Activator Compositions for Cyanoacrylate Adhesives

(19) United States US 20030191248A1 (12) Patent Application Publication (10) Pub. No.: US 2003/0191248 A1 Ryan et al. (43) Pub. Date: Oct. 9, 2003 (54) ACTIVATOR COMPOSITIONS FOR CYANOACRYLATE ADHESIVES (76) Inventors: Bernard Ryan, Dublin (IE); Hanns Roland Misiak, Dublin (IE); James A Houlihan, Dublin (IE); Gerard R MacCann, Dublin (IE) Correspondence Address: Steven C Bauman Henkel Loctite Corporation 1001 Trout Brook Crossing Rockyhill, CT 06067 (US) (21) Appl. No.: 10/276,287 (22) PCT Filed: May 11,2001 (86) PCT No.: PCT/IE01/00063 (30) Foreign Application Priority Data May 12, 2000 (IE) ......................................... .. 2000/0367 Apr. 27, 2001 (IE) ......................................... .. 2001/0422 Publication Classification (51) Int. Cl.7 ................................................... .. C08F 22/22 (52) U.S.Cl. ........................................................ .. 525/328.2 (57) ABSTRACT An activator composition for the accelerated hardening of cyanoacrylate adhesives, wherein the activator comprises a member selected from the group consisting of: aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) such as pyridines, quinolines and pyrimidines and substituted on the ring(s) with at least one electron- Withdrawing group which decreases the base strength of the substituted compound compared to the corresponding unsubstituted compound, mixtures of any of the foregoing with each other, and/or with N,N-dimethyl-p-toluidine, and mixtures of any of the foregoing and/or N,N-dimethyl-p- toluidine with an organic compound containing the struc- tural element, —N=C—S—S—, such as dibenzothiazyl disulfide. 6,6'dithiodinicotinic acid, 2,2'dipyridyl disulfide, and bis(4-t-butyl-1-isopropyl-2-imidazolyl) disulfide. An activator composition may comprise a solution of one or more activators in a solvent mixture which comprises a volatile hydrocarbon and a cyclic ketone. US 2003/0191248 A1 ACTIVATOR COMPOSITIONS FOR CYANOACRYLATE ADHESIVES FIELD OF THE INVENTION [0001] This invention relates to activator compositions, particularly well suited for accelerating the hardening of cyanoacrylate adhesives. The invention also relates to novel mixtures of chemical compounds and to the use of the activator compositions and the novel mixtures for the accel- erated hardening of cyanoacrylate adhesives. The invention further relates to a process for the accelerated bonding of substrates using cyanoacrylate adhesives. BRIEF DESCRIPTION OF RELATED TECHNOLOGY [0002] Adhesive compositions based upon cyanoacrylate esters are well known and have found extensive use, because of their rapid cure speed, excellent long-term bond strength, and applicability to a wide variety of substrates. They generally harden after only a few seconds, after which the joined parts exhibit at least a certain degree of initial strength. [0003] It is known that certain cyanoacrylate (CA) adhe- sives typically harden by an anionic polymerisation reaction. If the adhesive is conventionally applied in a relatively thick layer in the joint gap or relatively large amounts of adhesive are applied so that relatively large drops of adhesive pro- trude from between the parts to be joined, rapid hardening throughout the adhesive may rarely be achieved, i.e. cure- through-gap or cure-through-volume (CTV) performance may be unsatisfactory. [0004] With certain substrates, particularly substrates hav- ing acidic surfaces, such as wood or paper, the polymerisa- tion reaction may be retarded, oftentimes to an unmanage- able extent. Moreover, unless the adhesive is gelled or rendered thixotropic by appropriate additives to confer such properties, the wood or paper substrates, due to their poros- ity, tend to draw the adhesive out of the joint gap by capillary action before hardening has taken place in the gap. [0005] Heretofore efforts have been made to accelerate the polymerisation of such CA adhesives by means of certain additives. Addition of accelerators directly to the adhesive formulation is possible to only a very limited extent, how- ever, since substances having a basic or nucleophilic action, which would normally bring about a pronounced accelera- tion of the polymerisation of the cyanoacrylate adhesive, are generally used at the expense of the storage stability of such formulations. [0006] Addition of such accelerators shortly before appli- cation of the adhesive results in virtually a two-component system. However, such method has the disadvantage that the working life is limited after the activator has been mixed in. In addition, with the small amounts of activator that are required, the necessary accuracy of metering and homoge- neity of mixing are difficult to achieve. Moreover, use of such a two-component system is often seen as cumbersome to the end user, and sometimes only modestly improves the intended result. [0007] Activators are also used in the form of a dilute solution which is either applied beforehand onto a substrate or part which is to be bonded, and/or is applied onto the Oct. 9, 2003 adhesive where it is still liquid after the substrates have been joined. The solvents used for such dilute solutions of acti- vators are generally low-boiling organic solvents, so that they may be readily evaporated, leaving the activator on the substrate/part or the adhesive. [0008] Japanese Patent Application No. JP-A-62 022 877 proposes the use of solutions of lower fatty amines, aromatic amines, dimethylamine and the like. Japanese Patent Appli- cation No. J P-A-03 207 778 proposes the use of solutions of aliphatic, alicyclic