Die Angewundtr Makromolrkirlare Chemie 38 ( 1 9 7 4 ) 91-102 (iVr. 5 7 1 )

From the Chemistry Department, University of Surrey, Guildford, England

Sorption of Nitroglycerine from Solution by Granular Nitrocellulose

By Peter Howard

(Received 5 November 1973)

SUMMARY: Two nitrocelluloscs of 12.7 and 14.0% nitrogen contents were prepared and fractionated

to give samples having R,, values in the range 2,200-388,000. The effect of chain-length of the nitrocellulose samples, having the same nitrogen content, on the sorption of nitroglycerine from n-hexane solution was investigated by means of partition equilibrium experiments. The separate effect of nitrogen content on sorption of nitroglycerine was determined by comparison of sorption isotherms for samples of similar molecular weight but different nitrogen contents. The amount of nitroglycerine taken up was greater for the lower nitrogen content samples, but sorption was not influenced by the molecular weight of the nitrocellulose. The results are discussed in terms of the molecular structure of the nitrocellulose samples, which is determined by the orientation and spacing of the chains in the crystalline and amorphous regions.

ZUSAM M ENFASSUNG: Zwei NitrocelluloseprBparate mit einem Stickstoffgehalt von 12,7 und 14,0% wurden

zu Proben mit m,-Werten von 2,2. lo3 bis 388. lo3 fraktioniert. Der EinflulJ der Kettenlan- ge der Nitrocellulose bei konstantem Stickstoffgehalt auf die Sorption von Nitroglycerin aus n-Hexan wurde in Verteilungsexperimenten untersucht. Ebenso wurde der EinflulJ des Stickstoffgehalts bei Chnlichem Molekulargewicht ermittelt. Die Menge des von der Nitrocellulose aufgenommenen Nitroglycerins war bei niedrigerem Stickstoffgehalt gr6lJer; dagegen wurde die Sorption nicht vom Molekulargewicht der Nitrocellulose beeinflulJt.

Introduction

Earlier studies - l o on the sorption of various compounds from the vapour phase and from solution have concentrated on the nature of the compound and its role in the gelation of nitrocellulose. In some cases the relation between degree of nitration of the nitrocellulose and extent of sorption has been examined, and in the few cases where different molecular weight nitrocellu- loses were compared this was solely on the basis of solution viscosities. The term “sorption” is used here in the sense originally proposed by McBain

91

P. Howard

to include surface adsorption, absorption to form a solid solution or chemical reaction (chemisorption).

The degree of sorption of nitroglycerine from solution by solid nitrocellulose is of considerable practical importance in the manufacture of Cordite. In spite of this, very little work has been published on this system, probably because of the hazardous nature of glycerol trinitrate. The first reliable sorption studies appear to have been carried out by Desmaroux" which involved measuring the partition of nitroglycerine between nitrocelluloses of different nitrogen contents and an inert solvent. This work followed his earlier X-ray examination of nitrocellulose films containing nitroglycerine 1 2 . The inability to completely remove nitroglycerine from impregnated nitrocellulose films by heating was noted by ChedinI3, who reported a residual 5-6 wt.-% which was independent of the degree of nitration. Rates of gelation of nitrocellu- lose suspended in nitroglycerine-ethylene glycol dinitrate solutions have been studied by Fensom14 using a photometric method. His results were partly inconclusive due to variable solubility of nitrocellulose in the mixed solvent, a "skin effect" associated with the fibres, and a final sorption equilibrium not always having been reached. The process ofgelation has also been examined by viscosity and viscoelasticity measurements on nitrocellulose-nitroglycerine mixtures 1 5 . 1 h . An investigation of sorption of nitroglycerine from chloroform solution by nitrocellulose films carried out by Dubar l 7 suggests that nitrogen content, degree of crystallinity, and to a lesser extent chain-length influence the sorption equilibrium.

In the work to be described partition coeficients and sorption isotherms for the distribution of nitroglycerine between granular nitrocellulose and n-hexane were compared for samples having similar molecular weights and different nitrogen contents, and those with the same nitrogen content but differing molecular weights. The solvent n-hexane was chosen so as to be chemically inert towards nitroglycerine and nitrocellulose while having a limited solubility for nitroglycerine. The experiments were carried out under conditions so as to ensure that true sorption equilibrium had been reached.

Experimental

Preparation of High Nitrogen Content Nitrocellulose

20g of an industrial nitrocellulose (12.2%N) was renitrated for Shrs. at O'C in 400 ml of reaction mixture of weight composition 50% HNO,, 25% acetic acid, and 25",,

92

Sorpfion of N itroylycerinr hy Nitrocellitlose

acetic anhydrite. The product was washed free of acid, stabilised by extraction with boiling ethanol and finally dried at 50"C and stored in a desiccator. Analysis (described later) gave 13.97 '/O N corresponding to virtually fully nitrated cellulose (theoretical 14.14%, N).

