Study on Mixed Ester Gun Propellent Plasticized with Supercritical Carbon Dioxide during Screw Extrusion: In-line Rheometer Design and Experimental Measurement

GU Han, YING San-jiu, HU Qi-peng, WANG Chao

(1.School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; 2.Key Laboratory of Special Energy Materials, Ministry of Education, Nanjing 210094, China)

Abstract：In order to improve the processing fluidity of mixed ester gun propellent, supercritical carbon dioxide (SC-CO2) as a plasticizer was injected during extrusion. For better observation of plasticization, an in-line slit die rheometer was constructed to measure the shear viscosity and rheological parameters of mixed ester gun propellent. The flow curves demonstrated that both of gas-free propellant and SC-CO2/propellant mixture are non-Newtonian pseudoplastic fluids. The power model is appropriate to describe the rheological behavior of SC-CO2/propellant mixture. Experimental measurements of shear viscosity as a function of processing temperature and plasticizer content were conducted. Higher processing temperature was beneficial for reducing shear viscosity. The activation energy to flow decreased about 20kJ/mol after SC-CO2 injection. SC-CO2 was shown to be an effective plasticizer that significantly improved fluidity. More SC-CO2 leaded to a more ideal fluidity during extrusion, but continuously increasing plasticizer content could not lead to better plasticization without limit.

Keywords：physical chemistry; mixed ester gun propellant; supercritical carbon dioxide; rheometer; plasticization; plasticizer

Introduction

Screw extrusion process is a common continuous gun propellant manufacturing method, which has the obvious advantages of high production efficiency and good product quality[1]. However, with the development of propellant manufacturing technology, the screw extrusion molding process has gradually exposed some deficiencies[2]. The main component of the gun propellant is nitrocellulose (NC) which is a kind of polymer with poor flexibility. In order to solve the problem of poor processing fluidity, energetic plasticizers or volatile solvents are usually used. On one hand, the commonly used energetic plasticizers include nitroglycerin (NG), triethylene glycol dinitrate, diethylene glycol dinitrate (DEGN) and so on[3], but excessive addition of energetic plasticizers leads to deteriorating production safety. On the other hand, during the processing, a large number of volatile organic solvents are often added to increase the plasticity, reduce the viscosity of the gun propellant system and improve the fluidity. However, most of these organic solvents are harmful to human body. In addition, these solvents eventually need to be removed from the product. When the solvent content is high, it will cause size shrinkage of the gun propellant product[4]. Although using solvent-free preparation method can aviod the shrinkage of gun propellant from the source, ahigher temperature and pressure during the molding process could bring issues to the processing safety. Additionally, it is difficult to break through the limitation of not being able to prepare large size gun propellant by solvent-free preparation method. Therefore, it is necessary to find an ideal solvent replacement or partial replacement of organic solvent, which can not only reduce the viscosity of gun propellant in extrusion molding process and improve the fluidity, but also does not have the shortcomings of organic solvent.

Besides the conventional energetic plasticizers and low molecular weight volatile solvents mentioned above, an alternative lies in the use of a supercritical fluid (SCF). In general, SCFs offer mass transfer advantages over low molecular weight plasticizers because of their gas-like diffusivity, liquid-like density, low viscosity and surface tension[5]. In particular, supercritical carbon dioxide (SC-CO2), which is considered as a green solvent, has emerged as an important SCF[6]. In addition to its non-toxicity, non-flammability, and chemical inertia, it displays accessible supercritical conditions. In the supercritical domain, it is relatively easy to finely tune its physicochemical properties from vapor-like to liquid-like limits by varying pressure and temperature[7]. Most important of all, SC-CO2is also known for being soluble in large proportions in many polymers[8], and has already been used in polymer processing, including screw extrusion[9]and injection molding[10].

SC-CO2is dissolved in the material as a plasticizer, which can change the physical properties of the material and decrease the constraints undergone by the material. The glass transition temperature, or the viscosity of various polymers in the barrel of the extruder could be modified by SC-CO2, without changing the viscoelastic behavior of the polymer matrix[11]. In the field of industrial plastic manufacturing, the injection of SC-CO2can adjust the rheological properties of polymer melt during the screw extrusion of poly lactic-acid[12], polyethylene terephthalate[13]and polyacrylonitrile[14], so as to obtain a better processing window. In the direction of military industry, Ding[15]couples SC-CO2and extrusion to improve fluidity of gun propellant for safer processing.

