GB2074995A

GB2074995A - Stabilization of Red Phosphorus

Info

Publication number: GB2074995A
Application number: GB8109673A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-03-28
Filing date: 1981-03-27
Publication date: 1981-11-11
Also published as: GB2074995B

Abstract

A method of protecting particulate red phosphorus used in smoke-producing compositions against atmospheric oxidation and hydrolysis comprises coating the phosphorus with a surface layer of a rigid thermoplastic material, preferably polyvinyl butyral. The coating is applied by mixing red phosphorus with the thermoplastic material in the presence of a solvent, eg dichloromethane, which softens or dissolves the thermoplastic, or by contacting the phosphorus with a solution or gel of the thermoplastic material, and evaporating the solvent. The resultant composition may also include an anti-oxidant added before or during the coating procedure. In addition to the protection granted by the coating, the rigidity of the thermoplastic layer allows the coated phosphorus material to be used as a filling in eg artillery shells and/or mortar bombs with a reduced risk of "set back" of the shell filling during firing from a gun.

Description

SPECIFICATION Stabilisation of Red Phosphorus This invention relates to coatings for the protection of particulate red phosphorus against oxidation and hydrolysis due to atmospheric exposure, and to methods of applying such coatings.

Amorphous red phosphorus is one of the best smoke producing materials known, as it forms copious clouds of phosphoric acids on combustion and subsequent hydrolysis. Red phosphorus is however very prone to gradual oxidation in air, and if allowed to remain in contact with moist air will become moist and eventually semi-liquid with the formation of phosphoric acids and phosphine. This problem is accentuated by the use of red phosphorus in a particulate form, when a large surface area is exposed to the atmosphere, and is catalyzed by the presence of metal ions, especially copper.

UK Patent No 1,458,194 describes a method of stabilising red phosphorus against atmospheric oxidation and hydrolysis by coating the individual grains of particulate amorphous phosphorus with an elastomer. It has been found however that although phosphorus coated with elastomers which are soft and resilient is suitable for use in smoke flares and grenades, phosphorus coated in this way is unsuitable for use in artillery shells and mortar bombs, since the high acceleration forces on firing leads to set back of the shell filling and resultant instability in flight.

According to the invention there is provided a method for protecting particulate red phosphorus against atmospheric oxidation and hydrolysis comprising coating the phosphorus with a surface layer of a rigid thermoplastic material. The coating of rigid thermoplastic material serves both to protect the phosphorus from the action of the atmosphere and to bind the phosphorus together, thus conferring rigidity on red phosphorus when used as shell filling. The invention also includes compositions containing red phosphorus coated with such a thermoplastic material.

The thermoplastic material is preferably applied to the phosphorus by mixing the phosphorus and the themoplastic material in the presence of a solvent which will dissolve or soften the thermoplastic material, allowing the thermoplastic material to dissolve or soften in the presence of the solvent, and subsequently removing the solvent by evaporation. Other variants of this method will be apparent. For example, the thermoplastic material may be formed into a solution or gel with a solvent and then mixed with the phosphorus, the solvent being evaporated as before.

Suitable solvents for use in such a method should not dissolve or react in any way with red phosphorus. In addition suitable solvents should preferably be capable of dissolving or gelling the thermoplastic material at ambient temperatures, should be sufficiently volatile to allow speedy evaporation after application to the phosphorus but not so volatile that it is difficult to maintain the mixed phosphorus and solution or gel in a plastic condition prior to any necessary moulding, and should desirably be non flammable and of low toxicity. Suitable solvents include chlorinated hydrocarbons having a boiling point within the range 300 to 1 00C (especially 30 50 C or 90o,1 100C). Preferred solvents include trichloroethylene, trans-dichloroethylene and especially methylene dichloride.Such solvents may be used singly or as mixtures.

