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Significance and Use
5.1 The yield stress of a material is a measure of the amount of force required to initiate movement of that material in a pipe, through a pump, or from nozzle. The yield stress also characterizes the ability of the material to maintain particles in suspension. Along with viscosity measurements, yield stress measurements have been useful in establishing root causes of flow problems such as excessive orange peel and sagging and in explaining resistance to such problems. After a coating has been applied, flow and leveling tends to be inversely related to yield stress and sag resistance tends to be directly related to yield stress. The ability of an automotive basecoat to keep aluminum and/or mica flakes oriented has been related to yield stress (direct relationship).
1.1 These test methods cover three approaches for determining yield stress values of paints, inks and related liquid materials using rotational viscometers. The first method uses a rotational viscometer with coaxial cylinder, cone/plate, or plate/plate geometry. The second method uses a rheometer operating in controlled stress mode with similar geometries. The third method uses a viscometer with a vane spindle.
1.2 A non-rotational technique, the falling needle viscometer (FNV), also can be used to measure yield stress values in paints, inks and related materials. See Test Methods D5478, Test Method D, Yield Stress Determination for details.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D3925 Practice for Sampling Liquid Paints and Related Pigmented Coatings
D5478 Test Methods for Viscosity of Materials by a Falling Needle Viscometer
ICS Number Code 87.040 (Paints and varnishes); 87.080 (Inks. Printing inks)
UNSPSC Code 31211500(Paints and primers); 12171703(Inks)