Adhesive material with carbon material and method for its production and use

Abstract

The present invention relates to an adhesive material, especially an electrically and/or thermally and/or radiation-curing or curable adhesive material, having at least one adhesive constituent and/or adhesive matrix, and moreover having at least one additive in the form of a carbon material, especially based on carbon nanomaterials and/or carbon micromaterials, present in the adhesive constituent and/or adhesive matrix. The invention further relates to a method for producing, activating and/or curing an adhesive material. Finally the invention also relates to a method for adhesively bonding two substrates.

Claims

1. An adhesive material comprising at least one adhesive constituent, adhesive matrix, or both in which one additive in the form of an aqueous carbon nanotube dispersion is dispersed within the adhesive material, wherein the aqueous carbon nanotube dispersion is stabilized with an anionic surfactant, wherein the carbon nanotubes are present in the adhesive material at a concentration of between 0.5 wt % and 3.0 wt %, and wherein the adhesive material is cured by microwave radiation or a combination of microwave radiation and thermal curing.

2. The adhesive material of claim 1 wherein the adhesive constituent or adhesive matrix is polyurethane-based.

3. The adhesive material of claim 1 wherein the carbon nanotubes are multiwalled carbon nanotubes (MWNTs).

4. The adhesive material of claim 1 wherein the adhesive constituent or adhesive matrix is a hot melt adhesive.

5. The adhesive material of claim 1 further comprising a hardener.

6. The adhesive of claim 1 wherein the carbon nanotubes are present in the adhesive material at a concentration of between 0.5 wt % and 2.0 wt %.

7. The adhesive of claim 1 wherein the carbon nanotubes are present in the adhesive material at a concentration of between 0.5 wt % and 1.5 wt %.

Description

(1) The invention is now described below with preferred embodiments, also referring in particular to the accompanying drawings. These show

(2) FIG. 1 various CNT-containing adhesive systems;

(3) FIG. 2 the schematic structure of a microwave belt dryer for carrying out methods according to the invention;

(4) FIG. 3 the absolute temperature profile in various heating tests;

(5) FIG. 4 the plot of the heating over a defined period of time; and

(6) FIG. 5 an evaluation table.

(7) As an example, various investigations for dispersion of CNTs in an adhesive matrix of a PU-based adhesive are described belowas a first embodiment, not limiting the whole invention. The investigations were carried out as follows:

(8) 1. Preparation of reference samples (without CNTs) the following are adhesive-bonded: PC ABS (polycarbonate/acrylonitrile-butadiene-styrene copolymer) Planware thin (leather) as top material

(9) 2. Improved dispersion of the CNTs

(10) 3. Surface-modified CNTs for improved dispersion

(11) 4. Incorporation of redissolving CNT granules for further improvement of the dispersion of the CNTs in the adhesive

(12) 5. Testing dispersants

(13) 6. Incorporating various CNT concentrations, for example 0.5 wt %, 1.0 wt %, 1.5 wt %, 2.0 wt %, 2.5 wt %, 3.0 wt %.

(14) 7. Use of Carbodis, i.e.: predispersed, stabilized CNT dispersions, for better dispersion of the CNTs in the adhesive.

(15) The results of the dispersion tests showed, firstly, that, at times, the CNTs used, regardless of the respective mixture ratio, could only be incorporated in the adhesive with difficulty and still showed agglomerates even when using established mixing technologies. These mixing difficulties could be attributed to the fact that even after preliminary purification, CNTs are still rather to be regarded as hydrophobic, which greatly hampers dispersion for example in diols.

(16) A further improvement of dispersion could be achieved in such cases by transmitting high shearing forces to the CNT agglomerates in the highly viscous systems, so that the agglomerates are distributed more finely in the adhesive matrix.

(17) In order to improve the dispersibility of the CNTs in the matrix, further tests were carried out with dispersants, but the viscosity of the adhesives/CNT system increased further as a result. This increase can be attributed to the better distribution of the CNTs in the matrix, as finely-divided CNTs possess extraordinary binding capacity for liquids.

