Effects of Polymers on Phase Transition of ε-CL-20 and CL-20-Based PBXs

Hang Li, Xueyong Guo,, Qingjie Jiao, Jingyuan Zhang, Honglei Zhang， >Zhenghong Wang and Shengquan Chang

(1.State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China;2.Liaoning Qingyang Special Chemical Co. Ltd, Liaoyang 111002, Liaoning, China)

Abstract: The powder X-ray diffraction technique is applied to investigate phase transitions of ε-CL-20 coated with additives (polymers and wax) and of ε-CL-20 heated in petroleum ester at different temperature points. First, CL-20-based plastic-bonded explosives (PBXs) were prepared by mechanical kneading. Characteristics of different polymer binders were measured and the relationship between polymer properties and sensitivity of CL-20-based PBXs was analyzed. The results indicated that ε-CL-20 coated with or without additives both showed high stability in the crystal phases at different temperature in petroleum ether. Compared with thermal conductivity, friction efficient and specific heat capacity, hardness of polymers has the most dominant effect on the sensitivity of CL-20-based PBXs. Butadiene rubber (BR) showed the best desensitizing effect among the chosen binders. Combination of EPDM and other several polymers (C-EPDM) as the polymer binder has higher press density and a lower expansion rate.

Key words: CL-20; PBX; phase transition; sensitivity; charge density

The development in the field of high energy density compounds (HEDCs) is based on preparation of new materials with high performance, good thermal stability and low sensitivity[1]. Hexanitrohexaazaisowurtzitane (HNIW or CL-20) is currently one of the most important HEDCs with a cage structure[2-3]. Under ambient conditions of pressure and temperature, CL-20 has four polymorphs, α, β, γ and ε. Among these phases the ε-phase has the highest density (2.04 g/cm3), the best thermo-dynamical and mechanical stability, and is the preferred phase for high energy applications[4-8]. The performance of ε-CL-20-based PBXs is approximately 14% greater than HMX-based PBXs[9]. The high performance of CL-20 is due to a strained cage structure carrying 6 NO2substituents, which makes it a potential candidate in replacing HMX and RDX[10-11].

Plastic-bonded explosives (PBXs) are a type of composite material in which energetic materials are dispersed in a polymer matrix[12-14]. PBXs have been commonly used in weapon equipment because of their improved safety, enhanced mechanical properties, and reduced vulnerability during storage, transportation and application[15-17]. Pressed PBXs have high energy, which is due to the higher solid loadings and superior charge densities. Molding powder of pressed PBXs can be prepared using an aqueous slurry process, mechanical kneading and other techniques. These procedures are all preferredo coat energetic material particles with a polymer binder and agglomerate them into small granules or prills, which are suitable for compression molding[18].

The type of binder has a significant effect on the properties and safety of the PBXs[19-21]. Compatibility with the explosives is an absolute prerequisite for substances to be used as binder. Hardness, friction coefficient, specific heat capacity, thermal conductivity and compression set are other important factors determining whether a polymer qualifies as binder material. For polymers with lower hardness, they can reduce the impact sensitivity of formulations, as a soft binder provides a shock absorber or diverter for the impact energy. For polymers with lower friction coefficient and thermal conductivity, friction generates less heat and therefore the heat is not easily transmitted to explosives. For polymers with higher specific heat capacity, their temperature changes less when absorbing certain amount of energy and they transmit less energy to explosives. In addition, polymers used for pressed explosives should have a high compression set so as to ensure higher press density and lower expansion rate. The purpose of the work is to study the effects of different polymers on phase transitions of ε-CL-20 and on the property of ε-CL-20-based formulations.

1 Experimental

1.1 Material

ε-CL-20, previously purified by recrystallization, was provided by the Qingyang Chemical Industry Corporation. Petroleum ether was provided by Beijing Chemical Reagent Company.Ethylene propylene diene monomer (EPDM), butyl rubber (IIR), butadiene rubber (BR) and wax were purchased from respective suppliers. C-EPDM and C-IIR (combination of IIR and other several polymers) were provided by Beijing Institute of Technology (Beijing, China).

1.2 Preparation of ε-CL-20 coated with additives

ε-CL-20 crystals coated with polymers and wax (mass ratio 10/1) were prepared for PXRD experiments. First, polymers and wax were dissolved in petroleum ether. Then the ε-CL-20 crystals were added to the binder solution with a constant speed of mixing. Following this addition, the solvent was evaporated by heating under vacuum. This process coated the surfaces of the ε-HNIW crystals with a continuous and homogeneous layer of the chosen additives.

