Rubber, vulcanized or thermoplastic -- Determination of ageing characteristics by measurement of stress relaxation in tension

Caoutchouc vulcanisé ou thermoplastique -- Détermination des caractéristiques de vieillissement par mesurage de la contrainte de relaxation en traction

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INTERNATIONAL ISO
STANDARD 6914
Fifth edition
2021-02
Rubber, vulcanized or thermoplastic —
Determination of ageing
characteristics by measurement of
stress relaxation in tension
Caoutchouc vulcanisé ou thermoplastique — Détermination des
caractéristiques de vieillissement par mesurage de la contrainte de
relaxation en traction
Reference number
ISO 6914:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 6914:2021(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 6914:2021(E)
Contents Page

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

Introduction ..................................................................................................................................................................................................................................v

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

6 Calibration .................................................................................................................................................................................................................. 2

7 Test pieces ................................................................................................................................................................................................................... 3

7.1 Type and preparation of test pieces ..................................................................................................................................... 3

7.2 Measurement of test piece dimensions ............................................................................................................................. 3

7.3 Number of test pieces ....................................................................................................................................................................... 3

8 Storage and conditioning ............................................................................................................................................................................. 3

9 Test conditions ....................................................................................................................................................................................................... 3

9.1 Duration of test ....................................................................................................................................................................................... 3

9.2 Temperature of exposure ............................................................................................................................................................... 4

10 Procedure..................................................................................................................................................................................................................... 4

10.1 Method A ...................................................................................................................................................................................................... 4

10.2 Method B ...................................................................................................................................................................................................... 4

10.3 Method C ...................................................................................................................................................................................................... 5

11 Expression of results ........................................................................................................................................................................................ 5

12 Test report ................................................................................................................................................................................................................... 6

Annex A (normative) Calibration schedule ................................................................................................................................................... 8

Bibliography .............................................................................................................................................................................................................................10

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 6914:2021(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 45, Rubber and rubber products,

Subcommittee SC 2, Testing and analysis.

This fifth edition cancels and replaces the fourth edition (ISO 6914:2013), which has been technically

revised to further align with ISO 3384-1.

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 ----------------------
ISO 6914:2021(E)
Introduction

When a constant strain is applied to rubber, the force necessary to maintain that strain is not constant

but decreases with time; this behaviour is called “stress relaxation”. Conversely, when rubber is

subjected to a constant stress, an increase in the deformation with time takes place; this behaviour is

called “creep”.

The process responsible for stress relaxation can be physical or chemical in nature, and under all normal

conditions both types of process will occur simultaneously. However, at normal or low temperatures

and/or short times, stress relaxation is dominated by physical processes, while at high temperatures

and/or long times chemical processes are dominant.

Stress relaxation tests in compression are suitable for obtaining a measure of the functional property of

sealing force and procedures are given in ISO 3384 (all parts). Stress relaxation tests in tension can be

used as a measure of ageing as chemical processes will dominate in the case of a thin test piece exposed

to an atmosphere containing oxygen at an elevated temperature for relatively long periods of time.

There are two variants of the technique. Measurements of stress can be made under either

a) continuous strain conditions, or
b) intermittent strain conditions.

In the case of a), continuous strain conditions, the test piece is held in extension throughout the ageing

period in the oven. In the case of b), intermittent strain conditions, the test piece is aged in the oven in

the unstressed state but, at periodic intervals, it is stretched to a fixed extended length for a short time

in order to determine the stress.

NOTE 1 The terms “continuous stress relaxation” and “intermittent stress relaxation” are commonly used

to describe the two principal variants of the technique. The latter term, “intermittent stress relaxation”, is a

misnomer since no true relaxation of stress occurs and indeed the measured stress can increase with time. For

this reason, the use of this term has been avoided in this document although it is fairly well established in the

literature.

In a second version of the intermittent test, the test piece is periodically removed from the accelerated

ageing atmosphere and the stress is measured under normal laboratory conditions. The advantage of

this method is that it does not require the use of special apparatus since a conventional tensile-testing

machine can be used for the measurement of stress.

Measurements made in accordance with the methods described in this document provide information

about the structural changes that occur in the rubber during ageing, and are complementary: under

continuous strain conditions, provided physical relaxation processes are not dominant, the decay of

stress provides a measure of the degradative scission reactions in the network. Any new networks

formed as a result of crosslinking reactions are considered to be in equilibrium at the test strain with

the main network and, therefore, do not impose any new stresses. Under intermittent strain conditions,

the decay of stress provides a measure of the net effect of both degradative scission and crosslinking

reactions.

