US3476530A - Iron based conductive filler for plastics - Google Patents

Description

OR 3,476 530 6R Nov. 4, 1969 J. E. EHRREICH ETAL IRON BASED CONDUCTIVE FILLER FOR PLASTICS Original Filed Oct. 2, 1962 IRON POWDER COPPER 5g COPPER SULFATE PLATING SOLUTION 7 WASHING 2WATER Qil Z6 SILVER Hz SILVER CYANIDE" 5 PLATING SOLUTION I32 Io I5 WASHING LWATER I'r m DRYING we 'LIQ v CONDUCTIVE IRON BASED POWDER United States Patent 3,476,530 IRON BASED CONDUCTIVE FILLER FOR PLASTICS John E. Ehrreich, Arlington, and Donald H. Avery, Boston, Mass., assignors to Chomerics, Inc., Cambridge, Mass., a corporation of Delaware Continuation of application Ser. No. 227,755, Oct. 2, 1962, which is a continuation-in-part of application Ser. No. 153,078, Nov. 17, 1961. This application June 10, 1966, Ser. No. 556,777

Int. Cl. B23p 3/20; C23b 5/00 US. Cl. 29-192 3 Claims ABSTRACT OF THE DISCLOSURE A process for preparing conductive metal powders comprising replacement plating an iron powder by immersion in a copper sulfate solution to deposit an adherent, thin, continuous film of copper, and thereafter immersing the copper coated powder in an aqueous solution of silver cyanide to deposit an adherent thin continuous film of silver.

This application is a continuation-in-part of Ser. No. 153,078, Conductive Metal Filler for Plastics, filed Nov. 17, 1961, by the present inventors, now abandoned, and a continuation of application Ser. No. 227,755, filed Oct. 2, 1962, now abandoned.

The present invention pertains to a conductive metal filler for rendering plastics electrically conductive and magnetically responsive, and to plastic compositions prepared therefrom. It is more particularly concerned with the preparation of an iron powder coated with silver in such a manner as to form a stable, electrically conductive filler that has utility for forming electrically conductive plastics of various forms such as adhesives, solders, coatings, moldings and R. F. caulking compounds.

Nob-1e metal powders, such as silver, have been incorporated in plastic systems to make them electrically conductive. The volume resistivities of the filled plastic systems are in the order of 0.1 ohm-centimeter or less which is sufficient to suit the plastics for such uses as conductive adhesives to replace metal solders, and conductive paints for shielding. The noble metals must be used because insulating oxide coatings do not form on the particles as is the case with other metal powders, such as copper and aluminum. Conductivity of a plastic mass filled with a conductive metal powder depends upon the particle-toparticle contact between the metal particles. The electric current must be able to flow from particle-to-particle with desirably the lowest amount of contact resistance possible. With the non-noble metals the oxide coating that forms on the particles, while perhaps only a few atoms thick, has a high resistivity and prevents the ready how of current between contiguous articles.

Non-noble metal particles can be protectively coated with a noble metal to give the necessary noble metal outer surface. It has been found, however, that it is difiicult to lay down a protective coating that is properly bonded to the core metal so that the coated metal powder is stable and adequately conductive, particularly when using an economical amount of noble metal. It is difficult to secure coated particles that are electrically stable as determined by the heat, oxidation and moisture stability of the coating. If the noble metal coating is not properly electrically and physically attached to the metal core, current fiow into and through the core is hindered or lost.

The present invention is based on the finding that by 3,476,530 Patented Nov. 4, 1969 careful and proper control of processing conditions, a thin silver coating can be replacement plated on iron powder that has been pre-coated with copper. The conductive powder obtained is inexpensive, very stable, electrically conductive and magnetically responsive.

The ratio of silver to copper deposited on the iron powder is a key feature of this process.

The amount of silver that can be deposited is dependent upon the amount of copper layed down. A small amount of copper must remain on the iron cores as a bonding agent between the iron and silver. Without it the coated powder is not stable. Enough silver must be deposited, however, to protect the copper or otherwise the coated powder will not be stable. The gram moles of silver deposited cannot exceed the gram moles of copper originally deposited nor should it be less than 40% thereof. Preferably, the gram moles of silver deposited is in the range of 50 to of the amount of copper deposited so that 15 to 50% of the copper originally layed down remains on the iron cores.

Several other features are of some importance. The iron powder used should be clean and free from lubricants and waxes. Desirably a powder is used that has been prepared in such a way as to be inherently free from surface contamination, although the surface of the powder can be cleaned if this is necessary.

The copper is replacement plated on the iron powder from a copper sulphate solution. While the reaction is fairly straight forward it is autocatalytic and should be properly handled. It is advisable for the plating solution to be free from small amounts of acids, such as acetic or sulphuric because the heat stability of the end product may be adversely affected.