1.3 Preparation of ε-CL-20 samples heated in petroleum ether

2.0 g ε-CL-20 was added into the 40 mL of petroleum ether. With a reflux condenser, the mixture was agitated for 2 h under the temperature of 60 ℃, 70 ℃, 80 ℃, and 90 ℃ respectively.

1.4 Preparation of molding powder

The reasonable particle gradation of coarse and fine CL-20 will lead to higher press density, which will affect the properties of explosives. The weight ratio of coarse CL-20 (50-300 μm) to fine CL-20 (10-50 μm) was fixed to 4/1 according to theoretical calculation and tap density research and the tap density is 1.66 g/cm3. The procedures are listed as follows:

① The binder was prepared by mixing small pieces of rubber with petroleum ether under vigorous agitation within the temperature range of 50-55 ℃ for about 3 h.

② After the rubber was dissolved in the petroleum ether, melted wax was added in and mixed homogeneously. This is the binder matrix.

③ CL-20 powder (94 wt%) was mixed with the binder matrix (5.5 wt%) by a computerized mixer within the temperature range of 50-55 ℃ for 60 min and then the petroleum ether was removed under vacuum.

④ The prepared samples were granulated and lubricated with lubricant (0.5 wt%). For all formulations, the only difference is the type of binder, and all other factors remained unchanged.

1.5 Characterization of polymers

Thermal conductivity of polymers was analyzed by DZDR-S. Heat capacity of polymers was tested using TA Q2000 at the heating rate of 10 ℃/min under N2atmosphere. Friction coefficients of polymers were measured by TMI MONITOR/SLIP & FRICTION. Hardness of polymers was measured by the Shore durometer. The compression set was measured by a TY compression set apparatus.

1.6 Powder X-ray diffraction (PXRD)

All phase transitions were studied on a Bruker D8 Advance powder X-ray diffractometer using Cu Kαradiation (λ=0.154 180 nm) without a monochromator, and the voltage and current applied were 40 kV/40 mA. The 2θrange measured was 5-40° with steps of 0.02°/0.1 s.

1.7 Vacuum stability tests

The compatibility of components was assessed in vacuum stability tests at 100 ℃ for 48 h. If the overall amount of gases released is less than 2 mL/g, the components are compatible. If the volume of the gases released reached 5 mL/g, the components are incompatible.

1.8 Mechanical sensitivity tests

Impact sensitivity was measured with a WL-1 type impact apparatus. The testing conditions were: drop weight, 10±0.01 kg; sample mass, 50±1 mg. Each sample was tested for 50 times to obtain an explosion probability (%) at 25 cm.

Friction sensitivity was measured with a WM-1 friction apparatus. The testing conditions were: angle, 90±1°; pressure, 3.92±0.07 MPa; sample mass, 20±1 mg. Each sample was tested for 50 times and an explosion probability (%) was obtained.

1.9 Molding property and expansion rate

The explosive charges 50 mm in diameter were pressed with a 30-ton hydraulic press. The cylinders were pressed with different pressures and the specific pressure-relative density (P-ρ0/ρTMD,ρ0was the pressing density,ρTMDwas the theoretical maximum density) curves were obtained.

The expansion rate of grains 50 mm in diameter and 50 mm in height were measured within the military specified temperature cycling range of -54 ℃≤T≤71 ℃.

2 Results and Discussion

2.1 Characterization of polymers

Polymer binders significantly affect properties of PBXs. Physical and chemical properties such as thermal stability, sensitivity, molding property, density and expansion rate will affect the application of PBXs.

Polymer properties like thermal conductivity, specific heat capacity, friction coefficient, shore hardness and compression set are listed in Tab.1.

Tab.1 Polymer properties

2.2 Effects of additives and petroleum ether on phase transition of ε-CL-20

2.2.1Phase transition of ε-CL-20 coated with additives under constant temperature conditions

CL-20-based formulations with additives as binders were prepared at the temperature range of 50-55 ℃. Therefore studies should be conducted over phase transitions of ε-CL-20 coated with additives for a long time and under high temperatures. PXRD was used to study the phase transitions of ε-CL-20 coated with additives. The PXRD patterns of ε-CL-20 coated with additives before and after being heated at 70 ℃ for 60 h are shown in Fig.1.

