Plastics — Determination of the transient extensional viscosity of polymer melts

This document specifies the general principles of a method for determining the transient extensional viscosity of polymer melts. The procedure details the measurement of polymer melt specimens stretched uniaxially under conditions of constant strain rate and constant temperature. The method is capable of measuring the transient extensional viscosity of polymer melts at Hencky strain rates typically in the range 0,01 s–1 to 1 s–1, at Hencky strains up to approximately 4 and at temperatures up to approximately 250 °C (see NOTEs 1 and 2). It is suitable for measuring transient extensional viscosity values typically in the range from approximately 104 Pa⋅s to 107 Pa⋅s (see NOTE 3). NOTE 1 Hencky strains and strain rates are used (see Clause 3). NOTE 2 Values of strain, strain rate and temperature outside these limiting values can be attained. NOTE 3 The operating limit of an instrument, in terms of the lowest transient extensional viscosity values that can be measured, is due to a combination of factors, including the ability of the specimen to maintain its shape during testing and the resolution of the instrument.

Plastiques — Détermination de la viscosité élongationelle transitoire des polymères à l'état fondu

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Published
Publication Date
06-Apr-2021
Current Stage
6060 - International Standard published
Start Date
07-Apr-2021
Completion Date
07-Apr-2021
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INTERNATIONAL ISO
STANDARD 20965
Second edition
2021-04
Plastics — Determination of the
transient extensional viscosity of
polymer melts
Plastiques — Détermination de la viscosité élongationelle transitoire
des polymères à l'état fondu
Reference number
ISO 20965:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 20965:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 20965:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 General principles ............................................................................................................................................................................................... 3

5 Apparatus ..................................................................................................................................................................................................................... 3

5.1 General description ............................................................................................................................................................................. 3

5.2 Silicon oil bath/temperature-controlled chamber .................................................................................................. 5

5.3 Temperature measurement and control ........................................................................................................................... 6

5.4 Strain and strain rate measurement ..................................................................................................................................... 6

5.5 Force measurement ............................................................................................................................................................................ 6

5.6 Calibration .................................................................................................................................................................................................. 6

6 Sampling and specimen preparation .............................................................................................................................................. 7

6.1 Sampling ....................................................................................................................................................................................................... 7

6.2 Specimen preparation ...................................................................................................................................................................... 7

6.3 Specimen mounting ............................................................................................................................................................................ 7

7 Procedure..................................................................................................................................................................................................................... 8

7.1 Specimen loading .................................................................................................................................................................................. 8

7.2 Pre-conditioning of the specimen........................................................................................................................................... 8

7.3 Testing ............................................................................................................................................................................................................ 8

8 Analysis of extensional flow measurements............................................................................................................................ 9

8.1 Symbols used ............................................................................................................................................................................................ 9

8.2 Analysis of extensional flow ........................................................................................................................................................ 9

8.2.1 General...................................................................................................................................................................................... 9

8.2.2 Analysis for type A and B instruments (rotating clamps) .........................................................10

8.2.3 Analysis for type C and D instruments (translating clamps) ..................................................11

9 Precision ....................................................................................................................................................................................................................11

10 Test report ................................................................................................................................................................................................................12

Annex A (informative) Checking for swelling of specimens due to immersion in silicone

oilor other fluids ................................................................................................................................................................................................13

Annex B (informative) Uncertainties in transient extensional viscosity testing .................................................14

Bibliography .............................................................................................................................................................................................................................19

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 20965:2021(E)
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 5, Physical-

chemical properties.

This second edition cancels and replaces the first edition (ISO 20965:2005), which has been technically

revised.
The main changes compared to the previous edition are as follows:
— figures have been updated and figure keys have been introduced;
— calibration period in 5.6 has been clarified.

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 © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO 20965:2021(E)
Plastics — Determination of the transient extensional
viscosity of polymer melts
1 Scope

This document specifies the general principles of a method for determining the transient extensional

viscosity of polymer melts. The procedure details the measurement of polymer melt specimens

stretched uniaxially under conditions of constant strain rate and constant temperature.

The method is capable of measuring the transient extensional viscosity of polymer melts at Hencky

–1 –1

strain rates typically in the range 0,01 s to 1 s , at Hencky strains up to approximately 4 and at

temperatures up to approximately 250 °C (see NOTEs 1 and 2). It is suitable for measuring transient

4 7

extensional viscosity values typically in the range from approximately 10 Pa⋅s to 10 Pa⋅s (see NOTE 3).

NOTE 1 Hencky strains and strain rates are used (see Clause 3).

NOTE 2 Values of strain, strain rate and temperature outside these limiting values can be attained.

