chapter 5 mechanical and morphological characterisation of...

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Chapter 5

Mechanical and Morphological Characterisation of GAP and GAP Based Blends

5.1 Introduction

Solid rocket motors have to withstand a wide range of mechanical loads for

successful performance during their mission. The various mechanical loads are

imposed on it during storage, handling and operational phase. The loading

environments include thermal cycling, vibration and ignition pressurisation.1

Quantitative measurements of mechanical characteristics are important as the

propellant is utilized as a material of construction in case bonded configuration. The

different loads may sometimes be experienced in combination. Structural integrity is

one of the most important factors that is to be considered for propellant grain design.

The mechanical properties of solid propellants depend both on intrinsic and extrinsic

variables.2 These intrinsic variables are molecular weight, effect of crosslinking,

branching and crystallinity, plasticisation, quality and quantity of fillers. The external

variables are temperature, time frequency or strain rate, pressure, amplitude of stress

or strain, mode of deformation and nature of surrounding atmosphere.

Composite solid propellants display viscoelastic characteristics as they show

behaviour similar to both viscous liquids in which rate of deformation is proportional

to applied forces and to elastic solids, in which magnitude of deformation is

proportional to applied forces. The polymeric binder is the main contributor to the

viscoelastic nature of solid propellants. The mechanical behaviour is mainly

controlled by the polymer matrix. Fundamental contribution to the field of

thermodynamics of rubber elasticity made by Saunders,3 Mullins4 and Treolar5 forms

the basics of this area of study. The theoretical approaches provide relationship

between the material characteristics and its macroscopic behaviour. Another

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important contribution in this field was by William et.al6 in the study of

time–temperature superposition method. The mechanical properties of propellant are

strongly dependant on quality of bonding at the interface between solid fillers and the

resin matrix. Propellant dewetting takes place before mechanical failure. In the

dewetted condition, the mechanical strength essentially depends on the binder

characteristics. Higher crosslink density leads to decrease in elongation of the binder

matrix. Increase in strain rates leads to increase in stress at which failure occurs in

the binder matrix. These factors imply that the mechanical integrity of the propellant

system is strongly dependant on strain capability of binder matrix.7, 8 It is important

to correlate mechanical properties as a function of their composition during

propellant development. The mechanical properties are mainly determined by the

crosslink density of the binder matrix.9 An analysis of the interrelation between

modulus, ultimate tensile strength and elongation at break and the specific

deformation energy can be used as basic data for the preparation of propellant

formulations. The mechanical properties or the crosslink density of the binder matrix

can be tailored by quantitative variation of polymer diol, curative and triol.10 In the

case of GAP based high burn rate propellant compositions, it has been reported that

GAP concentration in the range of 33 to 60% has been used.11, 12

5.2 Chemistry of GAP based polyurethane network

Polyurethane based matrix systems find application as engineering materials

due to their good thermal, physical and mechanical properties. Polyurethanes based

on HTPB and isocyanate curatives are widely used as binder for composite solid

propellant application.13 Detailed studies on the use of polyfunctional groups to tailor

the properties of HTPB based polyurethane network have been presented in literature

by many authors.14, 15 The impact of variation of NCO/OH ratio on the properties of

the polymers has been reported.16 Long chain polyurethane network formation

reaction between hydroxyl terminated GAP and polyisocyanate have been reported.17

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Use of organo metallic catalysts like dibutyl tin dilaurate was found to have strong

influence on the rate of cure reaction.17 The equivalent ratio of NCO and OH groups

and the triol to diol ratio were varied to adjust the crosslink density.10 The use of long

chain diols as binder helps to reduce the problems of cure shrinkage, exotherm due to

curing and non elastomeric properties. The urethane formation reaction can be

represented schematically as shown in scheme 5.1. The most favoured mechanism in

the scheme is the nucleophilic attack of the hydroxylated compound to the isocyanate

group.18, 19

Scheme 5.1 Urethane formation reaction

The urethane link is chain extended by 1, 4-butanediol (BD) and crosslinked by

trimethylol propane (TMP) through OH functional groups and isocyanate. The

urethane reaction leads to crosslinking. The poly condensation reaction between the

polymer functional groups, crosslinking agent and the isocyanate which leads to the

crosslinked network structure is shown schematically in scheme 5.2.

OHHO OHHO HO OH

OH

+ +GAP BD TMP

+OCN NCODiisocyanate

NHCOO NHCOO NHCOOOOCHN OOCHNOOCHN

OOCHN OOCHNOOCHN NHCOONHCOONHCOO

OO

CHN N

HC

OO

NHC

OO O

O

CHN

Scheme 5.2 Poly condensation reaction of GAP diol, BD and TMP with diisocyanate forming polyurethane network

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5.3 Studies on gumstock properties of crosslinked GAP

The various applications of plastics, elastomres and fibers depend mainly on

their mechanical strength than their chemical characteristics. The various

classification of polymers used for most of the practical applications can be classified

as follows.20

i. Soft, weak which have low modulus, tensile strength with moderate

elongation at break.

ii. Hard, brittle with high modulus and tensile strength with low elongation

iii. Soft, tough with high values of tensile strength, elongation and low modulus

iv. Hard, tough with high tensile strength, elongation and modulus

v. Hard, strong with properties intermediate between hard and brittle and hard

and tough.

The mechanical properties of polyurethane network prepared with GAP and

GAP based blends including that of interpenetrating network structure with GAP and

HTPB were evaluated from the stress-strain curves in terms of tensile strength,

elongation at break and modulus.

5.3.1 Experimental

GAP resin and the crosslinker used for the matrix preparation were dried in

rotary vacuum evaporator to remove moisture and volatiles to the extent of less than

0.1%. Presence of moisture in GAP causes formation of large extent of blowholes

while curing. This is mainly due to the dominating reaction between isocyanate and

moisture compared to the lower reactivity of the secondary hydroxyl functional in

GAP.

