prEN ISO 1628-2

• Abstract:

  1.1 This document specifies conditions for the determination of the reduced viscosity (also known as viscosity number) and K-value of PVC resins. It is applicable to resins in powder form which consist of homopolymers of the monomer vinyl chloride and copolymers, terpolymers, etc., of vinyl chloride with one or more other monomers, but where vinyl chloride is the main constituent. The resins may contain small amounts of unpolymerized substances (e.g. emulsifying or suspending agents, catalyst residues, etc.) and other substances added during the course of the polymerization. This document is not applicable, however, to resins having a volatile-matter content in excess of 0,5 % ± 0,1 %, when determined in accordance with ISO 1269. In addition to this, it is not applicable to resins which are not entirely soluble in cyclohexanone.; 1.2 The reduced viscosity and K-value of a particular resin are related to its molecular mass, but the relationship varies depending on the concentration and type(s) of other monomer(s) present. Hence, homopolymers and copolymers having the same reduced viscosity or K-value might not have the same molecular mass.; 1.3 The values determined for reduced viscosity and K-value, for a particular sample of PVC resin, are influenced differently by the concentration of the solution chosen for the determination. Hence the use of the procedures described in this document only gives values for reduced viscosity and K-value that are comparable when the concentrations of the solutions used are identical.; 1.4 Limiting viscosity number is not used for PVC resins.; 1.5 The experimental procedures described in this document can also be used to characterize the polymeric fraction obtained during the chemical analysis of a PVC composition. However, the values calculated for the reduced viscosity and K-value in these circumstances might not indicate the actual values for the resin used to produce the composition because of the impure nature of the recovered polymer fraction.

• Status: Not Published
• Publication Date: 05-Dec-2027
• ICS: 83.080.20 - Thermoplastic materials
• Technical 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 1628-2:2026 - Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 2: Poly(vinyl chloride) resins (ISO/DIS 1628-2:2026)

• Revises: EN ISO 1628-2:2020 - Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 2: Poly(vinyl chloride) resins (ISO 1628-2:2020)
• Effective Date: 18-Dec-2024

prEN ISO 1628-2 is a CEN/ISO draft standard for measuring the reduced viscosity, also called viscosity number, and the K-value of poly(vinyl chloride) (PVC) resins in dilute solution. It is written for laboratories and resin users who need a consistent way to compare PVC grades by capillary viscometry, using either an efflux time method or a differential pressure method.

What does prEN ISO 1628-2 specify?

prEN ISO 1628-2 specifies the test conditions for determining reduced viscosity and K-value for PVC resins. It applies to powder-form resins where vinyl chloride is the main constituent, including homopolymers and copolymers, and it sets the limits for when the method may be used.

The document is organized into 12 clauses and one informative annex. The clauses cover scope, normative references, terms and definitions, principle, materials, apparatus, sampling, number of determinations, procedure, expression of results, precision, and test report.

Annex	What it covers
Annex A	Informative conversion of viscosity ratio (VR) to reduced viscosity and K-value

Two practical exclusions matter in use: the resin must be entirely soluble in cyclohexanone, and the volatile-matter content must be within the stated limit checked by ISO 1269. That matters because the test depends on preparing a true solution, not a suspension or partially dissolved sample.

What are the key requirements of prEN ISO 1628-2?

1) Use the document only for suitable PVC resins

prEN ISO 1628-2 is intended for PVC resins in powder form, including homopolymers, copolymers, terpolymers and similar materials where vinyl chloride is the main constituent. It also notes that small amounts of residual process materials may be present.

In practice, this means the method is for resin characterization, not for every plastic containing PVC. If the material is not fully soluble in cyclohexanone, the result is outside the intended use of the document.

2) Prepare a solution at the specified concentration and temperature

prEN ISO 1628-2 calls for a solution concentration of 5 kg/m3 with a tight tolerance, prepared at 25 °C. The sample is dissolved in cyclohexanone after being weighed into a 50 mL flask, then heated and mixed before making up to volume.

