{"id":9400,"date":"2019-04-12T08:00:05","date_gmt":"2019-04-12T14:00:05","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=9400"},"modified":"2022-08-08T07:50:09","modified_gmt":"2022-08-08T13:50:09","slug":"pc-sedimentation-and-settling-during-storage","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/9400\/pc-sedimentation-and-settling-during-storage\/","title":{"rendered":"How to Prevent Sedimentation and Settling During Storage"},"content":{"rendered":"

Introduction<\/h3>\n

\"photoMost paints and inks contain solid particles, being pigments and\/or fillers. Two instability problems, flocculation<\/strong> (the gluing together of separated particles) and sedimentation<\/strong>, can occur with solid particles that have been separated from each other and that are distributed in a paint or ink. In this article, learn about the basics of sedimentation and how to prevent it.<\/p>\n

Sedimentation defined<\/h3>\n

Sedimentation is the process of solid particles sinking during storage because of gravity. In extreme cases, particles can cluster together on the bottom of the can because of sedimentation. This highly undesired phenomenon, resulting in the formation of a hard or rubbery sediment, is called settling<\/strong>.<\/p>\n

The basics of sedimentation<\/h3>\n
\"Figure
Figure 1. Sedimentation of a solid particle in a liquid caused by gravity.<\/figcaption><\/figure>\n

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 surrounding liquid\u2019s density. The process of particles sinking (sedimentation<\/strong>) causes problems during storage. The speed of sedimentation is governed by a few properties, shown in Figure 1.<\/p>\n

The sedimentation velocity of a solid particle having perfect spherical shape in a Newtonian liquid, shown in Equation 1, was derived by Sir George Stokes.<\/p>\n

\"Equation
Equation 1. The sedimentation velocity of a spherical particle in a Newtonian liquid.<\/figcaption><\/figure>\n

In this equation ‘g<\/strong>‘ is the gravitational acceleration.<\/p>\n

The most important factor, governing how fast a particle will sink, is the density<\/strong> of the particle, or, to be more precise, the difference between the density of the particle and the density of the surrounding liquid (\u03c1p<\/sub> – \u03c1l<\/sub>).<\/p>\n

The second factor is size<\/strong>: big particles sink faster than small particles. The sedimentation velocity of a spherical particle is proportional to the square of the radius of the particle (r2<\/sup>).<\/p>\n

The third factor is the viscosity<\/strong> of the surrounding liquid (\u03b7). A liquid with low viscosity results in faster sedimentation.<\/p>\n

The equation is valid for smooth particles of spherical shape. A fourth factor, governing how fast a particle sinks, is the shape<\/strong> of the particle.<\/p>\n

Preventing sedimentation<\/h3>\n
\"Figure
Figure 2 – Physical network preventing sedimentation<\/figcaption><\/figure>\n

In order to understand how sedimentation can be prevented, take into consideration that sedimentation is mainly a problem that occurs during storage of a paint or ink. During storage, the system stands still and the main force acting on the system is gravity. The only way to prevent particles from sinking is by assuring that a three-dimensional physical network is present in the system during storage.<\/p>\n

The physical network must be strong enough to give the material a so-called yield stress<\/strong>, also called yield value or yield point. When a material has a yield stress, it implies that during storage, at low shear stress, the material behaves as an elastic solid. This implies that the material has an infinite viscosity during storage. The solid particles are \u2018frozen-in\u2019 and sedimentation is prevented.<\/p>\n

Rheology additives<\/h3>\n

A physical network can be created in a paint or ink by using a suitable rheology additive<\/strong>, often referred to as anti-settling agent<\/strong>. The additive is used to prevent sedimentation and settling. 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 sufficient shear force is applied.\u00a0Many rheology additives are developed for a wide variety of paints and inks.<\/p>\n

A few examples of rheology additives, used to prevent sedimentation:<\/p>\n