and, especially, tertiary aromatic amines; particular examples which are mentioned are N,N-dimeth- ylbenzylamine, N-methylmorpholine and N,N-diethyltolui- dine. [0009] Japanese Patent Application No. 59-66471 dis- closes a hardening accelerator for use with cyanoacrylate adhesives comprising an amine compound, with a boiling point of between 50° C. and 250° C., together with a deodorizer and a solvent. Examples of suitable amines are triethyl amine, diethyl amine, butyl amine, isopropyl amine, tributyl amine, N,N-dimethyl aniline, N,N-diethyl aniline, N,N-dimethyl-p-toluidine, N,N-dimethyl-m-toluidine N,N- dimethyl-o-toluidine, dimethyl benzyl amine, pyridine, picoline, vinyl pyridine, ethanolamine, propanolamine and ethylene diamine. [0010] U.S. Pat. No. 3,260,637 of von Bramer discloses the use of a range of organic amines (excluding primary amines) as accelerators for cyanoacrylate adhesives, particu- larly for use on metallic and non-metallic substrates. [0011] N,N-dimethyl-p-toluidine has been widely used as an accelerator for the accelerated hardening of cyanoacrylate adhesives. A crucial disadvantage of the use of that sub- stance is the short duration of the surface activation, which does not permit long waiting times between application of the accelerator solution beforehand to the substrates to be bonded and the subsequent bonding process. In addition, the use of N,N-dimethyl-p-toluidine in some countries often- times involves rigorous regulatory labelling requirements. [0012] Basicity of an accelerator substance is not a suffi- cient criterion for identifying solutions which are acceptable in practice in terms of application technology. Many sub- stances, such as alkylamines, 1,2-di-(4-pyridyl-ethane), 4,4‘- dipyridyl disulfide, 3-(3-hydroxypropyl)pyridine, 1,2-bis- (diphenylphosphino)-ethane, pyridazine, methylpyridazine or 4,4‘-dipyridyl, are so basic or nucleophilic that sponta- neous superficial hardening takes place at the adhesive interface (shock hardening) before the activator is able to initiate polymerisation throughout the liquid adhesive layer by convection and diffusion. The result is that an often cloudy polymerisation occurs at the surface only. With other compounds, such as oxazoles, the basicity is evidently too low, and the hardening is often too slow for practical purposes. [0013] German Patent DE-A-22 61 261 proposes accel- erator substances containing the structural element —N=C—S—, including 2,4-dimethylthiazole. However, that compound has a very high volatility, so that activator solutions based thereon are unsuitable for application beforehand since the active ingredient also evaporates off with the solvent. [0014] It is desired to find new activator compositions for use with cyanoacrylate adhesives, which activator compo- US 2003/0191248 A1 sitions have a pronounced accelerating action and low volatility, so that application beforehand is also possible. It is also desired to find activators which are subject to regulatory labelling requirements less rigorous than is N,N- dimethyl-p-toluidine currently. [0015] European Patent Specification No. 0 271 675 A2 of Three Bond Co. Ltd. describes a primer for CA resin compositions for use in bonding non-polar or highly crys- tallized resins such as polyolefins, polyethyleneterephtha- lates, nylons, fluorine-containing resins, soft PVC films etc. The primer comprises (A) a compound selected from the group consisting of benzene ring compounds having alde- hyde group and nitrogen or oxygen atom-containing hetero- cyclic compounds having aldehyde group, and (B) an organic amine compound. Nitrogen-atom containing hetero- cyclic compounds having aldehyde group in the component (A) include 2-pyridine carboxylaldehyde, 2,6-pyridine car- boxylaldehyde and pyrrole 2- carboxylaldehyde. The speci- fication states that in bonding non-polar or highly crystal- lized resins using a CA adhesive the primer instantaneously exhibits a high bonding strength at ambient temperature by a simple procedure which comprises applying such primer onto a surface of one of the resins, applying the CA adhesive onto a surface of the other resin and bringing both the surfaces into contact with each other. The Three Bond specification is directed to priming the surfaces of substrates which are difficult to bond. As described, the surfaces of the two substrates are brought into contact with one another (i.e. with “zero gap”) and a high bonding strength is achieved instantaneously. The Three Bond specification is not con- cerned with an activator which is able to initiate polymeri- sation throughout a layer of adhesive (e.g. in a joint gap) by convection and diffusion without spontaneous superficial hardening taking place at the adhesive/substrate interface. In other words, the Three Bond specification is not directed to good CTV performance. A good CTV initiator should be a sufficiently slow initiator to allow effective initial mixing of the activator through the adhesive prior to polymerisation. [0016] British Patent Specification No. 1 230 560 of International Chemical Company Limited (ICC) describes CA adhesive compositions containing certain substituted heterocyclic compounds as accelerators. The compositions may be presented in a two-part form, the first part compris- ing the CA adhesive and the second part comprising at least one of the substituted heterocyclic compounds, preferably in solution in an organic solvent. In the compositions in which the heterocyclic compound is an iminoethylene-substituted triazine or pyrimido-pyrimidine, the heterocyclic compound is invariably present in one