Common knowledge of coatings: analysis of how coatings form films

Core tip: the purpose of coating is to form a tough film on the surface of substrate Generally speaking, the coating is a kind of flowing liquid first, and the solid film is formed after coating, so there is a process of glass transition temperature rising The main ways of film formation are as follows 1 Film formation mode of solvent evaporation and hot melting: Generally, polymers show better physical properties only under higher molecular weight, but they have high molecular weight and high glass transition temperature In order to make them coated, it is necessary to use enough solvent to reduce the glass transition temperature of the system and make the value of T-Tg Paint knowledge how about Akzo Nobel Powder Coating Recommended introduction: powder coating is a kind of coating with different form from general coating, which is in powder form It is a new type of coating, because of its own advantages and more and more known by people Let's introduce Akzo Nobel Powder coating Akzo Nobel Powder coating is a cost-effective coating solution, which can be applied to different kinds of products After the treatment of static electricity and fire baking, a tough and durable coating surface will be formed, with the least impact on the environment Since the introduction of this technology, powder coating has been able to continue to maintain the Paint Home News: the purpose of using paint is to form a tough film on the surface of the substrate Generally speaking, the coating is a kind of flowing liquid first, and the solid film is formed after coating, so there is a process of glass transition temperature rising The main ways of film formation are as follows 1 Solvent evaporation and hot-melt film forming method: generally, polymer only shows better physical properties at a higher molecular weight, but the molecular weight is high, and the glass transition temperature is also high In order to make them coated, it is necessary to use enough solvent to reduce the glass transition temperature of the system, so that the value of T-Tg is large enough to make the solution flow and coating When the solution is close to 0.1pA · s at room temperature, it can be used for spraying After coating, the solvent volatilizes, thus forming a solid film, which is the film-forming form of general plastic coating In order to make the film smooth and smooth, it is necessary to choose a good solvent If the solvent volatilizes too fast and the concentration rises quickly, the paint on the surface may lose its fluidity due to the high viscosity, resulting in uneven paint film; in addition, if the volatilization is too fast, due to the excessive heat loss during solvent evaporation, the surface temperature may drop to zero point, which will cause water to condense in the film, leading to the loss of transparency of the paint film and whitening or the decrease of the film strength; different solvents will affect the polymer in the paint film Molecular form As mentioned before, polymer molecules in bad solvents are curled and agglomerated, while polymer molecules in good solvents are stretched and relaxed With different solvents, the microstructure of the final film is also very different The former molecules are less entangled while the latter are tightly entangled The former often has much higher strength This film forming method can be explained by the vinyl chloride paint on the inner wall of the can When the PVC solution is dissolved in the mixed solvent of butanone and toluene, the viscosity of the PVC solution can reach about 0.1pA · s at 25 ℃ After coating, the solvent volatilized gradually and TG increased continuously Three days later, TG can reach room temperature, i.e T-Tg = 0, which means that the free volume has reached the lowest level, which can not provide enough holes for molecular movement, and the solvent is not easy to escape from the membrane, but at this time, about 3-4% of the solvent is bound in the membrane, and these solvents must be heated at 180 ℃ (i.e increase the value of T-Tg) for more than 2min to be removed In order to make the polymer film, in addition to adding solvent to reduce the Tg of the system, the method of increasing temperature can also be used to increase the T-Tg (that is, to increase the free volume), so that the polymer can flow, that is to say, heating makes the polymer melt After the flowing polymer film is formed on the surface of the substrate and cooled, the solid paint film can be obtained This is another form of thermoplastic coating film formation, namely hot-melt film formation For example, polyethylene coated on milk paper bottle is formed by this method Powder coating is also hot-melt into film: polyethylene, PVC, polyacrylate and other plastic polymers can be crushed into powder, and then it is attached to the surface of the base material by electrostatic or thermal method, and heated to the melting temperature above After the molten polymer viscous fluid flows flat, the solid paint film can be obtained by cooling The powder coating is mainly a thermosetting comminuted powder coating, which has crosslinking reaction in the process of heating, melting and film forming The content of powder coating will be discussed in the future 2 After the polymer with molecular weight is coated on the surface of the substrate, the reaction between the molecules will further increase the molecular weight or cross-linking to form a tough film under heating or other