The kneading process in extrusion involves plasticizing and melting polymers, adding dissimilar polymers, fillers, and additives, and creating a uniform mixture. This process includes two distinct mixing modes: dispersive mixing and distributive mixing.

• Dispersive Mixing: Breaks down fillers and polymer gels into finer particles.
• Distributive Mixing: Homogenizes the composition by thoroughly mixing the materials.

A balance between these two actions is crucial for achieving a uniform mixture of dissimilar polymers and fillers.

　

Dispersive and Distributive Mixing

Dispersive Mixing

Dispersive mixing applies shear and elongational forces to break materials into finer particles, reducing their size. Strong dispersive forces are necessary for breaking up highly cohesive fillers or substances prone to agglomeration.

This process, known as dispersive blending, ensures the uniform integration of immiscible liquids or solid particles into the polymer matrix. In this context, stress generated within the system plays a more critical role than strain, as controlling the flow is vital for efficiently distributing solid agglomerates or liquid droplets.

Key applications include polymer blends and compounds where tasks like droplet fragmentation, filler dispersion, and fiber disentanglement are essential. Dispersive blending helps achieve fine dispersion, improving the homogeneity and performance of the final product.

Distributive Mixing

Distributive mixing focuses on stirring the mixture to reduce variations in composition and physical properties. This action involves elongation, splitting, and rearranging, resulting in complex material flow. Distributive mixing is driven by the repeated splitting and merging of streams, which facilitates the exchange of material positions and enhances uniformity.

In distributive blending, one component is dispersed as droplets or layers within another. When strain is applied, the interfacial area between components expands, and droplets or layers become smaller and thinner, leading to a more uniform mix.

This type of mixing is particularly important for fillers like glass fibers and carbon fibers that retain their shape while imparting functional properties. Effective distributive mixing ensures these fillers are evenly distributed throughout the material, maximizing their effectiveness.

Shear and Elongational Flow

Shear and elongational flow are key concepts that describe how fluids and materials deform under force. They are especially relevant when dealing with non-Newtonian fluids like viscous polymers. Material flow is broadly categorized into two flow types: shear flow and elongational flow.

Shear Flow

Shear flow occurs when different layers of a material move parallel to one another at varying speeds, creating shear stress. This deformation resembles a sliding motion and is characterized by a velocity gradient across the layers.

Elongational Flow

Elongational flow arises when a material is stretched, either in one direction or multiple directions, causing a change in volume. Unlike shear flow, deformation is concentrated along the stretching direction, resulting in a uniform velocity gradient.

Simple Shear and Simple Extension

In extrusion, both simple shear flow and extension play important roles. Shear flow is generally dominant in twin screw extruders, while elongational flow is crucial for effective dispersive mixing. Elongational flow can occur in various forms, including uniaxial, planar, and biaxial extension. Each flow type affects dispersion efficiency differently.

Flow Conditions in the Clearance

Material flow within the extruder includes both shear and elongational components. The Tip clearance —the narrow gaps between the screws and the barrel or between the screws themselves—are critical zones.

As materials pass through these clearance regions, they experience high shear stresses, promoting dispersive mixing. Additionally, elongational flow plays a vital role in achieving fine dispersion. Complex-shaped elements may be used to generate intricate flow patterns, enhancing distributive mixing and promoting compositional uniformity.

By optimizing shear and elongational flow in these critical regions, the extrusion process achieves efficient mixing and high-quality final products.

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