NOTE 3 The operating limit of an instrument, in terms of the lowest transient extensional viscosity values

that can be measured, is due to a combination of factors, including the ability of the specimen to maintain its

shape during testing and the resolution of the instrument.
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 472, Plastics — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 472 and the following apply.

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/
3.1
Hencky strain

strain given by the natural logarithm of the elongation ratio given by Formula (1)

ε =In ll/ (1)
where
l is the specimen length and
l is the original specimen length
Note 1 to entry: It is also referred to as the natural or true strain.
© ISO 2021 – All rights reserved 1
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ISO 20965:2021(E)
Note 2 to entry: It is dimensionless.
3.2
Hencky strain rate
rate of change of Hencky strain with time, given by Formula (2)
ε=×1/ll∂∂/ t (2)
where t is the time
Note 1 to entry: It is independent of the original specimen length l .
Note 2 to entry: It is expressed in reciprocal seconds.
3.3
net tensile stress
for tensile uniaxial extension, stress given by Formula (3)
σσ=−=−σσ σσ=−σ (3)
Er11 22 11 33 zz r
where σ is a stress tensor in either rectangular or axisymmetric co-ordinates
+ +

Note 1 to entry: The tensile stress growth function is indicated by σ where the indicates start-up of flow.

Note 2 to entry: Net tensile stress is expressed in pascals.
3.4
tensile stress growth coefficient
ratio of the net tensile stress to the Hencky strain rate, given by Formula (4)
η ()t,/εσ= ε (4)
for tensile uniaxial extension, where t is time and indicates start-up of flow

Note 1 to entry: It is also known for the purposes of this document as “transient extensional viscosity”.

Note 2 to entry: It is a transient term.
Note 3 to entry: It is expressed in pascal seconds.
3.5
tensile viscosity
term given by Formula (5)
lim +
η tt,,εη=  ε  (5)
() ()
EEt→∞  

Note 1 to entry: It is the limiting tensile stress growth coefficient value and represents an equilibrium extensional

viscosity if a steady value is achieved. However, for materials that do not exhibit a steady-state behaviour, the use

of an “equilibrium extensional viscosity” such as this is not appropriate.
Note 2 to entry: It is expressed in pascal seconds.
2 © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
ISO 20965:2021(E)
4 General principles

In contrast to shear flow where reference is normally made only to steady shear flow behaviour,

extensional flow behaviour is best described as being transient. In describing the transient behaviour

of materials in extension at constant strain rate, they may exhibit either an unbounded stress growth

behaviour in which the stress continually increases with increasing strain until the material fails,

or the stress reaches a steady value with increasing strain thus yielding a tensile or equilibrium

extensional viscosity. The latter occurs typically at large strains. An equilibrium extensional viscosity

is thus dependent on strain rate but not on strain or time. Normally, the extensional viscosity varies as

a function of both strain and strain rate as well as temperature.

In describing and modelling plastics processing, the use of Hencky strain is preferred. The rate of

Hencky strain of an element of fluid within a flow is independent of its original length and is determined

only from the velocity field of that element. It is thus a more suitable characteristic of the flow. Strain

and strain rate are taken by default herein to be Hencky values.

Stretching flow methods can be used to generate quantitatively accurate data on the extensional

viscoelasticity of polymer melts. In carrying out extensional flow measurements, there are four types

of measurement that are normally made: constant strain rate, constant stress, constant force and

constant speed. This document describes the first of these: constant strain rate. In this method, the

strain rate is uniform throughout the specimen and is held constant with time.

The basic principle behind stretching flow measurements is to subject a specimen to a tensile stretching

deformation. By measurement of the force and deformation of the specimen, the stresses and strains

and hence strain rate can be determined.
5 Apparatus
5.1 General description

The measuring apparatus shall consist of one of the following types, shown in Figure 1 to Figure 4. These

types define the various instrument configurations. The notation used in these figures is defined in 8.1.

Type A: Two rotating clamps. Each clamp shall consist of either a single rotating element or a pair of

rotating elements – only the pair arrangement is shown. The force exerted on the specimen can be

measured at the fixed or rotating end.

NOTE It is likely that the force will be easier to measure, and will be measured with greater accuracy, on a

fixed clamp rather than on a moving clamp as there will be fewer complications due, for example, to vibration

and the inertia of the clamp which can introduce noise and errors into the force signal.

Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 angular speed of rotating clamps of side 1, ω , expressed in rad⋅s
5 angular speed of rotating clamps of side 2, ω , expressed in rad⋅s
Figure 1 — Schematic diagram of type A test instrument
© ISO 2021 – All rights reserved 3
---------------------- Page: 7 ----------------------
ISO 20965:2021(E)

Type B: A single rotating clamp and a fixed clamp. The rotating clamp shall consist of either a single

rotating element or a pair of rotating elements. The force exerted on the specimen is normally measured

at the fixed end.
Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 angular speed of rotating clamps, ω , expressed in rad⋅s
Figure 2 — Schematic diagram of type B test instrument
Type C: Two translating (non-rotating) clamps.
Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 speed of the ends of the specimen of side 1, V , expressed in m⋅s
5 speed of the ends of the specimen of side 2, V , expressed in m⋅s
Figure 3 — Schematic diagram of type C test instrument
Type D: Single translating (non-rotating) clamp.
4 © ISO 2021 – All rights reserved
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ISO 20965:2021(E)
Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
–1 –1
4 speed of the ends of the specimen, m·s , V , expressed in m·s
Figure 4 — Schematic diagram of type D test instrument

In each of these configurations, the specimen is mounted between the clamps and is stretched uniaxially.

The requirements for the apparatus are that it shall permit the measurement or determination of the

force acting on the specimen, and the strain and strain rate of the specimen subjected to a constant

strain rate under isothermal conditions. The strain and strain rate of the specimen shall either be

derived from the displacements and/or speeds of the clamp or clamps, or be measured directly from

the dimensions and/or local velocities of the specimen.
5.2 Silicon oil bath/temperature-controlled chamber

Heating may be provided by placing the specimen in a silicone oil bath or in a temperature-controlled

chamber with a forced gas flow through it.

NOTE 1 When heating using forced gas, a gas can be used in the chamber surrounding the test specimen to

provide the required test environment, for example nitrogen to provide an inert atmosphere.

NOTE 2 The use of a silicone oil bath can permit more rapid heating of the specimen.

For low-viscosity materials, it is essential to support the specimen during heat-up and testing (to avoid

it sagging under the influence of gravity).

NOTE 3 The use of a silicone oil bath results in the specimen being supported by the silicone oil due to its

buoyancy, particularly if the densities of the silicone oil and specimen are matched at the test temperature. If a

forced-gas oven is used, then support of the specimen can be obtained by the cushioning effect provided by the gas.

Silicone oil can be absorbed by some polymers. A check should preferably be made to see if the immersion

time affects the measured properties of the polymer by varying the length of the immersion time (see

Annex A and NOTE 1 in 7.1). When quantitative results are required, then this check shall be made.

Even if the silicone oil does not affect the shape of the tensile stress growth coefficient versus strain (or

time) plot, it can affect the point of failure of the specimen. Thus, assessment of the effect of the silicone

oil on the measured properties should consider both of these aspects.

NOTE 4 Alternative methods for checking the effect of immersion in silicone oil on the specimen can also

be used. Such methods include the measurement of the mass or dimensions of the specimen before and after

immersing in silicone oil and identifying whether a change has occurred due to that immersion – see Annex A.

© ISO 2021 – All rights reserved 5
---------------------- Page: 9 ----------------------
ISO 20965:2021(E)
5.3 Temperature measurement and control

The test temperature should preferably be measured using a device that is mounted close to the

specimen. Contact of the device with the specimen is not permitted. It is essential to mount temperature

sensors in at least two positions to monitor temperature uniformity along the length of the specimen.

NOTE The uniformity of the temperature along the specimen length is critical to the measurement of the

transient extensional flow properties of polymer melts. Localized hot spots result in excessive strain in those

regions. This can lead to premature failure, particularly for materials that do not exhibit a high degree of strain

hardening.
The spatial temperature variation shall be within ±0,75 K.

The temporal temperature variation shall be within ±1,0 K of the set temperature.

The temperature-measuring device shall have a resolution of 0,1 K and shall be calibrated using a device

accurate to within ±0,1 K.
5.4 Strain and strain rate measurement

The strain and strain rate of the specimen shall be determined either from measurement of the

displacements and/or speeds of the clamp or clamps, or measured directly from the dimensions and/or

local velocities of the specimen.

NOTE The diameter of the specimen can be measured during the test by use of optical or cutting methods

to derive strains and strain rates and to assess the uniformity of deformation. The cutting method results in the

test being terminated once the cuts have been made and thus prevents data to failure from being obtained. Local

velocities can be measured using optical methods.

Corrections for slippage of the specimen at the clamp or clamps may be applied, obtained through

independent measurement of the strain of the specimen during testing through measurement of its

diameter or local velocities by other methods.

The apparatus shall have an accuracy of strain determination or measurement to within ±3 % of the

absolute value.

The apparatus shall have an accuracy of strain rate determination or measurement to within ±3 % of

the absolute value.
5.5 Force measurement

The force on the specimen shall be measured during the test by an appropriate means, for example a

leaf spring arrangement (see NOTE).

