Jan. 4, 1966 w. T, JONES ETAL 3,227,588 CRYSTALLINE EXPLOSIVES IN A VISCO~ELASTIC BINDER OF SHEET FORM

Filed March 11, 1964

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United States Patent 0 1

3,227,588 CRYSTALLINE EXPLOSIVES IN A VISCO

ELASTIC BINDER 0F SHEET FORM Walter Thomas Jones, Petts Wood, Kent, and John Wilby,

Eltham, London, England, assignors to Her Britannic Majestys Principal Secretary of State for the War Department, London, England

Filed Mar. 11, 1964, Ser. No. 351,138 Claims priority, application Great Britain, Mar. 14, 1963,

10,115/63 15 Claims. (Cl. 14918)

The present invention relates to explosive compositions that are particularly useful for the production of explosive sheets, which are used, for example, to harden metal surfaces and to form, shape or demolish metal articles.

Explosive compositions in accordance with the present invention provide explosive sheets that have su?icient mechanical strength to retain an imposed shape without support and su?icient ?exibility to be bent through acute angles without materially cracking or breaking. Further more, these explosive sheets do not require coating either to provide strength or to remove surface stickiness.

In accordance with the present invention, an explosive composition that is suitable for the production of explosive sheets comprises between 85% and 90% by weight of a crystalline high explosive, such as R.D.X. (cyclo trimethyl ene trinitramine), H.M.X. (cyclo tetramethylene tetrani tramine), P.E.T.N. (pentaerythritol tetranitrate) or tetryl (2,4,6-trinitrophenylmethylnitramine), in a ?ne particu late form with a bimodal particle size distribution bound

7 in a visco-elastic binder of which between about 3 and 25% by weight is a highly ?uorinated polymeric halo carbon. The term highly ?uorinated throughout the speci?cation and claims includes fully ?uorinated. If there is less than 85% by weight of the explosive there is van increasing chance (particularly for thin sheets) that the sheet will not propagate a detonation. If there is more than 90% of explosive the sheet is brittle and lacks mechanical strength. The proportion of explosive is preferably between 87% and 89% by weight. To produce an explosive composition containing these

high proportions of explosive powder and yet having ' suitable rheological properties, the explosive must be I present as a ?ne powder (i.e. at least 95% of the particles are less than 100 microns diameter) which has a bimodal particle size distribution so that a high degree of particle packing can be achieved with separation of the particles in the binder matrix. The measured particle size distribu tion of a suitable explosive powder showed one peak between 20 and 25 microns and another between 8 and 10 microns while practically all of the powder had a particle size of less than about 50 microns. The visco-elastic binder used in the explosive composi- .

1tion is a Bingham material whose response to stress conforms to the superposition of elements which obey Hookes elastic law and elements which obey Newtons viscosity law. Visco-elastic substances which are prefer ably used as the main constituent (i.e. comprising more than 50% by weight of the viscoelastic binder) are aliphatic hydrocarbon polymers, such as polyisobutylene or polypropylene, having a substantially linear polymer

chain with lower alkyl side-groups. Polyisobutylene is a satisfactory binder when its molecular weight is at least of the order of 5,000 (when measured by S'taudingers viscosity method); too low a molecular weight results in a sticky explosive composition that lacks mechanical strength. When the binder has a relatively high molecular weight it may be too viscous for ready incorporation with the explosive even in a water slurry, in which case the viscosity may be reduced by blending with a suitable amount of either a lower molecular weight polymer or a

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3,227,588 Patented Jan. 4, 1906

2 plasticiser. For example, it has been found that poly isobutylene with a molecular weight of about 15,000 (Staudinger) requires the addition of about 20% of a plasticiser before it can be readily incorporated with the explosive.

