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CHAPTER 3
MATERIALS & EXPERDfENTAL METHODS
32
A brief description of the various materials, techniques, methods of
preparation as well as characterization of the prepolymers, oligomers,
crosslinked elastomers and solid propellants is presented in this chapter.
3.1 MATERIALS
Benzene
Benzene (BDH, Bombay) ,AnalaR' was of high purity. The traces of
moisture present were removed by, treating it with anhydrous calcium chloride
and keeping in contact with sodium wires. Distillation was done prior to use
and the distillate boiling at 80-81°C was collected and stored over molecular
sieves of 4A0•
Toluene
High purity toluene was obtained from the manufacturer (SRL, Bombay).
The purification procedure adopted was the same as that used for benzene.
The fraction boiling at 109-11 DoC was collected and stored over molecular
sieves of 4A0•
Methanol
The traces- of moisture, which could be present in the high purity
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methanol (Ranbaxy, Madras) was removed by distilling it through a
fractionating column and standing it over molecular sieve of 4Ao.
Glacial acetic acid (HAc)
Glacial acetic acid (BDH, Bombay) was 'AnalaR' grade and aldehyde
free. The traces of moisture present were removed by keeping the acid in
contact with dry silica gel. Finally the pure acid was obtained by distillation and
the fraction distilling at 118-119°C was collected.
Periodic acid (PIA)
Pure crystal1ine periodic acid (H5106 ) (Fluka, Switzerland) was used from
a freshly opened bottle. Crystals were quickly weighed out into stoppered
amper coloured conical flasks, as PIA was reported to be light sensitive (135).
Perbenzoic acid (PBA)
150 ml of pure methanol was taken in a conical flask of 500 ml capacity
fitted with a reflux condenser and kept cooled at -5°C in a Julabo immersion
bath. 5.5 g of sodium metal was added in small portions to this. After the
reaction subsided, the condenser was removed and 220 ml of 25% CHCI3
solution of benzoyl peroxide was slowly added (20 ml/min) into it under
agitation. After the addition was over, the cold reaction mixture was
transferred to a separating funnel and extracted the sodium perbenzoate
34
formed with 500 ml of ice cold water. The methyl benzoate present as an
impurity in the aqueous layer is removed (from the aqueous layer) by extracting
it twice with 120 ml of cold chloroform. The aqueous layer is acidified with
250 ml of ice cold 0.5 M. H2S04 and the liberated PBA is extracted thrice with
125 ml of benzene. The PBA solution thus obtained was transferred to
polyethylene bottles and kept cooled inside a refrigerator.
Acetone
Acetone (SRL, Bombay) was 'AR' grade. Traces of impurities were
removed by shaking it with silver nitrate solution, followed by 1 M-sodium
hydroxide solution (136). This mixture was then filtered, dried with anhydrous
calcium sulphate and finally distilled. The fraction distilling between 56-5rC
was collected.
n-Propanol
n-Propanol (SRL, Bombay) was pure when obtained from the
manufacturer. It was used after distillation.
Gy252, and 1,2,7,8 Diepoxy octane
Gy252 (CIBA, Bombay) and 1,2,7,8 diepoxy octane (Aldrich, USA) were
very pure when received from the manufacturers and used without further
purification.
35
Aluminium (AI)
Pure Aluminium powder (MEPCO, Madurai) of average particl~ size
15-20 microns, was used as the metallic fuel in all the propellant formulations.
Tris (2-methyl aziridinyl-1) phosphine oxide (MAPO)
The crosslinking agent MAPO (Arsynco, Inc,USA) was very pure and
used without any purification.
Ammonium perchlorate (AP)
The oxidizer used for the various propellant formulations were pure
white crystalline ammonium perchlorate (WIMCO, Bombay) of two different
particle sizes; the coarse variety with a particle size of 300 microns and the
fine variety with a particle size of 50 microns. The fine ammonium perchlorate
was prepared by grinding the coarse in a pin mill grinder under a blanket of
nitrogen. Both the varieties of ammonium perchlorates were dried thoroughly
in an electrically heated oven at 80aC for 8 hours before use, to remove the
traces of surface moisture.
Copper chromite
Copper chromite (Spectrochem, Bombay) was very pure and used as
received from the manufacturer.
36
Ferric oxide
Ferric oxide (Glaxo, Bombay) was very pure and used as received from
the company.
Cis 1,4 polybutadiene rubber (PB)
PB of number average molecular weight 0.3012 x 106 (Synthetics and
Chemicals, Bareily) was purified as per the adopted method (Sec. 3.2.1.1) and
used for the oxidative degradation reaction.
Poly (isobutene-isoprene) rubber (PIPP)
PIPP of number average molecular weight 0.2914 x 106 (VSSC,
Trivandrum) was purified as per the adopted method (Sec. 3.2.1.1) and used
for the oxidative degradation reaction.
Poly (isobutene-butadiene) rubber (PIPB)
PIPB of number average molecular weight 0.3071 x 106 (VSSC,
Trivandrum) was purified as in the case of PIPP and used for the oxidative
degradation reaction.
Cis 1,4 polyisoprene rubber (PP)
PP of number average molecular weight 0.3124 x 106 (IS-NR 5, Rubber
Board, Kottayam) was purified as per the method given (Sec. 3.2.1.1) and used
for the oxidative degradation reaction.
37
HC-434
HC-434 prepolymer of number average molecular weight 41 00 (Thiokol,
USA) was used after removing the antioxidant by washing 150 g of this resin
four times with 500 ml of pure distilled methanol. After decanting out the
supernatent liquid, the entrapped methanol was removed by flash evaporating
the resin under vacuum at 70DC. Pure HC-434 resin thus obtained was stored
in polyethylene containers.
Phenyl-B-naphthyl amine (PBNA)
PBNA (ICI, Ltd, Bombay) was very pure and used as received.
Zinc octoate
Zinc octoate (Amrut Chemicals, Bombay) a viscous straw yellow liquid
used as the curing catalyst, was used as received.
