LOW-TEMPERATURE PLASMA TREATMENT

Abstract

Method for bonding a substrate surface of a substrate to an adhesive surface of an adhesive by generating a low-temperature plasma in a low-temperature plasma generator, activating the substrate surface and/or the adhesive surface with the low-temperature plasma, and thereafter layering the substrate surface and the adhesive surface atop one another to form a bonded assembly.

Claims

1. A method for bonding a substrate surface (2) of a substrate layer (1) to an adhesive surface (4) of an adhesive (3), by generating a low-temperature plasma in a low-temperature discharge configuration, under atmospheric pressure, activating the substrate surface (2) and/or the adhesive surface (4) with the low-temperature plasma, and thereafter layering the substrate surface (2) and the adhesive surface (4) atop one another to form a bonded assembly.

2. The method as claimed in claim 1, wherein the adhesive used comprises a pressure-sensitive adhesive.

3. The method as claimed in claim 2, wherein the pressure-sensitive adhesive used comprises an acrylic adhesive.

4. The method as claimed in claim 1, wherein a temperature of the plasma emerging from a plasma discharge space is at most 70 C.

5. The method as claimed in claim 4, wherein the plasma discharge space is moved at a distance of less than 15 mm over the surface to be treated.

6. The method as claimed in claim 1, wherein a substrate layer (1) with a substance selected from the group consisting of PTFE, PE, PP, EPDM, ClearCoat, PET, ABS, CRP, CEC, glass and steel is used.

7. The method as claimed in claim 1, wherein the adhesive surface (4) and the substrate surface (2) are treated with the same low-temperature discharge configuration at identical plasma temperature.

8. The method as claimed in claim 1, wherein the plasma is generated by passing a process gas in front of a piezoelectric electrode (101, 102) and thereby exciting a voltage field which forms between the piezoelectric electrode (101, 102) and a grounded electrode, and cooling the piezoelectric electrode (101, 102).

9. A method for activating surfaces of a bonded assembly having an adhesive surface (4) and a substrate surface (2), wherein said surfaces are activated with a low temperature plasma discharge configuration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention is described with a number of exemplary embodiments in 14 figures, wherein:

[0043] FIG. 1a shows the activation of a substrate surface of a bond,

[0044] FIG. 1b shows the activation of an adhesive surface of the bond,

[0045] FIG. 1c shows the activation of the substrate surface and the adhesive surface of the bond,

[0046] FIG. 2 shows a graph on the plasma activation of the ACX.sup.plus 7074 core

[0047] FIG. 3 shows a graph on the potential of a plasma treatment with different adhesives and ACX.sup.plus cores

[0048] FIG. 4 shows a graph on the resistance of a plasma-activated bond without humidity effects

[0049] FIGS. 5a, 5b show resistance of plasma-activated bond at 40 C.180% relative humidity

[0050] FIG. 6 shows peel adhesion measurement of ACX.sup.plus 7812 on piezoelectric plasma activation on LSE paint

[0051] FIG. 7 shows peel adhesion measurement of ACX.sup.plus 7812 on piezoelectric plasma activation on polypropylene

[0052] FIG. 8 shows peel adhesion 90 comparison chemical primer vs. Corona vs. Plasma-ACX.sup.plus 7074 on LSE paints from PPG

[0053] FIG. 9 shows activation efficiency Corona vs. Plasma

[0054] FIG. 10a shows a schematic view of the operating principle of a low-plasma-temperature plasma generator

[0055] FIG. 10b shows directions of polarization occurring within the low-plasma-temperature plasma generator of FIG. 10a

[0056] In-house Tesa adhesive units are evaluated for their behavior under plasma conditions. For this purpose, different substrate layers 1 with associated substrate surfaces 2 are selected. Plasma treatments are carried out first with the Plasmatreat technology (Open-Air Plasma). This is done using a Plasmajet from Plasmatreat, Steinhagen. The Plasmajet is a plasma cannon for generating an atmospheric pressure plasma. A substrate surface and/or an adhesive surface 2 is treated with the atmospheric pressure plasma.

[0057] In the context of applying a layer 3 of adhesive to the substrate layer 1, there are in principle three options for the plasma treatment. Firstly, only the substrate surface 2 may be activated, as per FIG. 1a. Secondly, as per FIG. 1b, only an adhesive surface 3 of a layer 4 of adhesive can be activated, or, thirdly, as per FIG. 1c, both the substrate surface 2 and the adhesive surface 4 can be activated. The three possibilities are represented in FIGS. 1a, 1b and 1c.

