Nitro esters, such as nitrocellulose (NC), have been extensively used as propellants due to some unique characteristics, such as their high energetic potential. However, over time, several catastrophic accidents resulting from the NC decomposition, have showed that the chemical stability is an important aspect to ensure a safe use and storage of propellants (de Klerk, 2015; Defanti et al., 2020). This NC degradation was identified as being intrinsic to the nitro esters, depending on temperature and time, and happens mainly due to denitration reactions based on the chemical detachment of –NO₂ groups caused by thermal decomposition and hydrolysis reactions involving the residual moisture present in the propellant mass (Bohn, 2007). The products formed during this decomposition are mostly NO₂, NO, HNO₃ and HNO₂, which promote, by their turn, consecutive hydrolysis reactions in an auto-catalytic process of degradation. As the decomposition process is exothermic in nature, the rate of degradation of the propellant increases with temperature (Vogelsanger, 2004), and such synergetic processes can even lead to the spontaneous combustion of the material. Therefore, chemical stabilizers are necessary to ensure an adequate shelf-life of NC-based propellants, by capturing the generated radicals or by interrupting, over a period of time, the auto-catalytic process of decomposition. The most commonly used stabilizers are diphenylamine (DPA), ethylcentralite (EC), methylcentralite, akardite-II (AK-II) and 2-nitrodiphenylamine (2-NDPA). However, as these compounds are amines and secondary amides, the degradation process leads to the formation of R–N–N=O (where R is an alkyl radical), which are suspected of being carcinogenic (Lin, 1990).
DPA is the most used stabilizer for single-base gunpowder and is quite standard for small guns ammunition. It has been cited by the European Union as a recalcitrant pollutant, leading to the development of standards to evaluate and control the environmental risks associated with its use (Mendonça-Filho et al., 2019). Substances produced within the stabilization process, especially N-nitroso-DPA, which is a major product formed during the degradation of NC-based propellants, have carcinogenic, mutagenic and toxic to reproduction properties, according to toxicological tests (Jain et al. 2020; Rodrigues et al. 2018).
Mestankova et al. (2014) reported that N-alkyl-nitrosamines are known to have a common mutagenic mechanism, triggered by the enzymes of the cytochrome P450, which increase the polarity and water solubility of the substances, through the hydroxylation of the carbon adjacent to the N-nitroso group (Figure 1), which is the case of DPA and 2-NDPA:
Araújo et al. (2000) also reported that N-nitroso-amides, which are the degradation products of EC and AK-II, undergo a hydrolysis process which is similar to the N-nitroso-hydroxylamine decomposition (Figure 2), producing diazo-alkanes that are able to interact with DNA.
The use of natural substances as stabilizers for energetic materials comes up as an excellent alternative to reduce environmental impacts, besides the fact that they are prone to the generation of products that are less harmful to humans. This is particularly important in training facilities, destruction sites and, of course, in manufacturing plants, where ammunition is, respectively, employed, decommissioned and produced for decades. In this context, some studies have suggested the complete replacement of traditional stabilizers by "green" alternatives, especially those that can be found in natural products (Langlet et al., 2007; Dejeaifve et al., 2018).
Moving on this direction, Krumlinde and coworkers (2017) investigated new stabilizers that were not prone to form by-products containing nitrosamines, concluding that bis(2,6-dimethoxyphenyl)triethyleneglycol was the most promising one. Cherif and co-workers (2020a, 2020b) also studied natural stabilizers, lignin in particular, due to its availability via sustainable processes. The aromatic rings present in this amorphous biomacromolecule, exhibit great potential to sequester NO₂ groups during NC degradation, being an attractive option. Chelouche and coworkers (2019) studied a binary eutectic mixture of N-(2-methoxyethyl)-p-nitroaniline (MENA) and DPA, aiming to minimize the number of toxic products resulting from the decomposition of DPA. This molecule was considered compatible and functional as a stabilizer for NC-based propellants. Dejeaifve and co-workers (2020), in adition to their previous works, analyzed potential (natural) substances, such as alpha-tocopherol, alpha-ionone and curcumin for replacing DPA, using Heat Flow Microcalorimetry (HFC) to predict their shelf life when compared to AK-II. In previous works of our research group, we also found promising results for NC-based propellants stabilized with 1.0% of curcumin (Rodrigues et al., 2019, 2021), while very recent research on the use of alpha-ionone as stabilizer (Damseaux et al., 2021), has demonstrated its effectiveness, with the main derivative products identified as: 3-oxo-α-ionone, 4-oxo-β-ionone and 4,5-epoxy-α-ionone.
