US2461582A - Nitramines and their preparation - Google Patents

Description

Patented Feb. 15, 1949 UNITED STATES 2,4615% PATENT OFFICE NITRAMINES AND THEIR PREPARATION No Drawing. Application December 30, 1944, Serial No. 570,813

8 Claims.

The present invention relates generally to nitramines and more particularly to certain new nitramines having a plurality of nitroxy groups, and to their preparation from readily available raw materials.

Broadly speaking, the object of the present invention is to provide a new series of nitroxyalkylnitramines suitable for various industrial purposes but of particular interest in connection with the production of military and industrial explosives.

A more particular object is the provision of a new series of nitroxyalkylnitramines of great explosive power and brisance.

consists of bis-nitroxyethylnitramine having the .structura1 formula:

This compound may also be named dinitroxydiethylnitramine, for which reason it may conveniently be referred to by the trivial designation, DINA.

As described in detail hereinafter, DINA may be prepared by a variety of different methods as briefly characterized below:

(1) By treating diethanolamine nitrate under conditions effecting the elimination of the elements of water;

(2)by oxidizing bis-nitroxyethylnitrosamine to the corresponding nitramine; and

(3) by treating diethanolamine with nitric acid and a lower fatty acid anhydride, in the presence of a suitable catalyst. The conversion of secondary amines to the corresponding nitramines in the presence of chlorine-containing catalysts is disclosed and claimed in our application Serial Number 570,814 filed of even date herewith.

The product (DINA) obtained by any of the foregoing methods possesses the following important explosive properties:

(1) Very high explosive power (1.48 TNT in the ballistic mortar);

(2) High brisance and high rate of detonation (ca. 6000 m./sec. at a density of 1.07 and 7300 m./sec. at a density of 1.45);

(3) The ability to gelatinize nitrocellulose;

(4) Relatively low melting point (52 C.), enabling the material to be cast-loaded;

(5) High thermal stability in relation to nitroglycerine.

(6) High heat of combustion (2403 calories per gram);

(7) Good ballistic characteristics: adiabatic flame temperature of 3700 K. (calculated from Ei (2500 K.) :4832 calories per gram and average heat capacity at constant volume of 0.3653 calorie per gram per degree) (8) Satisfactory sensitivity to impact, friction, etc.;

In order clearly to disclose the nature of the present invention, a number of specific examples will hereinafter be described in considerable detail. For purposes of convenience, the preparation of DINA will first be discussed in detail and thereafter the properties of DINA will be described. It should be clearly understood, however, that the following examples are given purely for illustrative purposes and are not intended to delineate the scope of the invention in its broad aspects.

I. THE PREPARATION OF DINA EXAMPLE 1 (Preparation of DINA from dinitromydiethylamine nitrate) (a) THE PREPARATION OF DINITROXYDIETHYL- AMINE NITRATE In a 5 l. flask provided with a mercury sealed stirrer was placed 3780 g. (60 moles) of 99-100% nitric acid. The nitric acid was cooled to 10-12" C. and agitated whil 420 g. (4.0 mo1es) of diethanolamine (M. P. 26.5 C.) was added over a 2 /2 hour period. After the addition was complete the reaction mixture was stirred for one hour at 40 C., moisture being excluded from the flask at all times. The reaction mixture was then poured on 6 kg. of ice and the resulting white crystalline precipitate was filtered immediately at the lowest possible temperature and then washed with 500 cc. of ice water, then with 300 cc. of ethanol and finally with 200 cc. of ether. The dried product melted at 118-119 C. and weighed 846 g., corresponding to a. yield of 82% of the theoretical.

Dinitroxydiethylamine nitrate is fairly soluble in acetone, methanol, hot solutions of ethanol, water and nitromethane. It is less soluble in cold ethanol and water and insoluble in benzene, chloroform, ether and petroleum ether. It shows some tendency to be hygroscopic and tends to hydrolyze when moist. It may be ignited and burns to leave a deposit of carbon. It may be detonated by impact. Repeated crystallizations of the crude salt from methanol, ethanol, acetone and. glacial acetic acid raised the melting point to 120.5.

Analysis: Calcd for C4H1009N4: Carbon, 18.6; hydrogen, 3.91; nitrogen 21.7. Found: Carbon 18.8; hydrogen, 3.90; nitrogen 21.7.