[0058] FIG. 2 illustrates an experimental series. Tesa ACX.sup.plus 7074 is selected as substrate layer 1 and adhesive layer 2. Different substrates are selected, identified in FIG. 2 by their usual codes. The 10 bars per treatment option correspond, from left to right, to the 10 codes to the right of the graph, from top to bottom.

[0059] It is evident from FIG. 2 that the activation of both bonding surfaces acts synergistically in almost all cases. This means that in the relevant cases tested, the activation of the adhesive surface 4 and of the substrate surface 2 is the best interface for improving adhesive properties.

[0060] It can also be ascertained that the peel adhesion of an adhesive bond between substrate layer 1 and adhesive layer 3 reaches the level of the double-sided treatment only in exceptional cases when only the substrate is activated. Treatment of adhesive alone may show, in specific combinations of materials, that the quality of a double-sided treatment can be achieved.

[0061] Determining the peel adhesion of an adhesive tape on a steel test plate takes place under testing conditions of 23 C.+/1 C. temperature and 50%+/5% relative humidity. The adhesive tapes are cut to a width of 20 mm as test specimens and are adhered to a steel plate. Prior to the measurement, the test plate is cleaned and conditioned. For this purpose, the steel plate was wiped down first with acetone and left to stand in the air for 5 minutes to allow the solvent to evaporate. The side of the single-layer test specimen facing away from the test plate is then lined with 36 m etched PET film, thereby preventing the adhesive tape from stretching during measurement. This is followed by the rolling of the test specimen onto the steel substrate. For this purpose, the tape is rolled down five times back and forth with a 4 kg roller at a rolling speed of 10 m/min. 20 minutes after roller application, the steel plate is inserted into a special mount, which allows the test specimen to be peeled vertically upward at an angle of 90. The peel adhesion measurement takes place using a Zwick tensile testing machine. The results of measurement are reported in N/cm and are averaged from three individual measurements.

[0062] An important finding is that the activation of bonding surfaces of which one is a TesaACX.sup.plus surface of a TesaACX.sup.plus adhesive tape is able to achieve a significant improvement in the peel adhesion. In the case of the ACX.sup.plus adhesive tapes, these are commercially available adhesive tapes from Tesa. The ACX.sup.plus adhesive tapes have a viscoelastic carrier and two adhesive surfaces opposite one another on the carrier, these surfaces consisting of the same or a modified chemical structure. Hence the peel adhesion-boosting effect also extends to pure viscoelastic carrier systems. It is typically the viscoelastic carriers which are responsible for the desired properties in the finished product (thickness, damping properties, etc.), these carriers not having been developed primarily for the adhesive properties. The carrier systems are therefore frequently laminated with dedicated functional adhesive layers in order to generate the adhesive properties.

[0063] ACX.sup.plus carrier systems feature a single-layer construction composed of an acrylate layer. In the great majority of cases, the performance properties of the plasma-activated viscoelastic ACX.sup.plus carrier systems as per FIG. 2 are comparable with plasma-activated three-layer constructions, composed of a carrier layer on which adhesive layers have been applied to both surfaces. The peel adhesion, however, may also be well above these.

[0064] FIG. 2 shows the peel adhesion, measured in the standard method, of an adhesive bond between the ACX.sup.plus 7074 adhesive without functional compound, which in this case is a resin-modified acrylate adhesive, on ten different substrate surfaces 2. The substrate surfaces are PTFE (polytetrafluoroethylene), PE (polyethylene), MOPP (monoaxial oriented polypropylene films), PU (polyurethane), EPDM (ethylene-propylene-diene rubber), ClearCoat from BASF, PET (polyethylene terephthalate), ABS (acrylonitrile-butadiene-styrene), CRP (carbon fiber-reinforced plastic), CEC (cathodic electrocoat), and steel. Three treatment options by means of plasma treatment are selected. The left-hand bar group represents the peel adhesion of an ACX.sup.plus 7064 adhesive surface on the ten aforementioned different substrate surfaces without plasma treatment of one of the two bonding surfaces 2, 4.