part of a two-part composition because iminoethylene-substituted triazines and pyrimido- yprimidines accelerate the polymerisation so rapidly they must be kept apart from the CA composition before use. In example 1 of the ICC specification triallylcyanurate as accelerator is mixed with the CA monomer and there is no suggestion that it or other accelerators (apart from the iminoethylene-substituted triazines and pyrimido-pyrim- idines) need to be presented in a two-part form. In example 2, triethylanemelamine as a 1% solution in acetone is used as a primer on two steel surfaces, after which the monomer- containing composition is applied onto the primed surfaces and the surfaces are placed in mutual contact (i.e. with “zero gap”). An effective adhesive bond is obtained. However the ICC Specification, like the Three Bond Specification dis- Oct. 9, 2003 cussed above, is not concerned with an activator which is able to initiate polymerisation throughout a layer of adhesive (e.g. in a joint gap). [0017] Japanese Patent Abstract Publication No. 62018485 of Alpha Giken KK also describes a primer for a CA adhesive and is not concerned with an activator for good CTV performance. [0018] Notwithstanding the state-of-the-art, it would be desirable to provide further activator 30 substances and combinations of activator substances with different proper- ties from the activator substances used up to now. It is particularly desirable to use a mixture of activators in order to obtain a combination of properties, some at least of which would not be expected. [0019] Activator solutions are often applied by spraying. However there is a demand for activator solutions which can be applied in excess volumes (e.g. as drops) onto an adhe- sive already present on a substrate (e.g. in the form of a bead or fillet). [0020] Aliphatic hydrocarbons such as heptane are often used as the solvent for CA activators. However if the activator solution is applied onto a bead or fillet of CA adhesive which is already on a substrate (“post-applica- tion”), particularly if the solution is applied in excess volumes, a white “halo” may be formed on the substrate around the adhesive bead in the course of curing. Although the present invention is not limited by any theory, it is thought that the solvent heptane, which is substantially insoluble in the CA adhesive composition, dissolves small amounts of CA monomer and that some of the heptane phase (i.e. the activator solution with traces of CA monomer dissolved therein) runs off the adhesive bead onto the substrate. The traces of CA monomer then polymerise, and after evaporation of the solvent a thin layer of whitish amorphous material is left on the substrate around the adhesive bead, creating a “halo”. This is visually unattrac- tive and is undesirable, particularly when the substrate is of a dark colour, such as black or dark shades of colours such as blue, red, brown or green, as well as transparent substrates like glass or polycarbonate, on or through which a whitish layer would be clearly visible. [0021] While it may be possible to use a solvent which is miscible with the CA adhesive composition, so that the solvent would mix with the adhesive, a miscible solvent would likely make the adhesive composition undesirably soft and bulky. In addition the solvent in the adhesive composition would likely take a longer time to evaporate than n-heptane which remains as a separate phase. Miscible solvents such as acetone, ethyl acetate or acetylacetone also have a stronger odour than n-heptane, and they would be unappealing to the end user. If one or both substrates is of plastics materials, a solvent which is miscible in the adhe- sive composition (such as acetone, toluene etc.) would also likely attack the plastics material. [0022] One aspect of the invention according to the present application reduces the problem of the “halo” effect and provides activator solutions with different properties from the activator solutions used to date. US 2003/0191248 A1 SUMMARY OF THE INVENTION [0023] According to one aspect, the present invention provides a composition comprising a mixture of: [0024] (A) a member selected from the group con- sisting of: [0025] aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) and substi- tuted on the ring(s) with at least one electron- withdrawing group which decreases the base strength of the substituted compound compared to the corresponding unsubstituted compound, [0026] N,N-dimethyl-p-toluidine, [0027] [0028] (B) an organic compound containing the structural element, —N=C—S—S—. and mixtures of any of the foregoing, with [0029] According to another aspect, the present invention provides a composition comprising a mixture of: [0030] (A) a member selected from the group con- sisting of: [0031] aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) and substi- tuted on the ring(s) with at least one substituent selected from the group consisting of halo, CN, CF3, COOR, COR4, OR, SR, CONR1R2, NO2, SOR, SOZR3, SO3R3, PO(OR3)2 and optionally substituted C6-C20 aryl, wherein R, R1, R2 and R4 (which may be the same or different) are H, optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, and R3 is optionally sub- stituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, N,N-dimethyl-p-toluidine, [0032] [0033] (B) an organic compound containing the structural element, —N=C—S—S—. and mixtures of any of the foregoing, with [0034] According to a special feature, the present inven- tion provides an activator composition for the accelerated hardening of cyanoacrylate adhesives, wherein the activator comprises a mixture