5.3 - FRICTION MATERIALS MANUFACTURING
Friction materials, such as brake linings or brake pads, are usually manufactured by variations of the same dry process. This involves material preparation resulting in the mixing of dry components, preforming this mixture in cold presses, hot pressing and curing the preformed pieces, and cutting, grinding, drilling as well as finishing. The pieces can be shaped during or after hot pressing. (There are also so-called wet methods and extrusion methods.)
The dry mixture consists of three major components:
1) the binder, usually phenolic resin including extenders,
2) the reinforcing material, normally chrysotile asbestos fibres, and
3) functional fillers to improve the properties or performance of the final product.
The latter are a mixture of metallic materials (e.g. lead, brass), non-metallic materials (e.g. alumina, barite), and carbonaceous materials (e.g. graphite, gilsonite).
Chrysotile asbestos, the main constituent in most friction products, is received in 50 kg bags on pallets of 20 bags. (Other types of asbestos, i.e. crocidolite or blue asbestos, or amosite or brown asbestos are not used in the manufacture of friction materials.) For receiving and warehousing asbestos, the same precautions must be applied that have been discussed for asbestos-cement manufacture. Other materials, e.g. resin or functional fillers are generally received in bags or drums and must be stored in a warehouse or in the plant. Some of these materials are toxic and care must be taken not to generate dust during storage and handling.
Material Preparation
Initially, the chrysotile asbestos is debagged as in other processes. This is done by a variety of methods ranging from manual operations without hoods to automated debagging stations. Debagging of asbestos presents a special problem and appropriate precautions must be taken to avoid creation of dust. This involves the establishment of a proper debagging station.
Debagging is followed by a dry fiberizing step which requires an enclosed fiberizer to avoid the creation of dust. The dry methods used for this purpose, such as cage milling and hammer milling, unfortunately have a tendency to be dusty. This can be avoided by connecting the fiberizer to a bag house that provides negative pressure.
Various asbestos grades are often used for manufacturing friction materials. These grades may be combined prior to fiberizing to achieve thorough blending. Alternatively, the different fibre grades may be fiberized separately.
Asbestos and other raw materials are then weighed and blended. The resulting moulding compound is collected in drums. Depending on the procedure and equipment used, this can be an extremely dusty process.
Debagging, fiberizing and blending can be carried out in a single process. However, in actual practice, a batch process is often used for these steps. This means that the various ingredients used in each step are weighed out separately, usually in the open without special precautions. This includes the individual grades of asbestos as well as the fiberized asbestos. Similarly, the feeding of all materials to the blender is carried out in the open without the use of hoods.
The entire material preparation procedure can be potentially very dusty. Obviously, the process should be changed to eliminate or reduce the number of manual handling steps which take place in the open. One possibility is to automate the entire process. For this purpose, all materials including asbestos, should be debagged automatically, and stored in sealed bins. All materials should then be weighed and blended automatically, and the resulting mixture should again be collected in closed containers. These containers should be mobile so that they can be transported to the preforming presses without creating dust. Alternatively, the compound may be transferred by sealed conveyor to the presses.
Preforming
Under normal practice, the dry blended asbestos-resin-filler (moulding compound) is delivered in open drums to the preforming presses. It is then weighed and transferred to the mould for the pressing operation that produces dry-pressed units. All steps are usually carried out by hand and in the open. Obviously, this is a potentially dusty operation, which can be avoided if the procedures are changed. For example, the moulding compound should be delivered in closed containers and stored in a glove-box style hood adjacent to the press. The hood should be directly connected to the press so that the weighing of the material and the filling of the moulds can be carried out inside the hood. It should be possible to move the filled mould to the press without removing it from the hood. For this purpose, the space of the press that houses the piston and the base for the mould should be completely enclosed and connected to the hood. Appropriate windows and access doors must be provided and the hood must be connected to a bag house.
Another approach to avoid the creation of dust is to automate delivery, weighing, mould filling and pressing. That is, the compound could be weighed and placed into the mould automatically without the need for manual intervention. Likewise, the mould would be placed under the piston and pressing would proceed automatically. This would allow the process to be carried out under a completely closed hood. The glove-box type hood or the enclosure for the automatic press should be connected to a bag house to receive proper exhaust ventilation.
Hot Pressing and Curing
The next step in the process is hot pressing of the preformed pieces. Usually, these pieces are moved manually from the preforming presses to the hot presses and placed into moulds. This operation can be dusty and proper precautions must be taken by the worker involved with these tasks when dust levels exceed the permissible exposure limit. In these cases, it is necessary to wear personal HEPA respirators and protective clothing (disposable overalls, or overalls that are laundered periodically).
After hot pressing and curing, the moulded pieces are removed from the mould and moved either by hand or automatically to the next process step. Although the asbestos fibre is locked into the cured resin at this stage, workers performing these tasks may still be required to take precautions, i.e. when the dust levels rise above the permissible exposure limit.
