DE102012018630A1 - Heat-activated structural pressure-sensitive adhesive tape - Google Patents

Abstract

Heat-activated, structural adhesive tape based on an epoxy-functional acrylate with improved resistance to moisture infiltration and therefore particularly suitable for. B. for gluing attachments to the window of a vehicle.

Description

The present invention is a heat-activated structural adhesive tape with improved resistance to moisture infiltration.

In industrial applications, especially in the automotive industry, high demands are placed on adhesive joints. Depending on the purpose of the requirements z. B. with regard to breaking strength, temperature, climatic or moist heat resistance or high resistance to solvents and fuels very specific. For many applications such. B. when bonding a Spiegelfußhalters on the windshield of a vehicle, a pressure-sensitive adhesive tape is not able to meet all such demands on such a connection such. B. to meet the above in the best way. Currently, such applications are represented by liquid adhesives. The disadvantages of a liquid adhesive, however, are the long cycle time, frequent outflow on the edge and generally the more difficult handling compared to an adhesive tape, which can be brought in the form of a stamped part in the required form and then only has to be glued. Remedy could create a commercially available heat-activatable film, but this is not pressure-sensitive when cold, so the processing is difficult. In addition, these films are usually also thin and inflexible and thus unable to control the thickness variations in the compound, the z. B. caused by the curvature of the windscreens, compensate. Surprisingly, the special properties of the reactive composition according to the invention make it possible to meet these requirements in the form of an adhesive tape or a stamped part.

Reactive pressure-sensitive adhesive tapes are adhesive tapes made of reactive adhesives that undergo a solid, almost insoluble structural bond under the influence of temperature and pressure.

The term "structural adhesive tape" describes a tape which has no adhesive force at room temperature, this is achieved only after the heat activation and to such an extent that the adhesive itself is finally an integral part of the new structure. Due to the missing at room temperature bond strength and depending on the heat sensitivity of the material to be bonded such a band can not be used in every application. Pressure-sensitive adhesive tapes on the other hand have adhesive strength at room temperature, but can not be cured later and could so far the high demands on the bond strength z. B. the known structural liquid adhesives do not do justice. The reactive semi-static pressure-sensitive adhesive tape described in the present invention now combines the advantages of pressure-sensitive adhesive tapes with those of the structural adhesives.

Heat-activated films, films or tapes in general have long been known both from industrial applications and from the literature. With regard to the adhesive systems used, either thermoplastic, heat-activated systems or heat-activatable elastomer / reactive component systems are used.

A particular requirement of the adhesive tape for the purpose intended here in the first place is now to be resistant to moisture infiltration. Due to the special properties of the composition, it is possible to meet these requirements. The tape consists of a heat-activated, epoxy-based adhesive layer, which is located on a release liner. The adhesive tape is already in the non-activated state slightly pressure-sensitive, and can be treated during processing as a normal pressure-sensitive adhesive tape. Stamped parts can be produced from the adhesive tape, which can be pre-applied to the respective parts to be bonded. The adhesive tape can be produced in a thickness of 300 to 1500 μm, depending on the thickness tolerances which may have to be compensated in the respective application. The adhesive contains, in addition to the components typical for a structural epoxy-based adhesive tape, an epoxy silane, which acts as a coupling agent between the glass and the adhesive. This coupling effectively reduces the moisture infiltration and thereby causes a much higher residual holding force after a wet heat storage.

Neither in the literature state of the art nor from the practical application, a system according to the invention is known. So describes JP 2012046738 A as a patent in adhesive technology closest prior art, which can be used in a variety of applications heat-curable adhesive film, wherein the adhesive is an epoxy-functional acrylate copolymer with, inter alia, acrylic rubber microparticles in a core-shell rubber component. JP 08157793 A claims an epoxy-modified "acrylic foam" equipped on one side with a nonwoven fabric which is impregnated with an epoxy-modified acrylate pressure-sensitive adhesive. The entire construction is used as heat-curable double-sided adhesive tape in the attachment of moldings on car doors. EP 311370 B1 teaches a thermosetting epoxy resin adhesive to make a prepreg in which the adhesive is reinforced with fibers. EP 899106 B1 describes a method and composition for bonding components to glass, including mounting a mirror base on a windscreen by means of a thermosetting, heat- or radiation-curable acrylic ester-epoxy resin mixture.

