Viscometric Determination Standard
An international testing methodology specifies the determination of the viscosity number and intrinsic viscosity of thermoplastic polyester homopolymers and copolymers in dilute solution using capillary viscometers under ISO 1628-5. The protocol governs polyethylene terephthalate, polybutylene terephthalate and related saturated polyesters dissolved in specified mixed solvents. Solvent mixtures include phenol combined with 1,2-dichlorobenzene in equal mass ratios, or alternatively ortho-dichlorobenzene combined with phenol.
Analytical Dissolution Procedure
Preparation mandates drying the polyester resin to prevent hydrolytic degradation during dissolution at elevated temperatures between one hundred and one hundred forty degrees Celsius. Accurately weighed polymer samples dissolve inside volumetric flasks, followed by cooling and filtration through sintered glass filters to remove insoluble additives or particulate contaminants. Dilute solutions and pure solvent blanks flow through calibrated Ubbelohde glass capillary viscometers immersed in a thermostatic water bath regulated strictly at twenty-five degrees Celsius plus or minus 0.05 degrees.
Viscosity Calculation Relationship
Flow times for pure solvent and dilute polymer solution are measured with electronic timers accurate to within one hundredth of a second. The ratio of solution flow time to pure solvent flow time yields relative viscosity, from which specific viscosity and reduced viscosity numbers derive across standardized concentration equations. Intrinsic viscosity correlates mathematically with polymer molecular weight, providing a reliable proxy for polymer chain length without resorting to complex gel permeation chromatography.
Testing Operational Constraint
Insoluble particulate fillers, pigment dispersion masterbatches and crosslinked polymer fractions disrupt capillary fluid passage, producing invalid viscometric measurements. The standard ceases to yield reliable intrinsic viscosity values if solution moisture content exceeds specification limits or if polymer degradation occurs from prolonged heating during initial dissolution steps.