Cutting, Grinding and Drilling
During these steps, the moulded pieces are cut to proper dimensions, shaped and bevelled by grinding and if necessary, fitted with holes for fastening to brake shoes. Grinding, in particular, is a very dusty procedure. The hoods provided for these operations are usually inadequate to prevent the release of dust. These operations should be automated or semi-automated to allow a redesign of the hoods to accommodate as much enclosure as possible without interfering with the operator.
If manual operations are retained, in most cases, a redesign of hoods is necessary. Hoods must be sufficiently close to the operation so that all dust is caught. They must also be sufficiently large so that there is no interference with manual operations. It is preferred that during the actual operation, i.e. pressing, cutting, grinding or drilling, the front of the hood is closed by a transparent door. This would ensure that dust does not escape from the interior of the hood. Also, this would reduce the required bag house capacity for each of these operating stations, considerably improving the effectiveness of the dust control of the plant.
Finishing
This process involves polishing, painting, riveting, etc., followed by packaging. The procedures must be carefully monitored and personal HEPA respirators and protective clothing (disposable or washable overalls) must be worn if the dust levels rise above the permissible exposure limit.
Wet Processes and Extrusion
The so-called wet process for friction materials is a misnomer. It refers to the use of solvents to prepare a blend of raw materials that is wet rather than dry as in the conventional method. This allows the use of binders, such as specific resins, that cannot be used otherwise. Usually, the blended compound is present in the form of a dough that is dried and fluffed to be used as other blends. Solvents are costly and they are usually not safe, both from a health or fire hazard standpoint. In addition, the fluffing operation is a dusty process and consequently, it may negate all dust control benefits that were gained initially.
There is, however, some advantage to be gained with respect to dust control if the compound is extruded to exact size requirement, followed by a combination of pressing and solvent removal. Such a process could be completely automated. Of course, the other operations that tend to generate dust, such as fiberizing of asbestos, cutting, grinding and drilling of the friction material, are not positively affected.
Impregnation
Some brake linings are produced by soaking a tape of woven asbestos in resin, followed by curing. The asbestos yarn used for weaving the tape contains other materials, such as brass wire, to improve the performance of the lining. The tape can be cut to appropriate length and.
is often used for special applications or by smaller brake repair shops that serve a large variety of cars and trucks.
Impregnation is also used for the manufacture of clutch facings, whereby asbestos yarn is soaked in resin and wound onto a spindle prior to curing.
These installation and manufacturing methods may require cutting, grinding and drilling. Therefore, the worker involved in these activities can be exposed to asbestos dust and must take necessary precautions, such as wearing a personal HEPA respirator and protective clothing (disposable or washable overalls). HEPA vacuum equipment should be used for clean-ups.
Waste Disposal
During the manufacture of friction materials, bag houses are used to provide exhaust ventilation and to collect dust and debris produced during grinding, etc. The solid material collected is deposited in a container located underneath the bag house. Ideally, these containers should be self-contained so that they can be removed by forklift truck and shipped to an approved disposal site for burial of the waste. Water can be added to these containers to suppress dust during emptying.
Alternatively, the containers used for collecting solid material should be lined with a plastic liner or bag. These bags should be sealed and transported to a suitable disposal site, where the entire bag should be buried. The collection or transport of solid waste in open bags or containers should not be permitted. The worker concerned with waste disposal activities can be exposed to dust and, therefore, must take the necessary precautions.
5.4 - AUTOMOBILE BRAKE REPAIR AND INSTALLATION
Brake linings, brake pads, clutch facings, etc., consist essentially of three major components, i.e.
1) a binder (usually phenolic resin modified with additives,
2) a fibre reinforcing agent (usually chrysotile asbestos, about 50 % by weight), and,
3) a property modifier (metallic, non-metallic and carbonaceous fillers).
Asbestos fibre may be found in the dust deposited on brake assemblies and as a result, measurable concentrations of asbestos fibre exist in the work environment of workers engaged in repair operations. Therefore, precautions must be taken to prevent exposure of the worker to asbestos fibre that may occur during automotive brake repair and installation.
The purpose of this discussion is to describe the different methods which can be used by workers to assure that their potential exposure to asbestos during brake lining repair or installation is avoided, or kept to as low as level as is practicably possible.
It is normal practice in automobile repair shops to start the repair process by removing the tire and wheel rim assembly from the automobile, truck, etc. In the past, the wheel and brake assembly was cleaned by using a compressed air hose and/or various types of brushes. Needless to say, this operation generates dust which is released into the workplace atmosphere. Because of the risk to workers, use of compressed air is prohibited and should never be condoned.
Several alternative methods exist. One consists of spraying the wheel/brake assembly with a fine mist of water to thoroughly soak the dust. This is followed by a stronger jet of water to wash the wetted dust off the assembly. A sufficiently large container is positioned underneath the assembly to collect the contaminated water. This water can be discarded into the sewage system.
