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This International Standard specifies a method for the determination of the viscosity number of dilute solutions of polyamides in certain specified solvents. Polyamide samples must be completely soluble in the solvents mentioned. Additives such as flame-retardants and modifiers often interfere with the viscosity measurement, having an increasing effect on the viscosity number in formic acid and a decreasing effect on the viscosity number in sulfuric acid. The extent of the effect for polyamide compounds depends on the additive, the quantity of the additive, the presence of other additives and the compounding conditions. For pure polyamides or polyamides containing additives that do not interfere with the viscosity measurement, the determination of the viscosity number of a polyamide provides a measure of the molecular mass of the polymer. The viscosity number of pure polyamides or polyamides which contain additives that do not interfere with the viscosity measurement can be converted from one solvent to another. The viscosity number of polyamides containing additives that do interfere with the viscosity measurement is specific to the solvent used and the material composition. In this case, the measured viscosity number cannot be converted from one solvent to another. The method is applicable to the polyamides designated PA 46, PA 6, PA 66, PA 69, PA 610, PA 612, PA 11, PA 12, PA 6T/66, PA 6I/6T, PA 6T/6I/66, PA 6T/6I, PA 6I/6T/66 and PA MXD6 as defined in ISO 1874-1, as well as to copolyamides, compounds of polyamides and other polyamides that are soluble in one of the specified solvents under the specified conditions. The method is not applicable to polyamides produced by anionic polymerization of lactams or produced with cross-linking agents; such polyamides are normally insoluble in the specified solvents. The viscosity number is determined by the general procedure specified in ISO 1628-1, observing the particular conditions specified in this International Standard.