Title of Invention	METHOD FOR PRODUCING A FRICTION MATERIAL FOR A FRICTION GEAR IN A CLUTCH
Abstract	The invention relates to a method of making a friction material, more particularly a friction crown for a clutch friction disc for dry operation, and comprising an operation of impregnating a filler which is based in particular on mineral fibres such as glass fibre, by means of an aqueous impregnating cement containing SBR latex and at least one compound of the anionic surfactant type.

Full Text

FORM 2 THE PATENTS ACT 1970 [39 OF 1970] & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See Section 10; rule 13] "METHOD FOR PRODUCING A FRICTION MATERIAL FOR A FRICTION GEAR IN A CLUTCH" VALEO, a French company, 43 rue Bayen, F-75017 Paris, France, The following specification particularly describes the invention and the manner in which it is to be performed: The present invention relates to a method of making a friction material, and more particularly a friction crown for a clutch friction disc, and also to a friction crown. More particularly, the friction crown is for a clutch friction disc and is of the dry friction type. Such a friction crown is made from a filler based in particular on mineral fibres such as glass fibres, which provide strength against centrifugal force, rubber in order to obtain a good coefficient of friction, various additives, and a binder, which is in practice a phenolic resin, in order to render the whole.cohesive. During manufacture of the crown it is usual to make use of chlorinated solvents, in particular to dissolve the rubber. The problem that arises in the use of such chlorinated solvents is their noxious character, which consequently makes it necessary to arrange for confinement and recuperation in order to avoid any contact with the operators and any escape into the atmosphere. In order to overcome this problem it is known to replace the chlorinated solvents with water. This makes it necessary to use latex rather than dry rubbers. In practice an aqueous cement is made which results from the mixture, in water, of phenolic resins, additives, particularly in the form of powders, and latex. This cement is then used to impregnated a filler which consists of mineral and other fibres which serve to form a blank, which is subsequently cured under pressure to constitute a friction crown. The Applicant has found that the aqueous cement has a viscosity which increases rapidly over time. The result of this is firstly, that the time during which the cement can be used is short, and secondly, there are variations in the friction and wear resistance characteristics of the friction crowns eventually produced. Addition of phenolic resin in powder form into the mixture containing latex causes the water of constitution in the latex to be absorbed, leading to rapid increase in viscosity and making the cement unsuitable to impregnate a filler. n the document GB-A-2 054 626, the aqueous impregnating cement contains at least one compound of the surfactant type. Nevertheless, a problem can arise because the particles of the cement may aglomerate. In order to overcome this problem, the invention proposes the use of anionic surfactant compounds to stabilise the agueous mixtures for dutch crowns containing mixturres.of latex.phenoiic resins. formol melamine and various organic powders and minerals. More precisely, the present invention is directed to a method of making friction material, more particularly a friction crown for a clutch friction disc for dry operation, comprising an operation of impregnating a filler, based in particular on mineral fibres such as glass fibres, with an aqueous impregnating cement. In accordance with the invention, the aqueous cement contains at least one compound of the anionic surfactant type. Thanks for the invpnlion the anionic surfactant compound carries negative actlge charges and thus prevents the various negatively charged powder particles from aggregating. According to further features of the invention which may be taken separately or in all their technically possible combinations: - the aqueous cement contains a surfactant compound of the ionic type and a surfactant compound of the non-anionic type. - the compound of the anionic surfactant type is chosen from the group consisting of phosphates, polyphosphates, pyrophosphates, sulphates, sulphonates, and carboxylates. - the cation of the compound of the anionic surfactant type is chosen amojarng the elements sodium+ , potassium+ , ammonium+ , calcium + + and amines. - the compound of the non-ionic surfactant type is chosen from the group consisting of hydroxyls and ethers. - the compound of the non-ionic surfactant type is a polyglycol ether. - the compound of the anionic surfactant type is a mixture of polyphosphates of potassium. - the compound of the anionic surfactant type is a mixture of polyphosphates of sodium. - the compound of the anionic surfactant type is a mixture of pyrophosphates of potassium. - the compound of the anionic surfactant type is a mixture of pyrophosphates of sodium. - the making of the impregnating cement includes formation of a preliminary mixture including additives, phenolic resin, the compound of the surfactant type, and water, and adding styrene butadiene rubber latex, usually called SBR latex, to this preliminary mixture. - the latex is of the SBR type. r - the percentage by weight of compounds of the anionic