Coating film is the process of transforming a coating (liquid or powder) into a continuous complete coating, a complex physical and chemical process that is carried out in accordance with a rigorous construction process by selecting the appropriate coating method. The control of the film-forming process determines the quality and performance of the coating. The process by which the powder coating is heated by electrostatization, thermal spray, and fluidized bed to melt the powder into a film and cross-link it into a film will be discussed in the powder coating
. This section focuses on film-forming liquid coatings. The film-forming system of different film-forming objects is different and is related to the composition of the coating. Moreover, the film-forming process is affected by -—— temperature, humidity, ventilation, film thickness, time, etc., which determines the choice of coating method and coating process. Usually the film is divided into physical film-forming mode before and after the chemical structure does not change (thermoplastic resin solvent evaporation or hot-melting film, non-cross-linked emulsion film), as well as the chemical film-forming mode - film-forming by chemical reaction into three-dimensional large molecule film. In fact, many modern coatings are a combination of a variety of film-forming methods, for example, solvent-based two-part epoxy or polyurethane coating film is a combination of physical and chemical methods. Self-interlinking acrylic emulsions are physically filmed and then chemically cross-linked. In particular, it should be emphasized that there are dynamic control, multi-part compatible mixing, diffusion control and other factors in the film-forming process, which directly affect the quality of film-forming.
, the basic concepts related to the film-forming process
1. The viscosity
of the fluid coating's fluid properties. The fluidity of the coating is different, and its measurement methods and parameters are different. Generally to ensure that the paint good wetting, leveling, anti-flow hanging, its construction viscosity according to the coating method is different, in the high shear should be 0.05 to 1Pa.s. Liquid coating film is the low viscosity of the liquid "wet film" into a solid "dry film", commonly known as the drying process. The viscosity of the coating gradually increases during film-forming process, Burrell and others think that the viscosity is greater than 103Pa, s when the hand touch drys, and to achieve the anti-adhesion requirements of its viscosity is about 107Pa-s. The change in coating viscosity of the thermoplastic film depends on the rate of solvent volatilization and its glassy temperature, and the situation of reacting to cross-linked film is much more complex, which is discussed in more detail below. Changes in viscosity directly reflect changes in the free volume within the coating, i.e. the freedom of movement of polymer chains, which in turn is closely related to film quality. Adjusting the proper construction viscosity, strictly controlling the film conditions, and ensuring the normal growth of viscosity in the film process is one of the basic requirements of the coating process.
2. Drying time
the time it takes for a liquid coating to be converted into a solid coating. China's standard , GB/T 1728-1979 (1989) , divides it into three stages: surface drying, actual drying and complete drying, i.e. table drying, do-drying and hard-drying. In fact, only the measurement table dry and do-it-do, hard drying takes too long, except for special requirements generally do not test. ASTMD1640-95 divides the drying process into eight stages: finger-drying, non-stick dust drying, finger-drying, dry to touchable, hard-drying, dry through, dry re-coatable, dry to no indentation. The drying time of the coating is restricted by the drying conditions, the standard conditions for normal temperature drying are 23 degrees C, the relative humidity is 50%, and the high temperature baking has the corresponding temperature range. The drying time of the solvent hair-forming process is directly related to the ventilation conditions. The thickness of the coating is also an important factor, it is important to determine the thickness of the dry and wet film, otherwise the drying time is meaningless. Solvent-based coatings often through the adjustment of solvent system to achieve drying time control, and the reaction cross-linked film process should also control the reaction dynamics, the degree of cross-link drying is often through its solvent-resistant swelling or solvent-resistant scrubbing to do intuitive and rapid judgment. The degree of reaction crosslinking can be monitored by using infrared, nuclear magnetic resonance, differential thermal analysis and other instrument analysis methods.
3. The glucification temperature Tg of the film and the minimum film temperature MFT
reaction cross-linked film are small molecule lymer, Tg is very low, the large molecule after cross-linking increases with the crosslink density Tg to more than 100 degrees C. Thermoplastic film has a certain glass temperature, and the film must be higher than 25 degrees C under normal temperature conditions in order to form a coating with a certain strength. However, it is not possible to form a film above the temperature Tg. Only coatings below temperature have the necessary fluidity and film-forming. Solvents and plasticizers, emulsion polymer film-forming additives can reduce the film temperature, coating film-forming solvents and film-forming additives volatile, film-forming material gradually close to its Tg value, that is, cured into film, plasticizer remains in the coating. Film-forming Tg is a characteristic parameter related to the physical mechanical properties of the coating and can be measured by standard methods. The minimum film-forming temperature is the parameters related to the film-forming process control, which can be adjusted in a large range according to the requirements.
