LOW-TEMPERATURE PLASMA TREATMENT

Description

[0001] The invention relates to a method for bonding a substrate surface of a substrate to an adhesive surface of an adhesive.

[0002] Fundamentally, when bonding surfaces together using adhesives, the problem arises of permanently and firmly applying the adhesive to the substrate surface. This requires particularly strong adhesion of the pressure-sensitive adhesive to the surface. Adhesion is typically defined as the physical effect that causes two contacting phases to bond together at their interface due to intermolecular interactions. Adhesion thus determines the adhesion of the adhesive to the substrate surface, which can be measured as tack and adhesive strength. To specifically influence the adhesion of an adhesive, plasticizers and / or adhesive strength-enhancing resins (so-called "tackifiers") are often added to the adhesive.

[0003] A simple definition of adhesion can be "the interaction energy per unit area" [in mN / m], although this is not measurable due to experimental limitations such as a lack of knowledge of the true contact areas. Furthermore, the surface energy (SFE) is often described with "polar" and "nonpolar" components. This simplified model has become established in practice. This energy and its components are often measured by measuring the static contact angles of different test liquids. Polar and nonpolar components are assigned to the surface tensions of these liquids. The polar and nonpolar components of the surface energy of the test surface are determined from the observed contact angles of the drops on the test surface. This can be done, for example, using the OWKR model. An industrially common alternative method is the determination using test inks according to DIN ISO 8296.

[0004] In the context of such discussions, the terms "polar" and "high-energy" are often equated, as are the terms "nonpolar" and "low-energy." This is based on the realization that polar dipole forces are comparatively strong compared to so-called "dispersed" or "nonpolar" interactions, which are established without the involvement of permanent molecular dipoles. The basis of this model of interfacial energy and interfacial interactions is the notion that polar components interact only with polar ones, and nonpolar components only with nonpolar ones.

[0005] However, a surface can also exhibit small or medium polar components of the surface energy without the surface energy being "high." As a guideline, if the polar component of the surface energy is greater than 3 mN / m, the surface can be considered "polar" for the purposes of this invention. This roughly corresponds to the practical lower detection limit.

[0006] In principle, there are no hard limits for terms such as high- and low-energy. For the purposes of this discussion, the limit is set at 38 mN / m or 38 dyn / cm (at room temperature). This is a value above which, for example, the printability of a surface is usually sufficient. For comparison, consider the surface tension (=surface energy) of pure water; this is approximately 72 mN / m (depending, among other things, on temperature).

[0007] Particularly on low-energy substrates such as PE, PP or EPDM, but also on many paints, there are major problems in achieving satisfactory adhesion, both when using pressure-sensitive adhesives and other adhesives or coatings.

[0008] Physical pretreatment of substrates (e.g., by flame, corona, plasma) to improve bond strength is common, especially with liquid reactive adhesives. Physical pretreatment can also include cleaning the substrate, for example, to remove oils, or roughening it to increase the effective surface area.

[0009] Physical pretreatment is usually referred to as "activation" of the surface. This usually implies a nonspecific interaction, as opposed to, for example, a chemical reaction based on the lock-and-key principle. Activation usually implies an improvement in the wettability, printability, or anchoring of a coating.

[0010] With self-adhesive tapes, it is common practice to apply a bonding agent to the substrate. However, this is often an error-prone, time-consuming, and manual step.

[0011] The success in improving the adhesion of pressure-sensitive adhesives through physical pretreatment of the substrate (flame, corona, plasma) is not universal, since non-polar adhesives such as synthetic rubber typically do not benefit from this.

[0012] Corona treatment is defined as a surface treatment with filamentary discharges generated by a high alternating voltage between two electrodes, whereby the discrete discharge channels impinge on the surface to be treated, see also Wagner et al., Vacuum, 71 (2003), pages 417 to 436. Without further qualification, ambient air is assumed to be the process gas.

[0013] The substrate is almost always placed or passed through the discharge chamber between an electrode and a counter electrode, which is defined as "direct" physical treatment. Web-shaped substrates are typically passed between an electrode and a grounded roller.

[0014] In industrial applications, the term "corona" is often used to describe a "dielectric barrier discharge" (DBD). In this case, at least one of the electrodes consists of a dielectric, i.e., an insulator, or is coated or covered with one. The substrate can also act as a dielectric.

