Robust propellant liner and interfacial propellant burn rate control

A robust polyurethane liner is disclosed for solid propellant rocket motors which is insensitive to large variations in stoichiometry, exhibits increased cohesive strength and, when used in combination with a unique two coat (wet coat) liner process described below, modifies the ballistic properties of the adjacent interfacial propellant layer. The liner compositions include a polymeric binder having a plurality of reactive hydroxyl functional groups and a curing agent having a plurality of reactive isocyanate functional groups. The quantity of binder and curing agent is selected such that the NCO/OH ratio is in the range from 1.5 to 4.5. Diisocyanate curing agents are particularly useful. A bonding agent, such as a polyfunctional aziridine compound, is typically present in the liner composition in an amount ranging from about 3% to about 5%, by weight. An oligomer catalyst is included in the liner composition in an amount ranging from about 15% to about 50%, by weight. Diammonium phosphate ((NH.sub.4).sub.2 HPO.sub.4) functions as an oligomer catalyst and as a ballistic modifier to inhibit propellant burn rate when the liner is cured concurrently with the propellant. A colorant, such as rhodamine B, may optionally be included to assist in determining coverage or thickness of the liner coating.

FIELD OF THE INVENTION 
The present invention relates to a robust polyurethane liner for solid 
propellant rocket motors which is insensitive to large variations in 
stoichiometry, exhibits increased cohesive strength and, when used in a 
wet coat process, is capable of modifying the ballistic properties of the 
adjacent interfacial propellant layer. The present invention also provides 
an optional labeling technique which provides a visual and/or ultraviolet 
fluorescent method for monitoring coverage and controlling thickness of 
the wet coat liner. 
BACKGROUND OF INVENTION 
Rocket motors employing solid propellants typically include a rigid outer 
casing or shell; a heat insulating layer (insulation) bonded to the inner 
surface of the casing; a liner layer (liner) bonded to the insulating 
layer; and a solid propellant grain bonded to the liner. The insulation is 
generally fabricated from a composition capable of withstanding the high 
temperature gases produced when the propellant grain burns, thus, 
protecting the case. The liner is an elastomeric composition which must 
bond the propellant grain to the insulation and to any uninsulated 
portions of the case, as well as inhibit interfacial burning. 
Polyurethane liners, used in a large number of rocket motors, are very 
sensitive to variations in stoichiometry, i.e., the isocyanate/hydroxyl 
equivalents ratio (NCO/OH). They are typically formulated at an NCO/OH 
ratio between 1.0 and 1.5. A slight excess of isocyanate (NCO) is required 
to compensate for curative diffusion into the insulation, the presence of 
moisture which is reactive with isocyanates, and other process variables 
which may reduce the isocyanate presence. The liner becomes very soft 
and/or does not cure at NCO/OH ratios of less than 1.0 or greater than 
1.5. Process variables such as moisture contamination, relative humidity, 
ingredient migration (diffusion), weighing errors, misformulation and 
other parameters which may directly or indirectly affect the NCO/OH ratio 
can greatly alter liner properties. NCO/OH ratio variations as small as 
0.10 from nominal can result in a soft, degraded propellant to liner to 
insulation bondline, bond failure and potential motor malfunction. Very 
rigorous process controls must therefore be imposed with polyurethane 
liners in order to maintain critical bondline integrity. 
A second function of the liner is to inhibit the burning surface of the 
propellant grain when the interface is exposed to the flame front. There 
are several basic propellant grain configurations. The two most commonly 
used configurations are the center perforated grain and the end burning 
grain. In the center perforated grain configuration, the flame front 
advances radially from the center perforation to the outer casing. The 
insulating layer and liner are not exposed to the flame front or hot gases 
until near the end of motor firing in this configuration. In the end 
burning grain configuration, the flame front advances axially from the 
nozzle end of the motor to the forward dome. The insulation and liner are 
directly exposed to the hot combustion gases as the flame front advances 
in this configuration. The insulation in the aft section of the motor has 
the longest exposure time and the insulation in the forward section has 
the least exposure time. 
End burning propellant grains are particularly sensitive to interfacial 
burn rate gradients since the flame front advancement is perpendicular to 
the interface. These interfacial burn rate gradients cause the propellant 
to burn at a different rate near the liner bondline. Interfacial 
ballistics are complex in nature but are believed to be a function of 
several factors including propellant ingredient diffusion, particle 
alignment and/or particle size stratification during propellant casting, 
and localized radiant/convective heat transfer conditions at the liner 
interface. 
Propellant ingredient diffusion into the liner changes the propellant 
composition and therefore the burn rate at the liner interface. Curative 
and plasticizer diffusion, for example, results in a higher solids and 
therefore a higher burn rate propellant at the interface. Conversely, burn 
rate catalyst diffusion results in a catalyst deficient and, therefore, a 
slower burning propellant at the interface. Particle size distribution 
changes at the interface and preferential alignment of solid particulate 
ingredients with a length to diameter ratio greater than one during 
propellant casting typically result in a propellant burn rate gradient 
near the liner interface. Localized heat transfer conditions can also 
affect propellant temperature and burn rate at the interface. 
It is undesirable to have large and uncontrolled propellant burn rate 
gradients in a rocket motor design. Burn rate gradients result in 
progressive changes in propellant burning surface area, chamber pressure, 
motor thrust and, in the case of end burning grains, unpredictable 
insulation exposure times. Motor design and performance are therefore 
somewhat unpredictable and exhibit a high degree of variability. In 
extreme cases, severe burn rate gradients have resulted in motor 
malfunction. 
From the foregoing, it would be a significant advancement in the art to 
provide a solid rocket motor propellant liner which is insensitive to 
large variations in stoichiometry, which is insensitive to adverse process 
conditions including high relative humidity, residual moisture in liner 
ingredients and residual moisture in the insulation substrate, and which 
is able to modify and/or control the ballistic properties of the adjacent 
interfacial propellant layer. It would also be a significant advancement 
in the art to provide an in situ labeling technique for liner application 
monitoring and control. 
Such solid rocket motor propellant liner compositions are disclosed and 
claimed herein. 
SUMMARY OF THE INVENTION 
The present invention is directed to a robust polyurethane or 
polythiourethane liner for solid propellant rocket motors which is 
insensitive to large variations in stoichiometry, exhibits increased 
cohesive strength and, when used in combination with a unique two coat 
liner process described below, has the ability to modify the ballistic 
properties of the adjacent interfacial propellant layer. 
The currently preferred liner compositions within the scope of the present 
invention include a polymeric binder having a plurality of reactive 
hydroxyl functional groups and a curing agent having a plurality of 
reactive isocyanate functional groups. The quantity of binder and curing 
agent is selected such that the NCO/OH ratio is in the range from 1.5 to 
4.5, preferably from about 2 to 4, and most preferably from about 2 to 3. 
