Appl. Rheol. 2020; 30:14–26 Research Article Le Qi, Zhongliang Ma*, Jiahao Liang, and Zhongliang Xiao Effect of Solid Additives on the Rheological Property of Nitroglycerin Plasticized Nitrocellulose https://doi.org/10.1515/arh-2020-0002 equations will form a contribution to the research on ex- Received Jul 02, 2019; accepted Feb 21, 2020 trusion process of this energetic material containing Cy- clotrimethylenetrinitramine (RDX) and graphene, and the Abstract: The rheological properties of energetic materials results obtained by this research have certain practical sig- comprising nitroglycerin plasticized nitrocellulose were nificance of the extrusion process for this energetic mate- studied using rheological tests in a parallel plate rheome- rial. ter. The Carreau-Yasuda equation was applied to calcu- late the zero-shear viscosity, and the dependence of solid Keywords: Rheometer, Zero-shear viscosity (ZSV), Viscous additives, temperature and solvent content on zero-shear flow activation energy, Carreau-Yasuda equation, master viscosity was developed. One can study flow character- curve istics of the energetic materials by observing the zero- shear viscosity instead of the effect of solid additives, tem- perature and solvent content. Additionally, the relation- ship between zero-shear viscosity and additives concen- 1 Introduction tration was studied. The Kissinger-Akahira-Sunose (KAS) Energetic materials can be defined as metastable materi- method was used to obtain the viscous flow activation als with high energy density containing a high amount of energy, and the equation to describe the relationship be- stored chemical energy that can be released. Nitrocellulose tween solid additives concentration and viscous flow ac- (NC) is an important energetic material for some specific tivation energy was represented. The Zero-Shear Viscos- applications, such as explosive and propellant. Its com- ity (ZSV) test showed that temperature was the predomi- bination with nitroglycerin (NG) makes it possible for a nant effect on the ZSV value at low solvent content, asthe broader range of applications [1–4]. Mixtures of NC and NG concentration of solid additives increased, the ZSV value show good performance in terms of thermal stability and decreased at low solvent content but increased at high energy performance [5]. Extrusion molding is the preferred one, however, there is an opposite trend when graphene method for processing mixtures of NC and NG, and numer- concentration is above 0.1%. The viscous flow activation ous works have been completed to study the process [6, 7]. energy showed different changing trends with solid con- While most previous works have studied the process from centration that increased at different solvent content. The the extrusion step to forming, only a few studies have fo- master curves were obtained by Time-Temperature Equiv- cused on the flow properties of the materials. Rheology is alence Principle, the viscosity prediction model has been affected by factors such as temperature, shear rate andthe established and showed a good agreement with the ex- concentration of additives. In order to explore the relation- perimental data, compared with the test results, the vis- ship between rheological properties of materials and these cosity prediction model is more accurate at low tempera- factors during extrusion, rheological method is used to in- ture (15∘C-25∘C). The obtained knowledge of the different vestigate the viscosity change of this material during the extrusion process. While apparent viscosity of a NC and NG mixture, as *Corresponding Author: Zhongliang Ma: School of Environment a non-Newtonian fluid, is one of the important factors in and Safety Engineering, The North University of China, Taiyuan the operability of the extrusion process, for solid-like en- 030051, China; Email: [email protected] ergetic materials it is in most instances replaced by com- Le Qi, Jiahao Liang: School of Environment and Safety Engineering, plex viscosity [8–10]. The relationship between tempera- The North University of China, Taiyuan 030051, China ture and viscosity is very well described by the Arrhenius Zhongliang Xiao: School of Chemical Engineering, The Nanjing University of Science and Technology, Nanjing 210094, China equation which has been shown to be valid for a broad Open Access. © 2020 L. Qi et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License Effect of Solid Additives on the Rheological Property of Nitroglycerin Plasticized Nitrocellulose Ë 15 range of non-Newtonian fluids and solid-like (viscoelas- nitroglycerin plasticized nitrocellulose. Adding graphene tic) fluids. The Zero-shear viscosity (ZSV) is a phenomeno- into a propellant can improve its mechanical