ISO 1628-5:1998

Overview

ISO 1628-5:1998 specifies a standardized method for determining the viscosity number (reduced viscosity) of thermoplastic polyester (TP) homopolymers and copolymers in dilute solution using capillary viscometers. The procedure provides a reproducible measure of relative molecular mass for polyesters such as PET, PBT, PCT, PEN and related copolyesters. Measurements are performed at 25 °C on dilute solutions (nominal concentration 0.005 g·ml−1) and use capillary viscometry principles defined in ISO 1628-1.

Key topics and technical requirements

• Scope and materials: Applies to thermoplastic polyesters (polyethylene terephthalate PET, polybutylene terephthalate PBT, poly(cyclohexylenedimethylene terephthalate) PCT, polyethylene naphthalate PEN and copolyesters soluble in specified solvents).
• Sample prep: Representative sample, predried at 120 °C under vacuum for 3 hours to avoid hydrolytic degradation; account for inorganic fillers/additives >0.5% (ISO 3451-2 referenced).
• Solvents: Several hazardous solvents are specified; common choices include phenol/1,2-dichlorobenzene (50/50), phenol/1,1,2,2-tetrachloroethane (50/50 or 60/40), o‑chlorophenol, m‑cresol, dichloroacetic acid and phenol/2,4,6‑trichlorophenol. Solvent purity, preparation ratios and storage (protected from light) are defined.
• Apparatus: Ubbelohde suspended-level capillary viscometers (No. 1B, 1C or 2 per ISO 3105) are specified; other equivalent viscometers may be used but Ubbelohde is mandated in case of dispute.
• Measurement: Flow times of solvent and polymer solution are measured in a thermostatic bath at 25 ± 0.05 °C. Viscosity number is calculated from the measured flow times and solution concentration (per ISO 1628-1). Repeatability and reproducibility data from round-robin tests (Annex A) are reported for common solvent systems.
• Safety: Solvents are hazardous - avoid skin contact and inhalation; follow lab safety procedures.

Practical applications and users

ISO 1628-5 is used for:

• Quality control and acceptance testing of polyester resins in manufacturing.
• R&D and material characterization to estimate relative molecular mass.
• Supplier and specification verification for molding and extrusion grades. Primary users include polymer producers, plastics test laboratories, materials engineers, QA/QC teams and standards laboratories performing capillary viscometry on TP polyesters.

Related standards

• ISO 1628-1 - General principles for dilute-solution capillary viscometry
• ISO 3105 - Glass capillary kinematic viscometers (specifications)
• ISO 3451-2 - Determination of ash content (for filler analysis)
• ISO 7792-1 - TP designation system and basis for specifications

Keywords: ISO 1628-5:1998, viscosity number, thermoplastic polyester, PET viscosity, capillary viscometer, Ubbelohde, dilute solution viscometry, polymer molecular mass.