Inactive pigments are presented as a key alternative in the formulation of paints, and can offer additional benefits in the interaction with other components.
by Julián Restrepo*
An active pigment, in its most basic definition, is a solid material, in the form of a fine, inert powder that has the property of providing color and opacity (covering power) to a paint [1], and differs from a dye (another material in the form of a fine powder that provides color), because it has the characteristic of being insoluble in the medium in which it is located, in this case the vehicle (mixture of resin and solvents), while the dye is soluble in it; therefore, it is logical to expect that paints formulated with pigments present sedimentation, while those formulated with a good dye will not have a tendency to present this phenomenon.
We must also say that, compared to pigments, dyes are characterized by their high dyeing effort and low opacity, while pigments provide greater opacity, but have less dyeing effort.
Thus, the basic difference between pigment and dye is that the first is insoluble and the other is insoluble in the medium in which it is located, but other factors must be taken into account. This is how we can raise some special cases, where confusions can occur, for example:
- You have a paint formulated with a dye soluble in organic solvents, but insoluble in water; if at any given time this water-based paint were formulated, the dye would already behave like a pigment, since it would become insoluble in the medium.
- Suppose (example for illustrative purposes),1 that you have a pigment that is soluble in acids (such as zinc oxide, ZnO); This pigment in paints, either water-based or solvent-based, will be insoluble, since these are normally found in alkaline pHs, but if one of these paints were formulated at acidic pHs, this pigment would solubilize in the medium and behave like a dye.
As for their classification, pigments are divided into active and inactive, as well as organic and inorganic, and also in turn can be classified as natural, synthetic and artificial. We say that a pigment is active when it provides color and opacity to the paint, while the inactive (sometimes referred to, unfairly only as loads), fail to provide color and opacity to the paint. Strictly speaking, the concept of PVC (pigment concentration in volume) as a formulation parameter comes into play here, since an inactive pigment can contribute to the coverage of PVCs near or above the CPVC (critical PVC), but delving into this topic is not among the objectives of this work.
At this point it is necessary to make several comments:
- In simple technical terms, in case the inactive pigments cannot provide color or covering to the paint, it is due to the fact that they have a refractive index (IR) similar to that of resins, which is between 1.45-1.55 approximately. Thus, because when the coating film formation occurs, the pigment is embedded in the polymer matrix, an active pigment provides color because its IR is outside the range of 1.45-1.55, while the inactive pigments cannot provide color, because its IR is within this range. Titanium dioxide (rutile type), for example, has an IR of 2.71, making it an active pigment; while talc has an IR between 1.54-1.55, so it is considered an inactive pigment.
- When we have a dry inactive pigment, we observe that it has a whitish color (grayish, creamy, etc., depending on the type of pigment and its purity), this is by the medium in which it is: the air, which has an IR of 1.0, with which although the inactive pigment has an IR between 1.45 and 1.55, it has color in the air, since its IR is greater than that of the medium in which it is located (remember that we are talking about a fine dust and between these particles there is air).
- The situation begins to change a little when we wet the pigment, for example with water, since we will see that the pigment will become a little more transparent, that is, it will lose part of its color; this is because the water has an IR of 1.33, with which said IR begins to approach the IR range of the inactive pigment (there are even inactive pigments that have IR < 1.33, con lo cual son transparentes en agua). Es decir, a medida que aumentamos el IR del medio en que se encuentra el pigmento inactivo, éste se hará cada vez más y más transparente, hasta que el IR del pigmento inactivo se hará igual al IR del medio, con lo cual se hará transparente para el ojo humano.
- In some applications it may be advantageous to use inactive pigments that are transparent in the medium, so that they make it possible to make the system cheaper, because they allow to include "a transparent product volume", which increases the performance of the product. In the case of wood paints, for example, when matting agents are used, it is sought that these reduce the brightness appreciably, without affecting the transparency of the finish; that is, that its effect on the properties is very noticeable, but that its presence is not noticed.
- We must also look back at the definition of pigment, where it is said that it is an inert and insoluble material, since there are particular situations: calcium carbonate (CaCO3, of calcite origin) is an inactive pigment but has some reactivity with acids; In addition, it is not completely insoluble in water (15 μg/L, 25ºC), and can be said to contribute to the increase in the ion content of the vehicle (water), since a small part of calcium and carbonate ions will be in the aqueous medium (remember that water is the universal solvent).
- Finally, although inactive pigments by definition do not provide color to the coating, it is worth mentioning that they contain a certain level of impurities that modify the color of the final coating. This is more applicable in the case of natural loads, which present a relatively low cost, such as calcium carbonate, kaolin and talc, which contain known characteristic impurities. Although this will depend on the degree of benefit to which the natural material is subjected in its processing.
References
[1] Restrepo, J.A., "New developments in pigments". Inpralatina, Vol. 14, No. 1, January/February 2008, pp. 28-33.
1 This clarification is made, bearing in mind that the vast majority of substrates on which we paint have alkaline pHs, therefore the paints intended to protect them must have very close pHs. Thus, it is not common to propose a painting of an acidic nature.
*M.Sc. Ph.D. Technical Marketing at Insucolor S.A.S. [email protected] Medellín, Colombia.
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