prEN ISO 1628-5

• Status: Not Published
• Publication Date: 11-Jan-2028
• ICS: 83.080.20 - Thermoplastic materials
• Technical Committee: CEN/TC 249 - Plastics
• Drafting Committee: CEN/TC 249 - Plastics
• Current Stage: 4060 - Closure of enquiry - Enquiry
• Start Date: 08-Sep-2026
• Completion Date: 08-Sep-2026
• Ref Project: oSIST prEN ISO 1628-5:2026 - Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 5: Thermoplastic polyester (TP) homopolymers and copolymers (ISO/DIS 1628-5:2026)

• Revises: EN ISO 1628-5:2015 - Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 5: Thermoplastic polyester (TP) homopolymers and copolymers (ISO 1628-5:1998)
• Effective Date: 18-Dec-2024

Overview

prEN ISO 1628-5:2026 is a European draft international standard developed by CEN, aligning with ISO 1628-5. The standard specifies methods for the determination of the viscosity of thermoplastic polyester (TP) homopolymers and copolymers in dilute solution using capillary viscometers. This part of ISO 1628 focuses specifically on materials such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(cyclohexylenedimethylene terephthalate) (PCT), poly(ethylene naphthalate) (PEN), and related copolyesters that are soluble in designated solvents under defined conditions.

This standard provides a reliable approach to assess the viscosity number (also termed "reduced viscosity"), a measure that offers valuable insights into the relative molecular mass of thermoplastic polyesters. Accurate viscosity measurements are essential for quality control, product specification, and research in polymer production and processing.

Key Topics

• Capillary Viscometry Methods
  The document details two primary methods for viscosity determination:

  • The Efflux Time Method, where the flow time of a dilute polymer solution and solvent through a capillary is measured.
  • The Differential Pressure Method, comparing the differential pressure across capillaries for solvent and solution simultaneously.

• Solvent Selection and Preparation
  Guidance is provided regarding solvent types (including phenol/1,2-dichlorobenzene, o-chlorophenol, m-cresol, dichloroacetic acid, and others) and stringent requirements for purity and storage to ensure accurate results.

• Apparatus Requirements
  Specifies required equipment, including types of viscometers (Ubbelohde types), thermostatic baths, mills for sample preparation, analytical balances, and filtering equipment.

• Sample Preparation
  Detailed steps for drying, milling, dissolving, and filtering the polyester samples, addressing different crystallinity levels to achieve consistent solutions for analysis.

• Test Procedure
  Instructions for performing both the efflux time and differential pressure methods, including temperature control and flow time measurements.

• Calculation and Reporting
  Provides formulas for calculating the viscosity number and outlines the necessary information for a comprehensive test report.

Applications

prEN ISO 1628-5:2026 is widely applicable in the plastics industry and polymer research laboratories for:

• Quality Control: Verifying the molecular mass of thermoplastic polyesters used in processes such as injection moulding, extrusion, and film production.
• Product Specification: Ensuring that polyester materials meet the required standards for mechanical properties and processing performance.
• Material Development: Supporting the development and innovation of new polyester resins and copolymers by enabling precise comparison of molecular structure.
• Regulatory and Certification Compliance: Satisfying the requirements for material certification and traceability in line with ISO and European standards.

Related Standards

• ISO 1628-1:2024 - Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 1: General principles
• ISO 3105:1994 - Glass capillary kinematic viscometers - Specifications and operating instructions
• ISO 3451-2:1998 - Plastics - Determination of ash - Part 2: Poly(alkylene terephthalate) materials
• ISO 20028-1:2019 - Plastics - Thermoplastic polyester (TP) moulding and extrusion materials - Part 1: Designation system and basis for specification
• ISO 760:1978 - Determination of water - Karl Fischer method (General method)

These standards provide the necessary methodologies, equipment specifications, and reference materials to ensure comprehensive and harmonized approaches to polymer viscosity determination.

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Keywords: prEN ISO 1628-5, ISO 1628, thermoplastic polyester, viscosity number, capillary viscometer, PET, PBT, polyester molecular mass, polymer quality control, plastic standard, CEN, polymers in dilute solution.