To the authors knowledge, there have been no detailed studies about the effect of process parameters on plasticization during mixed ester gun propellant extrusion assisted with SC-CO2. In this study, SC-CO2was used to be plasticizer for improving the processing fluidity of mixed ester gun propellant. Propellant extrusion assisted with SC-CO2can solve the defect of size shrinkage caused by traditional solvent preparation method. Additionally, compared with the solvent-free preparation method, propellant extrusion assisted with SC-CO2can improve the safety of extrusion process, because the addition of SC-CO2can reduce processing temperature and pressure. The purpose of this work is to conduct a complete, comprehensive, and accurate study of the effect of SC-CO2on the plasticization of gun propellant from different extrusion process conditions, including processing temperature and SC-CO2concentration. In this paper, both the high-pressure extrusion in-line slit die rheometer and the SC-CO2plasticization extrusion device were used. The shear viscosity of the fluid and the pressure drop at the die will be researched by the in-line slit die rheometer. The gathered data will be analyzed to reflect the plasticizing effect of SC-CO2. These analysis results will be of reference value to optimize the screw extrusion process of mixed ester gun propellant.

1 Method and Materials

1.1 Materials preparation

In this study, experiments were performed on certain formula mixed ester gun propellant. The main component of this gun propellant includes NC, NG, DEGN, titanium dioxide and centraliter. Other materials are acetone, ethanol and CO2. Acetone (analytically pure, AR) and ethanol (analytically pure, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. Industrial CO2(purity≥99.9%) was supplied by Nanjing Wenda Special Gas Co. Ltd.

Before continuous processing, gun propellant was preliminarily kneaded in the mixed solvents of acetone and ethanol by the opposite rotor kneader at the set kneading temperature. In mixed solvents, the volume ratio of acetone to ethanol was 1∶1. Kneaded gun propellant was stored in a desiccator to reduce the solvents volatilization loss for later use. The ratio of the volume of mixed solvents to the mass of gun propellant is an important parameter, which is usually referred to as the solvent ratio.

1.2 Extrusion processing

Fig.1 is a schematic diagram of the SC-CO2plasticization extrusion device used, equipped with an injection system and a single screw extruder. The injection system injected CO2into the extruder under precisely controlled condition, which contains a gas cylinder, a cooling circulation unit, a syringe metering pump (260D, ISCO, Inc.), and a high-pressure anti-backflow valve. This valve was used to maintain the constant injection pressure (higher than the critical pressure of CO2) and to prevent the backflow of gun propellant. The amount of CO2was controlled by the syringe pump which was operated at constant flowrate according to the percentage of CO2required in the final SC-CO2/propellant mixture.

Fig.1 Schematic diagram of SC-CO2 plasticization extrusion device

The extruder uses a copper plated screw with a diameter of 30mm. The length/diameter ratio of screw is 37. As shown in Fig.2, the screw is designed into four functional sections, which are feeding, injection, mixing and metering. The increase in the groove depth at the injection point can facilitate a drop pressure, allowing pressurized CO2to be more easily injected. The injection section prior to this drop is a region of higher pressure that allowed the formation of a melt seal to help prevent CO2from escaping up the channel and out of the hopper. The suitable length of mixing section can homogenize the mixture better. In addition, the extruder also comes with funnel type forced feeder and water bath heating jacket. After gun propellant entered the barrel through the forced feeder, it shall wait until the material was filled in the barrel and a certain pressure was established in the barrel before opening the valve and injecting CO2. This operation can effectively avoid the separation of the two phases.

Fig.2 Diagram of screw structure

The CO2injection process is divided into three steps. Firstly, the gas came out from the CO2cylinder and was cooled by the cooling circulation unit. Then, the flowrate of CO2was regulated by the syringe metering pump as required. Finally, the high-pressure CO2entered the barrel through the high-pressure anti-backflow valve to mix with the gun propellant. Due to its internal construction, the injection port acts as a pressure regulator, allowing the continuous input of the corresponding amount of CO2as soon as the pressure of whole system reaches the steady state condition.

1.3 Rheometer design

The experimental in-line measuring apparatus used is based on the rheometers developed first by Han[16]and later used by Lee et al[17]. The configuration of the designed in-line slit die rheometer is shown in Fig.3. The internal flow channel of the rheometer is divided into three zones, namely the transition zone, the fully developed zone and the nozzle zone. Three zones are respectively responsible for lead-in fluid, measurement and maintaining pressure.

