Conductive color-changing ink

Details Conductive color-changing ink Conductive color-changing ink

1998-10-28

2001-02-13

Mack, Ricky (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

Reexamination Certificate

active

06188506

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to thermochromic compositions. The present invention also relates to electrically conductive compositions such as conductive inks and the like.
BACKGROUND OF THE INVENTION
Thermochromic dyes are known for use in various applications including electrical systems applications. For example, thermochromic dyes have been used as a voltmeter in the prior art to determine the signal strength of batteries. In such an application, a thermochromic dye containing layer may be arranged in contact with an electrically conductive layer which extends between positive and negative electrodes of a battery. A switch would be used to open and close a current flow path so that when the switch is in a closed position, current from the battery is permitted to flow between the positive and negative electrodes through the conductive layer. Heat is generated as the current flows through the conductive layer thus activating the thermochromic dye in the adjacent layer so that a color change is obtained. Such a prior art system has shortcomings as it requires entirely separate layers of thermochromic ink and conductive materials. The separate layers contribute to increased manufacturing costs and printing steps. Further, problems may arise due to separation of the various layers. Examples of such prior art systems are disclosed in U.S. Pat. Nos. 5,059,895 to Capaldi et al.; 5,015,544 to Burroughs et al.; 4,835,476 to Kurosawa; and 4,835,475 to Hanakura et al.
The prior art also discloses the use of thermochromic materials for coating color changing designs onto substrates. For example, U.S. Pat. No. 4,028,118 to Nakasuji et al. teaches that color changing thermochromic compositions may be applied to the surface of a circuit board or machine and activated by heat generation caused by overloads in the electrical circuits or electrical machines to provide a warning of an overload condition. It is also disclosed that color changes could be used to hide or reveal a background for a display, exhibition, advertisement, teaching material, toy, magnetic device, or the like. As with the other prior art patents discussed above, the '118 patent requires a separate thermochromic layer in contact with a conductive material. All of the foregoing prior art systems have the shortcoming of requiring thermochromic compositions to be used in conjunction with at least one additional separate layer of conductive material. The present invention overcomes this shortcoming.
SUMMARY AND OBJECTS OF THE INVENTION
In one aspect of the present invention, a conductive composition is provided. The conductive composition preferably comprises a thermochromic material and a conductive material dispersed in a resin. The resin, the thermochromic material and the conductive material can therefore be applied as a single composition layer. It should be appreciated that as used herein, the term “resin” is intended to include materials conventionally known as resins, binders or adhesives.
Preferably, the thermochromic material is present in an amount sufficient to affect a noticeable color change in response to a predetermined temperature when heat is applied thereto. In another preferred embodiment, the amount of heat required to activate the color changing properties of the thermochromic material is greater than room temperature. In another preferred embodiment, the amount of heat required to activate the thermochromic material to change colors is greater than normal skin temperature. In yet another preferred embodiment, the amount of heat required to activate the thermochromic material to change colors is greater than about 99 degrees F. In still another embodiment, the conductive composition is capable of changing a plurality of colors in response to a predetermined number of different temperatures. Alternatively, either discreet areas or expanding areas will respond with a color change responsive to an increased resistance and increased heat.
The conductive material of the composition is preferably present in an amount sufficient to permit electrical current to flow therethrough so that an associated circuit component can be activated. At the same time, it is desirable for the conductive composition to have a preselected resistivity selected so that heat will be generated as electrical current flows through the conductive material. The heat generated by the flow of electrical current through the conductive material would preferably be sufficient to activate the thermochromic material to change color.
It is also preferable for the resin of the conductive composition to be selected from the group consisting of urethanes, acrylics, phenolics, epoxies and other oxidizing materials. The thermochromic materials may be encapsulated or nonencapsulated and may include cholosteric liquid crystal. The combination of the properties of the resin and the conductive materials, as well as their respective properties and the properties and proportion of the thermochromic material, can be used to modify the resistivity of the composition and, therefore, the amount of heat generated when current flows therethrough. This can be used, along with the selection of the appropriate thermochromic dyes, to adjust the temperature/color response of the resulting materials.
The conductive material of the present invention may include various conductors including precious metals, non-precious metals, conductive polymers and the like. Various conductive materials are disclosed in U.S. Pat. No. 5,626,948, such disclosure being incorporated by reference herein. The conductive composition of the present invention may also include a pigment. Suitable pigments may be organic or inorganic and are also disclosed in the '948 patent, which has been incorporated by reference herein.
In another preferred embodiment, the conductive composition of the present invention may include a defoamer such as silicone or non-silicone surfactants or solvents.
In yet another preferred embodiment, the present conductive composition may comprise a thickener which is described herein as any product that can change the flow or rheological characteristics of the subject composition.
Preferable ranges by weight of the materials which may be present in the conductive composition are as follows:
Resin—from 5% to 60%
Defoamer—from 0% to 5%
Thermochromic—from 2% to 75%
Conductor—from 10% to 60%
Solvents—from 0% to 50%
Thickener—from 0% to 10%
Reducer—from 0% to 50%
Pigment—from 0% to 10%
In a particularly preferred formulation, the resin may comprise about 38.7% of the conductive composition. The thermochromic material may comprise 31% of the conductive composition. The conductive material may comprise about 22.6% of the conductive composition, and the pigment may comprise about 7.7% of the conductive composition.
In another preferred formulation, the resin may comprise about 20% of the conductive composition while the thermochromic and conductive materials may comprise about 40% of the conductive composition.
The conductive composition of the present invention may also include a reducer, which is described herein as any product that thins the composition, such as water or other compounds.
In accordance with another aspect of the present invention, an electrical system is provided. The electrical system comprises a substrate, a conductive composition arranged on a substrate, a power source having a positive terminal and a neutral terminal wherein the conductive composition is arranged between the positive and neutral terminals of the power source. In a preferred embodiment, the electrical system may also comprise a switch for selectively opening and closing a circuit that will permit current to flow from the power source through the conductive composition. In yet a further embodiment, the electrical system may comprise a current operated module responsive to current flow generated from the power source. The conductive composition in accordance with this aspect of the invention, may include the materials and features discussed above.
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