Carbon tetrachloride
Carbon tetrachloride (Glaxo, Bombay) was pure. However, to remove
traces of moisture, it was dried over anhydrous calcium chloride and distilled.
The first few ml were rejected and the fraction distilling between 77-78DC, was
collected.
38
Chloroform
Chloroform (SRL, Bombay) was sufficiently pure. To remove the traces
of ethanol, which could be present as the stabiliser, chloroform was thoroughly
shaken with small quantities of water and the water and chloroform layers
were separated out. The final traces of moisture were removed by keeping it
in contact with anhydrous calcium chloride for 12 hours. Finally, the pure
chloroform was collected by distillation at 61-62°C. Chloroform thus obtained
was kept in dark in stoppered bottles.
1,4-Dioxane
1A-Dioxane (Ranbaxy, Madras) was extra pure and was used just after
distillation at 101°C over sodium.
Hydrochloric acid
High purity hydrochloric acid (BDH, Bombay) was used as received.
Pyridine
Pyridine (BDH, Bombay) was of ' AR' grade and was further purified by
refluxing it over KOH pellets, for 8 hours and distilling the contents. The
fraction distilling in the temperature range 11 5-116°C was collected and stored
over Fisher type 4A° molecular sieves.
39
Dichloromethane
Dichloromethane (SRL, Bombay) received from the manufacturer was
further purified, by shaking it vigorously with a 6% aqueous solution of sodium
carbonate and washing several times subsequently with distilled water. It was
then dried with anhydrous calcium chloride and finally distilled. After rejecting
the first few ml, the fraction boiling at 40-41°C was collected and stored in
stoppered bottles.
Dioctyl adipate (DOA)
Dioctyl adipate (Indo-Nippon Company, Bombay) used as the plasticizer
in the propellant formulations, was very pure and used as received.
Tetrahydrofuran
Tetrahydrofuran (Spectrochem, Bombay) was distilled prior to use. The
pure fraction distilling at 65- 67°C was collected.
Boron trifluoride etherate
Boron trifluoride etherate (Fluka, Switzerland) was pure when received
from the manufacturer. The final purification was done by the procedure
recommended (137). Accordingly, 10 ml of pure ether was added to 500 ml
of boron trifluoride etherate and distillation was done in presence of 2g of
calcium hydride, which removed volatile acids and avoided bumping, in an all-
40
glass apparatus at 46°CI1 0 mm. Pure colourless boron trifluoride etherate thus
obtained was collected in amper coloured bottle and stoppered after rejecting
the first fraction.
Potassium Iodide
Potassium Iodide (BDH, Bombay) extra pure (GPR) was used for all
iodometric estimations.
3.2 HOMO PREPOLYMER, OLIGOMER, BLOCK PREPOLYMER, NITRATO
PREPOLYMER SYNTHESES AND CROSSLINKED ELASTOMER
PREPARATION
3.2.1 Homoprepolymers
3.2.1.1 Purification of PB, PP, PIPP and PIPB
200 gm of PB was chopped into small slices with a rubber cutter and
added in portions to 2.5 Iitres of toluene, kept agitated with a glass stirrer in
a round bottom flask. The agitation was continued till PB, completely dissolved
in it. The viscous solution was divided into two parts and each part was
separately precipitated with 5 Iitres of propanol which being the nonsolvent for
PB was taken in excess to ensure complete precipitation (138). Care was taken
41
to add the PB solution as a thin stream into propanol under constant agitation.
Dissolution and precipitation were repeated three times to ensure the complete
expulsion of all the antioxidant and other impurities present. The solution
containing the precipitated polymer was quickly filtered using a buckner funnel
to separate the polymer from the toluene-propanol mixture. The polymer was
further washed several times with fresh quantities of propanol and again
filtered. The soft fluffy polymer thus obtained was transferred to a glass tray
and dried at 50°C in a vacuum oven to remove the last traces of propanol.
The same procedure was adopted for the purification of PP, PIPP and
PIPB. The non solvent used for precipitation was methanol instead of propanol
(139).
3.2.1.2 Epoxidation of PB, PP, PIPP and PIPB
For carrying out the epoxidation, an excess of PB polymer solution
(8%) in benzene (about 2 litre) was taken in a glass trough in which was
immersed the impeller pump. The pump was first operated to calibrate the rate
of pumping of the (polymer) solution. Accordingly, the quantity of PBA solution
(0.4 moles/litre) was taken in the oxidant dispenser which was connected to
the impeller pump. The polymer solution was maintained at 25±1°C
temperature range and PBA mixing was found to proceed smoothly (when the
pump was operated), resulting in the desired extent of epoxidation (140-142).
42
During the reaction PBA attacked the double bonds, present in the
macromolecular chain and rapidly got consumed as expected (143).
The epoxidised cis 1,4 polybutadiene (EPB) was precipitated by excess
of propanol as per the procedure adopted (Sec. 3.2. 1.1) for the precipitation
of PB. Epoxidized PP, PIPP and PIPB (designated as EPP, EPIPP and EPIPB)
were also prepared as stated above. But the polymers were precipitated with
methanol instead of propanol. Care was taken during drying of all the
epoxidized polymers, in the vacuum oven to prevent any crosslinking reaction
(144) by maintaining the drying temperature at 40°C.
3.2.1.3 Lactone terminated cis 1,4 polybutadiene (LPB)
LPB prepolymer was prepared in a 5 litre round bottom flask, equipped
with a teflon stirrer, thermometer and a dropping funnel. The apparatus was
kept in a water bath, maintained at 25°C and was purged with a stream of dry
nitrogen gas. 2 litres of a 8% EPB solution in toluene was transferred to the
flask and kept stirred, followed by 1.066 litre Qf a 0.18 mole/litre solution of
PIA in glacial acetic acid. Care was taken to add the solution in two portions
from the dropping funnel at an interval of 5 minutes. The stirring 'was stopped
for a while, the dropping funnel was replaced with a water condenser and the
stirring restarted. The solution was stirred for another 5 more minutes. The
reactants were then refluxed for 55 minutes, and allowed to cool to room
43
temperature. The contents of the flask were precipitated with large excess (8
to 9 liters) of propanol and allowed to stand overnight. The supernatent
propanol solution was decanted out, to isolate the liquid prepolymer and it was
repeatedly washed with fresh quantities of pure propanol till the washings were
free from acid. The liquid prepolymer thus obtained was flash evaporated under
vacuum at 900 e for half an hour to remove the traces of propanol and other
impurities. LPB prepolymer, thus synthesized was poured into a weighed
beaker and the final weight recorded. From these values the weight of the
prepolymer was obtained and the yield computed. The prepolymer was
transferred to a labelled and stoppered amber coloured bottle.
