ASTM D6746 Determination of Green Strength and Stress Relaxation of Raw Rubber 

Scope:

The ASTM D6746 test method determines the stress-strain properties of rubber compounds. These properties are named green strength and are very important in the performance evaluation of rubber compounds. Green strength is characterized by the physical and chemical properties of polymers. The weight, crystallizing ability, degree of branching, the compound formulation such as filler and plasticizer content are some of those properties which determine the processing behavior of rubber compounds. 

Procedure:

In ASTM D6746, the tensile stress-strain properties are measured for the raw or unvulcanized rubber by using a tensile testing machine in which the moving jaws are maintained at a specific distance from each other—this way, the composition and elongation in the rubber compounds are detected. For the second method, a dumbbell or other specimen is tested through a tensile machine which reports the tensile stress after the specified time.

Video 01: Stress Relaxation Test Systems for the Determination of Rubber Properties

Specimen Size:

The specimen size should be taken as per the ASTM D6746 test requirements. Raw rubber or unvulcanized compounds should be 2.5 mm thick, whereas the polyester film should be 0.05 mm thick.

Data:

Calculate the tensile green strength (yield stress) as follows:

Y (stress) = F(y)/A 

Where: 

Y (stress) = yield stress, that stress level where the yield point occurs, MPa

F(y) = magnitude of force at the yield point, MN

A = cross-sectional area of the unstrained specimen, m2

Conclusion:The ASTM D6746 test method determines the properties of rubber compounds by measuring the green strength. This strength plays an important role in the performance evaluation of raw rubber and unvulcanized rubber compounds.