PET, Thermal History

Sample

Polyethylene terephthalate from the neck of a bottle for soft drinks 

Conditions

Measuring cell: DSC821e with air cooling and sample changer

Pan: Aluminum standard 40 , pierced lid

Sample preparation: Flat piece cut from neck of bottle, 23.16 mg

DSC measurement: Heating from 30 to 300 at 10 K/min. At 300 C the sample changer removes the molten sample within a few seconds and places the pan on the cold tray. This results in reproducible quench-cooling with a mean cooling rate of approx. 50 K/s or 3000 K/min.

As soon as the cell reaches 30 C the pan is inserted and measured again at 10 K/min. After quench-cooling, it is annealed at 65 C for 10 h for enthalpy relaxation. The sample is then measured again from 30 to 300 C at 10 K/min. The final cooling is at 10 K/min to allow time for recrystallization. Finally, it is measured again at 10 K/min. 

Atmosphere: Nitrogen, 50 cm3/min

The heats of crystallization have been integrated by using a straight baseline between 120 and 180 C, and the heats of fusion between 210 and 270 C. For the sake of clarity, the evaluation results are not shown in the diagram.

Conclusions

The properties of many materials are influenced by their thermal history, e.g. metallic glasses, the elements sulfur and selenium and simple organic compounds such as acetylsalicylic acid or ethanol (sub-ambient measurement). The glass transition temperatures and melting temperatures are, of course, individually different as are the cooling rates necessary to prevent crystallization when the samples are quench-cooled from the melt.

The second heating run should be used to determine the properties of amorphous materials free from the influence of unknown thermal or mechanical history. To do this, the sample from the heating run is quench-cooled from the melt. If, on the other hand, the greatest possible degree of crystallinity is of interest, the molten sample should be cooled very slowly (maximum rate 10 K/min).

The steps of the relatively complex temperature program described above can be carried out automatically using the sample changer. 

^{PET, Thermal History | Thermal Analysis Handbook No.HB235 | Application published in METTLER TOLEDO TA Application Handbook Thermoplastics}