US 5,587,236 deals with a device for mounting a windshield wiper using a thermosetting, modified epoxy transfer adhesive tape. EP 828648 B1 Finally, as the application technology closest to the state of the art claimed a method and apparatus for the bubble-free attachment of mirror feet to windshields using a thermosetting adhesive, the mirror is surrounded with a sealed space in which the pressure was reduced.

However, none of the protective rights mentioned addresses the claimed in the present invention structure of a tape or film-shaped adhesive system with slight tackiness already at room temperature for heat-activatable structural connection of two components, in particular for mounting a Spiegelfußhalters on the disc of an automobile.

In its simplest form, the adhesive tape according to the invention is constructed as follows: heat-activated adhesive layer, release liner.

The thickness of 300 to 1500 microns is achieved by a solvent-free UV band polymerization of the adhesive. Support materials such. As films or fabric accounts for the preferred application here, but their use is quite conceivable for other possible applications. The necessary thickness of the overall system is essentially determined by possible thickness tolerances in the application. So z. B. to compensate for the variations in thickness of a substrate to be bonded such as a disc curvature or the substrates to be bonded or it is a perfect adaptation to curved / curved surfaces strive to thereby effect an optimal adhesive wetting of the parts to be bonded. In this way, no weak point in the composite arises. In addition, the reactive adhesive system described here, which is expediently usable here, exhibits slightly pressure-sensitive adhesive properties even in the non-activated state, which entails the advantages of processing as in the case of a "normal" pressure-sensitive adhesive tape, before a structural connection is finally produced by heat activation.

In principle, a reactive pressure-sensitive adhesive tape based on epoxy resin, which is distinguished by the following properties, is preferred for the intended use because of its high ultimate strength and very good adhesion to metal or glass.
For epoxides, an oxygen atom together with two carbon atoms forms a three-membered ring, a sterically very unfavorable arrangement, very unstable and thus highly reactive.

In the case of the epoxy adhesive used according to the invention, the epoxide components and the crosslinker are present directly next to one another. They are mixed together during the manufacture of the adhesive. For the crosslinker to react, it requires high temperatures from about 130 ° C. In the preparation of the epoxy adhesive, the individual known epoxide-functional components are crosslinked with a crosslinking agent in the form of a latent amine hardener, preferably a dicyanopolyamide, and more preferably a dicyandiamide, to form a macromolecule. Dicyandiamide is very well suited as a crosslinker because of its tetrafunctionality. This reaction takes place only from a temperature of about 160 ° C. Instead, at room temperature dicyandiamide shows virtually no reactivity towards epoxy resins. Further suitable latent amine hardeners are, for. B. diaminodiphenylsulfone or mixtures of dicyandiamide and diaminodiphenylsulfone. In addition, an accelerator / catalyst for epoxy resins is added, which is tailored in its mode of action exactly on the crosslinker. Known catalysts for epoxy adhesives are z. B. 2,6 or 2,4-isomers of Toluolbisdimethylharnstoff and mixtures thereof, but also other known thermally initiated catalyst systems are conceivable. As a result of the use of such a catalyst, the mentioned crosslinking reaction already takes place at 130.degree. The initiator system used according to the invention is a mixture of substances consisting of two different substances, namely 25% of bis (2,6-dimethoxybenzoyl) -2,4,4-trimethylpentylphosphine oxide and 75% of 1-hydroxycyclohexylphenyl ketone. By irradiation with UV light, the components decompose into radicals. UV-active initiators have the great advantage that they decompose into their radicals even at room temperature. These radicals then start the radical polymerization, as monomers while acrylates and epoxifunctional acrylates are used. The reaction is stopped at a certain degree of conversion (20 to 50%) and there is a pre-thickened acrylate prepolymer.