This matters because the result depends strongly on concentration. The document also states that results are only comparable when the same solution concentration is used, so labs need to control weighing, dilution, and temperature carefully.

3) Choose between two measurement principles

prEN ISO 1628-2 offers an efflux time method and a differential pressure method. The efflux time method uses a Ubbelohde viscometer and compares the flow times of solvent and solution, while the differential pressure method uses a relative viscometer and compares pressure differences.

In practice, this gives labs a choice between a classic manual route and a more automated route. The document says the alternative method was added for consistency with the general principles in ISO 1628-1.

4) Follow the apparatus and temperature conditions closely

prEN ISO 1628-2 sets specific apparatus needs, including an analytical balance, temperature-controlled bath, thermometer, timing device, and suitable viscometer arrangement. For the efflux time method, the viscometer temperature must be held at 25 °C within tight limits.

This matters because viscosity results are very sensitive to temperature and flow conditions. The document also stresses filtering the solvent and solution when filling the viscometer and keeping the viscometer clean.

5) Carry out two complete determinations on fresh test portions

prEN ISO 1628-2 requires two complete determinations, each starting with a fresh test portion. The sample must be representative and large enough for at least two determinations.

In practice, this is the basic check on repeatability within the laboratory. If the two values differ too much from the mean, the results are rejected and the test is repeated with fresh portions.

6) Use the stated calculation and acceptance rules

prEN ISO 1628-2 gives formulas for reduced viscosity and K-value, and the mean of the two determinations is reported. The document also provides a conversion table in Annex A for the special case of a 5 kg/m3 solution, so some results can be read directly from the viscosity ratio.

This matters because the report must be based on the correct calculation route for the chosen method. For the efflux time method, the document also notes that a K-value above 85 can give a viscosity ratio above the usual limit, and that this is ignored for PVC to keep the testing uniform.

7) Record the method and test details in the report

prEN ISO 1628-2 requires the test report to identify the standard used, the material, the result, the method, any viscometer difference from the reference type, the test date, deviations, and unusual observations.

In practice, this gives enough information to trace the result and compare it with other labs or later re-testing. For the differential pressure method, the report also needs the procedure used to obtain the viscosity ratio increment.

What terms does prEN ISO 1628-2 define?

• Reduced viscosity - A viscosity-based measure calculated from the solution and solvent response in the viscometer; the document also calls it viscosity number.
• K-value - A PVC viscosity index calculated from the measured solution behavior; it is used as a practical resin characterization number.
• Viscosity ratio (VR) - The ratio used in Annex A to convert measured flow behavior into reduced viscosity and K-value.
• Efflux time method - The procedure that determines viscosity from the flow time of solvent and solution in a Ubbelohde viscometer.
• Differential pressure method - The procedure that determines viscosity from the differential pressure between solvent and solution in a relative viscometer.
• PVC - Poly(vinyl chloride), the resin family covered by this document.
• Cyclohexanone - The solvent specified for dissolving the PVC resin before measurement.

Who uses prEN ISO 1628-2?

prEN ISO 1628-2 is used by polymer testing laboratories, PVC resin producers, quality control teams, and technical buyers who need a standard way to compare resin batches. It is also relevant in contract laboratories that perform incoming inspection, product release testing, or dispute testing.

The document is useful when a buyer or quality manager needs a viscosity-based identification of a PVC resin grade, or when a lab characterizes the polymeric fraction recovered during chemical analysis of a PVC composition. The method helps users compare results only when the same solution concentration and test conditions are used.

What changed in prEN ISO 1628-2 from the previous edition?

prEN ISO 1628-2 is the fourth edition and it cancels and replaces the third edition. The main change stated in the document is the addition of the differential pressure method, aligned with the same change made in ISO 1628-1:2024.

Which standards are used with prEN ISO 1628-2?

• ISO 1269:2006 - Used to determine volatile matter, which is part of the applicability check for the resin.
• ISO 1628-1:2024 - Provides the general principles, terms and definitions, and the calculation and procedural basis used throughout this part.

