Assessing Dynamic Viscoelastic Properties of Vulcanized Rubber Using Rheological Analysis

Vulcanized rubber, which is treated with sulfur and organic polysulfides, is extensively utilized as an industrial material. The process of vulcanization creates crosslinks within the molecular structure of raw rubber, reducing its thermoplasticity and enhancing its elasticity, tensile strength, and abrasion resistance, making it suitable for various industrial applications. Additionally, vulcanization improves the rubber's resistance to solvents, heat, and cold. The degree of crosslink density and its distribution in vulcanized rubber varies depending on several factors, including the amount of vulcanizing agent, vulcanizing catalyst, temperature, and duration of the vulcanization process.

The variations in the rate of vulcanization, which correspond to differences in the degree of crosslinking, can be studied through dynamic viscoelasticity measurements. This Application Brief outlines the use of dynamic viscoelasticity to assess vulcanized rubber with different crosslink densities.?

Experiment Conditions

For the experiment, three types of fluoro-rubber (A, B, and C) were prepared with varying crosslink densities by altering the amount of vulcanizing agent. Sample A contains the least amount of vulcanizing agent, while Samples B and C have progressively higher amounts.

The measurements were performed using an SDM5600H Rheol. Station connected to a DMS200 Dynamic Mechanical Spectrometer (Tension Module). The measurement frequency was set to 1Hz, with the temperature ranging from -100°C to 200°C, and the temperature ramp rate was 2°C/min.

Measurement Results

Figures 1 to 3 illustrate the dynamic viscoelasticity spectra of the vulcanized rubber samples A, B, and C. These figures show the E', E", and tanδ curves at a 1Hz measurement frequency. Peaks on the tanδ curves, representing the main dispersion (glass transition), are observed between -15°C and -10°C. The E' curve shows a decrease in elasticity during the glass transition phase.??

Figure 4 compares the E' curve measurements for each sample. Sample C, which has the highest crosslink density due to the largest amount of vulcanizing agent, shows the highest E' curve in both the transfer area and the viscous state (flat region of the curve). Samples B and A exhibit progressively lower curves in the same range. Previous studies have demonstrated that the storage elasticity rate along the flat section of the curve correlates with crosslink density, with higher storage elasticity indicating higher crosslink density. This report confirms that the sample with the highest crosslink density (Sample C) has the highest stored elasticity. Additionally, other experiments have demonstrated how to calculate crosslink density and inter-crosslink molecular weight from the storage elasticity rates in the flat section of the curves.

Figure 5 shows a comparison of the tanδ curve results for each sample. Sample A, which has the lowest crosslink density, exhibits the highest tanδ values in the flat section of the curve, followed by Samples B and C, which show progressively lower values. As crosslink density increases, the main dispersion (glass transition) peak shifts to higher temperatures, and the peak range widens. The tanδ curve is lower for samples with higher crosslink densities. This behavior, indicating that the tanδ peak widens and shifts to higher temperatures as crosslink density increases, is also supported by other studies and is confirmed by the results in Figure 5.

Conclusion

This Application Brief has examined the effect of crosslink density on the dynamic viscoelastic properties of vulcanized rubber. More broadly, this method can be applied to other crosslinked polymers to determine their physical properties. Dynamic viscoelasticity measurement is a widely used and effective technique for understanding the impact of crosslinking rates on various crosslinked polymers. The experiment can be conducted with greater precision and efficiency using Hitachi's NEXTA? DMA200, a technologically advanced and sophisticated successor to the DMS200 Dynamic Mechanical Spectrometer.

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Supplementary Note:

We express our gratitude to FUJIKURA RUBBER LTD. for their contribution of fluoro-rubber in connection with the work contained in this Application Brief.

Reference: Hitachi