ZA200106456B

ZA200106456B - Soluble adhesives.

Info

Publication number: ZA200106456B
Application number: ZA200106456A
Authority: ZA
Inventors: Christian Kirsten; Andreas Ferencz; Michael Hirthammer
Original assignee: Henkel Kgaa
Priority date: 1999-02-08
Filing date: 2001-08-06
Publication date: 2002-11-06
Also published as: BR0008038A; CA2362502A1; MXPA01007993A; JP2002536527A; AU3151100A; CZ20012877A3; WO2000047686A1; EP1161507A1; DE19904835A1; TR200101933T2

Description

ST 2001645¢ ®

Dissolvable Adhesives

This invention relates to adhesive compositions based on binders which contain disulfide or polysulfide bonds and which are suitable for the production of dissolvable adhesive bonds. The present invention also relates to splitting reagents for dissolving adhesive bonds and to a process for forming and dissolving adhesive bonds.

In many branches of industry, particularly in the metal-processing industry, for example the motor industry, in the manufacture of utility vehicles and the associated supplier industries or even in the production of machines and domestic appliances and in the building industry, identical or different, metallic and non-metallic substrates are being increasingly joined together by adhesives or sealants. This method of joining structural components is increasingly replacing conventional joining techniques, such as rivetting, screwing or welding, because bonding/sealing offers a number of technological advantages. In contrast to traditional joining techniques, such as welding, rivetting, screwing, the problem of dissolving adhesive bonds and separating the bonded components has not yet been satisfactorily solved.

EP-A-735121 describes an adhesive film section for a residue-free, damage-free and dissolvable adhesive bond consisting of a double-sided adhesive film with a grip tab projecting from the adhesive film at which the adhesive bond can be separated by pulling in the direction of its plane.

However, this method can only be applied where the adhesive layer of the adhesive film is a contact adhesive. Unfortunately, adhesive bonds produced by this method have very poor tensile and peel strengths, with the result that this method can only used to fix small articles, such as hooks and the like, in the home.

)

DE-A-4230116 describes an adhesive composition containing a mixture of an aliphatic polyol with an aromatic dianhydride. This adhesive composition enables the adhesive bond to be dissolved in water/alkali systems, more specifically soda solutions or alkali metal hydroxides.

According to the document in question, these water/alkali-soluble adhesives are suitable for the efficient production of magnet components and other small parts, the adhesive only being used for temporary bonding during processing of the materials. Very similar adhesives are also known as labelling adhesives which enable the labels to be removed from beverage bottles and similar containers in aqueous or aqueous/alkaline medium.

DE-A-4328108 describes an adhesive for floor coverings and a process for taking up the bonded floor coverings using microwave energy.

To this end, the adhesive is said to be electrically conductive and softenable by a microwave unit. Solventless contact adhesives based on (aqueous) polymer dispersions containing copper powder or aluminium powder are specifically mentioned. According to the teaching of this document, the adhesive bond securing the pieces of floor covering can be dissolved by application of a microwave unit to soften the adhesive layer so that, after the layer of adhesive has softened, the pieces of floor covering can be manually removed.

WO 94/12582 describes a contact adhesive based on a mixture of an aqueous polymer dispersion, an adhesive dissolved in an organic solvent, tackifiers and finishing agents. This contact adhesive has constant adhesive strength over a broad temperature range and enables the adhesive bonds to be mechanically separated. According to the document in question, the adhesive is suitable for bonding insulation and/or parts of decorative surfaces, for example insulating materials or plastic films.

EP-A-521825 describes a dissolvable adhesive bond where the parts joined to one another are bonded by a strip of adhesive applied

® between them. This strip of adhesive contains a flat thermoplastic separating element. When the adhesive bond is heated by electrical current or heat, this thermoplastic separating layer is softened so that the parts joined to one another can be mechanically separated. According to the document in question, these dissolvable adhesive bonds are suitable for direct glazing in car manufacture.

