Optimization of DSC Measurement Conditions - METTLER TOLEDO

Optimization of DSC Measurement Conditions

Introduction

Semicrystalline polymers are normally characterized using DSC heating curves. The numerical values obtained with the technique are:

  • Peak temperature (“crystallite melting point”)
  • Peak area (heat of fusion)

The results depend to some extent on the actual measurement parameters used. But even under apparently identical conditions, certain variations in the results can be observed. The aim of this work was to establish the optimum measurement parameters using an elegant set of measurements.

 

Experimental Details 

The measurements were performed according to ISO 11357-1 1997.

Measuring cell: DSC30

Crucible: Al standard 40 µl, pierced lid

Samples: Two closed-cell polyolefin foams: - PE-LD (90.7 kg/m3 ), - PE-LD-EVA (87.6 kg/m3 ) Polyamide film (1272 kg/m3 ).

Sample preparation: Disk-shaped samples taken from points close to each other. 

DSC measurement: Heating from -40 °C to 200 °C (foam), or from -40 °C to 320 °C (polyamide). For other measurement parameters, see Table 2.

 

Preliminary Experiments 

The PE-LD-EVA foam was measured three times under the same conditions (2.5 mg sample, 10 K/min, 100 ml/min nitrogen). The PE-LD peak temperature and the heat of fusion were evaluated; the following 95% confidence intervals were obtained

  • 0.15 °C for the peak temperature.
  • 4.8% for the heat of fusion.

The reproducibility of the heat of fusion is not good enough to characterize the samples. For this reason, we began an improvement project to identify the measurement parameters that affect reproducibility and which should therefore be optimized.

 

The Taguchi Procedure 

The Taguchi procedure [1, 2] was used to identify the optimum measurement parameters for the heat of fusion and the peak temperature with relatively few measurements. Particularly advantageous is the fact that measurements are very robust after Taguchi optimization, i.e. the results are relatively insensitive to external factors such as changes in the ambient conditions.

The Taguchi procedure is, by the way, also used to improve the quality of products such as automobiles. In principle, any process can be optimized with this method.

The Taguchi procedure calculates the influence of all the factors on a process. In the case at point here, these factors consist of all the possible parameters that could influence the measurement. They are divided into

  • “Control” factors, i.e. ones that can be kept under perfect control such as the heating rate, and
  • “Noise” factors, i.e. ones that have undesirable effects such as non-reproducible heat transfer between the crucible and the sample or changes in the ambient temperature.

The various factors involved were worked out in thermal analysis team meetings (Table 1).

Summary

The Taguchi procedure identifies and weights the parameters that influence a measurement. Better understanding of these factors allows the measurement conditions to be optimized. The accuracy of the heat of fusion was significantly improved. Besides this, the measurement was significantly more robust.

Optimization of DSC Measurement Conditions | Thermal Analysis Application No. UC 144 |