chapter 7 studies on gap based propellant formulations...

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Chapter 7

Studies on GAP Based Propellant Formulations, Cure Kinetics and Rheology

7.1 Introduction

The search for more energetic and eco-friendly propellants has lead to studies on

large number of compounds like GAP, POLYGLYN, BAMO, AMMO, ADN, HNF

and CL-20.1-3 The important considerations in the development of new formulations

include higher performance parameters like density specific impulse, minimum

signature, eco-friendliness and low friction and impact sensitivity. The cost involved

and reliability are also important. The operational solid motors widely use

HTPB/AP/Al based propellant, which has reached a saturation level in terms of the

above mentioned requirements. The high content of HCl in the exhaust of AP based

propellant is considered as a major concern.4-7 New generation propellant

formulations with energetic binders like GAP and oxidiser systems like ADN, HNF

and CL-20 show lot of promise in this respect.8 GAP has been reported to be

compatible with high energy oxidisers.9 GAP based propellant system and its

characteristics have been dealt with in literature.10-12 Ballistic properties of GAP

based energetic composites involving NC/NG, HMX and AP has been presented by

Kubota et.al.13, 14 A comparison of GAP based propellant with that of HTPB for gun

propellant application has been presented by Schedlbauer.15 GAP/AN propellant

system have been studied for chemical stability, combustion behaviour and

sensitivity by Menke et.al.16 GAP crosslinked with TDI and IPDI has been evaluated

as integrated ram rocket propellants by Sahu et.al.17 Studies on the pyrolysis of

GAP/RDX/BTTN propellant formulation have been presented by Ross et.al.18

Investigation of the GAP based propellant for gas generator application has been

presented by Helmy.19

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In this study, evaluation of theoretical performance parameters like specific

impulse and density impulse of various GAP based propellant formulations and

propellant characteristics like burn rate, mechanical properties and rheological

behaviour were carried out.

Rheological evaluation of the binder and propellant slurry are important steps for

assessing the processability of the propellant system.20 The rheological behaviour of

the polymeric binder and that of uncured propellant slurry is to be understood to

optimise the process parameters like mixing time, mixing temperature, rate of casting

or slurry feed rate and pot life of propellant. Realisation of defect free propellant

grain depends on thorough understanding of the nature and effect of rheological

behaviour of propellant binder and slurry. For the binder, the viscosity depends

largely on the molecular weight, temperature and rate of shear. The relationship

between molecular weight and viscosity was given by Bueche,21 which was later

modified by a number of interpretations. The effect of temperature on viscosity was

first enunciated by Andrade.22 Study of rheological characteristics of the system

could help in the selection of the processing technique for a particular propellant

system.23, 24 Rheology of highly solid loaded system is influenced by a number of

factors like particle size, size distribution and shape of solid additives, rate of shear,

binder viscosity, rate of cure reaction and process temperature. A large number of

studies were presented by many authors on the effect of various parameters on

rheology of suspensions with low solid loading.25-28 Study on the kinetics of polymer

network formation could throw light on the rheological characteristics.29

Reproducibility of ballistic and mechanical properties of propellant requires

uniform distribution of solid additives in the matrix. This is possible only if the

propellant slurry has controlled rheological characteristics and also good

processability. Rheological characterisation could help to understand the flow pattern

of the propellant slurry, measurement techniques required, effect of compositional

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variables, effect of process variables and requirement of processing techniques.

Propellant flow characteristics change as the cure reaction proceeds. Though a large

volume of studies on the synthesis, characterisation and energetic formulations based

on GAP are available in the literature, data on rheological evaluation of GAP and

GAP based formulations are scarce. Hence, there is scope to study the rheological

characteristics of GAP.

In this study, first the effect of plasticiser content on the viscosity of GAP was

evaluated. Different types of plasticisers were evaluated for this purpose. The

viscosity build up of GAP resin due to curing reaction with different diisocyanates

was evaluated at different temperatures. GAP based propellant system was studied to

understand the effect of different variables on the rheological behaviour of the

system. A selected formulation of GAP-HTPB blend based propellant was also

evaluated for this purpose. Chapter 7 comprises of two parts viz Part I and Part II.

Part I Studies on GAP Based Propellant Formulations

7.2 Theoretical performance evaluation of GAP based propellant formulations

Theoretical computations were carried out with the help of computer codes to

evaluate various combinations of energetic ingredients to arrive at an optimum result.

NASA-SP-27330 is one of the most widely used programme code for the purpose.

The inputs required for the evaluation of performance parameters by NASA-SP-273

include concentration of oxidiser, binder, metallic fuel, molecular formulae of the

constituents and heat of formation.

For the purpose of comparison of various propellant formulations, the

evaluation was done for standard conditions of rocket operation at 70 ksc pressure

with isentropic expansion to 1 atmospheric pressure and vacuum conditions. The

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expansion of the combustion products are assumed to be at equilibrium and a nozzle

expansion ratio of 10 was employed for the study. The theoretical computations were

carried out with different formulations. Figure 7.1 shows the results of evaluation of

various combinations of high energy oxidisers with GAP.

Figure 7.1 Effect of the solid loading on specific impulse of GAP based propellant formulations with 18% Al.

Comparison of data shows that, HTPB based compositions with AP and Al

(18%) could contribute to a peak specific impulse (Isp) value of 265 seconds with

86% solid loading. Further increase in solid loading was found to have no influence

on Isp. GAP with ADN and Al with solid loading of 82% was found to provide a

peak Isp of 275 seconds which indicate a good performance characteristic. The near

plateau region observed in the Isp profile in the range of 76 to 86% solid loading

indicate an important advantage in terms of flexibility to tailor the propellant

formulation to meet processing requirement of propellant. GAP with HNF and Al

was found to provide highest Isp value of 280 seconds at 84% solid loading.

However, incompatibility of HNF with diisocyanate curatives and particle shape of

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HNF are difficult problems for propellant formulations. The relative low value of Isp

(260 seconds) seen for GAP with CL-20 and Al at 86% solid loading could be due to

the comparatively low oxygen balance of (11%) of CL-20. The comparison shows

that GAP with advanced oxidiser systems could provide significant improvement in

performance parameters compared to conventional propellants.

For the purpose of comparison with conventional propellant formulations, the

specific impulse of GAP-AP propellant was also determined for both aluminised and

non aluminised formulations. Figure 7.2 shows the results of theoretical

computations. The data show that the specific impulse of aluminised propellant is

higher than that of non aluminised propellant for both sea level and vacuum

conditions.

Figure 7.2 Comparison of Isp of GAP-AP propellant with and without Al

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The higher Isp of aluminised propellant results from the higher energy out put

from the combustion of aluminium. It is also noted that for all the formulations, the

peak Isp is observed in the range of 75 to 80% solid loading. The peak vacuum Isp

value of 291 s was observed for aluminised GAP-AP formulation.

7.2.1 Vacuum specific impulse of GAP based propellant formulations

The vacuum specific impulse of GAP based propellant formulations with

advanced oxidisers was also estimated for comparison. Figure 7.3 shows the vacuum

Isp of GAP based propellant with advanced oxidiser systems in comparison with

HTPB based propellant. The vacuum Isp values are also found to be significantly

higher than that of conventional propellant formulations based on HTPB and AP.