What does the prEN ISO 1628-2 document contain?

prEN ISO 1628-2 contains two measurement routes, the efflux time method and the differential pressure method, each tied to specific apparatus and operating conditions. It includes solution preparation steps, temperature requirements, filtration and cleaning instructions, and rules for repeating measurements until the results are stable enough.

The document also contains the formulas for reduced viscosity and K-value, plus acceptance rules for comparing duplicate determinations. Annex A provides a conversion table from viscosity ratio (VR) to reduced viscosity and K-value, which is useful for routine work at the specified concentration.

A precision section reports interlaboratory results for K-value and reduced viscosity, giving users a practical sense of method variability. The final clause sets out the contents that should appear in the test report, which is important for audits, quality records, and laboratory traceability.

SLOVENSKI STANDARD
01-september-2026
Polimerni materiali - Ugotavljanje viskoznosti polimerov v razredčenih raztopinah
s kapilarnimi viskozimetri - 2. del: Polivinilklorid (ISO/DIS 1628-2:2026)
Plastics - Determination of the viscosity of polymers in dilute solution using capillary
viscometers - Part 2: Poly(vinyl chloride) resins (ISO/DIS 1628-2:2026)
Kunststoffe - Bestimmung der Viskosität von Polymeren in verdünnter Lösung unter
Verwendung von Kapillarviskosimetern - Teil 2: Vinylchlorid-Polymere (ISO/DIS 1628-
2:2026)
Plastiques - Détermination de la viscosité des polymères en solution diluée à l'aide de
viscosimètres à capillaires - Partie 2: Résines de poly(chlorure de vinyle) (ISO/DIS 1628-
2:2026)
Ta slovenski standard je istoveten z: prEN ISO 1628-2
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 1628-2
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 2:
Poly(vinyl chloride) resins
Plastiques — Détermination de la viscosité des polymères en
solution diluée à l'aide de viscosimètres à capillaires —
Partie 2: Résines de poly(chlorure de vinyle)
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.
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BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
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STANDARDS MAY ON OCCASION HAVE TO
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RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 1628-2:2026(en)
DRAFT
ISO/DIS 1628-2:2026(en)
International
Standard
ISO/DIS 1628-2
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 2:
Poly(vinyl chloride) resins
Plastiques — Détermination de la viscosité des polymères en
solution diluée à l'aide de viscosimètres à capillaires —
Partie 2: Résines de poly(chlorure de vinyle)
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
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Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 1628-2:2026(en)
ii
ISO/DIS 1628-2:2026(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 Materials . 2
6 Apparatus . 2
6.1 Efflux time method .2
6.2 Differential pressure method .3
7 Sampling . 3
8 Number of determinations . 3
9 Procedure . 3
9.1 Preparation of solution .3
9.2 Efflux time method: determination of efflux times .4
9.3 Differential pressure method .4
10 Expression of results . 4
10.1 Reduced viscosity .4
10.2 K-value .5
11 Precision . 5
12 Test report . 6
Annex A (informative) Conversion of viscosity ratio (VR) to reduced viscosity (I) and K-value . 7

