Propellants Manufacture, Hazards, and Testing

6 Solid Propellants Based on Polybutadiene Binders

Downloaded by CORNELL UNIV on June 7, 2012 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0088.ch006

E. J. MASTROLIA and K. KLAGER Aerojet-General Corp., Propulsion Division, P.O. Box 15847, Sacramento, Calif. 95813

Butadiene prepolymers containing carboxyl functional groups are widely used to make the binder matrix for solid composite propellants. The prepolymers used most extensively are the copolymer of butadiene and acrylic acid (PBAA), the terpolymer of butadiene, acrylic acid and acrylonitrile (PBAN), and carboxyl-terminated polybutadiene (CTPB). These prepolymers are compared; the problems arising from side reactions of the curing agents used in these propellants are a major contribution to the postcuring and softening phenomena. CTPB propellants are the most desirable from the viewpoint of mechanical behavior and solids loading capability. PBAA has been replaced by PBAN or CTPB because of improvements in storage stability and mechanical behavior which they provide. The reproducibility of butadiene propellants is satisfactory when the lot qualification technique is used.

he most recent development in solid composite propellants makes use of liquid butadiene prepolymers which provide 2-3 seconds' higher specific impulse than is realized from most other ammonium perchlorate composites. Higher concentrations of solids and the greater fuel values of the butadiene binder are responsible for the increase i n energy. Satisfactory mechanical properties over the temperature range —75° to + 1 8 0 ° F . are obtained, and the propellants can also tolerate exposure to high levels of humidity for prolonged periods with minimum change i n mechanical behavior. These attributes, plus the simplicity of compounding the propellants and the ready processability of these composites A

122 In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

6.

MASTROLiA A N D K L A G E R

Ρolybutadiene

Binders

123

containing between 82 and 88% solids have extended the capability of solid propellant technology markedly. A range of compositions for typical high energy propellants is shown i n Table I. This paper discusses the three butadiene prepolymers which have been used most extensively i n solid rocket propellants—i.e., the copolymer of butadiene and acrylic acid ( P B A A ) , the terpolymer of butadiene, acrylic acid, and acrylonitrile ( P B A N ) , and the carboxyl-terminated poly butadiene ( C T P B ). Since the chemistry of all of these carboxyl-containing prepolymers is essentially the same, the discussion of butadiene propel lants i n this paper is concerned mainly with those based on C T P B .


Table I.

Compositions of High Energy Butadiene Propellants Ingredient

Weight %

Ammonium perchlorate Aluminum Butadiene prepolymer Stabilizers Curing agent(s)

60-84 2-20 12-16 0-1 0.2-1.0 100

Historical Development PBAA. The first butadiene prepolymer to be used i n a rocket motor application was the liquid copolymer of butadiene and acrylic acid ( P B A A ) . It is synthesized by a free radical emulsion polymerization technique to an average molecular weight of 3000 and an average func tionality of 2. Owing to the method of preparation, the functional groups are distributed randomly along the chain, and the number of functional groups per molecule varies over a wide range. The liquid prepolymer is therefore a mixture of nonfunctional, monofunctional, difunctional, and polyfunctional molecules which exhibits a range of molecular weights. As a result, the epoxide-cured binders and propellants prepared with P B A A show poor reproducibility of mechanical properties. The low viscosity of the liquid P B A A prepolymer at the propellant processing temperature permitted the preparation of propellants with higher solids than had been possible with other binders. However, the postcure during storage and the poor mechanical behavior of the propellants ( caused by the poor spacing of functional groups and functionality distribution of the prepolymer) led to the discontinuation of this material i n solid rocket propellants. PBAN. The mechanical behavior and storage characteristics of buta diene propellants were improved by using terpolymers based on buta-

