Propellants, Explosives, Pyrotechnics 21, 43-50 ( 1 996) 43

Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder for Composite Solid Propellants Akira Iwama, Kazuo Hasue, and Terue Takahashi

The Institute of Space and Astronautical Science, Ministry of Education, Culture and Science, 3- I-1-Yoshinodai, Sagamihara, Kanagawa 229 (Japan)

Kiyoshi Matsui

Harima Plant, Daicel Chemical Industries, Co. Ltd., 805 Umaba Ibokawa-cho, Ibogun, Hyogo 671-16 (Japan)

Kazunari Ishiura

Kashima Laboratories, Kuraray Corporation, 36 Higashi-wada, Kamisumachi, Kashima-gun, Ibaragi 3 14-02 (Japan)

Hydriertes Polyisopren mit endstiindigen OH-Gruppen als Brennstofminder fur Komposit-Festtreibstdfe

Versuche zur Synthese von hydriertem Polyisopren mit endst&- digen OH-Gruppen (HHTPI) zur Anwendung als Binder fur Kom- posit-Festtreibstoffe wurden durchgefihrt. Ein HHTPI-Prapolymer wurde hergestellt durch Hydrierung von Polyisopren mit endstiindi- gen OH-Gruppen (HTPI) in Gegenwart von Nickel- und Zirco- nium-Katalysatoren an Kieselgur mit Wasserstoff bei einem Druck von 2.0 MPa, bei 443 K-453 K warend 24 Stunden. Ein Prapoly- mer mit einem mittleren Molekulargewicht von 2500-3800 er- brachte den f i r den Einsatz als Binder erforderlichen Viskositlits- bereich, so daB damit die Festtreibstoffe wahrscheinlich direkt im GieRprozeB hergestellt werden konnen. Die thennische Stabilitat und Alterung der HHTPI-Elastomeren gegenuber Umgebungsein- fliissen ist der von HTPB-Treibstoffen uberlegen. Einige Weichma- cher und Haftvermittler konnen brauchbare mechanische Ei- genschaften hervorbringen bei Treibsatzen, die hauptsachlich aus HHTPI, Ammoniumperchlorat und Aluminiumpulver bestehen. Die lineare Abbrandgeschwindigkeit von HHTPI-haltigen Treibstoffen hat dieselbe GroBe wie bei den HTPB-haltigen. Jedoch verschiebt sich die Zusammensetzung der HHTPI-Treibstoffe bei maximaler Leistung um 1-2 Gew.% auf die Brennstoffseite im Vergleich zu dem entsprechenden Brennstoffgehalt bei HTPBl APl A1 mit 12 Gew.%. Die HHTPI-Treibstoffe zeigten anliche lineare Ab- brandgeschwindigkeiten wie die HTPB-Treibstoffe trotz der ver- gleichsweise geringen Zundf&igkeit. Dennoch wurden statische Tests bei Forschungsraketen mit einem Durchmesser von 100 mm erfolgreich unter Druck durchgefuhrt. Die ballistischen Leistungen sind nicht niedriger als bei den HTPB-Treibstoffen.

Polyisoprhe hydrogin6 B terminaison OH c o m e liant com- bustible pour des propergols solides composites

Des essais ont Ctt rtalists en vue de synthttiser du polysioprhe hydrogtnt B terminaison OH (HHTPI) utilist en tant que Iiant pour des propergols solides composites. Un prtpolym&re HHTPI a t t t synthktist par hydrogtnation de polyisoprhe 8 terminaison OH (HTPI) en prtsence de catalyseurs nickel et zirconium sur de la diatomite dans de l’hydrogtne B 2,O MPa, ?i 443 K-453 K pen- dant 24 heures. Un prtpolym2re posstdant un poids moltculaire moyen de 2500-3800 a donnt le niveau de viscosit6 ntcessaire pour I’utilisation en tant que liant, B partir duquel les propergols solides peuvent &tre vraisemblablement fabriquts par un processus de coulte directe. La stabilitt thennique et le vieillissement des tlastomtres HHTPI contre les agressions exttrieures sont suptr- ieurs B ceux des propergols HTPB. Certains plastifiants et agents adhksifs peuvent donner des proprittts mtcaniques utiles aux prop- ergols composts essentiellement de HHTPI, perchlorate d‘ammo- nium et poudre d’aluminium. La vitesse de combustion lintaire de propergols ?i base de HHTPI se situe au m2me niveau que celle des propergols 8 base de HTPB. Toutefois, h puissance maximale, la composition des propergols HHTPI est d6caEe de 1-2% en poids du cStt combustible par rapport 8 la teneur en combustible correspondante de 12% en poids pour HTPBIAPIAl. Les proper- gols HHTPI ont donne des vitesses de combustion linkaires sembl- ables 8 celles des propergols HTPB en dtpit de leur amorqabilitt proportionnellement plus faible. Toutefois, des tests statiques ont Ct6 rtalists avec succi.s sous pression sur des fusees-sondes de diamktre 100 mm. Les performances balistiques ne sont pas inf6r- ieures B celles des propergols HTPB.

