A curable epoxy composition

Abstract

A cationically curable epoxy composition comprising: [i] a curable epoxy component; [ii] a cationic polymerisation initiator; and [iii] a Norrish type Il photoinitiator; wherein the cationic polymeris

Description

Field of the Invention

[0001] The present invention relates to a curable epoxy composition. Of particular interest are curable epoxy composition which are UV activated. Curable epoxy composition which are UV activated but which cure quickly after activation are of interest. One-part (also referred to as “1k”) curable epoxy compositions are of most interest. Such compositions are used as adhesives. Background to the Invention

[0002] Curable epoxy compositions are well known. One-part curable epoxy compositions are well known.

[0003] Such epoxy compositions can be cured by different mechanisms including heat cured, UV-cured, or possibly both.

[0004] With heat-curable epoxies they are stored at ambient temperatures or possibly in cold storage below ambient temperatures and then heated when it is desired to cure.

[0005] UV-curable epoxies are exposed to actinic UV so as to cure. Such actinic UV may be from a UV light source.

[0006] In many cases with curable epoxies it may take some time for a desirable bond strength to be achieved.

[0007] Also when considering bond strength one major factor is the substrates being bonded together. It is well known that a given curable composition may perform very differently when bonding different substrates. Bond strengths ranging from zero bond strength (i.e. failure to bond) to high bond strength may be achieved by the same curable composition (and under the same conditions) with different substrates.

[0008] It is known in the adhesives industry that certain substrates, are difficult to bond. This may be due to various factors such as low surface energy / low surface tension, or other properties of the materials from which the substrate(s) to be bonded are made.

[0009] Various approaches are generally used to bond such difficult to bond materials. For example, adhesive compositions have been specifically formulated for use to bond such substrates. Additionally, or alternatively, primers have been used. Primers are applied to the substrate before subsequent over application of the adhesive. This requires an additional primer formulation and an additional step of application of a primer. Additionally, or alternatively, surface treatment of the substrates has been employed to make them more susceptible to bonding. Such treatment is often due to a physical effect such as roughening of a surface of the substrate thus making it more susceptible to bonding or a chemical treatment such as with acid.

[0010] Where speed of achieving a desirable bond strength is a factor, compositions that cure faster such as cyanoacrylate adhesives can be used to achieve the speed. However there are other factors to consider such as the integrity of the bond over time in an end-use application.

[0011] For example achieving a bond that has a high temperature performance required for the end-use application. It is desirable that a curable composition cures to form a bond that retains bond strength even though exposed to high temperatures over time.

[0012] Initial bond strength should be sufficient to allow the substrates to be handled even if cure continues and bond strength subsequently improves.

[0013] One challenging type of substrate to bond is a substrate with a coating thereon. This may mean that, while the material from which the substrate is formed may be easily bonded, the coated material may be more difficult to bond as the coating has different properties.

[0014] Of particular interest in relation to the present invention is where the coating is a coating applied as an electrically insulating coating. In the present invention the term “insulation” will refer to electrical insulation.

[0015] For example, the substrate may be formed of a metal, such as steel, which is relatively easy to bond. However, the coating makes the substrate much less easy to bond. Furthermore, while removal of at least a part of the coating (for example using techniques described above) can allow bonding, this is undesirable as desirable properties imparted by the coating are lost.

[0016] Such coated steel materials are used in may electronic / electrical components including for example motors. For example there may be a plurality of layers. Such layers may be considered laminated or stacked. In the industry they may be referred to as stacked layers or sometimes e-stack laminated layers.

[0017] In particular it is undesirable to remove a coating that has been applied for the purpose of electrical insulation. The loss of such insulation can lead to potential electrical shorts, danger of electrical shock, and associated hazards such as fire, degradation of performance or injury.

[0018] Instead of using adhesive to bond such coated metal substrates a range of mechanical fixing techniques are more commonly used.

[0019] In the manufacture of electrical / electronic products including electrical / electronic motors, using such coated steel materials theses substrates may be physically attached to each other, for example by welding or using ties such as metal ties. These methods can be problematic as they can increase the electrical conduction between the substrates. This in turn may decrease the energy performance / efficiency of products such as motors.

[0020] Such materials can be referred to as “e-coat” materials or “e-coated” materials for example as e-coat steel or e-coated steel.

[0021] Notwithstanding that state-of-the-art proposed solutions to these issues exist, it is desirable to provide alternative solutions, so the end user has more choices available. Summary of the Invention

[0022] In one aspect, the present invention provides a cationically curable composition comprising: (i) a curable epoxy component; (ii) a cationic polymerisation initiator; and (iii) a Norrish type II photoinitiator; wherein the cationic polymerisation initiator is present in an amount of from about 3 wt.% to about 10 wt.% and the Norrish type II photoinitiator is present in an amount of from about 1 wt.% to about 5 wt.%, based on the total weight of the cationically curable composition.

[0023] The composition of the invention may be formulated as a one-part composition. It is a curable epoxy-based composition.

[0024] At least one substrate to be bonded to another may be an e-coated steel substrate. Both substrates may be e-coated steel substrates. Examples of such substrates are set out below.

[0025] The high speed of cure of the composition of this new invention allows accelerated production processes in areas where strength and heat resistance are important, e.g. bonding of e-stack laminations for electric motors.

