Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2001-02-22
2003-10-07
Buttner, David J. (Department: 1712)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S514000, C523S210000, C524S440000, C200S265000
Reexamination Certificate
active
06630080
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductive resin composition having both excellent abrasion resistance and conductivity and an encoder switch having a contact part made from this conductive resin composition.
2. Description of the Related Art
Conventional encoder switches have sliding brushes made from phosphor bronze and contact parts made by punching out radial-shaped parts from phosphor bronze strips and coating the parts with nickel plating and silver plating in succession, wherein the sliding brushes contact the contact parts intermittently when the encoder switches are actuated.
However, the method wherein contact parts are punched from phosphor bronze strips requires production of dies for each shape of contact part, making it difficult to enable flexible production in which a wide variety of products are manufactured in low volume.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a long-life encoder switch which can enable the flexible production in which a wide variety of products are manufactured in low volume by making a contact part from a conductive resin composition having both excellent conductivity and abrasion resistance.
The conductive resin composition of the present invention comprises a binder resin and conductive particles comprising a conductive powder of carbon (hereinafter referred to as “carbon beads”) with silver-coated surfaces and a sliver powder, wherein the ratio of the conductive particles to the total is 74 to 88% by weight.
In such conductive resins, the silver coating acts to impart conductivity to the conductive resin composition and high hardness of the carbon beads, which is a base material, supports load applied to the conductive resin composition and acts to improve the abrasion resistance of the conductive resin composition. The conductive powder is based on carbon beads and is generally spherical without sharp edges, and therefore it does not abrade sliding brushes even if the sliding brushes which slide in contact with the conductive resin composition contact the conductive powder.
The silver powder exists between the conductive powders to increase contact points between the conductive particles and therefore acts to improve the conductivity of the conductive resin composition. The silver powder also acts to smooth the surface of the conductive resin composition by filling gaps between the conductive powders on the surface of the conductive resin composition.
The binder resin disperses the conductive powder and the silver powder of the conductive particles evenly and increases the contact pressure between the conductive particles by hardening and shrinking to reduce the contact resistance between the conductive particles to improve the conductivity.
When the conductive particles constitute 74% by weight or less of the total, the conductive particles are scarce, which decreases the conductivity and abrasion resistance of the conductive resin composition. On the other hand, 88% by weight or more of the conductive particles make patterning of the conductive resin composition difficult by screen printing.
In the conductive resin composition of the present invention, the conductive powder constitutes 1 to 9% by weight of the total.
When the conductive powder constitutes below 1% by weight of the total, the conductive powder cannot fully support load, which results in a decrease in the abrasion resistance of the conductive resin composition. On the other hand, when the ratio of the conductive powder to the total exceeds 9% by weight, the conductive resin composition becomes too hard so that it may abrade the sliding brush that slides in contact with the conductive resin composition.
Furthermore, the above-mentioned sliver powder is a mixture of globular particles and arborescent particles in the conductive resin composition of the present invention.
As for silver powder used in a conductive resin composition, globular particles have lower specific surface area and oil absorption than arborescent particles, therefore more silver powder can be added to a conductive resin composition.
When a lot of silver powder is added, a change in conductivity of the silver powder by sulfidization can be reduced, and sulfidization can be slowed when the powder is left in a sulfidizing atmosphere. On the other hand, the conductive resin composition has less projections and depressions, which reduces contact points between particles and increases the specific resistance of the conductive resin composition.
In such a resin composition, the contact points between silver powder are increased and superior conductivity of the conductive resin composition is obtained while containing more silver powder and maintaining its sulfidization resistance, by mixing globular particles with arborescent particles having a complex shape.
In the conductive resin composition of the present invention, the weight ratio of the globular particles to the arborescent particles is 0.8 to 1.6.
In such a conductive resin composition, since the weight ratio of the globular particles to the arborescent particles is 0.8 or more, the ratio of the arborescent particles having a complex shape to the silver powder is low and therefore more silver powder can be added in the conductive resin, which improves sulfidization resistance. Furthermore, the conductivity of the conductive resin composition can be improved by increasing the ratio of the silver powder in the conductive resin composition. In addition, the conductive resin composition does not need to contain a large quantity of the conductive powder as conductive particles, which prevents the conductive resin composition from being too hard.
Moreover, since the weight ratio of the globular particles to the arborescent particles is 1.6 or less, an appropriate quantity of the arborescent particles is present between the globular particles, which increases contact points between the silver powder having high conductivity to further improve the conductivity of the conductive resin composition.
In the conductive resin composition of the present invention, the binder resin is a thermosetting resin.
In such a conductive resin composition, the thermosetting resin has an appropriate quantity of hydrogen-binding functional groups (e.g., hydroxyl group and amino group) in the resin structure, which allows the conductive resin composition to adhere to a substrate. Use of a thermosetting resin as a binder resin also prevents the conductive resin composition from softening due to ambient temperatures and frictional heat.
In the conductive resin composition of the present invention, the thermosetting resin is a phenolic resin.
In such a conductive resin composition, the phenolic resin undergoes a dehydration condensation and causes sufficient hardening and shrinking to yield a higher contact pressure and a lower contact resistance between the conductive particles to improve the conductivity of the conductive resin composition, compared to when an epoxy resin or the like is used as a binder resin. Examples of the phenolic resin include, for example, resol-type phenolic resins, novolak-type phenolic resins, phenol aralkyl resins, xylene-modified phenolic resins, cresol-modified phenolic resins, furan-modified phenolic resins, epoxy-phenol resins, phenol-melamine resins, phenol-carboimide resins, and resorcinol-modified phenolic resins.
The encoder switch has a sliding brush made of a metal and a contact part which comprises the above-mentioned conductive resin composition and contacts with the sliding brush intermittently.
The conductive resin composition can be patterned by screen printing to provide such an encoder switch and thus facilitates the flexible production system in which a wide variety of products are manufactured in low volume by simply changing masks for screen printing when contact parts of different shape are manufactured. Furthermore, when a contact part is manufactured by screen printing, more complex shapes can be produced than when a metal plate is punc
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Buttner David J.
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