Stabilisation of isocyanate cross linked polybutadiene binder

Abstract

Isocyanate cross-linked hydroxy terminated polybutadiene is used as a
binder for solid rocket propellant. Rocket motors containing this propellant
require a storage life of at least 20 years. During storage it has been found that
the important rubbery properties of the binder can be lost due to oxidative cross-linking of the polybutadiene chains. This could cause catastrophic failure when the rocket motor is required. At present the bis-hindered phenol Calco 2246 is used as a thermal oxidative stabiliser, but it's performance is only adequate. This has led to the search for a more efficient stabiliser system.
To hasten the evaluation of new antioxidant systems the use of dynamic
thermal analysis was investigated. Results showed that a tentative relationship
existed between predictions by thermal analysis and the long term oven ageing
for simple single antioxidant systems. But for more complex systems containing
either autosynergistic or mixed antioxidants no relationship was observed
suggesting that results for such an "accelerated" technique cannot be used for the
purpose of extrapolation for long term performance. This was attributed to the
short time and more aggressive condition used (hjgher temperature and oxygen
rich atmosphere in thermal analysis) altering the mechanism of action of the
antioxidants and not allowing time for co-operative effect of the combined
antioxidant system to form.
One potential problem for the binder system is the use of an diisocyanate as a cross-linking agent. This reacts with the hydroxyl hydrogen on the polymer
as well as other active hydrogens such as those contained in a number of
antioxidants, affecting both cross-linking and antioxidant effectiveness. Studies
in this work showed that only antioxidants containing amine moieties have a
significant affect on binder preparation, with the phenolic antioxidants not
reacting. This is due to the greater nucleophilicity of the amines.
Investigation of a range of antioxidant systems, including potentially
homo, hetero and autosynergistic systems, has highlighted a number of systems
which show considerably greater effectiveness than the currently used
antioxidant Calco 2246. The only single antioxidant which showed
improvement was the partially unhindered phenol y-Tocopherol. Of the mixed
systems combinations of the sulphur containing antioxidants e.g. DLTP with
higher levels of chain-breaking antioxidants, especially Calco 2246, were the
most promising. Also the homosynergistic mix of an aromatic amine and a
phenol was seen to be very effective but the results were inconsistent. This
inconsistency could be explained by the method of sample preparation used. It
was shown that the efficiency of a number of antioxidant.s could be dramatically
improved by the use of ultrasound during the mixing stage of preparation. The
reason for this increase in performance is unclear but in the case of the
homosynergistic amine/phenol mix both more efficient mixing and/or the
production of a novel mechanism of action are suggested

Date of Award	Apr 1998
Original language	English
Supervisor	Sahar Al-Malaika (Supervisor) & A. Davis (Supervisor)