NOTE 2 Even under conditions conducive to chemical processes, some physical relaxation can occur. The

extent to which it does so will depend on the viscoelastic characteristics of the rubber and on the test conditions

and care should be exercised in the interpretation of the results. Physical relaxation is increased by fillers and

will be more evident at short times and at lower temperatures. It is often found to be proportional to logarithmic

time and is less temperature sensitive than chemical relaxation.

The validity of the methods described in this document depends on the uniformity of degradation in

the rubber. For this reason, the thickness of the test pieces used is 1,0 mm to minimize the effect of

oxygen diffusion on ageing.

The change in stress can be of direct interest, but the relative resistance of rubbers to ageing will depend

on the properties being measured or required by the application. This document should, therefore, be

regarded as complementary to ISO 188.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
ISO 6914:2021(E)

The lifetime of the material, if this is to be investigated, can be determined using the procedures

described in ISO 11346.

The most important factor in achieving good repeatability and reproducibility when making these tests

is to keep the temperature and the elongation constant during all measurements.
vi © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 6914:2021(E)
Rubber, vulcanized or thermoplastic — Determination of
ageing characteristics by measurement of stress relaxation
in tension

WARNING 1 — Persons using this document should be familiar with normal laboratory practice.

This document does not purport to address all of the safety problems, if any, associated with its

use. It is the responsibility of the user to establish appropriate safety and health practices and to

determine the applicability of any other restrictions.

WARNING 2 — Certain procedures specified in this document can involve the use or generation

of substances, or the generation of waste, that could constitute a local environmental hazard.

Reference should be made to appropriate documentation on safe handling and disposal after use.

1 Scope

This document describes three methods for the measurement of the change of stress in a test piece at a

given elongation for the purpose of determining the ageing characteristics of a rubber.

— Method A is intended for measurement under continuous strain conditions.

— Method B is the preferred method for measurement under intermittent strain conditions.

— Method C is an alternative to method B for measurement under intermittent strain conditions in

which the test piece is removed from the ageing environment for measurement of the stress at

standard laboratory temperature.

Measurements at a single elevated ageing temperature can be used for quality control purposes

as a measure of heat-ageing resistance. Measurements at a number of temperatures can be used for

research and development purposes to estimate long-term ageing characteristics in accordance with

the procedures described in ISO 11346.

Tests under continuous and intermittent strain conditions measure the results of different processes,

and hence do not give the same results. The results of the intermittent methods B and C also cannot be

expected to be the same as they use different test conditions.
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 188:2011, Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests

ISO 5893:2019, Rubber and plastics test equipment — Tensile, flexural and compression types (constant

rate of traverse) — Specification
ISO 18899:2013, Rubber — Guide to the calibration of test equipment

ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test methods

3 Terms and definitions
No terms and definitions are listed in this document.
© ISO 2021 – All rights reserved 1
---------------------- Page: 7 ----------------------
ISO 6914:2021(E)

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

A test piece of vulcanized or thermoplastic rubber is elongated to a predetermined tensile strain and

the force measured as a function of time in combination with exposure of the test piece to an elevated

temperature.
In method A the extension is appl
...

INTERNATIONAL ISO
STANDARD 6914
Fifth edition
Rubber, vulcanized or thermoplastic —
Determination of ageing
characteristics by measurement of
stress relaxation in tension
Caoutchouc vulcanisé ou thermoplastique — Détermination des
caractéristiques de vieillissement par mesurage de la contrainte de
relaxation en traction
PROOF/ÉPREUVE
Reference number
ISO 6914:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 6914:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

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 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 6914:2020(E)
Contents Page

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

Introduction ..................................................................................................................................................................................................................................v

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

6 Calibration .................................................................................................................................................................................................................. 2

7 Test pieces ................................................................................................................................................................................................................... 3

7.1 Type and preparation of test pieces ..................................................................................................................................... 3

7.2 Measurement of test pieces dimensions ........................................................................................................................... 3

7.3 Number of test pieces ....................................................................................................................................................................... 3

8 Storage and conditioning ............................................................................................................................................................................. 3

9 Test conditions ....................................................................................................................................................................................................... 3

9.1 Duration of test ....................................................................................................................................................................................... 3

9.2 Temperature of exposure ............................................................................................................................................................... 4

10 Procedure..................................................................................................................................................................................................................... 4

10.1 Method A ...................................................................................................................................................................................................... 4

10.2 Method B ...................................................................................................................................................................................................... 4

10.3 Method C ...................................................................................................................................................................................................... 5

11 Expression of results ........................................................................................................................................................................................ 5

12 Test report ................................................................................................................................................................................................................... 6

Annex A (normative) Calibration schedule ................................................................................................................................................... 8

Bibliography .............................................................................................................................................................................................................................10

© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO 6914:2020(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 45, Rubber and rubber products,

Subcommittee SC 2, Testing and analysis.