surfactant type with respect to the SBR dry latex is less than 6% - the percentage by weight of compounds of the anionic surfactant type with respect to the dry mixture is less than 5%. - the percentage by weight of compounds of the anionic surfactant type with respect to the finished liner is less than 3%. - the percentage by weight of compounds of the non-ionic surfactant type with respect to the SBR latex is less than 3%. - the percentage by weight of compounds of the non-ionic surfactant type with respect to the dry mixture is less than 2%. - the percentage by weight of compounds of the non-ionic surfactant type with respect to the finished liner is less than 1.2%. - the filler comprises at least one textured elementary glass thread. - the texture of the textured glass thread is 600 to 5000 tex. - the filler inlcudes metallic thread. - the friction crown for a clutch friction disc for dry operation contains filler consisting basically of mineral fibres such as glass fibres, organic fibres such as fibres of polyacrylonitryl or derivatives, an aqueous cement containing additives, and at least one compound of the surfactant type of the anionic type. It has been found that, thanks to the invention, the period of time over which the cement is able to be used in the impregnation of the filler which constitutes the skeleton and the blank for the crown, is considerably increased: it is multiplied by a factor of the order of 10. In addition, stability is noted in the physical-chemical characteristics of the cement during the whole of the utilisation time, which has the beneficial result that the final product, that is to say the friction crown, is of improved and constant quality. Equally, the surfactant compounds, acting as a blocker, protect the system by emulsifying the SBR latex while emulsifying the other additives of the cement. Further features and advantages of the invention will appear in the description of examples of practical applications of the invention with reference to the attached drawings, in which: - Figure 1 shows the variation in the viscosity of an SBR latex base mixture maintained at 20o c - Figure 2 shows the variation in the viscosity of an SBR latex base mixture maintained at 45°C. - Figure 3 shows the application of a surfactant compound on a negatively charged powder. Crowns for a dry friction clutch disc, for a motor vehicle, are made in the way that will be described below. An aqueous cement is made by mixing various compounds. Preferably the mixture is made in two steps. A preliminary mixture is first made by mixing additives, especially in the form of powders of phenolic and melamine-formol resins with water. Also introduced into this preliminary mixture are anionic surfactant compounds such as for example phosphates, sulphates, sulphonates or carboxylates. The cations used may be sodium+ , potassium+ , With reference to Figure 3, since the greater part of the powders 1 which are used are negatively charged, the anionic surfactant compound 2, carrying a negative active charge 5, thereby produces a barrier 3 which prevents aggregation of the various powder particles. Similarly, this surfactant compound acts by being adsorbed on the powders, in particular those with a high specific surface Isuch as lamp black, finely ground resin and so on). In this way it forms a new powder-water interface 4, and thereby prevents- absorption of water by the compounds with high specific surface, and particularly latex, so that it thereby reduces the risks of destabilisation of the latex. Preferably, the surfactant compound also acts as a blocker, by maintaining the pH of the mixtures. The latex, which is sensitive to any variation in pH, thus has its stability reinforced. The said anionic surfactant components are introduced in an amount of 3% maximum by weight of the aqueous mixture. This percentage depends on the quantity of powders, on their electrical charges, and on their specific surfaces, and also on the quantity of latex and the pH of the latex. The friction crown obtained in this way has a percentage by weight of anionic surfactant compound with respect to the dry SBR latex which is less than 6%, either a percentage by weight of anionic surfactant compound with respect to the dry mixture, that is to say dry cement without the filler, which is less than 5%, or, a percentage by weight of the anionic surfactant compound with respect to the finished liner, including the fibre and the dry cement, which is less than 3%. Preferably, non-ionic surfactant compounds can also be introduced into the preliminary mixture, for example surfactant compounds having hydroxyl (R-OH) and ester (R-O-R) groups. These non-ionic surfactant compounds act, above all, to reduce the mixing time of the cement, so as to obtain homogeneous dispersion. They are perfectly compatible with anionic surfactant compounds, with which they have a strong synergy. These non-ionic surfactant compounds are preferably introduced in a maximum amount of 1 % by weight of the aqueous mixture. The friction crown thus obtained has a weight percentage of non-ionic surfactant compound with respect to the dry SBR latex which is less than 3%, or a weight percentage of non-ionic surfactant compound with respect to the dry mixture, that is to say dry cement without the filler, which is less than 2%, or again, a percentage by weight of non-ionic surfactant compound with