(18) The question of viscosity has an influence on homogeneous dispersion of CNTs in solvent and matrix systems: depending on the dispersion techniques used, the transmission of shearing forces improves with increasing viscosity. As a result of corresponding shearing of CNT agglomerates, this can result in successive comminution of the latter and can lead to homogeneous distribution of the CNTs in the adhesive. On the other hand if the viscosity is too high, processability can again be more difficult and functionality of the adhesive may be poorer; it is thus a parameter requiring optimization.

(19) In the sense of the binding capacity of the CNTs described and of the viscosity increase, by adding surface-oxidized CNTs, starting from a concentration of 1.5 wt % CNTs a better distributiondetermined opticallycould be achieved. In this case the adhesive became highly viscous.

(20) The results for processability of the adhesive mixtures with the various proportions by weight are presented below. 0.5 wt %, low viscosity 1.0 wt %, low viscosity 1.5 wt %, can still be processed well 2.0 wt %, upper limit 2.5 wt %, no longer free-flowing 3.0 wt %, no longer free-flowing

(21) A very good distribution could be achieved by adding soluble CNT granules, which have good dispersibility primarily in aqueous systems. The additive used for this is a granulated product dispersible in polar systems, which is characterized by an especially low agglomeration tendency (in contrast to conventional CNTs). However, in the concrete case a surfactant contained herein has an electrically insulating action in the total system, so that at higher concentrations the electrical resistance of the adhesive increases, which prevents electrical curing of the samples. In the end, all attempts to introduce CNTs in the adhesive system in powder form led to at least moderately homogeneous distribution of the CNTs, or else they did not display sufficient electrical conductivity owing to the insulating effect of the surfactant used (in the case of soluble granules, when these contain surfactants of that kind). According to the present invention it is possible in particular to disperse CNTs directly in the adhesive. For example, the CNTs can be used in powder form. Preferably, predispersed CNTs can be used, which in particular are easier to handle.

(22) For these reasons, in the present invention the further new approach was also adopted, which operates with already excellently dispersed CNTs in aqueous or organic or solvent-based media.

(23) Several corresponding products were used. FIG. 1 shows CNT-containing adhesive systems with an improved quality of dispersion through the use of CarboDis materials. The CarboDis materials are aqueously predispersed CNTs.

(24) The greatest success in dispersing the CNTs was achieved by means of CarboDis TA. CarboDis TA is a dispersion of anionically stabilized CNTs, which can be easily metered. There was no difficulty with compatibility between hardener and the water-containing dispersion for the previously prepared CNT dispersion that was used.

(25) Various tests were conducted for electrical curing, which were carried out as follows:

(26) 1. Sample processing: knife-coating of the adhesive drying for 20 min at 40 C. crosslinking for 20 min at 80 C.

(27) 2. Measurement of resistance and electrical curing with prepared mixtures (adhesive/CNT/hardener) on contacted sample plates

(28) The results of the investigations with respect to electrical curing of the adhesive matrix were as follows:

(29) The plates coated with the adhesive/CNT/hardener mixture were contacted on two opposite sides with copper metal strips and the respective resistances were measured with a digital multimeter. The respective values are presented below.

(30) TABLE-US-00001 Specific resistance Adhesive without CNTs 844 cm Adhesive with 0.5% CNTs 705 cm Adhesive with 1.0% CNTs 677 cm Adhesive with 1.5% CNTs 654 cm Adhesive with 2.5% CarboGran 741 cm Adhesive with 3.5% CarboGran 770 cm Adhesive with 1% surfactant and 1.5% CNTs 728 cm Adhesive with 40% CarboDis TA 600 cm (at 2.5% CNTs and 2.5% surfactant), equivalent to CNT content in adhesive: 1%

(31) Then a voltage of 80 V was applied to the contacts, in order to cure the adhesive.

(32) Next, various testsas second embodimentwere carried out for microwave curing.

(33) The tests for heating the test specimens were carried out in a microwave arrangement provided with a conveyor belt (microwave belt dryer), as is used in a larger version in industrial processes for drying products. The constituents of the device, their function and the schematic structure of the microwave belt dryer can also be seen in FIG. 2.

