The past decades have seen a significant increase in the use of plastic and composite materials in society. From the proliferation of single use plastic drink bottles, to the lightweighting of aircraft for increased performance and reduced environmental impact. In order to deliver these game changing materials, engineers and scientists have depended on a series of cutting edge analytical tools. One category of such analytical tools which have had a significant impact in developing these materials is thermal analysis.
Thermal analysis is a blanket term given to a series of instruments used to measure the properties of materials when subjected to changes in heat. These come in a number of forms, the most common of which are Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and Thermomechanical Analysis (TMA). Each of these tools perform a different role in order to understand the properties of materials under temperature. Other thermal analysis techniques also used include Thermogravimetric Analysis (TGA), Thermal Conductivity and Heat deflection temperature, which will not be covered in this article.
Differential Scanning Calorimetry is probably the most versatile of the thermal analysis tools. The DSC works by placing a sample within a crucible on to a sample holder which has an integrated temperature sensor. A reference crucible (usually empty) is then placed on a second sample holder and as the furnace is heated through the area of interest, the DSC measures the heat quantity which is emitted or absorbed excessively by the sample on the basis of a temperature difference between the sample and the reference material.
Figure 1 sample pan inserted into DSC furnace
Based on the mechanism of operation, DSCs can be classified into two types: heat-flux DSCs and power-compensated DSCs. In a heat flux DSC, the sample material, enclosed in a pan, and an empty reference pan are placed on a sensor plate surrounded by a furnace. The furnace is heated at a linear heating rate, and the heat is transferred to the sample and reference. However, owing to the heat capacity (Cp) of the sample, there would be a temperature difference between the sample and reference pans, which is measured by area thermocouples, and the consequent heat flow is determined by the thermal equivalent of Ohm’s law.[1]
In a power-compensated DSC, the sample and reference pans are placed in separate furnaces heated by separate heaters. The sample and reference are maintained at the same temperature, and the difference in thermal power required to maintain them at the same temperature is measured and plotted as a function of temperature or time [1].
As a rule, a material should undergo two heating cycles during analysis. The first will heat beyond the glass transition temperature or beyond the melt of the sample, cooling back to the start temperature followed by another heating cycle. This is necessary to erase the thermal history of the material to allow the molecules to achieve a near optimum orientation. The first heating cycle analyses the material in the as received condition, which will include the effects of the processing conditions and any environment the material has been subjected to. The second heating curve provides information on the ideal behaviour of the material in relation to temperature and therefore makes it a better curve for analysing the material in comparison with other materials. [2]
Figure 2 – Example of DSC trace showing two heating cycles for analysis
Using the output from this analysis, a number of different properties of the polymeric material can be determined. These are: Glass Transition Temperature (Tg), Crystallisation Temperature (Tc), Melting temperature (Tm), Recrystallisation temperature (Tm) and Specific Heat Capacity (Cp). In addition to this, the degree of cure in a resin system can also be determined. This is done by comparing the magnitude of the exothermic transition between an uncured and cured polymer. With the capability of measuring so many properties, it highlights the versatility of the DSC.
The second most common thermal analyser is the Dynamic Mechanical Analyser. This machine works by vibrating the material at a given strain or stress and with a defined frequency and increasing the temperature the material is subjected to. The LVDT then measures the travel and measures the change in stiffness (modulus) with temperature. The primary property DMA is used for is determining the glass transition temperature of polymeric material. Given that DSC can also measure Tg, why would you also invest in a DMA? Well the short answer is that the DMA is more sensitive in measuring Tg than DMA. The change in gradient on a DMA can be as much as 1000 times greater than detected by DSC [4]. This is because the changes in dimensions, which the DMA measures, are significantly greater at Tg than the changes in heatflow measured by the DSC. In addition to this, the DMA gives information on the change of thermal properties as a function of different frequencies, which would be impossible to understand by DSC use.
Figure 3 – Sample being loaded into TMA at R-TECH
Finally, Thermomechanical Analysis (TMA) is used for measuring the coefficient of thermal expansion of materials. This is done by placing a quartz probe on a material and measuring the dimensional changes of the material via an LVDT attached to the other end of the probe, which is away from the furnace and therefore undisturbed by temperature effects. Coefficient of thermal expansion is an important characteristic for design engineers, particularly when considering what effect dimensional changes in the material may have on the overall structure when subjected to temperature. As we have already discussed with DSC, polymeric materials can undergo several phase changes as a result of heating, which means that the measured material may exhibit several different coefficients of thermal expansion, depending on the temperature ranges between each phase transition. As the phase changes are readily visible, this means that the TMA can also be used for detecting the Tg of materials, however as the Tg profiles measured by TMA are quite broad and can be affected by the probe loading conditions, the measurements will be less sensitive than DMA.
In future blog posts, we will look into more specifics of what the glass transition of materials is along with further information on each of the three different thermal analysis techniques outlined above, starting with DSC. If you are interested in carrying out any of the above techniques, R-TECH offers UKAS accredited DSC, DMA and TMA testing of polymeric materials with a unique cryogenic capability.
References:
- Gill P, Moghadam TT, Ranjbar B. Differential scanning calorimetry techniques: applications in biology and nanoscience. J Biomol Tech. 2010 Dec;21(4):167-93. PMID: 21119929; PMCID: PMC2977967.
- The Madison Group (2020) Back to basics: Differential Scanning Calorimetry, YouTube. Available at: https://www.youtube.com/ (Accessed: 05 July 2023).
- Mulligan D, Gnaniah S and Sims G Thermal Analysis Techniques for Composites & Adhesives, NPL Measurement Good Practice Guide No.32 2000 Sept
- Mettler-Toledo International Inc. all rights reserved (2019) Webinar – Determination of Glass Transition Temperature, METTLER TOLEDO Balances & Scales for Industry, Lab, Retail – METTLER TOLEDO. Available at: https://www.mt.com/ (Accessed: 05 July 2023).
- Formean J, Sauerbrunn S R & Marcozzi C L, Exploring the Sensitivty of Thermal Analysis Techniques to the Glass Transition, TA Instruments (TA-082)