Silicone-based friction lining mixture with metal fibres and friction lining

Abstract

The invention relates to a friction lining for use in motor vehicle braking systems, made of silicone, metal fibres made of a soft iron compound and / or a soft steel compound, a filler and friction particles, having high heat resistance and mechanical resistance.

Description

Technical Field

[0001] This invention relates to silicone resin-based friction lining mixtures and friction linings, particularly for disc brake linings, and also to their manufacturing methods and applications. Background Technology

[0002] Silicone-based bonding systems and friction pads are known in the prior art.

[0003] EP 2 310 714 B1 discloses a brake pad for a disc brake having a friction section made of a ceramic substrate material. The ceramic substrate material is made of silicon ceramic raw material, hard material particles used as abrasives, material particles suitable as lubricants, and metallic material particles.

[0004] A brake pad composition based on silicone resin and nitrile rubber (NBR) is described in DE 11 2009 000 893 T5.

[0005] US 5,984,055 relates to friction linings comprising fiber-reinforced ceramic substrate materials, the ceramic substrate materials comprising ceramizable resins and fibers.

[0006] US 8,960,384 B2 discloses a method for manufacturing a ceramic substrate material for friction linings, wherein a silicon-containing ceramic precursor, abrasive, lubricant, and metal particles are used as raw materials.

[0007] DE 101 30 395A1 discloses a friction material in which the skeleton component is formed of a fibrous material (such as copper or brass). Additional permeation components may consist of, for example, phenolic resins, soft metals, or glass.

[0008] DE 697 18 346T2 relates to a friction material having an iron sintered body in which graphite particles are dispersed. The sintered body comprises both iron fibers and iron particles.

[0009] Finally, WO 2013 / 076744 A1 describes a friction material for disc brakes comprising 1-8% ceramizable resin and 2-10% organic resin. Silicone resin can be used as the ceramizable resin.

[0010] These silicone-based bonding systems, or friction linings, generally offer advantages over conventional friction linings described in the prior art. However, even with these linings, there is still room for improvement in their performance characteristics in so-called high-performance applications, i.e., especially under high pressure and high temperature conditions. Summary of the Invention

[0011] Therefore, the object of the present invention is to provide friction linings and friction lining mixtures that, compared with the prior art, are particularly improved in terms of parameters such as wear, coefficient of friction, compressive strength, and high temperature resistance.

[0012] Research has found that specific silicone-based friction pad mixtures or friction pads made from them can meet these higher requirements, while requiring no additional requirements or modifications to manufacturing methods known in the prior art.

[0013] To achieve the above objectives, the friction lining must be physically highly dense, and its components must have a high chemical density of chemical bonds and polar interactions. The formation of a high chemical density of chemical bonds between the different components of the friction lining is particularly important, and therefore special measures are required to ensure this.

[0014] The high-wear-resistant and friction-optimized silicone-based friction liner mixture or friction liner according to the present invention preferably has the following composition. Unless otherwise stated, all contents described herein and hereinafter are weight percentages of the finished friction liner mixture:

[0015] Metals (such as fibers, which mostly contain iron): 5-90%

[0016] Friction particles / abrasives: 25-45%

[0017] Filler (especially coke): 5-40%

[0018] Lubricant: 0-20%

[0019] Crosslinking agent / catalyst (e.g., organozinc compounds): 0.1-4%

[0020] Silicone resin: 6-16%

[0021] Metals (such as iron, copper, or mixtures, alloys, or sintered metals) ensure a high coefficient of friction even at elevated or high temperatures, while also guaranteeing good wear performance. A preferred component is metal fiber, which preferably constitutes 5-90%, particularly 5-30%, of the friction lining mixture according to the invention. Iron or steel fibers are preferred here, as they enhance the strength of the mixture or friction lining while preventing individual components from detaching from the friction lining. Copper or copper fiber is less preferred due to its poor environmental performance. Therefore, the invention also covers copper-free friction lining mixtures or friction linings.

[0022] Advantageously, by using specific iron or steel fibers in silicone-based mixtures ( Figure 1Using M2 or M3 can reduce unnecessary premature wear of friction materials. Here, the fibers are made of relatively soft iron or steel. Studies have shown that specific iron fibers (M2) made with reduced alloying element content have particularly good performance in terms of wear. In particular, the carbon content, manganese content, and silicon content are mentioned (see Table 1). Therefore, these fibers are also referred to as pure iron fibers, or according to the invention, soft iron compounds or soft iron fibers (preferably compounds of type M2 according to Table 1).

