Method of manufacturing a friction lining

0001The present invention relates to the method of manufacturing a friction lining, in particular for clutches or brakes of motor vehicles. Such linings are usually composed of fibers, binders and fillers. They are generally manufactured in the form of crowns with a rectangular section or fragments of such crowns.

0002A friction lining comprises a friction face intended to come into contact with another part and a bearing face making it possible to fix the lining to a support, in particular to a clutch disc.

0003Usually, such fillings are manufactured by mixing the components and then performing a “baking” which most of the time corresponds either to a hot pressing step or to an injection step. The parts obtained are then machined, generally by grinding both sides, to obtain a part with parallel faces. Under certain operating conditions of a vehicle, it is observed that such linings can lead to undesired behavior, such as in particular chattering (phenomenon of jerks or jerks), for example when cold. Such chatter can be observed, for example, during the first kilometers when the vehicle is started after having spent a night in the cold.

0004In order to remedy this drawback, a friction lining and its production method have been disclosed in the application. <patcit id="pcit0001" dnum="FR2742503"><text>FR 2 742 503</text></patcit>. According to the method described, a friction lining, in particular for friction clutches or for brakes, consists of fibers, binders and fillers and is obtained by mixing these materials and then by the following sequence of processing steps:<ul id="ul0001" list-style="dash" compact="compact"><li>a forming step, also called a “curing step”, such as for example a hot pressing or an injection molding,</li><li>a crosslinking step (also called “post-curing”, “post-crosslinking” or else “post-polymerization”), for example a curing process,</li><li>a surface treatment step such as a process of grinding at least the friction surface of the lining,</li><li>a crosslinking step (ie a second “post-crosslinking” step) such as a curing process.</li></ul>

0005It can be seen that such a process, although advantageous in terms of the property of the fillings obtained and in particular with regard to cold chattering, adds at least one post-crosslinking step with respect to a usual process for manufacturing a lining. This results in longer cycle times and a significant additional cost.

0006This elongation is important because the post-crosslinking step is generally the longest in the process and usually reaches several hours.

0007The documents <patcit id="pcit0002" dnum="DE4205645"><text>FROM 42 05 645</text></patcit> and <patcit id="pcit0003" dnum="FR2706556"><text>FR 2 706 556</text></patcit> also disclose a method of manufacturing a friction lining. However, the temperatures used during the various stages do not make it possible to obtain a simplified process and to obtain sufficient carbonization of the friction lining.

0008The aim of the present invention is to remedy the above drawbacks and to provide a less complex and more economical process.

0009This object is achieved by a method of manufacturing a friction lining comprising a friction face and a bearing face, in particular for clutches or for brakes, characterized in that it comprises the following successive steps:<ol id="ol0001" compact="compact" ol-style=""><li>a) mixture of fibers, binders and fillers,</li><li>b) forming the mixture at a temperature T_F between 180 and 220 ° C, so that the binders are not completely polymerized, followed without significant modification of the structure of the binder by:</li><li>c) removal of material on at least the friction face, this step c) comprising at least one rectification.</li><li>d) heat treatment of post-crosslinking of the binders at a temperature T_T greater than temperature T_F of step b), the temperature difference between the temperature T_T and T_F being at least 30 ° C</li></ol>and in that it does not include a subsequent step of sizing after step d) of post-crosslinking.

0010Thanks to this process, fillings are obtained where the problems of cold chatter are very significantly reduced with cycle times lower than those of the disclosed process. <patcit id="pcit0004" dnum="FR2742503"><text>FR 2 742 503</text></patcit>.

0011Indeed, the inventors have surprisingly found that by moving the surface treatment step before the post-crosslinking step, one could do without one of the two crosslinking steps disclosed by <patcit id="pcit0005" dnum="FR2742503"><text>FR 2 742 503</text></patcit>.

0012According to the invention, the forming step b) is directly followed, without significant modification of the structure of the binders, by a step of removing material c), unlike the process disclosed by <patcit id="pcit0006" dnum="FR2742503"><text>FR 2 742 503</text></patcit> wherein the material removal step follows a crosslinking step which profoundly changes the structure of the binder after the forming step.

