Лако-красочные материалы — производство

It is difficult, if not impossible, to accurately define the difference between a dispersant and a wetting aid. One simple, but general, definition is that dispersants keep the dispersed particles apart, whereas wetting aids reduce surface tension which may permit dispersion. A wetting aid can be a dispersant, but a dispersant cannot be a wetting aid.

Dispersing aids are almost invariably a necessity in aqueous systems, since the polymers have an inherently poor ability to wet and disperse pigments.

They fall into two distinct types, namely :

• Additives to improve dispersing

• Resins (solution) into which pigments or extenders can be dispersed.

The first category is used to enhance the performance of the second category. However, there are additives which can be used without a resin. For example, some pigment dispersions use propylene glycol as the binder with additives to help disperse the pigments.

The most usual ’in house’ method would be to disperse the pigments/extenders into a suitable solution dispersed resin with the use of additives to improve efficiency and increase loadings. The resins available for this kind of dispersion are usually solubilised by the addition of bases (amines are the most common). Some come with the amine added. Others require addition on site to make them soluble. They can then be treated in many respects as if they were solvent based systems.

It may be necessary to add wetting and dispersing agents and a defoamer to the millbase. If formulated correctly, pigments and extenders can be dispersed easily.

Care has to be taken to ensure that the equipment is suitable for use with water, as rusting will occur very rapidly in mild steel equipment, particularly with the heat generated in the milling process. Stainless steel is recommended for use throughout.

Dispersants prevent re-agglomeration and maintain stability by one of three general mechanisms

• Cationic

• Anionic • Nonionic

The first two utilise a principle of charge repulsion whilst the third works through steric hindrance. In most waterborne systems, anionic dispersants are preferred due to their greater efficiency. Cationic dispersants are only used in specialised instances. Of the many types that are available, the following are most commonly used;

• Phosphates

• Inorganic alkali

• Polyamines

• Aminoalcohols

• Polyalcohols

• Organic polyacid alkali salts

‘Difficult’ pigments such as carbon black or phthalo blue are often best dispersed using a resin-based dispersant, although ultimately these may require more efficient mixing, such as bead or sand milling.

Incorporation of dry powders and substrate wetting are assisted by the use of a surfactant-based wetting agent. With both dispersants and surfactants, it is important to select products with low acid values in order not to compromise the water resistance of the final film. Often a blend of surfactants and dispersants has proven to be the most efficient. The wetting agents can also affect other properties, such as sheen, viscosity, flow and film formation, and they can have an adverse or beneficial effect. The correct evaluation and choice can have wide ranging repercussions. Wetting agents can stabilise foams but they can also create severe aeration problems.

Some common dispersants and surfactants are given in Table 7-9.

Suppliers include Zeneca Resins, Johnson Polymers, Allied Colloids, Vinamul, Rohm & Haas, Akzo, Alfa, Croda, Henkel, I. C.P., and Rhone-Poulenc.