ASTM D1646-19

5.1 Viscosity—Viscosity values determined by this test method depend on molecular structure, molecular weight, and non-rubber constituents that may be present. Since rubber behaves as a non-Newtonian fluid, no simple relationship exists between the molecular weight and the viscosity. Therefore, caution must be exercised in interpreting viscosity values of rubber, particularly in cases where molecular weight is very high. For example, as the molecular weight increases, the viscosity values for IIR polymers (butyl rubbers) reach an upper limit of about 80, at 100°C (212°F) using a large rotor at a rotation speed of 2 r/min, and may then decrease to considerably lower values. For these higher molecular weight rubbers, better correlation between viscosity values and molecular weight is obtained if the test temperature is increased.

5.2 Stress Relaxation—The stress relaxation behavior of rubber is a combination of both an elastic and a viscous response. Viscosity and stress relaxation behavior do not depend on such factors as molecular weight and non-rubber constituents in the same way. Thus both of these tests are important and complement each other. A slow rate of relaxation indicates a higher elastic component in the overall response, while a rapid rate of relaxation indicates a higher viscous component. The rate of stress relaxation has been found to correlate with rubber structure characteristics such as molecular weight distribution, chain branching, and gel content.

5.3 Pre-Vulcanization Characteristics—The onset of vulcanization can be detected with the Mooney viscometer as evidenced by an increase in viscosity. Therefore, this test method can be used to measure incipient cure (scorch) time and the rate of cure during very early stages of vulcanization. This test method cannot be used to study complete vulcanization because the continuous rotation of the disk will result in slippage when the specimen reaches a stiff consistency.

1.1 These test methods cover procedures for measuring a property called Mooney viscosity. Mooney viscosity is defined as the shearing torque resisting rotation of a cylindrical metal disk (or rotor) embedded in rubber within a cylindrical cavity. The dimensions of the shearing disk viscometer, test temperatures, and procedures for determining Mooney viscosity are defined in these test methods.

1.2 When disk rotation is abruptly stopped, the torque or stress on the rotor decreases at some rate depending on the rubber being tested and the temperature of the test. This is called “stress relaxation” and these test methods describe a test method for measuring this relaxation.

1.3 When compounded rubber is placed in the Mooney viscometer at a temperature at which vulcanization may occur, the vulcanization reaction produces an increase in torque. These test methods include procedures for measuring the initial rate of rubber vulcanization.

1.4 ISO 289 Parts 1 and 2 also describes the determination of Mooney viscosity and pre-vulcanization characteristics. In addition to a few insignificant differences there are major technical differences between ISO 289 and this test method in that ISO 289 does not provide for sample preparation on a mill, while this test method allows milling sample preparation in some cases prior to running a Mooney viscosity test. This can result in different viscosity values for some rubbers.

1.5 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.