Gravity pulls us and objects to the ground, but also pulls down particles of dispersed pigments and fillers, creating a deposit which over time can be very difficult or impossible to mix and distribute again in the paint. This phenomenon is also present in water-borne paints based on acrylic polymer dispersions, and in this article we will discuss the causes of its formation, effects and how to reduce and eliminate the formation of sediment.
Reasons for settling and methods of evaluation
Quoting from Paolo Nanetti’s dictionary “Coatings from A to Z”:
Settling is a tendency of certain pigmented coating systems to form, during storage, a more or les compact sediment. The sediment consist of pigment or filler particles, enwrapped by film former which, due to their density, in the course of time sink to the ground. If the film former wrapping around the particles is stable and the mechanism of electrostatic rejection is sufficiently pronounced (zeta potential), the sediment can normally be easily stirred up and thus is repsent no real disadvantage. Conversely, they so called solid sediment may have a negative effect on the application properties, because in the includes the rist that, in spite of intensive stirring up, the pigment particles cannot be dispersed any more without speck formation. The cause of solid sediment is mostly instable wetting of the pigment particles, often in combination with insufficient addition of anti-settling additives.
Sediment is a solid deposit on the bottom of a container, caused by separation of pigments and fillers, during paint storage. The reason of sedimentation is mostly found in the insufficient suspension behaviour of the solid particles in the coating, but at times in defective wetting of pigments as well. The only remedy is provided by the addition of suitable additives.
In order to properly classify the intensity of the deposit occurrence at the bottom of the paint, the classification and evaluation procedure described in ASTM D869-21 Standard Test Method for Evaluating Degree of Settling of Paint are used. The assessment is made by lowering the metal spatula with square-end blade into the can. It is assessed whether the spatula has fallen to the bottom, whether it is easy to swipe it with the blade surface, whether there is any resistance, and whether the spatula can be moved along the bottom with the front surface, and whether the sediment can be mixed by hand mixing with the spatula.
The ASTM D869 standard provides the rating used for the settling rating and is as follows:
In order to eliminate settling in latex paints, there are two ways to proceed. The first is to check whether the dispersing additives are correctly selected in terms of the dose, and whether the grinding process was carried out correctly. Settling is largely due to wetting additives, the proper dosing of which ensures the replacement of air between pigment aggregates and fillers, and then enabling the dispersing additives to separate the particles properly. The particle disintegration process is also very important, be it in cowles dissolver (dispersing) or in a bead-mill (grinding).
The second direction is to make sure that the wetting and dispersing additives are correctly selected and that the grinding process is also performed correctly. If settling continues in this case, it may be due to the incompatibility of the raw materials in terms of settling stability. It is often difficult to change key raw materials at an advanced stage of the project, especially when other paint and coating parameters have already been approved. In this case, anti-settling add-ons should be included in the tests. Typical anti-settling additives are typically surfactants, such as alkylol ammonium salt of a higher molecular weight acidic polymer, a wetting and dispersing additive to control flocculation and improve stabilization during in-pack storage of the paint. There is also a cause of settling which is caused by too low viscosity of the paint under low-shear forces. However, we omit this aspect here because the elimination of the sediment with the use of effective thickeners in low-shear forces and mid-shear forces, because we assume that the viscosity of the paints is set at the required level, and yet settling still occurs.
Anti-settling additives in the form of surfactants enhancing the stability provided by dispersants are additives that should be added at the beginning of the production process, to the millbase before pigments and dispersants, and their dose should be determined by ladder use of technical materials developed by TS&D, based on e.g. Spektrochem’s technical reports , the data of which are presented later in this article.
Settling in latex paints
In water-based latex paints based on dispersions of acrylic polymers, the deposit is relatively rare if the formulations are properly prepared. This means that the additives responsible for the proper distribution of pigment particles and fillers (dispersants and wetting additives) are selected efficiently and work well with rheological additives. However, in order for the effectiveness of these additives to be high, dosing recommendations are prepared for them, the doses recommended for use with various pigments and fillers are determined, and it is such research work that is the subject of the daily work of the Spektrochem Paint Technical Center. There are, however, formulations in which some raw materials disrupt the stabilization provided by the dispersing additives, or the relationship between the raw materials and the formulation is not sufficient to ensure non-settling.
