Due to the existence of a large number of interfaces in coating resins, the discussion and study of the interface properties of coating resins plays an important guiding role in understanding the characteristics of coating systems and solving the problem of paint film defects in the coating process. In this paper, the surface tension of resin, the interfacial tension between resin melts, the surface tension of resin solution and the interfacial tension of resin solid solution are discussed, and the causes and corresponding countermeasures of paint film shrinkage porosity are analyzed.

For solvent-based coatings, solvents can usually dissolve film-forming substances, so there is a film-forming substance solution in the system. The film-forming material is often composed of a variety of polymer resins, and each polymer resin has different molecular weights, end groups, branching forms, bonding structures, etc., and this solution itself has the property of polydispersion. Pigments have very little solubility in solvents and are usually in suspension, so pigments form a complex system composed of polydispersed colloids and coarse dispersion systems mixed in solvents. For such a complex polydispersion as coatings, there will be many interface phenomena in the coating construction project, so the detailed discussion and research Therefore, the discussion and study of the interface properties of the coating resin have an important guiding role in understanding the characteristics of the coating system and solving the problem of paint film defects in the coating process. Therefore, this paper discusses the generation and influencing factors of interfacial tension between coating resins, and focuses on the causes and corresponding countermeasures of interfacial tension on paint film defects and shrinkage.

1. Interface of liquid resin

1.1 Surface tension of resin

Paint resin belongs to objects with low surface tension. The surface tension of most resins is less than 50 mN/m. In practical applications, it is necessary to study the interface phenomena and interactions between resins and gases such as air and water vapor, various organic solvent liquids, and other solid materials such as resins and pigments. The content is quite extensive and the objective is complex. It is necessary to discuss the surface of the resin itself and analyze some characteristics of its surface tension.

Due to the fact that polymers do not have a gaseous state, the vapor pressure of the resin gas phase is zero. Therefore, only the gas phase coexists with liquid resin. The determination of surface tension of solid resins is very difficult, therefore, the surface tension of solids is generally measured indirectly. For coatings, the surface tension of the resin melt can be measured first, and then extrapolated based on the relationship between surface tension and temperature. Table 1 lists the surface tension values of some resins when extrapolated from liquid to solid.

Table 1 Surface tension of some solid resins

In the viscous flow state, the relationship between the surface tension of resin and temperature varies linearly. With the change of temperature, the conformational changes of polymer chains are affected by the entanglement of long-chain molecules, and the changes are relatively small, so the temperature gradient of their surface tension is also small. The surface tension of homologues often increases with the increase of molecular weight, but when the molecular weight reaches a certain value, the surface tension no longer changes. When a mechanical state transition occurs, the surface tension of the resin will also change significantly.

1.2 Surface tension between resin melts

When the two resins that form two phases have exactly the same polarity, the surface tension between the resins will decrease, such as polymethyl methacrylate and poly (n-butyl methacrylate); As the difference in polarity between the two phases increases, the surface tension between the resins will also increase, such as the significant difference in polarity between polyethylene and polyvinyl acetate. The magnitude of interfacial tension mainly depends on the difference in polarity between the two phases. The greater the polarity difference, the greater the interfacial tension.

The interfacial tension of the two resin melts decreases with increasing temperature, with a small amplitude generally only 0.01 mN/(m • k). The interfacial tension between resin melts can generally range from a few to tens of millinewtons per meter, and increases with the increase of molecular weight. If two resins are completely compatible, their interfacial tension is zero. Generally speaking, resins should be completely compatible with each other, and their molecular weight should not be too large. The molecular weight of resins is generally high, and it is unlikely to achieve complete compatibility between resins with significant structural differences. But if grafting or block polymers are used as additives, which contain chain segments similar to the structures of the two resins. This can greatly reduce the interfacial tension between them.

1.3 Surface tension of resin solution

The surface tension of resin solution is the same as that of other solutions and is related to its composition. When the surface tension of the resin is lower than that of the solvent, the solution surface will be enriched with resin; On the contrary, it enriches solvent molecules.

Resin solutions will undergo phase separation under certain conditions. There is also interfacial tension between the two separated phases. If the temperature for critical phase separation is T *, and T>T *, it is because phase separation has not yet occurred. Therefore, there is no separated phase and the interfacial tension is zero. As the temperature decreases, phase separation becomes increasingly apparent, and the interfacial tension also increases with an approximate linear relationship. At the same time, the larger the molecular weight, the smaller the interfacial tension. The interfacial tension between typically layered resin solutions is much smaller than that between resin melts, only on the order of 10-2mN/m. It can be seen that the higher the temperature. The lower the interfacial tension. When the temperature reaches the phase transition temperature T *, the two phases merge and the interfacial tension disappears. But there are also some systems that undergo phase separation with increasing temperature, where the effect of temperature is exactly the opposite.