Fig.3 Schematic diagram of in-line slit die rheometer

The dimension of slit in the rheometer is designed to be 2mm high and 24mm wide, because measurement errors due to boundary effects can be ignored for the width/height ratio greater than 10[18]. In the transition zone, the slit die rheometer whose inlet length is greater than 20 times the height of channel has been used to allow for flow rearrangement in this region[14]. The distance between the beginning of rectangle channel and the first pressure transducer is 25mm, which is designed to ensure steady flow[19]. In the fully developed zone, there are four transducers installed along the slit path, each of them 65mm apart, to measure temperature and pressure. The pressure transducer taps are mounted flush with the slit wall. Pressure data adopted from four transducers will be calculated and analyzed to study viscosity change and pressure drop. In the nozzle zone, there is an extended slit channel. The distance between the last pressure transducer and the outlet of slit is 70mm. Such design is to elevate pressure within fully developed zone above the critical pressure of CO2. The monitor of temperature by circulator water bath ensures that the temperature of the flow in the slit stays the same as the temperature of material in the barrel.

Additionally, the existence of a single-phase mixture is crucial to the viscosity measurement of SC-CO2/propellant system. A homogenous single-phase of SC-CO2/propellant mixture must be present under fully developed flow condition. The use of four pressure transducers in our design allows us to measure pressure drop. Each measurement is under a constant screw speed, which means that the flowrate of the material is determined. A linear pressure drop within the slit can confirm the presence of a single-phase mixture during viscosity measurements[20].

1.4 Viscosity calculation

Shear viscosity is an important parameter to describe the flowing properties of gun propellant during extrusion, and the efficiency of plasticizer can be determined by the change of viscosity. Therefore, the shear viscosity needs to be calculated by the volumetric flowrate and pressure readings of the substitute as it passes through the rheometer.

With the volumetric flowrate and the pressure values, fluid viscosity can be determined by the following equations. The shear stress of the wall of slit die rheometer (τw) can be calculated by Equation (1):

(1)

Where ΔPis the differential pressure value between two adjacent pressure transducers.H(0.002m) andL(0.065m) are the height of the slit and the distance between two adjacent pressure transducers, respectively.

(2)

WhereQis the volumetric flowrate of the extrudate, andw(0.024m) is the width of the slit. (2+b)/3 is the Weissenberg-Rabinowitsch correction factor, and it compensates the loss of shear rate between Newtonian fluid and shear-thinning fluid.

The constantbcan be calculated by the shear rate and shear stress[21]:

(3)

The real shear viscosity (η) can be obtained by Equation (4):

(4)

To obtain a real shear viscosity curve over several different shear rates, the above procedures were repeated at different screw speeds.

Under specific process condition, pressure data can be adopted only when the pressure is in a stable state, which the fluctuation range is less than 0.3MPa. The experiments showed that the maximum pressure difference was about 0.3MPa when different parts of the screw rotated through the pressure measuring point. The final pressure values used were taken from the average data of each transducer in one minute, because the minimum screw speed of the device is 1r/min. At the same process condition, the volumetric flowrate of the extrudate was measured at least three times and its average value was taken for calculation.

2 Result and Discussion

2.1 Experimental verification of rheometer design

The viscosity values of the binary system under process conditions are very difficult to be obtained in conventional rheometers, especially SCF/polymer mixture. In order to ensure the accuracy of the viscosity measurement, it is necessary that the fluid in the measurement area is a single-phase mixture.

Fig.4 displays pressure drops along slit die length during measurement at different conditions and Table 1 shows the linear correlation coefficient of all curves in Fig.4.

Fig.4 Pressure drops along slit die length

As can be seen from Fig.4(a), the experimental data points for all samples show a good linear pressure drop across the length of the slit die at various screw speeds, and the lowest linear correlation coefficient is 0.99911 (in Table 1). Moreover, Fig.4(b) and Fig.4(c) show pressure drops along slit die length at various processing temperatures and injection flowrates respectively. There is also a good linear relationship between pressure and die length shown on the two figures, and the linear correlation coefficients are all greater than 0.99900 (in Table 1). It should be stated that a linear pressure drop is maintained in all cases, indicating that no phase separation occurs in the system. Furthermore, it can also be known from Figure 4(a), 4(b) and 4(c) that the pressures of all transducers in the fully developed area are all above 7.38MPa, which means that the data is adopted when CO2is in supercritical state. The measured data in the above state are closer to the viscosity results in real processing state.

