The pH value of mineral fibre

Category Industry Insights Published 2026-04-02 Source www.boerfibre.com

Abstract

As asbestos-free conversion accelerates in the friction and sealing industry, engineers are watching substitute-fibre indicators closely — especially whether reinforcement fibres corrode brake-mating hubs or sealing flange faces in humid environments. The article, by Wen Yi of Qing Yuan Boer Fibre, sorts out two pH indexes: the cation index pH and the pH of the fibre's water extract.

The cation index pH measures chemical stability (weathering and water resistance, i.e. ageing resistance) via the formula pH = –0.0602·W(SiO2) – 0.120·W(Al2O3) + 0.232·W(CaO) + 0.120·W(MgO) + 0.144·W(Fe2O3) + 0.21·W(Na2O). By this index, mineral fibres are graded into five classes: below 4 (most stable), below 5 (stable), below 6 (moderately stable), below 7 (less stable), above 7 (least stable); fibres made from pure rock generally score below 4.

The water-extract pH reflects water resistance and possible attack on metal faces: more modifier ions (alkali and alkaline-earth) leach out and make the extract alkaline, so users generally want the extract pH above 7. Test conditions: 2 g fibre sample in 100 ml distilled or purified water, measured immediately, after 3 h and 6 h at room temperature, and after 0.5 h at 100°C then cooled. Glass fibre, rock wool, slag wool and aluminium silicate fibre differ in extract pH because of their chemistry; the authors verified samples with a PHS-25 laboratory pH meter from Shanghai Weiye Instrument Factory. Conclusion: the two pH values are key fibre indicators, and the friction and sealing industry watches the water-extract pH most closely.

Key facts

  • Why pH matters: whether reinforcement fibres corrode brake hubs or sealing flange faces in humid environments
  • Index 1 — cation index pH: pH = –0.0602·W(SiO2) – 0.120·W(Al2O3) + 0.232·W(CaO) + 0.120·W(MgO) + 0.144·W(Fe2O3) + 0.21·W(Na2O)
  • Stability classes: pH <4 most stable; <5 stable; <6 moderate; <7 less stable; >7 least stable
  • Index 2 — water-extract pH: desired above 7; evaluated for water and medium resistance
  • Test method: 2 g fibre + 100 ml distilled water; measured immediately, 3 h, 6 h at room temperature, and 0.5 h at 100°C then cooled; PHS-25 pH meter
  • Comparison: glass fibre, rock wool, slag wool and aluminium silicate fibre differ in extract pH by composition
  • Conclusion: the two pH values are key indicators; the friction/sealing industry watches the water-extract pH closely

The original article is in Chinese; see the Chinese version for the full text. This page provides the English title, abstract and key facts only — translated and compiled from the original, without full verbatim translation.