GB2144406A - Production of an electrically conducting molding and device comprising such molding - Google Patents

Abstract

This invention relates to a process for manufacturing an electrically conducting resin molding, e.g., for replacement of a metallic electrode in telephone receivers. The molding comprises carbon particles and a thermosetting resin, and an improvement in its fabrication involves a two-stage heat treatment resulting in desirably low electrical resistance, high physical strength and integrity, and low noise in electroacoustic use.

Description

SPECIFICATION Method for making an electrically conducting molding and device comprising such molding The invention is concerned with the manufacture of an electrically conducting shaped component and, more particularly, with the heat treatment of a thermosetting resin molding.

As gold remains expensive, efforts continue towards reducing or even eliminating its use in the manufacture of industrial and commerical products. For example, in the interest of eliminating the use of gold in telephone transmitter electrodes, U.S. patent 4,205,206, issued to T. Eishima et al. on May 27, 1980 discloses replacement of a metallic electrode by a conductive resin molding in which conductivity is due to carbon particles dispersed in the resin material.

Among properties of a desired electrode material are adequate physical integrity, suitably low electrical resistance, and low noise, the latter being of particular importance in electroacoustic applications. Accordingly, means are sought for making electrically conducting resin moldings which meet or exceed strength, resistance, and noise criteria.

According to the present invention there is provided a method for making an electrically conducting molded article from a molding material comprising a mixture including thermosetting resin binder material and carboniferous particles, said particles being present in said article in an amount in the range of from 50 to 80 weight percent of said article, said method comprising a heat treatment of said article, wherein in order to produce an article substantially free of pores and microfissures said heat treatment comprises a first stage and a second stage, said first stage comprising heating said article in a first temperature range of from 150 to 250 degrees C for a duration which is sufficient to substantially cross-link said binder material, and said second stage comprising heating said article in a second temperature range of from 800 to 1600 degrees Cfor a duration which results in a significant amount of carburization.

In an embodiment of the invention, an electrically conducting component is made as a molding comprising a thermosetting resin binder material and carboniferous particles, the latter preferably representing 50 to 80 weight percent of the component. After molding, the component is treated by heating first at a relatively low temperature in a preferred range of from 150 to 250 degrees Celcius and, subsequently, at a higher temperature in a preferred range of from 800 to 1600 degrees Celcius. As compared with properties obtained after a less sophisticated, single-temperature heat treatment, mechanical properties are enhanced and electrical resistance is reduced to meet or exceed, e.g., specifications of a telephone transmitter electrode.

For a better understanding of the invention, reference is made to the accompanying drawing, in which: Figure 1 is an enlarged cross-sectional schematic of a telephone transmitter microphone comprising an electrode made in accordance with an embodiment of the invention; and Figure 2 is an enlarged cross-sectional schematic of a voltage surge arrestor block assembly comprising first and second electrodes made in accordance with another embodiment of the invention.

FIG. 1 shows a microphone transmitter unit comprising frame 1, insulator 2, washer 3, chamber cap 4, dome electrode 5, granular carbon 6, molded resin back electrode 7, clamping ring 8, washer 9, compliant chamber closure 10, grid 11, spider 12, diaphragm 13, membrane 14, acoustic resistance 15, and ferrule 16; and FIG. 2 shows porcelain holder 21 and electrodes 22 and 23.

Electrodes such as, e.g., those numbered 7, 22, and 23 in the drawing are made, in accordance with the invention, from a molding material comprising a thermosetting resin and carboniferous particles. The thermosetting resin may be an epoxy resin or a phenolic resin such as, e.g., a phenol formaldehyde, and carboniferous particles may be obtained, e.g., as lampblack, coke, or graphite. In the interest of desired electrical conductivity., carboniferous particles are comprised in the molded article in a preferred amount of at least 50 and preferably at least 65 percent by weight; in the interest of desired moldability, this preferred amount is less than or equal to 80 weight percent and preferably less than or equal to 75 weight percent.In the interest of enhanced strength of a molded electrode, mixtures of different forms of carbon are preferred in which particle size varies; e.g., a mixture of graphite, lampblack, and coke is preferred in this respect.

Preferred phenolic resin materials suitable for the purpose of the invention have a melting point in the vicinitY of 200 degrees C, and their dryblend compound temperature preferably does not exceed 120 degrees C. In the interest of ease of molding, lubricants such as, e.g., stearic acid, G-70, Loxiol and suitable mixtures thereof are preferably included in a mixture so as to constitute a preferred amount in the range of 1 to 4 weight percent of the blend. Amounts less than 1 percent are considered insufficient to appreciably enhance moldability, and amounts greater than 4 percent are difficult to incorporate in a blend. A preferred range for lubricants is from 2 to 3 weight percent of the mixture.

Especially preferred processing is by blending of ingredients, fluxing, molding, and firing (pyrolysing) under a protective atmosphere; such atmosphere may be, e.g., pure hydrogen or a partially reducing and/or inert atmosphere. For example, when the atmosphere consists essentially of hydrogen mixed with nitrogen, hydrogen is preferably used in an amount representing at least 4 volume percent of the mixture. Pyrolysis may also be effected under vacuum.

