By: Arnold Anderson
Background South San Francisco Bay was declared an impaired water body by the California State WaterResources Control Board in 1989 under Section 304(1) of the federal Clean Water Act. USEPA water quality criteria were periodically exceeded for nine heavy metals (cadmium, chromium, copper, lead, mercury, nickel,selenium, silver, and zinc). Recent data (Chen '96) shows that the South Bay has high total and dissolved copper levels, especially in the winter. Copper concentrations throughout the estuary are known to rise with distance from the ocean, both North and South. However, south of the Dumbarton Bridge (toward Silicon Valley), copper measurements rise sharply. Water districts in the impaired region have reduced copper levels from point sources. USGS monitoring data indicates a 90% decrease of wastewater copper between 1980 and 1990. Non-point sources of copper remain aproblem. The Santa Clara Valley Non-Point Source Pollution Control Program funded studies which estimated in 1994 that 7,717 pounds of copper enters the South Bay each year from disc brake pad wear (WCC '94). If correct, this would be quite significant since the estimated total copper load from all sources was 14,600 pounds (WCC '91) and 17,400 pounds (Lacy '93). In February of 1994, an agreement was reached that required non-point sourcerunoff controls which would reduce copper loadings into the bay by 20%, with an extra 950 pounds of reductions for South Bay loadings. The 950-pound copper reduction was targeted through stormwater controls. The entire bay area has had protracted problems with heavy metals, some dating back to the gold rush era. Copper levels in the Bay are known to vary with time and location. For example, high dissolved copper levels are endemic to the South Bay, primarily in the winter. North Bay regions have much lower dissolved copper. Some North Bay locations at times have higher levels of suspended copper. Copper concentration data (e.g., time, form, and distribution) does not appear to have been used to diagnose possible brake lining copper effects. Introduction It is essential to have some understanding of where copper is used in friction materials, why it isused, and what happens to copper in the wear processes. Copper-containing friction materials are employed in brakes and clutches of vehicles and machinery used everywhere. However, three applications produce most of thecopper wear particulate in the region surrounding the Estuary. These are sintered aircraft disc brakes, heavy duty drum brakes, and passenger car/light truck disc brakes. Applications that may use copper-containing friction materials, but with minimal annual wear, are motorcycle brakelinings, light aircraft organic disc brake linings, light rail disc brake linings, and heavy-duty clutch facings. Other friction material applications, such as railroad tread brake shoes, have inconsequential copper content. These will not be discussed. To provide a basis for discussion, a description of the three main applications follows, then reasons why copper is used in these friction materials, and finally what happens to the copper, or copper-containing constituents, during friction and wear processes. Existing reports are reviewed, then consequences of friction material wear on copper level are discussed for the entire region, but with primary focus on the South Bay.
Disc Brake Linings (Pads) for Passenger Cars and Light Trucks
Over the past twenty years, changing vehicle design requirements have brought about changes to the compositionof disc brake linings. Most of the early disc brake linings were 'organic.' Organic originally meant that the friction material was held together with an organic resin. At that time, all organic brake linings used chrysotile asbestos asthe primary constituent. As passenger cars increased in performance, and light trucks increased in both performance and payload weight, new brakes were developed. Federal brake standards of the early 1970s led to increased usage of disc brakes, especially on front axles. By the late 1970s, brake lining compositions also started to change as aconsequence of fuel economy improvements and widespread concern about asbestos toxicity.
One new friction material class was called resin bonded metallic. These became widely known as 'semi metallic' or 'semimet brake linings.' Most of the original semimet brake linings used steel wool and iron powders as the major constituents, but also contained graphite, binder resins, and minor other ingredients. In the US, semimet pads became the dominant disc brake lining material for OEM (original equipment manufacturer) passenger cars and light trucks. One shortcoming of semimet brake linings was their high wear rate during high-speed brake applications. For example, a single hard stop from 100 mph was found to provide the same amount of brake liningwear as 120 similar stops from 50 mph. This provided no major concern for US passenger cars and light trucks with existing highway speed limits.
High performance vehicles, race cars, sports cars, and high-performance touring cars, evolved different disc brake linings, based on earlier 'type B' asbestos-based formulations. These employed substantial amounts of copper, in powder, wire, sulfide, and oxide forms. Some formulations also used copper alloys, generally in the form of brass chips.
High copper content brake lining formulations became popular for cars in European countries, where some roads had no speed limits. Performance sedans and performance image cars from Asian manufacturers also developed similar disc brake linings of high copper content. The usage of high copper brake linings provided manyperformance benefits for high temperature braking, especially from high speeds.
Every brake lining in the Woodward-Clyde study (WCC '94) with copper content over 0.25% was manufactured inAsia or Europe. The Ford Escort disc pad, 9% copper, was originally developed for the Mazda 323 vehicle series. It replaced a US material that contained no copper but used asbestos.
Light truck disc brakes were not included in the WCC study. In California, and throughout the US, light trucks continue to grow in popularity. Their brakes tend to be similar to those used in many US passenger cars, except forsome large utilitarian trucks. Semimet disc brake linings (essentially copper free) continue to be used in most US light trucks. However, Asian sport utility vehicles use copper containing non-asbestos brake linings.
Arnold Anderson, known as Arne in the brake technology world, has devoted most of his professional life to studies in tribology, the science of friction and wear. Much of this was done as a researcher at the Scientific Laboratory of Ford Motor Company. After his retirement in 1987, Arne was a consultant to vehicle manufacturers, brake manufacturers, and regulatory groups. He recently retired after having accumulated 12 patents, 42 technical papers, as he performed diagnostic studies of car, truck, aircraft, and aerospace brakes and clutches.