[0065] The middle bar group shows the peel adhesion if only the adhesive surface 4 is activated with the atmospheric pressure plasma, and the right-hand bar group represents the peel adhesion if both the adhesive surface 4 and the respective substrate surface 2 are activated.

[0066] FIG. 3 includes, in an overview, the results of the peel adhesion testing of different plasma-treated adhesives on PE (polyethylene) surfaces or a steel surface.

[0067] The first bar group relates to the peel adhesion measurements on untreated PE surface, and the second bar group to peel adhesion measurements on PE surfaces when both the adhesive surface and the substrate surface are activated. The third bar group relates to peel adhesion measurements on a steel surface without plasma treatment of one of the two bonding surfaces, and the fourth bar group relates to the peel adhesion measurements of various adhesives on a steel surface when both bonding surfaces are plasma-activated.

[0068] The adhesives are ACX.sup.plus 7476, MOPP, PU (polyurethane), ACX.sup.plus 705x from Tesa, an adhesive from 3M, which is a VHB grade from 3M, ACX.sup.plus with glass or Fillite cores, and ACX.sup.plus 68xx single-layer, foamed.

[0069] The results show that the plasma treatment on all adhesives possesses a positive effect, but that the absolute peel adhesion figures are differently pronounced. A moderate increase in the peel adhesion is recorded for the adhesive tape with ACX.sup.plus 7476 and also for the pure PU adhesive, in part limited via cohesive failures and mixing breakages. It is observed, however, that the tesa acrylate cores without adhesive, ACX.sup.plus core with hollow glass beads and ACX.sup.plus core with Fillite that were investigated respond strongly to the plasma treatment, which is able to bring about a significant boost in peel adhesion on PE and steel. The 3M product as well (straight acrylate, single-layer, with hollow glass beads) profits from the treatment. Single-layer acrylate cores have a high potential for plasma activations.

[0070] A fundamental potential evaluation is shown in table 1:

TABLE-US-00001 TABLE 1 Strong improvement by ACXplus Investigation of . . . plasma possible? Properties Peel adhesion Yes Shear strength Yes Instantaneous peel adhesion Yes Substrates EPDM, PP, PE, PET, . . . Yes Steel, aluminum, . . . Yes Finishes Yes Teflon No Compositions Acrylate adhesives Yes Natural rubber Yes Synthetic rubber Yes PU Yes Ac-SBC blends/HPSR Yes Construction Conventional adhesive tapes Yes with film carrier ACXplus cores: straight acrylate, Yes foamed, filled d/s foam fixing tabs Yes

[0071] The resistance of double-sided, plasma-activated bonds of ACX.sup.plus 6812 adhesive on ASTM steel and PP after pure temperature storage, at temperatures of 30 C., 40 C. and 70 C. over 4 weeks, proved to be extremely stable, as per FIG. 4. There was no surface combination where the peel adhesion could be found to have reduced over time. In many cases, higher values relative to untreated references are obtained.

[0072] Long-term aging stability under moisture is critically influenced by the quality of the bonding interfaces. The aim of a plasma treatment is to create appropriate reactive centers on the adhesive surface in order to increase the bond to the substrate and to alleviate or to eliminate aging phenomena caused for example by storage conditions of heat plus humidity.

[0073] As described above, a plasma does not act in the volume region of an adhesive, but may, via plasma-induced hydrophilization, give rise to or promote the advance of a water front into the interface. The moisture that is absorbed triggers physical and chemical changes in the interface. In this case it is possible, via suitable parameters of the Plasma treatment, such as distance of the nozzle from the bond surface, and the speed, to eliminate heat-plus-humidity weakness or reduce it, as shown by the results according to FIG. 5a and FIG. 5b.

[0074] FIG. 5a shows the peel adhesion of an ACX.sup.plus 7070 adhesive on two automobile finishes after seven days of storage of the bond at room temperature and at 40 C. and 80% relative humidity. FIG. 5b saw a second measurement carried out in relation to an ACX.sup.plus 6812 adhesive under the same climatic conditions set out above. The left-hand pair of bars in each of FIG. 5a and FIG. 5b relates to a Ford finish, and the right-hand pair of bars in each of FIG. 5a and FIG. 5b relates to a Daimler finish. In all of the experimental arrangements, both bond surfaces 2, 4 were activated with a Plasmajet.

