The preparation of latex paint is roughly the same as the traditional paint production process, generally divided into pre-dispersion, dispersion, blending, filtration, packaging and so on. However, as far as traditional coatings are concerned, paints, as dispersing media, meet pigments and fillers in the pre-dispersion stage, and pigments and fillers are directly dispersed into paints. However, for latex paints, since emulsions are usually more sensitive to shear stress, in In the low-shear mixing stage, it is safer to make it meet the pigment and filler dispersion slurry. Therefore, pigments and fillers are only dispersed in water during the dispersion stage. The viscosity of water is low and the surface tension is high, so it is difficult to disperse. Therefore, wetting agents, dispersants, and thickeners need to be added in the dispersion operation. Because there is a large amount of surfactant in the dispersion system, it is easy to generate air bubbles and hinder the production. Therefore, in the dispersion operation, it is also necessary to add a defoamer. Obviously, the modulation of latex paint is more complicated.
The main products directly produced on the latex paint production line are white paint and base paint, and the color paste is generally prepared separately. The production line mainly considers the dispersion of white pigments such as titanium dioxide and fillers. Among modern titanium dioxides, those used in latex paints are very easy to disperse, and the commonly used fillers are generally ultrafine. In addition, architectural latex paint does not require high fineness, so a high-speed disperser is usually only needed to be installed on the latex paint production line. The high-speed disperser is preferably equipped with a speed regulating device, so that the dispersion and paint mixing can be completed in a high-speed disperser.
Of course, under certain conditions, in order to meet the higher requirements for fineness, or to adapt to the thicker pigments and fillers that may encountered, in addition to high-speed dispersers, some latex paint workshops are also equipped with equipment such as sand mills and ball mills. .
The order of feeding in latex paint production is very important, generally as follows [2]:
First, add water, preservatives, fungicides, wetting and dispersing agents, about 1/2 of defoamers, thickeners, and co-solvents into the mixing tank , and mix well. If necessary, part of the emulsion can also be added. For preservatives and antifungal agents with poor thermal stability, they should be added after letdown to prevent them from decomposing and becoming invalid when the temperature is too high during pulping.
Then, place the center of the dispersing disc close to the bottom of the mixing tank, rotate at a low speed, and gradually add the pigments and fillers into the deep vortex, first add the finer pigments and fillers, and then add the thicker pigments and fillers. This addition is not only conducive to dispersion, but also conducive to defoaming. With the addition of pigments and fillers, the abrasive becomes thicker, the position of the dispersion disc should be raised to make the vortex shallower, and the speed should be increased accordingly. When all the pigments and fillers are added, increase the rotational speed so that the peripheral line speed of the dispersion disc is about 20~25m/s.
It is generally believed that the dispersion of pigments and fillers is better at this speed. The grinding and dispersing time is generally about 15 minutes, and it should be based on the requirement of dispersion fineness. For mercerized, semi-gloss and glossy latex paint abrasives, the fineness should generally be less than 2oµm. For flat interior wall latex paint abrasives, the fineness Generally, it can be controlled below 40µm; for flat exterior wall latex paint abrasives, the fineness can even be placed below 100µm (testing instrument: Fineness Meter). Attention should be paid to the temperature of the abrasive during dispersion. If the temperature is too high, if the temperature exceeds 45°C, the viscosity will drop and the dispersion will not be possible. You can stop it and disperse after cooling.
After the dispersion fineness is qualified, under the condition of low-speed stirring, add the emulsion, film-forming aid, part of the thickener and about 1/2 of the defoamer.
As for the pH adjuster, if it is AMP-95, it can be added before the pigment and filler are dispersed; if it is ammonia water, it can be added after the emulsion is added; if it is NaOH, KOH, it can be added after the pigment and filler are dispersed, but before the emulsion is added.
It is also possible to add film-forming aids before the dispersion of pigments and fillers, which is relatively safe for emulsions, but it may be absorbed by pigments and fillers.
This is the general feeding sequence, which can be determined according to the performance of raw materials, dispersing equipment, actual operation conditions and requirements for dispersing.
Never leave the prepared pigment and filler slurry without emulsion for more than 24 hours, especially the pigment and filler slurry of glossy latex paint, to prevent flocculation, agglomeration and instability. When preservatives and antifungal agents are not added during pulping, due to the high temperature, it may even be polluted by bacteria and be scrapped.
Due to the rapid development of latex paint and the sharp increase in output, it has become an important paint variety, and the titanium dioxide industry has also developed titanium dioxide water slurry accordingly. The production of latex paint with water slurry not only saves the energy consumption of drying, jet crushing and grading operations in the titanium dioxide industry, but also shortens the working hours of the coating industry, improves work efficiency, and saves energy consumption in dispersed operations.
The process mode of a modern latex paint professional production plant is roughly like this: send the emulsion, titanium dioxide paste, and bulk filler into the factory with a tanker, and send it to the storage tank and powder silo with a pump. When batching, add water and additives, and use pumps and other conveying equipment to send the titanium dioxide slurry and fillers to the high-speed disperser through the meter, and use the high-speed gear to carry out the dispersion operation. After the dispersing operation is completed, reduce the speed, pump the emulsion to the high-speed dispersing machine through the flow meter, add other additives, and stir evenly. White latex paint and base paint are often prepared in this way.
After passing the inspection, the white latex paint and base paint are sent to storage tanks for filling and color matching.
