ADHESIVE FILM THAT CAN BE WOUND AND STAMPED

Abstract

An adhesive film that can be wound and stamped having an epoxy-based adhesive compound that can be activated thermally or by UV radiation, and a dye and/or pigment mixed into the adhesive compound for producing a first color change after the activation of the adhesive compound and a second color change after the curing of the adhesive compound.

Claims

1. An adhesive film configured to be wound and stamped, comprising an epoxy-based adhesive compound adapted to be activated thermally or by UV radiation, and a dye and/or pigment mixed into the adhesive compound for producing a first color change after the activation of the adhesive compound and a second color change after a curing of the adhesive compound.

2. The adhesive film of claim 1, wherein 0.001 to 0.2 percent by weight of the dye or a pigment mixed into the adhesive compound.

3. The adhesive film of claim 1, wherein the dye or the pigment is an azo dye or an azo pigment.

4. The adhesive film of claim 3, wherein the azo dye or the azo pigment are comprised by the azo substances that exhibit a color change subject to an effect of an acid.

5. The adhesive film of claim 1, wherein the adhesive compound further comprises: a. 2 to 50 percent by weight of film formers, b. 10 to 70 percent by weight of aromatic epoxy resins, c. 0.5 to 7 percent by weight of a cationic initiator, d. 0.001 to 0.2 percent by weight of the dye or pigment, e. cycloaliphatic epoxy resins, wherein the cycloaliphatic epoxy resins do not exceed 35 percent by weight, f. 0 to 50 percent by weight of epoxidised polyether compounds, and g. 0 to 20 percent by weight of a polyol, wherein the proportions add up to 100%.

6. The adhesive film of claim 1, wherein the adhesive film is a UV-activatable transfer adhesive tape without backing.

7. The adhesive film of claim 6, wherein the adhesive film comprises different adhesive compound systems, at least one of which is a UV-activatable system.

8. The adhesive film of claim 1, wherein the adhesive film comprises a UV-transparent backing.

9. The adhesive film of claim 8, wherein the adhesive film comprises at least one UV-activatable or thermally activatable adhesive compound.

10. The adhesive film of claim 2, wherein 0.01 to 0.07 percent by weight by weight of the dye or the pigment is mixed into the adhesive compound.

11. The adhesive film of claim 10, wherein 0.015 to 0.04 percent by weight of the dye or the pigment is mixed into the adhesive compound.

12. The adhesive film of claim 1, wherein the adhesive film comprises a UV-non-transparent backing.

13. The adhesive film of 2, wherein the dye or the pigment is an azo dye or an azo pigment.

14. The adhesive film of claim 13, wherein the azo dye or the azo pigment are comprised by the azo substances that exhibit a color change subject to the effect of an acid.

15. The adhesive film of 5, wherein the dye or the pigment is an azo dye or an azo pigment.

16. The adhesive film of claim 15, wherein the azo dye or the azo pigment are comprised by the azo substances that exhibit a color change subject to the effect of an acid.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0060] Embodiments of the disclosure are illustrated in further detail by the subsequent description of the Figures. In this:

[0061] FIG. 1 schematically shows a UV-activatable adhesive compound prior to activation.

[0062] FIG. 2 schematically shows a UV-activation of the UV-activatable adhesive compound of FIG. 1;

[0063] FIG. 3 schematically shows a color change of the UV-activatable adhesive compound immediately after activation; and

[0064] FIG. 4 schematically shows a color change of the UV-activatable adhesive compound 24 h after activation.

DETAILED DESCRIPTION

[0065] Various embodiments now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments by which the innovations described herein can be practiced. The embodiments can, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense.

[0066] Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “herein” refers to the specification, claims, and drawings associated with the current application. The phrase “in an embodiment” as used herein does not necessarily refer to the same embodiment, though it can. Furthermore, the phrase “in another embodiment” as used herein does not necessarily refer to a different embodiment, although it can. Thus, as described below, various embodiments can be readily combined, without departing from the scope or spirit of the invention.

[0067] In addition, as used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. The term “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.”

[0068] Described hereinafter is the manufacture of an adhesive film and its UV-activation with reference to the accompanying Figures. Samples were produced using the manufacturing method described herein. The samples were subjected to various examinations to test their properties. The results of the tests are described in detail below.

[0069] The adhesive compound has the composition as described above.