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5.3.1.1 Materials

GAP resin with molecular weight 2000 (by VPO) and hydroxyl value 45 mg

KOH/g was prepared in VSSC as mentioned in section 4.3.5 for the experiments.

The crosslinker used was a mix of 1, 4-butanediol and trimethylol propane in the

weight ratio of 1:2. Curatives used include toluene diisocyanate (TDI), isophorone

diisocyanate (IPDI) and methylene bis (cyclohexyl isocyanate) (MDCI) available

from commercial sources. The catalyst (0.006 g per drop) used was a 10% solution

of DBTDL in toluene.

5.3.1.2 Sample preparation and testing

For preparation of crosslinked GAP samples, first GAP was mixed with

crosslinker and dried at 1000C for 30 minutes in a vacuum flash evaporator. After

cooling the mix, weighed quantity of curative was added and mixed thoroughly. In

this preparation, the stochiometric ratios of curatives and crosslinker were varied to

study the effect on the gumstock properties. After addition and mixing of curatives,

weighed quantity of catalyst was added and mixed. The mix was evacuated to

remove entrapped volatiles. The resin mix was then poured into mould and

evacuated. The initial curing was done under nitrogen blanket for 24 hours at

ambient conditions. This was followed by curing at 600C for 48 hours in air oven.

From the cured slabs, dumbbells were prepared and tested as mentioned in section

2.5.

5.3.1.3 Results and discussion

In this study, effect of concentration of reactive species, curing temperature,

curing time, and effect of different type of curatives on the gumstock properties of

GAP were evaluated.

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The influence of the NCO/OH ratio (R value) on the gumstock properties of

GAP matrix was investigated experimentally by varying the NCO/OH ratio in the

range of 0.7 to 1.75 for a constant crosslinker content of 5%. This evaluation was

also carried out with different curatives like TDI, IPDI and MDCI. Figures 5.1 to 5.4

shows the variation of tensile strength, elongation, stress at 100% elongation and

shore A hardness with NCO/OH ratio of the crosslinked network with different

curatives. Samples prepared with NCO/OH ratio below 0.9 were found to be of too

low strength to be tested. The increase in tensile strength, stress at 100% elongation

and shore A hardness and reduction in elongation with increase in NCO/OH ratio

could be related to the increase in crosslink density of the network. The results show

that increasing the NCO content improves the tensile strength. Elongation is found to

decrease up to a NCO/OH ratio of 1, beyond which no significant change was

observed.

Figure 5.1 Variation of tensile strength with NCO/OH ratio

Figure 5.2 Variation of elongation

with NCO/OH ratio

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Tensile strength, stress at 100% elongation and hardness for the three

curatives were found to be in the order of MDCI > IPDI > TDI. The change from

aromatic diisocyanate to aliphatic diisocyanate is found to significantly improve the

mechanical strength. This could be due to the high reactivity of aromatic

diisocyanate leading to side reactions forming more of biuret and allophanate

linkages compared to aliphatic diisocyanate.

The effect of the crosslinker content on crosslinked networks was evaluated

by testing the gum stock properties of the network prepared with varying

concentration of the crosslinker. The crosslinker content in the GAP mix was varied

from 3 to 9% for a NCO/OH ratio of unity. When the crosslinker content was

reduced below 3%, the cohesive strength of the crosslinked slab was affected

severely. The low cohesive strength of the network could be due to the low crosslink

density resulting from the lower functionality (less than 2) of the GAP resin. Hence,

for GAP based networks, to achieve sufficient crosslink density, a minimum

crosslinker content of 3% is necessary for NCO/OH ratio above 0.90. The test results

show that increase in crosslinker content improves the tensile strength, stress at

100% elongation, shore A hardness and reduce the elongation due to increase in

Figure 5.3 Variation of stress at 100% elongation with NCO/OH ratio

Figure 5.4 Variation of hardness with NCO/OH ratio

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crosslink density of cured network. Figures 5.5 to 5.8 show the variation of the

properties with crosslinker content.

Figure 5.5 Effect of crosslinker content on tensile strength

Figure 5.6 Effect of crosslinker content on elongation

Figure 5.7 Effect of crosslinker content on stress at 100% elongation

Figure 5.8 Effect of crosslinker content on hardness

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Cure cycle studies were done on GAP binder at ambient condition and at

600C and the properties were compared. The effect of curing time was also evaluated

by curing the samples at extended periods of time. Figures 5.9 to 5.12 and figures

5.13 to 5.16 show the effect of curing time at ambient and at 600C on mechanical

properties respectively. The test results show that optimum mechanical properties

were obtained at ambient curing for 96 hrs. beyond which no significant changes in

mechanical properties are observed.21

Figure 5.9 Effect of ambient curing on tensile strength

Figure 5.10 Effect of ambient curing on elongation

2.52.72.93.13.33.53.73.9

0 100 200 300Curing time (hrs)

Stre

ss a

t !00

% e

long

atio

n (k

sc)

Figure 5.11 Effect of ambient curing on stress at 100% elongation

15

20

25

30

35

0 100 200 300Curing time (hrs)

Sho

re A

har

dnes

s

Figure 5.12 Effect of ambient

curing on hardness

Tens

ile st

reng

th (k

sc)

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In the case of accelerated curing, curing of GAP is almost complete within 48

hours itself and extended curing does not increase the cross-link density appreciably

as against the drastic change in mechanical properties seen on variation of curing

agent or crosslinker content.

