CHAPTER 4

EXPLOSIVE PROPERTIES OF POLYVINYL NITRATE AND EFFECT OF

ADDITIVES THEREON

INTRODUCTION

Several compounds containing C-NO_ groups (the nitro compxjunds),

N-NOp groups (the nitramines), and 0-N0_ groups (the nitrates or nitric esters) are knovm to exhibit explosive properties, i.e. they decompose rapidly producing heat and gaseous products under the

influence of suitable mechanical and/or thermal stimuli. Polyvinyl nitrate (PVN), being a nitrated polymer, is expected to exhibit explosive properties depending on its degree of nitration

(expressed in terms of % N by weight). The propensity of PVN tovreu'ds explosive decomposition is attributable to the presence of the relatively weaJc N-0 bond (bond energy 53 kcal/mol) in large number throughout the chain. It vras, therefore, considered desirable, in this study, to investigate a few selected explosive properties of

PVN which would have a bearing on its practical application, viz. the deflagration (or autoignition) temperature, sensitiveness to impact loads, heats of combustion and explosion, and thermal stability. (Detonation characteristics, like detonation velocity, critical diameter, power, etc, were not included in the study due to lack of experimental facilities). It is intended to investigate the influence of certain variables on the explosive properties of

PVN, as, for example, the type (fibrous or gelatinized) as well as nitrogen content of PVN vis-a-vis its deflagration temperature; the type, nitrogen content and sensitizer additive in PVN vis-a-vis its impact sensitiveness; the type, stabilizer additive and its concentration in PVN vis-a-vis its heat stability. Thermochemical properties of PVN were determined, najnely, heats of combustion and explosion by calorlmetrlc experiments, and heat of formation by

calculation, In order to obtain an Idea of Its energetic potential.

MATERIALS AND METHOD

PVN Fibrous

Fibrous PVN with different nitrogen content (11.76%, 13.34%,

14.95% and 15.71%) was prepared by nitration of granular PVA

(Gohsenol NM-11) at low temperature by varying the concentration of nitric acid and time - temperature conditions, as described In

Chapter 3.

PVN Gelatinized

Fibrous PVN (15.71% N) was gelatinized by dissolving It in acetone. A thick slurry was obtained which was • dried at room temperature by using high vacuum.

Additives

The following compounds, In CP grade, were used ais additives by incorporating them separately with gelatinized PVN in weight concentrations ranging from 0.25% to 1% :

(a) Additives used in 1% concentration for Impact Sensitivity test-

(I) Ceu'bon black

(II) Lead stearate

(ill) Lead salicylate

(b) Additives used in 0.25%, 0.5%, 0.75% and 1% concentrations for

Stability test

(1) Diphenylajnine

(li) 2 Nitro diphenylamine

(ill) Sym. diethyl diphenyl urea (carbamite) (iv) 1,3 dihydroxy benzene (resorcinol)

The following properties of PVN samples, without and with

additives, were determined using the staJidEird apparata and 38 procedures :

(1) Deflagration (or ignition) temperature and activation energy.

(2) Impact sensitivity.

(3) Stability (Abel heat test and Vacuum stability test).

(4) Calorific and calorimetric Values.

All these properties are considered relevant to the potential

usefulness of PVN as a high energy material. The experimental

procedure (in brief outline) euid results are presented in the

following paragraphs.

RESULTS AND DISCUSSION

/ Deflagration temperature and activation energy

Deflagration (or ignition) temperature tests of PVN

sajnples, with various nitrogen contents, were carried out in a

special appeiratus shown in Fig 4.1. It consists of a cylindrical

copper block of diajneter 10 cm, which has a cavity of diajneter 2.5

cm ajid depth 7.5 cm, containing Wood's metal (composition : bismuth

50%, lead 25%, tin 12.5% and cadmium 12.5%) as the bath material.

Wood's metal has a low melting point of 65 C. The molten metal bath

can be heated upto a temperature of 300°C. A thermometer is inserted

in the bath to measure the temperature.