[0026] Such coated substrates often form part of electrical devices, including motors, generators, transformers, transmitters, sensors, and other devices, such as those that function by electromagnetic induction. The coatings impart suitable electrical insulation and have sufficient structural integrity to allow operation of the electrical device.

[0027] Initial bond strength is sufficient to allow substrates bonded by a composition of the invention to be handled. Typically with compositions of the invention cure continues and bond strength subsequently improves.

[0028] In the building of such electrical devices using adhesives, there are three priorities: speed, performance, temperature resistance. In the main, such electrical devices have not been manufactured using adhesives, as there are no suitable one-part adhesives to match the performance requirements.

[0029] This present invention allows all three of these priorities to be achieved simultaneously.

[0030] The present inventors believe that a composition of the invention can be stored at room temperature and will reach full bond strength in less than 20 minutes at room (ambient) temperature following UV activation.

[0031] A composition of the invention will also give desirable bond strengths at elevated temperatures. A composition of the invention will maintain desirable bond strengths despite exposure to elevated temperatures over time.

[0032] A composition of the invention will also provide desirable bond strengths with “blocking” substrates - those that are not UV transmissive.

[0033] The composition of the invention is not UV-cured.

[0034] The composition of the invention is UV-activated.

[0035] This means that the composition of the invention is activatable by exposure to actinic UV radiation. It subsequently progresses to full cure in the absence of actinic radiation.

[0036] This means that curing can be started by an initial short exposure to actinic UV radiation and then cure will continue after that exposure ends.

[0037] With a composition of the invention there is sufficient open-time of the curable composition, post-activation (post-exposure), to allow the substrates to be bonded together to be assembled.

[0038] This means that activation can be achieved before assembly of substrates for bonding.

[0039] With a composition of the invention there is a rapid RT (room-temperature / ambient) cure (for example less than 20 minutes to a full strength bond).

[0040] A composition of the invention progresses to full cure in the absence of actinic radiation.

[0041] The composition of the invention provides a novel technology for one-part epoxy adhesives that allows for fast fixtures and fast cure times at room temperature that also gives good initial performance and good performance at elevated temperatures with fast activation. For example with a composition of the invention activation is achievable in less than about 10 seconds of exposure to actinic radiation. For example a composition of the invention is activable in about 2 seconds or less on various e-coated steels and is activable in about 5 seconds or less on grit blasted mild steel.

[0042] The composition of the invention provides a one-part UV activable adhesive that will allow for easy and efficient assembly and bonding together of substrates.

[0043] The composition of the invention will enable high speed manufacture of devices such as electrical devices including motors with a bond that performs well at elevated temperatures. The composition of the invention will enable high speed manufacture of devices which comprise e-coated steel.

[0044] With adhesive bonding using a composition of the invention, the coating on a coated substrate, such as an e-coat, is not degraded which will ensure that there is minimal impact on the properties of the coating. For example there may be minimal impact on induction between layers in the product. This means that the product, such as a motor, can be made at high speed and still achieve higher efficiencies.

[0045] The curable epoxy component may be present in an amount of from about 40 wt.% to about 70 wt.%, based on the total weight of the cationically curable composition.

[0046] The curable epoxy component may be an epoxy resin component. The curable epoxy component may comprise an alicyclic epoxy compound. The curable epoxy component may comprise an oxetane compound.

[0047] The curable epoxy component may comprise a glycidyl ether-based epoxy compound may be an aromatic glycidyl ether-based epoxy compound, for example 2-vinyloxyethylglycidyl ether.

[0048] Suitable epoxy components are set out in more detail below.

[0049] Suitably the cationic polymerization initiator comprises a photocationic polymerization initiator. The photocationic polymerization initiator may be a tetrakis(pentafluorophenyl)borate compound such as (4-lsopropylphenyl)(4-methylphenyl)iodonium tetrakis(pentafluorophenyl)borate.

[0050] The cationic polymerization initiator optionally comprises a thermal cationic polymerization which may be a tetrakis(pentafluorophenyl)borate compound initiator for example a tetrakis(pentafluorophenyl)borate compound.

[0051] Desirably the thermal cationic polymerization initiator comprises the ammonium salt of tetrakis(pentafluorophenyl)boric acid.

[0052] The weight ratio of the cationic polymerisation initiator to the Norrish type II photoinitiator may be from about 3:1 to about 5:1 such as from 7:2 to about 9:2. A weight ratio within this range may provide a suitably fast speed of cure, for example a fixture time of less than 1 minute. Epoxy component

[0053] The curable epoxy component may be an epoxy resin. The compositions of the invention may be epoxy-based compositions. This component may be the main component (being present in the largest amount based on percentage by weight) present. Suitable curable epoxy components are set out below.

