DIN 53240-3:2016-03

The usual method for determining the hydroxyl value (according to DIN 53240-1 or DIN EN ISO 4629-1) involves acetylation of the hydroxyl groups with acetic anhydride in pyridine. However, this method has some disadvantages: - the sample shall be boiled for 1 h under reflux; - the method cannot be automated; - small hydroxyl values cannot be accurately determined; - the unpleasant and toxic pyridine shall be used. For these reasons, the much simpler method (for automation) was chosen. The (primary and secondary) hydroxyl groups are reacted with toluenesulfonyl isocyanate to form acidic carbamate groups. The carbamates can be titrated with the strong base tetrabutylammonium hydroxide (TBAOH) under non-aqueous conditions, that is, in an organic solvent. This method is mainly suitable for neutral media. Acidic products yield higher values, while neutral products yield lower values due to neutralization of the acidic carbamates. For these products tests shall be carried out in advance to ensure the suitability of the method. The committee responsible for this standard is NA 002-00-02 AA "Lackrohstoffe" ("Raw materials for paints and varnishes") at DIN Standards Committee Coatings and Coating Materials (NAB).