Formulation for a stable electrically conductive polymer

Abstract

The present invention is an electrically conductive polymer that is stable with respect to both time and environmental conditions. Most electrically conductive polymers have bulk resistance that varies (increases) over time. The current electrically conductive polymers also vary when they are exposed to harsh environments. The time and environmental variability is attributable to both the type of fiber and the type of coating used. The present invention uses stainless steel fibers that have an outer most coating that is one of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, and silver-palladium. The coating comprises 5%-40%, by weight, of the coating fiber. The coated fiber comprises 25%-35%, by weight, of the electrically conductive polymer. The bulk polymer is at least one of polypropylene (“PP”), polycarbonate (“PC”), acrylonitrile butadiene styrene (“ABS”), polyethylene (“PE”), polyether ether ketone (“PEEK”), and polyethylene terephthalate (“PET”).

Claims

1. An electrically conductive polymer comprised of a bulk polymer; and a plurality of coated stainless-steel fibers; wherein the stainless-steel fibers have an outermost coating that is one of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, and silver-palladium; wherein the outermost coating is between 5% and 40%, by weight, of the coated stainless-steel fiber; and wherein the coated stainless-steel fibers make up between 25% and 35%, by weight, of the electrically conductive polymer.

2. The electrically conductive polymer of claim 1, wherein austenitic stainless-steel fibers are used.

3. The electrically conductive polymer of claim 1, wherein martensitic stainless-steel fibers are used.

4. The electrically conductive polymer of claim 1, wherein precipitation hardened stainless-steel fibers are used.

5. The electrically conductive polymer of claim 1, wherein duplex stainless-steel fibers are used.

6. The electrically conductive polymer of claim 1, where ferritic stainless-steel fibers are used.

7. The electrically conductive polymer of claim 1, wherein the bulk polymer is one of polypropylene (“PP”), polycarbonate (“PC”), acrylonitrile butadiene styrene (“ABS”), polyethylene (“PE”), polyether ether ketone (“PEEK”), and polyethylene terephthalate (“PET”).

8. The electrically conductive polymer of claim 1, wherein the stainless-steel fibers have an inner coating.

9. The electrically conductive polymer of claim 8, wherein the inner coating is nickel.

10. The electrically conductive polymer of claim 9, wherein the inner coating of nickel is over-coated with copper.

11. The electrically conductive polymer of claim 8, wherein the inner coating is copper.

12. The electrically conductive polymer of claim 11, wherein the inner coating of copper is over-coated with nickel.

13. The electrically conductive polymer of claim 1, wherein the electrically conductive polymer has over 3.4 million coated stainless-steel fibers per cubic inch.

14. The electrically conductive polymer of claim 1, wherein the outermost coating is inductively heated in order to create solder bonds between the coated stainless-steel fibers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is illustrated with 11 drawings on 10 sheets.

(2) FIG. 1 is a magnified perspective cross-section of a single coated stainless-steel fiber used in the formulation for a stable electrically conductive polymer.

(3) FIG. 2 is a cross-section through the bulk of the formulation for a stable electrically conductive polymer showing that the fibers have a common orientation.

(4) FIG. 3 is a graph of the bulk resistance of a representative electrically conductive polymer.

(5) FIG. 4 is a graph showing the various families of stainless steel as a percentage of nickel and chromium.

(6) FIG. 5 is a front view of a plaque for testing electrically conductive polymers.

(7) FIG. 6A is a 100× magnification of a test plaque showing the bulk material and stainless-steel fibers.

(8) FIG. 6B is a 100× magnification of a test plaque showing the edge of the test plaque, along with the bulk material and stainless-steel fibers.

(9) FIG. 7A is a 100× magnification of a test plaque showing the bulk material and stainless-steel fibers.

(10) FIG. 7B is a 100× magnification of a test plaque showing the edge of the test plaque, along with the bulk material and stainless-steel fibers.

(11) FIG. 8 is a 40× magnification of a test plaque showing the bulk material and stainless-steel fibers after a destructive ash test.

(12) FIG. 9 is an X-ray image of a printed circuit board shielding case fabricated using the present invention.

(13) FIG. 10A is a diagram showing a chain of fibers and electro-mechanical contacts.

(14) FIG. 10B is a diagram showing the length and width of a fiber.

