Silicone resin-based friction lining mixtures and friction linings containing metal fibers

Abstract

Friction linings for use in motor vehicle braking systems, which are produced using silicone resins, metal fibers from soft iron compounds and / or steel compounds, fillers and abrasive grains are characterized by high thermal and mechanical resistance.

Description

[0001] Description: Silicone resin-based friction lining compounds and friction linings with metal fibers

[0002] Description

[0003] The present invention relates to silicone resin-based friction lining mixtures and friction linings, in particular for disc brake linings, a process for their production and their use.

[0004] Silicone resin-based binder systems and friction linings are known in the state of the art.

[0005] EP 2 310 714 B1 discloses a brake pad for disc brakes comprising a friction section made of a ceramic matrix material. This ceramic matrix material is made from a silicon ceramic starting material, particles of hard materials that serve as abrasives, particles of substances suitable as lubricants, and particles of metal materials.

[0006] DE 1 1 2009 000 893 T5 describes compositions for brake pads based on silicone resin and NBR rubber.

[0007] US 5,984,055 relates to friction linings comprising a fiber-reinforced ceramic matrix material which comprises a ceramizable resin and fibers.

[0008] US 8,960,384 B2 discloses a process for producing ceramic matrix material for friction linings, wherein a silicone-containing ceramic precursor, abrasives, lubricants and metal particles serve as starting material.

[0009] DE 101 30 395 A1 discloses a friction material in which the framework component is made of fiber material such as copper or brass. An additional infiltration component can be made of, for example, phenolic resin, soft metal, or glass.

[0010] DE 697 18 346 T2 refers to a friction material comprising a sintered iron mass in which graphite particles are dispersed. The mass contains iron as fibers and particles.

[0011] Finally, WO 2013 / 076744 A1 describes a friction material for disc brakes comprising 1-8% of a ceramizable resin and 2-10% of an organic resin. A silicone resin can serve as the ceramizable resin.

[0012] These silicone resin-based binder systems or friction linings generally represent an advance over conventional friction linings described in the state of the art. However, there is still potential for improvement with these linings with regard to performance properties in so-called high-performance applications, i.e. particularly at high pressure and at high temperatures.

[0013] The object of the present invention was therefore to provide friction linings and friction lining mixtures which are improved in particular with regard to the parameters of abrasion, coefficient of friction and pressure and temperature resistance with regard to the state of the art.

[0014] It was found that special silicone resin-based friction lining mixtures or friction linings made from them can meet these increased requirements and at the same time do not impose any additional requirements or modifications on the production processes known from the state of the art with regard to their manufacture.

[0015] To achieve these objectives, the friction lining must be physically highly dense and, at the same time, exhibit a high chemical density of chemical bonds and polar interactions between its components. Achieving this high chemical density of chemical bonds between the various components of the friction lining is particularly important and therefore requires special measures to ensure this.

[0016] The highly abrasion-resistant and friction-optimized silicone resin-based friction lining mixtures or friction linings according to the invention preferably have the following composition. Unless otherwise stated, the following information here and below is in wt. % based on the finished friction lining mixture:

[0017] Metals (e.g. fibers, mostly iron-containing) 5-90%

[0018] Friction grains / friction media 25-45%

[0019] Fillers (especially coke) 5-40%

[0020] Lubricants 0-20%

[0021] Crosslinking chemicals / catalyst (such as zinc organyls) 0.1-4%

[0022] Silicone resins 6-16%

[0023] Metals such as Fe or Cu, or their mixtures, alloys, or sintered metals, ensure a high coefficient of friction even at elevated or high temperatures and simultaneously ensure good wear behavior. A preferred component is metal fibers, with a proportion of the friction lining mixture according to the invention of preferably 5-90%, in particular 5 to 30%. Iron or steel fibers are preferred here, as they increase the strength of the mixture or friction lining and prevent individual components from breaking out of the friction lining. Copper or copper fibers are less preferred due to their unfavorable environmental properties. Cu-free friction lining mixtures or friction linings are therefore also encompassed by the present invention.

[0024] Advantageously, unwanted premature wear of the friction material can be reduced by using special iron or steel fibers in the silicone resin-based mixture (Figure 1 Use of M2 or M3). These are fibers made of relatively soft iron or steel. It has been shown that special iron fibers produced with a reduced amount of alloying elements have particularly good properties with regard to abrasion (M2). The carbon, manganese and silicon content are particularly noteworthy here (see Table 1). For this reason, they are also referred to as pure iron fibers or, according to the invention, as soft iron compounds or soft iron fibers (compounds of type M2 according to Table 1 are preferred).