Preparation 01 L o w Nitrogen Content Nitrocellulose

A preliminary attempt to nitrate cellulose with a nitric acidiacetic acid/acetic anhydride mixture gave a product partially soluble in acetone and has been reported elsewhere18. 18g of this partially soluble material was nitrated for 5 hrs. at 0 C in 600ml of reaction mixture of weight composition 4 3 O / 0 HNO,, 40% H2S04, and 16% H 2 0 . The product was treated as before and an analysis gave a nitrocelldose of 12.68% N which was completely soluble in acetone.

Fractional Precipitation

Preparation of nitrocellulose samples having different molecular weights was carried out by fractional precipitation of a 1 YO solution in acetone by addition of water at

1 25 C. Precipitatesfirst formed were redissolved by warming and the solution left overnight to rcach equilibrium. Fractions were separated by decantation of the clear supernatant liquid or else centrifuged off. Some refractionation of large fractions was nccessary to prepare samples having an adequate range of molecular weights. Each fraction was precipitated again from acetone solution using a fine jet of water or sometimes 0.1 M sodium acetate solution in order to produce a finely divided nitrocellulose which settled rapidly. Precipitates were well washed with water, dried, and then stored in the usual way. Altogether ten fractions of the 13.97% N nitrocellulose (wts. 0.36-2.52~) and nine samples of 12.68% N nitrocellulose (wts. 0.28-3.30~) were obtained, from which a selection was made for the sorption studies.

Nitrogen Anulysis and Molecular Weight Determination

A modification of the standard method for nitrate analysis, by reduction with Dcvarda's alloy followed by acidimetric determination of ammonia, was used for the analysis of nitrocellulose and has been described earlier'". Number average molecular weights of the nitrocellulose fractions were determined viscometrically in n-butyl acetate at 25 'C. The limiting viscosity number/molecular weight relationships of Moore and Edge'" and of HarlandZ1 were used for the low and high nitrogen content fractions, respectively.

Estimution of Nitrogljcerine in Solution

The method was essentially that described by Sandi and Flanquet22 for nitrate deter- mination in which the sample was dissolved in concentrated sulphuric acid and the liberated nitric acid titrated potentiometrically with ferrous sulphate. Reduction of nitrate to nitrite proceeds rapidly and gives a sharp end point (large change in potential), corresponding to the reaction H N 0 3 + 2 F e S 0 4 + H 2 S 0 4 = H N 0 2 + F ~ 2 ( S O J ) J + H 2 0 , provided the sulphuric acid concentration does not fall below 55 YU during the reaction.

93

P. Howard

In the prcsent work the electrodes consisted of a bright platinum wire (cathode) and a reference electrode consisting of a Pt spiral coated with P b 0 2 immersed in 98% H1SO4. Titrations were carried out in an ice-bath to minimise loss of HNO, using 0.1 M FeS04 in 42% H2S04 standardised with potassium nitrate. Titres were in the range of 2-81111 and were read to kO.01 ml using a calibrated burette, and a jump of about 2OOmV was observed at the end point.

For nitroglycerine the procedure was to transfer three 5ml aliquots of n-hexane solution to three 150ml beakers. The n-hexane was evaporated off in a stream of air to leave the nitroglycerine in the form of tine droplets. This was then dissolved in 25 ml of 98 O/O H2S04 while the beakers were cooled in ice; the nitroglycerine concentra- tion was then evaluated from the mean of the three ferrous sulphate titres.

Sorption Equilibrium Studies

Sorption experiments were carried out at 20°C by shaking up known weights of nitrocellulose with known initial concentrations of nitroglycerine in n-hexane until equili- brium was reached, the time for which had been determined in a previous experiment. The amount of nitroglycerine taken up by the solid phase was obtained from the difference between the initial and final (equilibrium) solution concentration. The thermo- stat, controlled at 20°C 50.1‘. had a rotating steel rod passing centrally through it via water-tight seals in the end walls. Clamps attached to the rod enabled a number of lOOml flasks to be fastened symmetrically along its length, so that the rotation provided a shaker for the flasks and a stirrer for the water-bath.

A special type of stopper was required for the flasks containing nitroglycerine solution, since friction between a normal stopper and the ground-glass neck was liable to detonate any entrapped nitroglycerine. Corks impregnated with gelatin and treated with paraformal- dehyde to give them an inert, tough, resilient skin of “tanned” gelatin and which fitted tightly into a plain-neck flask were finally adopted for this purpose.