Table 1 The linear correlation coefficient (pearson correlation coefficient) of all curves in Fig.4

The in-line slit die rheometer designed above is suitable for the viscosity measurement of SC-CO2plasticizing gun propellant process. The reliable viscosity data obtained with this in-line slit die rheometer are discussed and analyzed below.

2.2 Flow curve

The flow curve of polymer fluid is a method which can comprehensively show the variation of shear viscosity and shear rate of materials under different process conditions. The rheological properties of polymer fluids vary greatly at different shear rates, and the flow curves can describe the rheological properties of polymer fluids over a wide range of shear rates. In this study, the shear rate was adjusted by changing the screw speed and the flow curves weredrawn according to the experimental results measured by in-line slit die rheometer. The flow curves of gas-free propellant and SC-CO2/propellant mixture through the slit die rheometer are shown in Fig.5(a) and Fig.5(b) respectively, which illustrate the shear viscosity variation with a gradual increase in shear rate. Fig.5(a) is based on the measurement results for gas-free propellant under the condition that the processing temperature is 35℃ and the solvent ratio is 0.31mL/g. The data of Fig.5(b) came from the same processing temperature and solvent ratio, and the injection flowrate of CO2is 0.2mL/min.

Fig.5 Logarithmic relationship between shear viscosity and shear rate

As can be seen from Fig.5, with the increase of shear rate, the shear viscosity of fluid decreases, presenting a distinct phenomenon of shear thinning. Obviously, both samples show a pseudoplastic behavior. In addition, the shear viscosity of SC-CO2/propellant mixture decreases significantly compared with gas-free propellant at the same shear rate. When the shear rate is low (range from 14 to 15s-1), the viscosity difference before and after SC-CO2injection is about 1kPa·s. The above phenomenon is related to the plasticization of SC-CO2. In practice, this means that the flow encounters less resistance at higher shear rates. The structure of nitrocellulose consists of entangled long molecular chains, which have irregular internal order that forms high resistance against flow. With increasing shear rates, molecule chains are disentangled, stretched, and reoriented parallel to the driving force. Molecular aligning mentioned above allow the molecules to slip past each other more easily[22].

When shear rate and viscosity curve plotted in double logarithmic scales gives an approximate straight line, it is a good indication that the Ostwald-de-Waele Equation (Power Law) is a suitable viscosity model of the mixture[23]. This can be written as:

(5)

(6)

(7)

According to Equation 6 and 7, the consistency coefficients of the fluid before and after adding SC-CO2are 76.62×103Pa·snand 54.87×103Pa·snrespectively. From the equations, we can also know that, after CO2injection, the power law index drops down from 0.15 to 0.08, which indicates that SC-CO2/propellant mixture is stronger shear thinning fluid. Nevertheless, the power law indices of both systems are much less than 1, which is a clear indication of the pseudoplastic behavior. Moreover, when SC-CO2is mixed with propellant, the consistency index of system decreases, which means SC-CO2acts as a plasticizer working.

2.3 Effect of processing temperature

Processing temperature is an important technological parameter to control the production process and product quality during extrusion, especially for the polymer which is very sensitive to temperature. Processing temperature is strongly linked to the viscosity of polymer fluid and affects the viscosity to a certain extent. Fig.6 shows the shear viscosity change of SC-CO2/propellant mixture at different processing temperatures when the solvent ratio is 0.34mL/g and the injection flowrate of CO2is 0.1mL/min.

Fig.6 The flow curves of SC-CO2/propellant mixture at different processing temperatures

As can be seen from Fig.6, at the same shear rate, the shear viscosity of SC-CO2/propellant mixture decreases with the increase of processing temperature. Especially at lower temperatures, the reduction of shear viscosity is more significant. This is because when the temperature is low, the degree of disentanglement between nitrocellulose macromolecules in the material is high, and the movement ability of molecular chain segments is restricted, showing high shear viscosity. With the increase of temperature, the thermal motion energy of each unit in the system increases, which makes the nitrocellulose molecular chain gains enough energy to overcome the energy barrier, hence the activity of the chain segment is enhanced and the motion of the nitrocellulose molecular chain is intensified. Furthermore, it can be found that at 40℃ the flow curve of gas-free propellant is above on the curve of CO2/propellant mixture. This demonstrates that gun propellant has been plasticized with SC-CO2and obtains better liquidity.