iii
ISO/DIS 1628-2:2026(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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 9, Thermoplastic
materials, in collaboration with the European Committee for Standardization (CEN) Technical Committee
CEN/TC 249, Plastics, in accordance with the Agreement on technical cooperation between ISO and CEN
(Vienna Agreement).
This fourth edition cancels and replaces the third 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.
A list of all parts in the ISO 1628 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
DRAFT International Standard ISO/DIS 1628-2:2026(en)
Plastics — Determination of the viscosity of polymers in
dilute solution using capillary viscometers —
Part 2:
Poly(vinyl chloride) resins
1 Scope
1.1 This document specifies conditions for the determination of the reduced viscosity (also known
as viscosity number) and K-value of PVC resins. It is applicable to resins in powder form which consist of
homopolymers of the monomer vinyl chloride and copolymers, terpolymers, etc., of vinyl chloride with one
or more other monomers, but where vinyl chloride is the main constituent. The resins can contain small
amounts of unpolymerized substances (e.g. emulsifying or suspending agents, catalyst residues, etc.) and
other substances added during the course of the polymerization. This document is not applicable, however,
to resins having a volatile-matter content in excess of 0,5 % ± 0,1 %, when determined in accordance
with ISO 1269:2006. In addition to this, it is not applicable to resins which are not entirely soluble in
cyclohexanone.
1.2 The reduced viscosity and K-value of a particular resin are related to its molecular mass, but the
relationship varies depending on the concentration and type(s) of other monomer(s) present. Hence,
homopolymers and copolymers having the same reduced viscosity or K-value can have different molecular
masses.
1.3 The values determined for reduced viscosity and K-value, for a particular sample of PVC resin, are
influenced differently by the concentration of the solution chosen for the determination. Hence the use
of the procedures specified in this document only gives values for reduced viscosity and K-value that are
comparable when the concentrations of the solutions used are identical.
1.4 The experimental procedures specified in this document can also be used to characterize the polymeric
fraction obtained during the chemical analysis of a PVC composition. However, the values calculated for the
reduced viscosity and K-value in these circumstances can deviate form the correct values for the resin used
to produce the composition because of the impure nature of the recovered polymer fraction.
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 1269:2006, Plastics — Homopolymer and copolymer resins of vinyl chloride — Determination of volatile
matter (including water)
ISO 1628-1:2024, Plastics — Determination of the viscosity of polymers in dilute solution using capillary
viscometers — Part 1: General principles
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1628-1:2024 apply.

