DE102010002501A1 - Method for bonding substrates - Google Patents

Abstract

The invention relates to a method for bonding two parts to be joined by means of a heat-activatable adhesive film, the heat-activatable adhesive film being initially arranged between the parts to be bonded, so that a workpiece is produced, the workpiece is brought into the contact area of a sonotrode, and the heating of the adhesive mass film by means of ultrasound activation in this way is caused that ultrasonic vibrations are transmitted to the workpiece under pressure by means of the sonotrode, characterized in that a layer of a protective material is provided between the sonotrode and the workpiece.

Description

The present invention relates to the bonding of plastic elements from the electronics industry by means of heat-activatable films and ultrasound.

Adhesive tapes are used regularly to join two joining partners. In particular, pressure-sensitive adhesive tapes (also referred to as self-adhesive tapes) are used, which frequently have adhesive forces already in contact with the respective surface to be bonded, but in particular under the effect of a certain pressure, in order to effect the joining of the joining partners. However, the pressure-sensitive adhesive tapes usually have only limited adhesive forces. In particular, in cases where only a very small bond area is available, the bond strengths are often no longer sufficient to effect a stable adhesive bond. For example, in the electronics sector, this problem occurs due to a progressive miniaturization of electronic devices and equipment, for example those from the consumer goods sector.

As an alternative to achieving higher bond strengths become increasingly heat-activated. Sheets used. Such adhesive films have no or very low bond strengths at room temperature, but develop by activation with heat significantly higher bond strengths, the bond strength after preparation of the adhesive bond and after cooling is maintained. Therefore, compared to conventional pressure-sensitive adhesive tapes significantly higher bonding strengths are achieved in the products. The heat required for activation is usually introduced by a metal contact. Therefore, such heat-activatable films are used in the electronics industry in particular for the bonding of metal / plastic components. The use of plastic / plastic bonds is difficult and currently heat-activatable films are not used in the electronics industry. In addition, plastics act as thermal insulators and thus prevent rapid heat input.

From the prior art ( DE 10 2005 037 662 A1 ; DE 10 2005 037 663 A1 ), the use of an adhesive tape is known, which receives only by a thermal welding process, a permanent adhesive bond with a polymeric substrate on one side and another piece of the same adhesive tape on the other side.

Another method for assembling two joining partners (substrates) is the so-called ultrasonic welding. At least one of the substrate surfaces is softened by the action of ultrasound and thus welded to the other substrate surface. Thus, in particular the welding of plastics together is possible, especially in plastics, which have relatively similar melting temperatures. Unfortunately, this is rarely the case in the electronics industry. Often different plastics are connected.

As with all other welding processes, the material is also melted by applying heat at the welding point during ultrasonic welding. In ultrasonic welding, the necessary heat is generated by a high-frequency mechanical vibration. The main feature of this method is that the heat required for welding between the components is created by molecular and boundary surface friction in the components. Thus, ultrasonic welding belongs to the group of friction welding.

• • Generator
• • Schwinggebilde (Sonotrode)
• • Amboss

The ultrasonic welding device required for this purpose consists essentially of the assemblies:

• • Generator
• • oscillating structure (sonotrode)
• • anvil

The ultrasonic frequency is generated by means of the generator. This converts the mains voltage into a high-frequency high voltage. The electrical energy thus generated is transmitted to an ultrasonic transducer, the so-called converter. The converter operates in particular according to the piezoelectric effect, in which the property of certain crystals, which expand and contract when applied alternating electric field, is used. This results in mechanical vibrations, which are transmitted via an amplitude transformation piece on a stamp, the so-called sonotrode or welding horn. The amplitude of the oscillation can be influenced by the amplitude transformation piece in size. The vibrations are transmitted under pressure, in particular in the range of 2 to 5 N / mm ² , to the workpiece clamped between the sonotrode and an anvil, whereby the necessary heat for the welding process is produced by molecular and boundary surface friction. Due to the local temperature begins to soften at least one of the joining partners or its surface, and the Damping coefficient increases. The increase of the damping factor leads to further heat generation, which ensures the effect of a self-accelerating reaction. This process is characterized by very low welding times and thus often high efficiency.

After cooling, the welded joint is tight. Since the sonotrode is permanently exposed to ultrasonic vibrations, the demands on the material are very high. In most cases, carbide-coated titanium is therefore used.

If the plastics to be joined soften significantly different melting temperatures, the welding is problematic because the welding temperature must at least equal the temperature of the higher of the two melting points. Thus, the plastic with the lower melting point is exposed early to a high temperature load, which can lead to undesirable structural changes. Frequently, the joining partners are not heated until melting, but the connection is effected by a mechanical toothing of the joining partners with each other, but this causes lower stabilities of the joint assembly.

A modification of the ultrasonic welding is given by the fact that not soften the joining partners themselves, but between the joining partners, a connecting element is provided which melts by ultrasonic activation and thus causes the joining between the joining partners. This is of particular interest if the joining partners do not melt or if the temperature is too high or are temperature-sensitive.

Heat-activatable adhesive films can also be activated to effect the bonding by means of the aforementioned technique. For example, the bonding of joining partners by means of heat-activatable films, which are activated by ultrasonic technology, has already been described for automotive applications ( WO 2009/021801 A ).

However, the method according to the prior art is not suitable for the bonding of a large number of joining partners, since they are not equal to the mechanical influence of the sonotrode and either their surface or the joining partner itself would be destroyed. This problem arises in particular in joining partners of very brittle, fragile materials, from too soft materials, from temperature-sensitive materials or in cases where the bond area between the joining partners is very small and therefore high bond strengths must be achieved, which regularly preferred can be achieved at higher temperature activatable adhesive systems.

