Using Differential Scanning Calorimetry (DSC) for Material and Failure Analysis of Plastics

Components fabricated from polymer materials are widely utilized in all fields that require cost effective production and weight reduction. Though injection molding components made of thermoplastic materials have been used in the automotive industry for quite a while, there is still a continuous increase in demand for lightweight solutions for use in modern automobiles. Due to the reduction of carbon dioxide emissions becoming a prominent concern acoupled with the advancement of electric vehicles, automotive parts made of light materials are being increasingly used.

Significance of Material Analysis

Ensuring the quality and stability of the components has become imperative due to the increased use of plastics. Material analysis plays a key role in such instances. The mechanical properties of components are greatly affected by many process steps. Hence, it is crucial to ascertain the constant quality of materials from start through end of the production process. Thermal analysis techniques such as DSC are suitable for such analyses.

Experiment and Results

In this example, a housing component fabricated from glass fiber-reinforced Polyamide 6 exhibited embrittlement at the clip hook when connected with the clip joints. The clip broke during the installation of the component. For such failures, examining all possible influencing factors throughout the production chain is highly important.

DSC investigation of the damaged component (niO part) and an iO control part rapidly determined the reason behind the failure. The DSC curves are depicted in Figure 1. For analyzing the material composition, the second heating curves are always assessed because of the absence of any effects of the thermal history.

DSC results of the second heating of the iO part (green curve) and the niO part (blue curve)

Figure 1. DSC results of the second heating of the iO part (green curve) and the niO part (blue curve)

In conjunction with the glass transition of the sample at 50.9 °C, the control component (green curve) exhibited a melting endotherm at 221 °C and a melting enthalpy of 53.7 J/g (typical for pure PA 6). On the other hand, the damaged component exhibited noticeably different behavior, with an enthalpy of 45.2 J/g and a peak temperature at 215 °C. The melting profile of the damaged part exhibits a second peak at 239 °C, as illustrated in Figure 2.

Enlarged scaling of the DSC results from Figure 1

Figure 2. Enlarged scaling of the DSC results from Figure 1

From the results of the DSC measurements, it is clear that the material of the damaged component is now a mixture of Polyamide 6 and Polyamide 66 instead of pure Polyamide 6. These two parts can create a eutectic, which is responsible for the change in the melting temperature from 221 °C (pure PA 6) to 215 °C (PA 6 + PA 66). The variations between the two parts can also be observed from their different crystallization profiles in the event of cooling (Figure 3). In the DSC investigation, crystallization is seen as an exothermic effect.

DSC results of the cooling curves for the iO part (green curve) and the niO part (blue curve)

Figure 3. DSC results of the cooling curves for the iO part (green curve) and the niO part (blue curve)

The enlarged scaling in Figure 4 further illustrates a greater onset temperature for crystallization of the material of the iO component at 217 °C, when compared to the temperature of 203 °C for the pure PA6 sample. Moreover, the area of the peak for the iO part is smaller.

Enlarged scaling from Figure 3

Figure 4. Enlarged scaling from Figure 3

Conclusion

From the results, it is evident that the properties of a finished component are greatly affected its material composition and it is possible to avoid such failures by monitoring the raw material quality using thermal analysis. It is much easier to achieve quality control using DSC thermal analysis.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.

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