Лако-красочные материалы — производство

The pigmentation of latices is different from that of most resin systems. In most systems, the resin (binder) is soluble in the liquid system. Pigment is dispersed in the solution which can be considered as a varnish (for inks). Resin can be absorbed onto the pigment. Partially soluble resins can coat the pigment, but there is a significant amount of soluble resin present in the pigmented system, and evaporation (or loss) of solvent results in the resin being deposited over the surface in a layer which may vary in thickness, depending upon physical forces and inter-reactions during drying.

In a latex system there is essentially no soluble binder resin. The pigment particles must be dispersed with polymer particles. The polymer particles and pigment particles must stick together. Too much sticking and flocculation occurs. Pigment particles can induce charge destabilisation, particularly some of the modem pigments which are surface treated.

Thus, a pigmented latex consists of agglomerates of pigment and polymer particles. However, the pigment acts as a non-deformable particle, similar to a monomer of high Tg. Thus, during film formation the polymer particles must be capable of deforming more than in a non-pigmented system. This also requires the loss of water to be slow enough to allow this to happen, otherwise incomplete film formation occurs. Not only must the polymer particles flow into each other, they must flow around intervening pigment particles.

Obviously the nature, surface and surface area of the pigment particles have important effects on latex stability and subsequent film formation. However, for any given pigment, the polymer particle size, Tg and surfactant level of the latex are critical. Determination of the CPVC must take into account the particle size of the polymer as well as the pigment void volumes.

One of the important properties is for the pigment not to flocculate. The smaller the polymer particles the higher the CPVC, and this also applies for softer particles which can deform and stick more readily.

However, the pigment binding power is lower in emulsions that solution polymers. The situation is further exacerbated by the poor wetting characteristics of water, making high pigment loadings difficult to achieve. Typically, a white alkyd would have a pigment to binder ratio of 1:1, whereas the corresponding water-based acrylic would be in the region of 1:1.5 without a commensurate decrease in gloss.

Most of the common pigments and extenders can be used with emulsion polymers. The main problem is that pigments can adsorb surfactant from polymer particles and destabilise the latex. Calgon is often used as a dispersant and a surfactant or benzene sulphonic acid may also be added. These additives ensure thorough wetting of pigments and prevent destabilisation of the latex. A colloid is also added as an aqueous solution to assist pigment dispersion and also to increase the viscosity. Hydroxyethyl cellulose is a commonly used colloid for this purpose, but methyl cellulose, ammonium salts of poly(acrylic acid) and poly(vinyl alcohol) have also been used. Both vinyl acetate and acrylic ester polymers have refractive indices below 1.55 which enhances the opacity of pigments used with them.

The majority of pigment types are compatible with the acrylic emulsions, though certain organic pigments, such as toluidine reds, are incompatible with some of the more common coalescing solvents, e. g. ethylene glycol butyl ether.

Factors worthy of consideration include:

• The degree of dispersion will make a major contribution to the gloss level of an emulsion system.

• PVCs will be lower for a full gloss acrylic emulsion than for the corresponding conventional acrylic system.

• Carbon black pigments may require dispersion at a high pH (e. g. ~9) to ensure good wetting, no flocculation and a satisfactory gloss level.

• Inhibited aluminium pigments are also available, and these should be added after the necessary pH adjustment of the formulation. Optimum stability for aluminium pastes is at pH ~7.

• Interference pigments, micas, should be considered as a more stable alternative to inhibited aluminiums for certain colours.

If an acid or amine functional inverted resin is present, then care must be taken to ensure that reactions do not occur between the functional group and the pigment. This could easily lead to destabilisation or change in colour or shade or flocculation.

In recent years, polymer emulsions with inherent pigment wetting capability have been developed, for the production of in-house mixing schemes. These, in common with their solvent based counterparts, have one major limitation in that they have a negative effect on the final physical properties of the dried film.

The reader is recommended to consult a paint technology text book for more detailed descriptions of the effects of pigment concentrations, below and above, the CPVC on the performance of the film.