Wet friction material with coated filler particles

A method of making a wet friction material layer includes adding a media in liquid form to a material base including filler particles embedded in a matrix of fibers; drying the media to solidify the media on the filler particles such that the media plugs holes in the filler particles; adding a binder to the material base; and unplugging at least some of the holes in the filler particles by removing at least some of the media from the material base.

The present disclosure relates generally to friction clutches and plates used in torque converters and motor vehicle transmissions and more specifically to wet friction material.

BACKGROUND

The friction material in wet-type friction clutches generally operates in an oil submerged environment and is often paper-based material used to form friction material rings. It is known to spray or sprinkle diatomaceous earth sold under the trade name CELITE on top of the base materials directly during the paper making process in a Fourdrinier machine at the wet end of the machine when the paper base material is being moved along a conveyor.

SUMMARY OF THE INVENTION

A method of making a wet friction material is provided. The method includes adding a media in liquid form to a material base including filler particles embedded in a matrix of fibers; drying the media to solidify the media on the filler particles such that the media plugs holes in the filler particles; adding a binder to the material base; and unplugging at least some of the holes in the filler particles by removing at least some of the media from the material base.

In some embodiments of the method, the media may be an emulsion. The emulsion may be latex. The latex may be an acrylic emulsion. The filler particles may be diatomaceous earth particles. The binder may be phenolic resin. The media may be added to the material base in liquid form during a pulping process for forming the material base. The removing of at least some of the media from the material base may include burning the media. The burning of the media may be performed by heat that cures the binder. The heat that cures the binder may be provided by a heat plate that is pressed against an outer surface of the wet friction material layer. The removing of at least some of the media from the material base may form voids in the wet friction material layer along the filler particles. The media may form 3 to 6% by percentage weight of the wet friction material layer before the removing at least some of the media from the material base.

A method of making a part of a friction clutch is also provided that includes making the wet friction material. The removing of at least some of the media from the material base may include burning the media by heat that cures the binder to fix the wet friction material to a metal part of the friction clutch.

A wet friction material is also provided that includes a base material including a matrix of fibers and filler particles embedded in the matrix of fibers. The filler particles each include a plurality of holes. The wet friction material also includes a binder embedded in the base material, at least some of the holes in the filler particles being unplugged.

In some embodiments of the wet friction material, the wet friction material includes remnants of a media. The media may be latex. The remnants may include carbonized residue. The latex may be an acrylic emulsion. The filler particles may be diatomaceous earth particles. The binder may be phenolic resin.

A clutch assembly is also provided that includes a metal part and the wet friction material fixed on the metal part.

DETAILED DESCRIPTION

The present disclosure provides a method that includes saturating diatomaceous earth particles with a temporary media, in the form of a liquid, before friction material is saturated with phenolic resin to prevent plugging of diatomaceous earth particles with phenolic resin. By preventing the plugging of diatomaceous earth particles with phenolic resin, the porosity of the friction material is increased, providing an improved frictional material that can absorb oil quickly between reaction plate and the wet friction material layer during clutch engagements.

The impregnating of the wet friction material with phenolic resin is performed in order to be durable in high temperature and high pressure in vehicle application. Since phenolic resin is carried through a liquid media and during wet friction material saturation the phenolic resin penetrates inside diatomaceous earth particles and then hardens during the curing process, making diatomaceous earth particles harder and less porous.

The present disclosure provides a method of forming wet friction material that includes adding latex liquid in wet friction material (raw paper) pulping process and so the latex plugs diatomaceous earth particles. The latex liquid is an emulsion system that is capable of gel formation/solidification/hardening). Latex dries during the paper drying stage and shapes a thin rubbery barrier around diatomaceous earth that prevents phenolic resin from penetrating into holes in diatomaceous earth particles. In contrast to the phenolic resin, the latex is not hardened during the phenolic resin curing process. Latex on the wet friction material surface is removed, by the latex wearing off or being burnt off during wet friction material bonding to steel plates, and the remaining resin on surface rubs off during clutch engagement during use, to help keep DE particle pores open and fresh. It may be particularly advantageous to add latex up to a specific percentage to improve the performance of wet friction material, as no improvement is seen above such a specific percentage.

The latex used may also help cause the stronger binding of diatomaceous earth particles to the network of fibers so that during the application of the clutch, the diatomaceous earth particles become more effective and provide better friction characteristics.

FIGS.1ato1dandFIG.2schematically illustrate a method of forming a wet friction material layer and a clutch assembly in accordance with an embodiment of the present disclosure.

A wet friction material layer12may be formed of fibers, filler material, a binder and a media. The fibers can be aramid fibers, organic fibers, carbon fibers and/or fiberglass. The organic fibers may include cellulose fibers or cotton fibers. The filler material may be particles of diatomaceous earth. The binder may be a phenolic resin. The media may be an emulsion in the form of latex. Optionally a friction modifier such as graphite may also be included in wet friction material layer12. The fibers of layer12may have a mean diameter of 45 to 55 microns and a mean length of 1 to 2 millimeters.

In one preferred embodiment, wet friction material layer12may include, by percentage weight, 25 to 40% fibers, 30 to 40% filler material and 25 to 40% binder. More specifically, wet friction material layer12may include, by percentage weight, 30 to 35% fibers, 30 to 35% filler material and 30 to 35% binder.

