Overcuring of brake lining

p0001The present invention relates to a friction lining, particularly for clutches or motor vehicle brake. Such liners are usually composed of fibers, binders and fillers. They are usually made in the form of rings with a rectangular section or fragments of such rings.

p0002A friction lining includes a friction face intended to come into contact with another part and a support face for securing the liner to a support, especially to a clutch disc.

p0003Usually, such linings are made by mixing the components and then performing a "cooking" which corresponds mostly to be a hot-pressing step is in an injection step. The parts obtained are then machined, usually by grinding both faces, to obtain a part with parallel faces. In certain operating conditions of a vehicle, it is found that such fillings can lead to unexpected behavior, such as a chatter (juddering or jerking), eg cold. Such chatter can be seen for example in the first kilometers when the vehicle is started after spending a night in the cold.

p0004To overcome this disadvantage, a friction lining and its method are disclosed in the application <patcit id="pcit0001" dnum="FR2742503"><text>FR 2742503</text></patcit>. According to the method described, a friction lining, in particular for friction clutches or brake, consists of fibers, binders and fillers and is obtained by mixing these materials and by the following sequence of processing steps:<ul><li>a step of forming, also known as "firing step", such as for example hot pressing or injection molding,</li><li>a crosslinking step (also called "post-baking", "post-cross-linking" or "post polymerization"), for example a curing process,</li><li>a surface treatment step such as a grinding process at least the friction surface of the lining,</li><li>a crosslinking step (or a second stage of "secondary crosslinking") such as a curing process. It is noted that such a method, although advantageous in terms of property obtained fillings and particularly in terms of cold chatter, add at least one post-crosslinking step compared to a conventional method of packing manufacture. This results in a longer cycle times and a significant additional cost. This extension is important because the post-crosslinking step is generally the longest of the process and usually reaches several hours. The object of the present invention is to overcome the above drawbacks and to provide a less complex and more economical process. This object is achieved by a method of manufacturing a friction lining comprising a friction face and a bearing face, in particular for clutches or brakes, comprising the following successive steps: <ol><li>a) mixing of fibers, binders and fillers,</li><li>b) forming the mixture to a temperature T_F between 150 and 250 ° C, preferably between 180 and 220 ° C, so that the binders are not fully polymerized, followed without significant change in the structure of the binder by:</li><li>c) removing material from at least the friction face, said step c) comprising at least one rectification.</li><li>d) treatment of post-crosslinking binders.</li></ol>By this method is obtained where the linings cold chatter problems are very significantly decreased with time cycles less than those of the disclosed method of <patcit id="pcit0002" dnum="FR2742503"><text>FR 2742503</text></patcit>. Indeed, the inventors have surprisingly found that by moving the step of surface treatment prior to post-crosslinking step, one could dispense with one of the two steps of crosslinking disclosed by <patcit id="pcit0003" dnum="FR2742503"><text>FR 2742503</text></patcit>. According to the invention, the shaping step b) is directly followed, without significant change in the binding structure, a material removal step c), unlike the method disclosed in <patcit id="pcit0004" dnum="FR2742503"><text>FR 2742503</text></patcit> wherein the material removal step follows a crosslinking step which profoundly modifies the binder after the step of forming structure. According to one embodiment of the invention, the material removal step c) follows without intermediate step of step b). According to another embodiment, the intermediate product from step b) is stored temporarily before the implementation of step c). In the context of the invention, the term "a forming at a temperature T_F chosen so that the binders are not fully polymerized ", a forming step after which a significant proportion of binders crosslinked, but where further processing can still lead to a rearrangement of the structure of binders in particular by postcrosslinking , dissemination or other mechanism. Such a rearrangement is particularly identifiable by observation under a scanning electron microscope (SEM) by measuring by gas chromatography (GC) or differential scanning calorimetry (DSC) of the surface and the core of the gasket or any other technique known to the the skilled person. The skilled person is able to identify simple conditions for forming cycle after which the binders are not fully cured. The primary outcome of such a cycle is the temperature T_FBut parameters such as the time of the forming cycle, the applied pressure, possible degassing during the shaping step may also play a role. The postcrosslinking treatment of step d) leads to a significant rearrangement of the binder structure, observable by the techniques mentioned above. The inventors have found that the structure of the material is substantially identical when producing a liner with one or the other of these processes; in both cases there is the formation of a layer of granular surface which takes the form of a crust. The grains formed typically have a size of 5 to 20 microns and may coalesce them. The crust may not cover the entire surface of the liner. Such a structure is easily observed in a scanning electron microscope (SEM). Generally such a layer reached between 50 and 100 microns thick. The inventors were particularly surprised to find that the trim obtained with the process according to the invention retained their flatness after the post-cure cycle, while it was recognized that a post-cure cycle, followed by a surface treatment and a second post-cure cycle, were needed to get a good trim, including flat and having a favorable behavior in cold chatter. As a result, a characteristic of the process of the invention is that it typically does not include a subsequent step of the ribs after step d) post-crosslinking. According to embodiments of the invention, taken separately or in combination.</li><li>step a) comprises impregnating son of continuous fibers in a cement comprising the binders dissolved and fillers, followed by the production of a preform, and wherein step b) comprises hot pressing. Such a wire generally comprises textured glass fibers. It can also include metal son, including copper son and / or brass, as well as organic fibers, such as polyacrylonitrile fibers or derivatives and / or aramid fibers. The impregnation is done in general by dipping the wire in the cement, in a manner known per se. In general, the prepreg wire is disposed in the form of lobes on a substrate before hot pressing step, for example according to the teaching of document<patcit id="pcit0005" dnum="WO2005040633A"><text>WO 2005/040633</text></patcit> ; </li><li>the binder is dissolved with an aqueous solvent. Such a method spring eg teaching of Application<patcit id="pcit0006" dnum="WO00063577A"><text>WO 00/063577</text></patcit> ;</li><li>the binder is dissolved with an organic solvent such as trichlorethylene;</li><li>step b) and optionally step a) is (are) carried out (s) during an injection process in which the fibers are staple fibers. Such a method spring eg teaching of Application<patcit id="pcit0007" dnum="WO02075175A"><text>WO 02/075175</text></patcit> or application <patcit id="pcit0008" dnum="WO03074899A"><text>WO 03/074899</text></patcit> ;</li><li>the binders include one or a mixture of resin (s) chosen (s) among the list epoxy, polyester, melamine, polysiloxane, phenolic, polyamide-imide, polyimide, polybenzimidazole, silicone;</li><li>it also removes material on the support surface during the step c);</li><li>during step c) is removed between 0.3 and 1 mm thickness of material on the entire treated surface;</li><li>the postcrosslinking treatment is a heat treatment at a temperature T_T greater than the temperature T_F of step b);</li><li>the temperature difference between the temperature T_T and T_F is at least 30 ° C, preferably about 50 ° C;</li><li>temperature T_T is between 200 and 300 ° C, preferably between 220 and 280 ° C;</li><li>the treatment of post-crosslinking is radiation treatment, for example the Ultra Violet (UV), infrared (IR) or visible light.</li></ul>

