An improved plastic-bonded explosive composition which includes from 2 wt. % up to 30 wt. % of a nitrocellulose binder and comprises incorporating into the composition during preparation from about 0.0025 wt. % up to a value less than 2 wt. % of fibrillatable polytetrafluoroethylene (PTFE) and mixing the composition thoroughly and with sufficient shearing action whereby the PTFE fibrillates and becomes substantially uniformly distributed throughout the finished composition.

BACKGROUND OF THE INVENTION 
The present invention relates to plastic-bonded explosive (PBX) 
compositions, and, more particularly, to an improvement in such PBX 
compositions which comprises incorporating therein from about 0.0025 wt. % 
up to a value of less than 2 wt. % of fibrillated polytetrafluoroethylene 
(PTFE) whereby the coherency of the resulting composition is enhanced, and 
the resulting formulation is extrudable and formable into desired shapes, 
such as, for example detonating cords. The present invention is 
particularly useful in improving the extrudability and formability of PBX 
formulations in which the nitrocellulose component is a non-dynamite 
grade, i.e., low-viscosity grade, nitrocellulose. The present invention 
also relates to a process for improving the tensile strength and the 
elongation properties of such PBX compositions in which a grade of 
nitrocellulose other than dynamite grade nitrocellulose is employed as a 
binding agent which comprises incorporating into the composition from 
about 0.0025 wt. % up to a value less than 2 wt. % of fibrillatable PTFE, 
and then mixing the composition with sufficient shearing action to 
fibrillate the PTFE and distribute it uniformly throughout the finished 
composition. 
Nitrocellulose of a "high" viscosity is normally required when forming PBX 
compositions, as described, for example, in U.S. Pat. Nos. 2,992,089; 
3,317,361; 3,400,025; and 3,943,017. Such "high" viscosity nitrocellulose 
is commonly referred to as "dynamite grade nitrocellulose" or "blasting 
soluble nitrocellulose" in contrast to industrial nitrocellulose grades 
which are inherently weaker because of a lower relative tensile strength 
and bonding strength. The coherency of PBX compositions, i.e., 
formulations, which are based on a non-dynamite grade nitrocellulose, 
makes them generally not formable into useful explosive products using 
conventional pressing, molding, sheet forming, and extrusion techniques. 
It has now been found according to the invention that PBX products can be 
successfully formulated with non-dynamite grade nitrocellulose when 
fibrillated PTFE resin is uniformly distributed throughout the 
composition. 
SUMMARY OF THE INVENTION 
The present invention is an improvement in a PBX composition of the type 
which consists essentially of a crystalline high explosive compound and 
from about 2 wt. % to about 30 wt. % of a nitrocellulose binder, the 
improvement comprising incorporating into the composition from about 
0.0025 wt. % up to a value less than 2 wt. % of fibrillated PTFE whereby 
the tensile strength of the finished composition is improved. The present 
invention provides a plastic-bonded explosive composition consisting 
essentially of: 
(a) from about 44 wt. % up to about 90 wt. % of a crystalline high 
explosive compound having a maximum particle dimension within the range of 
0.1 and 50 micrometers, the average particle dimension being no greater 
than about 20 micrometers; 
(b) from about 2 wt. % up to about 14 wt. % of a nitrocellulose binder 
having a nitrogen content in the range of from 10% to 14%; 
(c) from about 15 wt. % up to about 35 wt. % of a plasticizer; and 
(d) from about 0.0025 wt. % up to a value which is less than 2 wt. % of 
fibrillated PTFE. 
Fibrillatable PTFE, useful according to the invention, is any "Teflon" 
fluorocarbon resin, such as, for example, "Teflon" K, which is capable of 
forming microscopic to submicroscopic fibers or strands when worked 
vigorously, i.e., mixed homogeneously under high shear. High shear mixing 
action causes fiber formation and then aids in distributing the fibers 
throughout the explosive composition. The fibers of PTFE then tend to 
interlock and add strength to the resulting mixture. 