Those skilled in the art will appreciate that the binder may include a 
plurality of reactive thiol functional groups to perform the same function 
as hydroxyl groups. Thus, the NCO/OH ratios discussed herein also apply to 
NCO/SH ratios. The combination of binder and curing agent usually 
represents about 35% to 60%, by weight, of the liner composition. Cure 
catalysts are often included to facilitate curing of the binder and curing 
agent. 
The curing agent is a polyfunctional isocyanate, preferably a diisocyanate 
curing agent. Currently preferred diisocyanate curing agents include 
m-tetramethyl xylene diisocyanate (TMXDI), isophorone diisocyanate (IPDI), 
dimeryl diisocyanate (DDI), and toluene 2,4-diisocyanate (TDI). 
A bonding agent is preferably provided to help strengthen the polymeric 
matrix which strengthens the propellant to liner interface. The bonding 
agent is preferably present in the liner composition in an amount ranging 
from about 3% to about 5%, by weight. Currently preferred bonding agents 
are polyfunctional aziridine compounds. 
An oligomer catalyst is preferably included in the liner composition in an 
amount ranging from about 5% to about 50%, by weight. An oligomer catalyst 
which has provided very good results is diammonium phosphate 
((NH.sub.4).sub.2 HPO.sub.4). Diammonium phosphate also functions as a 
ballistic modifier to inhibit propellant burn rate. Other known ballistic 
modifiers may also be included. 
An inert filler, such as titanium dioxide (TiO.sub.2), carbon black, or 
silica oxide, is preferably present in the liner composition in an amount 
ranging from about 5% to about 40%, and more preferably from about 10% to 
about 30%, by weight. A colorant, such as rhodamine B, may optionally be 
included to assist in determining coverage or thickness of the liner 
coating. 
The robust, stoichiometry insensitive liner composition containing a 
ballistic modifier, when used in a unique two coat or wet coat process, 
provides interfacial propellant burn rate control. The first coat of liner 
is applied to the interior surface of the rocket motor chamber covering 
all propellant contacting surfaces. The first coat of liner is partially 
or fully cured prior to application of a subsequent coat. The second coat 
(approximately 5 mils thick) of liner, or "wet coat," is applied over the 
first coat but not cured prior to propellant casting. The second coat of 
liner is preferably formulated to contain the appropriate ballistic 
modifier(s) in place of all or a portion of the total filler content. It 
will be appreciated at least one wet coat is applied over any number of 
cured coats to produce the desired results. The propellant is cast into 
the motor chamber under vacuum to promote intimate contact and intermixing 
of the wet coat liner with the adjacent propellant layer. This process 
results in dispersement of the wet coat liner and contained ballistic 
modifier into the adjacent propellant layer and a corresponding 
modification of interfacial propellant burn rate.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to a urethane or thiourethane liner 
composition containing an oligomer catalyst to form a robust, 
stoichiometry insensitive liner and a ballistic modifier which, when used 
in a unique two coat process, will also provide interfacial propellant 
burn rate control. A colorant added to the second coat will provide visual 
and/or UV fluorescence coverage and thickness detection methods. Currently 
preferred robust liner formulations are summarized below. 
ROBUST LINER FORMULATION 
______________________________________ 
Composition (parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
hydroxy terminated polymer binder 
35-60* 45-60* 
isocyanate curing agent 
bonding agent present 3-5 3-5 
reactive filler (oligomer catalyst/ 
5-40 20-50 
burn rate suppressant) 
inert filler 10-30 -- 
thixotrope 2-3 -- 
cure catalyst 0-0.5 0-0.5 
colorant -- 0.001-0.01 
______________________________________ 
*NCO/OH ratio = 1.5-4.5 
The robust liner composition described above comprises the reaction product 
of a prepolymer binder which is terminated with hydroxyl or other 
functional groups which react with a isocyanate, a diisocyanate, 
polyisocyanate or oligomer of an isocyanate curing agent which will react 
with the binder to form a urethane or analogous linkage, a bonding agent, 
such as a conventional aziridine bond promoter, a reactive filler which 
may function as an oligomer catalyst and/or a propellant burn rate 
modifier, an inert filler, a thixotropic agent, a cure catalyst, and a 
colorant. The formulation shown above is a presently preferred for 
baseline formulation. Many of the ingredient substitutions and 
alternatives obvious to one skilled in the art are available, several of 
which have been successfully demonstrated. The nature and function of each 
major ingredient, alternates and substitutions are discussed in the 
following paragraphs. 
Binder. The prepolymer binder used herein include organic compounds having 
at least two active hydrogen providing moieties, preferably hydroxyl or 
thiol moieties, capable of reacting with a polyisocyanate to form urethane 
or thiourethane linkages. The presently preferred prepolymers are 
compounds having the formula: 
EQU HO--R--OH 
EQU HS--R--SH 
where "R" is a divalent organic radical. The hydroxyl or thiol groups may 
be of any type suitable for forming urethane or thiourethane linkages with 
isocyanate groups. Hydroxy terminated polybutadiene (HTPB) polymers are 
currently preferred binders. 
Isocyanate Curing Agent. The isocyanates useful as curing agents herein 
include di-, tri-, and polyfunctional organic isocyanates. Diisocyanates 
are currently preferred. Both alkylene and arylene isocyanates are 
suitable. Currently preferred isocyanates include m-tetramethyl xylene 
diisocyanate (TMXDI), isophorone diisocyanate (IPDI), dimeryl diisocyanate 
(DDI), and toluene 2,4-diisocyanate (TDI). 
Bonding Agent. Bonding agents or bond promoters are typically mobile, 
reactive ingredients which diffuse from the liner into the interfacial 
propellant and react with the ammonium perchlorate oxidizer, propellant 
binder, or other propellant ingredients to enhance bondline properties. 
Conventional, state of the art, bond promoters include di- and 
tri-functional aziridine (i.e., cyclic ethylene imines) compounds. One 
well known bonding agent, 1,1-1,3-phenylene dicarbonyl 
bis(2-methylaziridine!, known in the industry as HX-752 (manufactured by 
3M), is a currently preferred bonding agent. 
Reactive Filler(s). A reactive filler or mixture of reactive fillers are 
included to function as an oligomer catalyst and/or a propellant burn rate 
modifier, described below. 
