Improvements relating to epoxy resin compositions

Abstract

An epoxy resin composition is described which is the product of a reaction mixture comprising an epoxy-containing component which comprises 35 to 60 parts by weight of a cycloaliphatic epoxy compound and 40 to 65 parts by weight of a bisphenol epoxy compound. This product constitutes 35 to 65 wt% of the total weight of the epoxy resin composition. Also present are 35 to 65 wt% hardener, 0.2 to 5 wt% accelerator, and 1 to 30 wt% toughening agent (in each case based on weight of the epoxy resin composition). The epoxy resin compositions of this invention have a matrix of properties which make them highly suitable for use in high temperature vehicular applications. These properties include high glass transition temperature, high fracture energy, good thermal ageing properties, low shrinkage on curing, and good adherence to low surface energy engineering thermoplastic polymers, for example polyaryletherketones.

Description

[0001] IMPROVEMENTS RELATING TO EPOXY RESIN COMPOSITIONS

[0002] This invention relates to epoxy resin compositions. It relates in particular to epoxy resin compositions which are suitable for use in high temperature vehicular applications. In particular epoxy resin compositions of the present invention are suitable for use as insulating materials for encapsulating wires in motors that can be used in electrical apparatus used in automotive, aerospace, power generation, transmission and storage, and marine and naval industries. The epoxy resin compositions of the invention have been found to exhibit a valuable range of properties, including surprisingly good adhesion to low surface energy polymers used in high temperature vehicular applications. Low surface energy polymers include polymers of the polyaryl ether ketones (PAEK) class, which includes PEEK (claimed Trade Mark of Victrex pic), polyether ether ketone. Further low surface energy polymers are known to the person skilled in the art.

[0003] US 8,742,018 B2 describes a thermoset resin which is the reaction product of an epoxy resin mixture which includes a cycloaliphatic epoxy resin, a cycloaliphatic resin hardener, and an accelerator. It is stated in US 8,742,018 B2 that there exists a need for epoxy compositions having a relatively low viscosity and a relatively high reactivity, and where the resulting thermoset resin has a good balance of a high glass transition temperature (Tg), tensile strength, tensile modulus, and elongation properties. Consistent with this, the thermoset resins of US 8,742,018 B2 are said to be of low viscosity and high reactivity, and are further defined in the claims by Tg of at least 210°C, by tensile modulus of at least about 260000 psi, by elongation at break of at least 5%, and by peak stress of at least about 7000 psi.

[0004] The disclosure of US 8,742,018 B2 is very extensive both in terms of available chemistry options and the stated uses of the resulting epoxy resins. The wide range of stated uses includes adhesives, structural and electrical laminates, coatings, castings, structures for the aerospace industry, as circuit boards and the like for the electronics industry, as well as for the formation of composites, pultruded composites, pultruded rods, skis, ski poles, fishing rods, and other outdoor sports equipment, electrical varnishes, encapsulants, semiconductors, general molding powders, filament wound pipe, storage tanks, liners for pumps, corrosion resistant coatings and resin coated foils.

[0005] CA2858840C describes a process for preparing a cured composite material useful for a specific use: radio frequency filter applications. The process comprises the steps of: (a) providing a curable thermoset epoxy resin composition comprising (i) at least one epoxy resin; (ii) at least one toughening agent; (iii) at least one hardener; and (iv) at least one filler; (b) curing the curable thermoset epoxy resin composition of step (a) to form a cured composite; wherein the curable thermoset epoxy resin composition upon curing provides a cured composite product with a balance of properties comprising Tg, coefficient of thermal expansion, tensile strength, thermal conductivity; and having a density of less than 2.7 g / cc; and (c) coating at least a portion of the surface of the cured composite of step (b) with an electrically conductive metal layer to form a metalized coating on at least a portion of the surface of the cured composite. The cured composite material may be useful as a radio frequency cavity filter body housing for radio frequency filter applications. Unlike US 8,742,018 B2, the use proposed for the cured composite materials of CA2858840C is limited.

[0006] We have examined the use of epoxy resin compositions in the industries and technologies mentioned above and have found that existing products are not optimal for use in this area. We have sought to develop epoxy resin compositions which have enhanced properties for use in the industries and technologies mentioned above.