Consistency coefficient is an important metrics of polymer fluid viscosity, which is related to temperature, but it is not equal to viscosity. For the same polymer fluid, the higher the value of consistency coefficient is, the more viscous the fluid is in that condition. Through linear fitting of rheological data, we found that the value of consistency coefficient decreased from 53.99kPa·snto 40.91kPa·snwith the increase of processing temperature, as shown in Table 2.

Table 2 The rheological parameters of flow curves in Fig.6

According to the segmental transition mechanism of polymer fluid flow, during the flow of nitrocellulose molecular chain, the center of gravity of macromolecular chain shifts along the flow direction, and mutual slippage occurs between chain segments. Slippage is a kind of motion process, which requires certain space and energy, hence temperature has a significant influence on the slippage behavior. With the increase of processing temperature, the free volume increases, which enhances the activity of chain segment, weakens the intermolecular force, and strengthens the flexibility of molecular chain[24]. The increase of processing temperature also provides enough energy for the slip of nitrocellulose molecular chain, hence consistency coefficient decreases with the increase of processing temperature.

The shear viscosity of the gun propellant with and without SC-CO2at different processing temperatures is illustrated in Fig.7. In order to make the data comparable, all the data in Fig.7 were taken from same injection flowrate, screw speed and solvent ratio. Fig.7 can reflect the effect of processing temperature on plasticization. According to the data shown in Fig.7, the calculation displays that the shear viscosity decreases 36.65% (35℃), 35.59% (40℃), 33.31% (45℃), 28.68% (50℃), 21.41% (55℃) respectively after adding SC-CO2at different processing temperatures. It indicates that the plasticizing effect of SC-CO2on the gun propellant is weakened with the increase of processing temperature.

Fig.7 Shear viscosity of the gun propellant with and without SC-CO2 at different processing temperatures

The effect of processing temperature on the viscosity of different polymers is different, and the sensitivity of viscosity to temperature is also different for the same polymer in different temperature ranges[25]. The temperature dependence of rheological properties of polymers under certain constraints is usually evaluated by the Arrhenius equation:

η=Aexp(Eη/RT)

(8)

Take the log of both sides of this equation:

lnη=lnA+(Eη/RT)

(9)

whereAis the viscosity constant;Eηis the activation energy to flow;Ris the universal gas constant (8.314J·mol-1·K-1) andTis the processing temperature (K).

Fig.8 is drawn by Equation 9, reflecting the relationship between the natural logarithm of shear viscosity and the reciprocal of temperature. It is not difficult to find that the linear relationship between lnηand 1000/Tis highly correlated, and the linear correlation coefficients are 0.9864 (10s-1), 0.9862 (15s-1), 0.9861 (20s-1), 0.9860 (25s-1) and 0.9860 (30s-1) respectively. Hence, the activation energy to flowEη, can be calculated by linear fitting.

Fig.8 Effect of processing temperature on shear viscosity of SC-CO2/propellant mixture

Eηis a physical quantity which describes the viscosity-temperature dependence of a material. The higherEηis, the greater the influence of temperature on the viscosity of SC-CO2/propellant is, otherwise, the less the influence of temperature on the viscosity of system is. A lower value ofEηimplies a lower energy barrier for the movement of an element in the fluid. In the case of the fluid here under analysis, this barrier can be related to the interaction between chain segments and can be determined by polymer entanglements[26]. Fig.9 shows the activation energy of flow at different shear rates, when the solvent ratio is 0.34mL/g and the processing temperature is 40℃. As shown in this figure, with the increase of shear rate, the activation energy of flow in both systems presents a downward trend. When the shear rate increases from 5s-1to 30s-1, the activation energy of flow decreases by 0.07kJ/mol (gas-free propellant) and 0.05kJ/mol (SC-CO2/propellant). The activation energy to flow of propellant is calculated as about 35.3kJ/mol, however, with the addition SC-CO2, the activation energy is considerably reduced to between 15.46 and 15.51kJ/mol. This reduction is created by the increased free volume afforded by the polymer on the absorption of SC-CO2[27]. The sharp decline ofEηmeans that the chain segments of nitrocellulose are easier to move. Obviously, the plasticizing effect of SC-CO2is remarkable.

