US3669924A

US3669924A - Stabilized polyvinyl nitrate and process

Info

Publication number: US3669924A
Application number: US857234A
Authority: US
Inventors: Eduard Daume; Jurg Breitenmoser
Original assignee: Werkzeugmaschinenfabrik Oerlikon Buhrle AG
Current assignee: Rheinmetall Air Defence AG
Priority date: 1968-09-19
Filing date: 1969-09-11
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: CH504482A; SE348742B; DE1938902B2; DE1938902A1

Abstract

A novel stabilized polyvinylnitrate and a process for the production thereof including the precipitation of the compound from organic solutions containing same.

Description

United States Patent Daume et a1.

STABILIZED POLYVINYL NITRATE AND PROCESS Eduard Daume; Jurg Breitenmoser, both of Zurich, Switzerland Werkzeugmaschinenfabrik Oerlikon-Buhrle AG, Zurich, Switzerland Filed: Sept. 11, 1969 Appl. No.: 857,234

Inventors:

Assignee:

Foreign Application Priority Data Sept. 19, 1968 Switzerland 14042/68 US. Cl. ..260/45.9 R, 260/77.5 A, 260/78.4 R,

260/791, 260/913 VA, 260/942 R, 260/220, 149/88 [4 1 June 13, 1972 Primary Etaminer.loseph L. Schofer Assistant Examiner-Stanford M. Levin Att0rney--Wenderoth, Lind & Ponack [5 7] ABSTRACT A novel stabilized polyvinylnitrate and a process for the production thereof including the precipitation of the compound from organic solutions containing same.

12 Claims, No Drawings STABILIZED POLYVINYL NITRATE AND PROCESS The present invention relates to stabilized polyvinylnitrate (PVN), and to methods for producing same.

In the development of fuels for rocket motors, it is endeavored to produce fuel compositions which are as energyrich as possible, in order thereby to be ableto deliver large payloads with small volumes of fuel. In the field of solid-fuel rockets, the so-called double-base" fuels are mainly used to this end. They consist essentially of a mixture of nitrocellulose and nitroglycerine, to which additives are admixed for modifying the combustion properties. Moreover, inorganic oxidizing agents such as ammonium perchlorate, ammonium nitrate as well as metal powder may also be admixed to these fuels in order to increase the energy content thereof. Apart from the double-base" fuels, the so-called composite fuels are also known, which are substantially based on the use of inorganic oxidizing agents with organic bonding agents. These bonding agents are for example polybutadiene, polyurethane, polysultide, polyester and the like. The oxidizing agents are mixed with the still fluid organic components for producing these fuels and are subsequently subjected to a curing process, this resulting in a solid resilient mass of predeterrninated fonn. Metal powder may also be added to these fuels in order to increase the energy content thereof.

When using rockets for military purposes, it is in many cases necessary that the propellants burn without the formation of smoke. This property exists, in the conventional fuels, on the basis of nitrocellulose and nitroglycerine, and, in the composite fuels, on ammonium nitrate. The limit of capacity, expressed as specific impulse, is about 230 to 240 seconds in the double-base fuels, without the addition of ammonium perchlorate and metal powder. However, in order to obtain such capacities, a fairly high proportion of nitroglycerine is necessary, which causes the fuel already to begin to deform plastically when the ambient temperature is increased, this having a disadvantageous effect on combustion at high temperatures. However, an addition of ammonium nitrate to the double-base fuels does not bring a substantial increase in energy, so that this possible solution can only be applied in rare cases. Composite fuels based on ammonium nitrate are only used for special purposes, e.g. for gas generators, since the adiabatic flame temperature is very low and therefore only specific impulses in the order of 160 to I80 seconds can be achieved. All other fuels of the composite class produce a considerable amount of smoke during combustion, which smoke is composed, according to the choice of the oxidizing agent, of solid combustion products, e.g. potassium chloride, potassium oxide, sodium oxide and lithium chloride. When using ammonium perchlorate as oxidizing agent, gaseous hydrochloric acid is produced which forms heavy fumes particularly when the atmosphere is damp. The hydrochloric acid is also disadvantagous due to its corrosive effect on the ground installa tion. The addition of metal powder, in particular aluminum or magnesium, also increases the formation of smoke since the corresponding metal oxides are produced during combustion.

High-energy fuels which do not present these disadvantages of smoke formation and the corrosive effect on ground installations, may be produced, on the basis of the present invention, from stabilized polyvinylnitrate. This purely synthetically produceable material has good thermoplastic properties and may be gelatinized with many organic nitric esters or other known plasticizers. In this form, it is capable of absorbing larger amounts of known explosives in pure or mixed form, such as for example trimethylenetrinitroamine, nitropenta, trotyl or even ammonium nitrate. From such mixtures, propellants may be formed, which deliver completely smoke-less combustion products and high specific impulses.