Such an epoxy-functional, pre-thickened acrylate prepolymer forms the essential component of the adhesive system according to the invention, which is thus produced by partial polymerization of a mixture of various components without solvent. The epoxy-functional acrylate prepolymer provides for its tackiness in this system and is responsible for the fact that the adhesive system according to the invention in the non-activated state is slightly pressure-sensitive. It is finally incorporated after curing in the structural composite and is then no longer sticky. Epoxy is less viscous than most adhesives, so it will flow out of crevices and joints if it is not mixed with a thickener or filler. Fillers usually have no chemical effect on the mixture, they simply make the epoxy just thicker, the amount of filler used varies depending on the application. In principle, for example, colloidal silica, polymers, hollow microspheres, phyllosilicates, fiber materials, mica, concrete, kaolin, wollastonite, aluminum oxide, metal powder, pigments and dyes, silicones, waxes or stearates can be used as thickeners. The pre-thickened epoxy-functional acrylate prepolymer is present in the finished adhesive solution in a proportion of 20 to 80%, preferably in a proportion of 40 to 60%.

Tackifiers in the form of solid, liquid or so-called "semi-solid", d. H. high viscosity, resins. One of the resins used is an unmodified epoxy resin with a softening point between 55 and 65 ° C, good solubility and a narrow molecular weight distribution. This resin contributes significantly to the good final strength of the adhesive system and its level of adhesive formulation is from 1 to 50%, preferably from 10 to 30%.

A resin added in an amount of 10 to 40%, preferably 15 to 25% in the form of a silicone rubber is for impact modification. The silicone rubber particles used have an organic shell structure that has reactive groups (they are so-called "core-shell" particles), they have a very good resistance to high temperatures up to 200 ° C. as well as against ozone and UV.

In order to obtain a heat resistance in the uncured state, a diacrylate in small amount (<1%) is added, which causes a slight Vorvenetzung.

The component that distinguishes this invention from other structural adhesive tapes is silane modification. As possible substances here Epoxi- or aminosilanes in question, they are able to bind with your epoxy or amino group to the adhesive matrix, and to couple with your silane group to the glass, so that the infiltration of the glass by water at a To effectively prevent damp heat storage. Thus, the adhesive solution contains a silane-based coupling agent in an amount of one to 5%.

The adhesive system according to the invention moreover contains further adjuvants customarily added in the production of adhesives, for example a reactive diluent in an amount of 2 to 20%, preferably of 5 to 10% and a thermal crosslinker in an amount of 2 to 20%, preferably 3 to 5%.

Further added are 2 to 10%, preferably 3 to 5% of a catalyst acting as an accelerator and 0.5 to 3% of a UV initiator for the radical polymerization and thus the film formation of the acrylate polymer. In addition, the adhesive solution may in principle contain other adjuvants commonly used in the production of adhesives or adhesive tapes, such as. As plasticizers, rheology additives, surface-active substances, wetting and dispersing additives, defoamers, UV stabilizers or antioxidants.

Finally, the finished adhesive tape is a 100% solids system, which was brought from the viscous state of an adhesive film by radical polymerization of the acrylate polymer from the viscous state.

As a rule, such reactive pressure-sensitive adhesive tapes are cured after application to the substrate. This hardening can occur as a result of conventional heating in the oven. Disadvantages in this method are that a high energy input is necessary and that the substrate itself is also very hot, also it is associated with long heating and cooling phases. For this reason, the hardening of the adhesive is advantageously carried out by means of IR or induction, a much faster process compared to the oven curing and not associated with the other disadvantages described. It can in the IR process to improve the absorption of IR rays z. B. red dyes are added to the recipe. In inductive hardening, metallic substrates / support materials are used or metallic particles are added to the adhesive.