The resolution of the force-measuring device should preferably be no greater than 0,1 % of the full-

scale value.

The apparatus shall have an accuracy of force measurement to within ±2 % of the full-scale value.

NOTE Typical peak forces measured in testing of polyethylenes are estimated to be up to approximately 1 N

for specimens approximately 3 mm in diameter.
5.6 Calibration

The force, displacement, rate of displacement and temperature functions of the rheometer shall be

calibrated at least once in 6 months.

It is preferable that calibration be carried out at the test temperature as measurement of these functions,

in particular that of force, can be temperature sensitive.

No traceable standard reference materials are known to exist for checking the calibration of such

instruments. Where a reference material is used for checking the instrument, it is preferable that the

6 © ISO 2021 – All rights reserved
---------------------- Page: 10 ----------------------
ISO 20965:2021(E)

transient extensional viscosity of the reference material, and the dimensions of the specimen produced

using it, have values that are similar to those encountered or used during normal operation of the

instrument.
6 Sampling and specimen preparation
6.1 Sampling

The sampling method, including any special methods of specimen preparation and introduction into

the rheometer, shall be as specified in the relevant materials standard or otherwise by agreement.

If samples or specimens are hygroscopic or contain volatile ingredients, then they shall be stored to

prevent or minimize any effects on the measurements. Drying of samples can be required prior to

preparing test specimens.

As the test specimens are typically small, being of the order of a few grams, it is essential that they be

representative of the material being sampled. Repeat testing may be used to identify batch-to-batch or

within-batch variation.
6.2 Specimen preparation
The specimen shall be either cylindrical or rectangular in cross-section.

Test specimens in the form of a cylinder may be produced by extrusion or by injection, transfer or

compression moulding.

Test specimens in the form of a strip may be produced by extrusion or by injection or compression

moulding or by cutting from sheet.

The length-to-diameter ratio of cylindrical specimens should preferably be at least 10.

NOTE A length-to-diameter ratio of at least 10 is required to minimize end-effects. However, a longer

specimen results in a reduction in the maximum strain rate that can be achieved. The magnitude of the end-

errors can be assessed by using specimens of different length or diameter to produce different length-to-

diameter aspect ratios. The effect on measured values can then be determined.

The specimen shall not contain any visible impurities, voids or air bubbles. The specimen shall not show

any obvious discoloration prior to or after testing.

For cylindrical specimens, measure the diameter D of the specimen in at least three positions along its

length. Repeat these measurements after rotating the specimen by 90°. Calculate an average value for

the diameter from these measurements.

For rectangular specimens, measure the width and thickness in at least three positions along its length.

Calculate average values for the width b and thickness h from these measurements.

Calculate the cross-sectional area of the specimen from the measurements.

The diameter, or width and thickness, of the specimen, as appropriate, shall be determined to and be

uniform to within ±2 % of their average value.
6.3 Specimen mounting

Specimens may be either gripped by the clamps or attached using adhesive to studs that are then

clamped into the instrument.

Attachment by a suitable high-temperature epoxy adhesive has been found suitable. Treat the ends of

the specimen by passing them through a butane flame and then dipping them into concentrated sulfuric

acid for 30 s. Prevent any other part of the specimen, except that to be bonded, from being exposed to

either the flame or the acid. Dip the ends of the specimen into the adhesive and then attach them to

© ISO 2021 – All rights reserved 7
---------------------- Page: 11 ----------------------
ISO 20965:2021(E)

the studs. Place the specimen with its studs into an oven and cure the adhesive using a suitable time-

temperature cycle (100 °C for 1 h has been found suitable). Allow the specimen to cool before handling.

7 Procedure
7.1 Specimen loading
Mount a specimen in place in the rheometer.

Measure the length of the specimen between the clamps to within 1 % of its absolute value.

After mounting the specimen in the instrument, immerse it in the silicone oil bath or place it in the

temperature-controlled chamber (see 5.2). Where possible, bring the bath or chamber to the test

temperature before inserting the specimen to reduce the time spent by the specimen reaching and

equilibrating at the test temperature. Allow the specimen and apparatus to reach thermal equilibrium

at the test temperature. This period of time is referred to as the equilibration time.

NOTE 1 The adequacy of the time allowed for the specimen to reach thermal equilibrium and the effects of the

silicone oil, degradation, crosslinking and other time-dependent phenomena on the specimen can be checked by

varying the time for which the specimen is in the oil bath or environmental chamber before testing. The effect

on the measured values can then be assessed. For measurements in silicone oil of specimens approximately

3 mm in diameter, an equilibration time of approximately 5 min has been found to be sufficient for testing at a

temperature of 150 °C.