Oligomer Catalyst. The oligomer catalyst causes excess isocyanates present 
in the liner to form dimer, trimer, and tetramer structures of the 
isocyanate as shown below: 
##STR1## 
The presently preferred catalyst evaluated in this invention is diammonium 
phosphate, (NH.sub.4).sub.2 HPO.sub.4. Other known catalysts include, but 
are not limited to, calcium acetate, sodium formate, sodium carbonate, 
sodium methoxide, tertiary amines such as triethylamine, oxalic acid, 
sodium benzoate in dimethyl formamide, soluble metal compounds of iron, 
sodium, potassium, magnesium, mercury, nickel, copper, zinc, aluminum, 
tin, vanadium, and chromium, titanium tetra butyrate, oxygen and 
Friedel-Crafts type reagents. 
Propellant Burn Rate Modifier. A propellant burn rate modifier causes a 
decrease or an increase in propellant burn rate to compensate for 
interfacial burn rate changes. The currently preferred burn rate modifier, 
which also functions as a catalyst, is diammonium phosphate, 
(NH.sub.4).sub.2 HPO.sub.4. Diammonium phosphate is intended for use in 
bondlines where curative migration, plasticizer migration, oxidizer 
particle alignment, oxidizer particle stratification or other variables 
cause significant increases in interfacial propellant burn rates. Other 
burn rate suppressants which may be used in the practice of the present 
invention include, but are not limited to, dicyanoguanidine, NH.sub.2 
COCONH.sub.2 (oxamide), Dechlorane plus (a chlorinated hydrocarbon made by 
Occidental Chemical), NH.sub.4 F, NH.sub.4 Cl, NH.sub.4 Br, NH.sub.4 I, 
NH.sub.4 PF.sub.6, (NH.sub.4).sub.2 SiF.sub.6, NH.sub.4 BF.sub.4, 
ZnF.sub.2, ZnBr.sub.2, n-bromosuccinimide, hexabromocyclododecane, 
pentabromodiphenyl oxide, decabromodiphenyl oxide, Firemaster 836, 
tetrabromophthalate diol, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, triphenyl 
antimony, diammonium bitetrazole, 5-aminotetrazole, aluminum hydroxide, 
calcium oxalate, ammonium sulfate, ammonium oxalate, ammonium 
polyphosphate, and other flame retardants, ballistic modifiers and other 
materials obvious to one skilled in the art. Burn rate catalysts, for use 
in bondlines where the interfacial propellant burn rate has been 
significantly decreased, include ferric oxide, ferrocene, n-butyl 
ferrocene, catocene, aluminum oxide, copper chromite, and other catalysts 
known in the art. Selection of the optimum burn rate modifier must be 
tailored to the specific propellant and interfacial conditions. 
Other Additives. Other liner ingredients may include but are not limited to 
inert fillers, thixotropic agents, cure catalysts, colorants, 
plasticizers, coupling agents, reinforcing agents and other specialty 
additives. 
Two Coat Liner Process. 
The robust liner may be used in a conventional single or multicoat process 
in which the liner is cured or partially cured prior to application of the 
subsequent coat or prior to propellant casting. However, a unique two coat 
process is required for interfacial propellant burn rate control. The 
first coat of liner is applied to the interior surface of the rocket motor 
chamber covering all propellant contacting surfaces. The first coat may be 
applied using a conventional application process, such as by spray, 
centrifugal disc, brush, slush, or screed. A centrifugal Disc is a high 
speed rotating disc which atomizes and expels liner by centrifugal force. 
Slush means to pour an excess of liner into the case allow excess to drain 
prior to cure. The liner thickness is controlled by viscosity. Screed uses 
a template or guide to contour and control thickness. 
The first coat of liner is partially or fully cured prior to application of 
a subsequent coat. The second coat (approximately 5 mils thick) of liner, 
or "wet coat," is applied over the first coat but not cured prior to 
propellant casting. The second coat of liner is preferably formulated to 
contain the appropriate ballistic modifier(s) in place of all or a portion 
of the total filler content. A two coat system is preferred although a 
minimum of one wet coat over any number of cured coats will produce the 
desired results. The propellant is cast into the motor chamber under 
vacuum to promote intimate contact and intermixing of the wet coat liner 
with the adjacent propellant layer. 
This process results in dispersement of the wet coat liner and contained 
ballistic modifier into the adjacent propellant layer and a corresponding 
modification of interfacial propellant burn rate. The interfacial 
propellant burn rate may be decreased by incorporating burn rate 
suppressants into the wet coat liner formulation or increased by 
incorporating burn rate catalysts. The degree of burn rate tailoring may 
be controlled by the type of modifier selected, modifier content in the 
liner, liner thickness, liner viscosity and other process variables 
affecting the degree of dispersement in the adjacent propellant layer. 
The robust liner and liner process described herein provide several 
important features. For instance, it has been found that diammonium 
phosphate functions both as an oligomer catalyst and as a burn rate 
suppressant. This catalyst causes excess isocyanate curative to 
oligomerize into dimer, trimer and tetramer structures. The isocyanate 
groups on the oligomers also react with the hydroxyl groups on the polymer 
forming the conventional urethane linkages. Variations in the NCO/OH ratio 
affect the degree of oligomerization but have little effect on propellant 
to liner bond properties. The result is a stoichiometric insensitive 
formulation. Oligomer structures of some isocyanates also enhance liner 
mechanical properties. When diammonium phosphate is used in a unique two 
coat (cured coat and wet coat) liner process, it has the capability to 
alter the interfacial propellant burn rate and function as a burn rate 
suppressant. 
A two coat liner process is used to modify and control the interfacial 
propellant burn rate. The first coat of robust liner is applied to the 
interior of the case and cured or partially cured. A second, wet (uncured) 
liner coat is applied over the first cured coat. The uncured liner in the 
second coat, containing the burn rate modifier additive, intermixes with 
the adjacent propellant layer during vacuum casting. This intermixing 
provides physical transport and dispersion of the burn rate modifier into 
the adjacent propellant layer and affords a method to tailor and control 
the interfacial propellant burn rate. 
A colorant is preferably used to label the second liner coat and affords a 
visual and/or a ultraviolet ("UV") fluorescence detection method for 
verification of complete coverage and thickness control. 
The following examples are offered to further illustrate the present 
invention. These examples are intended to be purely exemplary and should 
not be viewed as a limitation on any claimed embodiment. 