Nitroglycerine solutions for sorption experiments were made up from a stock saturated solution in n-hexane at 20°C. Four separate solubility determinations gave values of 0.327, 0.326, 0.327, and 0.329g/IOOml respectively, from which a mean value of 0.327 g/100ml was taken as the solubility of nitroglycerine in n-hexane at 20°C.

Results

A preliminary experiment to determine the sorption equilibrium time was carried out with a 12.2OhN nitrocellulose which had been precipitated from acetone solution as described earlier. The wt.-% of nitroglycerine (NG) in the solid phase as a function of time for 0.3853 nitrocellulose in 75ml of saturated n-hexane solution is shown graphically in Fig. 1 and the data shown in Table 1. I t is clear that a minimum for fourteen days is* required to establish sorption equilibrium, and in all subsequent experiments with fractionated samples (except fraction B2) at least twenty days was allowed.

94

Sorption of Nirroglycerine by Nitrocellulose

Wt.-% NG in solid

W In < I a 0

z d t

-I

51

N

IS0 250 H O U R S

13.9 16.7 18.9 19.4 20.0 20.1 20.1

Fig. 1. The amount of nitroglycerine taken up with time from n-hexane solution by granular nitrocellulose (12.2% N content) at 20°C.

The first series of sorption experiments were carried out with O.lG0.16g nitrocellulose in 25 ml of saturated nitroglycerine solution in order to compare equilibrium partition coefficients for each of the fractions under closely similar conditions. The results of these partition studies are shown in Tables 2 and 3.

In a second series of experiments an attempt was made to examine the separate effects ,of molecular weight and of nitrogen content on the shape of the sorption isotherms. Results are recorded in Tables 4 and 5. Isotherms for different molecular weight fractions .with the same nitrogen content are shown in Fig. 2 and 3, and for similar molecular weights but different nitrogen contents in Fig. 4. No change in nitrogen content had occurred during fractiona- tion of the 12.68% N nitrocellulose as shown by analysis of fractions 1.1 (12.70% N), 1.2 (12.65 O h N), and 1.4 (12.62 O h N) while only a slight change occurred for the 13.97OhN nitrocellulose as indicated by fractions B 1 (14.03 O h N), C 1 (1 3.74 O/O N), and F (13.56 YO N).

Only fractions having a fine granular structure, similar to that used to determine the equilibrium time (Table 1 and Fig. 1) were selected for sorption

95

Tab

le 2.

Pa

rtitio

n of

nitr

ogly

cerin

e be

twee

n 13

.97%

N n

itroc

ellu

lose

frac

tions

and

n-h

exan

e at

20-

C.

! Eq

uilib

rium

NG

con

c.

; Pa

rtitio

n cz

co

effic

ient

1 I-

(wt.

-%

in s

olid

) (g

;lOO

ml

liqui

d)

ClI

CZ

Frac

tion

Tim

e al

low

ed

for e

quili

briu

m

(day

s)

El" x

10-

A

20

BI

20

B2

18

c2

22

D

20

E

23

G

24

H

25

22.5

4 20

.93

22.6

8 23

.63

22.8

9 23

.42

23.8

4 22

.46

0. I4

66

0.16

14

0.15

07

0.17

25

0.13

73

0.15

46

0.1 34

9 0.

1455

1 54

130

150

137

167

151

177

154

205.

8 16

3.8

89.7

51

.4

38.9

16

.6

6.9

2.2

0

E; a

Tab

le 3

. Pa

rtitio

n of

nitro

glyc

erin

e be

twee

n 12

.68%

N n

itroc

ellu

lose

fra

ctio

ns a

nd n

-hex

ane

at 2

0 'C

Frac

tion

Tim

e al

low

ed

1 cI Eq

uilib

rium

NG

c

2 co

nc.

1 Partiti

on

, ~,x 10-3 fo

r eq

uilib

rium

co

effc

ien t

~ (d

ays)

(w

t.-o/

, in

solid

) (g

/100

ml

liqui

d)

CII

CZ

1.1

28

1.21

29

1.

3 20

1.

22

33

1.4

22

5 28

25.8

4 25

.87

21.8

5 24

.96

27.0

4 30

.64

0.1 1

91

0.11

22

0.15

36

0. I2

79

0.10

46

0.13

39

217

23 1

142

195

259

229

388.

2 34

2.2

2 15.

8 18

7.9

117.

2 38

.5

Table 4. Sorption of nitroglycerine from n-hexane solution by 13.97 % N nitrocellulose fractions at 20 C.