DE-A-19526351 describes a dissolving gel for lacquers, paints and adhesives based on organic solvents containing additions of wetting agents, thickeners and other typical auxiliaries. The use of the gel as a remover in the stripping of two-component lacquers is mentioned as a specific application. Although it is stated that the mixtures in question may also be used for two-component adhesives, there is no specific reference to the dissolution of the adhesive bonds. Similarly, WO 87/01724 describes a composition for removing hardened polysulfide sealants or coatings. In this case, an alkali metal or ammonium thiolate based on alkyl or phenyl thiolates is dissolved in a solvent or solvent mixture consisting of dimethyl formamide or dimethyl acetamide or a mixture thereof with aromatic solvents, such as toluene or xylene, and the resulting solution is applied to hardened polysulfide sealants or coating materials so that they may subsequently be removed from their substrates, such as aircraft tanks for example. Particulars of the dissolving of adhesive bonds are not disclosed.

WO 97/00283 describes a process for regenerating cured or partly cured polysulfide and/or polymercaptan compositions. In this process, cured polysulfide materials are depolymerized in a solution of a depolymerizing agent based on vulcanization accelerators in a non-volatile liquid so that they may be re-used as part of a hardener component of two- component polysulfide and/or polymercaptan adhesives/sealants or coating materials. There are no references in the document in question to the dissolving of adhesive bonds by this method.

®

In an article entitled “Reversible Crosslinking in Epoxy Resins” published in Journal of Applied Polymer Science, 39, 1429 to 1457 (1990), V.R. Sastri and G.C. Tesoro describe epoxy resins with various epoxy equivalents which are crosslinked with 4,4'-dithioaniline. The crosslinked resin is said to be ground into particles 600 um in size. The fine-particle powder obtained is then refluxed in a solution of diglyme, hydrochloric acid and tributyl phosphine until the ground resin has dissolved. Similar disclosures are made by the same authors in US-A- 4,882,399. There is no specific reference in this article to dissolvable adhesive bonds.

The dissolvable adhesive bonds described in the prior-art literature cited above all have very narrow fields of application. In particular, there are no adhesive compositions which combine the ready and rapid dissolvability or simple removability of the adhesive bond with high bond strength and stability to outside influences.

Hitherto unpublished PCT/EP98/04667 describes adhesives of which at least one structural component contains disulfide or polysulfide bonds and which can be redissolved after curing by applying solutions of splitting agents based on mercapto compounds. In this way, bonded parts can be chemically separated at the glue line. According to the teaching of this document, the splitting agent may also be added to the adhesive formulation in a form where it is inert at room temperature, in which case splitting can take place after activation of the reagent at elevated temperature. Actual embodiments of this inert form of the splitting agent are not mentioned. Although the use of solvent-containing splitting agents enables adhesive bonds to be redissolved, it is desirable to avoid the use of solvent-containing splitting agents because this procedure « is very time-consuming on account of the diffusion-based contact time of the splitting agents and

® « the handling of solvent-containing splitting agents should be avoided on environmental grounds.

Accordingly, the problem addressed by the present invention was to provide adhesive compositions which could be redissolved as required and where the use of externally applied solvent-containing splitting agents could be avoided.

The solution to this problem as provided by the invention is defined in the claims and lies essentially in the provision of adhesive compositions based on binders which contain at least one disulfide or polysulfide bond per molecule and which, in the adhesive composition, contain a splitting agent that is inert at room temperature or at the service temperature but, when suitably activated, is capable of breaking these disulfide or polysulfide bonds so that the joined parts can be separated.

In the context of the invention, the expression “inert at room temperature or at the service temperature” in relation to the splitting agent means that, although this splitting agent is dispersed in the adhesive matrix at room temperature or at the service temperature of the bonded unit, it is unable to break the disulfide or polysulfide bonds of the adhesive matrix at temperatures around that temperature. Instead, this breaking and hence dissolving of the adhesive bond is only supposed to occur after an initiating step, i.e. after activation of the splitting reagent.

The present invention also relates to a process for forming and dissolving adhesive bonds essentially comprising the following steps: ee Assembling and joining the parts with an adhesive composition of which the binder contains at least one structural component containing at least one disulfide or polysulfide bond per molecule. The adhesive composition may be a one-component system which the user can apply directly without having to mix components. However, the adhesive system may also consist of two or more components which are stored separately from one another and which are only mixed together immediately before application. e Curing the adhesive at room temperature, i.e. by reaction of the individual components with one another in the case of multicomponent systems or by reaction of the one-component system with atmospheric moisture and/or atmospheric oxygen. The adhesive may also be cured by heat, UV light or electron beams. The particular curing process used is governed by the crosslinking mechanism of the components. e The bond is dissolved by application of a splitting reagent dispersed in the adhesive mixture. e The bond dissolving process may optionally be further accelerated by heating the bonded parts or the adhesive bond.

