METHOD FOR CONNECTING TWO JOINING ELEMENTS

Abstract

The present invention relates to a method for connecting two joining elements, these elements being connected by means of a thermally activatable adhesive with a flat heating element arranged therein, by suitable heating of the adhesive. The invention also relates to an assembly produced in this way from two joining elements and to an arrangement designed for carrying out a corresponding method.

Claims

1. A method for connecting two joining elements, comprising the steps of: a) providing the two joining elements, b) providing a thermally activatable adhesive, c) providing a flat heating element, d) arranging the heating element in the adhesive, e) arranging the heating element and the adhesive between the joining elements, f) heating the adhesive by applying electrical power to the heating element while at the same time measuring the temperature of the adhesive, the extent to which electrical power is applied to the heating element being controlled in dependence on the temperature measured, the measurement of the temperature of the adhesive being performed by determining the resistance of the heating element.

2. The method as claimed in claim 1, wherein the resistance is determined by means of measuring the voltage and current intensity, with a time difference between the measurement of the current intensity and the measurement of the voltage of 10 ms, preferably 1 ms, more preferably 100 s and particularly preferably 10 s.

3. The method as claimed in claim 1, wherein the control of the electrical power is performed in dependence on a temperature-resistance dataset, which comprises data materially and geometrically specific to the heating element used.

4. The method as claimed in claim 3, wherein the geometry-dependent characteristic data of the heating element required for the temperature calculation are determined by means of an automatic resistance measurement, before or during the heating.

5. The method as claimed in claim 1, wherein the control of the electrical power is performed in dependence on a time-temperature curve.

6. The method as claimed in claim 1, wherein the control of the electrical power takes place with a maximum delay of 1000 ms, preferably 500 ms, more preferably 250 ms.

7. The method as claimed in claim 1, wherein the adhesive is solid in the non-activated state.

8. The method as claimed in claim 1, wherein the adhesive is a thermoset in the cured state.

9. The method as claimed in claim 1, wherein at least one of the joining elements, preferably both joining elements, is/are a plastic or consists of other non-metallic materials.

10. The method as claimed in claim 1, wherein at least one joining element is selected from the group consisting of bolts, angles, plates, holders and fittings.

11. An arrangement designed for carrying out the method as claimed in claim 1, comprising (i) contacts for contacting a heating element for applying electrical power, (ii) a thermally activatable adhesive in which a flat heating element is arranged and (iii) a control unit for controlling the electrical power in dependence on the temperature of the adhesive.

Description

EXAMPLES

Example 1

Determination of the Temperature Coefficient of the Electrical Resistance for a Stainless Steel Foil

[0090] A stainless steel foil (steel: 1.4310, width: 12.7 mm, length: 25 mm, thickness: 0.01 mm) was provided with holes by analogy with FIG. 3c (hole diameter: 0.5 mm). The resistance of the foil was measured with the aid of a 4-wire measurement in the temperature range between room temperature and 200 C. The measuring device used (HP 34401A) had a resolution of 0.1 mOhm.

[0091] By the measurement, the linear temperature coefficient of the electrical resistance was determined as 78020 ppm/K (also compare FIG. 4: Change of the resistance of a stainless steel foil (steel: 1.4301) with temperature). With the given arrangement comprising the measuring device and the steel foil, a temperature measurement can therefore be carried out with an accuracy of 3 K.

Example 2

Rapid Heating of a Test Arrangement Comprising a Heating Foil and PTFE Substrates

[0092] A stainless steel foil as described in Example 1 was positioned between two sheets of PTFE (25254 mm.sup.3). In direct contact with the stainless steel foil was a thermocouple of type K for temperature detection. The foil was supplied with power by a controllable power supply unit (Delta Elektronika SM7020). The maximum current intensity was in this case limited to 7.5 A. For detecting the temperature and controlling the heating power, a controller developed at the Fraunhofer IFAM (IFAM IHC) was used.

[0093] For the rapid heating test, a setpoint temperature of T.sub.set=180 C. was prescribed. Altogether, the temperature was recorded over a time period of 180 s, in which the heating was only activated in the first 60 s.

