Overview
ISO 15038:1999 specifies a laboratory method for determining the total active‑oxygen content of organic perester crosslinking agents used with unsaturated‑polyester thermosetting materials. The standard covers a titrimetric procedure in which the perester is reacted with potassium iodide in an isopropanol/acetic acid medium (with CuCl2 catalyst) and the liberated iodine is quantified by titration with standardized sodium thiosulfate. A normative annex (Annex A) gives a complementary procedure to determine tert‑butyl hydroperoxide (TBHP) when present as an impurity.
Keywords: ISO 15038:1999, organic perester, active‑oxygen, unsaturated‑polyester, crosslinking agents, titration, TBHP, sodium thiosulfate.
Key topics and technical requirements
- Purpose: Measurement of active‑oxygen (%) in organic peresters that initiate crosslinking of unsaturated‑polyester resins.
- Chemical principle: Perester oxidizes iodide to iodine; iodine is titrated with sodium thiosulfate to quantify active oxygen.
- Reagents and media: isopropanol, glacial acetic acid, potassium iodide (fresh), copper(II) chloride catalyst, standardized Na2S2O3, inert gas (N2 or CO2).
- Apparatus: 200 ml conical flask (ground glass), analytical balance (0.1 mg), pipettes (1 ml, 5 ml), 50 ml burette, magnetic stirrer.
- Procedure highlights: weigh ~2 mmol test portion, dissolve in isopropanol/acetic acid with CuCl2, add KI, allow reaction (30 min typical), dilute with water and titrate to colorless endpoint with Na2S2O3; perform blank and duplicate determinations.
- Calculations: formulae provided for total active‑oxygen (%) and conversion to perester content using molecular mass (with di‑peresters adjusted accordingly).
- TBHP correction: Annex A describes a SO2/pyridine reduction and iodine back‑titration to quantify TBHP; perester active oxygen obtained by subtraction.
- Precision data: for a 10% active‑oxygen level-within‑lab std dev (s) = 0.017, repeatability (r) = 0.05; between‑lab std dev = 0.035, reproducibility (R) = 0.1 (values from round‑robin study).
Applications and users
- Quality control and acceptance testing of organic perester crosslinkers in the manufacture of unsaturated‑polyester resins and composites.
- R&D and analytical laboratories testing peroxide initiators for correct active‑oxygen dosing in thermoset curing.
- Suppliers verifying product specifications and impurity levels (e.g., TBHP traces).
- Standards bodies and certification labs performing method validation or inter‑laboratory comparisons.
Safety note: The standard warns that peroxides and hydroperoxides are dangerous; wearing safety glasses and appropriate peroxide handling precautions are essential.
Related standards
- ISO 385‑1 (burettes), ISO 648 (pipettes), ISO 3696 (water for analytical use) - referenced normative documents in ISO 15038:1999.
This summary helps QC engineers, analytical chemists, and manufacturers locate and apply ISO 15038:1999 for reliable measurement of active‑oxygen in organic peresters used with unsaturated‑polyester thermosets.
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