CHAPTER 3
PREPARATION, CHARACTERIZATION AND PHYSICAL PROPERTIES OF
POLYVINYL NITRATE
For the present study, it vra.s intended to prepare samples of
polyvinyl nitrate (PVN) differing in their nitrogen content, so
that their properties could be determined and correlated with
nitrogen content. In a nitrated polymer like PVN, it is expected
that higher degree of nitration would ensure greater availability
of oxygen (that is, higher oxygen balance), which, in turn, should
improve the performance potential of PVN as a high energy material.
Literature survey has shown scanty reports of investigation in this area.
MATERIALS AND METHOD
Polyvinyl alcohol (PVA)
Commercial variety of polyvinyl alcohol, procured from
trade and having the following characteristics, was used for
preparation of PVN :
Designation of PVA : Gohsenol NM-11
Manufacturer : M/S Nippon (Japan)
Supplied by
M/S Merubeni Corporation,
Bombay.
Degree of polymerization : 1100 (approx.)
Degree of hydrolysis : 99 - 100°/
Viscosity (of 4% aqueous solution at 20°C) : 13-16 cp
Volatiles , max : 5%
24 Nitric acid
The following three grades of nitric acid were used for nitration
of PVA, viz.
(a) 98-99% HNO ("Strong nitric acid" or SNA, supplied by
High Explosives Factory, Khadki, Pune)
(b) 70.7+1% HNO ("Concentrated Nitric Acid", supplied by
M/S Glaxo)
(c) 63.7+ 1% HNO„ (Moderately Cone. Nitric Acid, prepared
locally by diluting above acid).
Acetic anhydride
Glaxo, Special quality (SQ) grade
Density (at 20°C ) : 1.080 gm/cc
Preparation of PVN
Several small batches of polyvinyl nitrate (PVN) were
prepared by nitration of granular or powdered polyvinyl alcohol
(PVA) 50 gm, by addition of excess of cooled strong nitric acid
(SNA) to PVA at a temperature of -10 + 2°C. The reaction, being highly exothermic and fast, needs careful attention to avoid local
ignition of PVA particles. A brief outline of the procedure is given below.
SNA (300gm), initially cooled to 0 c, was gradually added to a pre-cooled suspension of Gohsenol NM-11 grade of PVA (50 gm), in acetic anhydride (572.4gm) in a three-necked flask. The reaction mixture was maintained at -10 to -5°C, with constant stirring, for approx 2 hrs (acylation of alcoholic group is then avoided). Then
the mixture was kept at -5° to 10°C for approx 2 hrs and at 10°C to
20°C for another 3 hrs, with constant stirring, the final
temperature was gradually raised to 20°C, which was required for
complete nitration. Thus, a total reaction time of 7 hrs was
25 allowed, after which the yellowish reaction mixture was poured
slowly to excess of ice cold water. The resulting white fibrous solid was filtered off .
After filtration, the product was washed repeatedly with plenty of hot water (at 60°C) to remove acidity. It was then washed with 12% solution of sodium bicarbonate and followed by washing with cold water to neutrality. After filtering off, the moist product was dissolved in acetone and reprecipitated with 47. solution of sodium carbonate and again washed with water until neutral, and finally washed with distilled water.
After drying, the product was dissolved in acetone and the solution diluted with water to regenerate PVN in a pure white fibrous form. The yield of PVN was approximately 86% of theory.
Several batches of PVN, prepared by this method, were mixed thoroughly to form a "lot" and used for the present study.
The above mentioned method was used for preparation of PVN with relatively high percentage (> 15.0%) of nitrogen. Due to very high reactivity of PVA with HNO , it was very difficult to prepare lower nitrated grades of PVN. Therefore, to obtain different percentages of nitrogen in PVN, starting with 13 gm of PVA in all experiments, some reaction conditions were changed, namely, reaction temperature, reaction time and strength (concentration) of acid.
A batch of PVN was prepared by addition of excess of
ntrated nitric acid (70,7 + 1% HNO„) into PVA+acetic anhydride conce 3 suspension at a temperature of -10 to -5°C, followed by gradual rise to -5 to 15°C (2 hrs) and 15 to 20°C (2 hrs). Thus, a total reaction time of 6 hrs was allowed. After necessary processing
16.17% yield of PVN was obtained with 14.95% N content.
