ISO 21561-2:2024

Overview

ISO 21561-2:2024 defines a standardized FTIR‑ATR (Fourier transform infrared spectrometry with attenuated total reflection) procedure to quantitatively determine the microstructure of the butadiene portion and the styrene content in solution‑polymerized styrene‑butadiene rubber (S‑SBR). Results report styrene as a mass fraction of the S‑SBR and vinyl / trans / cis microstructures as mol fractions relative to the butadiene portion. The method applies to raw rubbers only and includes calibration, sampling and spectrum‑acquisition requirements.

Key topics and technical requirements

• Measurement principle: ATR‑FTIR spectra are used to obtain absorbances characteristic of styrene and each butadiene microstructure; regression formulae convert absorbance ratios into composition.
• Instrumentation and settings:
  • FTIR detector: DTGS or TGS
  • Resolution: 2 cm−1
  • Number of scans: 32
  • Spectral range: 600–1 800 cm−1
  • ATR: single‑bounce diamond crystal, incidence angle 45°, with a pressure clamp to ensure consistent contact
• Sampling and sample prep: prepare test piece per ISO 1795; sample surface must be flat and roughly the size of the ATR crystal.
• Peak measurement points (examples from the standard):
  • Styrene: 695–700 cm−1 (or 720–730 cm−1 when shifted)
  • Styrene (secondary): 755–761 cm−1
  • Cis: commonly at 726 cm−1 when masked by styrene
  • Vinyl: 905–912 cm−1 and 991–996 cm−1
  • Trans: 962–967 cm−1
  • Baseline reference: 1 200 cm−1
• Data processing: baseline correction, calculation of absorbance ratios and second‑order terms, then substitution into the published regression formulae (derived from interlaboratory data) to obtain styrene and microstructure values.
• Controls and limitations:
  • Maintain consistent sample chamber atmosphere to avoid CO2 interference (peaks at ~668 and 723 cm−1).
  • Precision (Annex A) may deteriorate for S‑SBR containing polystyrene blocks or with styrene content >45 %.
• Supplementary material: Annex B (regression acquisition), Annex C (NMR alternative) and precision data in Annex A.

Applications

• Quality control and batch verification of S‑SBR in rubber and tire manufacturing
• Polymer synthesis monitoring and process optimization for solution‑polymerized SBR
• Supplier qualification, acceptance testing and material specification compliance
• R&D and failure analysis where microstructure affects performance (rolling resistance, wear, etc.)

Who should use this standard

• Rubber and tire manufacturers, QC laboratories, polymer chemists, materials characterization labs, and regulatory/standards bodies involved in rubber testing and specification.

Related standards

• ISO 1795 - Rubber, raw natural and raw synthetic - Sampling and preparative procedures
• ISO 1382 - Rubber - Vocabulary
• Other parts of the ISO 21561 series covering complementary methods (e.g., NMR described in Annex C)

Keywords: ISO 21561-2:2024, SBR microstructure, S‑SBR, FTIR ATR method, styrene content, vinyl trans cis, rubber testing, ATR‑FTIR rubber analysis.