Approximately 5 mg sample was taken in an aluminium cup of dia

5 mm and height 2 mm. The cup was covered with an aluminium cap. The

Wood's metal bath was electrically heated (the heating rate was

controlled by a dimmerstat). The bath temperature was monitored by

thermometer. When a desired temperature was reached, the current was controlled. The aluminium cup containing the sample was

^•^ ALUMINIUM LID

ALUMINIUM SAMPLE CUP

METALLIC CONE

WOOD'S METAL

KANTHAL WIRE 23 SWG

COPPER BLOCK

ASBESTOS BOX

- ASBESTOS WOOL INSULATION

Fig 4.1. APPARATUS FOR DETERMINATION OF IGNITION TEMPERATURE (SCHEMATIC)

54 introduced into the bath and a stop watch was started. After a

certain time interval the sample ignited or exploded with flaSh and

sound and the stop watch was instantly stopped. This reading was

noted as the ignition delay or deflagration time lag.

In the above majiner, readings were taken at successive

temperature intervals of 5C till a temperature was reached at

which the ignition delay was less than 5 sec. The temperature

corresponding to 5 sec ignition delay is usually referred to as the

"ignition temperature" (T ) or "explosion temperature" or

"deflagraration temperature".

The data on ignition delay corresponding to different temperatures, ranging from 468 K (195°C) to 518 K (245''C), for fibrous PVN (15.71% N) are presented in Table 4.1.

Table 4.1. Ignition Delay of Fibrous PVN (15.71% N) at different

temperatures

SNo Temperature I/Txio"* Ignition delay Log of D

K K D (sec)

1. 468 21.4 23.5 1.37

2. 473 21. 1 19.0 1.28

3. 478 20.9 15.5 1. 19

4. 483 20.7 13.0 1. 11

5. 488 20.5 12.5 1. 10

6. 493 20.3 9.0 0.95

7. 498 20. 1 7.5 0.88

8. 503 19.9 6.0 0.78

9. 508 19.7 5.5 0.74

10 513 19.5 5.0 0.69

11 518 19.3 4.6 0.66 similarly, three other sets of experiments were conducted on

deflagration temperature vs ignition delay of fibrous PVN with

14.95%, 13.34% and 11.76% nitrogen content at temperature rsinging from 473 K (200°C) to 553 K (280°C). The results are presented in

Table 4.2.

Table 4.2. Ignition Delay of Fibrous PVN (14.95%, 13.34% & 11.76% N)

at different temperatures

PVN (14.95% N) PVN (13.34% N) PVN(11.76% N)

Temp. 1/TxlO * Ign. Log of Ign. Log of Ign. log of delay delay delay D D D SNo. D D D K K sec sec sec sec sec sec

1 473 21. 1 - - 34.0 1.53 - -

2 478 20.9 14.0 1. 15 27.0 1.43 37.5 1.57

3 483 20.7 13.8 1.14 26.0 1.41 33.0 1.52

4 488 20.5 13.0 1.11 17.0 1.23 27.5 1.44

5 493 20.3 12.0 1.08 12.0 1.08 24.0 1.38

6 498 20.1 9.0 0.95 11.0 1.04 21.5 1.33

7 503 19.9 8.0 0.90 10.0 1.00 18.5 1.27

8 508 19.7 6.5 0.81 9.0 0.95 15.0 1. 18

9 513 19.5 6.0 0.78 8.5 0.93 13.5 1.13

10 518 19.3 5.5 0.74 8.0 0.90 11.0 1.04

11 523 19.1 5.0 0.69 7.0 0.84 10.0 1.00

12 528 18.9 4.5 0.65 6.5 0.81 9.0 0.95

13 533 18.8 - - 5.0 0.69 8.0 0.90

14 538 18.6 - - 4.5 0.65 7.0 0.84

15 543 18.4 - - - - 5.9 0.77

16 548 18.2 - - - - 5.0 0.70

17 553 18.1 - - - - 4.5 0.65 It would be seen from the data given in Table 4.1 and 4.2

that the ignition temperatures (correspxinding to 5 sec delaiy time,

as per definition) for the fibrous PVN SEunples are as follows

PVN (15.71% N) 513 K (240*C)

PVN (14.95% N) 523 K (250°C)

PVN (13.34% N) 533 K (260°C)

PVN (11.76% N) 548 K (275°C)

The ignition temperature range (240-275*'c) for PVN appears

to be quite satisfactory for all practical purposes.

