{"id":21951,"date":"2026-07-08T06:00:02","date_gmt":"2026-07-08T12:00:02","guid":{"rendered":"https:\/\/ulprospector.ul.com\/?p=21951"},"modified":"2026-06-09T12:40:17","modified_gmt":"2026-06-09T18:40:17","slug":"pc-how-to-prevent-sedimentation-settling-during-storage","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/21951\/pc-how-to-prevent-sedimentation-settling-during-storage\/","title":{"rendered":"How to Prevent Sedimentation & Settling During Storage?"},"content":{"rendered":"

\"\"Most paints and inks contain solid particles, mostly pigments and\/or fillers. Two instability problems, flocculation<\/strong> (the spontaneous gluing together of separated particles because of particle-to-particle attraction)1,2<\/sup> and sedimentation<\/strong><\/a> (the sinking of particles because of gravity), can occur with solid particles that have been separated from each other and distributed in a liquid ink, paint or pigment concentrate. This article describes the basics of sedimentation and how to prevent this undesired phenomenon.<\/p>\n

Sedimentation defined<\/strong><\/p>\n

Sedimentation is the process of solid particles sinking in a liquid 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>3<\/sup>.<\/p>\n

The basics of sedimentation<\/strong><\/p>\n

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 surrounding liquid. The process of particles sinking, called sedimentation<\/strong>, causes problems during storage and transport. The speed of sedimentation is governed by a few properties.<\/p>\n

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Sedimentation of a solid particle in a liquid caused by gravity.<\/figcaption><\/figure>\n

The equation that describes sedimentation velocity v<\/strong> of a solid particle of perfect spherical shape in a Newtonian liquid was derived by Sir George Stokes.<\/p>\n

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

The Stokes equation shows that three factors govern how fast a particle, that is surrounded by liquid, sinks. 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, is important. 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>). The third factor is the viscosity<\/strong> of the surrounding liquid. A particle goes faster down in a liquid of low viscosity. The Stokes equation is valid for smooth particles of spherical shape.<\/p>\n

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A fourth factor, governing how fast a particle sinks, is the shape<\/strong> of the particle.<\/p>\n

Prevent sedimentation<\/strong><\/p>\n

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

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Reversible physical network.<\/figcaption><\/figure>\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. The network is broken down, and the material becomes fluid again, when enough force is applied, for example by stirring.<\/p>\n

Check sedimentation stability<\/strong><\/p>\n

Some companies have a sophisticated rheometer that is capable of measuring yield stress. Most paint and ink producers, however, must rely on simple storage tests, in which samples are stored in an oven at elevated temperature, like 50 or 60 \u00b0C. An attention point is that during such standing-still tests, stability under movement, like in a truck or on a ship, is not take into account. This can give unpleasant surprises.<\/p>\n

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Sedimentation testing at elevated temperature.<\/figcaption><\/figure>\n

Additives to prevent sedimentation<\/strong><\/p>\n

A reversible physical network can be created in a liquid paint or ink by using an additive, often referred to as anti-settling agent<\/strong>. The additive is designed to prevent sedimentation and settling. The physical network must be strong enough to resist the gravitational force during storage as well as the combination of gravity and shaking during transport. On the other hand, the network must be weak enough to be broken down as soon as sufficient shear force is applied. A range of additives has been developed for a wide variety of paints and inks.<\/p>\n

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