The present invention according to another aspect is a method for improving 
the tensile strength and elongation characteristics of an explosive 
composition of the type which comprises a plastic-bonded explosive and 
from about 2 wt. % to about 30 wt. % of industrial grade nitrocellulose 
binder which is not dynamite grade nitrocellulose in which the method 
comprises adding to the explosive composition during preparation from 
about 0.0025 wt. % up to a value which is less than 2 wt. % of 
fibrillatable PTFE, and mixing the composition thoroughly and with 
sufficient shearing action whereby the PTFE will fibrillate and become 
substantially uniformly distributed throughout the finished composition. 
Thereafter, the composition can be formed by extruding, rolling, or other 
means into cords, rods, sheets and other shapes as desired. The formed 
composition can then be processed into final products, such as, for 
example, detonators, initiators, downlines, trucklines, boosters, cutting 
charges and shaped charges. 
According to yet another aspect, the invention is an improved low energy 
detonating cord of the type which includes a cap-sensitive crystalline 
high explosive compound selected from the group consisting of organic 
polynitrates and polynitramines admixed with a nitrocellulose binding 
agent which is not dynamite grade nitrocellulose. The improvement 
comprises incorporating into the admixture of explosive compound and 
nitrocellulose binding agent from about 0.0025 wt. % up to a value which 
is less than 2 wt. % of fibrillatable PTFE and thoroughly mixing it with 
sufficient shearing action that the PTFE fibrillates and becomes 
distributed uniformly throughout the explosive mixture. 
DETAILED DESCRIPTION OF THE INVENTION 
As described in greater detail in U.S. Pat. No. 2,992,087, the teachings of 
which are incorporated herein by reference, dynamite grade nitrocellulose 
is the term used to differentiate a generally high viscosity 
nitrocellulose having an average degree of polymerization within the range 
of 2000 and 3000 from non-dynamite grades of nitrocellulose. Dynamite 
grade is also known as a "soluble type" nitrocellulose and has a nitrogen 
content of from about 7% up to about 13%. 
Alternative grades of nitrocellulose are generally of higher quality than 
dynamite grade nitrocellulose, but they do not posses the same physical 
characteristics, i.e., generally they tend to be weaker and are not 
capable of imparting the same or equivalent tensile strength and 
elongation properties to the nitrocellulose-based explosive composition of 
which they are a component. When dynamite grade nitrocellulose is not 
available, therefore, it becomes necessary to employ an additive which is 
compatible with the other ingredients of the composition and which resists 
degradation over long storage periods. 
PBX formulations to which the invention is particularly applicable comprise 
from about 44 wt. % up to about 90 wt. % of a crystalline high explosive, 
such as, for example, PETN, RDX, HMX, and mixtures thereof. The explosive 
is combined with from about 2 wt. % up to about 14 wt. % of nitrocellulose 
and from about 15 wt. % up to about 35 wt. % of a plasticizer for the 
nitrocellulose. Suitable plasticizers include, for example, the trialkyl 
esters of 2-acetoxy-1,2,3-propanetricarboxylic acid wherein each alkyl 
group contains from 2 to 8 carbon atoms, dioctyl sebacate, triethylene 
glycol di(2-ethylbutyrate), trimethylolethane trinitrate (TMETN) and other 
similar materials. PBX formulations are prepared typically by: 
(a) combining the crystalline high explosive with the nitrocellulose; 
(b) adding the plasticizer for the nitrocellulose to the combination; and 
then 
(c) adding from about 0.0025 wt. % up to a value less than 2 wt. % of 
fibrillatable PTFE, although the PTFE can be added to the formulation at 
any convenient point in the preparation; and 
(d) mixing the ingredients thoroughly with sufficient shearing action to 
fibrillate the PTFE and distribute it throughout the composition. 
Thereafter, the formulation can be formed by rolling, extruding or other 
convenient means into cords, rods, sheets and other shapes for final 
processing. 
The crystalline high explosive and the nitrocellulose are normally wetted 
with water and an antifreeze solvent (alcohol) to decrease hazards in 
storage, handling, and processing. 
The order of addition of the components is not critical, and the 
composition may be mixed by any procedure that is consistent with the 
processing of plastic-bonded explosives, such as by dry processing or wet 
processing. The temperature of mixing is not critical, although 
temperature may be elevated as desired to remove excess water from the 
composition. 