[0007] In accordance with a first aspect of the present invention there is provided an epoxy resin composition which is the product of a reaction mixture comprising: an epoxy-containing component which comprises from 35 to 60 parts by weight of a cycloaliphatic epoxy compound and from 40 to 65 parts by weight of a bisphenol epoxy compound, wherein the epoxy-containing component constitutes from 35 to 65 weight percent of the total weight of the epoxy resin composition; a hardener able to react with the epoxy-containing component, wherein the hardener constitutes from 35 to 65 weight percent of the total weight of the epoxy resin composition; an accelerator for the reactions(s) between the epoxy-containing component and the hardener, wherein the accelerator constitutes from 0.2 to 5 weight percent of the total weight of the epoxy resin composition; and a toughening agent, wherein the toughening agent constitutes from 1 to 30 weight percent of the total weight of the epoxy resin composition.

[0008] Optionally the epoxy-containing component comprises from 40 to 55 parts by weight of a cycloaliphatic epoxy component and from 45 to 60 parts by weight of a bisphenol epoxy component. In embodiments the epoxy-containing component comprises from 41 to 47 parts by weight of a cycloaliphatic epoxy component and from 53 to 59 parts by weight of a bisphenol epoxy component.

[0009] Optionally the epoxy-containing component constitutes from 40 to 60 weight percent of the total weight of the epoxy resin composition.

[0010] In embodiments the epoxy-containing component constitutes from 45 to 65 weight percent of the total weight of the epoxy resin composition.

[0011] Suitably the cycloaliphatic epoxy component may include a diepoxide of a cycloaliphatic ester of a dicarboxylic acid. Examples include bis(3,4-epoxycyclohexylmethyl)oxalate (known as ECC), bis(3,4-epoxycyclohexylmethyl)adipate, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, vinylcyclohexene diepoxide; limonene diepoxide, bis(3,4-epoxycyclohexylmethyl)pimelate and dicyclopentadiene diepoxide.. Other diepoxides of cycloaliphatic esters of dicarboxylic acids are described, for example, in U.S. Pat. No. 2,750,395.

[0012] Alternatively or additionally the cycloaliphatic epoxy component may include a compound of the 3'4'-epoxycyclohexanemethyl-3,4-epoxycyclohexylcarboxylate class. Examples of such compounds include 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate; 3,4-epoxy-l- methylcyclohexyl-methyl-3,4-epoxy-l-methylcyclohexanecarboxylate; 6-methyl-3,4-epoxycyclo- hexylmethylmethyl-6-methyl-3,4-epoxycyclohexanecarboxylate; 3,4-epoxy-2-methylcyclohexyl- methyl-3,4-epoxy-2-methylcyclohexanecarboxylate; 3,4-epoxy-3-methylcyclohexyl-methyl-3,4- epoxy-3-methylcyclohexanecarboxylate; 3,4-epoxy-5-methylcyclohexyl-methyl-3,4-epoxy-5- methylcyclohexane carboxylate, di- or poly-glycidyl ethers of cycloaliphatic polyols such as 2,2- bis(4-hydroxycyclohexyl)propane, and the like. Other suitable 3,4-epoxycyclohexylmethyl-3,4- epoxycyclohexanecarboxylates are described, for example, in U.S. Pat. No. 2,890,194.

[0013] Suitably the bisphenol epoxy component includes one or more epoxidised derivative of 4,4'- dihydroxydiphenyl dimethyl methane (known as bisphenol A) and / or epoxidised derivatives of bis(4-hydroxyphenyl) methane (known as bisphenol F). Such derivatives may include the diglycidyl ether of bisphenol A) known as DGEBA or BADGE), the diglycidyl ether of bisphenol-F (known as BFDGE) and the diglycidyl ether of tetrabromobisphenol A (known as TBBPA). A bisphenol epoxy compound of particular interest in the present invention is DGEBA. In embodiments of the invention DGEBA constitutes at least 60 wt% of the bisphenol epoxy component, suitably at least 80 wt%, or at least 90 wt%. DGEBA may suitably make up substantially all of the bisphenol epoxy component. Optionally the hardener constitutes from 40 to 60 weight percent of the total weight of the epoxy resin composition.

[0014] In embodiments the hardener constitutes from 45 to 55 weight percent of the total weight of the epoxy resin composition.

