Overview
EN ISO 4629-2:2016 - "Binders for paints and varnishes - Determination of hydroxyl value - Part 2: Titrimetric method using a catalyst" - is a CEN-endorsed ISO method for measuring the hydroxyl value of resins, binders, primary alcohols, glycols and fats used in paints and varnishes. The standard specifies a titrimetric acetylation procedure (acetic anhydride) with a catalyst (4‑N‑dimethylaminopyridine in N‑methylpyrrolidone) and back-titration of excess reagent with alcoholic potassium hydroxide. It is part of ISO 4629 (Part 1 covers a non‑catalyzed pyridine method).
Key topics and technical requirements
- Scope: Intended for resins, binders, primary alcohols, glycols, fats; applicability to hydroxycarboxylic acids, phenolic OH, certain polyols and Friedel–Crafts‑activated aromatics is decided case-by-case. Castor oil and derivatives can be analyzed under appropriate conditions.
- Principle: Acetylation of hydroxyl groups with acetic anhydride; excess anhydride titrated with standardized alcoholic KOH to calculate hydroxyl value (mg KOH/g).
- Reagents: N‑methylpyrrolidone (NMP) solvent, 10% acetic anhydride in NMP (acetylating reagent), 1% DMAP catalyst in NMP, 0.5 mol/L KOH in methanol (standardized), methyl ethyl ketone (rinse), thymolphthalein/phenolphthalein indicators.
- Apparatus & procedure essentials: Analytical balance, conical flask, magnetic stirrer/hot plate, automatic titrator or burette. Determinations in triplicate; sample mass calculated from expected hydroxyl value (m = 300 / HV_expected). Typical reaction times: at least 15 min (ambient) - secondary OH groups require ≥60 min reaction and ≥30 min hydrolysis. End point detected colorimetrically or potentiometrically.
- Quality: References to reagent purity (ISO 3696 water), standardization of KOH, blank tests and acid‑value determination are required. Precision and reporting requirements are defined.
Applications and users
- Who uses it: Quality control and R&D laboratories in coatings and paints manufacturers, resin and binder producers, raw material suppliers, contract testing labs, and regulatory/testing bodies.
- Practical uses:
- Determining hydroxyl functionality for stoichiometry in crosslinking agents (isocyanates, blocked isocyanates, melamines).
- Characterizing polyols, glycols and fatty binders for formulation and performance prediction.
- QC batch release and incoming raw material verification.
- Advantages (compared to non‑catalyzed method): less hazardous solvents, lower solvent consumption, faster reactions, clearer titration end point and improved solubility for polyols.
Related standards
- ISO 4629-1 (titrimetric method without catalyst)
- ISO 660 (acid value for fats & oils)
- ISO 2114 (partial and total acid value)
- ISO 3696 (water for analytical use)
- ISO 4618 (paints & varnishes terms)
- ISO 15528 (sampling of paints, varnishes, raw materials)
Keywords: EN ISO 4629-2:2016, hydroxyl value determination, titrimetric method, binders for paints and varnishes, acetylation, DMAP catalyst, N‑methylpyrrolidone.