EXAMPLE 1 
Robust Liner 
A robust liner was formulated having the ingredient composition set forth 
below in Table 1A, with NCO/OH ratios ranging from 1.5 to 4.5: 
TABLE 1A 
______________________________________ 
Ingredient Weight % 
______________________________________ 
HTPB 
58.7 
DDI 
HX-752 4.9 
(NH.sub.4).sub.2 HPO.sub.4 
19.6 
TiO.sub.2 14.6 
SiO.sub.2 2.0 
Triphenyl bismuth 0.2 
______________________________________ 
The liner was cast into sheets for uniaxial tensile specimens, used to bond 
steel tensile adhesion buttons and used to bond a 88 percent solids 
HTPB/Al/AP propellant and a 90 percent solids HTPB/Al/AP/HMX propellant to 
an insulator based on an EPDM (ethylenepropylenediene monomer) polymer 
system. The uniaxial tensile specimen sheets and tensile adhesion buttons 
were cured for 336 hours at 135.degree. F. prior to test. The liner for 
the propellant to liner to insulation (PLI) samples was cured for 48 hours 
at 135.degree. F. prior to propellant cast and then co-cured with the 
propellant for an additional 288 hours at 135.degree. F. prior to test. 
The liner uniaxial tensile properties are presented in Table 1B, below. 
The robust liner exhibited uniaxial tensile values of 900 to 1400 psi with 
elongations of 430 to 610 percent. 
TABLE 1B 
______________________________________ 
Liner Uniaxial Mechanical Properties 
Corrected Tensile 
Elongation 
NCO/OH Stress (psi) 
(%) 
______________________________________ 
1.5 1090 430 
2.0 1170 450 
2.25 1160 530 
2.5 1290 460 
3.0 1400 480 
4.5 900 610 
______________________________________ 
Liner to steel adhesion data are presented in Table 1C, below. Tensile 
adhesion to steel ranged from 430 to 550 psi. 
TABLE 1C 
______________________________________ 
Liner-to-Steel Adhesion 
NCO/OH Tensile (psi) 
______________________________________ 
1.5 430 
2.0 480 
2.5 530 
3.0 550 
______________________________________ 
The PLI bond data are presented in Table 1D, below. The tensile strength of 
the PLI bond was 178 to 184 psi for the 88 percent solids HTPB/Al/AP 
propellant and 221 to 227 psi for the 90 percent solids HTPB/Al/AP/HMX 
propellant with cohesive propellant failure modes. Ninety degree peel 
values ranged from 21 to 33 and 12 to 18 pli, respectively, also with 
cohesive propellant failure modes. 
TABLE 1D 
______________________________________ 
Propellant/Liner/Insulation Bond Data 
Tensile 
Stress 
90.degree. Peel 
Propellant Type 
Insulation 
NCO/OH (psi) (pli) 
______________________________________ 
HTPB/Al/AP EPDM 1.5 178 33 
(88% solids) 2.25 176 21 
3.0 184 29 
HTPB/Al/AP/HMX 
EPDM 1.5 221 12 
(90% solids) 2.0 226 18 
2.5 226 16 
3.0 227 18 
______________________________________ 
The foregoing data demonstrate robustness and process insensitivity over a 
wide range of stoichiometry over which conventional liners fail to cure. 
EXAMPLE 2 
Robust Liner With TMXDI Substitution 
A robust liner was formulated having the ingredient composition set forth 
in Table 1A, except that m-tetramethyl xylene diisocyanate was used as the 
curing agent. The NCO/OH ratios formulated were 1.50, 2.25 and 3.00. The 
liner was cast into sheets for uniaxial tensile specimens and used to bond 
a 88 percent solids HTPB/Al/AP propellant to an EPDM-based insulator. The 
uniaxial tensile specimen sheets were cured for 336 hours at 135.degree. 
F. prior to test. The liner for the propellant to liner to insulation 
(PLI) samples was cured for 48 hours at 135.degree. F. prior to propellant 
cast and then co-cured with the propellant for an additional 288 hours at 
135.degree. F. prior to test. The liner uniaxial tensile properties are 
presented in Table 2A. The robust liner exhibited uniaxial tensile values 
of 1760 to 2630 psi with maximum enhanced uniaxial tensile properties at a 
NCO/OH ratio of 2.25. Elongation ranged from 360 to 480 percent. 
TABLE 2A 
______________________________________ 
Liner Uniaxial Mechanical Properties 
Corrected Tensile 
Elongation 
NCO/OH Stress (psi) 
(%) 
______________________________________ 
1.5 1760 480 
2.25 2630 410 
3.0 1820 360 
______________________________________ 
The PLI bond data are presented in Table 2B. The tensile strength of the 
PLI bond was 184 to 188 psi with a cohesive propellant failure mode. 
Ninety degree peel values ranged from 24 to 36 pli also with cohesive 
propellant failure modes. 
TABLE 2B 
______________________________________ 
Propellant/Liner/Insulation Bond Data 
Tensile 
Stress 
90.degree. Peel 
Propellant Type 
Insulation 
NCO/OH (psi) (pli) 
______________________________________ 
HTPB/Al/AP EPDM 1.5 184 36 
(88% solids) 2.25 188 28 
3.0 186 24 
______________________________________ 
The foregoing data demonstrate liner robustness and process insensitivity 
over a wide range of stoichiometry and enhanced mechanical properties with 
an alternate isocyanate curative. 
EXAMPLE 3 
Robust Liner with IPDI Substitution 
A robust liner was formulated having the ingredient composition set forth 
in Table 1A, except that isophorone diisocyanate (IPDI) was used as the 
curing agent. The NCO/OH with NCO/OH ratios of 1.50, 2.25 and 3.00. The 
liner was cast into sheets for uniaxial tensile specimens. The uniaxial 
tensile specimen sheets were cured for 336 hours at 135.degree. F. prior 
to test. The liner uniaxial tensile properties are presented in Table 3A. 
The robust liner exhibited uniaxial tensile values of 2450 to 3710 psi 
with elongations of 380 to 590 percent. 
TABLE 3A 
______________________________________ 
Liner Uniaxial Mechanical Properties 
Corrected Tensile 
Elongation 
NCO/OH Stress (Psi) 
(%) 
______________________________________ 
1.5 2450 590 
2.25 3710 510 
3.0 3570 380 
______________________________________ 
These data also demonstrate enhanced mechanical properties, robustness and 
process insensitivity over a wide range of stoichiometry. They also 
demonstrate the ability to substitute isocyanate curatives in the robust 
liner formulation. 
EXAMPLE 4 
Robust Liner with Inert Filler Substitution 
A robust liner was formulated having the ingredient composition set forth 
in Table 1A and a NCO/OH ratio of 2.25, except that two alternate fillers, 
carbon black and silicon dioxide, were substituted for the titanium 
dioxide. The liner was cast into sheets for uniaxial tensile properties 
and cured for 336 hours at 135.degree. F. prior to test. Tensile 
properties, presented in Table 4A, indicate nominal liner properties and 
demonstrate the ability to substitute inert fillers in the robust liner 
formulation. 