Separation of the bonded parts may optionally be further accelerated by subjecting the adhesive bond to a mechanical load.

Accordingly, a key constituent of the structural components of the binder according to the invention are compounds which contain at least one disulfide or polysulfide bond corresponding to the following general formula:

X-R'-8,-R%-Y (1) in which R' and R? are branched alkyl and/or aryl groups, i.e. in the most simple case a Cy. alkylene group, or a difunctional aromatic radical such as, for example, 1,2-, 1,3- or 1,4-phenylene, diphenylene, naphthylene or similar aromatic radicals. X and Y independently of one another may represent any functional group capable of reacting, preferably primary or

[J secondary amino groups, hydroxyl groups, carboxyl groups. X and/or Y may also be mercapto groups, epoxy groups, isocyanate groups, alkoxysilyl groups or even olefinic double bonds. In the latter case, R' and/or R? may be replaced by a covalent bond. x is an integer of 2 to 8 and, in a particularly preferred embodiment, has a value of 2.

Examples of structural components corresponding to formula | which contain olefinic double bonds may be prepared as follows: in a first step, dithiodialcohols or dithiodiamines are reacted with diisocyanates to form isocyanate-terminated disulfide compounds, i.e. the diisocyanate component is used in more than the stoichiometric quantity in relation to the dithiodialcohol or dithiodiamine component. In a second step, these

NCO-terminated disulfide compounds are reacted with hydroxyalkyl acrylates or hydroxyalkyl methacrylates, so that (meth)acrylate-terminated disulfide bonds are formed. Examples of suitable hydroxyalkyl (meth)acrylates are the corresponding ethyl, propyl or butyl compounds.

The resulting (meth)acrylate-terminated disulfide compounds may be combined as usual with corresponding copolymerizable compounds and may be cured by a radical or ionic mechanism. Examples of these known copolymerizable compounds can be found in DE-C 19545123, column 5, lines 24 to 47. The comonomers mentioned there are an integral part of the present invention.

Similarly, epoxyfunctionalized disulfides or polysulfides can be produced from the NCO-terminated disulfide or polysulfide compounds by reaction with hydroxyfunctional epoxy compounds. Examples of such hydroxyfunctional epoxides are glycidol and the various glycidyl ethers of bisphenol A which generally carry free hydroxy groups.

Similarly, epoxyfunctionalized disulfide or polysulfide compounds can be obtained by reacting COOH-terminated disulfide or polysulfide compounds with difunctional or polyfunctional epoxy compounds.

[J

The corresponding alkoxysilane-terminated products can be produced from the NCO-terminated disulfide or polysulfide compounds by reaction with aminofunctional alkoxysilanes.

Particularly preferred structural components corresponding to formula | are cystamine, dithiodiethanol and dithiodipropionic acid.

Another structural component of the adhesive composition according to the invention may consist of one or more compounds corresponding to the following general formula:

X-R%-Y (1) where X and Y may be as defined above and R? is an at least difunctional organic radical. The components corresponding to formula Il are normally so-called prepolymer compounds with a molecular weight in the range from 300 to 20,000 and preferably in the range from 700 to 10,000.

Particularly preferred components corresponding to formula Il are epoxy resins, isocyanate-containing polyurethane prepolymers, novolak resins, phenolic resins or unsaturated polyesters. However, the components corresponding to formula lI may also be replaced by the copolymerizable olefinically unsaturated compounds mentioned above.

Suitable epoxy resins are various polyglycidyl ethers of polyols, such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, pentane-1,5-diol, hexane-1,2,6-triol, glycerol, 2,2-bis-(4- hydroxycyclohexyl)-propane and polyalkylene glycols, such as polypropylene glycol. Other suitable epoxy resins are the polyglycidyl esters of aliphatic or aromatic polycarboxylic acids, such as oxalic acid, succinic acid, glutaric acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid and dimer fatty acid. Other suitable epoxy compounds are the polyglycidyl ethers of polyphenols, such as bisphenol A, 1,1-bis-(4- hydroxyphenyl)-ethane, 1,1-bis-(4-hydroxyphenyl)-isobutane, 1,5-

® dihydroxynaphthalene and novolak resins. Preferred epoxy compounds include relatively high molecular weight resins, such as the chain-extended diglycidyl ethers of bisphenol A, diglycidyl ethers of dimer fatty acid- extended bisphenol A and bisphenol A glycidyl ether-terminated polyether polyurethanes. The adducts of epoxy resins with carboxy-, amino- and/or hydroxyfunctional nitrile rubbers (Hycar types) may also be used as the epoxy component.

