Influence of Pretreatment on the Melting of EPDM

Purpose

The melting behavior of elastomers depends not only on their chemical structure, but also on storage and processing conditions. This is illustrated using uncross-linked EPDM as an example.

Sample

Unvulcanized EPDM (EPG 6170).

Conditions

Measuring Cell: DSC822e with liquid nitrogen cooling option

Pan: Aluminum 40 µl, pierced lid

Sample preparation: Cube of approx. 5 mg cut from the starting material

DSC measurement: Heating from –120 °C to 100 °C at 10 K/min (1st run). Then cooling to –120 °C at 5 K/min and repeating the first heating program (2nd run). The measurement was repeated (3rd run) after storing at room temperature for 20 days.

Atmosphere: Nitrogen, 50 ml/min. 

Interpretation 

The three measurement curves show a step at –45 °C that is due to the glass transition. Melting begins immediately afterward and ends at approx. 70 °C. The relatively broad melting region has to do with the wide distribution of crystal size. The smallest crystallites melt at low temperatures, while the larger and relatively perfect crystallites melt at higher temperatures.  

In the first run, the melting range consists of three peaks. The first peak is broad and has a maximum at 14 °C. There then follows a narrower peak with a maximum at 43 °C and then a smaller peak at 52 °C. This complex melting behavior is a result of the storage and processing conditions. Prior to the measurement, the sample had been stored for a long time at room temperature. Afterward, it was cooled in the DSC to the starting temperature. During cooling, small crystallites were formed which then, on heating, melted as soon as the glass transition was exceeded. These crystallites formed on cooling are therefore the cause of the first peak. Through the storage at room temperature, the crystallites that melt in the storage temperature range had sufficient time to grow or improve in quality. The number of crystallites that actually melt at room temperature is therefore relatively low so that a minimum occurs in the DSC curve. The crystallites that actually grew during storage melt at a temperature somewhat higher than the storage temperature. The melting of these crystallites is the cause of the second peak. During processing, the material was mechanically treated in the extruder. The internal stresses applied in this process cause stress-induced crystallization. These internal stresses lead to a greater degree of alignment of macromolecules and thus to the formation of crystallites with of higher quality; these crystallites then melt at higher temperatures. This effect is the cause of the third peak. In the second measurement there was no longer any storage or stress-induced crystallization. All the crystallites present were formed during cooling. The result is a broad melting peak without any structure due to different types of crystallites. The width of the melting peak of about 100 K indicates a wide size distribution of the crystallites. The third measurement was performed after storing the sample at room temperature for 20 days. During this time, crystallites of better quality were formed through the melting and recrystallization process. The DSC curve is similar to that of the first run except that no stress-induced crystallization is observed. 

Evaluation

As has already been shown in Section 3.1.2 the degree of crystallinity, α_c, can be determined from the normalized area of the melting peak, Δh,:

In the case of EPDM, the value of polyethylene of 290 J/g can be used for the enthalpy of fusion of the 100% crystalline material, Δh_c. An evaluation of the measurement curve yielded the following values:

The value for the degree of crystallinity refers to the crystallinity of the sample after cooling to below the glass transition temperature of about –30 °C. At room temperature the degree of crystallinity is about 6% lower because the smallest crystallites have then already melted. The measurements show that the crystallization of the sample prior to the second heating run is smaller than that of the original sample. The reason for this is the stress-induced crystallization caused by mechanical processing. This crystallization is not reproducible in the DSC and is the cause of the peak at 52 °C in the first measurement curve.

_{Influence of pretreatment on the melting of EPDM  | Thermal Analysis Handbook No.HB433 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}