It is essential, after addition of the PTFE, that the composition be mixed 
thoroughly with sufficient shearing action to fibrillate the PTFE 
throughout the composition. Methods for fibrillating PTFE which can also 
be used practicing this invention are discussed in U.S. Pat. No. 
3,838,092, the teachings of which are incorporated herein by reference. 
Crystalline high explosives particularly useful for forming PBX to be used 
in applications such as detonating cord are PETN, RDX, and HMX. For use as 
low-energy detonating cord, the particles of the crystalline high 
explosive should have their maximum particle dimension in the range of 
from about 0.1 to 50 micrometers, the average maximum particle dimension 
generally being no greater than about 20 micrometers, because the smaller 
the explosive particles the more sensitive the explosive is to 
propagation. Preparation of such finely divided high explosives is 
disclosed in U.S. Pat. No. 3,754,061, the teachings of which are 
incorporated herein by reference. 
As is realized by those skilled in the art, the explosive content of PBX is 
a function of the crystalline high explosive, the shape into which the PBX 
is formed, and the purpose and requirements of the product into which it 
is formed. In the present invention the amount of explosive can vary from 
a low of about 44% to up to about 90%. 
Non-dynamite grade nitrocelluloses include both nitrocellulose made for use 
in explosives as well as industrial nitrocelluloses made for use in 
coating applications. Nitrocelluloses with a nitrogen content in the range 
of about 10 to about 14 are contemplated for use according to the 
invention. 
Plasticizers compatible with nitrocellulose and suitable for use in PBX 
include the trialkyl esters of 2-acetoxy-1,2,3-propanetricarboxylic acid, 
dioctyl sebacate, triethylene glycol di(2-ethylbutyrate), and other 
similar materials having pour points of -40.degree. C. or below. When it 
is desired that the plasticizer be an explosively active ingredient, a 
liquid nitric ester, such as trimethylolethane trinitrate (TMETN), may be 
used as the plasticizer as described in greater detail in U.S. Pat. No. 
3,943,017 the teachings of which are incorporated herein by reference. 
Plasticizers particularly useful in PBX compositions with nitrocellulose, 
according to the invention, are the trialkyl esters of 
2-acetoxy-1,2,3-propanetricarboxylic acid described in U.S. Pat. No. 
2,992,087, the disclosure of which is incorporated herein by reference. 
Useful trialkyl esters include those wherein each alkyl group contains 2 
to 8 atoms, such as the triethyl, tripropyl, tributyl, tripentyl, 
trihexyl, triheptyl esters and their isomers, as well as 
tri(2-ethylhexyl). The tributyl ester, referred to as acetyl tributyl 
citrate, is particularly preferred because it does not adversely affect 
the crystalline high explosive. 
Additives for explosive compositions known in the art to impart 
characteristics such as increased efficiency, camouflage, stability, and 
detectability may be added to the plastic-bonded explosives of this 
invention as long as the performance of the composition is not adversely 
effected. 
Polytetrafluoroethylene (PTFE) is a polymeric fluorocarbon resin. As used 
throughout this specification, "fibrillatable PTFE" refers to those types 
of PTFE that will fibrillate, that is, under conditions of working by 
mixing to impart a shearing action, the PTFE particles will form a network 
of fibers throughout the composition with which they are mixed. The type 
of PTFE known as fine powders or as coagulated dispersions readily 
fibrillate and are preferred in the compositions of the present invention. 
The fine powders are actually agglomerates of PTFE particles which have an 
average size of about 275 to 855 micrometers. Fine powders are defined by 
ASTM D-4895-89. Fibrillatable PTFE may be used as a dry powder or as an 
aqueous dispersion. Aqueous dispersions of fibrillatable PTFE also readily 
fibrillate and are defined by ASTM D-4441. These dispersions may contain 
surfactants. In aqueous dispersions the PTFE particles are not 
agglomerated, and the average particle size is about 0.05 to 0.5 
micrometers. Aqueous dispersions may be used in the composition of the 
present invention as long as the performance of the final composition is 
not adversely effected by any surfactant that may be present.