(15) FIG. 11 is an X-ray image of a wire harness shield fabricated using the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(16) The following descriptions are not meant to limit the invention, but rather to add to the summary of invention, and disclose the present invention, by offering and illustrating various embodiments of the present invention, a formulation for a stable electrically conductive polymer. While embodiments of the invention are illustrated and described, the embodiments herein do not represent all possible forms of the invention. Rather, the descriptions, illustrations, and embodiments are intended to teach and inform one skilled in the art without limiting the scope of the invention.

(17) Traditional electrically conductive polymers are comprised of a large plurality of conductive fibers dispersed in a bulk polymer. Likewise, the present invention is comprised of a large plurality of conductive stainless-steel fibers dispersed in a bulk polymer. FIG. 1 shows a single coated stainless-steel fiber 100. The single coated fiber 100 is comprised of a raw stainless-steel fiber 105 and a coating 101. The stainless-steel fiber 105 material of the present invention is austenitic stainless steel, although other families of stainless steel also work. The stainless-steel fiber 105 has a stainless-steel interior 105 and an outer surface 201. The stainless-steel fiber 105 has an outer diameter 103.

(18) The coating 101 has an outer diameter 104. The coating 101 has an outermost layer of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium. Tin, silver, and palladium are soft materials with low resistance oxides. They conform easily to make an electro-mechanical contact. The coating 101 is between 5% and 40%, by weight, for the fibers 100. In other words, between 5% and 40% of the weight of the fibers 100 is made up of the coating 101. Coating 101 done with only tin, tin-silver, and tin-palladium are at the lower end of the spectrum, near 5% by weight (the stainless-steel fiber 105 accounting for 95% of the weight and the coating 101 accounting for 5% of the weight). Coating 101 with nickel and then over-coating with tin-lead is towards the upper end of the spectrum at nearly 40% by weight.

(19) The coating 101 of the coated fiber 100 is necessary in order to achieve sufficient conductivity within the bulk polymer. FIG. 3 is a graph 17 showing the bulk resistance for the present invention. The resistance per unit length is on the y-axis 15 and the percentage of overall weight attributable to the coating is on the x-axis 18. The resistance per unit length 15 falls 16 as the percentage of coating 18 rises. This relationship holds true regardless of the type of stainless-steel fiber (e.g., austenitic and martensitic) and the type of coating (e.g., copper, nickel, and tin).

(20) Referring now, also, to FIG. 2, a cross-section of a the formulation for a stable electrically conductive polymer 12. The formulation for a stable electrically conductive polymer 12 is comprised of coated stainless-steel fibers 11, 100 in a bulk polymer 10. The bulk polymer 10 can be any commercially viable polymer. The formulation for a stable electrically conductive polymer 12 can be made with most commonly used bulk polymers 10 such as polypropylene (“PP”), polycarbonate (“PC”), acrylonitrile butadiene styrene (“ABS”), polyethylene (“PE”), polyether ether ketone (“PEEK”), and polyethylene terephthalate (“PET”). These polymers are all attractive due to their cost, physical characteristics, and wide-spread acceptance. The cross-sections 11 of the fibers 100 is apparent in FIG. 2, as the fibers in an electrically conductive polymer 12 will typically possess the same orientation.

(21) FIG. 4 shows a chart 20 with the primary families of stainless steel: austenitic 27, duplex 25, ferritic 23, martensitic 24, and precipitation hardened 26. The families of stainless steel 27, 25, 23, 24, 26 are defined by their percentage of nickel 21 versus their percentage of chromium 22. Austenitic stainless steel 27 has between 16% and 25% chromium by weight; and between 7% and 20% nickel by weight. Although the stainless-steel fiber 105 of the present invention 12 can be fabricated with any stainless steel 27, 25, 23, 24, 26, austenitic 27 stainless steel has advantages in terms of ease of injection molding and dispersion.

(22) FIG. 10A is a simplified diagram showing a plurality of coated stainless-steel fibers 792, 793, 794, 795, 796, 797, 798, 799. The plurality of coated stainless-steel fibers 792, 793, 794, 795, 796, 797, 798, 799 make a plurality of electro-mechanical contacts 701, 702, 703, 704, 705, 706, 707.