3.2.1.4 Lactone terminated polyisobutene (LPI) and cis 1,4 polyisoprene
(LPP)
The prepolymers lactone terminated polyisobutene (LPn and lactone
terminated cis 1,4 polyisoprene (LPP) were synthesized as per the procedure
given for LPB, by starting from the corresponding epoxidised polymers (EPIPB,
EPP) and subjecting them to oxidative degradation with PIA. The final
purifications of LPI and LPP were done by repeatedly washing them with
methanol. The traces of methanol were removed by flash evaporating the
prepolymers at 700 e for half an hour under vacuum.
44
3.2.2 Oligomers
The experimental set up for the syntheses of oligomers of PB, PI and
PP with lactone terminals (OlPB, OlPI and OlPP) was similar to that used for
the synthesis of lPB, except for the batch size. So 2 litres of the 4% EPB
solution was transferred to a 5 litre round bottom flask and kept stirred. The
solution was further treated with 2.02 litre of a 0.18 moles/litre solution of PIA
in glacial acetic acid, added in portions at an interval of 5 minutes and then
afterwards followed the same procedure, as given for lPB synthesis.
However, for the synthesis of OlPI, the polymer used ,was EPIPP
instead of EPIPB, as the latter polymer was of higher molecular weight and also
consisted of longer blocks of polyisobutene, which could not result in OlPI of
lower molecular weight during the degradation reaction.
3.2.3 Block prepolymers
3.2.3.1 lactone terminated cis 1,4 polybutadiene-polyisobutene (lPBPI)
The experimental set up consisted of specially fabricated five necked
reaction kettle of 5 litre capacity, fitted with a mechanical stirrer and an
inlet-outlet connection for the continuous flow of dry nitrogen gas. A dropping
funnel and a thermometer were the other accessories of the apparatus: The
45
kettle was maintained at a temperature of 1O.±.1°C in a low temperature bath,
constructed employing Julabo FT 401 immersion cooler, provided with an
integrated electronic temperature control and an immersion probe.
The reaction kettle was first purged with nitrogen and 550 ml of a 0.7
moles/litre solution of 1,2,7,8 diepoxy octane in CCI4 and methanol (2: 1) was
taken in it. To this was added 1 litre of a 0.2 moles/litre solution of (Mn =
660) OlPB in CCI4 containing 5.5 ml of boron trifluoride etherate, (taken in the
dropping funnel) (145). The addition was dropwise over a period of 30 minutes
and under vigorous agitation such that the reaction temperature has never
risen beyond 13°C. The reaction mixture was agitated for two more hours at
the same temperature. The dropping funnel was now filled with a freshly
prepared 1 litre (0.2 moles/litre) solution of OlPI (Mn = 728) in CCI4 containing
5.5 ml of boron trifluoride etherate. The experiment was repeated as detailed
above with OlPI also, to get the block prepolymer (lPBPI). After completion of
the reaction, the catalyst was destroyed (146). The viscous reaction mixture
containing the precipitated lPBPI block prepolymer was diluted by adding 500
ml of CCI4-CH30H mixture (2:1) and stirred well. After fifteen minutes, the
block prepolymer was separated by decanting out the supernatent solution.
lPBPI, thus obtained, was extracted repeatedly with a warm solution of
methanol-benzene mixture (5: 1) to remove the unreacted oligomers, inorganic
impurities and the traces of homopolymers, if any, present in it (147) . Finally,
the block prepolymer was flash evaporated at 80°C and transferred to a
46
weighed beaker and the final weight noted. The weight of the prepolymer and
its yield were recorded. lPBPI, thus, prepared was stored in labelled and
stoppered amber coloured bottle.
The procedure given above, was extended to prepare lPIPP block
prepolymer, by substituting OlPB with OlPP. For preparing lPBPP, the
procedure followed was same as in the case of lPBPI, except that OlPI was
replaced by OlPP.
3.2.4 Nitrato prepolymers
The experimental set up for the preparation of the nitrato prepolymers
consisted of a 5 litre glass reaction kettle with a tight fitting lid having five
holes. An inlet-outlet connection for the continuous flow of dry nitrogen, a
glass stirrer, a thermometer and a dropping funnel were the other accessories
of the apparatus. The reaction kettle was kept immersed in a water bath. The
bath temperature was maintained at 15,±,1°C by employing Julabo FT 401
immersion cooler. After the reaction kettle attained the bath temperature, 0.5
litre of a 30% solution of the epoxidized prepolymer lPB in CCI4 was
transferred into it and kept stirred for 15 minutes under a blanket of nitrogen.
The dropping funnel was subsequently filled with 0.55 litre of a 0.9 moles/litre
solution of N20 6 in CH 2CI2 (148) which was then added in portions of 20 ml,
at an interval of 10 minutes to the epoxidised prepolymer solution. After the
47
addition was completed, the reaction mixture was kept stirred for another half
an hour more and precipitated with 5 litre of cold propanol. The solution thus
precipitated was kept 1 h immersed in a water bath, maintained at 10aC to
ensure complete precipitation and the supernatent solution was decanted out.