Preferred pyrolysis in one embodiment involves heating at temperatures in two distinct temperature ranges, namely first at a temperature or temperatures in a range of from 150 to 250 degrees C and then at a temperature or temperatures in a range of from 800 to 1600 degrees C. The first heating step is understood to result in substantial cross-linking of a thermosetting resin binder material; the second step is understood to result in a significant amount of carburization of such material. Typical heating times and temperatures are 1 hour at 180 degrees C and 1 hour at 1000 degrees C, higher temperatures corresponding to shorter times and conversely. Heating from first to second temperatures and final cooling to room temperature may each take approximately 1 hour.

Processing in the embodiments was determined to produce high-strength, high-density electrodes.

Furthermore, desirably low dynamic resistance and minimized noise figures were measured. Efficient pyrolysis of modifiers used in a fluxed blend is considered to result in reduced surface carbon residue and thereby to contribute to minimization of noise. Noise figures less than or equal to 17.5 dBRNC and dynamic resistance less than or equal to 65 ohms are readily realized in the case of a molded telephone transmitter electrode. (Units of dBRNC, decibel reference noise curve-weighted, express noise generation relevant to telephonic transmission and relative to reference noise as produced in a 600-ohm resistor at a power dissipation of 10-'2 watts) Example I.

A dry mixture was prepared containing approximately 23 weight percent phenolic resin designated as "1436" as obtained from the Plastic Engineering Company, approximately 57.5 weight percent coke, approximately 15.1 weight percent lampblack, approximately 2.6 weight percent hexa, approximately 1.2 weight percent stearic acid, and approximately 0.6 weight percent "Loxiol G-40" as obtained from the Henkel Corporation. Mixing was in a high-speed, internal-type Henschell mixer and consisted in first mixing phenolic resin, carbon black, hexa, and stearic acid for approximately 4 minutes and after adding "Loxiol", mixing for an additional 2 minutes approximately. The mixture was compression-molded into the shape of a telephone receiver back electrode.The molded article was placed in a furnace which had been purged with pure hydrogen, and heated to a temperature of approximately 180 degrees C. After heating at this temperature for approximately one hour, temperature was raised to approximately 1000 degrees C and maintained there again for approximately 1 hour. After cooling, the component was determined to have a dynamic resistance of approximately 60 ohms, a noise figure of approximately 17 dBRNC, and satisfactory mechanical properties.

Example 2.

A dry mixture was prepared as described above in Example 1. The mixture was fluxed, compression-molded, and then pyrolized by heating as described above in Example 1. The cooled component was determined to have a dynamic resistance of approximately 47.6 ohms, a noise figure of approximately 16.5 dBRNC, and even better mechanical properties than the component of Example 1 where fluxing had not been used prior to molding.

Component properties of Examples 1 and 2 are contrasted with corresponding properties obtained upon pyrolyzing an otherwise essentially identical molding by heating directly to a temperature of approximately 1100 degrees C and holding there for approximately 1 hour. In this case dynamic resistance of the cooled component was determined to be approximately 70 ohms which is considered to be unacceptably high for telephone transmitter usage. Also, high porosity and a large number of microcracks rendered such component less suitable for telephone transmitter assembly and usage.

Claims (9)

1. Method for making an electrically conducting molded article from a molding material comprising a mixture including thermosetting resin binder material and carboniferous particles, said particles being present in said article in an amount in the range of from 50 to 80 weight percent of said article, said method comprising a heat treatment of said article, wherein in order to produce an article substantially free of pores and microfissures said heat treatment comprises a first stage and a second stage, said first stage comprising heating said article in a first temperature range of from 150 to 250 degrees C for a duration which is sufficient to substantially cross-link said binder material, and said second stage comprising heating said article in a second temperature range of from 800 to 1600 degrees C for a duration which results in a significant amount of carburization.

2. Method in accordance with claim 1, further comprising optionally fluxing said molding material prior to molding.

3. Method in accordance with claim 2, wherein molding material is mixed prior to fluxing.

4. Method in accordance with claim 3, wherein the mixing of the thermosetting binder material and said carboniferous particles is in combination with one or more lubricating modifiers representing an amount in the range of from 1 to 4 weight percent of the mixture.

5. Method in accordance with claim 4, wherein said lubricating modifiers represent an amount in the range of from 2 to 3 weight percent of the mixture.

6. Method in accordance with claim 1, wherein said carboniferous particles are present in said article as a mixture of at least two types selected from coke, lampblack, and graphite.

7. Method in accordance with claim 1, wherein carboniferous particles are present in said article in an amount in the range of from 65 to 75 weight percent of said article.

8. A method of making an electrically conducting molded article, substantially as hereinbefore described with reference to Example 1 or 2.

9. Device comprising an article made in accordance with the method of any one of the preceding claims.