[0054] Epoxy resins that may be included in a composition of the present invention include aliphatic and aromatic epoxy resins. Epoxy resins which may be included in a composition of the invention may be polyglycidyi epoxy compounds or epoxy novolaks. Suitable polyglycidyI epoxides include, but are not limited to, polyglycidyi ethers, poly(P-methylgiycidyl) ethers, polyglycidyi esters and poly(p-methylglycidyi) esters. Examples of such polyglycidyi ethers and poly(^-methylglycidyl) ethers are those based on monocyclic phenols, such as resorcinol or hydroquinone, and polycyclic phenols such as bis(4-hydroxyphenyl)methane (bisphenol F), 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), bis(4-hydroxyphenyi)sulfone (bisphenol F), whereby the aforementioned can optionally be substituted , for example with alkoxy or halogen residues, as well as phenol and cresol novolaks. Suitable polyglycidyi esters and poly(p-methylglycidyl) esters can be prepared by reacting epichlorohydrin, 1,3-dichlorohydrin or p-methylepichlorohydrin with aromatic polycarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid and pyromellitic acid. Another epoxy resin that may be included in a composition of the invention is poly[(phenyl glycidyl ether)-co-formaldehyde. Mixtures of the aforementioned aromatic epoxy resins are also suitable. The epoxy resins may have on average more than one, and desirably at least 2, epoxy groups per molecule.

[0055] The at least one aromatic epoxy resin may be selected from diglycidyl ethers based on bisphenol A, F and / or S and epoxy novolaks and mixtures thereof, in particular from bisphenol A digylcidyl ethers and bisphenol F digylcidyl ethers and mixtures thereof.

[0056] The aromatic epoxy resin may have an epoxy equivalent weight of 120 to 1000 g / eq, such as 120 to 600 g / eq, for example 150 to 250 g / eq (determinable according to DIN 16945).

[0057] The curable epoxy component may impart the bulk properties of the cured adhesive to the bond formed, e.g. strength, heat resistance, oil resistance, etc. Cationic polymerisation initiator

[0058] The compositions of the invention are cationically polymerized with the help of a cationic polymerisation initiator. This may be a cationic photoinitiator, which forms cations when a certain wavelength of light is absorbed.

[0059] The cationic polymerisation initiator may be one of those commonly used for cationic polymerization. Examples include onium salts with anions of low nucleophilicity. such as halonium salts, iodonium salts, sulfonium salts, sulfoxonium salts or diazonium salts. Suitable anions include, for example, hexafluoroantimonate, hexafluorophosphate or tetrakis (pentafluorophenyl) borate. Sulfonium and iodonium salts are suitable, the counterion being selected from hexafluoroantimonate, hexafluorophosphate and (tetrakis-(pentafluorearyl))borates, in particular from triarylsulfonium and bis(aikyiphenyl)iodonium salts.

[0060] Examples of cationic polymerisation initiators useful in the present invention include, (4-isopropylphenyl)(4-methylphenyl)iodonium tetrakis(pentafluorophenyl)borate, bis(dodecylphenyl)iodonium hexafluoroantimonate, bis(4-methylphenyl)iodonium hexafluorophosphate), triarylsulfonium borate, triarylsulfonium hexafluoroantimonate.

[0061] Concentrations of the cationic polymerisation inhibitor within the claimed range may improve both the activation energy required to cure the composition and the fixture time achieved by the cured composition. Reactive Diluents

[0062] Reactive diluents, insofar as they are not among the flexibilizers, can be employed in a composition according to the invention. Such diluents may be employed in amounts of 10 to 40% by weight, particularly preferably 5-10% by weight, based on total weight of the composition. These concentrations of the reactive diluent may provide a viscosity low enough that the composition can be applied easily and in a suitable thickness (for example below about 7 pm), while avoiding any deleterious effects (e.g. lowered temperature resistance) on the properties of the cured adhesive that may be observed when higher concentrations are used. Reactive diluents may be selected from monoglycidyl ethers of aliphatic or aromatic alcohols, in particular C12 / C14 fatty alcohols and alkylphenols, such as para-tert-butylphenol, and oxetanes, including trimethylolpropane oxetane (TMPO). Monoglycidyl ethers of aromatic alcohols and particularly low molecular weight oxetanes such as TMPO are suitable. 1,4-butanediol diglyceryl ether; 3-ethyl~3-[(2"ethylhexyloxy)methyl]oxetane are specific examples of reactive diluents. Reactive diluents may help control the viscosity of the end formulation and react to become part of the cured adhesive matrix. Adhesion promoters

[0063] Adhesion promoters may be employed in a composition of the invention. Epoxymodified silanes, including glycidoxyalkyl-modified silanes, such as trimethoxysilanes, including glycidoxypropyltrimethoxysilane, (3-giycidoxypropyl)trimethoxysilane can be used as adhesion promoters. Polyalkyleneoxide modified heptamethyltrisiloxane; or (tris[2-(2-methoxyethoxy)ethoxy]-[3-(oxiran-2-ylmethoxy)propyl]silane) may be used. Such silanes are commercially available, for example, as Silquest ™ A-187. The adhesion promoter can alternatively or additionally be chelate-modified epoxy resins.

[0064] Such adhesion promoters may be employed in amounts of 0.8% to 2 % by weight, based on the total weight of the composition. Suitably these values are sufficient to ensure good adhesion both at room temperature and at elevated temperatures, e.g. 180 °C. Norrish type II photoinitiator

[0065] Norrish type II photoinitiators are those that form radicals by hydrogen abstraction. Norrish type II photoinitiators require a hydrogen donor acting as a source of free radicals: since initiation is based on bimolecular reactions, Norrish type II photoinitiators are generally slower than free radical based unimolecular formation Norrish type I photoinitiators. On the other hand, Norrish type II photoinitiators have better optical absorption properties in the near ultraviolet spectral region. A Norrish type II radical photoinitiator undergoes the Norrish type II reaction when exposed to actinic radiation: that reaction is defined by I URAC as the photochemical abstraction of a y-hydrogen by an excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct. Norrish type II photoinitiators suitable for use in the present invention may act as photosensitisers.