EXAMPLES 
Superfine PETN as used herein in the following examples is characterized as 
having a maximum particle dimension within the range of 0.1 and 10 
micrometers, the average maximum particle dimension being within the range 
of 0.1 and 2 micrometers. 
"Teflon" K-20 is a fibrillatable PTFE product manufactured and available 
from E. I. du Pont de Nemours and Company, Wilmington, Del. It is an 
aqueous suspension of fluorocarbon particles. The suspended particles are 
negatively charged, ranging in size from 0.05 to 0.5 micrometers. Active 
ingredients are a nominal 33% by weight, and the suspension is stabilized 
with approximately 1% by weight of a nonionic surfactant. 
EXAMPLE 1 
Each of the nitrocelluloses listed in Table I was mixed according to the 
following procedure both with and without Teflon K-20; thus, 10 batches 
were mixed. 
A slurry coat was prepared by adding 37 g, dry basis, of water/alcohol wet 
superfine PETN (about 30% solids) to a 250 mL beaker containing 150 mL of 
water while the beaker was stirred at about 150 RPM by a small electric 
impeller. After 2 minutes of stirring, 2.5 g, dry basis, of water/alcohol 
wet nitrocellulose (about 30% solids) was added to the stirred slurry. Two 
minutes after the addition of nitrocellulose, 10.5 g of acetyl tributyl 
citrate (ATC) was added slowly to the stirred slurry. The slurry coated 
PETN was stirred for 5 more minutes. For the slurry coated PETN mixes 
containing "Teflon" K-20, 0.125 g, dry basis, Teflon K-20 was added to the 
stirred slurry after the addition of the nitrocellulose. 
After the five minutes of stirring, the slurry coated PETN was neutched 
(vacuum filtered) to remove about 2/3 of the total volume of water then 
dried in a vacuum oven at 160.degree. F. to a moisture content of less 
than 0.3%. After drying, the slurry coat was kneaded in a small Atlantic 
Research Twin Cone Mixer (to provide kneading and shearing action) for 5 
minutes and expelled from the mixer. The mixing and expelling operation 
was repeated 4 more times to assure homogeneity of the mix. The final 
product was a cohesive mass. 
The product was extruded using a piston and a cylinder apparatus which 
could be equipped with different orifices or dies so that different 
diameter cords or different thickness of sheets could be extruded. Two 
cords, each 30 mil, were extruded. Prior to the second extrusion, the 
batch was remixed for about 20 minutes using the Twin Cone Mixer. 
The procedure was repeated for Hercules 9000 Series nitrocellulose 
incorporating 1.0 g, dry basis, of "Teflon" K-20 instead of 0.125 g of 
"Teflon" K-20. The incorporation of 1/4% PTFE into Hercules 9000 Series 
nitrocellulose did not result in a composition that was suitable for 
extrusion; thus the results of the Hercules 9000 Series with PTFE is based 
on the incorporation of 2% of PTFE. The Hercules 9000 Series was prepared 
for use by soaking and stirring the nitrocellulose in a 
water/alcohol/acetone mixture over night. 
The experimental results for each nitrocellulose both with and without 
Teflon K-20 are shown in Tables II, III, and IV. For each batch two cords 
of 30 mil were extruded and tested for elongation and tensile strength. 
The cord extruded the second it was tested for its shooting reliability. 
Elongation results are given in Table II. Elongation of the cords was 
measured by attaching a piece of the cord to the jaws of a dial caliper 
and manually opening the caliper slowly until the cord broke. The 
elongation is reported as the percent (%) elongation. 
Tensile strength results are given in Table III. Tensile strength was 
measured by attaching the cord to a tension meter using a spring type 
digital dial and manually pulling the cord until the cord broke. The 
tensile strength is reported in grams (g). 
Shooting reliability of the cord was determined by coating the cord with a 
plastic oversleeve and shooting a 10 foot length of the coated cord as a 
detonating cord. The shooting reliability is reported as the number of 
feet which detonated. In general the shooting reliability improved by the 
addition of PTFE. 
The explosive compositions of the Examples are particularly applicable for 
use in low-energy detonating cords of the type described in U.S. Pat. No. 