5

6

7

8

9

0 50 100 150 200 250

Curing time (hrs)

Tesi

le s

treng

th (k

sc)

Figure 5.13 Effect of cure time

at 600C on tensile strength

150

170

190

210

230

0 50 100 150 200 250Curing time (hrs)

% E

long

atio

n


at 600C on elongation

2.5

3

3.5

4

0 50 100 150 200 250

Curing time (hrs)

Stre

ss a

t 100

% e

long

atio

n (k

sc)

Figure 5.15 Effect of cure time at 600C on stress at 100% elongation

05

101520253035

0 50 100 150 200 250Curing time ( hrs)

Sho

re A

har

dnes

s


at 600C on hardness

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5.4 Studies on the gumstock properties of GAP-HTPB blends

The use of GAP as an energetic additive or plasticiser with other polymeric

binders have already been reported.17 In this study, crosslinked GAP-HTPB blends

with varying percentage of the binders were prepared. TDI, IPDI and MDCI were

used for preparation of the crosslinked networks. The effect of NCO/OH ratio on

gumstock properties of GAP-HTPB blend prepared with a 50:50 ratio of GAP and

HTPB was also evaluated.

5.4.1 Experimental

GAP and HTPB were found to be immiscible. The two resins were found to

phase separate when kept for a while after mixing. However, crosslinked networks of

GAP-HTPB blends could be prepared without phase separation by increasing the

reaction rate and gelling of the mix by adjusting the catalyst content. The catalyst

concentration was varied between 0.036 to 0.1% in the trials.

5.4.1.1 Materials

GAP, TDI, IPDI, MDCI, crosslinker and catalyst used were obtained from

sources as mentioned in section 5.3.1.1. HTPB resin with molecular weight 2500 (by

VPO) and hydroxyl value of 43 mg KOH/g, produced in VSSC by free radical

polymerisation of butadiene gas was utilised for the study.

5.4.1.2 Sample preparation

GAP-HTPB blends were prepared with GAP content ranging from zero to

100% and the mechanical properties of the crosslinked networks were evaluated. The

crosslinker concentration was maintained at 5% level with R value of 1. For

preparation of the blend, first GAP and HTPB were mixed as per required ratio.

Required amount of crosslinker was added and then the mix was evacuated and dried

at 1000C using rotary vacuum flash evaporator. After cooling, weighed quantity of

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curative was added and stirred well. After addition of catalyst and thorough mixing,

it was poured into a mould and allowed to cure initially under nitrogen atmosphere at

ambient temperature for 24 hours and then at 600C in an air oven for 48 hours.

During the experiment, care was taken to make sure that the initial gelling of the mix

takes place without any phase separation. From the slab prepared, dumbbells were

cut and tested using Instron testing machine as mentioned under section 2.5.

Crosslinked samples of GAP-HTPB blend with a ratio of 50:50 was also prepared

with varying NCO/OH by the same procedure. In these experiments, the TDI was

used as curative and the NCO/OH ratio was varied from 0.85 to 1.2.


The gum stock properties GAP-HTPB blends prepared with various ratios of

GAP and HTPB and crosslinked with different curatives are shown in Figures 5.17 to

5.20. The test results of the blends show that the tensile strength and stress at 100%

elongation increase with increase in the percentage of HTPB up to 70%, beyond

which a decreasing trend was seen. The tensile strength was increased to 25 ksc and

elongation reduced to 120% by increasing HTPB content to 70%. The results show

higher values for tensile strength and stress at 100% elongation for crosslinked

HTPB than for the crosslinked GAP as expected due to the higher functionality of

HTPB binder (F = 2.3) compared to GAP (F = 1.7). However, GAP–HTPB blends of

50:50 to 30:70 ratios show superior properties over the virgin networks of GAP and

HTPB due to the synergetic effect of interlocking of the two networks resulting in

interpenetrating network structure (IPN).22 The formation of IPN structure was

further studied using optical micrography and scanning electron micrography. The

details of the studies are presented in section 5.6.

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Figure 5.17 Effect of GAP content on tensile strength of GAP-HTPB blend

Figure 5.18 Effect of GAP content on elongation of GAP-HTPB blend

Figure 5.19 Effect of GAP content on

stress at 100% elongation of GAP-HTPB blend

Figure 5.20 Effect of GAP content on hardness of GAP-HTPB blend

Shor

e A

had

ness

130

The three curing agents show a similar pattern with respect to the gumstock

properties of the blends. MDCI was found to give relatively higher strength

compared to TDI and IPDI as in the case of GAP gum stock properties.

The effect of NCO/OH ratio on the gum stock properties of GAP-HTPB

blend was found to be similar to that of crosslinked GAP formulation. It was

observed that for the GAP-HTPB blend, a minimum NCO/OH ratio of 1 is required

for achieving satisfactory mechanical strength for the cured slab. The test results are

shown in table 5.1.

Table 5.1 Effect of variation of NCO/OH ratio on gumstock properties and hardness of crosslinked GAP-HTPB network

NCO/OH ratio

Tensile strength (ksc)

Elongation (%)

Stress at 100 % elongation (ksc)

Hardness (Shore A)

0.85 2.4 21 - -

0.95 3.4 25 - 35

1.0 19.4 130 17 64

1.1 22.7 97 23 65

1.2 23.2 92 24 68

Non-curing was observed at a NCO/OH ratio of 0.8 even for extended periods

of curing. However, when the NCO/OH ratio was increased above 0.95, tensile

strength, stress at 100% elongation and hardness were found to increase remarkably.

5.5 Effect of plasticiser content on gumstock properties of crosslinked GAP

The mechanical properties of crosslinked GAP with plasticiser content was

determined to evaluate the compatibility of different plasticiser systems with GAP.

Energetic plasticiser systems like trimethylol ethanetrinitrite (TMETN),23

bis(2, 2-dinitropropyl) formal/acetal (BDNPF/A),24 derivatives of glycidyl nitrate viz.

