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Analysis Of The Causes Of Shrinkage Of Paint Film Based On The Surface Tension Of Paint Resin

Jun 12, 2020

Due to the large number of interfaces in the coating resin, discussion and research on 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. This paper discusses the generation and influencing factors of the surface tension of resin, interfacial tension between resin melts, surface tension of resin solution and interfacial tension of resin solid solution, and focuses on the analysis of the causes of paint surface shrinkage and the corresponding countermeasures.

For solvent-based coatings, the solvent can usually dissolve the film-forming substance, so there is a film-forming substance solution in the system. The film-formed product is often composed of multiple polymer resins, and each polymer resin has different molecular weight, terminal group type, branching form, bonding structure, etc. This solution itself has polydispersity Nature. The solubility of the pigment in the solvent is extremely small, usually in a suspended state, so the pigment formed in the solvent is a complex system composed of a polydisperse colloid and a coarse dispersion system. For such a complex polydispersion of paint, many interface phenomena will appear in the paint construction project. Therefore, detailed discussion and research are carried out to discuss and study the interfacial properties of the paint resin to understand the characteristics of the paint system and solve the problems in the paint coating process. The problem of paint film defects has an important guiding role. Therefore, this article discusses the generation and influencing factors of the interfacial tension between coating resins, and focuses on the analysis of the causes of the interfacial tension of the coating on the film film defects-shrinkage and the corresponding countermeasures.

1 Liquid resin interface

1.1 Surface tension of resin

Coating resins are 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 interfacial phenomena and interactions between resins, air, water vapor and other gases, various organic solvent liquids, and other resins, pigments and other solid materials. Their contents are quite numerous and complicated. It is necessary to discuss the surface of the resin itself and analyze some characteristics of its surface tension.

Since the polymer has no gaseous state, the vapor pressure of the resin gas phase is zero. Therefore, only the gas phase coexists with the liquid resin. It is very difficult to measure the surface tension of solid resins. Therefore, the surface tension of solids is generally measured by indirect methods. For coatings, the surface tension of the resin melt can be measured first, and then based on the relationship between surface tension and temperature. Extrapolation. Table 1 lists the surface tension values when some resins are extrapolated from liquid to solid.

In the state of viscous flow, the relationship between the surface tension of the resin and the temperature changes linearly. With the change of temperature, the conformational change of the polymer chain is affected by the entanglement of long-chain molecules, and the change is small, so the temperature gradient of its surface tension is also small. The surface tension of homologues tends to increase with increasing 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 polarities of the two resins forming the two phases are exactly the same, the surface tension between the resins will become smaller, such as polymethyl methacrylate and polybutyl methacrylate; as the difference in polarity between the two phases increases, The surface tension between the resins will become larger, such as polyethylene and polyvinyl acetate, the polar difference is large. The magnitude of the interfacial tension mainly depends on the difference between the polarities of the two phases. The greater the difference in polarity, the greater the interfacial tension.

The interfacial tension of the two resin melts decreases with increasing temperature, and its amplitude is very small, generally only 0.01 mN/(m•k). The interfacial tension between the resin melts can generally range from a few to a few milliN/m, and increases with increasing molecular weight. If the two resins are completely compatible, the interfacial tension is zero. Generally speaking, if the resins are to be completely compatible, their molecular weight cannot be too large. The molecular weights of resins are generally better than others, and it is not possible to achieve complete compatibility between resins with large structural differences. However, if a graft or block polymer is used as an additive, it contains segments similar in structure to the two resins. It can greatly reduce the interfacial tension between them.

1.3 Surface tension of resin solution

The surface tension of the resin solution is the same as the surface tension of other solutions and is related to the composition. When the surface tension of the resin is lower than the surface tension of the solvent, the surface of the solution will enrich the resin; otherwise, it will enrich the solvent molecules.

Under certain conditions, the resin solution will undergo phase separation. Interfacial tension also exists between the separated two phases. If the temperature for critical phase separation is T*, T>T*, because phase separation has not yet occurred. Therefore, there is no separated phase, and the interfacial tension is also zero. As the temperature decreases, the phase separation gradually becomes obvious, and the interfacial tension also increases, and has an approximately linear relationship. At the same time, the larger the molecular weight, the smaller the interfacial tension. Generally, the interfacial tension between layered resin solutions is much smaller than the interfacial tension 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. However, there are some systems where phase separation occurs as the temperature rises. At this time, the effect of temperature is just the opposite.