Fig.1 PXRD patterns of CL-20 coated with additives before and after being heated at 70 ℃ for 60 h

By comparing these PXRD patterns with the standard PXRD pattern of ε-CL-20, it shows that such conditions have no effects on the phase transition of ε-CL-20. Therefore, these additives can be used as binders for CL-20-based formulations.

2.2.2Effects of petroleum ether on polymorphic transition of ε-CL-20

ε-CL-20 is soluble in solvents with carbonyl group like acetone, ethyl acetate, DMF etc., difficult to dissolve in alcohols and ethers；and insoluble in hydrocarbons, halogenated hydrocarbons, and water. After dissolving in a solvent, ε-CL-20 will have phase transition in crystallization, which will change the density and enthalpy of formation of CL-20 crystals, and thus affect the performance of explosives. Therefore, solvents used for applying, transporting, and storing ε-CL-20 should be carefully selected to make sure ε-CL-20 crystals are insoluble or difficult to dissolve in the solvents and ε-CL-20 crystals have no phase transition within the solvents. This study analyzed the effect of petroleum ether (used in composite explosives) on phase transition of ε-CL-20.

It can be observed from Fig.2 that ε-CL-20 had no phase transition after being heated in petroleum ether at 60 ℃, 70 ℃, 80 ℃, and 90 ℃ respectively for 2 h while being agitated. ε-CL-20 molecules are the most symmetrical and have the smallest dipole moment among the four crystal phases (μβ->μα->μγ->με-)[22]. As ε-CL-20 won’t be easily polarized in petroleum ether that has zero dipole moment (μ=0), it can exist in ε-phase, which is more stable.

Fig.2 PXRD patterns of ε-CL-20 heated in petroleum ether at different temperature points

2.3 Compatibility of components

Compatibility with explosives is an absolute prerequisite for polymer binders. To ensure safety in PBXs storage and usage, different components in the explosive should be compatible with each other. Vacuum stability test is the most widely used and reliable method to test chemical compatibility. Compatibility between CL-20 and polymers or wax were analyzed in the vacuum stability test. The vacuum stability data are presented in Tab.2.

Tab.2 Results of vacuum stability test

Based on the standard of vacuum stability, if the volume of gas released from 1.0 g sample is less than 2.0 mL at 100 ℃ for 48 h, then CL-20 is compatible with the polymer binders. In the tests of the six groups, volumes of gas released were all less than 2.0 mL, which means that CL-20 is compatible with BR, IIR, EPDM, C-EPDM, C-IIR and wax.

2.4 Mechanical sensitivity of CL-20-based PBXs

Sensitivity is one of the most important properties, which determines the application of composite explosives. Therefore, it is essential to study the effects of polymer properties on sensitivity of CL-20-based PBXs.

The impact sensitivity and friction sensitivity of CL-20-based PBXs are shown in Tab.3.

Tab.3 Sensitivity of CL-20-based PBXs

2.4.1Relationship between hardness and sensitivity

Polymers with high hardness will sensitize explosives and raise mechanical sensitivity. Fig.3 shows that as hardness of polymers grows, the impact sensitivity increases; and the higher the hardness is, the sharper the increase in impact sensitivity becomes. But for the friction sensitivity, there is a deflection point. The hardness of C-IIR is lower than IIR, but the friction sensitivity of explosives with C-IIR mixture as binder is higher than explosives with IIR. The reason is that compared with IIR, the friction coefficient and thermal conductivity of C-IIR are higher, and the specific heat capacity is lower. Compared to C-IIR and IIR, BR has lower hardness, although the friction coefficient of BR is higher than that of C-IIR and IIR, formulations with BR as binders still have lower mechanical sensitivity.

Fig.3 Relationship between hardness and sensitivity of CL-20-based PBXs

From the relationship between hardness and sensitivity, it is obvious that hardness of polymers plays a major role on sensitivity of CL-20-based PBXs. When the two types of polymer binders have similar hardness, the impact of friction coefficient, thermal conductivity, and specific heat capacity on sensitivity become evident.

2.4.2Relationship between thermal conductivity and sensitivity

Fig.4 Relationship between thermal conductivity and sensitivity of CL-20-based PBXs

In the CL-20-based PBXs, as thermal conductivity of polymers increases, heat can be transmitted to explosive particles more easily and the explosives are more likely to form hot spots; the mechanical sensitivity will thus rise. Such correlation can be verified in Fig.4. But for BR, although it has higher thermal conductivity than C-IIR and IIR, CL-20-based PBXs with BR as binders still have low sensitivity. The reason is that BR has low hardness and absorbs more energy under mechanical stimuli, so it can curb the forming of hot spots.