Polyvinyl nitrate was already under consideration in the 40's as component in fuels and explosives, but its use in this field has, up to the present, not been able to penetrate. The main disadvantage is stated to be the difficulty in the stabilization process. PVN decomposes more or less quickly during storage and/or heating.

Polyvinylnitrate may be produced by nitration (esterification) of polyvinyl alcohol with pure concentrated nitric acid or with a so-called mixed acid, consisting of concentrated nitric acid and concentrated sulfuric acid. Nitration (ester-iiication) is preferably carried out at 0 C. The polyvinyl nitrate formed is thereby completely or partly soluble in the nitrating acid depending upon the selected excess thereof. After nitration, the product is precipitated in water. During this precipitation process, a considerable heat of dilution is produced, which causes the polyvinylnitrate precipitating in the water to coagulate.

Although the particle size of the polyvinyl nitrate formed may be relatively well controlled according to the conditions of precipitation, an acid inclusion cannot be avoided. The conventional methods used in the explosives and fuel industry for washing out the nitration product formed and subsequently treating with alkaline agents such as sodium bicarbonate or ammonium bicarbonate and the like, do not lead to the target, since the included acid cannot be reached by the alkalis. It was also proposed to treat the polyvinylnitrate with an alkaline solvent-water mixture and in this way to remove the residual acid. Also, this method does not produce good stability values. Finally, it is known that it was attempted to dissolve the polyvinylnitrate in acetone and to reprecipitate the lacquer solution after its neutralization in water.

In this way, good stability values may be obtained, but the process is only to be carried out with considerable dilution and is therefore uneconomical. The precipitate coagulates at higher concentrations.

It is therefore an object of this invention to procure a new and improved process to prepare stabilized PVN from the crude product which may in turn be prepared by conventional means.

It is a further object of the invention to produce a stabilized PVN having outstanding properties of purity and ability to be stored for comparatively long periods of time, substantially without decomposition.

Finally, it is another object of the invention to develop solid fuel compositions superior to the known ones for use as propellants in rocket motors.

It has now been found that pure and stabilized polyvinylnitrate, usually with a nitrogen content of more than 15 percent (theoretical maximum value about 15.7 percent), can be produced from the raw, decomposable PVN. The new stabilized polyvinylnitrate generally forms free flowing, non caking powders which was not possible until now.

Briefly spoken, the process according to the invention comprises the steps of a. providing a solution of raw acid containing polyvinylnitrate in an organic solvent,

b. neutralizing or rendering slightly alkaline said solution,

c. separating said solution from non-dissolved products of the neutralization,

d. introducing said purified solution of step (c) into a hot precipitating bath in which said organic solvent is insoluble or only partially soluble, whereby said solvent is stripped and stabilized polyvinylnitrate is precipitated,and whereby the rate of introduction of said purified solution corresponds to the stripping rate of said solvent so that steady-state conditions are maintained, and whereby a solvent is selected in step (a) the pure boiling point of which or the azeotropic boiling point of which with the precipitating bath is below the boiling point of the pure precipitating bath.

As precipitating bath, either pure water or an aqueous medium which contains certain additives which are given hereinbelow by way of example, is preferably used. With aqueous precipitating baths, such solvents are preferably used in the PVN-solution which form with the water an azeotrope with minimum boiling point, this usually being immediately the case when using solvents which are not or only partially miscible with water. Examples of such solvents are esters and ketones, such as ethyl and butyl acetate or methylethylketone and methylisobutylketone. Ethyl acetate is preferred.

As PVN starting product, a direct product of nitration of polyvinyl alcohol after precipitation with water, filtration and washing-out with water until neutrality is reached, is advantageously used. This product can immediately be dissolved in humid state without drying, e.g. in ethylacetate.

One proceeds best by agitating the solution of the raw PVN with an aqueous solution of an alkaline agent, e.g. NaOH, NaHCO or Na CQ or other alkali metal hydroxides, carbonates or bicarbonates until neutralization or weak basicity is reached, this necessitating considerable amounts of the alkaline agent, according to the residual acid content which was included in the PVN particles.

The term weakly basizity" or slightly alkaline as used herein is to be understood as comprising pH values in the range of about 7 to about 9, preferably between about 7 and about 8.5.

