ReviewRecent research progress in the synthesis and properties of burning rate catalysts based on ferrocene-containing polymers and derivatives
Graphical abstract
An excellent solid propellant can be achieved by adding burning catalyst into the propellant. In this review, the recent progress in synthesis of ferrocene based polymer and derivatives, several pioneering methods, catalytic and migration mechanism along with migration as a barrier in this field is discussed.
Introduction
Rocket technology research is of great significance to the development of aerospace industry. Propellants are one of the most important ingredients to provide a driving force for rockets to escape the earth gravity. An excellent solid propellant should have an extremely stable burning rate and a low pressure exponent. To achieve this aim one of the best ways is to add a burning rate catalyst into the propellant [1]. Nowadays, burning rate catalysts mainly include transition metal oxides [2], [3], [4], [5], nano-metal particles [6], [7], [8], metal chelates [9], [10], Fc-based polymers and derivatives [11], [12], [13], [14], [15], [16]. Among them, Fc-based polymers and derivatives have been widely used because they have extraordinary effects in enhancing the burning rates of butyl hydroxide propellants containing ammonium perchlorate (AP) which is the most common oxidizer and has been widely used in composite solid propellants (CSPs) [17], [18], [19]. Recently, the thermal decomposition of AP and the combustion properties of solid propellants have been extensively studied by taking advantages of the catalytic activities of Fc-based polymers and derivatives [11], [12], [13], [14], [15], [16]. However, the previously synthesized BR catalysts display undesired properties of anti-migration, which leads to uneven combustion and deteriorate the resistance to aging with the formation of highly sensitive boundary layers [12], [13]. Much attention has been paid to obviate these problems and to improve the efficiency of Fc-based BR catalysts [14], [15], [16], [19], [20], [21].
Fc, with an iron atom linked to two cyclopentadienyl (Cp) ligands, was discovered in 1951 [22], [23]. Fc is apt to derivatize and has attracted increase interest in polymer field since early 1970s [24]. Due to the unique structure and aromatic character of Fc, Fc-based polymers and Fc derivatives have wide applications, such as burning rate catalysts [11], [12], [13], [14], [15], [16], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Therefore, the number of publications related to the synthesis and applications of Fc-derivatives and Fc-based polymers exhibits an exponential increase in the past 20 years [28], [29], [30], [31], [32], [33].
Much attention has been paid for the synthesis of novel Fc-based polymers and derivatives serving as BR catalysts, and the catalysis mechanism of these materials has been discussed [11], [12], [13], [14], [15], [16], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Therefore, this contribution mainly reviewed recent research progress in the synthesis of Fc-based polymers and Fc derivatives serving as BR catalysts. In addition, some approaches recent developed, to improve the BR catalytic properties of Fc-based polymers and Fc derivatives, also is reviewed. The catalytic properties and the anti-migration mechanism of Fc-based polymers and derivatives are discussed as well.
Section snippets
Synthesis of Fc-based polymers and their burning rate catalytic properties
Fc-based polymers used as BR catalysts are generally classified to two classes. One is main-chain Fc-based polymers (with Fc unit as an integral part of the polymer backbone) and another one is side-chain Fc-based polymers (with Fc unit as a pendant group) [34].
Synthesis of Fc derivatives and their burning rate catalytic properties
Early researchers have discovered the BR catalytic properties of several Fc derivatives such as HTPB and carboxyl-terminated polybutadiene (CTPB) propellant. HTPB and CTPB were found to be more effective BR catalysis than Fe2O3 and ferric ferrocyanide accelerant [2], [3], [4], [5], [15], [16]. Recently, the functionalization of Fc with reactive groups such as hydroxyl or amino groups increased the polarity of the resulting compounds to overcome the migration phenomena.
In 1969, Pittman firstly
BR catalytic mechanism of Fc-based polymers and Fc derivatives
The BR catalytic mechanism of Fc-based polymers and derivatives is not clear till now [82], [83], [84], [85], [86], [87], [88], [89], [90]. Generally, there exist four catalytic mechanisms: electron transfer mechanism, proton transfer mechanism, forming transition metal materials mechanism and acid–base interaction mechanism. Fc and Fc cation are a good redox pair and their redox potential depends on the properties of the substituent on the Fc ring.
K. Kishoreand co-workers [87] studied the
Anti-migration mechanism of Fc-based polymers and Fc derivatives
Fc-based polymers and Fc derivatives, as BR catalysts, are prone to migrate in the propellant system, which leads to uneven distribution of BR in propellant grains and thus affects the performance of the propellant. There are two possible mechanisms for the migration: (a) diffusion mechanism; (b) surface migration mechanism. For the first case, macroscopic directional migration is caused by Fc derivatives diffusion (entropy increase process) due to the presence of concentration gradient (Fig. 1
Summary
Fc and its derivatives are promising candidates for BR catalysts and are widely used in propellant systems. This contribution mainly reviewed the recent research progress in the synthesis, functionalization and applications of BR catalysts of Fc-based polymers and Fc derivatives. Though notable advances have been achieved in the synthesis of Fc-based polymers and Fc derivatives serving as BR catalysts, defects of migration and easy volatilization, hampered the catalytic performance of the
Acknowledgments
Financial support by International Science and Technology cooperation Project of Ministry Science and Technology of China (2009 DFR 40640), National Natural Science Foundation of China (21072175), Natural Science Foundation of Zhejiang Province (Z4100291), Science and Technology Program of Ningbo (2009D10005), Science and Technology innovation team of Ningbo (2011B82002) and Foundational Research Funds for the Central Universities are gratefully acknowledged.
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