Rapid addition of the copper clad iron powder to and through dispersing of it in the silver plating solution is desirable.

The amount of copper deposited can be varied by a factor of four or five. Generally at least 8 mole percent of copper on the iron particles is deposited. This can be as high as 40 mole percent.

The silver is deposited from a silver cyanide solution. Most effective use must be obtained from the. silver layed down because of the relatively large surface area of the particles. The thinnest silver coating possible is desired so that an undue amount of silver will not be consumed. Both the copper and silver plating reactions are carried to the point of depletion of the copper or silver ions, which controls thicknesses of the coatings deposited.

Oxide films form very rapidly on cleanedcopper surfaces. It is very difiicult to keep them from forming. It is possible that in the present process some oxide film forms on the surface of the coated iron powder during the time it is transferred from the cleaning step to the plating solution. It is believed, however, that the abnormally high concentration of the cyanide ion in the plating solution has the effect of removing or somehow modifying any copper oxide as to eliminate or overcome any deleterious effects it may have.

No effect of surface area or particle size on the process has been observed. The amount of silver consumed increases with an increase in the available surface area of the powder. The copper precoat, however, will fill in porous particles which helps to decrease silver consumption.

It is much preferred, however, to work with relative 1y coarse iron powder. The surface area of a powder varies as the square of the diameter and substantial silver economies are realized by working with large sized particles,

3 A coarser plated powder also enhances the performance of the filled plastics in several respects. Higher and more reliable electrical conductivity and better heat transfer and moisture resistance result from the use of a coarser conductive filler.

solution admitted by line 11. The copper clad powder is preferably admitted as rapidly as possible to the plating solution. If the time interval over which the particles admitted were extended, the particles first entering would acquire more of the silver than the later entering particles.

Three types of iron powders have been successfully 5 The coating would be uneven and some of the copper on coated so far: A powder spongy in nature, grindings or the first entering particles might be depleted. After addifilings and a spherical atomized powder. tion of the powder a black haze may be present initially The following table describes suitable base powders: but will disappear.

Trade Name Ferroflame C Grade H LID-180 Vasco 4000 Supplier Easton Metal Belmont Metals Vanadium- Powder Co. smelting & Distintealloys Steel Refining grating Co.

Pre-alloyed Type Atomlzed Ground Sponge atomized Apparent Density, grn./ce 2. 7-3. 2 2. 402. 80 3. 34 Iron, Percent min 98 Screen Analysis (Percent +100 mesh 0. 50 5 0 2. 1 +200 mesh- 40-50 95 2035 39. 9 +325 mesh. -40 -50 27. 3 -325 mesh 20-25 20-40 30. 7

The iron of the base powder can contain various in- A very high level of mechanical agitation as by an imgredients such as carbon, chromium and tungsten to impeller is used to keep the heavy solids well dispersed. prove its properties and still remain susceptible to copper 30 Being as heavy as they are they would settle rapidly in plating. Generally speaking, it is preferred to work with the absence of good agitation. Firm cementation might iron having a purity of st 90 weight percent. occur at this point if the solids were allowed to settle.

The drawing attached td amforming a part of this If this occurs and the solid particles are later pulled apart specification schematically illustrates thepresent\inventhe coatings will tend to be pulled off the particles. tion. I The plating solution is based on silver cyanide. An