Dinitroxydiethylamine picrate may be prepared from aqueous solutions of dinitroxydiethylamine nitrate and picric acid. After successive recrystallizations from water. isopropyl alcohol and 1:1 toluene-ethyl acetate, it melted at 136-137".

(D) CONVERSION OF DINITROXYDIETHYLAMINE NITRATE TO DINA 25.8 g. (0.10 moles) of dinitroxydiethylamine nitrate was added to a solution of 0.63 g. (0.01 mole) of 99-100% nitric 'acid and 0.54 g. (0.004 mole) of zinc chloride in 25.5 g. (0.25 mole) of acetic anhydride. The salt dissolved after the mixture was heated to 55 C. for five minutes.

The solution was then poured into 60 cc. of water I sponding to a yield of 92% of the theoretical.

Purification by successive crystallizations from methanol, methanol-ether and benzene-petroleum ether (GO-70) raised the melting point to 51-52 C.

Analysis: Calcd for C4HsOsN4: Carbon 20.0;

hydrogen, 3.36; nitrogen, 23.3. Found: Carbon 20.2; hydrogen, 3.31; nitrogen, 23.0.

EXAMPLE 2 (Preparation of DINA from dinitromydiethylnitrosamine) (a) PREPARATION OF DINITROXYDIETHYL NITROSAMINE 17.3 g. (0.067 mole) of dinitroxydiethylarninenitrate in 240 cc. of water at C. was mixed with 4.25 g. (0.067 mole) of 70% nitric acid and 9.25 g. (0.134 mole) of sodium nitrite in 20 cc. of water. After mixing, the reaction mixture was cooled to 0 C. and held 'at this temperature for minutes. The solid product was filtered ofi, washed with water and dried in a vacuum desiccator. The yield was 14.7 g. or 97.7% of the theoretical. I

Dinitroxydiethylnitrosamine is soluble in most organic solvents but quite insoluble in water and petroleum ether. It gives a violet-red coloration in the Liebermann nitrosamine test. It is rapidly decomposed upon heating in boiling water and even pure samples will decompose merely on continued standing. It ignites readily, burning to leave a brown residue. It can be detonated under impact, its sensitivity being about 2.6 TNT. Its explosive power as measured in a ballistic mortar is about 1.50 TNT.

The crude dinitroxydiethylnitrosamine melted at 46 C. It was purified by successive crystallizations from ether and from ether-petroleum ether (28-38"). The purified material melted at 46-47 C.

Analysis: Calcd for C4HaO7N4: Carbon, 21.4; hydrogen, 3.59; nitrogen 25.0. Found: Carbon, 21.7; hydrogen, 3.60; nitrogen 24.9.

(b) OXIDATION OF DINITROXYDIETHYLNITROS- AMINE TO DiNA 100 g. (0.0042 mole) dinitroxydiethylnitrosamine was dissolved in 10.0 g. (0.16 mole) of 99-100% nitric acid cooled to 2-3 C. To this solution was gradually added 2.74 g. (0.0125 mole) of ammonium persulfate along with a trace of silver nitrate. The mixture was maintained at 0 C. for 15 minutes and then at 25 C. for one hour. The reaction mixture was poured on 20 g. of ice producing a precipitate which was filtered, washed with water and dried; the product weighed 0.59 g. and melted at 47-49 C. The crude material was dissolved in 7.0 cc. of 70% nitric acid and precipitated by dilution with 2 volumes of water. The recovered material melted at 50.5-51 C. and weighed 0.24 g., corresponding to a yield of 22% of the theoretical. A mixed melting point with DINA showed no depression. The DINA prepared by oxidation gave a green coloration with the Liebermann nitrosamine test rather than a violet-red color produced with the nitrosamine. A slightly improved yield (32%) was obtained by replacing the silver nitrate with 0.01 mole of zinc chloride per mole of nitrosamine and then decomposing the nitrosamine remaining in the crude product by heating the latter in boiling water.

