Influence of Carbon Black on the Glass Transition

Purpose

To demonstrate the influence of carbon black on the glass transition of elastomers using an EPDM as an example. 

Sample

EPDM elastomers with different carbon black contents  

EPDM: Buna EPG 5450  

Carbon black: N550  

Composition:

Conditions

Measuring cell: DSC822 with liquid nitrogen cooling option

Pan: Aluminum 40 µl, pierced lid 

Sample preparation: Cubes of approx. 20 mg were cut from the starting material. Before the measurement the samples were cooled to the starting temperature at 10 K/min. 

DSC measurement: Heating from 90 °C to 10 °C at 10 K/min  

Atmosphere: Nitrogen, 50 ml/min

The glass transition is observed at about –55 °C. Immediately after the glass transition step, the polymer crystallites begin to melt. The melting peak is relatively small due to the low degree of crystallinity. Melting is completed by about –10 °C. 

Evaluation

With EPDM, the melting process begins directly after the glass transition. This makes it more difficult to determine the characteristic values for the glass transition. The tangents needed for the evaluation are therefore drawn in a temperature range above the glass transition in which the material has already melted (see Section 3.1.2. Glass transition and melting of EPDM). The results show that the glass transition temperature is independent of the carbon black content. The step height, Δcp , does of course decrease with increasing carbon black content.  

The following diagram shows a plot of the step height at the glass transition against polymer content, αpolymer. For comparison purposes, the step height of the pure elastomer (αpolymer=100%) is also given. 

Conclusions

While the glass transition temperature itself remains unchanged, the relationship between the step height and the polymer content is a straight line through the origin. The decrease in the step height with increasing carbon black content is therefore due to a dilution effect. Information about the interaction between the carbon black and the polymer matrix cannot be obtained from these measurements. 

_{Influence of Carbon Black on the Glass Transition | Thermal Analysis Application No. HB452 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}