ASTM-D6746:15(R2020) pdf free download – Standard Test Method for Determination of Green Strength and Stress Relaxation of Raw Rubber or Unvulcanized Compounds.
9. Test Specimen
9.1 The preferred test specimen is the dumbbell Die C specimen in accordance with Test Methods D412. Other types of test specimens may also be used. Since surface conditions can affect the results, there may be cases where a wider specimen would he preferred, such as Die A from Test Methods D412.
9.2 Since diflérent types of test specimens do not necessariiy give the same values, comparison of results from different specimen types should be avoided.
10. Preparation
10.1 Raw rubber and unvulcanized compounds shall be sheeted out to approximately 2.5-mm thickness and placed in the mold with the grain direction oriented so as to have the grain direction parallel with the length of the dumbbells. It is also permissible to test against the grain, hut this must he noted in the report section. (See Section 16.) Only fresh stock shall he used.
10.2 Polyester film (0.05-mm thickness) shall be placed between the mold walls and the rubber in order to promote mold release. The sample shall he compressed for 5 ± 1 mm at 1(X) ± 2°C under a minimum of 2.5 MPa platen pressure, allowed to cool to laboratory temperature for 30 ± 2 mm under adequate pressure to keep test specimen flat, then removed. The cooling shall be done out of the press using room temperature metal plates.
10.3 For Some raw rubbers, longer times or higher temperatures may he necessary in order to obtain a smooth sheet free from porosity. For some compounds, a lower temperature may be necessary when there is a danger of scorch at the preferred temperature.
10.4 A sample may also he acquired during processing by sheeting off a mill, calender, or by extrusion and then cut with a die. Special care should be taken to ensure that the sample is free of trapped air.
10.5 Cut five test specimens from the sheet, using a suitable die. (See 9.1.)
11. Calibration and Standardization
11.1 The tensile testing machine shall he calibrated according to the procedure in Test Methods D4l2.
12. Conditioning
12.1 The test specimens shall be conditioned at a temperature of 23 ± 2°C for at least 16 h and not more than 24 h before testing.
12.2 Some slow crystallizing polymers. such as polychloroprene, may need a longer conditioning period in order to accurately measure the maximum green strength.
13. Part A Test Procedure
13.1 Place the test specimen in the grips of the testing machine in accordance with the procedure in Test Methods 1)412. Adjust the rate of displacement of the moving jaw to 100
± 10 mm/min and start the tensile test. If the sample breaks at the grips, that result shall be discarded and a retest carried out.
13.1.1 The preferred rate of separation of the jaws is 100 ± 10 mm/min. In special cases, other rates may be used, but only tests carried out at the same rate can he compared. Rollover or self-tightening grips may help prevent slippage.
13.2 The test shall be carried out on three test specimens. If the standard deviation is greater than 20 % of the mean value, then five specimens should be tested.
13.3 Unless otherwise specified, the standard test temperature shall be 23 ± 2°C.
13.4 When it is necessary to test at other temperatures. use one of the standard temperatures listed in Practice D 1349. Specimens shall he conditioned at these test temperatures for 15 ± 2 mm prior to testing.