[0075] But even without optimization and use of standard parameters such as 12 mm distance, 5 m/min Plasmajet treatment speed, combinations of materials are frequently already resistant to heat-plus-humidity conditions. In this regard, see table 2.

TABLE-US-00002 TABLE 2 Heat plus humidity resistance varies by substrate PP EPDM PP Test Conditions PA 90 T30 GF30 plate 3 d/RT N/cm 66* 61* 63* Climate alternation 54* 52* 11** 1000 h 38 C./95% r.h. Climate alternation 57* 55* 9** 10 d 85 C./40 C.; 85% r.h. BMW climate alternation PR303.5 d 63* 62* 16** 240 h + 85 C./60% r.h.; 30 C. *= cohesive fracture/cohesive near to surface **= adhesive fracture

[0076] Table 2 presents peel adhesion measurements for ACX.sup.plus 6812 on three different substrate surfaces. The first column relates to a peel adhesion measurement on the adhesive bond after three days at room temperature; the second column relates to the peel adhesion measurement after 1000 h at 38 C. and 95% relative humidity. The third column describes the peel adhesion measurement after 10 days with climate alternation, and the fourth column describes a peel adhesion measurement after 5 days with climate alternation.

[0077] The thermal influence of the Plasmatreat treatment is held definitively responsible for the other unwanted side-effects, producing low-molecular-weight oxidizing materials (LMWOMs) on PP substrate and on the adhesive. Polymer or oligomer layers highly oxidized accordingly are not sufficiently bonded to the polymers in the volume of adhesive and, in addition, they are swellable or soluble in water.

[0078] It is found that the discharge technology of a plasma treatment occupies an essential role with regard to the humidity resistance. In the case of a Plasmajet, typically, the afterglow is generated via an electric arc or an arc-like discharge.

[0079] An alternative technology, from Reinhausen Plasma GmbH, generates the plasma by way of a piezoelectric effect, made possible by opposite directions of polarization of the crystal. The result of this discharge technology relative to an electric arc is a cold, non-thermal plasma. The temperatures are virtually at room temperature. Accordingly, thermal overtreatments and hence the formation of LMWOMs can be prevented or at least reduced. As a result, stable heat-and-humidity resistance of the adhesive can be demonstrated on LSE automotive finishes and low-energy polymers, in accordance with FIG. 6 and FIG. 7. In the case of bonding to finishes, a strong rise in the adhesive properties with plasma activation is a positive outcome.

[0080] FIGS. 10a and 10b show, schematically, the functioning of the plasma cannon based on a piezoelectric effect. A preferentially oriented piezoceramic in this case is, for example, lead, zirconate-titanates. Known materials having piezoelectric properties are quartz as a piezoelectric crystal, and piezoelectric ceramics such as the aforementioned lead, zirconate-titanates are also conceivable.

[0081] In the exemplary embodiment as per FIG. 10a, 10b oppositely oriented piezoceramics are arranged alongside one another in a secondary region 10, while in a primary region 11 there is a condenser 12 having two opposing condenser plates, with each of the condenser plates being firmly connected to one of the piezoelectric elements 101, 102. Application of an alternating voltage U to the condenser plates produces mechanical vibration of the condenser plates of the condenser 12 by reversal of polarity. The mechanical vibration is transmitted to the piezoelectric elements 101, 102 and, in the condenser-facing end thereof, produces an alternating potential difference which corresponds in its frequency to the mechanical vibration of the condenser plates. The electrical field E generated by the potential difference is shown in FIG. 10b.

[0082] The piezoelectric elements 101, 102 themselves comprise an insulator, meaning that the safety requirements to be met are low. The frequency of the low-volt alternating voltage U at the condenser plates corresponds to the piezoelectric resonance frequency and is situated in the order of magnitude between 10 kHz and 500 kHz. Accordingly, a low-volt alternating voltage at the condenser is converted into a mechanical deformation which in turn generates a high-volt electrical alternating voltage at the free ends of the piezoelectric element 101, 102. The principle of the piezoelectric element is shown for example in EP 2 168 409 B1. Particularly in conjunction with cooling arrangements provided on piezoelectric elements, such elements are suitable, and so the plasma generated by the alternating electrical field can be subsequently cooled and what is called a low-plasma-temperature plasma can emerge from an exit nozzle of the plasma cannon, which is not explicitly shown.

