] In order to determine parameters for the adhesive strength on FRE, tensile shear tests are carried out according to DIN EN 1465 (2009) at 23° C.±2° C. and 50%±5% relative humidity at a testing speed of 2 mm/min. The substrates are cleaned with isopropanol and joined afterwards. The curing achieved using UV-light, and the mechanical check is performed 24 h after activation. The results are indicated in MPa (N/mm.sup.2). The figures stated are the mean value based on five measurements including standard deviation.

[0104] c) Peel Test

[0105] The peel resistance of the cured adhesive tapes on a typical automotive paint, e.g. PPG 2K-ApO Klarlack 1.2 [A-6203512] determined according to DIN EN 1939 (1996) at 23° C.±2° C. and 50%±5% relative humidity at a testing speed of 100 mm/min and a peel-off angle of 90°. The samples are cured using UV-light and tested 24 h after activation. The results are indicated in N/mm. The figures stated are the mean value based on five tear resistance measurements including standard deviation.

[0106] d) Tensile Test

[0107] In order to determine parameters for the strength of the adhesive film alone in its cured state, tensile tests are carried out according to DIN EN 527 (2012) at 23° C.±2° C. and 50%±5% relative humidity at a testing speed of 10 mm/min. To this end, strips of a width of 19 mm and of a length of 100 mm are cut out of completely cured adhesive films. In the results illustrated, the layer thickness amounts to 0.2 mm. The samples are cured using UV light and tested 24 h after activation. The results are indicated in MPa (N/mm.sup.2). The figures stated are the mean value based on five measurements including standard deviation.

[0108] e) Expansion

[0109] Expansion after drying is carried out by measuring the layer thickness using a thickness gauge. A sample without fillers is used as reference that was produced with the same coating parameters. Measurements were taken once immediately after coating and drying of the adhesive compound as described above in the section on production of the pressure-sensitive adhesive compounds. Expansion derives from the following formula:

[00001]$\mathrm{Expansion}\left[\%\right]=\frac{100\%.Math.\mathrm{Thickness}\mathrm{of}\mathrm{the}\mathrm{adhesive}\mathrm{film}\mathrm{modified}\mathrm{with}\mathrm{filler}\mathrm{agents}}{\mathrm{Thickness}\mathrm{of}\mathrm{the}\mathrm{reference}\mathrm{without}\mathrm{filler}\mathrm{agents}}$

[0110] f) Compression

[0111] Compression is measured using a microscope of the type Keyence VHX-5000 with a magnification of 20×100. To this end, an adhesive film is clamped between two metal substrates, and film thickness is measured in the unloaded state. Subsequently, the clamped-in adhesive film is exposed to a load of 100 N and film thickness is measured once more. Compression derives from the following formula:

[00002]$\mathrm{Compression}\left[\%\right]=\frac{100\%.Math.\mathrm{Thickness}\mathrm{of}\mathrm{the}\mathrm{adhesive}\mathrm{film}\mathrm{subject}\mathrm{to}\mathrm{load}\left[\mathrm{mm}\right]}{\mathrm{Thickness}\mathrm{of}\mathrm{the}\mathrm{adhesife}\mathrm{film}\mathrm{without}\mathrm{load}\left[\mathrm{mm}\right]}$

[0112] Samples:

[0113] Table 3 summarizes samples regarding the compositions in respect of the selection of the expandable filler agents, with the volume specifications indicating parts by weight. K1 to K3 are formulations according to the disclosure with expandable filler agents. V1 is an adhesive transfer film without added expandable filler agent:

TABLE-US-00003 TABLE 3 Sample: K1 K2 K3 V1 Phenoxy Resin PKHH-25-B 40.0 40.0 40.0 40.0 D.E.R 331 17.5 17.5 17.5 17.5 D.E.R. 736 13.4 13.4 13.4 13.4 Struktol Polycavit 3550 25.0 25.0 25.0 25.0 Araldite DY 3601 4.1 4.1 4.1 4.1 Omnicat 432 1.5 1.5 1.5 1.5 Dynadylan GLYEO 0.7 0.7 0.7 0.7 Microsphere F-35D 6.3 — — — Microsphere F-36D — 6.3 — — Expancell 920 DU 40 — — 6.3 — Methyl ethyl ketone 55.2 55.2 55.2 55.2

[0114] Table 4 summarizes the results of the tensile shear, tensile and peel tests as well as the associated expansion and compression.

TABLE-US-00004 TABLE 4 Sample: K1 K2 K3 V1 Tensile shear 4.1 ± 0.6 4.4 ± 0.7 6.2 ± 0.9 6.3 ± 1.0 strength (AF/CF) (AF/CF) (AF/CF) (AF/CF) [MPa] Peel resistance 1.64 ± 0.08 1.78 ± 0.12 1.24 ± 0.09 1.31 ± 0.06 [N/mm] (AF) (AF) (AF) (AF) Tensile 1.9 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 2.1 ± 0.5 strength [MPa] Expansion after 180 ± 10  200 ± 10  0 0 drying [%] Compression approx. 30 approx. 30 0 0 not-cross- linked @ 23° C. [%] Key: AF: Adhesion Failure; CF: Cohesion Failure

[0115] Adhesive films K1 and K2, K3 and V1 all feature the same UV-activatable adhesive compounds. Only the expandable filler agent is varied to illustrate differences in the selection of the expandable filler agent.

[0116] The adhesive films according to K1, K2, K3 and V1 are not significantly different in terms of tensile shear strength within the range of the standard deviation. Thus it can be shown that the use of the expandable filler agent to achieve an adhesive film that is compressible prior to curing has no negative effect on these mechanical parameters. Also the tensile shear strength of the adhesive films according to K3 and V1 remains within the range of the standard deviation without significant differences. Here, the tensile shear strength values of the formulations K1 and K2 are slightly decreased, which is caused by the expansion.

[0117] Adhesive films K1 and K2 exhibit higher peel resistance than the films K3 and V1, which is due to the positive peel properties of the compressible adhesive film.

[0118] Expansion following drying at 90° C. for 10 minutes differs significantly for each different filler agent used. Adhesive compounds K1 and K2, where similar expandable filler agents were used, exhibit expansion of ca. 180% and 200%, respectively, whereas adhesive film K3, just like the reference without filler agent, exhibits no expansion, which in K3 is due to the different properties of the expandable filler agent employed in comparison to K1 and K2.

[0119] Due to the testing method used, compression could not be measured exactly. It very much depends on the filler agent concentration, the adhesive film thickness and previous expansion. For adhesive films K1 to K2 compression after pre-cross-linking at 90° C. for 10 minutes amounted to ca. 30%. Due to the different type of expansive filler agent or the absence of such filler agent, adhesive compounds K3 and R1 are not compressible.

[0120] As far as applicable, all individual features shown in the sample embodiments can be combined to and/or exchanged without leaving the scope of the invention.