A correction for thermal expansion of the specimen during heating can be necessary. A correction

for effects arising from stress relaxation of the specimen, if unclamped during the temperature

equilibration period, can also be required. Both of these effects can result in a change in the critical

dimensions of the specimen (i.e. thickness and width, or diameter) that may need to be taken into

account.

NOTE 2 As an example, measurements of a PE-HD indicate a 20 % decrease in density on heating from 25 °C

to 150 °C which, if accommodated solely by a change in the diameter of a cylindrical specimen, results in an

approximately 10 % increase in diameter of the specimen. Furthermore, stress relaxation of extruded specimens

can result in a recovery (shrinkage) in length with a corresponding increase in the specimen’s critical dimensions

(e.g. diameter) if unclamped during the temperature equilibration period.
7.2 Pre-conditioning of the specimen

The specimen may be pre-conditioned by applying a known strain prior to testing and allowing the

induced stresses to relax to zero before commencing the test.
7.3 Testing
Subje
...

INTERNATIONAL ISO
STANDARD 20965
Second edition
Plastics — Determination of the
transient extensional viscosity of
polymer melts
Plastiques — Détermination de la viscosité élongationelle transitoire
des polymères à l'état fondu
PROOF/ÉPREUVE
Reference number
ISO 20965:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 20965:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 20965:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 General principles ............................................................................................................................................................................................... 3

5 Apparatus ..................................................................................................................................................................................................................... 3

5.1 General description ............................................................................................................................................................................. 3

5.2 Silicon oil bath/temperature-controlled chamber .................................................................................................. 5

5.3 Temperature measurement and control ........................................................................................................................... 6

5.4 Strain and strain rate measurement ..................................................................................................................................... 6

5.5 Force measurement ............................................................................................................................................................................ 6

5.6 Calibration .................................................................................................................................................................................................. 6

6 Sampling and specimen preparation .............................................................................................................................................. 7

6.1 Sampling ....................................................................................................................................................................................................... 7

6.2 Specimen preparation ...................................................................................................................................................................... 7

6.3 Specimen mounting ............................................................................................................................................................................ 7

7 Procedure..................................................................................................................................................................................................................... 8

7.1 Specimen loading .................................................................................................................................................................................. 8

7.2 Pre-conditioning of the specimen........................................................................................................................................... 8

7.3 Testing ............................................................................................................................................................................................................ 8

8 Analysis of extensional flow measurements............................................................................................................................ 9

8.1 Symbols used ............................................................................................................................................................................................ 9

8.2 Analysis of extensional flow ........................................................................................................................................................ 9

8.2.1 General...................................................................................................................................................................................... 9

8.2.2 Analysis for type A and B instruments (rotating clamps) .........................................................10

8.2.3 Analysis for type C and D instruments (translating clamps) ..................................................11

9 Precision ....................................................................................................................................................................................................................11

10 Test report ................................................................................................................................................................................................................12

Annex A (informative) Checking for swelling of specimens due to immersion in silicone

oilor other fluids ................................................................................................................................................................................................13

Annex B (informative) Uncertainties in transient extensional viscosity testing .................................................14

Bibliography .............................................................................................................................................................................................................................19

© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO 20965:2021(E)
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/d irectives).

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/p atents).

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/f oreword. html.

This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 5, Physical-

chemical properties.

This second edition cancels and replaces the first edition (ISO 20965:2005), which has been technically

revised.
The main changes compared to the previous edition are as follows:
— figures have been updated and figure keys have been introduced;
— calibration period in 5.6 has been clarified.

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/m embers. html.
iv PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO 20965:2021(E)
Plastics — Determination of the transient extensional
viscosity of polymer melts
1 Scope

This document specifies the general principles of a method for determining the transient extensional

viscosity of polymer melts. The procedure details the measurement of polymer melt specimens

stretched uniaxially under conditions of constant strain rate and constant temperature.

The method is capable of measuring the transient extensional viscosity of polymer melts at Hencky

–1 –1

strain rates typically in the range 0,01 s to 1 s , at Hencky strains up to approximately 4 and at

temperatures up to approximately 250 °C (see NOTEs 1 and 2). It is suitable for measuring transient

4 7

extensional viscosity values typically in the range from approximately 10 Pa⋅s to 10 Pa⋅s (see NOTE 3).

NOTE 1 Hencky strains and strain rates are used (see Clause 3).

NOTE 2 Values of strain, strain rate and temperature outside these limiting values can be attained.

NOTE 3 The operating limit of an instrument, in terms of the lowest transient extensional viscosity values

that can be measured, is due to a combination of factors, including the ability of the specimen to maintain its

shape during testing and the resolution of the instrument.
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 472, Plastics — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 472 and the following apply.