[0015] Suitably the hardener can react with the epoxy-containing component to form the epoxy resin. Suitably the hardener is an anhydride compound. Suitably the anhydride compound is a partially or completely hydrogenated phthalic anhydride, which may be substituted by one or more 1 substituents, for example Cl-4 alkyl group(s). In embodiments the hardener may be a partially or completely hydrogenated phthalic anhydride optionally substituted in the ring by a single methyl group.

[0016] Specific examples of hardeners include methyltetrahydrophthalic anhydride (known as MTHPA), methylhexadyrophthalic anhydride (known as MHHPA), hexahydrophthalic anhydride (known as HPPA), nadic methyl anhydride, trimellitic anhydride, dodecenyl succinic anhydride and phthalic anhydride.

[0017] The accelerator is sometimes called a catalyst but since it may be consumed in the curing reaction we prefer the term 'accelerator'. Optionally the accelerator for the reactions between the epoxy-containing component and the hardener constitutes from 0.5 to 3 weight percent of the total weight of the epoxy resin composition.

[0018] In embodiments the accelerator constitutes from 0.75 to 2 weight percent of the total weight of the epoxy resin composition.

[0019] When the hardener is an anhydride compound we have found that there is benefit when epoxy moieties supplied by the epoxy-containing compound are present in molar excess over anhydride moieties supplied by the anhydride compound. In some embodiments the molar ratio of such epoxy moieties to such anhydride moieties is at least 1.02 to 1, preferably at least 1.06 to 1, more preferably at least 1.08 to 1, and in some embodiments at least 1.1 to 1. Suitably the molar ratio of such epoxy moieties to such anhydride moieties is up to 1.3 to 1, preferably up to 1.2 to 1, more preferably up to 1.15 to 1, and in some embodiments up to 1.12 to 1.

[0020] Compounds that may be useful as accelerators include amines, for example dicyandiamide, tertiary amines, and imidazole or imidazole derivatives; metal halide Lewis acids, including tin stannous chloride and zinc chloride; metal carboxylates, for example stannous octoate; boron containing accelerators for example boron trifluoride, for example as a complex with an amine, for example piperidine or monoethylamine or methylethylamine.

[0021] Examples of amines useful as accelerators include benzyl dimethylamine; dimethyl aminomethyl phenol; methyldiethanolamine; triethanolamine; diethylaminopropylamine; benzyldimethyl-amine; m-xylylenedi(dimethylamine); N,N'-dimethylpiperazine; N- methylpyrolidine; N-methyl-hydroxypiperidine; N,N,N'N'-tetramethyldiaminoethane; N,N,N',N',N'- pentamethyl-diethylene-triamine; tributyl amine; trimethylamine; diethyldecyl amine; triethylene diamine; N-methyl morpholine; N,N,NN'-tetramethylpropanediamine; N-methyl piperidine, N,N'- dimethyl-l,3-(4-piperidino)propane; pyridine; l,8-diazobicyclo-5.4.0-7-ene; 1,8-diazabicyclo- [2.2.2]octane, 4-dimethy laminopyridine; 4-(N-pyrolidino)pyridine; triethylamine; and 2,4,6- tris(dimethyl-aminomethyl)phenol.

[0022] As note above the accelerator may be imidazole or a derivative of imidazole. Suitable derivatives of imidazole are ring-substituted by one, two or three optionally substituted alkyl groups, for example by one or two Cl-4 alkyl or Cl-4 cyanoalkyl groups. Examples of derivatives of imidazole which could be used as accelerators are 1-methylimidazole, 2-methylimidazole, 2- ethylimidazole, 2-ethyl-4-methyl-imidazole, phenylimidazole, l-(2-cyanoethyl)-2-ethyl-4(5)- methylimidazole and 1-cyano ethyl-2-undecylimidazolium trimellitate.

[0023] Optionally the toughening agent constitutes from 2 to 20 wt% of the total weight of the epoxy resin composition.

[0024] In embodiments the toughening agent constitutes from 3 to 12 wt% of the total weight of the epoxy resin composition, for example from 4 to 10 wt% of the total weight of the epoxy resin composition.

[0025] Suitably the toughening agent forms a secondary phase within a matrix formed by the epoxycontaining component, the hardener and the accelerator. The toughening agent is suitably elastomeric, and is capable of inhibiting or limiting crack growth, thereby providing improved toughness.

