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    Home > Coatings News > Paints and Coatings Market > Coating knowledge: film forming mechanism of latex paint

    Coating knowledge: film forming mechanism of latex paint

    • Last Update: 2020-04-03
    • Source: Internet
    • Author: User
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    It is suggested that latex paint is widely used as the base material of water-based paint The particle size of latex is generally 50-500 μ m, and the solid content is usually more than 50% Latex is prepared by emulsion polymerization of monomers such as acrylic acid, butyl acrylate, vinyl acetate, styrene and butadiene In addition to adding pigments and fillers, corresponding dispersants should also be added to prepare emulsion paint In order to increase the film forming ability, coalescence solvent should be added Coalescing solvent is an organic substance with high boiling point, which will remain in the paint film Its basic function is to promote the coalescence of individual polymer particles to form a continuous film and reduce Paint knowledge | precautions for wall paint color selection Recommended introduction: when painting the wall, the owner sometimes has difficulty in choosing what color of the wall paint to paint In fact, there is no ugly color in the wall paint, only the disharmonious color matching The use of wall color contains the knowledge of health Too strong color will stimulate people's perception, make people have a sense of fidgety, affect people's mental health Grasp the basic principles of wall color and paint a colorful space 1、 Wall paint color selection notice 1 Light color is generally used for the top surface Light colors make people feel light, dark colors make people feel heavy Usually, the treatment of rooms is from top to bottom, from shallow to deep For example, the ceiling and walls of rooms are white and Paint Home News: latex paint is widely used as the base material of waterborne paint The particle size of latex is generally 50-500 μ m, and the solid content is usually more than 50% Latex is prepared by emulsion polymerization of monomers such as acrylic acid, butyl acrylate, vinyl acetate, styrene and butadiene In addition to adding pigments and fillers, corresponding dispersants should also be added to prepare emulsion paint In order to increase the film forming ability, coalescence solvent should be added Coalescing solvent is an organic substance with high boiling point, which will remain in the paint film Its basic function is to promote the coalescence of individual polymer particles to form a continuous film, and reduce the internal stress in the dry paint film, so as to maintain the integrity and good adhesion of the paint film when the temperature or water content changes, leading to the rise and contraction of the matrix and other changes The film forming process of emulsion paint is more complicated than that of solution paint Once the emulsion paint is coated on the surface of the substrate, it will enter the first stage of film formation, namely the evaporation of water, which is controlled by the diffusion of water vapor phase When the water evaporates, only polymers, pigments and other major components remain in the coating The second stage of film formation is the flattening and coalescence of polymer particles At this stage, the polymer and the pigments and fillers will be fully combined to form a paint film with good adhesion There are many factors affecting this important stage, including: (1) the hardness / viscoelasticity of the polymer itself; (2) the properties of the coalescing solvent used; (3) the difficulty of overcoming the repulsive force on the polymer particles The hardness of polymer, the first of the above factors, has been studied by the dynamic viscoelastic method Through these studies, the film forming process of latex paint has been investigated in depth, which is discussed in detail later in this paper All factors listed above are related to temperature, so the film forming process itself is also related to temperature When the temperature is lower than a certain critical temperature, the paint cannot form a complete film, which is called the minimum film forming temperature The lowest film forming temperature is not the property of polymer itself, but the property of the whole latex paint formula Some factors that affect the glass transition temperature will also affect the minimum film formation temperature, such as the inherent soft monomer in the polymer will reduce the glass transition temperature and the minimum film formation temperature at the same time On the other hand, the properties and concentrations of non polymer components in the formulation, such as coalescer and dispersant, also affect the minimum film formation temperature 2 The coalescence of colloidal particles shows that the mechanism of coalescence has been put forward a lot, among which there are three most representative ones: the theory of fusion, the theory of capillary action and the theory of mutual diffusion According to the theory, after all the water evaporates, the individual latex particles are in close contact with each other In order to minimize the surface energy, the polymer viscous flow makes the particles fuse together The problem of this theory is that it assumes that the water has evaporated completely before the film is formed, and the experiment proves that the agglomeration of colloidal particles begins before the water has completely lost However, in terms of the morphology of the particles in the coalescence process, the fusion plays a certain role in forming a complete paint film (2) The theory of capillary action was put forward by brown, who thought that when there was still water in the paint film, the particles began to coalesce Brown analyzed the stress of the particles in this stage, and proposed that there are two main forces on the particles, i.e capillary force between the particles, which is caused by the existence of water to form a negative bending surface of the particles, expressed by FC Deformation resistance of spherical rubber