The activations described in particular do not meet the requirements of the electronics industry, since the bond areas are significantly smaller and thus higher bond strengths are required. In addition, optical components are often glued together in the electronics industry. An example is z. PMMA windows on polycarbonate / acrylonitrile-butadiene-styrene-capolymer plastics, (PC / ABS), or glasses, or on glass fiber reinforced polyamide housings. Damage can occur here due to the ultrasonic activation (mechanically caused), which then no longer make the materials appear optically clear or in the case of glasses, for example. B. a complete destruction of the material occur.

The object of the invention is to provide a method which enables ultrasound activation for a larger number of heat-activatable bonds and in particular to allow adhesions in the electronics sector via such a method.

This object is achieved by a method for bonding two parts to be joined by means of a composite element, in particular a heat-activated adhesive film, wherein the composite element is first arranged between the adherends to be bonded, so that a workpiece is formed, the workpiece is brought into the contact region of a sonotrode and the heating the adhesive sheet is caused by means of ultrasonic activation such that ultrasonic vibrations are transmitted under pressure to the workpiece by means of the sonotrode, wherein according to the invention a layer of at least one protective material ("protective material layer") is provided between the sonotrode and the workpiece.

The inventive method is preferably carried out in such a way that the protective material layer is removed after bonding by means of the heat-activated adhesive film back from the workpiece, for example, together with the sonotrode. Advantageously, the protective material layer is therefore used only temporarily, ie not as a permanent layer.

Surprisingly, when using a protective material layer, the ultrasonic activation can also be used for a large number of such applications of a heat-activatable adhesive film, for which this is not possible by the prior art processes. In this case, the protective material can take on a surface-protecting function and / or a dampening function. The protective material layer does not have to be excessively strong, in unexpected ways a significant effect could already be determined by layer thicknesses of a few 10 μm.

In an advantageous procedure, a plastic film or a film made of a plastic composite is used as the protective material layer. Such films are also referred to as sacrificial films, since they - as stated above - preferably after the bonding process is removed again from the joining compound.

The sacrificial foil can in the simplest case z. Example of polyethylene or polypropylene and has a thickness of preferably 20 to 100 microns. Damage to the sacrificial foil can occur here. It may therefore be advantageous to use sacrificial films, which have a higher mechanical stability, such. As those of or based on polyurethane, polyimide and / or polyester, more preferably polyethylene terephthalate (PET). For temporary fixation, it can also be advantageous if these films are provided on one side with a weakly adhesive PSA. According to the invention, the bond strength of these single-sided pressure-sensitive adhesive tapes according to PSTC-101 is preferably less than 1.5 N / cm. The PSA should be removable residue-free in a preferred design.

In the event that brittle-sensitive materials are bonded, it is advantageous to use softer films. These should preferably have a Shore hardness D (15 sec. Value) of less than 60, preferably less than 50. The Shore hardness is a material characteristic for elastomers and plastics and is in the standards DIN 53505 and DIN 7868 established. To reduce the Shore D hardness in polyethylene (PE) z. B. the density can be modified. Particular preference is given to using low-density polyethylene (LDPE, high-pressure polyethylene) having a density in the range from 0.910 to 0.940 g / cm ³ . In another very preferred embodiment, LLDPE (Linear Low Density Polyethylene) is used as the film material.

In a further development of the invention, the protective material layer, in particular the sacrificial foil, is reversibly self-adhesive, either inherently self-adhesive and / or comprising at least one layer of self-adhesive compositions. This allows a good positioning of the protective layer and a good fate to achieve the desired position during the process. The stickiness of the protective layer is advantageously low, so that the protective layer can be easily removed from the workpiece again.

Designs are also conceivable in which the protective layer is permanently bonded to the workpiece and remains permanently on the workpiece after the end of the process. However, since the protective layer in the process is intended to absorb the stresses that otherwise would affect the workpiece, particularly process-stable materials would have to be used for this embodiment.

For ultrasonic activation, the method known for ultrasonic welding, as shown in the introductory part of this document, is advantageously used, wherein it is modified by the inventive use of the protective material layer. The composite element is heated and brought to its application temperature (equal to or greater than the activation temperature).

An advantageous embodiment of the method according to the invention is characterized in that the workpiece itself is mounted damping so as to reduce the mechanical stress of the workpiece caused by the vibrations. This can be achieved in particular by a damped mounting of the workpiece. Also, the workpiece for holding the component of a molded plastic, such. As a cast phenolic resin or polyurethanes, be made. The materials are slightly softer than the commonly used metals, such as aluminum or stainless steel, and so can also have a dampening effect.

For the damped storage, a foam or a blanket can be used in an advantageous embodiment. Printing blankets are usually characterized by a fabric layer, a pore layer and a rubber-like layer, the latter also being a mixture with EPDM (ethylene-propylene-diene terpolymers). An essential feature is the Shore hardness. The Shore hardness is preferably between 20 and 150, very preferably between 50 and 100.

Very advantageously, for example, one or more blankets can be used, as they are known in particular from the printing industry. In addition to the blankets but also foams based on z. For example, rubbers, silicones, polyethylenes, polyethylene / EVA mixtures or polyurethane-based. For these, too, a Shore hardness between 20 and 150, very preferably between 50 and 100, is advantageously selected. The material should preferably have at least a thickness of 1 mm.

As heat-activatable films can be monolayer systems based on a heat-activated adhesive; optionally an additized heat-activatable adhesive. In a first embodiment, the heat-activatable film consists of a layer of the (optionally additivated) heat-activatable adhesive.

As a further embodiment, heat-activatable films are used which are composed of two or more interconnected layers (optionally each independently optionally additivated) heat-activatable heat-activatable adhesives used. In the simplest embodiment, such a heat-activatable film consists of two laminated (or otherwise interconnected) heat-activatable adhesive layers. In this case, the individual adhesive layers, in particular the outer adhesive layers, can be excellently adapted to the respective joining partner to be bonded.

For example, two elastomer-based films, two thermoplastic-based films, or a composite elastomer-based film and a thermoplastic-based film may be used. Also, a composite of two acrylate-based films or a composite of an acrylate-based film with an elastomeric or thermoplastic-based film may be excellent for the particular application.