of an aromatic heterocyclic compound as described above and an organic compound having the structural element, —N=C—S—S—. According to a fur- ther feature, the present invention provides an activator composition for the accelerated hardening of cyanoacrylate adhesives, wherein the activator comprises a mixture of a 3,5-dihalopyridine and an organic compound having the structural element, —N=C—S—S—, more particularly the structural element, —N=C—S—S—C=N—, more espe- cially —N=C—S—S—C=N— wherein the N=C and C=N double bonds are parts of aromatic heterocyclic rings. [0035] According to a further aspect, the present invention provides an activator composition for the accelerated hard- ening of a cyanoacrylate adhesive throughout the adhesive, wherein the activator comprises a member selected from the group consisting of: [0036] aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) and substituted on the ring(s)with at least one electron-withdrawing group which decreases the base strength of the Oct. 9, 2003 substituted compound compared to the correspond- ing unsubstituted compound, [0037] mixtures of any of the said aromatic hetero- cyclic compounds with each other, and/or with N,N- dimethyl-p-toluidine, [0038] and mixtures of any of the said aromatic heterocyclic compounds and/or N,N-dimethyl-p- toluidine with an organic compound containing the structural element, —N=C—S—S—. [0039] This aspect of the invention is particularly directed to good CTV performance, in particular accelerated hard- ening throughout the adhesive, in drops of adhesive or relatively large layers of adhesive in a joint gap which is larger than the “zero gap” achieved when two substrate surfaces are in contact. In general a gap having a greater width than 10 microns is of interest. The depth of the adhesive drop or layer perpendicular to the substrate surface is suitably in the range 0.5 mm-2 mm, particularly 0.75 mm-1.25 mm. [0040] In the said organic compound containing the struc- tural element —N=C—S—S—, the N=C double bond may optionally be part of an aromatic system, which may suitably be monocyclic, bicyclic or tricyclic. For example, the N=C double bond may suitably be part of an aromatic heterocyclic ring having one or more N hetero atoms in the ring, optionally with one or more other hetero atoms selected from S and O. The heterocyclic ring may be substituted. [0041] Desirably the said organic compound contains the structural element —N=C—S—S—C=N—, in which case both the N=C double bond and the C=N double bond may optionally be part of aromatic systems as described above, suitably two similar aromatic systems. More desir- ably the said organic compound is selected from dibenzothi- azyl disulfide, 6,6‘-dithiodinicotinic acid, 2,2‘-dipyridyl dis- ulfide, and bis(4-t-butyl-1-isopropyl-2-imidazolyl) disulfide. Of course, combinations of these organic compounds may also be used. Organic compounds having structural element —N=C—S—S—, which are useful as accelerators for accelerating the curing of CA adhesives if diluted in a solution, are described in WO 00/39229 and the correspond- ing U.S. Patent of Henkel KGaA, both of which were 20 published after the first priority date of this application and the contents of which are incorporated herein by reference. [0042] Desirably the activator comprises a member selected from the group consisting of pyridines, quinolines, pyrimidines and pyrazines substituted on the ring(s) with at least one electron-withdrawing group which decreases the base strength of the substituted compound compared to the corresponding unsubstituted compound. [0043] An aromatic heterocyclic compound may suitably be monocyclic, bicyclic or tricyclic. The N hetero atoms(s) may be present in one or more of the rings. Two or more heterocyclic 30 rings may be fused, or a heterocyclic ring may be fused to one or more carbocyclic rings. A hetero- cyclic ring may suitably be a 5- or 6-membered ring and may suitably have one or two N-atoms in the ring. A 6-membered heterocyclic ring is particularly suitable. In the case of two fused heterocyclic rings, the total number of N-atoms is suitably not more than three. The aromatic heterocyclic compounds are suitably substituted on the ring carbon atoms. A carbocyclic ring fused to a heterocyclic ring may US 2003/0191248 A1 suitably have 6 carbon atoms and/or may be an aromatic ring. A compound comprising a heterocyclic ring fused to a carbocyclic ring may be substituted by electron-withdraw- ing group(s) on either or both of the heterocyclic and carbocyclic rings. [0044] Suitably the at least one electron-withdrawing group is selected from the group consisting of halo, CN, CF3, COOR, COR4, OR, SR, CONR1R2, NO2, SOR, SOZR3, SO3R3, PO(OR3)2 and optionally substituted C6-C20 aryl, wherein R, R1, R2 and R4 (which may be the same or different) are H, optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, and R3 is optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl. Suitably the number of electron-withdrawing group(s) (which may be the same or different) may be from 1 to 3 groups per ring, for example 1 or 2 groups per ring. [0045] For example, the electron-withdrawing group(s) may be selected from the group consisting of halo, CN, COOR and COR4. Desirably R4 is optionally substituted C -C10 alkyl or optionally substituted C6-C20 aryl. R, R1, R2, R5 and R4 may suitably be optionally substituted C1-C5 alkyl, for example unsubstituted C1-C5 alkyl. [0046] The criterion that the electron-withdrawing group decreases the base strength of the substituted compound compared to the corresponding unsubstituted compound