Overview
EN ISO 1628-5:2015 - Plastics: determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 5 covers test methods for thermoplastic polyesters (TP) (homopolymers and copolymers). It specifies how to determine the viscosity number (reduced viscosity) of dilute polymer solutions - a standard metric used as an indicator of relative molecular mass for PET, PBT, PCT, PEN and related copolyesters. The procedure follows the general principles of ISO 1628‑1 and was adopted by CEN without modification (ISO 1628‑5:1998 → EN ISO 1628‑5:2015).
Key topics and technical requirements
- Scope: Applicable to TP polyesters soluble in specified solvents under defined conditions (e.g., PET, PBT, PCT, PEN and copolyesters).
- Measurement principle: Determine flow times of solvent and polymer solution at 25 °C using capillary viscometers; calculate viscosity number from flow times and solution concentration per ISO 1628‑1.
- Solvents (examples & ratios):
- Phenol/1,2‑dichlorobenzene (50/50)
- Phenol/1,1,2,2‑tetrachloroethane (50/50 or 60/40)
- o‑Chlorophenol, m‑cresol, dichloroacetic acid
- Phenol/2,4,6‑trichlorophenol (60/40) for crystalline PEN
- Safety note: solvents are hazardous - avoid inhalation/skin contact and store protected from light.
- Apparatus highlights:
- Ubbelohde suspended‑level capillary viscometers (No. 1B, 1C or 2) per ISO 3105 (Ubbelohde required in disputes)
- Thermostatic bath maintained at 25 ± 0.05 °C
- Steam bath (90–100 °C) and oil bath (135–140 °C) for dissolution when needed
- Analytical balance (high precision), sieves/filters, vacuum dryer
- Sample preparation:
- Predry samples (e.g., 120 °C under vacuum for 3 h) to avoid hydrolysis
- Account for inorganic fillers/additives (>0.5 % m/m) in solution concentration
- Two preparation procedures (weighing and volumetric dilution) are provided
- Measurement tolerances:
- Repeat flow times until two successive readings agree within 0.25 %
- Monitor solvent flow time daily; discard if it drifts >1 %
Applications - who uses it and why
- Plastics manufacturers (PET/PBT/PEN processors) for incoming material verification and lot acceptance
- Quality control and analytical laboratories for routine monitoring of molecular weight changes after processing, aging or recycling
- R&D teams for polymer characterization and process development
- Certification and specification preparation, where viscosity number is referenced as a performance indicator
Practical uses include assessing polymer chain degradation, verifying consistency between batches, and correlating viscosity number with mechanical properties.
Related standards
- ISO 1628‑1 - General principles for dilute‑solution viscosity measurement
- ISO 3105 - Glass capillary viscometers specifications (Ubbelohde)
- ISO 3451‑2 - Determination of ash (fillers) in polyalkylene terephthalates
- ISO 7792‑1 - Designation system and basis for specifications for thermoplastic polyesters
Keywords: EN ISO 1628-5:2015, viscosity number, thermoplastic polyester, capillary viscometer, Ubbelohde, PET viscosity testing, PBT, PEN, dilute solution viscosity, polymer molecular mass.