Finally, heat-activatable films comprising at least one carrier or reinforcing film or a plurality of carrier and / or reinforcing films which have at least two outer layers of heat-activatable adhesives can be used. The outer heat-activatable adhesive layers may be identical or different, with the same applies to the adhesive layers, which has already been shown above for the carrierless two- or multi-layer heat-activated adhesive film.

A carrier-free training, for example as an adhesive tape with two different adhesives or comprising only one layer of adhesive (so-called transfer adhesive tapes) is particularly useful if the composite element should have as small a height as possible, such as in bonding in the miniature area. Also transfer adhesive tapes are very useful when glued on rougher plastic surfaces, since then the heat-activatable film has a much better flow and thus better wetting takes place.

In contrast, the training with an additional carrier, for example, particularly advantageous when a particularly high mechanical stability of the composite element itself is required, such as highly loaded compounds and to improve the punchability when using stamped parts as a composite element. In addition, the flow behavior is minimized for very small diecuts, so that the diecuts remain dimensionally stable during the ultrasonic activation.

Alternatively, two or more (not interconnected), identical or different heat-activatable films, in particular each of the films according to one of the aforementioned embodiments, be used for bonding the joining partners.

As heat-activatable adhesive film (s), it is possible in principle to use all known films based on heat-activatable adhesives which are known from the prior art and which are suitable for bonding the respective joining partners (thus the heat-activatable films should advantageously also be selected in the application according to the invention Be sure that none of the joining partners both the respective activation energy of the heat-activatable film (s) used damage).

Thus, according to the invention, it is advantageously possible to use thermoplastic heat-activatable films and / or elastic heat-activatable films. Outstandingly suitable are reactive heat-activatable films, here in particular the heat-activatable films, which comprise at least one elastomeric component and one or more reactive components, in particular reactive resins, or consist of the abovementioned components.

In a preferred embodiment, the adhesive mass (s) used for or for the heat-activatable film (s) are those which permit the shortest possible process times. In combination With the ultrasound activation, the entire process of joining the plastic elements can be done very quickly and efficiently. It has been shown that it is favorable if heat-activatable adhesives are selected which have process times of below 10 s, preferably below 5 s, more preferably below 2 s for activation. The cooling phase under pressure preferably proceeds in a time frame between 1 and 30 seconds, particularly preferably below 10 seconds, particularly preferably below 5 seconds. The pressure depends on the type of heat-activatable adhesives used. Thus, between 1.5 and 8 bar are preferably used for thermoplastic heat-activatable film in the ultrasonic phase and in the cooling phase. For reactive heat-activatable films, preferably between 2 and 15 bar, very preferably between 3 and 12 bar are applied in the ultrasonic phase and in the cooling phase. It is also possible that the pressure level differs in the ultrasonic phase and in the cooling phase.

Heat-activatable films are used very advantageously, which can be heated particularly well by ultrasound. A preferred group of heat-activatable films used is that of the elastic - and therefore vibration-inducing - materials, in particular the rubbers. It is very advantageous to use films based on (heat-activatable) nitrile rubber compositions, very particularly preferably films based on reactive systems. These are in particular films in which the (elastic) adhesive base component reactive resins such as epoxy resins, novolak resins, melamine resins, terpene phenolic resins, phenolic resins and / or polyisocyanante are added.

As elastomers can be used advantageously nitrile butadiene rubbers. Suitable nitrile butadiene rubbers are available, for example, under Europrene ^™ from Eni Chem, or under Krynac ^™ and Perbunan ^™ from Bayer, or as Breon ^™ and Nipol N ^™ from Zeon. Hydrogenated nitrile-butadiene rubbers are available under Therban ^™ from Bayer and Zetpol ^™ from Zeon. Nitrile butadiene rubbers are polymerized either hot or cold.

The nitrile rubber (s) preferably have an acrylonitrile content of 15 to 45% by weight. With acrylonitrile contents of more than 15% by weight, complete phase separation with the reactive resins can usually be avoided. Another criterion for the nitrile rubber is the Mooney viscosity. Since high flexibility at low temperatures is to be ensured, the Mooney viscosity should be below 100 (Mooney ML 1 + 4 at 100 ° C). Commercial examples of such nitrile rubbers are e.g. Nipol ^™ N917 from Zeon Chemicals.

The elastomer component may advantageously comprise carboxyl, amine, epoxy or methacrylate-terminated nitrile-butadiene rubbers, preferably with a proportion of the abovementioned modified nitrile-butadiene rubbers of not more than 20% by weight. Such elastomers are particularly preferably used with a molecular weight of M _w <20,000 g / mol and an acrylonitrile content of 5% to 30%. With an acrylonitrile content of at least 5% or more, in turn, a good miscibility of the components can be realized. Commercial examples of such terminated nitrile rubbers are e.g. B. Hycar ^™ from Noveon.

As carboxy-terminated nitrile-butadiene rubbers, preference is given to using rubbers having a carboxylic acid number of from 15 to 45, very preferably from 20 to 40. The carboxylic acid number corresponds to the amount of KOH (value in milligrams) needed to completely neutralize the carboxylic acid.

As the amine-terminated nitrile-butadiene rubbers, particularly preferred are rubbers having an amine value of from 25 to 150, more preferably from 30 to 125. The amine value refers to the amine equivalents determined by titration against HCl in ethanolic solution. The amine value is based on amine equivalents per 100 grams of rubber divided by 100.

Reactive resins are advantageously added as a further component to the elastomer component. The proportion of reactive resins in the heat-activable adhesive is preferably between 25 and 75 wt .-%.

As reactive resins can be used very advantageously epoxy resins, wherein the resins may be present in particular oligomeric and / or polymeric. The molecular weight M _w (weight average) of the epoxy resins used is preferably in the range from 100 g / mol up to a maximum of 10,000 g / mol for epoxy resins.