may be determined by pKa measurement in water under standard conditions (e.g. 25° C. and zero ionic strength) by conventional means or using a software package which calculates pKa of the base such as “ACD/pKa Calculator” available from Advanced Chemistry Development, 133 Richmond Street West, Suite 605, Toronto, ON N5H 2LS, Canada. A decrease in base strength is indicated by a reduction in pKa value. [0047] The aromatic heterocyclic compound substituted with electron-withdrawing groups may also be substituted on the ring with one or more electron-releasing groups, provided that overall the base strength (i.e. the pKa) is reduced compared to the corresponding unsubstituted com- pound. [0048] According to a particular aspect, the aromatic het- erocyclic compound is selected from: [0049] pyridines having one or more electron-with- drawing groups in the 3-, 3,4- or 3,5-position on the ring, suitably 3,5-dihalopyridines, such as 3,5- dichloropyridine or 3,5-dibromopyridine, or 3-cyano pyridine, a lower alkyl 3,5-pyridine dicarboxylate, or a 5-halo nicotinic acid such as 5-bromo nicotinic acid, [0050] pyridines having an electron-withdrawing group in the 2 position on the ring, suitably a COOR or COR4 group, such as 2-acetyl pyridine, [0051] pyridines having an electron-withdrawing group in the 4-position on the ring, suitably 4-nitro- pyridine, [0052] pyrimidines having an electron-withdrawing group in the 4- or 5-position on the ring, suitably 4- or 5 -halo pyrimidines, such as 4-bromopyrimidine or 5-bromopyrimidine, nitroquinolines, suitably 5-nit- roquinoline, polyhalogenated quinolines, suitably 4,7-dihalo quinolines such as 4,7-dichloro quinoline, Oct. 9, 2003 [0053] pyrazines having an electron-withdrawing group in the 2-position on the ring, suitably 2-meth- oxy pyrazine or 2-methylthio pyrazine. [0054] and aromatic heterocyclic compounds which are substantially iso-electronic to any of the forego- ing compounds. [0055] According to one feature, the present invention provides an activator composition for the accelerated hard- ening of cyanoacrylate adhesives, wherein the activator comprises a member selected from the group consisting of: [0056] aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) and substituted on the ring(s)with at least one substituent selected from the group consisting of halo, CN, CF3, COOR, COR4, OR, SR, CONR1R2, NO2, SOR, SOZR3, SO3R3, PO(OR3)2 and optionally substituted C6-C20 aryl, wherein R, R1, R2 and R4 (which may be the same or different) are H, optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, and R3 is optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, mixtures of any of the foregoing with each other, and/or with N,N- dimethyl-p-toluidine, and mixtures of any of the foregoing and/or N,N-dimethyl-p-toluidine with an organic compound containing the structural element, —N=C—S—S—. [0057] According to a further aspect, the present invention includes the use of a composition as defined above for the accelerated hardening of a cyanoacrylate adhesive. The composition may be applied to a substrate before application of the cyanoacrylate adhesive thereto, and/or the composi- tion may be applied to the cyanoacrylate adhesive after application of the adhesive to a substrate. [0058] According to a further aspect, the present invention provides an adhesive system comprising a cyanoacrylate adhesive together with a composition as defined above. Suitably, the composition as defined above is held separately from (i.e. does not contact) the adhesive prior to application on a substrate. [0059] According to another aspect, the present invention provides a process for the bonding of substrates or parts, characterised by either of the following series of steps: [0060] (a) dispensing an activator composition as defined above onto at least one surface of the sub- strates or parts to be joined; [0061] (b) optionally exposing solvent or other liquid vehicle in the activator composition to air, optionally with heating or with the aid of a fan; [0062] (c) optionally holding the substrate or part having the activator composition thereon for a reten- tion or shipping period, [0063] (d) applying a cyanoacrylate adhesive to at least one substrate or part; [0064] (e) joining the substrates or parts, optionally with manual or mechanical fixing, and [0065] optionally subsequently dispensing the activator composition onto adhesive exposed from a joint gap; or US 2003/0191248 A1 [0066] applying a cyanoacrylate adhesive onto at least one surface of the substrates or parts to be joined; [0067] joining the substrates or parts, optionally with manual or mechanical fixing; [0068] (iii) dispensing an activator composition as defined above onto the adhesive before or after the step of joining the substrates or parts, and [0069] (iv) optionally exposing solvent or other liquid vehicle in the activator composition to air, optionally with heating or with the aid of a fan. [0070] Suitably the retention or shipping period in step (c) may be in the range from several minutes to several days, for example from two minutes to forty-eight hours. Optionally the activator composition may be applied onto parts prior to their shipping, forwarding or delivery to an end-user, cus- tomer or contractor. [0071] The present invention includes a bonded assembly of substrates or parts bonded by a process as defined above. The present invention also includes as an article of com- merce a substrate or part having a composition as defined above applied thereto. [0072] According to another aspect, the present invention provides an activator composition for