When neutralization is finished, the neutralized or slightly alkaline PVN solution is separated from the salts formed, i.e. generally from the alkali-nitrates or their solution, if an aqueous neutralizing solution was used. To this end, the solution is generally allowed to stand and the separation of the salts or the salt solution is awaited. The separation usually occurs by itself after a short time, the supernatant PVN solution has clarified and may be drawn off for further processing. If it ap pears advantageous, the PVN solution may be subjected to a post-sedimentation, e. g. by centrifugation.

From the beginning, such solutions are generally used which are good to process. For neutralization, a percent by weight solution of PVN in ethyl acetate is for example particularly suitable. The content of the solutions will vary depending upon the molecular weight or viscosity of the crude PVN. Too strongly diluted solutions represent volumes which are difficult to handle, while too concentrated solutions are very viscous and are difficult to neutralize.

For further processing of the solution, i.e. for the precipitation of the solutes, the solution may be diluted after neutralization.

Solutions which are more diluted than those described above generally produce purer products. After dilution, the solid content of the solution is advantageously assayed, e.g. by evaporating a sample until it is dry. Before precipitation, a stabilizer for the PVN may be added to the solution, preferably 2- nitrodiphenylamine; however, the process according to the present invention mainly consists in a stabilization by purifying the PVN.

The precipitating bath used consists, in the simplest case, of water. However, it has proved advantageous if the precipitating bath contains certain additives which increase the quality, i.e. in particular the stability, of the precipitating PVN. Also coming into question in particular are surface active agents, preferably such of anion-active nature, e.g. the sodium salt of disulfonated dodecyldiphenyloxide; electrolytes, e.g. sodium sulfate; pH regulators, preferably for adjusting or maintaining slightly basic conditions, e.g. sodium bicarbonate; protective colloids, e.g. bone glue, etc. If the PVN solution contains a solvent or solvent mixture which is partially miscible with water, it is recommended to saturate the precipitating bath with this solvent or mixture or to add so much to these liquids that saturation practically occurs at precipitation temperature. In this case, the boiling temperature of such a precipitating bath corresponds to the temperature at which the continuous precipitating process proceeds.

The precipitating bath is preferably strongly stirred at least during the introduction of the PVN solution, perhaps with the aid of a vibrator-mixer. In this way, a uniform and fine precipitation is obtained.

It has proved advantageous to strip ofi about one-fifth of the PVN solvent used per hour. After the end of the feed-in of the PVN solution, stripping is generally continued until the vapor temperature has risen by about in order to be sure that practically all solvent has been stripped off.

Further processing may be undertaken in any way. One generally proceeds in the following manner: The precipitating bath is allowed to come to room temperature, agitation being continued. Then the stirring device is switched 05. The precipitated, stabilized PVN settles in the reaction vessel. The supernatant liquid is drawn off and replaced by the same quantity of water. Thereupon, stirring is again continued for about 1 hour. Subsequently, the product is filtered off, washed and dried in vacuo.

A polyvinylnitrate purified in this way, produced from polyvinyl alcohol with a saponification number of 99 mole to which 0.5 percent 2-nitrodiphenylamine were added, is thermally stable and satisfies for example the Heat-Test according to US-MIL-Standard at 120 C. in 65 min., at l34.5 C. in 25 min. and the Abel-Test at C. in 40 min.

The nitrogen content of such a product, determined according to the Dumas method, gives 15.5 percent and corresponds to the theoretical value in which the saponification number of the starting product and the proportion of stabilizer are taken into account.

This product has an explosion heat of 909 cal/g and a combustion heat of 2,963 cal/g.

The process according to the present invention is further illustrated by the following non-limiting Examples. All parts are by weight unless otherwise stated.

EXAMPLE 1 20.4 parts of crude humid polyvinylnitrate, produced by the nitration (ester-ification) of a polyvinylalcohol with a saponification number of about 99 mole and an approximate molecular weight of 100,000 (Mowiol N -99 sold by Farbwerke Hoechst AG), subsequent dilution with water and filtration (water content about 50 percent by weight) were dissolved in 40.8 parts of ethyl acetate. (The crude product contains nitrating acid still included therein. It would therefore be senseless to determine the nitrogen content.)

The solution obtained was mixed, with stirring, with a 5 percent by weight aqueous solution of NaHCO until the CO evolution had stopped and the mixture had a pH-value of 7 to 7.5. To this end, 1.5 parts of the NaHCO solution were necessary. After the mixture had been allowed to stand for 20 minutes, two layers had formed. The upper organic layer was separated off. 1.0 parts thereof were dried in vacuo, and the residue amounted to 0.2 parts. The neutralized PVN solution was therefore still 20 percent by weight. By the addition of 50 parts of ethyl acetate, an about 10 percent by weight PVN solution was produced. 0.05 parts of Z-nitrodiphenylamine were added to this solution.