4,232,606, the teachings of which are incorporated herein by reference. 
EXAMPLE 2 
Six production batches (150 pounds each) were mixed according to the 
following plant procedure. A slurry coat was prepared by stirring about 
105 pounds, dry basis, of water wet superfine PETN into about 10,000 
pounds of water in a tank equipped with a double bladed stirrer. After 
stirring for about 5 minutes, about 10 pounds, dry basis, of water/alcohol 
wet nitrocellulose (Hercules dynamite grade) was stirred into the tank. 
After about 5 more minutes, about 36 pounds of ATC (acetyl tributyl 
citrate) was gravity fed into the tank, over a period of about 20 minutes, 
after which mixing continued for 20 more minutes. For the batch containing 
"Teflon", about 3 ounces, dry basis, of "Teflon" K-20 was added prior to 
the addition of the ATC. The slurry coated PETN was transferred to a 
neutching (vacuum filtering) tank and the water was removed to 1/3 content 
by weight, then was transferred to a centrifuge, and the water was removed 
to 1/6 water content by weight. 
The slurry coated PETN was put in a steam heated Baker Perkins mixer and 
mixer for about 4 hours to a moisture content of less than 0.3%. In this 
process the nitrocellulose was masticated in the ATC to bind the PETN. The 
composition was analyzed by liquid chromatography; the composition for 
each batch is given in Table V. 
Each batch was slugged into cylinders about 2.25 inches in diameter by a 
length of about 4 inches. Cords of 30 and 25 mil were extruded and tested 
for elongation and tensile strength as in EXAMPLE 1; the results are shown 
in Tables VI and VII. Elongation and tensile strength for the 23 and 21 
mil cords was so low that it could not be accurately measured. 
The mixes were extruded into detonating cords according to the methods of 
U.S. Pat. No. 4,369,688, the teachings of which are incorporated herein by 
reference. Three cords of each diameter, 21, 23, 25, and 30 mil were 
extruded then enclosed in a plastic sheath with multifilament yarns for 
reinforcement lying between the cord and sheath. The addition of Teflon to 
the mix improved the runability of the detonating cords. The shooting 
reliability (SR) results are given in Table VIII and are the average of 
the three cords for each diameter. The SR was calculated according to 
Equation I: 
##EQU1## 
wherein,the initial length of cord was 2700 feet for the 30 and 25 mil 
cords, 1000 feet for the 23 mil cords, and 500 feet for the 21 mil cords. 
TABLE I 
______________________________________ 
List of nitrocelluloses used in Example 1. 
______________________________________ 
NC1: Dynamite Grade 
Source: Hercules 
% Nitrogen: 12.15-12.4 
Viscosity: 20-99 seconds in a 4% solution* 
NC2: dynamite Grade C.A.2 
Source: Societe Nationale des Poudres et Explosifs 
% Nitrogen (max): 12.6 
Viscosity: 48 seconds** 
NC3: RS 1000-1500 
Source: Hercules 
% Nitrogen: 11.8-12.2 
Viscosity: 1000-1500 seconds in 12.2% solution.sup. 
NC4: Smokeless Series 2000 Grade A Type II 
Source: Hercules 
% Nitrogen: 12.45-12.70 
Viscosity: 8-20 seconds in a 10% solution* 
NC5: Smokeless Series 9000 Grade C Type II 
Source: Hercules 
% Nitrogen: 13.1-13.2 
Viscosity: 9-15 seconds in a 10% solution* 
______________________________________ 
*Viscosity was measured by a 5/16 inch steel ball falling 10 inches in a 
inch diameter tube through a solution of specified nitrocellulose 
concentration in a solvent composed of 8 parts of acetone and 1 part ethy 
alcohol. 
**Method employed for viscosity determination was not available. 
.sup. Viscosity in seconds as measured by a 3/32 inch diameter steel ball 
falling through a column of a solution of 12.2% nitrocellulose in a 
solvent composed, by weight, of 25 parts ethyl alcohol, 55 parts toluene, 
and 20 parts ethyl acetate. 
TABLE II 
______________________________________ 
Elongation results (%) for 30 mil cord for each nitrocellulose. 