Suitable plasticisers are aliphatic esters containing at least 15 carbon atoms and the esters of dibasic acids, such as sebacic acid and phthalic acid,.are particularly suitable. Examples of suitable plasticisers include ethyl oleate, dibutyl phthalate and di(2-ethylhexyl) sebacate. The presence of extremely ?ne particles (i.e. particles

of less than 1 micron in diameter) of a polymeric highly ?uorinated halocarbon in the binder of the explosive com position, confers the ability on explosive sheets made of the composition to remain in contact with a surface with out having excessive surface stickiness and generally en hances their mechanical and ?exing properties. In par ticular, the resistance to tearing is increased and the chances of cracking during ?exing are reduced.

Suitable ?uoro-poly-mers are aliphatic polymeric ?uoro carbons that do not contain any hydrogen but may contain another halogen, for example, chlorine and which are made by a suspension process so that they can separate in extremely ?ne particles in the composition. Such com pounds are exempli?ed by polytetra?uoroethylene, poly hexa?uoropropylene and polytri?uorochloroethylene. The preferred ?uoropolymer is polytetra?uoroethylene

(P.T.F.E.), which is available commercially (for example, grade CD3 supplied by Messrs. I.C.I. Ltd.) as a powder consisting of aggregates (about 500 to 600 microns in diameter) composed of individual particles, each about 0.1 micron diameter, so that the aggregates can be broken down in the ?nal manufactured explosive composition.

If the ?uoropolymer is present in only a small propor tion (at least 3%) of the visco-elastic binder there is some improvement in the properties of the explosive composition but proportions between 5 and 20% by weight, and preferably between 7 and 15% by weight must be added to obtain the best-results.

There is no overall improvement in the properties of the composition as the proportion of ?uoropolymer is increased beyond 20% by Weight of the binder, and as the mixing and rolling of the composition in manufacture

_ becomes increasingly ditlicult as the proportion increases beyond 20%, the proportion should not exceed 25%.

Furthermore, the ?uoropolymer sti?'ens the composi tion so allowance should be made for this when deter mining the viscosity of the binder, especially for additions of more than 10% say. A suitable overall viscosity value for the binder with the fluoropolymelr added is between 40,000 and 200,000 poises determined 'by the extrusion method using Mooneys equation at a rate of shear of 5sec._1 at25 C. - V '

Crystalline high explosives are not produced in the form of ?ne particles as required for the explosive com position and any treatment such as crushing the neat high explosive to produce ?ne particles is preferably avoided. However, in accordance with a feature of the invention, an explosive composition as hereinbefore de scribed may be readily manufactured by forming'a mixed composition of the constituents in the required propor tion and then repeatedly passing the mixture through differential rollers having a separation which is less than 0.01 inch, and is preferably about 0.002 to 0.003 inch.

In accordance with the invention, therefore, a process for the manufacture of an explosive composition com~ prises intermixing a composition containing 85 to 90%

_ by weight of a crystalline high explosive in a visco-elastic

70 binder containing between 3 and 25% by weight of a highly ?uorinated polymeric halocarbon as extremely ?ne particles made by a suspension process, and repeatedly

3,227,588 =3

passing the mixed composition through differential rollers having a separation less than 0.01 inch, whereby the high explosive is ground into ?ne particles having a bimodal size distribution and the polymeric halocarbon is dis persed in the binder.

It is generally impractical, especially in large scale processes, to incorporate the explosive with the binder after the ?uoropolymer has been added, and the explosive is then incorporated with the plasticized binder before the ?uoropolymer is added. In practice a Water slurry of the high explosive is intermixed with the binder which is then drained before the addition of the ?uoropolymer. The differential rollers in practice should be knurled

as otherwise it is di?icult to pass the moist composition through the narrow gap. The number of passes required to grind the explosive particles to the required size is reduced by decreasing the gap between the rollers and by increasing the difference between the peripheral speeds of the rollers which causes shearing of the particles. Between 10 and 30 passes should be su?cient if a gap of about 0.002 to 0.003 inch is used with a differential peripheral speed of the rollers of the order of three feet/sec. This differential rolling also breaks down the aggregates of ?uoropolymer particles and disperses the particles through the composition.

The explosive composition is then dried and can be formed into an explosive sheet simply by a ?nal rolling between rollers separated by the desired thickness of the sheet which may in practice be down to about 0.08 inch.