FIG.1aschematically shows wet friction material layer12before the binder is added. Wet friction material layer12includes a material base formed by a plurality of diatomaceous earth particles14imbedded in a matrix of fibers16between a first outer surface12aand a second outer surface12bof wet friction material layer12.

FIG.1bshows an enlarged view of wet friction material layer12illustrating a section of wet friction material layer12including fibers16and diatomaceous earth particles14coated with latex22. In the embodiment shown inFIG.1b, diatomaceous earth particles14have a hollow cylindrical shape and each includes a hole18passing axially therethrough. Particles14are also porous and include a plurality of holes20passing radially therethrough from an inner circumferential surface to an outer circumferential surface of the respective particle14. In other embodiments, particles14may have a different shape, but still include a plurality of holes and are porous. As shown inFIG.1b, each of diatomaceous earth particles14is covered in latex22such that holes18,20are plugged by the latex22.

Latex22may be added as a 3 to 6% add-on to the raw material (in weight %, referring to the solid latex in the liquid in comparison to the weight of the other solid material) during the formation of wet friction material layer12during the pulping process as a water-based liquid along with a base liquid, in which fibers16and diatomaceous earth particles14are submerged. After being formed into sheets during the pulping process, the wet friction material layer12may be then dried at for example at 300° F. to remove the base liquid, causing latex22to dry and solidify to cover the diatomaceous earth particles14, plugging holes18,20and creating a thin rubbery barrier around each of the diatomaceous earth particles14to penetrate holes18,20.

As shown inFIG.1c, after latex22is solidified in wet friction material layer12inside of holes18,20of diatomaceous earth particles14, the binder24is added to wet friction material layer12such that voids26(FIG.1b) in wet friction material layer12between a matrix formed by fibers16and diatomaceous earth particles14are saturated with the binder. The coating of diatomaceous earth particles14with solidified latex22prevents or minimizes the penetration of the binder into holes18,20in diatomaceous earth particles14.

As shown inFIG.2, wet friction material layer12is then placed on top of a metal part30and layer12and part30are joined together to form a friction assembly. Prior to joining of layer and part30, the binder is subject to initial curing to a level called B-stage, where the layer12is somewhat flexible. The joining of layer12and part30together includes pressing wet friction material layer12against metal part30with a heat plate32to complete curing of the binder24in wet friction material layer12, fixing wet friction material layer12and metal part30together. The force of pressing of heat plate32against outer surface12aof wet friction material layer12, while inner surface12bof wet friction material layer12rests on an outer layer30aof metal part30, causes the binder to accumulate at an interface of inner surface12bof wet friction material layer12and outer surface30aof metal part30, while the curing of the binder by the heat of heat plate32creates a permanent connection between metal part30and wet friction material layer12. Binder24shown inFIG.1csolidifies and hardens in wet friction material layer12in contact with solidified latex22and fibers16. In one preferred embodiment, the heat at a surface32aof plate32that contacts outer surface12aof outer layer is 375 to 425 degrees F.

The heat from heat plate32unplugs at least some of the holes18,20in particles14by removing at least some of the latex by burning off the latex22surrounding diatomaceous earth particles14and unclogs holes18,20in particles14. In some embodiments, at least 50% of the holes (by number or volume) are unplugged, for example, at least 75% or at least 90%. As shown inFIG.1d, the burning of the latex22creates voids34in regions occupied by the latex22before the burning of the latex22. Voids34allow fluid to flow from outer surface12athrough wet friction material layer12to inner surface12balong diatomaceous earth particles14, allowing particles14to absorb fluid via holes18,20. Remnants of latex in the form of carbonized residue due to thermal degradation may be present after the burning of the latex.

Any latex22that possibly remains under surface12aafter the application of heat plate32rubs away during clutch engagement of wet friction material layer12with a further clutch part and keep particles14unclogged.

It is advantageous to add 3 to 6% of latex as an add on (wt % in reference to the solid weight of the latex solution) with respect to raw friction material weight to improve the performance of the layer12, but adding latex in amounts above 6% does not appear to further help performance. In some embodiments, latex may added as an add on below 3% wt or greater than 6% wt.

Latex is in the form of butadiene acrylonitrile copolymer improves performance of layer12to a certain degree, but latexes in the form of acrylic emulsions have a more dominant effect on the performance of layer12.

FIG.3shows a friction versus speed graph illustrating the effect of higher acrylic based latex on the material. As noted above, the percentage of latex is an add on (wt %) with respect to raw friction material weight, considering the solid weight of the latex solution for the percentage.

FIG.4shows wet friction material layer12bonded to both sides of a metal part in the form of a clutch plate40of lockup clutch assembly42of a torque converter44. A piston46of lockup clutch assembly42forces clutch plate40against an inside surface48aof a front cover48of torque converter44. Piston46contacts the surface12aof the rear piece of wet friction material42to force the surface12aon the front piece of wet friction material layer12against inside surface48aof front cover48. The forcing of clutch plate40against front cover48by piston46locks the lockup clutch assembly42such that a torque path in torque converter44to a transmission input shaft bypasses an impeller50and a turbine52of torque converter44, and instead flows from front cover48to clutch plate40and through a damper assembly54to a transmission input shaft that is connected to an output hub56of torque converter44.

In the preceding specification, the disclosure has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of disclosure as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.

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