[0094] In the test, the prescribed setpoint temperature was reached within 20 seconds, which corresponds to an average heating-up rate of about 8 K/s (compare FIG. 5a: Temperature curve during the controlled heating up of the test arrangement. The target temperature was at 180 C. After 60 seconds, the heating was switched off). A maximum heating power of about 20 W was applied to the stainless steel foil. In the steady-state phase of the test, the temperature was maintained with great accuracy (FIG. 5b: Deviations of the actual temperature from the setpoint temperature). The maximum deviation from the setpoint temperature was below 1 K.

Example 3

Rapid Curing of Adhesive Bonding Specimens (Temperature Control by Means of Thermocouple)

[0095] Adhesive bonding specimens were produced with a tensile lap-shear geometry in accordance with DIN EN 1465. Substrates of glass-fiber reinforced epoxy resin (GRP) were used in each case for this. Serving as the adhesive was IFAM PASA-EH1, which at room temperature has the consistency of a plastic film with low tackiness. For the rapid curing, patches in the format 2512.50.5 mm.sup.3 were produced from the adhesive with a perforated stainless steel foil and a thermocouple as the middle layer. As shown in FIG. 6 (tensile lap-shear specimen before and after the rapid curing of the adhesive by electrical resistance heating), these patches were placed in the overlapping region (2512.5 mm.sup.2) of the GRP joining parts. Curing by resistance heating was performed in the same test arrangement and with the same temperature program as described in Example 2. As a control, tensile lap-shear specimens with the adhesive PASA EH1 were conventionally cured in a circulating-air oven without a heating/sensor element (30 minutes at 180 C.).

[0096] During the curing test, the setpoint temperature of 180 C. was likewise reached after about 20 seconds and was maintained during the steady-state phase of the curing cycle with an accuracy of 1 K (FIG. 7a: Temperature profile during the rapid curing of the adhesive PASA EH1 between GRP joining parts, FIG. 7b: Deviation from the setpoint temperature (180 C.)). The heating first caused the adhesive to liquefy and partially come out of the bonding gap. After removing the tensile lap-shear specimen from the mount, the adhesive appeared to be completely cured. This also applies to the regions that had left the bonding gap, which are not in such intensive thermal contact with the heating element as the material in the bonding joint. Bubbling in the adhesive and outgassing of volatile components (smoke emission) only occurred to a very slight extent.

[0097] With conventional oven curing, the adhesive achieved a tensile lap-shear strength of about 15 MPa (FIG. 8: Tensile lap-shear strength of the adhesive PASA EH1 between GRP joining parts after conventional and rapid curing). Although the strength of the specimens rapidly cured by electrical resistance heating was well below this value at about 10 MPa, the strength is already sufficient for many applications (even for structural connections). The reason for the reduced strength is probably not to be sought in the different curing conditions but in the fact that the temperature sensor used in the tests described weakens the structure of the adhesive joint due to a relatively thick supply lead wire (diameter about 0.5 mm). Better integration of the heating element and the sensor provides possibilities for improvement here.

Example 4

Rapid Curing of Two Different Adhesives

(Temperature Control by Means of the Change in Resistance in the Heating Element)

[0098] The performance capability of rapid curing by means of resistance heating of the adhesive joint was to be demonstrated by the example of two adhesives with very different sets of properties:

Adhesive 1:

[0099] The adhesive AF 163_2U (3M) at room temperature takes the form of a flexible, substantially non-tacky film, which by heating first melts and thereby cures to form a thermosetting material. The adhesive represents a latent one-component system, i.e. the adhesive contains both an epoxy resin and the necessary catalyst in a premixed form. At normal ambient temperature, the curing only takes place very slowly (latency), so that at room temperature the adhesive film can be handled for several hours. On the other hand, it must be refrigerated at 18 C. for storing over longer time periods. The adhesive is intended for thermal curing (for example 90 minutes in the circulating-air oven at 120 C.).