Activation energy of PVN samples having different nitrogen

contents, was calculated by using Semenov equation D = C e^/«T

In D = In C + E / RT

where, D = Deflagration time lag or ignition delay,

E = Activation energy,

T = Absolute temperature,

C = a constant (depends on the compxssition of material),

R = Universal gas constant (1.986 cal/g).

The above expression cam be rewritten as

log D = B + E/(2.303 x 1.986) T

where B is a constant

On plotting log D against 1/T, a straight line is obtained.

The slope of the plot is equal to E/R, frpm which the activation energy (E) for self-ignition (deflagration) of PVN samples could -4 be found. Plots between log D vs 1/TxlO K for the present work are shown in Fig 4.2 to 4.5 . From these plots the following values of activation energy of PVN samples were obtained :

PVN (15.71% N) = 16.076 kcal/mol,

PVN (14.95% N) = 13.299 kcal/mol,

PVN (13.3% N) = 16.017 kcal/mol.

C7 Another standard equation was also used for calculating activation energy of PVN

(log D -log D ) E = 4.576 = — (1/T -1/T )

(log Dg-log Dj) TjX Tg or, E = 4.576 (T - T )

where D and D„ are Ignition delays corresponding to the absolute temperatures T. and T„, respectively.

Using this formula, activation energy of various fibrous PVN samples was calculated and the results are given below :

PVN (15.71% N) = 16.9 kcal/mol, PVN (14.95% N) = 12.6 kcal/mol, PVN (13.34% N) = 16.7 kcal/mol. PVN (11.76% N) = 19.063 kcal/mol Activation energy values obtained from calculations by both methods mentioned above show close agreement for PVN samples having identical nitrogen content. The activation energy, calculated from ignition delay data, is the energy required for initiating the decomposition of the explosive compound and it roughly indicates the thermodynajnic stability level of the compound.

Results show that the activation energy, and hence the inherent stability, of PVN is rather low and varies with its nitrogen content. The activation energy is minimum for PVN with 14.95% N and higher for PVN with 11.76%, 13.34% and 15.71% N.

Thus, there is no linear relation between nitrogen content and activation energy of PVN. The reason for the minimum E value of PVN containing 14.95% N is not clear. <

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Impact sensitivity measvirements on gelatinized and fibrous

(ungelatinized) samples of PVN, without ajid with additives, were 38 carried out using a standard "Fall Hajnmer apparatus" , shown in Fig

4.6, and Bruceton Staircase method . The apparatus has two parallel

bases. The rods have grooves running through the entire inside

length to facilitate and guide the fall of the hajnmer/weight. The

impact of falling weight is transferred to the striker-ajivil-collar

assembly through a plug. The rods are held in position by a clamp

at the top. The falling weight can be held in a fixed position by

two knobs and can be released from a preselected height by a mechajiical arrangement. The maximum height from which the weight can be dropped is 180 cm. The striker, collar and anvil are made from special hardened steel.

20 mg of the SEunple was placed on the anvil and a 2 kg weight was dropped on it from einy eirbitrary height. If the sample exploded, the next trial was done from a lower height, but if it did not explode, the next trial was done from a greater height. Successive trials with PVN ajid RDX seunples were done at height intervals of 5 cm and continued until sufficient data was collected. For PVN (test) samples and RDX (standard) samples, the median heights of fall for their 50% explosions were obtained by calculation.

From these data, "Figure of Insensitivity" (F of I) of PVN etc was calculated using the following simple formula : "2 Figure of Insensitivity = X 100 "1 where H = median height of fall for 50% explosions in the reference standard explosive (RDX).