SLOVENSKI STANDARD
01-september-2026
Polimerni materiali - Ugotavljanje viskoznosti polimerov v razredčenih raztopinah
s kapilarnimi viskozimetri - 5. del: Plastomerni poliestri (TP) homo- in kopolimerov
(ISO/DIS 1628-5:2026)
Plastics - Determination of the viscosity of polymers in dilute solution using capillary
viscometers - Part 5: Thermoplastic polyester (TP) homopolymers and copolymers
(ISO/DIS 1628-5:2026)
Kunststoffe - Bestimmung der Viskosität von Polymeren in verdünnter Lösung durch ein
Kapillarviscosimeter - Teil 5: Thermoplastische Polyester (TP) Homopolymere und
Copolymere (ISO/DIS 1628-5:2026)
Plastiques - Détermination de la viscosité des polymères en solution diluée à l'aide de
viscosimètres à capillaires - Partie 5: Homopolymères et copolymères des polyesters
thermoplastiques (TP) (ISO/DIS 1628-5:2026)
Ta slovenski standard je istoveten z: prEN ISO 1628-5
ICS:
83.080.20 Plastomeri Thermoplastic materials
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 1628-5
ISO/TC 61/SC 9
Plastics — Determination of
Secretariat: KATS
the viscosity of polymers in
Voting begins on:
dilute solution using capillary
2026-06-16
viscometers —
Voting terminates on:
2026-09-08
Part 5:
Thermoplastic polyester (TP)
homopolymers and copolymers
Plastiques — Détermination de la viscosité des polymères en
solution diluée à l'aide de viscosimètres à capillaires —
Partie 5: Homopolymères et copolymères des polyesters
thermoplastiques (TP)
ICS: 83.080.20
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
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NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED
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NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 1628-5:2026(en)
DRAFT
ISO/DIS 1628-5:2026(en)
International
Standard
ISO/DIS 1628-5
ISO/TC 61/SC 9
Plastics — Determination of
Secretariat: KATS
the viscosity of polymers in
Voting begins on:
dilute solution using capillary
2026-06-16
viscometers —
Voting terminates on:
2026-09-08
Part 5:
Thermoplastic polyester (TP)
homopolymers and copolymers
Plastiques — Détermination de la viscosité des polymères en
solution diluée à l'aide de viscosimètres à capillaires —
Partie 5: Homopolymères et copolymères des polyesters
thermoplastiques (TP)
ICS: 83.080.20
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
or ISO’s member body in the country of the requester.
NATIONAL REGULATIONS.
ISO copyright office
RECIPIENTS OF THIS DRAFT ARE INVITED
CP 401 • Ch. de Blandonnet 8
TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
Phone: [phone]
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 1628-5:2026(en)
ii
ISO/DIS 1628-5:2026(en)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
4.1 Efflux time method .2
4.2 Differential pressure method .2
5 Solvents . 2
5.1 General .2
6 Apparatus . 3
6.1 Efflux time method .3
6.2 Differential pressure method .4
7 Solvent and solution . 4
7.1 Selection of the solvent.4
7.2 Determination of inorganic materials or other additives in the sample .5
7.2.1 General .5
7.2.2 Determination of the content of inorganic materials .5
7.2.3 Determination of the content of other additives .5
7.3 Sample .5
7.4 Preparation of solution .6
7.4.1 General .6
7.4.2 Procedure A .6
7.4.3 Procedure B .7
8 Procedure . 7
8.1 Efflux Time method.7
8.1.1 Preparatory operations .7
8.1.2 Measurement of flow time .7
8.2 Differential pressure method .7
9 Expression of results . 8
10 Test report . 8
Annex A (informative) Round-robin testing . 10

iii
ISO/DIS 1628-5:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are circulated to the member bodies for
voting. Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
International Standard ISO 1628-5 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee
SC 9, Thermoplastic materials.
This third edition cancels and replaces the second edition ), which has been technically revised.
The main change is the incorporation of an alternative procedure, the differential pressure method
(following same change in ISO 1628-1:2024), based on comparing the differential pressure in capillary
tubing due to the flow of polymer solution and neat solvent simultaneously.
ISO 1628 consists of the following parts, under the general title Plastics — Determination of the viscosity of
polymers in dilute solution using capillary viscometers:
— Part 1: General principles
— Part 2: Poly(vinyl chloride) resins
— Part 3: Polyethylenes and polypropylenes
— Part 4: Polycarbonate (PC) moulding and extrusion materials
— Part 5: Thermoplastic polyester (TP) homopolymers and copolymers
— Part 6: Methyl methacrylate polymers
Annex A of this part of ISO 1628 is for information only.