the accelerated hard- ening of cyanoacrylate adhesives, wherein the composition comprises a solution of one or more activators in a solvent mixture which comprises a volatile hydrocarbon and a cyclic ketone. Cyclic ketones as co-solvents have shown better results in reducing the “halo effect” discussed above than linear ketones such as acetone, butanone, pentanone, hex- anone, 4-methyl-2-pentanone, or octanone; than cyclic ethers such as dioxane or tetrahydrofuran; or than adhesive- miscible solvents such as ethyl acetate. [0073] Suitably, the cyclic ketone is present in an amount of up to about 15%, especially up to about 12%, particularly up to about 10%, by weight of the solvent mixture. If an amount substantially greater than 10%, and particularly greater than 15% is used, there may be a risk that a plastic substrate will be attacked. Desirably, the cyclic ketone is present in an amount of at least about 2.5% by weight of the solvent mixture. Below this amount the reduction in the “halo effect” may not be sufficient for full visual satisfaction. Preferably, the cyclic ketone is present in an amount of at least about 3% by weight of the solvent mixture. At or above this level the presence of cyclic ketone is seen to be beneficial. Desirably, the cyclic ketone is present in an amount in the range of 3% to 75% by weight of the solvent mixture, particularly an amount in the range of 4% to 7% by weight of the solvent mixture. [0074] A cyclic ketone may suitably be monocyclic or bicyclic. [0075] Suitably the cyclic ketone is an optionally-substi- tuted cyclic ketone, desirably an alicyclic ketone, having 3-10 carbon atoms in the ring. A substituted cyclic ketone may be mono- or di-substituted on the ring with C1-C5 alkyl, more particularly C1-C2 alkyl. [0076] One particular example of suitable cyclic ketone is cyclohexanone. Other examples include cyclobutanone, cyclopentanone, cycloheptanone and 2-methyl cyclopen- Oct. 9, 2003 tanone. Examples of bicyclic ketones include 2-nor- bomanone, bicyclo[3.2.1]octan-2-one and bicyclo[2.22] octanone. [0077] Desirably, the volatile hydrocarbon is an aliphatic hydrocarbon. Suitably, the volatile aliphatic hydrocarbon may have from 4 to 10 carbon atoms, particularly from 5 to 8 carbon atoms, and may be straight chain, branched or cyclic. One particular example of a suitable hydrocarbon is n-heptane. [0078] In one aspect, the present invention relates to use of an activator composition as defined above for the acceler- ated hardening of a cyanoacrylate adhesive, particularly when the activator composition is applied to the cyanoacry- late adhesive after application of the adhesive to a substrate. [0079] In an activator composition for the accelerated hardening of cyanoacrylate adhesives, the activator may suitably comprise a member selected from the group con- sisting of: [0080] organic amines including: lower fatty amines, aromatic amines, dimethylamine and the like; ali- phatic, alicyclic and, especially, tertiary aromatic arnines; such as N,N-dimethylbenzylamine, N-me- thylmorpholine and N,N-diethyltoluidine; [0081] amine compounds with a boiling point of between 50° C. and 250° C. such as triethylamine, diethylamine, butylamine, isopropyl amine, tributyl amine, N,N-dimethyl aniline, N,N-diethyl aniline, N,N-dimethyl-p-toluidine, N,N-dimethyl-m-tolui- dine, N,N-dimethylo-toluidine, dimethyl benzyl amine, pyridine, picoline, vinyl pyridine, ethanola- mine, propanolamine and ethylene diamine; [0082] organic compounds containing the structural element, —N=C—S—S— (as described above). [0083] aromatic heterocyclic compounds having at least one N hetero atom in the ring(s) and substituted on the ring(s)with at least one electron-withdrawing group which decreases the base strength of the substituted compound compared to the correspond- ing unsubstituted compound (as described above), [0084] and mixtures of any of the foregoing with each other. [0085] According to one aspect, the present invention includes the use of an activator composition as defined above for the accelerated hardening of a cyanoacrylate adhesive. The composition may be applied to a substrate before application of the cyanoacrylate adhesive thereto, but more suitably the composition is applied to the cyanoacry- late adhesive after application of the adhesive to a substrate. [0086] According to a further aspect, the present invention provides an adhesive system comprising a cyanoacrylate adhesive together with an activator composition as defined above. Suitably, the activator composition as defined above is held separately from the adhesive prior to application on a substrate. [0087] According to another aspect, the present invention provides a process for the bonding of substrates or parts, characterised by the following series of steps: US 2003/0191248 A1 [0088] applying a cyanoacrylate adhesive onto at least one surface of the substrates or parts to be joined; [0089] (ii) joining the substrates or parts, optionally with manual or mechanical fixing; [0090] (iii) dispensing an activator composition com- prising a solution of one or more activators in a solvent mixture which comprises a volatile hydro- carbon and a cyclic ketone onto the adhesive before or after the step of joining the substrates or parts, and [0091] (iv) optionally exposing the solvent mixture in the activator composition to air, optionally with heating or with the aid of a fan. [0092] The process of the invention is particularly advan- tageous when at least one of the substrates has a surface of a dark colour or is transparent and/or at least one of the substrates is of a