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/
3.1
Hencky strain

strain given by the natural logarithm of the elongation ratio given by Formula (1)

ε =In ll/ (1)
where
l is the specimen length and
l is the original specimen length
Note 1 to entry: It is also referred to as the natural or true strain.
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Note 2 to entry: It is dimensionless.
3.2
Hencky strain rate
rate of change of Hencky strain with time, given by Formula (2)
ε=×1/ll∂∂/ t (2)
where t is the time
Note 1 to entry: It is independent of the original specimen length l .
Note 2 to entry: It is expressed in reciprocal seconds.
3.3
net tensile stress
for tensile uniaxial extension, stress given by Formula (3)
σσ=−=−σσ σσ=−σ (3)
Er11 22 11 33 zz r
where σ is a stress tensor in either rectangular or axisymmetric co-ordinates
+ +

Note 1 to entry: The tensile stress growth function is indicated by σ where the indicates start-up of flow.

Note 2 to entry: Net tensile stress is expressed in pascals.
3.4
tensile stress growth coefficient
ratio of the net tensile stress to the Hencky strain rate, given by Formula (4)
η ()t,/εσ= ε (4)
for tensile uniaxial extension, where t is time and indicates start-up of flow

Note 1 to entry: It is also known for the purposes of this document as “transient extensional viscosity”.

Note 2 to entry: It is a transient term.
Note 3 to entry: It is expressed in pascal seconds.
3.5
tensile viscosity
term given by Formula (5)
lim +
η tt,,εη=  ε  (5)
() ()
EEt→∞  

Note 1 to entry: It is the limiting tensile stress growth coefficient value and represents an equilibrium extensional

viscosity if a steady value is achieved. However, for materials that do not exhibit a steady-state behaviour, the use

of an “equilibrium extensional viscosity” such as this is not appropriate.
Note 2 to entry: It is expressed in pascal seconds.
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4 General principles

In contrast to shear flow where reference is normally made only to steady shear flow behaviour,

extensional flow behaviour is best described as being transient. In describing the transient behaviour

of materials in extension at constant strain rate, they may exhibit either an unbounded stress growth

behaviour in which the stress continually increases with increasing strain until the material fails,

or the stress reaches a steady value with increasing strain thus yielding a tensile or equilibrium

extensional viscosity. The latter occurs typically at large strains. An equilibrium extensional viscosity

is thus dependent on strain rate but not on strain or time. Normally, the extensional viscosity varies as

a function of both strain and strain rate as well as temperature.

In describing and modelling plastics processing, the use of Hencky strain is preferred. The rate of

Hencky strain of an element of fluid within a flow is independent of its original length and is determined

only from the velocity field of that element. It is thus a more suitable characteristic of the flow. Strain

and strain rate are taken by default herein to be Hencky values.

Stretching flow methods can be used to generate quantitatively accurate data on the extensional

viscoelasticity of polymer melts. In carrying out extensional flow measurements, there are four types

of measurement that are normally made: constant strain rate, constant stress, constant force and

constant speed. This document describes the first of these: constant strain rate. In this method, the

strain rate is uniform throughout the specimen and is held constant with time.

The basic principle behind stretching flow measurements is to subject a specimen to a tensile stretching

deformation. By measurement of the force and deformation of the specimen, the stresses and strains

and hence strain rate can be determined.
5 Apparatus
5.1 General description

The measuring apparatus shall consist of one of the following types, shown in Figure 1 to Figure 4. These

types define the various instrument configurations. The notation used in these figures is defined in 8.1.

Type A: Two rotating clamps. Each clamp shall consist of either a single rotating element or a pair of

rotating elements – only the pair arrangement is shown. The force exerted on the specimen can be

measured at the fixed or rotating end.

NOTE It is likely that the force will be easier to measure, and will be measured with greater accuracy, on a

fixed clamp rather than on a moving clamp as there will be fewer complications due, for example, to vibration

and the inertia of the clamp which can introduce noise and errors into the force signal.

Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 angular speed of rotating clamps of side 1, ω , expressed in rad⋅s
5 angular speed of rotating clamps of side 2, ω , expressed in rad⋅s
Figure 1 — Schematic diagram of type A test instrument
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Type B: A single rotating clamp and a fixed clamp. The rotating clamp shall consist of either a single

rotating element or a pair of rotating elements. The force exerted on the specimen is normally measured

at the fixed end.
Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 angular speed of rotating clamps, ω , expressed in rad⋅s
Figure 2 — Schematic diagram of type B test instrument
Type C: Two translating (non-rotating) clamps.
Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
4 speed of the ends of the specimen of side 1, V , expressed in m⋅s
5 speed of the ends of the specimen of side 2, V , expressed in m⋅s
Figure 3 — Schematic diagram of type C test instrument
Type D: Single translating (non-rotating) clamp.
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Key
1 specimen
2 force, F, expressed in N
3 original specimen length, l , expressed in m
–1 –1
4 speed of the ends of the specimen, m·s , V , expressed in m·s
Figure 4 — Schematic diagram of type D test instrument

In each of these configurations, the specimen is mounted between the clamps and is stretched uniaxially.

