PBAA Rubber
Polybutadiene Acrylic Acid.
Almost concurrently with the evolution of the polyurethane propellants, a new
type of binder based on long-chained polybutadiene backbone gained the attention
of rocket manufacturers. The selection of polybutadiene binder for propellants
to be used over wide temperature ranges was a natural one, since most butabiene
copolymers (butadiene-styrene, butadiene-acrylonitrile, butadiene- methylvinyl
pyridine) had glass-transition temperatures near or below -100°F. This was
advantageous, since the mechanical behaviour of a propellant during periods
of strain was related to its properties at different temperatures. For example,
the ability of a propellant to withstand high strain rates such as those encountered
on ignition of a large-diameter rocket motor was directly related to low-temperature
properties such as elongation and brittle point. Therefore, the polybutadiene
propellants were attractive, both for large motors and for those requiring wide
temperature ranges of operation.
Probably the most widely used polybutadiene polymer (1967) had been PBAA, a
copolymer of polybutadiene and acrylic acid. One of the added benefits of PBAA
over polyurethane was that the binder system was less complex, consisting essentially
of the liquid polymer and a single curative chemical such as an epoxide resin.
In certain formulations where a large amount of oxidiser and an auxiliary metal-powder
fuel such as aluminium was needed to provide high energy, it was necessary to
add to the binder a liquid hydrocarbon or other low-viscosity fluid that acted
as a plasticiser to aid in processing.
By contrast, the polyurethane system normally consisted of a main chain polymer such as a polyether diol, a shorter chain cross-linking agent (perhaps a trifunctional polyol), a curing agent (isocyanate) and a curing catalyst; polyurethanes also employ plasticisers where necessary, usually in the form of aliphatic esters such as dioctyladipate or dioctylsebacate.
