When tested under atmospheric conditions or by the rapid method, the change of the state of the coating film is judged by the visual step. Therefore, of course, it cannot be considered that the amount of coating film identified in this way is very objective. Quantitatively indicating the degree of paint film damage is only possible in individual damage cases; therefore, identification is often carried out on a broad scale, while trying to synthesize a variety of different damage degrees into a certain system.
According to the standard (OCTl0086-39 test method 38), the paint film damage can be divided into 9 categories, namely: loss of gloss, discoloration, film relaxation, reticulation, color cracking, weathering, rust contact, shedding and powdering. (Related instrument: Colorimeter)
Gloss loss can be expressed as an approximate percentage when compared to a standard. Discoloration can be indicated by the following four characteristics: yellowing, darkening, whitening, fading, and the degree of discoloration (strong, slight, etc.) must also be noted.
The mesh strands that appear can be expressed by comparing the photos of the coarse mesh pattern with the photos of the fine mesh strands. Only three items of cracking, shedding and weathering need to be confirmed by inspection. To determine the degree of chalking, OCT has developed scales that indicate varying degrees of such damage.
However, this identification method cannot fully distinguish various types of damage to the coating film, and it cannot accurately indicate the damage.
It is also possible to use a graph to illustrate each damage condition, that is, the test time is shown on the abscissa axis, while the ordinate axis expresses the change of the coating film in percentage or fraction.
In this way, the kinetics of the individual processes of film damage can be studied during the test52. The disadvantage of this method is that it is quite clumsy and does not give a general picture of the overall damage.
The standard method is of great significance to the objectivity of the assessment. This standard is the photographs and micrographs that best represent the degree of damage of a certain type. Compared with these standards, the following conditions can be shown: different shapes and degrees of electrical cracking, shapes and degrees of reticulation, state and degree of shedding, state and degree of rusting, state and degree of bubble formation, wrinkling and dusting. State and degree of transformation.
When studying the state of the coating film during the test, it is necessary to use a magnifying glass and a microscope (32 to 100 times magnification). If there is no obvious damage (mainly cracking) of the coating film under the microscope, but the original color and gloss are still maintained, it can be considered that the "appearance" of the coating film has not changed. In order to identify the damage that occurred in the corrosion resistance and weather resistance tests of the coating film, Yakubovich and Grozowska sisters have proposed a ten-point system that has been widely used in many laboratories and research institutes. Practice has proved that this ten-point system can objectively identify the characteristics of coating film damage. Table 32 lists the ten-point identification method for THNNH-4; photographic criteria for individual damage types.
When determining the score, it should be noted that if there are several kinds of damage and the damage should be classified into different categories, the coating film can be scored according to the most serious damage and category, that is, the lowest possible score can be given.
When using HNNJIK photoelectric measuring instrument to measure the gloss, the degree of loss of gloss is determined according to the percentage of the standard sample of the tested coating film, and the gloss of the standard sample is considered to be 100%. (Related instrument: gloss meter)
The damage category can be simply indicated by fractions with appropriate labels, for example: 5B for cracks reaching deep to the metal surface; 4T for rust not greater than 10% of the surface area; 6E for large air bubbles, etc. wait.
In order to fully express the condition of the coating film after the test, a 5-point scale can be used
