ISO 3384-2:2019 ゴム、加硫または熱可塑性プラスチック—圧縮における応力緩和の決定—パート2：温度サイクルによるテスト | ページ 2

Foreword

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This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products, Subcommittee SC 2, Testing and analysis.

This second edition cancels and replaces the first edition (ISO 3384-2:2012), which has been technically revised.

The main changes compared to the previous edition are as follows:

• the requirement for compression device (5.1) has been harmonized with other International Standards;
• other changes have been made to keep the consistency with ISO 3384-1 throughout the document.

A list of all parts in the ISO 3384 series can be found on the ISO website.

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 takes place with time; this behaviour is called “creep”.

Tests in compression are normally made under continuous stress conditions (i.e. the test piece remains strained throughout the test), and are hence a measure of sealing force. Note that the terms continuous and discontinuous used in this standard refer to whether the measure of force is made continuously of at intervals.

Tests to use stress relaxation in tension as a measure of ageing are given in ISO 6914.

The processes 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.

If the life-time of a material is to be investigated, it can be determined using the method described in ISO 11346.

In addition to the need to specify the temperatures and time intervals in a stress relaxation test, it is necessary to specify the initial stress and the previous mechanical history of the test piece since these can also influence the measured stress relaxation, particularly in rubbers containing fillers.

The most important factor in achieving good repeatability and reproducibility when making stress relaxation tests is to keep the compression constant during all measurements.

The two cycling test methods specified are designed to carry out the following:

• age the test piece by stress relaxation and determine the sealing force at low temperatures (method A);
• introduce thermal stress by stress relaxation and determine the sealing force at low temperatures (method B).

For products used in outdoor applications where the temperature can cycle between a low temperature (e.g. −40 °C) and a high temperature (e.g. 150 °C), it is important to also consider the shrinking of the rubber at low temperatures when assessing performance in the anticipated application and life-time.

For polymers that crystallize at low temperature, the crystallization will add to the shrinking of the rubber. For example, for hoses and seals in automotive applications, the product might work satisfactorily at the normal working temperature, but might leak at a low temperature.

1 Scope

This document specifies two methods for determining the decrease in counterforce exerted by a test piece of vulcanized or thermoplastic rubber which has been compressed to a constant deformation and then undergoes temperature cycling.

Method A: The temperature is cycled at intervals between a high temperature for ageing and a low temperature for checking the sealing force at this low temperature.

Method B: The temperature is cycled continuously between a high temperature and a low temperature to introduce thermal stress in the test piece.

The counterforce is determined by means of a continuous-measurement system.

Two forms of test pieces are specified in this document: cylindrical test pieces and rings. Comparison of results is valid only when made on test pieces of similar size and shape.

The use of ring test pieces is particularly suitable for the determination of stress relaxation in liquid environments.

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.

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

Bibliography