The detection method of cation electro deposition coating performance is discussed and discussed

With the rapid development of the automobile industry, the requirements for cation electro-deposition coatings are becoming more and more high. The author thinks that some problems in the testing method and standard of the coating need to be put forward for discussion and discussion by experts.
1 swimming transmission force
there are many methods to determine the swimming transmission force, currently in the Ford box method, four box method, their test results can objectively reflect the quality of paint penetration, and has a better reproducible
China
. However, these methods have the shortcomings of large amount of test paint, more complex operation, so some people are reluctant or unable to adopt the method, and habitually adopt FAW steel pipe method. FAW steel pipe method using less paint, easy to operate, but because today's coating than the 1990s electrophoresis paint swimming transmission has a greater improvement, with this method of detection, often swimming transmission of different electrophoresis paint measurement results are full plate, that is, 100%, so can not identify the advantages and disadvantages of the paint swimming transmission. After the test using electrophoresis test board end and lower thickness determination, calculate the upper and lower thickness gradient, according to this value can identify the advantages and disadvantages of paint swimming penetration. If the ratio of the upper end to the bottom thickness is closer to l, the higher the paint penetration. If the upper end of the measurement value of the sample is 11.2 m and the lower end is 18.5 m, the thickness gradient δ is 0.6054, and if the measurement value of the sample is 12.3 m at the top and the lower end is 19.4 m, the calculation of δ is 0.63, then the 2 swimming transmission is better than 1. If the measured value of the sample is l5.7 m at the top, 21.6 m at the lower end, and 0.7268 at the δ, then 3 s > 2 s > 1 s. This method is also applicable for coatings with high swimming permeability, but requires accurate and repetitive measurements by the paint film thickness gauge used. To reduce measurement errors, thickness can be measured 3 times per sample and then averaged. FAW steel pipe method combined with this method for evaluation, then not only to maintain a small amount of paint, easy to operate advantages, but also to accurately judge the difference between the strength of various coatings. The author thinks this is a good way to popularize the application.
2 Salt mist resistance test (SST)
coating salt spray test is usually using the U.S. ASTM method, that is, cross on the coating model, put into a neutral salt water spray tank, with the one-sided spread of the cross 2 mm time as SST time, and then depending on the foaming situation in other parts (graded) to evaluate the paint's salt spray resistance. CED coatings can basically reach more than 1,000 h. This data is an important technical indicator of CED coatings and a decisive data on corrosion resistance. However, the author observed that after the test is completed, the test piece will be taken out, washed with water, dried, and then put into a plastic closed bag to save, some models after a period of time, especially in the summer, when the temperature is about 30 degrees C, relative humidity is more than 75% At about 1 month, some models will appear silk rust at the cross corrosion, like cobwebs, but some with the above SST
at the same time, there is no silk rust, or even continue to be placed for a longer period of time without silk rust. In the early years of the author has done the following experiments on AED paint model, in order to speed up the production of silk rust, will do a good job of the model in the cigarette factory's wire storage room, only one or two days can appear silk rust. AED coating on a steel plate that has been phosphated will take longer to rust. In addition, the solvent-based coating rust-proof better iron epoxy red primer sprayed on the steel plate to do the above-mentioned test, one or two days after the film fell off.
from the above test, it can be seen that AED coating through the electro-deposition coating formed by the denseness of the coating film is good, salt fog test resulting in local rust points can only crawl from the plate, forming silky rust. The solvent-based coating as a whole is far less attached than ED, so the pieces fall off. CED is better than AED corrosion resistance and takes much longer to rust. Remove the silk rust coating layer to see the rust state of the substrate. Using the current test method, several CED coatings, which can reach 1,000 h, appear to have consistent corrosion resistance, but the appearance of silk rust may vary greatly. In practice, the difference in corrosion resistance is very noticeable. This issue is of concern. Of course, the deviation of the salt-resistant fog test can be corrected by the cross-change test, but its differences can not be fully reflected. What's more, in reality can do alternating testing units after all, there are very few, so how to correctly evaluate the corrosion resistance of coatings is debatable.