[0026] Many types of toughening agents have been proposed for use in epoxy resins. These include reactive liquid rubbers, core-shell rubbers, block copolymers, thermoplastics, organopolysiloxane resins, linear polybutadiene-polyacrylonitrile copolymers, oligomeric polysiloxanes, polysulfides, amine-terminated butadiene nitriles, polythioethers, organic rigid particles and inorganic and organic rigid particles. Whilst in principle any toughening agent could be considered for use in the present invention we have found unexpected stand-out benefit in using reactive liquid rubbers. This is one of the earliest methods of toughening structural epoxies, originating in the 1960s. Typically, reactive liquid rubbers are butadiene-acrylonitrile polymers terminated with reactive groups, for example carboxyl, epoxy, amine, hydroxyl or vinyl groups. Reactive liquid rubbers include derivatives of polybutadiene, and suitably include carboxyl-terminated butadiene-acrylonitrile (CTBN), amine- terminated butadiene-acrylonitrile (ATBN), epoxy-terminated butadiene-acrylonitrile (ETBN), hydroxyl-terminated butadiene-acrylonitrile (HTBN), hydroxyl-terminated polybutadieneacrylonitrile (HTPB), and vinyl-terminated butadiene-acrylonitrile (VTBN) butadiene-acrylonitrile rubber. The liquid rubbers are introduced into the epoxy component, in which it is typically initially miscible. Typically it pre-reacts with the epoxy component to form an adduct. The rubber then phase-separates during the curing process via reaction-induced phase separation to form a rubber phase within the matrix. In embodiments of the invention the rubber phase comprises discrete particles within the matrix, for example micron-scale rubber particles within the matrix.

[0027] It has been reported in the scientific literature, for example by YJ. Lim et al in J. Mater. Sci. 51 (18) (2016) 8631-8644 that high temperature performance is reduced when using reactive liquid polymers but we have found the high temperature performance of the epoxy resins of the invention to be excellent, and in addition for the epoxy resins to exhibit a range of further valuable properties.

[0028] We have found that an epoxy resin composition of the invention has properties which make it highly suitable for use in high temperature vehicular applications. In this regard significant properties of the epoxy resin composition of the invention are set out below.

[0029] An epoxy resin composition of the invention suitably has a glass transition temperature (Tg) of at least 160 °C, for example at least 180 °C. In embodiments it may have a Tg of at least 190 °C. In embodiments the Tg may be up to 220°C, or higher. Suitably the Tg is less than 210 °C, and preferably less than 205 °C.

[0030] An epoxy resin composition of the invention suitably has a fracture energy of at least 150 J / m2. In embodiments it may have a fracture energy of at least 200 J / m2, or at least 300 J / m2.

[0031] An epoxy resin composition of the invention suitably has good thermal ageing properties. Suitably it survives thermal ageing at 200 °C for at least 100 days with loss of less than 10% of its weight. In embodiments it may survive thermal ageing at 200 °C for at least 300 days with loss of less than 10% of its weight. An epoxy resin composition of the invention suitably has low shrinkage on curing, compared with commercially available resins recommended for high temperature vehicular applications. Suitably the shrinkage on curing of epoxy resin composition of the invention is less than 5% of the starting (pre-cure) volume. In embodiments the shrinkage on curing of epoxy resin composition of the invention is less than 3%.

[0032] The uncured precursor of an epoxy resin composition of the invention suitably has a dynamic viscosity in the range from 150 mPa.s to 1000 mPa.s. In embodiments it may have a dynamic viscosity in the range 200 to 400 mPa.s, or in the range 200 mPa.s to 300 mPa.s. If the viscosity of the uncured precursor is regarded as too low to facilitate easy handling or application a viscosity increasing component can be added, for example fumed silica. We have found that use of 0.2 to 1 wt% of the total weight of the epoxy resin composition, for example 0.3 to 0.7 wt%, for fumed silica gives a useful increase in viscosity without significantly compromising materials properties of the epoxy resin composition.

[0033] An epoxy resin composition of the invention suitably adheres strongly to low surface energy engineering thermoplastic polymers, for example PAEK polymers. The PAEK class includes polyetherketones (PEK), polyetheretherketoneketones (PEEKK), polyetheretherketoneketones (PEKK), as well as polyetheretherketones (PEEK). Suitably, good adhesion with low surface energy engineering thermoplastic polymers, including PAEK polymers, is maintained at elevated temperatures, for example up to at least 120 °C. In embodiments good adhesion may be maintained up at least 150 °C, or up to at least 180 °C. This is an important benefit in high temperature automotive application, as more manufacturers are turning to PEEK coated wires for their motors for use in high voltage applications, for example in stators, rotors, wires, cables, busbars and motors that can be used in electrical apparatus used in automotive, aerospace, power generation, transmission and storage, and marine and naval industries. Our methodology used to assess the strength of bonding is described in the later examples.

