University of Twente Student Theses


Reliable specific heat capacity measurements of thermoplastic composites with differential scanning calorimetry

Farkas, Marton (2023) Reliable specific heat capacity measurements of thermoplastic composites with differential scanning calorimetry.

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Abstract:Carbon fiber reinforced composites have gained more attention in recent decades due to their high strength in the direction of the fiber reinforcement, along with their low density compared to metals. These properties render these materials particularly attractive to the aviation industry, where weight reduction is one of the most important goal for improving fuel efficiency and extending travel range. Specifically, thermoplastic composites are attracting more interest due to their faster processing compared to thermoset composites, as no secondary curing step is required. Moreover, their remeltability allows them to be welded together. Consequently, various processes have been developed for these novel materials. A common feature among these processes is that they all rely on heating and cooling the material. Ultimately, the thermal properties of the composites are crucial for developing optimal process windows. Specific heat capacity is one of the necessary thermal properties and is most commonly measured by differential scanning calorimetry (DSC). However, to date, there are no well-established standards for the measurement of composites. This has led to considerable variability in the reported values for the same composite materials. Therefore, the objective of this master assignment is to investigate the existing methods and propose guidelines for reliable specific heat capacity measurements for thermoplastic composites. Firstly, the working principle of the DSC and its application in measuring specific heat capacity is discussed. It is followed by a comprehensive literature review, encompassing previous attempts to measure the specific heat capacity of composites and also including recommendations from the DSC manufacturer. Ultimately, specific heat capacity measurements were carried out with carbon fiber reinforced polyether-etherketone (CF/PEEK) composites, neat PEEK, and dry carbon fibers. The choice to test neat PEEK and dry fibers was done, to test the validity of obtaining the composite specific heat capacity by the rule of mixtures. The influence of sample mass, shape and form were investigated. Moreover, the impact of thermal cycles on the specific heat capacity were also explored. Additionally, various heating rates were tested to evaluate their influence and to gain information about the thermal gradient accumulation within the samples. The importance of frequent baseline measurements was highlighted, as it can have a strong influence on the results. It was concluded that a sample mass of at least 15 mg is needed to minimize susceptibility to baseline deviations and achieve results with low variability. The recommendations included the use of disk-shaped specimens, in order to ensure good thermal contact between the samples and the crucibles. Heating rates between 10 and 5 K/min were advised to avoid significant temperature gradient accumulation within the samples. Employing two thermal cycles were recommended in order to erase the thermal history of the samples and to introduce one which is the same for all specimens, allowing meaningful comparisons. The careful selection of the maximum applied temperature, considering the material-dependent thermal degradation, was also emphasized.
Item Type:Essay (Master)
Thermoplastic Composites Research Center, The Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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