The requirements for the apparatus are that it shall permit the measurement or determination of the

force acting on the specimen, and the strain and strain rate of the specimen subjected to a constant

strain rate under isothermal conditions. The strain and strain rate of the specimen shall either be

derived from the displacements and/or speeds of the clamp or clamps, or be measured directly from

the dimensions and/or local velocities of the specimen.
5.2 Silicon oil bath/temperature-controlled chamber

Heating may be provided by placing the specimen in a silicone oil bath or in a temperature-controlled

chamber with a forced gas flow through it.

NOTE 1 When heating using forced gas, a gas can be used in the chamber surrounding the test specimen to

provide the required test environment, for example nitrogen to provide an inert atmosphere.

NOTE 2 The use of a silicone oil bath can permit more rapid heating of the specimen.

For low-viscosity materials, it is essential to support the specimen during heat-up and testing (to avoid

it sagging under the influence of gravity).

NOTE 3 The use of a silicone oil bath results in the specimen being supported by the silicone oil due to its

buoyancy, particularly if the densities of the silicone oil and specimen are matched at the test temperature. If a

forced-gas oven is used, then support of the specimen can be obtained by the cushioning effect provided by the gas.

Silicone oil can be absorbed by some polymers. A check should preferably be made to see if the immersion

time affects the measured properties of the polymer by varying the length of the immersion time (see

Annex A and NOTE 1 in 7.1). When quantitative results are required, then this check shall be made.

Even if the silicone oil does not affect the shape of the tensile stress growth coefficient versus strain (or

time) plot, it can affect the point of failure of the specimen. Thus, assessment of the effect of the silicone

oil on the measured properties should consider both of these aspects.

NOTE 4 Alternative methods for checking the effect of immersion in silicone oil on the specimen can also

be used. Such methods include the measurement of the mass or dimensions of the specimen before and after

immersing in silicone oil and identifying whether a change has occurred due to that immersion – see Annex A.

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5.3 Temperature measurement and control

The test temperature should preferably be measured using a device that is mounted close to the

specimen. Contact of the device with the specimen is not permitted. It is essential to mount temperature

sensors in at least two positions to monitor temperature uniformity along the length of the specimen.

NOTE The uniformity of the temperature along the specimen length is critical to the measurement of the

transient extensional flow properties of polymer melts. Localized hot spots result in excessive strain in those

regions. This can lead to premature failure, particularly for materials that do not exhibit a high degree of strain

hardening.
The spatial temperature variation shall be within ±0,75 K.

The temporal temperature variation shall be within ±1,0 K of the set temperature.

The temperature-measuring device shall have a resolution of 0,1 K and shall be calibrated using a device

accurate to within ±0,1 K.
5.4 Strain and strain rate measurement

The strain and strain rate of the specimen shall be determined either from measurement of the

displacements and/or speeds of the clamp or clamps, or measured directly from the dimensions and/or

local velocities of the specimen.

NOTE The diameter of the specimen can be measured during the test by use of optical or cutting methods

to derive strains and strain rates and to assess the uniformity of deformation. The cutting method results in the

test being terminated once the cuts have been made and thus prevents data to failure from being obtained. Local

velocities can be measured using optical methods.

Corrections for slippage of the specimen at the clamp or clamps may be applied, obtained through

independent measurement of the strain of the specimen during testing through measurement of its

diameter or local velocities by other methods.

The apparatus shall have an accuracy of strain determination or measurement to within ±3 % of the

absolute value.

The apparatus shall have an accuracy of strain rate determination or measurement to within ±3 % of

the absolute value.
5.5 Force measurement

The force on the specimen shall be measured during the test by an appropriate means, for example a

leaf spring arrangement (see NOTE).

The resolution of the force-measuring device should preferably be no greater than 0,1 % of the full-

scale value.

The apparatus shall have an accuracy of force measurement to within ±2 % of the full-scale value.

NOTE Typical peak forces measured in testing of polyethylenes are estimated to be up to approximately 1 N

for specimens approximately 3 mm in diameter.
5.6 Calibration

The force, displacement, rate of displacement and temperature functions of the rheometer shall be

calibrated at least once in 6 months.

It is preferable that calibration be carried out at the test temperature as measurement of these functions,

in particular that of force, can be temperature sensitive.