[0034] In this specification:

[0035] Glass transition temperature (Tg) is determined by dynamic mechanical thermal analysis using a cuboidal test piece of nominal dimension 45 X 10 X 3 mm oscillated at a frequency of 1 Hz and subjected to a temperature ramp rate of 2 C° / min. The glass transition temperatures defined here are identified as the temperature location of the peak of the tan(delta) curve (tan(delta) = E" / E'. We chose this method as it allows consistent comparison with the majority of the academic literature. Fracture energy is determined at ambient (laboratory) temperature by the Single Edge notch Beam method of ISO-13586, sample size 80 x 16 x 8 mm, beam span of 64 mm with nominal 8 mm pre-crack in centre. Fracture energy was determined via the energy method outlined in ISO-13586.

[0036] Dynamic viscosity is determined using a rotational rheometer equipped with 25 mm diameter parallel aluminium plates, 1 mm gap, strain of 1%, rotational velocity of 2 rad / s, at a test temperature of 25°C.

[0037] Thermal ageing is the time taken to lose 10% of the initial weight at an elevated temperature and was determined by test standard: IEC 60216. Tests were done on 50 x 50 x 3 mm specimen size. A range of elevated test temperatures was used.

[0038] We have found that epoxy resin compositions of the invention have unexpectedly excellent adhesion to PAEK polymers, including at elevated temperatures, such as are found in high voltage electrical environments requiring excellent dielectric properties. Such environments may include automotive, aerospace, power generation, transmission and storage, and marine and naval industries, and other vehicular or power generation, transmission and storage environments. Examples of articles that can benefit from the invention include stators, rotors, wires, cables, busbars, motors and capacitors.

[0039] In accordance with a second aspect of the invention there is provided a process for making an epoxy resin composition of the first aspect, which process comprises the steps of mixing the epoxy component, the hardener and the accelerator to form a precursor composition at a temperature at which curing does not take place, holding the precursor composition in a desired conformation, and subjecting the precursor composition to a curing temperature. The curing temperature may typically be in the range 50 °C to 120 °C, for example 60 to 100 °C, suitably 70 to 90 °C. Curing is allowed to take place for 1 to 6 hours, for example 2 to 4 hours. There may be a post-curing step. Typically a post-curing temperature may be in the range 120 °C to 200 °C, for example 140 to 180 °C, suitably 150 to 170 °C. Post-curing may take place for 1 to 12 hours, for example 2 to 10 hours. As mentioned above the dynamic viscosity of the precursor composition is low; shrinkage on curing is low; and the cured epoxy resin composition has values of Tg, fracture energy and thermal ageing which are very suitable for high temperature automotive uses. Further, the adhesion between the epoxy resin composition of the invention and a PAEK polymer is strong.

[0040] In accordance with a third aspect of the invention there is provided an article in which an epoxy resin composition of the present invention is in contact with a PAEK polymer, for example PEEK. Such an article could be, for example, a wire used for the winding of a motor, in which the epoxy resin composition and the PAEK polymer together form the casing for the conductive core. Many other uses of the invention are possible, as described above.

[0041] In accordance with a fourth aspect of the invention there is provided a method of making an article of the third aspect, by carrying out the process of the second aspect while the precursor composition is in contact with already-formed PAEK article. In some embodiments the contacting surface of the PAEK polymer is subjected to a corona discharge treatment prior to coming into contact with the epoxy resin precursor composition. The purpose is to still further enhance the adhesion between the epoxy resin composition and the PAEK.

[0042] The invention will now be further described, by way of example, with reference to the accompanying figures, in which:

[0043] Description of figures

[0044] Figure 1 shows diagrammatically the set-up for wire pullout tests examining the adhesive properties of an epoxy resin composition of the invention under tensile load, in comparison to a competitor epoxy resin product;

[0045] Figure 2 shows the results of the wire pullout tests; and

[0046] Figure 3 shows the results of lap shear tests examining the adhesive properties of a composition of the invention under shear load, in comparison to a competitor product.