EXAMPLE 3 (One-step preparation of DINA from methanolamine) A Z-gal. stainless steel jacketed kettle fitted with a thermometer and pedal stirrer rotating at 150 R. P. M. was equipped with 2 graduated dropping funnels One of the funnels was filled with 823 g. (7.84 mole) of diethanolainine, the other with 1646 g. (25.6 moles) of 98% nitric acid (nitrous acid content 0.11%) 2754 g. (27 moles) of acetic anhydride was placed in the kettle and cooled to 5 C. by running tap water through the jacket of the kettle. 62 g. of nitric acid was run into the kettle followed by 23 g. (0.16 mole) of diethanolamine hydrochloride. With the reaction mixture maintained at 15-13 C., the remainder of the nitric acid and diethanolamine was added at proportional rates during 45 minutes. When the addition was complete the temperature of the reaction mixture was raised to 40 C. for 10 minutes after which the reaction mixture was permitted to flow into 6480 cc. of water cooled at 5 C., the water being agitated during the addition of the reaction mixture which took about 10 minutes. The product was filtered at 22 C. and washed with 25-30 1. of water until neutral to bromocresol green. An aliquot of the damp crude product was dried in a vacuum pistol above refluxing ether. The computed crude yield was 1737 g. or 90.5% of the theoretical. The filtrate remaining after separation of the precipitate was cooled to 0 C. whereupon 25 g. of DINA was precipitated, raising the total yield to 1762 g. 'or 91.8%. The crude product melted at 49.5-5 .5". Similar yields were obtained when nitric acid was used provided enough additional acetic anhydride was added to compensate for the water present in the 95% nitric acid.

EXAMPLE 4 (Preparation of DINA from diethanolamine using nitric and hydrochlorid. acids) g. (1.0 mole) of diethanolamine was added dropwise to 954 g. (15 moles of 99% nitric acid maintained at 20 C'. The resulting solution 'was cooled to 0-5 C., and hydrochloric acid gas passed into the solution until the increase in weight was 36.5 g. (1.0 mole of hydrochloric acid). The reaction mixturewas then heated to 55 C. for 20 minutes and poured onto 1900 g. of ice. The crude DINA was filtered off; it melted at 49-51" C. and weighed 100.8 g., corresponding to a yield of 42% of the theoretical.

Small yields of DINA were obtained when the hydrochloric acid was replaced by phosphorus pentachloride, phosphorus trichloride phosgene and chlorine while no yield was obtained with sulfuryl chloride or thionyl chloride.

EXAMPLE (Purification ofDINA) the water which was violently stirred with a Lightnin mixer (1725 R. P. M.). As the initial portion of the acetone solution was added to water, previously purified DINA was introduced in order to induce crystallization. During dilution'the temperature of the water was maintained at 5-8 C. by cooling with tap water. The process of dilution required about one hour. The precipitated DINA was filtered from the acetonewater liquor using mild suction. The product was repeatedly washed-with cold water until further washing failed to remove color. The purified DINA was dried in a forced draft drier operating at room temperature, 48 hours in this drier reducing the moisture content to 0.13-0.07% by weight. The usual weight of purified material was 1650 g., corresponding to a recovery of 85% of the theoretical. The purified DINA melted at 50.5-52 C. and gave an Abel heat test at 100 C. of 12-16 minutes.

An alternative method of crystallization which produces distinct crystals of DINA having a bulk density reliably higher than that produced by the process described above may be carried out where adequate stirring facilities and temperature regulation are available. This slight modification of the procedure described above depends on the formation of a finely divided DINA fast dispersed by violent agitation in wateracetone at 25 C. and is accomplished by mixing water with the DINA-acetone solution as quickly as possible, the heat of mixing being removed by means of a cooling jacket. The order of the addition of solution and diluent is not important.

In carrying out this alternative purification I procedure, crude DINA was melted in a jacketed aluminum vessel and poured into acetone (0.104 Imp. gal/lb. DINA). The solution at 77 F. was filtered into gal. cylindrical aluminum kettle 12" in diameter by 22" high and equipped either with a Lightnin propeller type stirrer (1750 R. P. M.) or with one fitted with four flat blades 11 5'" wide staggered at right angles along the shaft and sweeping a total diameter of 6 There were no bafiles in the kettle. The agitation was started at 1750 R. P. M. and water at 77 (twice the volume of the acetone employed in preparing the DINA-acetone solution) was added'. as rapidly as possible. The temperature rise was 3-9 F. depending on whether the kettle was one-fourth or completely filled. The heat of dilution was dissipated by cooling water in the jacket in order to hold the temperature riseto below 10 F.; if the temperature rise exceeds 10 F., the rate of crystallization is inordinately slow. Crystallization was complete in about 7 minutes as evidenced by clearing of the liquorwhich was then cooled to 60 F. to complete crystallization and prevent caking of the product on the nutsch. The mixture was filtered and the crystals on the nutsch were washed until acid-free and then dried in a forced air drier at 104 F.

