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Further requirements of modern friction linings are a sensitive reaction of the brake even with low-force application, a uniform braking progression, a very low-noise emission, and a reduced environmental load due to abrasion and odor emission.
Neither the friction pad nor the brake disk should be subject to rapid wear. In addition low weight and a good cost/benefit ratio are required.
Already in 1984 the Borg-Warner company applied for a patent (Albertson and Nibert 1984) on a “pad-type friction material for brake and clutch linings containing an inorganic fibrous material, a binder, zinc oxide and molybdenum sulfide.” The wollastonite was used as a fibrous material replacing the asbestos used in those applications till that date. “In the past the fibrous material has invariably been chrysotile asbestos. Asbestos fiber imparts strength to the friction material and, at the same time, has a beneficial effect on friction properties. Moreover, in the past, it has been relatively cheap.
However, asbestos deposits throughout the world are rapidly being depleted, with the result that its cost is increasing. Moreover, the continued use of asbestos has recently come under a cloud because of the realization that it presents a serious cancer hazard for those who work with it. There has been an increasing concern about such hazard with the result that the expense of health precautions now required for the safe handling of asbestos has become a significant burden. Accordingly, it has become desirable to eliminate asbestos from frictional material formulations and to find a replacement.” The preferred fibrous material replacement was wollastonite or a mixture of wollastonite and glass fibers.
The mode of operation of the HAR wollastonite in comparison to the raw material magnesium carbonate and mineral fiber used as a rule in brake linings was investigated by HPF (HPF - The Mineral Engineers 2008) for passenger car and truck brake pads as published in detail.
The investigations were performed on a standard brake pad recipe. In the four series of tests (Table 7), the basic raw materials magnesium carbonate and the mineral fibers were successively replaced by the HAR wollastonite.
The influence of HAR wollastonite in friction linings was assessed in accordance with the following parameters: bulk volume after 4 min-mixing time, green strength, shear strength, friction values in accordance with the AK master test program (brake- program simulation test), and wear of pad/disk (Table 9).
After mixing for 4 min in a Papenmeier mixer, the materials were compressed at 300 daN/cm2 and cured at finally 155 °C with a curing program lasting for 390 s in total.
Table 9 Test series
“Mineral fiber with a fiber length from 125 ± 25 |rm bMagnesium carbonate with a grain size of 6% > 40 |rm
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