BS ISO 9026:2021 pdf download – Raw rubber or unvulcanized compounds — Determination of green strength
6.2 Preparation
6.2.1 General Standard test conditions shall be followed where determination of green strength of raw rubber or unvulcanized compounded rubber is to be made with no reference given to any particular process (see Clauses 7 to 9). Raw rubber shall be homogenized in accordance with ISO 1795.
6.2.2 Preparation of moulded test pieces Raw rubber and unvulcanized rubber shall be sheeted out to approximately 2,2 mm thickness and placed in the mould with the grain direction oriented so as to have the grain direction along the length of the test pieces, care being taken that a suitable film is placed between the mould walls and the rubber compound in order to promote mould release. Polyester or PTFE film 0,25 mm thick has been found suitable. The sample shall be compressed for 5 min at 100 °C under 2,5 MPa platen pressure, then removed after cooling to a chosen standard laboratory temperature under pressure.
NOTE 1 For some raw rubbers, longer times or higher moulding temperatures can be necessary in order to obtain a smooth sheet free from porosity. For some compounds, a lower temperature can be necessary when there is a danger of scorch at the preferred temperature.
NOTE 2 The cooling time depends on the apparatus used. The test piece shall be cut from the sheet using a suitable die. When it is necessary to characterize the behaviour of compounds intended to be used in a given application, the test pieces shall be prepared in such a way that the properties of the rubber are not altered.
6.2.3 Preparation of test pieces from calendered sheet The dumbbell test pieces shall be prepared directly by die-cutting the test pieces from a sheet calendered from 2 mm to 4 mm thickness.
6.3 Number of test pieces The test shall be carried out on at least five test pieces.
6.4 Measurement Thickness shall be measured, using a micrometer gauge, in accordance with ISO 23529:2016, method A, with a pressure of 10 kPa ± 2 kPa on the rubber. The result shall be taken as the median of three measured values. The width shall be assumed to be equal to the width between the cutting edges of the central part of the die.
7 Conditioning After suitable preparation, the test pieces shall be conditioned at the chosen standard laboratory temperature (see ISO 23529) for a fixed conditioning period between 24 h and 72 h. The same conditioning period shall be used throughout the test and throughout a series of tests intended to be comparable.
8 Temperature of test The test shall normally be carried out at the chosen standard laboratory temperature (see ISO 23529). Where other temperatures are used, take the preferred test temperatures as given in ISO 23529. The same temperature shall be used throughout the test and throughout a series of tests intended to be comparable.
9 Procedure After removal of the mould-release film, when applicable (see 6.2.2), insert test pieces with beaded ends in the fixture illustrated in Figure 3. Adjust the rate of displacement of the moving jaw to 100 mm/min and start the tensile test. If the test piece breaks at the grips, that result shall be discarded and a retest carried out. NOTE The preferred rate of separation of the jaws is 100 mm/min. In special cases, other rates can be used, but only tests carried out at the same rate can be compared.
10 Expression of results
Using the typical stress-strain curves given in Figure 1, determine the yield stress or maximum stress in megapascals. Other parameters can be determined, such as yield elongation (ε 1 ), or the stress at a definite reference elongation corresponding to the deformation entailed by a subsequent processing operation.
The stresses are calculated from the initial cross-sectional area of the parallel-sided portion of the dumbbell. The stresses and elongation shall be calculated using the procedures and equations given in ISO 37.BS ISO 9026 pdf download.