In addition to the physical properties of the coating, the more important is the service life, that is, the durability of the coating to the atmosphere. It represents the true value of the coating and is a comprehensive expression of various technical properties. Therefore, improving the weatherability of coatings is the key to improving the quality of coatings.
General conditions
The atmospheric aging test of coatings is to study various factors in various climate types, such as: sunlight, wind, snow, rain, dew, temperature, humidity, oxygen, chemical gases, etc. Visual inspection of the test panel to identify its durability.
It is divided into rural atmosphere, industrial atmosphere, and marine atmosphere according to the type of atmosphere; it is divided into temperate, cold, tropic, and humid tropical areas according to the area; it is divided into 45 ° south latitude, vertical angle, and horizontal exposure according to the exposure method. According to China's climatic conditions, exposure can be performed in the following areas;
· Wet tropical climate-Hainan Island, Leizhou Peninsula and southern Taiwan.
· Subtropical climate-south of the Yangtze River, Sichuan, basin, northern Taiwan.
· Temperate standard climate-the Yellow River Basin and the southeast.
· Cold and temperate climates-most of Heilongjiang, Inner Mongolia and northern Xinjiang.
The exposure angle also has an influence on the durability of the coating. In atmospheric aging, light is a very important factor. You should try your best to maximize the use of solar energy at the most suitable angle. In order to accelerate the destruction of samples during short-term exposure, the hottest angle in spring and summer (ie -25 °; where 叩 is the local latitude) can be adopted in the first half of each year, and can be adjusted to the hottest angle in autumn and winter (0.893 γ + 24 °). For those who need continuous exposure for several years or do not consider adjusting the angle back and forth, the local latitude (γ) should be the most suitable, so that the sample can be exposed to more time light than other angles.
In China, the damage of the sun board is the fastest in spring and the slowest in autumn. The sequence is: spring → summer → winter → autumn. Therefore, it is reasonable to test paint in spring and construction in autumn.
Test piece size：
The standard sample size is 70 × 150 × 0.8 ～ 1.5 mm. Paint thickness can be tested thickness or actual construction thickness according to test requirements. It is recommended that the surface be sanded with No. 1 or 2 abrasive cloths or rusted with an electric abrasive cloth wheel, and then wiped with solvent. The back side is of a slower variety than the front paint film.
Paint film inspection standards
· Loss of light
During the atmospheric aging test, as time goes by, the gloss of the coating film will gradually decrease, which is generally called matt. This indicator can be measured by DFH-66 photoelectric gloss meter, expressed as the percentage of loss of light, and its grade is divided as follows:
level 0 - not lost light - percentage of light's loss(<5%)
level 1 - slightly - percentage of light's loss(6%~30%)
level 2 - obviously - percentage of light's loss(31%~60%)
level 3 - seriously - percentage of light's loss(61%~90%)
level 4 - completely - percentage of light's loss(>91%)
· Discoloration
This refers to the phenomenon of fading or darkening or lightening of the paint film due to aging in the atmosphere. This indicator can generally be measured quantitatively using a photoelectric color difference meter, or a "colorfastness and fade sample card" recommended by the International Organization for Standardization, whose color difference is measured by a spectrophotometer 'and calculated according to the Atmus color formula.
· Powdering
After a long period of natural exposure to the paint film surface, due to the effects of atmospheric factors, the pigment on the surface cannot stay firmly in the paint film and fall off from the paint film surface to form a powder layer. This phenomenon is called powder . Methods for measuring pulverization include a pulverization tester, a gravimetric method, a gloss method, and a finger method.
· Crack
This is a typical failure phenomenon of the paint film in the atmospheric aging test, which is caused by the deformation force generated by the external atmospheric damage exceeding the strength limit of the paint film itself. The shapes of cracks are stripe cracks, fork cracks, web cracks, cracks, bends and irregular cracks. The depth of the crack is shallow, moderate, deep and severe. The density of cracks is divided into four grades, dense, dense, medium, and sparse.
· Foaming
This is a phenomenon caused by bubbles in the paint film due to gas or liquid. There are three reasons for blistering: ① the paint film absorbs water and swells, ② the sunlight irradiates, the volatile components in the paint film are thermally expanded, and ③ the corrosion of the metal base plate increases the volume of the bubble. There are four types of bubbles, with the smallest diameter being 1 mm or less, followed by 2 mm, 8 mm, 5 mm, and more.
· Rust:
This is a phenomenon in which iron hydroxides and oxides are the main compounds generated on the surface of painted steel. In the atmospheric aging test, there are the following types of rust: ① rust is generated on the metal surface and is not shown on the surface of the paint film; ② rust is caused by the surface of the paint film being damaged to generate cracks. There are also other forms of rust. The assessment of corrosion is mainly expressed by the percentage of the tested area of the sample. Generally divided into 5 levels, namely o.1%, 0.2%, 0.5%, 1%, 2%. Standard photos are attached as a reference.
The above are several single inspections, which are more important and often encountered in atmospheric aging tests. However, they still have certain problems in the inspection instruments and methods, which need to be further improved and improved.
