Coating film formation mechanism, non-transforming Coating and transforming Coating film formation
涂料涂覆于物体表面以后，由液体或疏松粉末状态转变成致密完整的固态薄膜的过程，即为涂料的成膜，也称为涂料的干燥和固化。
涂料成膜主要靠物理作用和化学作用来实现。例如：挥发性涂料和热塑性粉末涂料等，通过溶剂挥发或熔合作用，便能形成致密涂膜；热固性涂料需要通过化学作用才能形成固态涂膜。因此涂料成膜机理依组成不同而有差别。
一、非转化型涂料
仅靠物理作用成膜的涂料称之非转化型涂料。它们在成膜过程中只有物理形态的变化而无化学作用。此类涂料包括挥发性涂料、热塑性粉末涂料、乳胶漆及非水分散涂料等。
（一）挥发性涂料
挥发性涂料的品种有硝基漆、过氯乙烯漆、热塑性丙烯酸漆及其他烯基树脂漆等。这类涂料的树脂分子量很高，靠溶剂挥发便能形成干爽的硬涂膜，在常温下表干很快，故多采取自然干燥方法。
（二）热塑性乳胶涂料
乳胶涂料的成膜过程可参见图
此类涂料的干燥成膜与环境温度、湿度、成膜助剂和树脂玻璃化温度等相关。环境湿度极大地制约着成膜湿阶段水的蒸发速率，提高空气流速可大大加快涂膜中水的蒸发；当乳胶粒子保持彼此接触时，水的挥发速率降至湿阶段的5~10%。此时如果乳粒的变形能力很差，将得到松散不透明且无光泽的不连续涂膜。乳胶漆膜为了赋予应用性能，树脂的玻璃化温度都在常温以上，故加入成膜助剂来增加乳粒在常温下的变形能力，使乳胶漆的最低成膜温度达到10℃以上，彼此接触的乳粒将进一步地变形融合成连续的涂膜。在乳粒融合以后，涂膜中水分子通过扩散逃逸，释放非常缓慢。
Generally speaking, the surface of latex paint is dry within 2 hours, it takes about 24 hours to dry, and it takes about two weeks to dry completely. The volatilization rate of film-forming aids from the coating film decreases in order of ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol butyl ether acetate, ethylene glycol, and diethylene glycol monobutyl ether. Ethylene glycol monomethyl ether evaporates too quickly and is completely lost before reaching the dry film; glycol ether acetate is essentially completely distributed in the resin phase. These two additives have little effect on the evaporation of water in the stage. Ethylene glycol butyl ether tends to partition between the water phase and the resin phase, and water evaporation is affected by its partition ratio. The presence of ethylene glycol makes it form a continuous swelling hydrophilic network structure, which makes it easy for the polar coalescence agent to diffuse and escape. However, ethylene glycol is more hygroscopic than propylene glycol, and the coating film dries slowly. The latex paint film added with propylene glycol retains very little water or coalescent after a few weeks, so that the coating film (especially outdoor paint) will not produce Negative Effects.
(3) Hot melt film forming
Thermoplastic powder coatings and thermoplastic non-aqueous dispersion coatings need to be heated above the melting temperature to make the resin particles fuse to form a continuous and complete coating film. At this time, film formation depends on melt flow temperature, melt viscosity and melt surface tension.
2. Conversion coating
Coatings that cross-link to form films by chemical reactions are called conversion coatings. The resins of this type of coating have a low molecular weight, and they are cross-linked into a network-like macroscopic solid-state coating film through condensation, polyaddition or oxidative polymerization. Since the condensation reaction uses heat to obtain the energy of the chemical reaction to cure the coating, this type of coating is called a thermosetting coating. Phenolic paints, amino baking varnishes, polyester paints, acrylic baking varnishes, etc. are cured into films through condensation reactions; unsaturated polyesters, two-component epoxy, two-component polyurethanes, etc. are cured into films through polyaddition reactions ; Oily paints, alkyd paints, and epoxy ester paints are cured into films through oxidative polymerization. Therefore, the types of conversion coatings can be divided into the following three categories.
(1) Air-drying coating
Air-drying paint is a paint that uses oxygen or moisture in the air to cure and form a film.
1. Oxidative polymerization coating
Coatings containing drying oil are formed into films by oxidative polymerization, and the drying performance is related to the nature of the oil, oil degree, drier, etc. The oxidative polymerization reaction of dry oil base is extremely complicated, and there are always decomposition products of the coating film during the drying process , which is difficult to express with a certain chemical reaction. However, for oil bases containing conjugated double bonds , the generation of hydrogen peroxide is not significant during the drying process, and hydrogen peroxide is generated
2. Moisture curing coating
Moisture-curing coatings are mainly moisture-curing polyurethane and moisture-curing epoxy coatings. Moisture-cured polyurethane is the use of terminal isocyanate of polyurethane resin to react with water molecules in the air
(2) Curing agent curing coating
Most of these coatings are two-component coatings, and there is a high chemical activity between the two components, so they can be cured to form a film at room temperature , and the use period is only 4-8 hours after mixing. The main varieties are epoxy, polyurethane and unsaturated polyester. The mixing ratio between the components has a great influence on the film performance and drying.
(3) Baking cured coating
Each group in the resin of this type of coating has weak chemical reactivity at normal temperature, but when heated to a higher temperature, a chemical reaction will quickly occur between the groups to make the coating film cross-linked and cured. The main varieties are amino baking varnish, acrylic baking varnish, polyester paint, thermosetting polyurethane, epoxy baking varnish and silicone coating and so on.