Document Zp7X5MEG4Qd7NeeOxL0bQOjV

EPA-560/ 3-80-001 November I960 PROCEEDINGS OF THE NATIONAL WORKSHOP ON SUBSTITUTES FOR ASBESTOS Arlington, VA, July 14-16, 1980 Sponsored by:' United States Environmental Protection Agency United States Consumer Product Safety Commission Interagency Regulatory Liaison Croup Arlene Levin, Co-editor Contract No, 68-02-3168 Task 17 EPA Project Officer and Co-editor: Hope Plllsbury Office of Pesticides and Toxic Substances U.S. Environmental Protection Agency_________ . Prepared for U.S. ENVIRONMENTAL PROTECTION AGENCY Office of Pesticides and Toxic Substances Washington, D.C. 20460 S-DC-140 U^h SCF-ALLF-08300 S NON-ASBESTOS FRICTION MATERIALS by ' * Michael G. Jacko, Ph.D. Bendix Advanced Technology Center Southfield, Michigan 1 and Mr. Charles M. Brunhofer and Mr. F. William Aldrich* Bendix Friction Materials Division Troy, New York ABSTRACT Friction materials for automotive orakes are complex composites containing three general types of ingredient materials: reinforcing fibers; modifiers that adjust or maintain friction level, wear- rate, and noise properties; and organic resin binders. Historically, the foundation or major constltutent of automo tive friction materials has been asbestos fiber, so chosen because of thermal stability, friction level, reinforcing properties, availability, and relatively low cost. .... Numerous substitutes for asbestos in conventional organic materials have been evaluated, including both naturally occurring and synthetic materials. Direct substitution of these alternative materials in conventional formulations has resulted in poor friction levels, friction instability, roughness, structural failure, increased noise, mating surface deterioration and/or front-to-rear vehicle brake Imbalance. Complete reformulation, not simple substitution, is necessary to meet the numerous, complex performance requirements of consumers, manufacturers, and government standards, such as FMVSS 105-75 and FMVSS 121. In the 1960's, a new class of friction materials, called semlmetallies, was developed to meet severe braking requirements, primarily In heavy-duty disc brake and extreme duty truck block applications. Semlmetalllcs operate satis factorily against the ventilated cast-iron rotors in the smaller brakes of downsized cars, as well as against the solid rotors found in the lighter brakes of new front wheel drive vehicles. Semlmetalllcs rely on steel fiber and pow der metallurgy techniques for reinforcement, and do not require asbestos. The Improved performance of semlmetalllcs compensates for their higher costs due to more expensive ingredients, higher specific gravity, and more costly pro cessing requirements. Overall development took more than 10 years from intro duction to significant customer acceptance. Presented by Mr. Charles M. Brunhofer 9 .i-t.iiiM aih Um mnwl> iUflolt and costly i 'iipwat, CwhvmUi, an tUltload ev class of sr fmltfont, ipieMfaU; lor irm liming iffUtf swot effort la atcmilfi not nlf to confirm the oi of these oo substitute fiber formulations, feat tailor technique!, tfeaoo pew .type friction matsrtala ever, doe to the In(rodlent! end new proteanInf u be viewed, quite simply, so energy trsnsformsrs. ton. the friction materiel (etetor) makes eootect with ac (rotor)t creating a friction force reflating the the two bodlea. The energy of notion la transformed < la dlaalpated, primarily through the rotating aenber. . :t, the friction material neat operate In a rather hostile oak. toperetures in eaceas of 400*C (750*F) ete not ree at the contact Interface can exceed tS0C (15(0F). he-road operating envlroorient (dust! aid, oalt, water, problem. The friction material most peeeeaa an optimized itlca, and maintain thoee characterlatlce throughout a of vehicle operation. characterlatlca *ef friction materlala are Hated In ml nust he adequate end atable over a vide range of plication pressurea, and temperatures, regardleas of the of the material. Of particular interest are the fade/ ticsI l.e, the ability to resiet friction level deterloid to extreme elevated temperatures (the fade) and then to de friction level on cooling (the recovery). The_frlctlon good wear properties lor long life, hut It moat else'net - r or grooving on the mating disc or dnn. excessive comi and roughness (chatter, vibration, pulsation) must be tlvlty to moisture must be minimised.' Finally, the friction tpablc of being manufactured with consistency at a reasonable nltlona of these characterlatlce, end their Interaction and rave been dlacuseed at length by Aldrich end Jacks.1 In gento Improve upon one characteristic often reeulce In the deterloharacterlatlca. The development of frletloo materials la zi, Interactive process seeking an optimized combination of israeterlatlcs. ce of numerous brake designs provides another level of com yilng friction materials. Linings for drum brakes require a -cpertlce. The duo-servo drun broke (the most popular U.S. two different types of linings, designated primary and , .' ` )' , . r ' _ ' 10 I I TABLE 1. CHARACTERISTICS OF FRICTION