Overview
ISO 20368:2017 defines a standardized FTIR method to determine the degree of crosslinking of crosslinked epoxy resins by measuring the disappearance of the epoxy functional group. The technique uses Fourier Transform Infrared (FTIR) spectroscopy in transmittance mode to compare epoxy absorption before and after curing, producing a quantitative crosslinking value useful for research, process development and quality control.
Key topics and requirements
- Measurement principle: Degree of crosslinking Cr is calculated from the peak-height ratio of the epoxy absorption to an internal standard in the uncured (X) and cured (Y) states using Cr = (1 − Y/X) × 100.
- Spectral features:
- Epoxy group absorption: around 900–915 cm⁻¹.
- Recommended internal standard: methylene band near 2 930 cm⁻¹ (unchanged by curing).
- Apparatus & settings:
- FTIR spectral range: 4 000–650 cm⁻¹; cumulative scans > 16.
- Use FTIR in transmittance mode.
- Silicon wafers: single-side polished, ~625 µm thick; test sample area ~15 mm square.
- Sample delivery: 5–10 µL (micropipette) or 5–10 mg (analytical balance).
- Sample preparation:
- Place test resin between two 15 mm square polished silicon wafers (polished faces together), wipe excess, and record background on blank wafer stack.
- Preliminary applicability test:
- Two uncured samples (measured within 5 minutes) must produce consistent peak-height ratios for the method to be applicable to a given epoxy system.
- Results & reporting:
- Report spectra, instrument details, sample formulation, curing conditions (temperature/time), peak-height method and calculated Cr.
- Precision:
- Interlaboratory data (specified in the standard) provide repeatability and reproducibility benchmarks; analysis was performed in accordance with ISO 5725‑2.
Applications and users
ISO 20368 is practical for:
- Polymer scientists and R&D teams studying cure kinetics and reaction rates.
- Production and QC laboratories monitoring batch-to-batch cure of adhesives, coatings, composites and varnishes.
- Process engineers optimizing curing conditions (temperature/time) and catalysts.
- Material characterization labs needing a fast, low-sample-volume method to quantify epoxy conversion.
Benefits include simple sample prep, small sample requirements, and suitability for systems containing competing active groups when applicability is verified.
Related standards
- ISO 5725-2 (precision of measurement methods and results) - referenced for precision analysis.
- ISO/IEC Directives referenced in the standard’s development and editorial rules.
Keywords: ISO 20368, epoxy resins, degree of crosslinking, FTIR spectroscopy, transmittance FTIR, epoxy cure, quality control, polymer characterization.
Open preview (PDF)Plastics — Epoxy resins — Determination of degree of crosslinking of crosslinked epoxy resins by Fourier Transform Infrared (FTIR) Spectroscopy