Summary

The synthesis and application of hydrogenated hydroxy-termi- nated polyisoprene (HHTPI) to a fuel binder of composite solid propellants were attempted. An HHTPI prepolymer was synthe- sized through the hydrogenation for the hydroxy-terminated poly- isoprene (HTPI) in the presence of nickel and zirconium catalysts over kieselguhr in 2.0 MPa hydrogen and at 443 K-453 K for 24h. A prepolymer of a number-averaged molecular weight 2500-3800, provided a viscosity level required for the use of a fuel binder from which solid propellant can be possibly made by means of direct casting method. Thermal stability and aging char- acteristics of HHTPI elastomer against environmental attacks are superior to those of HTPB. Some plasticizers and bonding agents

can bring about the acceptable mechanical properties to the propel- lant grains mainly composed of HHTPI, ammonium perchlorate and aluminium powder. The linear burning rates of HHTPI-based propellants are at the same level with that of HTPB-based propel- lants. However, the composition that gives the maximum perfor- mance with HHTPI-based propellants, shifts to 1-2 wt% fuel-rich side from the most adequate fuel content 12 wt% in HTPBIAPIAl. The HHPTI propellants indicated the similar burning rate as HTPB-based propellants in the linear burning rates in spite of the comparatively poor ignitability. Nevertheless, the static tests of 100 mm dia. sounding rocket motors are successfully performed by an ignition operation at the pressurized condition. The ballistic performances are not inferior to those of the HTPB-based propel- lants.

0 VCH Verlagsgesellschaft, 0-6945 1 Weinheim, 1996 0721-3 1 15/96/01/02-043 $10.00+.25/0

44 A. Iwama, K. Hasue, T. Takahashi, K. Matsui, and K. Ishiura Propellants, Explosives, Pyrotechnics 21, 43-50 (1996)

1. Introduction

Full saturation of the double bonds in the diene polymer type fuel binders would lengthen the service life, and im- prove the thermal and aging stability. It may increase in the ballistic performance and lower the vulnerability in the ap- plication to solid propellants and plastic bonded explosives. Staudinger(') and Pummerer(2) early in 1922 tried a hydro- genation for natural rubber of which ingredient is mostly cis-polyisoprene. Thomas(3) pointed out that chemical unsa- turation, in spite of its importance from a stand point of permitting the vulcanization to take place, induces a great weakness of the natural rubber concerning the aging pro- blem. Long time exposure of unsaturated fuel binder in air would cause the addition of oxygen atoms or hydroxyl groups to the double bonds; this would bring about the de- terioration in mechanical properties and court the combus- tion instability on making the motor firing. The inclusion of any anti-aging agent in such a propellant as practiced for butadiene polymer -based propellants are indispensable.

Joned4) first reported that, when non-functional polybuta- diene and copolymers of butadiene and the other com- pounds are hydrogenated leaving the residual unsaturation to the extent not higher than 50%, the polymers and copo- lymers convert to the thermoplastics having a strong oil- resistance and high tensile strength with their glass transi- tion point maintained at low values. Nearly 100% hydroge- nation for the polybutadiene including high fractions of trans- and cis-microstructures would form the polybuta- diene inelastic even at room temperature. Jones also studied the hydrogenation of non-functional polybutadiene and de- scribed the synthetic process and the results of the analysis and mechanical property measurements of hydrogenated polybutadiene. His study avoided a complete hydrogenation because the product would become a waxy state. At pre- sent fully hydrogenated products of 1,4-polybutadiene (HTPB) are provided by Mitsubishi Chemical Industries Co. in Japan.