If the treated phosphorous is to be subsequently moulded into biscuits or pellets suitable for use in a shell or mortar bomb for example, then it is advantageous not to evaporate the solvents completely, as complete evaporation has been found to lead to the formation of weak, crumbly moulded products. If a small amount of residual solvent is left in the treated phosphorus prior to moulding, this will assist moulding and lead to firm biscuits or pellets which may be handled without fear of breakage. Excessive quantities of residual solvent tend to lead to mould sticking, and acceptable upper and lower solvent limits should be determined experimentally for particular applications, compositions, and mould pressures.

The thermoplastic material may be chosen from any such materials which will not react with or be affected by either the phosphorus or the surrounding atmosphere and should be sufficiently rigid to withstand the stresses of firing a shell or mortar bomb. The thermoplastic material should preferably be soluble in or softened by a suitable readily available solvent. A particularly preferred thermoplastic material is polyvinyl butyral, which is soluble in methylene dichloride, but other commonly available materials, for example polystyrene, polycarbonate, polyvinyl acetate, acrylic polymers especially polymethyl methacrylate, acrylonitrile/butadiene/styrene copolymers or silicone resins may be suitable.

The minimum amount of thermoplastic material required for a useful degree of protection and binding is that which substantially covers the surface of the phosphorus. It will be appreciated that this minimum amount will depend upon the surface area of the phosphorus and hence the particle size. It has been found that the inclusion of about 5% by weight of a thermoplastic material may markedly reduce oxidation reactions, increase stability and provide adequate binding.

There is no theoretical upper limit to the proportion of thermoplastic material which may be added to the phosphorus. This limit may be determined in practice by its particle size, the time of storage, the economics of the process and the use to which the coated phosphorus is to be put, for example larger shells or higher muzzle accelerations will require a greater degree of binding as a result of the higher shear stresses imposed upon the shell filling. As the thermoplastic material makes no significant contribution to the amount of smoke produced on combustion, the advantages of better binding and protection must be weighed against the disadvantages of incorporating a larger quantity of effectively inert material into a shell or mortar bomb charge in any particular application of the treated phosphorus.

In a further embodiment of the invention additional anti-oxidant materials are incorporated into the mixture of the phosphorus and thermoplastic material. These anti-oxidants may either be incorporated into the thermoplastic material or the phosphorus prior to mixing, or may be incorporated during the mixing operation.

Suitable anti-oxidants are most conveniently handled in solution form, and may thus either be mixed into a solution of the thermoplastic material or applied in a volatile solvent to the phosphorus prior to mixing with the thermoplastic material. Suitable antioxidants include NN'-di-(1ethyl-3 methylpentyl)-p-phenylenediamine and NN'-di-beta-naphthyl-p-phenylenediamine.

Since traces of metal, especially copper, greatly accelerate the oxidation of red phosphorus by moist air, the coating at red phosphorus should preferably be carried out with copper-free apparatus, for example stainless steel. Brass sieves should be especially avoided.

The invention will now be illustrated by way of example only by reference to the coating of red phosphorus with polyvinyl butyral in the ratio 95:5 by weight and to the use of such coated material as smoke fillings in shells, mortar bombs and the like.

1900 g of oiled amorphorus red phosphorus and 100 g of powdered polyvinyl butyral were dry blended for 2 minutes in a stainless steel bowl of a food mixing machine equipped with a "dough hook" mixing blade. 750 ml of methylene dichloride were added and mixing was continued under a stream of air from a compressed air line for a further 20 minutes. During this period this polyvinyl butyral was swolien and softened by the methylene dichloride and the composition changed from a stiff paste to a lumpy powder as the solvent content was reduced by evaporation in the air stream from 33% to 14% by weight. The composition was then hand rubbed through a stainless steel sieve having 8 meshes to the linear inch and stored in airtight tins.

