The hardness of the coating film is closely related to the glass transition temperature (Tg) of the film-forming substance. Synthetic emulsions with hard monomers or rigid segments can be used to produce products with high glass transition temperatures. Emulsion films with high T g have high hardness. Low temperature film formation may not be good. Although the contradiction between high Tg (high hardness) and low-temperature film-forming properties can be resolved to a certain extent by selecting a suitable film-forming aid, a better way is to change the structure of the emulsion particles and synthesize a core-shell structure by a two-step method. Emulsion can balance coating film hardness and low-temperature film-forming property.
Crosslinking is a good way to increase the hardness of the coating film. The cross-linking (including chemical cross-linking and physical cross-linking) between polymer molecules restricts the movement of chain segments, which increases Tg. The relationship between polymer T g and crosslink density ν is:
Tgc=Tg+νC
In the formula, Tgc——is the glass transition temperature of the polymer after crosslinking;
Tg——is the glass transition temperature of the uncrosslinked polymer;
ν——is the crosslinking density;
C - is a constant.
The higher the crosslinking density, the higher the T g will be, and the higher the hardness of the coating film will be. A study on a solvent-based polyether polyurethane found that as the crosslinking density ν increases, the hardness of the coating film increases accordingly, and there is approximately a linear relationship, as shown in Figure 2. For water-based coatings, the situation is similar. Compared with uncrosslinked coatings, the hardness of the coating film will be significantly improved after adding a crosslinking agent. Mild cross-linking is often used in emulsion synthesis to improve its performance. To prevent gelation caused by excessive cross-linking, the number of cross-linking points should not be too large, so the effect on the increase of T g and the improvement of coating film hardness will not be great. Additives, especially powdery fillers, can change the hardness of the coating film, so the hardness of the base paint is often higher than that of the varnish of the same base material. In addition, additives such as nano-colloidal silicon can also improve the hardness and scratch resistance of the coating film.
Fig.2 The relationship between the pendulum hardness H of the coating film and the crosslink density v
For more information, please refer to the full text of "Hardness of Waterborne Wood Coatings" (Zhu Wanzhang)
