The Chrysotile Content of the Wear Debris of Brake Linings

Our plain-English summary of The Chrysotile Content of the Wear Debris of Brake Linings, by D.M. Rowson (1977).

The study

• Author: D.M. Rowson
• Published: 1977
• Type of article: Journal article
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This was a laboratory study by a single author of the dust produced when disc brake pads containing chrysotile asbestos wear. A disc brake was run on an inertia dynamometer inside a sealed box to simulate normal car braking, mainly at a brake application temperature of 175 °C and also at 40 °C. More than 80% of the wear debris was collected on a filter. Electron microscopy showed many asbestos fibres and fibrils on used and unused pad surfaces. The wear debris instead showed particles of 10 - 20 nm and larger clumps. A normal fibril appeared in only one field of view, suggesting a fibril concentration below 0.5 wt.%. X-ray diffraction found crystalline chrysotile in the pad material but not in the debris. Debris heated to 1100 °C showed the mineral forsterite. Thermal analysis showed none of the changes typical of chrysotile. The author concluded that the chrysotile was present in some chemical form, but not in its original fibril form, and suggested it had become amorphous or forsterite. The author cautioned that the forsterite finding was not definite evidence of what happened during wear.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment: controlled brake wear tests on a disc brake mounted on an inertia dynamometer. The collected wear debris was examined by electron microscopy, X-ray diffraction, thermal analysis (DTA and TGA) and neutron activation analysis.
• Who was studied: No people were studied. The material was wear debris from disc brake pads (plus iron worn from the brake disc), produced in a sealed box under conditions the author describes as typical of normal saloon car driving. Brake application temperature was 175 °C, with additional debris produced at 40 °C. The author's affiliation was The Higher Petroleum Institute, Tobruk (Libya). No study dates are given.
• Exposure: Chrysotile asbestos in fibrous form in moulded brake friction material. The author states that friction materials in general contain some 20 - 50 wt.% asbestos. Asbestos content of the friction material and the debris was traced through scandium, present at 4.2 ppm in raw chrysotile fibre.
• Main results:
  • More than 80% of the wear debris produced was collected and analysed.
  • Electron micrographs of used and unused pad surfaces showed a profusion of asbestos fibres and fibrils, with no significant difference between used and unused surfaces.
  • Electron micrographs of the wear debris showed particles of size 10 - 20 nm together with larger agglomerates. The results section states no fibres were present.
  • Across many fields of view of the wear debris, a normal fibril was found in only one, suggesting a fibril concentration in the debris of less than 0.5 wt.%.
  • X-ray diffraction identified crystalline chrysotile asbestos in the pad material but not in the wear debris. When the debris was heated in vacuo at 1100 °C, the mineral forsterite was positively identified.
  • In thermal analysis of debris containing the equivalent of 50 mg of chrysotile, neither the endothermic reaction at 650 °C nor the exothermic reaction at 780 °C typical of chrysotile was observed. There was a mass loss at about 780 °C that the author describes as unexplained.
  • Neutron activation analysis indicated that all other components of the friction material were present in the wear debris in the same proportions as in the original friction material. Some 20 wt.% of the debris was iron from the disc.
  • Examination of debris produced at 175 °C and at 40 °C brake application temperatures showed no significant differences.
• The authors' conclusion: The author concluded that the chrysotile worn from the pad was present in some chemical form in the debris, but not in its original fibril form. The author suggested the structure had been reduced to an amorphous state or to forsterite. The author added that the elements of chrysotile are present in the debris, and that if the health hazard of chrysotile is found to be associated with its chemical behaviour in body tissue rather than its physical form, the implications of these findings should be considered.
• Limitations the authors state:
  • The presence of forsterite in debris heated to 1100 °C is not definite evidence that forsterite formed during wear.
  • The absence of a chrysotile X-ray diffraction pattern may be due to very small, highly strained particles rather than to the absence of chrysotile material.
  • There is insufficient information about how small particles of chrysotile or forsterite behave to be certain of the interpretation of the thermal analysis data.
  • A mass loss at about 780 °C on the thermogravimetric trace of the debris remained unexplained.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Brake Lining, Fibril, X-Ray Diffraction (XRD), Crystalline, Forsterite.

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