H = median height of fall for 50% explosions in the test explosive. / 1 1 s ~-1—

1 1

1 STEEL RODS 2 GROOVE 3 WEIGHT L FIXED CLAMP S STRIKER 6 ANVIL 7 COLLAR 8 PLUG

Fig 4.6 FALL HAMMER APPARATUS FOR IMPACT SENSITIVU.Y The Figure of Insensitivity for "standard RDX" is arbitrarily taJcen as 80. (It may be noted that the figiore characteristic of the explosive under test is so calculated as to become smaller for the more sensitive explosives. That is why it is called Figure of

Insensitiveness and not Figure of Sensitiveness).

The percentage of explosions for each height was calculated by the following equation : Z E + Z e Percentage explosion = x 100 ZE + Ze + ZN + Zn where, E = Explosion,

e = Assumed explosion,

N = No explosion,

n = Assumed no explosion. Total No. of explosion or simply , % explosion = • x 100 Total No. of trials From a graph plotted between the height of fall and percentage of explosion, it was possible to determine the height of fall corresponding to 50% explosion, or median height for 50% explosion for any explosive substance. Therefore, "median height for 50% explosion" was employed as a criterion for comparison of Impact sensitiveness of different samples.

Impact sensitivity results for PVN samples having different nitrogen contents, without ajid with additives, are plotted in Fig

4.7 to 4.14.

Fig 4.7 and 4.8 show that for fibrous PVN (15.71% N) and gelatinized PVN (15.71% N), the median heights of fall for 50% explosion are 81 cm and 82 cm, respectively. Thus, it may be concluded that PVN containing 15.71% N shows almost the sajne sensitiveness to impact loads for fibrous as well as gelatinized sajmples.

Fig 4.9, 4.10 and 4.11 show that for fibrous PVN sajnples containing 11.76%, 13.34% and 14.95% nitrogen, the median heights of fall for 50% explosion are 90 cm, 85 cm and 82.5 cm, respectively.

Thus, it is observed that with increasing percentage of nitrogen content in fibrous PVN samples, the median heights of fall for 50% explosion decreases, that is, the impact sensitivity increases. It is concluded, therefore, that the impact sensitivity of PVN depends on its % nitrogen or degree of nitration.

Fig 4.12, 4.13 and 4.14 show that for gelatinized

PVN (15.71% N), intimately mixed with 1% by weight of additives, nsunely carbon black, lead stearate and lead salicylate (which have also been used as burn rate modifiers in another series of experiments), the median heights for 50% explosion are 62 cm, 55 cm, and 46 cm, respectively, as compau'ed to 82 cm for PVN alone. These results lead to the interesting conclusion that carbon black, lead stearate, and lead salicylate are very effective as additives in small dosage (1%) in increasing the impact sensitiveness of PVN.

Also, an aromatic lead salt, viz. lead salicylate, is more effective as a 'sensitizer' than an aliphatic lead salt, viz. lead stearate.

In a separate series of experiments, using the same apparatus, median heights for 50% explosion were determined for some other typical explosives. With RDX, p HMX, tetryl and nitrocellulose

(13.1% N), the median heights were found to be 51 cm, 64 cm, 81.5 cm and 61 cm, respectively. Thus, it is observed that PVN (15.71% N)

(81-82 cm) ir, as sensitive to impact loads as the well known

"intermedieury" high explosive tetryl (81.5 cm) and less sensitive than nitrocellulose (13.1% N) (61 cm). The median height of fall for 50% explosion are shown in Table 4.3.

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From the impact sensitivity data in Table 4.3, it is evident

that :

(a) With increasing % N in fibrous PVN sajnples, the median height

of fall for 50% explosion decreases, that is, impact sensitivity

increases.

