GB/T 19466.4-2025Plastics - Differential scanning calorimetry(DSC) method - Part 4: Determination of specific heat capacity (English PDF)

4.1 Overview

Each measurement is performed at the same scan rate in three tests as follows (see Figure 1).

a) Blank test (both the specimen end and the reference end are empty crucibles);

Where,

deltaQsp---the total heat consumed within the temperature interval deltaT of the specimen test, expressed in (kJ) or (J);

deltaQcal---the total heat consumed within the temperature interval deltaT of the calibration test, expressed in (kJ) or (J);

deltaQbl---the total heat consumed within the temperature interval deltaT of the blank test, expressed in (kJ) or (J);

deltaT---the temperature interval, in seconds (s);

P---the subscript that indicates isobaric process.

5.1 DSC Instrument

In accordance with the provisions of GB/T 19466.1.

5.2 Crucible

In accordance with the provisions of GB/T 19466.1.

The crucible for holding the specimen and the crucible for holding the reference material (calibration material) shall have the same shape and texture, and their masses shall be similar, with a difference not exceeding 0.1 mg.

NOTE. if the instrument is sufficiently stable, the mass difference between the crucible with calibration material and the blank crucible can be corrected, allowing the same blank test and calibration test results to be used for multiple measurements. Correction can be achieved by adding cp,crucible(T)betadeltam to the heat flow rate of the calibration test, where cp,crucible(T) is the temperature-dependent specific heat capacity of the crucible with calibration material, beta is the heating rate, and deltam is the mass difference between the crucible with calibration material and the blank crucible. The same method can be used to correct the mass difference between the specimen test crucible and the blank test crucible.

5.3 Analytical Balance

In accordance with the provisions of GB/T 19466.1.

9.1 Instrument Preparation

In accordance with the provisions of GB/T 19466.1.

9.2 Crucible Selection

Prepare three sets of crucibles and lids. Weigh each set of crucibles and lids together. The total mass difference between sets shall not exceed 0.1 mg (see 5.2). Furthermore, the texture, size, and type (open or sealed) of the crucibles shall be consistent.

NOTE. in the blank test and calibration test, if the crucible lid is simply placed on the crucible, then, the same crucible can be used in the specimen holder for the blank test, calibration test, and specimen test. For the specimen test, the crucible can be sealed.

9.3 Instrument Setup and Isothermal Baseline Adjustment

9.3.1 Respectively place a pair of empty crucibles with lids at the specimen end and reference end of the DSC instrument.

9.3.2 Continuous temperature-rise scanning method.

a) Set the starting and final temperatures (Ts and Tf). The initial temperature Ts should ideally be at least 30 K lower than the temperature point of the first data.

For more accurate results over a wide temperature range, the entire range can be divided into two (or more) smaller ranges, each with a temperature span of 50 K to

100 K. The starting temperature Ts of the second temperature range should ideally be

30 K lower than the final temperature Tf of the first temperature range to ensure adequate coverage.

b) Set the scan rate.

c) Set the isothermal times for isothermal stages I and II (see Figure 1) to stabilize their respective isothermal baselines. The isothermal times are typically between 2 min and

10 min.

NOTE. for some calorimeters, for example, the Calvet calorimeter, baseline stabilization may take more than 30 minutes.

9.3.3 Step temperature-rise scanning method.

When the specific heat capacity of the specimen does not significantly change with temperature, then, the step temperature-rise scanning method can be used. The step temperature-rise scanning method integrates the heat flow over a small temperature interval, obtaining a series of individual specific heat capacity values within the considered temperature range. The following points shall be noted.

a) The time interval of the isothermal stage shall meet the requirements to obtain a stable baseline.

b) This method shall not be applied to temperature ranges where first-order phase transitions occur.

The step temperature-rise scanning method is performed as follows.

---Set the starting and final temperatures (Ts and Tf);

---Set the temperature increment, typically 5 K or 10 K;

---Set the temperature scan rate to 5 K - min^-^1 or 10 K - min^-^1;

---Set the isothermal time of the isothermal stage, typically between 2 min and 10 min.

9.3.4 Set the coordinate axis range for the heat flow rate, so that the vertical span of the curve is at least 80% of the coordinate axis range (see Figure 1).

9.3.5 Adjust the instrument, so that the isothermal baselines before and after the heating stage are at similar vertical coordinate positions.

Check the repeatability of the DSC baseline after adjustment. If the baseline repeatability is poor, re-adjust the instrument and repeat the test.

NOTE. poor baseline repeatability can be caused by factors such as contamination of the specimen crucible, the position of the crucible lid, the stability of the gas flow rate, sample decomposition, volatilization, and chemical reactions between the specimen crucible and the specimen, etc.

9.3.6 See 9.2.2 and 9.2.3 for temperature program settings. Figure 2 shows a typical DSC curve of continuous scanning, and Figure 3 shows a typical DSC curve of step scanning.

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