Scope

• This document specifies general conditions for the analysis of polymers using thermogravimetric techniques. It is applicable to liquids or solids. Solid materials can be in the form of pellets, granules or powders. Fabricated shapes reduced to appropriate specimen size can also be analysed by this method. 

• This document establishes methods for the investigation of physical effects and chemical reactions that are associated with changes of mass.

• This document can be used to determine the temperature(s) and rate(s) of decomposition of polymers, and to measure at the same time the amounts of volatile matter, additives and/or fillers they contain. 

• This document is applicable to measurements in dynamic mode (mass change versus temperature or time under programmed temperature conditions) or isothermal mode (mass change versus time at constant temperature).

• This document is applicable to measurements at different testing atmospheres, such as separation of decomposition in an inert atmosphere from oxidative degradation.

Principle

A test specimen is heated at specified rates with a controlled temperature programme, and the change in mass is measured as a function of temperature (dynamic mass-change determination). Alternatively, the specimen is kept at a given constant temperature and the change in mass is measured as a function of time over a given period (isothermal mass-change determination).

During measurement, the test specimen is held in a controlled inert, oxidising or otherwise reactive atmosphere.

In general, the reactions which cause the mass of a test specimen to change are decomposition or oxidation reactions or the volatilisation of a component. In some cases, measurements with special reaction gases may also be carried out. The change in mass vs. temperature and/or time is recorded as a thermogravimetric (TG) curve.

The change in mass of a material as a function of temperature and the extent of this change are indicators of the thermal stability of the material. TG data may therefore be used to evaluate the relative thermal stability of polymers of the same generic family and polymer-polymer or polymer-additive interactions, using measurements made under the same test conditions.

NOTE TG data can be used for process control, process development and material evaluation. Long-term thermal stability is a complex function of service and environmental conditions. TG data alone cannot be able to describe the long-term thermal stability of a polymer.

Apparatus

A number of commercial instruments suitable for thermogravimetric measurements are available. The basic apparatus consists of the following.

Thermobalance, meeting the following requirements:

— capability to generate constant heating and cooling rates suitable for intended measurements;

— capability to maintain the test temperature constant (to within ±0,3 K or less for the duration of measurement);

— capability to maintain a constant purge gas flow rate controllable to within ±10 % over the range of flow rates (e.g. 10 ml/min to 150 ml/min) required for intended measurements;

— temperature and mass range in line with experimental requirements;

— recording device capable of automatically recording the measured curve of mass versus temperature and time;

NOTE Some instruments can also display differential thermogravimetric curves (DTG curves) for improved evaluation of results.

— measurement of temperature signals with an accuracy of ±1 % of the absolute temperature measured in K or better;

— measurement of time with an accuracy of ±1 s or better;

— measurement of mass with an accuracy of ±20 μg or better.

Purge gas, dry air or oxygen (oxidizing conditions) or a suitable inert gas with an oxygen content of 0,001 % by volume or less (non-oxidizing conditions). In either case, the water content of the purge gas shall be less than 0,001 % by mass.

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