[0025] Structurally, the fibers are characterized by the fact that they predominantly contain only ferrite phases. Steel fibers with alloying elements exhibit distinct pearlite structures in addition to the ferrite phases. These pearlite structures make the iron particularly strong. These iron-carbon alloys are also referred to as strong steel.

[0026] According to the invention, steel fibers of type M3 according to Table 1 are preferred, which are referred to here as soft steel fibers or soft steel compounds. In addition to the proportion of alloying elements, the strength of the steel is also increased by its stretching. The stretching can be recognized by elongated crystalline structures. If the fibers are then hot-strengthened, the resulting strength is lost. The use of special hot-strengthened steel is therefore particularly advantageous according to the invention. This is because excessive strength is rather detrimental to reducing abrasion. The differences can also be determined from the differences in hardness, cf. (M 1 ) and (M3) in Figure 1 .

[0027] Figure 1 shows images at different magnifications, which show the structure of the materials M 1 , M2 , M3 according to Table 1.

[0028] Table 1 : Composition and dimensions of iron fibers or steel fibers M1 , M2, M3

[0029] Fig. 1 Image of the steel fibers M1 (left), M2 (middle) and M3 (right) with approximate scale

[0030] M1 : STAX steel fiber (unalloyed) Manufacturer: DEUTSCHES M ETAL L FASE RWE RK Dr. Schwabbauer GmbH & Co. KG.

[0031] REPLACEMENT SHEET (RULE 26) M2: STAX pure iron fibers Manufacturer: DEUTSCHES METALLFASERWERK Dr. Schwabbauer GmbH & Co. KG.

[0032] M3: annealed steel fibers; American Metal Fibers, Inc. Table 2: Wear / abrasion of pads with different

[0033] Metal fiber composition

[0034] Wear is determined by measuring the thickness of the lining at several points before and after the measurement.

[0035] For rubbers with only M1 fibers, the abrasion value is the same as for the reference (Ref), i.e., a conventional rubber. Rubbers with additional proportions of M2 and M3 fibers exhibit lower abrasion; see Table 2.

[0036] Preferably, the metal fibers are made predominantly, i.e., more than 50%, from soft iron and / or soft steel compounds. These can be, in particular, pure iron fibers or heat-strengthened steel fibers. It is also conceivable to mix pure iron fibers or heat-strengthened steel fibers. Particularly preferably, the metal fibers used according to the invention are made from more than 60% soft iron and / or soft steel compounds.

[0037] The friction lining mixtures / friction linings according to the invention contain a high proportion of friction agents, preferably in the form of abrasive grains, collectively also referred to as abrasives. Preferred representatives are SiC (6.4 pm), ZnO (2.5 pm), MgO (12.7 pm), Al2O3 (5 pm), and various silicates, to name the most important. The preferred d50 values ​​of the particle sizes are shown in parentheses. The d50 value means that 50% of the particles are smaller. This value is determined by light scattering. The particle size is determined by dynamic light scattering from a suspension of the particles in suspension.

[0038] The coke listed in the description is referred to as an active filler. Active fillers are understood to be materials whose surfaces exhibit the best possible affinity to the crosslinking chemicals used, especially to the silicone resin, thereby achieving greater crosslinking between the friction lining components and ultimately a higher density of the finished friction linings. Examples of preferred active fillers according to the invention include petroleum coke (calcined) and carbon black.

[0039] Active fillers are classified based on their ability to form a chemical bond with the silicone resin. Active fillers are characterized by their ability to react with the resin due to their functional groups on the surface and their relatively high specific surface area (the coke used has a specific surface area of ​​preferably > 0.9 m 2 / g) have many possibilities to bond with the resin The specific surface area of ​​the particles is determined by BET measurements.

[0040] The remaining materials of the friction lining mixture must be enclosed by a highly cross-linked matrix, since the active, ie inherent chemical, bonding of these remaining materials to the matrix is ​​only insufficiently possible due to their chemically less active surfaces.

[0041] The lubricants used are primarily so-called high-temperature lubricants. Examples include antimony, molybdenum, and zinc sulfides, as well as graphite.

[0042] The crosslinking chemicals or catalysts are preferably organozinc compounds (catalysts).

[0043] Silicone resin-based friction linings, as is known in the prior art, must undergo a curing reaction (a polycondensation reaction), which can be carried out uncatalyzed or, preferably, with appropriate catalysts. According to the invention, organozinc compounds (organylzinc compounds) are used for this purpose. Zinc acetylacetonate, zinc pentadioneate, zinc acetate, or their derivatives are particularly suitable.

