Color filler is the most used powder material in the field of coating, which can make the coating have the functions of coloring, covering, protecting, decorating, anti-corrosion, temperature resistance, enhancing, weathering, beautifying and improving the living environment.
The surface drying of latex paint is relatively fast, generally no more than 2h under standard conditions. However, the complete film formation time of latex paint is relatively long, which takes several weeks to one month. Free volume is the main factor affecting the film formation rate, synthetic wax dispersion and the free volume is largely determined by the temperature difference (T-Tg) .
Based on the well-known WLF equation, Win nik et al. derived an equation for evaluating the plasticizing efficiency of film-forming additives.
1g[D(W;) T/D(T+AT)] =CAT/(Cz+△T)(18-4) where T-—temperature, K; D(W)—temperature is T, film formation The amount of auxiliary agent is W; the diffusion coefficient when; C, C2-constant.
AT is defined as the plasticizing effect that promotes the diffusion of polymers. Obviously, adding a film-forming aid is equivalent to providing free volume, thereby providing conditions for the inter-diffusion and permeation of polymers, and finally achieving the result of film-forming.
Requirements for film forming aids
In fact, the film-forming assistant is a polar solvent with high boiling point, low glass transition temperature and very slow volatilization that is added to latex paint to help polymer film formation.
High film-forming ability
The film-forming aids are required to have high film-forming properties. A polymer has its own characteristic T value due to its different composition and structure. This polymer emulsion also has an MFT value corresponding to its ?, value. If a film-forming aid is added to this emulsion, an effective film-forming aid must be a good solvent for the emulsion polymer. Therefore, the solubility parameter has guiding significance for understanding
discriminating and selecting film-forming aids. When the solubility parameter of the emulsion polymer is the same or similar to the solubility parameter of the film-forming aid, the film-forming aid can have better film-forming properties. The solubility parameters of the emulsion introduced by K.Ala happe ruma are shown in Table 18-2, and the solubility parameters and molar volumes of the film-forming aids are shown in Table 18-3.
It can be seen from the table that the solubility parameters of the two do not match well. In latex paint, the film-forming aid at the interface of polymer particles and water softens the surface of latex particles due to dissolution, so that the particles are easily deformed and can be brought closer together under a small force. Also because the film-forming aid is low T. Yes, at the construction temperature, it can provide more self-mounting volume: make polymer molecular chains diffuse each other and fuse to form a film. That is to say, a polymer with a higher T value can form a film at a lower temperature when a film-forming aid is used, so that higher film performance and lower application temperature can be unified. . At the same time, it is required that the film-forming assistant can effectively reduce the MFT of the system, that is, the amount is as low as possible.
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