Improving conductivity of plastics containing metallic fillers

Abstract

 The electrical conductivity of plastic materials is improved by dispersing highly electrically conductive filler elements throughout the volume of the plastic coupled with dispersion of low ionization potential materials in the space separating the conductive fillers, the low ionization potential materials providing a source of electrons which increases the conductivity of the material.

Description

[0001] The present invention relates to electrically conductive plastic composite materials and particularly to methods for making such materials. More particularly, this invention relates to a method for increasing the electrical conductivity of such composite materials and to the products of such a method. While the invention relates primarily to solid or foamed resinous composites, it is believed that the principles of the invention may be applicable to materials other than resinous materials including various other high resistivity composite materials such as ceramics, wood composites, and concrete, for example.

[0002] It has long been a goal to provide plastics with improved conductivity for various purposes. Techniques for increasing the conductivity of molded thermoplastic articles to a level such that they are useful for various purposes as replacements for metals are well known. Two different approaches have been taken to achieve this result. The first consists of coating the plastic with a conductive layer of some sort. The second approach relies on impregnating the material with conductive fillers, e.g., carbon black, aluminum flake and stainless steel fibers. In one of the latter techniques, conductive or metal coated filaments, such as metallized glass fibers, are incorporated into the plastic materials to dissipate electrostatic charge and to provide shielding against electromagnetic radiation, among other benefits. The improved electrical conductivity of these materials is based on the fact that the metal fibers or filaments, which are of relatively short length are either in direct contact with each other at enumerable points throughout the volume of the plastic or are so closely adjacent each other that despite the small separation by the plastic, improved conductivity nevertheless results.

[0003] In making such thermoplastics, the short lengths of metal coated fibers or conductive filaments are combined with fluid polymerizable material. In such a fabrication technique, the many individual lengths of conductive filaments develop a thin film of the insulating plastic thereon. This thin insulating film may increase the distance between adjacent conductive fibers in the finished material to the extent that the desired electrical conductivity is not achieved. Since the metallic or conductive fibers are interspersed throughout the volume of the plastic in a three dimensional random network, the overall conductivity of the plastic is the cumulative effect of the conductivity of the many various individual current paths between adjacent metallic elements. One obvious way to increase conductivity in such composites is to increase the ratio of metallized particles or fibers carried in the material. But as a practical matter, there is a limit to the extent to which such materials may be increased without adversely affecting the desirable properties of the plastic. Moreover, it would be desirable to reduce, rather than increase, the ratio of conductive material required to achieve a given level of conductivity of the plastic. In addition to adverse effects on the physical properties of the plastic, the use of such conductive fillers usually adds significantly to the cost of the final product.

[0004] It is, therefore, a primary object of the invention to provide a method for increasing the conductivity of conductive composite materials, particularly conductive plastic composit.

[0005] It is a further object of the invention to provide for a method of improving the conductivity of plastic material impregnated with conductive filler of the above-noted type without significantly increasing the percentage of conductive material contained therein tc achieve a given conductivity.

[0006] A yet further object of the invention is to produce a conductive plastic composite having a given electrical conductivity with a lower weight percentage of conductive material therein.

[0007] It is yet a further object of the invention to reduce the cost of producing plastic composite materials of the above-noted type by reducing the amount of relatively expensive metal conductive fillers incorporated into such composites to achieve a given conductivity.

[0008] These and other objects of the invention are accomplished by the inclusion of low ionization potential materials into the conductive plastic composite material, the effect of such low ionization potential materials being to increase the electrical conduction between adjacent but separated metallic or conductive filler elements in the material.

[0009] The product of the above-noted process is highly useful for various applications such as enclosures for electronic equipment where conductivity, shielding, and grounding are important including, for example, cabinetry for electronic equipment such as communications equipment, instruments and computers. The intended improvement in the electrostatic shielding capability of plastic composite materials may be sufficiently great to allow their use as a low cost alternative to metallic enclosures.

[0010] The method of the present invention relates primarily to a wide variety of conductive resinous composite materials, such as conductive plastic composite materials where the plastics include both thermoplastics and thermosets. Examples of suitable thermoplastics include, but are not limited to polycarbonates, polyesters, oxide polymers, polyurethanes, polyamides, acrylics, polyvinylchlorides and hydrocarbon polymers. Examples of suitable thermosets include, but are not limited to polyesters, epoxies, ureas and silicones.

[0011] Suitable conductive materials useful for incorporation into the plastic composites include various types of metal fibers and ribbons and metal particles of various shapes, metallized glass, metallized graphite or other conductive or nonconductive fibers, and carbon fiber and carbon particles of various shapes. For given overall desired conductivity characteristics, the choice metal is wide; however, for practical reasons the choice is usually narrowed to inexpensive metals that have good conductivity and are easily formed. Aluminum is the most preferred as fitting the above criteria. However, the processes are equally applicable to composites having other metals such as stainless steel, silver, copper, zinc, iron, nickel, carbon steel, etc. and mixtures thereof. Likewise, metallized glass fibers are preferably coated with aluminum due to the above criteria and due to its low melting point, which facilitates its manufacture.