[0023] Structurally, these fibers are characterized by preferably containing only a ferrite phase. Steel fibers containing alloying elements, in addition to the ferrite phase, also exhibit a pronounced pearlitic structure. This pearlitic structure contributes to the exceptionally high hardness of iron. Such iron-carbon alloys are also known as hard steel.

[0024] According to the present invention, it is preferred to use steel fibers of type M3 according to Table 1, which are referred to herein as soft steel fibers or soft steel compounds.

[0025] In addition to the proportion of alloying elements, the strength of steel is also increased by its vertebral extension. This extension can be identified by its elongated, oriented crystal structure. This strength derived from vertebral extension is lost if the fibers are subsequently thermosetting. Therefore, according to the invention, it is particularly advantageous to use specific heat-workable steels, as excessively high strength is detrimental to reducing wear. This difference can also be determined by differences in hardness, see [reference needed]. Figure 1 (M1) and (M3) in the middle. Attached Figure Description

[0026] Figure 1 Images at different magnifications are shown, which reveal the structure of the materials M1, M2, and M3 in Table 1. Detailed Implementation

[0027] Table 1: Composition and dimensions of iron or steel fibers M1, M2, and M3

[0028]

[0029]

[0030] Figure 1 Images of steel fiber M1 (left), M2 (middle), and M3 (right) with approximate scale.

[0031] M1: STAX steel fiber (non-alloy), manufacturer: DEUTSCHES METALLFASERWERK

[0032] Dr.Schwabbauer GmbH & Co.KG

[0033] M2: STAX pure iron fiber, manufacturer: DEUTSCHES METALLFASERWERKDr.Schwabbauer GmbH&Co.KG

[0034] M3: Annealed steel fiber, manufacturer: American Metal Fibers, Inc.

[0035] Table 2: Wear of linings composed of different metal fibers

[0036]

[0037] The amount of wear is determined by measuring the thickness of the lining at multiple locations before and after the test.

[0038] In the liner containing only M1 fibers, the wear value was the same as the reference sample, i.e., the conventional liner. However, the liner with additional M2 and M3 fibers showed lower wear; see Table 2.

[0039] Preferably, the metal fibers are mostly, i.e., more than 50%, made of soft iron compounds and / or soft steel compounds. Here, pure iron fibers or heat-worked steel fibers are particularly suitable. A mixture of pure iron fibers and heat-worked steel fibers may also be considered. Particularly preferably, the metal fibers used according to the invention are more than 60% made of soft iron compounds and / or soft steel compounds.

[0040] The friction liner mixture / friction liner of the present invention contains a high proportion of abrasive, preferably in the form of friction particles, which are also collectively referred to as abrasive materials. Preferred examples include silicon carbide (6.4 μm), zinc oxide (2.5 μm), magnesium oxide (12.7 μm), aluminum oxide (5 μm), and various silicates. Here, the preferred particle size d50 value is given in parentheses. The d50 value refers to the particle size of 50% of the particles being smaller than this value. This value is determined by light scattering. The particle size is determined by dynamic light scattering of the particle suspension.

[0041] The coke mentioned in the specification is referred to as an active filler. Active fillers specifically refer to materials whose surfaces have the highest possible affinity for the crosslinking agent used (especially silicone resin), thereby increasing the degree of crosslinking between the components of the friction lining and ultimately resulting in a higher density of the finished friction lining. Examples of preferred active fillers used according to the invention include (calcined) petroleum coke or carbon black.

[0042] Here, active fillers are classified according to their ability to form chemical bonds with silicone resins. The active fillers are characterized by: first, their ability to react with the resin due to the functional groups on their surface; and second, their relatively high specific surface area (preferably >0.9 m²). 2 The particle size ( / g) provides ample possibilities for bonding with the resin. The specific surface area of ​​the particles was determined by BET measurement.

[0043] Here, the remaining materials of the friction lining mixture must be encapsulated by a highly cross-linked matrix because these remaining materials have low surface chemical activity, making it difficult for them to form an effective active chemical bond, i.e., self-chemical bond, with the matrix.

[0044] The lubricants used are especially so-called high-temperature lubricants. For example, antimony sulfides, molybdenum sulfides, zinc sulfides, and graphite can be mentioned.

[0045] The crosslinking agent or catalyst is preferably an organozinc compound (catalyst).

[0046] In the prior art, silicone-based friction pads require a curing reaction (polymerization reaction), which can be carried out under catalyst-free conditions or preferably with a suitable catalyst. According to the invention, organozinc compounds (organozinc reagents) are used in particular for this purpose. Zinc acetylacetonate, zinc pentylene, zinc acetate, or their derivatives are especially noteworthy here.