Adhesive 2:

[0100] The adhesive EC 7256 (3M) cures already at room temperature after a short time when the two components have been mixed (processing time according to technical data sheet: 12 minutes, handling stability: about 80 minutes). The curing can be accelerated by heating.

General Experimental Methods

[0101] The bonding strengths of adhesively bonded specimens were measured by tensile lap-shear tests in accordance with DIN EN 1465. For both adhesive systems, the strengths of specimens cured conventionally in ovens were compared with those of rapidly cured specimens. [0102] Joining parts: glass-fiber reinforced epoxy (GRP) (10025 mm.sup.2) [0103] Preparation: superficial grinding of the joining parts, followed by cleaning with 2-propanol [0104] Oven curing: 2 hours at 120 C. in a circulating-air oven [0105] Tensile lap-shear testing at room temperature, testing rate: 10 mm/min, for every 5 test pieces

Heating Element

[0106] Used as the heating element for the electrical resistance heating of the adhesive joint was a laser-perforated stainless steel strip (alloy: 1.4310) with a thickness of 10 m, analogous to FIG. 2c. The heating element has at room temperature a typical resistance of 0.5 Ohm. The heating element was embedded in the middle of the adhesive layers.

Measuring the Temperature in the Adhesive Joint by Means of the Resistance of the Heating Element

[0107] The temperature dependence of the electrical resistance of the heating strip, which in the temperature range of interest is linear (cf. FIG. 4: Change of the resistance of a stainless steel foil (steel: 1.4301) with temperature) was used for measuring the temperature in the adhesive joint. The resistance measurement itself was performed by simultaneously measuring the voltage applied to the heating strip and the current flowing (cf. FIG. 1b), with the aid of two highly resolving 24-bit analog-digital converters. This ultimately allows a temperature measurement with an accuracy of about 1 K (Table1).

TABLE-US-00001 TABLE 1 Characteristic values for the data acquisition of current, voltage, resistance of the heating element and temperature. rms accuracy of 100 V voltage measurement rms accuracy of 200 A current measurement rms accuracy of 300 Ohm resistance measurement rms accuracy of 0.9 K temperature measurement Data sampling rate 4 Hz

Temperature Control

[0108] Before the actual heating run, each heating element was individually calibrated against a thermocouple. This was a 1-point calibration at room temperature. The slope of the resistance characteristic of the heating element was determined in a previous measurement and then treated as a material constant applicable to each of the heating elements used.

[0109] The temperature measured by means of the heating element served as an input variable for a control circuit. A software PID controller, which activated a DC voltage source (maximum voltage: 5 V, maximum current 10 A), was used for the control.

[0110] A particular feature of the method used for temperature detection and control is that the base variables for the temperature measurement (current and voltage) are measured absolutely synchronously. This is important because, in the present arrangement, the heating element serves at the same time as a temperature sensor and changes of the current flow must not have any retroactive effects on the resistance measurement. If the detection of current and voltage were not synchronous, there would be excessive systematic errors in the resistance measurement and also in the temperature measurement. This applies in particular when there are rapid changes over time in the manipulated variable (i.e. the heating voltage), as are necessary for efficient and quick control.

Temperature Program for the Rapid Curing of the Adhesives

[0111] The temperature program for the adhesive curing comprised a rapid linear heating ramp, and an isothermal curing phase and subsequent cooling down without further energy input by the heating element. The parameters for the curing are compiled in Table 2.

TABLE-US-00002 TABLE 2 Temperature programs for rapid curing. Heating ramp Isothermal phase Adhesive 1 30 K/s (duration <10 s) 280 C. (duration 60 s) Adhesive 2 20 K/s (duration <10 s) 180 C. (duration 60 s)

Carrying Out the Rapid Curing

[0112] The adhesive bonding device described in Example 3 was used for the mounting and electrical contacting. In order to obtain additional information about the heating through of the joining parts, a thermocouple of type K was attached on the surface of the joining parts in the region of the adhesive bond.

Results (Temperature Profile in the Adhesive Joint and at the Surface of the Joining Part)

[0113] FIGS. 9 and 10 show the temperature profiles in the adhesive joint and on the surface of the joining part during the curing of the two model adhesives.