II, physical method - solvent volatile into a film
traditional thermoplastic solvent-based coatings, such as chlorinated polyolefins, nitroculose, acrylic resin, CAB and polyethylene alcohol deformaldehyde and other film-forming substances dissolved in a certain solvent system prepared into a coating less than 50% solid parts, after coating by solvent volatile curing into a film. In fact, the film-forming process is much more complex than thought. Changes in the fluid properties of coatings caused by solvent volatility are balanced with leveling and anti-flow retention, and solvent retention has a significant impact on coating performance and even coating structure.
polymer large molecules, usually the molecules of the line structure in the solution in the form of wire entanglement, in the dissolving force of different solvents in different forms. When the solvent evaporates, the polymer molecular line group movement is reduced, especially when using the strong solvent and weak solvent mixing system, the difference between the evaporation rate of different solvents will inevitably affect the large molecular line group and the shape of each other, resulting in differences in coating structure and performance.
it is generally believed that solvent evaporation is divided into two stages. At the beginning of the first stage, the large molecules of the film have less effect on solvent evaporation, which is mainly determined by the vapor pressure of the solvent or the relative volatilization rate of the solvent. With the evaporation of the solvent, the mucousity of the coating film increases to a certain extent, the free volume decreases, the solvent diffuses from the coating to the surface is blocked, the solvent evaporation changes from the surface volatilization control of the coating to the diffusion control, the volatilization rate is significantly slower, that is, the second stage. This phase can last a long time. For example, some chlorinated polyolefin coatings still have 2% to 3% residual solvents after 2 years, called solvent retention. In fact, they've been turned into plasticizers. The diffusion rate depends on the free volume, the most important of which are T and Tg. The diffusion control does not work if the drying temperature is higher than Tg, and if Tg is higher than T, the solvent volatility is controlled at the diffusion rate. Therefore, to remove the solvent completely from the coating, it must be baked at tg above the film temperature. Although a lot of work has been done in recent years on the quantitative treatment of solvent volatile models, no satisfactory results have been achieved. The evaporation rate control of mixed solvent systems with different solubility and relative volatility rates and the effect on film-forming quality are more complex and can only be determined by practice.
solvents in high-solid sub-coatings are much lower than conventional coatings, and their solvent evaporation rate is more important for the effect of coating fluidity. In general, high solid sub-coating solvents evaporate more slowly, not only because most of the high-pressure airless spray construction and less atomization losses, but also mainly by diffusion control solvent volatilization. High solid sub-coating is mainly chemical film, cross-linking causes the mucousity of the coating film to increase, and the decrease of free volume is also an important factor.
3. The film-forming
polymer dispersion system of the polymer dispersion system includes the emulsion of water as the dispersion medium, and the organic sol of non-water dispersion, so that the polymer is insoluble in the medium and is distributed steadily in the dispersion medium in the particle state. Dispersed medium volatility when forming a film, driven by capillary force and surface stress, emulsion particles are tightly stacked, and deformation occurs, particle shell layer breaks, the interface between particles gradually disappears, polymer molecular chains penetrate and winding each other, thus forming a continuous and equal coating film. Lotion film-forming process has been carried out a lot of research, put forward several theories, some divided into three stages, and some put forward four stages, somewhat arbitrary. The driving force of today's film is the capillary force, or whether the surface can be reduced is still under dispute.
good physical mechanical properties and stain resistance of the coating requires that the film has tg above normal temperature, and the film coating needs the lowest possible minimum film temperature MFT. This contradiction is currently solved by membrane additives. They are a class of high boiling point solvents with strong solubility against the film, which evaporates slowly after filming. Different film-forming objects should choose different additive combinations, and their efficiency varies greatly. Hydro-hydro-strong polymers can absorb water solubility, water can be used as film-forming additives, can reduce up to Tg5 degrees C.
film additives are the main component of VOC in emulsion polymers, and with the increasingly stringent environmental regulations, the development of ultra-low VOC or zero VOC emulsions has become an important direction. In recent years, considerable progress has been made in the preparation of low VOC emulsions using high Tg as core and low Tg as shell, or different Tg emulsions, using nano pigments to enhance low Tg emulsions, synthesizing low Tg emulsions, and cross-curing to improve coating Tg when forming film. The ultra-low VOC emulsion prepared with liquid epoxy and alcoholic acid as active film-forming additives and acrylic monosomes using hybrid emulsion polymerization process has environmentally friendly and performance advantages.
Emulsion film-forming process involves the migration of emulsifying agents, i.e. the membrane-forming process of small molecule emulsizers and the migration of film-forming back to the substrate and coating surface, which adversely affects the adhesion, water resistance and soil resistance of the coating. Work is under way to develop soap-free emulsions (without emulsions) and the use of polymerized emulsizers and non-migrating polymer emulsizers, with many issues to be resolved.