ISO/DIS 1628-2:2026(en)
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle
4.1 Efflux time method
A test portion shall be dissolved in a solvent. The reduced viscosity and the K-value shall be calculated from
the efflux times for the solvent and the solution in a Ubbelohde viscometer.
4.2 Differential pressure method
A test portion shall be dissolved in a solvent. The reduced viscosity and the K-value shall be calculated from
the differential pressures for the solvent and the solution in a relative viscometer.
5 Materials
−6 2 –1
Cyclohexanone, having a viscosity/density ratio (kinematic viscosity) between 2,06 × 10 m s and
−6 2 –1 2 –1 2 –1
2,33 × 10 m s (2,06 mm s and 2,33 mm s ) at 25 °C. The specified boiling point shall be 155 °C.
The solvent shall be stored in the dark in a dark-coloured bottle fitted with a ground-glass stopper. The
kinematic viscosity shall be checked before use.
6 Apparatus
6.1 Efflux time method
6.1.1 The apparatus required to carry out viscosity measurements on polymers in dilute solution shall be
in accordance with ISO 1628-1:2024, Clause 5. In addition, the following particular items are required for the
procedures specified in this document.
6.1.2 Viscometer. The viscometer model 1C specified in ISO 1628-1:2024, shall be used as the reference
viscometer.
Other viscometers specified in ISO 1628-1:2024 may be used provided the correlation between the chosen
viscometer and the reference viscometer has been established over the range of reduced viscosities and
K-values to be measured, and the results are corrected accordingly.
6.1.3 Graduated flask (one-mark volumetric flask), class A, as specified in ISO 1042, with a volume of
50 ml.
NOTE The use of a flask calibrated at a temperature of 20 °C, as specified in ISO 1042, causes a systematic error
which can, however, be neglected.
6.1.4 Filter funnel, with fritted-glass filter disc of medium porosity (pore size 40 µm to 50 µm), or glass
funnel with paper filter.
6.1.5 Mechanical agitator, equipped with a heating device to keep the flask (6.1.3) and its contents at a
temperature between 80 °C and 85 °C.
As an alternative, a rotary agitator or shaker may be placed in an oven at a temperature between 80 °C and
85 °C.
ISO/DIS 1628-2:2026(en)
6.1.6 Analytical balance, accurate to 0,1 mg.
6.1.7 Temperature-regulated bath, capable of being set at 25,0 °C ± 0,5 °C in steps of 0,1 °C and
maintaining a stability of ± 0,05 °C around the set temperature.
6.1.8 Thermometer, capable of reading to 0,05 °C in the range to be used and in a known state of
calibration, is suitable. Other thermometric devices of at least equal precision may be used.
6.1.9 Time-measuring device, with a sensitivity of 0,1 s.
6.2 Differential pressure method
6.2.1 Analytical balance, accurate to 0,1 mg.
6.2.2 A schematic diagram of a relative viscometer used in this method is depicted in ISO 1628-1:2024.
The reference/solvent capillary and the measurement/polymer solution capillary shall be kept at the same
temperature of 25 °C, enclosed in a proper temperature-controlled device with variations from the specified
temperature lower than ± 0,2 K. The nominal inside diameter of the capillaries shall be 0,5 mm, or 0,75 mm,
and length 80 mm to 600 mm.
7 Sampling
A sample shall be taken which is representative of the resin whose properties are to be determined and
which is large enough for at least two determinations.
8 Number of determinations
Two complete determinations shall be carried out, starting each with a fresh test portion.
9 Procedure
9.1 Preparation of solution
General requirements for the dissolution of polymer in solvent are given in ISO 1628-1:2024. A solution shall
3 3
be prepared with a concentration of 5 kg/m ± 0,1 kg/ m at 25 °C ± 1 °C, as follows.
0,250 g ± 0,005 g of resin shall be weighed to the nearest 0,2 mg, and transferred quantitatively to the
50 ml flask (6.1.3). About 40 ml of cyclohexanone shall be added (Clause 5) to the flask, swirling the flask by
hand to prevent coagulation or the formation of lumps. The dissolution shall be continued by agitating for
1 h between 80 °C and 85 °C using the agitator (6.1.5). The completion of the dissolution shall be checked
visually. If gelatinized particles are still visible, the procedure shall be started again with a new portion of
the resin. The solution shall be cooled to 25 °C ± 1 °C and made up to the mark with cyclohexanone at the
same temperature. The solution shall be mixed thoroughly by shaking.
The actual concentration of the solution shall be checked to an accuracy of ± 0,1 %.
If a mass of 0,250 g ± 0,000 25 g is taken and made up to 50 ml of solution as described above, Table A.1 in
Annex A may be used to read off the reduced viscosity and K-value from the ratio of the efflux time of the
solution to that of the solvent (the so-called viscosity ratio).
Alternative methods for the preparation of the solution may be used, for example the addition of a measured
volume of solvent to a measured mass of test portion, provided that the values obtained for the reduced
viscosity and K-value can be shown to be equivalent to those obtained with the method of solution
preparation described above. Such alternative methods of solution preparation requires the amounts of