TABLE 4A 
______________________________________ 
Liner Uniaxial Mechanical Properties 
Corrected Tensile 
Elongation 
Inert Filler 
NCO/OH Stress (psi) 
(%) 
______________________________________ 
Carbon Black 
2.25 1490 450 
SiO.sub.2 2.25 1830 570 
______________________________________ 
EXAMPLE 5 
Robust Liner with Cure Catalyst Substitution 
A robust liner was formulated in accordance with Table 1A with NCO/OH 
ratios of 1.5, 3.0 and 4.5. Dibutyl tin dilaurate (DBTDL) was substituted 
for the triphenyl bismuth cure catalyst. The liner was cast into sheets 
for uniaxial tensile specimens and used to bond a 90 percent solids 
HTPB/Al/AP/HMX propellant to an EPDM-based insulator. The uniaxial tensile 
specimen sheets were cured for 336 hours at 135.degree. F. prior to test. 
The liner for the propellant to liner to insulation (PLI) samples was 
cured for 48 hours at 135.degree. F. prior to propellant cast and then 
co-cured with the propellant for an additional 288 hours at 135.degree. F. 
prior to test. The liner uniaxial tensile properties are presented in 
Table 5A. The robust liner exhibited uniaxial tensile values of 890 to 
1430 psi with elongations ranging from 470 to 650 percent. 
TABLE 5A 
______________________________________ 
Liner Uniaxial Mechanical Properties 
Corrected Tensile 
Elongation 
NCO/OH Stress (psi) 
(%) 
______________________________________ 
1.5 1200 470 
3.0 1430 600 
4.5 890 650 
______________________________________ 
The PLI bond data are presented in Table 5B. The tensile strength of the 
PLI bond was 208 to 213 psi with a cohesive propellant failure mode. 
Ninety degree peel values ranged from 11 to 16 pli also with cohesive 
propellant failure modes. 
TABLE 5B 
______________________________________ 
Propellant/Liner/Insulation Bond Data 
Tensile 
Stress 90.degree. Peel 
Propellant Type 
Insulation 
NCO/OH (psi) (pli) 
______________________________________ 
HTPB/Al/AP/HMX 
EPDM 1.5 208 11 
(90% solids) 3.0 208 16 
4.5 213 13 
______________________________________ 
These data demonstrate liner robustness and process insensitivity over a 
wide range of stoichiometry with an alternate cure catalyst. 
Some of the foregoing results of Examples 1 through 5 are graphically 
illustrated in FIGS. 1 through 3. FIG. 1 shows liner uniaxial tensile 
strength as a function of liner isocyanate/hydroxyl equivalents ratio 
(NCO/OH) for three liner compositions. The illustrated liner compositions 
are based on DDI (Examples 1 and 5), TMXDI (Example 2), and IPDI (Example 
3) diisocyanate curing agents catalyzed with either triphenyl bismuth 
(TPB) or dibutyl tin dilaurate (DBTDL) cure catalysts. The results 
illustrated in FIG. 1 suggest that good liner uniaxial tensile strength 
may be obtained with the DDI-cured liner over an extended NCO/OH range 
from 1.5 to 4.5. Enhanced uniaxial properties, i.e., significantly 
increased uniaxial tensile strength, are obtained with TMXDI-cured and 
IPDI-cured liners over an extended NCO/OH range of 1.5 to 3.0. 
FIG. 2 shows 90-degree peel strength of a propellant-to-liner-to-insulation 
bondline as a function of liner NCO/OH ratio for two propellants (88 
percent solids and 90 percent solids) and two liner compositions within 
the scope of the present invention. The illustrated liner compositions are 
based on DDI (Examples 1 and 5) and TMXDI (Example 2) diisocyanate curing 
agents catalyzed with either triphenyl bismuth (TPB) or dibutyl tin 
dilaurate (DBTDL) cure catalysts. The results illustrated in FIG. 2 
suggest excellent propellant-to-liner-insulation bond integrity as 
measured by 90-degree peel strength over an extended NCO/OH range of 1.5 
to 4.5 with DDI-cured liners and from 1.5 to 3.0 with TMXDI-cured liners. 
Failure modes were cohesive in the propellant, indicating that the 
interfacial strength exceeds the strength of the propellant. 
FIG. 3 shows tensile strength of a propellant-to-liner-to-insulation 
bondline as a function of the liner NCO/OH ratio for two propellants (88 
percent solids and 90 percent solids) and two liner compositions within 
the scope of the present invention. The illustrated liner compositions are 
based on DDI (Examples 1 and 5) and TMXDI (Example 2) diisocyanate curing 
agents catalyzed with either triphenyl bismuth (TPB) or dibutyl tin 
dilaurate (DBTDL) cure catalysts. The results illustrated in FIG. 3 
suggest excellent propellant-to-liner-insulation bond integrity as 
measured by tensile strength over an extended NCO/OH range of 1.5 to 4.5 
with DDI-cured liner and from 1.5 to 3.0 with TMXDI-cured liners. Failure 
modes were cohesive in the propellant, indicating that the interfacial 
strength exceeds the strength of the propellant. 
These figures graphically demonstrate the robustness of the disclosed liner 
compositions and propellant-to-liner-to-insulation bondlines over an 
extended range of stoichiometry over which conventional liners fail to 
cure, and the enhanced mechanical properties attainable with certain 
curatives. 
EXAMPLE 6 
Robust Liner with Reactive Filler Concentration Variation 
A robust liner was formulated having the ingredient composition set forth 
below in Table 6A, and with a NCO/OH ratio of 2.25. The diammonium 
phosphate reactive filler and carbon black inert filler concentrations 
were inversely varied in 5 part increments from 5 to 20 parts and from 30 
to 15 parts, respectively, as shown. 
TABLE 6A 
______________________________________ 
Composition 
Ingredient (Parts) 
______________________________________ 
HTPB 
58.7 
IPDI 
HX-752 5.0 
(NH.sub.4).sub.2 HPO.sub.4 
5-20 
Carbon black 30-15 
SiO.sub.2 2.5 
DBTDL 0.003 
______________________________________ 
The liner was cast into sheets for uniaxial tensile specimens and used to 
bond a 88 percent solids HTPB/Al/AP propellant to an EPDM rubber 
insulation. The uniaxial tensile specimen sheets were cured for 336 hours 
at 135.degree. F. prior to test. The liner for the propellant to liner to 
insulation (PLI) samples was cured for 48 hours at 135.degree. F. prior to 
propellant cast and then co-cured with the propellant for an additional 
288 hours at 135.degree. F. prior to test. 
