Acid Value Determination in Powder Coating Resins
Method developed according to DIN EN ISO 2114 (200)
This application describes the determination of the acid value, expressed in mg KOH/g resin, of powder coating resins by potentiometric titration.
Background:
Powder coating is nowadays a widely applied technique that shows remarkable advantages over other coating processes. Even thick coatings can be produced without the need of solvents or finishers that are costly, and are often detrimental to the environment. The basis of the powder coating formulation is the resin. Depending on the application, the resin has to satisfy different requirements. Hence, there is a large variety of resins available. The properties of these resins are mainly affected by the sort and number of functional groups on the resin.
One important characteristic of the powder coating resins is the total number of acidic functional groups (i.e. carboxyl groups). Mainly relevant here are free carboxyl groups as well as anhydrides, which are hydrolyzed to carboxyl groups during the analysis. The so-called acid value (AV) is represents the amount of mg KOH/g sample that is consumed for the neutralization of the acidic groups contained in the sample. Anhydrides add double to the acid value as they react with water to form two equivalents of carboxylic acids.
Neutralization of carboxyl groups:
RCOOH + KOH → RCOO- + K+ + H2O
Hydrolyzation of anhydrides
R(C=O)O(C=O)R + 2 KOH → 2 RCOOK + H2O
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