The isocyanate-containing polyurethane prepolymers are made up of aromatic, cycloaliphatic or aliphatic polyisocyanates and diols and/or polyols. The following are examples of suitable aromatic polyisocyanates: any isomers of toluene diisocyanate (TDI) either in the form of pure isomers or in the form of mixtures of several isomers, naphthalene-1,5-diisocyanate, diphenylmethane-4,4'-diisocyanate (MDI), diphenylmethane-2,4'-diisocyanate and mixtures of 4,4-diphenylmethane diisocyanate with the 2,4-isomer or mixtures thereof with oligomers of higher functionality (so-called crude MDI). Examples of suitable cycloaliphatic polyisocyanates are the hydrogenation products of the above-mentioned aromatic diisocyanates, for example 44- dicyclohexylmethane diisocyanate (H:2MDI), ~~ 1-isocyanatomethyl-3- isocyanato-1,5,5-trimethyl cyclohexane (isophorone diisocyanate, PDI), cyclohexane-1,4-diisocyanate, hydrogenated xylylene diisocyanate (HeXDI), m- or p-tetramethyl xylene diisocyanate (m-TMXDI, p-TMXDI) and dimer fatty acid diisocyanate. Examples of aliphatic polyisocyanates are hexane-1,6-diisocyanate (HDI), 1,6-diisocyanato-2,2,4-trimethyl hexane, 1,6-diisocyanato-2,4,4-trimethyl hexane, butane-1,4-diisocyanate and 1,12- dodecane diisocyanate (C12Dl).

Preferred diols and/or polyols are liquid polyhydroxy compounds containing two or three hydroxy! groups per molecule, for example difunc- tional and/or trifunctional polypropylene glycols with molecular weights in the range from 200 to 6,000 and preferably in the range from 400 to 3,000.

®

Statistical and/or block copolymers of ethylene oxide and propylene oxide may also be used. Another group of preferred polyethers are the polytetra- methylene glycols which are obtained, for example, by the acid polymer- ization of tetrahydrofuran, the molecular weights of the polytetramethylene glycols being in the range from 200 to 6,000 and preferably in the range from 400 to 4,000.

Other suitable polyols are liquid polyesters which may be obtained by condensation of dicarboxylic or tricarboxylic acids, for example adipic acid, sebacic acid, glutaric acid, azelaic acid, hexahydrophthalic acid or phthalic acid, with low molecular weight diols or triols, for example ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, butane-1,4-diol, hexane-1 6-diol, decane-1,10-diol, glycerol or trimethylol propane.

Another group of polyols which may be used in accordance with the invention are polyesters based on e-caprolactone which are also known as “polycaprolactones”.

However, polyester polyols of oleochemical origin may also be used.

Oleochemical polyester polyols may be obtained, for example, by complete ring opening of epoxidized triglycerides of a fatty acid mixture containing at least partly olefinically unsaturated fatty acids with one or more alcohols containing 1 to 12 carbon atoms and subsequent partial transesterification of the triglyceride derivatives to form alkyl ester polyols containing 1 to 12 carbon atoms in the alkyl group (see, for example, DE-A-3626223). Other suitable polyols are polycarbonate polyols and dimer diols (Henkel KGaA) and castor oil and its derivatives. Hydroxyfunctional polybutadienes of the type commercially available, for example, as “poly-bd” may also be used as polyols for the compositions according to the invention.

In addition, the low molecular weight hydroxyfunctional (meth)acrylate polymers disclosed in EP-A-205846 may be used as polyols.

Suitable novolak resins or phenolic resins are the universally known condensation products of phenol and/or resorcinol with formaldehyde. The co-condensates with terpenes, i.e. the terpene/phenol resins, may also be used.

