① If the Newtonian fluid flows under the action of shear force at a certain temperature, its flow rate increases proportionally with the increase of the external force, but its viscosity remains unchanged. This fluid Called Newtonian fluid, water, organic solvents, solvent-based varnishes, and low-viscosity paints (such as nitrocellulose paint) used in the coatings industry are all sub-Newtonian fluids.
② Non-Newtonian fluid If the viscosity of the fluid changes when it flows under the action of shear force, it is called non-Newtonian fluid. Concrete can be divided into several different situations. If it flows under the action of an external force and as the external force (shear force) increases, its viscosity decreases, which is called a pseudoplastic fluid, and if the viscosity increases, it is called a dilatant fluid. When the shear force is applied, it does not flow, and only when the applied shear force exceeds a certain value (this value is called the yield point) does it start to flow, and the viscosity decreases with the increase of the shear force, which is called plasticity fluid. Most of the paints used in the paint industry are non-Newtonian fluids, many of which are pseudoplastic fluids, and their viscosity can be reduced by shearing and agitation, so this is often reduced by mechanical methods in the spraying process. Pseudoplastic paints have high viscosity during storage and are not easy to stratify or settle. When spraying, the viscosity becomes smaller due to the shear force, which is conducive to atomization and leveling. When forming a coating, Due to the removal of the shearing force and the rapid increase of the viscosity, the sagging of the coating film is inhibited, which is very convenient for spraying construction. However, latex paint and water-based paint are dilatant fluids, and the more they are stirred, the higher their viscosity will be. Its viscosity increases, which is not conducive to construction. Try to avoid stirring it, and try not to use pumps, otherwise, the more you pump, the higher the viscosity, and sometimes it will even solidify and block the pipeline and nozzle. Since the Spray Gun is also a source of shearing force, in order to prevent changes in the viscosity of the material, the number of triggering the gun should be minimized. In the paint delivery process, the pressure vessel is used instead of the booster gauge to connect the Spray Gun. The pipeline for conveying the paint should be as large as possible in diameter and short in length, and pipe fittings made of stainless collars should be used when necessary. These measures are all in order to avoid changes in the viscosity of the coating caused by the shear force.
