US3420695A

US3420695A - Process for desensitizing metallic powders

Info

Publication number: US3420695A
Application number: US410031A
Authority: US
Inventors: George B Rice; Robert L Dow
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1964-11-09
Filing date: 1964-11-09
Publication date: 1969-01-07
Anticipated expiration: 1986-01-07

Description

United States Patent Ofiice 3,420,695 PROCESS FOR DESENSITIZING METALLIC POWDERS George B. Rice, Trout Creek, Utah, and Robert L. Dow,

China Lake, Calif., assignors to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Nov. 9, 1964, Ser. No. 410,031 US. Cl. 117-100 6 Claims Int. Cl. C23c 3/00 ABSTRACT OF THE DISCLOSURE A method for making particulate metal safe to handle in large quantities by coating the metallic particles with an organic elastomeric material. The desensitized metal powders thus formed are relatively insensitive to electrostatic discharge and practically non-toxic to handlers.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a process for desensitizing metallic powders and to the product thereof.

Many powdered metals suitable for use as additives for solid propellant grains, explosives, pyrotechnics and the like, are sensitive to electrostatic discharge, are pyrophoric or exhibit toxic properties which make them hazardous to handle safely. These metallic powders are limited in their application to use in small quantities and are usually handled with expensive, bulky equipment necessary to protect the individuals involved in working with them. Handling large quantities of non-desensitized metal powders are extremely dangerous because of possible fire and explosion. Other methods for desensitization of metallic powders are known, such as oxidizing a coating on the particles, and displacement coating with less active metals. Both of these methods produce metal powders with lower reaction rate and energy output. The present invention provides a simple process for desensitizing metallic powders which produces a product having improved physical and chemical properties.

It is an object of the present invention to provide a process for desensitizing metallic particles in an economical and efficient manner.

Another object is to produce metallic particles which are safe to handle in large quantities.

A further object is to provide a process for making metallic powders relatively insensitive to electrostatic discharge.

Still another object is to provide metallic particles which are relatively non-toxic to handlers.

Yet another object is to provide coated metallic particles which have more energy available for oxidizer-fuel reactions.

A still further object is to provide coated powders having a relatively low air oxidation rate.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following detailed description.

In accordance with the present invention a basic process is provided for coating metallic powders with an organic elastomeric material thereby forming an improved desensitized metal powder. The process generally comprises mixing moisture-free metallic powder with an acetone or other suitable solution of a polymer, such as a copolymer of vinylidene fluoride and perfluoropropylene (known to the trade as Viton A) to form a slurry, washing the slurry 3,420,695 Patented Jan. 7, 1969 with a nonsolvent, such as hexane, to precipitate the Viton on to the metal powder and repeating the hexane wash once or twice to harden the polymer on the powder particles. The invention will be described more particularly with reference to coating zirconium, uranium, beryllium and magnesium; however, the process is applicable to other metal powders such as hafnium and thorium and mixtures thereof. The following examples are presented to illustrate the invention and should not be considered as limiting thereof.

EXAMPLE I In order to desensitize zirconium powder the following materials should be on hand: Moisture-free zirconium, a quantity of 18-22% Viton A in solution, and a quantity of hexane. Zirconium powder is procured commercially as a water-wet powder because it is highly flammable in the dry state. It is important to remove the moisture so that the polymeric material, Viton A herein used, will adhere to the zirconium particles. The moisture was removed by treating about 35 pounds of water-wet zirconium with acetone. This was accomplished in three operations. Three gallons of technical grade acetone was mixed with the zirconium powder, and after a few minutes the powder settled and the acetone was decanted or siphoned olf; then, another three gallons of the same grade acetone was mixed into the powder residue, and again decanted off; finally, a two gallon quantity of reagent grade acetone was stirred into the zirconium powder residue which was decanted oif leaving moisture-free zirconium powder ready for the desensitization process. A quantity of Viton A solution (previously prepared by dissolving suflicient Viton A in acetone to form an 1822% Viton A content) was added to the moisture-free Zirconium powder and stirred vigorously for several minutes until a slurry or suspension formed. This slurry was washed with hexane with continuous agitation for from 5 to 10 minutes, the volume of hexane being from one to four times the volume of slurry. After the slurry settled the supernatant hexane was decanted or siphoned off. This step precipitates the Viton on to the zirconium powder particles. The Viton-coated powder is washed with hexane in about the same amount as before to harden the Viton on to the metallic powder. After the residue settles, the hexane is decanted or siphoned ofi" and the residue is air dried. The product produced is a fine, free-flowing Viton-coated zirconium powder which may be safely handled in large amounts without danger of fire or explosions.