[0047] Epoxy resin compositions tested

[0048] In the tests described below an epoxy resin composition of the invention was prepared, along with a commercial epoxy resin composition DAMISOL 3500 LoV (Trade Mark), available from Synflex Group, of Germany ('competitor resin'), and their properties were assessed and compared.

[0049] The composition of the epoxy resin composition of the invention is as follows:

[0050] Epoxy-containing component: Hardener component:

[0051] To the composition described above was added a phase separating carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber, as toughening agent, in the following weight percentage based on the total composition.

[0052] Thus the epoxy resin composition contained 6.6 wt% toughening agent; and 93.6 wt% of epoxy-containing component and hardener component in combination.

[0053] The composition prior to curing has a dynamic viscosity of 230 mPa.s under the testing conditions described above.

[0054] Curing may be carried out under the following conditions: a cure cycle of 3 hours at 80 °C, followed by a post-cure for 8 hours at 160 °C. The 8 hour post-cure can be shortened if necessary.

[0055] Following curing the resulting epoxy resin composition was found to have the following properties (using test methods as described above):

[0056] Glass transition temperature (Tg) - 182.3 ± 1 °C

[0057] Fracture energy - 342 ± 29 J / m2

[0058] Thermal Ageing - survives thermal ageing for at least 300 days at 200 °C

[0059] Low cure shrinkage (2-part system) - less than 5% of the starting (pre-cure) volume - can be assessed by a volumetric displacement method or other simple available methods

[0060] Adhesion to a low surface energy polymer, polyether ether ketone (PEEK). The epoxy resin composition has very good adhesion to PEEK, and which is maintained at high temperature, up to 180 °C. If wished the adhesion can be enhanced by appropriate preparative treatment of the polymer, such as corona treatment or plasma treatment. The tests described below examine the adhesion property.

[0061] The competitor resin DAMISOL 3500 LoV is sold for the impregnation of medium and low voltage industrial motors. The supplier Synflex states that due to the very good electrical, mechanical and thermal properties of DA ISOL 3500 LoV it is also suitable for traction motors and transformers. DAMISOL 3500 LoV is a homologous curing epoxy resin based on DGEBA as the major component. Further components present in lesser amounts are 1,6-hexanediol diglycidyi ether (HDDGE) and p-tert butylphenyl glycidyl ether (reactive diluents) and trichloro(n,n- dimethyloctylamine)boron (hardener). The datasheet states that the glass transition temperature of DAMISOL 3500 LoV is 122°C.

[0062] Example Set 1 Single wire pullout tests

[0063] 1.1 Preparative method.

[0064] 75 mm samples of PEEK coated copper wire were cut from a spool of wire provided by Victrex pic, UK. The ends of the wire were then flattened and squared using a belt sander. The wire had a nominal cross section of 4.2 mm X 3.5 mm. The surface of the wire was wiped clean with acetone solvent. Wires were held in the centre of a cylindrical polytetrafluoroethylene (PTFE) mould with an internal diameter of 25 mm and a height of 20 mm. Care was taken to ensure that the bottom of the wire touched the bottom of the cylindrical mould. Resin (i.e. 'epoxy resin composition' as defined and discussed above) prepared according to the present invention and a competitor resin prepared in accordance with the manufacturer's recommendation, successively, was then carefully decanted into the cylindrical mould. The moulds were then transferred to an oven and the resin was cured according to the relevant cure cycle.

[0065] 1.2. Test protocol:

[0066] Single wire pullout tests were conducted. The test method, which is based on methods developed to determine adhesion of carbon fibres in composites, is shown in Figure 1. The wire is pulled out under quasi-static loading conditions using a universal tensile testing machine. The maximum pullout load was recorded for each test. A temperature calibrated environmental test chamber was used to test specimens at different temperatures. Samples were tested at 20°C i.e. lab conditions; at 150°C, as an indication of normal motor operating conditions, i.e., and at 180°C, as an indication of extreme motor operating condition. The environmental test chamber used can maintain a prescribed temperature within ±1°C. Samples were preheated in the environmental chamber and preliminary investigations indicated that allowing 10 minutes between test set up and test was sufficient to ensure thermal equilibrium throughout the chamber.

