Learn how to get more out of these substances to raise the quality of your paints.
Julian A. Restrepo*
An active pigment, in its most basic definition, is a solid, inert, fine powder-forming material that has the property of providing color and opacity (covering power) to a paint, and differs from a dye (another material in the form of fine powder that provides color), because it has the property that it is 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; of course, this taking into account other factors.
Dyes
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 found, but there are some special cases where you can have confusions, 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 is insoluble in the medium.
Suppose (example for illustrative purposes), that you have a pigment that is soluble in acids; this pigment in water-based or solvent-based paints will be insoluble, since these are normally found in alkaline pHs, but if one of these paints is formulated at acidic pHs, the pigment will solubilize in the medium and behave like a dye.
We must say that dyes are characterized by their high dyeing effort and low opacity, compared to pigments that provide greater opacity, but have less dyeing effort. Strictly speaking, pigments are classified into active and inactive, as well as organic and inorganic, and can also be classified as natural, synthetic and artificial.
Active pigments
We say that a pigment is active when it provides color and opacity to the paint, while the inactive ones 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, the fact that the loads cannot provide color or covering to the paint is due to the fact that the loads have a refractive index (IR) similar to that of resins, which is between 1.45 and 1.55 approximately. Because when evaporation of the solvent occurs, the pigment is embedded in the polymer matrix, and an active pigment has to provide color by the fact that its IR is outside the range of 1.45 and 1.55, while the charges cannot provide color, because its IR is within this range. Titanium dioxide, for example, has an IR = 2.7.
When we have a dry inactive pigment, we observe that it has a whitish color (grayish, yellow, etc., depending on the type of pigment and its purity), this is by the medium in which it is: The air has an IR = 1.0, so although the inactive pigment has an IR between 1.45 and 1.55, it has color in the air, since your IR is greater than that of the medium in which you are.
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 = 1.33, which begins to approach the IR range of the inactive pigment (there are even inactive pigments that have IR <1.33, which are transparent in water). That is, as we increase the IR of the medium in which the inactive pigment is located, it will become more and more transparent, until the IR of the inactive pigment = IR of the medium, which will become transparent to the human eye.
In other areas it can be advantageous to find inactive pigments that are transparent in the medium, so that it allows to cheapen the system, because it allows to include "a volume of transparent product" which increases the performance of the product. In the case of 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) is an inactive pigment but has some reactivity with acids; In addition, it is not completely insoluble in water 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 relatively low-cost natural fillers, such as calcium carbonate, kaolin and talc, which contain known characteristic impurities.
Inactive pigments
We can say that inactive pigments are those that "have no optical activity" in the coating, since by definition they do not provide color and opacity, being insoluble in the vehicle, but that have a much lower price than that of active pigments (such as titanium dioxide), and are mainly used to lower the cost of paints.
The general name used to designate the type of pigments that do not provide color is that of inactive pigments, inert pigments or simply fillers.
But despite being simply called fillers, in some cases, by careful selection of a specific type of inactive pigment for each particular type of paint, certain properties of it can be improved; that is, the proper selection of the inactive pigment at the formulation stage, can improve different properties, such as viscosity, fluidity, rheology, permeability, hardness, washability and abrasion resistance; as well as conveniently favoring the sedimentation conditions in the container during the storage time, in order to avoid or reduce it to the maximum. Certain loads mechanically reinforce the structure of the dry film, while others increase its resistance and especially to the penetration of moisture through it.
In these cases where inactive pigments are not only limited to lowering the cost of paints, but also provide additional effects and properties, they are called "functional loads"; that is, its function is not limited to reducing the cost of painting, but to providing additional beneficial properties to it.
Such is the case of extenders, as a partial substitute for the active pigment; but there are other functional loads, such as some anti-decanting agents (bentonite clays), matting agents (silicas), corrosion inhibitors (phosphates, phosphososilicates), among others; but the cost of these functional loads is, in many cases, higher than that of active pigments; that is, although they do not provide color, they provide other advantages that are highly appreciated and for which they are hardly surpassed by other materials.
Extenders
Among the inactive pigments are the extenders which, as already said above, are a type of functional loads.
Extender is a word, translation from English that means extender and that denotes, among others, amplifier, enlarger or developer, and that contextualized to the formulation of coatings, indicates that the extenders, being loads, are not only used in order to reduce the cost of paints, because they are relatively inexpensive raw materials and their specific gravity is generally lower than that of an active pigment.
In addition, the extenders due to their characteristics are used to replace part of the titanium dioxide of the formulation (the white pigment par excellence used in the paint industry [1]); thus they allow to reduce a part of the titanium dioxide used, which is replaced by a pigment that is cheaper than titanium dioxide, which finally allows to reduce the cost of painting, maintaining the coating and color, and of course, without appreciably modifying the rest of the properties.
There are cases where the use of an extender causes a favorable impact on other properties of the paint, such as the viscosity, rheology or pH of the product, which in turn allows to reduce the use of other raw materials, in many cases additives that are relatively expensive (pH regulators type AMP, thickeners, flow regulators...).
That is, in this case we have that extending not only works as a "functional charge", but not only allows "extending part of the titanium dioxide", but interacts synergetically with other components of the formulation; we could then talk about having a "functional extension", but it would already overload the term a little.
It should be noted that, due to the complex relationships that occur between the final properties of a coating and the raw materials used, it is expected that effects will be obtained in the different properties of the paint when replacing part of the active pigment. What happens strictly speaking is that the formulator always focuses his attention on a certain number of properties that "the end user will see", with which the others lose importance.
The inactive pigments usually used as extenders are found in pigments such as: Calcium carbonates, kaolins, talcs and silicas (note that they are of mineral origin).
It is clear, therefore, that the suppliers of these raw materials, by determining that their loads not only fulfill the function of lowering the cost of painting, but also provide other additional advantages (in this case, they work as extensions), raise the price of their product; this can be explained, in part, by the fact that manufacturers subject their product to a greater number of purification operations, to provide their product with greater added value.
Conclusion
It has therefore been seen that inactive pigments become key in the formulation of coatings, because their functions are not only limited to the fact that they function as "transparent raw materials" and as reducing factors of the cost of paints, but in many cases (in reality, most), additional beneficial effects can be obtained, they can even interact synergetically with other components of the formulation.
Although, it should be borne in mind that, unwanted effects and antagonistic interaction can also occur, these are smaller compared to the benefits obtained. The selection of the appropriate type of inactive pigment is therefore a factor that must be taken very seriously from the paint formulation process to obtain the best possible results.
References
[1] Restrepo, Julián A., "Titanium dioxide: Give your application the ideal pigment". Revista INPRALATINA, Vol. 10, No. 1, January/February 2005, pp. 22-23.
* I.Q. Depto de Química Inorganic y Orgánica, Universitat Jaume I, Castellón, Spain. . [email protected]
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