[0066] Non-limiting examples of photoactive compounds that undergo a Norrish II reaction and thus may be used in a composition of the invention as Norrish type II photoinitiators include but are not limited to: bis(2,6-dimethylbenzoyl)-2,414-trimethylpentylphosphine oxide (Irgacure 1800, 1850, and 1700); 2,4,6-trimethylbenzoyl- diphenylphosphine oxide (Lucerin TPO, available from BASF); ethyl (2,4,6-trimethylbenzoyl)-phenylphosphinate (Lucerin TPO-L,); bis(214,6-trimethylbenzoyl)-phenylphosphine oxide; isopropyl thioxanthone; diethyl thioxanthone (for example 2,4-diethyl-9H-thioxanthen-9-one); and, 2-chlorothioxanthone.

[0067] The Norrish type II photoinitiator may comprise isopropyl thioxanthone (ITX). This may be used in a mixture of isomers or as a single isomer. For example, the Norrish type II photoinitiator may comprise 2-isopropylthioxanthone (CAS 5495-84-1, also known as 2-isopropyl-9H-thioxanthen-9-one), 4-isopropylthioxanthone (CAS 83846-86-0, also known as 4-isopropyMhioxanthene-9-one), or any combination thereof.

[0068] Norrish type II photoinitiators may be employed in amounts of 1% to 5 % by weight, based on the total weight of the composition. Concentrations within this range may improve both the activation energy required to cure the composition and the fixture time achieved by the cured composition. Chain Extenders

[0069] Chain extenders that may be included in a composition according to present invention include diols such as 1,4 butanediol, 1,1,1-trimethylolpropane, glycerol, 1,2,6-hexanetrio and polyester diol, and includes triois.

[0070] Usually, chain extenders are low molecular weight molecules used to modify the backbone of a given polymer. Suitably, the chain extender has a molecular weight Mn of 60 to 600 g / mol, and for example from 60 to 500 g / mol.

[0071] As above the chain extender may be a diol, in particular a linear or branched diol with 9 or less carbon atoms, in an especially preferred embodiment, the chain extender is selected from the group consisting of 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediel, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-prentanediol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, neopentyl glycol, polybutylene glycol, cyclohexane-1,2-diol, cyclohexane-1,4-diol, 1,4-bis(hydroxymethyl)cyclohexane, and dimer diols (such as Pripol series from Croda) and mixtures thereof.

[0072] The chain extender is typically present in amounts from about 4% to about 15%, by weight based on the total weight of the composition. The concentration of the chain extender may affect the final properties of the adhesive, including the observed shear strength. Concentrations within this range may lead to cured compositions having a high shear strength at room temperature while avoiding any decrease in shear strength at elevated temperatures that may be observed at higher concentrations of the chain extender. Antioxidants

[0073] Any suitable antiexidant(s) may be used in a composition of the present invention. Suitable antioxidants include substituted diphenyl amine or phenols such as butylated hydroxytoluene. The antioxidant(s) is typically present in amounts from about 0.01% to about 0.25%, for example from about 0.01% to about 0.15% by weight based on the total weight of the composition. Such a concentration may increase the rate of cure of the composition. Substrates

[0074] It will be appreciated that at least one of the substrates to be bonded together may be an e-coated steel substrate having one of the following coatings as classified according to AISI-ASTM A 976-9 standards: CO, C2, C3, C3A, C4, C4A, C4AS, C5, C5A, C5AS, C6. This is a difficult to bond substrate and is a substrate with a coating applied for the purposes of electrical insulation. In the present invention such a substrate / coating may be referred to as e-coat, e-coated, e-coating etc.

[0075] CO, C2, C3, C3A, C4, C4A, C4AS, C5, C5A, C5AS, C6 are coating names as set out in AISI-ASTM A 976-9, which describes the classification of insulating coatings for electrical steels and characterises the coating names CO, C2, C3, C3A, C4, C4A, C4AS, C5, C5A, C5AS, C6.

[0076] In the electrical industry such coatings are often applied to parts for electrical / electronic components. The composition that is applied to form the coating is often referred to as a varnish. In this respect the present invention is directed to bonding substrates, and in particular electrical substrates which have been coated with such a varnish. Often times the varnish imparts electrical insulation to the substrate. Typically, the electrical substrates are made from metal such as steel and have been insulated using such a coating.

[0077] Of particular interest in the present invention are coated substrates that function utilising electromagnetic induction.

[0078] Such coated substrates often form part of electrical devices, including motors, generators, transformers, sensors, and other devices that function by electromagnetic induction. The coatings impart suitable electrical insulation and have sufficient structural integrity to allow operation of the electrical device.

[0079] Such insulating coatings may be applied by encapsulation, casting or potting to a suitable substrate.

[0080] The coatings are typically epoxy resins, phenolic resins, including phenol / formaldehyde resins, and polyurethane resins.

