Understanding The Moving Die Rheometer Cure Curve

Rahco Rubber uses a Moving Die Rheometer (MDR) to evaluate the curing properties of elastomers and give insights into the viscoelastic components of the material. When it comes to standardization and precision, ASTM D5289, the “Standard Test Method for Rubber Property—Vulcanization Using Rotorless Cure Meters,” defines the framework for MDR testing, ensuring consistency and reliability across the rubber industry. This standard outlines the procedures for measuring cure characteristics, making it an indispensable instrument for quality control.

During the test, an uncured sample is pressurized and heated in the sealed test chamber while being sheared by oscillation of the lower die. As the test progresses, rubber molecules within the sample form chemical bonds called cross links, which bind the rubber molecules together. The cross-linked rubber molecules develop a net-like structure that reinforces the sample and increases its stiffness, also known as curing on vulcanization.

This increase in stiffness is measured by the instruments torque transducer which determines the materials torque response, or how much resistance it has to being sheared by the oscillating dye.

The main line on an MDR tests graph shows the amount of torque response a material has at the peak strain amplitude of the lower dies oscillation, which is plotted against time. This is called the “S Prime” curve.

Since the material stiffens as it cures, the line will curve upwards until the material is fully cured. The MDR also records the torque value at zero strain amplitude which is called “S Double Prime,” indicating the viscous response of the material.

The values for S Prime and S Double Prime can be used to calculate what is called the Tan Delta value, which expresses the ratio of viscosity — which is flow or fluid like movement to elasticity — which is spring or solid like movement.

The material’s Tan Delta value can be used to make predictions about molecular properties, processability, and cure performance characteristics.

By examining the data from the test results, it can be determined when a material begins curing or its scorch time, as well as how long it takes the material to fully cure at the temperature used for the test or its cure time. Predictions about the materials cure durometer hardness can be made from S Prime Max and can be useful in determining appropriate filler loading.

TC times indicate the rate of cross-linked formation at various stages of the reaction, and can be useful in determining appropriate quantities of chemicals used to speed slope or otherwise control the curing process.

These data points can be used in quality management to ensure compounds are within the limits of production specifications. Gates can be set up in moong controls test parameters to automatically determine if a material has passed or failed meeting of thresholds required for production.

Furthermore, statistics from multiple tests can be examined at once to look at uniformity and data distribution among different batches of material.