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GLYN dimer25 have been synthesised for use with GAP. Low molecular weight

GAP26 and GAP polyester having acyl residue of an organic carboxylic acid27 have

also been reported for use as energetic plasticisers. Gumstock properties of GAP

plasticised with BDNPF/A has been reported.28 In this study, the compatibility of

GAP was evaluated with isodecylpelargonate (IDP), dioctyladipate (DOA),

dioctylphthalate (DOP) and paraffin oil. It was observed that, with IDP and paraffin

oil, GAP shows phase separation when kept for a while after mixing. In the case of

paraffin oil, the phase separation was found to be very fast. No phase separation was

found to occur when DOA and DOP were used as plasticisers.

5.5.1 Experimental

Prior to determination of the gum stock properties, miscibility of different

plasticiser systems (DOA, DOP, IDP and paraffin oil) with GAP was studied by

preparing mix of GAP and plasticisers with varying concentration in the range of

0 to 50%. Phase separation between different layers was visually checked after

different intervals.

5.5.1.1 Materials

GAP resin with molecular weight 2000 (by VPO) produced in VSSC was

used for the trials. Toluene diisocyanate, (Bayer, Germany) which is an 80:20

mixture of 2, 4 and 2, 6 isomers was used as the curing agent. Dibutyl tin dilaurate

(DBTDL) in toluene solution was used as cure catalyst. Cross-linking agent used was

a diol-triol mixture (a mixture of 1,4-butanediol and 1,1,1-trimethylolpropane in the

weight ratio of 1:2) and was prepared by mixing 1,4-butanediol and 1,1,1-

trimethylolpropane and the mixture was dried under vacuum for extended periods to

remove volatiles and moisture. DOA, DOP, IDP and paraffin oil were used as

available from commercial sources.

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Gum stock properties of crosslinked GAP network plasticised with DOA and

DOP were prepared. Plasticiser content was varied from 0 to 20%. In these trails,

TDI was used as curative. GAP was first mixed with plasticiser and then mixed with

other ingredients and crosslinked slabs were prepared as mentioned under 5.3.1.2.

The specimens were tested using Instron testing machine as mentioned in section 2.5.


During the miscibility experiments, it was found that DOA and DOP are

completely miscible in all the proportions used for experimentation. This may be due

to the polar nature of GAP and ester linkage in DOA/DOP. Miscibility is found to

decrease considerably and phase separation is found to occur when plasticiser was

changed to IDP, due to the long hydrocarbon chains of IDP compared to DOA/DOP.

Miscibility was totally absent when paraffin oil was used. This may be due to the

non-polar nature of the hydrocarbon chain.

Plasticiser content was found to strongly influence the gumstock properties. It

was noted that, increasing the plasticiser content from 0 to 20%, reduced the tensile

strength from 7.5 to 4 ksc and increased the elongation from 235 to 300%. Shore A

hardness was found to decrease with increasing plasticiser content. Figures 5.21 to

5.24 show the variation of the properties with palsticiser content.

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5.6 Effect of plasticiser content on the gumstock properties of GAP-HTPB blend GAP-HTPB blends were prepared without separation and the mechanical

properties were evaluated. For use as propellant binder, the resin is required to be

plasticised for improving the processability. GAP-HTPB blend with 50:50 ratio was

Figure 5.21 Effect of plasticiser on tensile strength of crosslinked GAP

Figure 5.22 Effect of plasticiser on elongation of crosslinked GAP

Figure 5.23 Effect of plasticiser on stress at 100% elongation of

crosslinked GAP

Figure 5.24 Effect of plasticiser on hardness of crosslinked GAP

Shor

e A

had

ness

134

selected for the study. DOA was found to be miscible with the blend. The DOA

content was varied from 0 to 20% in this study.

5.6.1 Experimental

For preparation of the blend, first GAP was mixed with HTPB at weight ratio

of 50:50. Weighed quantity of plasticiser was added and mixed thoroughly.

Crosslinker content of 5% was used in the experiments. TDI was used as the curative

and a NCO/OH ratio of unity was employed. Catalyst used was a 10% solution of

DBTDL in toluene. The curing of the samples and preparation of dumbbells were

done as mentioned in section 5.3.1.

5.6.1.1 Materials

GAP processed in VSSC, with molecular weight 2000 (by VPO) and hydroxyl

value of 45 mg KOH/g was used for the experiments. HTPB with molecular weight

of 2500 (by VPO), produced in VSSC was used. DOA available from commercial

sources was used as such. The crosslinker and curative were used from sources as

mentioned in section 5.3.1.1.


For preparation of the blends, first GAP and HTPB were mixed and then

desired quantity of plasticiser was mixed. Mixing of crosslinker, curative, catalyst

and preparation of slabs were done as explained in section 5.3.1.2.


Crosslinked GAP-HTPB blends were prepared with plasticiser content up to

20% for the evaluation. The effect of plasticiser content in crosslinked GAP-HTPB

blend was found to be similar to that for crosslinked GAP. The tensile strength was

found to decrease from 19.4 to 4.4 ksc for an increase in plasticiser content from 5 to

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20%. Hardness was also found to decrease from 68 to 25 shore A with increase in

plasticiser content as mentioned above. The elongation was found to increase to

132% for a plasticiser content of 10%. Further increase in plasticiser content was

found to decrease the elongation. Figures 5.25 to 5.28 shows the graphical

representation of the results.

Figure 5.25 Effect of plasticiser on tensile strength of crosslinked blend

Figure 5.26 Effect of plasticiser on elongation of crosslinked blend

Figure 5.27 Effect of plasticiser on stress at 100% elongation of

crosslinked blend

Figure 5.28 Effect of plasticiser on hardness of crosslinked blend

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5.7 Study on the morphology of GAP-HTPB blends

From the study of the gumstock properties of GAP-HTPB blends, it was

found that when the GAP-HTPB blends were crosslinked without phase separation, it

leads to formation of interpenetrating network structure (IPN). IPN structure forms

when a pair of polymeric networks are synthesised in intimate contact with one

another. When the two networks are totally compatible, one network fully

interpenetrates the other and forms catenated structure without any chemical

crosslinks.29 However, in most cases a certain degree of phase separation in the blend

could lead to substantially pure domains of each network. The physical interlinking

could be localised at phase boundaries and to optimise the IPN structure, the phases

should be as discontinuous as possible.30

GAP and HTPB are immiscible and phase separation occurs when the blend

is uncured and left undisturbed. When GAP-HTPB blend is mechanically agitated

and gelling of the blend is accelerated by tailoring the catalyst content, IPN structure

was found to form. Gumstock properties of the blend evaluated at definite

proportions indicate formation of IPN structure. In order to verify this observation,

morphological study was carried out.