[0047] The silicone resins applicable to this invention are typically industrially common crosslinked polymethylsiloxanes, polymethylphenylsiloxanes, or polyphenylsiloxanes, which have the structural elements shown in Formula I:

[0048]

[0049] In the formula, A and B can represent different organic groups (the same or different), such as methyl, ethyl, or phenyl. These types of silicone resins are typically sold in pre-polymerized form. The silicone resin portion may also be blended with phenolic resins and polyester resins, for example, to further improve surface hardness, heat resistance, and chemical resistance. All of the silicone resins mentioned above are, in principle, suitable for use in the purposes of this invention.

[0050] Silicone resins with a high inorganic content are particularly preferred here. The inorganic content can reach up to approximately 82% by weight. Here, inorganic content refers to the percentage of functional groups after calcination of the pure resin sample. An organic content in the resin is necessary because the resin needs to be melted during the pressing process. This is ensured by a certain amount of organic groups in the resin. The organic groups in the resin consist of phenyl and / or alkyl groups. Alkoxy groups can also be a component of the organic content of the resin. Here, ethoxy and methoxy groups are preferred as organic side groups of the resin.

[0051] The resin content (silicone content) in the mixture is preferably 6% to 16% by weight (based on the friction liner mixture produced). Suitable resins of this type are, for example, those from Wacker Chemie AG in Germany. Sold under a brand name. Here, the preferred resin model is Silres MK.

[0052] The preferred low organic content according to the present invention ensures that the matrix of the friction liner (i.e., the bonding system and filler) remains as intact as possible even if the organic components degrade. Here, silicone resins with significantly higher organic content exhibit significantly greater weight loss at elevated temperatures, indicating significant matrix degradation and consequently, an unfavorable increase in wear values. Degradation of the organic matrix can be easily and reliably monitored and determined using thermogravimetric analysis (TGA).

[0053] Another advantage of the silicone resin to which this invention is applicable is its low softening temperature, preferably between 35-55°C. This lower softening temperature is advantageous, for example, when pressing friction liner mixtures into pre-crosslinked friction liner preforms, as it allows for the selection of lower temperatures during the molding process. Lower temperatures result in lower internal air pressure within the liner during molding.

[0054] However, according to the present invention, it is preferred to use the above-mentioned catalyst, and the amount thereon is preferably 0.25 to 4%.

[0055] Silicone-based systems for high-temperature applications are known in principle. For this purpose, these systems require a curing reaction, which can be carried out under catalyst-free conditions or by using a suitable catalyst. During the curing reaction, the polycondensation reaction that begins when the friction liner mixture is pressed continues. Simultaneously, organic groups are shed. However, due to the preferred low curing temperature (<300°C) according to the invention, methyl groups in the resin remain in the resin, and the hydrolysis reaction may not be 100% complete. This means that residual ethoxy groups remain in the system.

[0056] The polycondensation reaction of silicone resin used in brake pads is preferably catalyzed by organozinc compounds. During this polycondensation reaction, low-molecular-weight substances are released, which may cause bubbles and cracks to form inside the friction pad during the pressing of the friction pad mixture. Gases formed in the material to be pressed can prevent the pad from becoming sufficiently dense. All of these factors adversely affect the wear performance and coefficient of friction of this type of friction pad. Therefore, gas generation should be minimized as much as possible during the pressing process.

[0057] According to the present invention, the following measures, either individually or in combination, contribute to further improvement of the obtained friction lining.

[0058] Lower curing temperatures allow residual reactive components on the filler surface to remain, and these components improve the bonding between the filler and the resin system. Furthermore, during curing, covalent bonds form between the active filler and the resin system.

[0059] This reduces the tendency of the friction lining to loosen and expand after pressing by increasing the number of residual active groups on the surface, thereby improving the degree of crosslinking of the material. This also increases the overall crosslinking density of the system. Therefore, the curing temperature is preferably <361°C. Above this temperature, the oxygen-containing groups / residual parts present on the surface of the calcined petroleum coke will decompose. Therefore, according to the present invention, the curing temperature is particularly preferably selected as 300°C or lower. Studies have shown that this curing temperature is sufficient to achieve crosslinking of the silicone resin.

[0060] During the pressing process, the mixture should be densified to the maximum extent possible. This can be achieved by omitting the venting step during pressing.

[0061] Furthermore, the pressing time of the friction lining is preferably >8 minutes, which has proven to be advantageous. The relatively long pressing time allows for further densification of the lining material.