Table 4.3, Results of Impact Sensitivity Test on PVN without and

with Additives Median Seunple Comjxasition height of F of I fall for (RDX=80) 50% explo­ PVN type sion Additive cm

PVN (11.76% N),fibrous none 90.0 141.2

PVN (13.34% N),fibrous none • 85.0 133.3

PVN (14.95% N),fibrous none 82.5 129.4

PVN (15.71% N),fibrous none 81.0 127. 1

PVN (15.71% N),gelatinized none 82.0 128.6

carbon black 1% 62.0 97.2

lead stearate 1% 55.0 86.3

lead salicylate 1% 46.0 72.2

« Tetryl none 81.5 127.8

Nitrocellulose (13.1% N) none 61.0 95.7 p HMX none 64.0 100.4

RDX none 51.0 80.0

* included for comparison.

** Reference standard. (b) Fibrous and gelatinized samples of PVN (15.71% N) as well as the well known "intermediary" high explosive Tetryl are equally sensitive to impact loads (F of I rajige 127-129).

(c) Carbon black, lead stearate and lead salicylate are very effective as 'sensitizer' additives in small (1%) dosage.

III. Stability

The stability of ein explosive may be regarded as its capacity to remain unchajiged during a relatively long period of storage.

This depends on the principle that, in general, chemical reactions

( whether of spontaneous degradation or of interaction ) are accelerated by heat. Abel heat test eind Vacuum stability test are two stajidsird procedures which were adopted in this work for evaluating stability of PVN and PVN-based compositions. Stability can be distinct with two aspects, namely, chemical stability

(stability of pure chemical compound) and practical stability (a product stabilized by methods applied in practice).

38 (1) Abel heat test

It is widely used with nitric esters and gun propellajits

(based on nitrocellulose ajid nitroglycerine). It was devised by

Abel who had noticed that an early sign of degradation of NC was the appearance, in the air above it, of the brown fumes of nitrogen tetroxide, N„0 . This test depends on the reaction of N„0. with potassium iodide to liberate iodine (the potassium iodide is present in a moistened starch-iodide paper which gives a characteristic blue coloiiration when iodine is liberated in sufficient concentration).

1.6 gm of the sample was taJcen in a 10 ml test tube and heated in a water bath at 150+2°F (65.5+l°C) or 180+2°F (82+l°C) with a starch-iodide paper (tip wetted with 1:1 glycerine-water mixture).

Time required for development of colouration (due to evolved oxides of nitrogen) was noted and was considered as a measure of relative stability of the sample.

Abel heat test values obtained for various PVN sajnples, with and without additives, at temperatures 65.5+l°C and/or 82+l°C, are presented in Table 4.4.

From the Abel heat test data, the following observations are made :

(a) PVN (15.71% N) in gelatinized form is less stable than in fibrous form at higher temperature (82 + IC).

(b) Interestingly, with increasing percentage of nitrogen content in fibrous PVN sajnples, the heat stability generally decreases at higher temperature (82+l°C) and becomes unsatisfactory for PVN (15.71% N).

(c) Diphenyl amine, 2-nitro diphenyl amine, sym diphenyl urea

(capbeimite), and resorcinol are effective as stabilizers even in 0.25% concentration in PVN, at 82+l*'C.

(d) At 82+l°C, the effectiveness of diphenyl amine (DPA) 'I w / increases with concentration upto 1% whereas that of carbamite ij •" ' decreases with concentration. \V

(e) Effectiveness of 2-nitro diphenyl amine (2NDPA), however,

increases with concentration upto 0.75% and then decreases considerably.

(f) Effectiveness of resorcinol increases with concentration upto 0.5% and then decreases.

(g) On overall appraisal of the Abel heat test data, diphenyl amine (DPA) is rated superior to 2NDPA, carbamite and resorcinol as a stabilizer for PVN in concentration upto 1%