Figure 7.3 Comparison of vacuum Isp of GAP and HTPB based propellants

7.2.2 Density impulse of GAP based propellant formulations

Density specific impulse is the product of density and specific impulse of

propellant. Figure 7.4 shows comparison of density specific impulse of different

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propellant formulations with that of HTPB-AP based systems with different solid

loadings.

Figure 7.4 Comparison of density Isp of GAP and HTPB based formulations

Density specific impulse is an important parameter for comparison of

different propellants in terms of their volume limited performance capability. From

the data it is seen that GAP with CL-20 shows maximum density specific impulse of

525 s g/cc at 90% solid loading. The higher density Isp is the result of higher density

and energetics of CL-20 compared to HNF or ADN. However, such a high solid

loaded formulation is difficult to work with. The GAP and advanced oxidiser based

propellant compositions with peak performance characteristics are shown in table

7.1. With Al content of 14%, CL-20/Al/GAP based propellant could provide a

density Isp of 545 s g/cc. However, a total solid loading of 90% could pose difficulty

in propellant processing.

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Table 7.1 Peak performance characteristics of propellants based on GAP with ADN, HNF and CL-20

Propellant ADN/Al/GAP HNF/Al/GAP CL-20/Al/GAP

Composition 64/18/18 60/18/16 76/14/10 Peak Isp (s) 275 280 274 Peak Vac.Isp (s) 300 305 298 Peak density Isp (s g/cc)

492

514

545

Density (g/cc) 1.79 1.84 1.99 Flame temp (K) 3787 3985 3749 Molecular weight of products 28.4 29.3 29.3

7.3 Evaluation of propellant formulations

Composite propellant formulations with GAP as binder were prepared for the

study. AP was used as the oxidiser in all the propellant formulations. The propellant

formulations were selected based on the optimum performance characteristics arrived

at from the theoretical performance evaluation and also based on the gumstock

property evaluations done as explained in chapter 5. NCO/OH ratio of unity and

crosslinker content of 5% with respect to binder content was selected for all the

trials. The propellant formulations were prepared in such a way as to process defect

free specimens for evaluation of mechanical and ballistic properties. In order to study

the effect of particle size of AP on the properties of the propellant, propellant mix

with combination of coarse and fine AP and with fine AP alone were prepared. A

solid loading of 75% was utilised for the study. Propellants with optimum aluminium

content of 18% and with low aluminium content of 2% were also prepared for the

study. In order to modify the slurry flow characteristics, DOA was used as the

plasticiser. DOA content of 15% with respect to binder content was used in the

propellant formulations.

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7.3.1 Propellant experimentations

7.3.1.1 Materials

GAP resin with molecular weight 2000 (by VPO) and hydroxyl value of

45 mg KOH/g was used for the propellant processing. The crosslinking agent used

was a 2:1 mix of TMP and butane diol, mixed and dried for extended periods under

vacuum to remove moisture to the extent of 0.1%. TDI with purity higher than 99%

as available from commercial sources was used as the curing agent. AP with purity

higher than 99% produced in VSSC was used. Two grades of AP namely coarse and

fine grades with average particle size of 300 µ and 40 µ respectively were used for

the study. Aluminium powder (of average particle size 10-20 µ) with purity higher

than 99%, as available from commercial sources was used. Copper chromite

available from commercial source was used. Curing catalyst used for the study was

prepared by mixing DBTDL in toluene with a volumetric ratio of 1:10.

7.3.1.2 Equipments

Propellant processing was carried out using a small scale horizontal sigma

blade mixer. A photograph of the sigma mixer used is shown in figure7.5. The mixer

is equipped with hydraulic arrangement for tilting the mixer bowl for loading and

unloading and all safety measures to carry out safe processing of propellant.

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Figure 7.5 Horizontal sigma mixer used for propellant mixing

The propellant specimens were prepared by casting the propellant slurry

using vacuum casting set up. The vacuum casting set up consists of a hopper and

casting chamber which can be evacuated to a vacuum level of 5-10 mm Hg. A

photograph of the vacuum casting set up is shown in figure7.6.

Figure7.6 Vacuum casting set up used for propellant processing

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The propellant slurry was fed into the hopper of the vacuum casting set up.

The propellant is slowly fed through the valve into an evacuated container. After the

casting was completed, the propellant was kept in an air oven and cured at 600C for

15 days. Mechanical properties of the propellant samples were determined using

Instron testing machine. The dumbbell specimens were prepared as mentioned in

section 2.4 in chapter 2. The burn rate measurements were done for the propellant

samples sliced from the cured block. The measurements were done using the acoustic

method as mentioned in section 2.9 at different pressures, namely 20, 40 and 60 ksc.

7.3.1.3 Propellant processing

GAP based propellant slurry was prepared with TDI as curing agents for the

study. AP with coarse to fine ratio of 2:1 was selected for the study. The estimation

of stochiometric quantities of resin and curing agent for preparing the propellant

formulations were done as shown below.

(i) Hydroxyl number + Acid number of binder = x

(ii) Hydroxyl number of crosslinking agent = y

(iii) Purity of diisocyanate (TDI) = C

(iv) Purity of TDI, NCO/OH equivalent ratio = R

(v) Quantity of resin = D gm

(vi)

(vii) Quantity of crosslinker = E gm

(1:2 by weight ratio of 1,4-butanediol and 1,1,1- trimethylol propane)

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x) TDI required for D gm of resin =

AD x x R561

87C = F ---------- 8.16

xii) Catalyst added ( DBTDL in toluene solution = H gm (0.326% by weight of binder). Weight percentage of the ingredients for a typical propellant formulation prepared

is shown in table 7.2.

Table 7.2 Typical propellant formulation prepared for the study

Propellant ingredient Materials Percentage by weight (%) Binder composition

NCO/OH ratio =1

GAP DOA

Crosslinker TDI

Catalyst

17.77 3.14 0.88 3.15 0.06

Oxidiser AP (coarse) AP fine

38 19

Metal fuel Aluminium 18

(7.5)

(7.4)

(7.3)

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For processing the slurry, first GAP resin was mixed with DOA and charged

into the mixer. This was followed with addition of crosslinking agent. Aluminium

powder was added next and mixed. The oxidiser was added in small lots and mixed

thoroughly. In one formulation, AP fine alone was used. Mixing of AP was followed

by addition and mixing of curing agent. Finally DBTDL in toluene solution was

added and mixed. The mixing schedule followed for the propellant slurry was as

shown in table 7.3. A process temperature of 500C was maintained during mixing.

Table 7.3 Mixing schedule followed for preparation of propellant

Propellant ingredient Mixing time (min)

Premix (GAP+DOA+Crosslinker) 5

Aluminium powder 5

½ AP coarse 5

½ AP coarse 5

½ AP fine 5

½ AP fine 5

Mixing 30

TDI 20

Catalyst 20

Total 100

The mix was evacuated and then fed into a vacuum casting setup. The

propellant was vacuum cast at room temperature and cured at 600C for 15 days.

Samples were machined out from the cured propellant for evaluation of mechanical

and ballistic properties. The effect of AP content in the propellant on the properties

was evaluated by preparing formulations with different grades of AP. In the

formulations prepared with fine grade AP, low aluminium content of 2% was used.

