Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-10-25
2003-09-09
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S233000, C524S296000, C524S377000, C524S439000, C524S440000, C524S495000, C524S496000
Reexamination Certificate
active
06617377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to polymer thick film conductive compositions containing nanomaterials. In particular, the invention is directed to such compositions, which are suitable for making variable resistive elements such as those used in position sensing elements.
2. Description of the Related Art
Electrically resistive polymer thick film compositions have numerous applications. Polymer thick film (PTF) resistive compositions are screenable pastes which are used to form resistive elements in electronic applications. Such compositions contain conductive filler material dispersed in polymeric resins which remain an integral part of the final composition after processing.
Resistive compositions are used as resistive elements in variable resistors, potentiometers, and position sensor applications. A resistive element is, in most cases, printed over a conductive element which acts as a collector element. In position sensing applications, a metallic wiper slides over the resistive element. The wiper can slide back and forth for several million cycles over the collector and resistive elements during the lifetime of the electronic component. For accurate position sensing, the wiper should give continuous electrical output throughout the life of the sensor.
The durability of these position sensing elements depends on the mechanical properties of both the resistor and the conductive film. The polymer thick films tend to wear out after several million cycles of sliding with a metallic contactor over the elements at extreme temperature conditions typically seen in an environment such as an automotive engine compartment. Therefore, polymer resistive and conductive compositions having excellent mechanical properties and wear resistance are required for performance and signal output in these applications.
In addition to good mechanical properties, these materials should also have good thermal properties. Polymer thick films show a decrease in storage modulus as temperature is increased. A sharp decrease in mechanical properties is observed near the glass transition temperature. In addition to loss in modulus, these materials also tend to show an increase in coefficient of thermal expansion, which increases significantly above the glass transition temperature (Tg). When used in, for example, motor vehicles, a position sensor is exposed to high temperatures in under the hood applications. At these temperatures resistive elements show a high rate of wear due to a decrease in modulus properties. In addition to the surrounding temperature, a still higher temperature is observed at the interface between the metallic wiper and the resistive element surface due to frictional heating. In some cases, these temperatures can approach the glass transition temperature (Tg) of the resistive material and can cause loss of the material's mechanical properties, which adversely affect signal output.
A prior art resistor composition is as follows:
Prior Art Composition
Component
Weight (%)
Polyamide imide
21.0
Carbon black
5.3
N-methyl pyrrolidone
73.7
One way to improve mechanical properties of a resistive film is to incorporate fillers, such as short fibers, in these films. The presence of fibers of relatively large dimension creates an electrically heterogeneous surface. This results in non-linear electrical output in contact sensor applications. Even when the size of the fibers is in the order of a few microns, the surface is still electrically and mechanically heterogeneous. A dither motion at high frequency on a surface region where these fibers are absent can create large wear. Another problem with using fibers with greater than 10 volume percentage is that it can significantly wear the metallic contactor. This wear is accelerated if these fibers are protruding from the surface. Therefore, there is a need in the art for resistor elements with enhanced mechanical and thermal properties while exhibiting homogeneous surface electrical characteristics.
SUMMARY OF INVENTION
According to a preferred embodiment of the invention, a resistive composition for screen printing onto a substrate is provided. The resistive composition, based on total composition, has a) 5-30 wt. % of polymer resin, b) greater than 0 up to and including 10 wt. % of thermosetting resin, c) 10-30 wt. % conductive particles selected from the group consisting of carbon black, graphite and mixtures thereof, and d) 1-20 wt. % carbon nanoparticles, wherein all of (a), (b), (c) and (d) are dispersed in a 60-80 wt. % organic solvent.
The present invention relates to an improved nanocomposite resistive composition comprising a polymeric resin and dispersed nanomaterials having conductive fillers and potentially anti-friction additives, with the dispersed nanomaterials being present in an amount less than 30% by weight of the cured nanocomposite films. The nanomaterials are preferably selected from carbon nanotubes, vapor grown nanofibers, milled carbon fibers, nanoclays, and molecular silica.
The invention provides increased mechanical, wear, electrical, and thermal properties of the resistor materials by incorporating the nanomaterials into the resistive composition. The large surface to volume ratio of the materials imparts significant interfacial strength to the composites. The functions of nanoparticles and nanofibers are to increase the polymer-filler interactions. The large surface area of these nanomaterials significantly interacts with functional groups in the macromolecular chains. These interactions in the molecular and nanoscale increases the microhardness and nano-hardness properties of these materials. These micro and nanohardness properties are very important for the sliding contact applications. The homogeneity of the nanocomposite film increases the toughness and hardness uniformly. Forming a resistor surface with molecularly dispersed fibers or other so called nanomaterials of submicron size in accordance with the invention can create an electrically and mechanically homogeneous surface which enables a consistent and durable electrical output to be established. The molecular silica materials and nanoclay can provide increased thermal properties. The carbon fibrils provide increased electrical and mechanical properties. A composition containing carbon nanofibers and molecular silica materials provide enhanced wear resistance, enhanced thermal properties, and enhanced electrical properties.
The invention provides a decrease in contactor wear by either avoiding the use of relatively large carbon fibers or by using a very small concentration of very finely milled carbon fibers in conjunction with nanoparticles and nanofibers. Due to the large surface to volume ratio, nanoparticles and nanofibers need to be used in less than 5 volume percentage. This significantly reduces the tendency of the contactor to prematurely wear.
The invention creates a resistor surface with a homogeneous electrical and mechanical surface in nanoscale. During a high frequency small stroke dither test, the contactor will always be sliding on a mechanically tough nanocomposite surface. In contrast, the high frequency small stroke dither test on a composition of prior art can gouge and pit a resistor surface where the carbon fibers are absent.
The invention decreases the coefficient of thermal expansion (CTE) of the resistor material. Wear of resistor materials typically is significantly increased at high temperature. One of the reasons for this phenomenon is the increased expansion of the material. By incorporating molecular silica, nanoclay, and nanofibers, molecular scale interactions with the polymer matrix are achieved. These strong interactions in nanoscale decrease the CTE of the material. In contrast, significantly large amount of fibers would be needed to be added to a polymer matrix to decrease the matrix's thermal expansion coefficient. As mentioned earlier, adding a large amount of carbon fibers to the matrix can significantly wear the associated metallic contactor.
The invent
Bourgeois Mark P.
CTS Corporation
Szekely Peter
LandOfFree
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