DIN Standards Committee Coatings and Coating Materials
DIN SPEC 91143-2
Modern rheological test methods - Part 2: Thixotropy - Determination of the time-dependent structural change - Fundamentals and interlaboratory test; Text in German and English
Moderne rheologische Prüfverfahren - Teil 2: Thixotropie - Bestimmung der zeitabhängigen Strukturänderung - Grundlagen und Ringversuch; Text Deutsch und Englisch
Procedure
Technical Report
Overview
The aim of this work is to find a method for the determination of the thixotropic behaviour of coating materials and their raw materials and to check its applicability successfully. It is not possible in each case to cover all measuring samples by only one generalized measuring specification. Already in the year 2005 first ideas and preliminary tests have been elaborated in order to establish the best possible conditions for the measuring procedures and to find samples suitable to cover a range relevant for the respective industry. So, in a first comparative testing programme five samples (three waterborne basecoats, a pasty sealing compound and a dispersion) together with a Newtonian reference substance of the PTB were measured with four different methods, to prove the fundamental suitability of these measuring procedures for the determination of the thixotropic properties of coating materials. In the second comparative testing programme (2009, 2010) special emphasis was given to the determination of precision data (repeatability precision, reproducibility precision). For that purpose a waterborne basecoat and a conventional clearcoat were investigated. The following methods have been applied: - flow curves, with evaluation of the hysteresis area (rotational test) - shear step with recovery, as rotational test with preset of shear rate - shear step with recovery, as rotational test with alternating preset of shear stress and shear rate - shear step with recovery, as combined oscillatory/rotational test with preset of shear deformation or shear rate respectively In principle, it can be said that for unambiguous characterization of various products of different substance classes, the respective elaboration of detailed test specifications makes sense and is necessary. In order to compare results, the adherence to previously exactly specified measuring conditions is required. The recording of flow curves for the determination of thixotropy is less suitable to obtain quantitative findings (hysteresis area as a measure for thixotropy). In this respect flow curves can yield only a first overview about the general flow behaviour of a substance. In comparison, methods based on shear steps with recovery, for instance as rotational test with preset of the shear rate, show more evidence concerning the thixotropic behaviour - and especially to structure recovery after reducing load. Here, well suitable values for the repeatability precision and reproducibility precision (for instance, structure recovery index, thixotropy index) can be observed. The committee responsible for this Technical Report is NA 002-00-07-16 AK "Rheologie" ("Rheology") at DIN.
