JPS6320467B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a paint for applying electromagnetic shielding treatment to plastic housings of OA equipment, etc., and was made with the aim of developing a paint that has a shielding effect that is higher and more stable than conventional ones, and whose effect is less likely to deteriorate over time. It is something that Traditionally, the housings of electronic devices such as personal computers, workstations, and office automation equipment such as word processors have been mostly made of metal.
They are gradually being replaced by plastic ones. However, along with this, leakage of electromagnetic waves has become a problem. In other words, when the casing is made of metal, the casing itself absorbs and reflects electromagnetic waves, blocking electromagnetic waves inside and outside the casing, whereas when the casing is made of plastic, the plastic allows the electromagnetic waves to pass through. In order to
There is a risk that electromagnetic waves generated by the device may leak to the outside, or external electromagnetic waves may enter the housing, causing the device to malfunction. Furthermore, electromagnetic waves leaking from office automation equipment usually have a frequency of 30 to 1000 MHz, and preventing this leakage is essential for the health of workers. For these reasons, it has become necessary to apply electromagnetic shielding treatment to the plastic housings of electronic devices. Conventional electromagnetic shielding treatment for plastics involves
There are two methods: one method is to mix a conductive filler into plastic to give the plastic itself a shielding property, and the other method is to attach a shielding material to the surface of the plastic. Currently, the latter method is the mainstream, and methods include pasting metal films (Al, Cu, Fe, etc.), metal spraying (Zn), plating, and forming a conductive film by applying conductive paint. The present invention pertains to the last-mentioned paint used for forming a conductive film. The electromagnetic shielding paint (Japanese Patent Application No. 112478/1982; Japanese Patent Application Laid-open No. 4569/1983) that the present inventors developed earlier is a coating containing carbon black or graphite in nickel powder of 0.5 to 0.5%.
With a composition containing 20% by weight of filler, the paint exhibited a better electromagnetic shielding effect than conventional filler containing only metal powder. By the way, compared to when the filler component is only metal powder,
When a small amount of graphite is added to this, the degree of attenuation increases rapidly. This effect becomes significant when the amount added to the metal powder is very small, 0.5% or more, and after reaching a maximum value around 2%, it shows a tendency to decrease as the amount added further increases, and at 30%, the amount added to the metal powder alone becomes significant. Only a lower degree of attenuation can be obtained. Therefore, the gist of the present invention is to set the amount of graphite and carbon black added in a range of 0.5 to 20% as a range in which the effect of addition is certainly large. Subsequently, as a result of further improvements to this electromagnetic shielding paint, while following the gist of the previous invention, a combination of graphite and carbon black was used as a filler.
In addition, the present invention was achieved by discovering that when the blending ratio is limited to a specific range, a higher electromagnetic shielding effect than before can be obtained, and the effect can be maintained for a long time. That is, the present invention provides a paint containing a filler in which a total of 0.5 to 20% by weight of graphite and carbon black is added to nickel powder.
The basic idea is to keep the ratio between 90:10 and 50:50. In addition, keeping the paint in a stable suspension state for a long period of time,
In order to form a strong coating film, it is necessary to make the solid particles in the coating composition fine. For this reason, nickel powder with an average particle size of 5μ or less is
In the case of flaky nickel, it is preferable that the average particle size is 50 μm or less, and there is no problem in using a mixture of both. Further, it is preferable that the graphite powder has an average particle size of 10 μm or less, and the carbon black has an average particle size of 1 μm or less. Combining these selections not only increases the pot life of the paint during application, but also
Fine carbon black particles fill the gaps between coarse graphite particles, forming a dense and strong coating. The present invention will be explained in more detail below with reference to Examples. Example First, as a material for conductive filler, the average particle size is 5μ.
graphite powder, carbon black with an average particle size of 0.1μ, carbonyl nickel with an average particle size of 2.5μ, and average particle size