[0114] FIG. 9: Temperature/time diagram for 5 curing runs of the film adhesive AF 163_2U. The setpoint temperature for the curing of the adhesive was 280 C. Temperature curves shown as light were measured by means of the heating element in the middle of the adhesive layer, curves shown as dark were measured with the aid of a thermocouple on the surface of a joining part.

[0115] FIG. 10: Temperature/time diagram for 4 curing runs of the 2-component epoxy adhesive 3M Scotchweld EC 7256. The setpoint temperature for the curing of the adhesive was 180 C. Temperature curves shown as light were measured by means of the heating element in the middle of the adhesive layer, curves shown as dark were measured with the aid of a thermocouple on the surface of a joining part.

[0116] It is clearly evident how quickly the temperature signal from the adhesive joint reacts to the energy introduced by electrical resistance heating. The temperature on the surface of the joining part follows it with a significant time delay. For the control of rapid heating-up processes, direct temperature measurement in the adhesive joint is therefore ideal. By contrast, control by means of the surface temperature involves a significant dead time between the changing of the heating power and the temperature response. Therefore, control that relies only on the surface temperature of the joining parts is not advisable for rapid curing.

[0117] In the comparison of the curing runs for the various adhesive bonding specimens, it is found that the desired temperature profile is maintained very precisely and reproducibly. On the other hand, significant deviations occur in the surface temperature, attributable to the fact that the dissipation of heat to the surroundings is difficult to control (it is determined for example by the prevailing air flow). This problem is especially relevant in the case of high curing temperatures, because here there are great differences from the ambient temperature.

[0118] With slow cooling down of the specimens, the temperatures of the adhesive joint and the surface increasingly approach one another. This is also to be expected, because a temperature equalization takes place over time in the adhesively bonded specimen.

Results (Tensile Lap-Shear Testing of the Adhesive Bonding Specimens after Rapid Curing)

[0119] After conventional curing in a circulating-air oven, both model adhesives investigated achieved structural strengths that make them suitable for use in the construction sector. Adhesive 1 achieved a tensile lap-shear strengths of 23 MPa, adhesive 2 a strength of 33 MPa. In the latter case, the strength of the GRP substrate represented the limiting factor for the bonding strength (FIG. 11: Bonding strengths in the tensile lap-shear test of two different adhesive systems after conventional oven curing and after electrical rapid curing).

[0120] Under the conditions of rapid curing, adhesive connections are obtained with strengths which, though markedly below the conventionally cured specimens, at 11 MPa (adhesive 1) and 17 MPa (adhesive 2) are sufficient for many structural tasks.

[0121] The analysis of the fracture micrograph suggests that the loss of strength is at least partly caused by the inserted heating element. In the case of all the adhesive bonds, failure of adhesion with respect to the perforated stainless steel foil was observed. An improvement is probably still possible here, because the stainless steel foils used for the tests had not been specifically pretreated, and stainless steel without pre-treatment is considered to be a material that is difficult to bond adhesively.

[0122] In spite of the sometimes very high curing temperatures, the adhesive bonding specimens did not show any visible signs of decomposition of the adhesive or joining parts (discoloration or bubbling). This shows that, with the method used, very uniform and controlled heating of the adhesive layer is possible.

SUMMARY OF THE RESULTS

[0123] By controlled electrical resistance heating, it was possible to cure two different model adhesives within 70 seconds. The adhesives thereby achieved structural strengths of >10 MPa. The control allowed very high heating rates and reproducible maintenance of the prescribed temperature profile. The tests show that it is possible with the newly developed electronics for measurement data acquisition to use the heating element that is used for heating the adhesive layer at the same time as a precise and quickly responding temperature sensor. It is therefore possible to dispense with additional temperature sensors (for example thermocouples). In comparison with the temperature measurement on the surface of the parts to be joined corresponding to the prior art, the new method offers the advantage of a much shorter response time. This is the basic prerequisite for a controlled rapid curing of adhesives.