Latex paint production in slurry process

Depending on the scale and automation of the latex paint production line, production is carried out in one dissolver or in a production line. In Europe, small production companies manufacture paints from start to finish in one dissolver with different tank capacities. Usually it is a ladle dissolver with a movable ladle (300 – 1000 L). Larger companies use stationary dissolvers with larger tank volumes (up to 5,000 L).

Dissolver station with one engine and two vessels

Two-stage latex paints production process are very common in Canada and the US. It involves preparing a suspension of pigments and reinforcements in water with additives in the dissolver. The suspension prepared in this way is pumped into let-down tanks with very large capacity, e.g. 5000 gallons (18,927 L). The suspension in this way is usually used immediately for paint production. It is added to another tank containing latex, and the process is called “let-down” or “egalization”.

This process is very optimal, however it can be further modified and optimized to prepare slurries containing mono-pigment or mono-filler (or mono-reinforcement) and add them as single slurries by volume, adding individual amounts of slurries to the latex tank and mixing together with the remaining post-additives ( e.g. for consistency correction).

This technology requires the development of stable slurries for individual pigments and fillers. Of course, storage tanks for slurries should have agitators to mix them from time to time, but there must be no sedimentation or syneresis in slurries.

The challenge for our laboratory was to develop standardized formulations for many slurries containing a single filler or pigment and in high concentration, so that there would be no problem with introducing a large amount of water into the recipe, which at low concentrations of slurries could lead to the inability to prepare paint from such a formulation.

Spektrochem laboratory during research work on slurry preparation

The process involves preparing a pigment slurry or filler using the dissolver from ingredients resulting from the final paint formulation. In addition, ingredients that stabilize the slurry itself during storage are used (in-can biocides, substances that hold suspended particles, agents that prevent surface water syneresis, etc.). The slurry concentration is always chosen so that it is high so that the slurry contains as little water as possible.

Preparation of such stable suspensions requires a lot of research and laboratory tests, however, as an independent R&D laboratory for formulation development, we deal with such issues on a daily basis. We have developed a number of stable slurries dedicated to the NAFTA market.

Below is a schematic process for preparing latex paints according to the developed slurry process.

Slurry process – stage I – dispersing and slurries storage in the tanks

A single dry ingredient is dispersed in the dissolver, e.g. titanium white – so that each slurry has a separate pigment or filler. The dispersion is carried out in a specific manner while maintaining the flow parameters to obtain the appropriate degree of grinding measured at the end of the process. Stabilizing components are added, which allow slurries to easily pump or drain by gravity, as well as ensuring stability during storage in buffer tanks – in the slurries storage section.

In slurries storage tanks, we usually recommend installing anchor stirrers that make it easier to break up the thixotropic consistency of slurries before pumping them into the process, however, for some types of slurries, mixers are not needed. Of course, the stirrers additionally ensure homogenization, which is an additional protection in case of slight disturbance of homogeneity. Slurries prepared in this way can be stored for a period of several months to a year depending on the pigment / filler.

At this stage, the “dirty” process of handling dry ingredients ends. From now on, pigments, fillers and reinforcements are only in the form of slurry and can be dosed together with individual ingredients in the volume system. Stable viscosity and density ensures the possibility of repeatable dosing. The next diagram presents the stage II – let-down, i.e. the process of combining the polymer dispersion with slurries and other additives.

Slurry process – stage II – let-down

In the let-down process, a latex binder is pumped into the tank by volume, the amount of which results from the formulation. Such tanks can prepare up to 5,000 gallons of ready paint at one time. Slurries containing e.g. calcium carbonate, titanium white, kaolin clay as well as nepheline syenite are dosed into the latex tank at the same time or sequentially. At this stage, slurries are dosed into the latex because the higher density results in easier mixing. This process is slow and does not generate heat (as in the case of production in one container in the dissolver).

Other additives that do not tolerate high temperature (e.g. biocides) are added and the whole is homogenized to obtain the right consistency that can be adjusted at the process stage.

At a later stage, the finished paint after passing the quality control goes to the filtering and packaging line.

Slurries storage tanks and let-down tanks with pipelines in one of the paint factories in the USA

This process allows you to dispense individual ingredients faster, since dispensing takes place by volume. The use of pipelines eliminates the need for multiple scales, and the entire process takes place in a closed installation without access to the surroundings.

The production process using slurry technology allows the rapid production of paints and dosing in a large-scale volume system. Of course, the use of such a process on a small scale is also possible, but may not be optimal. The greatest benefits are visible only with a large production scale, several lines and series of paints produced in this way.

Today’s design and execution capabilities of the dissolver manufacturers are so large that it is possible to adapt existing ladle and stationary dissolvers to lines with additional storage tanks and let-down tanks.

Automatic dissolvers (picture from European Coating Show 2015, Nürnberg)

Stable slurries were developed in the Spektrochem laboratory and are now functioning solutions in the USA. They allow the production of paints from pre-prepared dispersed pigments and fillers, for which producers can prepare before the season, as well as perform operations in stages, which allows more complete quality control and better mixing of ingredients, at least due to the final consistency of paints obtained immediately. In some factories, the paint production process has been shortened by almost 38%.

Laboratory sample – 65% slurry of Minex S-7 reinforcement (photo from Spektrochem Paint Laboratory)

The Spektrochem Laboratory develops formulation modifications for individual needs to adapt recipes to prepare the slurry technology process, maintaining a repeatable tinting system and the quality of latex paints. This system has worked well in the US, it can also be implemented in other parts of the world for the production of large-scale paints.

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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