DINA sometimes contains an impuritycapable I of forming a deep red water soluble derivative with ammonia. This impurity may or may not result in a product of inferior quality with respect to vacuum stability. In any event, however, if the purification procedure described immediately above is modified by adding an equal volume (0.25 volume per cent) of concentrated aqua ammonia to the acetone and if desired also to the water, the purification improves the vacuum stability of the DINA and the resulting solutions vary in color from deep red to brown. Since the addition of ammonia does not adversely affect the crystallization procedure in other respects,

its inclusion in all crystallization purification of DINA is to be recommended.

EXAMILE 6 (Recovery of acetone from the purification of DINA) The purification of DINA on the scale described in Example 5 requires 1900 cc. (1.50 kg.) of -98% acetone which is diluted with 3800 cc. of water to produce 5.31 kg. of 27-28% acetone solution. The recovery of acetone from the resulting solution presents few difliculties and may be accomplished by distillation of the ,filtered solution either with or without the addition of a small amount of lime to the acetone solution before distillation. If distillation is carried out on an acetone solution that has not been treated with lime, a very small amount (about 0.23 g./1. of dilute acetone solution) of DINA may remain in the still. If the dilute acetone solution is treated with a small amount of lime (for example 1% by weight of lime) no DINA will be found in the residual water.

EXAMPLE 7 The preparation of DINA according to the method described in Example 3 produces about 1.96 parts of acetic acid per part of DINA, this acetic acid being obtained as a 30-33% solution containing nitric acid and organic impurities. Assuming that the DINA has been filtered from this solution at 22 C., the solution contains about 3.2 g. of DINA per liter. If the acetone solution is cooled to 0 C. before filtration, the DINA content may be reduced to about 0.8 g./l.

From the standpoint of safety it is desirable to destroy the residual DINA before the 30% aqueous acetic acid soluti-on is concentrated. It is also desirable to remove the other impurities such as and if the distillation is carried too far a violentv explosion occurs.

The residual DINA inthe dilute acetic-aci'dmaybe destroyed either by refluxing the dilute acetic. acid or by digesting it with 2% concentrated sulfuric acid or with 2 /2 sodium chloride-or with sulfuric acid and either iron filings or manganese dioxide. However, the distillate obtained in all these cases contains nitric acid.

A much more satisfactory method of recovering the acetic acid is to heat the dilute acetic acid to"200' C. in a stainless steel pressure vessel for about 30 minutes, prior to distillation. The pressure digestion completely destroys the DINA and subsequent distillation of the pressure digested solution gives at leasta 95% recovery of acetic acid free from significant amounts of nitric, nitrous and formic acids. Corrosion of the pressure vesselv may be reduced by adding about-3' cc. of ethanol per 50 cc. of dilute acid prior to pressure digestion. Since an ester of acetic acid is customarily used as an entrainer in the concentration of dilute acetic acid, addition of an alcohol to the liquor does not introduce an undesirable contaminant. The recovery procedure is conveniently carried out by passing the liquor, heated at 200 C., through a stainless steel tube column under pressure, releasing the liquor through a degassing heat exchanger and then distilling the liquor using the heat dissipated in the heat exchanger to efiect distillation.

II. THE PROPERTIES OF DINA (a) CHEMICAL PROPERTIES DINA is very soluble in acetone; quite soluble in nitric acid, glacial acetic acid, methanol,

ethanol, benzene and-ether; and insoluble in water, carbon tetrachloride and petroleum ether.

DINA gives a red coloration with the Franchimont reagents (Rec. trav. chim. 16, 227, 1897'), and a green color With the Liebermann test. It may be reduced with sodium amalgam to give a small yield of hydrazine which was identified as salicyl aldazine. v

The stability of DINA in water depends upon the temperature. Thus 5 g. of. DINA were shaken with cc. of water for. six weeks atv 25 (3.; a

99.2% recovery of DINA was obtained, the water remaining neutral to indicators. A 39% loss occurred when DINA Washeated in boiling water for six hours. DINA was completely destroyed after heating for one hour: and: 10 minutes inan equal weight of boiling. 5% aqueous sodium hydroxide.