plastics material. However the invention is also useful with substrates of other materials such as card- board, paper, or wood, particularly if the surface is of a dark colour. [0093] The present invention includes a bonded assembly of substrates or parts bonded by a process as defined above. [0094] Desirably, an activator composition comprises an amount of activator effective to accelerate hardening of a cyanoacrylate adhesive, the activator being carried in a suitable solvent mixture in accordance with the invention. [0095] The solutions of the activator(s) may suitably con- tain the activator compound(s) in concentrations of from 0.01 to 10 g per 100 ml of solvent mixture; for example, from 0.05 to 5 g of activator substance are dissolved per 100 ml of solvent mixture. [0096] Various conventional organic solvents are suitable as the hydrocarbon solvent (in the solvent mixture) for the activator(s) according to this aspect of the present invention, provided they have a sufficiently high volatility. Desirably, the boiling point of the solvent is below about 120° C., suitably below about 100° C., at ambient pressure. Although aromatic solvents such as toluene or xylene may be used, the hydrocarbon solvent is desirably an aliphatic hydrocarbon. Suitable solvents include specialized boiling point gasolines, but especially n-heptane, n-hexane, n-pentane, octane, cyclohexane, cydopentane, methyl cyclopentane, methyl cyclohexane and isomers of them like isooctane, methyl- hexanes, methylpentanes, 2,2-dimethyl butane (neohexane), or mixtures thereof, as well as petroleum benzines and ligroin. DETAILED DESCRIPTION OF THE INVENTION [0097] An alkyl group may be straight-chained or branched and may be unsaturated, i.e. the term alkyl as used herein includes alkenyl and alkynyl. A C1-C10 alkyl group may for example be a C1-C5 alkyl group. A lower alkyl group may suitably be a C1-C5 alkyl group. An aryl group includes phenyl and naphthyl groups, either of which may be substituted with an alkyl group, more particularly a lower alkyl group. Halo includes chloro, bromo, fluoro and iodo, as well as pseudohalo-radicals such as CN, SCN, OCN, NCO, NCS. Oct. 9, 2003 [0098] An optionally substituted alkyl, alkoxy or aryl group may be substituted with a substituent selected from the group consisting of halo, CN, CF3, COOR, COR, OR, SR, CONR1R2, NO2, SOR, SOZR3, SO3R3, PO(OR3)2 and optionally substituted C6-C20 aryl, wherein R,R1 and R2 (which may be the same or different) are H, optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, and R3 is optionally substituted C1-C10 alkyl, or option- ally substituted C6-C20 aryl. [0099] In an organic compound containing the structural element —N=C—S—S—, in which the N=C double bond is part of a heterocyclic ring, the heterocyclic ring may be substituted for example with optionally substituted C1-C10 alkyl, optionally substituted C1-C10 alkoxy, optionally sub- stituted C1-Clo alkoxyalkyl, halo, CN, CF3, COOR, COR, OR, SR, CONR1R2, NO2, SOR, SOZR3, SO3R3, PO(OR3)2 and optionally substituted C2-C20 aryl or aryloxy, CSOR3, COONR32, NRCOOR, N=N—R3, OOR3, SSR3, OOCOR3, NOR32, ON(COR3)2, S-aryl, NR32, SH, OH, SiR33, Si(OR3)3, OSiR33, OSi(OR3)3, B(OR)2, P(OR3)2, SOR3, OSR3, wherein R,R1 and R2 (which may be the same or different) are H, optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl, and R3 (which may be the same or different) is optionally substituted C1-C10 alkyl, or optionally substituted C6-C20 aryl. [0100] Desirably, an activator composition comprises an amount of activator effective to accelerate hardening of a cyanoacrylate adhesive, the activator being carried in a suitable vehicle. [0101] Suitably the activator composition is an activator solution of the activator in a solvent. Alternatively the composition may be a dispersion of the activator in a suitable vehicle, particularly a liquid vehicle. [0102] Desirably, the activator(s) are dissolved in readily volatile organic solvents, such as hydrocarbons, carboxylic acid esters, ketones, ethers or halogenated hydrocarbons and carbonic acid esters or acetals or ketals. The solutions of the activator(s) may suitably contain the activator compound(s) in concentrations of from 0.01 to 10 g per 100 ml of solvent; for example, from 0.05 to 5 g of activator substance are dissolved per 100 ml of solvent. [0103] When the activator composition contains a mixture of two activator compounds, the respective amounts of the activator compounds may vary and are only limited by respective amounts which will no longer be effective for the desired combination of properties. More particularly, when the activator composition contains a mixture of an aromatic heterocyclic compound substituted with at least one elec- tron-withdrawing group and an organic compound having the structural element —N=C—S—S—, the activator com- pounds may suitably be present in amounts of about 0.1% to about 10% by weight of the said aromatic heterocyclic compound and about 0.01% to about 5% by weight of the said organic compound, more particularly about 0.05% to about 1%, of the said organic compound (c), based on the total weight of the activator composition. [0104] Various conventional organic solvents are suitable as solvents for the activator(s) according to the present invention, provided they have a sufficiently high volatility. Desirably, the boiling point of the solvent is below about 120° C., suitably below about 100° C., at ambient pressure. US 2003/0191248 A1 Suitable solvents include specialized boiling point gasolines, but especially n-heptane, n-bromopropane, alcohols, for example isopropyl