The separated resin NLPB was further washed several times with fresh
quantities of cold propanol and flash evaporated at 90aC under vacuum, to
remove the traces of propanol. The prepolymer NLPB thus obtained, after
recording the weight, was kept stored in labelled amber coloured bottle. The
nitrato prepolymers NLPP, NLPBPI, NLPBPP and NLPIPP were synthesized from
epoxidised LPP, LPBPI, LPBPP and LPIPP following the above procedure, except
that the nonsolvent used for the precipitation was rnethanol and the prepolymer
drying was done at BOaC.
3.2.5 Preparation of crosslinked elastomers
200 g of the individual prepolymer and the requisite quantities of Gy252
and MAPO were taken in a 2 litre glass kettle equipped with a thermometer, a
vacuum-sealed stirrer and inlet-outlet connections for passing dry nitrogen gas.
The contents of the kettle were thoroughly mixed under the blanket of nitrogen
for 5 minutes and at the end, zinc octoate catalyst was added. The syrupy
resinous liquid, thus, prepared was degassed by applying vacuum and poured
into glass trays coated with silicon grease, the releasing agent. These trays
48
were kept for 48 hours in an oven maintained at 70±1°C for the completion
of the curing process, at the end of which a solid elastomer was obtained.
3.2.5.1 Mechanical property measurement
A series of crosslinked elastomeric samples were made from each
synthesised prepolymer (with various Gy252 and MAPO concentrations) to
study the effect of curing agent concentrations on their mechanical properties.
Dumb-bell shaped samples were made out of them and by employing INSTRON
Model 1122, their mechanical properties were measured.
3.3 CHARACTERIZATION OF HOMO PREPOLYMERS, OliGOMERS,
BLOCK PREPOLYMERS, NITRATO PREPOLYMERS AND
CROSSLINKED ELASTOMERS
3.3.1 Lactone value
Lactone values of the prepolymers were estimated by the standard
method (149), taking advantage of their reaction with alkalies even at low
temperatures, unlike ~he usual noncyclic esters (150). In the estimation
procedure, about 2 g of the synthesised prepolymer sample was taken in a 250
ml stoppered conical flask and dissolved in a mixture of toluene and alcohol
49
(2:1 v/v), which was precooled to 5°C. To this was added a known excess of
0.1 N alcoholic potassium hydroxide solution. After thoroughly mixing, the
content of the flask was maintained at 5°C for half an hour and the excess
alkali was back titrated with a standard 0.1 N alcoholic hydrochloric acid.
Phenolphthalein was used as the indicator and the end point of titration was
just disappearance of the pink colour. A blank titration was carried out without
the polymer. The titration experiments were repeated, to get concordant results
and the following equation was used to calculate the lactone value (LV) of the
prepolymer.
LV =56.1 x N x (V1-V2 )
w- CV
Where
N =
V, =
V 2 =
The strength of alcoholic hydrochloric acid solution
Volume of alcoholic hydrochloric acid consumed by the blank
Volume of alcoholic hydrochloric acid consumed by the sample
w = Weight of the dissolved prepolymer
CV = Carboxyl value of the sample
3.3.2 Carboxyl value
Standard alkali titration method (151) was used to find out the carboxyl
50
value (CV). In the adopted procedure for the estimation, about 2 9 of the
prepolymer was dissolved in 25 ml of benzene-acetone mixture. The ratio of
benzene to acetone was 1: 1 (v/v) and the indicator used was phenolphthalein.
The prepolymer solution prepared as given above in the anhydrous medium
was quickly titrated with 0.02 N alcoholic potassium hydroxide solution. The
appearance of a permanent pink colour was taken as the end point of the
titration. The titration experiment was done again, but without the prepolymer
to obtain the blank value. The titrations were repeated to get concordant
results and the carboxyl value was computed from the titre results based on
the equation:
CV =w
Where
N = The normality of the alcoholic potassium hydroxide solution
V3 = Volume of the potassium hydroxide solution consumed by the sample
V4 = Volume of the potassium hydroxide solution consumed by the blank
w = Weight of dissolved prepolymer.
3.3.3 Iodine value
The Iodine value (IV) was determined by the standard procedure (152)
51
by making use of iodine monochloride (ICI) in CCI4 and starch indicator. 2 g of
the prepolymer was weighed out into an iodine flask containing 25 ml of CCI4
and the prepC'lymer was completely dissolved in it. To this was addpd excess
(250%) of 0.2N ICI solution in CCI4 " The content in the iodine flask was
thoroughly shaken and kept in dark for half an hour. Afterwards 20 ml of KI
(15%) solution and 50 ml of distilled water were added to it. The content in the
flask was then titrated against a O.IN sodium thiosulphate solution and the
Iodine value was computed employing the equation.
IV =w
where
N = The normality of the sodium thiosulphate
V5 = Volume of thiosulphate (m!) consumed for the blank,
Va = Volume of thiosulphate (ml) consumed for the experiment
w = Weight of sample taken.
3.3.4 Epoxy value
Pyridine-HCI reagent was made use of, for the estimation of the epoxy
value (EV). The reagent was prepared by the addition of 10 ml of con HCI into
52
500 ml of pyridine contained in a stoppered conical flask. The solution thus
obtained, was mixed well and kept aside to make it homogeneous. 20 ml of
this reagent was pipetted out into a clean and dry iodine flask. Weighed out
about O.4g of sliced epoxy sample into this and kept overnight. Afterwards, the
content was refluxed on a hot plate for an hour to complete the dissolution as
well as the reaction of the epoxy sample. After cooling, the condenser was
washed with 10 ml of butanol and the solution was titrated with a standard
0.1 N NaOH solution. The indicator used was phenolphthalein and the
appearance of a faint pink colour was taken as the end point of the titration.
A blank experiment was also done and the epoxy value computed from the
equation.