77 Table 4.4 Abel Heat Test values of PVN samples without and with

Additives

Heat test values, min Sajnple Composition Test temperature

PVN type Additives Cone.of 65.5+1 c 82+l°c additive

PVN (11.76% N),fibrous none 0.00 >75 60

PVN (13.34% N),fibrous none 0.00 38 15

PVN (14.95% N).fibrous none 0.00 >60 20

PVN (15.71% N),fibrous none 0.00 >65 10

PVN (15.71% N),gelati­ nized none 0.00 >85 06 DPA 0.25 >90 50

0.50 67

0.75 67

1.0 >90 >70

2NDPA 0.25 90 20

0.50 90

0.75 >90

1.0 >90 35

Carbajnite 0.25 >90 45

0.50 >40

0.75 35

1.0 >90 20

Resorcinol 0.25 >90 50

0.50 >60

0.75 40

1.00 >90 45 op

(2) Vacuum Stability

Explosive compounds containing C-nitro and 0-nltro groups generally undergo only a little decomposition in the solid state, even at temperatures approaching their melting points. However, gradual decomposition with evolution of gas occurs at temperatures between the melting ajid ignition points and the rate is accelerated by the presence of impurities.

Vacuum stability test of gelatinized eind ungelatinized PVN samples (15.71% N), without and with "stabilizer" additives in two different concentrations (0.25 and 1.0% by wt. ) were carried out at

ERDL, Pune. One gram of each sample was heated in a glass tube under vacuum at 100 C for 40 hrs. The volume of evolved gases was measured; the gas volume is inversely proportional to the stability of the explosive. Results are given in Table 4.5.

Table 4.5. Vacuum Stability Test results of PVN without and with Additives

Sajnple Composition Volume of gas evolved(at 100°C, Cone.of 40 hrs.) PVN type Additive additive % ml/g

PVN (15.71% N),fibrous. none 0 00 6.74

PVN (15.71% N),gelatinized none U.OO 7.98

DPA 0.25 8. 18

1.00 7.00

2NDPA 0.25 8.84

1.00 6.25

Carbamite 0.25 8.34

1.00 7.22

Resorcinol 0.25 7.54

1.00 8.75 From vacuum stability data, the following observations can be

summarized :

Fibrous PVN (15.71% N) appears to be a little more stable than

gelatinized PVN (15.71% N).

Diphenyl ajnine, 2nitro diphenyl amine and carbajnlte at 0.25%

concentration in gelatinized PVN are found to have a marginally

negative effect on the vacuum stability of PVN. However, when used

in 1% concentration, these additives are found to have a favourable

effect. Amongst these additives, 2NDPA is the most effective

stabilizer at 1% concentration.

Interestingly, resorcinol has a m2u:'ginally positive effect at

0.25% concentration but a negative effect at 1% concentration.

It may be mentioned here that a value of more than 11 cc of gas

evolved diiring vacuum stability test on PVN at 100°C for 16 hrs has 13 been cited in AMCP . Although this work has shown better vacuum

stability of PVN, without and with stabilizers, yet it would be

desirable to obtain even lower volumes of evolved gas (for practical

applications as a propellant) by use of more effective additives at

appropriate concentration. Therefore, there is a need for extensive

and intensive work involving a wide spectrum of potential 13 stabilizers at different concentrations. According to AMCP report,

the volume of gas evolved during vacuum stability test on NC at

100 C, for different nitrogen content varies as follows :

NC (13.45% N) for 40 hrs. = 1.5 cc.

NC (14.14% N) for 14 hrs. = 11+ cc.

IV. Calorific and Calorimetric Values

Heat of combustion (calorific value) and heat of explosion

(calorimetric value) of PVN were determined in a Gallenkajnp 39 adiabatic bomb calorimeter in accordance with stajidard procedures. THERMOMETER STIRRER MOTOR

AOIABATIC BOMB CALORIMETER

CONKECTION To TERMINALS ON COVER^ / MAIN OXYGEN INOUT SUPPLY

FIRINQ UNIT

(A)

PIPING TERMINALS

CUfYCEH INLET

LID

BOHa

^—C«UCI81-£

WATER

(B)

Fig 4.15 APPARATUS FOR DETERMINATION OF CALORIFIC VALUE ( SCHEMATIC ) A diagrammatic sketch of the adiabatic bomb calorimeter is

given in Fig 4.15. Benzoic acid was used as a standard reference

material for calculating the water equivalent of calorimeter, which

is essential before calorific value of an unknown fuel is estimated.

The outline of the procedure is as follows.