In another experiment, PVN was prepared by addition of
26 strong nitric acid (98-99% HNO ) to PVA+acetic anhydride suspension
at a temperature of -10 to -5°C, followed by a regulated rise to
19 C. A reduced total reaction time of 5 hrs was allowed. Yield of
70% PVN was obtained with 13.34% nitrogen content.
In another experiment, PVN was prepared by addition of diluted
commercial nitric acid (63.7+1% HNO ) to PVA+acetic anhydride
suspension at a temperature of -10 to -5°C, followed by a regulated
rise to 20°C. A total reaction time of 6 hrs was allowed. 15.4% yield of PVN was obtained with 11.76% nitrogen content.
A summary of experimental variables for preparation of PVN
lots, which were subsequently used for studies under the present dissertation, is given in Table 3.1.
Table 3.1. Reaction conditions for preparation of PVN samples
having different Nitrogen content
Condition during Condition after Overal1 Nitro Reagents add it;: on of addition of reaction gen in reagents reagents time PVN * Temperature Time Temperature Time hrs. °C hrs. hrs. %
PVA + -10 to -5 2 -5 to 10 2 98-99% HNO 7 15.71 10 to 20 3
PVA + -10 to -5 2 -5 to 15 2 70.7+1% HNO 6 14.95 15 to 20 2
PVA + -10 to -5 2 -5 to 8 2 5 13.34 98-99% HNO^ 8 to 19 1
PVA + -10 to -5 2 -5 to 15 2 63.7+1% HNO 6 11.76 o 15 to 20 2
* details given on page 29
27 Before standardizing the above procedure, other possible
procedures were also used on the basis of trial and error. For example, addition of polyvinyl alcohol (PVA) into cooled strong nitric acid (SNA), or addition of PVA+ acetic anhydride slurry into to cooled nitric acid, or addition of nitric acid into a suspension of PVA+carbon tetrachloride were tried out. However, the addition of nitric acid into a suspension of PVA+acetic anhydride (which was 18 essentially similar to that metioned in U.S. Patent) was better than other procedures due to the following advantages : better control over the rate of mixing of reactants, avoidance of local ignition of polyvinyl alcohol, use of small excess quantity of SNA, better yield of product, and better quality of product.
CHARACTERIZATION OF PVN
The above product (PVN) was characterized by following studies:
(a) Nitrogen content
(b) Infra-red (IR) spectra
(c) Scanning electron microscopy (SEM)
(d) X-ray diffraction
(a) Nitrogen Content
Nitrogen content in PVN is a convenient indicator of the degree of substitution (DOS) of hydroxyl groups in the original PVA chain by nitric ester (ONO ) groups. PVA contains secondary alcoholic groups, and when nitrated completely, it should form PVN with a theoretically maximum nitrogen content of 15.73%.
It is reasonable to expect that several properties of PVN, especially its energetic and explosive properties, would bear correlation with its "nitrate" content, or, simply, its % N.
However, it should be remembered that " % N " indicates only the
28 average nitrogen content in the PVN chains and not the actual distribution pattern of -ONO groups.
Amongst 13 "lots" of PVM, prepared for the present work, eight were micro-analysed for nitrogen content only, while the other five were analysed for carbon and hydrogen also. The results showed values of % N varying from 11,76% to 15.73%, as shown in Table 3.2.
IK Table 3.2. Elemental composition of PVN Lot No. Carbon % Hydrogen % Oxygen % Nitrogen %
I 30.35 3.79 51.88 13.98
II - - - 15.62
III - - - 14.74
IV - - - 14.51
V - - - 14.76
VI - - - 15. 13
VII - - - 15.73
VIII - - - 15.35
IX - - - 15.33
X 27.05 3.42 53.82 15.71
XI 28.38 3.55 53. 12 14.95
XII 31.80 3.97 50.89 13.34
XIII 35.05 5. 16 48.03 11,76
Note
(a) Lot I was prepared by adding PVA to SNA + CCl^, and Lot II was prepared by adding PVA + Ac 0 slurry to SNA. Lots III to X and
XII were prepared by adding SNA to PVA + Ac 0 slurry. Lot XI was prepared by adding concentrated HNO^ (70,7+1%) to PVA + Ac^O slurry, while Lot XIII was prepared by adding moderately concentrated HNO„
(63.3 + 1%) to PVA + Ac 0 slurry. Lots IV to IX represent mixtures of different batches..