Ivy, chemical form of film-forming
film-forming in the process of film-forming chemical reactions, inter-molecular cross-linking to produce a three-dimensional structure of large molecules of a continuous coating called chemical form of film-forming. The chemical reactions that may occur cross-linking include almost all chemical reactions in the film, according to the film conditions and the different requirements of the construction process, there are normal temperature curing, heating curing, ultraviolet curing type, there are single-part and double-group film-splitted. The design of cross-link group and film structure and cross-link density is multi-ended according to the final coating performance and construction process requirements. In general, the combined properties of chemically formed coatings are superior to those of physically formed coatings. Such film-forming materials are often referred to as thermoolytic resins, in addition to powder coatings, they are low molecular weight of low polymer, low construction viscosity, with cross-link density increase, viscosity increases, free volume decreases, Tg increases, until the production of a continuous solid coating.
1. Single-part thermo-solid film-forming system
single-part coating construction facilities, labor-saving, time-saving, material-saving, very popular with the market. Aolic acid and modified alcoholic acid, epoxy, urethane oil, i.e. polyurethane modified alcoholic acid, etc. by absorbing oxygen in the air caused by unsaturated fat lateral chain oxidation crosslink is typical representative. Single-component moisture-cured polyurethane absorbs water from the air and reacts with an excess of one NCO in the film to produce a polyurethane coating. High-mode potassium silicate, lithium silicate absorption of carbon dioxide in the air into silicol shrinkage cross-linking, etc. are temperature-cured cross-linking type. The amino resin film system consisting of melamine formaldehyde resin and acrylic, alcoholic acid, epoxy and polyester containing hydroxyl and pyrethroid is typical of high temperature baking curing. There is also a closed isocyanate film-forming system, which is stable enough at room temperature to heat and release an NCO rapid reaction cross-linked membrane under the action of a catalyst. Reaction cross-linked powder coatings can also be classified as single-part coatings.
The greatest technical challenge in developing such coatings is to ensure the stability of products produced, stored and transported for a considerable period of time, to take various measures to suppress cross-linking reactions to acceptable limits, and to ensure that the cross-linking of reactions is sufficiently fast and adequate in the film-forming process. In recent years, in the process of developing water-based acrylic self-interlinking emulsions and coatings, a variety of means, such as hydroxymethyl acrylamide, pyridolic acrylic monosome, unsaturated sioxane and other functional monosomals, will be discussed in detail in the next chapters.
2. Free-based polymerization reacts into a membrane
a film made up of unsaturated polyester, acrylic or acrylic-based epoxy, polyurethane, polyester and epoxy compounds and active diluents, etc. under the action of free-based agents, or high-energy beams such as ultraviolet light, electron beams, which trigger the polymerization of free agents or active ions, and the whole process is completed in a matter of seconds to a few minutes. The film-forming process has almost no organic solvent volatilization, environmental friendliness and energy savings, which is one of the fastest growing areas of the coatings industry. Free-based triggers are generally packaged separately from unsaturated polyester in two parts, while photo-curing coatings are single-packed. Oxygen in the air has a blocking effect on polymerization reaction, and the problem of oxygen resistance must be solved.
assessment of occupational safety and health with volatile and irritant active thinners has yet to be completed.
3. Film-forming process of two-part coatings
Epoxy resin and amine curing agent, polymer polyol or polyamine and polycyanate curing agent between the addition of polymerization cross-linking film, they are two-part packaging, before use proportionally mixed, coated into a film. Two-part coating generally does not have storage stability, but isocyanate curing agent is sensitive to moisture, production, packaging storage should be careful.
many factors that affect the membraneization process of two-part coatings, first of all, the mixing and solubility of two-part coatings. For example, epoxy resin and low molecular weight of fat polyamine, lipid ring polyamine molecular structure and molecular size are very different, poor solubility affects mixing and diffusion efficiency, after mixing need to be placed for a certain period of time called "ripe" period. Epoxy prepolymer curing agent or cashew phenol phenolic amine curing agent is much more soluble with epoxy resin. The choice of solvent system is equally important to improve the two-part solubility, of course, the influence of solvent polarity, electro-negativeity and parent protonivity on cross-linking reaction should also be considered. The ratio of two-parts should not be too different, and the formula should be adjusted appropriately to improve mixing efficiency.
reaction rate between functional groups is mainly controlled by reaction dynamics and film diffusion rate. Dynamic factors are mainly reaction concentration and reaction rate constants, which are closely related to reaction temperature. Unlike the reactions between small molecules, the function groups on the polymer molecular chain are distributed according to a certain structure, their reaction activity is affected by stereoscopic configuration, etc., and the polymerization chain must have the necessary mobility to match the reaction groups together, so the diffusion rate of film-forming matter is very important. When the diffusion rate is greater than the reaction rate (the reaction starts and the low viscosity state), the reaction is controlled by dynamics. With the increase of crosslink density, the mucousity of the coating film increases, the free volume decreases, the diffusion rate decreases gradually lower than the dynamic reaction rate, and the film-forming process becomes diffusion control. Once the film's Tg is above room temperature, diffusion is no longer possible and the reaction actually stops. This is why some room temperature curing coatings take weeks or even months to fully cure. Temperature has an important effect on both control factors