Nisso Chemical Industries Co. is providing almost com- pletely hydrogenated hydroxy-terminated polybutadiene (HHTPB) with vinyl microstructure. This prepolymer is a highly viscous fluid as it indicates a viscosity of 55 Pa. s at 318 K. This value is near the critical viscosity to make the direct casting of propellant dough possible barely. Thus, vinyl group pendant from every monomer retains a better fluidity and lowers the glass transition temperature, and results in a better processability of the dough made from 1,ZHHTPB. Linear polymers of fully saturated hy- drocarbons have a higher glass transition temperature, and the addition of a side arm plays a role to lower the glass transition temperature. Therefore, we expected that the in- troduction of hydrogen atoms to the double bonds in hy- droxy-terminated polyisoprene (HTPI) should not make viscosity increase till the directs casting becomes difficult.

polymerization of isoprene monoiners was conducted : hy- drogen peroxide, 1.2 g-12 g for isoprene monomer, 100 g, and isopropyl alcohol being used as reaction initiator and solvent, respectively. The reaction temperature of 400 K and reaction time of 6 h yielded a polymer of the number- averaged molecular weight, M,, 3800, hydroxyl group con- tent of 0.73meq/g and hydroxyl number of 2.19. The mean molecular weight was varied by adjusting the initia- tor fraction for monomer and reaction temperature. The vi- nyl-microstructure content less than 7.2% in the product can insure such a liquid property as HTBP prepolymer. The hydrogenating operation of HTPI under 2.0 MPa and 443 K-453 K for 24 h in tetrahydrofuran solvent has satu- rated up to 98% in the presence of nickel-over-kieselguhr catalyst and zirconium co-catalyst.

Varying the synthesis condition, we can provide the fob lowing prepolymers available for propellant and explosive binders,

CH3 I

OH-(CH2-CH-CH2-CH2),-OH n = 28-54.

3. Characteristics and Modification of HHTPI Elastomer for the Application to Liner/Insulation

Table 1 shows the fundamental properties of HTPI and HHTPI prepolymers that are suitable for propellant applica- tions at present. Herein, TH-1 and TH-21 are Kurary Com- pany's products, and recently, Atochem-Idemitsu Petro- chem Company, Japan participated in the production of HHTPI, EP-25 that has a longer curing time with a combi- nation of isophorone diisocyanate.

The HTPI and HHTPI (TH-21) prepolymer indicate the viscosities higher by 6 and 16 times, respectively, than that of HTPB prepolymers for the similar mean molecular weight. However, the propellant dough whose fuel binder is either HTPI or HHTPI (TH-21) prepolymer, has not dis- turbed the mixing and casting operation even in case of high solid filler 86-88.5 wt%. It is because a better dough fluidity is obtained owing to the more uniform distribution of atomized solid fillers and the addition of conventional or particular plasticizers. Further, at elevated temperatures, 340 K-350 K, we noted the viscocity of TH-21 lowers down to the similar value to that of HTPB as shown in Fig. 1. The HHTPI prepolymer which we aim to use for extrudable propellants, expresses favorable viscosity also for the application to plastic bonded explosives.

Remarkable characteristic of HHTPI consists in a high H/C atom number ratio 2.0 whereas H/C of HTPB is 1.5. The high hydrogen content contributes to an increase of the specific impulse effectively without an elevation of com- bustion temperatures. As shown in Table 2, the perform- ance calculation according to NASA standard computer program(5) demonstrates that the suitable HHTPI fuel-bin- der content is 13-14 wt% for AP-based propellants with fixed aluminum content 18-20 wt%. These composition shifts apparently to fuel rich side compared to the HTPB

Hydroxy-terminated polyisoprene (HTPI) was synthe- propellant for which the most suitable fuel-binder content is around 12 wt%. Therefore, to some degree a casting dif-

2. Production of HTPI and HHTPI Prepolymers

sized through a similar procedure to that of HTPB: radical

Propellants, Explosives, Pyrotechnics 21, 43-50 (1996)