29 (b) The analysis of PVN samples was carried out partly at
National Chemical Laboratory (NCL), Pune and partly at Explosive
Research and Development Laboratory (ERDL), Pune.
(c) Lot Nos. X, XI, XII and XIII were prepared in sufficient
quantities and used for detailed studies.
(b) Intra-red Spectra
Infra-red spectra of thin film of PVN samples were taken in
a Perkin Elmer double beam spectrophotometer. Typical IR spectra of
PVN samples containing 15.71%, 13.34%, and 11.76% N are shown in
Fig 3.1. The spectra show absorption peaks at the following
frequencies which are assigned to characteristic structural po "^c^ '^R features in accordance with literature ' '
Frequency cm Relative intensity Assignments
690 strong -NO , bending
750 strong -NO , wagging
850-•870 broad -NO, streching
1275 very strong -0N0„, bending
1430 strong -CH , bending
1675 very strong -0N0„, streching
1700 weak impurities
As can be seen in IR graphs, the absorption peaks due to various
structural features depend upon % N in PVN samples. It is
noteworthy that -OH group absorption is absent in most of the
samples. These observations are supported by Fourier Transform
Infrared (FTIR) spectra for PVN, containing 15.71% N and 11.76% N,
which are shown in Fig 3.2.
30 CD Z z < I— z o o z > a.
=3 O cc
a: o ca
00 Di
NOlSSIWSNVbi Vo KD
o v^ 2: > 2: Q- ^ U~) ZD vO 0 r- c: ^ CCl '^ Li_ CQ a: **-• 0 a_ rr < ^^ 1— ;:: UJ Q- i-n
(/•) r— cr
no (c) Scanning Electron Microscopy (SEN)
The morphology of PVN was determined by using a
JSMT, T-200 scanning electron microscope (SEM), fitted with a
Majniya (6x7) camera. Other specifications of the instrument are as
follows :
Maximum magnification 100000
Maximum resolution 70 A
Maximum working distance 48 mm
Maximum acceleration voltage 25 kv
Specimen size 50 mm
PVN (15.71% N and 11.76% N) samples, ungelatinized and gelatinized (a thin film of the latter placed on a round glass piece), were mounted on a silver coated stud. Then vacuum was applied and gold was plated by a cathode spurting coating before examination. Fig 3.3 and 3.4, show some of the SE micrographs of
PVN (15.71% N and 11.76% N), obtained under magnifications of 75,
150, 350, 500, and 750. It is observed that : —
(i) Fibrous PVN (15.71% N) contains streamlined and orderly arrangement of fibre bundles, unlike fibrous PVN (11.76% N),
(ii) Fibrous PVN (15.71% N) shows a greater surface porosity than fibrous PVN (11.76% N),
(ill) PVN (15.71% N) undergoes gelatinization, by acetone treatment, to a greater extent than PVN (11.76% N). (The fibrous and porous nature of PVN is lost due to its gelatinization into a homogeneous colloidal mass).
(d) X-ray Diffraction
Configurational studies of PVN samples were carried out by X-ray diffraction method. All measurements were carried out by vertical goniometer method, on a Philips PW 1730/10 difractometer
33 mounted on a Philips PW 1050/70 (40kv, 20mA) generator. The
diffractometer vra.s equipped with slits in the incident beam and a
Nickel (Ni) manometer (filter) in reflected beam. Copper CukA
radiation (X = 1.5418 A or 0.15418nm) was used in all experiments.
The sample was aligned approximately parallel to the scattering
vector. 29 was varied around 10 to 60° or 10 to maximum intensity.
Typical scan times of diffractograjns were 1° (2e)/min. for
measurements at room temperature.
Fig 3.5 shows a typical diffractogram for PVN containing
15.71% N. A sharp intense peaJc was observed at the 2 6 angle between 16-18°.
Fig 3.6 shows a diffractogram for PVN containing 11.76% N.