properties, logical parameter that is derived, actually extrapolated, such as tensile strength; while adding RDX to a propel- from the shear viscosity as a function of shear rate curves lant can improve its energy performance. There is no sys- (rheogram). The ZSV value can be calculated from shear tematic research on the rheological properties of graphene rate or strain rate curves. In addition, ZSV value is calcu- and RDX added to nitroglycerin-plasticized nitrocellulose. lated under the same shear rate or shear stress, while fre- The purpose of this work was to (a) provide a funda- quency or shear rate scanning are more suitable for solid- mental investigation of the rheological properties of nitro- like materials. ZSV is a time-independent parameter that cellulose material plasticized by nitroglycerin, and (b) ob- reflects the microscopic properties of a fluid and isanim- tain a viscosity predictive model as an analytical reference portant basic parameter for evaluating fluid viscosity and for the extrusion process of such material. The ZSV and vis- chemical properties of polymer solutions. ZSV is a com- cous flow activation energy parameters were investigated mon parameter for evaluating the end-use of polymers is to define the flow characteristics of the energetic mate- in food, road construction and with other polymeric ma- rial, while the master curve was obtained by a rheological terials to characterize the flow characteristics during their method, useful in investigating the viscosity-temperature processing, such as obtaining starch intrinsic viscosity at dependence of the material. The analytical method was different temperatures or evaluating the conditions of us- chosen to get valuable information to optimize the extru- ing asphalt [11–16]. In the evaluation system for the per- sion process and the end-use properties of the final prod- formance of asphalt at different shear rates and tempera- uct. tures, the zero-shear viscosity is used to replace the pre- vious G*/ tan δ as the evaluation criterion [17]. Some re- searchers have explored starch solutions at different con- 2 Theoretical background centrations, hence the values of ZSV were obtained by the Carreau-Yasuda equation, providing a fundamental back- Viscometry is one of the most widespread methods to in- ground on the starch-ionic liquid rheology [12]. Since zero- vestigate and characterize the flow characteristics of ener- shear viscosity is difficult to measure directly, many re- getic materials, yielding significant information for their searchers have proposed different models to calculate the extrusion process in engineering. Viscometry and rheolog- zero-shear viscosity, such as the Power Law, Cross, and ical measurements have been widely used in investigating Carreau-Yasuda equations. Because the models have differ- engineering and polymeric liquid materials, while more ent scope of application and precision, one should choose and more works focus on the applications of energetic ma- the appropriate model for calculations of ZSV according to terials, hence operation of extrusion processes is mani- their needs [18–25]. Also, the viscous flow activation en- fested by their rheological parameters, such as zero-shear ergy can be used to characterize the dependence of viscos- viscosity (ZSV) and viscous flow activation energy. ity on temperature, reflecting the viscous flow characteris- From the rheology point of view, the energetic mate- tics of a material and its temperature dependency [26, 27]. rial in our study belongs to the pseudoplastic fluids class, From viscosity curves and the viscous flow activation en- for which, when flow is very slow, shear viscosity iscon- ergy, master curves can be obtained. It is possible to obtain stant, and its value representing the ZSV. Subsequently, flow information over a wide temperature and shear rate as shear rate increases up to a certain value, viscosity de- range with a small amount of experimental data using mas- creases. The flow curves are generally divided into three ter curves, which is very advantageous for material charac- stages: at the initial phase stage, shear rate approaches terization [28–32]. Although beneficial trials showed that zero (훾˙ ! 0), shear viscosity (ηa) is constant (η ) and η is master curves give an advantage in studying the rheologi- 0 0 the zero-shear viscosity. This stage is called the first New- cal properties and processing of materials, little research tonian region. As shear rate increases, the apparent shear effort has so far been given to the rheological properties viscosity decreases, defining the second shear-thinning re- of nitroglycerin plasticized nitrocellulose with solid addi- gion. When shear rate further increases (훾˙ ! ∞), the ap- tives.