Table 1. Liquid Prepolymers of Hydroxy-Tenninated Polyisoprenes

Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder 45

Name TL-20 TH- 1 TH-21 EP-25

CH3 Molecular Structure I

Densit at 289 K, 910

Number Averaged 2600 Molecular Weight(') OH Value(2' 62.8

OH Number per one 2.9 Molecule Viscosity at 308 K, 15 [Pa . s](~) Heat of Formation, +18.1 AHf [kJ/kg] Glass Transition 218 Temperature, [K]

(1) Vapor pressure osmosis (2) Acetylation method (3) Broodfield/Lockwood type viscometer

HO-(CH2-C=CH-CH2),-OH

Y [kglm 1

(ms KOH/g)

OH- 860

3800

32.8

2.2

310

-20

218

-(CH2 -CH-CH2 -CH2), -OH 860 860

2700 2500

56.1 50.1

2.6 2.25

40 38

-560 -350

218 21 8

Table 2. Performance Comparison among HHTPI-, HTPI- and HTPB-Based Propellants

Mixing Ratio by Combustion Combustion Mean Characteristic Nozzle Specific Weight Pressure Temperature Molecular Velocity Coefficient Impulse

Weight [MPal IKI Wmo11 [m/sl [SI

HHTPI HHTPI/AP/Al = 13/69/18

HHTPI/ AP/AI = 14/68/18

HTPI HTPI/ AP/ A1 = 12/70/18

HTPI/AP/ A1 =12/68/20

HTPB HTPB /APIA1 = 12/70/18

HTPBl AP/ A1 = 12/68/20

5.0 7.0

5 .O 7 .O

5.0 7.0

5.0 7.0

5.0 7.0

5.0 7.0

3364 3389

3287 3309

3440 3469

3369 3395

3486 3517

3514 3546

27.00 27.08

26.30 26.37

27.90 27.99

27.23 27.31

28.85 28.95

29.17 29.29

1581 1585

1582 1586

1574 1578

1574 I579

1560 1.564

1559 1563

1.567 1.616

1.565 1.613

1.594 1.634

1.566 1.614

1.570 1.619

1.572 1.621

252.7 261.2

252.5 260.8

251.6 260.1

25 1.4 259.8

249.7 258.2

249.8 258.3

ficulty due to the poor fluidity of HHTPI can be relaxed in practice.

For surveying the appropriate compositions for linerlin- sulation applications, aluminum oxide filled and unfilled HHTPI elastomers were tentatively produced and tested. At first, since OH functionality of 2.6 in TH-21 is too many to apply to the linerhsulation, the four different diols are blended in the HHTPI prepolymer: 1,4-butanediol (BD), 3-methyl-l,5-pentanediol (MPD), 1,9-nonanediol (ND), and 2-ethyl-1,3-hexanediol (EHD). Figure 2 shows the stress-strain curves of the dumbbells made from the mix- tures of prepolymer(TH-21)-isophorone diisocyanate-diol, 1 : 3 : 2 by mole ratio. The mixing operation was conducted by one shot method in vacuum condition less than 1 Tom and the curing for one hour at 393 K. As a diol for smoke- less liner or insulation EHD can be recommended, and

polybutylene adipate (M,, = 500) gives a good mechanical property but generates smoke slightly. Conventional plasti- cizers such as dioctyl adipate, dioctyl phthalate, polybu- tene, non-functional HHTPI prepolymer can improve the elasticity at low temperatures. A composition of the HHTPI-based elastomer has such a good bonding strength as its peeling test for the sheet sample bonded to a stain- less steel plate indicated 1000-1500 g/in at 300 K and 300mm/min of peeling rate. As many kinds of hydrocar- bons, ethers, esters, and ketones dissolve HHTPI elasto- mers, it is not difficult work to remove the liner/insulator from motor case.

An emphasized merit of HHTPI polymer is the possibi- lity that it can provide a binder for smokeless liner/insula- tor. Figure 3 is thennogravimetry curve to show the prop- erty that HHTPI decomposes completely at the

46 A. Iwama, K. Hasue, T. Takahashi, K. Matsui, and K. Ishiura

- 1 1 I

200 r

80 Heating Rate : lO%/min

-

Figure 1. Viscosity of HHTPI, HHTPB and HTPB prepolymer.

