Hard sediment

Mechanical aids to resolution of an emulsion are important. Although no chemical assistance is required to form a hard sediment of closely packed emulsion, stubborn emulsions require application of a chemical agent for the final film-rupture stage. 

Many properties of liquid paints can be measured with considerable accuracy. Samples must be homogeneous. They must also be sufficiently large to be representative for a given batch. Impurities and permanent material defects (e.g., skin formation, a hard sediment, or gelling of the paint) can be detected. The most common investigations to which a sample is subjected before starting the tests are described in ISO 1513. Only those tests that are important for paint storage, transportation, and application are described here. 

In some cases, particularly with large and asymmetric particles, flocculation into the secondary minimum may occur. This flocculation is usually weak and reversible and may be advantageous for preventing the formation of hard sediments. 

With state (b), the particles are uniform and they will settle under gravity, forming a hard sediment (technically referred to as clay or cake ). The repulsive forces between the particles allow them to move past each other until they reach small distances of separation (that are determined by the location of the repulsive barrier). Owing to the small distances between the particles in the sediment it is very difficult to redisperse the suspension by simple shaking. 

With case (c), consisting of a wide distribution of particle sizes, the sediment may contain larger proportions of the larger size particles, but a hard clay is still produced. These clays are dilatant (i.e. shear thickening) and they can be detected easily by inserting a glass rod in the suspension. Penetration of the glass rod into these hard sediments is very difficult. 

States (b) and (c) are physically unstable (showing settling and formation of hard sediments), even though the system is colloidally stable. In this case the gravity force exceeds the Brownian diffusion. 

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