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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
210
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
211
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
212
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
213
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
214
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
215
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.
216
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.
217
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.
218
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
219
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)
220
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)
221
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
222
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.
223
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
224
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
225
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.
226
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.
227
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.
7.5.1.4 Results and discussion
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
228
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
propellant Propellant is not burned.
GAP with 2% KN and 0.05%
Cu Cr2 O4
Igniter fired. No firing of
propellant Propellant is not burned.
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.
Propellant is consumed and
charring of the cavity seen.
GAP with 10% AP and 0.05%
Cu Cr2 O4
Ignition of propellant and
sustained combustion seen.
Smoke seen.
Propellant is consumed and
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%
229
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
230
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
231
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
232
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
233
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.
7.8.1.3 Results and discussion
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.
7.9.1.2 Instrumental
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.
7.9.1.4 Results and discussion
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
7.10.1.2 Instrumental
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.
7.10.1.3 Results and discussion
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.
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