Basically, the adhesive compositions according to the invention contain at least one structural component of general formula | containing disulfide or polysulfide bonds and at least one structural component corresponding to general formula Il. The two components must of course be compatible with one another. The following are examples of useful combinations:

Functionality component | | Functionality component li

Hydroxy, epoxy, amino Isocyanate, epoxy

Phenol, novolak, amine

Acrylate, methacrylate Unsaturated polyester, acrylate, methacrylate

Epoxy, isocyanate, mercapto

Alkoxysilyl Alkoxysilyl

However, a suitable combination of the structural components corresponding to formulae | and Il may also lie in binders based on liquid rubbers polymerized with vulcanization systems containing sulfur and/or sulfur compounds. Actual examples of the liquid rubbers used for this purpose are given on page 5 of WO 96/36660. Sulfur-containing vulcanizing agents suitable for this purpose are disclosed on page 6, last paragraph to page 7, first paragraph of WO 96/36660.

The adhesive compositions according to the invention may additionally contain plasticizers. Basically, any conventional plasticizers

® . may be used, including for example the Ce.14 dialkyl esters of phthalic acid, alkylbenzyl esters of phthalic acid, benzoates of difunctional or trifunctional polyols such as, for example, dipropylene glycol dibenzoate, alkyl sulfonic acid esters of phenol and cresol, aryl phosphates, alkyl phosphates, Ce-14 diesters of aliphatic C410 dicarboxylic acids and/or polymer plasticizers based on diols and dicarboxylic acids and mixtures thereof.

The adhesive compositions may also contain fillers in quantities of 5 to 60% by weight. Examples of suitable fillers are limestone powder, natural ground chalks (calcium carbonates or calcium magnesium carbonates), precipitated chalks, heavy spar, talcum, mica, clays, carbon black and pigments, for example titanium dioxide or iron oxides.

The adhesives according to the invention may additionally contain other auxiliaries and additives, for example antiagers and stabilizers, flow aids, such as pyrogenic silicas, bentones, castor oil derivatives, and catalysts, accelerators and optionally tackifying resins.

According to the invention, the adhesive compositions contain splitting agents which are inert in normal use to dissolve the adhesive bond. The function of the splitting agents is to break the disulfide or polysulfide bonds incorporated in the polymer system. The splitting agents are either mercapto compounds, optionally with additions of inorganic or organic basic compounds or other accelerators, or reducing agents for reductive cleavage of the S-S bonds of the adhesive. The splitting agents may optionally contain other auxiliaries, more particularly swelling agents which facilitate and accelerate the action of the splitting agents by softening the crosslinked polymer matrix of the adhesive system.

Actual examples of the mercapto compounds suitable for use as splitting reagent in accordance with the invention are 2-mercaptobenzoic acid (thiosalicylic acid), 2-mercaptobenzothiazole (2-benzothiazole thiol), 2- mercaptobenzoxazole (2-benzoxazole thiol), D,L-mercaptosuccinic acid (thiomalic acid), mercaptopyruvic acid sodium salt, 2-mercapto-4(3H)-

® quinazoline, 2-mercaptoquinoline (2-quinoline thiol), 2-mercapto-1-methyl imidazole, 5-mercapto-1-methyl-1H-tetrazole, 2-mercapto-5-methyl-1,3,4- thiadiazole, 3-mercapto-4-methyl-4H-1,2,4-triazole, 4-mercaptophenol, 5- mercapto-3-phenyl-1,2,4-oxadiazole, 5-mercapto-1-phenyl-1H-tetrazole, N- (2-mercaptopropionyl)-glycine, 2-mercapto-2-thiazoline, triphenyl methyl mercaptan, 4-toluene thiol, 2-mercaptopyrimidine, 3-mercapto-1,2,4- triazole, 2-mercapto-4-methyl pyrimidine hydrochloride, 2- mercaptobenzimidazole. Actual examples of the reducing agents are trialkyl or triaryl phosphines solid at room temperature.

There are various ways of making the splitting reagents inert at room temperature so that they allow a bond to be established which is stable at all service temperatures and which is only activated as required. One possibility is to select crystalline splitting reagents of the above-mentioned type which have a sufficiently high melting point so that they are dispersed in largely inert form in the adhesive matrix at normal service temperatures.

Another possibility is to encapsulate the splitting reagents which can be done in two ways. On the one hand, the splitting reagent may be encapsulated in a separate inert capsule material; on the other hand, the particles of splitting reagent may be encapsulated by an in situ surface reaction. Another possibility is to use chemically blocked splitting reagents and to use topologically or sterically inactivated splitting reagents or to use kinetically inhibited splitting reagents.

