Measurements of the Crystallization and Melting of Chloroprene Rubber (CR) - METTLER TOLEDO

Measurements of the Crystallization and Melting of Chloroprene Rubber (CR)

Purpose

To investigate the crystallization behavior of chloroprene rubber with DSC.

 

Sample

Unvulcanized CR (Baypren 210).

 

Conditions

Measuring cell: DSC822e  with IntraCooler cooling option

Pan: Aluminum 40 Pl, pierced lid  

Sample preparation: Cube of approx. 20 mg cut from the starting material.  

DSC measurement: Heating from –60 °C to 60 °C at 10 K/min  

Atmosphere: Nitrogen, 50 ml/min 

The uppermost curve is that of the 1st heating run. The sample was then cooled to the starting temperature and immediately measured again (2nd run). The sample was then stored for different periods of time at 0 °C, cooled to the starting temperature and again measured. The measurement curves obtained are shown in the same diagram.

 

Evaluation

The first measurement shows a glass transition at –38 °C. In addition, there is an endothermic peak at 44.6 °C with an area of 4.81 J/g. In the second run, the glass transition appears at practically the same temperature, but no peak is observed.

The endothermic peak is a melting peak that occurs with some CR materials. The crystallization process can be investigated by measuring the melting behavior after crystallization under specific conditions. To do this, the sample was stored for different periods of time at 0 °C and then measured. In the storage process, it is important that the sample is not heated to temperatures greater than the crystallization temperature before the measurement

It can be seen that with longer storage times at 0 °C, a melting peak with a maximum at 28.9 °C is formed; the peak becomes progressively larger the longer the storage time. The temperature of the peak maximum, however, remains the same. 

The diagram displays the enthalpy of fusion as a function of crystallization time. The dotted line corresponds to the enthalpy of fusion obtained in the first measurement. 

 

Evaluation

The CR elastomer used crystallizes at temperatures between the glass transition temperature and the equilibrium melting temperature (Tgand Tm). The material crystallizes on storage at 0 °C. The crystallization can be detected in the following DSC measurements by means of the melting peak. After a crystallization time of approx. 200 h, the enthalpy of fusion was about 40% of the original enthalpy of fusion, i.e. that of the first measurement. The degree of crystallinity is therefore correspondingly low.

The melting temperature of the first measurement was 45 °C. This material had apparently undergone crystallization at room temperature during storage. The melting temperature of the material that crystallized during storage at 0 °C is about 30 °C. Since the melting temperature of polymer crystallites depends on their size, it follows that smaller crystallites are formed during crystallization at 0 °C than during crystallization at room temperature. The rate of crystallization of CR is very slow compared with that of EPDM. 

 

Conclusion

When CR is stored, crystallization can occur depending on the molecular structure (see Section 3.1.2). The peak observed at about 45 °C is due to the melting of the crystallites that were formed during storage at room temperature. The origin of the peak can be investigated by means of crystallization experiments. CR in fact crystallizes relatively slowly. 

 

Measurements of the Crystallization and Melting of Chloroprene Rubber (CR) | Thermal Analysis Handbook No.HB432 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2