(23) FIG. 10B shows is a close-up of one of the stainless-steel fibers 792, showing its length 751 and diameter 752. The resistance of the stainless-steel fiber 792 is given by the standard formula:

(24) $r_{792}=\frac{\rho l}{A},$
where ρ is resistivity of stainless steel, l is the length 751 of the fiber 792, and A is the cross-sectional area of the fiber, which is πd, where d is the diameter 751. All of the plurality of coated fibers 792, 793, 794, 795, 796, 797, 798, 799 are assumed to have the same resistance. Therefore, the resistance of the eight fiber-long string 792, 793, 794, 795, 796, 797, 798, 799 is given by r.sub.bstring=kr.sub.bfibers=8*r.sub.792. Again, cumulative resistance of the conductor made from electrically conductive polymer is the sum of bulk fiber resistance and fibers contact resistance, thus R=Σr.sub.B+Σr.sub.C=Σr.sub.bstring/m+Σnr.sub.ci. The important part, here, is that the quantity

(25) $r_{792}=\frac{\rho l}{A}$
does not vary with time or environmental condition. Any change in overall resistance, R=Σr.sub.B+Σr.sub.c=Σr.sub.bstring/m+nr.sub.ci is not caused by the resistance of the stainless-steel fibers 792, 793, 794, 795, 796, 797, 798, 799, it is caused by change in the electro-mechanical contact 701, 702, 703, 704, 705, 706, 707.

(26) The present invention improves on the prior art and solves the problem with time- and environmentally-caused change in bulk resistance in the current generations of electrically conductive polymers. The coating 101 has an outermost layer of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium, which is mechanically soft and which is far superior to nickel and copper with respect to oxidation, sulfidation, and contamination.

(27) In the current generations of electrically conductive polymers, the most frequently used fibers are nickel-plated carbon fibers and copper-plated carbon fibers. In some special applications, carbon fibers are plated first with nickel then with copper, although this clearly add cost. Nickel- or carbon-plated carbon fibers are externally rigid, and therefore do not necessarily create the best electro-mechanical contacts. It is important to note that the electrically conductive polymer coatings of the current generations of electrically conductive polymers do not melt and bond. They are merely held together in the bulk polymer 10 as an electro-mechanical contact. Nickel and copper both have high surface resistance, also. The high surface resistance is a material attribute which is exacerbated by oxidation, sulfidation, and/or contamination. Environmental cycling also tends to raise the surface resistance in the presence of oxidation, sulfidation, and contamination.

(28) The inventors of the current generations of electrically conductive polymers were concerned with thermal expansion within the bulk polymer 10. Carbon fibers have a very low coefficient of thermal expansion. Copper- and nickel-plating have lower coefficients of thermal expansion than tin. Table 1 shows the relative coefficients of thermal expansion, a, in SI units of (μm/m−C°):

(29) TABLE-US-00001 TABLE 1 Coefficient of Thermal Expansion (a) Coefficient of Thermal Material Expansion (a) Carbon Fiber ~1 Nickel 13 Copper   16-16.7 Austenitic Stainless Steel 14-17 Tin 20-23 Lead-Tin Solder 25

(30) But the issue of thermal expansion within the bulk polymer 10 was not the real issue. Surface resistance and its reaction to oxidation, sulfidation, and contamination of the electro-mechanical contacts 701, 702, 703, 704, 705, 706, 707 within the bulk polymer 10 was the issue.

(31) The present invention fixes the problem of high surface resistance for the contacts of the fiber by using stainless steel fibers with an outmost coating of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium. Tin, silver, and palladium are soft materials with low resistance oxides. They conform easily to make an electro-mechanical contact. By using a coating 101 of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium over an austenitic stainless-steel fiber 105, the present invention was able to provide constant bulk resistance over both time and environmental exposure. The surface resistance of a coating 101 of tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium is relatively impervious to oxidation, sulfidation, and contamination.

(32) Electrically conductive polymers 12 are useful when their internal fiber mat creates a Faraday Cage. The Faraday Cage prevents electromagnetic signals from passing through the structure. If lesser amounts of fiber 100 are used in a formulation, bundling, tending toward clumping, occurs. When using a low percentage, by weight, of fibers, clumping can impair the homogeneity of the Faraday Cage formed by the fibers 100, and therefore, performance.