0013According to one embodiment of the invention, the material removal step c) follows step b) without an intermediate step. According to another embodiment, the intermediate product resulting from step b) is stored transiently before the implementation of step c).

0014In the context of the invention, the term "forming at a temperature T_F chosen so that the binders are not completely polymerized ”, a forming step after which a significant part of the binders has crosslinked, but where a subsequent treatment can still lead to a rearrangement of the structure of the binders, in particular by post-crosslinking , diffusion or any other mechanism. Such a rearrangement can in particular be identified by observation under a scanning electron microscope (SEM), by measurement by gas phase chromatography (GC) or differential scanning calorimetry (DSC) of the surface and the core of the lining or any other technique known to the industry. skilled in the art. Those skilled in the art are thus able to identify in a simple manner the conditions of a forming cycle after which the binders are not completely polymerized. The main parameter of such a cycle is the temperature T_F, but parameters such as the time of the forming cycle, the pressure applied, possible degassing during the forming step can also play a role. The post-crosslinking treatment of step d) leads to a significant rearrangement of the structure of the binder, observable by the techniques mentioned above.

0015The inventors have been able to observe that the structure of the material is substantially identical when a filling is produced with one or the other of these methods;

0016In both cases, there is the formation of a granular surface layer which takes on the appearance of a crust. The grains formed typically have a size of 5 to 20 µm and can coalesce with each other. The crust may not cover the entire surface of the filling. Such a structure is easily observed with a Scanning Electron Microscope (SEM). In general, such a layer reaches between 50 and 100 μm in thickness.

0017The inventors were particularly surprised to find that the linings obtained with the process according to the invention retained their flatness after the post-crosslinking cycle, whereas it was commonly accepted that a post-crosslinking cycle, followed by a post-crosslinking cycle. surface treatment followed by a second post-crosslinking cycle, were necessary to obtain a lining of good quality, in particular flat, and exhibiting favorable cold chatter behavior.

0018According to embodiments of the invention:<ul id="ul0002" list-style="dash" compact="compact"><li>step a) comprises the impregnation of threads of continuous fibers in a cement comprising the binders dissolved and the fillers, followed by the production of a preform, and where step b) comprises hot pressing. Such a yarn generally comprises textured glass fibers. It can also comprise metallic threads, in particular copper and / or brass threads, as well as organic fibers, for example fibers of polyacrylonitrile or derivatives and / or aramid. Impregnation is generally carried out by dipping the wire in cement, in a manner known per se. In general, the prepreg yarn is arranged in the form of lobes on a support before the hot pressing step, for example according to the teaching of the document.<patcit id="pcit0007" dnum="WO2005040633A"><text>WO 2005/040633</text></patcit> ;</li><li>the binder is dissolved with an aqueous solvent. Such a method emerges for example from the teaching of the application<patcit id="pcit0008" dnum="WO00063577A"><text>WO 00/063577</text></patcit> ;</li><li>the binder is dissolved with an organic solvent, in particular trichlorethylene;</li><li>step b) and optionally step a) is (are) carried out during an injection process where the fibers are staple fibers. Such a method emerges for example from the teaching of the application<patcit id="pcit0009" dnum="WO02075175A"><text>WO 02/075175</text></patcit> or request <patcit id="pcit0010" dnum="WO03074899A"><text>WO 03/074899</text></patcit> ;</li><li>the binders comprise one or a mixture of resin (s) chosen from the list of epoxy, polyester, melamine, polysiloxane, phenolic, polyamide-imide, polyimide, polybenzimidazole, silicone;</li><li>material is also removed from the bearing face during step c);</li><li>during step c) between 0.3 and 1 mm of material thickness is removed from the whole of the treated face;</li><li>the post-crosslinking treatment is a heat treatment at a temperature T_T greater than temperature T_F of step b);</li><li>the temperature difference between the temperature T_T and T_F is of the order of 50 ° C;</li><li>temperature T_T is between 200 and 300 ° C, preferably between 220 and 280 ° C.</li></ul>

0019An operation to rectify the bearing face can be carried out at the end of the cycle, after step d), for example if the tolerances on the parallelism of the two faces are very severe.