This fifth edition cancels and replaces the fourth edition (ISO 6914:2013), which has been technically

revised to further align with ISO 3384-1.

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 PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 6914:2020(E)
Introduction

When a constant strain is applied to rubber, the force necessary to maintain that strain is not constant

but decreases with time; this behaviour is called “stress relaxation”. Conversely, when rubber is

subjected to a constant stress, an increase in the deformation with time takes place; this behaviour is

called “creep”.

The process responsible for stress relaxation can be physical or chemical in nature, and under all normal

conditions both types of process will occur simultaneously. However, at normal or low temperatures

and/or short times, stress relaxation is dominated by physical processes, while at high temperatures

and/or long times chemical processes are dominant.

Stress relaxation tests in compression are suitable for obtaining a measure of the functional property of

sealing force and procedures are given in ISO 3384 (all parts). Stress relaxation tests in tension can be

used as a measure of ageing as chemical processes will dominate in the case of a thin test piece exposed

to an atmosphere containing oxygen at an elevated temperature for relatively long periods of time.

There are two variants of the technique. Measurements of stress can be made under either

a) continuous strain conditions, or
b) intermittent strain conditions.

In case a), continuous strain conditions, the test piece is held in extension throughout the ageing period

in the oven. In case b), intermittent strain conditions, the test piece is aged in the oven in the unstressed

state but, at periodic intervals, it is stretched to a fixed extended length for a short time in order to

determine the stress.

NOTE 1 The terms “continuous stress relaxation” and “intermittent stress relaxation” are commonly used

to describe the two principal variants of the technique. The latter term, “intermittent stress relaxation”, is a

misnomer since no true relaxation of stress occurs and indeed the measured stress can increase with time. For

this reason, the use of this term has been avoided in this document although it is fairly well established in the

literature.

In a second version of the intermittent test, the test piece is periodically removed from the accelerated

ageing atmosphere and the stress is measured under normal laboratory conditions. The advantage of

this method is that it does not require the use of special apparatus since a conventional tensile-testing

machine can be used for the measurement of stress.

Measurements made in accordance with the methods described in this document provide information

about the structural changes that occur in the rubber during ageing, and are complementary: under

continuous strain conditions, provided physical relaxation processes are not dominant, the decay of

stress provides a measure of the degradative scission reactions in the network. Any new networks

formed as a result of crosslinking reactions are considered to be in equilibrium at the test strain with

the main network and, therefore, do not impose any new stresses. Under intermittent strain conditions,

the decay of stress provides a measure of the net effect of both degradative scission and crosslinking

reactions.

NOTE 2 Even under conditions conducive to chemical processes, some physical relaxation can occur. The

extent to which it does so will depend on the viscoelastic characteristics of the rubber and on the test conditions

and care should be exercised in the interpretation of the results. Physical relaxation is increased by fillers and

will be more evident at short times and at lower temperatures. It is often found to be proportional to logarithmic

time and is less temperature sensitive than chemical relaxation.

The validity of the methods described in this document depends on the uniformity of degradation in

the rubber. For this reason, the thickness of the test pieces used is 1,0 mm to minimize the effect of

oxygen diffusion on ageing.

The change in stress can be of direct interest, but the relative resistance of rubbers to ageing will depend

on the properties being measured or required by the application. This document should, therefore, be

regarded as complementary to ISO 188.
© ISO 2020 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
ISO 6914:2020(E)

The lifetime of the material, if this is to be investigated, can be determined using the procedures

described in ISO 11346.

The most important factor in achieving good repeatability and reproducibility when making these tests

is to keep the temperature and the elongation constant during all measurements.
vi PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 6914:2020(E)
Rubber, vulcanized or thermoplastic — Determination of
ageing characteristics by measurement of stress relaxation
in tension

WARNING 1 — Persons using this document should be familiar with normal laboratory practice.

This document does not purport to address all of the safety problems, if any, associated with its

use. It is the responsibility of the user to establish appropriate safety and health practices and to

determine the applicability of any other restrictions.

WARNING 2 — Certain procedures specified in this document can involve the use or generation

of substances, or the generation of waste, that could constitute a local environmental hazard.

Reference should be made to appropriate documentation on safe handling and disposal after use.

1 Scope

This document describes three methods for the measurement of the change of stress in a test piece at a

given elongation for the purpose of determining the ageing characteristics of a rubber.

— Method A is intended for measurement under continuous strain conditions.

— Method B is the preferred method for measurement under intermittent strain conditions.

— Method C is an alternative to method B for measurement under intermittent strain conditions in

which the test piece is removed from the ageing environment for measurement of the stress at

standard laboratory temperature.

Measurements at a single elevated ageing temperature can be used for quality control purposes

as a measure of heat-ageing resistance. Measurements at a number of temperatures can be used for

research and development purposes to estimate long-term ageing characteristics in accordance with

the procedures described in ISO 11346.

Tests under continuous and intermittent strain conditions measure the results of different processes,

and hence do not give the same results. The results of the intermittent methods B and C also cannot be

expected to be the same as they use different test conditions.
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 188:2011, Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests

ISO 5893:2019, Rubber and plastics test equipment — Tensile, flexural and compression types (constant

rate of traverse) — Specification
ISO 18899:2013, Rubber — Guide to the calibration of test equipment

ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test methods

3 Terms and definitions
No terms and definitions are listed in this document.
© ISO 2020 – All rights reserved PROOF/ÉPREUVE 1
---------------------- Page: 7 ----------------------
ISO 6914:2020(E)

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

A test piece of vulcanized or thermoplastic rubber is elongated to a predetermined tensile strain and

the force measured as a function of time in combination with exposure of the test piece to an elevated

temperature.
In method A the extension is applied and the force measured ov
...

DRAFT INTERNATIONAL STANDARD
ISO/DIS 6914
ISO/TC 45/SC 2 Secretariat: JISC
Voting begins on: Voting terminates on:
2020-04-14 2020-07-07
Rubber, vulcanized or thermoplastic — Determination of
ageing characteristics by measurement of stress relaxation
in tension

Caoutchouc vulcanisé ou thermoplastique — Détermination des caractéristiques de vieillissement par

mesurage de la contrainte de relaxation en traction
ICS: 83.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
This document is circulated as received from the committee secretariat.
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 6914:2020(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2020
---------------------- Page: 1 ----------------------
ISO/DIS 6914:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/DIS 6914:2020(E)
Contents Page

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

Introduction ..................................................................................................................................................................................................................................v

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

6 Calibration .................................................................................................................................................................................................................. 2

7 Test pieces ................................................................................................................................................................................................................... 3

7.1 Type and preparation of test pieces ..................................................................................................................................... 3

7.2 Measurement of dimensions of test pieces .................................................................................................................... 3

7.3 Number of test pieces ....................................................................................................................................................................... 3

8 Storage and conditioning ............................................................................................................................................................................. 3

9 Test conditions ....................................................................................................................................................................................................... 3

9.1 Duration of test ....................................................................................................................................................................................... 3

9.2 Temperature of exposure ............................................................................................................................................................... 4

10 Procedure..................................................................................................................................................................................................................... 4

10.1 Method A ...................................................................................................................................................................................................... 4

10.2 Method B ...................................................................................................................................................................................................... 4

10.3 Method C ...................................................................................................................................................................................................... 5

11 Expression of results ........................................................................................................................................................................................ 5

12 Test report ................................................................................................................................................................................................................... 6

Annex A (normative) Calibration Schedule ................................................................................................................................................... 8

Bibliography .............................................................................................................................................................................................................................10

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/DIS 6914:2020(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 45, Rubber and rubber products,

Subcommittee SC 2, Testing and analysis.

This 5th edition cancels and replaces the 4th edition (ISO 6914:2013), which has been technically

revised to increase alignment with ISO 3384-1.

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 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/DIS 6914:2020(E)
Introduction

When a constant strain is applied to rubber, the force necessary to maintain that strain is not constant

but decreases with time; this behaviour is called “stress relaxation”. Conversely, when rubber is

subjected to a constant stress, an increase in the deformation with time takes place; this behaviour is

called “creep”.

The process responsible for stress relaxation can be physical or chemical in nature, and under all normal

conditions both types of process will occur simultaneously. However, at normal or low temperatures

and/or short times, stress relaxation is dominated by physical processes, while at high temperatures

and/or long times chemical processes are dominant.

Stress relaxation tests in compression are suitable for obtaining a measure of the functional property

of sealing force and procedures are given in ISO 3384. Stress relaxation tests in tension can be used as

a measure of ageing as chemical processes will dominate in the case of a thin test piece exposed to an

atmosphere containing oxygen at an elevated temperature for relatively long periods of time.

There are two variants of the technique. Measurements of stress can be made under either

a) continuous strain conditions, or
b) intermittent strain conditions.

In the case of a), continuous strain conditions, the test piece is held in extension throughout the ageing

period in the oven. In the case of b), intermittent strain conditions, the test piece is aged in the oven in

the unstressed state but, at periodic intervals, it is stretched to a fixed extended length for a short time

in order to determine the stress.

NOTE 1 The terms “continuous stress relaxation” and “intermittent stress relaxation” are commonly used

to describe the two principal variants of the technique. The latter term, “intermittent stress relaxation”, is a

misnomer since no true relaxation of stress occurs and indeed the measured stress can increase with time. For

this reason, the use of this term has been avoided in this document although it is fairly well established in the

literature.

In a second version of the intermittent test, the test piece is periodically removed from the accelerated

ageing atmosphere and the stress is measured under normal laboratory conditions. The advantage of

this method is that it does not require the use of special apparatus since a conventional tensile-testing

machine can be used for the measurement of stress.

Measurements made in accordance with the methods described in this document provide information

about the structural changes that occur in the rubber during ageing, and are complementary: under

continuous strain conditions, provided physical relaxation processes are not dominant, the decay of

stress provides a measure of the degradative scission reactions in the network. Any new networks

formed as a result of crosslinking reactions are considered to be in equilibrium at the test strain with

the main network and, therefore, do not impose any new stresses. Under intermittent strain conditions,

the decay of stress provides a measure of the net effect of both degradative scission and crosslinking

reactions.

NOTE 2 Even under conditions conducive to chemical processes, some physical relaxation can occur. The

extent to which it does so will depend on the viscoelastic characteristics of the rubber and on the test conditions

and care should be exercised in the interpretation of the results. Physical relaxation is increased by fillers and

will be more evident at short times and at lower temperatures. It is often found to be proportional to logarithmic

time and is less temperature sensitive than chemical relaxation.

The validity of the methods described in this document depends on the uniformity of degradation in

the rubber. For this reason, the thickness of the test pieces used is 1,0 mm to minimize the effect of

oxygen diffusion on ageing.

The change in stress might be of direct interest, but the relative resistance of rubbers to ageing will

depend on the properties being measured or required by the application. This document should,

therefore, be regarded as complementary to ISO 188.
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ISO/DIS 6914:2020(E)

The lifetime of the material, if this is to be investigated, can be determined using the procedures

described in ISO 11346.

The most important factor in achieving good repeatability and reproducibility when making these tests

is to keep the temperature and the elongation constant during all measurements.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 6914:2020(E)
Rubber, vulcanized or thermoplastic — Determination of
ageing characteristics by measurement of stress relaxation
in tension

WARNING 1 — Persons using this document should be familiar with normal laboratory practice.

This document does not purport to address all of the safety problems, if any, associated with its

use. It is the responsibility of the user to establish appropriate safety and health practices and to

determine the applicability of any other restrictions.

WARNING 2 — Certain procedures specified in this International Standard might involve

the use or generation of substances, or the generation of waste, that could constitute a local

environmental hazard. Reference should be made to appropriate documentation on safe

handling and disposal after use.
1 Scope

This document describes three methods for the measurement of the change of stress in a test piece at a

given elongation for the purpose of determining the ageing characteristics of a rubber.

— Method A is intended for measurement under continuous strain conditions.

— Method B is the preferred method for measurement under intermittent strain conditions.

— Method C is an alternative to method B for measurement under intermittent strain conditions in

which the test piece is removed from the ageing environment for measurement of the stress at

standard laboratory temperature.

Measurements at a single elevated ageing temperature can be used for quality control purposes

as a measure of heat-ageing resistance. Measurements at a number of temperatures can be used for

research and development purposes to estimate long-term ageing characteristics in accordance with

the procedures described in ISO 11346.

Tests under continuous and intermittent strain conditions measure the results of different processes,

and hence do not give the same results. The results of the intermittent methods B and C also cannot be

expected to be the same as they use different test conditions.
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 188:2011, Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests

ISO 5893:2019, Rubber and plastics test equipment — Tensile, flexural and compression types (constant

rate of traverse) — Specification
ISO 18899:2013, Rubber — Guide to the calibration of test equipment

ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test methods

3 Terms and definitions
No terms and definitions are listed in this document.
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ISO
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