[0015] The treatment intensity of a corona treatment is given as "dose" in [Wmin / m 2< ], with the dose D=P / b*v, with P=electrical power [W], b=electrode width [m], and v=track speed [m / min].

[0016] The substrate is almost always placed or guided through the discharge chamber between an electrode and a counter electrode, which is defined as "direct" physical treatment. Web-shaped substrates are typically guided between an electrode and a grounded roller. The term "blown-out corona" or "single-sided corona" is sometimes used. This is not comparable to atmospheric-pressure plasma, as highly irregular discharge filaments are "blown out" along with a process gas, and stable, well-defined, efficient treatment is not possible.

[0017] FR 2 443 753 discloses a device for surface treatment using a corona discharge. The two electrodes are arranged on the same side of the object surface to be treated, with the first electrodes consisting of a plurality of points along which a curved arrangement of a second electrode is provided. An alternating voltage of several kV with a frequency of 10 kHz is applied between the two electrodes. The corona discharge along the field lines influences the surface being passed by and leads to a polarization of the surface, which improves the adhesion properties of a pressure-sensitive adhesive to the surface treated by the corona effect.

[0018] A disadvantage of the device, however, is that the surface treatment is difficult to control due to the corona effect.

[0019] To enable a more uniform, intensive corona treatment of materials of different types, shapes, and thicknesses, the corona effect on the surface of the material to be treated can be completely avoided by selecting a double-pin electrode according to EP 0497996 B1, with each pin electrode having its own pressurization channel. A corona discharge is generated between the two tips of the electrodes, which ionizes the gas flowing through the channels and converts it into a plasma. This plasma then reaches the surface to be treated, where it carries out, in particular, surface oxidation, which improves the surface's wettability. This type of physical treatment is referred to (here) as indirect because the treatment is not carried out at the site where the electrical discharge is generated.The surface treatment takes place at or near atmospheric pressure, although the pressure in the electrical discharge chamber or gas channel may be elevated. Plasma is defined here as an atmospheric-pressure plasma, which is an electrically activated, homogeneous, reactive gas that is not in thermal equilibrium, with a pressure close to ambient pressure in the effective area. Generally, the pressure is 0.5 bar higher than ambient pressure. The electrical discharges and ionization processes in the electric field activate the gas, creating highly excited states in the gas components. The gas and gas mixture used are referred to as the process gas. In principle, gaseous substances such as siloxane, acrylic acids, solvents, or other components can also be added to the process gas.Components of atmospheric-pressure plasma can include highly excited atomic states, highly excited molecular states, ions, electrons, and unaltered components of the process gas. Atmospheric-pressure plasma is not generated in a vacuum, but usually in an air environment. This means that the escaping plasma, if the process gas itself is not air, at least contains components of the surrounding air.

[0020] In a corona discharge as defined above, the applied high voltage creates filamentary discharge channels with accelerated electrons and ions. The light electrons, in particular, hit the surface at high speeds with energies sufficient to break most molecular bonds. The reactivity of the reactive gas components that are also formed is usually a minor effect. The broken bond sites then react further with components of the air or the process gas. A crucial effect is the formation of short-chain degradation products through electron bombardment. Higher-intensity treatments also result in significant material removal.

[0021] Through the reaction of a plasma with the substrate surface, the plasma components are directly "incorporated" to a greater extent. Alternatively, an excited state or an open bonding site and radicals can be generated on the surface, which then react further, for example, with oxygen from the ambient air. With some gases, such as noble gases, no chemical bonding of the process gas atoms or molecules to the substrate is to be expected. In these cases, the activation of the substrate occurs exclusively through secondary reactions.

[0022] The key difference is that plasma treatment does not involve the direct impact of discrete discharge channels on the surface. The effect is therefore homogeneous and gentle, primarily via reactive gas components. In indirect plasma treatment, free electrons may be present, but they are not accelerated, as the treatment takes place outside the generating electric field.

[0023] Plasma treatment is therefore less destructive and more homogeneous than corona treatment, as no discrete discharge channels impinge on the surfaces. Fewer short-chain degradation products of the treated material are produced, which can form a layer with a negative impact on the surface. Therefore, better wettability can often be achieved after plasma treatment compared to corona treatment, with the effect lasting longer.

[0024] The reduced chain degradation and the homogeneous treatment by using plasma treatment contribute significantly to the robustness and effectiveness of the taught process.