DINA is. rapidly decomposed in concentrated sulfuric acid even at O C. On the other hand when DINA was maintained in solution in glacial acetic acid or 99% nitricacid for three hours at 25 almost no loss of DINA occurred.

Pure DINA shows no gassing when heated to 100 C. About 155 C. slow evolution of gas-begins and above about 180-185 C. the formationof nitrogen oxides is vigorous. No ignition was obtained when a small quantit of DINA. was heat ed to 2 10" C. at the rate of 5 per minute; the sample, however, decomposed between about 180 and. 200 C. Ignition was obtained when DINA A train of DINA was laid in an iron trough of semi-circular section in diameter. The trough was heated at one point until ignition of DINA took place at the point immediately above the fiameg the material did not burn beyond the point of ignition. I

(b) PHYSICAL PROPERTIES Pure DINA melts at iii-52 C; It may bev cast to a. density of 1.67.. Its heat of fusion is.23.5:0.2 cals/g. and its specific heat is 03810.02 cal/g.

A determination of hygroscopicity at room temperature showed no gain at relative humidity and a.0.(1021%v gainat relative humidity. 1

DINA crystallizes from acetone in the form of flattened prisms DINA exhibits. polymorphism and four different polymorphs may exist.

(0) PHYSIOLOGICAL Errscrs or DINA DINA appears to have no significant toxic properties. When fed to a dog it caused some formation of methaemcglobin but there was no evidence of haemoglobin destruction. It was applied to rabbits in a salve without effect.

(d) ExPLosIvE PROPERTIES DINA was not detonated when struck on stone or wood with a, rawhide mallet. Detonation was obtained however when DINA on steel or stone was struck with a steel hammer. The impact sensitivity of DINA appears-to be about3.2 No detonations could be obtained on a sliding shot friction. impact machine at a maximum fall with a 12 lb. shot.

Rifle. tests showed DINA to be sensitive. A block 2" 2 4" (density, 1.60) was facedwlth A;" mild steel and backed by mild steel; It detonated completely when struck squarely by a .303 rifle bullet. A second block which was backed by 1" mild steel but without facing was shattered by a rifle bullet with no indication of explosion.

The sensitivity of DINA to influence was determined by placing a sample of well-cast DINA (density 1.6) at a measured distance from 4" lengths of 1" diameter 60 polar dynamite; The DINA fired when the distance was 0, /2"and 1" and failed at 1". In a similar test with granular DINA, the sample fired at 1-"; 2" and3 and no failures were obtained.

The explosive power of DINA as measured in the ballistic mortar is approximately 1.48' TNT for well-cast DINA and 1.45 for granular DINA.

20 g. cast blocks of DINA were fired with a No; 8 E. B. cap upon 4 4" A mild steel plates; Clean holes similar to that produced by high density cycionite were blown through plates laid on clean Ottawa sand.

The rates of detonation of DINA at difierent densities lies slightly below those of cyclonite.

(c) USE or DINA InDEMoLruoN Four 2" 2 X4" bloeksofsDINA (density 1.60) were exploded on top of.' a 2" cube of concrete. Three of the DINA blocks werelaid side by side and the fourth block. placed on top', the upper block being fired by a No; 8 d etonator. The con crete was completely reduced to rubble 2"-3" in diameter together withmuch. dust and fines.

In another, experiment a. MK-lll. shaped charge was filled by-pouringinto itia mixtureof water andmolten DINA, the excess water being run on to leave a container full'of'DINA which solidified'overnight. The shaped charge wasfired on.10" of mild"steel',.No; 8 cap and a 1102.- CE

7 primer being 'used to obtain detonation.

The steel plate was completely pierced and the scab on the steel was driven into the earth and rock.

(1) Use or DINA IN PROPELLANT COMPOSITIONS DINA offers several properties which will recommend it as a component for high power propellants. Its calorific value is about 1260-1350 cals.