A weighed quantity of benzoic acid was pressed into a pellet

with a cotton thread by means of a briquette press ajid burnt in

exess supply of oxygen in a stainless steel bomb surrounded by a

known mass of water in a calorimeter. Water equivalent was then

calculated from the rise in temperature, calorific value of benzoic

acid and mass of water in the vessel. Details of the procedure are

described below.

2. 1 kg of water was taJcen in the calorimeter vessel which was

placed in the Jacket of the calorimeter. The initial temperature of

calorimeter water and jacket water are adjusted to the seime

temperature with the help of a temperature control unit, with

continuous stirring.

1.2 gm of dried benzoic acid, 6" length of cotton thead

(supplied separately), and 3" length of fuse wire were weighed

accurately. The entire weighed quantity of benzoic acid was

transferred to the mould of briquette press alongwith cotton thread

and pressed into a pellet. The pellet with thread was removed amd

reweighed for correct weight.

The pellet with the thread was placed in the pyrex cup on a

circular ring support attached to the terminals of a bomb lid. The fuse wire was attached across the electrodes and the thread was

tied to the wire. 10 ml of distilled water was pipetted out into

the bomb and it was closed with the lid prepared as described above.

The bomb was tightened with a collar nut by hand only. From an oxygen cylinder, oxygen was admitted slowly into the bomb till a pressure of 25 atm. was recorded on the gauge. The bomb thus made ready vra.s then placed into the calorimeter vessel and the lid of the vessel was closed. Thermistor, steuidard thermometer etc, were placed in their proper position.

The calorimeter and water Jacket tenperature was ceu^efully adjusted. After waiting for 5 minutes, the charge was fired by closing the ignition cicuit. The rise in temperature after 10 min. was recorded ajid two more readings, each after 5 minutes interval, were also recorded. The thermometers were removed carefully. The vessel was opened eind the bomb was removed carefully and allowed to cool for 30 minutes.

The gas pressure inside the bomb was released slowly. The interior of the bomb was rinsed with distilled water 2-3 times ajid the washing was collected in a beaker. The volume was made to about

250 ml and the formation of acidity due to HNO„ and HpSO. was estimated by titration against N/10 NaOH. Then, water equivalent was calculated as per the following formula :

H + Cj + C^ + C3 + C^ - Q M

Q

where E = Water equivalent.

H = Cal. val. per gm. of benzoic acid . (standard value 6324 cal/g) C. = Heat released due to cellulose, 4190 cal/g.

Cp = Heat released due to acidity formation,1.45 cal/cc

of N/10 NaOH.

C„ = Heat released due to fuse wire, 335 cal/gm.

C. = Constant heat due to stirring, 30 cal/gm .

Q = Correct rise in temperature (final - initial).

M = Mass of water in kg.

Similar procedure as above was applied for the "unknown" PVN sample, but due to explosive property of PVN the quantity taken was less th£ui 1.6 gm. The heat of combustion (calorific value) was calculated as cal/g using the following equation :

( Q X E ) - Cj - C2 - C3 - C^ H = U

where, H = Heat of combustion in cal/g.

The meanings of E, Q, C., C_, C^, & C. are described earlier.

W = Weight of PVN taken.

Heat of explosion (calorimetric value) was determined by repeating the above test using nitrogen at 25 atm or air at 1 atm pressure (instead of using excess oxygen at 25 atm). The value of

H is now regarded as the heat of explosion, which is expected to be lower than the corresponding heat of combustion. The results are presented in Table 4.6.

Table 4.6. Calorific and Calorimetric Values of PVN.

Specimen Calorific value Calorimetric value cal/g cal/g

PVN (11.76% N),fibrous. 3744 456

PVN (15.71% N),fibrous 3023 987

PVN (15.71% N),gelatinized 3065 864

The heats of combustion and explosion for each specimen of PVN, reported in Table 4.6, are averages of at least two readings. The reproducibility of readings was, generally, very good.

