GB/T 33047.1-2025Plastics - Thermogravimetry (TG) of polymers - Part 1: General principles (English PDF)

5.1 Thermobalance

It shall meet the following requirements.

-- It shall have a constant heating or cooling rate;

-- The test temperature shall remain constant (temperature variation not exceeding ±0.3 K during continuous measurement);

-- The purge gas flow rate shall be stable, with fluctuations not exceeding ±10% (e.g., 10 mL/min ~ 150 mL/min);

-- The temperature range and mass range shall meet the test requirements;

-- It shall automatically record the mass change curves against temperature and time;

Note. Some instruments can also display the derivative thermogravimetric (DTG) curve, which is helpful for data analysis.

-- The accuracy of temperature signal measurement shall be ±1% of the measured absolute temperature or better;

-- The accuracy of time measurement shall be ±1 s or better;

-- The accuracy of mass measurement shall be ±20 µg or better.

5.2 Purge gas

The purge gas under oxidizing conditions shall be dry air or oxygen; the purge gas under non-oxidizing conditions shall be a suitable inert gas with an oxygen content below

0.001% (volume fraction). The moisture content in the purge gas shall be less than 0.001% (mass fraction).

6.1 General

The test sample may be in the form of a liquid, powder, sphere, granule, sheet, or any other suitable solid form and size. For finished products, the test sample shall be obtained directly from the product by machining.

6.2 Preparation of test samples from finished products

Cut the test sample using a slicer or a blade. The dimensions of the sample shall be compatible with the crucible used.

Note. The dimensions and shape of the test sample generally depend on the crucible.

The surface area of the test sample may affect the test results. For test samples of the same mass, those with a larger surface area generally exhibit a higher rate of change than those with a smaller surface area.

6.3 Sample conditioning

Unless otherwise specified, the test sample shall be conditioned in accordance with

GB/T 2918 at a temperature of (23 ± 2) °C and a relative humidity of (50 ± 10) %, or under other conditions as agreed upon by the interested parties.

6.4 Sample mass

The sample mass shall be appropriate for the selected test method and shall be within the weighing capacity of the thermobalance.

7.1 Mass calibration

Calibrate the thermobalance under static purge gas conditions. Select a calibration mass within the measurement range, and calibrate the thermobalance as follows.

Calibrate the thermobalance using calibrated weights in accordance with the instrument's instruction manual, and record the temperature at which the calibration is performed. If mass calibration is performed by a calibration service provider, the validity of the mass calibration may be confirmed by means of a valid calibration certificate issued by that provider.

Note. Static purge gas conditions are used to eliminate interference caused by buoyancy resulting from gas flow.

7.2 Temperature calibration

Calibrate the temperature using the same gas, gas flow rate and heating rate as those used in the actual measurement (see Chapter 8).

If the thermobalance is not used in combination with another thermal analysis method, calibrate as follows.

a) Select two or more calibration materials with Curie temperatures close to the temperature range to be measured. Try to ensure that the temperature range to be measured is between the Curie temperatures of the two selected calibration materials.

Note 1.The Curie point is the critical temperature at which a ferromagnetic material changes to a paramagnetic material during heating. This transformation is reversible. Apply magnetic field to the calibration material by placing a magnet either above or below it, thereby exerting an upward or downward force on the material. When the calibration material is heated above its Curie temperature, a distinct apparent increase or decrease in mass shall occur.

More detailed information on the evaluation of the apparent mass change associated with the Curie point can be found in the literature.

Certified reference materials traceable to a metrology laboratory should be used.

Suitable calibration materials may be obtained from the instrument manufacturer or the national metrology authority. Examples of suitable magnetic calibration materials are given in Appendix A.

If the thermobalance is used in combination with single-sensor differential thermal analysis (SS-DTA), differential thermal analysis (DTA) or differential scanning calorimetry (DSC), the thermobalance should be temperature-calibrated in accordance with the procedure specified in the relevant standard, for example, GB/T 19466.1 for

DSC.

Note 2.In the single-sensor differential thermal analysis (SS-DTA) method, the DTA signal is obtained by comparing the temperature of the test sample with a calculated reference temperature.

Note 3.The melting temperature of a calibration material is defined as the intersection of the extrapolated baseline and the tangent drawn at the inflection point of the endothermic curve (i.e., the extrapolated initial temperature).

Note 4.Calibration is the most critical step in obtaining reliable TG data. The interaction between the temperature sensor, sample shape, gas type and gas flow rate may influence the calibration of the measurement system. In the absence of time-lag correction, the temperature calibration also depends on the heating rate. Some instruments provide automatic calibration for time lag and/or temperature calibration independent of the heating rate.

The rate of mass loss depends on the oxidation rate of the sample, the atmosphere to which the sample is exposed, and the gas flow rate. Therefore, it is important that the same atmosphere and gas flow rate as specified in Chapter 8 be used during calibration.

8.1 General

Select an appropriate instrument configuration according to the measurement requirements. Three test modes may be used.

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