• – 2 Wochen bei 70°C. und 98% relativer Feuchte
• – 2 Wochen Klimawechseltest nach Norm PV 1200 (80°C., 98% relative Feuchte im Wechsel mit –40°C.)
• – 400 h Temperaturschocks mit 100°C. und –20°C. im Wechsel
• – 250 h Alterung unter Dauerlast mit 3 kg bis 5 kg bei 10 mal 16 h 40°C. und 95% relativer Feuchte, dann 3 h bei –20°C. und schließlich 6 h bei 100°C. und trockener Hitze
• – eine Woche bei 70°C. und 100% relativer Feuchte, dann 2 h bei –40°C.
• – Schwitzwasserbeständigkeit bei 40°C. über 1.000 h

The most important test criterion for proving the excellent suitability of the strip for the exemplary application mentioned is the measurement of torsional strength before and after aging tests. The following aging tests were carried out with the tape according to the invention:

• - 2 weeks at 70 ° C. and 98% relative humidity
• - 2 weeks climate change test Standard PV 1200 (80 ° C., 98% relative humidity alternating with -40 ° C.)
• - 400 h temperature shocks at 100 ° C. and -20 ° C. alternately
• - 250 h aging under continuous load with 3 kg to 5 kg at 10 x 16 h 40 ° C. and 95% relative humidity, then 3 h at -20 ° C. and finally at 100 ° C for 6 h. and dry heat
• - one week at 70 ° C. and 100% relative humidity, then 2 h at -40 ° C.
• - condensation resistance at 40 ° C. over 1,000 h

A significant difference in torsional strength on glass was not detectable between a tape according to the invention and known tapes already used for this purpose prior to aging. However, after the aging tests, the bands not according to the invention had a strong decrease in torsional strength, e.g. B. at two weeks at 70 ° C. and 98% relative humidity by more than 50% of the original holding power. This was due to the apparent infiltration of the adhesive on the glass surface by moisture.

Finally, a so-called "cleavage" test was carried out: In this test, the uncured adhesive tape must be able to maintain the dead weight of the attachment (10 to 50 g) in a horizontal position at temperatures of typically 60 ° C. up to 140 ° C. to wear for 2 h. There must be no displacement, slipping or shearing of the attachment.

The heating of the adhesive tape takes place in this exemplary application of gluing a mirror base on the window of a vehicle in a few seconds by induction. This is possible because the Spiegelfußhalter to be bonded consists of metal. All other possible types of warming are quite conceivable, but they are usually more time consuming. The composite is loadable after a short time and does not slip anymore, so attachments such. B. vehicle mirror according to the described "Cleavage" test held without slipping. Complete curing and loading is possible after approx. 24 h.

The composition and structure of the tape have been exemplified for the application mentioned, but there are quite a variety of other applications conceivable.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• JP 2012046738A [0007]
• JP 08157793 A [0007]
• EP 311370 B1 [0007]
• EP 899106 B1 [0007]
• US 5587236 [0008]
• EP 828648 B1 [0008]

Cited non-patent literature

• Standard PV 1200 [0023]

Claims (8)

A heat-activated structural adhesive strip, characterized in that the adhesive of the strip is a reactive epoxy resin system consisting essentially of a mixture of an epoxy-functional acrylate polymer, an epoxy resin and a silicone rubber. Heat-activatable structural adhesive strip according to claim 1, characterized in that its thickness is between 300 and 1500 μm. Heat-activatable structural adhesive strip according to claim 1, characterized in that the adhesive is produced as a ribbon by UV polymerization in a layer. A heat-activatable structural adhesive strip according to claim 1, characterized in that the heat-activatable adhesive layer is at a temperature of 120 ° C. up to 180 ° C. and activated by pressure, preferably at a temperature of 150 ° C. up to 170 ° C. A heat-activatable structural adhesive strip according to claim 1, characterized in that the heat-activatable adhesive layer without heat activation at room temperature are slightly pressure-sensitive. A heat-activatable structural adhesive strip according to claim 1, characterized in that the heat-activatable adhesive layer is capable, even before curing, of attachments having a weight of typically 10 to 50 g, even at a temperature of 60 ° C. up to 140 ° C. to keep in a vertical position without slipping. A heat-activatable structural adhesive strip according to claim 1, characterized in that the adhesive contains a silane component which serves as a coupling agent to a glass surface and prevents submergence by water in a moist heat storage. Heat-activatable structural adhesive strip according to one of the preceding claims, characterized in that it is in the form of a strapless adhesive strip for bonding attachments on the window of a vehicle.