Nitrocellulose colloids containing DINA have greater force than comparable propellant containing nitroglycerine. Moreover, the tensile strength of propellant containing, for example,

. 40% DINA is 1225 lbs/sq. in. as compared with a comparable propellant containing 40% nitroglycerine with tensile strength of 800 lb./sq. in. The stiffness of the former is 29 "Smith-Tabor units as compared with 15 units for the latter. Because of such mechanically advantageous properties the composition .of propellant containing DINA can be varied conveniently over wider limits than can those containing nitroglycerine and, especially than those containing diethyleneglycol dinitrate. Moreover compositions containing DINA are safer to manufacture than those containing nitroglycerine and are more stable in storage. Examples of such propellant compositions are:

(1) 49% nitrocellulose (13.1% N), 48% DINA and 3% oarbamite (ethyl centralite).

(2) 57% nitrocellulose (12.6% N), 35% DINA and 8% carbamite.

In connection with this particular application of DINA reference is made to the copending application of John F. Kincaid, Ser. No. 570,808, filed of even date herewith, which describes and claims a fiashless, high velocity propellant composition containing nitrocellulose plasticized with DINA together with nitroguanidine as a coolant.

Certain nitroxyalkylamine nitrates such as that obtained by treatment of various ethanolamines with nitric acid have been disclosed in the prior art. Thus Herz (German Patent 630,079) describes the pentanitrate of the quaternary base prepared from triethanolamine and ethylene oxide. Likewise, Dynamit act Gesellshaft (French Patent No. 639,632; German Patent No. 500,407 and 513,635; and British Patent 357,581) described the nitration of monoethanolamine. The first of these DAG patents also mentions a trinitrat of diethanolamine as having a melting point of 120 C. which identifies it as bis(nitroxyethyl)amine nitrate, Moreover, Aubry in Men. poudres, 25, 189-193 (1932-33) in discussing the preparation of Monoethanolamine dinitrate states that the nitrates of diand tri-ethanolamines are unstable, thus indicating that he has prepared bis (nitroxyethyl) amine nitrate which is of course merely a starting material for the preparation of DINA according to one method of the present invention. It is therefore apparent that the compounds of the present invention are new products having decidedly different properties from those of the unstable amine nitrates of the prior art.

It will be apparent to those skilled in the art that many variations may be made from the above detailed description of the preferred embodiment of the present invention. Thus, for example, as an obvious variation of the preparative method described in Example 1, the diethanolamine may be mixed with nitric acid in the proper proportions necessary to form the nitrate, the resulting mixture cooled and then used as a liquid feed to a vessel containing acetic anhydride and the chloride catalyst, this modification obviating the isolation of the amine salt as a solid material. Other lower fatty acid anhydrides, such as those of propionic and butyric acids may be used although acetic anhydride is preferred under existing market conditions. Many other variations and modifications will be readily apparent to those skilled in the art. It is to be clearly understood that all such modifications and variations are embraced within the scope of the appended claims.

We claim:

1. As a new composition of matter, a member of the group consisting of the nitramines and nitroso-amines having the structural formula wherein R1 and R2 are alkylene radicals andn is a whole number less than 3.

2. A method of preparing bis-nitroxyethylnitramine which comprises treating diethanolamine with nitric acid to form a nitrate and treating the product with nitric acid and a lower fatty acid anhydride in the presence of a chloride catalyst.

3. A method of preparing bis-nitroxyethylnitramine Which comprises treating diethanolamine with nitric acid and a lower fatty acid anhydride in the presence of a small proportion of a chloride salt as a catalyst.

l. A method of preparing bis-nitroxyethylnitramine which comprises treating diethanolamine with nitric acid and acetic anhydride in the presence of a small proporton of diethanolamine hydrochloride, said reactants being employed in the proportions of at least 3 moles of nitric acid and at least 4 moles of acetic anhydride per mole of diethanolamine.

5. A method as defined in claim 2 wherein the product is purified by treating the same with boiling water, passing steam through the molten nitramine, separating the molten nitramine from the supernatant water, dissolving the nitramine in a water-miscible solvent and then diluting the solution with water whereby to precipitate purified bis-nitroxyethylnitramine.

6. A method as defined in claim 2 wherein the catalyst is zinc chloride.

7. A new composition of matter as defined in claim 1 in which the alkylene radicals are CH2CH2 and n is 2.

8. A new composition of matter as defined in claim 1 in which the alkylene radicals are CHz-CI-Tz and n is 1.

GEORGE F. WRIGHT. WALTER JOHN CHU'I'E.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS