ISO 188:1998
(Main)Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests
Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests
Caoutchouc vulcanisé ou thermoplastique — Essais de résistance au vieillissement accéléré et à la chaleur
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 188
Third edition
1998-04-15
Rubber, vulcanized or thermoplastic —
Accelerated ageing and heat resistance
tests
Caoutchouc vulcanisé — Essais de résistance au vieillissement accéléré et
à la chaleur
A
Reference number
ISO 188:1998(E)
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ISO 188:1998(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.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 188 was prepared by Technical Committee ISO/TC 45, Rubber and rubber products,
Subcommittee SC 2, Physical and degradation tests.
This third edition cancels and replaces the second edition (ISO 188:1982), which has been technically revised.
Annex A of this International Standard is for information only.
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
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Printed in Switzerland
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© ISO
ISO 188:1998(E)
Introduction
Accelerated ageing and heat resistance tests are designed to estimate the relative resistance of rubber to
deterioration with the passage of time. For this purpose, the rubber is subjected to controlled deteriorating
influences for definite periods, after which appropriate properties are measured and compared with the
corresponding properties of the unaged rubber.
The purpose of the accelerated ageing test may be to assess the deterioration of the rubber
a) either during prolonged periods at normal or elevated temperatures in air;
b) or during prolonged periods at elevated temperatures and at elevated oxygen pressure.
In accelerated ageing, the rubber is subjected to a test environment intended to produce the effect of natural ageing
in a shorter time.
In the case of heat resistance tests, the rubber is subjected to prolonged periods at the same temperature as that
which it will experience in service.
Three types of method are given in this International Standard, namely an air-oven method using a low air speed,
an air-oven method using forced air ventilation and an oxygen pressure method.
The selection of the time, temperature and atmosphere to which the test pieces are exposed and the type of oven to
use will depend on the purpose of the test and the type of polymer.
In the air-oven methods, deterioration is accelerated by raising the temperature and, in the oxygen pressure
method, by increasing the oxygen concentration and the temperature. The degree of acceleration thus produced
varies from one rubber to another and from one property to another.
Degradation can also be accelerated by air speed. Consequently, ageing with different ovens can give different
results.
Consequences of this are:
a) Accelerated ageing does not truly reproduce under all circumstances the changes produced by natural
ageing.
b) Accelerated ageing sometimes fails to indicate accurately the relative natural or service life of different
rubbers; thus, ageing at temperatures greatly above ambient or service temperatures may tend to
equalize the apparent life of rubbers which deteriorate at different rates in storage or service. Ageing at
one or more intermediate temperatures is useful in assessing the reliability of accelerated ageing at high
temperatures.
c) Accelerated ageing tests involving different properties may not give agreement in assessing the relative
lives of different rubbers and may even arrange them in different orders of merit. Therefore, deterioration
should be measured by the changes in property or properties which are of practical importance, provided
that they can be measured reasonably accurately.
Air-oven and oxygen pressure ageing should not be used to simulate natural ageing which occurs in the presence
of either light or ozone when the rubbers are stretched.
To estimate lifetime or maximum temperature of use, tests can be performed at several temperatures and the
results can be evaluated by using an Arrhenius plot. This method is described in ISO 11346.
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©
INTERNATIONAL STANDARD ISO ISO 188:1998(E)
Rubber, vulcanized or thermoplastic — Accelerated ageing and
heat resistance tests
WARNING – Persons using this International Standard should be familiar with normal laboratory practice.
This standard does not purport to address all of the safety problems, if any, asssociated with its use. It is
the responsibility of the user to establish appropriate safety and health practices and to ensure compliance
with any national regulatory conditions.
1 Scope
This International Standard specifies accelerated ageing or heat resistance tests on vulcanized or thermoplastic
rubbers. The methods are:
Method A: air-oven method using a cell-type oven or cabinet oven with low air speed and a ventilation of 3 to 10
changes per hour;
Method B: air-oven method using a cabinet oven with forced air circulation by means of a fan and a ventilation of 3
to 10 changes per hour; and
°
Method C: oxygen pressure method at 2,1 MPa and 70 C.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
ISO 37:1994, Rubber, vulcanized or thermoplastic – Determination of tensile stress-strain properties.
Rubber, vulcanized or thermoplastic – Determination of hardness (hardness between 10 IRHD and
ISO 48:1994,
100 IRHD).
ISO 471:1995, Rubber – Temperatures, humidities and times for conditioning and testing.
ISO 11346:1997, Rubber, vulcanized or thermoplastic – Estimation of life-time and maximum temperature of use
from an Arrhenius plot
.
3 Principle
Test pieces are subjected to controlled deterioration by air at an elevated temperature and at atmospheric pressure
(for 3.1 and 3.2) or at an elevated temperature and an elevated oxygen pressure (for 3.3), after which the physical
properties are measured and compared with those of unaged test pieces.
The physical properties concerned in the service application should be used to determine the degree of
deterioration but, in the absence of any indication of these properties, it is recommended that tensile strength,
stress at intermediate elongation, elongation at break (in accordance with ISO 37) and hardness (in accordance
with ISO 48) be measured.
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ISO 188:1998(E)
3.1 Accelerated ageing by heating in air
In this method, the oxygen concentration is low and, if oxidation is rapid, oxygen may not diffuse into the rubber
quickly enough to maintain uniform oxidation. This ageing method is therefore liable to give misleading results with
poor-ageing rubbers when the normal thickness specified in the International Standard appropriate to the test
method is used.
3.2 Heat resistance test
In this method, the test pieces are subjected to the same temperature as they would experience in service and,
after definite periods, appropriate properties are measured and compared with those of the unaged rubber.
3.3 Accelerated ageing by heating in oxygen
In this method, the increased oxygen concentration promotes rapid diffusion and so helps to ensure uniform
oxidation. On the other hand, the artificial promotion of oxidation may overemphasize oxidative changes relative to
those caused by after-vulcanization, e.g. reversion, so that the total effect may not resemble that of natural ageing.
4 Apparatus
4.1 Air oven (for 3.1 and 3.2)
To achieve a good precision when doing ageing and heat resistance tests, it is very important to keep the
temperature uniform and stable during the test and to verify that the oven used is within the temperature limits with
regard to time and space. Increasing the air speed in the oven improves temperature homogeneity.
However, air circulation in the oven and ventilation influence the ageing results. With a low air speed, accumulation
of degradation products and of evaporated ingredients, as well as oxygen depletion, can take place. A high air
speed increases the rate of deterioration, due to increased oxidation and volatilization of plasticizers and
antioxidants.
The oven shall be of such a size that the total volume of the test pieces does not exceed 10 % of the free space in
the oven. Provision shall be made for suspending test pieces so that they are at least 10 mm from each other and,
in cabinet ovens and ovens with forced air circulation, at least 50 mm from the sides of the oven.
The temperature of the oven shall be controlled so that the temperature of the test pieces is kept within the
specified tolerance for the specified ageing temperature (see clause 7) for the whole ageing period. A temperature
sensor shall be placed inside the heating chamber to record the actual ageing temperature.
No copper or copper alloys shall be used in the construction of the heating chamber.
For the ovens specified in 4.1.1 and 4.1.2, provision shall be made for a slow flow of air through the oven of not less
than three and not more than ten air changes per hour. The air speed shall depend on the air change rate only, and
no fans are allowed inside the heating chamber.
Care shall also be taken to ensure that the incoming air is heated to within ±1 °C of the temperature of the oven
before coming in contact with the test pieces.
The ventilation (or air change rate) can be determined by measuring the volume of the oven chamber and the flow
of air through the chamber.
4.1.1 Cell-type oven, consisting of one or more vertical cylindrical cells having a minimum height of 300 mm. The
cells shall be surrounded by a thermostatically controlled good-heat-transfer medium (aluminium block, liquid bath
or saturated vapour). Air passing through one cell shall not enter other cells.
4.1.2 Cabinet oven, comprising a single chamber without separating walls.
4.1.3 Oven with forced air circulation, with an air speed of 0,5 m/s to 1,5 m/s. The air flow through the heating
chamber shall be as uniform and laminar as possible. The test pieces shall be placed with the smallest surface
facing towards the air flow direction to avoid disturbing the air flow.
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ISO 188:1998(E)
The air shall be changed at a rate of not less than three and not more than ten air changes per hour.
The air speed near the test pieces can be measured by means of an anemometer.
4.2 Apparatus for heating in oxygen (for 3.3)
4.2.1 Oxygen pressure chamber, consisting of a vessel, made of stainless steel or another suitable material,
designed to contain an atmosphere of oxygen under pressure, with provision for placing rubber test pieces within it
and subjecting them to a controlled, uniform temperature. The size of the vessel is optional, but shall be such that
the total volume of the test pieces does not exceed 10 % of the free space in the vessel.
No copper or copper alloy parts shall be inside the pressure chamber or in the construction of the tubing leading
from the oxygen reservoir to the pressure chamber.
1)
4.2.2 Thermostat, for controlling the temperature of the heating medium surrounding the pressure vessel so that
the temperature of the test pieces in the pressure chamber is kept at 70 °C ±1 °C.
4.2.3 Thermocouple, or other suitable device, located near the centre of the test pieces to record the actual
ageing temperature.
2)
4.2.4 Reliable safety valve, set at a gauge pressure of 3,5 MPa .
4.2.5 Pressure gauge.
5 Test pieces
It is recommended that the accelerated ageing or heat resistance test be carried out on test pieces prepared and
conditioned as required for the appropriate property tests, and not on complete products or sample sheets, and that
their form be such that no mechanical, chemical or heat treatment will be required after ageing.
Only test pieces of similar dimensions and having approximately the same exposed areas shall be compared with
each other. The number of test pieces shall be in accordance with the International Standard for the appropriate
property tests. The test pieces shall be measured before heating but, whenever possible, marking shall be carried
out after heating as some marking inks can affect the ageing of the rubber.
Care shall be taken to ensure that the markings used to identify the test pieces are not applied in any critical area of
the test piece and are not such as to damage the rubber or to disappear during heating. Care shall also be taken to
ensure that the test pieces have a good smooth finish and are free from blemishes and other flaws.
Avoid simultaneous heating of different types of compound in the same oven, to prevent the migration of sulfur,
antioxidants, peroxides or plasticizers. For this purpose, the use of individual cells is highly recommended. In order,
however, to give some guidance for cases where it is not practicable to provide equipment with individual cells, it is
recommended that only the following types of material be heated together:
a) polymers of the same general type;
b) vulcanizates containing the same type of accelerator and approximately the same ratio of sulfur to
accelerator;
c) rubbers containing the same type of antioxidant;
d) rubbers containing the same type and amount of plasticizer.
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1)
For safety reasons, a
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