Advantageously used epoxy resins include epichlorohydrin, glycidyl esters, the reaction product of bisphenol A and epichlorohydrin, the reaction product of epichlorohydrin and p-aminophenol.

Advantageously used according to the invention commercial examples of epoxy resins are, for. Araldite ^™ 6010, CY-281 ^™ , ECN ^™ 1273, ECN ^™ 1280, MY 720, RD-2 from Ciba Geigy, DER ^™ 331, DER ^™ 732, DER ^™ 736, ^TM 432, ^TM 438, ^TM 485 from Dow Chemical, Epon ^™ 812, 825, 826, 828, 830, 834, 836, 871, 872, 1001, 1004, 1031 etc. from Shell Chemical and HPT ^™ 1071, HPT ^™ 1079 also from Shell Chemical. Examples of commercial aliphatic epoxy resins are e.g. Vinylcyclohexanedioxides such as ERL-4206, ERL-4221, ERL 4201, ERL-4289 or ERL-0400 from Union Carbide Corp.

As reactive resins, novolak resins can furthermore be used alone or in combination with other reactive resins, in particular those mentioned in this document. Novolak resins are commercially available as Epi-Rez ^™ 5132 from Celanese, ESCN-001 from Sumitomo Chemical, CY-281 from Ciba Geigy, DEN ^™ 431, DEN ^™ 438, Quatrex 5010 from Dow Chemical, RE 305S from Nippon Kayaku, Epiclon ^TM N673 from DaiNipon Ink Chemistry or Epicote ^™ 152 from Shell Chemical.

Furthermore, can be used as reactive resins alone or in combination with other - in particular the reactive resins mentioned in this document - melamine resins such. Cymel ^™ 327 and 323 from Cytec.

Furthermore, as reactive resins alone or in combination with other - in particular the reactive resins mentioned in this document - terpene phenolic resins, such as. For example, NIREZ ^™ 2019 from Arizona Chemical.

Preferably, alone or in combination with other - in particular the reactive resins mentioned in this document as reactive resins and phenolic resins, such as. YP 50 from Toto Kasei, PKHC from Union Carbide Corp. and BKR 2620 from Showa Union Gosei Corp. deploy. Furthermore, phenolic resole resins can also be used as reactive resins in combination with other phenolic resins.

Furthermore, as reactive resins alone or in combination with other - in particular the reactive resins mentioned in this document, polyisocyanates, such as. For example, use Coronate ^™ L from Nippon Polyurethane Ind., Desmodur ^™ N3300 or Mondur ^™ 489 from Bayer.

In an advantageous embodiment of the reactive activatable adhesive, adhesive-increasing (tackifying) resins are also added as a further component; very advantageous to a proportion of up to 30 wt .-%, based on the total mixture of the activatable adhesive. As the tackifying resins to be added, the previously known adhesive resins described in the literature can be used. Mention may be made representative of the pinene, indene and rosin resins, their disproportionated, hydrogenated, polymerized, esterified derivatives and salts, the aliphatic and aromatic hydrocarbon resins, terpene resins and terpene phenolic resins and C5, C9 and other hydrocarbon resins. Combinations of two or more of the aforementioned and / or other resins can be used to adjust the properties of the resulting adhesive as desired. In general, all compatible with the nitrile rubbers (soluble) resins can be used, in particular reference is made to all aliphatic, aromatic, alkylaromatic hydrocarbon resins, hydrocarbon resins based on pure monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins and natural resins. On the representation of the knowledge in the "Handbook of Pressure Sensitive Adhesive Technology" by Donatas Satas (van Nostrand, 1989) be explicitly noted.

The adhesive for the heat-activatable adhesive film may optionally be added further additives.

To accelerate the reaction between the two components, crosslinkers and accelerators can also be optionally added to the mixture. For example, imidazoles commercially available as 2M7, 2E4MN, 2PZ-CN, 2PZ-CNS, PO550, L07N from Shikoku Chem. Corp. are suitable as accelerators. or Curezol 2MZ from Air Products. Further suitable crosslinkers are HMTA (hexamethylenetetramine) additives. Furthermore, amines, in particular tertiary amines can be used for acceleration.

Furthermore, plasticizers can also be used. For example, plasticizers based on polyglycol ethers, polyethylene oxides, phosphate esters, aliphatic carboxylic acid esters and benzoic acid esters can advantageously be used according to the invention; furthermore aromatic carboxylic acid esters, relatively high molecular weight diols, sulfonamides and adipic acid esters.

The adhesives are optionally filled with fillers (eg fibers, carbon black, zinc oxide, titanium dioxide, chalk, solid or hollow glass spheres, microspheres of other materials, silicic acid, silicates), nucleating agents, blowing agents, adhesive-enhancing additives and thermoplastics, compounding agents and / or aging inhibitors, For example, in the form of primary and secondary antioxidants or in the form of sunscreens added. Other additives can be added advantageously, such as. As polyvinyl formal, polyvinyl butyrals, polyacrylate Rubbers, chloroprene rubbers, ethylene-propylene-diene rubbers, methyl-vinyl-silicone rubbers, fluorosilicone rubbers, tetrafluoroethylene-propylene copolymer rubbers, butyl rubbers, styrene-butadiene rubbers. By adding these additives, the elastic and / or viscoelastic behavior of the adhesive can be influenced, the temperature stability can be increased and / or the solvent resistance can be improved.

Polyvinyl butyrals are available, for example, under the name Butvar ^™ from Solucia, under Pioloform ^™ from Wacker and under Mowital ^™ from Kuraray. Polyacrylate rubbers are available under Nipol AR ^™ from Zeon. Chloroprene rubbers are available under Baypren ^™ from Bayer. Ethylene-propylene-diene rubbers are available under Keltan ^™ from DSM, under Vistalon ^™ from Exxon Mobile and under Buna EP ^™ from Bayer. Methyl vinyl silicone rubbers are available from Silastic ^™ from Dow Corning and Silopren ^™ from GE Silicones. Fluorosilicone rubbers are available under Silastic ^™ from GE Silicones. Butyl rubbers are available on Esso Butyl ^™ from Exxon Mobile. Styrene-butadiene rubbers are available from Buna S ^™ from Bayer, Europrene ^™ from Eni Chem, and Polysar S ^™ from Bayer. Polyvinylformals are available on Formvar ^™ from Ladd Research.