Such situations can be, for example, the relationship between the pigments and fillers used, PVC (pigment volume concentration) and the desired viscosity which is not able to keep the particles from falling downwards. Such a situation can often be found with low PVC formulations in latex paints with a high content of titanium dioxide (high pigment density) and the required relatively low viscosity and good flow (e.g. in acrylic latex enamels). Certain raw materials are often found in such formulations. for example, coalescents can disturb the stability of the particulate distribution and create a deposit in a relatively short time
The photo above shows the sediment that formed in a short time on the bottom of the latex paint in a formulation based on two different styrene-acrylic polymer dispersions. On the right side you can see no sediment, in the preparation sample on polymer dispersion B. On the left side the same formulation but with a changed polymer dispersion and the sediment is already clearly visible on the spatula. The sample on the right was prepared on another polymer dispersion marked as A. As a result of the investigation carried out on the prepared samples in our laboratory, it was found that the coalescent is responsible for the formation of the deposit, which, as a result of disturbing the stability of the formulation based on polymer dispersion A, quickly leads to sediment formation, and the same coalescent with polymer dispersion B in the same formation is not the cause of the sediment formation.
The elimination of the sediment in the case of the sample on the left side (with polymer dispersion A) can be carried out by ladder studies of anti-settling additives, which may constitute a solution of an alkylol ammonium salt of a higher molecular weight acidic polymer and have a stabilizing effect on pigment and filler particles. Anti-settling is added before the fillers and pigments to the millbase and grinded along with the rest of the wetting and dispersing additives. For an example of ladder studies of anti-settling additives, see the slurries section below.
Settling in slurries (and pigment concentrates)
Pigment concentrates for tinting, as well as slurries (suspensions of fillers, pigments with all required additives) used in the preparation of latex paints, both without binder and highly associative with thickeners, are also a common example of the presence of sediment as well as a relatively low desired viscosity for good pumpability of both colorants (pigment concentrates) and slurries. In these types of formulations, the appearance of a precipitate in a short time or even if it does occur and is not easy to mix and redistribute in the colorant or slurry will leave the customer unhappy.
This applies especially to pigment concentrates, where the lack of an appropriate amount of pigment in the dispersion will prevent the color from repeating itself, and in the slurries, an insufficient amount of solids will enter the recipe, thus disrupting all solid recipes (PVC, volume solids, solids by weight, etc.) followed by paint and coating parameters such as opacity, scrub resistance, etc.
The above photo shows the initial action of anti-settling additives in slurries, which are suspensions of high solid content of calcium carbonate and titanium dioxide with wetting and dispersing additives, in-can biocide and grinding aid in the form of a Newtonian polyurethane thickener. Slurry has a low viscosity at low shear rates and is not stabilized with a thickener in this shear forces range. The sediment assessments were carried out after 24 hours and after 3, 5, 7, 10 and 14 days. The photo shows the action of anti-settling additives after 24 hours in doses of 0% (control), 1%, 1.5% and 2% in terms of solids in slurry.
It can be seen that a slightly visible deposit is already forming on the spatula in the control sample (in the case of slurries, wooden spatulas were used for the photo), but the evaluation procedure was performed according to ASTM D869, the data and test results of which are presented in the table below.
The data in the table shows that the effectiveness of the tested additive as an anti-settling additive is visible, especially after up to 10 days for the 2% dose. As mentioned before, the prepared slurries were not stabilized with low-shear forces thickeners and a short shelf life is normal in this case. However, the use of anti-settling in doses ranging from 1 to 2% yields a result of 6 in samples after 14 days, which means that settling has occurred, however the sediment is easy to mix. Compared to the control sample, the sediment after 14 days from slurry preparation is already difficult to re-suspend.
The presented results show, on the example of slurries, how the ladder selection of the dispersing additive dose allows to determine its effectiveness as anti-settling additive. Additives of this type are very important because they allow to extend the durability of suspensions, which in the case of slurries not stabilized with low-shar thickeners show a high tendency to settling. Their use in latex paints is also helpful, especially when the selection of wetting additives and dispersants is not completely correct.