[0012] For details regarding the shaping and forming of the metallic fibers, process steps for their inclusion into the plastic material, and range of percentages by weight or volume to be included in the plastic in order to achieve desired electrical conductivity characteristics, reference may be had made to various prior art patents and publications well known in the art.

[0013] In order to illustrate the present invention, various examples are set forth below. It will be appreciated that these examples are set forth in order to further illustrate the invention, but are not intended to limit it. In each of the below noted examples, the test samples were cut from molded plaques of the designated matrix or base material. The samples were typically 6 cm long, 0.5 cm wide and 0.33 cm thick. The small sides of each sample (0.5X0.33) were painted with Dupont conducting paint 4817 and the resistance between them measured before and after treatment, using a Beckman RMS3030 multimeter. From the measured resistance the resistivity was then calculated and is reported in the Tables.

[0014] The large range of values of resistivity for the samples before treatment in each of the following Tables is primarily a result of the differing density and orientation of the conductive fillers throughout a particular molded plaque. Also in the following tables R_B and R_A represent resistivity before and after treatment in ohm-cm.

Example 1

[0015] A plurality of samples cut from molded plaques of a polymer that is sold under the trademark NORYL and which are designated as SE90-960 and containing 6% by weight stainless steel fibers were exposed in a bell jar at atmospheric pressure to a vapor of N, N' dimethylaniline (DMA). The amount of DMA absorbed into the samples was determined from its increase in weight. Resistivity measurements were made before and after exposure to the DMA according to the technique noted above.

[0016] The table below relates the resistivity of each sample before exposure to the DMA with the resistivity after absorption of the listed % by weight of DMA.

Example 2

[0017] Samples of Noryl® resin designated as SE90-GH100, also containing 6% by weight of stainless steel fibers were tested as in Example 1 above and TABLE II reflects the results of such tests.

Example 3

[0018] Another experiment similar to Experiments 1 and 2 was conducted using test samples cut from a plaque of LEXAN® polycarbonate with measurement of resistivity taken before and after absorption of the listed % by weight of DMA. Treatment of the sample was performed in a bell jar in air at room temperature. TABLE III reflects the results of these experiments.

Example 4

[0019] Same as Example 3 except that the test samples were exposed to vapors of tetrathia fulvalene (TTF) under a vacuum at 130°C. Measurements were taken as in the previous examples with Table IV below reflecting the values measured.

Example 5

[0020] Same as Example 4 except that the test samples were exposed to vapors of N, N, N', N' tetramethyl p-phenylene diamine (TMPD) under vacuum at 60°C. Measurements were taken as in the previous examples and are reflected in Table V below.

Example 6

[0021] Again a procedure similar to Example 5 was performed with the exception that Lexan polycarbonate samples were exposed to vapors of 2-methyl naphthalene (2 MN) in air at atmospheric pressure at 50°C. Table VI shows the values of resistivity measured before and after addition of 2MN to the test samples at stated concentrations of 2MN.

Example 7

[0022] Again a procedure similar to Example 5 was performed with the exception that the Lexan polycarbonate contained 26% by weight aluminum flakes and was exposed to DMA in air in a bell jar at atmospheric pressure. Table VII shows the values of resistivity measured before and after the treatment.

[0023] 

[0024] It is apparent from the above examples that the addition of low ionization potential materials to composite plastic materials of the above noted type results in a significant decrease in the resistivity of such composite materials.

[0025] The low ionization materials used in the examples above are but a few of those which could be used to achieve a similar result. As used herein, the ionization potential of a material refers to the energy required to remove an electron from the highest filled orbital (most loosely bound electron) of a neutral molecule of such material in its ground state. A low ionization potential material is, therefore, generally characterized by the fact that, when added to the base plastic/metallic composites, it operates to supply electrons upon the application of a relatively small electrical potential, and thereby increases the overall conductivity of the composite. These materials must, of course, be selected, in part, based on their compatability with the matrix or base plastic material employed.

[0026] The gas phase ionization potentials for the materials used in the examples discussed above are as follows: N N' DMA, 7.14 eV; 2MN, 7.96 eV; TMPO 6.33 eV and TTF, 7.0 eV. The base or matrix polymer materials have gas phase ionization potentials which range between 8.3 eV and 9.3 eV. Thus, materials having an ionization potential in the range between approximately 15%-30% lower than the matrix plastic material have been shown to produce significant reductions in resistivity. It is believed that a broader range of matrix/low ionization potential materials may be utilized to produce similar effects. Reference may be made to Blaunstein and Christophorou, On Molecular Parameters of Physical, Chemical and Biological Interest, 3 Radiation Res., 69-118 (1971), for materials which may be suitable candidates, the final judgement on a specific low ionization material being made based on its compatibility with the base or matrix material.