[0081] Siliconised Steel, also known as electrical steel, is steel with silicon added to it. Adding silicon to steel increases its electrical resistance, improves the ability of magnetic fields to penetrate it, and reduces the steel’s hysteresis loss. Silicon steel is used in many electrical applications where electromagnetic fields are important, such as electrical stators / rotors and motors, coils, magnetic coils and transformers.

[0082] Steel used in electrical applications may also be known as: lamination steel, silicon electrical steel, silicon steel, core plate steel, C5 core plate, or transformer steel.

[0083] Some types of steel include: GO Grain Oriented I NGO Non-Grain Oriented I CRML Cold Rolled Motor Lamination.

[0084] Electrical insulation coatings are coatings that insulate steel such as silicon steel and they are often pigmented.

[0085] Electrical steel insulation coatings are classified according to AISI-ASTM A 976-9 standards. Some Insulation Classes for electrical steel insulation coatings are set out below:

[0086] C31 EC-3: These are unfilled, organic based varnishes that deliver increased punch ability and have exceptional insulation properties. Typical applications for these coatings are small motors, transformers and transmitters. A special C3 coating is a selfbonding varnish, which shows the highest level of adhesive properties thanks to its adherence over the whole cross-section, even of complex geometries. It enables electrical device manufacturers to insulate and to adhere steel sheet stacks in one step while still retaining the magnetic properties and the excellent mechanical strength of the electrical steel.

[0087] C51 EC-5 coatings are filled organic and inorganic based varnishes ideally used for increased insulation properties, resistance against annealing and improved weldability. Typical applications for these coatings are machines undergoing treatments like welding, Al-die casting or annealing.

[0088] C61 EC-6: These are highly-filled organic and inorganic based varnishes that deliver increased insulation properties as well as the required resistance against pressure. Typical applications for these coatings are medium and large machines with high resistance against pressure and temperature. Electrical Steel Coatings - Insulation classes Classification Description For Rotors / Stators Anti-stick treatment CO Natural oxide formed during mill processing No No C2 Glass like film No No C3 Organic enamel or varnish coating No No C3A As C3 but thinner Yes No C4 Coating generated by chemical and thermal processing No No C4A As C4 but thinner and more weldable Yes No C4AS Anti-stick variant of C4 Yes Yes C5 High-resistance similar to C4 plus inorganic filler No No C5A As C5, but more weldable Yes No C5AS Anti-stick variant of C5 Yes Yes C6 Inorganic filled organic coating for insulation properties Yes Yes

[0089] The composition of the invention is suitable for use with all of the classes of 5 insulation coating given above. Detailed Description

[0090] Compositions according to the present invention were tested as set out below.

[0091] Adhesive bonds were assembled using the following methods. to

[0092] The adhesive composition was applied to the chosen substrate (mild steel, or ecoated steel) as a 20 pm to 100 pm thick film. They were then UV activated with the appropriate actinic UV radiation dose (circa 5 J / cm2) from a 405nm LED. Within the open time the bonds were assembled to a Yd' (1.27 cm) overlap lap shear specimen and clamped for the appropriate amount of time prior to testing. While clamped the excess adhesive in the bond line is squeezed out to give a bond line thickness of <7pm.

[0093] The strength tests were carried out as per ASTM D1002 -05 “Strength Properties of Adhesives in Shear by Tension Loading (Metal-to-Metal)”, carried out on a calibrated tensile testing machine.

[0094] The strength tests were carried out at both room temperature and at raised temperatures. For raised temperatures the test were carried out inside a calibrated oven.

[0095] To determine full strength the bonds were allowed to cure for at least 24 hours at room temperature, to ensure full cure.

[0096] Tests were also conducted to measure the “time to full strength”.

[0097] To determine fixture time the bonds were unclamped and tested from 1 second to 1 minute after assembly and tested to determine at which point the bond strength had achieved 0.1 MPa.

[0098] Definitions: • Open Time’. The time between the end of UV exposure and the time in which the material has already reached the stage of cure that the bond cannot be assembled, typically as the viscosity has built enough, via the cure reaction, that the formulation will not wet (and therefor will not bond to) the incoming substrate. • Fixture Time’. The time between the end of the UV exposure and the time in which the adhesive has cured sufficiently to create a bond with a strength of at least 0.1 MPa. Within this time period is the open time and the time taken to assemble the bond. • Time to full strength’. The time between the end of the UV exposure and the time in which the adhesive has cured sufficiently to create a bond with the same bond strength seen after 24 hours of room temperature cure. Within this time period is the open time, the fixture time and the time taken to assemble the bond. • Bondline thickness: The thickness of the adhesive layer between the bonded specimen, measured by measuring the thickness of each substrate without adhesive and then measuring the thickness of the bonded assembly.

[0099] In all of the examples of the invention (below) and irrespective of the formulation or the substrate used, all bonds were allowed to cure at RT (room temperature) overnight before testing took place before determining “Lap Shear Strength” or “Lap Shear Strength at 180°C”.

[00100] In all of the examples of the invention (below) all percentages are percentages by weight based on the weight of the entire composition / based on the total weight of the composition.

[00101] In the examples of the invention (below) the Backlack Suralack 9 and Backlack 2x PE 75W substrates are both e-coated steel substrates and fall under C5 as set out above and as classified according to AISI-ASTM A 976-9 standards.