[0025] The plasma device described in EP 0 497996 B1 has quite high gas flows in the range of 36 m³ per hour, with an electrode width of 40 cm per gap. The high flow velocities result in a short residence time of the activated components on the surface of the substrate. Furthermore, only those components of the plasma that are sufficiently long-lived and can be moved by a gas flow reach the substrate. Electrons, for example, cannot be moved by a gas flow and therefore play no role.

[0026] A disadvantage of this plasma treatment, however, is the fact that the plasma impinging on the substrate surface reaches high temperatures, ideally at least 120 °C. However, the resulting plasma often reaches temperatures of several hundred °C. Common plasma guns result in high thermal input into the substrate surface. These high temperatures can lead to damage to the substrate surface, producing, in addition to the activating materials, unwanted byproducts known as LMWOM (low-molecular-weight oxidized materials). This highly oxidized and water-soluble polymer scrap, which is no longer covalently bonded to the substrate, results in low resistance to humid, warm climate conditions.

[0027] Surprisingly, it has now been shown that a significant increase in adhesive strength can also be achieved by treating adhesive surfaces and substrate surfaces before bonding with low-temperature plasma nozzles, whereby the surfaces are strongly activated and the bonded joints are resistant to warm and humid conditions after bonding.

[0028] WO 2012 / 152713 A1 discloses a method in which a corona or plasma pretreatment is used for a double-sided adhesive tape having a first outer pressure-sensitive adhesive side and a second outer heat-activatable side, comprising an at least two-layer product structure consisting of layers A and B that are in direct contact with one another. Specifically, the surface of layer A, which is in direct contact with layer B, is corona or plasma pretreated, wherein the corona or plasma pretreatment is carried out in an atmosphere of nitrogen, carbon dioxide, or a noble gas, or a mixture of at least two of these gases.

[0029] WO 2012 / 152714 A1 discloses a process for increasing the adhesive properties of pressure-sensitive adhesives on substrates by means of plasma treatment and describes the treatment of pressure-sensitive adhesives and substrates by means of various plasma sources.

[0030] WO 03 / 86031 A1 discloses an atmospheric-pressure plasma arrangement comprising a first and a second pair of vertically arranged, parallel, spaced-apart planar electrodes with at least one dielectric plate between the first pair, an adjacent electrode, and at least one dielectric plate between the second pair adjacent to an electrode, thus resulting in two plasma regions. The atmospheric-pressure plasma arrangement further comprises a device for transporting a substrate through a first and a second plasma region and an atomizer suitable for introducing an atomized liquid or a solid coating material into one of the plasma regions.

[0031] From DE 10 2008 018 827 A1 a device for generating an atmospheric pressure plasma by means of an arc discharge is known, wherein the arc discharge is generated by means of a piezo element acting as a cathode with a primary and secondary side.

[0032] US 2009 / 068375 A1 relates to a method for generating a non-equilibrium atmospheric pressure plasma in a process gas and to a method for plasma treatment of a surface with a non-equilibrium atmospheric pressure plasma generated in this way.

[0033] US 2008 / 118734 A1 describes a method for incorporating one or more active materials into coating compositions obtained by plasma polymerization or plasma-enhanced chemical vapor deposition (PE-CVD).

[0034] From DE 20 2008 008 980 U1 a device for generating an atmospheric pressure plasma is known.

[0035] It is an object of the invention to provide a bonding method as mentioned above, in which the resulting bonded joint has a higher moisture-heat resistance.

[0036] This object is achieved by a method having the features of claim 1.

[0037] It has surprisingly been found that for bonding a substrate surface of a substrate layer to an adhesive surface of an acrylate pressure-sensitive adhesive, an increase in the adhesive strength can also be achieved by low-temperature plasma, in particular at atmospheric pressure, which is generated in a low-temperature discharge configuration by activating the substrate surface and the adhesive surface with the low-temperature plasma and, after activation, layering the substrate surface and the adhesive surface on top of one another to form an adhesive bond.

[0038] A low-temperature discharge configuration, for example, is understood to be a configuration that generally generates low-temperature plasma. In this case, a process gas is passed through an electric field, generated, for example, by a piezo element, and is thereby excited to form a plasma. A plasma discharge chamber is the space in which the plasma is excited. The plasma exits from the plasma discharge chamber through an outlet.