A 40 μm flake of nickel was prepared, an acrylic resin whose main component was methyl methacrylate with a molecular weight of about 80,000 as a resin component, and a solvent consisting of a mixture of toluene and methyl ethyl ketone (7:3) as an organic solvent. Next, these were blended in the proportions shown in Table 1 below and mixed in a ball mill to prepare Paints 1 to 16. The blending ratios in the table are parts by weight. Next, each of these paints was applied to an ABS resin plate with a size of 150 x 150 x 2 mm so that the film thickness after drying would be 50μ, air-dried for 24 hours, and a sample for shielding effect measurement (sample Sample 16) was prepared from Sample 1. Note that the sample number corresponds to the paint number. Measuring shielding effectiveness: Place a resin plate in the middle of a pair of antennas placed at a predetermined distance so that the plane is perpendicular to the straight line connecting the antennas.
This was done by emitting electromagnetic waves with a frequency of 500 MHz from one side of the antenna, and measuring the intensity of the electromagnetic waves received by the other side (attenuated depending on the type of paint applied to the resin plate). The attenuation degree of each sample obtained is shown in the initial shielding effect column of Table 1. Note that when the resin plate is a blank sample, that is, no paint is applied, the attenuation degree (shielding effect) is 0 decibel. Changes in shielding effectiveness over time: Each sample whose initial shielding effectiveness was measured in the previous section was subjected to 30 heat cycles, and then tested in the same manner as in the previous section to measure the degree of attenuation of the received electromagnetic waves. The results are shown in Table 1 "After heat cycle"
The difference from the initial value is shown in the "Time-dependent change value" column. FIG. 1 is a graph of this data, showing the influence of the blending ratio of carbon black and graphite on the change (deterioration) of the shielding effect over time. One heat cycle is as follows: [85°C, 1 hour] ~ [23°C, 50% relative humidity, 1 hour] ~ [-29°C, 1 hour] ~ [23°C, 50% relative humidity] , 1 hour] These test conditions apply to metal coatings on plastics intended for electromagnetic shielding.
This is one of the environmental test conditions specified in the UL standard (UL746C), and is the condition where changes over time are most noticeable. Further, the number of repetitions in this standard is 3 cycles, but in this example, it was set to 30 cycles, which is more severe. From the above experimental results, the following can be found. That is,
Looking at Samples 1 to 6, which have the same conditions other than the blending ratio of graphite and carbon black in the filler,
First, between plain graphite (sample 1) and pure carbon black (sample 6), the latter has a better initial shielding effect, and the former has slightly less deterioration over time, but it is noticeable that the degree of deterioration is large in both cases. . On the other hand, in the case of samples 2 to 5 in which graphite and carbon black were used in combination, although the initial shielding effect was good, the major feature was that there was little deterioration over time (samples 2 to 4). This means that the electromagnetic shielding effect of electronic equipment is maintained for a long time.
This is extremely important for the reliability of shield paint. It should be noted that this phenomenon shows a similar tendency in both the group of samples 12 to 16 using flaky nickel and the group of samples 7 to 11 in which nickel is reduced. As described above, the change in shielding effect over time is significantly reduced by the combined use of graphite and carbon black, but this effect is related to the blending ratio of both.
That is, as is clear from the graph in Figure 1, the degree of deterioration exhibits a downward convex curve with a minimum value when the ratio of graphite and carbon black is 2:1, and increases suddenly when the ratio of graphite exceeds 90%. , the case of 1:1 is almost equal to the case of 9:1. Therefore, in order to guarantee a stable shielding effect with little deterioration over time, the blending ratio of graphite and carbon black is
90:10 to 50:50 is preferred.

[Table] As explained above, the electromagnetic shielding paint of the present invention not only has a high electromagnetic shielding effect, but also has little deterioration of the shielding effect over time. can be applied, and is extremely useful in practice.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the blending ratio of carbon black and graphite in the filler on the change in shielding effect over time.

Claims (1)

[Claims] 1. Graphite powder and carbon black in weight ratio
1. An electromagnetic shielding paint characterized by containing a filler having a composition in which a mixture in a ratio of 90:10 to 50:50 is added in an amount of 0.5 to 20% by weight based on nickel powder. 2. The electromagnetic shielding paint according to claim 1, wherein the graphite powder has an average particle size of 10 μm or less, and the carbon black has an average particle size of 1 μm or less. 3. The electromagnetic shielding paint according to claim 1 or 2, wherein the nickel powder is carbonyl nickel powder with an average particle size of 5 μm or less. 4. The electromagnetic shielding paint according to claim 1 or 2, wherein the nickel powder is flaky nickel powder with an average particle size of 50 μm or less. 5. The electromagnetic shielding paint according to claim 1 or 2, wherein the nickel powder is a mixture of carbonyl nickel powder with an average particle size of 5 μm or less and flaky nickel powder with an average particle size of 50 μm or less.