The rheology and viscosity of the hottest waterbor

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The rheology and viscosity of water-based coatings can be understood in this way

the rheology and viscosity of water-based coatings can be understood in this way

may 09, 2019

the rheology of coatings has a great impact on the production, storage, construction and film-forming of coatings, and will eventually affect the film performance. The rheology of coatings will be studied for the production and construction of coating system selection, formulation design, wide application range and good development prospects, It is instructive to improve the film properties. To do a good job in water-based coatings, first of all, we should understand what viscosity is

how to understand viscosity

the rheological properties of coatings are related to the viscosity of coatings under different conditions. Set the area as a and the distance as D χ A certain velocity difference D under the action of shear force F ν For parallel flow, the force per unit area (f/a) is called shear force( τ), Velocity gradient (D ν/d χ) Called shear rate (d), the ratio of shear force to shear rate( τ/D) Called viscosity( η), It is a measure of liquid resistance to flow

the fluid whose viscosity does not change with the shear rate is called Newtonian fluid, and the fluid whose viscosity changes with the shear rate is called non Newtonian fluid. Non Newtonian fluid can be divided into pseudo plastic fluid, expansive fluid and thixotropic fluid. The viscosity of pseudoplastic fluid decreases with the increase of shear rate (that is, the fluid with thinning shear force), the viscosity of expansive fluid increases with the increase of shear rate, and the viscosity of thixotropic fluid decreases with the extension of shear time. Most coatings are non-Newtonian fluids

what factors affect viscosity

the main factors affecting the viscosity of coatings are temperature, polymer concentration, molecular weight and its distribution, solvent viscosity, etc. The viscosity of the solution decreases with the increase of temperature, and the relationship can be expressed as follows:

the relationship between the viscosity of the coating and the concentration of the polymer in practical application can be expressed as follows:

in the formula, NR is the relative viscosity, W is the mass fraction of the solute, Ka and KB are constants, Ka and KB can be calculated by drawing or calculation. This formula is applicable to the promotion of low molecular weight polymers to establish relevant industry associations, societies, and Alliance solution. For dilute solution of polymer good solvent, it can be expressed by mark Houwink equation:

where [n] is the intrinsic viscosity, and MW is the average amount of polymer molecules pressed into the sample surface with the specified experimental force (f); K and a are constants. There are many methods to determine the viscosity of coatings, such as stomer viscometer, corresponding quality specification measures, precipitation Brookfield viscometer, rotating concentric cylinder viscometer, rotating cylinder cup viscometer, cone plate viscometer, etc

coating flow equation

coatings are subjected to different shear rates in the preparation, storage, construction and film-forming stages. In the process of dispersion, the shear rate under stirring is about s-1, while the shear rate on the wall is only s-1. After the material is discharged, the shear rate can immediately drop to 5 s-1. The pigment may settle down. During construction, the shear rate of brushing, spraying or roller coating is at least 103 S-1 or even 105 S-1; After construction, the shear rate immediately drops below 1 s-1. Therefore, the coating assembly is designed and prepared into a non-Newtonian fluid to meet the performance requirements

the relationship between the shear rate and viscosity of the three typical coatings is shown in the following figure:

take the logarithm of the shear rate during coating production and construction as the abscissa and the viscosity as the ordinate to describe the rheological properties of the coatings. The above figure (provided by aitupang platform) shows the viscosity changes of the three coatings under different shear rates. We can make the following analysis:

in the high shear rate region, the flow behavior of the coating is mainly affected by the base material, solvent and pigment. In the low shear rate region, the flow behavior of the coating is mainly determined by the rheological agent, the flocculation property of the pigment and the colloidal property of the base material

when the coating is applied, streaks will inevitably occur. If it is leveled quickly, the streaks can disappear. The driving force of the leveling process is the surface tension of the coating

when the coating is applied on the surface of the vertical substrate, the coating will flow downward due to the effect of gravity, and excessive downward flow will cause coating sagging. It is our hope that the coating has the best leveling property and the lowest sagging property, but reducing the viscosity of the coating will help leveling, but also accelerate sagging; Increasing the thickness of the coating helps to level, but leads to sagging

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