The inert splitting reagents may be activated in the adhesive matrix by melting, a change of conformation, reaction acceleration (especially in the case of catalysts), initiation of chemical reactions by removal of protective groups or by in situ production of active centers or by capsule bursting (bursting of the inert outer layers of the microencapsulation or by bursting of the capsule core) or by activation of splitting catalysts inert at the service temperature.

) :

Various processes may be used to activate the inert splitting reagents or inert catalysts. On the one hand, the bond may be heated; on the other hand, the splitting reagents may be activated by exposure to thermal radiation (IR radiation), by exposure to particle radiation, by passage of an electrical current (in the case of electrically conductive adhesive formations) and by exposure to electrical and especially electromagnetic fields. Where electromagnetic or magnetic fields are selected, various so-called IMS frequencies are available (industrial, medical, science applications). Electromagnetic radiation is understood to be, on the one hand, the permitted IMS frequencies in the radiofrequency range up to about 100 MHz and, on the other hand, microwaves which are normally in the 0.9 to 10 GHz range. Another method of activating the inert splitting reagents or inert catalysts is to use ultrasonic energy or high- energy pressure or shock waves.

Another embodiment of the dissolvable adhesive bond according to the invention is characterized in that normal prior-art adhesives are used for assembly and in that a dissolvable primer is used. It is well known that, in many adhesive bonds, adhesion-promoting primers still have to be applied to at least one of the substrates to be joined. According to the invention, the primer may contain a structural component corresponding to general formula |. In this way, the primer layer can be dissolved by the splitting agents according to the invention on the same action principle as described above, so that the joined parts are separated in accordance with the invention without the adhesive as such being subjected to the splitting reaction.

The following Examples are intended to illustrate the invention without limiting its scope in any way. In the Examples, all quantities are percentages by weight, based on the composition as a whole, or parts by weight.

Examples

DIN 1541/ST 1206 steel plates measuring 1.5 mm x 25 mm x 100 mm (sand-blasted or rubbed with emery cloth, degreased with dichloromethane) were used for the bonding and debonding tests. The adhesive used was a two-component insulating glass adhesive based on a polysulfide polymer (Terostat 998 R of Henkel Teroson). Before application, varying amounts of a finely powdered splitting reagent were dispersed in the adhesive.

The adhesive was applied to the steel plates in such a way that a 20 x 25 x 0.5 mm glue line was formed between the overlapping steel plates.

After curing to the manufacturer's instructions and storage for 7 days at room temperature, the debonding properties were tested. To this end, the bonded plates were clamped at one end in a stand and placed in a oven and a 1.3 kg weight was attached. The temperature and time at which the bonded test specimen separated were determined. The test specimen was heated from room temperature to 50°C at a rate of 2.5°C/minute; between 50°C and 200°C, the heating rate was 0.5°C/minute.

Comparison Example 1 and Examples 1 to 4:

Splitting reagent used: 2-mercapto-2-thiazoline. The test results are set out in Table 1:

Table 1. ompereont [0 | emo |e

) :

In the Comparison Example with no splitting reagent in the adhesive, it can be seen that the test specimens are still in tact after more than 5 days, even at temperatures above 200°C, i.e. the adhesive bond had not been broken. As can clearly be seen from Examples 1 to 4, both the splitting temperature and the time required to break the bond can be controlled through the quantity of splitting agent dispersed in the adhesive.

Both decrease distinctly with increasing quantity of splitting agent.

The following tests were carried out to show that a certain minimum temperature must be maintained as the “trigger temperature” to break the bond:

Adhesive bonds containing 1% of splitting agent were stored at a constant 80°C under a load of 1.3 kg. Even after 10 days under that load, the bond showed no sign of breaking. Even bonds containing 2.5% of splitting agent were in tact after 5 and a half days.

To determine the trigger temperature, the test specimens which had already been loaded for 10 days at 80°C without breaking were stored under load at 100°C and 120°C. Again, the bond did not break. The same test specimens were then stored under load at 145°C. The bond broke after 2.5 hours. In other words, a bonded test specimen where the adhesive contained 1% by weight of the splitting reagent had to be heated for 2.5 hours at 145°C under a load of 1.3 kg in order to split the adhesive matrix so that the bond broke.

In the same way as in Examples 1 to 4, 2-mercaptobenzoxazole was dispersed as splitting reagent in the adhesive formulation.

The test results are set out in Table 2:

Table 2.