- 5 2 60- U

4 40 0

VI 0 K

20

nn

HHTPI(TH-21)/ IPDI = l / l (mo le)

- HTPB(P-41)/IPDI=l/l(mole)

-

I I I

Sample : TH-21/IPDI/Short Chain Diol=1/3/2(mole) Diols : BD 1,4-butanediol

MPD 3-methvl-l,5-~entadiol ND 1,9-noianedioi EHD 2-ethyl-1,3-hexanediol 15 -

Tensile Rate : 500mm/min, 298K ,-. a 5

b 10-

2 3

., Lo

500 Strain, E (%)

Figure 2. Stress-strain curves of some HHTPI elastomers (num- bers in parenthesis indicate shore hardness).

Temperature, T (‘C)

Figure 3. Thermogravimetry curves of an HHTPI and HTPB poly- mer.

Propellants, Explosives, Pyrotechnics 21, 43-50 (1996)

temperatures less than 770 K, and no carbonaceous residue is left. On the other hand, HTPB generates smoke and leaves 3%-4% residue to the original sample amount. Ac- cordingly, HHTPB may be a material to develop smokeless liner/insulator and HHTPI-based propellant would not gen- erate a great deal of soot even in fuel rich compositions or when used as a fuel composition of hybrid propellant.

4. Thermal and Aging Properties of HHTPI

It is presumed that HHTPI exhibits an intensive resis- tance against thermal attack and degradative oxidation be- cause of its saturated molecular structure. We compared HHTPI and HTPB elastomers by exposing test pieces in air at 423 K and 473 K. The ingredients of test pieces are a mixture of prepolymer - IPDI-BD, 1 : 3 : 2 by mole. The HTPB, Poly-BD, supplied by Atochem-Idemitsu Petro- chemical Co. is equivalent to HTPB, R-45 HT, produced by Elf-Atochem, France. The difference of HHTPI from HTPB elastomer subjected to an accelerated thermal attack appeared in the color aspect. The addition of oxygen atoms and/or hydroxyl groups to the double bond of HTPB made the test piece brownish and caused numerous cracks to in- troduce a poor elongation in the material test.

Table 3 is an evaluation result on the deterioration prop- erty for three polymers having the saturated hydrocarbons as the main chain. The test pieces were soaked in distilled hot water at 373 K. In this case, methylene bis(4-phenyl diisocyanate) is used as a common currant for three differ- ent polyols; polybutyl adipate (M, = 2000), polytetra- methylene glycol (M, = 1000). The ingredient ratio of the test piece is prepolymer-MDI-BD, 1 : 3 : 2 by mole and the mixing and curing are done at 393 K by one shot method. The dumbbell configuration and test procedure are pro- duced and performed according to the Japan Industrial Spe- cification K6301-1962 for rubbers. On soaking these dumb- bells in hot water the PTG and HHTPI elastomer would induce an increase in elongation without a remarkable de- crease in the tensile strength. The PBA lost rapidly the me- chanical integrity to come back to the liquid state 168 h after. As for HTPB the attack of hot water at the first stage is directed to the OH addition to the double bond, keeping pace with the cleavage of the urethane bond. If the main

Table 3. Deterioration of Saturated Hydrocarbon Polymers in Hot Water

Sample Compositions Polyol TH- 1 PBA PTG Isocyanate MDI MDI MDI Short Chain Diol 1,4-BD 1,4-BD 1,4-BD

o b Eb ob Eb o b Eb Blank 100 100 100 100 100 100 4 days 100 113 65 130 74 122 8 days 90 111 16 110 68 126

12 days 93 118 unmeasurable 66 126

ob; Ratio to the tensile strength of virgin samples to that of pro- cessed samples at break point. &b; Ratio to the elongation of virgin samples to that of processed samples at break point.

Propellants, Explosives, Pyrotechnics 21, 43-50 (1996) Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder 47

Table 4. Aging Property of HHTPI and HTPB ~~-

Process- Sample ing Time HHTPI HTPB * [hl Mechanical Properties

o b Emax El00 OP Emax EIOO [MPal [%I [MPal [MPal [%I [MPa]

0 2.1 580 0.19 5.0 250 1.8 100 1.9 610 0.22 2.2 30 1.6 300 1.8 610 0.21 1.9 0 0 500 1.7 550 0.23 1.8 0 0

~~-

* No anti-aging agent included.

chain is the saturated hydrocarbon the action of hot water is primarily directed to the cleavage of the urethane bonds rather than against the fission of prepolymer chains.