Propellant formulation with copper chromite as burn rate modifier and formulations

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with variable GAP resin content were also prepared and evaluated. Samples were cut

from the cured propellant and evaluated for mechanical and burn rate properties

7.3.1.4 Results and discussion

The mechanical properties determined for propellant samples prepared with

different formulations are shown in table 7.4.

Table 7.4 Mechanical properties of different GAP based propellant formulations

Properties

Propellant formulation Solid loading -75% AP content - 57% AP coarse to fine ratio - 2:1 Al content - 18%

Solid loading -70% AP content - 68% AP fine only Al content - 2%

Solid loading -70% AP content - 67% AP fine only Al content - 2% C C content -1%

Tensile strength(ksc) Elongation (%) Modulus (ksc) Shore A hardness

6.5 38 27 65

7.5 31 38 68

7.1 30 40 50

The burning rate of the propellant samples were evaluated at different

pressures as mentioned in section 2.9 in chapter 2. From the data generated, the burn

rate law was derived for specific propellant formulations. For GAP propellant with

18% aluminium content, the burn rate was found to increase with increasing pressure

as expected for composite propellants.13, 17 Table 7.5 shows the burn rate data

generated for aluminised GAP propellant at three different pressures.

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Table 7.5 Burn rate data generated for aluminised GAP propellant

Propellant formulation Burn rate at different pressures (mm s-1) 30 ksc 40 ksc 60 ksc

GAP -17 % Solid loading -75%

AP – 57 % coarse to fine ratio – 2:1

Al - 18%

7.46

8.00

8.73

The burn rate law was determined from a logarithmic plot of burn rate versus

pressure. The burn rate law for the aluminised propellant derived from the plot

(figure 7.7) can be represented as shown in equation 7.6

Where r is the burn rate in mm s-1 and P is the pressure in ksc.

Figure 7.7 Logarithmic plot of burn rate vs pressure

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The effect of GAP content on the burn rate of low aluminised GAP-AP

propellant was evaluated. The GAP resin content was varied from 18.3 to 26.3% by

weight in the experiments. AP content was varied from 62 to 70% with particle size

of 40 µ, Aluminum content of 2% and copper chromite content of 1% were used for

the study. The burn rate was determined at 70 ksc pressure. Table 7.6 shows the

variation of burn rate with GAP resin content in the propellant. The study showed

that high burn rate of 29.4 mm s-1 at 70 ksc pressure could be achieved for GAP

propellant with a solid loading of 67%.

Table 7.6 Effect of GAP resin content on burn rate of propellant

GAP content ( %)

AP content (%)

Solid loading (%)

Burn rate at 70 ksc (mm s-1)

18.3 70 73.0 20.1

20.0 68 71.3 21.3

22.4 67 70.0 29.4

24.0 64 67.3 26.3

26.3 62 65.0 21.6

7.4 Studies on the ignitability of GAP based propellant formulation

Ignitability of GAP based propellant formulations have been studied and

reported for advanced applications such as microthruster propellants.31-33 In this

study, different GAP based propellant formulations were evaluated for ignitability

using low power nichrome wire based ignition systems.

7.5 Propellant system for microthrusters

The propellant system that can be used for this application should meet

specific requirements. The important factors are

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i. The propellant should be energetic enough to meet the mission needs.

ii. Propellant should be processable and it should form defect free grain when

filled and cured inside the cavity.

iii. It should have low ignition temperature.

iv. The flame temperature should not be too high.

v. It should be stable in the space environments for long time of storage.

vi. Above all, it should be safe and should function reliably.

Different types of propellant charges including pyrotechnic materials like

sodium azide, lead styphanate and composite propellant based on GAP and

ammonium perchlorate have been reported in the literature for micro thrusters. Since

the GAP based composite propellant was found to meet almost all the requirements

satisfactorily, it was considered as the primary choice.

7.5.1 Experimental

GAP polymer contains energetic azide group in the molecule, which increase

the gaseous content of combustion products during thermal decomposition. GAP can

also perform as a monopropellant without any oxidiser. Also, the flame temperature

of GAP based propellant is comparatively lower than many other composite solid

propellants. In the trials carried out, GAP based propellant system was used. Many

propellant trials were carried out for GAP based formulations with different oxidiser

combinations. The AP propellant was made by mixing GAP with other ingredients

and curing it after filling it in the microthruster cavities. For curing, the resin was

mixed with crosslinker and curing agent composition. The oxidisers used for the

trials included AP and Potassium Nitrate. In order to minimise the chlorine

compounds and other corrosive elements in the combustion products, the oxidiser

content was minimised.

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7.5.1.1 Materials

The sources of GAP, crosslinker, TDI, AP, copper chromite and catalyst used

were same as mentioned in section 7.3.1.1. Fine grade of AP of average particle size

40 µ was used for the study. Potassium nitrate with average particle size 40 µ as

available from commercial source was also used as oxidizer.

7.5.1.2 Sample preparation

For preparing the propellant, GAP resin was first vacuum dried. Resin was

then mixed with 5% crosslinker. The oxidiser was dispersed thoroughly by hand

mixing. A small quantity (0.05%) of copper chromite was also mixed. TDI was

added as curing agent. After curator addition, 2 mg of catalyst was added and mixed

thoroughly. The mix was then degassed and then filled into the microthruster cavities

using a nitrogen jet. Filling was done repeatedly to ensure that the cavities are fully

filled with propellant. The assembly was then placed inside a hot air oven at 600C for

48 hrs for curing.

The micro thruster array was prepared by drilling 2 mm size cavities at 2 mm

apart on a hylam substrate of size 50 x 50 cm2. A nichrome wire based ignition

system was developed. The nichrome wire was soldered on to Cu strips, which are

adhesively bonded on both sides of the microthruster array. The nichrome wire was

placed across the thruster cavity. Figure 7.8 shows the thruster arrays along with

electrical contact for ignition system.

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Figure 7.8 Micro thruster assembly prepared with 2 mm size thrusters 7.5.1.3 Testing

A novel ignition system was developed to ignite the propellant by using a

nichrome wire segment as a heat source on the surface of the cured propellant. While

propellant filling, it was made sure that the nichrome wire and propellant are in

contact. The nichrome wire bridge was tested using current supplied from a 6 Volt

battery. In the ignition circuit, electrical power supply was established by connecting

the Cu strips to 6 Volt battery through contact switches. Each thruster was provided

with independent ignition system. The electrical contact was designed to supply the

necessary electrical power independently to each thruster.


The independent ignition and sustained burning of the individual thruster was

observed visually. The details of the propellant formulation tested and observations

made are presented in table 7.7. After the trials, further tests were carried out with an

optimised AP content of 5%. In all the trials carried out with AP, ignition and the

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sustained burning of the propellant could be seen without affecting the adjacent

thrusters.

Table 7.7 Test results of the GAP based propellant formulations

Propellant formulation Observation during test Post test observation

Cured GAP without AP and

Cu Cr2 O4

Bridge wire fired.

Propellant not ignited

Propellant is intact after

test. Igniter bridge wire

broken.

GAP with 2% KN and

0.05% Cu Cr2 O7

Igniter fired. No firing of

propellant Propellant is not burned.