A four-necked, round-bottomed flask with distilling attachment and cooler, thermometer, dropping funnel and agitator is charged with parts of an aqueous solution, which contains 1 part of disodium dodecyldiphenyloxide disulfonate, 2.5 parts of sodium sulfate, 0.4 parts of sodium bicarbonate, 2 parts of bone glue and 5 parts of ethyl acetate. The agitator (vibrator-mixer) is set into action and the flask is heated until the contents thereof are boiling (about 71 C.). Immediately after the beginning of boiling, the above described PVN solution is made to drop in. Heating of the flask and speed of feed-in are regulated so that the liquid level in the flask remains practically constant. About 20 parts of an ethylacetate-water azeotrope with about 92 percent ethylacetate and 8 percent water distil per hour with a vapor temperature of about 71 C. The precipitation of PVN is to be observed in the flask. After 5 hours, the dropping in of the PVN solution is stopped and distillation is continued until the vapor temperature has risen to 100 C.; there are no more noteworthy amounts of ethyl acetate in the distillate. The contents of the flask are cooled, while being stirred, to 25 C., the agitator device is switched off and the precipitated PVN is allowed to settle. The supernatant liquid is drawn off and replaced by 100 parts of de-ionized water. Stirring is continued for 1 hour at room temperature, and the product is drawn off, washed with water and dried in vacuo. Yield: 9.4 parts of PVN (about 94 percent of the amount used).

EXAMPLE 2 26.0 parts of crude, humid polyvinylnitrate, produced by the nitration (esterification) of a polyvinylalcohol. with a saponification number of about 98 mole and an approximate molecular weight of 41,000 (Mowiol N 50-98" sold by Hoechst AG), subsequent dilution with water and filtration (water content about 40 percent), were dissolved in 36.4 parts of methylethylketone. The solution obtained was mixed, with stirring, in a 5 percent by weight aqueous NaHCO solution, until the CO, stopped forming and the mixture had a pH-value of 7. To this end, 2.5 parts of the NaHCO solution were necessary. After the mixture was left to stand for 2 minutes, two phases were formed. The upper organic phase was separated off. 2.0 parts thereof were dried in vacuo. The residue amounted to 0.6 parts. The neutralized PVN-solution EXAMPLE 4 Components, percent by weight W r 'lv, L, (l lVN AN ll'lN K. golr. (lnw. 5

35 50 1.5 2,785.3 245. l 240. l 42 l l8 2,701). 7 2 15.3 2401 60 30 10 2,773. l 245. 8 240.3

EXAMPLE 10 Components, percent by weight n, (ll. PVN AN lhN (lvw.

EXAMPLE 6 Components, percent by weight 1100, Stability 6| PVN PETN RDX AN HTN Russ Pb.St. 7L min/s it 120 C ;I. 't111.

10 l 1 0. 4'2 12. 5 240 l. 645 10 l 1 0. 52 12. 4 230' 1.614 10 1 1 U. 78 11.0 150 1. 551

parts of methylethylketone, a 15 percent by weight PVN solution was produced. 0.075 parts of 2-nitrodiphenylamine were added to this solution.

A four-necked roundbottomed flask with distillating attachment and cooler, thermometer, dropping funnel and agitator is charged with 100 parts of an aqueous solution which contains 1 part of disodium dodecyldiphenyloxydisulfonate, 2.5 parts of sodium sulfate, 0.4 parts of sodium bicarbonate, 2 parts of bone glue and 5 parts of methylethylketone. The agitator (vibrator-mixer) is set into action and the flask is heated until the contents thereof are boiling (about 74 C). immediately after the beginning of boiling, the above described PVN solution is made to drop in. Heating of the flask and rate of feed-in are controlled so that the liquid level in the flask remains practically constant. About parts of a methylethylketone-water azeotrope with about 88 percent MEK and 12 percent water are stripped per hour with a vapor temperature of about 74 C. The precipitation of PVN is to be observed in the flask. After 5 hours, the dropping in of the PVN-solution is stopped and distillation is continued until the vapor temperature has risen to 100 C.: there are no more noteworthy amounts of MEK in the distillate. The contents of the flask are cooled, while being stirred, to C., the agitator device is switched off and the precipitated PVN is allowed to settle. The supernatant liquid is drawn off and replaced by 100 parts of de-salted water. Stirring is continued for 1 hour at room temperature, and the product is drawn ofi, washed with water and dried in vacuo. Yield: 14.4 parts of PVN (about 96 percent of the amount used).