3 MEQ
is determined using an automatic bit titrator (from the animation out of the middle and curve and shows the equivalent point). The specific practice is to use potassium hydroxide-ethanol standard solution titration added to the electrophoresis coating of tethydrofuran, calculate the amount of l00 g solid electrophoresis coating consumption of the liquid, the value is MEQ (acid). MEQ (alkali) is measured in the same way as MEQ (acid) and simply replaces potassium hydroxide-ethanol with hydrochloric acid-ethanol standard solution.
MEQ (VN/GS) ×100
: V——— Titration consumes potassium hydroxide (or hydrochloric acid) - the volume of the standard solution of ethanol, mL;
N——— Potassium Hydroxide (or hydrochloric acid) - concentration of standard ethanol solution, mol/L;
G——— electrophoretic tank liquid sample quality, g;
S——— electrophoretic tank liquid sample solid score.
At present, CED coating resin is basically a polydamine resin, added acid to make its protons (commonly known as neutral), it constitutes water dispersion, water emulsification resin, the amount of acid added is not the polyamine resin amino all protonization, but only a part of protonization, its pH has become acidic. This is determined by the structure and properties of its polymers, i.e. the cause of the formation of MEQ values. In this paper, it is believed that the characteristics of the coating can be better reflected if the acid is used to characterize it. That is, MEQ (acid)/MEQ (alkali), through which it can be indicated in the process of electrical deposition of the resin protonization degree is how much, it is able to investigate the resin immediate water dispersion, water solubility of the stability. Therefore, the determination of MEQ is of great significance.
coating is made up of paint-based and pigments, which leads to the concept of pigment-based ratio, i.e. the ratio of pigment (including fillers) to resin (solids). During the use of electro-deposition, it is not deposited on the sample according to this basis. Further, the various parts of the pigment are not deposited in proportion to the preparation time. The pigment ratio and pigment composition in the process of electro deposition were analyzed in the early stage, and the results confirmed that the pigment ratio was changed and the pigment composition was also changed. The former is decreasing with electrical deposition, while the latter is related to the characteristics of pigment fillers. If the same iron oxide, one is type A iron red, the other is y type iron red, the latter in electrical deposition more uniform. For these reasons, it is undoubtedly difficult to determine the yan-base ratio in the process of electro-deposition. It is a common practice to measure ash and then calculate it by pigment correction coefficient. But this is only the correction factor obtained when the original paint was made, which has already changed during the electro-deposition process. What's more, the determination of yanjibi is calculated on the basis of the determination of solid content, and the solid content is determined under certain conditions of non-volatile points. Therefore, the system error is obvious. The current MEQ value is wise to calculate based on the solid content of the coating.
4 Additives
often add various types of additives to CED coatings, such as plasticizers, by reducing the resistance of electro-deposited wet film, to increase the thickness of the film for electro-deposition coating, improve leveling and improve the materiality of the coating film. For example, in order to improve the curing degree of the coating film or reduce the curing temperature, a variety of additives such as curing catalysts can be added. It is understood that the current abuse of additives in CED coatings is very serious, the use of additives are in itself a traditional poison, some in use will release toxic substances, even carcinogens. It is recommended that methods for testing these toxic and harmful additives should be established and that the current limits be clarified in order to promote the development and application of innovative technologies conducive to environmental protection.
5 Time to flash after washing of electro-deposited plate water
Due to the electrical seepage performance of various CED coatings there are many differences, electro-deposited plate washing, if immediately into the oven can see the episodural differences after drying coating. When the flash time is short, the baked model will produce water marks, traces of water and other ills on the coating film. However, this may improve after a long time of flashing (or drying) before entering the oven.
the electro-permeability of the coating is also one of the important energys of the electro-deposition coating. In the actual coating line, washing to the oven but 7 min, so to investigate the flash time of the coating to evaluate the electro-permeation performance of the coating is an easy and easy way to do.
these issues have been encountered and considered in our practice for many years, and this is put forward to the experts for advice and consultation.