conditions This film forming method is a common film forming method of thermosetting coating, including photosensitive coating, powder coating, electrophoretic coating, etc For example, dry oil and alkyd resin form film through the action of oxygen, amino resin and hydroxyl alkyd resin, polyester and acrylic resin form film through ether exchange reaction, epoxy resin and polyamine cross-linked film, polyisocyanate and hydroxyl oligomer react to form polyurethane Film formation and photosensitive coating film formation by free radical polymerization or cationic polymerization, etc these contents will be discussed one by one in the future It should be noted that before or at the same time of chemical reaction, there is also a solvent evaporation process 3, the formation of latex film before discussing the formation of latex film, we must clearly distinguish between latex and emulsion: latex is solid particles dispersed in continuous phase water, while emulsion is liquid dispersed in water General latex is prepared by emulsion polymerization The characteristic of latex is that its viscosity has nothing to do with the molecular weight of polymer, so when the solid content is over 50%, even if the molecular weight is very high, it has a lower viscosity After coating, with the evaporation of water, the latex particles are close to each other, and finally transparent, tough and continuous films can be formed However, some latex can only get powder but not tough films after drying Whether latex can form film or not depends on the properties of latex itself, especially its glass transition temperature, as well as the drying conditions Because latex is widely used in coatings and other fields, and latex film-forming is mostly needed, it is very important to understand the mechanism of latex film-forming The process of latex film formation is complex, and the current views are not the same Here is a brief introduction After coating, latex particles can still move freely in the form of Brownian motion When the water evaporates, their movement is gradually limited, and finally latex particles close to each other to form a close accumulation Due to the protection of the double electric layer on the surface of the latex particles, the polymer in the latex can not contact directly, but at this time, the space with a small curvature radius can be formed between the latex particles, which is equivalent to a small "capillary" The capillary is filled with water The capillary force caused by the surface tension of water can exert great pressure on the latex particles The magnitude of the pressure (P) can be estimated by Laplace formula: where p = τ (1 / R1 + 1 / r2), τ is the surface tension (or interfacial tension), R1 and R2 are the main curvature radius of the surface respectively When the water evaporates further, the surface pressure increases continuously, which eventually leads to overcoming the resistance of the double electric layer and making the polymer in the latex directly contact with each other The contact between polymers forms the polymer water interface, and the interfacial tension causes new pressure, which is also related to the curvature radius The same can be calculated by Laplace formula The capillary force and the interfacial tension between polymer and water complement each other This combined force can deform polymer particles and lead to the formation of membrane The size of pressure is related to the size of particles The smaller the particles are, the greater the pressure is The above discussion only shows the source of the force to promote the film-forming of the latex Whether the latex particles can form the film under this force also depends on the properties of the latex particles themselves If the latex particles are rigid and have high glass transition temperature, they will not deform or fuse with each other even under high pressure The fusion of particles requires the mutual diffusion of polymer molecules, which requires that the glass transition temperature of latex particles is low, so that they have a large free volume for molecular movement Diffusion fusion, also known as self adhesion, can finally make the particles fuse into a uniform film and remove the insoluble emulsifier from the surface Therefore, on the one hand, whether the latex film is formed depends on the pressure caused by the surface (or interface) tension, and this force is related to the particle size; on the other hand, the particle itself is required to have a large free volume, if the temperature at the time of film formation is t, the glass transition temperature of the latex particle is TG, T-Tg must be large enough, otherwise the film cannot be formed For example, PVC latex can not form film at room temperature In order to make it film-forming, it must be heated to a certain temperature, which is called the minimum film-forming temperature Plasticizer can also be added to the emulsion to reduce the Tg of the emulsion, so as to reduce the "minimum film-forming temperature" to room temperature In the coating, some volatile plasticizers (solvents) are often added to reduce the minimum film-forming temperature This kind of volatile plasticizers, also known as auxiliary film-forming agents, can volatilize after the latex film-forming, so that the film can recover to a higher Tg Home of coatings is specialized in coatings, diatom mud, paint coating, coating technology, fire retardant coating news 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