Temperature °C Filling level % ew Te

In this case, too, it can be seen that the trigger temperature for breaking the bond depends on the amount of splitting agent in the adhesive formulation. : In the following Examples, varying amounts of an inert crystalline substance (sodium chloride) were dispersed in the adhesive Terostat 998

R and the bonds were observed to determine whether their heat resistance was affected by the addition of this filler. The results are set out in Table 3:

Table 3. ] Temperature °C Filling level % seme | m0 0

It can be seen that the adhesive is stable to about 250°C without the dispersion of foreign substances. The dispersion in the adhesive of increasing amounts of an inert crystalline substance reduces this heat resistance slightly to about 170 to 175°C. This clearly shows that the incorporation of the solid splitting agents in accordance with the invention in Examples 1 to 7 reduces the temperature at which the adhesive bond breaks by initiating chemical separation of the bond.

Both the separation temperature and the time required can be adjusted within wide limits to meet particular requirements.

Claims

® CLAIMS

1. An adhesive composition based on at least one binder containing disulfide or polysulfide bonds for forming adhesive bonds, characterized in that the adhesive bond can be broken with a splitting reagent, the splitting reagent being dispersed in the adhesive composition in crystalline, encapsulated, chemically blocked, topologically or sterically inactivated or kinetically inhibited, finely dispersed form.

2. An adhesive composition as claimed in claim 1, characterized in that the structural components of the binder are selected from the group of epoxy resins combined with disulfide/polysulfide bond-containing di- or polymercaptans, di- or polythioalkanols, di- or polythiodicarboxylic acids or di- or polythiodi- or polyamines or mixtures thereof, polyurethane systems made up of monomers or prepolymers, di- or polyisocyanates with di- or polythiodi- or polyamines, di- or polythio-di- and/or polyols, liquid polyenes (liquid rubber) with vulcanization systems containing sulfur and/or sulfur compounds.

3. A splitting agent for dissolving adhesive bonds based on the adhesives claimed in either of the preceding claims, characterized in that the splitting reagent is selected from one of the following compounds: 2- mercaptobenzoic acid (thiosalicylic acid), 2-mercaptobenzothiazole (2- benzothiazole thiol), 2-mercaptobenzoxazole (2-benzoxazole thiol), D,L- mercaptosuccinic acid (thiomalic acid), mercaptopyruvic acid sodium salt, 2-mercapto-4(3H)-quinazoline, 2-mercaptoquinoline (2-quinoline thiol), 2- mercapto-1-methyl imidazole, 5-mercapto-1-methyl-1H-tetrazole, 2- mercapto-5-methyl-1,3 4-thiadiazole, 3-mercapto-4-methyl-4H-1,2,4- triazole, 4-mercaptophenol, 5-mercapto-3-phenyl-1,2,4-oxadiazole, 5- mercapto-1-phenyl-1H-tetrazole, N-(2-mercaptopropionyl)-glycine, ~~ 2- mercapto-2-thiazoline, tripheny methyl mercaptan, 4-toluene thiol, 2- mercaptopyrimidine, 3-mercapto-1,2,4-triazole, 2-mercapto-4-methyl pyrimidine hydrochloride, 2-mercaptobenzimidazole.

20 PCT/EP00/00719

4. A process for forming and dissolving adhesive bonds essentially comprising the following steps: a) assembling and joining the parts with the adhesive composition claimed in claims 1 to 3, the adhesive composition optionally being mixed from two or more components before application, b) curing the adhesive at room temperature or by heating, c) dissolving the adhesive bond by activating the splitting reagent dispersed in the adhesive matrix, the splitting reagent being inert at the service temperature of the bonded parts, d) optionally followed by mechanical stressing.

5. A process as claimed in claim 4, characterized in that the inert splitting reagent is activated by melting, a change of conformation, removal of blocking protective groups, production of active centers, bursting of encapsulations or by catalytic action.

6. A process as claimed in claim 4 or 5, characterized in that activation is achieved by heating (thermal conduction), thermal radiation, particle radiation, passage of an electrical current, electrical or electromagnetic fields, ultrasound, pressure or shock waves.

7. A composition as claimed in claim 1, substantially as herein descried and illustrated.

8. An agent as claimed in claim 3, substantially as herein described and illustrated.

9. A process as claimed in claim 4, substantially as herein described and illustrated.

10. A new composition, a new splitting agent, or a new process for forming and dissolving adhesive bonds, substantially as herein described. AMENDED SHEET