Analysis of Anti-Degradant Additive (6PPD) and its Transformation Product 6PPD-Quinone in Tire Rubber Using an Integrated HPLC System

Significance of the topic

Rubber products, particularly tire compounds, are formulated with anti-degradant additives to protect polymer chains from oxidative and ozone-induced damage and to extend service life. One commonly used additive is 6PPD (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine). Atmospheric oxidation of 6PPD produces 6PPD-quinone, a transformation product reported to be highly toxic to aquatic organisms (notably coho salmon). Reliable measurement of both parent 6PPD and 6PPD-quinone in tire rubber and particulate matter is therefore essential for environmental risk assessment, product quality control, and development of lower-impact additive alternatives.

Objectives and overview of the study

This application note demonstrates a simple, high-sensitivity HPLC method for simultaneous quantitation of 6PPD and its oxidation product 6PPD-quinone in commercial tire rubber. The goals were to establish sample pretreatment compatible with ISO 23075:2021, define chromatographic conditions using an integrated i-Series HPLC, evaluate calibration linearity and limits of detection, and assess method accuracy by spike recovery tests.

Methodology

• Sample preparation followed ISO 23075:2021. Rubber pieces (~1 g cut to ~2 mm squares) were subsampled (~200 mg) into a 20 mL flask and extracted with 10 mL of a 1:1 (v/v) mixture of HPLC-grade methanol and 0.01 mol/L aqueous ammonium acetate by ultrasonic bath for 3 hours at temperatures not exceeding 30 °C. Extracts were filtered through PTFE prior to analysis.
• Calibration standards were prepared by diluting commercially available 6PPD and 6PPD-quinone in the same methanol/ammonium acetate solvent. Calibration ranges: 6PPD: 0.5–50 mg/L; 6PPD-quinone: 0.1–10 mg/L.
• Chromatographic conditions: Shim-pack GISS C18 (150 × 4.6 mm, 5 μm); mobile phases: Eluent A = methanol/0.01 mol/L ammonium acetate (1:1), Eluent B = methanol. Gradient: hold 30% B from 0.00–25.00 min, ramp to 100% B 25.01–30.00 min, then re-equilibrate (0% B 30.01–35.00 min). Flow rate 1.0 mL/min, column 40 °C, injection 10 μL.
• Detection: dual UV monitoring with channel 1 at 290 nm for 6PPD and channel 2 at 351 nm for 6PPD-quinone.

Used instrumentation

• Shimadzu i-Series integrated LC system (LC-2070).
• Column: Shim-pack GISS C18, 150 mm × 4.6 mm, 5 μm (P/N: 227-30061-06).
• Vials: Shim-vial S Glass (P/N: 227-34500-02).
• UV detector with dual channels (set to 290 nm and 351 nm).
• Ultrasonic bath for extraction and PTFE syringe/filtration equipment for sample clean-up.

Main results and discussion

• Calibration linearity: Both analytes exhibited excellent linear response across the tested ranges, with correlation coefficients (r2) exceeding 0.999, demonstrating robust quantitation by UV detection within those ranges.
• Detection limits: Method detection limits (S/N = 3) in solution were established as 0.1 mg/L for 6PPD and 0.05 mg/L for 6PPD-quinone, supporting sensitive screening-level analyses of tire extracts.
• Spike recovery and reproducibility: Duplicate analyses and spike-recovery experiments showed satisfactory accuracy and precision. Representative recoveries: 6PPD ~96% (spike concentration 5 mg/L), 6PPD-quinone ~92% (spike concentration 1 mg/L). Results indicate efficient extraction and reliable quantitation under the described conditions.
• Chromatographic performance: Dual-wavelength UV detection provided selective monitoring for each analyte. Chromatograms of standards and spiked/unspiked extracts demonstrated clear peak detection and adequate separation for routine analysis with the specified gradient.

Benefits and practical applications of the method

• Simplicity: The extraction protocol is straightforward and conforms to an international standard (ISO 23075:2021), enabling easy adoption in routine QC and research laboratories.
• Sensitivity: Low detection limits and linear calibration support both screening and quantitative work for typical concentrations found in rubber extracts.
• Simultaneous analysis: The method enables concurrent determination of 6PPD and the environmentally relevant transformation product 6PPD-quinone in a single run, improving throughput.
• Applications: Method is suitable for monitoring additive levels in tire manufacturing and quality control, environmental fate studies of tire wear particles, assessment of alternative anti-degradant candidates, and laboratory-scale ecotoxicology support.

Future trends and potential uses

• Mass spectrometric detection: Coupling the separation with LC-MS/MS would increase selectivity and sensitivity, enable lower detection limits, and provide structural confirmation and detection of additional degradation products and isomers.
• Environmental monitoring: Routine application to road runoff, urban stormwater, sediments, and microplastic-associated extracts to quantify fluxes of 6PPD and 6PPD-quinone into aquatic systems.
• Higher throughput and automation: Online extraction, plate-based sample preparation, or accelerated solvent extraction can increase throughput for large monitoring campaigns.
• Standardization and interlaboratory studies: Wider adoption of harmonized protocols (e.g., ISO-based workflows) will improve comparability of concentration data used in risk assessments and regulatory decision-making.
• Alternative additive screening: Analytical methods like this support comparative studies that evaluate whether novel anti-degradants maintain performance while reducing formation of toxic transformation products.

Conclusion

The described HPLC approach using an integrated i-Series LC system and Shim-pack GISS C18 column provides a simple, reproducible, and sensitive procedure for simultaneous quantitation of 6PPD and 6PPD-quinone in tire rubber extracts. Compliance with ISO 23075-based extraction, excellent calibration linearity, low detection limits, and acceptable spike recoveries demonstrate the method's suitability for routine screening, quality control, and preparatory studies informing environmental impact assessments and additive development.

References

• Tian, Z.; et al., Acute toxicity of 6PPD-quinone to coho salmon reported in Science, 2021, 371, 185–189.
• ISO 23075:2021, Vulcanized rubbers - Determination of antidegradants by high-performance liquid chromatography.