ISO 4629-2:2016

Overview

ISO 4629-2:2016 specifies a titrimetric method using a catalyst to determine the hydroxyl value (hydroxyl number) of resins and binders for paints and varnishes. The method measures milligrams of KOH equivalent per gram of sample by acetylating hydroxyl groups with acetic anhydride and titrating the excess reagent with alcoholic KOH. It applies to resins, paint and varnish binders, primary alcohols, glycols and fats; applicability to certain polyols, phenolic OH, hydroxycarboxylic acids or aromatics is decided case-by-case. Under suitable conditions the method can be used for castor oil and derivatives.

Key topics and technical requirements

• Principle: Acetylation of OH groups with acetic anhydride; titration of excess anhydride with alcoholic KOH; hydroxyl value calculated from titrant volumes and concentration, corrected for acid value.
• Catalyst and solvents: Uses a catalyst solution of 4‑N‑dimethylaminopyridine (DMAP, 1% in NMP) and acylating reagent as 10% acetic anhydride in N‑methylpyrrolidone (NMP). NMP improves solubility and reduces hazardous solvent use versus pyridine methods.
• Reagents and quality: Analytical-grade reagents; water meeting ISO 3696 quality. Methanolic (or ethanolic) KOH standard (~0.5 mol/L) is prepared and calibrated against potassium hydrogen phthalate.
• Apparatus: Standard laboratory glassware plus titrimeter or automatic burette, analytical balance, magnetic stirrer and heating plate.
• Procedure highlights:
  • Sample mass calculated from expected hydroxyl value.
  • Reaction times: typically ≥15 min at 23 ± 2 °C; products with secondary OH require longer acetylation (≥60 min) and hydrolysis times (≥30 min).
  • Endpoint detection potentiometrically or with indicators (thymolphthalein or phenolphthalein).
  • Daily blank determination required.
• Results & precision: Hydroxyl value reported in mg KOH/g (individual values and mean, rounded to 1 mg KOH/g). Repeatability and reproducibility limits are specified (examples: repeatability ≤0.84 mg KOH/g for HV <20).

Applications and users

• Who uses it: Quality control and analytical laboratories, paint and varnish manufacturers, binder and resin producers, raw material suppliers, R&D chemists and regulatory testing labs.
• Practical uses:
  • Characterizing polyols, binders and resins for formulation control.
  • Verifying raw-material specifications and batch consistency.
  • Supporting process control and R&D when selecting or modifying binders.
• Benefits: Faster reaction times, lower solvent hazards and clearer titration endpoints compared with non‑catalyzed pyridine methods (ISO 4629-1), and improved solubility for many polyols.

Related standards

• ISO 4629-1:2016 - Titrimetric method without catalyst
• ISO 2114:2000 - Acid value determination (used to correct HV)
• ISO 660 - Acid value for fats and oils
• ISO 3696 - Laboratory water quality
• ISO 15528 - Sampling of paints and varnishes
• ISO 4618 - Terms and definitions for paints and varnishes

Keywords: ISO 4629-2, hydroxyl value, hydroxyl number, binders for paints and varnishes, titrimetric method, DMAP catalyst, NMP solvent, quality control, paint binder analysis.