[0039] A low-temperature plasma is defined here as a plasma that has a maximum temperature of 50 °C upon impact with the surface. Due to the low temperature, the surfaces are less damaged and, in particular, no unwanted byproducts, so-called LMWOMs (low-molecular-weight oxidized materials), are formed. These LMWOMs lead to a reduction in the adhesive strength of the adhesive on the substrate surface, especially in humid and warm ambient conditions.

[0040] The low temperature of the plasma also has the advantage that a plasma nozzle of the plasma generator can be moved at a very close distance of less than 2 mm above the surface to be treated, and this distance can be maintained constant regardless of the surface properties. In particular, the substrate surface can be activated at the same distance from the plasma nozzle as the adhesive surface, which significantly accelerates the process. Previously, when using high-temperature plasma nozzles, the distance of the plasma nozzle outlet from the substrate surface had to be adjusted to each material. According to the state of the art, this is achieved by increasing or decreasing the treatment distance from the material surface. However, this is associated with increased time expenditure and a complication of the activation process.

[0041] Atmospheric pressure is understood here to mean the ambient pressure, whereby a maximum deviation from the prevailing ambient pressure of at most 0.1 bar, preferably 0.05 bar, is subsumed under the term ambient pressure according to the invention. This atmospheric pressure is present at least in the active and / or discharge range.

[0042] According to the invention, it is provided that the active and / or discharge area is not directly encapsulated or structurally enclosed.

[0043] Because the active and / or discharge area is not enclosed, the plasma treatment of the individual surfaces can be carried out continuously. There is no need to remove the part to be treated from a vacuum or low-pressure chamber, insert the new part into the vacuum or low-pressure chamber, and create a negative pressure in the vacuum or low-pressure chamber, as was previously the case.

[0044] Pressure-sensitive adhesives (PSAs) from the acrylate group are used for the process according to the invention. Plastics such as polypropylenes or LSE coatings such as Apo 1.2 are used as substrates.

[0045] The low-temperature plasma is advantageously generated by a plasma nozzle based on a piezoelectric effect. A process gas is guided past a piezoelectric material in a plasma discharge chamber. The piezoelectric material, acting as the primary region, is oscillated by a low-voltage alternating current via two electrodes. The oscillations are transmitted to the further secondary region of the piezoelectric material. Due to the opposing polarization directions of the multilayer piezoceramic, electric fields are generated. The resulting potential differences enable the generation of plasmas with low temperatures of up to 50 °C. Minimal heat generation can only be achieved through the mechanical work in the piezoceramic. This cannot be achieved with conventional plasma nozzles with arc-like discharges, as the discharge temperature required to excite the process gas exceeds 900 °C.

[0046] In a variant of the invention, the plasma is used with a plasma nozzle unit without additional introduction of one or more precursor materials into the working gas stream or into the plasma jet.

[0047] The problem is also solved by using a low-temperature plasma generator to activate surfaces of a bonded assembly comprising an adhesive surface and a substrate surface.

[0048] As a low-temperature plasma generator, the plasma generator from the Reinhausen Plasma GmbH has provided Piezobrush PZ1 and the Piezobrush PZ2 be used.

[0049] The invention is described using several embodiments in 14 figures, showing: Fig. 1a the activation of a substrate surface of a bond, Fig. 1b the activation of an adhesive surface of the bond, Fig. 1c the activation of the substrate and the adhesive surface of the bond, Fig. 2 graphic for plasma activation of ACX plus< 7074 core Fig. 3Graph showing the potential of plasma treatment for different adhesives and ACX plus< cores Fig. 4: Graph showing the resistance of a plasma-activated bond without exposure to humidity. Fig. 5a, 5b: Resistance of plasma-activated bonding at 40°C / 80% relative humidity. Fig. 6: Adhesive strength measurement of ACX plus< 7812 with piezo plasma activation on LSE varnish. Fig. 7: Adhesive strength measurement of ACX plus< 7812 with piezo plasma activation on polypropylene. Fig. 8: Activation efficiency. Corona vs. Plasma Fig. 9a a schematic view of the operating principle of a low plasma temperature plasma generator, Fig. 9b in the low plasma temperature plasma generator according to Fig. 9a occurring polarization directions,

[0050] The behavior of tesa ®< adhesive components under plasma treatments is evaluated. For this purpose, various substrate layers 1 with corresponding substrate surfaces 2 are selected. Plasma treatments are initially carried out with the Plasmatreat -technology (OpenAir Plasma). For this purpose, a plasma jet from Plasmatreat, Steinhagen. The plasma jet is a plasma gun for generating atmospheric-pressure plasma. A substrate surface and / or an adhesive surface 2 is treated with the atmospheric-pressure plasma.