[0044] The silicone resins suitable according to the invention are generally crosslinked polymethylsiloxanes or polymethylphenylsiloxanes or polyphenylsiloxanes commonly used in industry with the structural element of the formula I

[0045] (I), where A and B can represent various organic radicals (identical or different), such as methyl, ethyl, or phenyl. Such silicone resins are generally sold in precondensed form. They are sometimes combined with phenolic and polyester resins to further improve surface hardness, heat resistance, and chemical resistance, for example. All of these silicone resins can, in principle, be used for the purposes of the invention.

[0046] Silicone resins with a high inorganic content are particularly preferred. The inorganic content can be up to approximately 82 wt.%. The inorganic content is defined as the portion remaining after calcining the pure resin sample. The organic content in the resin is necessary because the resin must melt during pressing. This is ensured by a certain amount of organic residues in the resin. The organic residues in the resin consist of phenyl and / or alkyl groups. Alkoxy groups can also be part of the organic portion of the resin. Ethoxy groups and methoxy groups are preferably used as organic side groups of the resin.

[0047] The resin content (silicone resin content) in the mixture is preferably 6

[0048] Wt.% to 16 wt.% (based on the finished friction lining mixture). Suitable resins of this type are marketed, for example, by Wacker Chemie AG, Germany, under the brand name Silres®. The resin designated Silres MK is preferred.

[0049] The relatively low organic content preferred according to the invention ensures that the matrix (i.e., binder system and fillers) of the friction lining is largely preserved even when the organic components degrade. Silicone resins with a significantly higher organic content exhibit significantly greater weight loss at elevated temperatures, which means that the matrix is ​​significantly degraded, resulting in a disadvantageous increase in abrasion values. The degradation of the organic matrix can be easily and reliably monitored and determined using thermogravimetric analysis (TGA).

[0050] A further advantage of the silicone resins suitable according to the invention is their low softening temperature, which is preferably between 35 and 55 °C. This low softening temperature is advantageous, for example, when pressing the friction lining mixture into the pre-crosslinked friction lining compact, since it allows a lower temperature to be selected during molding.

[0051] A low temperature results in low gas pressure within the coating during forming.

[0052] However, according to the invention, the use of the catalysts mentioned is preferred, preferably in an amount of 0.25 to 4%.

[0053] The use of silicone resin-based systems for high-temperature applications is generally known. For this purpose, these systems must undergo a curing reaction, which can be realized uncatalyzed or with the use of appropriate catalysts. During the curing reaction, the polycondensation reaction that begins during the compression molding of the friction lining mixture continues. At the same time, organic residues are eliminated. However, due to the low curing temperature of <300°C preferred according to the invention, the methyl groups of the resin remain in the resin, and it is also likely that the hydrolysis reaction is not 100% complete. This means that residual ethoxy groups remain in the system.

[0054] The polycondensation reaction of silicone resins for use in brake pads is preferably catalyzed by organozinc compounds. During this polycondensation reaction, low-molecular-weight substances are released, which can lead to blistering and cracking in the friction lining during the pressing process of the friction lining mixture. The gases formed in the material being pressed result in insufficient compaction of the lining. All of this negatively impacts the abrasion and friction coefficient of such a friction lining. Therefore, gas formation during the pressing process should be minimized as much as possible.

[0055] According to the invention, the following measures alone or in combination contribute to the further improvement of the friction linings obtained.

[0056] A lower curing temperature of the coating ensures that some of the reactive components remain on the surface of the fillers, resulting in improved bonding of the fillers to the resin system. Curing also forms covalent bonds between the active fillers and the resin system.

[0057] This increases the crosslinking density of the material, firstly due to the lower resulting porosity, and secondly, the increased crosslinking due to the increased number of active groups remaining on the surface reduces the tendency of the friction lining to relax again after the pressing process and subsequently expand. This also increases the overall network density in the system. A curing temperature of <361 °C is therefore preferred. Above this temperature, for example, the oxygen-containing groups / residues present on the surface of the calcined petroleum coke decompose. For this reason, a curing temperature of 300 °C or lower is particularly preferably selected according to the invention. It has been shown that this curing temperature is sufficient to achieve crosslinking of the silicone resin.

[0058] During pressing, maximum compaction of the mixture should be achieved. This can be achieved by eliminating the need for aeration during pressing.

[0059] A friction lining pressing time of preferably > 8 minutes has also proven advantageous. This comparatively long pressing time results in additional compaction of the lining material.