Advantageously according to the invention, one or more thermoplastic materials and / or mixtures thereof may also be added to the heat-activatable adhesive composition based on elastomers, for example those from the group of the following polymers: polyurethanes, polystyrene, acrylonitrile-butadiene-styrene terpolymers, polyesters, hard Polyvinyl chlorides, soft polyvinyl chlorides, polyoxymethylenes, polybutylene terephthalates, polycarbonates, fluorinated polymers, such as. As polytetrafluoroethylene, polyamides, ethylene vinyl acetates, polyvinyl acetates, polyimides, polyethers, copolyamides, copolyesters, polyolefins, such as. As polyethylene, polypropylene, polybutene, polyisobutene, poly (metha) crylate. By adding these monomers, the degree of hardness of the resulktierenden adhesives can be influenced. For example, the thermoplastic materials have significantly less damping, but tend to flow better onto the substrate after heat reaction. Also, the melting point of the adhesive can be varied. Fluorinated polymers have a significantly higher temperature resistance. By admixing a proportion of thermoplastic components to the elastomer component, these properties can be at least partially transferred to the resulting adhesive.

It is also possible to add synthetic rubbers, such as, for example, styrene isoprene, di and triblock copolymers (SIS), styrene-butadiene, di- and triblock copolymers (SBS), styrene-ethylene-butadiene, di- and triblock copolymers (SEBS).

The bond strength of the activatable adhesive can be increased by further targeted additivation. So can be z. As polyimine or polyvinyl acetate copolymers also use as adhesion-promoting additives.

In a further embodiment of the invention, heat-activatable films are those based on adhesives which in turn are based on thermoplastic materials.

Thermoplastic heat-activatable films are based on at least one thermoplastic component, in particular in such a way that the heat-activatable film also behaves thermoplastically. In this case, the composition of the adhesive of the heat-activatable film and / or the heat-activatable film can itself be limited to the one thermoplastic component and / or to a mixture of thermoplastic components; However, other (non-thermoplastic) components and / or additives may also be added. In principle, all amorphous and partially crystalline thermoplastics which are suitable for heat-activating the bonding of the respective joining partners, in particular of metal parts on plastics, metal parts or glasses, can be used for the invention.

In a preferred procedure, thermoplastics having a melting temperature T _S in the range of -10 ° C to + 40 ° C (crystalline and partially crystalline systems) and / or a glass transition temperature T _G in the range of 85 ° C and 150 ° C (partially crystalline and amorphous Systems) are used. The determination of the static glass transition temperature or the melting temperature is carried out by differential scanning calorimetry DIN 53765 ,

The information on the glass transition temperature T _G refer to the glass transition temperature value T _g DIN 53765: 1994-03 unless otherwise stated in individual cases. The information on the melting temperature T _S refers to the peak maximum value T _{SP of} the melting temperature measurement DIN 53765: 1994-03 unless otherwise stated in individual cases.

According to the invention suitable thermoplastics are, for example, polyester or copolyester, polyamides or copolyamides, thermoplastic polyurethanes, polyolefins, such as. B. polyethylene (Hostalen ^®, Hostalen Polyethylen GmbH), polypropylene (Vestolen P ^®, DSM). Furthermore, it is also possible to use mixtures ("blends") of different thermoplastics, as well as heat-activatable films based on at least two different thermoplastics (eg double-sided coating or different coating on both carrier nonwoven sides).

In a further embodiment, poly-α-olefins are used as thermoplastic coponents. Different heat-activatable poly-α-olefins are commercially available from Degussa under the trade name Vestoplast ^™ .

To optimize the adhesive properties and the activation range, the thermoplastic component (s) can optionally be added with adhesion-increasing resins or reactive resins. The proportion of the resins is advantageously between 2 and 30 wt .-% based on the thermoplastic or the thermoplastic blend. As tackifying resins and as reactive resins, the compounds already described in the elastomeric films can be used.

Optionally, one or more of the further additives which have already been mentioned in the case of the elastomer components can be added to the thermoplastic heat-activated films.

As a heat-activatable film according to the present invention, it is also possible to use films based on heat-activatable acrylate-based adhesives; in the simplest embodiment such films consisting of a layer of a (optionally additivated) heat-activatable adhesive based on acrylate

• a1) 40 bis 95 Gew.-% Acrylsäureester und/oder Methacrylsäureester mit der folgenden Formel CH₂ = C(R₁)(COOR₂), wobei R₁ = H und/oder CH₃ und R₂ = H und/oder Alkylketten mit 1 bis 30 C-Atomen sind.
• a2) 5 bis 30 Gew.-% eines copolymerisierbaren Vinylmonomers mit zumindestens einer Carbonsäure und/oder Sulfonsäure- und/oder Phosphonsäuregruppe
• a3) 1 bis 10 Gew.-% eines copolymerisierbaren Vinylmonomers mit zumindestens einer Epoxygruppe oder einer Säureanhydridfunktion
• a4) 0 bis 20 Gew.-% eines copolymerisierbaren Vinylmonomers, welches mit der funktionellen Gruppe zur Kohäsionssteigerung, der Erhöhung der Reaktivität der Vernetzung, oder zur direkten Vernetzung beitragen kann,
• und b) 5–50 Gew.-% eines Epoxy-Harzes oder einer Mischung aus mehreren Epoxy-Harzen.