MATERIALS Friction Level (coefficient) Stability - speed - pressure - temperature - conditioning - age Fade/recovery Wear a Friction material * Drum or disc * Noise Roughness Moisture sensitivity Manufacturability Processlblllty Uniformity secondary, each of which needs different properties of strength, wear resistance, friction level and friction stability. The non-servo drum brake (used on many sub-compact vehicles) requires a friction material which encompasses the best characteristics of the primary and secondary in a single formulation, with emphasis on low-temperature properties and static friction capability. The large hydraulic and air-operated drum brakes utilised on medium and heavy trucks require. In general, the maximum properties of the smaller vehicle linings but at significantly higher operating temperatures. The arcuate form of drum brake linings places additional restrictions on the formulation, because of processing requirements. . Disc brakes demand a totally different set of operating conditions for the friction materials. Disc brakes generally operate at significantly higher tem peratures than equivalent drum brakes, and the front disc brakes run hotter than the rear drum brakes on the same vehicle (Table 2). The friction material for disc brakes must be specifically designed for these higher temperatures, and must possess a higher coefficient of friction and better wear characteristics across the temperature range. Friction-material formulations must also be tailored to the specific needs of the particular vehicle application. Numerous parameters such as vehicle weight, front-to-rear brake balance, actuating system design, and duty cycle affect the capability of a particular lining formulation to perform satisfactorily. The existence of numerous complex performance standards emanating from con sumers, associations, manufacturers, and government agencies provides an addi tional set of parameters that friction materials must meet. Significant dif ferences can exist between friction materials used as original equipment in new vehicles and friction materials available as replacement parts in the after market. Each vehicle manufacturer has a unique, extensive set of te&t and acceptance standards to .ensure the safety, durability, and performance of its products and the components used therein. Government-instituted requirements exist at the federal, state, and local levels. Federal requirements include those promulgated by the Department of Transportation (vehicle performance), the Occupational Safety and Health Administration (manufacturing work practices), and the Environmental Protection Agency (manufacturing practices and raw materials). In order to meet the many characteristics outlined thus far, friction mate rials for automotive brakes have developed as complex composites containing three general types of ingredient materials: reinforcing fibers; modifiers that adjust or maintain friction level; wear rate and noise properties; and . organic resin binders. Historically, the type of friction materials used In most automotive applications has been conventional organic friction material. The foundation or major constituent of conventional organic friction material has been asbestos fiber, so chosen because of its unique'combination of char acteristics. Asbestos fibers provide reinforcement, possess a high coefficient of friction, and more importantly, have excellent thermal stability. The open ness of the fiber, lt6 adsorptiveness, and its compactablllty enhance the pro cessing and uniformity requirements. Finally, asbestos fibers have been avail able in a variety of grades at a relatively low cost. V r TABLE 2. BRAKE FADE TiMPERATURES (*F) . (SUBCOMPACT FRONT WHEEL DRIVE VEHICLE) 1st SAE fadeb (10 stops) 2nd SAE fadeb (15 stops) Disc front Drum rear Disc front Drum rear Combination 1 780 300 870 360 Combination 2 985 235 1030 240 Combination 3 760 200 855 235 temperatures measured before the last stop. Actual temperatures for disc brakes are 80 to 180F higher, as measured below the rub bing surface. Surface temperatures' are In excess of 1600F (870C). bSAE J8A3c 13 C *. . . J Once the friction material has been cured, the asbestos fibers are locked into the matrix. During brake operation, the high temperatures generated at the Interface convert more than 99.7 percent of the asbestos to non-fibrous residues'(primarily olivine) In the wear debris,2 and less than 0.02 percent asbestos becomes airborne.