[0051] When applying an adhesive layer 3 to the substrate layer 1, there are basically three treatment options for the plasma treatment. Firstly, only the substrate surface 2 can be treated according to Fig. 1a Secondly, it can be activated according to Fig. 1b only one adhesive surface 4 of an adhesive layer 3 can be activated or thirdly, according to Fig. 1cboth the substrate surface 2 and the adhesive surface 4 are activated. The three possibilities are shown in the Fig.1a, 1b and 1c According to the invention, only the Fig. 1c variant shown.

[0052] In Fig. 2 A series of tests is shown. tesa ®< ACX plus< 7074 is chosen as adhesive layer 3. Various substrates are selected, which in Fig. 2 are named with their usual abbreviations. The 10 bars per treatment option correspond, from left to right, to the ten abbreviations on the right of the graph, from top to bottom.

[0053] It shows according to Fig. 2 that the activation of both bonding surfaces has a synergistic effect in almost all cases. This means that the activation of the adhesive surface and the substrate surface 2 is the best interface for improving adhesive properties in the relevant tested cases.

[0054] It can also be observed that the bond strength of an adhesive bond between substrate layer 1 and adhesive layer 3 only rarely reaches the level of a double-sided treatment when using substrate activation alone. In specific material combinations, a treatment with the adhesive alone can demonstrate that the quality of a double-sided treatment can be achieved.

[0055] The adhesive strength of an adhesive tape on a steel test plate is determined under a test environment of 23 °C + / - 1 °C and 50% + / - 5% relative humidity. The adhesive tapes are cut to 20 mm width as test samples and adhered to a steel plate. The test plate is cleaned and conditioned before measurement. For this purpose, the steel plate is first wiped with acetone and then left to air for 5 minutes to allow the solvent to evaporate. The side of the single-layer test sample facing away from the test plate is then covered with 36 µm etched PET film, which prevents the adhesive tape from stretching during the measurement. The test sample is then rolled onto the steel substrate. For this purpose, the tape is rolled back and forth five times using a 4 kg roller at a rolling speed of 10 m / min.Twenty minutes after rolling, the steel plate is slid into a special holder that allows the test specimen to be pulled vertically upwards at a 90° angle. The adhesive force is measured using a Zwick tensile testing machine. The measurement results are given in N / cm and are averaged from three individual measurements.

[0056] An important finding is that the activation of bonding surfaces, one of which is a tesa ®< ACX plus< surface of a tesa ®< ACX plus< adhesive tape, can achieve a significant improvement in adhesive strength. ACX plus< adhesive tapes are commercially available adhesive tapes from tesa ®< . ACX plus< adhesive tapes feature a viscoelastic carrier and two adhesive surfaces opposite each other on the carrier, which consist of the same or a modified chemical structure. This means that the adhesive strength-enhancing effect also extends to purely viscoelastic carrier systems. Typically, the viscoelastic carriers are responsible for the desired properties in the finished product (thickness, damping properties, etc.) and are not primarily developed for the adhesion properties. Therefore, the carrier systems are often laminated with their own functional adhesive layers to create the adhesive properties.

[0057] ACX plus< carrier systems have a single-layer structure consisting of an acrylate layer. In the majority of cases, the performance properties of the plasma-activated viscoelastic ACX plus< carrier systems are Fig. 2 comparable to plasma-activated three-layer structures, consisting of a carrier layer with adhesive layers applied to both surfaces. However, the adhesive strengths can be significantly higher.

[0058] Fig. 2shows the adhesive strength of an adhesive bond measured using the standard method between the ACX 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 2 are PTFE (polytetrafluoroethylene), PE (polyethylene), MOPP (monoaxially oriented polypropylene film), PU (polyurethane), EPDM (ethylene-propylene-diene rubber), ClearCoat from BASF, PET (polyethylene terephthalate), ABS (acrylonitrile butadiene styrene), CFRP (carbon fiber reinforced plastic), KTL (cathodic dip coating) and steel. Three treatment options using plasma treatment were selected. The left group of bars represents the adhesive strength of an ACX plus< 7064 adhesive surface on the ten different substrate surfaces mentioned above without plasma treatment of one of the two bonding surfaces 2, 4.

