BS ISO 1432:2021 pdf download – Rubber, vulcanized or thermoplastic — Determination of low-temperature stiffening (Gehman test)
7.2 Conditioning of test piece
7.2.1 The minimum time between forming and testing shall be 16 h. For non-product tests, the maximum time between forming and testing shall be 4 weeks and, for evaluations intended to be comparable, the tests should be carried out, as far as possible, after the same time interval. For product tests, whenever possible, the time between forming and testing should not exceed 3 months. In other cases, tests shall be made within 2 months of the date of receipt by the customer of the product.
7.2.2 Samples and test pieces shall be protected from light as much as possible during the interval between forming and testing.
7.2.3 Prepared test pieces shall be conditioned in accordance with ISO 23529 immediately before testing for a minimum of 3 h at a standard laboratory temperature, the same temperature being used throughout any one test or series of tests intended to be comparable.
8.1 Mounting of test piece Clamp each test piece used in such a manner that 25 mm ± 3 mm of the test piece is free between the clamps. The test piece clamp shall be located in such a position that the specimen is under zero torque or with a slight pre-load to keep the test piece straight when the temperature changes. If the absolute modulus is required, measure the test length of the test pieces to the nearest 0,5 mm, the width to the nearest 0,1 mm and the thickness to the nearest 0,01 mm.
8.2 Stiffness measurements in liquid media Make the reference measurement with the standard torsion wire (2,8 mN·m) at 23 °C ± 2 °C, either in air or in the liquid bath. Place the rack containing the test pieces in the liquid bath with a minimum of 25 mm of liquid covering the test pieces. Adjust the angle measuring device to zero.
Then turn all test pieces at the same time, or in sequence, quickly but smoothly, through 180° and record the torsion angle after 10 s. If the test pieces are measured in sequence, ensure that all the measurements are completed within approximately 2 min. If the reading at 23 °C does not fall in the range of 120° to 170°, the standard torsion wire is not suitable for testing the test piece.
Test pieces producing twists of more than 170° shall be tested with a wire having a torsional constant of 0,7 mN·m. Test pieces producing twists of less than 120° shall be tested with a wire having a torsional constant of 11,2 mN·m. Remove the test pieces from the liquid bath (assuming the reference measurement has been made in liquid) and adjust the temperature of the liquid to the lowest temperature desired. Place the test pieces in the bath with a minimum of 25 mm of liquid covering the test pieces and maintain them at this temperature for approximately 15 min.
Then increase the bath temperature by one of the two following methods:
a) stepwise by 5 °C intervals, each increase being made after approximately 5 min;
b) continuously with a heating rate of about 1 °C/min.
Make the stiffness measurements in the stepwise case after conditioning the test piece for 5 min at each temperature and in the continuous case at intervals of 1 min maximum. Continue the tests until a temperature is reached at which the angular twist is within 5° to 10° of the twist at 23 °C.
Record the angle of twist and the temperature at each measurement.
The relative modulus may be plotted on a graph during the test.
8.3 Stiffness measurements in gaseous media
Procedures with air, carbon dioxide or nitrogen differ from those with liquid media only in that the cooling is done with the test pieces in the medium and the length of the conditioning period is different.
With the test pieces in the test chamber, adjust the temperature of the chamber to the lowest temperature desired in approximately 30 min. After this temperature has been maintained constant for 10 min, make the measurements in a similar way to those in the liquid media, ensuring that all the test pieces in the rack are tested within 2 min.
Then increase the chamber temperature by one of the two following methods:
a) stepwise by 5 °C intervals, each increase being made after approximately 10 min;
b) continuously with a heating rate of about 1 °C/min.BS ISO 1432 pdf download.