Sulfur Determination in Rubber

Significance of the Topic

Monitoring sulfur levels in rubber compounds is critical because sulfur acts as the primary curing agent in vulcanization, directly influencing cross-link density and thus mechanical properties such as elasticity, plasticity and durability. Accurate sulfur determination enables manufacturers and researchers to optimize formulations, ensure product consistency and diagnose curing systems (e.g., conventional, semi-efficient or peroxide).

Goals and Study Overview

The primary objective of this application note is to demonstrate a robust analytical workflow for quantifying sulfur in rubber matrices using the LECO S832 sulfur analyzer. Key aims include:

• Establishing calibration and drift correction protocols with certified reference materials.
• Validating method precision and accuracy on natural and compounded rubber samples.
• Providing operational guidelines for routine QA/QC laboratories.

Methodology and Instrumentation

The method is based on high-temperature combustion and infrared detection. Core steps include:

1. Instrument setup and conditioning according to the operator’s manual, including two initial sample replicates to stabilize the system.
2. Blank determination with at least three replicates of 1.0 g Com-Cat™ to establish baseline correction.
3. Calibration using three levels of sulfur reference materials (LECO LCRM®, LRM®, NIST or equivalent), weighed at 0.10–0.15 g and mixed with 1.0 g Com-Cat.
4. Sample analysis by weighing 0.10–0.25 g of rubber into ceramic boats, adding Com-Cat, and combusting at 1350 °C in the S832 furnace.
5. Data acquisition parameters: baseline stability enabled, comparator set at 0.30 %, integration time 90–360 s.

Main Results and Discussion

Typical results for sulfur content:

• Natural rubber: average 0.024 % S (standard deviation 0.002 %).
• Compound Rubber 1: average 1.94 % S (s = 0.05 %).
• Compound Rubber 2: average 1.47 % S (s = 0.02 %).

Precision and accuracy were verified using a linear calibration through the origin with LECO BBOT LCRM (7.35 % S) and Mineral Oil standard (0.105 % S). Results demonstrate excellent repeatability and minimal systematic drift.

Practical Benefits and Applications

The described method delivers:

• High throughput with minimal sample preparation.
• Low detection limits suitable for natural rubber and high-sulfur compounds.
• Reliable QA/QC control in rubber manufacturing and R&D laboratories.

Future Trends and Opportunities

Emerging developments may include:

• Integration of automated autoloaders and LIMS connectivity for seamless data management.
• Miniaturized combustion cells for reduced reagent consumption and faster cycle times.
• Coupling sulfur analysis with simultaneous multi-element detection to broaden compositional profiling.

Conclusion

The LECO S832 method provides a precise, accurate and efficient approach for sulfur determination in rubber materials. Its straightforward calibration, robust instrumentation and validated performance make it an ideal tool for both routine QA/QC and advanced research applications.

References

1. LECO Corporation. Sulfur Determination in Rubber, Application Note, Form No. 203-821-578, April 2019.