[0067] 1.3. Results:

[0068] The results of these tests are shown in Figure 2. Several salient points arise from inspection of these results. Firstly, at 20°C, the pullout loads are high - between 1.5 and 2 kN for both resins. Secondly at 150°C and 180°C, indicative of normal and extreme motor conditions, there is a catastrophic, order of magnitude, loss in pullout load for the competitor resin. Indeed, a closer inspection of these samples indicated that there was likely a complete loss of adhesion and the load being recorded was merely that required to overcome the sliding friction between the wire and the resin. Finally, the resin of the invention maintains the same performance at 150°C, i.e. normal motor operating conditions as at 20°C. Under extreme motor operating conditions, 180°C, some loss in performance is noted. However, it should be noted that a significant debonding event was still observed in the load displacement, indicating that the resin is still bonded to the wire at this temperature.

[0069] Example Set 2 Single Lap Shear tests

[0070] Single Lap Shear tests are a commonly used method in industry to quickly discriminate between the performance of different resins and adhesives.

[0071] 2.1. Preparative method:

[0072] Steel-PEEK lap shear specimens bonded with epoxy resin of the present invention or by the competitor resin - in each case the same resins as described above and which were used for the single wire pullout tests - were prepared according to the following protocol.

[0073] Coupons of C45 tool steel with dimensions 100 mm X 20 mm x 6 mm were cut from stock. Any mill scale was removed, and the resultant surface was sanded and subsequently degreased using a commercially available steel degreasing agent. The samples were then wiped clean with methyl ether ketone (MEK) solvent. In parallel to this, coupons of 450G PEEK with the same dimensions were machined from plaques provided by Victrex pic, UK. These were cleaned using MEK solvent. To enhance the bondability of the PEEK, and to demonstrate retention of performance at elevated temperatures, each PEEK bonding surface was subjected to a corona surface treatment using a handheld BD-20AC Laboratory Corona Tester (Electro-Technic Products, USA) for 60 seconds.

[0074] Adhesive bonding of the steel and PEEK plaques using the respective epoxy resin took place within one hour of preparing both the steel and PEEK surface for bonding. It is important to note that the same surface pretreatment was carried out for both the resin of the invention and for the competitor resin, so there was no unfair advantage afforded when comparing the results.

[0075] Bonding lap shear specimens with the low viscosity resins proved initially problematic. Resin tended to flow out of the desired bonding area. To maintain the resin within the bond area, both the resin of the invention and the competitor resin were modified to behave in a thixotropic manner with the addition of 0.5 wt% fumed silica (Aerosil R-202 (Trade Mark), from Evonik, Germany). Previous research has indicated that such a low loading of fumed silica has a negligible impact on the mechanical properties of an epoxy resin.

[0076] Samples were bonded in a specially designed fixture to give a bond cross sectional area of 20 mm X 20 mm and a nominal bond-line thickness of 0.3 mm. The lap shear specimens were cured according to the recommended cure cycles. At least five specimens for each test temperature and substrate combination were prepared for testing.

[0077] 2.2. Test protocol:

[0078] Single Lap Shear tests were conducted based on ASTM D10022 using a universal testing machine and environmental chamber as described in Section 1.2. A cross-head displacement rate of 1 mm / min was prescribed. As for the single wire pullout, batches of samples were preheated in the oven to reduce waiting time between tests. Offset tensile grips were utilised to ensure the correct alignment of specimens under load, minimising the peeling load.

[0079] 2.3. Results:

[0080] The results of the PEEK-steel lap shear tests are presented in Figure 3. Tests were conducted at 20°C, 120°C and 150°C. It observed that the failure locus of the resin (adhesive) bond was always at the steel-adhesive interface. This implies that both the cohesive strength of the resin and the interfacial strength of the PEEK-adhesive interface is greater than the measured experimentally observed value. Secondly, it was noted that the performance of the resin of the invention at 20°C is far superior to that of the competitor resin. At a test temperature of 150°C, i.e. normal motor operating conditions, the measured lap-shear strength of the resin of the invention is still almost 1 MPa, while for the competitor resin a value of 0.015 MPa was measured for one sample. The rest of the specimens manufactured using the competitor resin failed either during transport to the test machine site despite being carefully packaged and transported, or during the preheat step prior to testing. It is important here to reflect that the lap shear specimens prepared using the resin of the invention were subjected to the exact same transportation and preheat protocols, and none of these samples failed prematurely. This is a demonstration of the durability and resilience of the resin of the invention when compared to the much more brittle competitor resin.

