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Types and Mechanisms of Waterborne Coating Additive Systems: Introduction to Substrate Wetting Agents

Water-based coatings use water as the medium, water-based coating additives are essential in the preparation and application of water-based coatings, water-based coating additives mainly include: substrate wetting agents, defoamers, levelling agents, dispersants, thickening agents, multi-functional additives, film-forming additives, biocides and so on. The following is the first introduction to the substrate wetting agent.

Substrate wetting refers to the process by which gases on the surface of a substrate are replaced by liquid paint. This process is critical to the successful application of the coating. Wetting can be described by the contact angle θ, the size of which depends on the amount of surface energy and surface tension.

If the contact angle θ < 90°, the coating can wet the substrate. the smaller θ, the better the wetting spread.

Substrate wetting agent consists of two parts, hydrophilic end and hydrophobic end, with small molecular weight. One part acts at the liquid-solid interface to form an intermediate sandwich, making it easier for the liquid to spread; the other part acts at the gas-liquid interface to reduce the surface tension of the liquid and promote the wetting of the substrate.

Types of substrate wetting agents for water-based coatings

There are four main categories:

(1) Polyether-modified dimethylsiloxane-based substrate wetting agent greatly reduces the static surface tension of the system, with excellent wetting and anti-cratering properties, and does not increase the surface slip, does not affect the interlayer adhesion, but there may be a disadvantage of stable foam. These substrate wetting agents include: Tego Wet 270, Tego Wet 280, BYK 346, BYK 349, BYK 3481 and so on.

(2) Organic biogenic polyether modified siloxane wetting agent contains at least two hydrophobic chain segments, two hydrophilic chain segments, a flexible bridging chain or bridging group. The molecular structure can strongly reduce the static surface tension of the system, a certain degree of reduction in the dynamic surface tension of the system, to improve the penetration of the very low energy surface, wetting and spreading, with excellent anti-shrinkage performance; also has a certain degree of antifoaming properties. These products are mainly: Tego Twin 4100, Tego Twin 4200 and so on.

(3) The wetting agent of the alkynediol bidentate surfactant type is an ethoxylated alkynediol surfactant with a molecular structure in which the spacer groups generally link amphiphilic and biphobic molecules, and the critical micelle concentration (CMC) is an order of magnitude lower than that of the “singlet” surfactant, meaning that the same or even better wetting effect as that of the “singlet” surfactant is achieved with a smaller amount. The critical micelle concentration (CMC) is an order of magnitude lower than that of “monomer” surfactants, which means that a smaller amount can achieve the same or even better wetting effect as “monomer” surfactants. However, because this type of surfactant does not contain silicon, its ability to reduce static surface tension is not very strong, wetting spreading is relatively weak, but has the ability to strongly reduce dynamic surface tension, the permeability of the substrate is also good, and has excellent anti-foaming and foam inhibition performance, does not affect the interlayer adhesion. These products mainly include: Surfynol 440, Surfynol 104E, Surfynol AD01, BYK-DYNWET 800 and so on.

Fluorocarbon surfactants are currently less commonly used in the actual production and application of waterborne coatings due to their high side effects.

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