critical surface tension
Pigment critical surface tension (γc) is an important index to measure the difficulty of wetting the pigment surface.
The critical surface tension of inorganic pigments is high, and its surface is a high-energy surface, so it is easier to disperse. The critical surface tension of organic pigments is low, and its surface is a low-energy surface, γc<100rnN/m, so it is difficult to wet and disperse. For example, the critical surface tension of cyanine blue and toluidine red are 31.3mN/m and 27.5mN/ m.
The critical surface tension of pigments can be determined by the Zisman method. Using several liquids with different surface tensions, measure their contact angles with the tested pigments respectively. The cosine of the contact angle is plotted against the surface tension, and a straight line is obtained, which is extrapolated to c0sθ=1, that is, the contact angle is 0, and the surface tension corresponding to this intersection point is called the critical surface tension of the tested pigment.
oil absorption and water absorption
Oil absorption refers to the mass of linseed oil absorbed by a certain amount of dry pigments bonded into a paste. Generally, it is expressed by the mass of refined linseed oil absorbed by 100g of pigment to reach the specified state, g/100g.
From a theoretical point of view, the significance of oil absorption includes adsorption, wettability, capillary phenomenon, and pigment particle size, shape, particle size distribution, surface area and surface treatment. In essence, the oil absorption is the amount of linseed oil required at the critical pigment volume concentration (CPVC), as shown in formula (3-2).
CPVC-1/[1 ten (0AXρ/93.5)] (3-2)
In the formula, 0A-oil absorption, g/100g;
ρ-pigment density;
93.5-linseed oil density X100.
It can be seen from the formula (3-2) that the lower the oil absorption of the pigment, the higher the CPVC of the coating can be, so that the coating with better performance can be obtained under the same raw material cost. But sometimes it's not always true.
The initial choice of linseed oil as the oil absorption medium was natural because at that time (1920s) linseed oil was the main paint base. The paint industry continues to use linseed oil to evaluate paint pigments, but some alternative liquids have emerged.
Since latex paint uses water as the dispersion medium, some pigments for latex paint are evaluated by water absorption. Some also add a certain amount of wetting and dispersing agent in water, such as sodium polyphosphate Calg0nN, to make the pigment more easily wetted by water, so that it is easy to grasp the drop to the end point, and at the same time, the surface tension of water is closer to that of the emulsion. But water is the dispersion medium, not the base material, so the concept of the relationship between water absorption and CPVIC is not clear.