Fig 3.7 shows a diffractogram for PVA (raw material used for preparation of PVN). A sharp peak was observed at the 2 9 angle between 18-20°
% crystal Unity of PVN samples was calculated by the 37 following stajidard formula
100 I Area underneath crystalline % crystal Unity = — or x 100 (1 +1 ) Total area c a or Area underneath crystalline ._^
(Area underneath crystalline+Area underneath amorphous)
The values obtained from above calculations are as follows :—
PVN (15.71% N) 47.62%
PVN (11.76% N) 41.78%
PVA (for comparison) 40.38%
Thus, according to above observations, it caji be concluded that the degree of crystal Unity (and, therefore, the orderly arrangement of fibres) increases after nitration of PVA and also increases with increasing % N in PVN samples.
3'^ !A) FIBROUS, X 75
'6) FIBROUS, X 150
Fig 3.3. SCANNING ELECTRON MICROGRAPH FOR PVN ( 15.71% N) (C) FIBROUS, X 350
(D) FIBROUS, X 500 (E) FIBROUS, X 750
fn GELATINIZED, x 350
37 (6) GELATINIZED, x 150
38 (A) FIBROUS, X 75
(B) FIBROUS, X 150
Fig 3.4. SCANNING ELECTRON MICROGRAPH FOR PVN ( 11.76% N )
no nh-^os'^
(C) FIBROUS, X 350
(D) FIBROUS. X 500
•1T).305S
40 ffJ FIBROUS, X 750
(^) GELATINIZED, x 350
41 (G) GELATINIZED, x 150
42 lA
a.
O
O U. z < u> o I— u <
Q£ I X in CO
Ai;SN3iNI a. 1/5
O
CQ
O
<1 <
CSI O <
< I X t
ro
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AllSN3iNI < a. Of o Ll. <
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AilSN3iNI PHYSICAL PROPERTIES OF PVN
The physical properties of polyvinyl nitrate have been studied as follows:
(1) Appearance
The PVN samples were, generally, white and fibrous in
appesu'zuice.
(ii) Solubility
PVN (15.71% N) was found to be insoluble in water,
ethanol and methanol". Its solubility in different solvents, at room
temperature (25°C), is as follows.
SN. Solvents Solubility of PVN (15.71% N) (•/. by weight)
1. Acetone 42.00
2. Ethyl acetate 4Q.74
3. Methyl ethyl ketone 40.00
4. Tetraihydrofuran 20.00
5. Isoamylacetate 20.00
6. Dimethylformamide 16.00
7. Ethylacetoacetate 10.00
8. Dimethylsulphoxide 04.00
9. Nitrobenzene 01.60
Thus, acetone, ethyl acetate and methyl ethyl ketone (MEK) were
found to be best solvents for PVN (15.71% N).
Interestingly, the solubility of PVN (11.76% N) was found to be
only 2% by wt. in acetone. This shows that there is a relationship
between degree of nitration (% N) and solubility. PVN with high % N
is more soluble in acetone.
46 (Hi) Gelatinization
When a fibrous nitrated polymeric substance is treated with
a suitable solvent, it passes into a colloidal solution losing its
fibrous and bulky nature. This process is known as gelatinization and the solvents used are called gelatinizing solvents. Certain compounds facilitate the process of gelatinization and these are known as gelatinizing agents. The gelatinized substance h£is a homogeneous non-porous structure which facilitates subsequent processing and fabrication In a suitable equipment. It has been found that PVN (15.71*/. N) undergoes gelatinization in acetone, ethyl acetate and MEK solvents, and also in the following solvent mixtures :
Acetone + ethyl alcohol (50:50)
Dibutyl phthalate + diethyl phthalate (50:50)
It has also been observed that, in a particular gelatinizing solvent, the process of gelatinization of PVN, becomes progressively more efficacious as the % N increases.
(iv) Plasticlzation
In order to facilitate the processing of propellemt compositions and subsequent fabrication of solid propellant grains through conventional procedures like extrusion, it is necessary to improve the plasticity of the composition by incorporation of suitable plasticizing agents. This process is known as pasticization. The following compounds, in CP grade, were used as plasticizers by mixing them uniformly with PVN SEunples, containing different '/. N, in different weight concentrations :
dioctyl adipate (DOA), dibutyl phthalate (DBP), diethyl phthalate
(DEP), dimethyl phthalate (DMP) and a mixture of DBP + DEP.