[0083] Low-plasma-temperature plasma cannons are marketed by Reinhausen Plasma GmbH. The Piezobrush PB1 generates plasma temperatures of only 70 C. The plasma of the Piezobrush PB2 has a temperature of 120 C.-250 C., depending on the exit nozzle.

[0084] The Piezobrush PZ2 produces a plasma having a plasma temperature of less than 50 C. Peel adhesion measurements result in FIG. 6 and FIG. 7.

[0085] The Piezobrush PZ2 is guided at a distance of 5 mm-10 mm and a speed of 5 m per minute over a substrate surface or a bonding agent surface, respectively, and so makes the surfaces ready for the bonding operation.

[0086] In view of the low plasma temperature of less than 50 C., the same plasma cannon can be used both to treat the substrate surface and to treat the bonding agent surface. The substrate surface in FIG. 6 is an LSE finish Apo1.2, while in FIG. 7 it is PP. The bonding agent surface is the surface of the ACX.sup.plus 7812 adhesive tape.

[0087] FIG. 6 and FIG. 7 relate to peel adhesion measurements in which bonding takes place between a substrate surface 2 and a bonding area 4 of the double-sided adhesive tape ACX.sup.plus 7812 from Tesa.

[0088] In a first step of the method of the invention, the substrate surface, such as a metal or plastic surface, is treated with the Piezobrush PZ2. In a second step of the method, an outer side of the ACX.sup.plus 7812 adhesive tape is activated with the same Piezobrush PZ2. The ACX.sup.plus 7812 adhesive tape consists of an acrylate layer whose two outer surfaces are pressure-sensitively adhesive. The two surfaces of pressure-sensitive adhesive are normally covered with a protective film, which is peeled off prior to the bonding operation. In accordance with the invention, the outside of one layer of pressure-sensitive adhesive is activated with the Piezobrush PZ2 in preparation for the bonding operation. The Piezobrush here is run over the outer side of the layer of adhesive at the same distance of around 2 mm-5 mm, after which the activated substrate layer 1 and the activated layer 4 of pressure-sensitive adhesive are pressed against one another.

[0089] FIG. 6 shows the results of a peel adhesion test as per test standard, in which an adhesive tape 1 cm wide is applied to a substrate surface in accordance with the method described above. The left-hand bar shown in graph 1 shows the force to be applied for the removal of the double-sided adhesive tape at an angle of 90 when both surfacesthat is, both the substrate surface 2 and the surface 4 of pressure-sensitive bonding agentare unpretreated. The second bar shows the pressure-sensitive adhesive tape in the test with activation only of the outer side of the layer of pressure-sensitive bonding agent; the third bar shows the peel adhesion on exclusive activation of the substrate layer, with the substrate being an LSE finish, namely APO 1.2. The fourth bar shows the force to be applied to remove the adhesive tape when both the substrate surface and the pressure-sensitive adhesive surface have been pretreated with the Piezobrush PZ2. The fifth bar shows the peel adhesion after storage (7 days, 40 C. at 100% relative humidity).

[0090] FIG. 7 shows the peel adhesion for the same test sequence for the double-sided adhesive tape ACX.sup.plus 7812 when adhered to a PP layer, i.e., a polypropylene layer (PP). Here again, the first bar denotes the peel adhesion for untreated surfaces. The second bar denotes the peel adhesion when only the outer surface of pressure-sensitive bonding agent has been treated.

[0091] The fourth bar shows the force to be applied for removing the adhesive tape when both the substrate surface and the pressure-sensitive adhesive surface have been pretreated with the Piezobrush PZ2. The fifth bar shows the peel adhesion after storage (7 days, at 40 C. and 100% relative humidity).

[0092] High peel adhesion values after heat-plus-humidity storage, after 7 days at 40 C. and 100% relative humidity and, respectively, at 85 C. and 85% relative humidity, can be achieved through the low-temperature plasma treatment relative to an RT storage (room temperature storage).

LIST OF REFERENCE SYMBOLS

[0093] 1 Substrate layer [0094] 2 Substrate surfaces [0095] 3 Adhesive layer [0096] 4 Adhesive surface [0097] 10 Secondary region [0098] 11 Primary region [0099] 12 Condenser [0100] P Direction of polarization [0101] U Alternating voltage [0102] 101 Piezoelectric elements [0103] 102 Piezoelectric elements