The liner uniaxial tensile properties are presented in Table 6B, below. The 
robust liner exhibited uniaxial tensile values of 2730 to 3280 psi with 
elongation of 400 to 470 percent. 
TABLE 6B 
______________________________________ 
Liner Uniaxial Tensile Properties 
Corrected Tensile 
Elongation 
(NH.sub.4).sub.2 HPO.sub.4 (pph) 
Stress (psi) 
(%) 
______________________________________ 
5 3280 470 
10 3180 420 
15 3180 260 
20 2730 400 
______________________________________ 
The PLI bond data are presented in Table 6C. The tensile strength of the 
PLI bond ranged from 193 to 207 psi with cohesive propellant failure 
modes. Ninety degree peel values ranged from 15 to 20 pli, also with 
cohesive propellant failure modes. 
TABLE 6C 
______________________________________ 
(NH.sub.4).sub.2 HPO.sub.4 (pph) 
Tensile Stress (psi) 
90.degree. Peel (pli) 
______________________________________ 
5 193 20 
10 218 20 
15 207 18 
20 207 15 
______________________________________ 
The foregoing data demonstrate a wide range of acceptable reactive filler 
contents over which oligomerization occurs and the liner functions as 
intended. 
EXAMPLE 7 
Process Insensitivity of Robust Liner 
A robust liner was formulated having the ingredient composition set forth 
in Table 6A, with a NCO/OH ratio of 2.25 and a diammonium phosphate (DAP) 
content of 10 pph. A conventional liner was formulated having the 
ingredient composition set forth in Table 12B, below, and a NCO/OH ratio 
of 1.5. Liner and liner bondline samples were prepared under adverse 
process conditions of high relative humidity and residual moisture in the 
inert liner filler ingredient and insulation substrate. The carbon black 
and the EPDM insulation were conditioned six days at a relative humidity 
of 90 percent to simulate adverse process conditions. Control samples of 
carbon black and insulation were vacuum dried prior to use for comparative 
purposes. The liner was used to bond tensile specimens of EPDM/liner/EPDM 
and to bond a 90 percent solids HTPB/Al/AP/HMX propellant to EPDM 
insulation. Samples were prepared to evaluate four combinations of adverse 
and dry process conditions, as shown below: 
______________________________________ 
Sample # Carbon Black EPDM 
______________________________________ 
A Vacuum dried Vacuum dried 
B Vacuum dried 6 days at 90% RH 
C 6 days at 90% RH 
Vacuum dried 
D 6 days at 90% RH 
6 days at 90% RH 
______________________________________ 
The tensile adhesion samples were cured for 336 hours at 135.degree. F. 
prior to test. The liner for the PLI samples was cured for 48 hours at 
135.degree. F. prior to propellant cast then co-cured with the propellant 
for an additional 288 hours at 135.degree. F. 
The tensile properties of the EPDM/liner/EPDM samples are presented in 
Table 7A. The conventional liner exhibited high variability and 
sensitivity to the adverse process conditions. The tensile strength of the 
conventional liner samples ranged from 90 to 480 psi. Cohesive insulation 
failure modes were obtained with dry insulation samples. However, cohesive 
liner failure modes were obtained with wet insulation samples indicative 
of degraded liner. In contrast, the robust liner demonstrated process 
insensitivity. The tensile strength of the robust liner samples was 
consistent ranging from 430 to 520 psi with cohesive insulation failure 
modes under all conditions. Some liner void failures were also observed. 
These voids were presumably caused by carbon dioxide generated from the 
isocyanate-residual moisture reaction. 
TABLE 7A 
______________________________________ 
Tensile strength of EPDM/Liner/EPDM 
Tensile 
Liner Type 
Sample # Stress (psi) 
Failure Mode 
______________________________________ 
Conventional 
A 480 CI 
B 90 CL 
C 480 CI 
D 120 CL 
Robust A 520 CI 
B 430 CI* 
C 510 CI 
D 450 CI* 
______________________________________ 
CI = cohesive insulation 
CL = cohesive liner 
*Some failure through voids in liner. 
Propellant/liner/insulation bondline samples test results are presented in 
Table 7B. The conventional liner hardness, as determined by penetrometer 
testing, was 26 penetrometer units under dry conditions, but softened to 
30 to 35 units with the addition of moisture to the carbon black and/or 
EPDM. The robust liner maintained a hardness of 22 to 23 penetrometer 
units under all conditions. The ninety-degree peel value obtained from 
conventional liner PLI samples varied form 17 to 27 pli with interfacial 
propellant to liner and cohesive liner failure modes which are indicative 
of a degraded bond. Robust liner PLI samples ranged from 32 to 37 pli with 
cohesive propellant failure modes under all conditions. The tensile 
strength of the PLI samples ranged from 201 to 247 psi and cohesive 
propellant failure modes for both liner samples when tested at 75.degree. 
F. Samples tested at 135.degree. F. failed predominately in the EPDM and 
did not discriminate bondline variations. However, the conventional liner 
samples did fail cohesively in the liner with combinations of wet carbon 
black and wet EPDM indicating liner degradation. 
TABLE 7B 
______________________________________ 
Propellant/Liner/Insulation Bond Strength 
Tensile at 
Tensile at 
Liner Hardness 90.degree. Peel 
75.degree. F. 
135.degree. F. 
type/ (penetro- 
Stress Failure 
Stress 
Failure 
Stress 
Failure 
Sample # 
meter) (pli) mode (psi) 
mode (psi) 
mode 
______________________________________ 
Conv./A 
26 17 IPL 239 CP 120 CP 
Conv./B 
34 27 IPL 207 CP 85 CI 
Conv./C 
30 21 IPL 234 CP 106 CI 
Conv./D 
35 27 CL 201 SBF 65 CL 
Robust/A 
22 32 CP 231 CP 70 CI 
Robust/B 
22 37 CP 240 CP 64 CI 
Robust/C 
22 32 CP 247 CP 80 CI 
Robust/D 
22 35 CP 239 CP 77 CI 
______________________________________ 
IPL= interfacial propellant/liner 
CI = cohesive insulation 
CL = cohesive liner 
CP = cohesive propellant 
SBF = secondary bond failure 
These data demonstrate the process insensitivity of the robust liner and 
it's ability to function under adverse process conditions of high relative 
humidity and residual moisture in liner ingredients and in the insulation 
substrate. Conventional urethane liners are degraded under similar process 
conditions. 
EXAMPLE 8 
Bond Durability of Robust Liner 
A robust liner was formulated in accordance with Table 1A with NCO/OH 
ratios of 1.50, 2.25 and 3.00. TMXDI and DDI were used as curative. The 
liner was cast into sheets for uniaxial tensile specimens and used to bond 
a 88 percent solids HTPB/ Al/AP propellant to an EPDM-based insulator. The 
uniaxial tensile specimen sheets were cured for 336 hours at 135.degree. 
