Detection of paint film adhesion and its influencing factors
Detection of paint film adhesion and its influencing factors Wen / Wang Ping Abstract: This paper theoretically introduces the detection method of paint film adhesion and its influencing factors, as well as the importance of determining the adhesion of paint film to the performance of paint film. 1 Introduction Adhesion refers to the ability of a paint film to adhere to each other by physical and chemical interactions between the paint film and the surface of the object to be attached. If, from a practical point of view, adhesion refers to the force required to remove the paint film from the sheet, this force is the bond between the paint film and the sheet. The adhesion of the paint film is one of the important indicators to evaluate the performance of the paint film. Only when the paint film has a certain adhesion, can it adhere to the surface of the coated object satisfactorily, and the decorative and protection of the paint will be exerted. effect. Achieve the purpose of applying the paint. Otherwise, the paint film does not have good adhesion on the surface of the object to be coated, and even if it has better decorative and protective properties, it has no practical significance. 2 adhesion mechanism of the paint film The adhesion mechanism of the paint film is divided into two types: mechanical adhesion and chemical adhesion: the mechanical adhesion depends on the properties of the painted plate (roughness, porosity) and the strength of the formed film. Chemical adhesion refers to the mutual attraction between the paint film and the plate molecules between the paint film and the plate. It depends on the physical and chemical properties of the paint film and the plate. When people explain the adhesion mechanism of the paint film, it is generally considered that the chemical adhesion is said. More realistic. Among the chemically attached cohesive forces, include electrostatic forces, van der Waals attraction, hydrogen bonding forces, and chemical affinity. These forces determine the adhesion of the coating to the surface of the object being coated. (1) the degree of wetting of the paint by the lacquer-forming material; (2) formation of a directional adsorption layer on the solid surface; (3) A double electron layer is caused on the boundary between the film formation and the solid surface during the formation of the paint film. Therefore, when studying the adhesion of the paint film, it is important to determine the type of film peeled off from the sheet, which can be roughly classified into three types. (1) Type of stick type: the paint film completely falls off the surface; (2) Cohesive type: the film is cracked or layered, and the adhesion is greater than the cohesion; (3) Mixing type: The paint film partially falls off from the surface, that is, the paint film itself is partially cracked, and the adhesion is close to cohesion. It is generally believed that the adhesion of the paint film depends on the polarity of the polymer (or lower molecular weight prepolymer) in the film-forming material, such as -OH, or -COOH and the polar group on the surface of the coated object. In order to combine these polar groups with each other, the polymer molecules are better wetted by the surface of the coated object, so that the polar groups of the polymer are close to the polar groups of the coated surface, when the two molecules The distance between them becomes very small (up to 1A), and the adhesion balance between the polar groups due to van der Waals force or hydrogen bonding. 3 Adhesion determination method There are two methods for determining the adhesion of paint film: one is a direct measurement method for the force required to separate the paint film from the coated surface, and the other is an indirect measurement method for the paint film in other properties. 3.1 Direct adhesion measurement method (1) Twisting method (peeling test method); (2) Pulling method; (3) Ultrasonic oscillation test method; (4) Ultracentrifugal adhesion test method; (5) BB-Jiala towel adhesion test method; (6) Adhesion test method. 3.2 Indirect adhesion determination method (1) measuring the adhesion of the paint film on a press; (2) Determination of the adhesion of the paint film by knife cutting: a. b. scraping method c. Micro knife method d. Bonding method e. Circle method 4 Factors affecting the adhesion of the paint film 4.1 Polarity of the paint film and the surface to be coated 4.1.1 From the viewpoints of molecular structure, molecular polarity and molecular interaction force, it is known that the adhesion of the paint film is caused by the orientation of the molecular polar group of the polymer in the coating and the polarity of the coated surface polar molecules. Mutual attraction between sex groups. Only when the polar basis of the two is adapted, will the paint film with good adhesion be obtained. Otherwise, a good polarity coating, applied to a non-polar sheet, or a non-polar coating applied to a polar sheet will not result in a good adhesion film. For example, a vinyl chloride paint is applied to a metal sheet with poor adhesion. The epoxy resin paint is applied to vinyl plastic sheets and has poor adhesion. Thus, in order to obtain good adhesion, it is necessary to select a sheet that is compatible with the polarity of the coating. 4.1.2 The degree of adhesion of the paint film to the surface to be coated will increase with the polarity of the film-forming material. Therefore, when various polar substances are added to the film-forming substance, the adhesion will be increased. 