ISO/DIS 1628-2:2026(en)
solvent and test portion taken to be determined by experiment, and can also require compensation for loss
of solvent by evaporation during the dissolution process.
With resins having K-values greater than 85, the ratio of the efflux time of the solution to that of the solvent
will exceed the maximum value of 2,0, which is contrary to the requirement specified in ISO 1628-1:2024.
In order to ensure uniformity of testing for PVC, this non-conformity shall be ignored, and all currently
available resins shall be tested using the same test-portion mass.
9.2 Efflux time method: determination of efflux times
The procedure is specified in ISO 1628-1:2024.
The temperature of the thermostat (6.1.7) shall be set such that the actual temperature which is measured
by the thermometer (6.1.8) lies in the range 25 °C ± 0,5 °C. The measured temperature shall be stable to
±0,05 °C around the temperature at which the thermostat has been set.
When filling the viscometer, the solvent and the solution shall be filtered using a filter funnel or a glass
funnel and paper filter (see 6.1.4).
Particular care shall be taken over viscometer cleaning, which shall be based on the procedure specified in
ISO 1628-1:2024. Efflux times with the control solvent cyclohexanone shall remain constant to within 0,2 s
for a given viscometer. With the solution, the measurement of the efflux time shall be repeated until two
successive measurements differ by less than 0,25 %. The first efflux time reading shall always be discarded.
NOTE This is a manual procedure. Proprietary equipment is available which will organize the charging of the
viscometer with solution and solvent and measure the respective efflux times automatically. The use of such equipment
is included in the scope of this document provided that all the procedures and verification checks described above are
followed by the automated procedure.
9.3 Differential pressure method
In general, automated sample preparation is used with this method. The guidelines for time and temperature
as in 9.1 shall be used.
The general procedure as specified in ISO 1628-1:2024 shall be followed, with reference to the manufacturer
instructions in case of automated systems, to obtain the value of the viscosity ratio increment for the tested
solution. A flow rate of 0,5 to 3,0 ml/min shall be applied for the determination.
10 Expression of results
10.1 Reduced viscosity
In the efflux time method the reduced viscosity, I, shall be calculated for each test portion as specified in
ISO 1628-1:2024, using Formula (1):
(1)
where
t and t are the efflux times, in seconds, of the solution and solvent, respectively;
3 3 3
c is the concentration of the solution, in 10 kg/m , i.e. g/cm .
The reduced viscosity I for the sample shall be calculated as the mean value of the two individual values
obtained in the two determinations, expressing the result to the nearest whole number. If the I values
obtained in the two determinations deviate by more than ± 0,4 % from the mean value, these results shall be
rejected and further determinations carried out with fresh test portions.
3 3
If the solution concentration is 5 kg/m ± 0,005 kg/m , it is more convenient to read off the values of I from
3 −3 3
Table A.1, expressing I in (m /kg) · 10 , i.e. cm /g, rounded to the first place of decimals.

ISO/DIS 1628-2:2026(en)
When the differential pressure method is applied, calculate the reduced viscosity I using the calculated
solution viscosity ratio increment according to Formula (2):
(2)
where
η is the viscosity ratio increment of the tested polymer solution;
sp
c is the concentration of the polymer, in grams per millilitre of solution.
Or from the viscosity ratio increment peak area according to Formula (3):
(3)
where
A(η (V)) is peak area of the continuous viscosity ratio increment signal as a function of elution volume,
sp
V;
M is the mass of polymer injected into the measurement capillary.
inj
10.2 K-value
For each test portion, the K-value shall be calculated as specified in ISO 1628-1:2024, using Formula (4):
(4)
where
is the ratio of the viscosities (efflux times) of the solution and solvent;
t and t are the efflux times, in seconds, of the solution and solvent, respectively;
3 3 3
c is the concentration of the solution, in 10 kg/m , i.e. g/cm .
The K-value for the sample shall be calculated as the mean value of the two individual K-values obtained in
the two determinations, expressing the result to the first place of decimals. If the K-values obtained in the
two determinations deviate by more than ± 0,4 % from the mean value, these results shall be rejected and
further determinations carried out with fresh test portions.
3 3
If the solution concentration is 5 kg/m ± 0,005 kg/m , it is more convenient to read off the K-value from
Table A.1, rounding to the second place of decimals.
11 Precision
Inter laboratory trials were conducted on three resins in 11 laboratories on four different dates. These inter
laboratory trials gave the following results for the repeatability standard deviation s (within the same
r
laboratory) and the reproducibility standard deviation s (among different laboratories).
R
See Table 1.
Table 1 — Results for s , s of K-value and reduced viscosity
r R
K-value Reduced viscosity
Approximately Approximately Approximately Approximately Approximately Approximately
50 70 90 61 124 227
s 0,132 0,115 0,120 0,313 0,458 0,742
r
s 0,420 0,291 0,495 0,984 1,202 3,042
R
ISO/DIS 1628-2:2026(en)
12 Test report
The test report shall include the following information:
a) a reference to this document, including the year of publication;
b) all details necessary
...