No traceable standard reference materials are known to exist for checking the calibration of such

instruments. Where a reference material is used for checking the instrument, it is preferable that the

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transient extensional viscosity of the reference material, and the dimensions of the specimen produced

using it, have values that are similar to those encountered or used during normal operation of the

instrument.
6 Sampling and specimen preparation
6.1 Sampling

The sampling method, including any special methods of specimen preparation and introduction into

the rheometer, shall be as specified in the relevant materials standard or otherwise by agreement.

If samples or specimens are hygroscopic or contain volatile ingredients, then they shall be stored to

prevent or minimize any effects on the measurements. Drying of samples can be required prior to

preparing test specimens.

As the test specimens are typically small, being of the order of a few grams, it is essential that they be

representative of the material being sampled. Repeat testing may be used to identify batch-to-batch or

within-batch variation.
6.2 Specimen preparation
The specimen shall be either cylindrical or rectangular in cross-section.

Test specimens in the form of a cylinder may be produced by extrusion or by injection, transfer or

compression moulding.

Test specimens in the form of a strip may be produced by extrusion or by injection or compression

moulding or by cutting from sheet.

The length-to-diameter ratio of cylindrical specimens should preferably be at least 10.

NOTE A length-to-diameter ratio of at least 10 is required to minimize end-effects. However, a longer

specimen results in a reduction in the maximum strain rate that can be achieved. The magnitude of the end-

errors can be assessed by using specimens of different length or diameter to produce different length-to-

diameter aspect ratios. The effect on measured values can then be determined.

The specimen shall not contain any visible impurities, voids or air bubbles. The specimen shall not show

any obvious discoloration prior to or after testing.

For cylindrical specimens, measure the diameter D of the specimen in at least three positions along its

length. Repeat these measurements after rotating the specimen by 90°. Calculate an average value for

the diameter from these measurements.

For rectangular specimens, measure the width and thickness in at least three positions along its length.

Calculate average values for the width b and thickness h from these measurements.

Calculate the cross-sectional area of the specimen from the measurements.

The diameter, or width and thickness, of the specimen, as appropriate, shall be determined to and be

uniform to within ±2 % of their average value.
6.3 Specimen mounting

Specimens may be either gripped by the clamps or attached using adhesive to studs that are then

clamped into the instrument.

Attachment by a suitable high-temperature epoxy adhesive has been found suitable. Treat the ends of

the specimen by passing them through a butane flame and then dipping them into concentrated sulfuric

acid for 30 s. Prevent any other part of the specimen, except that to be bonded, from being exposed to

either the flame or the acid. Dip the ends of the specimen into the adhesive and then attach them to

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the studs. Place the specimen with its studs into an oven and cure the adhesive using a suitable time-

temperature cycle (100 °C for 1 h has been found suitable). Allow the specimen to cool before handling.

7 Procedure
7.1 Specimen loading
Mount a specimen in place in the rheometer.

Measure the length of the specimen between the clamps to within 1 % of its absolute value.

After mounting the specimen in the instrument, immerse it in the silicone oil bath or place it in the

temperature-controlled chamber (see 5.2). Where possible, bring the bath or chamber to the test

temperature before inserting the specimen to reduce the time spent by the specimen reaching and

equilibrating at the test temperature. Allow the specimen and apparatus to reach thermal equilibrium

at the test temperature. This period of time is referred to as the equilibration time.

NOTE 1 The adequacy of the time allowed for the specimen to reach thermal equilibrium and the effects of the

silicone oil, degradation, crosslinking and other time-dependent phenomena on the specimen can be checked by

varying the time for which the specimen is in the oil bath or environmental chamber before testing. The effect

on the measured values can then be assessed. For measurements in silicone oil of specimens approximately

3 mm in diameter, an equilibration time of approximately 5 min has been found to be sufficient for testing at a

temperature of 150 °C.

A correction for thermal expansion of the specimen during heating can be necessary. A correction

for effects arising from stress relaxation of the specimen, if unclamped during the temperature

equilibration period, can also be required. Both of these effects can result in a change in the critical

dimensions of the specimen (i.e. thickness and width, or diameter) that may need to be taken into

account.

NOTE 2 As an example, measurements of a PE-HD indicate a 20 % decrease in density on heating from 25 °C

to 150 °C which, if accommodated solely by a change in the diameter of a cylindrical specimen, results in an

approximately 10 % increase in diameter of the specimen. Furthermore, stress relaxation of extruded specimens

can result in a recovery (shrinkage) in length with a corresponding increase in the specimen’s critical dimensions

(e.g. diameter) if unclamped during the temperature equilibration period.
7.2 Pre-conditioning of the specimen
The specimen may be pre-conditioned by ap
...

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