1.4 Surface tension of resin solid solution

Synthetic resins have polydispersity, so in the resin body, differences in structure, molecular weight, and density will result in varying surface tensions at different locations. The part with low surface tension will gradually migrate to the surface over time, causing the surface tension to continue to decrease. Generally, the surface tension of the amorphous phase, low molecular weight components containing alkyl or fluorinated groups, organic silicon and other functional groups is low. Therefore, a low molecular weight amorphous layer mainly composed of alkyl groups tends to accumulate on the surface of the material, resulting in a lower surface density and polarity of the polymer material than that of the bulk crystalline phase. Over time, these low surface tension components will gradually migrate towards the surface, resulting in a decrease in the surface tension of the material.

For systems such as polymer copolymers and blends containing multiple structural units, they can be regarded as solid solutions of polymers. Like small molecule solutions, in these systems, low surface tension components always preferentially adsorb on the surface to reduce the surface free energy of the system. For block or graft copolymers, their low-energy components exhibit significant surface activity due to the sufficient length of the low-energy components in the block or graft portion, which can accumulate on the surface independently of other parts of the molecule and form oriented structures. Thereby reducing the surface tension of the system.

When the resin is blended, whether it is compatible or incompatible, it exhibits significant surface activity, but the incompatible mixture has a more significant surface activity than the compatible system mixture. Incompatible mixtures due to their multiphase structure. Make its surface activity more complex. The surface activity increases with the increase of molecular weight, which is obviously due to the increased incompatibility.

There are often many additives in coatings. Low energy additives can greatly reduce the surface tension of coatings. For example, the addition of defoamers such as fluorocarbon surfactants and silicon containing surfactants, as well as leveling agents, can greatly reduce the surface tension of coatings. The speed at which additives migrate to the surface is controlled by diffusion.

Understanding these results is useful for studying the stability of polydisperse systems. For example, in coating formulations, pigments, base materials, and solvents are the three basic components, and their interaction ability should be similar, with good compatibility matching, in order to ensure the dispersion of pigments in the base material solution. If the surface tension is not well matched and the interaction is weak, the components with low surface tension will migrate to the surface, causing fogging or sweating, or pigment coagulation and sinking to the bottom.

2. Shrinkage and Formation Principle

In the production of high decorative automotive coatings, the coating defects that cause high repair rates are particles and shrinkage holes, which are one of the most likely coating defects on site. They not only affect the appearance quality of the coating, but also damage the integrity of the coating due to exposed shrinkage holes. Once shrinkage holes occur, they cannot be eliminated by general polishing and finishing methods. Severe exposed shrinkage holes need to be polished to the bottom layer before painting and drying. Large areas of shrinkage holes can also lead to rework of the vehicle body, seriously affecting production efficiency and quality.

2.1 Principle of Shrinkage Formation

Shrinkage is a general term for various irregular four depressions that appear on the surface of a coating film, which can be divided into flat, volcanic crater, point, exposed bottom, and bubble shapes in shape. It is usually centered around a drop or a small piece of impurity, forming a circular edge around it. This phenomenon is related to the low surface tension of the shrinkage donor. If its surface tension is high, it is impossible to expand and form shrinkage cavities, only when the surface tension is low.

In the coating formula, if the surface tension of each component does not match, shrinkage porosity may occur. During the coating process, due to the generation of a large fresh surface, components with low surface tension inside the coating will adsorb to the surface layer and drive some materials to migrate towards the periphery. The material flow driven by this flow may form shrinkage cavities. This adsorption process is a time process. If the viscosity of the system is very low, the system can quickly level off; If the viscosity of the system is high and the surface adsorption and material flow process are slow, the possibility of forming shrinkage pores is relatively small. Shrinkage will only occur when the viscosity is moderately low.

External physical disturbances also cause changes in surface composition, resulting in uneven distribution of surface tension on the coating surface and uneven surface tension at different locations. The low surface tension part will migrate towards the high surface tension area, and drive some of the paint to migrate together, resulting in local flow and the formation of shrinkage cavities. If the liquid film is thick enough, the liquid can be replenished into the depression from the bottom to close the shrinkage cavity. But if the liquid film is thin and there is no liquid to replenish, permanent shrinkage will form.

During the coating drying process, if a surface tension gradient is generated due to solvent evaporation, it may also cause shrinkage. Assuming the surface tension of the solvent is γ , the surface tension of the film-forming substance is γ 2, and the surface tension of the substrate is γ s. If γ  ＜ γ s ＜ γ 2, when the coating is applied, the surface tension of the coating at the beginning is γ c=γ  ＜ γ s, and the coating film can be spread out; As the solvent evaporates, the surface tension of the coating gradually increases. When the surface tension of the paint reaches or even exceeds γ s, shrinkage porosity may occur. But if the viscosity of the system is already extremely high and the surface tension is not enough to break the coating, shrinkage can be avoided. If γ 2<γ <γ s, when the solvent with high surface tension evaporates, the surface tension of the film-forming substance is lower than that of the substrate, so it will not affect the leveling. Moreover, the solvent content in the surface composition is small, and its evaporation will not produce a significant surface tension gradient, so the possibility of shrinkage is unlikely.