Thus, the saturated hydrocarbon chains implied a strong resistance against the hot water attack so that the deteriora- tion of HHTPI elastomer is delayed. However, the liberated prepolymer migrates in the matrix and takes a behavior like a plasticizer. Therefore, on soaking the test pieces in hot water the elongation of any polyurethane elastomer in- creases to some extent once. Although in case of HTPB elastomer the OH or 0 addition to the double bond at 373 K is limited on the surface, it should be noted the pre- polymer with only 1% loss of double bonds due to oxida- tion brings a vital effect on the mechanical property of the tailored elastomer(6). The addition of oxygen to the double bonds with a HTPB prepolymer that not included anti- aging agent and was stored more than ten years, would form a hard skin on the liquid surface and make the grain brittle remarkably.

An accelerated aging test, cyclic exposure in water and on hot panel witnessed to cause a rapid deterioration for HTPB because the loss of double bonds and cleavage of urethane bonds go on simultaneously. Saturated polymers, HHTPI and HHTPB revealed and excellent anti-aging prop- erty. This test was carried out by means of a xenon weather- ometer (Shimazu VW-6OV3, lamp output 4.5-6.5 kW); the test conditions were decided as the black panel temperature 335 K, the rain period 18 min for one cycle 102 min, 15%, and hot air exposure period 84min for 102min, 85% for the samples prepared from the same currant and short chain diol with the test pieces for the previous hot water dete- rioration test. Table4 compares the aging property of HHTPI with that of HTPB. No significant deterioration of HTPB elastomer is seen within the first 100 h but after 300 h both the elongation and tensile strength become nearly zero. Superiority of HHTPI and HHTPB in terms of the aging characteristics derives itself from the toughness of the main chain against the chemical attacks.

5. Modification of HHTPI Prepolymer for the Applica- tion to Fuel Binders

The predicted advantage of HHTPI is a possibility to become a fuel binder with which solid propellant can gen- erate a ballistic performance not inferior to HTPB-based propellant. Besides, the main chain saturated with hydrogen

gives HHTPI a high thermal stability and long shelf life. However, high viscosity of HHTPI prepolymer, exceeds by eight times the value of HHTPI having a similar M, of around 3000. Fortunately, the appropriate compositions that generate the maximum specific impulse, shift 1-2 wt% fuel rich side by weight from that of HTPB-based propel- lants. The addition of plasticizer or viscosity depressant is, however, required to accomplish the direct casting opera- tion of TH-21 based propellant dough.

We obtained a dough castable smoothly at 308 K by adding 10-23 phr of conventional plasticizers such as dioc- tyl adipate (DOA), dibutylphthalate (DBP), di- (2-ethylhex- y1)azelate (DOZ), isodecyl pelargonate (IDP) and non-func- tional saturated liquid olefin polymers to the HHTPI prepolymers. After many diisocyanates have been checked, isophorone diisocyanate was nominated most favorable. But, at present, commercially available HHTPB prepolymer of OH functionality 2.6 and M, 2700, indicated too fast curing rate to produce large propellant grains over 500 kg. For this purpose, it is required to add a curing rate retarder such as tetracycline(7). Another prepolymer EP-25 reduced to OH functionality down to 2.2 as indicated in Table 1, guarantees a pot life or zero flow time enough to take a long time casting operation.

6. Combustion Property of HHTPI-Based Propellants and Firing Results

We produced tentatively a series of HHTPI-based propel- lants aiming the final application to a sounding rocket. The compositions and mechanical properties are shown in Ta- ble 5. Herein, it must be stressed that without bonding agent HHTPI grain manifests a very poor mechanical prop- erty to reject the practical use. The inclusion of a proper bonding agent to realize a mechanical property level in- sures a stable motor combustion. Methylamino- bis (methyl- 2-aziridinyl-1) phosphoric oxide [MEBAPO] is one of the good bonding agents.

