Overview
ISO 15033:2018 - "Plastics - Determination of caprolactam and its cyclic and linear oligomers by HPLC" specifies validated high-performance liquid chromatography (HPLC) procedures for measuring caprolactam and its oligomers in polyamide 6 (PA6), caprolactam itself and mixtures of rearrangement products in water. The standard covers both cyclic oligomers (from 0.01% by mass and upwards) and linear oligomers (from 5 mg/kg and upwards) up to and including the hexamer (n = 6). Two methods are described: Method A (comprehensive determination of cyclic and linear oligomers) and Method B (a significantly faster procedure for caprolactam and the cyclic dimer).
Key technical topics and requirements
- Analytical principle: Reversed-phase HPLC with UV detection (200 nm) for cyclic oligomers; optional post‑column derivatization + fluorescence detection (excitation 330 nm / emission 420 nm) for linear oligomers.
- Scope limits: Quantitative determination up to hexamer (n = 6); sensitivity for cyclic oligomers consistent from tetramer upwards; linear oligomer calibration up to hexamer.
- Calibration: Method A calibration series for cyclic oligomers typically 100–1 500 mg/L; linear oligomer calibration typically 2–15 mg/L. Method B provides a faster calibration/determination for monomer and dimer.
- Sample preparation and stability: Test-sample size limits - max 0.5 g for PA6 and caprolactam, 1.25 g for aqueous mixtures. Dissolved samples are limited in stability (caprolactam hydrolyses to ε-aminocaproic acid); do not store longer than specified (document: up to 6 days).
- Instrumentation: HPLC pump and injector (auto-sampler with multi-component “sandwich” injection capability), C18 reversed‑phase column (3 mm ID × 250 mm, 40 °C, ~0.005 mm particle size), UV detector and optional reagent pump/reaction coil for post-column derivatization.
- Reagents: Formic acid, acetonitrile, phosphoric acid, 1,2-phthalic dicarboxaldehyde, 3‑mercaptopropionic acid and a short‑lived post‑column derivatization reagent (stability limited to ~3 days).
Practical applications and users
ISO 15033:2018 is used by:
- Polymer and analytical chemistry laboratories performing compositional quality control of PA6 and caprolactam feedstocks.
- Manufacturers and converters of nylon/PA6 assessing oligomer content for product performance, regulatory compliance and migration studies.
- R&D teams isolating or characterizing oligomer distributions and degradation/rearrangement products.
- Contract testing and certification bodies reporting oligomer concentrations in materials and aqueous extracts.
Benefits include reproducible quantification of monomer and oligomers, ability to detect both cyclic and linear species, and an accelerated option (Method B) for routine monomer/dimer screening.
Related standards
- ISO 472 - Plastics - Vocabulary (normative reference cited in ISO 15033:2018)
Keywords: ISO 15033:2018, caprolactam, oligomers, HPLC, polyamide 6, cyclic oligomers, linear oligomers, high-performance liquid chromatography, Method A, Method B, post-column derivatization, fluorescence, UV detection.
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