ISO 20965:2021

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

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ISO 20965:2021(E)

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© ISO 2021
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Published in Switzerland
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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
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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.
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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
ε
strain given by the natural logarithm of the elongation ratio given by Formula (1)
ε =In ll/ (1)
()
0
where
l is the specimen length and
l is the original specimen length
0
Note 1 to entry: It is also referred to as the natural or true strain.
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ISO 20965:2021(E)

Note 2 to entry: It is dimensionless.
3.2
Hencky strain rate
ε
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 .
0
Note 2 to entry: It is expressed in reciprocal seconds.
3.3
net tensile stress
σ
E
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
ii
+ +
Note 1 to entry: The tensile stress growth function is indicated by σ where the indicates start-up of flow.
E
Note 2 to entry: Net tensile stress is expressed in pascals.
3.4
tensile stress growth coefficient
+
η
E
ratio of the net tensile stress to the Hencky strain rate, given by Formula (4)
+
η ()t,/εσ= ε (4)
EE
+
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
η
E
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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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
0
–1
4 angular speed of rotating clamps of side 1, ω , expressed in rad⋅s
1
–1
5 angular speed of rotating clamps of side 2, ω , expressed in rad⋅s
2
Figure 1 — Schematic diagram of type A test instrument
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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
0
–1
4 angular speed of rotating clamps, ω , expressed in rad⋅s
1
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
0
–1
4 speed of the ends of the specimen of side 1, V , expressed in m⋅s
1
–1
5 speed of the ends of the specimen of side 2, V , expressed in m⋅s
2
Figure 3 — Schematic diagram of type C test instrument
Type D: Single translating (non-rotating) clamp.
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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
0
–1 –1
4 speed of the ends of the specimen, m·s , V , expressed in m·s
1
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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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
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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
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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 ap
...