[0027] No attempt has been made to identify the minimum effective percentages by weight of low ionization potential materials needed in order to produce a measurable reduction in resistivity. Indeed, these amounts may vary considerably based on the specific makeup of the composite. Generally, however, a minimum value of approximately .1% by weight appears plausible based on the above examples. The maximum amount of low ionization material will likely be governed in large measure by the impact such larger amounts of the low ionization potential material will have on the overall chemical and physical properties of the composite. In general, an upper limit of approximately 10% by weight is predicted.

[0028] Some low ionization materials may exhibit instability when combined with specific base plastics, however, it is believed that suitable stable compositions may be selected or, alternatively, known stabilizing techniques employed to improve the characteristics of the final composite.

[0029] While, in the above examples, the low ionization potential materials were added to a previously prepared base composite plastic/metal filler material, it is equally possible and preferable to prepare the final composite by using conventional blending equipment and processes. More specifically, the thermoplastic composition may be heated within an extruder to a temperature sufficiently high to provide a viscosity that allows the thermoplastic polymer to flow and be blended with other constituents. Preferably temperatures will, of course, depend on the thermoplastic selected. The low ionization potential adder material and metallic filler are injected into the extruder at a point where the thermoplastic composition is molten. During the above processing, conventional techniques are utilized to obtain a uniform dispersion of the constituents within the composite material. The resulting blended composition may then be passed through a die at the end of the extruder and pellitized. These pellets may be subsequently utilized in extrusion processes, injection molding processes, etc. to obtain finished products, suitable for a particular high conductivity application.

Claims

1. A method of forming a composite having increased electrical conductivity, said composite comprising an electrically insulating material having dispersed therein metallic elements, said method including the step of incorporating an adder material into said composite having an ionization potential which is low relative to said insulating material in an amount sufficient to increase the electrical conductivity of said composite relative to its electrical conductivity absent such adder material.

2. The method of claim 1 wherein the amount of said adder material is between .1% and 10% by weight of said insulating material.

3. The method of claim 1 wherein said metallic elements are selected from the group consisting of aluminum, stainless steel, silver, copper, zinc, iron, nickel, carbon steel and mixtures thereof.

4. The method of claim 1 wherein said insulating material is selected from the group consisting of thermoplastic and thermosetting materials.

5. The method of claim 4 wherein said insulating material is a thermoplastic material.

6. The method of claim 5 wherein said thermoplastic material is selected from the group consisting of polycarbonates, polyamides, polyesters, oxide polymers, polyurethanes, acrylics, polyvinylchlorides and hydrocarbon polymers.

7. The method of claim 1 wherein the insulating material is a resinous material.

8. The method of claim 1 wherein said metallic elements are selected from the group consisting of metal-coated glass fibers, metal-coated graphite fibers, and strips of aluminum.

9. The method of claim 1 wherein said low ionization adder material is selected from the group consisting of N,N' dimethylaniline, N, N, N', N' tetramethyl p-phenylene diamine, 2-methyl naphthalene, and tetrathia fulvalene.

10. An improved conductive composite comprising electrically insulating material and dispersed metallic filler elements and incorporated therein an amount of adder material having an ionization potential which is low relative to said insulating material in an amount sufficient to increase the electrical conductivity of said composite relative to its conductivity absent such adder material.

11. The composite of claim 10 wherein the amount of said adder material is between .1% and 10% by weight of said insulating material.

12. The composite of claim 10 wherein said metallic elements are selected from the group consisting of aluminum, stainless steel, silver, copper, zinc, iron, nickel, carbon steel and mixtures thereof.

13. The composite of claim 10 wherein said insulating material is selected from the group consisting of thermoplastic and thermosetting materials.

14. The composite of claim 13 wherein said insulating material is a thermoplastic material.

15. The composite of claim 14 wherein said thermoplastic material is selected from the group consisting of polycarbonates, polyamides, polyesters, oxide polymers, polyurethanes, acrylics, polyvinylchlorides and hydrocarbon polymers.

16. The composite of claim 10 wherein said insulating material is a resinous material.

17. The composite of claim 10 wherein said metallic elements are selected from the group consisting of metal-coated glass fibers, metal coated graphite fibers, and strips of aluminum.

18. The composite of claim 10 wherein said low ionization material is selected from the group consisting of N, N' dimethylaniline, N, N, N', N' tetramethyl p-phenylene diamine, 2-methyl naphthalene, and tetrathia fulvalene.