The heat of combustion values for PVN (15.71% N), whether gelatinized or ungelatinized (fibrous), are almost identical. (The minor difference may be considered as within acceptable limits of experimental error). However, the heat of combustion of PVN (11.76%

N) is appreciably higher than that of PVN (15.71% N); this is probably due to presence of a much larger number of oxidizable hydrogen atoms in the chains of the former and consequent

availability of extra heat when burnt in excess oxygen.

The heat of explosion of fibrous PVN (15.71% N) is higher

than that of gelatinized PVN (15.71*/. N). The heat of explosion of

PVN (11.76% N), is however, much less than that of PVN (15.71% N);

this is because of the presence of a much larger number of -0N0„

groups (and, therefore, higher oxygen balance) in the latter which

allows more complete combustion and greater heat release when

burnt in inert (nitrogen) atmosphere or limited oxygen supply.

The values of heat of combustion and heat of explosion of PVN,

cited in literature, are given below :

% Nitrogen Heat of Heat of Ref combustion explosion in PVN cal/g cal/g

15.60 2960 . 900 13

15.15 3016 960 5

It would be interesting to mention here the values of heat of

of combustion and heat of explosion of nitrocellulose (NC) having 40 different % N, as cited in literature , and to compare the same

with the corresponding values of PVN :

% N Heat of combution % N Heat of explosion in NC cal/g in NC cal/g 11.06 2612 12.62 973

12.46 2434 13.0 1025

13.53 2236 13.20 1055

14. 12 2208 13.45 1096

The above data for NC show that with increasing % N, the heat of combustion decreases, but the heat of explosion increases. Thus, from a comparison of the data on PVN and NC, it is concluded that—

(a) With increasing % N, both PVN and NC show the same trends,

i.e. the heat of combustion decreases and the heat of explosion increases, and

(b) Completely nitrated PVN gives a higher heat of combustion, but a lower heat of explosion, tham completely nitrated NC. (This

implies that, for a comparable degree of nitration, NC is slightly more energetic thein PVN).

Using the above data on heat of combustion and an empirical equation for combustion reaction of PVN, the heat of formation (AHf) of PVN was calculated, after applying the necessary corrections, and was found to be as follows :

Specimen AH (at 25°C)

PVN (11.76% N),fibrous = -751.25 kcal/kg.

PVN (15.71% N),fibrous = -241.25 kcal/kg. '

PVN (15.71% N),gelatinized = -205.12 kcal/kg.

As expected, with increasing % N in PVN, the heat of formation becomes smaller with a negative sign, and, therefore, more favorable from the viewpoint of performance potential (in combustion process).

CONCLUSION

Investigations of several explosive and related properties of PVN SEimples, having different nitrogen contents, lead to the following conclusions :

(1) Ignition temperature (corresponding to 5 sec delay time) for PVN is fairly high and decreases from 275°C to 240°C with

% N in PVN increasing from 11.76% to 15.71% .

(2) Activation energy of PVN samples containing 11.76% to 15.71% N, calculated from the ignition temperature/delay data, lies in the modest range of (approx.) 12-19 kcal/mol.

(3) Impact sensitivity of PVN increases with increasing

% N from 11.76% to 15.71% N. PVN (15.71% N) is as sensitive to impact as the intermediary explosive Tetryl. Addition of 1% carbon black,

lead stearate, and lead salicylate to PVN (15.71% N) increases its impact sensitivity considerably.

(4) Heat stability of PVN at 82+l°C decreases with increasing % N, and reaches an unsatisfactory level for PVN (15.71%

N). Addition of 0.25% wt/wt of DPA, 2NDPA, carbamite and resorcinol to PVN (15.71% N) improves its heat stability. DPA is, generally, the best stabilizer.

(5) Vacuum stability (at 100°C for 40 hrs) of PVN

(15.71% N) is reasonably satisfactory, with evolution of gas

® 7-8 ml/g. However, this is improved by addition of 1% wt/wt of

2NDPA to PVN.

(6) AH , ., decreases and AH , , increases combustion explosion with increasing % N in PVN (similar to the trends in NC). AH , , of PVN (15.71% N) is 987 cal/g, (whereas that of NC, explosion containing > 13.0% N, is more than 1000 cal/g).