Glass transition temperature (Tg) is an important parameter of polymer materials (such as rubber), which characterizes the temperature at which the material changes from the glassy state to the highly elastic state. This parameter directly affects the low-temperature performance, elastic modulus and application scenarios of rubber. Differential scanning calorimetry (DSC) is a technology widely used to determine Tg. It accurately captures the phase transition temperature by measuring the change in heat flow of the material during heating or cooling.
- Experimental Principle
Differential scanning calorimetry detects the thermal effects of materials by comparing the heat flow difference between a sample and a reference under programmed temperature control. When rubber undergoes a glass transition, its heat capacity (Cp) changes suddenly, which is manifested as a step-like baseline shift on the DSC curve (Figure 1). Tg is usually taken as the midpoint or inflection point of the curve shift.
- Experimental steps
1. Sample preparation
1.1 Sample selection: Take rubber sample (5-10 mg) to avoid contamination by impurities.
1.2 Pretreatment: If the sample contains plasticizers, fillers, etc., the composition needs to be recorded; if necessary, annealing treatment should be performed to eliminate the thermal history.
1.3 Sealed sample loading: Place the sample in an aluminum crucible and seal it tightly. Use an empty crucible or inert material (such as alumina) as the reference material.
2. Equipment selection
2.1 Instruments: DSC3000L differential scanning calorimeter
2.2 Instrument parameter settings
Temperature range: usually set to -100℃ to 50℃ (adjusted according to rubber type).
Heating rate: 10-20℃/min is recommended (too fast a rate may cause Tg shift).
Atmosphere control: Nitrogen atmosphere (flow rate 50-100 mL/min) to prevent oxidation reaction.
Baseline calibration: Use standard substances (such as indium, tin) to calibrate the instrument.
2.3 Testing Process
2.3.1 Blank Baseline Test (Empty Crucible).
2.3.2 Load the sample and run the heating-cooling-heating cycle (to erase the thermal history).
2.3.3 Record the second heating curve to analyze Tg.
2.4 Measurement reference standards
GB/T 19466.2-2004/IS011357-2:1999 Plastics Differential Scanning Calorimetry Part 2: Determination of Glass Transition Temperature
GB/T 29611-2013 Determination of glass transition temperature of raw rubber – Differential scanning calorimetry
- Data Analysis
Tg determination: On the DSC curve, the glass transition is a step-like change. According to GB/T19466.2-2004/IS011357-2:1999 standard, Tg is the temperature of the midpoint of the baseline deviation.
Software processing: Use the instrument’s supporting software to perform tangent analysis and determine the starting point, midpoint, and end point.
3.1 Factors affecting measurement results
- Heating rate: Too high a rate may cause Tg to shift to a higher temperature. It is recommended to select 10℃/min.
- Sample homogeneity: Uneven distribution of fillers or plasticizers can lead to multiple transition peaks.
- Elimination of thermal history: The initial heating needs to cover the material above the melting point, and then cool to a low temperature to eliminate the processing thermal history.
- Moisture interference: Water-absorbent rubber needs to be dried in advance to prevent the evaporation endothermic peak from masking Tg.
3.2 Practical Application Cases
Natural rubber (NR): Typical Tg is about -70°C to -60°C.
Styrene butadiene rubber (SBR): Tg ranges from -50°C to -30°C, affected by styrene content.
Fluororubber (FKM): Tg is relatively high (about -20℃ to 0℃), and the test range needs to be adjusted to -50℃~100℃.
Differential scanning calorimetry is a reliable method for determining the Tg of rubber. Through accurate measurement, it can provide key data for rubber formulation design, quality control and low temperature performance evaluation.