5.7.1 Optical micrographic study of GAP-HTPB blend

The morphology of GAP-HTPB network was investigated with the help of

optical micrography to study the interpenetrating network structure of the blend. The

details of the equipment used for the study is given under section 2.7 in chapter 2.

5.7.2 Scanning electron micrographic study of GAP-HTPB blends

The morphology of crosslinked GAP-HTPB network was also investigated

with scanning electron micrography (SEM) to confirm the observation of IPN

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structure of the blend with specific concentration of the polymers. The details of the

equipment used are given in section 2.7.

5.7.3 Experimental

The crosslinked GAP-HTPB samples prepared with varying compositions were

subjected to morphological evaluation by optical micrography and scanning electron

micrography.

5.7.3.1 Materials

GAP, TDI, crosslinker and catalyst used for the study were obtained from

sources as mentioned in section 5.3.1.1.


For Optical micrography and SEM evaluation, crosslinked GAP/HTPB

samples were prepared by mixing GAP and HTPB at 30:70 and 50:50 ratios. Virgin

GAP and HTPB samples were also prepared for comparison. 5% crosslinker was

added to the blend and the mix was dried in a rotary vacuum flash evaporator at

1000C for 30 minutes. After drying, TDI was added as curator and mixed. NCO/OH

ratio of unity was followed for the study. Finally catalyst was added and again

mixed. The catalyst concentration was varied (0.036 to 0.1%) to study the effect. The

mix was evacuated to remove entrapped air bubbles and then poured into teflon

coated moulds and cured under nitrogen envelope. The curing was done at room

temperature for 24 hrs followed by at 600C for 48 hrs.

For optical micrographic evaluation, fresh surface of the samples were

prepared by freeze fracturing the specimens as mentioned in section 2.7.1. The

micrographs were taken at a magnification of 600

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For evaluation under scanning electron micrography, crosslinked GAP-

HTPB samples of size 10 x 10 mm and thickness 2 mm were cut and made

conducting by gold coating using Denton vacuum sputtering unit. For scanning

electron micrography, the equipment used was Philips XL30 Scanning electron

microscope. The details of equipment used are provided in chapter 2. The

micrographs were taken at a magnification of 1460.


The micrographs show the intricate entanglements of both the networks of

HTPB and GAP and was found to be more pronounced when GAP:HTPB ratio was

50:50 and 30:70. The figures 5.29 and 5.30 show the micrographs of the crosslinked

GAP-HTPB blends prepared with GAP and HTPB with a ratio of 50:50 and 30:70

respectively.

When the crosslinking reaction was slower, phase separation was found to

result between the two networks. The extent of phase separation was found to vary

depending on the rate of the gelling of the two networks. Figures 5.31 and 5.32 show

different views of partial phase separation noted when low catalyst concentration

Figure 5.29 Optical micrograph of GAP-HTPB blend with 50:50 ratio

Figure 5.30 Optical micrograph of GAP-HTPB blend with 30:70 ratio

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(0.036%) was employed for crosslinking reaction for a GAP: HTPB blend with 70:30

and 50:50 ratio respectively.

The scanning electron micrographs (SEM) of GAP-HTPB blends show

entanglement of network featuring typical IPN structure. Figure 5.33 and 5.34 show

the SEM of crosslinked pure GAP and HTPB. Figures 5.35 and 5.36 show the SEM

of GAP-HTPB blends prepared with ratio of 50:50 and 30:70 respectively. The phase

contrasts of SEM images of the blends were compared with that of virgin networks

for establishing the phase morphology. Distinct morphological variation was noticed

in the SEM images of the blends prepared with specific concentration levels of GAP

and HTPB. The superior properties of GAP-HTPB blends over the virgin networks

could well be explained by the greater degree of topological entanglements of both

the networks at specific concentration levels. Phase separation was observed in GAP-

HTPB blend, prepared with low catalyst content (0.036%) due to low rate of

crosslinking reaction. Figure 5.37 show SEM of the blend prepared with 50:50 ratio

indicating phase separation.

Figure 5.31 Optical micrograph of

GAP-HTPB blend (70:30ratio) with phase separation

Figure 5.32 Optical micrograph of GAP-HTPB blend (50:50 ratio) with

local phase separation

140

Figure 5.33 Scanning electron micrograph of crosslinked GAP

Figure 5.34 Scanning electron micrograph of crosslinked HTPB

139 (a)

141

Figure 5.35 Scanning electron micrograph of GAP-HTPB blend with 50:50 ratio

Figure 5.36 Scanning electron micrograph of GAP-HTPB blend with 30:70 ratio

139 (b)

140

Figure 5.37 SEM of GAP-HTPB blend with 50: 50 ratio with phase separation

5.8 Characterisation of crosslinked GAP network structures by swelling and

mechanical methods

For the characterisation of crosslinked network of polymers, determination of

thermodynamic solution properties are important as it forms the basis for formulating

a structure property correlation. The average molecular weight between crosslinks

(Mc) or crosslink density of the network influence all the structural characteristics of

the material. These parameters are generally controlled by adjusting the weight ratio

of curatives, crosslinker, and binder.31 Different methods were utilised for

determination of the crosslink density of the polymer network. These include

swelling method32-34 and mechanical method based on stress strain measurement.5, 35

In this study, the crosslink density of crosslinked GAP network was determined from

swelling data and from Youngs modulus obtained from stress strain curve. For these

141

experiments, the crosslinked network based on GAP was prepared with TDI and

IPDI as curatives. The swelling ratios of GAP-HTPB blends prepared with varying

concentrations of GAP were determined for comparison purpose.

