Electrodeposition primer painting sex of bad accident analysis

Source: China coating network

1 the introduction

In order to ensure the vehicle especially rust in shipping quality chassis parts, at present each big factory in commercial vehicle on exports to spray wax protection at the same time, the coating of chassis parts also has a strict standard, in order to improve the quality of the coating. The company according to many years of experience in export car antirust and overseas market demand, to export car chassis parts request total coating thickness is not less than 40 microns, coating comprehensive performance to meet export quality standards. In order to meet the export standards, the factory of export car chassis part adopts "electrodeposition primer + high pressure electrostatic powder coating", its technological process is as follows: electrophoresis pretreatment (skim - water - > watch - phosphate - water - water drying) - after electrophoresis and flush - electrophoresis paint drying (180 ℃, 30 min) to turn hang - high pressure electrostatic powder spraying, powder curing (180 ℃, 25 min) using the production process, paint film thickness of the total for 70 ~ 100 microns, adhesion to zero level, more than 800 h neutral salt fog test, excellent comprehensive performance, coating and coating quality has been stable. But in the middle of October 2009, after powder painting parts appear serious shrinkage cavity large area, and mainly concentrated in the corner of the parts, the coating adhesion is poor. Paint film defects as shown in figure 1. In view of the problems of time and condition, through analyzing the operation condition of the production line for one by one, and in combination with related literature [1], there may be several causes of powder coating appear when painting shrinkage phenomenon: (1) the compressed air is not clean, oil, water, too much; (2) electrophoresis paint curing inadequate; (3) electrophoresis paint film contact outside dirt, contaminated; (4) in the electrophoresis paint some components released during drying adhesion in electrophoretic paint film surface, changed the paint film surface properties.

2 accident analysis and verification of the test

For the above may lead to powder coating appear shrinkage factors, through test analysis step by step.

2.1 not clean compressed air, oil, water, too

Our factory adopts WZG - 6/8 no hot regeneration air dryer, compressed air after degreasing, drying air treatment, all the parameters can meet the requirements of powder coating, the specific indicators are as follows: pressure is 0.6 ~ 0.8 MPa, dew point temperature is not more than to 40 ℃, oil, water and other impurities meet standard requirements. At the same time, the following tests: use the same parts, part of the electrophoresis after spraying powder, the rest parts after dealing with the former directly spray powder. Test result is: the electrophoresis parts, after spraying powder to poor adhesion of paint film on the surface of a massive shrinkage; Parts directly to the powder, paint film good appearance and excellent adhesion. Can be ruled out not clean compressed air, oil, shrinkage cavity caused by too much water.

2.2 electrophoresis paint curing is not sufficient

Electrophoretic coating process, if the drying process control is not strict, can not meet the minimum requirements, such as baking temperature or time can cause unlock after curing agent containing isocyanate group reaction did not completely and saved in electrophoretic paint film surface, change the surface properties of the electrophoretic paint film caused by poor with paint [1]. To validate the factors of electrophoresis paint film after quality inspection, its hardness is 2 h, adhesion to zero level, the quality of the paint film can meet the original paint technical indicators; At the same time, it by a cotton wool with acetone in the electrophoretic coating (9.8 N) repeat 10 times, no paint film coating on the surface to be in good condition, gauze; Then after electrophoresis paint chassis parts in powder coating line paint curing oven to 180 ℃, 25 min secondary curing, finally, powder painting, found the film there is still a large shrinkage cavity, poor adhesion. On the basis of electrodeposition primer could be eliminated due to inadequate curing shrinkage cavity.

2.3 electrophoresis paint film dirt contamination contact with the outside world

For validation of electrophoresis paint film contact outside dirt contamination, taking the validation plan as follows: (1) of electrophoresis paint film with xylene is wiped, dry powder coating of the reentry after; (2) take electrophoresis groove liquid in the laboratory of electrophoresis model, then carries on the powder painting. Xylene after wiping parts, painting paint film appearance quality has improved, but there is still a serious shrinkage cavity, basic no improve adhesion; Electrophoresis model made in laboratory, the effect after powder painting and electrophoresis production line of exactly the same: large shrinkage cavity, poor adhesion. On the basis of sewage can rule out the possibility of contact electrophoresis paint film outside the pollution caused by the shrinkage cavity.

2.4 electrophoresis paint release attached in the drying process

In the film surface, change the nature of film surface search reason failed in many cases, the communication with the electrophoresis paint manufacturers that recently used electrophoresis line is October 20 batches of raw lacquer, manufacturer in the case of the appearance quality of the inspection found that poor joined the Taiwan 435 flow ping agent of the company of modesty, and suspected may be the result of shrinkage cavity and the flow ping agent bad adhesion. To validate the factors, the following tests are carried out: October 20 batches were taken the original paint and September 10 batches the original paint (not including 435 flow ping agent), to produce electrophoresis paint sample, then powder painting, on October 20 batches of paint exist a large amount of shrinkage cavity, and poor adhesion, consistent with the situation of reflect; And September 10 batches of raw lacquer is normal, on the basis of the basic judgment is 435 flow ping agent after electrophoresis painting sex harmful, the results are shown in figure 2.

3 the results analysis and discussion

Flow ping agent is one of the coatings additives, it can give coating in the process of dry film formed a level off, smooth, uniform coating. The main components in 435 flow ping agent modified polysiloxane, as part of the organic silicone additives, the additive has a lower surface tension, through the side of the main chain base access with reactive functional groups of the hydroxyl of modified [2].

The flow ping agent can be used to solve the metal, polymer and wood such as remover and lubricating oil pollution caused by coating the surface shrinkage problem, add to the coating, can significantly reduce the surface tension of coating, reduce the film surface tension gradient, promote the coating flow leveling performance, prevent hair flower, orange peel, improve the wettability of substrate, adhesive bonding, reduces the coating surface defects such as shrinkage cavity, pinhole. But as a result of reactive functional groups were modified, and its in the process of film forming and crosslinking reaction resin and curing agent, concentration of methyl groups in the surface of paint film, film surface tension becomes very low. According to the theory of fluid distribution: when the surface tension is greater than the primer coating the surface tension of the film, will cause the wet membrane wetting distribution difficult, paint film produce ills, seriously affect the appearance of the coating and adhesion between the layers [3]. To understand the 435 system flow ping agent adding amount of powder coating painting, the influence of the different adding amount (from no to 0.3 ‰) of electrophoresis, powder coating observed recoat sex again.

4 conclusion

The influence factors of painting sex of there are many, including coating surface tension is a very important factor. If the surface tension of the primer is too low, will bring a big problem to subsequent painting sex of, can't even for painting. According to the quality accident and organic silicon class flow ping agent painting sex of research, the author thinks that: in the electrophoresis paint should do) is joined to a greater influence on the coating surface tension of additives, such as organic silicon flow ping agent, especially not modified polysiloxane, reactive functional groups of modified polysiloxane, because this kind of flow ping agent content in just 0.025 ‰, painting adhesion decreases obviously, paint film appearance became poor. , when users in the test of electrodeposition primer should also spot check the painting compatibility according to usage, paint manufacturers in the change of flow ping agent class additive formulation should communicate with the user in advance and prepare related process test, to ensure that the painting sex, so as to avoid irreparable loss.

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