20 g portions of this sieved composition were weighed into a suitable mould and pressed at 500 psi to form circular biscuits 24" (7 cm) in diameter and 1/8n (3.2 mm) thick, each biscuit having a central hole and eight radial channels. It was found that the quality of the moulded biscuits was sensitive to the solvent content of the sieved composition, a low solvent content giving a weak, crumbly biscuit, too high a solvent content causing mould sticking. Acceptable upper and lower limits for solvent content were found to be 2.5% and 6.5% by weight respectively. The moulded biscuits were stacked in an oven at 800C in batches of 10 surmounted by a 2 kg weight, left for 20 minutes to remove residual solvent and then stored in airtight tins.

The dried biscuits could be stacked in an artillery shell or mortar bomb, so providing a filling of sufficient rigidity to withstand the setback forces on firing. The filling could be base or nose ejected, ignited, fragmented and distributed over the ground by an ejection charge, and burned for some time.

Samples of phosphorus treated and dried as above have been exposed to air for 1 year without any visual change in appearance, and on analysis, only 1% by weight of the phosphorus had been oxidised.

Claims

1. A method for protecting particulate red phosphorus against atmospheric oxidation and hydrolysis comprising coating the phosphorus with a surface layer of rigid thermoplastic material.

2. A method according to claim 1 wherein the coating is applied by mixing the phosphorus and the thermoplastic material in the presence of a solvent which dissolves or softens the thermoplastic material, allowing the solvent to dissolve or soften the thermoplastic material and removing the solvent by evaporation.

3. A method according to claim 1 wherein the coating is applied by contacting the phosphorus with a solution or gel of the thermoplastic material in a suitable solvent and removing the solvent from the solution or gel by evaporation.

4. A method according to claim 2 or claim 3 wherein the evaporation is performed to leave a sufficient quantity of residual solvent to facilitate moulding of the coated phosphorus into pellets or biscuits.

5. A method according to any one of claims 2 to 4 wherein the solvent is one or more halogenated hydrocarbons.

6. A method according to claim 5 wherein the or each halogenated hydrocarbon has a boiling point in the range 300--1 0 C.

7. A method according to claim 6 wherein the halogenated hydrocarbon is trichloroethylene, trans-dichloroethylene or dichloromethane.

8. A method according to any preceding claim wherein the thermoplastic-coated phosphorus further comprises an anti-oxidant material.

9. A method according to claim 8 wherein the anti-oxidant is incorporated in the thermoplastic material or the particulate red phosphorus before coating.

10. A method according to claim 8 wherein the anti-oxidant is incorporated during the coating operation.

11. A method according to any preceding claim wherein the rigid thermoplastic material is a polyvinyl butyral, polystyrene, polycarbonate, polyvinylacetate, an acrylic polymer, polymethylmethacrylate, an ABS copolymer or a silicone resin.

1 2. A method according to claim 11 wherein the rigid thermoplastic material is polyvinyl butyral.

13. A method of protecting particulate red phorphorus substantially as herein before described with reference to the Example.

14. Particulate red phosphorus whenever coated by a method according to any one of claims 2 to 12.

1 5. A smoke producing composition comprising particulate red phosphorus coated with a surface layer of a rigid thermoplastic material.

1 6. A smoke producing composition according claim 1 5 wherein the thermoplastic material comprises 5% by weight of the composition.

1 7. A smoke producing composition according to claim 15 or claim 16 wherein the rigid thermoplastic material is a polyvinyl butyral, polystyrene, polycarbonate, polyvinyl acetate, an acrylic polymer, polymethylmethacrylate, an ABS copolymer or a silicone resin.

1 8. A smoke producing composition according to claim 1 7 wherein the rigid thermoplastic material is polyvinyl butyral.

1 9. A smoke producing composition according to any one of claims 1 5 to 1 8 and further comprising an anti-oxidant.

20. A smoke producing composition according to claim 19 wherein the anti-oxidant is NN'-di-(1 - ethyl-3 methyl pentyl)-p-phenylenediamine or NN'-di-P-napthyl-p-phenylenediamine.

21. A smoke producing composition substantially as herein before described with reference to the Example.

22. An artillery shell or mortar bomb including a smoke producing composition according to any one of claims 15 to 21.