*'* The conventional organic formulations and the processes by which they are made have been dependent upon and tailored to the physical and chemical proper ties of asbestos. Two courses of action are open for elimination of asbestos . from automotive friction materials: 1. Develop a new generation of friction materials, designed from the start-without asbestos in mind. 2. Attempt to substitute an alternative fiber system for the asbestos In conventional formulations, with subsequent modificatlon of composition and process techniques. Bendlx Is aggressively pursuing both courses of action. As Mr. William Agee, our Chairman and Chief Executive Officer, has stated, Bendlx Is committed to being asbestos-free at the earliest possible date within this decade. SEMIMETALL1C DISC PADS 1 Properties In the 1960s, a new generation of friction materials, called semlmetalllc, was developed to meet severe braking requirements which organics could not meet. Class A organics (typical U.S. materials), which perform well In low and mod erate temperature duty, are prone to fade and exhibit compressibility and poor wear resistance at high temperatures. Class B organics (typical European and Japanese materials) provide good high-temperature wear and friction levels, but have poor low-temperature wear resistance, produce rotor scoring and/or wear, and are prone to being noisy. Semlmetalllcs were initially develed for these extreme, high-temperature applications.8 Semlmetalllcs rely on steel fiber and powder metaUurgy techniques for re Inforcement. Various property modifiers are added to enhance performance to desired levels, with a resin binder holding the materials In a uniform solid mass. Semlmetalllcs may contain metallic powder, sponge iron particles, ceramic powder, steel fiber, rubber particles, graphite powder, and phenolic resin.87 Some manufacturers utilise a backing layer of a different composition which can contain asbestos. . Problems Overcome Inherent In the uniqueness of the semlmetalllc formulations and their per formance properties were a number of significant problems which required reso lution. Concentrated development effort was required to resolve both proces sing and performance related issues. Processing Issues Included: the unifor mity of the raw materials mixtures, the ability to form and handle the in process material, and the ability to manufacture high-quality parts consistently. Performance Issues Included: materials strength, cold friction properties, initial wear resistance, and attachment to the backing plate. The development effort on semiaetalllc friction material has been continuous, not only to fur ther Improve Its characteristics and properties, but also to overcome the prob lems inherent in accommodating new vehicle applications. Semlmetallics gained acceptance because they were able to solve some of the problems that could not be overcome using Class A or ClasB B organics. The improvements/advantages,are listed in Table 3. The key element is the attainment of overall excellent properties at both low and high temperatures. Semlmetallics cost more because of more expensive ingredients and a costlier process, but the improved performance capabilities offset these factors. An Increased usage of semlmetallics has occurred over the past few years. The downsizing of vehicles, with resulting smaller front brakes and higher oper ating temperatures has given impetus to increased use of semlmetallics.6 It is expected that the trend toward asbestos-free semlmetalllc disc pads will continue. SEM1METALLIC DRUM LININGS An obvious alternative to conventional, organic drum brake linings is the use of semlmetalllc material for drum linings. In fact, one of the first applications for semlmetallics was for air brakes on heavy-duty trucks used in the logging Industry--an extremely severe application. Significant development effort has been expended on semlmetalllc drum brake linings. However, the basic nature of semlmetallics does not lend Itself to the arcuate segment configuration required for small drum brakes. The semimetallic mix does not. possess the necessary green strength, is difficult to . bend into the arcuate shape, and is more brittle in its cured form and there fore subject to cracking. Modifications to the formulation to facilitate proces8ibillty generally result in a product that cannot achieve commercially acceptable performance characteristics. These difficulties present a clear challenge, and development work on semlmetalllc drum brake linings continues. ' ALTERNATE FIBERS/REINPORCERS . Properties __ - Alternative fiber systems in conventional organic formulations represent the second course of action open to friction-material manufacturers. Table 4 is a summary of the properties of some of the various materials which could be considered as alternate reinforcements. Since conventional organics and Bemimetallies have traditionally been reinforced with asbestos and steel fiber, respectively, these fibers are also Included in the table for comparative pur poses. The data in Table 4 were obtained from the material manufacturer's literature and extensive characterization data developed at Bendlx. Charac terization Included scanning electron microscopy and x-ray energy spectroscopy (SEM/XES). The selection of suitable alternate materials must also consider 15 TABLE 3. IMPROVEMENTS OFFERED BY SEMIMETALLIC DISC FADS Improved energy absorption Fade resistance' e Temperature insensitivity In-stop fade Speed spread Friction stability (FMVSS 105-75) Higher temperature capabilities (life) Rotor compatibility Scoring _ Heat cbecklng ._ Reduced noise Smaller brake sizing 16