Water Based Coatings
Additives - Dispersing Agents
Mr Veeramani shares his knowledge in this Column devoted exclusively to Water Based Coatings
NOW let me go back to the initial stage of ADDITIVES just to recall certain important aspects so that, the reader can understand the subject better and will have more clarity and retentivity, hence I decided to reverse the gear and go backward on the topic. An “ADDITIVE” is defined in Standards Terminology as “Any substance, added in small quantities to a coating material to improve or modify one or more properties”. They are not the major ingredients yet their role in the Formulation is vital. As the coating Industry responds to the relentless pressure for operational efficiency, it is increasingly important that Coating Formulators have a broad based understanding of technical principles of each component, and its interaction between them. Formulation may be described as “The science and technology of producing a physical mixture of two or more components, with more than one conflicting measure of product quality”
The role of a Formulator is to meet business needs by designing a product that has the required appearance, and a balance of properties suited to the envisaged application. Most coatings must satisfy aesthetic and functional considerations, which range from storage through application to film durability. Making an informed selection of components and then combining those in specific ratios and in the correct sequence at a specified state of sub-division meet these requirements. But the Formulator must also contend with the second of the key elements implicit in the Formulation definition, namely the certainty that there will be conflicting measures of product quality. This is true of any man made artefact.
Now let's go to a very important Additive known as DISPERSING AGENT. From the name one can easily make out that this is a Dispersing aid in the Water Based Paint system and no paint can be made without this additive or with improper quantity and will result in a bad paint.
The most difficult, time and energy consuming stage of the Paint manufacture is the Pigment/Extender Dispersion. This is because of the difference in the surface tension of the Liquids and Pigments/Extenders as the Extenders and Pigments are usually in the form of agglomerates, and are ground / dispersed in a High Speed Disperser. During this process Dispersants (Surfactants) are used mainly to prevent re-association of the particles. The dispersants are absorbed by the particles and prevent the close approach of the particles by charge repulsion effects. Dispersing agents help properly to de-flocculate the fillers and pigments and contribute to stabilize the particles, thanks to electrostatic and steric repulsions. Aggregates and agglomeration forming is therefore considerably limited thus imparting a very good stability to the formulation with no increase of the viscosity within time and prevents settling of Pigments and extenders. The proper choice of the Dispersant is a must depending upon the Inorganic and Organic Pigments in the Formulation.
Most preferred ionic (Hydrophilic) dispersant is Sodium polyphosphate. Potassium polyphosphate is also used as they are less soluble. The widely used Wetting; Surfactant & Dispersing Agent is alkyl phenol ethoxylates. Polymeric dispersants such as Poly acrylates are also used for steric stabilization. The level of dispersant is important for optimum amount of saturation of the pigment surface. It improves gloss, better compatibility with bases, good flow & levelling and enhanced colour strength.
A post-graduate in Chemistry from University of Madras, after a brief stint of teaching worked in a Food Industry for three years. Then shifted to Corrosion-Engineering in an Indo German joint venture and worked for 16 years. During this period was trained in Germany in the manufacture of Corrosion Resistant Mortars and Coatings. Subsequently shifted to Paint Industry in a well known all India brand and worked in various capacities viz: Projects, Operations and R & D. After superannuation started working as a free lance Paint Consultant and now attached to M/S VENLAC PAINTs, in Namakkal Tamil Nadu. Specialized in Formulating Water Based Paints to suit manufacturers’ needs.
Herein I would like to show a few illustrations from which the readers can understand the importance of a Dispersing Agent in a Paint system.
Dispersant's action in coatings
We have seen the action of Dispersant in Coatings and their advantages which necessitates Formulators to incorporate the right type of Dispersant to get optimum results. Dispersing agents help properly de-flocculate the fillers and pigments and contribute to stabilize the particles thanks to electrostatic and steric repulsions. Aggregates and agglomeration forming is therefore considerably limited thus imparting a very good stability.
In non-stabilized system, flocculation happens immediately after dispersion process, however, the degree of flocculation at that moment varies, depending on the dispersant used and the formulation. If a system is flocculated, pouring test can immediately detect this defect. Flocculation also can grow over time. For non-stable system it will be just a matter of time, which will eventually lead to very severe flocculation problem; some take days and some take months. The colour strength will be reduced and this will result in colour changing gradually with time. This could be very closely observed when a difficult pigment is used as a trade tinting dosage for a particular colour.
Let's see some more functions of a Dispersant in the Coatings. A Dispersant is an additive that increases the stability of a suspension of powders (Pigments & Extenders) in liquid medium. As said earlier, the pigment dispersing step is the most difficult and time /energy consuming part of the paint manufacturing process. This is because of the difference in surface tension between the liquids (Polymers & Solvents) and the solids (Pigments & the Extenders).
Pigments and Extenders are often received as agglomerates and then are subjected to a grinding process which incorporates the Pigment into the vehicle during paint manufacture. During the grinding the agglomerates are dissociated into a dispersion of particles. In the process, Dispersants (Surfactants) are used to mainly prevent re-association of the pigment and extender particles. The dispersants are absorbed on to the particles and hinder close approach of particles by charge repulsion effects (Ionic dispersants) or by steric effects (Non-ionic dispersants). Wetting and dispersing agents are used to stabilize pigment dispersions. They often use steric hindrance to avoid flocculation of the pigment particles.
Since a dispersant is used to help the suspension of fine particles of a solid in a liquid phase, it must be able to totally wet the solid particle and also be able to interact with the dispersing medium. Dispersants act by being absorbed on the surface of the Pigment particles with the other end of the molecule exposed and usually charged.
Since like charges repel, the individually coated pigment particles repel each other and stay uniformly dispersed, because the individual pigments are all so chemically different – some are attracted to water and some are not – the dispersants have to be chemically different. When changing pigmentation in a formula, the Dispersant needs to be checked quite carefully to see if it is stable in the formulation. Good dispersion provides the end-user with better hiding and color stability.
This is the most complex process and a good wetting and dispersing agent should have these properties. Poor stabilization will lead to many defects. Stabilization of solid particles in coating is defined as the ability to keep all solid particles separated in a certain distance and stop agglomerates, aggregates and flocculates after long period of storage. Solid particles in liquid will move around and collide with each other according to Brownian motion. If these particles are not well stabilized, due to the attraction forces (will be discussed in detail later) between the particles, these forces will re- agglomerate and flocculate them together and this is not desired by all formulators.
It can be concluded that, Dispersing is an act to move and separate an agglomerate particle to smaller particles. The ideal stage is to disperse the agglomerate particles to its primary particles size. However, for some pigments, it is impossible to do so. Most of the time, in coating systems, this action will mostly depend on the mechanical forces, which are applied to it such as stirring, grinding etc. A dispersing agent will only help to accelerate this process so that, the time needed is shorter.
Finally, as a thumb rule, SELECTION OF DISPERSING AGENT can be expressed as follows:
Water borne paints
Inorganic pigments and extender (e.g. TiO2 => very high surface tension):
Wetting and charging with dispersing agent (Ionic)
Organic pigments, carbon black (very low surface tension):
Wetting and charging with a wetting agent and dispersing agent or with a wetting and dispersing agent (non-ionic, ionic)
Email Id: email@example.com -To be continued