5.9 Swelling study on crosslinked GAP and GAP-HTPB blend

The crosslink density is defined as the moles of effective network chain per

unit volume. Swelling of a polymer by a solvent depends on the crosslink density of

the polymer apart from solubility aspect and temperature. In this study, crosslinked

GAP was swollen with tetrahydrofuran (THF) and toluene for evaluating the

swelling parameters. THF was found to be more effective as a swelling agent for

GAP, based on solubility criteria. In the swelling studies carried out, the swell ratio

of the polymer in the solvent was determined experimentally.

5.9.1 Experimental

The swelling experiments were carried out with THF and toluene.

Crosslinked GAP samples were prepared for the study by reacting GAP and

crosslinking agent with TDI or IPDI in the presence of a catalyst. The swelling

experiments were carried out for crosslinked GAP with different NCO/OH ratios.

The mixing of the materials and curing were done as mentioned in section 5.2.1.

Samples were also prepared with varying crosslinker content for the study. The

different crosslinker contents used were 3, 5, 7 and 9%. The swelling experiments

were carried out in THF and toluene at ambient temperature. Crosslinked GAP

samples of weight in the range of 0.3 to 0.35 g were kept in excess quantity of THF

and toluene for more than 10 hours and samples were taken out at 1 hour interval and

wiped with blotting paper to remove solvent on surface of the sample and weighed.

From the initial weight, final weight and densities of sample and polymer, the

swelling ratio of the sample was determined using equation 5.1.

142

where W1 is initial weight of specimen, W2 is the weight of the specimen

after swelling, ρ1 and ρ2 are the densities of the solvent and polymer respectively.

Volume fraction of the polymer in the swollen gel (V2) is given by equation 5.2.

Swelling experiments were also carried out for determination of sol gel

content of the polymer. The sol gel content was determined by solvent extraction.

Samples weighing 0.9 to 1.0 g prepared with different NCO/OH ratios and

crosslinker contents were used for the experiment. The samples were kept in excess

quantity of THF and toluene for 48 hours. The samples were deswollen in

chloroform and dried under vacuum at 600C. After drying the final weight of the

sample was taken. The sol fraction was calculated using the expression 5.3.

where W1 is initial weight of sample and W2 is the final weight of the sample

after drying.

5.9.1.1 Materials

GAP, crosslinker, TDI, IPDI and catalyst used for the study were same as

those mentioned in section 5.3.1.1. Toluene with purity greater than 99.5% and THF

with purity greater than 99.7% both obtained form commercial sources were used as

solvents.

143


Crosslinked GAP and GAP-HTPB samples were prepared with TDI and IPDI

as mentioned in section 5.3.1. Crosslinked GAP containing different crosslinker

content (from 3 to 9%) was also prepared with TDI as curative.


The swelling ratios of the polymer prepared with different NCO/OH ratios

show a similar pattern of variation in both THF and toluene. However, the swelling

ratios were found to be higher in THF. The higher solubility in THF could be a result

of higher polarity of THF. Figures 5.38 and 5.39 show the variation of swelling ratios

with time for GAP crosslinked with TDI with different NCO/OH ratios in THF and

toluene. Figure 5.40 show similar data generated for GAP crosslinked with IPDI in

THF solvent.

Figure 5.38 Swell ratio of GAP crosslinked with TDI with different NCO/OH ratios in THF

144

Figure 5.39 Swell ratio of GAP crosslinked with TDI with different NCO/OH ratios in toluene

Figure 5.40 Swell ratio of GAP crosslinked with IPDI with different NCO/OH ratios in THF

Figure 5.41 shows variation of swell ratio with time for different crosslinker

content for GAP–TDI system in THF solvent. The data show that the swell ratio

decreases with increasing crosslinker content. The increase in solvent resistance is

due to the higher crosslink density resulting from the increase in crosslinker content.

145

Figure 5.41 Swell ratio of crosslinked GAP-TDI system in THF with varying crosslinker content

The equilibrium swell ratio was found to decrease sharply with increase in

NCO/OH ratio initially in THF medium. No considerable variation was noticed in

toluene medium. Figure 5.42 shows the variation of equilibrium swell ratio with

NCO/OH ratio for GAP-TDI system in THF and toluene.

Figure 5.42 Variation of equilibrium swell ratio of GAP-TDI system with different NCO/OH ratio in toluene and THF

146

Among all the systems evaluated, GAP crosslinked with IPDI was found to

have highest swell ratio. This was found to be due to the lower crosslink density of

the network. The variation in NCO/OH ratio was found to influence the swelling

characteristics more effectively than the variation in crosslinker content. The swell

ratio determined for GAP-HTPB blends with varying GAP content shows low values

even with 10% GAP content in the blend. At 50 and 70% GAP contents, the

equilibrium swell ratio were only close to 50% of crosslinked pure GAP.

Figure 5.43 shows swell ratio of crosslinked GAP-HTPB blends with different GAP

content in THF. The large difference in the equilibrium swell ratio of GAP in THF

could be explained based on the polarity criteria. GAP being a polar system, a more

polar solvent like THF is able to swell the polymer to a higher extent compared to

less polar solvent, toluene. Also moderately hydrogen bonded solvent like THF have

a higher solubility parameter (δ = 19.6 J1/2 cm-3/2) compared to poorly hydrogen

bonded solvent, toluene (δ = 18.2 J1/2 cm-3/2).36 Higher polarity and solubility

parameter associated with THF compared to toluene explains the difference observed

in swelling characteristics.