[00102] Example 1 Component % epodil 750 25.8% DEN 431 (Press9) 51.1% Butyl hydroxyltoluene (BHT) 0.1% Speedcure 939 8.5% K-FLEX XM-332 11.4% Coatasil 77 1.2% ITX 1.9%

[00103] The above formulation was made and tested on Grit Blasted Mild Steel (GBMS) as follows: • Activation dose: 4.9 J / cm2 • Open time: 7s • Fixture time: 30s • Time for Full Strength to be obtained: 10 minutes • Lap Shear Strength (1 / 2” overlap): 12.1 MPa • Lap Shear Strength at 180°C (1 / 2” overlap): 2.5 MPa

[00104] This formulation was also tested on the following e-coated electrical steels (1 / 2” overlap) lap shear strength: Steel         Coating • M270-35AA Backlack Suralack 9 • NO20-15 Backlack 2xPE 75W

[00105] For testing on the e-coated electrical steels the following activation dose was used: 2.8J / cm2

[00106] Which gave the following results: • Open time: 10s • Fixture time: 45s • Lap Shear Strength (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 13.0MPa • Lap Shear Strength (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 5.6 MPa • Lap Shear Strength at 180°C (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 1.0 MPa • Lap Shear Strength at 180°C (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 2.84 MPa

[00107] Example 2: Component % epodil 750 27.4% DER 331 Epoxy Resin 54.2% Butyl hydroxyltoluene (BHT) 0.1% Speedcure 939 9.0% K-FLEX XM-332 6.0% Coatasil 77 1.3% ITX 2.0% • The above formu ation was made and tested on Grit Blasted Mild Steel (GBMS) • Activation dose: 7.8 J / cm2 • Open time: 7s • Fixture time: 30s • Time for Full Strength to be obtained: 5minutes • Lap Shear Strength (1 / 2” overlap): 12.1 MPa • Lap Shear Strength at 180°C (1 / 2” overlap): 2.32 MPa

[00108] This formulation was also tested on the following e-coated electrical steels (1 / 2” overlap) lap shear strength: Steel         Coating • M270-35AA Backlack Suralack 9 • NO20-15 Backlack 2xPE 75W

[00109] For testing on the e-coated electrical steels the following Activation dose was used: 2.8J / cm2

[00110] Which gave the following results: • Open time: 10s • Fixture time: 15s • Lap Shear Strength (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 12.6MPa • Lap Shear Strength (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 5.7MPa • Lap Shear Strength at 180°C (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 1.1 MPa • Lap Shear Strength at 180°C (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 0.9MPa

[00111] Example 3: Component % Oxetane 212 14.8% epodil 750 11.0% Epikote 154 25.6% DEN 431 (Press9) 25.6% Butyl hydroxyltoluene (BHT) 0.1% Speedcure 939 8.5% K-FLEX XM-332 11.4% Coatasil 77 1.3% ITX 1.9%

[00112] The above formulation was made and tested on Grit Blasted Mild Steel (GBMS) as follows: • Activation dose: 6.9J / cm2 • Open time: 7s • Fixture time: 25s • Time for Full Strength to be obtained: 10minutes • Lap Shear Strength (1 / 2” overlap): 9.56 MPa • Lap Shear Strength at 180°C (1 / 2” overlap): 1.8MPa

[00113] This formulation was also tested on the following e-coated electrical steels (1 / 2” overlap) lap shear strength. Steel         Coating • M270-35AA Backlack Suralack 9 • NO20-15 Backlack 2xPE 75W

[00114] For testing on the e-coated electrical steels the following activation dose was used: 1J / cm2

[00115] Which gave the following results: • Open time: 8s • Fixture time: 20s • Lap Shear Strength (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 13.3MPa • Lap Shear Strength (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 6.2MPa • Lap Shear Strength at 180°C (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 1.8MPa • Lap Shear Strength at 180°C (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 2.65MPA

[00116] Example 4: Component % epodil 750 25.8% DEN 431 (Press9) 51.1% Butyl hydroxyltoluene (BHT) 0.1% Speedcure 939 8.5% K-FLEX XM-332 11.4% ARALDYTE DY 1158 GB 1.2% ITX 1.9%

[00117] The above formulation was made and tested on Grit Blasted Mild Steel (GBMS) as follows: • Activation dose: 6.9J / cm2 • Open time: 9s • Fixture time: 35s • Time for Full Strength to be obtained: 10minutes • Lap Shear Strength (1 / 2” overlap): 14.87MPa • Lap Shear Strength at 180°C (1 / 2” overlap): 2.65MPa

[00118] This formulation was also tested on the following e-coated electrical steels (1 / 2” overlap) lap shear strength. • • Steel Coating M270-35AA Backlack Suralack 9 NO20-15 Backlack 2xPE 75W

[00119] For testing on the e-coated electrical steels the following Activation dose was used: 1.9J / cm2