The slurry may be washed with normal hexane or other low boiling liquid hydrocarbon two or three times as necessary.

The removal of the moisture from any water-wet metal powder is necessary for good adhesion of the polymeric coating. Other drying agents include any material such as ketones which will pickup water.

The table following shows the sensitivity test results of three batches of the Viton-coated zirconium powder prepared by this process as contrasted to the uncoated powder.

EXAMPLE II Uranium is highly pyrophoric and electrostatic sensitive was desensitized by the process using Viton A described in Example I. Uranium powder is shipped commercially under acetone for safety purposes. A weighed amount of uranium was mixed with an 18-22% solution of Viton for several minutes until a slurry formed which was subsequently washed with hexane to shock-gel the polymeric material, Viton, to the uranium particles. The product produced was a Viton-coated uranium powder which was free-flowing, and safe to handle in large amounts in the mixing of propellant materials.

EXAMPLE III Magnesium powder which is highly pyrophoric was also desensitized by the process described in Example I. It is shipped under heptane, hexane or kerosene, the excess of which is decanted off and Weighed amount mixed with an acetone solution of Viton A to form a slurry. The slurry was washed with hexane to precipitate or shock-gel the Viton onto the magnesium particles. Following a second hexane washing to harden the Viton coating the material was air dried. The resultant powder was fine, free flowing, and safe to handle.

EXAMPLE IV Beryllium was also desensitized by the process herein described to overcome the dusting danger (inhalation) of this very toxic material. Beryllium is procured as a dry powder (ranging from 12 to 15 1. particle size). A weighed amount was mixed with an 18-22% Viton solution to form a slurry which was washed with hexane according to the procedure hereinbefore set out. The Viton-coated beryllium particles were practically dustfree which greatly reduced the toxicity hazard encountered in handling the uncoated powder.

The powders produced herein consist essentially of from 4 to 6 percent by weight Viton and the remainder metal.

Nylon was used in place of Viton, but did not prove to be as effective as desensitizer as the Viton. The nylon was dissolved in a reagent grade methanol, mixed into the selected metal powders to form a slurry, and shocked out with hexane. From /2 to 4 percent by weight nylon was deposited onto the metal particles.

In preparing the 18 to 22 percent solution of the copolymer of vinylidene fluoride and perfluoropropylene a slab of Viton A was placed in a quantity of reagent grade acetone and left for about 16 hours. The solution was drained E and tested for Viton content because an 18 to 22% Viton solution was found to be successful.

Other polymeric materials which may be used in addition to the copolymer of vinylidene fluoride and perfluoropropylene (Viton A), include a similar copolymer of hexafluoropropylene and vinylidene fluoride, generally known as Fluorel, vinylidene fluoride and chlorotrifiuoroethylene, sold under the trade name Kel-F elastomer, and nylon which is a long chain polymeric amide.

What is claimed is:

1. A method for desensitizing particulate metal which comprises:

(a) dissolving a copolymer of vinylidene fluoride and perfluoropropylene in acetone to form a solution having a solids content of from 18 to 22 percent by weight of the copolymer;

(b) incorporating particulate metal into said solution to form a slurry;

(c) precipitating the copolymer from said solution onto said metal by adding a quantity of hexane to the slurry thereby forming a coated metal;

(d) separating said hexane from the coated metal; and

(e) drying the coated metal.

2. The method of claim 1 wherein the particulate metal is zirconium.

3. The method of claim 1 wherein the particulate metal is uranium.

4. The method of claim 1 wherein the particulate metal is beryllium.

5. The method of claim 1 wherein the particulate metal is magnesium.

6. The method of claim 1 wherein the coated metal is treated with a second quantity of hexane thereby hardening the coating on said metal.

References Cited UNITED STATES PATENTS Re. 25,277 10/ 1962 Toulmin.

1,915,788 6/1933 Hardy 117100 X 2,770,605 11/1956 Honn 117-16l X 2,793,200 5/1957 West 260-92.1 X 3,035,948 5/1962 Fox 14919 3,110,638 11/1963 Murphy et a1. 117-100 X 3,133,841 5/1964 Kuehl 149-5 3,202,557 8/1965 Kaufman 26092.1 X 3,300,329 1/1967 Orsino et al l17161 X WILLIAM D. MARTIN, Primary Examiner.

U.S. Cl. X.R.