1, Problem & Phenomenon
Mooney viscosity test results were qualified, but obvious differences in roll sticking occurred between batches. For the same cord fabric compound using different batches of natural rubber, roll sticking behavior varied significantly, resulting in mass rejection of semi-finished products, rework, repair, or even scrap.
2, Cause of Roll Sticking
Higher roll speed increases the shear rate, leading to more heat accumulation at the bonding area of the upper and lower rubber sheets. This intensifies thermal motion of molecular chains, macromolecular entanglement and segmental movement, causing viscous flow of the compound.Adhesion between the upper and lower sheets decreases, making it difficult to resist deformation toward the roll surface. After exiting the nip between Roll #2 and Roll #3, the rubber sheet expands toward the roll surface and is affected by the relatively low temperature on the roll side. As a result, the storage modulus of the rubber sheet near the roll increases, requiring greater force to detach from the roll surface.
3, Test Comparison
In conventional Mooney viscosity testing, Batch #A was only 2.59% higher than Batch #B, which was within the control range, so their calendering performance was normally considered equivalent.However, complex dynamic viscosity testing by RPA revealed clear differences in dynamic flow behavior between Batch #A and Batch #B, leading to inconsistent roll sticking and calendering quality.
| Compound Batch # | Process Characteristics (m/min) | Mooney Viscosity (MU) | RPA Test – Dynamic Viscosity | ||
|---|---|---|---|---|---|
| <22 | 22–28 | >28 | |||
| A | Normal | Roll sticking | Roll sticking | 77.20 | 2.79×10⁵ |
| B | Normal | Normal | Normal | 79.20 | 3.25×10⁵ |
| C | Normal | Normal | Normal | – | – |
| D | Normal | Occasional roll sticking | Roll sticking | – | – |
Conclusion
Higher complex dynamic viscosity (η) of the rubber compound indicates greater flow resistance and typically higher elastic characteristics. This parameter is far more sensitive than Mooney viscosity as specified in ASTM D1646, and can more directly reveal subtle differences among rubber compounds.