US3384688A - Manufacture of detonating fuse cord - Google Patents

Description

United States Patent 3,384,688 MANUFACTURE OF DETONATING FUSE CORD Robert Stewart Gow, West Kilbride, and James Hay London McAuslan, Largs, Scotland, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Dec. 13, 1965, Ser. No. 513,534 Claims priority, application Great Britain, Dec. 30, 1964, 52,844/ 64 Claims. (Cl. 2643) ABSTRACT OF THE DISCLOSURE Detonating fuse-cord is manufactured by feeding a special grade of free-flowing granular PETN through a die, continuously forming a sheath around the stream of PETN and subjecting the resulting fuse-cord to a pressure sufiicient to consolidate the granules into a core. The special grade of granular PETN is made by treating fine crystalline particles of PETN with a binding agent and aggregating into granules.

Disclosure This invention relates to granular pentaerythritol tetranitrate (PETN) and its manufacture and to the manu facture of detonating fuse-cord having a core comprising such PETN. PETN finds wide application in explosive devices including, inter alia, detonating fuse-cord and detonators.

PETN detonating fuse-cord is a cord-like product consisting of a core of fine particulate PETN surrounded by a protective sheath. The sheath may comprise textile wrappings and water-resistant coatings or may be of thermoplastic material such as polyvinyl chloride or polyethylene. In one method for the manufacture of the fusecord, dry PETN in a free-flowing form is fed in a continuous manner through a die whilst the protective sheath is spun or extruded around it. The PETN is consolidated as a core inside the sheath. In order to achieve desirable detonating characteristics in the PETN, it is advantageous for the core material to be of an average particle size finer than is satisfactory for the free-running of the PETN through the die. Consequently it is common practice to prepare the PETN in crystals of sufficiently large particle size to flow freely through the die, to feed this material into the protective sheath, and afterwards to subject the filled sheath to a crushing pressure suflicient to break down the crystals to the desired finely divided state. The explosive performance of the fuse-cord is determined to some extent by the amount of crystal break-down which occurs in this crushing operation. This is especially relevant at low core charge when very finely powdered PETN is necessary for reliable propagation of detonation. Since this crushing operation is difiicult to control, it is difficult to control accurately the properties of the fuse-cord.

It is an object of this invention to provide a grade of PETN which is superior, for use in the manufacture of detonating fuse, to that hitherto used and to provide a method of manufacturing detonating fuse-cord having a core of fine PETN in which the feeding of very finely divided PETN and the breakdown of PETN crystals in the fine core is avoided.

We have found that the fineness of the PETN in the finished fuse-cord can be more easily controlled if the PETN is first prepared as a fine powder average particle size approximating to that desired in the fuse-cord, and this powder is aggregated by the use of a binding agent to form free-flowing granules which are fed through the die into the protective sheath. The binding agent can readily be chosen so that the aggregated granules may be handled in normal manufacturing operations without substantial break-down and the granules can readily be pressed within the protective sheath to a sufficient density for reliable propagation of detonation.

Thus the invention consists in free-flowing granular PETN in which fine PETN crystals are bound into aggregates by means of a binding agent. In preparing the granular PETN for fuse-cord, fine crystalline particles of the size desired in the finished product are treated with a binding agent and aggregated into granules by known granulating methods, such as rotating the moist PETN in a revolving drum. The granular PETN of the invention is more free-running than the crystalline PETN hitherto used in the manufacture of PETN detonating fuse-cord.

While the binding agent used to prepare the free-flowing granules may be any binding agent capable of aggregating powdered materials, we have found aqueous dispersions of polyvinyl acetate or solutions of polyvinyl alcohol or cellulosic adhesive to be especially convenient. With these materials the quantity of binding agent required is so small as to have no substantial effect on the explosive properties of the PETN. Polyvinyl acetate and polyvinyl alcohol enhance the cohesion of the crushed core material in the final fuse-cord and thereby prevent PETN spillage during manufacture and use.

In accordance with a further aspect of the invention a detonating fuse-cord comprises a consolidated core of the granular PETN of the invention.

From a further aspect the invention consists in a method for the manufacture of'detonating fuse-cord comprising continuously feeding a stream of the free-flowing granular PETN of the invention through a die, continuously forming a protective sheath around said PETN stream to form a fuse-cord having a core of the granules, and subjecting the fuse-cord to a pressure sutficient to consolidate the granules into a core which propagates detonation. The consolidating pressure need not be such as to effect break-down of the granules to the original crystalline particles, but higher pressures will give beneficially higher packed densities in the core and will result in more crushing of the granules.

The fuse-cord of the invention may be varied in properties as desired by variation of the particle size of the PETN from which the aggregated granules are prepared. It is possible to prepare the granules from PETN of finer particle size than can normally be obtained by crushing larger size crystals and consequently fuse-cord may be prepared by the method of the invention which has a core of finer crystal size than core material of fuse-cord prepared by the hitherto employed manufacturing method. Thus fuse-cord may be prepared which has a higher velocity of detonation for any given loading density or the same velocity of detonation at lower loading density than the fuse-cord previously available. This fuse-cord with very fine core materials also has enhanced sensitivity to side initiation and enhanced ability to propagate detona tion. It is possible, therefore, to prepare satisfactory fusecord of lower loading density and lower core diameter and consequently of lower velocity of detonation than previous PETN detonating fuse-cord.