Meaning to continue contributing to the discover of new green stabilizers, the main goal of the present work was to propose a natural stabilizer as a substituent for synthetic stabilizers that would exhibit equal or superior stabilizing properties. Guaiacol was chosen due to the fact that this molecule does not exhibit amine or amide groups, which indicates that no carcinogenic, mutagenic or toxic to reproduction derivative will be formed during the stabilization process. Another important aspect is the possibility of improving the propellant shelf life, searching for formulations capable of lasting as long as 10-15 years, which is more elastic than the ones related to conventional ammunition. This can warrant resource savings in several aspects, once it lowers the replacement rates besides demanding lesser testing and destruction operations.
Guaiacol (Figure 3), the common name of 2-methoxyphenol, C6H4(OH)(OCH3), is an organic compound that, in its pure state, is found as an oily and colorless liquid, with molar mass of 124.137 g/mol and a melting point of 28°C. It is water soluble (17 g/L at 15 ºC) and can be found in guaco (Mikania glomerate) leaves and in wood-tar creosote.
It has been reported that the O-H and O-CH3 bonds within the guaiacol chemical structure present the weakest bond dissociation energies (BDE), ranging from 87 to 58 kcal mol-1, while the C-H bond in the aromatic ring present the higher BDE, of around 115 kcal mol-1. Thus, the more reactive groups (–OH and –OCH3) should allow bimolecular H-abstractions by free radicals, acting as potential stabilizers (Nowakowska et al., 2018).
Guaiacol has been cited (Hartman & Morton, 1981) as a potential NC stabilizer along with many other molecules exhibiting at least one aromatic ring substituted with alkoxy groups, such as: 1,3,5-trimethoxybenzene, 2-methoxy-naphthalene (nerolin), catechol, 1,2 dimethoxy benzene (veratrol), and 1,4 dimethoxy benzene (hydroquinone dimethyl ether). No direct investigation has been conducted yet on the products that could be formed during the degradation process of NC stabilized by guaiacol, but Kroflič and co-workers (2015) reported that the guaiacol originated from biomass burning, can interact with nitrite present in the atmosphere, resulting in 4-nitroguaiacol, 6-nitroguaiacol and, after a short interval of time, 4,6-dinitroguaiacol (Kroflič et al., 2015).
In order to investigate the substitution of DPA by guaiacol as a stabilizer we accomplished a set of six performance tests employed for comparison. The stability was first evaluated through HFC, three classical stability tests (Bergmann Junk test, German test and Storage Proof), pressure vacuum stability test (PVST), and ballistic parameter estimation using a closed vessel device along with a regression tool developed by our research group. Finally, the guaiacol concentration depletion after a 10/15-years artificial aging was analyzed using high performance liquid chromatography coupled with ultra-violet detection (HPLC/UV).
Aiming the best analytical scrutinization of the NC propellants under study, the techniques were employed and analyzed in conjunction in order to complement each other, once all present pros and cons. The PVST monitors the evolved gas from a fixed-volume system, being a reliable and accurate test for stability. HPLC/UV was used to determine the remaining amount of the stabilizer after an induced/artificial ageing protocol and a solvent extraction procedure, being crucial for inferring on the propellant shelf life. However, even though being expedite and effective techniques, there is room for misinterpreting the results, due to the presence of the products of the reaction between the stabilizer and any of the products of the NC degradation. The traditional stability tests, such as the Bergmann-Junk Test, the German Proof and the Storage Test are also based on analysis performed during an artificial ageing of the material that follow different protocols of time and temperature, according to the kind of propellant. These tests exhibit some intrinsic deficiencies such as exhibiting a long measuring period and being too dependent on the experimenter skill and on the quality of the indicator paper employed. Besides, the tests are not able to mimic the conditions in a real ammunition and unstable products and/or impurities can interfere in the results (Trache and Tarchoun, 2019).