The development of electromagnetic shielding materials, especially lightweight products, is of great significance. Absorbing electromagnetic shielding coatings have been valued for their convenient application, good conductive and absorbing properties, simple processes and easy adjustment, and have made great progress. They are moving towards thin and light textures, wide-band absorbing, sprayable, and thermal properties. 2. Stable performance. Increasing the carbon fiber content and nickel plating can improve the shielding effect of the shielding paper, but the improvement of this shielding effect will inevitably lead to the decline in the mechanical properties of the shielding paper. Therefore, a coating method is considered to improve its shielding effectiveness.
1 Effect of coating on shielding performance of carbon fiber shielding paper
The shielding effect of electromagnetic shielding coating mainly depends on the volume resistivity, that is, the better the conductivity of the coating, the better the electromagnetic wave shielding performance of the coating. Therefore, the factors that affect the conductivity of electromagnetic wave shielding coatings must be studied.
1.1 Effect of filler content
According to the tunnel effect mechanism in which the conductive filler particles in the polymer matrix realize electron transport and conduct electricity, as a simple blending system, the amount of conductive filler must be increased to a sufficient filling amount or a certain threshold value, which is the "diafiltration threshold ", The contact gap can be formed, and the electrical conductivity of the coating can increase sharply and change abruptly to form a conductive path. It can be seen from Figure 1 that when the filler content is about 65%, the coating has the best conductivity and the corresponding electromagnetic shielding effectiveness is also good.
1.2 Effect of film-forming resin
When the acrylic resin content is 1.0g, the resistivity is the lowest and the conductivity is the best. It can be seen that, for this system, when the ratio of filler to base resin is about 3: 1, the limit of the percolation threshold is reached. Beyond this mass ratio, the conductivity will not increase, but it will decrease. Affects other physical and mechanical properties of the coating.
1.3 Effect of coupling agent surface treatment
The resistivity of the coating made with the coupling agent decreases with the increase in the amount of the coating. This shows that the addition of the coupling agent allows the metal filler powder and the polymer resin to be combined with each other and increases the cohesion of the two. At the same time, the addition of the coupling agent also has the effect of dispersing the filler, thus increasing the conductive properties of the coating. It can also be seen from the figure that the optimal amount of coupling agent is 2.5%.
1.4 Effect of curing process
It can be known from the formation process of the coating that the curing temperature and time have a great influence on the contact state of the filler in the coating. For this coating system, the curing temperature is determined to be 50 ° C. As can be seen in Figure 4, the optimal curing time is 40 minutes.
1.5 Effect of coating thickness
The conductive coatings were prepared according to the above optimal conditions, and the effects of different thicknesses on coating properties were investigated. From the aspects of conductivity, adhesion and cost, the coating of this system is more suitable at 110 μm.
2 Shielding performance test of paint
The shielding effect of FeSiAl powder is better, which can reach 25dB. As the frequency increases, the shielding effectiveness gradually improves. When the frequency range is from 800MHz to 1GHz, the shielding effect is basically in a better stage. The conductivity and magnetic permeability of ferrosilicon aluminum powder are high, and the shielding effectiveness in the frequency band of 600 MHz to 1.2 GHz is better than that of other frequency bands, reaching more than 20dB. This shows that for electromagnetic shielding materials with high electrical conductivity and high magnetic permeability, both reflection loss and absorption loss are high.