In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


124

PROPELLANTS MANUFACTURE,

HAZARDS,

A N D TESTING

diene, acrylonitrile, and acrylic acid ( P B A N ) . This liquid terpolymer was prepared by the same emulsion polymerization technique used for P B A A ; however, the introduction of the acrylonitrile group probably improved the spacing of carboxyl species, which could be a factor i n the more reproducible propellant cures and mechanical properties. Propellants based on P B A N also showed a lesser tendency to surface harden, which is caused by oxidative attack at the double bonds and is known to be suppressed i n nitrile rubbers ( 1 ). Typical examples of the effects of surface hardening on the uniaxial tensile properties of solid propellants (JANAF-Instron (4) tensile specimens) can be seen i n Table II. The propellant based on P B A N shows little change in hardness upon exposure to 220°F. i n an oxygen atmosphere for 96 hours as opposed to the strong increase i n hardness observed for propellants based on a non-nitrile containing prepolymer ( C T P B ) . Table II.

1

Effect of 220°F. Storage on Propellant Properties"

„ Propellant Type

„ Storage Time, hrs.

Shore A Hardness Internal Cut

Surface

PBAN

0 96

40 49

40 47

CTPB

0 96

47 57

47 81

Oxygen atmosphere, 10 p.s.i.g.

Since the development of this terpolymer i n 1957, more solid composite rocket propellant has been produced from P B A N than from any other single prepolymer. Propellants based on this material have been used successfully i n applications ranging from small tactical motors to the 260-in. diameter motor containing greater than 2,000,000 lb. of propellant. P B A N propellants, therefore, have been and are expected to be a major factor i n making solid rocket motors during the next several years. The thermal stability which has been achieved, low temperature cycling characteristics, and low cost of propellants based on this prepolymer make this system attractive. CTPB. Ultimately, prepolymers were developed with carboxyl groups i n the terminal positions to take full advantage of the entire length of the polymer chain. These butadiene prepolymers were synthesized by a free radical- or lithium-initiation technique to an average molecular weight of 3500-5000 and a nearly bifunctional structure. These attributes provided for substantially improved mechanical behavior of highly loaded solid propellants, particularly at low temperatures. Pro-


6.

125

Polybutadiene Binders

AND KLAGER

MASTROLiA

pellants based on C T P B are therefore used i n rocket motor applications in preference to P B A A or P B A N propellants, wherever stringent mechani cal property requirements are imposed. Chemistry of Carboxyl-Containing Polybutadiene Prepolymers PBAA and

PBAN.

SYNTHESIS.

P B A A and P B A N are prepared by


an emulsion polymerization process initiated by a free radical mechanism. Using a quaternary ammonium salt as the emulsifier and azobisisobutyronitrile as the initiator, the reaction for the synthesis of P B A N proceeds according to: CH3

CH3

Γ

ι

N = C - C -

+ N

( M ) , + (N), + 2CHo=CH—COOH

2

Ν=C—C—N=N—C—C=Ν •

I

I

CH PBAN-

CH

3

3

CH

:i

chain - Ν == C — C — ( M ) — C H — C H — ( Ν ) — C H — C H · termination COOH COOH CH , 2

2

2

:

where M = χ moles of butadiene, N = y moles of acrylonitrile, and for P B A A , N = 0. In this process, the butadiene-acrylic acid-acrylonitrile ratio can be varied over a wide range. In general, there is an average of two carboxyl groups per molecule and an average of 6 % by weight of cyano groups. The acrylic acid and acrylonitrile are fed into the reactor containing the butadiene emulsion either intermittently or at a constant rate. The reaction is allowed to proceed until the molecular weight is i n the range 2000-4000, at which time chain termination is effected, usually with a mercaptan. After termination of the polymer, the emulsion is broken, the antioxidant is added, and the product is purified by a water wash and vacuum drying. Typical properties of P B A A and P B A N are shown i n Table III. [ A l l materials are used as received unless otherwise noted.] x

y

y

Table III.

