Research Progress on aging mechanism and life eval

Research progress of coating aging mechanism and life evaluation method

research progress of coating aging mechanism and life evaluation method

November 30, 2018

as a basic protective material, the service life of coating has a significant impact on the use of the protected structure, and its aging performance and service life directly affect the service performance and life of the protected body, Its aging performance represents the real use value of the coating to a certain extent, and is the comprehensive performance of the coating performance. The aging of coatings will lead to unexpected or premature product failures, making hundreds of millions of funds used to maintain and replace most of the products aged due to exposure to the outdoors every year. However, compared with the empirical formula for calculating the service life of rubber and metal materials, there is no general empirical formula for coatings due to their diversity. At present, there is no comprehensive and detailed research and summary on the aging mechanism and life evaluation of coatings at home and abroad. Therefore, studying the aging mechanism of coatings, the interaction between various components of coatings and their impact on the overall performance, and summarizing the research on the life evaluation of the coating industry at home and abroad have a very positive guiding role for the preparation and testing of various coatings

1 analysis of coating aging mechanism

1 1 factors affecting the service life of coatings

the factors affecting the service life of coatings can be mainly divided into the following: (1) internal factors: the chemical structure of the main chemical substances of the coatings, the aggregated structure and the formulation conditions of the coatings; (2) external factors: physical factors, including heat, light, high-energy radiation and mechanical stress; Chemical factors, including the effects of oxygen, ozone, water, acid, alkali, etc; Biological factors, such as microorganisms and insects. The aging of coatings is an extremely complex process affected by internal and external factors

coating is a complex mixture system. When the basic type of coating is determined, the main factors affecting its service life are shown in Table 1

among the above factors, although surface treatment, coating layer number and thickness, coating method and technology and other factors have a great impact on the service life of the coating, these factors are also easier to manually control and determine the specifications, and the quality of similar varieties of coatings also has strict quality control specifications. However, environmental factors are ever-changing and cannot be controlled manually. The most basic function of coatings is to protect the substrate to cope with the harsh changes of natural environment. Therefore, the mechanism of coating aging in natural environment is the most meaningful research direction

in addition to the above factors, there are many factors that affect the service life of the coating, such as the adhesion between the coating and the substrate, the resistance of the coating to medium penetration, the extent of the anti-corrosion effect of pigments in the coating, and the defects of the coating

1. 2 environmental factors that affect the service life of the coating

there are three main environmental factors that cause the aging of the coating: solar radiation (light energy), temperature and moisture (humidity). These factors, combined with secondary factors, such as air pollutants, biological phenomena and acid rain, will cause aging of the coating

1. 2.1 solar radiation (light energy)

solar radiation can generally be divided into three main wavelength ranges: ultraviolet, visible and infrared. The two important parameters characterizing solar radiation are radiation intensity and spectral distribution. Different latitudes, altitudes and atmospheric conditions have important effects on these two parameters. Among the solar radiation, ultraviolet radiation has the greatest damage to the coating, and the shorter the wavelength, the greater the absorption rate of the coating. At the same time, the shorter the wavelength, the greater the energy contained in its photons, and the greater the ability to destroy chemical bonds. However, ultraviolet light with a wavelength less than 290nm can be absorbed by ozone in the atmosphere, and solar radiation with a wavelength greater than 290nm reaches the ground. Nevertheless, the light energy with a wavelength of 300nm is equivalent to 95kcal/mol, which is 83% higher than the dissociation energy of C-C bond 1kcal/mol is large. If the wavelength of about 300nm is absorbed, it will still produce the fracture of - C - C bond in the polymer, destroy the chemical properties of the coating material, and cause the aging phenomenon of the coating

1. 2.2 temperature

when the coating is exposed to solar radiation, the temperature of its environment will affect the radiation effect, and the photochemical reaction rate will accelerate with the increase of temperature. Generally, the photochemical reaction rate will be doubled for every 10 ℃ increase in the temperature of the material. High temperature will also accelerate some chemical reactions inside the coating, and accelerate the occurrence of cross-linking, thermal degradation and other reactions. Moreover, the cold and heat changes caused by the temperature changes in the environment will cause the changes in the internal and external stress of the film, resulting in the deformation of the film, and the weak adhesion with the substrate, affecting the use of the coating. In the same environment, the surface temperature of different colors of coatings is also different. The darker the color, the higher the temperature. Therefore, the influence of temperature on the coating life is very complex, which should be comprehensively analyzed and judged according to the material, color, use environment, substrate and other conditions of the coating itself

