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

33

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.