JPS6048841B2 - Insulating material for embedding heating wires - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an insulating material used in heating plates such as stoves, hot plates, coffee makers, etc., and in particular, it is intended to prevent the loss of insulating powder and the occurrence of cracks, and to improve the insulating performance. There is something that says.

Generally, this type of embedded heater is made by forming a groove in a metal heating plate, and embedding a heating wire in the center of the groove with an insulating material.

Conventionally, the insulating materials used in the above-mentioned embedded heaters include insulating powders such as silica, talc, magnesia, and alumina, either singly or in a mixture of several powders, to which aluminum phosphate or silicone varnish is added. was used. However, since the insulating material is inorganic and the heating plate is made of aluminum or cast iron, cracks occur in the insulating material due to the difference in thermal expansion coefficient between the two. This can be improved to some extent by reducing the voids between the particles by adjusting the particle size and reducing the thermal expansion of the voids at high temperatures. Cracks begin to appear. Furthermore, if a large amount of silicone varnish is used, the shrinkage due to the burning compaction peculiar to silicone resin during firing will be large, and the difference between the coefficient of thermal expansion of the heating plate will be large, and cracks will occur in the insulating powder. Since the heating wire in the cracked area comes into contact with the atmosphere, oxidation of the heating wire progresses rapidly, causing wire breakage and a decrease in insulation resistance. In addition, the insulating material has the disadvantage that cracks cause it to break into pieces and easily peel off from the hot plate. The present invention eliminates the drawbacks of the insulating material for embedding heating wires in the conventional embedded heaters, etc., and embodiments of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, reference numeral 1 denotes a heating plate made of die-cast aluminum or cast iron, and a groove 2 having a substantially U-shaped cross section is formed on one side surface. 3 is an insulating material, and a heating wire 4 is buried in the center of the groove 2.

Note that the insulating material 3 has a ratio of 9/clt to 100 after filling, for example.
It was press-molded at a pressure of 380° C. for 5 hours. Next, the inventors studied insulating material 3. The material is talc with a scaly crystal structure (
3Mg0・4Si0.

・H. O), metal oxides (Al, O, or MgO), and mica (K.O-3A1'.0.

・6Si0.・Experiments were carried out with various blending ratios limited to 哄O) and silicone varnish. The amount of silicone varnish was varied with respect to the insulating powder mixed so that mica was 6% by weight with respect to the mixed powder of talc and alumina. As a result, when the resin content of the silicone varnish is less than 3% by weight based on the insulating powder, the amount of J silicone varnish is so small that it does not penetrate between all the insulating powder particles and does not act as a binder. As a result, although it was possible to mold it, when conducting a current test, the insulating material was missing, heat conduction was poor, and the heating wire broke, making it unusable. In addition, if the resin content of the silicone varnish exceeds 15% by weight based on the insulating powder, the shrinkage due to the burning compaction peculiar to silicone resin will increase during the baking process in the post-process, and the difference between the thermal expansion coefficient of the hot plate will become large. A crack has formed in the insulating material. From the results of the above experiments, it was found that the appropriate resin content of the silicone varnish is in the range of 3 to 15% by weight based on the insulating powder.
9% by weight was found to be optimal. Note that even if you change the insulating powder by talc alone or by changing the blending ratio of talc and alumina, or by changing the blending ratio of talc/alumina mixed powder and mica powder, which will be explained later, there is no need to change the amount of silicone varnish added. Nakatsuta. Next, the inventors investigated various blending ratios of mica in a mixed powder of talc and alumina. Silicone varnish was added to the mixed insulating powder so that the resin content was 9% by weight. The results are shown in Table 1. As is clear from the results shown in Table 1, the amount of mica added to the mixed powder of talc and alumina is 1 to 1% by volume.
The content was preferably 6% by weight.