The results of linear burning rate measurements for the strands are shown in Fig. 4. The pressure exponent is 0.46, 0.47 and 0.5 for A, B and C composition, respectively. This values are 5-8% higher than the pressure exponent of HTPB-based propellants, but acceptable enough for inside- burning motors. The level of the linear burning rate is nearly equal to that of HTPB-based propellants at the op- eration pressures of conventional motors for the same pro- pellant compositions. As the best performance composition of HHTPI-based propellants shifts to fuel-rich side, the composition A and B that is expected to issue the maxi- mum performance have the burning rates a few percent lower than would have a composition of 12 wt% HTPB, 70 wt% AP and 18 wt% Al.

As for the mechanical properties the propellant grains A and B in Table 5 could pass our specification. A rp 70 mm heavy wall motor loaded with the HHTP-based propellants 3.2 kg were static-fired. An example of pressure- and thrust-time histories are shown in Fig. 5. Through the firing tests, we noticed the startability of the motors loaded with

48 A. Twama, K. Hasue, T. Takahashi, K. Matsui, and K. Ishiura Propellants, Explosives, Pyrotechnics 21, 43-50 ( 1 996)

____.

Compositions

AP HHTPI A1 MEBAPO

[wt%] [wt%l [wt%] [phrl

A* 68 14 18 0 A 68 14 18 3.5 B 69 13 18 3.5 C 70 12 18 3.5

Max. Tensile

Strength omax

[MPaI

0.58 1.57 1.62 2.4 1

~~ ~

Mechanical Properties Tensile Elongation Strength at Break at Break Point

Point Eh

oh ["/.I ["/.I

-.+.-

Young's Modulus

E

[MPal

0.39 4.8 1.50 21.3 1.58 18.2 2.30 16.9

8.3 11.7 13.4 17.6

HHTPI-based propellants is considerably poor. If the igni- tion material amount is short, the conventional igniter B/ KNO3 granules and pellets, independently of the fuel-bin- der content of HHTPI 11.5-14.0 wt%, might take around one second from the ignition to the build-up of chamber pressure as seen in Fig. 5 if any measure is not made other- wise. A considerable increase in the amount of B/KNO3 igniter might make the pressure build-up time shorten with- out and allowable delay. The allyl carbon atoms in HTPB

I H H T P I ~ AP 1 AL IMEBAPO* - 1 14 I 6 8 I 1 8 I 3.5

12r

Time, t (s)

polymer are rich in reactivity, so no ignition problem has been noted except very low temperature conditions in HTPB propellants.

We already observed that the highly aluminum oxide filled HHTPI approached a self-extinguishing feature and the ignitability of HHTPI-based propellants is poor at around 0.1 MPa nitrogen through the static firing test and the burning rate measurement. In a nitrogen flow at the pressure less than 0.2 MPa, it is difficult to maintain self- sustaining combustion after the ignition with a Joule-heated nickel/chrome alloy wire, and a spontaneous burning inter- ruption was often observed. It has been proved also in mo- tor firings that the flame spreading along the surface is ex- tremely slow even though the igniter's burning products run over the grain channel. Krupp ignition test was made to know the ignitability when the bulk heating is imposed on the sample at given temperatures. Figure 6 indicates the ignition delay of HHTPI-based propellant is approximately twice longer compared to that of HTPB-based propellants. To observe the burning-interrupted surface we identified the presence of a thin liquid layer that is mostly liquid prepoly- mer coming back from the polymer.

The reactions HHTPI rendered as a fuel-binder, present a different aspect from HTPB in the preignition process. Be- fore the decomposition of main chain and the subsequent ignition occur, the following reactions are presumed (s. next page).

On heating or irradiating the polybutadiene elastomer strongly, the hydrogen atoms with methylene group adja- cent to the double bond unit are easily withdrawn from the main chain, and radicals having an allyl structure are

Figure 5. Pressure- and thrust-time history showing the de- layed pressure build-up of 3.2 kg HHTPI-based propellant grain (HHTPI 12 wt%, AP 70 wt%, cp 5 Frn Al 18 wt%).

