In many start point formulations, you can see raw materials divided into the grinding stage (in cowles dissolver or bead mill) followed by a process called let-down in which the raw materials such as polymer dispersion, coalescent, defoamer and thickeners and some other additives are listed. You can often find in the practice of paint production that after grinding pigments and fillers in a dissolver, the polymer dispersion is added to the prepared slurry, and then additional additives are added. The production of latex paint then takes place in one tank, and the mixing process takes place using a dissolver disc or other mixing elements mounted on an additional drive.
The charge, however, is not transferred to any other container, and the entire paint is created in a one-step process. It is not a completely correct process, and above all, not very economical and not ergonomic. But for what paints is such a process beneficial, and is it for all latex paints?
The latex paint production technology is an extremely fascinating process of combining ingredients that are not very compatible with each other. Pigments and fillers are not soluble in water or in the latex binder. Many additives do not mix with water at all, e.g. some defoamers (silicone, mineral oil base), coalescents. Acrylic thickeners work only in an alkaline environment (alkali swellable). On the other hand, many polyurethane thickeners are effective only in a certain polarity and require pre-dilution in a suitable glycol ether.
The situation is similar with combining slurry resulting from the grind process with a latex binder. The incompatibility in question is not due to the difference in polarity or the difference in solubility of the ingredients, because the slurry in the binder is not soluble, and vice versa. Slurry with a binder creates a suspension that is stabilized by surfactants and other raw materials present both in the slurry and in the polymer dispersion. However, due to the large difference in density (slurries usually above 2.3 g/cm3, and polymer dispersions usually around 1.03 – 1.05 g/cm3) and differences in viscosity, the combination of these ingredients is apparently not a simple operation.
Slurry usually consists of pigments (e.g. white titanium dioxide, color pigments), fillers (e.g. calcium carbonate, nepheline syenite, kaolin, talc, pyrophyllite, etc.) suspended in a form stabilized by surfactants (dispersing and wetting agents) together with the necessary amount of grinding aids to prevent the formation of foam (defoamers added at the grinding stage), thickeners to stabilize the viscosity, but not to increase it excessively, as well as in-can biocides to prevent the growth of bacteria and mold during slurry storage. Slurries can be a combination of different pigments and fillers, but single pigment and single filler suspensions are more commonly used, from which many types of paint can be made by dispensing individual slurries containing a specific pigment and filler. Slurry is usually prepared before the paint production in the same plant that produces the paints, e.g. at the beginning of the planned production season (provided that the slurries are stable in storage), and you can often find already prepared slurries of pigments (mainly titanium dioxide) and some fillers (e.g. calcium carbonate) offered by raw material producers. The concentration of pigments, such as TiO2, is usually around 70-75%, and for fillers it is dependent on grain size, mineral type, specific surface area and oil absorption, and ranges from 50 to 80%. The size of the dispersed primary particles of pigments and fillers is usually from 0.5 µm (for TiO2 pigments) to tens of micrometers for typical mineral fillers.
In turn, the polymer dispersion constitutes from 40 to 60% of solids by weight of the copolymer suspension in water, stabilized with ionic surfactants or a mixture with non-ionic surfactants (for dispersions of acrylic and styrene-acrylic copolymers, vinyl acetate copolymers) and additionally with protective colloids in the form of e.g. cellulose ethers (in case of VAE copolymer dispersion, vinyl-versatic copolymers, VeoVa, etc.) or polyvinyl alcohol for vinyl acetate homopolymers. The density of the polymer dispersion is much lower than that of the slurries, and their viscosity is sometimes much higher (especially in the case of vinyl acetate copolymers). The difference in the particle size of the polymer dispersion ranges from several dozen nanometers, through several hundred micrometers (mostly) to several micrometers for vinyl acetate copolymers.