respect to the finished liner comprising fibre and dry cement, which is less than 1.2%. In one preferred mode according to the invention, phosphate is introduced into the mixture (preferably in the form of a mixture of polyphosphates of sodium or potasium or in the form of a mixture of sodium or potassium polyphosphates) and water. The mixing time is determined in such a way that the mixture is homogeneous and the phenolic resin is fully dissolved. In order to form the final mixture, SBR latex is introduced into the preliminary mixture, in such a way as to obtain a homogeneous mixture. A filler consisting basically of mineral fibres such as glass fibres, organic fibres such as polyacrylonitryl or derivatives thereof, and metallic fibres in an amount which may be up to 75% of the weight of the crown, is plunged into the cement in order to become impregnated with it. The impregnated filler is then dried. A friction crown blank is obtained by distributing the impregnated filler in lobes between an outer diameter and an inner diameter. The blank is then cured under pressure. A post-curing step to stabilise the product is preferably carried out. Various machining operations such as rectification, drilling, and subsequent anti-dust treatment, are carried out. Crowns obtained in this way are adapted to be mounted on either side of an axially elastic support member of a clutch friction disc. More precisely, five batches of cement are made in the manner explained above, from constituents which are in proportions as indicated in the tables given below, first of all in parts by weight, and then in percentages by weight with respect to the SBR dry latex, the aqueous mixture and the dry mixture respectively,, and finally, in percentage by weight in the clutch liner. Measurements of the change in viscosity of the mixtures are performed using a "Brookfield" viscometer at temperatures of 20°C and 45°C. Measurements of changes in viscosity over time were carried out on these five mixtures. On the one hand, a Control mixture not containing any surfactant compounds in any form whatever. On the other hand, four mixtures containing surfactant compounds, here in the form of a mixture of sodium polyphosphates. The fifth mixture, represented by formula 4, contains in addition, as compared with the other mixtures, a non-ionic surfactant compound based on polyglycol ether. 5 Parts by weight of ingredients of the impregnating cement (base 100 for dry- latex) Ingredients Control Formula 1 Formula 2 Formula 3 Formula 4 SBR Latex 100 100 100 100 100 10 Powders 400 400 400 400 400 Sodium Polyphosphate 0 1 5 25 1 Polyglycol ether 0 0 0 0 1 15 Total Water in the mixture 210 210 210 210 210 20 Percentage by weight of surfactants with respect to the dry SBR latex Ingredients Control Formula 1 Formula 2 Formula 3 Formula 4 Sodium Polyphosphate 0 It 5% 2SV 1* 25 Polyglycol ether 0 0 0 0 1* Percentage by weight of surfactants with respect to the aqueous solution 30 Ingredients Control Formula 1 Formula 2 Formula 3 Formula 4 Sodium Polyphosphate 0 0.14* 0.7* 3.5* 0.14* Polyglycol ether 0 0 0 0 0.14* Percentage by weight of surfactants with respect to the dry mixture Ingredients Control Formula 1 Formula 2 Formula 3 Formula 4 Sodium Polyphosphate 0 0.2V 1* 5* 0.2% Polyglycol ether 0 0 0 0 0.2% Percentage by weight of surfactants in the clutch liner Ingredients Control Formula 1 Formula 2 Formula 3 Formula 4 Sodium Polyphosphate 0 0.12% 0.6% 3% 0.12% Polyglycol ether 0 0 0 0 0.12% Powder mixing time to obtain a homogeneous 15 min. 12 min. 12 min. 10 min. 8 min. dispersion As can be seen in the above table, Formula 4, containing non-ionic surfactant compound additional to the other formulae, requires a substantially smaller mixing time than the other formulae, which do . not have the non-ionic surfactant compound, to obtain a homogeneous dispersion. The results of these measures are shown on the graphs in Figures 1 and 2.; Given that in practice, correct impregnation of a filler of the type mentioned above is obtained with a cement having a viscosity of less than 80 poise, and optimally less than 40 poise, it will be seen in the graphs in Figure 1 that, for a mixture maintained at 20°C, the utilisation time of the Control mixture, counted from the time when it is made in accordance with the state of the art, that is to say using a phenolic resin and without addition of surfactant compound, we have a very short utilisation time, of less than 50 minutes (taking 80 poise as the maximum limit). For an optimum viscosity of less than 40 poise, and with a Control mixture held at 20°C, Figure 1 shows us that the utilisation time falls well below 40 minutes. By contrast, under the same conditions, the mixtures represented by Formulae 1, 2, 3 and 4 have a viscosity which is virtually unchanged at least during the four hours from the end of the preparation of the mixture. With reference to Figure 2, for a Control mixture held at 45°C and made in accordance with the state of the art, that is to say using a phenolic resin and without any surfactant compound, we have a utilisation time of the Control mixture, counted from the time it is made, which is very short and is less than 15 minutes (with a maximum limit of 80 poise). Considering a maximum viscosity of 40 poise, and with a Control mixture held at 45°C, Figure 2 shows us that the utilisation time falls to 5 minutes. By contrast, under