In a particularly preferred variant of the invention, the activatable adhesives consist of a base polymer a) comprising

• a1) 40 to 95 wt .-% of acrylic acid ester and / or methacrylic acid ester having the following formula CH ₂ = C (R ₁ ) (COOR ₂ ), where R ₁ = H and / or CH ₃ and R ₂ = H and / or alkyl chains having 1 to 30 carbon atoms.
• a2) 5 to 30 wt .-% of a copolymerizable vinyl monomer having at least one carboxylic acid and / or sulfonic acid and / or phosphonic acid group
• a3) 1 to 10 wt .-% of a copolymerizable vinyl monomer having at least one epoxy group or an acid anhydride function
• a4) from 0 to 20% by weight of a copolymerizable vinyl monomer which may contribute to the cohesion enhancing functional group, to increase the reactivity of the crosslinking, or to direct crosslinking,
• and b) 5-50% by weight of an epoxy resin or a mixture of several epoxy resins.

The polymer a) is itself or is a component of an activatable PSA which becomes tacky under the action of temperature and optional pressure and builds up a high bond strength after bonding and cooling as a result of solidification. Depending on the application temperature, these activatable PSAs have different static glass transition temperatures T _{G, A} or a melting point T _{S, A.}

In a very preferred procedure are used as monomers in the sense of component a1) acrylic and methacrylic acid esters with unbranched or branched hydrocarbon chains, in particular alkyl groups having 4 to 14 carbon atoms, preferably having 4 to 9 carbon atoms. Examples are n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, lauryl acrylate, stearyl acrylate, behenyl acrylate, their branched isomers, such as. B. 2-ethylhexyl acrylate. Further classes of compounds to be used, which can advantageously be added in small amounts under a1), are methyl methacrylates, cyclohexyl methacrylates, isobornyl acrylate and isobornyl methacrylates.

For example, itaconic acid, acrylic acid, methacrylic acid, vinylacetic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, β-acryloyloxypropionic acid, trichloroacrylic acid, vinylphosphonic acid, vinylsulfonic acid and vinylsulfonic acid can be used as monomers of component a2), to name but a few.

In a preferred manner, the monomers of component a3) are, for example, glycidyl methacrylate, maleic anhydride and itaconic anhydride.

Suitable monomers of component a4) are, for example, vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinyl compounds with aromatic rings and heterocycles in the α-position, to mention only a few here, but not by way of limitation. Concrete examples of these monomers are vinyl acetate, vinyl formamide, vinyl pyridine, ethyl vinyl ether, vinyl chloride, vinylidene chloride and acrylonitrile.

Furthermore, as monomers of component a4) alone or in combination with other monomers, in particular the abovementioned monomers of component a4), compounds comprising the functional groups hydroxy, acid amide, isocyanato or amino groups can be used.

Further particularly preferred examples of component a4) are hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, allyl alcohol, acrylamide, benzyl acrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate, t-butylphenyl acrylate, t-butylaphenyl methacrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate, 2-butoxyethyl methacrylate, 2-butoxyethyl acrylate, Dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl methacrylate, diethylaminoethyl acrylate, cyanoethyl methacrylate, cyanoethyl acrylate, 6-hydroxyhexyl methacrylate, N-tert-butylacrylamide, N-methylolmethacrylamide, N- (buthoxymethyl) methacrylamide, N-methylolacrylamide, N- (ethoxymethyl) acrylamide, N-isopropylacrylamide, tetrahydrofurfuryl acrylate , this list is not exhaustive.

For the purposes of component a4) it is also possible to use aromatic vinyl compounds, wherein the aromatic nuclei preferably contain from 4 to 18 carbon atoms and may also contain heteroatoms. Particularly preferred examples are styrene, 4-vinylpyridine, N-vinylphthalimide, methylstyrene, 3,4-dimethoxystyrene, 4-vinylbenzoic acid, but this list is not exhaustive.

The average molecular weights M _w (weight average) of the acrylate component of the heat-activatable adhesives are very preferably chosen such that they are in a range from 20,000 to 2,000,000 g / mol; specially prepared for further use as a heat-activable adhesive having average molecular weights M _w of 100,000 to 500,000 g / mol. The determination of the average molecular weight is carried out by size exclusion chromatography (SEC).

The acrylate component may advantageously be added to epoxy compounds, in particular epoxy resins. The Epoxyverbiindungen are chosen in principle from the entire group of epoxy compounds. Thus, the epoxy resins may be monomers, oligomers or polymers. Polymeric epoxy resins may be aliphatic, cycloaliphatic, aromatic or heterocyclic in nature. The epoxy resins preferably have at least 2 epoxy groups which can be used for crosslinking. The molecular weight of the epoxy resins used preferably varies from 100 g / mol up to a maximum of 25,000 g / mol for polymeric epoxy resins. In particular, the compounds already mentioned in the discussion of the elastomer systems can be used as epoxy resins.

The acrylate-based adhesive may contain other formulation ingredients such as fillers, pigments, rheological additives, adhesion enhancers, plasticizers, elastomers, anti-aging agents (antioxidants), sunscreens, UV absorbers, and other auxiliaries and additives such as desiccants (e.g. Example molecular sieve zeolites, calcium oxide), flow and leveling agents, wetting agents (surfactants) or catalysts.

As fillers, all finely ground solid additives such as chalk, magnesium carbonate, zinc carbonate, kaolin, barium sulfate, titanium dioxide or calcium oxide can be used. Further examples are talc, mica, silicic acid, silicates or zinc oxide. It is also possible to use mixtures of the substances mentioned.

Used pigments may be organic or inorganic in nature. There are all kinds of organic or inorganic color pigments in question, for example, white pigments such as titanium dioxide to improve the light and UV stability, and metal pigments.

Examples of rheological additives are pyrogenic silicas, phyllosilicates (bentonites), high molecular weight polyamide powders or castor oil derivative powders.

Additives for improving the adhesion may be, for example, substances from the groups of polyamides, epoxides or silanes.

Examples of plasticizers are phthalic esters, trimellitic esters, phosphoric esters, esters of adipic acid and other acyclic dicarboxylic acid esters, fatty acid esters, hydroxycarboxylic esters, alkylsulfonic esters of phenol, aliphatic, cycloaliphatic and aromatic mineral oils, hydrocarbons, liquid or semi-solid rubbers (for example nitrile or polyisoprene rubbers), liquid or semi-solid polymers of butene and / or isobutene, acrylic acid esters, polyvinyl ethers, liquid and soft resins based on the raw materials, which also form the basis for tackifier resins, wool wax and other waxes, silicones and polymer plasticizers such as polyesters or polyurethanes.

Suitable resins are all natural and synthetic resins, such as rosin derivatives (for example, by disproportionation, hydrogenation or esterification derived derivatives), coumarone-indene and polyterpene resins, aliphatic or aromatic hydrocarbon resins (C-5, C-9 , (C-5) ₂ resins), mixed C-5 / C-9 resins, hydrogenated and partially hydrogenated derivatives of the types mentioned, resins of styrene or α-methylstyrene and terpene-phenolic resins. Again, the adhesive resins already mentioned above can be used.

Suitable elastomers are, for example, EPDM rubbers (ethylene-propylene-diene copolymer) or EPM rubbers (ethylene-propylene copolymer), polyisobutylene, butyl rubber, ethylene-vinyl acetate, hydrogenated block copolymers of dienes (for example by hydrogenation of SBR (styrene Butadiene rubbers), BAN (butadiene-acrylonitrile copolymers), NBR (nitrile-butadiene rubbers), SBS (styrene-butadiene-styrene block copolymers), SIS (styrene-isoprene), cSBR (carboxylated styrene-butadiene rubber) Styrene block copolymers) or IR (isoprene rubbers), such polymers are known, for example, as SEPS (styrene-ethylene-propylene-styrene block copolymers) and SEBS (styrene-ethylene-butadiene-styrene block copolymers) or acrylate copolymers such as ACM (acrylate).

The preparation of the polymers for the heat-activable adhesives can be carried out by the customary processes known from the prior art.

The invention further relates to the use of ultrasonic activation for heating heat-activatable films, in particular the use of the method according to the invention and its variants, which are described in the context of this document, for the fields of application set out below.

This results in a particularly effective and easy way to permanently connect plastic elements together without damaging the plastic elements themselves in the process of joining.

Another area of application is the application in the electronic field, in particular in the field of electronic consumer goods. In particular, metals, plastics, glasses with metals, plastics and / or glasses can be bonded here. This high strength can be achieved without causing damage to the bonded joining partners. The inventive method is particularly suitable for bonding viewing windows, display windows, data displays or the like in electronic devices (in frame brackets, on the display or the like), without scratching the window material - or the housing - or otherwise take damage.

Finally, the object is achieved by a method for connecting two plastic elements, in particular for producing a plastic housing, by means of a composite element, in particular as described above. In this method, a composite element formed as a surface element is used which has at least one adhesive which is initially non-adhesive. This composite element is arranged between the plastic elements to be connected. First, the composite element and the two plastic elements are arranged in the desired position, then applied a sacrificial foil between the plastic surface and ultrasound sonotrode and then activates the adhesive. In this case, the activation of the adhesive creates a permanent connection between the plastic elements. Compared to welding methods without using a composite element, the method according to the invention offers the advantage that an occurrence of microstructural changes in the plastic elements can be reduced or completely avoided. Compared to the use of conventional adhesive tapes, the positioning of the plastic elements and the composite element itself and thus the entire handling is significantly simplified. Nevertheless, a permanent connection is achieved, which withstands even greater load forces.

It is also advantageous that two plastic elements can be connected to each other from different plastics. The composite element serves as a buffer, which compensates for a temperature difference between the plastic elements to be connected and thus reduces the structural change in the plastic elements.

The activation of the adhesive can be carried out in a particularly simple manner by means of ultrasonic welding. In particular, such welding operations enable activation of the adhesive in very short process times. For example, process times of less than 10 s are possible. Preferably, the activation of the adhesive takes place in a process time below 5 s, more preferably below 2 s.

Test Methods:

Bond strength A)

The bond strength is checked by a dynamic shear test DIN EN 1465 certainly. The bond area is 2 cm ² . A 3 mm thick PC plate with a width of 2 cm is connected to a PC plate with a width of 2 cm and a layer thickness of 3 mm by means of a heat-activatable film.

The test samples are torn apart with a tensile tester at a test speed of 10 mm / min. The result is given in N / mm ² and represents the maximum force related to the bond area, which is measured to separate the test specimens (PC and PC). The measurement is carried out at 23 ° C and 50% relative humidity.

Push-out test B)

A frame of 3 mm thick ABS (outside 50 × 40 mm, inside 30 × 24 mm) and a 1 mm thick PMMA window (45 × 35 mm) is centered with a rectangular, frame-shaped pattern of heat-activated film under ultrasonic heating and adhered to the inventive method. The heat-activated film is rectangular and has a web width of 2 mm. The entire bond area is 300 mm ² . After 24 h of conditioning at 23 ° C and 50% humidity, the bonded PMMA window is then pressed out with a Zwick Tensile Tester by a punch at 10 mm / min. The stamp pushes out the middle of the window and has the dimensions 20 mm × 25 mm. The maximum required force is measured.

Examples

Example 1)

Dynapol ^™ S1227 from Degussa was pressed between two layers of siliconized glassine release paper in a hot press at 140 ° C to 150 microns. The melting range of the copolyester is between 86 ° C and 109 ° C.

Example 2)

Grilltex ^TM 1442 E from EMS-Grilltech was pressed between two layers of siliconized glassine release paper in a hot press at 140 ° C to 150 microns. The melting range of the polymer is between 93 ° C and 121 ° C.

Example 3)

50% by weight of Breon N36 C80 (nitrile rubber) from Zeon, 40% by weight of phenol novolak resin Durez 33040 mixed with 8% HMTA (Rohm and Haas) and 10% by weight of the phenolic resole resin 9610 LW from Fa. Bakelites were prepared as a 30% solution in methyl ethyl ketone in a kneader. The kneading time was 20 h. The heat-activatable adhesive was then spread out of solution onto a glassine release paper and dried at 100 ° C for 10 minutes. After drying, the layer thickness was 100 μm. Two of these layers were then laminated together with a roll laminator at 100 ° C. Subsequently, the layer thickness was 200 microns.

bonding:

The bonding was carried out with the ultrasonic device, PS MPC (+) digital control from Hermann Ultraschalltechnik. The device operates at 35 kHz and with a welding power of 1000 W. Titanium carbide sonotrodes were used, which have the shape of the bonding surface for the two test specimens in test method A and B. The sonotrodes were used to apply pressure and ultrasound. In front The ultrasonic activation was a mechanical fixation, so that the ultrasound could be targeted introduced.

As a sacrificial film, a 40 micron thick PE film was selected, whose extension exceeded that of the frame and which was inserted between the sonotrode and the workpiece. The machine parameters used for bonding are listed in the results section. The lower fixation of the component was done with damping by a blanket. It was used by the company Phönix the model Sapphire ^® Carat. The blanket has a thickness of 1.96 mm and a total Shore hardness of 78 Shore A.

Results:

In the following, test specimens were produced by different methods. First, the bond strength was determined according to test method A (dynamic shear test).

The following machine parameters were set (abbreviation: US = ultrasonic). Table 1 Pressure [bar] US time [s] Print without US [s] US power [W] US Energy [J] Test A [N / mm ² ] example 1 3 bar 0.8 s - 513 349 5.2 3 bar 0.8 s 5 513 349 6.4 Example 2 3 bar 0.8 s - 513 349 5.5 3 bar 0.8 s 5 513 349 7.1

Table 1 shows that very high bond strengths are already possible in the dynamic shear test using ultrasound technology. However, the comparison with the post-cooling under pressure also clarifies that even higher bonding strengths are possible by the pressure application without ultrasound compared to the reference examples without subsequent pressure application.

Furthermore, other test methods are relevant for application-related tests in the electronics sector. So z. B. the push-out test (test method B) very often used in the mobile phone area. The experiments carried out are described in Table 2. For all bonds, the ultrasound was introduced from the side of the PMMA window. Table 2 Pressure [bar] US time [s] Time without US [s] US power [W] US Energy [J] Test B [N / window] Example 3 8 bar 0.8 s - 488 313 202 8 bar 0.8 s 5 488 313 267 Example 3 with sacrificial foil 8 bar 0.8 s 5 488 313 269 Example 3 8 bar 1.5 s - 503 301 223 8 bar 1.5 s 8th 503 301 298 Example 3 with sacrificial foil 8 bar 1.5 s 8th 503 301 294 Example 3 8 bar 0.5 s - 467 246 154 8 bar 0.5 s 8th 467 246 204 Example 3 with sacrificial foil 8 bar 0.5 s 8th 467 246 209

Table 2 shows that higher adhesion strengths can be achieved in all cases by the additional application of pressure. Also, the sacrificial foil had no negative impact on the bond strength in the push-out test. By varying the ultrasonic time, the bond strengths can be further increased.

The optical inspection of the PMMA window revealed that damage to the PMMA window surface was completely avoided by the PE sacrificial foil. All other examples, on the other hand, showed slight forms of thermal damage. An impression of the sonotrode was found.

In a further experiment, the PMMA window was replaced by a glass window of the same dimension. The bonding was carried out with Example 3. The bonding parameters chosen were:
Pressure = 8 bar
Time = 0.8 s
Time without ultrasound: 5 s
Ultrasonic power: 488 W
Ultrasonic energy: 313 J

In a first Verklebungsversuch was glued without PE film and without blanket. In the course of the ultrasonic activation, the glass broke. Subsequently, the identical test was carried out with the 40 μm PE film and the 1.96 mm thick blanket (see bonding). This time, the window did not break.

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

• DE 102005037662 A1 [0004]
• DE 102005037663 A1 [0004]
• WO 2009/021801 A [0012]

Cited non-patent literature

• DIN 53505 [0021]
• DIN 7868 [0021]
• "Handbook of Pressure Sensitive Adhesive Technology" by Donatas Satas (van Nostrand, 1989) [0053]
• DIN 53765 [0064]
• DIN 53765: 1994-03 [0065]
• DIN 53765: 1994-03 [0065]
• DIN EN 1465 [0097]

Claims (10)

Method for bonding two parts to be joined by means of a heat-activated adhesive film, wherein the heat-activatable adhesive film is first arranged between the adherends to be bonded, so that a workpiece is formed, the workpiece is brought into the contact area of a sonotrode and the heating of the adhesive sheet is caused by ultrasonic activation, the By means of the sonotrode ultrasonic vibrations are transmitted under pressure to the workpiece, characterized in that a layer of protective material is provided between the sonotrode and the workpiece. A method according to claim 1, characterized in that a plastic film is used as the layer of a protective material. A method according to claim 2, characterized in that a film of polyethylene or polypropylene is used as the plastic film. Method according to one of claims 2 or 3, characterized in that the plastic film has a thickness of 20 to 100 microns. Method according to one of the preceding claims, characterized in that the workpiece is mounted mechanically damping during the activation of the heat-activated adhesive film. Process according to one of the preceding claims, characterized in that the heat-activatable film used is one based on a heat-activable adhesive comprising 30-70% by weight of nitrile rubbers. A method according to claim 6, characterized in that the heat-activatable adhesive reactive resins are mixed. A method according to claim 6, characterized in that epoxy resins, novolak resins, melamine resins, terpene phenolic resins, phenolic resins and / or polyisocyanantes are used as reactive resins. Method according to one of the preceding claims for the bonding of components in consumer electronic goods. Method according to one of the preceding claims for the bonding of viewing windows and / or display devices in consumer electronic goods.