Referring to the drawing, the iron powder is admitted alkali metal cyanide, preferably sodium cyanide, is used by line 1 to the copper plating zone 2. For the purposes to bring the silver cyanide into solution and to provide of this illustration a batch operation will be described a high concentration of cyanide ions. The sodium cyanide but the process could well be continuous. The plating is first dissolved in the solution. The silver cyanide is done 2 contains the copper sulfate solution admitted by then added. Dissolution of the salts can be accomplished line 3. The copper sulfate solution is made up from pure at room temperature. copper sulfate and water and is almost a saturated solu- The amount of cyanide used is high enough to complex tion. The amount of copper sulfate solution used deterthe silver cyanide but not so high as to prevent dissolving mines the thickness of the copper layer deposited on the of the silver ion because of saturation. It is preferably at powder. The plating reaction is an autocatalytic reaction. least 40% of that concentration where saturation occurs When the iron is first admixed with the solution no reacand silver comes out of solution. If the saturation contion occurs but rather thorough or violent mixing should centration is exceeded, silver cyanide is deposited on the be carried on. The liquid remains blue until the start of copper ruining the effectiveness of the silver coating. the reaction. When the reaction takes off the solution The plating solution preferably does not contain plating turns brown. At this point agitation should be very thor- 5O additives or brightening agents normally used for such ough. Agitation is continued after the rather rapid repurposes as the prevention of dendrites. They may form action is apparently completed but not too long afteran insulating film on the copper surface. wards because the copper coat at this point is not too Agitation of the plating reaction is continued until the adherent to the iron cores and can be rubbed off or silver ions are just about depleted. At this point copious abraded. evolution of gas starts. While not critical, it is good prac- The temperature at which the copper plating is cartice to time the reaction to determine this point and stop ried out has some effect. If the starting solution is too just short of it. The time will depend on various factors cold it will not react. The initial temperature of the copsuch as the size of the reaction, concentration of ingreper sulfate solution is preferably at least F. The autoclients, and the like, but it is usually within a few minutes catalytic reaction releases heat and if the reaction tern- 0 for a batch operation. The reaction is stopped by ceasing perature becomes too high an inferior coating is obtained. agitation. The silver-coated particles drop rapidly. Liquid It is preferred that the reaction temperature does not exis then decantered or syphoned off from the particles. ceed 180 F., preferably F. It is believed that if the If gas evolution has started, when the solids drop the plating temperature is too high the copper comes out gas drives the liquid out of the settled mass. This auto- 21 solution before the sulfate has had the opportunity to 35 matically limits gas evolution. The settled solids tend to properly clean the iron surface. In this connection it might agglomerate, although not tightly, and should be promptly be noted that other acid radicals such as the acetate or removed and washed. cyanide are not effective. The sulfate radical seems to be The plated solids are transferred to washing zone 14 unique in this respect. by line 13. Wash water is added by line 15 and removed After the copper has plated out, particles are allowed 70 by line 16. The solids are thoroughly washed several times to settle. Spent plating solution is removed by line 4 and the coated solids are transferred by line 5 to a washing zone 6. Wash water is admitted by line 7 and spent wash water is removed by line 8. The washed solids are passed by line 9 to plating zone 10. Zone 10 contains a plating to remove cyanide ions. They are passed by line 17 to drying zone 18. They can be dried in any suitable manner such as by an acetone wash followed by air drying. The dried solids are passed by line 19 to packaging and dis tribution.

The powder particles do not have a specular surface but have instead a silver-white matte appearance. The conductive powder can be given a heat-oxidation stabiilty test to determine the effectiveness of the silver coating. This involves maintaining the solids in a tray in loose form at 400 F. for 24 hours in a circulating air atmosphere. The initial conductivtiy of the powder as determined by probes from a volt-ohmmeter or Wheatstone bridge will be under 10.0 ohm per centimeter, and this initiate conductivity should not change more than 50% after 48 hours. If the conductive powder is this stable in loose form it will perform adequately when incorporated into a protecive plastic matrix.

The following table gives pertinent conditions for the present process:

TABLE Range Broad Preferred 1 Example Iron Power:

Average particle size, mils.-.. $4 to 15. 1 to 6. 3 Surface area, ti/lb 2 to 1,500.... 25 to 500. 70 Percent Iron .4 85 to 100. 90 to 100- 98.0 Copper Plating Solution:

Copper Sulfate, wt. percent- 5 to 25 to 20. 17 Amount, gals/1b. powder. V to 8- )4 to 2. 54 Initial Plating temp., F- 40 to 200- 50 to 150- 70 Silver Plating Solution (Fresh) Silver ions, oz./gal 3 tot! 4to 6 5.3 Cyanide, oz./ga1-- 6 to 20. 10 to 19. 16. 5 Gallon/lb. powder V to 8--..-- %to 2---- Conductive Powder:

True Density, lbs/it 512 Silver, mole percent...-. to 50- 2 to 30. 12 Copper, mole percent- 34 to 25. 1 to 15- 6 Iron, mole percent... 25 to 99- 55 to 97. 82 Conductivity, ohm-c (loose form) 0.001 to 100.. 0.01 to 10.. 0. 3

1 Powder used was Ferroflame C.

The initial conductivity of the conductive ferromagnetic powder can be controlled by varying the conditions and the amount of copper and silver deposited. The initial conductivity during the heat stability test will change very little, however, even though the initial conductivity may be 0.1 of an ohm-cm. or 10 ohm-centimeter.

A gasket for sealing the base of a klystron tube (JANZK-ZS) was made from the conductive powder of the example and a polyvinyl chloride plastisol. The plastisol had a viscosity at room temperature in the uncured state of 160,000 cps. It could be cured at 330 F. in 10 minutes (Dewey and Almy Chemical Division, W. R. Grace and Company, Daxene A-60). This plastisol was loaded to the extent of 80 weight percent with the conductive powder. The heavy paste obtained was cast into an aluminum mold to form circular gaskets 2 /2" O.D., 2%" ID. and deep. The volume resistivity of the cured gaskets was about 0.05 ohm-centimeter and the permeability was in the order of 5 to 10.