EVw
Where
N = The normality of 'sodium hydroxide solution
V 7 = Volume of sodium hydroxide consumed by the blank
Va = Volume of sodium hydroxide consumed by the sample
w = Weight of the dissolved polymer
3.3.5 Estimation of PBA
Into a stoppered iodine flask, containing 20 ml of 2 N sulphuric acid, 5
53
ml of the PBA sample was introduced, followed by 15 ml of a 15% aqueous
KI solution. The contents in the flask was thoroughly shaken, kept for 5
minutes at room temperature and titrated against a standard sodium
thiosulphate solution. The indicator used was starch and the permanent
disappearance of the blue colour was taken as the end point. A blank
experiment was also conducted following the same procedure, but omitting the
PBA sample. From the titre values the PBA concentration was estimated as
follows:
The overall reaction could be represented as
Mol.wt 138Hence the equivalent weight of PBA = = = 69
2 2
Concentration of PBA =
where
v,gIl
v, = Volume of PBA solution
V2 = Volume of thiosulphate consumed by the sample
V3 = Volume of thiosulphate consumed by the blank
N = Normality of sodium thiosulphate solution
54
3.3.6 Estimation of PIA
A known weight of PIA sample was taken in a glass weighing bottle,
carefully transferred to a 100 ml volumetric flask and made up with distilled
water to mark. 10 ml of this solution was pipetted out into the iodine flask
containing 20 ml of 2N sulphuric acid. 15 ml of a 20% KI solution was added
to this, shaken vigorously and the flask was kept for 5 minutes at room
temperature. The contents were titrated against a standard sodium thiosulphate
solution, using starch as the indicator. The permanent disappearance of the
blue colour was taken as the end point. Adopting the same procedure, a blank
experiment was also conducted omitting the sample. From the results the PIA
concentration was estimated as given below.
PIA reaction with KI in the acid medium could be represented as
The equivalent weight of PIA =Mol.wt
8=
228
8= 28.5
Concentration of PIA =(V1-V2) x N x 28.5
w x 10xF
where
V 1 = Volume of thiosulphate consumed by the sample
55
V2 = Volume of thiosulphate consumed by the blank
N = Normality of sodium thiosulphate solution
w = Weight of the sample
F = Dilution factor
3.3.7 Estimation of dinitrogen pentoxide
The strength of dinitrogen pentoxide in CH2CI2 , (prepared as per the
reported HN03 dehydration method (153)), was determined by conv'erting it
into HN03 and estimating the acid concentration titrimetrically, as in the case
of dinitrogen tetroxide, with a standard sodium hydroxide solution (154).
3.3.8 Determination of [PIA] during degradation reaction
PIA consumption was measured by withdrawing 5 ml of the reaction
mixture, at an interval of 3 min during the oxidative degradation reaction of the
epoxy polymers. The reaction was arrested by precipitating the polymer sample
in 50 ml of a cold 2: 1 solution of methanol (propanol in the case of EPB) and
acetic acid, kept in the stoppered iodine flask. The flask was shaken well and
15 ml of a 20% aqueous KI solution was added. The iodine liberated was
titrated against 0.1 N thiosulphate solution, using starch as the indicator. A
56
blank experiment was also conducted and from the titre values, the PIA
concentration in the reaction mixture was estimated.
3.3.9 Molecular weight and distribution measurements
3.3.9.1 Gel permeation chromatograph (GPC)
The molecular weight distribution, the number and weight average
molecular weights of the various prepolymers were determined using WATERS
ALC/GPC 244 gel permeation chromatograph with a set of p styragel columns,
(104AO, 103AO, 500Ao, 100Ao) and Rl, UV (1 = 254 nm) detectors. The
solvent employed was THF and the flow rate was 2 ml/min. Molecular weight
calibrations were done using 6 polystyrene standards of molecular weights
ranging from 2.33 x 105 to 2900.
3.3.9.2 Vapour pressure osmometer (VPO)
Hewlett Packard model 302 B vapour pressure osmometer was used to
measure the number average molecular weights of the prepolymers and the
solvent employed for the measurement was toluene.
57
3.3.10 pH measurement
The solution pH measurements were done using a precalibrated,
Toshniwal pH meter with glass-calomel electrode system.
3.3.11 Elemental analysis
The elemental analysis of the various prepolymers were done with
Perkin-Elmer 2400 C, Hand N microelemental analyzer.
1 to 2 mg of the polymer sample underwent combustion in an
atmosphere of pure dry oxygen in the analyser. CO2 , H20 and the oxides of
nitrogen formed were passed through a reduction tube containing copper, to
convert the oxides of nitrogen into elemental nitrogen. The mixture of gases
were then sequentially separated with a chromatographic column, and the
percentage of individual elements recorded by employing a thermal conductivity
detector. Acetanilide was used to caliberate the instrument.
3.3.12 Volatile matter
The prepolymer of known weight was taken in a previously weighed dry
petridish and was kept in an air oven, maintained at a temperature of
105±.1 °C. After 2 h, the sample containing petridish was kept in a desiccator,
58
cooled and the weight was noted. Heating, cooling and weighing of the sample
containing petridish were repeated until constant weight was recorded. From
the difference in weight of the sample, the percentage of volatile matter was
computed.
3.3.13 Moisture content
The Karl-Fischer method (155) was employed to determine the moisture
content of the prepolymers. A known weight of the prepolymer sample was
dissolved in a mixture of toluene and methanol (7: 1). The solution was stirred
well and titrated against a standard Karl-Fischer reagent. A blank experiment
was also conducted using exactly the same quantity of solvent as was used for
the sample. From the titre values, the moisture content of the prepolymer was
estimated. Karl-Fischer reagent was standardised using sodium tartrate
dihydrate.
3.3.14 Functionality determination
The functionality of the prepolymers were determined by established
methods (156) wherein, the number average molecular weight was measured
using VPO and the functional equivalents for getting the equivalent weight,
estimated with the chemical analysis method (Sec. 3.3. 1). The following
59
equation was then used to compute the average functionality.
Fn = Mn / equivalent weight
where
Fn = the average functionality of the prepolymer
Mn = number average molecular weight of the prepolymer
3.3.15 Tg measurement
Tg measurements of the different samples were done with Dupont 942
TMA. The samples were precooled with liquid nitrogen and after attaining
sufficiently low temperature, they were heated at the rate of 1QOC/minute, to
evaluate the Tg values (157).
