Overview
ISO 7884-8:1987 - "Glass - Viscosity and viscometric fixed points - Part 8: Determination of (dilatometric) transformation temperature" specifies a dilatometric thermal-expansion method to determine the transformation temperature (tg) of bulk glass. The transformation temperature characterizes the glass transition range between the elastic/brittle (low-temperature) state and the viscous (high-temperature) state and is used for specifying cooling programmes and estimating useful temperature limits for glassware.
Key topics and technical requirements
- Measurement principle: record the relative change in length of a glass rod versus temperature (dilatometer curve) and determine tg graphically by the intersection of two tangents (low‑temperature branch and high‑temperature branch at the inflection point).
- Specimen: rod shape, cross-section about 10–25 mm², and length at least 5×10^4 times the extensometer resolution to avoid measurement artefacts. Cross-section chosen so dilatometer contact stress < 0.1 N/mm².
- Annealing: pre-anneal specimen ~30 °C above estimated tg, then cool to ~150 °C below tg at (2 ± 0.2) °C/min in draught-free air.
- Heating protocol: heat from 200 °C at (5 ± 1) °C/min, recording temperature and length change. Correct for specimen-to-thermocouple temperature differences if needed.
- Apparatus constraints: contact force ≤ 1 N (via spherical-faced contacts), furnace temperature uniformity so deviations along specimen < 5 °C, and furnace capacity up to ~50 °C above expected tg.
- Evaluation and repeatability: perform two runs; if tg values differ ≤ 5 °C, take the mean (round to nearest °C). If difference > 5 °C, repeat on a new sample. Reported precision: repeatability ≈ 3 °C, reproducibility ≈ 6 °C.
- Reporting: include sample description, sampling/preparation, apparatus, corrections applied, number of specimens, tg (°C), and any observed glass changes.
Applications and users
- Practical use: defining cooling programmes, estimating upper temperature limits for glassware, process control and quality assurance in glass manufacturing, and research on thermal properties of silicate and other bulk glasses.
- Typical users: materials and glass technologists, QA laboratories, research institutes, furnace/dilatometer equipment manufacturers, and standards bodies involved in glass testing and product certification.
- Relevant keywords: ISO 7884-8, dilatometric transformation temperature, glass transition temperature, dilatometry, thermal expansion, viscometric fixed points, glass testing.
Related standards
- Other parts of ISO 7884 (Parts 2–7) covering viscosity and viscometric fixed points (rotation viscometers, fibre elongation, beam bending, softening point, annealing and strain points, etc.).
- ISO 7991 - determination of coefficient of mean linear thermal expansion.
- IEC Publication 584-1 (thermocouples - reference tables).
This standard provides a reproducible, practical dilatometric procedure for characterizing glass transition behavior useful for manufacturing control, product specification, and comparative testing.
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