Rubber composite material mixed with short staple reinforcing fibers and method of manufacturing the same

Abstract: A composite material, suitable for use in a power transmission belt, having a rubber matrix and a plurality of short staple reinforcing fibers therein of methaphenyleneiso-phthalamide coated with one of a rubber latex and a rubber paste. The invention is also directed to a method of forming the composite material.

Description: This invention relates to rubber composite material as used in the formation of power transmission belts and, more particularly, to a rubber composite material with short reinforcing staple fibers of poly(m-phenyleneisophthalamide) uniformly dispersed in a rubber matrix. The invention is also directed to a method of manufacturing the composite material. Heretofore, to enhance the resistance of rubber to wear and compression, staple, reinforcing fibers have been dispersed in the rubber. It is known, for example, to uniformly distribute inorganic or organic, short reinforcing staple fibers in natural or synthetic matrix rubber. Commonly, the fibers have lengths of between 1 to 50 mm and make up 1 to 30 percent of the volume of the composite rubber material. The fibers are generally oriented in substantially parallel relationship and disposed with their lengths at an angle of from 20°-160°, and preferably 90°, to the frictional surface of the composite material, i.e. the side surfaces of a transmission belt. The fiber-filled rubber material has, in recent years, been particularly adaptable for use in V-ribbed belts in the automotive industry. The automotive environment is particularly demanding on V-ribbed belts, particularly when such belts are used in serpentine drive systems. The engine compartments in today's automobiles are quite compact and, with the engine operating, the temperature in the compartment is elevated to high temperatures. In this environment, and particularly with high horse power engines, it is essential that the V-ribbed belts exhibit high resistance to heat, bending, tension and wear. To meet these demands, designers of prior art transmission belt systems have commonly utilized aromatic polyamide fibers to reinforce the rubber and particularly to give the rubber greater resistance to heat and more rigidity. It is also known to improve the composite rubber material performance, and particularly the tensile strength and resistance to tearing, by adhering to the short reinforcing staple fibers, embedded in the matrix rubber, resorcin-formalin-rubber latex (RFL), isocyanate, and epoxy resin adhesive. While this treatment improves the performance of the rubber, the application of the adhesive by conventional techniques requires use of relatively expensive equipment and is a relatively time consuming, complicated and costly process. An additional problem is that the longevity of the belt may be dependant upon the survival of the adhesive, which time period may be shorter than desired. Further, it is difficult to uniformly adhesively treat the fibers, which may result in unpredictable belt performance, from one belt to the next. In composite rubber material having a rubber matrix with short reinforcing staple fibers therein, it is desirable, for the optimum belt performance, to use fine fibers. That is, the fibers take up space in the rubber matrix and, if they are too large, they significantly alter the belt characterisitics. The fibers are significantly larger than other additives in the matrix material, such as carbon particles, and it is thus important that their size be controlled. In the absence of the fibers being RFL treated, to prevent fibrillating of the filaments, the filaments are generally dipped in water, cut to length, and then dried. This treatment tends to undesirably entangle and expand the fibers to have an effectively larger diameter. Another problem with the formation of prior art belts incorporating composite material with dispersed fibers in a rubber matrix is that the fibers commonly become and remain entangled with each other. During manufacture, the cut yarns, in an entangled state, are kneaded with the rubber. During the kneading process, the yarns do not untangle. The tangled lumps of yarn remain intact in the end product, which detracts from the integrity of the end product. Certain parts of the rubber matrix have less fibers dispersed therein than desired, while other parts of the matrix material have clumps of tangled fibers which are not oriented as would produce the best quality belt. One solution to this problem is to repetitively knead the composite mixture. While this eliminates some of the fiber tangling, the kneading may also undesirably alter the characteristics of the fibers and/or rubber matrix. In US-A-4833191, there is disclosed a process for preparing a predispersed fiber composition useful for dispersing discontinuous fibers into a rubber or plastic polymer compound. The process comprises mixing at least about 15 percent by weight of a pourable binder composition with up to about 85 percent by weight of discontinuous fibers at a temperature of from about 40°C to about 150°C until a honogeneous predispersed fiber composition is obtained, The