Spotlight: How to... diagnose issues in polymer samples using DSC analysis

Materials World magazine
,
1 Jul 2019

Ruston Services Ltd, Staffordshire, UK, thermal analysis specialist, Phil Robinson, explains how to test for inconsistencies in polypropylene samples.

Problems in thermoplastic polymer processing are all too common – materials that should be the same are not and subsequent issues in production and missed deadlines affect product delivery. The main requirement for a thermoplastic polymer in a production process – either for extrusion or injection moulding – is a consistent feedstock. In this case, consistent refers to the ability of the polymer to solidify at the same speed and at the same process temperature.

A recent example I worked on was a polypropylene film material, where one material did not process in the same way as the other, although the polymer was supposedly identical and from the same supplier, but from different country sites. By using differential scanning calorimetry (DSC) to examine the two materials, a difference between them was quickly established, allowing the polymer convertor to go back to the supplier with solid evidence regarding the cause of the issue.

A closer look

DSC is a common analysis technique, which provides valuable insights into polymer melting and crystallisation parameters, which can cause issues during thermoplastic polymer processing. The addition or removal of additives by the polymer supplier or the masterbatch supplier can alter the processing characteristics and such changes affect the polymer nucleation – growing crystals to become solid.
The consequence is that the polymer is either liquid when it should
be solid, or solid when it should still be liquid. An example of the latter might be incomplete filling of a die during an injection moulding.

Attempting to resolve such issues by trial and error on the
process can be time-consuming and expensive, whereas a DSC experiment can often pinpoint a potential issue before it reaches production. By examining both the manner in which the polymer melts and how it crystallises from the melt, basic behavioural
data can be obtained to assess how the polymer will work in the production process.

Taking steps

The DSC test normally takes about 10mg of the polymer, which is encapsulated into a small aluminium sample pan for the analysis. The pan contains the sample and, because aluminium is a good thermal conductor, distributes the heat around the sample, increasing sensitivity. A typical DSC experiment for a semi crystalline thermoplastic would involve five steps of heating and cooling. In this example, polypropylene film was tested.

The first step is to heat the sample from -20°C to 200°C at 20°C/min. The second step is isothermal for one minute, and the third is to cool it from 200°C to -20°C at 20°C/min. The fourth step is isothermal for three minutes, and the fifth is heating to -20°C to 200°C at 20°C/min. This test programme takes around 45 minutes to complete, although if only the cooling step were examined, this could be reduced to around 16 minutes.

Each stage of the programme has a purpose. The first step is to examine the material previous thermal history resulting from the manufacturing process. In some cases, detail on this step can pinpoint a process temperature set incorrectly. The second is to allow the DSC analyser to stabilise at 200°C, but also allows time to destroy any remaining polymer crystal nuclei which might influence crystallisation behaviour during the following cooling step. The third step, involving cooling, shows the polymer as it crystallises. Two pieces of information are found from this – crystallisation onset, which is the sensitivity of the additives present, and crystallisation speed, the rate of crystal growth during crystallisation.

The next allows the DSC to fully stabilise. The final heating occurs at step five, giving a melting profile of the polymer, which can then be compared to other materials or other suppliers since the polymer now has a standardised thermal history.

The difference in characteristics between two polypropylene samples was identified after performing the same DSC analysis tests on them. In the first sample, data obtained from a test showed the melting peak during the first and the second heating showed the material had a melting point of 165°C.

When the data sets were overlayed, some apparent differences appeared. The peak maximum temperature for both curves was shown to be within 0.2°C, confirming the base material was polypropylene and equivalent. However, the size of the melting peaks curves in the graphs was different – the peak was smaller in the first sample than in the second, which showed a smaller crystallinity. This may have an impact on the material’s strength and flexibility.

Less obviously, an additional small peak was seen about 120°C, which related to the masterbatch carrier polymer, medium density polyethylene (MDPE). In sample one, the MDPE peak was less intense than that in sample two, implying that sample one contained less MDPE. From other data collected in this exercise, the MDPE content appeared to have had an influence on crystallisation speed, with higher MDPE content causing slower crystallisation.

The influence of additives and their effect on materials showed in the cooling data. During the test, there was a 2.1°C difference between the onsets of crystallisation, with sample one having the higher onset temperature. This implied better nucleation within the sample one polymer, meaning it would solidify more rapidly in a mould at a given temperature. More importantly, a change of a 2.1°C in the onset difference would mean the process would need to be adjusted to compensate if production is to be maintained without problems.

Another vital piece of information obtained from this part of the test was related to the speed of the crystallisation. Although the temperature difference at the onset of the crystallisation peak was 2.1°C, it had increased to 2.8°C at the peak maximum, showing that the speed of crystallisation was slower in sample two than in sample one. This has implications for the speed at which the process might be run.

Future implications

Such small differences in the polymer behaviour, detectable in a straightforward DSC test, could indicate a profound effect on the processing behaviour of the polymer. Identifying inconsistencies
in materials early through DSC testing can help reduce costs and improve efficiency.