Mooney viscosity is an important indicator for measuring the rheological properties of rubber materials and holds significant importance in the rubber industry. Precipitated silica generally causes an increase in the Mooney viscosity of rubber.
1. Significance of Mooney Viscosity
(1) Reflects Rubber Molecular Weight and Distribution:
Mooney viscosity is roughly positively correlated with rubber molecular weight; higher viscosity usually signifies longer molecular chains. Simultaneously, molecular weight distribution also affects Mooney viscosity. A narrower molecular weight distribution results in relatively stable Mooney viscosity, while a broader distribution may cause an increase in viscosity due to enhanced entanglement from long-chain molecules, or a decrease due to the plasticizing effect of short-chain molecules. Mooney viscosity allows for a rough inference of the molecular weight and distribution of the compound, thereby providing insight into its physical properties.
(2) Evaluates Rubber Processing Performance:
Mooney viscosity reflects the fluidity of rubber. A high viscosity value indicates thicker, more viscous rubber with poor fluidity, making it difficult to mix uniformly and process via extrusion. A low viscosity value indicates good fluidity, facilitating processing, but may lead to sticking to the mill rolls, and the cured product might exhibit lower tensile strength. Furthermore, Mooney viscosity is also related to processing safety. Compounds with high Mooney viscosity require greater shear force and energy input during processing, potentially causing excessive temperature rise and increasing the risk of scorch.
(3)Determines Curing Process Parameters:
The Mooney viscosity vs. time curve reflects the curing process characteristics of the compound. Parameters such as Mooney scorch time and Mooney cure time can be derived from the curve, helping to determine the optimal combination of curing temperature and time, avoiding under-cure or over-cure, and ensuring the quality of rubber products.
2.Effect of Precipitated Silica on Rubber Mooney Viscosity
(1) Increases Mooney Viscosity:
Precipitated silica has a large specific surface area. Its surface can form a physical adsorption network with rubber molecular chains, restricting their movement. Simultaneously, silica readily forms strong filler networks. Both factors increase intermolecular friction, thereby raising Mooney viscosity. As the amount of precipitated silica in the compound increases, its Mooney viscosity usually gradually increases.
(2)Affects Mooney Viscosity Stability:
If precipitated silica is not uniformly dispersed in the rubber and forms agglomerates, the effective interaction area between the filler and rubber molecules is reduced. This may lead to a decrease in Mooney viscosity and worsen the stability of Mooney viscosity, adversely affecting rubber processing performance and product quality.
3.What is the Specific Effect of Precipitated Silica on Mooney Viscosity?
The specific effect of precipitated silica on Mooney viscosity is primarily an increase. This is related to the structure and characteristics of silica, as detailed below:
(1)Large Specific Surface Area Forms Adsorption Network:
The specific surface area of precipitated silica is typically between 100-250 m²/g. This large specific surface area results in high surface energy, enabling the formation of a physical adsorption network with rubber molecular chains. This network restricts the movement of rubber chains, increases intermolecular friction, and consequently causes Mooney viscosity to rise.
(2)Effect of Surface Hydroxyl Groups:
Its surface contains a large number of hydroxyl groups. These hydroxyl groups facilitate hydrogen bonding between silica particles, leading to agglomeration. Agglomerated silica is difficult to disperse uniformly in rubber and forms strong filler networks, increasing the Mooney viscosity of the rubber system.
(2) Acidic Nature Affects Curing Rate:
Precipitated silica is generally weakly acidic, with a pH value around 5.0-7.5. This acidity can affect the activity of accelerators in the rubber curing system, leading to a slower curing rate. Changes in the curing rate affect the degree and progress of cross-linking of rubber molecular chains, indirectly impacting Mooney viscosity. It typically causes Mooney viscosity to increase somewhat and may also lead to poorer stability of Mooney viscosity. Additionally, if precipitated silica is not uniformly dispersed in the rubber and forms large agglomerates, the effective interaction area between the filler and rubber molecules is reduced. This might instead lower Mooney viscosity, but it will simultaneously severely affect other properties of the rubber.
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Post time: Jul-09-2025