binder composition comprises from about 50 to 100 percent by weight of a rubber or plastic polymer, from 0 to 50 percent by weight of a plasticizer or peptizer for the polymer, and from 0 to about 45 percent by weight of critical chemical additives, and is pourable at ambient temperature of up to about 135°C and has a Brookfield viscosity of less than about 1,300,000 centipoises at the temperature at which it is pourable. The present invention provides a rubber composite material comprising: a rubber matrix material; and a plurality of short reinforcing staple fibers dispersed in the rubber matrix material; characterized in that the short reinforcing staple fibers are poly(m-phenyleneisophthalamide) fibers produced by cutting poly (m-phenyleneisophthalamide) yarn into said short reinforcing staple fibers, in that the short reinforcing staple fibers have an irregular surface (i.e. a surface that has peaks and valleys), and in that the poly(m-phenyleneisophthalamide) yarn is coated with a coating formed by drying at least one of rubber latex and rubber paste prior to being cut to produce said short reinforcing staple fibers. It has been found, through extensive testing, that the rubber composite material of the invention exhibits excellent resistance to heat, bending, load, and wear. In addition, these characteristics are realized without having to subject the fiber to an adhesive treatment, which is conventionally carried out in a relatively time consuming and expensive process using expensive equipment. An example of suitable poly(m-phenyleneisophthalamide) fiber for use in the invention is CONEX fiber. CONEX fiber has an irregular surface, with peaks and valleys, as compared to other synthetic fiber such as 6,6 nylon, 6 nylon, polyester, polyvinyl alcohol fiber sold under the trademark VINYLON, and the like, which have relatively smooth cylindrical outer surfaces. The CONEX fiber tends to lock in the matrix rubber, obviating the need for adhesive treatment required with other types of synthetic fiber. It is common, to prepare conventional aromatic fiber for dispersion in a matrix, to dip the fiber in water. This often results in the fiber entangling, which prevents uniform dispersion in the rubber matrix. The water dipping step is unnecessary according to the present invention and therefore tangling of the fiber is minimized to thereby permit more uniform dispersion of the fiber in the rubber matrix, which accounts for more consistency of belt performance, from one to the next, and higher integrity belts. At the same time, manufacturing is simplified and the costs attendant thereto are reduced. Further, the need for repetitive kneading is avoided so that the characteristics of the fiber and matrix rubber are not altered detrimentally. In the composite material of the invention, the short reinforcing staple fibers are coated with at least one of a rubber latex and a rubber paste. Said at least one of the rubber latex and rubber paste and the rubber matrix can be the same or different materials. Preferably, said at least one of the rubber latex and rubber paste is provided in an amount from 1 to 10 weight percent with respect to the amount of yarn. The rubber latex is preferably at least one of styrene-butadiene copolymer rubber latex, natural rubber latex, vinylpyridine-styrene-butadiene-terpolymer latex, nitrilebutadiene latex, and chloroprene latex. The rubber in the rubber paste is preferably at least one of natural rubber, chloroprene rubber, styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, polysulfide rubber, isobutylene polymer rubber, alkylacrylate-chloroalkylvinyl-ether copolymer rubber, and chlorosulfonated polyolefin rubber. Preferably the rubber paste is made by mixing rubber with a solvent that is at least one of toluene, xylene, and methylethylketone. In a preferred form, the short reinforcing staple fibers are present in the amount of 5-50 parts per 100 parts by weight, and preferably 10-30 parts per 100 parts by weight, of the rubber matrix. The present invention also comprehends an improved power transmission belt of the type having a tension section, a compression section, and a load carrying section, wherein the improvement resides in the provision of the above composite material in at least part of the compression section of the belt. It is another object of the invention to provide a method of manufacturing a rubber composite material that provides excellent dispersion of short reinforcing staple fibers in a rubber matrix, without the need to adhesive treat the individual fibers. More particularly, the present invention provides a method of manufacturing a rubber composite material comprising the steps of: providing poly(m-phenyleneisophthalamide) yarn, having an irregular surface (i.e. a surface that has peaks and valleys), in a first predetermined length, and cutting the yarn into a plurality of short reinforcing staple fibres after the yarn has been coated with a coating formed by drying said at least one of rubber latex and rubber paste; and mixing the coated short reinforcing staple fibres in a rubber matrix material. In one embodiment of the method, the yarn is coated by dipping it in at least