The dissolver as a device equipped with a serrated disc is the most popular device for dispersing and grinding pigments and fillers in the latex paint industry. Dissolvers can be stationed (tank) and with mobile vats of different capacity. The production process of latex paint is often such that latex is slowly added to grinded slurry pigments and fillers (usually of high viscosity by picking up with cellulose ether added at the beginning). During this addition, the dispersing disc runs at a much slower speed than in grinding, or an additional drive in the form of a side knife (so-called scraper) is activated, which causes the latex to be collected from the edge of the tank (usually in dissolvers with movable vats). Adding the polymer dispersion in this order, i.e. to slurries, makes the paint homogenization process very difficult due to the difference in density. Most of the time during mixing, the polymer dispersion remains on the surface of the dissolver tank and requires a very long mixing time to be homogeneously incorporated into the slurry. As a result of differences in density and often differences in viscosity at the interface between the particles of the polymer dispersion and the particles of pigments and fillers, the so-called colloidal shock, which may result in coagulation of the binder particles and reagglomeration of pigments and fillers. This results in the necessity to filter a large part of the batch, which is disadvantageous for the recipe itself, as each raw material has been strictly weighed.
The process of incorporation of polymer dispersion into slurry, carried out by adding latex to slurry, causes that we add more than half the density (polymer dispersion) to slurry, which has at least twice the density. Such a process makes it necessary to homogenize the paint much longer in order to obtain a qualitatively appropriate combination of the binder with particles of fillers and pigments and to obtain homogeneity in terms of viscosity. In addition, mixing with a serrated disc causes additional shear of the polymer particles, which, in the case of many polymer dispersions, causes them to coagulate. The increased viscosity of the bottom stock makes it necessary to increase the speed to introduce the polymer dispersion on top. An additional side scraper does not help with this process and sometimes adds air to the paint.
On the other hand, pumping or overflowing the slurry from the dissolver into the let-down tank with the appropriate amount of latex and which is equipped with a propeller agitator causes the latex to mix spontaneously with the slurry by pushing the latex upwards and mixing caused by the difference in density, as the slurry falls on bottom due to the difference in density. The propeller agitator is only supportive and does not require high shear. This diagram is shown in the figure below (diagram on the right) for comparison with the previously described addition of latex to the slurry (diagram on the left).
The let-down process is the process of adding slurry to the latex. This can be done automatically by pumping the slurry into the equalization tank or by gravity draining the contents of the dissolver vat. This allows for a much more effective preparation of a homogeneous ink batch, and a much easier and faster obtaining of viscosity with the subsequent viscosity KU adjusting thickeners. As the case studies performed in our laboratory show, the let-down process performed in this way allows us to shorten the time needed for homogenization by up to half. Additionally, the next dissolver for the production of the next slurry can be loaded in the let-down tank during homogenization. In the case of the production of paints from slurries properly stabilized with surfactants that can be stored, the production of paints can be carried out by appropriately automated dosing of only liquid ingredients, all dry ingredients of which have already been grinded at the slurry production stage.
I wrote about how paints are produced in a two-stage slurry & let-down process on the blog here https://paintlaboratory.wordpress.com/2020/06/28/latex-paint-production-in-slurry-process/
This process, however, has its limitations and is dedicated mainly to paints in which the share of latex binder is high (low-medium PVC). In the case of cheap paints, in which the amount of dispersion binder is small and there is no risk of causing colloidal shock, dispersions can be added to the prepared slurry in the dissolver tank, however, subject also to the need for long homogenization. Paints with PVC above 85% can usually be produced using the one-step method, as they do not have to use the binder as efficiently as possible, and let-down slurry in such a small amount of binder will not have much purpose.
In the case of high-quality latex paints, especially paints with a high binder content, the let-down process carried out as described in this article allows not only more stable paints obtained by a better combination of ingredients, but also a higher gloss in the case of semi-gloss and gloss paints, where no a bead mill is used to prepare slurries.
The final stage is quality control, which allows for quick and efficient determination of the batch repeatability, which is confirmed by proper let-down process. We wrote about simple methods of controlling latex paints on the blog here: https://paintlaboratory.wordpress.com/2021/12/09/quality-control-of-latex-paints/