Figure 5.43 Variation of swell ratio of GAP-HTPB blends prepared with varying concentrations of GAP and HTPB

147

The sol fraction determined for the crosslinked polymer shows that the sol

content decrease with increasing NCO/OH ratio or increase in crosslink density. The

higher sol fraction seen at NCO/OH ratio below 1.2 could be due to the low crosslink

density, allowing solvent to easily swell the polymer chain and dissolve a higher

fraction. Figure 5.44 shows variation of sol content of GAP-TDI system with

NCO/OH ratio in toluene and THF.

Figure 5.44 Variation of sol fraction of crosslinked GAP with different NCO/OH ratios in THF and toluene

The reason for difference in sol fraction in THF and toluene is same as seen

for higher equilibrium ratio in THF. The equilibrium swell ratio was found to be

higher in THF than in toluene for crosslinked GAP. The difference in equilibrium

swell ratio was found to decrease for higher NCO/OH equivalent ratios. The swelling

studies proved that THF is a better solvent compared to toluene for crosslinked GAP

networks.

148

5.10 Evaluation of average molecular weight between crosslinks (Mc) by

solvent swelling and mechanical methods

Based on theory of elasticity by Flory37 relationships have been derived

between the deformation of elastic networks during swelling and length of the

polymer chain segments between crosslinks. This theory have been extended for

derivation of expression for determination of molecular weight between

crosslinks.5, 38 In this study, the average molecular weight between crosslinks (Mc)

was determined using data from swelling experiments and modulus of elasticity.

The equilibrium swell ratio as determined from the swelling experiments

as mentioned in section 5.14. was used for the evaluation. The equilibrium swell

ratio is related to the volume fraction of polymer in swollen gel (V2) and is given by

the equation, 5.2. The relation between V2 and the molecular weight between

crosslink is given by Flory- Rehner equation 39-41 as shown below.

where, V1 is the molar volume of the solvent, ρ is the density of the polymer

network, V2 is the volume fraction of polymer in the swollen gel as given in equation

5.2. χ is the Flory-Huggins polymer solvent interaction parameter. In this

experiment, the value of χ (0.25 at 450C) reported in the literature42 for GAP-THF

system was taken for the evaluation. GAP crosslinked with TDI and IPDI curatives

was subjected for the study. GAP-HTPB blends prepared with different GAP content

were also evaluated.

The Mc values of the crosslinked network were also determined by evaluation

of initial modulus determined from stress strain curve. The shear modulus is related

to the Mc by the following relationship.38, 43, 44

149

Where σ is the tensile stress, G is the shear modulus, λ is the extension ratio

in the elastic range. ρ is the density of the polymer, R is the universal gas constant

and T is the absolute temperature. The shear modulus and initial modulus (modulus

of elasticity or Young’s modulus) are related by the equation

where E is the initial modulus. By measuring the initial modulus from the

stress-strain curve, the Mc values of the polymer can be determined.

5.10.1 Experimental

Crosslinked GAP samples with different curatives were prepared as

mentioned in section 5.3.1. The equilibrium swell ratios and sol content were

determined for samples prepared with different formulations as mentioned in section

5.9. For the initial modulus measurement, sample slabs with different formulations

were prepared as mentioned in section 5.3.1. Stress-strain measurements were done

using Instron Universal Testing Machine as mentioned in section 2.4. A crosshead

speed of 50 mm/minute was employed for testing. GAP-HTPB blends prepared

with varying content of GAP were tested for the study.

5.10.1.1 Materials

Materials used for the study were same as mentioned in section 5.9.1.1.

150

5.10.1 2 Results and discussion

The Mc values were determined by swelling experiments for crosslinked

GAP prepared with different curatives, different NCO/OH ratios and crosslinker

content. It was noted that the Mc values of GAP-IPDI system was always higher than

the Mc values for GAP-TDI system for all NCO/OH ratios. The Mc values of

crosslinked GAP were found to decrease with increase in NCO/OH ratios,

irrespective of the curative used. Figure 5.45 shows variation of Mc with NCO/OH

ratio for GAP-TDI and GAP-IPDI systems.

Figure 5.45 Variation of average molecular weight between crosslinks (Mc) with NCO/OH ratio for GAP crosslinked with TDI and IPDI

The sol fraction was found to increase with increase in Mc values. The

variation of Mc values with sol fraction for different formulations are shown in

figure 5.46.

151

Figure 5. 46 Variation of average molecular weight between crosslinks with sol fraction for GAP crosslinked with TDI, using THF as solvent

Increase in crosslinker content was found to have a negative effect on Mc

values of the crosslinked network as in the case of NCO/OH ratio. Figure 5.47 show

the variation of Mc with crosslinker content.

Figure 5.47 Effect of variation of crosslinker content on the Mc of crosslinked GAP

The Mc values determined from measurement of initial modulus using

equations 5.6 and 5.7 were found to vary considerably from those determined from

152

swelling experiments. Table 5.2 shows comparison of Mc values determined by

swelling and Young modulus for GAP-TDI system.

Table 5.2 Comparison of Mc values determined by mechanical and swelling methods for GAP crosslinked with TDI

NCO/OH ratio

Mechanical method Swelling method

Initial modulus

(ksc)

Mc from initial modulus ( g mol-1)

Crosslink density

(mol m-3)

Mc from swelling (g mol-1)

Sol fraction (%)

0.9 5 19396.3 65.9 11654.6 49.5

1.0 8.9 10896.8 117.5 9760.3 45.5

1.1 10 9698.1 131.9 7668.4 39.3

1.2 12 8081.8 158.4 4668.7 11.9

1.5 25 3879.3 329.9 3675.1 7.9

1.75 48 2020.5 633.5 3366.6 7.8

The difference in the Mc values by the two methods could be due to the basic

difference in the methods of determination. The basic molecular dynamics of the

methods differ considerably. The visco-elastic process in stress-strain measurements

leading to dissipation of energy in the polymer network could affect results which

may be totally absent in the case of swelling experiments. Also the Mc values depend

on the time scale of the method.45 The higher Mc values seen in mechanical methods

could be a result of the network defects leading to fast disentaglement.46 For higher

values of NCO/OH ratios the difference between the Mc values were found to be

lower.