one of rubber latex and rubber paste. One embodiment of the method includes the step of controlling the amount of said at least one of the rubber latex and rubber paste on the yarn after the yarn is dipped in said at least one of the rubber latex and rubber paste. Preferably, the short reinforcing staple fibers have a length of between 1-50 millimeters. In one form of the invention, the controlling step is carried out by compressing the length of fiber between a cooperating pair of rollers. Preferably, this step is carried out so that said at least one of the rubber latex and rubber paste is present in an amount between 1-10 weight percent with respect to the amount of yarn. The invention also contemplates the step of incorporating the matrix material produced by the above method into a power transmission belt. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a schematic, side elevation view of a system for manufacturing rubber composite material according to the present invention; FIG. 2 is a cross-sectional view of a V-ribbed power transmission belt with a compression section having a composite material according to the present invention incorporated therein; FIG. 3 is a schematic, side elevation view of a machine for testing durability of a power transmission belt in a dynamic environment; FIG. 4 is a graph comparing the number of undispersed short reinforcing staple fibers in conventionally manufacture belts versus belts made according to the present invention; FIG. 5 is a graph comparing the crack generation time of conventional belts and belts made according to the present invention as tested on the machine of FIG. 3. In FIG. 1, a manufacturing system is shown at 10 for producing a rubber composite material, according to the present invention. The rubber composite material consists of a rubber matrix with short reinforcing staple fibers therein and is usable in different environments while being particularly adaptable for use in power transmission belts. The present invention contemplates the use of short reinforcing staple fiber made of poly(m-phenyleneisophthalamide), which is an aromatic polyamide fiber. A preferred form of this fiber in the form of yarn is sold commercially under the trademark CONEX. Through the system 10, the poly(m-phenyleneisophthalamide) yarn is treated to facilitate uniform dispersion in a rubber matrix. More specifically, a roll 12 of poly(m-phenyleneisophthalamide) yarn 13 is provided and is continuously fed consecutively around guide rollers 14, 16, 18, 20 to be immersed in a supply of processing liquid 22. The processing liquid 22 is one of rubber latex and rubber paste which may be the same as or different than the rubber making up the matrix material in which fibers cut from the poly(m-phenyleneisophthalamide) yarn are to be dispersed. The yarn 13 extends downwardly around guide roller 14 and into the liquid supply 22 around roller 16, which is partially immersed in the liquid 22. The yarn 13 passes upwardly around guide roller 18 and back downwardly around partially immersed guide roller 20. By this arrangement, the yarn 13 is twice-dipped into the processing liquid 22. After the yarn 13 is dipped/coated in the processing liquid 22, the yarn 13 is passed between a cooperating pair of squeeze rollers 26, 28, which control the amount of processing liquid 22 which is adhered to the yarn 13. The pressure between the rollers 26, 28 is set so that the processing liquid 22 is adhered in the amount of 1-10 weight percent with respect to the weight of the yarn 13. Downstream of the squeeze rollers 26, 28 the yarn 13 is severed at a cutting station 30. At the cutting station 30, a pair of guide rollers 32, 34 are provided between which the yarn 13 is drawn. The lower guide roller 34 acts as a backing surface for a cutting blade 36 which is directed downwardly to sever the advancing yarn 13 at preselected intervals to define individual short reinforcing staple fibers 38, generally between 1-50 mm in length, which fall down into and are collected in a container 40. Alternatively, a roll cutter could be employed. The aspect ratio of the short reinforcing staple fibers is of the order of 100-1000. In the container 40, the short reinforcing staple fibers 38 are dried at 50°-250°C, and preferably between 80°-150°C, by a hot air dryer, shown schematically at 42, or by such other device known to those skilled in the art which will produce the desired drying effect. Preferably, the treated short reinforcing staple fibers 38 are mixed with the rubber matrix in the amount of 5-50 weight parts, and preferably from 10-30 weight parts, per 100 parts of the matrix rubber. The mixed rubber matrix and fibers 38 are kneaded at a station 44 in the system 10 so that the fibers 38 are uniformly dispersed. The preferred form of rubber latex used for treating the short reinforcing staple fibers, according to the invention, is preferably one of, or a mixture of, styrene-butadiene copolymer rubber latex, natural rubber latex, vinylpyridine-styrene-butadiene-terpolymer latex, nitrilebutadiene latex and chloroprene latex. The rubber in the rubber paste, according to the present invention, preferably is one of, or a mixture of, natural