[0059] The middle group of bars shows the adhesive force when only the adhesive surface 4 is activated with the atmospheric pressure plasma and the right group of bars shows the adhesive forces when both the adhesive surface 4 and the respective substrate surface 2 are activated.

[0060] The Fig. 3 contains an overview of the results of the adhesive strength test of various plasma-treated adhesives on PE (polyethylene) surfaces or steel surfaces.

[0061] The first group of bars represents the bond strength measurements on an untreated PE surface. The second group of bars represents the bond strength measurements on PE surfaces when both the adhesive surface and the substrate surface are activated. The third group of bars represents the bond strength measurements on a steel surface without plasma treatment, one of the two bonding surfaces, and the fourth group of bars represents the bond strength measurements of different adhesives on a steel surface when both bonding surfaces are activated with plasma.

[0062] The adhesives are ACX plus< 7476 (according to the invention), MOPP, PU (polyurethane), ACX plus< 705x from tesa ®< (according to the invention), an adhesive from 3M, This is a VHB quality from the company 3M (according to the invention), ACX plus< with glass or Fillite cores (according to the invention) and ACX plus< 68xx single-layer, foamed (according to the invention).

[0063] The results show that plasma treatment has a positive effect on all adhesives, but the absolute bond strengths vary in intensity. A moderate increase in bond strength is observed for the adhesive tape with ACX plus< 7476 and for the pure PU adhesive, partially limited by cohesive failure and mixed fractures. However, it is noticeable that the tested tesa acrylic cores without adhesive, ACX plus< core with hollow glass spheres, and ACX plus< core with Fillite react strongly to plasma treatment and can cause a significant increase in bond strength on PE and steel. The 3M product (pure acrylic, single-layer, with hollow glass spheres) also benefits from the treatment. Single-layer acrylic cores exhibit a high potential for plasma activation.

[0064] A basic potential assessment is shown in Table 1: Table 1 ACXplus Investigation of ... Is significant improvement possible through plasma? Characteristics Adhesive strength Yes Shear strength Yes Instant adhesive strength Yes Substrates EPDM, PP, PE, PET, ... Yes Steel, aluminum, ... Yes varnishes Yes Teflon No Masses Acrylate adhesives Yes Natural rubber Yes Synthetic rubber Yes PU Yes Ac-SBC blends / HPSR Yes Construction Conventional adhesive tapes with foil backing Yes ACXplus cores: pure acrylate, foamed, filled Yes d / s foam fixes Yes

[0065] The resistance of double-sided plasma-activated bonds of ACX plus< 6812 adhesive to ASTM steel and PP after pure temperature storage at temperatures of -30°C, 40°C and 70°C for 4 weeks has been tested according to Fig. 4 proved to be extremely stable. No reduction in adhesive strength over time was observed in any surface combination. Higher values were often achieved compared to untreated references.

[0066] Long-term aging resistance under humid conditions is significantly influenced by the quality of the adhesive interfaces. The goal of plasma treatment is to create suitable reactive centers on the adhesive surface to enhance bonding to the substrate and mitigate or eliminate aging phenomena caused, for example, by warm and humid storage conditions.

[0067] As described above, a plasma does not act in the bulk of an adhesive, but can cause or promote the advance of a water front into the interface via plasma-induced hydrophilization. The absorbed moisture triggers physical and chemical changes at the interface. Here, a wet-warm weakness can be eliminated or reduced by appropriate plasma treatment parameters, such as the distance of the nozzle from the bonding surface and the speed, as shown by the results according to Fig. 5a and Fig. 5b show.

[0068] In Fig. 5aThe bond strength of an ACX plus< 7070 adhesive on two automotive paints is shown after seven days of storage of the bond at room temperature and at 40°C and 80% relative humidity. Fig. 5b A second measurement was carried out on an ACX plus< 6812 adhesive under the same climatic conditions as described above. The left pair of bars in Fig. 5a and Fig. 5b concern a Ford paint and the right pair of bars in Fig. 5a, Fig. 5b a Daimler paint. In all test setups, both bonded surfaces 2 and 4 are activated with a plasma jet.

