Summary of the mechanism analysis of different types of coating film forming methods
General coatings are composed of three components, namely film-formers, pigments and solvents. Sometimes it is necessary to add some functional additives to improve its special function.
The coating will form a thin film on the surface of the substrate. This film has certain mechanical properties, in addition to some decorative properties and other special functions are increasingly being pursued. For example: glow-in-the-dark, color changing, heat preservation, special touch, etc.
Generally speaking, the coating starts as a flowing liquid and forms a cured film only after the coating is completed, with the following main ways of film formation.
01 Film formation by solvent evaporation and hot melt
Polymers in general exhibit better physical properties only at higher molecular weights, but with high molecular weights, the glass transition temperature is also high. In order to make them coatable, the glass transition temperature of the system must be lowered with enough solvent to make the value of T-Tg large enough to allow the solution to flow and coat.
When the solution is close to about 0.1 Pa-s at room temperature, it can be used for spraying. After coating, the solvent evaporates and a solid film is formed, which is the general form of plastic coating film formation.In order to make the paint film flat and smooth, a good choice of solvent is required.
If the solvent evaporates too fast and the concentration rises quickly, the paint on the surface can lose fluidity due to high viscosity, and as a result, the paint film is not flat; in addition, evaporating too fast, the surface temperature may drop to zero point due to excessive heat loss during solvent evaporation, which will make water condense in the film, leading to the loss of transparency and whitening or making the film strength decrease; different solvents will affect the morphology of polymer molecules in the paint film. Translated with www.DeepL.com/Translator (free version)
As mentioned earlier, polymer molecules in bad solvents are curled into clusters, while those in good solvents are stretched and relaxed. The microstructure of the final paint film varies greatly from solvent to solvent, with the former molecules being less intertwined while the latter are tightly wound, and the former often having much higher strength.
This film-forming method can be illustrated by the can interior vinyl chloride paint, in which polyvinyl chloride is dissolved in a mixture of butanone and toluene solvents so that the viscosity of the resulting polyvinyl chloride solution reaches about 0.1 Pa-s at 25°C. After coating, the solvent gradually evaporates and Tg rises continuously. After three days, Tg reaches about room temperature, i.e., T-Tg=0, which means that the free volume has reached the minimum and cannot provide sufficient pores for molecular movement, and the solvent is not easy to escape from the membrane, but at this time there are about 3-4% of solvent bound in the membrane, and these solvents must be heated at 180°C (i.e., increase T -Tg value) for more than 2 min to be removed. Translated with www.DeepL.com/Translator (free version)
In order to make the polymer into a film, in addition to adding solvents to lower the Tg of the system, the temperature can also be increased to increase the T-Tg (i.e., to increase the free volume) so that the polymer reaches a flowable level, i.e., heating to melt the polymer. This is another form of thermoplastic coating film formation, i.e. hot melt film formation, such as polyethylene coated on milk paper bottles.
Powder coatings are also hot-melt film-forming: polyethylene, polyvinyl chloride, polyacrylate and other plastic polymers can be crushed into powder, which is then attached to the surface of the substrate by electrostatic or thermal means, and is heated to above the melting temperature, and after the molten polymer viscous fluid is leveled, cooling is the solid paint film. Powder coatings are mainly thermosetting crushed end coating, which is also accompanied by cross-linking reaction during the heating and melting process, the content of the powder coating will be discussed later.
02 Chemical film formation method
After the molecular weight polymer is coated on the surface of the substrate, the molecular weight is further increased or cross-linked into a tough film by the reaction between molecules under heating or other conditions. This is the common way of film formation for thermosetting coatings, including photosensitive coatings, powder coatings and electrophoretic paints.
These include dry oils and alkyd resins through the action of oxygen, amino esters and hydroxyl-containing alkyd resins, polyesters and acrylic resins through ether exchange reactions, epoxy esters cross-linked with polyamines, polyisocyanates and hydroxyl-containing oligomers to produce polyurethane films, and photosensitive coatings through free radical polymerization or cationic polymerization, etc., which will be discussed in turn. It is important to note that a solvent evaporation process is generally included before or at the same time as the chemical reaction.
02 Film formation of latex
Before discussing emulsion film formation, it is important to clearly distinguish the difference between emulsions and emulsions: emulsions are solid particles dispersed in continuous phase water,while emulsions are liquids dispersed in water. Generally emulsions are prepared by emulsion polymerization. The characteristic of emulsions is that their viscosity is independent of the molecular weight of the polymer, so when the solid content is above 50%, they have a low viscosity even with a high molecular weight. After the emulsion is coated, as the water evaporates, the particles get closer to each other and finally a transparent, tough and continuous film can be formed, but some emulsions only get powder after drying and do not get a tough film.
The ability of latex to form a film is related to the nature of the latex itself, especially its glass transition temperature, and to the drying conditions.Since latex is used in a wide range of coatings and other applications, and most of them require latex film formation, it is very important to understand the mechanism of latex film formation. The process of latex film formation is complex and the current views differ, so here is only a brief introduction.
After the latex is coated, the latex particles can still move freely in the form of Brownian motion. When the water evaporates, their movement is gradually restricted, and finally the latex particles are close to each other to form a tight accumulation. Due to the protection of the double electric layer on the surface of the latex particles, the polymer in the latex can not be in direct contact with each other, but at this time the latex particles can form a small radius of curvature between the gaps, equivalent to a small “capillary”, the capillary is filled with water. The capillary force caused by the surface tension of water can exert a large pressure on the latex particles, the size of the pressure (P) can be estimated by the Laplace formula: P = τ (1/r1 + 1/r2)where τ is the surface tension (or interfacial tension), r1 and r2 are the main radius of curvature of the surface.
Further evaporation of water, followed by increasing surface pressure, eventually leads to overcoming the resistance of the bilayer and to direct contact between the polymers within the latex. The contact between the polymers forms a polymer-water interface, and the interfacial tension causes a new pressure, which is also related to the radius of curvature and can also be calculated using the Laplace formula. Capillary forces plus the interfacial tension between polymer and water complement each other, and this combined force deforms the polymer particles and leads to the formation of a film. The magnitude of the pressure is related to the size of the particle, the smaller the particle, the higher the pressure.
The above discussion only illustrates the source of the force that induces latex to form a film. Whether latex particles can form a film under such a force is also determined by the nature of the latex particles themselves. If the latex particles are rigid and have a high glass transition temperature, they will not deform, much less fuse with each other, even under high pressure. Interparticle fusion requires interdiffusion of polymer molecules, which requires a low glass transition temperature of the emulsion particles, allowing a large free volume for molecular movement.
Diffusion fusion, also known as self-adhesion, by which the particles can eventually be fused into a uniform film and the immiscible emulsifier excluded from the surface. Therefore, on the one hand, whether the emulsion forms a film depends on the pressure caused by the surface (or interfacial) tension, which is related to the particle size; on the other hand, it requires a large free volume of the particles themselves, and if the temperature at film formation is T and the glass transition temperature of the emulsion particles is Tg, T-Tg must be large enough, otherwise the film cannot be formed.
For example, polyvinyl chloride emulsions cannot form a film at room temperature. In order to make the film, it must be heated to a certain temperature, which is called the minimum film-forming temperature; plasticizer can also be added to the latex to lower the Tg of the latex, so that the “minimum film-forming temperature” can be reduced to room temperature. In the paint is often added some volatile plasticizers (solvents) to reduce the minimum film-forming temperature, such volatile plasticizers are also called film-forming agents, they can be volatilized after the latex film, so that the film back to a higher Tg.
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