3.3. 16 Shelf life
The prepolymer samples were freely· exposed in flat-bottomed glass
dishes filled to 1 cm depth and the dishes were kept in ovens at the study
temperatures. To evaluate the shelf life, the individual sample viscosities were
determined with a Brookfield microviscometer having a cone and cup
arrangement. The resistance offered by the sample was metered which when
multiplied by the constant of proportionality, (due to the cone used and the rpm
60
at which the measurement was made), gave the viscosity of the sample. The
shelf life of the prepolymer was evaluated by following the change in viscosity,
Mn, Mw and polydispersity (D) over a period of time: Mn, Mw and D of the
individual samples were determined by GPC techniques.
3.3.17 Solubility
The prepolymer solubilities were investigated in several solvents at two
different temperatures. About 2 gm of the prepolymer sample was weighed out
into a conical flask of 50 ml capacity containing 20 ml of the solvent. The flask
was fitted with a condenser and also with a thermometer to measure the
temperature with an accuracy of ,,±,,1 DC. The flask was kept in a hot water bath
maintained at the required temperature and the contents of the flask agitated
with a magnetic stirrer for 8 hrs. The set up was to observe whether the
prepolymer at the study temperature was soluble, in a given solvent.
3.3.18 Bulk viscosity
Brookfield viscometer (Brookfield Engineering Laboratories, Inc, USA)
was used to measure the prepolymer bulk viscosity by the standard procedure
(1 58) at the study temperatures.
61
3.3.19 Solution viscosity
Ubbelhold type viscometer provided with guard tube, to prevent the
interference of atmospheric moisture was used to measure the solvent and
solution flow time, represented as to and t respectively at 30°C. From these
values, the relative viscosity ("r) and inherent viscosity ("inh) of the various\
polymer samples were determined based on the following relationships (159).
t
=
=
=
In "r--, dllg
c
2.303 log (t/to)
0.5dllg
3.3.20 Determination of v
The v values of the cured samples were measured using the standard
technique (160) from the swell ratio (0), employing the equation.
(0) =
where
WI = Weight of the swollen sample
Wd = Weight of the dried sample
-1
The sample swelling was achieved by keeping it in a good solvent
(161), where by the sample got enlarged until due to deformation stresses an
equilibrium was established. In this state, the rates of imbibition as well as the
driving out of the solvent would be the same. Usually for the swelling studies
with the rubber backbones, 10-12 hours at room temperature would be
sufficient to establish the equilibrium. In the study described here,
approximately 8x8mm specimen samples were cut and kept in benzene for
about 36 hours and were subsequently removed from the solvent and weighed
after removing the adhering solvents with a dry filter paper. The weight of the
dried sample was obtained by removing the absorbed solvent, by keeping it in
a vacuum oven for 4 hours at 70°C. Wp and W" the weight fractions of the
polymer and the solvent (in the swollen sample) were evaluated, with the help
of the following equation.
=1 + 0
and WI = 1 - W p
63
The volume fraction of the polymer in the swollen sample was
determined from the expression given below:
Here Vp is the volume fraction of the polymer, Ps and Pp were the
densities of the solvent and polymer respectively. Crosslink density v was
evaluated by using the Flory-Rhener equation (162,163), which relates Vs, the
molar volume of the solvent, Vp the volume fraction of the polymer, and X the
polymer solvent interaction parameter with v as
v =- [Ln (1-Vp) + Vp + XVp2]
Vs(Vp 1/3 - Vp/2)
Here X was computed by the equation:
VsX = 0.34 + -- (op - os)2
RT
Where op and Os were the solubility parameter of the polymer and the
solvent. R the gas constant and T, the room temperature. Again 0, the
solubility parameter is related to the cohesive energy density ECOh (164) as
64
Ecoh, in the case of low molecular weight products are computed from
the heat of vapourisation as a function of temperature. But indirect methods
were to be made use of in the case of polymers, for the cohesive energy
density determination, since polymers in general could not be evaporated. The
well known Vankrevelen and Hoftyzer method (164) used in this regard has
already been shown to give values very near to the experimental ones.
3.3.21 Thermal degradation of elastomers
Mettler TA 3000 system consisting of TG 50 thermobalance and TC
10A TA processor was employed to study the thermal degradation of the
prepared elastomers at a predetermined heating rate of 20oK/min. The
atmosphere used was air, purged at the rate of 50 ml/min. The extent of
degradation measured in percentage was recorded at various stages and the
temperature (Td) at which maximum decomposition occurred was registered
from the differential curve obtained from differential thermogravimetric
analysis.
65
3.3.22 Infrared spectroscopy (lR)
The Neat infrared spectra of polymers listed were recorded in
Perkin-Elmer PE 283 Infrared spectrophotometer in the range 4000-600 cm-1•
3.4 DEVELOPMENT, EVALUATION, PREPARATION AND
CHARACTERIZATION OF THE PROPELLANT
3.4.1 Propellant development
The prepolymers were treated with precalculated quantities of the
curing agents (Gy 252 and MAPD) and propellants were cast with 83% and
86% solid loadings. AP of two different particle sizes, coarse (300 J1) and fine
(50 J1) were used for the propellant development, keeping the coarse to fine
ratio at 3: 1. The metallic fuel used in all the propellant compositions were
aluminium powder with an average particle size of 18 J1. The 83% solid loaded
propellant composition consisted of 63% AP, 20% AI, and the rest 17% was
constituted by the prepolymer, Gy 252 and MAPD, in addition to the plasticizer
DDA and other minor additives like the bonding and wetting agents. The 86%
solid loaded propellant composition was made with 66% AP, 20% AI and the
remaining 14% was constituted by the prepolymer, Gy 252, MAPO and other
additives, as in the case of 83% solid loaded propellant.
66
3.4.2 Evaluation of propellant performance parameters
The propellant performance parameters were evaluated by the
variational method established by White, Johnson and Dantzig (165) and also
by the usual simplifying assumptions recommended in the literature (1 66).