GAP with 0.05% Cu Cr2 O4 Igniter fired. No firing of


GAP with 2% KN and 0.05%

Cu Cr2 O4

Igniter fired. No firing of


GAP with 2% AP and 0.05%

Cu Cr2 O4

Ignition of propellant and

sustained combustion seen.

Propellant is consumed and

charring of the cavity

observed.

GAP with 5% AP and

0.05% Cu Cr2 O4

Burning of propellant seen.

Jet seen clearly. Smoke

was seen in large

proportion.


charring of the cavity seen.

GAP with 10% AP and 0.05%

Cu Cr2 O4

Ignition of propellant and

sustained combustion seen.

Smoke seen.


charring of the cavity

noted.

7.6 Conclusion

Theoretical performance evaluation of GAP with different oxidiser systems

was carried out. Evaluation of the effect of solid loading on the performance showed

that formulation of GAP with HNF and aluminium with a solid loading of 78%

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could provide a peak specific impulse of 280 s. For aluminised GAP-HNF

propellant a peak vacuum Isp of 305 s was observed at a solid loading of 78%. The

vacuum Isp of GAP based propellant with ADN or HNF was found to be much

higher that that of GAP-AP propellant. Highest density impulse of the order of

545 s g/cc was observed for GAP–CL-20 based propellant. The study showed that

GAP with advanced oxidisers can significantly improve the performance capability

of the propellant.

As part of the study, GAP based propellant was processed and evaluated by

conventional means. The mechanical properties were evaluated for different GAP

based propellant formulations. The test results showed good mechanical properties

for the cured propellant.

Burn rate evaluation showed that, high burn rate of the order of 29.4 mm s-1

could be achieved for aluminized GAP-AP propellant with 70% solid loading.

Studies carried out for ignitability of GAP showed that GAP with 5 weight% fine

grade AP can be satisfactorily used for advanced propulsion application such as

microthruster systems.

Part II

Studies on Rheology and Cure Kinetics of GAP and GAP based Propellant

7.7 Basic concepts of rheological evaluation

The fundamental concepts of study of rheology include definition of flow

field under consideration, rheological model which describes the system under study

and experimental techniques for determination of rheological properties. A

rheological model establishes the relationship between shear stress, shear rate and

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shear strain. For Newtonian fluids, the shear stress-shear rate relationship is given by

the equation 7.7.

Where τ is shear stress, ηis the viscosity and du/dy is the rate of shear. In this

expression, the viscosity is a constant.

Majority of the non-Newtonian fluids which include polymeric fluids fall

under the category of pseudo plastic materials. The shear stress versus shear rate

relationship in such cases is given by the equation 7.8.

Where τ is the shear stress, K is consistency index in N s2 M-2 and ν is the

dimension less pseudo plasticity index.

The fluid viscosity is given by the equation 7.9.

Where η is the viscosity with units Pa.s.

These expressions describe non-Newtonian fluids which follow the Power

law. Proper selection of values of K and ν helps to predict the flow pattern of Power

law fluids. Detailed theoretical treatment of applications of flow theory to polymer

processing has been presented by number of authors.34-36

Viscosity data of the polymer compounds is an important input for the

rheological characterisation. A variety of viscometers are used for determination of

viscosity of fluids.37 The different types include rotational viscometers, capillary

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viscometer, orifice viscometer, rising bubble viscometer, pantographs and

plasticorders. Torque rheometers are used to test thermoplastic and thermosetting

elastomers.

Composite propellant slurry is a colloidal suspension of crystalline solids in a

polymeric binder. The shear stress versus shear rate curves of the slurry show a

hysterisis pattern. Studies have shown that, flow pattern of propellant slurry inside

the motor case during casting process could influence the orientation of the oxidiser

particles due to the shearing action generated, finally leading to a non uniform burn

front propagation.38 Relationship between flow along mandrel and walls, velocity

gradient and pattern generated inside motor case due to free fall of propellant paste

have been reported in literature.39 Propellant slurry with 86% solid loading has

shown to follow Herschel Bulkley40, 41 type rheological equation. The equation is

given by

Where τ0 is the yield stress and ν is the pseudo plasticity index.

The hysterisis shown by the shear stress versus shear rate curve of the slurry

is due to the thixotropic nature of the propellant slurry. The area under the hysterisis

loop represents the energy loss in destroying the structure of the system and is called

the thixotropic index. As the curing reaction progresses, the yield stress, consistence

index and thixotropic index increases. The yield stress denotes the shear stress

required to overcome the resistance offered by the slurry to flow.42 Once the yield

stress is overcome, the reduction in apparent viscosity of the slurry results as the

particles and polymer molecules orient in the direction of applied stress. This leads to

the pseudo plastic behaviour of the slurry. The thixotropicity or time dependant

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behaviour of the propellant slurry results from the hindrance offered to the process of

orientation of filler particles at higher filler loading due to inter particle interaction.

Propellant processability is strongly influenced by temperature. Temperature

increase accelerates the cure reaction, viscosity, yield stress and thixotropic index.

Initially, the fluidity is increased by temperature leading to a decrease in the

viscosity. With increase in rate of curing reaction, the fluidity decreases due to

polymer network formation resulting in the increase in viscosity, yield stress and

thixotropy. It is desirable for propellant slurry to have minimum yield stress,

minimum viscosity and minimum thixotropicity for good processability. The

processing temperature for the propellant slurry is optimised with these factors in

consideration. From the time required for rheological parameters to increase from an

initial value to a particular limit after curing agent addition, it is possible to have an

idea of the kinetics of curing reaction.43, 44 One method is to consider that the rate of

reaction is proportional to the reciprocal of time required to double the viscosity or

yield stress. From this consideration, the activation energy of the rate process can be

deduced using the modified Arrhenius relationship as shown below. Using this

expression a plot of ln (1/t) vs (1/T) can be made and from the slope of the straight

line, the activation energy can be determined.

Where t is the time to double the viscosity or yield stress, A is the pre-

exponential factor, E is the activation energy, T is the absolute temperature and R is

the universal gas constant.

7.8 Effect of plasticiser content on the rheological behaviour of GAP

The use of plasticiser in polymer systems has been dealt with in detail by

Sears et.al.45 Usual solid propellant formulations contain around 3 to 5% of

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plasticiser to enhance the processability.46 A detailed account of compatibility

studies carried out for GAP in terms of mechanical properties with ester and

hydrocarbon type plasticiser systems are provided in chapter 5. Studies were also

carried out with energetic plasticiser systems. It was found that, ester type

plasticisers like dioctyladipate (DOA) and dioctylphthalate (DOP) are compatible

with GAP. The effect of DOA, DOP and different energetic azido plasticiser systems

like 1,6-hexanediol bis (azidoacetate) (HDBAA), 2-ethyl-1,3-hexanediol bis

(azidoacetate) (EHDBAA) and diethylene glycol bis(azidoacetate) (DEGBAA) were

evaluated for this purpose.

7.8.1 Experimental

The effect of concentration of plasticiser on the viscosity of GAP resin was

evaluated by preparing mixes of GAP with varying concentration of DOA, DOP and

the three azido plasticisers. The plasticiser concentrations used for the study were 0,

10, 20, 30, 40 and 50 parts per 100 parts of GAP. The viscosity of the mix of each of

the formulations was evaluated at 300C. The shear rate employed for the

measurement was varied over a wide range.

