Any coating that uses water as a solvent or as a dispersion medium can be referred to as an aqueous coating. Category based adhesive coating, the aqueous coating is divided into two categories: natural materials or minerals (e.g., potassium silicate) aqueous coating natural and synthetic resins (e.g. acrylic resin) petrochemical aqueous coating material. Only the synthetic resin-based water-based paints are described here.
Water-based coatings include water-soluble coatings, water-reducible coatings, and water-dispersible coatings (latex coatings ) . The water-soluble paint is a water-soluble resin as a film-forming material, and is represented by polyvinyl alcohol and various modified products thereof, in addition to water-soluble alkyd resin , water-soluble epoxy resin , and inorganic polymer water-based resin. The water-thinning paint refers to a paint prepared by forming a post-emulsified emulsion as a film-forming material, so that the solvent-based resin is dissolved in an organic solvent, and then the resin is dispersed in water to form an emulsion by strong mechanical stirring with the aid of an emulsifier, which is called after Emulsified emulsion, the finished coating can be diluted with water during construction. The water-dispersible coating mainly refers to a coating prepared by using a synthetic resin emulsion as a film-forming material.
The viscosity and solid content of waterborne coatings are related to each other, but there is no absolute difference between them. Water-based coatings with high solid content do not necessarily have high viscosity, and coatings with high viscosity are not necessarily high in solid content. The viscosity of the water-based paint refers to the relative viscosity or the consistency, and the solid content of the water-based paint refers to the content of the active ingredient in the paint . In actual production, the viscosity of waterborne coatings is affected by the following materials.
1. A coating with a high molecular weight and a high resin content may have a high viscosity;
Second, the coating with high pigment content has high viscosity, such as the viscosity of water-based putty is very high;
Third, the coating with a large amount of thixotropic additives, the relative viscosity is higher, the thixotropic additive is also a thickener, and the viscosity of the large coating is also high;
The coating is generally viscous liquid or powder. It can be applied to the surface of the object by different construction techniques. After drying, it can adhere to the solid film with firm strength, continuous strength and continuous reading, and protect the smear. Other expected effects. Since the coating must pass through the fluid phase during the coating process, rheological properties are an important property of the coating. Rheological properties include not only viscosity but also elasticity and plasticity. When only the fluid flow law is discussed, the rheological properties can be approximated by viscosity.
The main rheological parameters most actual production coating viscosity, such as during storage, the system has a desired higher viscosity, prevent the precipitation of pigments and fillers; when the system requires the construction of a low viscosity, leveling conducive film, However, it is required that the viscosity of the coating film reaches a higher viscosity for a certain period of time to avoid sag and runny of the coating film .
Rheological properties and viscosity of coatings
The coating is generally a viscous liquid or a powdery substance, which can be applied to the surface of the object by different construction processes. After drying, it can form a solid film with strong adhesion, a certain strength and continuous, and protect and beautify the coated object. And other expected effects. Since the coating must pass through the fluid phase during the coating process, rheological properties are an important property of the coating. Rheology is the science of studying the laws of material deformation and flow . Rheology is actually a broader term that includes not only viscosity but also viscoelasticity and plasticity. When only the fluid flow law is discussed , the viscosity can be approximated to understand rheology. According to the general category, the fluid can be divided into Newtonian and non-Newtonian. Non-Newtonian fluids are further divided into shear rate dependent and time dependent types. It refers to the shear rate-dependence of the flow behavior of the fluid change with shear rate changes, including pseudoplastic, dilatant and plastic. Time dependent type refers to the flow characteristics of fluids changing with time at a certain shear rate, including thixotropic and seismic types. The actual coatings are all non-Newtonian. Paints are difficult to be clearly divided into shear rate dependent and time dependent types, but the degree of bias is discriminative. The flow behavior of the shear rate dependent and time dependent fluids is shown in Figures 1 and 2 .
One of the main rheological parameters of the paint zui in actual production is viscosity. In paint production, storage, construction and forming process, the force received can be divided into pure shear, tensile shear and simple shear. The coating focuses on simple shearing. When the coating is subjected to simple shearing and unidirectional laminar flow, there is a speed difference between the layers. If the shear stress is τ and the shear rate is D , the viscosity η =τ /D is called the power. viscosity, in units of Pa.s, a common unit of mPa.s. There are not many studies on pure shear and tensile shear. The viscosity required for the process should be achieved under all shear conditions . As during storage, the system has a desired higher viscosity, prevent the precipitation of pigments and fillers; at the start of construction requires lower the viscosity, facilitate leveling the coating, but requires a certain time to reach the viscosity of the coating film higher viscosity, In order to avoid sag and runny phenomenon in the coating film.
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