Thus, the thermochemical calculations were done for the various developed
propellants under frozen flow and equilibrium flow conditions. Besides the
accepted assumptions (167) like one-dimensional flow of gases ie., parallel to
the axis of the nozzle, their ideal behaviour, isentropic expansion through the
nozzle, absence of heat losses in casebonded motors complete chemical
equilibrium inside the chamber, it was further assumed (168) that the gases
were contained in a volume equal to the gas constant times the temperature
such that it satisfied the condition
where
Pi = the partial pressure of ith product species and
nj = the mole number of the ith product species
For the sake of convenience, the overall computation process was divided into
two parts, first part to arrive at the equilibrium composition of the products and
flame temperature. The second part to arrive at the exit temperature and the
remaining performance parameters.
67
Part I
The chemical reaction occurring in the combustion chamber could be
summarized as
A(NH4 CI04 + X. Cc Hh Nn 0 0 + Y. AI) = n1AI20 3 + n2 CO2 + n3CO + n4H20
+ n5H2 + n6HCI + n7N2 + naO
+ ngH + n lOOH + n 1,02 + n'2N
+ n'3NO + n,4CI + n,5C12
+ n,6A120 (1)
where the subscripts, c,h,n and 0 represent the numbers of atoms of carbon,
hydrogen, nitrogen and oxygen present in the prepolymer under consideration
and A is the number of formula weight of the reactants. The scheme
established by Barrere, Jaumotte, Veubeke and Vandenkerchove (169) was
employed for evaluating the equilibrium composition at a given temperature T.
According to the above scheme, the reaction products could be categorized as
principal and minor species. For the present evolution studies, the first seven
reaction products in equation (I), ie A120 3, COlt CO, H20, H2, HCI and N2 were
taken as the principal species and the remaining as minor species. The mass
balance equations were written as
2n1 + 2n'6 = A.V
68
(2)
= A.X.C (3)
2n] + n'2 + n'3 = A (X.n + 1)
(4)
(5)
3n1 + 2n2 + n3 + n4 + na + n,o + 2n" + n13 + n'6 = A (X.O+4) (6)
and
The pressure balance equation was given by
16
~ n i = operating pressure
Here the pressure being expressed in atm.
(7)
(8)
Hence the principal species were computed by neglecting the minor
species with the help of the mass and pressure balance equations and also by
employing one or two non-linear equilibrium equations, if necessary.
Subsequently the minor species were computed by substituting the values of
the principal species in the dissociation equations. The mass and pressure
balance equations were then further corrected with the help of the values
obtained for the minor species. This finally led to the computation of more
accurate values for the principal species. The iteration process was done
further to get the best possible results. Now the set of equilibrium equations
with the corresponding equilibrium constant were written as
70
At a given temperature T, In k being related to the total change in the entropy
(AS) and enthalpy (AH) through the universal gas constant R as
In k bS M= --
R RT
The series of equations [(2) to (9)] were adequate to solve for the gas
composition by the scheme described above. Tc, the adiabatic flame
temperature was defined by the preset condition.
where
Ho = Total enthalpy of the reactants at the initial condition
HT = Total enthalpy of the products at T = Tc
Tc in turn was obtained through a systematic trial procedure. For this, an
arbitrary approximation T = Tc (170), very well below the anticipated flame
temperature was made and at this temperature, the equilibrium composition
and hence HT, the total enthalpy of the reaction products, were evaluated. To
start with HT < Ho' then T the temperature was raised until the condition was
reached as shown below.
71
The linear il1t~rpolation method was maCle useot to evaluate T = Tc at which
Part II
As the process of expansion was taken to be isentropic,
When
Where
Sc = entropy of the system of combustion products at the chamber.
Se = entropy of the system of combustion products at the exit.
At any given temperature T, the entropy of the product system could be
expressed as
s s
S = ~ niSi - R L n i In n i~ P-tl.
where
nj = .mole number of the ith product species at temperature T
Sj = mole entropy of the ith product species at temperature T
S = The total number of product species in which the first P were the
condensed species.
72
The exit temperature Teat which Se = Se was computed employing the
trial procedure similar to the one described earlier in connection with the
evaluation of the flame temperature.
If He and He represented the chamber and exit enthalpies, the specific
Impulse (Isp) of the system could be related as
According to Barrere et al (169), a correction as shown below to be
incorporated into the equation for Ispt taking into account the thermal lag and
velocity lag imparted by the condensed species like AI20 3 present.
If E represent the weight fraction of the condensed phase, then the
specific heat at constant pressure and specific heat at constant volume are to
be written as
Cp' = (1-E)Cp + EC
and
Cv' = (1-E) Cy + EC
where
C = Specific heat of the condensed phase
73
Cp' and Cv' are the corrected values of the parameters. Thus, the corrected
equation for the specific impulse could be written as
=
where
r = (r) 1/4_2_) 2Ia-;':l)\ r+l
and r' represent the corrected value of r. C/ represent the corrected value of
CF, the thrust coefficient.
For the evaluation studies of the various propellants formulated, a
computer programme was written in FORTRAN. Standard tables (171) were
made use of to get the values for the various above written thermodynamic
functions. Cp the heat capacity for the product species was represented as a
cubic function of T, the absolute temperature as
The specific impulse values of the various propellants studied
experimentally on 20-kg control motors were shown to be very close to that
predicted by the above scheme.
The other parameters were computed by using the following equations.
Ejection Velocity
Characteristic velocity
74
= g x rap
= gCd
Vent temperature
Discharge coefficient
Expansion ratio
Velocity of the combustion
products at the throat
where
=
=
=
g.rrt_2_) 2T;_111
y. \ r+l
g = acceleration due to gravity
J = The mechanical equivalent of heat
The mean molecular weight of the product gases could be obtained as:
=
75
and for the propellant system
=
where
Wi = The molecular weight of the ith product species
By applying the formulae given above at chamber and exit conditions,
the mean molecular weights of the product gases and that corresponding to the
system at the chamber and exit conditions could be calculated.
