Differential Scanning Calorimetry for Determining the Melting Point and Glass Transition Temperature of Hot Melt Adhesives

Overview

Differential scanning calorimetry is a thermal analysis method that measures the relationship between the energy difference between the sample and the reference object with temperature under program temperature control. This method is widely used in the field of materials science and can accurately determine the physical transformations and chemical reactions that occur during the heating or cooling process of substances, such as melting, crystallization, and vitrification transitions. For polymer composites such as hot melt adhesives, their melting point and glass transition temperature are the key parameters to evaluate their process applicability and final properties. Measurement by differential scanning calorimeter can provide reliable data support for product development, quality control and process optimization.

Thermal transition characteristics of hot melt adhesives

The properties of hot melt adhesives are closely related to their thermal behavior. The melting point usually refers to the temperature range at which the material changes from solid to liquid, which determines the sizing temperature and opening time of the hot melt adhesive. The glass transition is the process of amorphous polymer transformation from glass state to high elasticity, and the corresponding glass transition temperature directly affects the flexibility, cohesion strength and adhesion properties of the material at low temperature. Accurate measurement of these two parameters is crucial for understanding the application performance of hot melt adhesives in different temperature environments.

Experimental methods

Before testing, the hot melt sample is made into uniform sheets or small particles and accurately weighed (usually 5-10 mg) in a standard aluminum crucible. For references, empty crucibles or crucibles containing inert materials such as alumina are used. Nitrogen is usually used as a protective atmosphere to eliminate oxidative reaction interference. The temperature program is typically set to start at a lower-than-expected transition temperature, warm up at a constant rate (e.g., 10°C/min) until complete melting, then cool down rapidly, and perform a second warm-up scan to eliminate thermal history. The key test parameters are shown in the table below:

Data analysis

The characteristic transition temperature can be determined by analyzing the DSC curve (heat flow rate with temperature curve) recorded by the instrument. The melting point is usually determined by extrapolating the starting temperature or peak temperature. For the melting peak, extrapolate the starting temperature (T_e) is widely adopted as the melting point, which is defined as the temperature at the intersection of the baseline extension line and the tangent line at the peak front. The glass transition temperature is determined by determining the midpoint temperature (T) at which the heat capacity changes step by step_{g, mid}) to characterize. On the DSC curve, the vitrification transition appears as a step-like change rather than a sharp peak. The specific determination method can refer to the following schematic description: the baseline before and after the transition is offset, and the temperature corresponding to the midpoint of the extension line before and after the baseline is T_{g, mid}。

Notes:

The accuracy and repeatability of test results are influenced by a variety of factors. Sample preparation uniformity, particle size, and close contact with the crucible bottom are key. The temperature rise rate directly affects the position and peak shape of the transition temperature, and too fast a rate may lead to temperature lag and resolution loss. In addition, the thermal history of the sample (e.g., cooling rate, annealing treatment) has a significant impact on the measurement of glass transition temperature, so it is often recommended to analyze the data from the second temperature scan. During the experiment, ensure that the instrument is calibrated with reference materials (such as indium and tin) to ensure the accuracy of temperature and thermal enthalpy measurement.

Conclusion

Differential scanning calorimetry provides an efficient and accurate technical means for determining the melting point and glass transition temperature of hot melt adhesives. Through standardized sample preparation, rigorous experimental condition control and standardized data analysis, thermal performance data with good repeatability and high comparability can be obtained. These data are valuable for guiding the formulation design of hot melt adhesives, evaluating their suitability under different climatic conditions, and optimizing the bonding process parameters.

References

International Standard ISO 11357-2: Plastics - Differential scanning calorimetry (DSC) Part 2: Determination of glass transition temperature.

International Standard ISO 11357-3: Plastics - Differential Scanning Calorimetry (DSC) - Part 3: Determination of melting and crystallization temperature and thermal enthalpy.

Thermal Analysis of Polymer Materials, by Liu Zhenhai, Chemical Industry Press.