4.1.3 The reduction of the polar basis of the paint film and either side of the surface is one of the reasons that affect the adhesion of the paint film. (1) The presence of dirt, grease, dust, etc. on the surface of the metal plate reduces the polarity of the metal surface and causes a decrease in adhesion. (2) The reduction of the polarity point in the paint film also reduces the adhesion. For example, when the amino alkyd paint is dried to form a film, the -OH of the alkyd resin is further cross-linked with the -CH2OH in the amino resin, and is continuously consumed, resulting in a continuous decrease in the adhesion polarity point, which is an amino alkyd paint. One reason for the decrease in adhesion after drying. Therefore, the longer the drying time of the paint, the lower the adhesion of the paint film. In addition, the polar groups in the polymer molecule self-bond, which also causes a decrease in the polarity point and reduces the adhesion. For example, epoxy resin has good adhesion to metal, mainly due to hydrogen bonding between epoxy resin and metal. -OH is dispersed at an appropriate distance, and it is difficult to attract each other, and the polar group is not reduced. The paint film produces good adhesion to the metal surface, not only depending on the polarity, but also on the mobility of the molecule. For macromolecules in polymer compounds, it is difficult to move, and their solutions are coated on the metal surface. Because of the poor orientation of macromolecules, polar groups are not easy to adsorb. This is nitrocellulose lacquer, perchloroethylene. The main reason for the low adhesion of paint. Conversely, when a film-forming substance having a lower molecular state is coated on the surface of the metal, the polar group of the low molecule is easily adsorbed on the surface of the metal to obtain better adhesion. For example, the adhesion of a soluble phenolic resin paint made of vegetable oil is good. In the film-forming material-pigment-metal surface system, there is no possibility of direct contact between the pigment and the metal surface due to the polar group of the molecule, which can be oriented not only on the metal surface but also on the surface of the pigment particles. Therefore, the non-polar portion of the molecule does not allow the pigment to contact the metal surface. 4.2 Correlation between adhesion and cohesion of paint film The force caused by cohesion between molecules of the same kind is called cohesion, and the greater the cohesion of the coating, the worse the adhesion. Conversely, the better the adhesion. Therefore, it is possible to reduce the cohesion to achieve the purpose of improving adhesion. (1) Reduce the thickness of the coating, reduce the cohesive force, and improve the adhesion strength of the paint film to the metal sheet. (2) Adding appropriate pigments to the paint to reduce the cohesive force and improve the adhesion strength of the paint film to the metal plate, so the paint has better adhesion than the varnish. During the drying process, as the solvent evaporates, cross-linking occurs, which causes the film to shrink and cause a decrease in adhesion. 4.3 Influence of surface tension and wetting on coating adhesion 4.3.1 The adhesion of the paint film is caused by the mutual attraction of the polar groups of the paint and the surface of the coated metal, and the mutual attraction of the polar groups depends on the wettability of the paint on the surface of the coated metal. The ability to wet the surface of the metal to be coated is in turn dependent on the surface tension. Therefore, reducing the surface tension can improve the wetting efficiency and increase the adhesion of the paint film to the metal surface. 4.3.2 In the application of the coating, it must be a good flow dynamic, even if the powder coating must reach the flow dynamics. The surface to be coated is wetted by the flow of the coating to achieve the purpose of adhesion. It is generally believed that the coating is not well wetted, the interface contact is small, and the adhesion is poor. On the contrary, the paint is wet well, the interface contact is large, and the adhesion is good. 4.3.3 Solvents have poor solubility in resins. The polymer is often formed into a crimped structure, which has poor wettability to the metal sheet and causes a decrease in adhesion of the paint film. 4.3.4 The presence of low molecular weight substances or additives (eg stearates, plasticizers, etc.) in the coating will form a weak interfacial layer between the coating and the interface of the object to be coated, reducing polarity , reduce adhesion. In addition, the metal surface is often not pure, often there are impurities such as water, dust, acid, alkali, etc., which will cause a weak interface layer and reduce adhesion. 4.4 Effect of thermal expansion coefficient on adhesion of paint film We know that all materials are affected by temperature changes, and their positive recovery and expansion are also different (due to different expansion coefficients), so when the coating is applied to the surface of the object, it is affected by thermal expansion and contraction, so that the coating and the coating are The bond points between the painted surfaces are damaged to varying degrees. Therefore, the smaller the thermal expansion system of the coating, the better the adhesion of the coating film. For example, the thermal expansion coefficient of epoxy resin is smaller than other resins, so the adhesion of epoxy resin is good. It is generally believed that the temperature expansion coefficient of the paint film is significantly larger than the corresponding coefficient of the metal, so when the temperature changes, the degree of expansion or contraction of the paint film is larger than that of the plate, causing corresponding deformation of the paint film, causing wrinkles, moiré, and the like. Reduces the adhesion of the paint film. 4.5 Effect of surface treatment of the sample on the adhesion of the paint film 4.5.1 The sandblasting of the sample surface and the sanding of the abrasive cloth can increase the adhesion of the paint film. This is because the rough surface of the surface is formed by rough and uneven surface, which increases the effective adhesion area. 4.5.2 Remove the dirt on the surface and obtain a polar surface. Therefore, the more thorough the surface treatment, the better the adhesion, but the treated surface must be applied immediately, and it should not be placed for too long, otherwise it will affect the adhesion of the paint film. 4.5.3 The material of the metal sheet also has an influence on the adhesion. Changing Bags With Insulated Pocket are becoming more and more popular as regular items for babies and parents. Whether it is a long-distance travel or a short trip, the Mummy bag provides important convenience for babies and parents. The popularity of mommy bags has caused new parents to pay attention to infant travel, not only psychologically but also physical changes, with great significance. Mummy Bag,Small Mummy Bag Backpack,Mummy Maternity Nappy Bag,Fashion Mummy Maternity Nappy Bag JOYBABIES PRODUCT CO.,LTD. , https://www.joybabiesproducts.com