Comparison of different types of EPDM by DSC

Purpose

It was shown in the example of chloroprene rubber (Section 3.1.2. DSC investigations on crystallinity) that the crystallinity depends on the structure of the macromolecule. Melting behavior therefore varies widely, especially with polyolefines.

This is demonstrated using uncross-linked EPDM as an example.

Sample

Unvulcanized EPDM with different ethylene contents (EPG 6170, EPG 8450 and EPG 3440).

Conditions

Measuring cell: DSC822e with liquid nitrogen cooling option

Pan: Aluminum 40 µl, pierced lid

Sample preparation: Cubes of approx. 10 mg were cut from the starting material. To eliminate thermal history, the samples were heated to 100 °C and then cooled at 5 K/min to –120 °C before measurement

DSC measurement: Heating from –120 °C to 100 °C at 10 K/min

Atmosphere: Nitrogen, 50 ml/min

Evaluation

The peak area was evaluated as described in Section 3.1.2. Glass transition and melting of EPDM. The tangents for the evaluation of the glass transition were constructed by extrapolating sections of the measurement curve after the melting peak. The degree of crystallinity was determined as described in Section 3.1.2. Determination of crystallinity. The following table summarizes the results for the three types of EPDM:

Interpretation

The three types of EPDM differ in their crystallization behavior. EPG 6170 shows a large melting peak with an area of 45.8 J/g. Melting in fact begins immediately after the glass transition.

The melting peak of EPG 8450 is about ten times smaller. The sample has only a relatively low degree of crystallinity after cooling. Melting again takes place immediately after the glass transition. This means that the crystallites formed on cooling are only very small. In contrast to the first two materials, EPG 3440 is completely amorphous. The different crystallization behavior has to do with in the structure of the macromolecule. EPDM is a copolymer in which the ethylene sequences crystallize. If the length of the sequences is relatively large, large crystallites form that have a higher melting temperature. With shorter average sequence lengths only very small crystallites form that melt immediately above the glass transition temperature. If the sequences consist of only a few CH2 groups, then crystallization is no longer possible and the sample remains amorphous.

The height of the glass transition step depends on the crystallinity. If the degree of crystallinity is greater, Δcp is smaller because the proportion of mobile amorphous material involved in the glass transition is lower. Crystallinity also influences the glass transition temperature. While the T_g of the two almost amorphous samples is approx. –53 °C, the glass transition temperature of the sample with the largest degree of crystallinity is significantly higher. This is a result of the decrease of molecular mobility in the amorphous regions due to the crystallites.

Conclusion

An analysis of the melting behavior of EPDM allows information to be obtained on the structure of the macromolecules.

^{Comparison of Different Types of EPDM by DSC | Thermal Analysis Handbook No.HB434 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}