Explosive compositions in accordance with the inven tion are essentially as follows:

Percent High explosive __________________________ _._ 85-90 Viscose-elastic binder _____________________ __ 9.5l2 Fluoropolymer __________________________ __ 0.5-3

By way of example, a typical general explosive com position is:

Percent High explosive _________________________ __ 87-89 Polyisobutylene+plasticiser ______________ __ 10.5-11 P.T.F.E. ______________________________ __ 11.5

The high explosive may be R.D.X., I~LM.X., P.E.T.N. or tetryl. A speci?c composition A which falls within this gen

eral composition is as follows: Percent

R.D.X. __________________________________ __ 88.0

Polyisobutylene (Oppanol B15) ______________ __ 8.4 Di(2-ethylhexyl) sebacate ___________________ __ 2.4 P.T.F.E. _________________________________ __ 1.2

An alternative composition B using a less viscous grade of Polyisobutylene is as follows:

Percent R.D.X. __________________________________ __ 88.0

Polyisobutylene (Oppanol B8) _______________ __ 9.6 Di(2-ethylhexyl) sebacate ___________________ __ 1.2 P.T.F.E. _________________________________ __ 1.2

All the above percentage proportions are by weight. By way of example, the manufacture of an explosive

composition having the speci?c composition A and the production of explosive sheets therefrom will now be described. The complete manufacture is illustrated by the accompanying diagrammatic drawing.

Polyisobutylene (70 parts by weight) is stirred and heated in an incorporator to a temperature between about 90 and 100 C. Di(2-ethylhexyl) sebacate (20 parts by weight) is added and the mixture stirred until a mix of uniform composition is obtained. A slurry of R.D.X. (88 lb.) and water (500 lb.) is heated to a temperature of about 95 C. and stirred at 200 r.p.m. in a mixing vessel. The plasticised polyisobutylene (10.8 lb.) is

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23. added and stirring continued for one hour, at the end of which time the particles of R.D.X. are uniformly coated. The suspension is ?itered through a oambric ?lter to remove excess Water and the ?ltered material con taining about 20% of water is thoroughly mixed with P.T.F.E. (1.2 lb.) in a Werner P?eiderer incorporator in the cold for one hour. The explosive composition is then passed repeatedly through a di?erential rolling ma chine having knurled rollers rotating at different periph eral speeds so that there is a pronounced shearing action on the composition as it .passes between the nip between the rollers. A suitable machine is a Torrance drum feed roll set

to a minimum separation between its knurled rollers of 0.003 inch and having a 12 in. diameter roll rotating at about 15 r.p.m. and a 15 in. diameter roll rotating at about 60 r.p.m. The peripheral speeds of the two rolls are thus about 50 and 250 ft./min. giving a differential speed of about 200 ft./min. Under these conditions, the composition is repeatedly passed through the rolls by the circulating drum feed for about ?ve minutes, this being theoretically equivalent to about 25 passes. The explosive composition is then ?nally dried in any

suitable drier. A Werner P?eiderer incorporator may be used, for example, treatment lasting for about four hours at 95 C. or the composition can be left as thin sheets and simply dried in an oven. As the binder composition is made more viscous by

the addition of the P.T.F.E. it is easier to incorporate the R.D.X. before mixing in the P.T.F.E., in the way already described. However, on a smaller scale (about 2 kg.) incorporation of R.D.X. with the complete visco-elastic binder composition containing the P.T.F.E. is practicable. The differential rolling machine may then be a Torrance Laboratory Roll through which the explosive composi tion is given ten passes with a minimum separation of about 0.003 inch. The explosive sheets are made by passing the dried

explosive composition through a rolling machine, having two smooth steel rollers of the same diameter, rotating at identical speeds, with the gap set to give the desired sheet thickness down to about 0.08 inch.

Vie claim: 1. A flexible explosive sheet composition which com

prises between 85 and 90% by weight of a crystalline high explosive in a ?ne particulate form with a bimodal particle size distribution which is bound in a visco-elastic binder of which between 3 and 25% by weight consists of extremely ?ne solid particles of a highly ?uorinated polymeric halocarbon having a diameter less than 1 micron through said binder.