2.4.3Relationship between friction coefficient and sensitivity

Theoretically, polymers with higher friction coefficients generate more heat in friction. Explosives with such polymers are more likely to form hot spots, and the sensitivity will rise. But such a trend is not clearly shown in Fig.5. CL-20-based PBXs with EPDM as binder have the highest sensitivity in spite of the second lowest friction coefficient of EPDM because EPDM has high hardness; CL-20-based PBXs with BR as the binder have the lowest sensitivity in spite of the highest friction coefficient of BR because BR have low hardness. This shows that effects of high hardness on mechanical sensitivity outweigh the effects of friction coefficient.

Fig.5 Relationship between friction coefficient and sensitivity of CL-20-based PBXs

2.4.4Relationship between specific heat capacity and sensitivity

If polymers have higher specific heat capacity, they can lower the sensitivity of CL-20-based PBXs. When taking in same amount of heat, polymers with higher specific heat capacity have a smaller temperature change. Theoretically, CL-20-based PBXs with such polymer binders are less likely to form hot spots and are less sensitive. But Fig.6 shows that as specific heat capacity increases, sensitivity does not change as predicted. Formulations with EPDM as binders have high sensitivity, but formulations with BR as binders have low sensitivity. The reason is that EPDM has high hardness, and BR has low hardness. This shows that effects of hardness on sensitivity outweigh the effects of specific heat capacity.

Fig.6 Relationship between specific heat capacity and sensitivity of CL-20-based PBXs

From the relationship between hardness, thermal conductivity, friction coefficient, and specific heat capacity of polymers and the mechanical sensitivity of CL-20-based PBXs, it can be seen that the hardness of binders is the most major influencial factor for sensitivity of CL-20-based PBXs, thermal conductivity is the second major factor, and then are the friction coefficient and specific heat capacity.

2.5 Molding property and expansion rate

2.5.1Effects of polymers on molding property

Polymers affect not only sensitivity, but also molding property, charge density, expansion rate and other properties of pressed explosives.

Fig.7 Specific pressure vs relative density curves of CL-20-based PBX

Under the same conditions, the charge density was investigated and the specific pressure vs relative density curves are presented in Fig.7. From Fig.7, it can be observed that formulations with IIR, C-IIR and C-EPDM mixture as binders have higher press density than formulations with BR as binder. The 50 mm explosive charges are pressed to achieve density of about 0.970, 0.970, 0.981, 0.950 of the theoretical maximum density (TMD) respectively. C-EPDM as binder can be molded by lower specific pressure and can increase charge density.

In this work, the charge density of JC-3 is not investigated because explosives with EPDM as binder have high sensitivity. Future work needs to consider how to reduce the sensitivity of IIR, C-IIR, EPDM and C-EPDM as binders and CL-20 based formulations.

2.5.2Effects of polymers on expansion rate

Expansion plays a prominent role in application of pressed explosives. One of the criteria pressed explosives must meet is that appearance doesn’t change evidently over the military specified temperature cycling range of -54 ℃≤T≤71 ℃. Otherwise, expansion will affect the shape of pressed explosive, making it unfit with the structure of warheads. The expansion rates of CL-20-based formulations with different polymer binders are shown in Tab.4.

Tab.4 Expansion rates of CL-20-based formulations

It can be deducted from Tab.4 that the higher the compression set of a polymer is, the lower the expansion rate of pressed explosives with such polymer binder.

3 Conclusions

Phase transitions of ε-CL-20 coated with additives and ε-CL-20 heated in petroleum ether at different temperatures under constant other conditions are investigated in the powder X-Ray diffraction experiments. PXRD results have confirmed that ε-CL-20 didn’t have phase transition in petroleum ether.

Polymer properties affect sensitivity of explosives. Hardness is the most major influencial factor, thermal conductivity is the second major factor, and then are the friction coefficient and specific heat capacity.

High compression set of polymers results in high charge density and a low expansion rate of pressed explosives.

Acknowledgment

We would like to express our gratitude to Dr. Bin Li and Dr. Jinjiang Xu for their enthusiastic help in this work.