An ordinary garden hose can be used for this cleaning operation, provided it is fitted with an adjustable nozzle that produces a fine spray as well as a concentrated jet of water. Attachments for garden hoses designed to spray insecticides or fertilizers are available for this purpose. These can be used with a non-foaming detergent (e.g. dish washer detergent) to improve the wetting of the dust.
Alternatively, it may be more convenient to use a manually pressurized spray container (or tank) such as those used for spraying garden insecticide. The nozzles for these containers usually have an excellent mechanism for the control of the water spray. Also, a non-foaming wetting agent may be added to the water in the container to assure rapid and thorough wetting of the dust.
This is one of the simplest and most efficient methods of preventing dust formation during brake repair. It requires no special equipment, and therefore, can be used practically anywhere. However, like most other manual operations, it requires a certain amount of awareness and skill.
A second method consists of a compressed air hose fitted at the end with a bottle of solvent that can be sprayed onto the brake assembly to loosen the deposited dust and to capture the resulting airborne dust in the solvent mist. The worker should begin spraying the parts that may be contaminated by asbestos with the solvent from a sufficient distance to ensure that the dust is not dislodged by the velocity of the solvent spray. After the dust is thoroughly wetted, the spray may be brought closer to the parts to remove grease and other materials. The parts sprayed by the solvent mist are then wiped clean with a rag that must be disposed of appropriately. Rags should be placed in a labelled plastic bag or other container while they are still wet. This assures that the asbestos dust will not become airborne again after the brake and clutch parts have been cleaned. If clean-up rags are being laundered rather than disposed of, they must be washed using methods appropriate for the laundering of asbestos-contaminated materials.
A variation of the compressed air/solvent mist procedure is said to have certain advantages, both in terms of cost and worker protection. This variation involves the use of pressurized spray cans filled with any of several solvent cleaners commercially available from automotive supply stores. Spray cans of solvents are inexpensive, readily available, easy to use, and they save time because the air hose/mister system does not have to be assembled. Also, the spray can will deliver solvents to the parts to be cleaned with considerably less force than the air hose/mister system, and will therefore produce less airborne dust.
These solvents can also be delivered from tanks that are pressurized manually.
The last, and most expensive, method uses the Enclosed Cylinder/HEPA Vacuum System. It consists of three components:
1) a drum-shaped steel cylinder with a hard plastic window, designed to cover and enclose the wheel assembly;
2) a compressed air hose and nozzle that fits through a port of the cylinder to facilitate cleaning of the brake parts inside the cylinder; and,
3) an HEPA vacuum cleaner used to evacuate airborne dust generated within the cylinder by the compressed air.
The cylinder is fitted with a pair of rubber gloves that permit the worker to reach inside the cylinder. At the rear of the cylinder, a triple pleated fabric forms a seal around the axle behind the wheel. The cylinder effectively isolates dust from the workers breathing zone.
Cylinders can be mounted on a stand to provide convenient brake installation on vehicles on garage lifts. They come in two sizes to fit brake drums in the 7-12 inch size common to automobiles and light trucks, and 12-19 inch size range common to large commercial vehicles.
To operate the system, the brake assembly is enclosed in the cylinder after removing the tire and wheel rim assembly from the vehicle. The worker then reaches into the cylinder using the gloves and cleans the brake assembly by discharging compressed air at the brake assembly components. The worker continues to use the compressed air to keep the residual dust airborne, so that it can be removed by the HEPA vacuum cleaner. The HEPA filtered vacuum remains in operation during the entire procedure.
The brake assembly is then dismantled, repaired or replaced using tools which had previously been placed in the bottom of the cylinder. When the operation has been completed, the worker cleans all remaining exposed parts with compressed air until no visible dust remains in the cylinder. The cylinder may then be removed safely.
The HEPA filter is capable of removing all particles greater than 0.3 microns from the air. When the vacuum cleaner filter is full, it must be replaced according to the manufacturer's instructions, and appropriate HEPA dual cartridge respirators must be worn during this process. The vacuum cleaner's filter is assumed to be contaminated with asbestos and should be handled carefully, wetted with a fine mist of water, placed immediately in a labelled plastic bag, and disposed of properly.
The HEPA vacuum cleaner can be disconnected from the cylinder when the cylinder is not in use. It can then be used for clutch facing work, grinding or other routine cleaning. In these cases, the material collected must be removed from the vacuum cleaner by using the plastic bag placed inside the cleaner. This bag should be placed into a labelled plastic bag or container for proper disposal at designated landfill sites.
It should be noted that many of the fibres used as substitutes for asbestos in the manufacturing of brake linings are also considered to be potentially hazardous to health and, therefore, the same precautions must be taken when handling these materials.
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