It was observed that in case of DOA, fibrous PVN (15.71% N) did
47 not show any chajige even after mech£inically working in agate
pastel.
After addition of DBP, fibrous PVN (15.71% N) broke up and on
continuous working in agate pastel the fibrous structure was
converted into a homogeneous sticky mass .
After addition of DEP, fibrous PVN (15.71% N) started breaking
up without gelatinization, and, therefore, particles of PVN remained
as such in DEP. However, after addition of DMP in PVN, it started
breaking up with gelatinization. A homogeneous mass was obtained
after continuous working in agate pastel which was less tacky than
in DBP and others.
A mixture of DBP and DEP was used in 1:1 ratio and after
addition of this mixture, fibrous PVN (15.71% N) gelatinized and a
homogeneous mass was obtained with less tackiness.
From the above experiments, it was observed that DMP and a
mixture of DBP+DEP (1:1) were suitable for use as plasticizers with
PVN (15.71% N), but it was very difficult to make a hard strand due
to somewhat tacky nature of plasticized PVN.
Similar results were obtained with the other PVN samples
(of lower % N), but the tackiness of plasticized PVN decreased with decrease in % N.
(v) Softening Temperature
PVN seunples were found to soften on heating (on a water bath) at different temperatures, depending on their nitrogen content, as mentioned below :
% N in PVN Softening Temp.(°C)
11.76 39
13.34 43
14.95 45 15.71 50
48 Thus, with increasing % N in PVN, its softening temperature
increeises, apparantly due to increase in crystallinity.
(vi) Viscosity
Viscosity values of 0,5% solutions of PVN samples (11.76%
to 15.71% N) in acetone were determined in a Ostwald viscometer at
25°C . The results are EIS follows; —
% N in PVN Viscosity, cP
11.76 0.643
13.34 0.513
14.95 0,451
15.71 0.443
Also, the viscosity of 0.5% solution of PVN (15.71% N) in acetone was found to be 0.494 cP at 23°C and 0.433 cP at 27°C.
From the above data, it is evident that with increasing % N
in PVN, as well ELS with increasing temperature, the viscosity (of
0.5% acetone solution) of PVN decreases. It is likely that the presence of more undissolved particles in acetone solution of a PVN sample having lower % N is responsible for its restricted flow rate and higher viscosity. And, with increasing temperature, the solubility of a PVN sample should increase and thus, cause
reduction in viscosity.
(vii) Molecular Weight The molecular weight of typical PVN samples containing
(15.71% N) was determined by employing gel permeation
chromatography (GPC) technique using polystyrene as a standard. The
molecular weight was found to be in the order of 100.000. This
corresponds to a degree of polymerization (n) approximately 1100, considering that the repeat unit (C^H^NO^) has a "molecular weight"
of 89.05. It appears, therefore, that the original chain length of
PVA (n=1100)is largely maintained in PVN despite the rigours of
nitration and subsequent washing with hot water, and oxidative
cleavage is negligible.
CONCLUSION
By varying the nitric acid concentration and reaction-time
schedule, four lots of PVN containing 15.71'/., 14.95%, 13.34% and
11.76% nitrogen were prepared. PVN seimples were characterized by
Infra-red spectra, Scannining electron microscope (SEM) eind X-ray
diffraction. IR sp>ectra confirm the presence of nitric ester group.
SEM shows that PVN has a porous surface which is lost on gelatinization with a solvent (acetone). The percent crystallinity of PVN, calculated from x-ray diffractograms, is greater for a sample with 15.71% N than one with 11.76% N.
PVN sajnples were, generally, white in appearance. The softening temperatures increased, with increasing '/, N, from 39°C to 50°C.
Mol.wt.of PVN (15.71% N), determined by Gel permeation chromatographic (GPC) technique, was approx. 100,000. PVN samples were soluble in polar solvents. When fibrous PVN was worked up with acetone, it formed a gelatinized homogeneous mass. Working up of
PVN, with dibutyl phthalate, diethyl phthalate and dimethyl phthalate, etc, at room temperature, brought about its plasticization giving a tacky mass. Viscosity of 0.5% solution of
PVN sajnples in acetone, showed a decrease with increasing % N in
PVN. Also, 0.5% solution of PVN (15.71% N) in acetone showed a decrease in viscosity with increasing temperature.
50