Traceable Temperature Calibration of Dynamic Mechanical Analysers (DMA)
TCI-2020-02-TCMM
The glass transition temperature (Tg), which is used to assess the cure of polymer composite systems, is a critical parameter to ensure the long-term performance of materials in service. Thermal analysis techniques, Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) have been widely used to determine the Tg as part of material qualification processes. DMA denotes the mechanical response of the material and the most sensitive for Tg determination and the preferred option by most users.
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Figure 1: Data comparison between DSC and DMA
An interlaboratory exercise conducted as part of prior research indicated the wide variation in the data for the same material measured using both DSC and DMA. Both techniques indicated good repeatability. However, the DMA had a significantly wider variability compared to the DSC. Forensic analysis into the data generated highlighted two critical factors affecting the reported values.
thermal lag inherent in some instrument systems
absence of traceable temperature calibration options
Fig 2: Carbon fibre reference specimen with metal embedded
DSC uses an established traceable temperature calibration procedure. Adapting this approach, a unique reference artefact replicating the specimen geometry used in a standard DMA test was manufactured using a carbon fibre epoxy, with the reference metal (indium and tin) embedded.
A multiple heating rate test method was developed to detect the melting transition of the embedded metal and thereafter extrapolating the data to the 0 °C/minute heating rate.
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Figure 3: New analysis option for
the melting transition
Figure 4: Melting point data extrapolation example (indium)
The project resulted in the following:
Measurement traceability to certified reference metals indium and tin
Improved data generation addressing thermal lag in the instrumentation
A new approach for consistent analysis for the temperature calibration
Improved specimen manufacturing process
Assessment of the reusability of the reference specimen
The next larger batch of specimens will form the basis for a new international intercomparison exercise that will help generate the precision and bias statements and a measurement best practice guide for temperature calibration of DMA.
Sam Gnaniah
sam.gnaniah@npl.co.uk