(34) FIG. 2 shows the schematic structure of a microwave belt dryer 10. This has a magnetron 11, a waveguide coupling 12, a directional coupler 13, a circulator 14, a tuner 15, a water load 16, a detector diode 17 and a cavity 18 with product load.

(35) This device 10 produces microwave radiation with a frequency of 2.45 GHz and a maximum power of 2 kW. The sample travels through the applicator on a Teflon-coated belt and in this way is resident in the electromagnetic field for 30 seconds per pass. When the sample leaves the cavity 18, its temperature can be measured by means of an infrared thermometer. The microwave radiation is produced in a magnetron 11.

(36) In addition, a tuning unit 15 is incorporated, for optionally carrying out adjustment of the field and thus maximizing the power absorbed by the sample. The applicator receives the product to be heated, the load, and, in addition to its process tasks, optionally reactor, furnace and the like, it is also adjusted for electromagnetic requirements, for example resonator.

(37) In the microwave belt dryer 10 used, a belt on which the samples have been placed passes through the applicator. Preliminary tests on the heating behavior were carried out in an adapted, steplessly adjustable laboratory microwave device.

(38) Various tests were carried out into how an adhesive material as described above can be activated and/or cured.

(39) 1. Sample Processing: knife-coating of the adhesive optionally, clamping the sample in the pressing plates

(40) 2. Microwave Drying at Different Powers, Nominal Total Power (3.2 kW):

(41) CNT Concentration [wt %] 0 0.5 1.0

(42) Power Level [%], Relative to 3.2 kW 20 30 40

(43) Results of investigations with respect to microwave-supported curing of the adhesive matrix were as follows.

(44) As preparation for the belt microwave, preliminary tests for microwave-supported curing were carried out with a laboratory microwave device. This already allows stepless power adjustment. Different CNT concentrations (0 wt %; 0.5 wt %; 1.0 wt %) were tested in conjunction with the PU-based adhesive with the power levels P=20%; P=30%; P=40% (corresponding to 640 W, 960 W, 1280 W).

(45) It was found that at a low power input in the microwave field, the CNTs bring aboutas an interim resulta respectable temperature increase of the matrix system within the short heating time and therefore cure the adhesive.

(46) In the last step, using CNT dispersions, this interim result was raised to a far better level, by increasing the heating rates further by a factor of 2-3, so that the attainable final temperatures were increased.

(47) With increasing power, the positive aspect of the CNTs decreases owing to the self-heating of the matrix system; the heating rates do not increase in proportion to the microwave power introduced and become less dependent on the CNT component. The system thus has a tolerant reaction to locally excessive proportions of CNT or inadvertently excessive microwave power, which is basically a technically useful self-limitation of the system.

(48) The total energy consumption required for curing the adhesive by microwaves can be lowered considerably by means of carbon-containing additives, especially CNTs.

(49) FIG. 3 shows the absolute temperature profile (temperature in C.) of the heating tests of the adapted adhesive in the microwave field at different powers.

(50) FIG. 4 shows the heating (temperature difference T) plotted against the time period coveredthe temperature before heating was taken as reference (=0).

(51) The diagrams shown here (absolute temperature T or temperature difference T as a function of the heating time) describe the adhesive system for a PU adhesive used in the test. Two different PU adhesives were used. There are no large differences between the two PU adhesives tested.

(52) The adhesive dispersions were prepared and cured under the same conditions. The samples were then subjected to curing tests in the belt microwave, which offers the possibility of a continuous process, in each case with and without the clamping force. It became clear that curing in the microwave, even without an applied clamping force, brings about very good adhesive bonding, which can be even further improved by applying the defined pressure. A corresponding evaluation table is shown in FIG. 5.

(53) Then various adherence samples were prepared.