2. A ?exible explosive sheet composition according to claim 1 in which said crystalline high explosive is a ni trarnine explosive.

3. A ?exible explosive sheet composition according to claim 1 in which the said polymeric halocarbon is poly tetra?uoroethylene.

d. A ?exible explosive sheet composition according to claim 3 in which the polytetra?uoroethylene is present in a proportion between 5 and 20% by Weight of the visco elastic binder.

5. A ?exible explosive sheet composition according to claim 4 in which the polytetra?uoroethylene is present as particles of the order of 0.1 micron diameter in a propor tion between 7 and 15% by weight of the visco-elastic binder.

6. A flexible explosive sheet composition according to claim 1 in which the high explosive is cyclo trimethylene trinitramine.

7. A flexible explosive sheet composition according to claim 1 in which the high explosive is cyclo tetramethylene tetranitramine.

8. A ?exible explosive sheet composition according to claim 1 in which the high explosive is pentaerythritol tetra nitrate.

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9. A ?exible explosive sheet composition according to claim 1 in which the high explosive is 2,4,6-trinitrophenyl methylnitramine.

10. A ?exible explosive sheet composition which com prises between 85 and 90% by weight of a crystalline high explosive in a ?ne particulate form having a bimodal particle size distribution which is bound in a visco-elastic binder comprising at least 50% by weight of an aliphatic hydrocarbon polymer having lower alkyl side chains, be tween 3 and 25% by weight of extremely ?ne solid par ticles of a highly ?uorinated polymeric halocarbon having a diameter less than 1 micron, and a plasticizer for the said hydrocarbon polymer.

11. A ?exible explosive sheet composition according to claim 10 in which the said hydrocarbon polymer is poly isobutylene.

12. A ?exible explosive sheet composition which com prises between 85 and 90% by weight of a crystalline high explosive in a ?ne particulate form having a bimodal particle size distribution which is bound in a visco-elastic binder comprising at least 50% by weight of polyiso butylene, between 3 and 25% by weight of extremely ?ne solid particles of a highly ?uorinated polymeric halo carbon having a diameter less than 1 micron, and a plas ticizer for the said polyisobutylene which is an ester hav ing at least 15 carbon atoms.

13. A ?exible explosive sheet composition which com prises between 87 and 89% by weight of a crystalline high explosive in a ?ne particulate form with a bimodal particle size distribution which is bound in a visco-elastic binder comprising polyisobutylene and a plasticizer for said polyisobutylene and polytetra?uoroethylene in the form of extremely ?ne solid particles having a diameter

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8 less than 1 micron, said polyisobutylene and plasticizer therefor constituting between 10.5 and 11% by weight of the composition and said polytetra?uoroethylene con stituting 1 to 1.5% by weight of the composition.

14. In a process for the manufacture of an explosive sheet composition the steps of intermixing a composition containing 85 to 90% by weight of a crystalline high ex plosive in a visco-elastic binder containing between 3 and 25 % by weight of a highly ?uorinated polymeric halo carbon as extremely ?ne particles made by a suspension process, and repeatedly passing the mixed composition through differential rollers having a separation less than 0.01 inch, whereby the high explosive is ground into ?ne particles having a bimodal size distribution and the halo carbon is dispersed throughout binder in the form of solid particles having a diameter less than 1 micron.

15. A process for the manufacture of an explosive sheet which comprises the process according to claim 14 followed by a ?nal rolling of the composition between rollers separated by the desired thickness of the explosive sheet.

References Cited by the Examiner UNITED STATES PATENTS

2,999,743 9/1961 Breza et a1. ________ __ 14919 3,102,833 9/1963 Schulz __________ __ 14919 X. 3,117,044 1/1964 Sauer _____________ __ 149-49 3,138,501 6/1964 Wright __________ __ 14919 X 3,173,817 3/1965 Wright __________ __ 149-19 X

CARL D. QUARFORTH, Primary Examiner. BENJAMIN R. PADGETT, Examiner.