[0070] FIG. 1 schematically shows a UV-activatable adhesive compound 1. Adhesive compound 1 comprises the following composition: [0071] a. 2-50 percent by weight of film formers, [0072] b. 10-70 percent by weight of aromatic epoxy resins, [0073] c. 0.5-7 percent by weight of a cationic initiator, [0074] d. 0.001-0.2 percent by weight of a dye or pigment [0075] e. cycloaliphatic epoxy resins, wherein the cycloaliphatic epoxy resins do not exceed 35 percent by weight, [0076] f. 0-50 percent by weight of epoxidised polyether compounds, and [0077] g. 0-20 percent by weight of a polyol,

[0078] wherein the proportions add up to 100%.

[0079] Following UV-activation, the adhesive compound has an open time of 10 seconds to 60 minutes, during which the film is tacky.

[0080] The dye and the pigment, respectively, preferably is an azo dye or an azo pigment and in particular such an azo substance that exhibits color change after exposure to acid. The table copied in below lists a few exemplary azo dyes:

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

[0081] Color change after 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 of less than 0.001 percent by weight, the coloration of the adhesive compound is no longer sufficiently visually detectable to provide satisfactory process liability, 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.

[0082] The experiments resulted in a range of 0.01 to 0.07 percent by weight and particularly preferably of 0.015 to 0.04 percent by weight of the dye or the pigment, respectively.

[0083] Subject to increased atmospheric humidity, the color change is less pronounced, because the resulting acid particles preferably bond to the OH.sup.− ions of the water and thus to a lesser extent to the dye or the pigment, respectively. Simultaneously, in these cases the cured adhesive tape is less densely cross-linked, which manifests itself in decreased strength values in the tensile test and associated increased elongation at break values.

[0084] The adhesive compound containing solvents is applied to a siliconised polyester film (thickness: 50 μm) using a coating knife. 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 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 during handling of the raw materials, the adhesive or for the coating. It is sufficient to work under regular laboratory conditions away from the UV-lamp. No further shielding is required.

[0086] FIG. 2 schematically shows the UV-activation of the UV-activatable adhesive compound 1 by means of a UV source 2. The UV-source 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 an irradiation chamber. The samples are irradiated in the irradiation 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] FIG. 3 shows the adhesive compound 1 after a first color change, triggered by the UV-activation. The cause of the color change is the dye or the pigment contained in the adhesive compound in the form of an azo dye or azo pigment. Dyes and pigments of the azo group are capable of changing color by protonation when the pH values fall below an appropriate level.

[0091] FIG. 4 schematically shows the color change of a UV-activatable adhesive compound 1 24 h after activation. This adhesive compound has fully reacted in terms of its cross-linking status in relation to the temperature provided during the cross-linking.

[0092] Below, the terms open time, time until handling strength and curing time are specified in further detail.

[0093] Open time is considered the maximum feasible period between the removal from the irradiation belt (UV-C) or removal from the irradiation chamber (UV-A), respectively, and the point in time of 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) during this period. It is determined by finger checking the tackiness 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 tangible tack decreases and continues to decrease until eventually the surface is non-tacky. The open time is determined as per the point in time when tack tangibly decreases so that afterwards no tack remains.

[0094] 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 advanced 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.

[0095] Additionally, by way of this new color change 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.

[0096] Curing time: The curing time is the period between the joining and the final strength. All sample formulations are fully cured after a maximum of 24 hours. Therefore, in most cases the waiting time was 24 hours before the quasi-static tensile shear strength was measured. When a value of more than 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. This state is shown in FIG. 4. Adhesive compound 1 has fully reacted in terms of its cross-linking status in relation to the temperature provided during the cross-linking.

[0097] For usage, a 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 omit fixing 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 the final load (permanent load or shock loads).

[0098] 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 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 the 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 irradiation type and intensity and duration as well as thermal management (temperatures) during the bonding process.

[0099] The time until handling strength means the period that elapses after the joining step until the strength of the bond is so high that bonded parts can 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 tolerance for the loads during an industrial manufacturing process.

[0100] Test Methods

[0101] a) Color Change

[0102] 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.

[0103] b) Quasi-Static Tensile Shear Test

[0104] In order to determine parameters for the adhesive strength on FRP, 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 is 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.

[0105] c) Peel Test

[0106] The peel resistance of the cured adhesive tapes on glass is 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.

[0107] d) Tensile Test

[0108] 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 from 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.

Examples

[0109] Table 2 summarises examples regarding the compositions in respect of the selection of the dyes or pigments, with the volume specifications indicating parts by weight. K1 to K4 are formulations according to the disclosure with dyes and pigments, respectively. V1 is an adhesive transfer film without added dye or pigment:

TABLE-US-00002 TABLE 2 Example: K1 K2 K3 K4 V1 Adhesive Compound (AC) AC 1 AC 1 AC 1 AC 1 AC 1 Fat Blue B01 0.02 0.01 — — — Orasol Yellow 081 — 0.02 0.02 — — Heliogen Green L8730 — — — 0.19 —

[0110] Table 3 summarises the results of the tensile shear, tensile and peel tests as well as the color change.

TABLE-US-00003 TABLE 3 Example: K1 K2 K3 K4 V1 Tensile shear 6.8 ± 1.0 7.1 ± 0.7 7.7 ± 0.9 0.3 ± 0.1 6.7 ± 1.0 strength [MPa] (CF) (CF) (CF) (AF) (AF/CF) Resistance to 0.73 ± 0.02 0.58 ± 0.12 0.44 ± 0.03 Not 0.79 ± 0.06 peel [N/mm] (AF) (AF) (AF) measurable (AF) Tensile strength [MPa] 1.9 ± 0.1 1.6 ± 0.1 1.7 ± 0.1 Not 2.1 ± 0.5 measurable Color change From cyan From green No color No color No color immediately after blue to pink- to colorless change change change activation (visual) violet Key: AF: Adhesion Failure; CF: Cohesion Failure

[0111] Adhesive films K1 and K2, K3, K4 and V1 all feature the same UV-activatable adhesive compounds. Only the respective dye or pigment, respectively, was varied to illustrate differences in the selection of the colorants.

[0112] The adhesive films according to K1, K2, K3 and V1 are not significantly different in terms of tensile shear strength and peel resistance within the range of the standard deviation. Thus, it is shown that the use of the colorant or the pigment for activation and curing control, respectively, has no negative effect on these mechanical parameters. The peel resistance of the adhesive films according to K1 and V1 is within the range of the standard deviation without significant differences, too. Here, the peel resistance values of the formulations K2 and K3 are slightly decreased, which is due to the influence of the yellow dye.

[0113] K4 exhibits no measurable tensile strength and peel resistance, the tensile shear strength is on the same level as in the non-cured adhesive film. Hence, it was possible to render proof of the upper dye or pigment concentration.

[0114] Table 4 shows the state where the adhesive films were conditioned in a different manner prior to activation. In example K5, the adhesive compound was applied on the adhesive surface without direct ingress of moisture. In example K6, the adhesive compound was conditioned at 70% relative atmospheric humidity for one hour and subsequently activated.

TABLE-US-00004 TABLE 4 Example: K5 K6 Adhesive Compound (AC) AC 1 AC 1 Fat Blue B01 0.02 0.02 Temperature [° C.] 23 ± 2 23 ± 2 Relative atmospheric humidity Without direct ingress 70 ± 5 [%] of moisture in respect of the adhesive surface

[0115] Table 5 summarises the results of the tensile tests and the color change immediately after activation.

TABLE-US-00005 TABLE 5 Example: K5 K6 Tensile strength [MPa] 1.4 ± 0.1 0.8 ± 0.1 Elongation at break [%] 200 ± 10 400 ± 40 Color change immediately after From cyan blue to From cyan blue to activation (visual) pink-violet light blue

[0116] The tensile strengths of the adhesive films according to K5 and K6 show a difference in terms of tensile strength and elongation at break within the range of the standard deviation. Elongation at break is twice as high at 400% for the adhesive film stored subject to increased atmospheric humidity for one hour. Tensile shear strength of the adhesive film stored in the absence of direct ingress of moisture is ca. 0.6 MPa higher compared to the adhesive film conditioned at 70% relative atmospheric humidity. The difference is significant in terms of color change, too. In adhesive film K5, a color change from cyan blue to pink-violet was detected immediately after activation, whereas in K6, subject to increased ingress of humidity, only a color change from cyan blue to light blue could be detected. Hence, it was possible to prove the influence of the ambient atmospheric humidity on color change and tensile strength and elongation at break, respectively.

[0117] Insofar as applicable, all individual features illustrated in the sample embodiments can be combined and/or exchanged without leaving the scope of the disclosure.

LIST OF REFERENCE NUMERALS

[0118] 1 adhesive compound [0119] 2 UV radiation source