Study on the Method of Coating Defect Elimination and Effect on Mechanical Properties

The casting method in coating preparation has the advantages of short sample preparation period, small raw material loss, low cost, but the paint film often has bubble defects.The defects were characterized by microscope observation and density balance test. The influence of solvent content and ultrasonic shock on coating defects and the influence of defects on coating mechanical properties were investigated. The results show that the density decreases linearly with the increase of the number of bubbles. With the increase of solvent, the viscosity of paint decreases and the content of bubbles decreases obviously. As solvent increased, coating drying time increased, sample preparation period increased.After ultrasonic shock, bubbles in the surface of the paint and the cured coating were significantly reduced, compared with spraying sample preparation, ultrasonic shock sample preparation period was shortened by more than 30%.The samples with defective mechanical properties were broken during bubble emergence, the tensile strength and elongation at break were significantly reduced, and the dispersion was large, which was invalid data.


Introduction
The main function of the radome is to protect the radar system inside the radome from any kind of damage and destruction without affecting the detection performance of the radome basically.The coating system is an important part of the radome.The main requirements for the coating are as follows: firstly, the influence of the coating on the electromagnetic radiation characteristics of the antenna should be minimized; secondly, when the aircraft is flying at high speed under complex meteorological conditions, the strong friction between the coating and the air will cause static electricity on the surface.If the static electricity cannot be released in time, the accumulated static electricity will be discharged to a certain extent, which may cause the radome or even the antenna inside the radome and other electronic equipment Destructive effects.In addition, the radome is easy to be scoured by raindrops and hailstones, and the coating will be damaged for a long time, which will affect the structural safety and electrical quality of the radome.Therefore, radome coating should not only have excellent dielectric properties, electrostatic conductivity, but also have good mechanical properties.In order to guarantee the coating quality of products, it is particularly important to test the coating performance before coating products.Among them, the mechanical properties of the coating were mainly characterized by the dumbbell type sample specified in GB/T 528 for tensile strength and elongation at break testing, which needed to be tested after coating film preparation.At present, there are mainly three methods of coating film sampling: spraying, brushing and casting.The advantages and disadvantages are shown in Table 1.

Table 1.Comparative advantages of the three sample preparation
Although the coating casting film has the advantages of short sample preparation period, small loss of raw materials, low cost, but the surface and interior of the coating film prepared by pouring often exist bubble defects.If the coating contains bubbles, in the tensile strength and elongation at break test, with the increase of the tensile force of the electronic tensile machine, it will cause stress concentration in the coating defect, resulting in the sample fracture at the defect, the result of abnormal dispersion is large, unable to characterize the mechanical properties of the material itself.Repeated sample preparation and repeated trials waste materials and manpower, and reduce efficiency.In view of the problems existing in casting film, this paper carried out a study on the characterization of defects and elimination methods.

Materials
The test materials were the base, curing agent and solvent of primer and antistatic.

Method
The base material and curing agent were mixed together in accordance with the prescribed proportions, and different proportions of solvent were added to investigate the influence of solvent content on coating defects.Ultrasonic oscillation was carried out on the coating, and the change of coating defects before and after ultrasonic oscillation was investigated.Then the influence of defects on the mechanical properties of the coating was evaluated.

Characterization
The coating surface was observed by KH7700 3D video microscope with a resolution of 0.001mm.BT224S density balance was used to measure the density of the coating with the accuracy of 0.1mg.The tensile strength, elongation at break of the coating were tested by WDW-5C electronic tensile machine at a speed of 100mm/min.

3.1.Characterization Methods of Coating Defects
The three-dimensional video microscope was used to observe the transparent primer with bubbles and the antistatic coating with filler.The results showed that the diameter of bubbles in the transparent primer was between 0.2mm and 0.7mm, and the size of bubbles on the surface of the antistatic coating was about 1mm to 2mm.

Figure1.3D video microscope observation image
The density balance drainage method was used for the antistatic treatment of clear primers and antistatic coating.The density test of the coating showed that the density of the primer without defects was 1.17g/cm 3 and that of the primer with defects was 1.28 g/cm3.As the number of bubbles in the antistatic coating increases, the density decreases linearly.The fitting linear equation is Y=1.2743-0.0167X(Y: coating density, X: number of bubbles).

3.2.1.Influence of solvent on defects
(1) The influence of solvent on the defects of primer Add different proportions of solvent to the primer respectively, as follows: 0%, 10%, 20%, 30%, preparation of primer coating.It can be seen that with the increase of solvent, paint viscosity (coated with 4 viscosity cup) decreased 20s, 17s, 15s, 14s, the content of bubbles in the coating significantly reduced.When the solvent is 30%, there is almost no bubble defect in the coating.At the same time, with the increase of solvent content, the coating drying time increases, the time to remove from the mold increases.When the additive is 30%, 32h can be removed.

Figure3. Coatings prepared by adding different amouts of solvents
Figure4.The amount of solvent added is related to the number of bubbles (2) The influence of solvent on the defects of antistatic paint Adding different proportions of solvent in the antistatic coating, respectively 10%, 30%, 40%, preparation of coating.It can be seen that with the increase of solvent, the content of bubbles in the coating shows a trend of decreasing first and then increasing.This is because with the increase of solvent, the viscosity of the paint decreases.Although it is conducive to the release of bubbles to the surface of the inclusion and rupture, excessive solvent will cause insufficient solvent volatilization before the coating surface dries, and residual bubbles will be formed.

Figure 5.Coatings prepared by adding different amouts of solvents
Figure6.The amount of solvent added is related to the number of bubbles

Influence of ultrasonic shock on coating defects.
Figure7 shows the bubble changes of the antistatic paint before and after ultrasonic shock.The results show that bubbles in the paint are all over the surface before ultrasonic shock, while only a few large bubbles are found on the surface after ultrasonic shock.This is because ultrasonic vibration will produce high-frequency vibration, vibration conduction into the paint, so that the bubbles in the paint are broken to produce a large number of small bubbles, separated from the accumulation of large bubbles and then rise to the surface of the paint.It can also be seen from the cured coating that there are more bubble defects in the coating before ultrasonic, but there are no bubble defects in the coating after ultrasonic.Compared with spraying for 60h, the period of pouring sample preparation for primer and antistatic paint after ultrasonic shock was 40h, respectively, which was shortened by 33%.Figure9 shows the antistatic paint is ultrasonic for 0min, 0.5min, 1min, 3min and 5min respectively.The bubble statistics showed that the number of bubbles decreased with the increase of ultrasonic oscillation time.Five dumbbell test specimens for tensile properties were randomly cut with primer coating prepared with solvent content of 0% and 30% in section 3.2.1 and antistatic coating before and after ultrasonic oscillation in section 3.2.2.The tensile strength and elongation at break were tested by an electronic tension machine.The results showed that the tensile strength of the primed sample without defects was 28.6MPa, and the fracture of the defective sample during bubble emergence was invalid data.The fracture tensile strength was 6.5MPa and the dispersion was large.

Figure 7 .
Figure 7.Changes of bubble on coating surface before curing before and after ultrasonic oscillation

Figure 8 .
Figure 8. Changes of bubble on coating surface after curing before and after ultrasonic oscillation

Figure 10 .
Figure 10.Test specimen for tensile properties of primer coating