In putting the method of the invention into practice it is convenient to feed the newly formed fuse-cord through a crushing die or crushing rolls in a continuous manner to effect the break-down and consolidation of the granules. The protective sheath may consist of one or more layers of material but it will be apparent that it is only necessary to apply one layer of protective material over the PETN core before subjecting the fuse-cord to the consolidating pressure. Subsequent layers may be added as desired. For example, it is sufiicient to apply only a single 3 layer of paper around the PETN core before the crushing operation and to apply outer textile and water-proofing layers afterwards.

The method is further illustrated by the following examples in which all parts and percentages are by weight.

Example 1 9 parts (dry weight) of water-wet PETN were washed with parts of a 3% aqueous dispersion of polyvinyl acetate (Mowilith D-registered trademark) and the PETN filtered in a vacuum filter. The wet PETN (10% water content) was loaded into a cylindrical polyethylene drum, 10 inches diameter and 18 inches long, and containing battles. The drum was rotated around its axis at r.p.rn. for minutes to aggregate the PETN into granules. The granulated PETN was then dried at 70 C. for 5 hours and sieved through a BS. 16 mesh sieve which retained about 5% of the PETN as oversize lumps which were not easily broken down. The properties of the PETN before and after granulation were as shown in Table 1.

The granulated PETN powder prepared in this example was fed in a continuous vertical stream through a die of diameter 0.110 inch into a paper tube as the tube was being formed continuously by the folding of 21 wide tape. The paper-wrapped column thus formed was passed through a spinning plate containing 10 spools of 6 lea jute, which were wrapped round the column at the rate of 8 turns per inch, and then pulled through a crushing die of internal diameter 0.106 inch. A further coating of cotton textiles was applied in the opposite hand to the jutes by passing through a spinning plate having 8 spools, each having 6 ends of 30 count cotton, revolving at the rate of 16 turns per linear foot of fuse. In a separate subsequent operation this partly finished fuse-cord was covered with a 0.010 inch thick coating of polyethylene by means of an extruder. The finished fuse-cord had a core loading of 10 grains (0.65 gram) of PETN per linear foot and detonated at a velocity of 4,500 metres/ sec.

Example 2 TABLE 2 Before After granulation granulation Bulk density (g./ec.) 0. 64 0. 62 Sieve analysis, percent pas Example 3 7 parts (dry weight) of water-wet PETN were washed with a 5% aqueous solution of sodium carboxymethyl cellulose (viscosity 35 centipoises) and the PETN filtered on a vacuum filter. The water-wet treated PETN (17% water content) was granulated, dried and sieved through a BS.

16 mesh sieve as described in Example 1, except that in the granulation the drum was rotated for 45 minutes. The amount of material in the form of hard lumps failing to pass through the 13.8. 16 mesh sieve was 510% of the original weight of PETN.

The properties of the PETN before and after granulation were as shown in Table 3.

TABLE 3 Before After granulation granulation Bulk density 0. 51 0.50 Sieve analysis, percent passing- Example 4 granulation are shown in Table 4.

TABLE 4 Before Alter granulation granulation Bulk density (g./ec.) O. 64 0.67 Sieve analysis, percent passing- 14135.8 100 100 36 13.5 S 99.8 98.6 5213.8 8.... 99.7 95.6 72 B.S.S. 06.2 72.0 100 B.S. 85.1 38.2 B.S.S 21.4 7.0

What we claim is:

1. Free-flowing PETN aggregates formed of PETN crystals of a fineness such that by themselves they are not readily free-flowing, said crystals being bound in aggregate form by means of a binder which has sufficient strength to permit handling of the aggregates during normal manufacturing operations without substantial breakdown, said binder rendering said aggregates capable of break-down into smaller particles and capable of consolidation under the' action of crushing forces subsequently applied to a mass of the aggregates.

2. PETN as claimed in claim 1 wherein the binder comprises polyvinyl acetate, polyvinyl alcohol or cellulosic adhesive.

3. PETN as claimed in claim 2 wherein the binder comprises water-soluble cellulose ether.

4. A detonating fuse-cord comprising a consolidated core of granular PETN as claimed in claim 1.

5. A method of manufacturing detonating fuse-cord comprising continuously feeding a stream of free-flowing aggregates of PETN as claimed in claim 1 through a die, continuously forming a protective sheath around said PETN stream to form a fuse-cord having a core of said aggregates, and subjecting the fuse-cord to a pressure sufiicient to break-down the aggregates into smaller particles of a size at least as large as the size of the fine crystals of which the aggregates were formed, whereby breakdown of the PETN crystals themselves is not required.

6. A process for the preparation of PETN as claimed in claim 1 wherein the fine crystalline particles are treated with the binder and aggregated into granules.

7. A process as claimed in claim 6 wherein the binder is applied to the PETN particles in the form of an aqueous dispersion or solution.

of paper is applied around the PETN core before subjecting the core to a consolidating pressure.

References Cited UNITED STATES PATENTS Gow 149-11 Brirnley et a1. 2643 Billard 149-93 X Schulz 149-11 X Paul 14993 X BENJAMIN R. PADGETT, Primary Examiner.