[0069] But even without optimization and using standard parameters such as a 12 mm distance and a 5 m / min plasma jet treatment speed, material combinations are often resistant to wet-hot treatments. See Table 2. Table 2: Humid-warm resistance varies depending on the substrate Conditions KK 90° EPDM T30 PP GF30 PP test plate 3d / RT N / cm 66* 61* 63* Climate change 1000h 38°C / 95% rH 54* 52* 11** Climate change 10d 85°C / -40°C; 85% rH 57* 55* 9** BMW climate change PR303.5d 240 h +85°C / 60% rH; -30°C 63* 62* 16** *=Cohesive failure / near-surface cohesive **=adhesive fracture

[0070] Table 2 presents bond strength measurements of the ACX plus< 6812 on three different substrate surfaces. The first column represents the bond strength measurement after three days at room temperature, the second column represents the bond strength measurement after 1000 hours at 38 °C and 95% relative humidity. The third column describes the bond strength measurement after 10 days of a climate change, and the fourth column represents the bond strength measurement after 5 days of a climate change.

[0071] The thermal influence of the plasma treatment is largely responsible for the other undesirable side effects, which it produces on both the PP substrate and the adhesive, low-molecular-weight oxidized materials (LMWOM). Accordingly, highly oxidized polymer or oligomer layers are not sufficiently bonded to the polymers in the adhesive bulk and are also water-swellable or soluble.

[0072] It has been found that the discharge technique used in a plasma treatment plays a significant role in moisture resistance. Typically, the afterglow in a plasma jet is generated via an arc or arc-like discharge.

[0073] Another technology of the company Reinhausen Plasma GmbH generates the plasma via a piezoelectric effect, which is enabled by the crystal's opposite polarization directions. This discharge technique results in a cold, non-thermal plasma, unlike an electric arc. Temperatures are close to room temperature. Thermal overtreatment and thus the formation of LMWOM can be prevented or at least reduced. This demonstrates stable wet-to-warm resistance of the adhesive on LSE automotive coatings and low-energy polymers, according to Fig. 6 and Fig. 7In the case of paint bonding, a strong increase in adhesive properties with plasma activation was positively observed.

[0074] In the Figures 9a and 9b The operation of a plasma gun based on a piezoelectric effect is schematically illustrated. A preferred piezoelectric ceramic, for example, is lead or zirconate titanate. Known materials with piezoelectric properties include quartz as a piezoelectric crystal; piezoelectric ceramics such as the aforementioned lead or zirconate titanate are also conceivable.

[0075] In the embodiment according to Fig. 9a9b oppositely oriented piezoceramics are arranged side by side in a secondary region 10, while in a primary region 11, a capacitor 12 with two opposing capacitor plates is provided, each of the capacitor plates being firmly connected to one of the piezo elements 101, 102. By applying an alternating voltage U to the capacitor plates, a mechanical oscillation of the capacitor plates of the capacitor 12 is created by polarity reversal. The mechanical oscillation is transmitted to the piezo elements 101, 102 and generates an alternating potential difference in the end applied to the capacitor, corresponding in frequency to the mechanical oscillation of the capacitor plates. The electric field E generated by the potential difference is in Fig. 9b shown.

[0076] The piezo elements 101, 102 themselves are insulators, so that low safety requirements must be met. The frequency of the low-voltage alternating voltage U at the capacitor plates corresponds to the piezo resonance frequency and is in the range of 10 kHz to 500 kHz. Thus, a low-voltage alternating voltage applied to the capacitor is converted into a mechanical deformation, which in turn generates a high-voltage alternating voltage at the free ends of the piezo elements 101, 102. The principle of the piezo element is described, for example, in EP 2 168 409 B1. Piezo elements are particularly suitable in conjunction with cooling arrangements provided on them, so that the plasma generated by the alternating electric field can be subsequently cooled and a so-called low-plasma temperature plasma can emerge from an outlet nozzle of the plasma gun, which is not explicitly shown.

[0077] Low plasma temperature plasma guns are manufactured by Reinhausen Plasma GmbH on the market. 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 outlet nozzle.

[0078] The Piezobrush PZ2 produces a plasma with a plasma temperature of less than 50 °C. Adhesive force measurements are obtained in Fig. 6 and Fig. 7 .

[0079] The Piezobrush PZ2 is moved over a substrate surface or an adhesive surface at a distance of 5 mm -10 mm and a speed of 5 m per minute, thus preparing the surfaces for the bonding process.

