The three-roll mill is a grinding and dispersing equipment . The three-roll machine can generate huge shearing force between the two mixing rollers rotating at different speeds, so it seldom depends on the solvent to wet the pigment, but relies on the huge extrusion force in the gap between the two sticks of the Grinder. The paint is coated on the surface of the pigment and penetrates into the pores of the pigment. At the same time, rely on the grinding force generated by the relative speed difference of grinding H to depolymerize the secondary particles of the pigment. Therefore, the characteristics of the ground mill base suitable for three-roll mill processing are: not only the use of high-viscosity paint, but also the addition of high volume fraction of pigments, so a rather thick mill base.
The research on the working principle of the three-roll machine shows that there are 9 key variables that affect the output of the three-roll machine, expressed in the following formula:
In the formula, θm is the production capacity of the three-roll machine; K is a variable determined by the size of the roller; D is the diameter of the roller; L is the length of the roller; r is the number of revolutions of the rear roller; n is the speed ratio of the middle and rear rollers and the front and middle rollers ; a, c are the transfer coefficients; η is the viscosity of the grinding mill; F is the force of the pressing roller.
Three-roll mill Click the picture to view
For any three-roll machine obtained by the paint factory, the five variables (K, D, L, r, n) in the formula are fixed , and the transfer coefficients a and c are also more or less fixed. Rewriting these invariants into a constant K', the formula can become:
It can be seen from the above formula that in order to increase the output of the three-roll mill, it can be realized by increasing the viscosity of the grinding mill slurry or reducing the pressure between H. However, reducing the pressure between the grinding rods will be accompanied by a decrease in the grinding dispersant t, so the correct way is to increase the viscosity of the grinding slurry. Either increasing the viscosity of the paint or increasing the amount of pigment.
Let's further analyze the factors that affect the shear force of the three-roll machine. It can be seen from the following formula:
In the formula, C——shear force;
T——the frictional force of dragging the pigment particles;
R——radius of series;
r' - the radius of the pigment particle.
a, the shearing force between the dry spokes is proportional to the frictional force of the dragging pigment particles, so if the viscosity of the paint is high, the shearing force of the three-roll machine on the grinding paint slurry can be increased, that is, at a distance from the stick gap can play a shearing role;
b. The larger the diameter of the grinding knuckle of the three-roller machine, the greater the shear force and the high dispersion efficiency; due to the strong tearing force of the high-viscosity paint on the pigment particles (low viscosity and easy rolling), and the high viscosity of the paint, It is easy to transfer from low-speed roller to high-speed H, so the three-roll machine should generally use high solid content (correspondingly high viscosity) paint and high pigment content grinding slurry for dispersion.
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Usually the solid content of the paint used in the three-mixer is 70%~100%, and the corresponding viscosity changes within the range of 2.0~10.0Pa·s. For example, 100% solid content polymerized linseed oil (η = 1.2Pa s) is a typical solvent-free paint; 70% solid content alkyd resin paint (η = 2.0Pa s) is solvent-containing Representative of paint. Then add as much pigment as possible to the above paint. As long as it does not affect the flow of the ground paint slurry from the pre-mixing basin into the feeding gap between the rear and middle parts of the three-chopper, and can be smoothly lowered from the scraper holder, it can flow into the paint mixing basin.
At this time, the ratio of the pigment volume fraction (PVC) to the critical volume concentration (CPVC) of the pigment calculated from the oil absorption value is generally between 55% and 65%. For example, when the above-mentioned 70% solid alkyd resin paint disperses titanium dioxide, when the PVC is 65%, an excellent grinding dispersion system can be obtained.