[0060] The material is densified by crosslinking the silicone resin. A catalyst is preferably required for this. To achieve the highest possible densification during the pressing process, all components that limit the catalyst's activity are avoided.

[0061] The production of the mixture, the pressing of the mixture, the grinding and grooving of the linings, and the hardening of the linings are generally carried out according to methods known in the art. According to the invention, the temperature during pressing is 100-170°C, in particular 140°C, and the temperature during hardening of the friction lining should not exceed 300°C.

[0062] All components of the friction lining mixture are first mixed in an internal mixer in the compositions described.

[0063] Mixing then continues for approximately 5 minutes at a medium speed for the trough and agitator. The mixture is then poured into a cavity of a high-pressure press. The cavity has the contours of the desired coating. The thickness of the coating is determined by the amount of mix added. The coating carrier plate is placed on top of the cavity. A rising ram compacts the mixture in the cavity and presses it against the coating carrier plate. A hold-down device holds the coating carrier plate in position on the cavity.

[0064] The specific pressure is preferably between 15 N / cm 2 and 200 N / cm 2 A typical pressing time is 2-14 minutes. To achieve maximum compaction, the pressure is kept constant throughout the entire pressing cycle.

[0065] The coating is then sanded to the desired thickness.

[0066] The curing process then begins at a temperature of preferably <300°C. The temperature is initially increased from room temperature to <300°C at a constant rate of 1°C / min. The temperature is then held at <300°C for 180 minutes. The furnace is then turned off, and the coatings are allowed to cool slowly in the furnace.

[0067] The resulting brake pads are subjected to a flywheel test bench to determine their wear resistance. The composition of the tested friction pads and the test results obtained are listed in Table 2. Ref. stands for a comparative brake pad or friction pads, and Examples 1-3 stand for brake / friction pads according to the invention. The described process for producing the friction pads according to the invention is generally applicable and not limited to Examples 1-3 (compositions in wt. %). The present invention thus encompasses the described friction pad mixtures, friction and brake pads produced therefrom, processes for producing the friction pad mixtures and the friction and brake pads, as well as the use of the friction pad mixtures for producing the friction and brake pads and the use of the friction and brake pads in high-performance applications, such as 24-hour car races, at elevated pressure and elevated temperature.

Claims

Patent claims 1 . Friction lining mixture for producing a friction lining with reduced abrasion, comprising at least one silicone resin, a filler, and friction grains and metal fibers, characterized in that the metal fibers consist of more than 50 wt.% soft iron and / or soft steel compounds.

2. Friction lining mixture according to claim 1, characterized in that the metal fibers comprise more than 50% by weight of pure iron fibers.

3. Friction lining mixture according to claim 2, characterized in that the pure iron fibers have a reduced proportion of alloying elements, the carbon content being below 0.01 wt.%, the manganese content being below 0.5 wt.%, and the silicon content being below 0.05 wt.%.

4. Friction lining mixture according to one or more of claims 2 to 3, characterized in that the pure iron fibers comprise essentially exclusively ferrite phases.

5. Friction lining mixture according to claim 1, characterized in that the metal fibers comprise more than 50 wt.% hot-strengthened steel fibers.

6. Friction lining mixture according to one or more of claims 1 to 5, characterized in that the mixture contains one or more silicone resins which have an inorganic content of 60 to 88 wt.%, in particular of approximately 82 wt.%.

7. Friction lining mixture according to one or more of claims 1 to 6, characterized in that the mixture contains one or more silicone resins in a total of 6 to 16 wt.%, based on the total mixture.

8. Friction lining mixture according to one or more of claims 1 to 7, characterized in that the mixture contains petroleum coke in a total amount of 10 to 20 wt.%, based on the total mixture.

9. Friction lining producible from a friction lining mixture according to one or more of claims 1 to 8.

10. Brake pad, characterized in that it has a friction lining according to claim 9.

11. Use of silicone resins and metal fibers in combination with fillers and friction grains for producing friction lining mixtures and friction linings according to one of claims 1 to 9.

12. A method for producing a friction lining from a friction lining mixture according to any one of claims 1-8, comprising the following process steps: a) providing the finished friction lining mixture, b) filling the friction lining mixture into a press, c) pressing the friction lining mixture at increased pressure, in particular at 15-200 N / cm 2 , and elevated temperature, in particular 100-170°C, for a duration of 8-14 min, d) removal of the friction lining from the press and e) hardening of the friction lining at a temperature of < 300°C.

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