3.4.3 Propellant preparation
3.4.3.1 Description of the propellant mixer
A Standard stainless steel 0.8 Kg sigma mixer was made use of to
disperse uniformly the various ingredients used to prepare the propellants. The
mixer had an outer jacket through which hot water at the required temperature
could be circulated. During mixing, the blades of the mixer rotated in opposite
directions with the leading and the trailing blades revolving with 25 rpm and
18rpm speeds respectively.
76
3.4.3.2 Propellant ingredient mixing
The prepolymer was weighed out into a beaker to which was added
dioctyl adipate, and other additives (like the bonding and wetting agents,
burning rate catalyst etc. as required in the individual experiments). After
mixing thoroughly with a PVC rod, the free flowing mix thus obtained was
charged into the sigma mixer and vacuum mixed at a temperature of 40°C. This
was followed by AI powder, added in a single lot and mixed for 15 minutes. AP
coarse variety was now added in three lots at an interval of approximately 8
minutes and the same sequence of addition was repeated with AP fine variety
lot also. The mixing was continued for 1 hour 20 minutes and calculated
quantities of Gy 252 and MAPO were added to the above slurry and finally
mixing was done under vacuum for 1 hour 20 minutes. The mixer was stopped
afterwards and a test sample of the propellant slurry was withdrawn for
homogeneity determination. Concentration of AP in the slurry was found out
by converting the ammonia present in AP into hexamethylenetetramine
complex by treating with formaldehyde and estimating the liberated perchloric
acid with standard alkali titration method (172). The concentration of AI
powder in the slurry was estimated by reacting it with an 1:1 hydrochloric acid
solution, whereby AI was transformed into aluminium chloride which on
reaction with excess EDTA was converted into a complex. The amount of
77
unreacted EDTA was determined with the standard zinc sulphate titration
method employing Eriochrome black-T indicator (173).
3.4.3.3 Propellant slurry casting
Vacuum casting technique was adopted for the consolidation of the
propellant slurry without blow holes (174). The compact packing of the slurry
was accomplished through a series of careful operations. The slurry was first
charged into a hot water circulated funnel, drawn through a slit-plate and
ultimately packed into the casting carton with the application of 750 mm Hg
vacuum.
3.4.3.4 Propellant slurry curing
Usually the slurry curing was a slow process and for its completion,
several days were required at a constant preset temperature. The cartons
containing the propellant slurry was kept in an electrically heated oven at the
study temperature for the required period and afterwards the cured solid
propellant blocks were released from these cartons and kept in desiccator to
prevent the moisture absorption.
78
3.4.4 Propellant characterization
3.4.4.1 Propellant slurry viscosity
Samples of the thoroughly mixed propellant slurries were taken out from
the sigma mixer and the End of Mix (EOM) viscosity determined at 40°C with
a Brookfield viscometer, Model HVT-71179 having Helipath stand (175) using
the "TD" spindle at 2.5 rpm.
3.4.4.2 Propellant heat of combustion
The heats of combustion AH of various samples were determined in an
isothermal bomb calorimeter (Toshniwal, India) by taking 1 g of the sample,
weighed to an accuracy of 0.1 mg. The samples were then ignited inside the
apparatus in air, at ambient conditions. The rise in temperature AT, for the
sample was measured to an accuracy ·of 0.01°C, using a Beckmann
thermometer. The bomb calorimeter was caliberated with benzoic acid as the
standard. AH value of the sample was then calculated from the following
expression.
AH =
79
where AH, and m, were the heat of combustion and weight of the standard. AT,
was the rise in temperature recorded in the case of the standard and m the
weight of the sample taken for combustion
3.4.4.3 Propellant mechanical properties
The cured propellant grains were cut with the help of non-sparking
cutting tools, into slabs of 10 x 10 x 0.6 (cm) size and the dumb-bell shaped
test specimens were punched out of them. INSTRON-Model, 11 22 was used
to determine the propellant mechanical properties like the tensile strength,
elongation and modulus, by the already reported evaluation techniques (176),
after conditioning the samples over night over CaCI2 •
3.4.4.4 Propellant burn rate
The propellant blocks were cut into strands of 6 x 6 x 85 mm size. Their
burning rates were evaluated by accoustic emission method inside the burning
rate apparatus, which consisted of a jacketed pressure vessel having provisions
for gas inlet and outlet, manifold for pressurisation, pressure· monitoring
system, ignition unit, constant temperature bath. accoustic emission detection
system with sensor and amplifiers. and a personal computer having data
acquisition set up with burn fate evaluation soft ware. The burning rate in
80
various cases were found out by burning the propellant strands under water in
a pressure vessel, pressurised with nitrogen gas. The sound produced during
the propellant strand burning was sensed with the accoustic emission sensor
positioned outside the pressure vessel. The sound energy generated was
transformed into electrical energy which was amplified and fed into the
computer. A plot of time vs voltage was made by the computer software, to
record the exact ignition and burn out points. From this the burn time was
determined. The burn rate (B.A.) was then computed from the following
equation.
B.A. .l mm/sect
where '1' was the burned length of the propellant strand and t the burn time.
3.4.4.5 Propellant cure shrinkage measurement
To determine the volume change during curing, a dilatometer was used.
The slurry was vacuum cast into the dilatometer and the remaining top unfilled
portion of the apparatus was filled with silicone oil. The apparatus was kept
immersed in a water bath, maintained accurately at the cure temperature with
the help of a contact thermometer, having an electronic relay circuit. After the
set up attained the bath temperature, the level of silicone oil in the capillary
81
was noted at frequent intervals, until the cure was over. Care was taken to
maintain the apparatus free from air bubbles and leak proof. The decrease AI
in the oil level was related to cure shrinkage as follows.
Cure shrinkage = L\V x 100V
Where V is the volume of slurry, found out from .its mass and density,
which were measured separately. The change in volume AV = AI xlTr2• Here
r was the radius of the capillary used in the dilatometer.
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