[00120] • • • • • • Which gave the following results: Open time: 6s Fixture time: 25s Lap Shear Strength (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 12.6MPa Lap Shear Strength (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 6.2MPa Lap Shear Strength at 180°C (1 / 2” overlap) M270-35AA, Backlack Suralack 9: 1.2 MPa Lap Shear Strength at 180°C (1 / 2” overlap) NO20-15, Backlack 2xPE 75W: 3.75 MPa Component commercial name non-commercial names Oxetane 212 3-Ethyl-3-[(2-ethylhexyloxy)methyl]oxetane epodil 750 1,4-Butanediol diglycidyl ether Epikote 154 Novalac resin / POLY[(PHENYL GLYCIDYL ETHER)-CO-FORMALDEHYDE] DEN 431 (Press9) Novalac resin / POLY[(PHENYL GLYCIDYL ETHER)-CO-FORMALDEHYDE] Butyl hydroxyltoluene (BHT) Butylated hydroxytoluene Speedcure 939 (4-lsopropylphenyl)(4-methylphenyl)iodonium tetrakis(pentafluorophenyl)borate K-FLEX XM-332 Polyester Diol Coatasil 77 adhesion promotor (Polyalkyleneoxide modified heptamethyltrisiloxane) ITX 2-lsopropylthioxanthen-9-one ARALDYTE DY 1158 GB adhesion promotor (tris[2-(2- m eth oxy ethoxy) eth oxy]- [3- (oxi ra n-2-ylmethoxy)propyl]silane) DER 331 Bisphenol A epoxy resin

[00121] Component Ranges Material Name(s) Description Concentration Ranges (%) A 1,4-Butanediol diglyceryl ether; 3- Ethyl-3-[(2- ethylhexyloxy)methyl]oxetane Reactive diluent, helps control the viscosity of the end formulation and reacts to become part of the cured adhesive matrix 10 to 40 B Epoxy resin: Novalac resin / poly[(phenyl glycidyl ether)-co-formaldehyde].; Bisphenol A epoxy Epoxy resin / main curable component of the formulation 40-70 resin or; Bisphenol F epoxy resin; or aliphatic epoxy resin c Butylated hydroxytoluene antioxidant 0.01-0.25 (preferably 0.00 to 0.15) D Polyester Diol Chain extending resin and finished property modifier, e.g. can increase flexibility 4-15 E Polyalkyleneoxide modified heptamethyltrisiloxane; or (tris[2-(2-methoxyethoxy)ethoxy]-[3-(oxiran-2-ylmethoxy)propyl]silane) adhesion promotor 0.8-2 F (4-lsopropylphenyl)(4-methylphenyl)iodonium tetrakis(pentafluorophenyl)borate Cationic Photoinitiator 3-10 G 2-lsopropylthioxanthen-9-one Norrish Type II photoinitiator / photosensitiser 1-5 H Ratio Cationic Photoinitiator to Norrish Type II photoinitiator This ration was found to be important for speed. 3:1 to 5:1 preferably 7:2 to 9:2

[00122] Effect of being outside target range:

[00123] A: Concentration of the reactive dilute is too low then the viscosity will be too high to easily apply the adhesive and cause difficulty in creating the bond line 5 adhesive thickness of <7pm. If the concentration is too high then the properties of the cured adhesive will be impaired, e.g.: the reactive diluents typically do not impart the same temperature resistance to the cured adhesive as the main epoxy resin.

[00124] B: Concentration of the epoxy resin: Imparts the bulk properties of the cured adhesive to the bond, e.g. strength, heat resistance, oil resistance etc. to

[00125] C: Concentration of the antioxidant: In the formulae tested increasing the concentration of the antioxidant above 0.15% decreases the rate of cure but conversely if the material is not contained in the formulation the rate of cure is also decreased. A concentration of 0.01% to 0.1% appears to be optimal.

[00126] D: The concentration of the polyester diol impact the final properties of the adhesive, including the observed shear strength. With an increase in shear strength at room temperature observed at higher polyester diol concentrations (19.4% was highest we went to give best initial strengths). Conversely, a decrease in shear strength at elevated temperatures was noted at the increased higher polyester diol concentrations with the best balance being approximately 11% of the higher polyester diol.

[00127] E: Concentration of the Adhesion Promotor: The concentration of this once above 0.8% did not have a significant effect on the properties of the adhesive. Below this level adhesion at both room temperature and at 180°C was negatively affected.

[00128] F, G and H: Concentrations and rations of the cationic photoinitiator and the Norrish Type II photoinitiator significantly impact the speed of cure. Both the required activation energy and the fixture times are increased significantly with lower concentrations of the materials. At the higher concentrations the ratio of the cationic photoinitiator and the Norrish Type II photoinitiator is important. At ratios below 3:1 (cationic to Type II) the speed of cure is too slow (fixture time less than 1 minute).

[00129] Comparative Example

[00130] Formula A (below) has 2.7% by weight cationic photoinitiator, but was considered too slow / not reactive enough requiring >30J / cm2 (35 seconds) of activation. Formula A % epodil 750 24.8% DER 331 Epoxy Resin 49.0% Butyl hydroxyltoluene (BHT) 0.14% Speedcure 939 2.7% K-FLEX XM-332 20.4% Coatasil 77 1.2% ITX 1.8%

[00131] The words “comprises / comprising” and the words “having / including” when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[00132] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in 5 combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

Claims

1. Use of a cationically curable composition comprising:(i) a curable epoxy component;(ii) a cationic polymerisation initiator; and(iii) a Norrish type II photoinitiator;wherein the cationic polymerisation initiator is present in an amount of from 3 wt.% to 10 wt.% and the Norrish type II photoinitiator is present in an amount of from 1 wt.% to 5 wt.%, based on the total weight of the cationically curable composition to bond first and second substrates to each other, wherein at least one of the first or the second substrate is an e-coated steel substrate.