[0062] Densification of the material is achieved through crosslinking of the silicone resin. For this purpose, a catalyst is preferably required. To achieve the highest possible densification during the pressing process, all components that limit catalyst activity should be eliminated.

[0063] The preparation of the mixture, the pressing of the mixture, the grinding and grooving of the lining, and the curing of the lining are all carried out in principle using methods known in the prior art. Here, according to the present invention, the pressing temperature is 100-170°C, especially 140°C, while the curing temperature of the friction lining should not exceed 300°C.

[0064] First, all components of the friction liner mixture are mixed in an internal mixer according to the stated composition. Then, the mixture is further mixed in a trough and a hydrocyclone at a medium speed for approximately 5 minutes.

[0065] Next, the mixture is filled into the mold cavity of the high-pressure press. Here, the mold cavity has the desired liner profile. The height of the liner is determined by the amount of mixture filled. The liner support plate is placed on the mold cavity from above. The mixture in the mold cavity is compressed by an upward-moving punch, pressing it against the liner support plate. The liner support plate is held in a specific position on the mold cavity by a clamping device.

[0066] Here, the specific pressure is preferably 15 N / cm. 2 Up to 200 N / cm 2Between 2 and 14 minutes. To achieve maximum densification, pressure is kept constant throughout the entire pressing cycle.

[0067] The liner is then ground to the required thickness.

[0068] Next, curing is carried out at a temperature preferably <300°C. Here, the temperature is first increased from room temperature to ≤300°C at a constant rate of 1°C / min. Then, the temperature is maintained at ≤300°C for 180 minutes. After that, the oven is closed and the liner is allowed to cool slowly in the oven.

[0069] To determine wear resistance, the obtained brake linings were subjected to flywheel mass test bench tests. The composition of the tested friction linings and the test results are shown in Table 2. "Ref." represents a comparative brake lining or friction lining, and "Examples 1-3" represent brake / friction linings according to the present invention.

[0070] The above-described method for manufacturing friction pads according to the present invention is universally applicable and is not limited to Examples 1-3 (compositions are by weight percentage).

[0071] Therefore, the present invention includes the above-mentioned friction lining mixture, friction linings and brake linings made therefrom, methods for manufacturing friction lining mixtures and friction linings and brake linings, applications of friction lining mixtures in manufacturing friction linings and brake linings, and applications of friction linings and brake linings in high-performance applications, such as 24-hour car endurance races (i.e., under high pressure and high temperature).

Claims

1. A friction liner mixture for manufacturing friction liners to reduce wear, comprising at least one silicone resin, filler, friction particles, and metal fibers, characterized in that, More than 50% by weight of the metal fiber consists of soft iron compounds and / or soft steel compounds.

2. The friction lining mixture according to claim 1, characterized in that, More than 50% by weight of the metal fiber comprises pure iron fiber.

3. The friction lining mixture according to claim 2, characterized in that, The pure iron fiber has alloying elements with reduced content, wherein the carbon content is less than 0.01% by weight, the manganese content is less than 0.5% by weight, and the silicon content is less than 0.05% by weight.

4. The friction liner mixture according to one or more of claims 2 to 3, characterized in that, The pure iron fiber consists primarily of the ferrite phase.

5. The friction lining mixture according to claim 1, characterized in that, More than 50% by weight of the metal fiber includes heat-hardened steel fiber.

6. The friction liner mixture according to one or more of claims 1 to 5, characterized in that, The mixture comprises one or more silicone resins, wherein the inorganic content of the silicone resins is 60 to 88% by weight, particularly about 82% by weight.

7. The friction liner mixture according to one or more of claims 1 to 6, characterized in that, The mixture contains one or more silicone resins in total weight of 6 to 16% with respect to the total mixture.

8. The friction liner mixture according to one or more of claims 1 to 7, characterized in that, The mixture contains 10 to 20% by weight of coke relative to the total mixture.

9. A friction liner capable of being made from a friction liner mixture according to one or more of claims 1 to 8.

10. A brake pad, characterized in that, The brake pad has the friction pad according to claim 9.

11. A combination of silicone resin and metal fibers with fillers and friction particles for use in the manufacture of friction lining mixtures and friction linings according to any one of claims 1 to 9.

12. A method for manufacturing a friction liner from a friction liner mixture according to any one of claims 1 to 8, comprising the following steps: a) Provide the mixture of friction pads to be manufactured; b) The friction lining mixture is fed into the press; c) Under increased pressure, especially 15-200 N / cm 2 The friction lining mixture is pressed for 8-14 minutes at elevated temperatures, especially 100-170°C. d) Remove the friction lining from the press; e) The friction lining is cured at a temperature of ≤300°C.

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