Most paints and inks contain solid particles, like pigments and/or fillers. Two instability problems can occur with solid particles that have been separated from each other and that are distributed in a liquid system. The first problem is called flocculation (EU), or the gluing together of separated particles. This article covers the second problem: solid particles sinking because of gravity in a process called sedimentation (EU).
The Basics of Sedimentation
In general, solid particles in a liquid are pulled down because of gravitational force. The cause of this phenomenon is that many solids have a density that is higher than the density of the liquid surrounding the particles. The process of particles sinking, called sedimentation, can cause problems during storage. In extreme cases, particles can cluster together on the bottom of the can. This phenomenon, resulting in the formation of sediment, is called settling (EU). The speed of sedimentation is governed by a few properties, as shown in Figure 1.
The sedimentation velocity of a solid particle having perfect spherical shape in a Newtonian liquid was derived by Stokes:
In this equation ‘g’ is the gravitational acceleration.
The most important factor, governing how fast a particle will sink, is size: big particles sink faster than small particles. The sedimentation velocity depends on the square of the radius of the particle. The second factor is the density (EU) of the particle, or, to be more precise, the difference in density of the particle and density of the surrounding liquid. The third factor is the viscosity (EU) of the surrounding liquid: a low viscosity of the liquid will result in faster sedimentation.
In order to understand how sedimentation in a liquid system can be prevented, it should be taken into consideration that sedimentation is mainly a problem occurring during storage of the system. During storage, the system stands still and the main force acting on the system is the gravitational force. An effective way to prevent particles from sinking is by arranging a physical network in the system, as seen in Figure 2.
A physical network can be created in a liquid system by using suitable rheology additives (EU). The physical network has to be strong enough to resist the gravitational force during storage. On the other hand, the network must be weak enough to be broken down as soon as shear force is applied. Most rheology additives are based on the principle of hydrogen bonding and specific commercial products are developed for a wide variety of paints and inks.
A few examples of rheology additives:
- Fumed silica (EU) consists of small solid particles building a physical network in a liquid at low shear. A wide variety of commercial fumed silica products are available, including Aerosil® (EU) products from Evonik (EU).
- Certain solid particles with a platelet shape can form a house-of-cards structure in a liquid during storage. Well-known are the smectite clays (EU). Check out the range of Bentone® (EU) products from Elementis (EU).
- Special polymers can also build a physical network in a liquid, for example: polyurethanes, polyacrylates, cellulose ethers and polysaccharides. A special polysaccharide rheology additive often used to prevent sedimentation is xanthan gum (EU). Commercial xanthan gum products are, for example, supplied by CP Kelco. The trade name of their products is Kelzan®.
Adding less than 1 % of a rheology additive will be enough to fully prevent sedimentation during storage of the liquid system, provided that the right additive is selected for the system.
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