1.4 Surface tension of resin solid solution

Synthetic resins have polydispersity, so in the resin body, due to differences in structure, molecular weight, and density, the surface tension will also be different. The part with low surface tension will gradually migrate to the surface with time, so that the surface tension continues to decrease. Generally, the non-crystalline phase part, low molecular weight components, containing hydrocarbon groups or fluorohydrocarbon groups, organosilicon and other groups have low surface tension, so they tend to gather on the surface of the material and be covered with a layer of low molecular weight, mainly hydrocarbon groups. The amorphous layer of the polymer makes the surface density of the polymer material always lower than the density of the bulk crystalline state, and the polarity is often lower. Over time, these low surface tension components will also gradually migrate to the surface, resulting in a reduction in the surface tension of the material.

For systems such as polymer copolymers and blends that contain several structural units, it can be regarded as a solid solution of polymers. As with small molecule solutions, in these systems, low surface tension components are always preferentially adsorbed on the surface to reduce the surface free energy of the system. For block or graft copolymers, the low-energy components exhibit significant surface activity, because the low-energy components in the block or grafted portion have sufficient length, and they can accumulate on the surface independently of the rest of the molecule And form a directional structure. As a result, the surface tension of the system is reduced.

When the resins are blended, whether they are compatible or incompatible, they all show significant surface activity, but the incompatible mixtures have more significant surface activity than the mixtures of compatible systems. Incompatible mixtures due to their multiphase structure. Make its surface activity more complicated. The surface activity increases with increasing 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 the coating. For example, the addition of defoamers such as fluorocarbon surfactants, silicon-containing surfactants, and leveling agents will greatly reduce the surface tension of the coating. The rate of migration of additives to the surface is controlled by diffusion.

Understanding these results is useful for studying the stability of polydisperse systems. For example, in paint formulations, pigments, binders and solvents are the three basic components. The interaction between the three should be similar and the compatibility is well matched to ensure the dispersion of the pigment in the binder 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 and cause fogging or sweating, or pigment flocculation and sinking.

2 Shrinkage and formation principle

In the production of highly decorative automotive coatings, the defects of the coating film that cause high repair rates are particles and shrinkage holes, which is one of the most likely paint film defects in the field. It not only affects the appearance quality of the coating film, but also damages the bottom shrinkage holes. In order to improve the integrity of the coating film, once the shrinkage phenomenon occurs, it cannot be eliminated by the general polishing and polishing modification methods. Serious exposed bottom shrinkage holes need to be polished to the bottom layer before painting and drying. Large-scale shrinkage holes will also cause the body to be reworked , Seriously affecting production efficiency and quality.

2.1 The principle of shrinkage cavity formation

Shrinkage is a general term for various irregular dents appearing on the surface of the coating film. It can be divided into plane type, crater type, point type, open bottom type and bubble type from the shape, usually centered on a drop or a small piece of impurities , Forming a ring-shaped edge around. This phenomenon is related to the low surface tension of the shrink cavity donor. If the surface tension is high, it is impossible to expand and form shrinkage holes, only when the surface tension is low.

In the coating formulation, if the surface tension of each component does not match, it is possible to cause shrinkage. In the process of coating the film, due to the huge fresh surface, the components with low surface tension inside the coating will be adsorbed to the surface layer, and some materials will be migrated to the periphery. The material flow driven by this flow may form shrinkage holes. This adsorption process is a time process. If the viscosity of the system is very small, the system can level quickly; if the viscosity of the system is large, the surface adsorption and material flow process is very slow, the possibility of forming shrinkage holes is small. Shrinkage occurs only when the viscosity is moderately low.

The external physical disturbance also changes the surface composition, resulting in uneven distribution of surface tension on the surface of the coating film, and the surface tension is not equal everywhere. The part with low surface tension will migrate to the place with high surface tension, and will drive part of the paint to migrate together, thereby causing localized flow and causing the formation of shrinkage cavities. If the liquid film is thick enough, the liquid can be replenished into the depression from the bottom to make the shrinkage hole close. However, if the liquid film is thin and there is no liquid to supplement, permanent shrinkage holes will be formed.

In the drying process of the coating, if the surface tension gradient is generated due to the evaporation of the solvent, it may also cause shrinkage. Let the surface tension of the solvent be γ, the surface tension of the film-forming substance be γ2, and the surface tension of the substrate be γs. If γ<γs<γ2, when the coating is applied, the surface tension of the coating starts γc=γ<γs, the coating film can be spread; as the solvent volatilizes, the surface tension of the coating film gradually increases. When γc reaches γs or even exceeds γs, shrinkage may occur. However, if the viscosity of the system is too great and the surface tension is not enough to break the coating, shrinkage can be avoided. If γ2 <γ <γs, when the solvent with high surface tension volatilizes, since the surface tension of the film-forming substance is lower than the substrate, it will not affect the leveling, and the surface composition has a small amount of solvent. Volatilization does not produce a significant surface tension gradient, and the possibility of shrinkage cavitation is unlikely.