0020In order to illustrate the invention in a non-limiting manner, the following examples are described: Clutch linings with an external diameter of 200 mm, an internal diameter of 137 mm and a thickness of 3.2 mm were produced from the constituents of the commercial reference F810 from VALEO MATERIAUX DE FRICTION where the fiber mixing step of binders and fillers is carried out according to the teaching of the patent application <patcit id="pcit0011" dnum="WO0063577A"><text>WO 00/63577</text></patcit>.

0021After impregnation with the aqueous cement, the threads are lobed on a support and placed in a mold to constitute a preform.

0022The step of forming the mixture is carried out by hot pressing the preform for 4 minutes at 200 ° C. During this step, high and low pressure cycles are applied alternating with degassing.

0023The lining EX1 is then surfaced to obtain parallel faces and its properties are measured.

0024The lining, EX2, has undergone a treatment of the type described in <patcit id="pcit0012" dnum="FR2742503"><text>FR 2 742 503</text></patcit>, comprising two post-crosslinking cycles of several hours between 240 and 260 ° C, separated by a surface grinding operation where 0.3 mm of thickness has been removed on each side.

0025The lining, EX3, underwent a treatment according to the invention, where a surfacing by grinding identical to that of EX2 was carried out after the forming step and followed by a post-crosslinking cycle of several hours between 240 and 260 ° C.

0026In these examples, the post-crosslinking cycle is carried out by passing through a tunnel oven where the linings circulate in stacks.

0027About ten tests were carried out for each type of seal.

0028It can be seen that all the linings thus obtained have flat and parallel faces.

0029Chatter tests were carried out for each type of seal according to a test procedure called "Global Judder test" during which the damping coefficient is measured during a vibration test for different temperature ranges, in particular between 40 and 60 ° C, between 80 and 140 ° C, between 150 and 250 ° C.

0030The test results are reported in the following table, which shows the average value (over 7 tests) and the maximum value of the damping coefficient for three temperature ranges and for the three examples of packing.<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="35mm" /><colspec colnum="2" colname="col2" colwidth="19mm" /><colspec colnum="3" colname="col3" colwidth="14mm" /><colspec colnum="4" colname="col4" colwidth="14mm" /><colspec colnum="5" colname="col5" colwidth="14mm" /><thead><row><entry align="center" valign="top">Temperature range</entry><entry align="center" valign="top">Value</entry><entry align="center" valign="top">EX1</entry><entry align="center" valign="top">EX2</entry><entry align="center" valign="top">EX3</entry></row></thead><tbody><row><entry morerows="1">40 - 60 ° C</entry><entry>average</entry><entry>- 0,01</entry><entry>- 0,07</entry><entry>- 0,05</entry></row><row><entry>maximum</entry><entry>0,17</entry><entry>0,10</entry><entry>0,01</entry></row><row><entry morerows="1">80 - 140 ° C</entry><entry>average</entry><entry>- 0,04</entry><entry>- 0,08</entry><entry>- 0,09</entry></row><row><entry>maximum</entry><entry>0,13</entry><entry>0,13</entry><entry>0,06</entry></row><row><entry morerows="1">150 - 250 ° C</entry><entry>average</entry><entry>0,12</entry><entry>0,11</entry><entry>0,20</entry></row><row><entry>maximum</entry><entry>0,36</entry><entry>0,25</entry><entry>0,36</entry></row></tbody></tgroup></table></tables>

0031Validation tests of the "Global Judder Test" have shown that the lower the damping coefficient in the 40 to 60 ° C range, the better the cold start behavior, and the more chattering behavior is avoided. cold.

0032There is thus a very significant improvement in a packing of type EX2 compared to a packing of type EX1. A lining according to the invention, of the EX3 type, has a further improved behavior compared to a lining of the EX2 type.

0033Tests on a vehicle have made it possible to confirm that the linings obtained with the process according to the invention have a particularly advantageous behavior with regard to the risks of cold chattering.

0034The invention is not limited to these types of embodiments.