7.8.1.1 Materials

The source of GAP used for the study was as mentioned in section 8.3.1.1.

DOA and DOP, as available from commercial sources were used for the study. The

azido plasticisers, HDBAA, EHDBAA and DEGBAA were synthesised in VSSC.

7.8.1.2 Instrumental

All the measurements were carried out using Brookfield viscometer as

mentioned in section 2.6. For resin-plasticiser combination, disc spindle no. 4 was

used. The spindle rpm was varied in the range from 0.5 to 100 rpm for varying the

234

shear rate. The shear stress and shear rate were estimated from the machine

parameters, geometrical parameters of the spindle, dial reading and viscosity.


An expected trend of reduction in viscosity was noted for GAP with increase

in plasticiser content. The reduction in the viscosity of GAP with ester type

plasticiser content is shown in figure 7.9 and reduction in viscosity with the three

azido plasticisers is shown in figure 7.10. A constant shear rate of 100 s-1 was

followed for the experiments.

Figure 7.9 Effect of DOA and DOP content on viscosity of GAP resin

235

Figure 7.10 Effect of azido plasticisers on viscosity of GAP resin

It was observed that the rate of reduction in viscosity decreases as the

plasticiser concentration exceeds 25% in the case of all the plasticisers. This could be

due to limited miscibility arising out of change over to secondary plasticiser

system.45 The viscosity evaluation showed that, the azido plasticisers are in general

far superior in modifying the flow characteristics of GAP. The observation of better

compatibility of azido plasticisers could be explained from the fact that the presence

of polar azido groups in both the systems lead to better chemical and thermodynamic

feasibility of mixing. From the figure 7.10, it can be noted that for a 50%

concentration of azido plasticiser HDBAA, the viscosity of GAP was reduced to

nearly 6.3% of initial value.

From the rheological measurements, the shear rate versus shear stress

relationship of GAP and GAP plasticised with DOA were evaluated. The shear stress

and shear rate was estimated from the instrument parameters and viscosity values

measured. Figure 7.11 shows a plot of shear rate versus shear stress relationship for

GAP and GAP with DOA content of 10 and 20%.

236

Figure 7.11 Shear stress vs shear rate relationship for GAP

and GAP with DOA

The shear stress versus shear rate relationship shows a good linear relationship

indicating the neat and plasticised resin follow behaviour close to Newtonian.

7.9 Studies on curing of GAP by viscometry and IR spectroscopy

Viscosity build up data of the polymer in the pre-gel stage could be used as a

valuable input to study the kinetics of the process and the temperature effect.47 The

chemical nature of the curatives, temperature and presence of crosslinker and catalyst

have strong influence on the viscosity build up of the network.

In this study, the curing reaction of GAP-TDI system was evaluated by

viscometry. The effect of temperature on the viscosity build up and kinetic

parameters of the process were evaluated. The cure reaction of GAP was also

followed by IR spectroscopy for comparison. Three different curing agents viz, TDI,

IPDI and MDCI were employed.

The influence of crosslinker and catalyst on the viscosity build up of GAP due to

cure reaction was also studied. The concentrations of crosslinker and catalyst were

237

selected based on the gum-stock property evaluation carried out as mentioned in

chapter 5. The study was done for GAP-TDI and GAP-IPDI systems.

7.9.1 Experimental

7.9.1.1 Materials

The sources of GAP, TDI, IPDI, MDCI, crosslinker and catalyst used for the

study were same as mentioned in section 7.3.1.1.


Viscosity during the cure reaction was measured using Brookfield viscometer

model RVDV II+. The curing mixture was degassed before charging into the sample

cell. The sample cell used was a small sample adapter of 10 ml capacity and the

spindle used was S-21. The sample adapter was placed in a constant temperature hot

water circulation bath. Viscosity of the curing polymer was measured at various

intervals.

The IR spectroscopy was done using Nicolet 510 P model FTIR

spectroscope. The intensity of the peaks in the spectra was derived using a built in

software available with the FTIR spectroscope. The samples were kept inside a

thermostat heated by an IR lamp for obtaining isothermal conditions.

7.9.1.3 Testing

For kinetic study, GAP resin was mixed with the curing agents and then

evacuated for 10 minutes before filling into the viscometer cup. The viscosity

measurement was done at four different temperatures viz 30, 40, 50 and 600C. The

viscosity data was recorded at regular intervals of time for 5 to 6 hours.

The IR spectra of the samples were recorded by using a drop of the sample

from the mix prepared as mentioned earlier. The samples were smeared between the

238

NaCl cells as a thin film for the measurement. The IR spectra were recorded at 30,

40, 50 and 600C at regular intervals of time for 5 to 6 hours for the study.

For viscosity build up study of GAP, crosslinker and catalyst combination,

the samples were made by mixing GAP first with cross linking agent followed by

curing agent. After that, the catalyst was added, mixed and evacuated for 10 minutes.

The mix was then filled into the viscometer cup for viscosity measurement. The

measurement was done at 30, 45 and 600C.


The rate of viscosity build up of GAP was found to be comparatively lower

than other binder systems like HTPB.29 The reason for the low rate observed could

be assigned to the secondary nature of hydroxyl groups of GAP. Figure 7.12 shows

the viscosity build up of GAP with TDI as curative at 30, 40, 50 and 600C. It was

noted that when IPDI and MDCI were used as curing agents, the viscosity build up

was very low even at elevated temperature of 600C.

Figure 7.12 Viscosity build up of GAP with TDI as curing

agent at different temperatures

239

The higher reactivity of TDI compared to IPDI and MDCI could be assigned

to the prominent electron withdrawing effect associated with the aromatic system.48

From the data, it was observed that at higher temperature, the viscosity of the mix

was lower due to the increase in mobility of the polymer molecule. The rate of

viscosity build up was found to steadily increase with temperature. It has been

reported that viscosity build up of polyurethane system follow exponential

relationship with time at constant temperature as the viscosity build up follows first

order kinetics.49 The exponential relationship is shown in equation 7.12.

Where, ηt is the viscosity at time t after curing agent addition, η0 is initial

viscosity and kv is the rate constant for viscosity build up. The rate constant can be

determined by linearising the exponential relationship as shown in equation 7.13.

From the slope of plot of ln (ηt) versus t, the rate constant k is determined.

Figure 7.13 shows the linearised viscosity time relationship with a straight line fit.

Figure 7.13 Plots of ln (viscosity) vs time at various temperatures

for GAP-TDI system

240

The plots show clearly a two stage pattern. The first stage is found to be

faster than the second. The stage separation may be due to the difference in the

reactivities of the two diisocyanate groups (ortho and para) of TDI. NCO group in

the ortho position is less reactive than the one in the para position due to steric

hindrance at the ortho position caused by the 1-methyl group.50 Both o- and p-NCO

groups are activated by each other through mesomeric electron withdrawing effect.

The depletion of p-NCO groups in the initial phase of cure reaction may further

cause deactivation of the o-NCO groups in addition to the steric hindrance. However,

the difference in the reactivities narrows down with increase in temperature. This

was confirmed by the observation that the ratio of the rate constants for the first and

second stage (k1/k2) decreases with temperature. It was also observed that there is no

stage separation at 600C. The rate constants determined for the two stages at each

temperature and the ratio of the rate constants are shown in table 7.8.