The powder was also used to prepare a conductive surface in a manner described in Conductive Plastic Solders and Shields, Electronics, June 15, 1962.

In this procedure a resin film is layed down on a surface such as XXX-P board and the loose powder is dusted onto the surface. The heavy powder sinks into the resin film and the particles come into electrical contact. The resin binder used in this case was a low viscosity epoxy (80% diglycidyl ether of bisphenol A, 20% orthocresyl glycidyl ether) mixed with 33 weight percent of a polyamide curing agent (Versamid 125, 100 parts to 14 parts triethylene tetramine). The room temperature viscosity of the mixture was 4,000 cps. and its pot life was 50 minutes. The mixture cured in 24 hours at room temperature. The thickness of the resin film deposited was 6 mils and after dusting with the powder the thickness was 12 mils. After curing the conductive surface obtained had a surface resistivity of ohm per square inch. The

solids loading in the resin by this method was approximately weight percent.

The magnetic properties of the conductive powder can be used to advantage in many applications. For example electrostatic and magnetic shielding of tubes, printed circuitry, transformers, caulking R.F. shielded containers and cabinets, and microwave gaskets.

The term plastic is intended to include resins and elastomers (rubbers) besides the conventionally accepted plastics such as polyethylene and the epoxies. The plastic matrix used can be thermosetting, thermoplastic or nonsetting depending upon the use to which the conductive plastic mass is to be put. Asphalts, polyvinyl chlorides, polyurethanes, epoxies, polyesters, polyamides, acrylates and natural rubber can all be used as the matrix to hold the powder of this invention in particle-to-particle contact.

The cured or set form of the conductive plastic can range from soft, flexible rubber-like materials to rockhard solids. Some formulations, such as a RF. caulking compound, can be non-setting pastes similar to window putty and useable over a wide temperature range.

The term matrix means something holding or capable of holding embedded within it another object to which it gives shape or form (The Winston Dictionary: college edition, the John C. Winston Company, Philadelphia, Pa., 1946).

The iron based powder of this invention, having been incapsulated with a protective coating, can be used in applications where only the iron properties are desired but corrosion resistance, that is, resistance to rusting, is required. There are several applications where only a magnetic responsive material is needed and not necessarily a conductive one but there has not been up to this time, so far as known, a commercially available iron powder that would not rust.

Having described this invention, what is sought to be protected by Letters Patent is succinctly set forth in the following claims.

We claim:

1. A process for preparing a conductive metal powder having a silver-white matte appearance comprising replacement plating an iron powder by:

(a) immersing said powder in a well agitated aqueous solution consisting essentially of water and copper sulphate and depositing over the entire surface thereof an adherent, thin, continuous film of copper, and thereafter (b) immersing the thus plated powder in a well agitated aqueous solution consisting essentially of water, silver cyanide and an alkali metal cyanide and depositing over the entire surface thereof an adherent thin continuous film of silver, the gram-moles of silver deposited being less the number of gram-moles of copper deposited in step (a), the amount of silver in said aqueous solution of silver cyanide being less than that which the copper-plated powder is capable of consuming and the concentration of cyanide ions therein being in the range of 40 percent of saturation point where silver comes out of solution, and continuing the plating until the silver ions in said aqueous solution of the silver cyanide are depleted.

2.; The process of claim 1, wherein said iron powder initially has a surface area in the range of 25 to 500 sq: ft./lb. and in the range of 0.01 to 0.15 lb. of copper and 0.02 to 0.30 lb. of silver are deposited thereon.

3. A conductive iron based filler for plastics having a silver-white matte appearance in loose form and consisting of an iron powder having an electrically adherent inner coating of copperdeposited thereon and outer electrically adherentmg of silver over said copper, the conductivity of said iron base filler varying less than 50% when heated in a tray in loose form at 400 F. for 24 hours in a circulating air atmosphere.

(References on following page) 7 8 References Cited Saubestre, Electroless Plating Today, Metal Finish- UNITED STATES PATENTS ing, June 1962 PP- 43,557 7/1864 Weil 106-1 HERBE T 2,771,380 11/1956 Coleman et a1. RT j Pnmary Ffxammer 2,853,403 9/1958 Mackiw et a1. 117100 5 JOHN WE H, A l nt Exammer 3,003,975 10/1961 Louis 252514 US. Cl. X.R.

OTHER REFERENCES 29-1963; 117-71, 100, 130, 227; 252 s12, 513; 260- Book of Formulas, Popular Sc1ence Monthly, Grosset- 10 37 Dunlap, N.Y. (1938), p. 159'.

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