1,4-Benzoquinone Dioxime: Cross-Linking & Curing Agent

1,4-Benzoquinone dioxime is an important organic cross-linking agent and curing aid. In ammonium perchlorate composite propellants based on nitrile rubber (NBR), it is commonly used as a highly effective curing agent. By cross-linking to form a dense three-dimensional network, it improves the mechanical properties and structural integrity of the propellant, whilst simultaneously reducing the curing temperature and shortening the curing cycle. This substance also possesses certain antioxidant and stabilising properties. 1,4-Benzoquinone dioxime is widely used in the preparation of composite solid propellants, specialised rubber products and high-temperature-resistant elastomers, and serves as a key functional additive in energetic materials and polymer curing systems.

Dehydrogenation of 1,4-benzoquinone dioxime

Sterically hindered phenols, widely used for the inhibition of various chemical processes, are also used in the vulcanization of rubbers. However, a detailed mechanism of their action remains un- clear. In the present paper it was shown on model reactions that phenolic inhibitors react with the agent used to vulcanize butyl rubber, namely 1,4-benzoquinone dioxime (QDO), to give various compounds, the amount and structure of which is quite dependent on the structure and stability of the corresponding phe phenoxyl radical. If the phenoxyl is capable of rapid transformations, then a complex mixture of products is formed. Employing TLC, a comparison of the composition of the reaction mixture, obtained by the reaction of QDO with 2,6-di-tert-buty1-4-methylphenol (or with 2-methyl-4-tert-butylphenol),with the results of the transformations of the individual components under the experimental conditions disclosed that the reaction mixture contains the corresponding phenoxyl radicals, as well as the unknown products of their re- action with QDO, as the oxidation products. If the phenoxyl radical is stable in the absence of oxygen and does not undergo either recombination of disproportionation, then, as a rule, only one reaction product is formed with 1,4-Benzoquinone dioxime.[1]

It is known that 2,4,6-tri-tert-butylphenoxyl easily dehydrogenates 1,4-Benzoquinone dioxime to give the quinol ether (I).A detailed study of this reaction disclosed that it obeys a strict stoichiometric ratio of the reactants equal to 4:1. The formed phenol is easily isolated from the filtrate after the recrystallization of(I).When the reactant ratio is 2:1 a half of the taken QDO is recovered unchanged.If an oxidizing agent (MnO2,PbO2) is added to the reaction mixture, then the formed phenol is oxidized anew to the phenoxyl radical and reacts with the 1,4-Benzoquinone dioxime. With this modification of the method the ratio of the starting components is 2:1. In the presence of an oxidizing agent the reaction proceeds easily at room temperature in either THF or dioxane, in an argon stream, to give (I)after recrystallization in ~70%yield.Other reaction products were not detected. When the reaction is run in the air the yield drops to 50-55%,which is as- sociated with the side consumption of the radical for the formation of the quinolide peroxide. The dehydrogenation of 1,4-benzoquinone dioxime by phenoxyl radicals was studied and it was shown that the quinol ethers are formed as the sole reaction product when stable phenoxyls are used. A universal method was developed for the synthesis of the indicated ethers.

Curing of Aluminized AP Propellant with 1,4-Benzoquinone Dioxime

The isocyanate-based curing agents used for polyurethane are toxic and hygroscopic in nature. In the present work, an alternate approach was adopted, a reaction between the unsaturated rubber having an α -methylene hydrogen atom and a dinitrosobenzene (DNB) ‒ generating system (quinol ether of 1,4-benzoquinone dioxime, QE) without a catalyst, thus generating a cured system. QE is a novel curing agent for propellant applications which imparts the necessary curing. The curing reaction between nitrile butatadiene rubber (NBR) and quinol ether (QE) was studied by FTIR and the results revealed the formation of anil groups (Ar–C=N). The anil group results from the reaction between NBR and DNB, generated on decomposition of 1,4-Benzoquinone dioxime. Propellant formulations were prepared with variation of the curing agent from 0.2 to 0.5%. The composition and rheological, mechanical, ballistic and thermal properties of the resulting cured systems were investigated. The viscosity and spreadability were suitable for casting. The tensile strength, modulus, and hardness show an increasing trend and the elongation decreases on varying QE from 0.2 to 0.5% in the propellant. However, all of the compositions showed nearly the same burning rate and pressure exponent. The 1,4-Benzoquinone dioxime based curing system is non-hygroscopic and has extremely low toxicity. The experimental results revealed that the proposed curing agent may find application in explosives and propellants.[2]

References

[1]Kashel'skaya, I. V., Sergeeva, A. A., Nesterenko, V. I., & Shifris, G. S. (1974). Dehydrogenation of 1,4-benzoquinone dioxime by stable phenoxyl radicals. Bulletin of the Academy of Sciences of the USSR, Division of Chemical Science, 23, 674–675.

[2]Singh, S., Raveendran, S., Kshirsagar, D., Gupta, M., & Bhongale, C. (2022). Studies on Curing of an Aluminized Ammonium Perchlorate Composite Propellant Based on Nitrile Butadiene Rubber Using a Quinol Ether of 1,4-Benzoquinone Dioxime. Central European Journal of Energetic Materials.

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