Principle of film formation and influencing factors of film performance

The paint film is a thin film adhered to the substrate. There are many factors that affect the mechanical properties of the paint film, not only the performance of the paint itself, the thickness of the paint film, but also the quality of the construction, whether there are defects, and the nature of the substrate. And the use environment, the length of use time. Therefore, the research on the mechanical properties of the paint film is different from other types of materials, and the corresponding test methods are also different.

The principle of paint film formation

(1) Free radical polymerization reaction film formation

① Oxidative polymerized vegetable oil absorbs oxygen in the air, forms hydrogen peroxide groups, decomposes to generate free radicals, triggers double bond polymerization in vegetable oil, and forms a network macromolecular structure. For example, long or medium oil alkyd resins, ester glue paints, rosin modified phenolic paints, pure phenolic resin paints, epoxy ester paints, polyurethane oils, etc., which are dried at room temperature, all use this mechanism. Quite a large proportion.

This type of coating is low in price and widely used, but it has poor color retention when baked, limited outdoor durability, and takes a long time to air dry. Even if a drier is added, it will take a long time to dry effectively at room temperature.

② Polymerization initiated by free radical initiator Both unsaturated polyester resin and solvent styrene contain double bonds, and the free radicals generated by the decomposition of the initiator initiate double bond polymerization to form a cross-linked coating film.

③ Energy-induced polymerization uses ultraviolet light or electron beams to initiate the polymerization of compounds containing double bonds. The photosensitizer in the photocurable coating generates free radicals or cations under the irradiation of ultraviolet light, and polymerizes the compound containing double bonds. The coating that uses electron radiation to generate free radicals to form a film is an electron beam curing coating.

(2) Condensation polymerization curing coating

①Amino resin is commonly used as a crosslinking agent, which needs to be baked and cured. Amino resin as curing agent mainly includes amino paint, saturated polyester paint and thermosetting acrylic paint. Small molecular compounds generated during film formation escape from the film. The blocked polyurethane coating releases the blocking agent under heating conditions and cross-links to form a film.

② Common two-component amine-epoxy resin coatings and two-component polyurethane coatings. There is also a moisture-curable polyurethane coating in the hydrogen transfer polymerization reaction. After coating, the wet film absorbs moisture in the external environment and reacts to form a film.

(3) Coagulation film-forming mechanism of polymer particles

The polymer particles are deformed during the process of close packing, the polymer molecules in the particles and between the particles diffuse each other, cross the boundary of the particles, and the polymer molecules are entangled to form a continuous film. Polymers on the surface of one particle only need to interdiffuse into the surface of another particle for a very small distance to form a high-strength film. This film-forming mechanism is used in waterborne polyurethane dispersions, organosols, latex paints, and powder coatings. Organosols and latex paints do not need to be cross-linked during the film-forming process, and water-based polyurethane dispersions and thermosetting powder coatings also undergo cross-linking during the film-forming process.

Polyester resin, the latter also has good weather resistance.

(4) Solvent volatile coating

In order to obtain a paint film with satisfactory performance, the molecular weight of the thermoplastic polymer is usually very high. To achieve the solution viscosity required for construction, a large amount of organic solvent needs to be added. The organic solvent volatilizes into the air after film formation, causing air pollution and Waste a lot of resources. This type of coating is mainly used for outdoor construction, which cannot be baked, and has special requirements for the performance of the paint film. The main varieties of application are nitro-lacquer, halogenated polymer paint (such as chlorinated rubber paint, perchlorethylene paint, vinylidene chloride copolymer resin paint, vinylidene chloride copolymer resin paint, organic fluorine paint), thermoplastic acrylic resin paint.

Other Factors Affecting Film Properties

The paint film is a thin film coated on the substrate, the substrate determines the degree of deformation of the paint film, and when the paint film has good adhesion, the substrate absorbs the energy of external impact and reduces the effect on the paint film. A thinner paint film is more impact-resistant than a thicker paint film, but a thinner paint film has poorer hiding power.

The paint film of pre-painted exterior wall panels is often compromised with a paint film with a thickness of 20-25 μm, which can increase the hardness of the paint film without cracking. In order to reduce the swelling of the paint film on the inner wall of fish cans by fish oil, it is necessary to use phenolic paint with a higher degree of cross-linking, and apply a pre-coated flat plate to make fish cans. This kind of paint film is very brittle due to its high degree of cross-linking. In order not to crack when processing pre-coated flat panels, the film thickness should be 5 µm or less.

The flexibility of paint films tends to deteriorate over time, especially with air-dried paints, because there is always some solvent left in the paint film. _{The T g} of most paint films is always close to or slightly higher than room temperature, and the solvent will evaporate very slowly at this time. Solvents generally have a plasticizing effect, so the solvent volatilizes, T _g increases, and the flexibility of the paint film decreases.

Hardening with time is also often observed in baked crosslinked paint films. When a polymer is heated above its T _g and then cooled rapidly (quenched), it tends to be less dense than when it is cooled gradually. Rapidly cooled ones have more free volume to freeze than slowly cooled ones, and thus have more opportunities for molecular movement. During storage, even if the temperature is lower than Tg _, the molecules in the quenched paint film will gradually move, causing the free volume to shrink and the density to increase. There is no chemical change in this process, so it is called physical aging. As free volume decreases and density increases, cracking is more likely during post-processing. This phenomenon may occur on the paint film on the metal that has been baked at high temperature and cooled rapidly after being out of the oven. This may also be a common cause of brittleness in the aging of baked lacquer films. The polyester/MF paint films baked at 180°C and then quenched at 30°C showed a modulus increase at 30°C. The rate of aging (ie, the rate at which the modulus increases) decreases with time. The sample was reheated at 180°C and re-quenched to 30°C, the modulus returned to a lower value, and physical aging was re-entered.