alcohol, alkyl esters of lower carboxylic acids, for example ethyl acetate, isopropyl acetate, butyl acetate, ketones, such as acetone, methyl isobutyl ketone and methyl ethyl ketone. Also suitable are ether solvents, ether esters or cyclic ethers, such as, especially, tetrahydro- furan. In the case of sparingly soluble activators, chlorinated hydrocarbons, such as dichloromethane or trichloromethane (chloroform), may also be used. [0105] The activator compositions according to the present invention are suitable for the accelerated hardening of conventional cyanoacrylate adhesives which contain as the fundamental constituent one or more cyanoacrylic acid esters, suitably with inhibitors of free-radical polymerisa- tion, inhibitors of anionic polymerisation and, optionally, conventional auxiliary substances employed in such adhe- sive systems, like fluorescence markers. [0106] The cyanoacrylic acid esters used in the adhesives are in the main one or more esters of 2-cyanoacrylic acid. Such esters correspond to the following general formula: H2C:C(CN)—CO—O—R5. [0107] In that formula, R5 represents an alkyl, alkenyl, cycloalkyl, aryl, alkoxyalkyl, aralkyl or haloalkyl or other suitable group, especially a methyl, ethyl, n-propyl, isopro- pyl, n-butyl, isobutyl, pentyl, hexyl, allyl, methallyl, crotyl, propargyl, cyclohexyl, benzyl, phenyl, cresyl, 2-chloroethyl, 3-chloropropyl, 2-chlorobutyl, trifluoroethyl, 2-methoxy- ethyl, 3-methoxybutyl or 2-ethoxyethyl group. The above- mentioned cyanoacrylates are known to a person skilled in the art of adhesives, see Ullmann’s Encyclopaedia of Indus- trial Chemistry, Volume A1, p. 240, Verlag Chemie Wein- heim (1985) and U.S. Pat. Nos. 3,254,111 and 3,654,340. Preferred monomers are the allyl, methoxyethyl, ethoxy- ethyl, methyl, ethyl, propyl, isopropyl or butyl esters of 2-cyanoacrylic acid. The monocyanoacrylic acid esters rep- resent the largest proportion by weight of the polymerisable monomers in the adhesive. [0108] The mentioned cyanoacrylic acid esters may suit- ably be present in the adhesives in amounts of from 99.99 to 90 wt. %. Preference is given to cyanoacrylic acid esters the alcohol radical of which is derived from alcohols having from 1 to 10 carbon atoms, which may also be cyclic, branched or perfluorinated. [0109] The cyanoacrylate adhesives according to the present invention may also contain an inhibitor of free- radical polymerisation. Such inhibitors are, for example, hydroquinone,p-methoxyphenol, but also sterically-hin- dered phenols, phenothiazine and the like. [0110] The cyanoacrylate adhesives according to the present invention may also contain thickeners as further DCP = 3,5-dichloro pyridine, NQ = 5-nitro quinoline DPDS = 2,2’-dipyridyl disulfide, DMPT = N,N-dimethyl-p-toluidine, Heptane = n-heptane, CNP = 3-Cyano Pyridine Oct. 9, 2003 auxiliary substances. That is desirable especially when there are to be bonded porous materials which otherwise readily absorb the low viscosity adhesive. Many types of polymer may be used as thickeners, such as polymethyl methacrylate, other methacrylate copolymers, acrylic rubber, cellulose derivatives, polyvinyl acetate or polyalphacyanoacrylate. A usual amount of thickener is generally about 10 wt. % or less, based on the total adhesive. In addition to or instead of the thickeners, the cyanoacrylate adhesives according to the present invention may also contain Reinforcing agents. Examples of such reinforcing agents are acrylic elastomers, acrylonitrile copolymers, elastomers or fluoroelastomers. Moreover, inorganic additives may also be used, for example silicates, thixotropic agents having a large surface area, which may be coated with polydi-alkylsiloxanes. [0111] The cyanoacrylate adhesives according to the present invention may also contain substances for increasing the thermal stability thereof. There may be used for that purpose, for example, the sulfur compounds mentioned in European Patent specification No. 579 476. [0112] In addition to or instead of the mentioned additives, the cyanoacrylate adhesives according to the present inven- tion may also contain plasticisers. These serve to protect the resulting adhesive bond from brittleness. Such plasticisers are, for example, C1-C10 alkyl esters of dibasic acids, especially of sebacic acid, phthalic acid or malonic acid. Other plasticisers are diaryl ethers and polyurethanes and the like. Furthermore, the adhesive preparations according to the present invention may also contain colorings, pigments, aromatic substances, extenders and the like, as well as fluorescing additives. Reference is directed to U.S.Pat. Nos. 5,749,956 (Fisher et al.), U.S. Pat. No. 4,869,772 (McDon- nell et al.) and U.S. Pat. No. 5,314,562 (McDonnell et al.), the contents of which are incorporated herein by reference. [0113] The activator compositions of the present invention are intended to be used with a wide variety of both metallic and non-metallic substrates, including substrates having acidic surfaces such as wood and paper or cardboard, and plastics substrates. [0114] In the aspect of the invention using a cyclic ketone as a co-solvent, the advantage of the activator solutions of the invention is particularly evident on dark-coloured sub- strates. [0115] The present invention will now be illustrated in greater detail. EXAMPLES [0116] In the Examples, the following abbreviations and terms are used: DBP = 3,5-dibromo pyridine, DCQ = 4,7-dichloro quinoline, BBID = Bis(4-t-butyl-isopropyl-2-imidazolyl) disulfide, NBP = n-bromopropane, CT V = cure through volume THF = tetrahydrofuran US 2003/0191248 A1 -continued