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Non-Newtonian Fluid: The Wonderful World of Polymer Melts
2024-09-14
In daily life, many substances we come into contact with, such as water and oil, follow the law of Newtonian fluids, that is, shear stress is proportional to shear rate. However, there is a special type of fluid whose flow behavior does not follow this simple law, which is non-Newtonian fluid. As a type of non-Newtonian fluid, polymer melts used in plastic processing have attracted much attention due to their unique physical properties and wide range of applications.
What is a non-Newtonian fluid?
Non-Newtonian fluids, as the name implies, are fluids that do not follow Newton's law of viscosity. Unlike Newtonian fluids, the viscosity of non-Newtonian fluids is not a constant, but changes with the shear rate. When this fluid is subjected to shear force, its internal molecular structure changes, causing the viscosity of the fluid to change.
Characteristics of non-Newtonian fluids
Shear-thinning: When a polymer melt is subjected to shear stress, its viscosity decreases as the shear rate increases. For example, when some polymer melts are injection molded, the viscosity of the melt can be reduced by increasing the injection speed, making it easier to fill the mold.
Shear-thickening: Although most polymer melts exhibit shear thinning, some polymer melts increase in viscosity when the shear stress increases, that is, shear thickening. This property may be advantageous in certain specific applications.
Viscoelasticity: Polymer melts are not only viscous, but also elastic. They can generate resilience, and if they are stretched and released, they will rebound to their original shape. This viscoelasticity gives polymer melts special flow behavior during processing
Properties of polymer melts
Polymer melt is a fluid formed by high molecular polymer in a molten state. Due to the long chain structure and entanglement of polymer chains, when polymer melt is subjected to shear force, the molecular chains will orient and slip, resulting in changes in viscosity. This unique rheological property gives polymer melt many unique advantages during processing.
The viscosity of polymer melts usually decreases with increasing shear rate, a phenomenon known as "shear thinning". This makes polymer melts easier to flow and process under high-speed shear. At the same time, polymer melts also have good elasticity and ductility, which allows them to better adapt to the shape of the mold during the molding process, thereby producing products with complex shapes and fine structures.
Application of polymer melts
Polymer melts have a wide range of applications in industrial production. In the production of plastic products, polymer melts are widely used in molding processes such as injection molding, extrusion, and blow molding. In addition, polymer melts also play an important role in rubber, coatings, adhesives and other fields.
Take the biaxially oriented film production line as an example. The polymer melt is heated to a molten state in the extruder and then injected into the mold through the melt pipe. Due to the shear thinning characteristics of the polymer melt, it can maintain a low viscosity during the high-speed shearing process of the screw, making it easier to fill every corner of the mold, and obtain a good film product after casting and biaxial stretching.
Conclusion
As a typical non-Newtonian fluid, the unique rheological properties of polymer melts have brought many conveniences to industrial production. By deeply understanding the physical properties and processing characteristics of polymer melts, we can make better use of this material and develop more innovative and practical products. With the continuous development of science and technology, the research on polymer melts and non-Newtonian fluids will continue to deepen, bringing more possibilities for human production and life.
