Rationale
Differential scanning calorimetry is a thermal analysis technique that obtains thermal characteristic information of a sample by measuring the heat flow difference required to maintain the temperature consistency between the sample and the reference object under programmed temperature control. When a sample undergoes a physical or chemical transition, such as a vitrification transition, its heat capacity undergoes a sudden change that manifests as a deviation from the baseline on the heat flow curve. For polymer materials such as epoxy resin, the glass transition temperature is one of the key thermodynamic parameters, which reflects the temperature point of the material's transition from glass state to highly elastic state, which is closely related to the application temperature range, mechanical properties and process conditions of the material.
The glass transition temperature of epoxy resin is the core index to evaluate its heat resistance and service performance. below the transition temperature, the material is in a rigid glass state; Above this temperature, the material transforms into a flexible and highly elastic state. This shift directly affects the modulus, strength, dimensional stability, and creep resistance of the material. Accurate measurement of this temperature has important guiding value for formulation design, process optimization and quality control of epoxy resins in electronic packaging, composite materials, coatings and other fields.
Test methodology
Testing is usually carried out according to the relevant standard methodology. Sample preparation needs to ensure its representativeness and uniformity, generally a few milligrams to more than ten milligrams. The instrument needs to be calibrated for temperature and heat flow before testing. Nitrogen is recommended as a purge gas to maintain a stable test environment. A typical temperature procedure is to warm up from room temperature at a constant rate (e.g., 10°C/min) to about 30°C above the expected transition temperature, followed by rapid cooling and a second temperature scan. The analysis usually takes a second warming curve to eliminate the thermal history effect.
Characteristic temperature determined
On the resulting heat flow-temperature curve, the vitrification transition appears as a step-like baseline offset. The characteristic temperature is usually determined by the tangent method: the tangent line is made on the extension line of the baseline before and after the transition zone, and the temperature corresponding to the midpoint between the two tangent lines is often defined as the glass transition temperature. In addition, the start and end temperatures of the transition can be determined by a similar method. The change in heat capacity before and after the transition can be calculated by the formula:
ΔCp = (ΔY / m) / β
Among them, ΔCpFor the heat capacity change, ΔY is the step height on the heat flow curve (unit: mW), m is the sample mass (in mg), and β is the temperature rise rate (in °C/min).
Influencing factors
The measurement results are affected by a variety of factors and need to be controlled during the test. The main factors and their impact are briefly described below:
| Rate of warming | Too fast a rate may result in a high measured temperature and a wider transition zone. |
| Sample quality and morphology | Excessive mass or uneven thickness can lead to temperature gradients, affecting resolution and accuracy. |
| Hot history | The preparation and pretreatment history of the sample has a significant impact on the results, and it is often recommended to analyze the second heating curve. |
| Atmosphere and sealing | Stable inert atmosphere prevents oxidative degradation; Sealing may affect volatile components. |
Notes:
The measured glass transition temperature is not a fixed point, but a temperature range. When reporting the results, the method used to define the characteristic temperature should be clearly defined. This data can be used to compare the differences in thermal properties of epoxies by formulation, degree of curing, or aging state. In practical applications, it is necessary to pay attention to the difference between the test conditions and the actual use conditions of the material, and combine it with other analysis methods (such as dynamic thermomechanical analysis) for comprehensive judgment. For partially cured systems or complex mixtures, there may be multiple transition steps on the heat flow curve, which need to be carefully analyzed.
References
Thermal Analysis Application Manual: Polymer Materials Booklet. International Association for Thermal Analysis and Calorimetry.
Plastics Differential Scanning Calorimetry Part 2: Determination of glass transition temperature. National standards.
A review of epoxy resin thermal performance characterization technology. Polymer Materials Science and Engineering.