Table 7.8 Rate constants for viscosity build up of GAP–TDI system

From the viscosity build up data, activation energy and activation enthalpy of

the process were also determined using the Arrhenius (7.14) and Eyring equations

(7.15) respectively.

Temp. (0C)

Rate constant (minute-1)

Ist stage (k1)IInd stage

(k2) k1/k2

30 3.80E-03 1.86E-03 2.04

40 4.79E-03 3.06E-03 1.56

50 5.79E-03 4.63E-03 1.25

60 6.95E-03 - -

241

Where k is the rate constant; E is activation energy; T is temperature in

Kelvin scale and R is gas constant, A is the pre-exponential factor or Arrhenius

frequency factor.

In the Eyring equation, ∆H* is activation enthalpy, kN is Boltzman constant, h

is Planks constant and ∆S* is activation entropy.

Arrhenius and Eyring plots are depicted in figures 7.14 and 7.15 respectively.

The corresponding activation parameters are listed in table 7.9.

Figure 7.14 Arrhenius plots for viscosity build up of GAP-TDI system

242

Figure 7.15 Eyring plots for viscosity build up of GAP–TDI system

Using the Arrhenius relationship, a plot of ln k against 1/T is made and from

the slope of straight line plot, activation energy is determined. From the Eyring

relationship, slopes of the straight line plots between ln (k/T) and 1/T, ∆H* is

obtained and the intercepts give activation entropy.

Table 7.9 Activation energy and activation entropy determined from Arrhenius and Eyring equations for viscosity build up of GAP-TDI system

Viscometric studies on GAP curing in the presence of crosslinker and catalyst

were carried out. Figures 7.16 and 7.17 show the viscosity build up profile for GAP-

TDI and GAP-IPDI systems respectively.

Reaction stage Activation energy (kJ mol-1)

Activation entropy (J mol-1K-1)

Stage 1 16.8 - 244.6

Stage 2 36.7 - 184.9

243

Figure 7.16 Viscosity build up profile for GAP-TDI system with crosslinker and catalyst

Figure 7.17 Viscosity build up profile for GAP-IPDI system with crosslinker and catalyst

Viscosity build up of GAP with TDI and IPDI as curatives in presence of

catalyst was evaluated and compared. The rate of viscosity build up of GAP with

IPDI was lower than that with TDI when the crosslinking was done in the presence

of catalyst. The data show that in the case of GAP-TDI at higher temperature, the

reaction rate increases rapidly after 100 minutes of curative addition, which leads to

244

higher rate of viscosity build up. In the case of GAP-IPDI system, the reaction rate

and viscosity follows an identical rate of build up even at higher temperature due to

the low reactivity of aliphatic diisocyanate.

Using the modified Arrhenius relationship (equation 7.11) for time to double

the viscosity of the curing polymer, the activation energy for viscosity build up was

determined for GAP-TDI and GAP IPDI systems. Figure 7.18 shows the kinetic plot

for GAP-TDI and GAP-IPD systems with crosslinker and catalyst. The activation

energy for viscosity build up noted for GAP-TDI system was 10.02 kJ mol-1 and that

for GAP-IPDI system was 19.32 kJ mol-1. It was observed that the presence of

catalyst helps to reduce activation energy for the crosslinking considerably.

Figure 7.18 Kinetic plot for viscosity build up of GAP-TDI and GAP-IPDI systems with crosslinker and catalyst

Study of the cure kinetics of GAP by IR spectroscopy was carried out. The

path of the reaction can be easily followed by recording FTIR spectra of the curing

mixture at various time intervals. For instance, the IR spectra of GAP-TDI mixture

immediately after mixing and after 3 hours are shown in Figures 7.19. It can be

245

observed that there is a sharp reduction in the absorbance of NCO peak (2273cm-1)

after 3 hours of reaction.

Figure 7.19 FTIR spectra of the GAP-TDI sample (a) immediately after mixing

the ingredients and (b) after 3 hours

Reduction in the intensity of the peak at 2273 cm-1 corresponds to the

consumption of NCO groups while increase in the intensity of the peak at 1726 cm-1

indicates the formation of urethane groups due to reaction between hydroxyl and

diisocyanate groups. The absorption bands at 2100 cm-1 due to stretching of azide

group and CH stretching at 2930 cm-1 remain almost unaffected throughout the

course of reaction. For this reason, the ratio between the absorbance of NCO (2273

cm-1) and that of CH stretching (2930 cm-1) is taken as a measure of concentration of

diisocyanate groups for the purpose of evaluating kinetic parameters. It has been

established that the reaction between hydroxyl and diisocyanate groups follow 2nd

246

order kinetics.51 The kinetic expression when the ratio between the equivalents of

NCO and OH groups is unity is as given below:

where, [CNCO]0 and [CNCO]t are concentrations of NCO groups at the start of

the reaction and at any given time t, and k is 2nd order rate constant. When the

absorbance is considered for concentration term, the kinetic equation may take the

form as

and

Thus, plotting 1/[A]t against t yields straight lines and slopes of which are the

2nd order rate constants for the reaction between GAP and the diisocyanate curative.

Figures 7.20 to 7.22 depict the 2nd order plots for GAP-TDI, GAP-IPDI and GAP-

MDCI respectively.

247

Figure 7.20 Second order kinetic plots for GAP-TDI system

Figure 7.21 Second order kinetic plots for GAP-IPDI system

248

Figure 7.22 Second order kinetic plots for GAP-MDCI system

The second order rate constants were obtained for various diisocyanates

namely TDI, IPDI and MDCI each at different temperatures viz: 30, 40, 50 and 60°C.

For all the cases, linear plots with good correlation coefficients were obtained,

indicating that the reactions between GAP and diisocyanate curatives follow a

second order kinetics as reported for other similar systems.48, 51, 52 The slopes of the

straight line plots are the rate constants for the reactions. The second order rate

constants thus obtained for the three diisocyanates are listed in table 7.10.

249

Table 7.10 Kinetic data for GAP crosslinking from IR spectroscopic study

Isocyanate Second order rate constant at different temperatures (k)

(mol-1minute-1)

300C 400C 500C 600C TDI Stage I Stage II

3.49 x 10-3 1.98 x 10-3

9.52 x 10-3

2.24 x 10-2

4.87 x 10-2

IPDI Stage I Stage II

2.20 x 10-4 5.53 x 10-4

7.43 x 10-4

2.36 x 10-3

7.92 x 10-3

MDCI kTDI/kIPDI kTDI/kMDCI kIPDI/kMDCI

2.85 x 10-5

16.1 122.3 7.6

1.23 x 10-4 12.8 77.0 6.0

6.80 x 10-4 9.5 32.9 3.5

3.39 x 10-3 6.1 14.3 2.3

It can be seen that at any given temperature, the rate constants for the three

diisocyanate compounds can be arranged in the order TDI > IPDI > MDCI. This is

very much in accordance with reported trend obtained with conventional chemical

kinetic approaches. A comparison of second order plot for reaction between GAP

and the three isocyantaes at 300C is shown in figure 7.23.