rubber, chloroprene rubber, styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, polysulfide rubber, isobutylene polymer rubber, alkylacrylate-chloroalkylvinyl-ether copolymer rubber and chlorosulfonated polyolefin rubber. The rubber paste is made by using a solvent on the rubber, which solvent preferably is at least one of toluene, xylene and methylethylketone. Following are experimental results which demonstrate the effectiveness of the present invention. Fiber Dispersion Test Text Sample A A 30,000 denier (2 denier X 15,000) poly(m-phenyleneisophthalamide) yarn (CONEX) was dipped in an aqueous dispersion of high saturation polymer rubber latex, containing a nitrile radical of 120 or less of iodine value (hereinafter referred to as "hydrogenated nitryl rubber latex" (H-NBR latex) having 40% solid content, currently manufactured by Nippon Zeon Co., Ltd.) to produce 20 weight percent of solid concentration, then operated on by squeeze rollers so that the adhering amount of rubber latex solid was approximately 5 weight percent with respect to the weight of the fibers, and cut by a guillotine-type cutter into short reinforcing staple fibers of approximately 3 mm in length. The cut fibers were dried at 150°C in a hot blast dryer until the water content was approximately 1-5%. The short reinforcing staple fibers, coated with H-NBR, were mixed at 15 volume % with the rubber matrix compound A (in the following Table 1) in a BR-type Banbury mixer to obtain a rubber composite material in which the short reinforcing staple fibers were dispersed in the rubber. Mixing Chemicals Mixing Ratio A (PHR) Z pole 2020 100 Zinc oxide 5 Stearic acid 1 Oil 5 Carbon Black 40 Antioxidant (3C) 2 Accelerator (TT) 1.5 Accelerator (CZ) 1 Sulfur 0.5 Z pole 202 is hydrogenated nitrile rubber (NBR) produced by Nippon Zeon Co. Ltd. Antioxidant (3C) is N-phenyl-N-isopropyl-p-phenylenediamine (IPPD) produced by Seiko Chemical Co. Ltd. Accelerator (TT) is tetramethyl thiuram disulfide (TMTD) produced by Sanshin Chemical Industry Co. Ltd. Accelerator (CZ) is N-cyclohexyl-2-benzothiazyl-sulphenamide (CBS) produced by Ouchi-Shinko Chemical Industry Co. Ltd. The rubber composite material of Test Sample A was compressed by 305 mm (12inch) rolls to a sheet having a 1 mm thickness to arrange the short reinforcing staple fibers in the direction of rolling. The four rubber sheets were adhered so that the lengths of the short reinforcing staple fibers were aligned in all sheets. The sheets were then cut to a predetermined length in a direction perpendicular to the rolling direction. The resulting composite material was then used to construct a V-ribbed belt, as shown at 50 in FIG. 2. The belt consists of a tension section 52, a compression section 54, and a load-carrying section 56. The compression section 54 was cut to define a plurality of longitudinally extending ribs 58, 60, 62. The ribs 58, 60, 62 define what is referred to as a "K shape" with a peripheral outer length of 1,280 mm. This particular belt configuration is identified in the industry as a 3PK1280 type belt. Test Sample B For a comparison belt, untreated fibers cut from poly(m-phenyleneisophthalamide) (CONEX) yarn having the same thickness and length as those cut from the treated yarn of Test Sample A, were mixed at 15 volume % with the rubber matrix compound A of Table 1 to be dispersed in the rubber matrix material. The composite material was used, as with test Sample A, to form 3PK1280 V-ribbed belts. Ten of the above-described belts (the 3PK1280) were selected at random. The ribs of the belts were visually observed to check the number of lumps of short reinforcing staple fibers having a diameter of at least 0.5 mm. The results are shown in FIG. 4. It can be seen that the number of lumps of short reinforcing staple fibers in belts made according to the present invention is dramatically reduced over belts made by conventional techniques, demonstrating the excellent dispersion of fibers realized through practice of the present invention. Belt Durability Test The above 3PK1280 belt sample was engaged in a reverse bending 4-shaft feeding testing machine, shown at 65 in FIG. 3. The machine 65 has a drive pulley 66, a driven pulley 68, a tensioning pulley 70 and an idler pulley 72. The pulleys 66,68,70,72 are all arranged to be rotatable about parallel axes 74,76,78,80, respectively. The drive pulley 66 had a diameter of 120 mm and was rotated at 4,900 rpm. The driven pulley 68 had a 120 mm diameter and was loaded to 5.4 kg (12 pounds). The tensioning pulley 70 had a diameter of 45 mm. The idler pulley 72 had a dimension of 85 millimeters. The idler pulley 72 was pressed against the back side 82 of the test belts 50 with sufficient force to produce a winding angle Θ of 120°. The system was operated until the bottom of the belts (that is, the inwardly facing side of the belt defined by the ribs 58, 60, 62) cracked at 120°C with an initial belt tension of 30 kg. The results of the test are shown in FIG. 5. The inventive belts ran considerably longer than conventional belts before cracking occurred. The performance of the conventional belts in terms of durability was erratic compared to the inventive belt.