Theoretically, water, as outer phase, may be replaced by another liquid compound in which PVN is not soluble. Suitable solvents which form the inner phase with the dissolved PVN, should not or only partly be soluble in the outer phase. Their boiling point must be lower than that of the outer phase.

The following examples illustrate the use of the stabilized PVN, obtained according to the present invention, in fuels. The parts are by weight.

In the above examples of application the abbreviations signify:

PVN: Polyvinlynitrate, stabilized according to the invention What we claim is:

l. A process for preparing a stabilized polyvinylnitrate from a crude starting material, comprising the steps of a. providing a solution of raw acid containing polyvinylnitrate in an organic solvent,

b. neutralizing or rendering slightly alkaline said solution,

c. separating said solution from non-dissolved products of the neutralization, and

d. introducing said purified solution of step (c) into a hot aqueous precipitating bath in which said organic solvent is insoluble or only partially soluble, whereby said solvent is stripped and stabilized polyvinylnitrate is precipitated, and whereby the rate of introduction of said purified solution corresponds to the stripping rate of said solvent so that steady-state conditions are maintained, and whereby a solvent is selected in step (a) the pure boiling point of which or the azeotropic boiling point of which with the precipitating bath is below the boiling point of the pure precipitating bath.

2. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of an alkali metal hydroxide, bicarbonate or carbonate.

3. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of NaHCO,,.

4. A process as in claim 1 in which in step (a) a solvent is used selected from the group comprising an ester and a ketone at most partially miscible with water.

5. A process as in claim 1 in which in step (a) ethyl acetate is used as a solvent.

6. A process as in claim 1 further comprising the step of diluting the solutions of step (b) with the same solvent as used in step a) 7. A process as in claim 1 in which a solution of about 20 parts by weight of crude polyvinylnitrate in about 80 parts by weight of ethyl acetate is prepared in step (a), and further comprising the dilution of the neutralized solution of step (b) with ethyl acetate to a concentration of about 10 percent by weight of polyvinyl nitrate.

8. A process as in claim 1, further comprising the step of adding to said solution after step (c) 2-nitrodiphenylamine as a stabilizing agent.

9. A process as in claim 1 in which the precipitating bath of step (d) is selected from the group comprising pure water and an aqueous medium containing at least one additive of the group consistir ifoTelectrolytes, protection colloids and pH regulators.

10. A process as in claim 1 in which an aqueous precipitating bath is used in step (d) which has been previously saturated with the organic solvent of step (a) in the range between normal room temperature and the azeotropic boiling point.

11. A process as in claim 1, further comprising the step of vigorously agitating the precipitating bath of step ((1) during and after the precipitating period.

12. A process as in claim 1, further comprising the step of continuing stripping off the solvent in step (d) after the feed of said polyvinylnitrate solution has been stopped until said precipitating bath is substantially solvent free.

4 IF t

Claims

2. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of an alkali metal hydroxide, bicarbonate or carbonate.
3. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of NaHCO3.
4. A process as in claim 1 in which in step (a) a solvent is used selected from the group comprising an ester and a ketone at most partially miscible with water.
5. A process as in claim 1 in which in step (a) ethyl acetate is used as a solvent.
6. A process as in claim 1 further comprising the step of diluting the solutions of step (b) with the same solvent as used in step (a).
7. A process as in claim 1 in which a solution of about 20 parts by weight of crude polyvinylnitrate in about 80 parts by weight of ethyl acetate is prepared iN step (a), and further comprising the dilution of the neutralized solution of step (b) with ethyl acetate to a concentration of about 10 percent by weight of polyvinyl nitrate.
8. A process as in claim 1, further comprising the step of adding to said solution after step (c) 2-nitrodiphenylamine as a stabilizing agent.
9. A process as in claim 1 in which the precipitating bath of step (d) is selected from the group comprising pure water and an aqueous medium containing at least one additive of the group consisting of electrolytes, protection colloids and pH regulators.
10. A process as in claim 1 in which an aqueous precipitating bath is used in step (d) which has been previously saturated with the organic solvent of step (a) in the range between normal room temperature and the azeotropic boiling point.
11. A process as in claim 1, further comprising the step of vigorously agitating the precipitating bath of step (d) during and after the precipitating period.
12. A process as in claim 1, further comprising the step of continuing stripping off the solvent in step (d) after the feed of said polyvinylnitrate solution has been stopped until said precipitating bath is substantially solvent free.