ADHESIVE TAPE WHICH CAN BE USED IN PARTICULAR IN A METHOD FOR CONNECTING TWO FIBER-REINFORCED PLASTIC COMPONENTS

Abstract

An adhesive tape useful in a method for connecting two fiber-reinforced plastic components, comprising a carrier material of polyethylene, a polyacrylate-based self-adhesive compound applied to one side of the carrier material, being formed from a reactant mixture comprising monomers of A1) acrylic esters and/or methacrylic esters of the formula CH.sub.2CH(R.sup.1)(COOR.sup.2), A2) acrylates and/or methacrylates whose alcohol component contains at least one primary hydroxyl or carboxyl group, and/or vinyl compounds which are copolymerizable with acrylates, A3) acrylates and/or methacrylates whose alcohol component contains at least one epoxy group, and/or vinyl compounds which contain at least one epoxy group, A4) and, if the fractions of A1), A2) and A3) do not add up to 100 wt %, olefinically unsaturated monomers having functional groups, with a proportion of 0 to 15 wt %.

Claims

1: An adhesive tape which may be used in a method for joining two fiber-reinforced plastic components, with a carrier material made of polyethylene, said carrier material bearing on one side a polyacrylate-based self-adhesive composition, which can be traced back to the following reactant mixture, comprising monomers of the following composition: A1) acrylic esters and/or methacrylic esters of the following formula
CH.sub.2CH(R.sup.1)(COOR.sup.2) where R.sup.1=H or CH.sub.3 and R.sup.2 is an alkyl radical having 1 to 14 carbon atoms, with a fraction of 50 to 95 wt %, A2) acrylates and/or methacrylates whose alcohol component contains at least one primary hydroxyl or carboxyl group, and/or vinyl compounds which are copolymerizable with acrylates and which contain at least one primary hydroxyl or carboxyl group, with a fraction of 5 to 30 wt %, A3) acrylates and/or methacrylates whose alcohol component contains at least one epoxy group, and/or vinyl compounds which are copolymerizable with acrylates and which contain at least one epoxy group, with a fraction of 0 to 5 wt %, A4) and, if the fractions of A1), A2) and A3) do not add up to 100 wt %, olefinically unsaturated monomers having functional groups, with a fraction of 0 to 15 wt %.

2: The adhesive tape as claimed in claim 1, wherein the carrier film comprises to an extent of 90 wt % polyethylenepolymers (based on the total composition of the carrier film), or the polymers forming the carrier film consist to an extent of 100 wt % of polyethylenepolymers (based on the total composition of the carrier film).

3: The adhesive tape as claimed in claim 1, wherein polyethylenepolymers used for the carrier film are HDPE, LDPE and/or LLDPE, the fraction of polyethylene in HDPE, LDPE and LLDPE being 99 wt % or more.

4: The adhesive tape as claimed in claim 1, wherein, the carrier film consists of a single film ply.

5: The adhesive tape as claimed in claim 1, wherein the thickness of the carrier film is between 30 and 200 m.

6: The adhesive tape as claimed in claim 1, wherein the coat weight of the adhesive composition on the carrier film is between 10 and 50 g/m.sup.2.

7: A method for joining two fiber-reinforced plastic components, comprising the following steps: producing a first fiber-reinforced plastic component by introducing a laid scrim of fiber mats which is impregnated with a resin/curing agent mixture, which in turn is subsequently cured; that part of the surface of the first fiber-reinforced plastic component which is later to be bonded to the second fiber-reinforced plastic component (also called joining surface below) is masked with an adhesive tape of claim 1 before the fiber mats are impregnated with the resin/curing agent mixture, producing a second fiber-reinforced plastic component according to the steps specified above; that part of the surface of the second fiber-reinforced plastic component that is later to be bonded to the first fiber-reinforced plastic component is likewise masked with an adhesive tape of claim 1 before the fiber mats are impregnated with the resin/curing agent mixture, removing the adhesive tapes from the two completed fiber-reinforced plastic components, contacting the two joining surfaces, introducing a resin/curing agent mixture between the two joining surfaces, curing the resin/curing agent mixture.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0023] FIG. 1 shows the adhesive tape in a lateral section

DETAILED DESCRIPTION

[0024] Suitable as carrier film, surprisingly, is polyethylene (PE). This is surprising because, for example, HDPE possesses a melting point of 130 to 145 C. and decomposes at temperatures of 230 C. In the absence of air, this decomposition process appears not to occur. The PE melts, with the melt remaining fixed in position by the vacuum bag during the autoclaving process, and solidifying again when the temperature is reduced.

[0025] Preference is given to using HDPE, i.e., polymer chains with little branching and therefore with a high density of between 0.94 g/cm.sup.3 and 0.97 g/cm.sup.3, LDPE, i.e., highly branched polymer chains, therefore with a low density of between 0.915 g/cm.sup.3 and 0.935 g/cm.sup.3; and/or LLDPE, i.e. a linear low-density polyethylene whose polymer molecule contains only short branches. These branches are prepared by copolymerization of ethene and higher -olefins (typically butene, hexene or octene).

[0026] The fraction of polyethylene in HDPE, LDPE and LLDPE is preferably 99 wt % or more.

[0027] The carrier film preferably comprises polyethylene polymers to an extent of 95 wt % (based on the overall composition of the carrier film).

[0028] The polymers forming the carrier film preferably consist to an extent of 100 wt % of polyethylene. The additives may, optionally additionally have been added to the polyethylene polymers. These additivesas statedare not mandatory, but instead may also not be used.

[0029] The polymers for forming the carrier film may be present in pure form or in blends with additives such as antioxidants, light stabilizers, antiblocking agents, lubricating and processing assistants, fillers, dyes, pigments, blowing agents, or nucleating agents.

[0030] The film preferably has none of said additives.

[0031] According to one preferred embodiment, the carrier film consists of a single film ply.

[0032] According to one preferred embodiment, the thickness of the carrier film is between 30 and 200 m, preferably between 40 and 100 m, more preferably between 40 and 60 m.

[0033] Use is made, as a self-adhesive composition, of a polyacrylate which can be traced back to the following reactant mixture, comprising monomers of the following composition:

[0034] A1) acrylic esters and/or methacrylic esters of the following formula

CH.sub.2CH(R.sup.1)(COOR.sup.2)

[0035] where R.sup.1=H or CH.sub.3 and R.sup.2 is an alkyl radical having 1 to 14 carbon atoms, with a fraction of 50 to 95 wt %,

[0036] A2) acrylates and/or methacrylates whose alcohol component contains at least one primary hydroxyl or carboxyl group, and/or vinyl compounds which are copolymerizable with acrylates and which contain at least one primary hydroxyl or carboxyl group, with a fraction of 5 to 30 wt %,

[0037] A3) acrylates and/or methacrylates whose alcohol component contains at least one epoxy group, and/or vinyl compounds which are copolymerizable with acrylates and which contain at least one epoxy group, with a fraction of 0 to 5 wt %,

[0038] A4) and, if the fractions of A1), A2) and A3) do not add up to 100 wt %, olefinically unsaturated monomers having functional groups, with a fraction of 0 to 15 wt %.

[0039] The alkyl radical may be linear, branched or cyclic.

[0040] The monomers are preferably selected such that the resulting polymers can be used as pressure-sensitive adhesive compositions at room temperature, more particularly such that the resulting polymers have properties of pressure-sensitive adhesiveness in accordance with the Handbook of Pressure Sensitive Adhesive Technology by Donatas Satas (van Nostrand, New York 1989, pages 444 to 514).

[0041] The polyacrylates described are acrylic acid-free. They are highly transparent. The particular feature of this kind of adhesive compositions is that they undergo post-crosslinking, or filming, at high temperatures. This happens primarily as a result of the copolymerized 2-hydroxyethyl acrylate. At elevated temperatures, the hydroxyl group is able to react intramolecularly and intermolecularly with the ester groups of the other monomers, in transesterification reactions.

[0042] The adhesive composition applied on the carrier film is a pressure-sensitive adhesive composition, i.e., an adhesive composition which provides a durable bond to almost any substrate even under relatively light pressure and is redetachable from the substrate after use essentially without leaving a residue. A pressure-sensitive adhesive composition is permanently tacky at room temperature, i.e. has a sufficiently low viscosity and a high initial tack, so it will wet the surface of the particular substrate by a minimal pressure. The adherability of the adhesive composition rests on its adhesive properties, and the redetachability on its cohesive properties.

[0043] The term acrylate adhesive composition here means that the polymers forming the basic framework of the adhesive composition (in other words without tackifier resins, plasticizers, or other adjuvants and additives) consist of the stated acrylate, to an extent of at least 50 wt %, preferably 75 wt %, more preferably to an extent of 90 wt %.

[0044] Use is preferably made of olefinically unsaturated monomers having functional groups selected from the following listing: hydroxyl, carboxyl, sulfonic-acid or phosphonic-acid groups, acid anhydrides, epoxides, amines.

[0045] Particularly preferred examples of these monomers are itaconic acid, maleic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, -acryloyloxypropionic acid, trichloroacrylic acid, vinylacetic acid, vinylphosphonic acid, itaconic acid, maleic anhydride, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, allyl alcohol, glycidyl acrylate, glycidyl methacrylate.

[0046] For further improvement of the properties, the adhesive compositions formulations may optionally be blended with light stabilizers or with primary and/or secondary aging inhibitors. Aging inhibitors used may be the products based on sterically hindered phenols, phosphites, thiosynergists, sterically hindered amines or UV absorbers. Preference is given to using primary antioxidants such as, for example, Irganox 1010 or Irganox 254, alone or in combination with secondary antioxidants such as, for example, Irgafos TNPP or Irgafos 168. The aging inhibitors here can be used in any desired combination with one another, and mixtures of primary and secondary antioxidants in combination with light stabilizers such as Tinuvin 213, for example, exhibit particularly good aging-inhibiting effects.

[0047] Having proven particularly advantageous are aging inhibitors which combine a primary antioxidant with a secondary antioxidant in one molecule. These aging inhibitors are cresol derivatives whose aromatic ring is substituted at any two different locations, preferably in ortho and meta positions to the OH group, by thioalkyl chains, where the sulfur acid may also be joined by one or more alkyl chains to the aromatic ring of the cresol building block. The number of carbon atoms between the aromatic moiety and the sulfur atom may be between 1 and 10, preferably between 1 and 4. The number of carbon atoms in the alkyl side chain may be between 1 and 25, preferably between 6 and 16. Particularly preferred here are compounds of the type of 4,6-bis(dodecylthiomethyl)-o-cresol, 4,6-bis(undecylthiomethyl)-o-cresol, 4,6-bis(decylthiomethyl)-o-cresol, 4,6-bis(nonylthiomethyl)-o-cresol or 4,6-bis(octylthiomethyl)-o-cresol. Aging inhibitors of these kinds are available for example from Ciba Geigy under the name Irganox 1726 or Irganox 1520.

[0048] The amount of aging inhibitor or aging-inhibitor package added ought to be in a range between 0.1 and 10 wt %, preferably in a range between 0.2 and 5 wt %, more preferably in a range between 0.5 and 3 wt %, based on the overall composition of the adhesive.

[0049] To improve the processing properties it is possible for the adhesive compositions to be admixed, for formulation, with further customary process auxiliaries such as rheological additives (thickeners), defoamers, de-aerating agents, wetting agents or flow control agents. Suitable concentrations are in the range from 0.1 to 5 wt %, based on the overall composition of the adhesive.

[0050] Fillers (reinforcing or nonreinforcing) such as silicon dioxides (spherical, acicular, lamellar, or irregular, such as the fumed silicas), glass in the form of solid or hollow beads, microballoons, calcium carbonates, zinc oxides, titanium dioxides, aluminum oxides or aluminum oxide hydroxides may serve both for adjusting the processing qualities and for adjusting the technical adhesive properties. Suitable concentrations are in the range from 0.1 up to 20 wt %, based on the overall composition of the adhesive.

[0051] The anchoring is customarily strong enough to allow an adhesive tape of this kind to be unwound easily from a roll, without the anchoring causing the adhesive composition to break and resulting in so-called transfer of the adhesive composition (in which case the adhesive composition is located on the reverse of the carrier).

[0052] According to one preferred embodiment, the coat weight of the adhesive composition on the carrier film is between 10 and 50 g/m.sup.2; preferably between 20 and 40 g/m.sup.2, more preferably between 25 and 35 g/m.sup.2.

[0053] The pressure-sensitive adhesive composition may be produced and processed from solution, from dispersion, and from the melt. Preferred preparation and processing procedures are accomplished from solution and also from the melt. Particularly preferred is the manufacture of the adhesive composition from the melt, in which case, in particular, batch methods or continuous methods may be used. The continuous manufacture of the pressure-sensitive adhesive composition by means of an extruder is particularly advantageous.

[0054] The pressure-sensitive adhesive composition thus produced can then be applied to the carrier by the methods that are common knowledge. In the case of processing from the melt, this may involve application methods using a nozzle or a calender.

[0055] In the case of processes from solution, coating operations with doctor blades, knives or nozzles are known, to name but a few.

[0056] In order to increase the adhesion between the adhesive composition and the carrier film, the carrier film may have its surface energy enhanced by undergoing corona treatment or plasma treatment, this representing a very preferred variant.

[0057] The use of a primer layer between carrier film and adhesive composition is advantageous for the purpose of improving the adhesion of the adhesive composition on the film and therefore for enhancing the residue-free removability after the application.

[0058] Etching of the film is advantageous, moreover, in order to allow the adhesive composition to be anchored.

[0059] The general expression adhesive tape for the purposes of this invention encompasses all sheet-like constructs such as two-dimensionally extended films or film portions, tapes of extended length and limited width, tape portions and the like, ultimately also die cuts.

[0060] The adhesive tape may be produced in the form of a roll, in other words wound up on itself in the form of an Archimedean spiral, or else may be lined on the adhesive side with release materials such as siliconized paper or siliconized film (also known as liners to the skilled person), which are removed from the layer of adhesive composition prior to use.

[0061] A suitable release material is preferably a nonlimiting material such as a polymeric film or a highly sized long-fibered paper.

[0062] The reverse side of the adhesive tape may carry an applied reverse-side lacquer, in order to exert a favorable influence over the unwind properties of the adhesive tape wound to the Archimedean spiral. This reverse-side lacquer may for that purpose be equipped with silicone or fluorosilicone compounds and also with polyvinylstearylcarbamate, polyethyleneimine stearylcarbamide or organofluorine compounds as abhesive substances or for nonstick coating.

[0063] The adhesive tapes in particular have running lengths of 1000 to 30.000 m. Typical widths selected for the rolls are 10, 15, 19, 25 and 30 mm.

[0064] The adhesive tape may particularly advantageously be used in a method which serves to join two fiber-reinforced plastic components.

[0065] Here, the method comprises the following steps: [0066] producing a first fiber-reinforced plastic component by introducing a laid scrim of fiber mats which is impregnated with a resin/curing agent mixture, which in turn is subsequently cured; that part of the surface of the first fiber-reinforced plastic component which is later to be bonded to the second fiber-reinforced plastic component (also called joining surface below) is masked with the adhesive tape of the invention before the fiber mats are impregnated with the resin/curing agent mixture, [0067] producing a second fiber-reinforced plastic component according to the steps specified above; that part of the surface of the second fiber-reinforced plastic component that is later to be bonded to the first fiber-reinforced plastic component is likewise masked with the adhesive tape of the invention before the fiber mats are impregnated with the resin/curing agent mixture, [0068] removing the adhesive tapes from the two completed fiber-reinforced plastic components, [0069] contacting the two joining surfaces, [0070] introducing a resin/curing agent mixture between the two joining surfaces, [0071] curing the resin/curing agent mixture.

[0072] For the fiber mats, glass fibers or carbon fibers are used with preference.

[0073] The fiber mats are usually laid scrims made from untwisted and untangled or from twisted or tangled filaments. The filaments consist in general of high-tenacity fibers with low elongation at break.

[0074] For the purposes of this invention, a filament refers to a bundle of parallel, linear individual fibers, often also referred to in the literature as multifilament. This fiber bundle may optionally be inherently strengthened by twisting, the filaments then said to be spun or twisted filaments. Alternatively, the fiber bundle may be given intrinsic strengthening by entanglement using compressed air or water jets.

[0075] The concept of the method is that the bare fibers (or the location at which subsequent joining to another component is to take place) are masked by the adhesive tape prior to the resin infusion process. Subsequently, as described above, the resin infusion process is carried out, preferably with subsequent autoclaving. Following removal of the component from the autoclave preferably used, and from the vacuum bag likewise preferably used, the applied adhesive tape is removed. Because the adhesive tape was adhered directly to the fibers, the fibers are visible again at the free surface after the adhesive tape has been removed. This means that the adhesive tape has masked these fibers throughout the procedure.

[0076] From the standpoints of technology and economics, the advantages of the method are immense. The method represents one alternative for obtaining bare fibers on the surface of a fiber-reinforced plastic component, without having to erode the surface or the matrix material chemically or physically subsequent to the completion of the component. The bare fibers of the laid fiber scrim (mono- or multifilament), and the location at which joining is subsequently to take place to another component, are bonded with the adhesive tape prior to the resin infusion process. Subsequently, as described above, the resin infusion process is carried out with the preferred subsequent autoclaving. After the component has been removed from the autoclave and from the vacuum bag, the applied adhesive tape is removed without problems (without residue). Since the adhesive tape has been adhered directly to the fibers, they can be seen at the surface again after removal of the adhesive tape. In other words, the adhesive tape has masked these fibers throughout the entire operation. The bonded fibers do not receive a flow of resin, and so the masked fibers are not surrounded by cured resin.

[0077] The component thus manufactured can be utilized in a further adhesive bonding operation without further readying and/or processing.

[0078] The method is quick, reliable, clean and efficient.

[0079] In the case of the known methods, particularly of laser ablation, there is a need first of all for capital investments to be made in corresponding laser equipment and the associated control instrumentation. Furthermore, high-energy laser radiation in particular (and, in the worst case, invisible laser radiation as well) is very dangerous and, in the event of the smallest reflections into the eyes, can lead to cases of blinding. It is therefore necessary to invest money into workplace protection as well. This begins with a complete encapsulation of the system. This system must be sited in a separate, absolutely impervious room (with walls that have been made extra hard in certain circumstances). Moreover, it is necessary to remove any object that may lead to reflections. Furthermore, all of the products (particulate, liquid, gaseous) must be drawn off/removed efficiently and disposed of efficiently. An expense of this kind of protection, and the necessary capital investments, are themselves associated with very high costs. Occasionally, however, even laser treatment, or an abrading or etching operation, lasts a long time, and downtimes are always associated with further costs, which can be avoided by virtue of the method of the invention.

[0080] The demands made of a relevant adhesive tape are exacting, since the adhesive tape is required to withstand the conditions that are usual in the production of fiber-reinforced plastic components. The adhesive tape withstands 230 C. for 24 hours in the absence of air (i.e., in the vacuum bag) in an autoclave at 18 bar. And the tape is able to be removed without residue.

[0081] Both the carrier and the adhesive composition resist the high temperature, the duration, and the pressure.

[0082] Residues of whatever kind critically affect any subsequent effective bonding. Additionally, during the infusion process, the adhesive tape receives a flow of resin and curing agent, which cures directly at the boundary layer to the adhesive tape.

[0083] The adhesive tape does not influence the properties of the composite material (by migration of the constituents or the like).

[0084] On account of the surprising properties, the adhesive tape of the invention may be used in the method described, without the need for special measures which should otherwise be taken in order to comply with workplace protection, in order to protect workers against liquids and/or gases and also against any (respirable) particles that may occur.

[0085] The invention is elucidated in more detail below with an example, without hereby wishing to restrict the invention in any way.

EXAMPLE

[0086] The vacuum and autoclaving operation as used customarily in the production of composite components is simulated by means of a vacuum heating press. The press compartment can be evacuated (vacuum) and the jaws of the press are heatable (temperature). In addition, a sample for analysis can be pressurized by the jaws of the press (pressure). Here it is possible to investigate the residue-free redetachability of the adhesive tape on a variety of substrates.

[0087] Additionally, glass/carbon fibers can be bonded with the adhesive tape, and impregnated with resin/curing agent, and cured. The residue-free redetachability on original substrate is therefore investigated.

Example 1

[0088] 30 g/m.sup.2 film-forming acrylate composition [0089] 20 wt % isobornyl acrylate [0090] 20 wt % 2-hydroxyethyl acrylate [0091] 40 wt % butyl acrylate [0092] 20 wt % ethylhexyl acrylate [0093] 50 m HDPE film (100 wt % HDPE) [0094] Activation of the carrier by corona treatment

Counter Example 1

[0095] 30 g/m.sup.2 film-forming acrylate composition consisting of [0096] 20 wt % isobornyl acrylate [0097] 20 wt % 2-hydroxyethyl acrylate [0098] 40 wt % butyl acrylate [0099] 20 wt % ethylhexyl acrylate [0100] 50 m BOPP film [0101] Activation of the carrier by corona treatment

[0102] The adhesive tape based in accordance with example 1 can be removed without residue according to the given conditions of 230 C., 24 h and 18 bar pressure.

[0103] In the case of counter example 1, it is observed when detaching the adhesive tape that the carrier cannot be removed in one piece, since it has become brittle.

[0104] Elucidated in more detail below is an adhesive tape by reference to a FIGURE, without wishing thereby to cause any restriction of whatever kind.

[0105] FIG. 1 shows in a section in the transverse direction (cross section) the adhesive tape consisting of a film carrier 1, bearing on one side an applied layer of a self-adhesive coating 2.