Claims: A rubber composite material comprising: a rubber matrix material; and a plurality of short reinforcing staple fibers dispersed in the rubber matrix material; characterized in that the short reinforcing staple fibers are poly(m-phenyleneisophthalamide) fibers produced by cutting poly (m-phenyleneisophthalamide) yarn into said short reinforcing staple fibers, in that the short reinforcing staple fibers have an irregular surface (i.e. a surface that has peaks and valleys), and in that the poly(m-phenyleneisophthalamide) yarn is coated with a coating formed by drying at least one of rubber latex and rubber paste prior to being cut to produce said short reinforcing staple fibers. A rubber composite material according to claim 1 wherein said at least one of rubber latex and rubber paste is made from the same rubber as the rubber matrix material. A rubber composite material according to claim 1 wherein said at least one of rubber latex and rubber paste is made from a different rubber than that of the rubber matrix material. A rubber composite material according to claim 1, 2 or 3 wherein said at least one of rubber latex and rubber paste coated on the short reinforcing staple fibers is provided in an amount from 1-10 weight % with respect to the amount of yarn. A rubber composite material according to claim 1, 2, 3 or 4 wherein the rubber latex is at least one of styrene-butadiene copolymer rubber latex, natural rubber latex, vinylpyridine-styrene-butadiene-terpolymer latex, nitrilebutadiene latex, and chloroprene latex. A rubber composite material according to claim 1, 2, 3, 4 or 5 wherein the rubber paste comprises at least one of natural rubber, chloroprene rubber, styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, polysulfide rubber, isobutylene polymer rubber, alkylacrylate-chloroalkylvinyl-ether copolymer rubber, and chlorosulfonated polyolefin rubber. A rubber composite material according to claim 6 wherein the rubber paste is made by mixing rubber with a solvent that is at least one of toluene, xylene, and methylethylketone. A rubber composite material according to any preceding claim wherein the short reinforcing staple fibers are present in the amount of 5-50, preferably 10-30, parts per 100 parts by weight of the rubber matrix material. An improved power transmission belt of the type having a tension section, a compression section and a load carrying section, the improvement wherein at least part of the compression section is provided by a rubber composite material according to any one of claims 1 to 8. A method of manufacturing a rubber composite material comprising the steps of: providing poly(m-phenyleneisophthalamide) yarn, having an irregular surface (i.e. a surface that has peaks and valleys), in a first predetermined length, and cutting the yarn into a plurality of short reinforcing staple fibres after the yarn has been coated with a coating formed by drying said at least one of rubber latex and rubber paste; and mixing the coated short reinforcing staple fibres in a rubber matrix material. A method of manufacturing a rubber composite material according to any one of claims 1 to 8 comprising the steps of: providing poly(m-phenyleneisophthalamide) yarn, having an irregular surface (i.e. a surface that has peaks and valleys), in a first predetermined length, and cutting the yarn into a plurality of short reinforcing staple fibres after the yarn has been coated with a coating formed by drying said at least one of rubber latex and rubber paste; and mixing the coated short reinforcing staple fibres in a rubber matrix material. A method according to claim 10 or 11 wherein the yarn is coated by dipping it in said at least one of rubber latex and rubber paste. A method according to claim 12 including the step of controlling the amount of said at least one of rubber latex and rubber paste on the yarn after the yarn is dipped in said at least one of rubber latex and rubber paste. A method according to claim 13 wherein the controlling step comprises the step of compressing the yarn with said at least one of rubber latex and rubber paste thereon between a cooperating pair of rollers. A method according to any of claims 10 to 14 wherein the short reinforcing staple fibers have a length of 1-50 mm. A method according to any of claims 10 to 15 including the step of incorporating the rubber composite material into a power transmission belt.

Inventors: HAMADA TAKASHI, HAMADA, TAKASHI
Applicants: MITSUBOSHI BELTING LTD, MITSUBOSHI BELTING LTD.
Application year: 1991
Main classification: C08J5
IPC codes: B29D29, C08J5, F16G5, B29K21, B29K105