F. The liner for the propellant to liner to insulation (PLI) samples were 
cured for 48 hours at 135.degree. F. prior to propellant cast and then 
co-cured with the propellant for an additional 288 hours at 135.degree. F. 
The specimens were then aged for 0, 2 and 6 months at 145.degree. F. and 6 
months at 75.degree. F. prior to test. The liner uniaxial tensile 
properties and the PLI bond data are presented in Table 8A. The robust 
liner exhibited uniaxial tensile values of 950 to 1870 psi with the DDI 
cured formulation and 1040 to 2910 psi with the TMXDI cured formulation. 
The tensile strength of the PLI bond was 176 to 208 psi for the 88 percent 
solids HTPB/Al/AP propellant with cohesive propellant failure modes. 
Ninety degree peel values ranged from 21 to 41 pli also with cohesive 
propellant failure modes. 
TABLE 8A 
______________________________________ 
Liner and PLI Bondline Data 
Liner 
Uni- 
Aging Conditions 
axial Prop./liner/insul. 
Cross- Time Temp. Tensile 
Tensile 
90.degree. Peel 
linker 
NCO/OH (months) (.degree.F.) 
(psi) (psi) (pli) 
______________________________________ 
DDI 1.5 0 N/A 1190 178 32 
2 145 950 186 36 
6 145 1150 202 39 
6 75 1480 191 35 
DDI 2.25 0 N/A 1160 176 21 
2 145 1370 183 27 
6 145 1870 203 28 
6 75 1410 191 27 
DDI 3.00 0 N/A -- 184 29 
2 145 -- 181 32 
6 145 -- 203 27 
6 75 -- 188 23 
TMXDI 1.5 0 N/A 1760 184 36 
2 145 1710 186 41 
6 145 2340 206 36 
6 75 2150 191 35 
TMXDI 2.25 0 N/A 2630 188 28 
2 145 2810 193 33 
6 145 1910 208 41 
6 75 2740 199 31 
TMXDI 3.00 0 N/A 1820 186 24 
2 145 2080 200 28 
6 145 1040 204 39 
6 75 2910 200 30 
______________________________________ 
These data demonstrate robustness, process insensitivity and bond 
durability of the robust liner over a wide range of stoichiometry and with 
two alternate isocyanate curatives. 
EXAMPLE 9 
Interfacial Propellant Burn Rate Control 
A robust liner formulated having the ingredient composition set forth below 
in Table 9A was used in a unique two coat process to demonstrate 
interfacial propellant burn rate control. 
TABLE 9A 
______________________________________ 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
57.0* 57.0* 
TMXDI 
HX-752 3.0 3.0 
(NH.sub.4).sub.2 HPO.sub.4 
40.0 20.0 
*NCO/OH = 1.5 
______________________________________ 
The first coat of liner was applied to EPDM insulation to a thickness of 20 
mils thick and precured for 48 hours at 135.degree. F. 
The second coat of liner was applied over the first coat to a thickness of 
approximately 5 mils. An 88 percent solids HTPB/Al/AP propellant, defined 
in Table 9B, was vacuum cast against the second, wet (uncured) liner coat. 
TABLE 9B 
______________________________________ 
Propellant composition 
Ingredient Weight % 
______________________________________ 
HTPB 
TMXDI 
TEPANOL (bonding agent) 8 
ODI (octadecyl isocyanate) 
TPB (triphenyl bismuth) 
Plasticizer 4 
Al 1 
Ammonium perchlorate (AP) 
87 
Ferric oxide 
______________________________________ 
A second set of samples was prepared using a single coat of a conventional 
liner, defined in Table 9C, for comparative purposes. 
TABLE 9C 
______________________________________ 
Conventional Liner Composition 
Composition 
Ingredient (Parts) 
______________________________________ 
HTPB 
57.0* 
TMXDI 
HX-752 3.0 
TiO.sub.2 40.0 
TPB 0.1 
*NCO/OH = 1.5 
______________________________________ 
The propellant and liner coats were co-cured for an additional 240 hours at 
135.degree. F. Propellant bondline samples, approximately 
1.5.times.1.times.0.25 inches, were prepared and ignited in a confined 
pressure chamber containing an observation port (window bomb) to visually 
observe and record (high speed film) the progression of the flame front 
and any propellant burn rate gradients present. The tests were conducted 
at a pressure of 1,700 psia and a temperature of 75.degree. F. Photographs 
of the tests conducted with the conventional liner indicate an extreme 
propellant burn rate gradient at the liner interface and extreme coning of 
the propellant during the test. Photographs of the tests conducted with 
the disclosed liner indicate a flat, neutral flame front and no propellant 
burn rate gradients present. These data demonstrate the ability to tailor, 
control and eliminate interfacial propellant burn rate gradients with the 
disclosed liner and a unique two coat liner process. 
EXAMPLE 10 
Interfacial Burn Rate Control with an Alternate Burn Rate Modifier 
A robust liner having the ingredient composition set forth below in Table 
10A was used in a unique two coat process to demonstrate interfacial 
propellant burn rate control. Oxamide was substituted for the diammonium 
phosphate burn rate modifier in the second, wet (uncured) liner coat. 
TABLE 10A 
______________________________________ 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
57.0* 57.0* 
TMXDI 
HX-752 3.0 3.0 
(NH.sub.4).sub.2 HPO.sub.4 
40.0 
Oxamide 20.0 
*NCO/OH = 1.5 
______________________________________ 
The first coat of liner was applied to EPDM insulation to a thickness of 20 
mils thick and precured for 48 hours at 135.degree. F. The second coat of 
liner was applied over the first coat to a thickness of approximately 5 
mils. An 88 percent solids HTPB/Al/AP propellant, defined in Table 9B, was 
vacuum cast against the second, wet (uncured) liner coat. The propellant 
and liner coats were co-cured for an additional 240 hours at 135.degree. 
F. Propellant bondline samples, approximately 
1.5.times.1.times.0.25-inches, were prepared and ignited in a confined 
pressure chamber containing an observation port (window bomb) to visually 
observe and record (high speed film) the progression of the flame front 
and any propellant burn rate gradients present. The tests were conducted 
at a pressure of 1,700 psia and a temperature of 75.degree. F. Photographs 
of the tests conducted with the disclosed liner indicate a flat, neutral 
flame front and no propellant burn rate gradients present. These data 
demonstrate the ability to tailor, control and eliminate interfacial 
propellant burn rate gradients with the disclosed liner,a unique two coat 
liner process and an alternate burn rate modifier used as a liner filler 
material. 