250

Figure 7.23 Second order kinetic plots for the reaction between GAP and various diisocyanate compounds at 300C

Due to electron withdrawing mesomeric effect, which is very important for

aromatic isocyanates,29 TDI is more reactive than the aliphatic isocyanates used in

the study. Between the two cyclo-aliphatic isocyanates used in the present study,

IPDI is expected to be more reactive than MDCI as one of the two isocyanate groups

in IPDI is primary in nature and the other is secondary. Both the isocyanate groups in

MDCI are secondary and can be expected to be far more sluggish in its reaction with

hydroxyl groups. A deviation from the general behaviour was observed with TDI and

IPDI at 300C. Both exhibit a two stage reaction pathway. With TDI, the first stage

was faster than the second, while with IPDI the second stage was faster than the first;

which is a typical example for autocatalysis. MDCI did not exhibit any stage

separation. It can be further noted that the ratio of the rate constants of higher

reactive to a lower reactive isocyanate steadily reduces with temperature, indicating

that rise in temperature narrows down difference between the reactivities. Highest

fall in the ratio occurs between TDI and MDCI.

251

As mentioned earlier, the activation energy and activation entropy for the

reaction between GAP and the three isocyanates were also determined using the data

generated by IR spectroscopic studies by means of Arrhenius equation (7.14) and

Eyring equation (7.15) respectively. Arrhenius and Eyring plots are depicted in

figures 7.24 and 7.25 respectively. The corresponding activation parameters are

listed in table 7.11. The values obtained for Activation energy (E) and activation

enthalpy (∆H*) are in conformity with the trend in the reactivities of the

diisocyanates with GAP.

Figure 7.24 Arrhenius plots for GAP with TDI, IPDI and MDCI

252

Figure 7.25 Eyring plots for GAP with TDI, IPDI and MDCI

Table 7.11 Activation parameters for the reaction between

GAP and various isocyanates

Diisocyanate Activation Energy (kJ mol-1)

Activation enthalpy (kJ mol-1)

Activation entropy (J mol-1 K-1)

TDI 73.2 71.0 -57.6

IPDI 100.3 97.7 6.8

MDCI 134.5 131.7 101.8

It is always desirable to draw a relationship between kinetic parameters and

viscosity of the curing mixture. Such a correlation would help to predict the viscosity

of the curing mixture at a given time during the pre-gel phase. Of the many attempts

made, the parameters 1/(1-p) and ln ηt give rise to linear correlations with fairly good

correlation coefficients, where p is the extent of reaction between NCO and OH

253

functional obtainable from rate expressions. The linear plots for all temperatures

except 600C exhibit a well defined digression. The discontinuity in the linear plots is

due to difference in the reactivities between the o- and p- NCO groups of TDI as

explained in the previous sections. 1/(1-p) vs ln ηt plots and a liner fit for the

relationship are depicted in figure 7.26.

Figure 7.26 Dependence of viscosity of curing mixture on the extent of reaction

7.10 Studies on the rheology of GAP based propellant

In this section, results of the rheological studies carried out on GAP based

propellant formulations are presented. GAP based propellant formulations with TDI

and IPDI as curatives were investigated. Effect of temperature on the rheological

parameters was also evaluated. The study was also extended to GAP-HTPB blend

based propellant formulations. The relevance of the data on rheological parameters

with respect to propellant processing have been discussed in the literature.20 Flow

254

behaviour of propellant slurry can be described by plotting the rheogram, which

shows the relationship between shear stress and shear rate. The chance of defect

formation in the propellant grain depends upon many factors like geometry of the

grain configuration, rate of viscosity build up, and mixing of propellant slurry within

the case. Rheological parameters have been found to provide an insight into the

processability of defect free propellant grain. In the present study, GAP propellant

with optimum solid loading of 75% has been employed. The oxidiser used was

ammonium perchlorate and the metallic fuel was aluminium. The formulation was

selected based on the gum-stock property evaluation, theoretical performance

evaluation and the studies on effect of plasticiser on GAP as described in previous

sections.

7.10.1 Experimental

GAP based propellant was processed with both TDI and IPDI as curatives

and AP coarse to fine ratio was kept at 2:1 in all the formulations. GAP-HTPB

propellant was prepared with GAP:HTPB weight ratios of 50:50. In all the

formulations, solid loading of 75% was maintained. The propellant samples were

subjected to rheological evaluation at three different temperatures, viz 45, 60 and

700C. The propellant formulation and mixing schedule are as mentioned in section

7.3.1.

7.10.1.1 Materials

The sources of GAP, AP, aluminium, crosslinker, TDI, IPDI and catalyst

used for the study are same as mentioned in section 7.3.1.1. HTPB resin with

molecular weight 2500 (by VPO) was used.

255


Propellant mixing was carried out using a small scale horizontal sigma blade

mixer as shown in section 7.3.1. For apparent viscosity measurements of the

propellant slurry, Brookfield viscometer model HBDVI+ was used. The rheological

studies were done with Rheometer model Rm 265. The details of the equipments are

provided in section 2.6. The rheological measurements were done at 45, 60 and 700C.


GAP propellant evaluated for the rheological characteristics exhibited

thixotropic nature. The thixotropy of the propellant slurry resulted in formation of

hysterisis loop in shear stress vs shear rate profile. Figures 7.27, 7.28 and 7.29 show

the shear stress vs shear rate and figures 7.30, 7,31 and 7.32 show viscosity vs shear

rate profiles of GAP-TDI propellant system generated at 45, 60 and 700C. Figures

7.33, 7.34 and 7.35 show the shear stress vs shear rate and figures 7.36, 7.37 and 7.38

show viscosity vs shear rate profiles of GAP-IPDI propellant system generated at 45,

60 and 700C.

Figure 7.28 Shear rate vs shear stress profile for GAP-TDI propellant at 600C

Figure 7.27 Shear rate vs shear stress profile for GAP-TDI propellant at 450C

256

Figure 7.29 Shear rate vs shear stress Figure 7.30 Shear rate vs viscosity profile for GAP-TDI propellant at 700C profile for GAP-TDI propellant at 450C

Figure 7.31 Shear rate vs viscosity Figure 7.32 Shear rate vs viscosity profile for GAP-TDI propellant at 600C profile for GAP-TDI propellant at 700C

Figure 7.33 Shear rate vs shear stress Figure 7.34 Shear rate vs shear stress profile for GAP-IPDI propellant at 450C profile for GAP-IPDI propellant at 600C

257

Figure 7.35 Shear rate vs shear stress Figure 7.36 Shear rate vs viscosity profile for GAP-IPDI propellant at 700C profile for GAP-IPDI propellant at 450C

Figure 7.37 Shear rate vs viscosity Figure 7.38 Shear rate vs viscosity profile for GAP-IPDI propellant at 600C profile for GAP-IPDI propellant at 700C

The higher reaction rate of TDI as explained in section 7.10.4 was found to

be the reason for the increased shear stress and viscosity build up of GAP-TDI

propellant compared to GAP-IPDI propellant. The lower shear stress and viscosity

indicate longer pot life for the propellant slurry. The hysterisis loss was found to be

lower at higher temperatures (600C and 700C) compared to that at 450C.