(54) The adhesive dispersion was prepared as follows: weighing the adhesive adding the hardener mixing by spatula weighing the CarboDis TA (2.5% CNTs/2.5% surfactant)=adding the CarboDis TA while stirring homogenizing/deaerating

(55) Coating and curing took place as follows:

(56) As the coating tests by knife-coating had better reproducibility than application with existing spraying equipment, the adhesive was knife-coated in all cases. However, spray application does not in principle pose any problem on an industrial scale.

(57) The samples were predried for 5 min at 30 C., so that the additional water that was introduced with the CarboDis was dried out again. Curing took no more than 5 minutes in the belt microwave at nominally approx. 1200 W (with reduced application amount, within 5 minutes 30 seconds).

(58) The tests and results as described above in their general form and in the form of the two aforementioned embodiments can be summarized as follows.

(59) First, a breakthrough was achieved with microwave heating of CNT-containing adhesive systems. It was successfully shown that by using carbon materials, especially carbon nanotubes (CNT), as additives in adhesive systems, it is possible for adhesive-bonded joints to be cured in a microwave field: carbon-based, microwave-absorbing materials can be added to the adhesives using suitable mixing technologies. In particular, carbon nanotubes, and CNT-containing mixtures of various carbon materials, are suitable for this, owing to the extraordinary absorption properties with respect to electromagnetic radiation. The high electrical conductivity of the CNTs and other carbon materials, but especially the attainment of electrical percolation in the adhesive system even with small proportions of additives, is decisive for success. Rapid and efficient heating takes place in the microwave field for the adhesive systems with additives, and this also applies to thin films of adhesive, with only a small total mass (of the order of 0.5-2 g/dm.sup.2) of the unhardened adhesive. Especially with high quality of dispersion (optimal distribution of the additive in the adhesive system), high but controllable heating rates can be achieved in microwave-induced heating. The resultant heating of the adhesive in the microwave field can effect the desired curing. Curing is uniform, and incompletely cured weak spots/zones can be avoided. Focusing the heating process on the adhesive-bonded joints alone can give clear advantages in terms of energy. The processability of the adhesives can be preserved. According to the results obtained so far, the adhesive action is not in any way impaired by the additives used, and is fully maintained. Furthermore, electrical conductivity of the adhesive-bonded joint is achieved, which is advantageous for avoiding static charges.

(60) A key result of the present invention is that a solution for microwave-based curing of adhesives that is suitable for industrial application is feasible using the aforementioned additivesall relevant principles for achieving this have been found and developed in the context of the present invention.

(61) An essential aspect is the quality of dispersion of the additive in the adhesive.

(62) The initial results showed that there is dependence of the conductivity and of the homogeneous microwave coupling on the addition of CNTs. This generally already applies with simple addition, which for CNT contents of approx. 2 wt % leads into the range of incipient percolation. However, the effect was further increased considerably, after uniform distribution of the CNTs in the matrixand therefore complete electrical percolationwas achieved. The microwave tests carried out showed that it is possible, by adding CNTs, for the present adhesive system to be cured even at low powers, and the desired effects, heating by microwaves, and the further accelerated heating by addition of CNTs, are obtained. In this case it was advantageous in particular to use predispersed CNT dispersions. The improvements could be achieved by preliminary dispersion of the CNTs in a water surfactant mixture (incorporating an aqueous dispersion) (CarboDis), which was already supplied to the adhesive in this form. This gave a mixture that is good with respect to manageability and functioning.

(63) The key point of the present invention is the method of curing by microwave, as this allows particularly uniform coupling to all available particles.

(64) Furthermore, it was found that the quality of the adhesive bond was slightly improved by microwave irradiation, relative to stove tests carried out for comparison.

(65) An essential result of the present invention is that it is possible for microwave curing to be used instead of the existing method, offering several advantages over the existing process (process time, heating of only the adhesive-bonded joint, energy aspects and so on). To summarize, the present invention shows high potential for converting the adhesive curing process to a microwave process. Preferably there is in particular shortening of process times, as primarily only the adhesive-bonded joint is heated, and consequently there are no long cooling times, of tools for instance (prior transmission of heat and clamping force). The heating itself can take place without any problem in the single-digit minutes range, which also already includes the cooling phase.