-

Fraction

A R, = 205.800

B2 R" = 89,700

II rn" = 2.200

Time allowed for eq uil i bri wn (days)

- . L .. . -

37 20 27 28

40 40 18

40 40 39 25 28 28

Equilibrium NG conc.

wt.-Yo in s o l i d ~ l ~ $ m l liquid

7.74 0.08 19 22.54 0.1466 32.62 0.2027 36.33 0.2363

~

~.

9.15 0.1022 13.00 0. I347 22.68 0.1507

11.62 0.0729 14.68 0.0858 17.68 0.1098 22.46 0. I455 29.86 0.2249 32.71 0.2460

.... - ~

Table 5. Sorption of nitroglycerinc from n-hexane solution by 12.68% N nitrocellulose fractions at 20 C.

Fraction Time allowed for Equilibrium NG conc. equilibrium (days) wt.-% in s o l i d - I g i l O O m l liquid

1.1 34

28 31

R" = 388,200 31

1.22 32

32 R,= 187,900 33

9.29 0.0890 20.14 0.0969 25.84 0.1 191 34.38 0.1780

16.70 0.1151 24.96 0.1279 32.05 0.1705

1.4 35 23.2 1 0.0858 &l = 1 17.200 22 27.04 0.1046

35 35.93 0.1776 - . - - - ~

equilibrium studies. For this reason the original fractions 2, 3, 4 and C 1 were not used for sorption studies, while too little of fraction F remained after nitrogen analysis.

97

P. Howard

t

1 1

. 0 5 40 .I5 .20 .25 .x) ~ N . G . IN LIQUID

Fig. 2. Sorption isotherms for 14.0% N nitrocellulose/nitroglycerine/n-hexane system at 20 C for samples A (R.=206x lo3), B 2 (R,=Wx lo3), and H (Rn=2.2x lo3).

5

Fig. 3. at 20 'C

I I I

Sorption isotherms for samples 1 . 1 (R.

P I I I

'05 '10 .IS .20 ,25 .30 ~ N . G . IN LIQUID

for 12.7 '10 N nitroccllulose/nitroglycerine/n-hexanc systcm =388x lo3), 1.22 (R,=188x 103),and 1.4 (i%lI,=117x 10').

98

L I I I

.05 .fO .I5 .20 ' .25 .30 ~ N . G . IN L I Q U I D

Fig. 4. Sorption isotherms for nitrocellulose samples of similar molecular weights but different nitrogen contents. 0 12.7% N samples 1.1 (R.=388x lo3) and 1.4 (a,= 117 x lo3); x 14.0% N samples A (Rn=206x lo3) and B 2 ( R n = 9 0 x lo3).

Discuss ion

I t is apparent that sorption of nitroglycerine by nitrocellulose takes place in two stages, an initial rapid uptake accounting for about 75% of the total amount sorbed followed by a much slower process until equilibrium is established. In precipitated nitrocellulose the crystallinity of the original material is largely destroyed resulting in a tanglcd mesh of chains in contact with each other only along part of their length. Thus spaces and interstices are present between aggregates of chains into which the nitroglycerine mole- cules can penetrate. The nature of the rate curve (Fig. 1 ) can perhaps be explained by assuming that the initial part corresponds to the filling up o f the free spaces already mentioned, when the slower process of penetrating the remaining aggregates of chains would then follow. The slowness of penetra- tion would be due, at least in part, to the low diffusibility of the bulky nitroglycerine moleculeand this effect has been recognised by other workers 5 . *.

From the results of the partition studies shown in Tables 2 and 3 there is clearly no obvious trend of miscibility with molecular weight for either

99

P. Howard

the high or low nitrogen content nitrocelluloses. In a homologous series of compounds, solubility in a given solvent usually decreases with increasing chain length. In fact it has been shown theoretically that an exponential decrease in solubility with chain length is to be expected23. However in the present system any preferential tendency of low molecular weight fractions to dissolve in nitroglycerine does not affect the amount taken up during the sorption process. From this it would follow that gelation of the nitrocellu- lose (involving partial dissolution in nitroglycerine) does not take place to any extent; probably a solid solution is formed with nitroglycerine filling the spaces within the network formed by molecular aggregates. This view is supported by earlier X-ray studies of' Petitpas' who found that sorption of alkyl nitrates by crystalline nitrocellulose never resulted in complete dissolu- ti& of the fibres. The amount of nitroglycerine taken up by precipitated nitrocellulose would thereforedepend to some extent on whether the aggregates of molecular chains were tightly or loosely packed in the sample. In turn the physical nature of the nitrocellulose'would be expected to depend on such factors as rate of addition of precipitant, concentration of solution, and rate of stirring during precipitation so that variations in these during preparation of the fractions may account for the varying partition coefficients observed (Tables 2 and 3).