The apparent viscosity build up of the GAP-TDI and GAP-IPDI propellants

were also evaluated with Brookfield viscometer. Figures 7.39 and 7.40 show the

258

viscosity build up of GAP-TDI and GAP-IPDI propellants respectively at 45, 60 and

700C. The low viscosity of the GAP-IPDI system is found to be well in agreement

with the observations made from the rheogram. The low rate of reactivity of GAP

with IPDI compared to TDI is reflected in the propellant viscosity build-up also.

Effect of increase in temperature on the rate of viscosity is found to be more for

GAP-TDI propellant compared to GAP-IPDI propellant. The viscosity of GAP-TDI

propellant is found to reach the threshold value of 16000 poise after 90 minutes at

700C, after 120 minutes at 600C and after 150 minutes at 450C. In the case of GAP-

IPDI propellant, the same level of viscosity build up was reached only after 360

minutes even at 700C. This clearly indicates higher pot life of GAP-IPDI propellant.

Figure 7.39 Viscosity build up of Figure 7.40 Viscosity build up of GAP-TDI propellant at GAP- IPDI propellant at different temperatures different temperatures

The rheological parameters such as yield stress and thixotropic index were

obtained from the rheograms generated at different temperatures. Evaluation of the

rheological parameters allow determination of optimum processing conditions for the

propellant. The rheological parameters were plotted against time to find an optimum

combination. Figures 7.41 and 7.42 show the variation of rheological parameters

259

with time at different temperatures for GAP-TDI and figures 7.43 and 7.44 show

those for GAP-IPDI propellants.

Figure 7.41 Variation of yield stress Figure 7.42 Variation of thixotropic index with time for GAP-TDI propellant with time for GAP-TDI propellant

Figure 7.43 Variation of yield stress Figure 7.44 Variation of thixotropic index with time for GAP-IPDI propellant with time for GAP-IPDI propellant

Yield stress is found to increase sharply in the case of GAP-TDI system

compared to GAP-IPDI system. This is expected due to the higher rate of cure

reaction of the aromatic isocyanate. Thixotropicity is found to increase with

temperature in both GAP-TDI and GAP-IPDI systems. It is found that compared to

conventional propellant systems GAP-TDI propellant has a low yield stress and

thixotropicity for 3 to 4 hrs after curing agent addition at 600C. A processing

temperature of 600C may be considered optimum for the system.

260

From the rheograms, the time for the propellant slurry to double the viscosity

was taken (at a uniform shear rate of 0.5 s-1) as the variable for determination of

activation energy for the curing process. Modified Arrhenius relationship as shown

in equation 7.11 was used for the purpose. A plot of ln (1/t) vs 1/T shows a straight

line and from the slope, the activation energy E was estimated. Figure 7.45 shows the

ln (1/t) vs 1/T for GAP-TDI and GAP-IPDI.

Figure 7.45 Arrhenius plot for viscosity build up for GAP-TDI

and GAP-IPDI propellants

From the slope, the activation energy estimated for GAP-TDI propellant was

95 kJ mol-1 and GAP-IPDI propellant was 120 kJ mol-1. The activation energy values

are higher than that was seen for pure GAP–TDI and GAP-IPDI systems as estimated

in section 7.10.4.2 due to the dilution effect of plasticiser and presence of number of

unreactive ingredients in the propellant formulation.

GAP-HTPB propellant with GAP:HTPB weight ratio of 50:50 is selected for

this study based on gum-stock experiment results reported in chapter 5. It was found

that unlike in the case of GAP propellant, GAP-HTPB propellant with a cross linker

content of 5% was not processable due to high rate of reaction. However, the higher

functionality of HTPB (f = 2.2) compared to GAP (f = 1.7) makes it possible to

261

achieve comparable properties even with a crosslinker content of 1.25%. Figure 7.50

shows the variation of viscosity with time for GAP–HTPB propellant in the ratio of

50:50 with TDI as curing agent at 450C and 600C.

Figure 7.46 Viscosity build up of GAP-HTPB (50:50 ratio) propellant with TDI as curative

Propellant slurry prepared with GAP:HTPB in the ratio of 50:50 was also subjected

to rheological evaluation. Figures 7.47 and 7.48 show the variation of yield stress

and thixotropic index with time for GAP-HTPB propellant. The data generated

shows that the rheological behaviour of GAP-HTPB propellant is comparable with

that of GAP propellant.

Figure 7.47 Variation of yield stress with Figure 7.48 Variation of thixotropic time for GAP-HTPB propellant index with time for GAP- HTPB with TDI as curative propellant with TDI as curative

262

7.11 Conclusion

The importance of rheological evaluation of binder and propellant

formulations were described. The basic concepts on non-Newtonian flow of highly

solid loaded formulations were presented. The mathematical expressions used for the

determination of kinetic parameters were also presented.

Effect of concentration of different plasticiser systems like DOA, DOP and

azido plasticisers like HDBAA, EHDBAA, DEGBAA on the viscosity of GAP was

evaluated. It was observed that, usually used ester type plasticisers like DOA and

DOP, and azido plasticisers like HDBAA, EHDBAA and DEGBAA are compatible

with GAP whereas, hydrocarbon plasticiser like paraffin oil and long chain

compound like isodecyl pelargonate (IDP) are not compatible with GAP.

Compatibility study shows that polar and chemical nature of GAP and plasticiser

influences the solubility strongly.

The reactivities of the isocyanates used in this study could be arranged in the

order: TDI > IPDI > MDCI. The difference between the reactivities narrowed down

with temperature. Both TDI and IPDI exhibited a stage separation in the kinetic plots

at 300C. Similar trend was observed with the viscosity build up for the reaction with

TDI. It has been shown that the chemical kinetic data obtained through FT-IR

spectroscopy was correlatable with viscosity build up data.

From the rheological studies, the activation energy obtained for the curing of

GAP-TDI propellant is 95 kJ mol-1 and that for GAP-IPDI propellant is 120 kJ mol-1.

Rheological characterisation of GAP propellant shows that an optimum temperature

of 600C can be followed for propellant processing with both TDI and IPDI curing

systems. Longer pot life could be obtained for GAP propellant with IPDI as curing

agent due to the low reactivity of the aliphatic isocyanate. GAP-HTPB propellant

shows rheological behaviour comparable to that of GAP propellant system.

263

7.12 References

1. Borman, S., Chemical and Engineering News, January (1994), 18.

2. Bottaro, J. C., Chem. Ind., April (1996), 249.

3. Golfier, M., Garindorge, H., Longevillale, Y., Maru, H., Proceedings of the 29th Institute for Chemical Technology Conference, Karlsruhe, Germany, (1998), 3/1.

4. Andrea, B. D., Lillo, F., Faure, A., Perut, C., Acta Austronautica, 47, (2000), 103.

5. Singh, H., Colloquim on Ammonium Perchlorate: Work-Horse Oxidiser for Solid Propellants, VSSC, Trivandrum, December (1996), 12.

6. Ward, M., New Scientist, 8, (1996), 14.

7. Muthiah, Rm., Varghese, T. L., Rao, S. S., Ninan, K. N., Krishnamurthy, V. N., Propellants, Explos., Pyrotech., 23, (1998), 90.

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chapter 7 studies on gap based propellant formulations...

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