(33) To get proper dispersion of the austenitic 27 stainless steel coated fiber 11 in the bulk polymer 10, the coated fiber 11 should be 30% by weight of the total electrically conductive polymer 12. In order to achieve proper dispersion and electrical conductivity, the coated fiber 11 should be no less than 25% by weight of the total electrically conductive polymer 12 and no more than 35% by weight.

(34) The proper dispersion was arrived at experimentally. FIG. 5 shows a test plaque 50. The electrically conductive polymer 12 was formed into plaques 50. Each plaque 50 had four 6″ sides 53, 52, 55, 54. For ease of handling, the test plaque 50 had rounded corners 59, 58, 57, 56. The surface 51 of the test plaque 50 was exposed electrically conductive polymer 12.

(35) FIG. 6A and 6B are images of a 100× magnification of the test plaque 60 constructed from ABS as the bulk polymer 62 with fibers 61 fabricated from austenitic stainless steel 27, which were first plated with nickel and copper, and then were plated with lead-tin. At 30% by weight of the total electrically conductive polymer 12, the fibers 61 are properly dispersed across the bulk ABS 12, even at the edge 64. FIG. 6B shows the lower left corner 57 of a test plaque 50, with the plaque 60 magnified 100× and the air 63.

(36) Likewise, FIG. 7A and 7B are images of a 100× magnification of the test plaque 70 constructed from a bulk polymer 72 with fibers 71 fabricated from austenitic stainless steel 27, which were plated with lead-tin. At 30% by weight of the total electrically conductive polymer 12, the fibers 71 are properly dispersed across the bulk polymer 12, even at the edge 74. FIG. 7B shows the lower left corner 57 of a test plaque 50, with the plaque 70 magnified 100× and the air 73. This is the type of dispersion that forms a Faraday Cage.

(37) Each test plaque 50 weighed, on average, about 3 ounces or 85 grams. Each test plaque 50 contained over 14,500,000 fibers with an aggregate fiber length of 12.86 miles or 20.75 km. There were over 3.4 million fibers per cubic inch or 210,000 fibers per cubic cm. The test plaques 50 were fabricated from electrically conductive polymer pellets. Each pellet had 12,000 fibers. Each ounce of resin requires 405 pellets.

(38) The test plaques 70 were run through demanding environmental testing, including an Ash test which charred the bulk polymer 72. FIG. 8 is a 40× magnification of a test plaque 70, showing only exterior fiber 71 and interior fiber 79. The fiber 71, 79 are still dispersed, even after the bulk polymer 72 has been charred away.

(39) FIGS. 9 and 11 show applications of the present invention. FIG. 9 is a representation of X-ray imaging of an RFID shield 200. An RFID shield 200 is typically used to shield printed circuit boards that generate significant radio frequency electro-magnetic interference. The RFID shield 200 has four sides 201, 202, 203, 204 connected by four filleted corners 205, 206, 207, 208. The RFID shield 200 has a plurality of tabs 209, 210 to secure the RFID shield 200 in place. The RFID shield 200 also has a plurality of holes 211. The image clearly shows that the plurality of fibers 71 fabricated from austenitic stainless steel 27, which were plated with lead-tin, are properly dispersed in a bulk 72 polymer.

(40) FIG. 11 shows an X-ray image of a wire shielding sleeve 300. The wire shielding sleeve 300 isolates particularly susceptible cables from the electro-magnetic environment around it. The wire shielding sleeve 300 has two tabbed ends 302 and a narrower cylindrical area 303. There is an opening along the width 301 of the tabbed ends 302. The X-ray image shows that the plurality of fibers 71 fabricated from austenitic stainless steel 27, which were plated with lead-tin, are properly dispersed in a bulk 72 polymer.

(41) The electrical conductivity of the present invention 12 can be enhanced by an inductive heat-treatment. Although conventional heat would tend to melt the bulk polymer 10, a large induced field would send high current through the coated stainless-steel fibers 11, 100. If properly controlled, this induction can melt the tin, tin-lead, tin-silver, tin-palladium, tin-silver-palladium, or silver-palladium coating 11, creating an impervious solder bond. This is only possible due to the relatively low melting point of tin and its eutectics.