Although sorption is apparently not affected by chain length of the nitrocellu- lose, the influence of nitrogen content is quite marked. All of the 12.68 Yo N fractions, excepting fraction 1.3, show a greater miscibility with nitroglycerine than any of those of 13.97'%N content (Tables 2 and 3, Fig. 4). One might perhaps expect that because of the chemical similarity between cellulose trini- trate and nitroglycerine, in that they are both completely nitrated hydroxy compounds, sorption would be greatest for the highly nitrated material. This was indeed found to be the case for sorption of ethyl, propyl, and butyl nitrates from the vapour phase as found by Petitpas'. In the same work sorption isotherms for, ketones and esters equilibrated with two crystalline nitrocelluloses of 11.4and 14.0% N contents showcd that at low concentrations sorption by the trinitrate was greater than for the less nitrated material, but at higher concentrations the dinitrate curve rose steeply and the reverse effect took place. A similar type of behaviour was observed in the present system as shown by the curves in Fig. 4. This effect may perhaps be explained by a difference in spacing between the nitrocellulose chains in the high and low nitrogen content materials based on the X-ray evidence of Petitpas. Spacing between adjacent chains of the dinitrate was found to be less than

in the trinitrate owing to steric effects. When sorbent molecules penetrate between the chains the ordered structure of the dinitrate is destroyed so that the chains can move apart and thus rapidly take up more sorbent. In the trinitrate, however, accommodation or sorbent molecules between molecular chains takes place with still some preservation of effective van der Waals forces between them which leads to a limited amount of sorption and a more gradual moving apart of the chains. In the present system of nitroglycerine and precipitated nitrocellulose the sorbent molecules are fairly large while the nitrocellulose is mainly amorphous with chains orientated at random. How far the above explanation, which is based on a more regular or crystalline structure, can really be applied here is uncertain. Its acceptance may have to await X-ray evidence to show that penetration of nitroglycerine molecules into crystalline cellulose dinitrate actually causes the molecular chains to move apart.

An alternative explanation of the increased sorption by the less nitrated cellulose is that i t might be due to some type of specific interaction between chemical groups. One such interaction might involve nitrate groups of the nitroglycerine and hydroxyl groups on the nitrocellulose chain. An interaction of this type has been suggested in the case of sorption of ketones by nitrocellu- l o s ~ ~ . ~ , but so far no evidence isavailable to suggest a similar type of interaction involving nitroglycerine.

I T. Petitpas, Mkm. Sew. chim. Etat 30 (1943) 201 W. R. Moore, Trans. Faraday SOC. 43 (1947) 543 J. Chedin and R. Vandoni, MCm. Scrv. chim. h a t 33 (1947) 205

W. R. Moore, J. Textile Inst. 40 (1949) T 731 P. Drechsel, J. L. Howard, and F. A. Long, J. Polym. Sci. 10 (1953) 241 P. C. Schcrer and N. J. Crookston, J. Polym. Sci. 14 (1954) 129

' H. Campbell and P. Johnson, J. Polym. Sci. 4 (1949) 247

- * W. R. Moore, J. Polym. Sci. 15 (1955) 305 '' F.. Calvct. Bull. Soc. Chim. Fr. 23 (1956) 75

I " P. Y. Hsieh, J. Appl. Polym. Sci. 7 (1963) 1743 J. Desmaroux, Mem. Poudres 24 (1931) 86, 211

l 2 J. Desmaroux and M. Mathieu, C. R. hebd. Seances Acad. Sci. 191 (1930) 786 I J J . ChCJin. MCm. I'oudrcs 29 (1939) 95 I' D. Fensom, Canad. J. Res. Sect. B 26 (1948) 59 l 5 T. Sakuri, J. ind. Explosives Soc. Japan 13 (1952) 33 I ' T. Sakuri and Y. Sato, J. ind. Explosives SOC. Japan 13 (1952) 288

J. Dubar, C. R. Acad. Sci. Ser. C 264 (18) (1967) 1532

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P. Howard and F. L. Usher, Nature 198 (1963) 179 l 9 P. Howard, Chem. Ind. London 1%3, 1031 2o W. R. Moore and G. D. Edge, J. Polym. Sci. 47 (1960) 469

2 2 S. Sandi and G. Flanquart, Chim. Anal. Paris 39 (1957) 20 23 P. J. Flory, Principles of Polymer Chemistry, 1. ed., Cornell University Press, New

W. G. Harland, J. Textile Inst. 49 (1958) T 478

York 1953, p. 559

102