The Mc values determined from the swelling study with THF were correlated

with the gum stock properties of GAP prepared with same NCO/OH ratios. The

crosslinker content was maintained at 5% by weight with respect to binder in the

experiments. The correlative equations could be employed to predict the tensile

153

properties of the crosslinked network system when used as binder for propellant

applications. Tensile strength and elongation were correlated with Mc values. The

correlations were determined by selecting a best fit for the data. Different fits were

attempted using computational software, Microcal Origin Version 5.0 and the best fit

was selected. The relationship between tensile strength and Mc values were best

described by a exponential decay function as given by expression 5.8 and the fit was

found to be as shown in figure 5.48.

Where Mc is in g/mol and σ is the tensile strength in ksc.

Figure 5.48 Correlation between Mc and tensile strength with best fit

A second order polynomial was found to be the best fit for relationship

between Mc values and elongation as shown in figure 5.49. The mathematical

relationship between Mc values and percent elongation was given by expression 5.9.

154

Figure 5.49 Correlation between Mc and % elongation with best fit

Mc= 54314.7 0.7 ε 2_ 441. 2 ε _+ Where ε is the % elongation.

The variation of tensile strength and elongation with Mc as given by the

expressions 5.8 and 5.9 are found to be in agreement with reported literature on

polyurethane networks.14, 46, 47 The decrease in tensile strength and increase in

elongation with increasing Mc values and the plateau observed in the curve at higher

values of Mc agree well with experimental observations. The MC values were

determined from initial modulus for GAP crosslinked with different curatives. The

data show that Mc values are in the order GAP-IPDI > GAP-TDI for crosslinked

samples with NCO/OH ratio of unity and a crosslinker content of 5%. Table 5.3

shows comparison of Mc values for GAP-IPDI and GAP-TDI systems.

(5.9)

155

Table 5.3 Comparison of Mc for GAP crosslinked with IPDI and TDI

Curative Initial modulus (ksc)

Mc from initial modulus (g mol-1)

IPDI 3.5 27709

TDI 8.9 10897

GAP-HTPB blends investigated showed comparatively lower values of Mc

than for pure GAP. GAP-HTPB blends with 50:50 ratio showed Mc values as low as

1329 g/mol for a NCO/OH ratio of 1.2 with TDI as curator. Table 5.4 shows Mc

values determined for GAP-HTPB blends prepared with TDI as curing agent. The

low values of Mc noted for GAP-HTPB blends explain the high mechanical strength

seen for the blends at 50:50 or 30:70 ratios.

Table 5.4 Mc values for GAP-HTPB blends prepared with different formulations

GAP:HTPB ratio

NCO/OH ratio

Initial modulus (ksc)

Mc (g mol-1)

Crosslink density (mol m-3)

50:50 0.95 16.9 4925.7 223.3

50:50 1.0 20.0 4167.2 263.9

50:50 1.2 62.7 1328.6 827.9

90:10 1.0 6.6 14234.9 87.1

70:30 1.0 13.1 6766.9 172.9

10:90 1.0 27 2682.7 356.4

5.11 Conclusion

Based on the evaluation of gumstock properties of GAP, it was noted that the

functionality of the polymer, NCO/OH equivalent ratio, concentration of the

crosslinker, and curing conditions significantly influence the mechanical properties

156

of the crosslinked GAP networks. Due to the difunctional nature of GAP, crosslinker

concentration was found to have more dominant effect on GAP gumstock properties

compared to cure temperature and cure duration. The type of curing agent was also

found to affect the curing and gumstock properties. Aliphatic diisocyanate curing

agents were found to improve the mechanical properties of crosslinked GAP more

readily than aromatic type curing agents. The order of improvement in mechanical

strength for curative is found to be MDCI > IPDI > TDI.

Though GAP and HTPB are immiscible, crosslinked polymer network of

GAP and HTPB could be prepared without phase separation by enhancing the cure

reaction rate over diffusion rate. GAP-HTPB blends with varying GAP content from

0 to 90% could be prepared without total phase separation. GAP-HTPB blends of

50:50 to 30:70 ratios show superior properties over the virgin networks of GAP and

HTPB due to the synergetic effect of interlocking of the two networks. The

formation of IPN structure seems to explain the higher mechanical strength noticed

at specific concentration levels of GAP and HTPB in the blend. The gumstock

properties of GAP-HTPB blends with 50:50 ratio was investigated to find the impact

of different curing agents and NCO/OH ratio. The effect of these variables on the

gumstock properties of GAP-HTPB blends were found to follow the pattern similar

to that of crosslinked virgin GAP network.

The morphology of crosslinked GAP-HTPB networks were investigated with

optical micrography and scanning electron micrography. The studies show greater

degree of topological entanglements of both networks at specific concentration

levels. The local phase separation occurring in the blends with slow rate of curing

was also identified in the morphological studies.

The swelling studies carried out showed that swelling behaviour is strongly

influenced by the NCO/OH ratio. Among THF and toluene, THF was found to be a

157

better solvent for the GAP network. GAP crosslinked with IPDI showed higher swell

ratio compared to GAP crosslinked with TDI. The data showed that the swell ratio

decrease with increasing NCO/OH ratios and also with increase in crosslinker

content. The molecular weight between crosslinks was determined from swelling

data and initial modulus. The Mc values determined by both the methods were found

to be comparatively closer at higher NCO/OH ratios. For GAP-HTPB blends, the

swelling ratios and the Mc values were found to be lower than that for virgin GAP

network. An exponential correlation was found to describe the relationship between

tensile strength and Mc values determined by swelling whereas for elongation, a

second order polynomial was found to be best suited.

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