the same conditions, the mixtures represented by Formulae 1, 2, 3 and 4 reach the permitted viscosity limit of 80 poise between 90 minutes and 110 minutes. Complementary tests, not described here, have made it possible to define a preferred range corresponding to a percentage by weight of the surfactant compounds with respect to the SBR dry latex, which is between 0.1% and 3%, enabling the constituent mixture of the cement to attain ideal emulsifying properties for good impregnation of the filler, with impregnation which is more regular and more thorough, which leads to more reliable performance. The advantages of the invention are, firstly, richer impregnation of the filler, and secondly, faster drying of the latter after impregnation. The invention has the advantage that it enables a composite filler, comprising at least one elementary textured glass thread (preferably from 1 to 3 textured threads of 600 to 5000 tex) to be impregnated. With the cement made in accorance with the invention, better impregnation is obtained with this type of thread, which results in greater strength of the clutch crown made from such an impregnated filler against the effects of centrifugal force. It can be seen that the invention gives a notable improvement to centrifugal strength, and that a particularly major feature is that it combines textured fibre and aqueous cement including surfactant compounds. WE CLAIM: 1. A method of making friction material, more particularly a friction crown for a clutch friction disc for dry operation, comprising an operation of impregnating a filler, based in particular on mineral fibres such as glass fibres, with an aqueous impregnating cement, wherein the aqueous cement contains at least one compound of the surfactant type together with additives, characterised in that the compound of the surfactant type is anionic. 2. A method of making a friction material as claimed in claim 1, wherein the aqueous cement contains a surfactant compound of the non-ionic type and a surfactant compound of the anionic type. 3. A method of making a friction material as claimed in claim 1 or Claim 2, wherein the compound of the anionic surfactant type is chosen from the group consisting of phosphates, polyphosphates, pyrophosphates, sulphates, sulphonates, and carboxylates. 4. A method of making a friction material as claimed in any-one of Claims 1, 2 or 4, wherein the cation of the compound of the anionic surfactant type is chosen among the elements sodium +, potassium +, ammonium +, calcium + + and amines. 5. A method of making a friction material as claimed in claim 1 or Claim 2, wherein the compound of the non-ionic surfactant type is chosen from the group consisting of hydroxyls and ethers. 6. A method of making a friction material as claimed in anyone of Claims 1, 2 or 5, wherein the compound of the non-ionic surfactant type is a polyglycol ether. 7. A method of making friction material as claimed in anyone of Claims 1 to 4, wherein the compound of the anionic surfactant type is a mixture of polyphosphates of potassium or a mixture of polyphosphates of sodium, or a mixture of pyrophosphates of potassium, or a mixture of pyrophosphates of sodium. 8. A method as claimed in anyone of the preceding Claim, wherein the making of the impregnating cement includes formation of a preliminary mixture including additives, phenolic resin, the compound of the surfactant type, and water, and adding SBR latex to this preliminary mixture. 9. A method of making a friction material as claimed in claim 8, wherein the latex is of an SBR type. 10. A method of making a friction material as claimed in any one of Claims 1, 2, 3, 4, 7, 8 or 9, wherein the percentage by weight of compounds of the anionic surfactant type with respect to the SBR dry latex is less than 6%. 11. A method of making a friction material as claimed in any one of Claims 1, 2, 3, 4, 7, 8 or 9, wherein the percentage by weight of compounds of the anionic surfactant type with respect to the dry mixture is less than 5%. 12. A method of making a friction material as claimed in anyone of Claims 1, 2, 3, 4, 7, 8 or 9, wherein the percentage by weight of compounds of the anionic surfactant type with respect to the finished liner is less than 3%. 13. A method of making a friction material as claimed in anyone of Claims 1, 2, 5, 6, 8 or 9, wherein the percentage by weight of compounds of the non-ionic surfactant type with respect to the SBR latex is less than 3%. 14. A method of making a friction material as claimed in anyone of Claims 1, 2, 5, 6, 8 or 9, characterised in that the percentage by weight of compounds of the non-ionic surfactant type with respect to the dry mixture is less than 2%. 15. A method of making a friction material as claimed in anyone of Claims 1, 2, 5, 6, 8 or 9, wherein the percentage by weight of compounds of the non-ionic surfactant type with respect to the finished liner is less than 1.2%. 16. A method as claimed in anyone of the preceding Claims, wherein the filler comprises at least one textured elementary glass thread. 17. A method as claimed in claim 16, wherein the texture of the textured glass thread is 600 to 5000 tex. 18. A method as claimed in anyone of the preceding wherein the filler includes metallic thread. Dated this 20th day of November,2000. [RITUSHKA NEGI] OF REMFRY & SAGAR ATTORNEY FOR THE APPLICANTS

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