% % 
______________________________________ 
NC1 18.9 38.4 
NC1 + PTFE 25.1 61.6 
NC1 + PTFE/NC1 1.33 1.60 
NC2 9.8 43.1 
NC2 + PTFE 23.1 52.5 
NC2 + PTFE/NC2 2.36 1.22 
NC2 + PTFE/NC1 1.22 1.37 
NC3 6.9 24.1 
NC3 + PTFE 25.7 49.7 
NC3 + PTFE/NC3 3.72 2.06 
NC3 + PTFE/NC1 1.36 1.29 
NC4 7.5 21.8 
NC4 + PTFE 20.8 63.5 
NC4 + PTFE/NC4 2.77 2.91 
NC4 + PTFE/NC1 1.10 1.65 
NC5 9.2 14.8 
NC5 + PTFE 17.0 19.8 
NC5 + PTFE/NC5 1.85 1.34 
NC5 + PTFE/NC1 0.90 0.52 
______________________________________ 
TABLE III 
______________________________________ 
Tensile Strength for 30 mil cord for each nitrocellulose. 
(Reported in g) 
g g 
______________________________________ 
NC1 23.5 10.3 
NC1 + PTFE 37.7 34.4 
NC1 + PTFE/NC1 1.52 3.34 
NC2 37.7 18.5 
NC2 + PTFE 47.4 25.8 
NC2 + PTFE/NC2 1.26 1.39 
NC2 + PTFE/NC1 2.02 1.80 
NC3 17.3 15.0 
NC3 + PTFE 48.9 31.1 
NC3 + PTFE/NC3 2.83 2.07 
NC3 + PTFE/NC1 2.08 3.02 
NC4 24.3 25.8 
NC4 + PTFE 48.0 31.9 
NC4 + PTFE/NC4 1.98 1.24 
NC4 + PTFE/NC1 2.04 3.10 
NC5 9.5 12.7 
NC5 + PTFE 25.1 30.3 
NC5 + PTFE/NC5 2.64 2.39 
NC5 + PTFE/NC1 1.07 2.94 
______________________________________ 
TABLE IV 
______________________________________ 
Shooting Reliability fo 30 mil cord for each nitrocellulose. 
(Reported in feet) 
______________________________________ 
NC1 2 
NC1 + PTFE 
1 
NC2 7 
NC2 + PTFE 
10 
NC3 0.1 
NC3 + PTFE 
10 
NC4 1 
NC4 + PTFE 
10 
NC5 0.1 
NC5 + PTFE 
5 
______________________________________ 
TABLE V 
______________________________________ 
Composition (%) 
Batch PETN NC ATC PTFE 
______________________________________ 
1 68.8 8.1 23.0 1/8 
2 67.8 8.2 24.0 0 
3 68.9 6.6 24.5 0 
4 68.9 7.0 24.1 0 
5 70.6 4.6 24.8 0 
6 73.4 4.4 22.2 0 
______________________________________ 
TABLE VI 
______________________________________ 
Elongation (%) 
Batch 30 mil 25 mil 30 mil 25 mil 
23 mil 
21 mil 
______________________________________ 
1 27 21 11 33 12 11 
2 12 12 22 32 20 x 
3 28 26 12 9 x x 
4 30 13 x 12 11 9 
5 13 16 7 8 4 x 
6 13 9 7 2 6 x 
______________________________________ 
TABLE VII 
______________________________________ 
Tensile Strength (g) 
Batch 30 mil 25 mil 30 mil 25 mil 
23 mil 
21 mil 
______________________________________ 
1 38 28 25 15 23 10 
2 27 20 20 20 24 x 
3 25 22 25 13 x x 
4 36 30 x 12 16 13 
5 22 22 14 19 4 x 
6 22 21 15 5 5 x 
______________________________________ 
TABLE VIII 
______________________________________ 
Shooting Reliability (SR) 
Batch 30 mil 25 mil 23 mil 
21 mil 
______________________________________ 
1 10 10 8 6 
2 10 2 0 0 
3 10 10 5 0 
4 10 10 8 0 
5 10 8 7 1 
6 10 10 10 2 
______________________________________