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We all know that PUD and PUA are secondary dispersions, while many single component lotion are primary dispersions. The primary and secondary dispersions we are discussing here mainly refer to water-based hydroxypropionic acid resins. These primary and secondary dispersions appear milky white in appearance, and their coating and application are also very similar.

But in reality, there are still many differences between the two.

The first one is that the synthesis method is different. The primary dispersion, also known as lotion, is an aqueous dispersion obtained by lotion polymerization. Secondary dispersion is generally obtained by polymerizing in a solvent and then dispersing in water.

The second one, check if it contains emulsifiers. In the primary dispersion, there is a secondary dispersion containing emulsifiers, usually without emulsifiers and requiring free emulsification.

The third one, check if it contains solvents. A primary dispersion is dispersed in water in combination with a liquid and generally does not contain solvents.

Secondary dispersions usually contain a small amount of solvent because they are polymerized in a solvent. The solvent content is generally around 3%.

The fourth one is also different in molecular weight. The molecular weight of polymer obtained by lotion polymerization with primary dispersant is very large.

Can be used for single component film-forming secondary dispersions. Synthesizing polymers with relatively small molecular weights in solvents makes it difficult to obtain polymers with high molecular weights.

The fifth one is that the molecular weight and particle size of primary dispersions, which have different dryness and fullness, are relatively large.

After film formation, its drying speed is relatively fast, and some low hydroxyl resins can even form films independently. The leveling gloss and fullness of the paint film are generally average. The tree value of the secondary dispersion, because its particle size is usually small, and the whole lotion is translucent. Film forming ability is better than first degree elasticity, but drying is relatively slower. But the leveling, gloss, fullness, and various durability of the paint film are better than those of primary dispersions.

The sixth one, which is the combination of curing agents, may also have differences. The primary dispersion, generally an emulsion lotion, is usually a curing agent that must be combined with water-based isocyanate. This special structural design can make the emulsified particle size of the curing agent smaller, resulting in better gloss and gloss stability of the paint film. The cross-linking of hydrophilic groups in the resin is also more complete, and its performance will be better. Has better stability and superior appearance.

From an application perspective, primary dispersions are usually used in systems that require high dryness and long activation periods, such as water-based wood coatings, general steel structure coatings, and general industrial coatings. Secondary production units are usually used in systems that require high gloss, fullness, durability, and so on, such as engineering machinery, rail transit, and so on.

Generally speaking, the price of primary dispersion is relatively cheap, especially in the early days when the price of secondary dispersion was very expensive. However, with the development of domestic products and increasingly fierce competition, the price of secondary dispersion has become more affordable. Another good secondary dispersion can be combined with oil-based curing agents, which further reduces costs. The primary dispersion basically needs to be combined with a water-based curing agent. So the application scope of primary dispersions is gradually narrowing.