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What is the flow properties of rubber?


At normal temperature, the rubber seal is in a highly e […]

At normal temperature, the rubber seal is in a highly elastic state. When the temperature drops below its crystallization temperature, the rubber enters the crystalline state. When the temperature rises to a certain critical point (different with different types of rubber), it changes from a high elastic state to a viscous flow state and becomes a viscous viscous body that can flow. At this time, under the action of external force, the rubber exhibits a fluid state to obtain fluidity. It is the fluidity that gives rubber a series of processing characteristics required for processing to meet the needs of processing such as calendering, extrusion, and injection molding.
The foundation of rubber fluidity is the characteristics of non-Newtonian fluids. Non-Newtonian fluids are different from Newtonian fluids of low molecular materials. It is subject to "Newton's flow law". Such fluids are of low viscosity, and the viscosity changes with temperature. Non-Newtonian fluids do not obey the "Newtonian flow law", and there is internal friction between the layers, which creates resistance to the flow of materials.
There are usually two ways to reduce this resistance.
First, apply shear force to the material;
Second, increase the processing temperature. This is exactly the effective method often adopted in rubber processing.
Factors affecting the fluidity of rubber usually include:
1. Rubber molecular structure Where the molecular chain is flexible and the glass transition temperature is low, the fluidity is generally better.
2. The molecular weight cannot be too high. If the average molecular weight is greater than 1 million, it will lead to increased molecular chain entanglement, which is not conducive to flow. Since natural rubber contains low-molecular components, it is advantageous for fluidity. Increasing the plasticizing time is also an effective measure to improve the fluidity.
3. The increase in processing temperature will inevitably lead to a decrease in viscosity, which is beneficial to the fluidity of the compound.
4. Elastic memory effect Once the rubber is removed from the stressed state during processing, there will be a strong tendency for elastic recovery to cause changes in shape and size. This recovery process should continue until equilibrium, which is also the characteristic of rubber fluidity. From the processing point of view, this is not conducive to the stability of shape and size, which needs to be solved through formula adjustment and plastic processing.