Claims

CLAIMS1. An epoxy resin composition which is the product of a reaction mixture comprising: an epoxy-containing component which comprises from 35 to 60 parts by weight of a cycloaliphatic epoxy compound and from 40 to 65 parts by weight of a bisphenol epoxy compound, wherein the epoxy-containing component constitutes from 35 to 65 weight percent of the total weight of the epoxy resin composition; a hardener able to react with the epoxy-containing component, wherein the hardener constitutes from 35 to 65 weight percent of the total weight of the epoxy resin composition; an accelerator for the reaction(s) between the epoxy-containing component and the hardener, wherein the accelerator constitutes from 0.2 to 5 weight percent of the total weight of the epoxy resin composition; and a toughening agent, wherein the toughening agent constitutes from 1 to 30 weight percent of the total weight of the epoxy resin composition.

2. An epoxy resin composition as claimed in claim 1 wherein the cycloaliphatic epoxy compound comprises a diepoxide of a cycloaliphatic ester of a dicarboxylic acid and / or a compound of the 3'4'-epoxycyclohexanemethyl-3,4-epoxycyclohexylcarboxylate class.

3. An epoxy resin composition as claimed in claim or 2 wherein the bisphenol epoxy compound comprises an epoxidised derivative of 4,4'-dihydroxydiphenyl dimethyl methane and / or an epoxidised derivative of bis(4-hydroxyphenyl) methane.

4. An epoxy resin composition as claimed in any preceding claim wherein the hardener is an anhydride compound, for example a partially or completely hydrogenated phthalic anhydride or a derivative thereof.

5. An epoxy resin composition as claimed in any preceding claim wherein the accelerator constitutes from 0.5 to 3 weight percent of the total weight of the epoxy resin composition.

6. An epoxy resin composition as claimed in any preceding claim wherein the accelerator comprises an amine, for example dicyandiamide, tertiary amines, and imidazole or an imidazole derivative; or a metal halide Lewis acid; a metal carboxylate; or a boron containing accelerator.

7. An epoxy resin composition as claimed in any preceding claim wherein the toughening agent constitutes from 3 to 12 wt% of the total weight of the epoxy resin composition.

8. An epoxy resin composition as claimed in any preceding claim wherein the toughening agent forms a secondary phase within a matrix formed by the epoxy-containing component; and is elastomeric, and capable of inhibiting or limiting crack growth.

9. An epoxy resin composition as claimed in any preceding claim wherein the toughening agents is a reactive liquid rubber; for example a reactive liquid butadiene-acrylonitrile polymer terminated with reactive group(s), for example selected from carboxyl, epoxy, amine, hydroxyl and vinyl.

10. An epoxy resin composition as claimed in any preceding claim, having a glass transition temperature (Tg) of at least 160°C as determined by a method as described herein.

11. An epoxy resin composition as claimed in any preceding claim, having a fracture energy of at least 150 J / m2as determined by a method as described herein.

12. An epoxy resin composition as claimed in any preceding claim, which survives thermal ageing at 200 °C for at least 100 days with loss of less than 10% of its weight as determined by a method as described herein.

13. An epoxy resin composition as claimed in any preceding claim, having shrinkage on curing of less than 5% of the starting (pre-cure) volume.

14. A process for making an epoxy resin composition as claimed in any preceding claim, which process comprises the steps of mixing the epoxy component, the hardener and the accelerator to form a precursor epoxy resin composition at a temperature at which curing does not take place, holding the precursor composition in a desired shape, and subjecting the precursor composition to a curing temperature.

15. A precursor epoxy resin composition as defined in claim 14, which has a dynamic viscosity in the range from 150 mPa.s to 1000 mPa.s as determined by a method as described herein.

16. An article in which an epoxy resin composition as claimed in any of claims 1 to 13 is in adhesive contact with a low surface energy engineering thermoplastic polymer, for example a polyaryl ether ketone polymer.

17. An article as claimed in claim 16 wherein adhesive contact is maintained up to at least 120 °C.

18. An article as claimed in claim 16 or 17 in which the article is for use in electrical equipment operative in the automotive, aerospace, power generation, transmission and storage, and marine and naval industries, and other vehicular or power generation, transmission and storage environments.

19. A method of making an article as claimed in claim 17 or 18 by carrying out a process as claimed in claim 14 while the precursor composition is in contact with an already-formed article of the low surface energy engineering thermoplastic polymer.

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