Measurement of the Stress-Strain Behaviour of Films in the TMA - METTLER TOLEDO

Measurement of the Stress-Strain Behaviour of Films in the TMA

Introduction

The shrinkage behavior of fibers or films can be easily investigated by TMA. 

In such experiments, the sample is subjected to a constant stress and the sample length measured as a function of temperature. In practice, several questions arise:

  • Can the low forces used be calibrated? 
  • How accurate is the force exerted on the sample? 
  • How reproducible is the measurement of the shrinkage process? 
  • How sensitive is the shrinkage behavior to the applied force?

These and other topics will be addressed in this article.

 

Experimental Details

The measurements were performed using a TMA/SDTA 840 equipped with the sample holder for film measurements. The sample material was a 10 µm thick PEHD film. The heating rate used for all experiments was 5 K/min. Before the actual experiments, the TMA/SDTA840e was adjusted and calibrated for temperature, length and force for film measurements. The procedures used and the preparation of the film samples are described in the following sections.

 

Preparation of Film Samples

Film samples are prepared in the following way. A sample is first punched out using the tool shown in Figures 1a) and 1b) (sample size: 16 x 6 x 0.01 mm). Two small screw clamps are then attached to the sample as illustrated in Figure 1c). The sample is then installed in the TMA film sample holder (Figure 1d).  

METTLER TOLEDO in fact provides a tool (Figure 1c) to help fix the clamps. Here, it is important to make sure that the two parts of the clamps are positioned correctly with respect to each other. The details are shown in Figure  2. These tools ensure that the sample length is exactly 10 mm. 

 

Temperature Calibration and Adjustment for Film Measurements

The procedure used for the calibration and adjustment of temperature for films is similar to that used for fibers [1]. For films, a sample is prepared as shown in Figure 3. First, two strips are punched out from a piece of aluminum foil (Figure 3a). Two indium pellets are then placed on one of the strips of foil (Figure 3b).

The pellets are covered with the second strip of foil and pressed lightly together (Figure 3c). The two small screw clamps are then attached (Figure 3d). The ready-prepared calibration sample is now installed in the TMA (Figure 3e) and can be used several times.

The thermocouple is positioned at about half the height of the sample (adjacent to the two indium pellets) and about 1 to 2 mm from the sample. The thermocouple must not touch the calibration sample or the samples that are measured afterward. Its position must not be changed after the calibration measurement.

Figure 4 shows the result of such a measurement. The melting of indium is observed on the SDTA signal and the length signal. The results obtained can be used for temperature adjustment. 

 

Length Calibration

The length measurement for the film sample holder can be conveniently checked by measuring the expansion coefficient of aluminum. To do this, a strip of aluminum foil is mounted in the TMA as described above and its thermal expansion measured.

Measurement of the Stress-Strain Behavior of Films by TMA | Thermal Analysis Application No. UC 336 | Application published in METTLER TOLEDO Thermal Analysis UserCom 33