Due to the special nature of polymer emulsions, the film-forming process is fundamentally different from ordinary coatings in which the resin is dissolved in a solvent, as well as water-soluble coatings. Because the emulsion is composed of polymer particle spheres, rather than individual polymer molecules, drying into a film must involve the fusion between these larger particles. This process can be divided into several steps: beginning with the evaporation of water, the overall volume of the coating shrinks, and the polymer particles approach each other. In order to achieve direct contact between the particles, the electrostatic repulsive force between the particles must first be overcome. This electrostatic repulsive force is the force that originally maintained the stability of the dispersion. After the particles are in contact with each other, the arrangement and deformation of the particles required for film formation so that they are fused with each other have not been effectively performed, and only when the capillary force and surface tension between the particles are greater than the deformation resistance of the particles. The following steps can be well described by the polymer glass transition temperature, and Tg is closely related to another important index of the coating, MFFF (minimum film formation temperature). The coating film can be formed only when the dispersion particles are on the polymer, specifically, on the MFFT. So people have to find the corresponding monomers for synthesis according to the molding temperature Tp, or add an external plasticizer, so that Tp ≥ MFFT.
Due to this limitation, both the low glass transition temperature and a certain degree of hardness must be guaranteed, so the needs of many applications cannot be met. Adding so-called film-forming aids and coagulants to the system makes this theory break through. This kind of material can also be called temporary plasticizer, which can soften the polymer particles, and after the film is formed, it will volatilize from the coating film. In this way, we can use a hard polymer with Tg> Tp to form a film at room temperature and obtain a hard coating film.
To be used as a film-forming aid, a product must meet certain requirements. First, the product used as a temporary plasticizer must be sufficiently chemically stable, for example, not to be saponified in alkaline formulations. Secondly, the odor should be small, and coatings used indoors have such requirements. Another requirement is that the film-forming aids must be compatible with these sensitive polymer dispersions, and there will be no instability after addition. This instability can be observed from a large increase in viscosity, or the most serious case is the same cause of gelation. Care must be taken when handling to prevent local vibration reactions (gelation). Generally, after the final product is prepared, it needs to be left for a period of time to achieve the distribution balance of the film-forming aid. It is also required that such temporary plasticizers have the best possible plasticizing effect on the polymer to reduce its amount of addition. In terms of volatility, the film-forming assistant is required to remain in the coating film during the film-forming process to play a plasticizing role, and once the film-forming is completed, the film-forming assistant is immediately evaporated. If the film-forming aid is left in the coating film for a long time, it can cause stickiness, lumping, and contamination. Therefore, we can think that the ideal film-forming aid does not exist. We must find an optimal compromise solution and use multiple products in combination.
A three-phase model can well describe the properties of film-forming aids. This model is based on the fact that there is a certain relationship between the distribution of this substance used as a film-forming aid in the three phases of water, particle interior and particle shell in the system. The presence in the particle shell is definitely the best, because film formation takes place here. If the film-forming aid is particularly water-soluble, then it is mainly distributed in water, and the plasticizing effect is naturally small. If the film-forming aid is more distributed inside the polymer particles, the effect on film formation is also moderate. In addition, this operation method can be adopted: that is, the use of a film-forming auxiliary with low water solubility, and the operation is in the aqueous phase. In this way, the film-forming aid will slowly enter the polymer particles from the water. However, there is a problem with this operation, because the film-forming aid is present in the water phase in the form of droplets, and if it is left for a long time, the coating film will be cloudy. One solution is to add a temporary plasticizer (dissolved into the monomer) during the polymerization reaction. To know if a product is suitable for use as a film-forming aid, it is necessary to know its distribution in the dispersion, its plasticizing effect on the polymer, and its volatilization rate. This evaluation is different for different dispersions and different for different applications, so there is no universal film-forming aid.
In summary, there are no fixed rules for the selection of film-forming aids, but only some basic points to consider. What is selected in the formulation is the adhesive that is most suitable for its use, the formulation that is most suitable for this formulation, and the film-forming aid that is most suitable for this application.