iv
DRAFT International Standard ISO/DIS 1628-5:2026(en)
Plastics — Determination of the viscosity of polymers in
dilute solution using capillary viscometers —
Part 5:
Thermoplastic polyester (TP) homopolymers and copolymers
1 Scope
This part of ISO 1628 specifies a method for the determination of the viscosity number (also referred to
as "reduced viscosity") of dilute solutions of thermoplastic polyesters (TPs) in certain specified solvents.
The method is applicable to poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly-
(cyclohexylenedimethylene terephthalate) (PCT), and poly(ethylene naphthalate) (PEN), as well as to
copolyesters and other polyesters, referred to in ISO 20028-1:2019, that are soluble in one of the specified
solvents under the specified conditions.
The viscosity number is determined by the general procedure specified in ISO 1628-1:2024, observing the
particular conditions specified in this part of ISO 1628.
The determination of the viscosity number of a thermoplastic polyester provides a measure of the relative
molecular mass of the polymer.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 760:1978, Determination of water — Karl Fischer method (General method)
ISO 1628-1:2024, Plastics — Determination of the viscosity of polymers in dilute solution using capillary
viscometers — Part 1: General principles
ISO 3105:1994, Glass capillary kinematic viscometers — Specifications and operating instructions
ISO 3451-2:1998, Plastics — Determination of ash — Part 2: Poly(alkylene terephthalate) materials
ISO 20028-1:2019, Plastics — Thermoplastic polyester (TP) moulding and extrusion materials — Part 1:
Designation system and basis for specification
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org

ISO/DIS 1628-5:2026(en)
4 Principle
4.1 Efflux time method
The times of flow of a solvent and a solution of resin in the solvent at a concentration of 0,005 g/ ml are
measured at 25 °C by conventional methods. The viscosity number is calculated from these measurements
and from the known concentration of the solution. Specific gravity difference and kinetic energy corrections
are small in this method and are not applied.
4.2 Differential pressure method
The differential pressure across each of the 2 capillaries connected in series, one of them receiving the
solvent having the differential pressure ∆p , the other receiving the polymer solution having the differential
pressure ∆p, are measured at 25°C. The viscosity number is calculated from these measurements and from
the known concentration of the solution.
5 Solvents
5.1 General
The viscosity number of saturated polyester is related to the solvent used, the test results between different
solvents cannot be directly compared. This standard specifies six different solvents.
Unless otherwise specified, only reagents confirmed as analytical grade should be used.
WARNING — The solvents used are hazardous. Avoid contact with the skin and inhalation of the
vapours.
WARNING — When using an automated instrument to apply the differential pressure method, use
only solvents compatible with the specific instrument used. In particular, some instruments are not
compatible with acids.
5.2 Phenol/1,2-dichlorobenzene
Weigh out 1 part by mass of phenol and dissolve in 1 part by mass of 1,2-dichlorobenzene . Work to an
accuracy of 1 % or better in the weighing.
5.3 Phenol/1,1,2,2-tetrachloroethane
Weigh out 6 parts by mass of phenol and dissolve in 4 parts by mass of 1,1,2,2-tetrachloroethane, at (25 ±
0.02) °C, its density range is (1.235 ± 0.003) g/cm³. Or weigh out 1 part by mass of phenol and dissolve in
1 part by mass of 1,1,2,2-tetrachloroethane. Work to an accuracy of 1 % or better in the weighing, at (25 ±
0.02) °C, its density range is (1.280 ± 0.003) g/cm³.
5.4 o-Chlorophenol
5.5 m-Cresol, meeting the following specifications:
— appearance: clear and colourless;
— m-cresol content: 99 % (m/ m) min.;
— o-cresol content: 0,3 % (m/ m) max.;
— water content: 0,3 % (m/ m) max.

ISO/DIS 1628-5:2026(en)
The purity of the solvent can be checked by gas chromatography.
NOTE A solvent of the appropriate purity can be obtained by distillation of chemically pure m-cresol, preferably
in vacuo, pressure compensation being effected with nitrogen in order to avoid oxidation.
5.6 Dichloroacetic acid
5.7 Phenol/2,4,6-trichlorophenol
Weight out 6 parts by mass of phenol and dissolve in 4 parts by mass of 2,4,6-trichlorophenol.
NOTE 2,4,6-trichlorophenol of appropriate purity can be obtained by distillation of chemically pure
2,4,6-trichlorophenol.
5.8 Storage of the solvents
Protect the solvents from light by storing them, for example, in stoppered brown-glass bottles. Maintain
phenol-containing solvents at an ambient temperature of (25 ± 2) °C to prevent crystallization. Strictly control
the water co
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