Physical Properties of PBAA and PBAN

Molecular weight Viscosity at 25°C, poise Density, grams/cc. AH , kcal./gram C

PBAN

PBAA

3000-4500 300-350 0.93-0.94 9.9-10.1

2500-4000 275-325 0.90-0.92 10.2-10.4


1

126

PROPELLANTS MANUFACTURE, HAZARDS, AND TESTING

C U R E REACTION. PBAA and PBAN are cured to a firm three-dimen sional structure by reaction with difunctional epoxides or aziridines: —CH —CH=CH—CH —CH —CH — 2

2

2

2

COOH + R— ( C H — C H ) 2

2

\ / Ο

— C H 2—CH=CH—CH — C H — C H — 2

2

2

C—Ο—CH — C H — R — C H — C H 2

I


II or

2

Ο

I

OH

OH

^ÇH —CH —CH=CH—CH 2—CH —CH 2 — COOH + R—( / N ^ I CH / ο 2

2

2

2

CH —CH=CH—CH —CH —CH — 2

2

2

2

C—O—CH —CH —N—R—N—CH —CH 2

2

O

2

Η

2

Η

where R is the curing agent radical. Because PBAN or PBAA prepolymers contain a sufficient quantity of poly functional species, a firm gel is formed without using a supplementary crosslinking agent. However, in preparing propellants, the presence of the ammonium perchlorate oxidizer causes side reactions involving the curing agents, which leads to the formation of a less complete three-dimensional network. This is discussed in greater detail in the section on curing agents for carboxyl-terminated polybuta diene prepolymers. C T P B . SYNTHESIS. Free Radical-Initiated Prepolymers. There are two principal methods for preparing the free-radical-initiated prepoly mers. The first method uses glutaric acid peroxide (2) as the initiator and follows the reaction scheme shown below: HOOC—R—C—O—O—C—R—COOH

CTPB-

->

'°™'"»»°" H O O C - R - ( M ) . coupling

2HOOC—R- + 2C0

^


2

6.

Ρolybutadiene

MASTROLiA A N D K L A G E R

127

Binders

where M is χ moles of butadiene, and R = C H G . This polymerization technique may be extended to produce functionally terminated copoly mers of butadiene and acrylonitrile. 8

x

The second method (12) uses 4,4'-azobis-4-cyanopentanoic the initiator according to the following synthetic scheme: CN

C N

H O O C — C H — C H — C — N = N — C — C H — C H — C O O H 2

acid as

2

2

CHÎJ

2

->

C H 3


CN

I 2HOOC—CH —CHa—C- + N 2

2

I

CH

3

CN CTPB •

2HOOC-CH -CH -C-(M),I

i o n

2

coupling

2

CH

3

where M is χ moles of butadiene. So far, most propellant work has been conducted with the glutaric peroxide-initiated prepolymer. Lithium-Initiated Prepolymers. The preparation of prepolymers b y the organolithium technique (11) follows the reaction: x

CH =CH—CH=CH 2

Li—R—Li 2

L i — ( C H o — C H = C H — C H ) — ( C H — C H )m — L I CH=CH 2

n

2

i

-

2

HOOC ( C H — C H = C H — C H ) — ( C H — C H ) — C O O H 2

2

n

2

m

^

CH^Co This process provides a prepolymer with a narrow molecular weight distribution, but the mean molecular weight can be varied over a wide range, and the process may also be adjusted to achieve various microstructures (cis, trans, and vinyl). O n the other hand, prepolymers pre pared by the free radical initiation technique generally exhibit a broad molecular weight range and a somewhat branched structure.


128

PROPELLANTS MANUFACTURE, HAZARDS, AND TESTING

Table IV.

Curing Agents for CTPB Propellants Properties

Epoxides Name ERLA-0510


Purity, %

Structure Ν— ( C H C H — C H ) \ / Ο 2

Epon X-801

2

2

CH CH—CEU 2

r V

Propellants Manufacture, Hazards, and Testing

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