2. Use according to claim 1, wherein the curable epoxy component is present in an amount of from 40 wt.% to 70 wt.%, based on the total weight of the cationically curable composition.

3. Use according to any preceding claim wherein the curable epoxy component is an epoxy resin component.

4. Use according to any preceding claim wherein the curable epoxy component comprises an alicyclic epoxy compound.

5. Use according to any preceding claim wherein the curable epoxy component comprises an oxetane compound.

6. Use according to any preceding claim wherein the curable epoxy component comprises a glycidyl ether-based epoxy compound such as an aromatic glycidyl ether-based epoxy compound, for example 2-vinyloxyethylglycidyl ether.

7. Use according to any preceding claim wherein the cationic polymerization initiator comprises a photocationic polymerization initiator.

8. Use according to Claim 7 wherein the photocationic polymerization initiator is a tetrakis(pentafluorophenyl)borate compound such as (4-lsopropylphenyl)(4-methylphenyl)iodonium tetrakis(pentafluorophenyl)borate.

9. Use according to any preceding claim wherein the cationic polymerization initiator comprises a thermal cationic polymerization which is optionally a tetrakis(pentafluorophenyl)borate compound initiator for example a tetrakis(pentafluorophenyl)borate compound.

10. Use according to Claim 9 wherein the thermal cationic polymerization initiator comprises the ammonium salt of tetrakis(pentafluorophenyl)boric acid.29 07 2511. Use according to any preceding claim, wherein the weight ratio of the cationic polymerisation initiator to the Norrish type II photoinitiator is from 3:1 to 5:1 such as from 7:2 to 9:2.

12. Use according to any preceding claim, further comprising a chain extender such as a polyester diol optionally wherein the chain extender is present in an amount of from 4 wt.% to 15 wt.%, based on the total weight of the cationically curable composition.

13. Use according to any preceding claim, further comprising an adhesion promoter such as a silane, optionally wherein the adhesion promoter is present in an amount of from 0.8 wt.% to 2 wt.%, based on the total weight of the cationically curable composition.

14. Use according to any preceding claim, further comprising an antioxidant such as substituted diphenyl amine or phenols such as butylated hydroxytoluene, optionally wherein the antioxidant is present in an amount of from 0.01 wt.% to 0.25 wt.%, based on the total weight of the cationically curable composition.

15. A method of bonding first and second substrates to each other the substrates having respective bonding surfaces to be bonded together wherein at least one of the first or the second substrate is an e-coated steel substrate, the method comprising:(a) applying to the bonding surface of the first substrate, and / or to the bonding surface of the second substrate, a cationically curable composition;(b) activating the cationically curable composition by exposing the bonding surfaces together with the cationically curable composition to actinic radiation;(c) mating the bonding surfaces together with the UV-activated cationically curable composition therebetween; and(d) allowing the UV-activated cationically curable composition to curewherein the cationically curable composition comprises:(i) a curable epoxy component;(ii) a cationic polymerisation initiator; and(iii) a Norrish type II photoinitiator;wherein the cationic polymerisation initiator is present in an amount of from 3 wt.% to 10 wt.% and the Norrish type II photoinitiator is present in an amount of from 1 wt.% to 5 wt.%, based on the total weight of the cationically curable composition.29 07 2516. A method according to Claim 15, wherein the energy of the actinic radiation to which the surface is exposed is in the range from 0.1 J / cm2 to 20 J / cm2.

17. A method according to Claim 15 or 16 wherein the actinic radiation of step (b) has a wavelength of from 300 nm to 500 nm, suitably from 365 nm to 405 nm.

18. A method according to any of Claims 15 to 17, wherein the duration of the exposure to the actinic radiation of step (b) is from 0.1 to 30 seconds, suitably from 1 to 10 seconds.

19. A method according to any of Claims 15 to 18 wherein at least one of the first and second substrates is a substrate with a coating thereon for example wherein the coating is a coating applied by curing a curable coating composition on the substrate.

20. A method according to any of Claims 15 to 19 wherein at least one of the first and second substrates forms a part of an electrical device such as an electrical motor.

21. A method according to Claim 19 or 20 wherein the coating is formed from an epoxy resin, a phenolic resin, including phenol / formaldehyde resins, or a polyurethane resin or combinations thereof.

22. A method according to any of Claims 15 to 21 wherein at least one of the first and second substrates in an e-coated steel wherein the e-coated steel has one of the following coatings as classified according to AISI-ASTM A 976-9 standards: CO, C2, C3, C3A, C4, C4A, C4AS, C5, C5A, C5AS, C6.

23. An assembly comprising first and second substrates bonded to each other by the cure product of a cationically curable composition and / or bonded to each other by the method of any of Claims 15 to 22, wherein the cationically curable composition comprises:(i) a curable epoxy component;(ii) a cationic polymerisation initiator; and(iii) a Norrish type II photoinitiator;and wherein the cationic polymerisation initiator is present in an amount of from3 wt.% to 10 wt.% and the Norrish type II photoinitiator is present in an amount of from 1 wt.% to 5 wt.%, based on the total weight of the cationically curable composition.s