1. 2.3 moisture (humidity)

water has a great impact on the aging of the film. Chemically, water molecules will participate in the breaking of the secondary bond of hydrogen bond and the decomposition of the primary bond of ester bond by adding water, so as to promote the degradation process of materials. Especially, coatings with polymers containing easily hydrolyzable groups (such as amide group, ester group, acetal group, etc.) as the main materials are prone to hydrolytic denaturation, which is particularly sensitive to the humid environment. Physically, when water vapor is absorbed, the volume of the coating surface will expand, and a certain stress will be applied to the dry bottom layer. After the surface layer is dried, the volume will shrink. In this way, the alternation of hydration and dehydration will make the coating surface crack or blister. If the ambient temperature is low and variable, the cycle of freezing and thawing will also lead to the peeling, cracking or falling off of the coating, resulting in the decline of the film performance, especially the polymer containing hydrophilic groups (such as hydroxyl, carboxyl, etc.) is prone to absorb water and moisture, which has a great impact on the film. Moreover, the penetration of water will send some oxygen into the interior of the coating, which will promote the oxidation and degradation reaction inside the coating and promote the pulverization of the coating

in addition, if the relative humidity is the same, the higher the temperature is, the greater the absolute humidity is, and the greater the influence of water vapor diffusion on the national measurement system deployed by the State Administration of molecular heat transport market supervision to provide a strong guarantee for the prevention and control of epidemic diseases, the greater the damage of the strengthened film; With the same absolute humidity, the lower the temperature is, the higher the relative humidity is, and the destructive force of water penetration and condensation on the film is enhanced. The test shows that the corrosion rate at 40 ℃ and 98% to 100% RH is 20 ~ 25 times that under natural conditions; The corrosion rate at 50 ℃ and 98% to 100% RH is 60 ~ 75 times that under natural conditions 10 〕 therefore, the laboratory can simulate the natural environment to conduct accelerated tests to calculate the service life of the coating

1. 2.4 other factors

the aging effect of some gases or pollutants in the atmosphere, such as Sox, NOx, ox, mold, etc., on the coating cannot be underestimated. Acid rain will cause brand-new reactions, and dust can change the absorption characteristics of the coating to solar radiation. Under appropriate temperature and humidity conditions, microorganisms such as mold and bacteria can make the coating moldy, and even degrade the polymer, resulting in the aging of the coating. The coating used in tropical and subtropical areas has more mildew. The test shows that PVC, melamine resin, polyurethane, epoxy resin and other coatings will have slight mildew, while polystyrene, polyethylene resin and other coatings are not easy to mildew

2 research progress of life assessment methods 2 1 difficulties in coating life evaluation

(1) the degree of life difference of different coating varieties. Among different kinds of coatings, the resin types are different, the structure of polymer materials has its own characteristics, and the tolerance to different environments is also very different. In addition, the difference of formula composition and production process, their ability to withstand corrosive environment or medium is very different. Therefore, their service life is different, and, This difference is difficult to summarize and study with a unified theory or formula, and it is impossible to calculate their service life with a unified test method

(2) the influence of the quality difference of similar coating products on the service life. Even for similar products, due to the difference of resin composition and molecular structure, formula, production process, storage time and so on, the service life of similar coatings is often different, and sometimes the difference is very huge. Therefore, even for similar coatings, without knowing their formula, resin structure, etc., it is necessary to design test comparisons that match their material conditions, and understand their use environment, so as to define their service life relatively accurately

(3) the difference of coating life under different environmental conditions. The same kind of paint may be used in different environments. It may be tested by freezing and thawing in Northeast China, or by sunlight exposure in southern China. If it is used in the coastal areas, it should also be tested for salt fog resistance, humidity and heat resistance, etc. Therefore, when calculating the service life of a coating, we must clearly understand the environment in which it will be used, and then we can carry out corresponding comparative tests according to its practical environment

2. 2 main characteristics of coating aging

there are many aging phenomena of coating, which can be summarized in the following four aspects: (1) changes in appearance, such as cracking, mildew, contamination, loss of light, discoloration, chalking, blistering, peeling, etc; (2) changes in physical and chemical properties, such as specific gravity, glass transition temperature, molecular weight, light absorption, temperature resistance, etc; (3) changes in mechanical properties, such as adhesion, tensile strength, bending strength, impact strength, wear resistance, etc; (4) changes in electrical properties, such as insulation resistance, dielectric constant, breakdown voltage, etc. During the aging process of the coating, it is impossible to have all the above changes and phenomena at the same time. In the aging test and evaluation of coatings, it is generally necessary to start from the needs of use, grasp the key indicators, and take the performance decline to a certain extent or the comprehensive rating reaches a certain level as the evaluation criteria

2. 3 laboratory coating life assessment test

there are two main methods for coating aging test: one is to carry out aging test under typical or harsh natural environment, such as atmospheric aging test, soil test, sea water test; The other is to use instruments and equipment in the laboratory to simulate specific environmental conditions, strengthen some factors as needed, and obtain test results in a short period of time, mainly designing test methods for solar radiation, temperature, humidity, harmful gases, mold, etc. The environment in which the coating is used outdoors includes salt spray, damp heat, light aging, chemical corrosion, etc. The life estimation should consider the influencing factors in the real environment as much as possible, and then compare the indoor corrosion accelerated test with the outdoor exposure test to find out the correlation between them. Therefore, we can simulate the outdoor aging environment in the laboratory and carry out accelerated tests, so as to infer the service life of the coating

2. 3.1 salt spray test

salt spray resistance refers to the ability of the coating film to resist the corrosion of simulated marine atmosphere containing salt spray. It is the basic requirement of the coating film for offshore or offshore facilities, and it is also the assessment index of the antirust performance of primer and antirust paint. Salt spray test is used to identify the electrochemical corrosion resistance of materials, which is mainly divided into neutral salt spray test and acetic acid salt spray test