Propellants, Explosives, Pyrotechnics 21, 43-50 (1996) Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder 49

HHTPI

7% heat, hv

yH3 H -(CH,-CH-CH,-CHJ,-O-C-N-R -- - (CH,-CH-CH,-CH,),- + - 0 - C - N - R

II 0

HTPB

-(cH, - CH = CH - CH,), - GT -(CH - CH =CH -cH,)- = 1%

- (CH - CH = CH -CH2)- I 0 I 0

A : H H T P I 12wt% A :HTPB 12wt%

At : I8w t% AP : Balance

I

A A

A

1.2 1.3 1.4 1.5 1.6 1.7

Reciprocal of Ambient Temperature , x 10’ (K”)

Figure 6. Krupp ignition test result showing a poor ignitability of HHTPI propellant.

formed(@. These free hydrogens and ally1 radicals react with the ambient oxidizing gases rapidly and make a trig- ger to ignition. Accordingly, there is no serious ignition delay problem with HTPB-based propellants. On the other hand, such a fully saturated hydrocarbon chain as HHTPI is chemically tough, hydrogens are not liberated from main chain at a very short stage while the propellant receives much enery from igniter. Rather a large part of the energy issued from igniter is consumed for the cleavage of ur-

+lo0 Motor Ignitor Grain

/

0

(CH = CH - CH - CH,)-

ethane bond and the surface is covered with a liquid layer of HHTPI prepolymer. This liquid layer impedes the quick flame propagation along the initial propellant surface and becomes the cause of a long delay till the build-up of the chamber pressure from the action of igniter.

This disadvantage imposed upon us to take some mea- sures in order to get quick pressure build-up. We have tried the installation of a nozzle closure to be removed upon the pressure rise beyond 2 MPa with a thin wall cp 100mm motor (Fig. 7). This motor was prepared as an upper stage motor that we have developed for the two-stage rocket for sounding a middle stratum 30- 100 km altitude. Figure 8 is a pressure- and thrust-time history ‘example. The delay from the igniter initiation to the build-up of thrust is within an allowable period 150 ms.

7. Conclusions

(1) Hydrogenated hydroxy-terminated polyisoprene is a promising fuel binder for producing thermally stable, high anti-aging and anti-humidity composite solid propellants, and it provides a smokeless insulator/liner. The viscosity of the prepolymer that has 2500-3800 of number-averaged mean molecular weight, and 2.2-2.6 of OH functionality, is not so high for us to undergo hardships to perform the direct casting operation of the highly filled propellant dough. For this purpose, the use of some plasticizers such as dioctyl adipate, isodecyl pelargonate, non-functional hy- drogenated polymers, etc. is indispensable and these may

Figure 7. A cp 100 mm motor nominated as the second stage of meteorological sounding rocket.

1 1 2 0 4 -

1394

(unit : mm)

50 A. Iwama, K. Hasue, T. Takahashi, K. Matsui, and K. Ishiura Propellants, Explosives, Pyrotechnics 21, 43-50 (1 996)

1000

14- 900-

12- 800- A m

Y z 10-"M a 700 -

5 600- a" 8 - 5 0 0 - 2 -

2 6 - f 2 400- a

300 - d .K E 4 - 2 0 0 -

Y)

L

0 2 - 100-

0- d

I I I I I I 0 -

the powerful igniter can overcome this disadvantage in practical motor applications. However, further investiga- tions are required on the ignition and combustion character- istics.

8. References

(1) H. Staudinger, Helv. Chim. Acta 5, 785-806 (1922). (2) R. hmmerer and P. A. Burkard, Ber: 55, 3458 (1922). (3) R. M. Thomas, I. E. Lightbown, W. J. Sparks, P. K. Frolich,

(4) R. V. Jones, C. W. Moberly, and W. B. Reynolds, Ind. Eng.

(5) S. Gordon and B. J. McBride, NASA SP-273 ( 1 97 1 ).

and E. V. Murphree, Znd. Eng. Chem. 32, 1283-1288 (1940).

Chena. 45, 1117-1122 (1953).

(6) H. Tokui and A. Iwama, J. Jupan Ind. Expl. Soc. 52, 2,

play a role to improve the mechanical property at low tem- peratures.

(2) The ballistic performance of HHTPI-based propel- lants is not inferior to that of HTPB-based propellants.

(3) The high thermal stability of the native polymer leads to a little poor ignitability of tailored grains. An ignition operation at pressurized conditions, or the use of (Received June 7, 1994; Ms 19/94)