Foreign pollutants are no different. If the surface tension of foreign pollutants γ3>γc, it will not spread on the surface of the wet coating film, so it will not cause a surface tension gradient; and if the pollutant is a low surface tension substance, it will be on a high surface tension coating Spreading and replacing the original surface, this irregular flow will cause shrinkage. Figure 1 shows the effect of different surface tension on the coating construction. When there is a surface tension gradient on the surface, the formation of shrinkage holes also depends on the fluidity of the coating itself. Fink and Jensen pointed out that under the effect of the surface tension gradient on the wet coating film, when the fluid flows from one point to another, shrinkage cavities will occur. In the shrinkage cavity area, if the flow volume is large, open bottom shrinkage cavity may also be formed, as shown in Figure 2. Therefore, under certain conditions, shrinkage is determined by the following formula (1): (1) where Q is the amount of paint flowing per unit time; h is the thickness of the wet film; η is the viscosity of the coating film; Δγ is the horizontal Surface tension gradient on the cross-section. It can be seen that to reduce shrinkage, the fluidity of the coating should be small, and the fluidity depends on the thickness of the coating film, the viscosity of the coating and the gradient of surface tension.

2.2 Causes of shrinkage

Shrinkage is always caused by the surface tension gradient, and the formation of surface tension is mainly due to the low surface tension active substances in the coating film during the coating construction process. Active substance contamination.

It can be summarized that the causes of shrinkage cavitation may be: (1) In the coating formulation, the surface tension of each component does not match, the low surface tension components in the system such as various surfactants are too much, and the surface tension of the solvent is lower than other substances Surface tension, etc., are likely to cause shrinkage; (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 coating by the coating; (3) oil stains on the substrate, causing local surface tension Low, making the coating wet and poor coating; (4) The process requirements of the coating construction are unreasonable, such as after the car body is cleaned, it is still allowed to set up contact with the car body, spraying if the low surface tension substance is not eliminated, the spraying film thickness is too thin, spraying When the air pressure/air flow rate of the equipment is too low (atomizing and forming air), and different colors of paint are mixed, the low surface tension paint mist causes pollution on the high surface tension paint; (5) During the automobile coating construction process, It is inevitable that someone needs to operate. From the addition of electrophoresis, PVC spraying, to polishing, cleaning, paint spraying, paint adjustment, etc., low surface tension substances on employees’ clothes and hands are brought to the body during the operation. It leads to shrinkage; (6) Oil and water are brought in when adjusting the viscosity; (7) Oil, water, dust and other impurities are mixed into the original paint and thinner during packaging, transportation or storage; (8) Wet film has passed Thickness, low viscosity, etc.

2.3 Prevention of shrinkage

Preventing the formation of shrinkage holes requires understanding the causes of this malaise. If it is caused by an external shrinkage donor, care must be taken to prevent surface contamination. If this method does not work, or if shrinkage occurs spontaneously, 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 a surfactant with low surface tension and a surface control agent with good compatibility to make it lower than the surface of the donor Tension; but if the added amount is too much, it will be counterproductive because of the surface tension mismatch in the system. Because the surface active substances in the coating component form a large amount of fresh surface during the coating process, the surfactant will be adsorbed from the bulk phase; at the same time, a large amount of surfactant may be incompatible with the coating during the coating drying process In its concentration, the concentration changes, beyond its solubility, the formation of the least insoluble droplets will cause shrinkage. For example, the excessive addition of silicone oil in the paint will easily cause shrinkage. The surface tension of the organic silicon compound is very different from the paint used. Large, even when diluted to a concentration of 10-10 mol/L, it will cause shrinkage.

3 Summary

In the process of coating film formation, with the spread of the coating film and the volatilization of the solvent, the interface characteristics of the coating also continuously 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 the coating, analyzing its causes, and studying the corresponding countermeasures will help to control the surface of the coating and prevent the formation of coating film defects.

There are many reasons for the shrinkage of paint film, and it is necessary to carry out detailed analysis and repeated tests according to the actual situation of the enterprise and their respective coating processes, before it is possible to find the real cause and thus determine preventive measures. The prevention of shrinkage defects is mainly based on prevention, strengthening the on-site management of painting and eliminating hidden dangers in the bud. For the shrinkage defects that have occurred, it should be analyzed and solved from various aspects such as paint, painting process, and painting environment.