The same applies to external pollutants. If the surface tension of the foreign pollutant γ 3>γ c, it will not spread on the wet coating surface, so it will not cause a surface tension gradient; If the pollutant is a low surface tension substance, it spreads on the high surface tension coating and replaces the original surface, and this irregular flow will cause shrinkage. Figure 1 shows the effect of different surface tension sizes on coating construction. When there is a surface tension gradient on the surface, the formation of shrinkage also depends on the flowability of the coating itself. Fink and Jensen pointed out that under the action of surface tension gradient on wet coating, shrinkage occurs when fluid flows from one point to another. In the shrinkage area, if the flow rate is large, exposed shrinkage holes may also form, as shown in Figure 2. So, under certain conditions, shrinkage is determined by the following equation (1): (1) where Q is the amount of paint flowing per unit time; H is the wet film thickness; η is the viscosity of the coating film; Δ γ is the surface tension gradient on the cross-section. From this, it can be seen that in order to reduce shrinkage, the flowability of the coating should be reduced, and the size of the flowability depends on the thickness of the coating, the viscosity of the coating, and the surface tension gradient.

2.2 Causes of Shrinkage

Shrinkage is always caused by surface tension gradient, and the formation of surface tension is mainly due to the presence of low surface tension active substances in the coating film during the coating construction process. To reduce shrinkage, it is necessary to eliminate the pollution of active substances during the coating construction process.

The possible reasons for the formation of shrinkage cavities can be summarized as follows: (1) In the coating formula, the surface tension of each component is not matched, and low surface tension components in the system, such as excessive content of various surfactants and solvent surface tension lower than that of other substances, are prone to shrinkage cavities; (2) The surface tension of the substrate itself is too low or the surface tension of the coating is too high, causing poor wetting of the substrate by the coating; (3) There is oil stains on the substrate, causing local surface tension to be too low, resulting in poor wetting and coating of the coating; (4) The process requirements for coating construction are unreasonable, such as allowing contact with the vehicle body after wiping, spraying without eliminating low surface tension substances, requiring too thin spraying film thickness, low air pressure/flow rate of spraying equipment (atomization and forming air), and pollution caused by low surface tension paint mist on high surface tension paint during mixed production of different color paints; (5) In the process of car painting construction, it is inevitable to have human operations, from adding materials for electrophoresis, PVC spraying, to polishing, wiping, paint spraying, paint adjustment, etc. During the operation, low surface tension substances on employees' clothing and hands can be carried onto the body of the car, causing shrinkage; (6) When adjusting viscosity, substances such as oil and water may be introduced; (7) Impurities such as oil, water, and dust have been mixed into the original paint and diluent during packaging, transportation, or storage; (8) Excessive wet film thickness, low viscosity, etc.

2.3 Prevention of Shrinkage

To prevent the formation of shrinkage cavities, it is necessary to understand the causes of this problem. If it is caused by external shrinkage donors, care should be taken to prevent surface contamination. If this method is not feasible or if the shrinkage is spontaneous, the formula needs to be readjusted. If the surface tension of the solvent is too low, the surface tension of the liquid film can be reduced by adding surfactants with low surface tension and surface control agents with good compatibility, making it lower than the surface tension of the shrinkage donor; But if the amount added is too much, it will have the opposite effect due to the mismatch of surface tension in the system. Because surface active substances in coating components adsorb surfactants from the bulk phase when a large amount of fresh surface is formed during the coating process; At the same time, a large amount of surfactants may become insoluble with the coating. During the drying process of the coating, their concentration changes, exceeding their solubility and generating the least soluble droplets, which can lead to shrinkage. For example, excessive addition of silicone oil to the coating can easily cause shrinkage. The surface tension of organic silicon compounds varies greatly from the coating used, and even when diluted at a concentration of 10-10 mol/L, shrinkage can occur.

3. Summary

During the process of coating film formation, as the coating spreads and the solvent evaporates, the interface characteristics of the coating constantly change, and the resulting material flow will affect the integrity of the coating film. Therefore, studying the impact of these dynamic processes on the surface characteristics of coatings, analyzing their causes, and researching corresponding countermeasures will help to control the coating surface and prevent the formation of coating defects.

There are many reasons for shrinkage in paint film, and detailed analysis and repeated testing must be conducted based on the actual situation of the enterprise and their respective coating processes in order to find the true cause and determine preventive measures. Prevention is the main focus for shrinkage defects, and on-site management of painting should be strengthened to eliminate hidden dangers in their early stages. For the shrinkage defects that have already occurred, they should be analyzed and solved from various aspects such as coating, painting process, and painting environment.