EXAMPLE 11 
Interfacial Burn Rate Control with Mixed Fillers 
A robust liner having the ingredient composition set forth below in Table 
11A was used in a unique two coat process to demonstrate interfacial 
propellant burn rate control. An inert filler, titanium dioxide, was used 
in combination with the diammonium phosphate burn rate modifier in the 
second, wet (uncured) liner coat. 
TABLE 11A 
______________________________________ 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
57.0* 57.0* 
TMXDI 
HX-752 3.0 3.0 
TiO.sub.2 7.0 7.0 
(NH.sub.4).sub.2 HPO.sub.4 
33.0 33.0 
TPB 0.1 0.1 
*NCO/OH = 1.5 
______________________________________ 
The first coat of liner was applied to EPDM insulation to a thickness of 20 
mils thick and precured for 48 hours at 135.degree. F. The second coat of 
liner was applied over the first coat to a thickness of approximately 5 
mils. An 88 percent solids HTPB/Al/AP propellant, defined in Table 9B, was 
vacuum cast against the second, wet (uncured) liner coat. The propellant 
and liner coats were co-cured for an additional 240 hours at 135.degree. 
F. Propellant bondline samples, approximately 1.5.times.1.times.0.25 
inches, were prepared and ignited in a confined pressure chamber 
containing an observation port (window bomb) to visually observe and 
record (high speed film) the progression of the flame front and any 
propellant burn rate gradients present. The tests were conducted at a 
pressure of 1,700 psia and a temperature of 75.degree. F. Photographs of 
the tests conducted with the disclosed liner indicate a flat, neutral 
flame front and no propellant burn rate gradients present. These data 
demonstrate the ability to tailor, control and eliminate interfacial 
propellant burn rate gradients with the disclosed liner, a unique two coat 
liner process and an inert substance and a burn rate modifier used as a 
liner filler material. 
EXAMPLE 12 
Interfacial Burn Rate Control in Extinguishment Motors 
A robust liner having the ingredient composition set forth below in Table 
12A was used in a unique two coat process to demonstrate interfacial 
propellant burn rate control. 
TABLE 12A 
______________________________________ 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
39.86* 44.68* 
TMXDI 
HX-752 5.0 4.98 
TiO.sub.2 20.0 -- 
(NH.sub.4).sub.2 HPO.sub.4 
35.0 49.8 
SiO.sub.2 -- 0.4 
TPB 0.14 0.14 
*NCO/OH = 1.5 
______________________________________ 
The liner was used to coat the interior of a 5-inch diameter.times.9-inch 
long rocket motor case consisting of an insulator on the inside diameter 
of the case and a center inhibitor. A conventional liner, defined in Table 
12B, was applied to a second rocket motor case for comparative purposes. 
TABLE 12B 
______________________________________ 
Conventional Liner Composition 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
60.59* 62.65* 
IPDI 
HX-868 3.28 3.39 
Carbon Black 32.75 33.86 
SiO.sub.2 3.38 0.10 
*NCO/OH = 1.5 
______________________________________ 
The first coat of liner was applied to insulation and inhibitor to a 
thickness of 20 mils thick and precured for 48 hours at 135.degree. F. The 
second coat of liner was applied over the first coat to a thickness of 
approximately 5 mils. An 88 percent solids HTPB/Al/AP propellant, defined 
in Table 9B, was vacuum cast against the second, wet (uncured) liner coat. 
The propellant and liner coats were co-cured for an additional 240 hours 
at 135.degree. F. The motors were static tested at a chamber pressure of 
approximately 1700 psia and extinguished prior to completion of burn. A 
replicate casting of the two extinguished propellant grains was made in 
order to document the effect of burn rate gradients. The replicate surface 
of propellant grain "A" containing the conventional liner had an uneven 
surface tapered towards the bondlines indicating moderate interfacial burn 
rate gradients adjacent to the insulator and center inhibitor bondlines. 
The replicate surface of propellant grain "B" containing the disclosed 
liner and liner process had a relatively flat combustion service 
indicating uniform flame front advancement and little or no burn rate 
gradients. These data demonstrate the ability of the disclosed liner and 
liner process to control and eliminate interfacial propellant burn rate 
gradients in subscale rocket motors. 
EXAMPLE 13 
Interfacial Propellant Burn Rate Control in Full Scale Tactical Motor 
A robust liner having the ingredient composition set forth below in Table 
13A was used in a unique two coat process to demonstrate interfacial 
propellant burn rate control. 
TABLE 13A 
______________________________________ 
Composition (Parts) 
Ingredient Cured Coat 
Wet Coat 
______________________________________ 
HTPB 
58.13* 44.68* 
TMXDI 
HX-752 4.84 4.97 
TiO.sub.2 14.53 -- 
(NH.sub.4).sub.2 HPO.sub.4 
19.38 49.65 
SiO.sub.2 2.91 0.63 
TPB 0.21 0.07 
Rhodamine B (dye) -- 0.005 
*NCO/OH = 1.5 
______________________________________ 
The liner was used to coat the interior of a full scale 9-inch 
diameter.times.105-inch long end burning tactical rocket motor case 
consisting of an insulator on the inside diameter of the case and a center 
inhibitor. The first coat of liner was applied to insulation and inhibitor 
to a thickness of 20 mils thick and precured for 48 hours at 135.degree. 
F. The second coat of liner was applied over the first coat to a thickness 
of approximately 5 mils. A Rhodamine B dye was added to the second coat of 
liner to provide visual detection, coverage and thickness control. An 88 
percent solids HTPB/Al/AP propellant, defined in Table 9B, was vacuum cast 
against the second, wet (uncured) liner coat. The propellant and liner 
coats were co-cured for an additional 240 hours at 135.degree. F. The 
motors were static tested at a chamber pressure of approximately 1700 
psia. A pressure versus time trace was obtained which showed a relatively 
neutral, constant pressure motor burn indicating a constant propellant 
burning surface, uniform flame front advancement, and the absence of 
interfacial propellant burn rate gradients. These data demonstrate the 
ability of the disclosed liner and liner process to tailor, control and 
eliminate interfacial propellant burn rate gradients in a full scale, end 
burning, tactical rocket motor. 
From the foregoing, it will be appreciated that the present invention 
provides a solid rocket motor propellant liner which is insensitive to 
large variations in stoichiometry and which is able to modify the 
ballistic properties of the adjacent interfacial propellant layer. 
The present invention may be embodied in other specific forms without 
departing from its essential characteristics. The described embodiments 
are to be considered in all respects only as illustrative and not 
restrictive. The scope of the invention is, therefore, indicated by the 
appended claims rather than by the foregoing description.