The amount of mica added is estimated as follows.
That is, as shown in FIG. 3, the insulating powder 3 is formed into many layers by press-molding a mixed powder to which talc having a scale-like crystal structure is added. Since the temperature near the heating wire 4 is high, cracks occur due to the difference in thermal expansion coefficient between the insulating material 3 and the heating plate 1. Furthermore, as shown in FIG. 2, cracks occur in the insulating powder 3a near the heating wire 4 due to thermal expansion of the gaps 6 between the particles 5. However, the difference in thermal expansion coefficient is alleviated due to the misalignment of the multiple insulating layers.
Cracks near the heating wire do not reach the insulating material surface 3b. Since talc having a scale-like crystal structure is very soft and lacks strength, cracks will appear on the insulating material surface 3b in about 10 hours if the current test is continued. Therefore, by adding mica powder having a strong layered structure, the strength of the multi-layered insulating material layer is improved. The main reason for this is that mica, which has a layered structure, becomes flat when it is powdered, and this mica powder is used as an insulating powder.
By mixing the mica powder and performing breath molding, this mica powder is also located in a layered manner, and as a result, the mica powder is mixed with the silicone varnish with a plurality of insulating powder particles 5 in contact with the periphery of the mica powder. This is because the particles and the particles 5 are bonded together. Note that in the above state, a tensile load is applied to the insulating powder due to thermal expansion, and a force that tries to separate the particles 5 acts, but mica powder is interposed between the particles 5 in a layered manner, and this mica powder The force required to break the insulating powder 3 is much greater than the force required to separate the particles 5 bonded by the silicone varnish, so the mica powder acts as a reinforcing material and is present everywhere. Cracks will no longer occur. However, the amount added is 1
If the amount is 1% by weight or more, the bulk specific gravity of mica is small, so the amount of insulating powder 3 will inevitably increase, and it will not act as a binder to allow the silicone varnish to penetrate between all powder particles. As a result, although it could be molded, it could not be used because the insulating material would be missing or the thermal conductivity would be poor and the heating wire would break when an electrical current test was carried out. Further, even if the amount of silicone varnish added is increased, no improvement can be made; if the amount added is 15% by weight or more, cracks will occur due to the difference in thermal expansion coefficient with the hot plate due to baking compaction. Furthermore, it was confirmed that there was no effect on performance even if talc alone or the blending ratio of talc and alumina was changed. Similar results were also obtained by using magnesia instead of alumina, and by using materials such as rouseite, kaolinite, nacrite, defukite, and halloysite, which have the same crystal structure, instead of talc, which has a scaly crystal structure. .
Figure 4 shows an embedded heater using the conventional insulating material B mixed with silica, talc, magnesia, etc. and the insulating material A of this example at a temperature of 40°C and a humidity of 90°C after an electrical current test.
This is a comparison of insulation properties in %. Since the embedded heater using the insulating material of this example does not generate cracks, there is almost no insulation deterioration even in high temperature and high humidity environments.

In addition, Figure 5 shows a silicone varnish with a resin content of 9% by weight added to an insulating powder made by mixing 6% by weight of mica with a mixed powder of talc having a scale-like crystal structure and alumina, a metal oxide. and after filling groove 2, 100k9/
This is a cross-sectional electron micrograph of an insulating material that was pressed under a pressure of 380°C and fired for 5 hours. As is clear from the above description, according to the present invention, mica powder having a strong layered structure is mixed with an insulating powder containing a material having a scaly crystal structure in an amount of 1 to 1% by weight, and It contains 3 to 15% by weight of silicone resin. Mica, which has a layered structure, becomes a flat shape when powdered, so this mica powder is mixed into insulating powder and press molded. Then, the mica powder is located in a layered manner, and the periphery of the mica powder is in contact with multiple insulating powder particles and bonded by silicone resin, and as a result, a tensile load due to thermal expansion is applied to the insulating powder. Even if a force tries to separate the insulating powder particles, there is a layer of mica powder between these particles, and the force required to break the mica powder separates the particles bonded with silicone resin. This mica powder acts as a reinforcing material and is present everywhere, so there is no cracking or chipping of the insulating powder, and there is no damage to the insulating powder even when conducting an energization test. Since cracks do not occur, it is possible to provide an insulating material for embedding heating wires that does not cause disconnection and has improved insulation performance. In addition,
It goes without saying that even when the insulating material of the present invention is used in a sheathed heater, similar effects such as preventing the occurrence of cracks can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of an embedded heater showing an embodiment of the present invention, Fig. 2 is a schematic diagram of the insulating material of the embedded heater, and Fig. 3 is an enlarged cross-sectional view of the main part of the embedded heater. 4 is a time characteristic diagram of insulation resistance of the same embedded heater and a conventional embedded heater, and FIG. 5 is an electron micrograph of an insulating material of an embedded heater showing an embodiment of the present invention. be. 3...Insulating material.

Claims (1)

[Claims] 1. 1 to 10% by weight of mica powder having a strong layered structure is mixed with an insulating powder containing a material having a scaly crystal structure, and 3 to 15% by weight of silicone resin is contained therein. Insulating material for embedding heating wires.