p0005A grinding operation of the bearing face can be made at cycle end, after step d), for example if the tolerances on the parallelism of the two faces are very severe.

p0006To illustrate the invention without limitation, the following examples are described:

p0007Clutch facings 200 mm in outer diameter, 137 mm in inner diameter and 3.2 mm thick were produced from the constituents of the F810 commercial reference VALEO FRICTION MATERIALS where fiber mixture of step binders and fillers is carried out according to the teaching of the patent application <patcit id="pcit0009" dnum="WO0063577A"><text>WO 00/63577</text></patcit>.

p0008After impregnation with the aqueous cement, son are lobed on a support and disposed in a mold to form a preform.

p0009The step of forming the mixture is carried out by hot pressing of the preform for 4 minutes at 200 ° C. During this step, applying high cycle and low pressure alternating with degassing.

p0010EX1 liner is then surfaced to obtain parallel faces and its properties were measured.

p0011Garnish, EX2, underwent treatment of the type described in <patcit id="pcit0010" dnum="FR2742503"><text>FR 2742503</text></patcit>Comprising two postcrosslinking cycles of several hours between 240 and 260 ° C, separated by a surfacing operation by rectification where 0.3 mm thick was removed on each of the faces.

p0012Filling, EX3, underwent a treatment according to the invention, where a facing by identical correction for that of the EX2 was performed after the forming step and followed by several hours of post-cure cycle between 240 and 260 ° C.

p0013In these examples, the post-cure cycle is performed by passing through a tunnel furnace where the linings circulate batteries.

p0014Ten trials was made for each type of trim.

p0015It is found that all the thus obtained fillings have flat and parallel faces.

p0016chatter tests were conducted for each type of packing following a test procedure called "Global Judder test" in which one measures the damping coefficient during a vibration test for different temperature ranges, particularly between 40 and 60 ° C, between 80 and 140 ° C, between 150 and 250 ° C.

p0017Test results are reported in the following table displaying the mean value (of 7 trials) and the maximum value of the damping coefficient for three temperature ranges and for the three examples trim.<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="13mm" /><colspec colnum="4" colname="col4" colwidth="13mm" /><colspec colnum="5" colname="col5" colwidth="13mm" /><thead><row><entry align="center">Temperature range</entry><entry align="center">Value</entry><entry align="center">EX1</entry><entry align="center">EX2</entry><entry align="center">EX3</entry></row></thead><tbody><row><entry morerows="1" valign="middle">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" valign="middle">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" valign="middle">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>

p0018Validation testing of the "Global Judder Test" have demonstrated that the higher the damping factor is low in the range 40 to 60 ° C, the better the cold start behavior and avoids behavior to chatter cold.

p0019It is thus observed a significant improvement of EX2 lining type EX1 compared to a type of filling. A gasket according to the invention, the type EX3 still a amé behavior Lioré against a trim EX2 kind.

p0020Vehicle Tests have confirmed that the toppings obtained with the process according to the invention have a particularly advantageous behavior to the risks of cold chatter.

p0021The invention is not limited to these types of embodiment and must be interpreted without limitation, encompassing any equivalent embodiment.