particles In FG In order to form film, the capillary force between particles must be greater than the anti deformation force: FC > FG Brown further analyzed the nature of anti deformation of particles, regarded the anti deformation force as a function of the time-dependent elastic modulus of polymer related to capillary force, expressed with G, and redefined the film forming conditions: GR, γ > 35 Among them, γ is the surface tension of polymer water interface, R is the radius of spherical particles In order to verify whether the judgment proposed by Brown actually exists, some experiments have been carried out, such as the film formation of n-butyl-methacrylic acid emulsion paint with GR, γ < 8.5 is very easy; on the other hand, the film formation of the formula with GR, γ exceeding the maximum value of 35 is not particularly difficult These experimental evidences show that the capillary force proposed by browm seems to play a certain role in the film formation, but whether it is crucial in the whole process of film formation is in great doubt (3) According to the mutual diffusion theory, the above two mechanisms assume that the film formation is only due to the effects of various forces on the polymer particles, while the actual paint formula will contain many other substances, which may affect the behavior of the particles, thus promoting the film formation In his theory of mutual diffusion, voyutskii clearly analyzed the influence of other components in the coating on the film formation He believed that when the water evaporated to a certain extent and the individual particles contacted each other more or less, the dispersant played a major role in promoting the film formation As a result, the dispersant either dissolves on the surface of the polymer or collects between the rubbers If the latter happens, a heterogeneous film will be formed It was observed that the non-homogeneous paint film produced by the existence of dispersant was easy to remove The dispersant gathered between the particles, eliminating the repulsion force between the particles As a result, the polymer particles were closer to each other, or even contacted, so that the polymer molecules could form a dense paint film through diffusion beyond the original interface This mechanism indicates that the degree of coalescence depends on the properties of dispersant and polymer and the conditions of film formation This has been proved by the film-forming experiments of a variety of latex paints In recent years, people are more and more interested in the measurement of viscoelasticity of latex paint, because the measurement of viscoelasticity can reveal the film-forming characteristics of latex paint The film formation of emulsion paint depends on the deformation of rubber particles, and the viscoelasticity of the polymer in the liquid emulsion paint will determine the mechanical properties of the dry film The basic method to study viscoelasticity is dynamic mechanical analysis Dynamic mechanical analysis needs to determine three parameters: storage modulus E ', loss modulus E' and their ratio e / E '(definition: Tan δ = e) / E') These parameters are used to map the temperature or frequency, and the samples are analyzed by the number of peaks and the location of peaks Fig 1 typical dynamic mechanical curve of carboxylate emulsion paint film This technology can be used to study the properties of paint film, such as the degree of formation of dispersed phase in the drying process If there are many components or structural particles in the latex, the film will produce dispersed phase, and at least two peaks on the curve of loss modulus E "- temperature T On the contrary, the homogeneous film only produces one peak In addition, the distribution of polar functional monomers (such as acrylic acid and methacrylic acid) was also studied There are many reasons for the existence of such functional monomers in latex, such as in order to improve the stability of latex, adjust viscosity and improve the adhesion of dry paint film These monomers are not uniformly distributed in the whole latex, but tend to be part of the polymer phase and water phase Increasing the content of carboxylic acid will produce a variety of functions: improving the stability and anti gel ability of latex At the same time, it also increases the storage modulus and loss modulus in the temperature range above the glass transition temperature, thus changing the difficulty of film formation These effects are due to the formation of hydrogen bonds between carboxyl groups and different molecules This phenomenon is especially obvious in the system with high concentration of carboxylic acid monomers Recently, the application of polymerizable surfactants in latex system is increasing Its advantage is that it can be firmly fixed in the paint film without migration or dispersion phase of dry paint film, so as to ensure that the paint film does not lose adhesion when contacting with water There is no doubt that the existence of Polymer Surfactants Combined with chemical bonds has changed the film formation process and the final structure of dry film The film-forming process of latex paint is very complex The whole formulation of latex polymer, dispersant and surfactant will affect the film-forming process With the development of the society, there will be more and more restrictions on the industrial application of the solvent, which is the highest boiling point coalescer At the same time, new surfactants will continue to appear, and the latex used in coatings will inevitably change These changes may lead to changes in the film forming process of latex paint, and the structure and viscoelasticity of the film will also change In order to make the water-based emulsion paint play the largest role in the coating industry, it is necessary to continue to carry out technical innovation Home of coatings is specialized in coatings, diatom mud, paint coating, coating technology, fire retardant coating news and coating, diatom mud, paint coating, coating technology, fire retardant coating decoration knowledge and 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