Fig.9 Effect of shear viscosity on the activation energy of flow

2.4 Effect of plasticizer content

In this study, the plasticizer content can be controlled by adjusting the injection flowrate of CO2. The set value of injection flowrate made SC-CO2concentration much less than the saturation solubility of SC-CO2in propellant, avoiding the phase separation of SC-CO2/propellant mixture in extruder. The effect of plasticizer content on rheological behavior is illustrated in Fig.10.

Fig.10 The flow curves of gun propellant at different injection flowrates of CO2

Fig.10 shows flow curves at different injection flowrates when the solvent ratio is 0.33mL/g and the processing temperature is 35℃. It can be observed from Fig.10 that the shear viscosity decreases with increasing injection flowrate of CO2at the same shear rate. Meanwhile, the shear viscosity decreases less and less when the injection flowrate higher. The flow curves in Fig.10 preserve same trend and shift to lower values with the increase of injection flowrate.

Table 3 The rheological parameters of flow curves in Fig.10

Through the linear fitting of the data in Fig.10, the rheological parameters can be obtained. It can be found from Table 3 that the power law index and the consistency coefficient decrease as the injection flowrate increases, which means that the fluid has a more ideal mobility due to the increase of SC-CO2. Obviously, the information presented from rheological parameters shows that increasing plasticizer content would be conductive to plasticization.

Calculated from the data in Fig.11, at a processing temperature of 35℃, for every 2% increase in SC-CO2content, the shear viscosity decreases by 8.14%, 18.74%, 6.13%, 3.61% and 3.05% successively. When the SC-CO2content rises from 2% to 4%, the shear viscosity drops most, and then the shear viscosity drops less and less as the content continues to increase.

Fig.11 Evolution of shear viscosity of SC-CO2/propellant mixtures as a function of SC-CO2 content at a constant screw speed of 9r/min

Similarly, when the processing temperature is 40℃, the shear viscosity decreases by 7.14%, 17.34%, 10.05%, 6.22% and 2.63% successively for every 2% increase in SC-CO2content. Predictably, as SC-CO2content continues to increase before reaching the saturation solubility of SC-CO2in propellant, better fluidity can be obtained. However, the positive influence of raising injection flowrate on plasticization will diminish gradually. This is because with the increase of injection flowrate, although the plasticizer content per unit mass also increases, but the proportion of SC-CO2molecules interacting with the groups on the main chain continues to decrease. This means that even below saturation solubility, not all SC-CO2molecules are effectively involved in plasticization, contributing to improved fluidity.

3 Conclusion

In this study, the plasticizing effect of SC-CO2on the mixed ester gun propellant was investigated by the measurements of a high pressure in-line slit die rheometer. The plasticization state was reflected by studying the change of shear viscosity and rheological behavior of SC-CO2/propellant mixture during extrusion.

(1) A linear pressure drop in rheometer is maintained at various process conditions, indicating that no phase separation occurs. Therefore, the viscosity data obtained with self-designed rheometer are reliable.

(2) The flow curves demonstrate that both gas-free propellant and SC-CO2/propellant mixture are non-Newtonian pseudoplastic fluids, and the Power model is an appropriate to describe the rheological behavior of SC-CO2/propellant mixture. With the addition of SC-CO2, the shear viscosity and consistency coefficient decrease 1kPa·s and 21.75×103Pa·snrespectively, indicating that SC-CO2is a good plasticize for the mixed ester gun propellant.

(3) The fluidity of SC-CO2/propellant mixture will be improved with the increase of processing temperature. TheEηdecreases about 20kJ/mol after CO2injection, which is evidence that SC-CO2had excellent plasticizing effect on the mixed ester gun propellant.

(4) The effect of plasticizer content on plasticization is pronounced, the increase of injection flowrate makes the plasticization state of gun propellant better. The shear viscosity is reduced by up to 35% when the plasticizer content is 10%. However, continue to increase plasticizer content leads to a result that the positive influence of reinforcing plasticization would diminish gradually and plasticizing effect should reach a limit eventually.

Inculsion, with the help of SC-CO2, the viscosity of mixed ester gun propellant was substantially reduced and the fluidity was significantly improved. As a plasticizer, SC-CO2has great potential for improving the processing performance of gun propellant. The study of rheometer design and the investigation of the plasticizing effect of SC-CO2on gun propellant during extrusion is fundamental and important for regulate and control the processing fluidity of gun propellant assisted with SC-CO2.