[0080] Due to the low plasma temperature of less than 50 °C, the same plasma gun can be used to treat both the substrate surface and the adhesive surface. The substrate surface is Fig.6 to a LSE_Lack Apo1.2 and in Fig. 7 PP. The adhesive surface is the surface of the ACX plus< 7812 adhesive tape.

[0081] Fig. 6 and Fig. 7 concern adhesive force measurements in which a substrate surface 2 is bonded with an adhesive surface 4 of the double-sided adhesive tape ACX plus< 7812 from tesa ®<.

[0082] In a first step of the method according to the invention, the substrate surface, for example a metal or plastic surface, is coated with the Piezobrush PZ2 In a second process step, the outer side of the ACX plus< 7812 adhesive tape is treated with the same Piezobrush PZ2The ACX plus< 7812 adhesive tape consists of an acrylate layer, both outer surfaces of which are pressure-sensitive adhesive. The two pressure-sensitive adhesive surfaces are normally covered with a protective film, which is removed before the bonding process. According to the invention, the outer side of one pressure-sensitive adhesive layer is coated with the Piezobrush PZ2 to prepare for the bonding process. Piezobrush at the same distance of approximately 2 mm - 5 mm over the outside of the adhesive layer, and then the activated substrate layer 1 and the activated pressure-sensitive adhesive layer 4 are pressed together.

[0083] Fig. 6shows the results of an adhesive strength test according to the test standard, in which a 1 cm wide adhesive tape is applied to a substrate surface according to the method described above. The left bar in graph 1 shows the force required to peel off the double-sided adhesive tape at an angle of 90° when both surfaces, i.e. both the substrate surface 2 and the pressure-sensitive adhesive surface 4, are untreated. The second bar shows the pressure-sensitive adhesive tape in the test only when the outer side of the pressure-sensitive adhesive layer is activated, the third bar shows the adhesive strength when only the substrate layer is activated, whereby the substrate is an LSE varnish, namely APO 1.2. The fourth bar shows the force required to peel off the adhesive tape when both the substrate surface and the pressure-sensitive adhesive surface are Piezobrush PZ2The fifth bar shows the adhesive strength after storage (7 days, 40 °C at 100% relative humidity).

[0084] In the Fig. 7 The adhesive strength for the same test sequence is shown for the double-sided adhesive tape ACX plus< 7812 when bonded to a PP layer, i.e., a polypropylene (PP) layer. Here, too, the first bar represents the adhesive strength for untreated surfaces. The second bar represents the adhesive strength when only the outer surface of the pressure-sensitive adhesive is treated.

[0085] The fourth bar shows the force required to remove the adhesive tape when both the substrate surface and the pressure-sensitive adhesive surface are in contact with the Piezobrush PZ2 The fifth bar shows the adhesive strength after storage (7 days, at 40 °C and 100% relative humidity).

[0086] High adhesive strengths after humid-warm storage after 7 days at 40 °C and 100% relative humidity or at 85 °C and 85% relative humidity can be achieved by low-temperature plasma treatment compared to RT storage (room temperature storage). List of reference symbols

[0087] 1Substrate layer 2Substrate surfaces 3Adhesive layer 4Adhesive surface 10Secondary area 11Primary area 12Capacitor PPolarization direction UAC voltage 101Piezo elements 102Piezo elements

Claims

1. A method for bonding a substrate surface (2) of a substrate layer (1) to an adhesive surface (4) of an adhesive (3), in which a low temperature plasma in a low temperature discharge configuration is generated at atmospheric pressure, the substrate surface (2) and the adhesive surface (4) are activated with the low temperature plasma, and thereafter the substrate surface (2) and the adhesive surface (4) are layered one on top of the other in order to form a compound bonded structure, 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 an earthed electrode, characterized in that the piezoelectric electrode (101, 102) is cooled and a temperature of the plasma emerging from a plasma discharge chamber is at most 50°C, wherein the adhesive surface (4) and the substrate surface (2) are treated with the same low temperature discharge configuration at the same plasma temperature, wherein a pressure-sensitive acrylate adhesive is used as the adhesive (3).

2. The method as claimed in claim 1, characterized in that the plasma discharge chamber is moved over the surface (2, 4) to be treated at a distance of less than 15 mm.

3. The method as claimed in claim 1 or claim 2, characterized in that a substrate layer (1) with a substance selected from the group: PTFE, PE, PP, EPDM, ClearCoat, PET, ABS, CRP, CEC, glass or steel, is used.