1. Introduction
It is well established that 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (C
6H
6N
12O
12, CL-20) is one of the most important energetic materials. CL-20 has emerged as a powerful alternative to hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) because of its superior oxygen balance, higher heat of formation, and higher density [
1,
2,
3,
4]. Composites based on nitramine such as RDX, HMX and CL-20, suffer from a problem known as dewetting, that is, the weak interfacial adhesion of nitramine crystals/binders may cause the binder to detach from the nitramine surface under stress, and thus induce further crack damages. Dewetting adversely affects the processability [
5], sensitivity [
6] and mechanical properties [
7,
8] of the composite propellants and plastic bonded explosives (PBXs). Therefore, it is crucial to strengthen the interfacial interaction between CL-20 and the binder for a wide application of CL-20. However, previous studies have shown that CL-20 is more difficult to wet than HMX [
9,
10,
11], in other words, dewetting is a more serious problem in CL-20-containing high energetic composites.
Many studies have shown that the addition of bonding agents significantly enhances the interfacial adhesion interactions of the nitramine/binder, thus effectively raising the mechanical properties of the propellants [
12,
13]. Bonding agents contain functional groups that have a favorable affinity for nitramine crystals and can chemically bond to the binder matrix during the curing process [
8]. Consequently, bonding agents play a vital role in improving the mechanical properties of high energetic composites. The commonly-used small molecule bonding agents are aziridine, alkanolamine, polyamine and their derivatives, etc. The boron trifluoride triethanolamine complex (TEA·BF
3), an alcoholic amine bonding agent, is a frequently-used bonding agent in propellants. It has been demonstrated that TEA·BF
3 had good interfacial interaction with CL-20, and it has been applied in hydroxyl-terminated polybutadiene (HTPB) propellant [
14,
15,
16]. Kim firstly proposed neutral polymeric bonding agents (NPBAs) in the 1990s. NPBAs not only solved the problem of small molecule bonding agents easily dissolving in the polar plasticizer, but also increased the number of polar groups in the bonding agents. Kim confirmed that NPBAs with cyano, ester and hydroxyl groups had a high affinity for the HMX particles, and the addition of NPBAs to HMX-filled poly(ethylene glycol) (PEG) binder remarkably increased the strength of the propellant [
12,
17]. Since then, many types of macromolecular bonding agents have been synthesized and applied [
18,
19,
20,
21], following the design idea of NPBAs. Chen [
22] synthesized a series of dodecylamine-based bonding agents with the end groups substituted with cyano, ester and hydroxyl groups, which showed a good affinity with RDX and enhanced the mechanical properties of the HTPB propellant.
Although previous studies have shown that the bonding agents with cyano, ester or hydroxyl groups had significant nitramine reinforcement, barely any studies have reported on the bonding agent of CL-20. Moreover, the affinity of different polar group to CL-20 is virtually unexplored. In this paper, a series of branched polyether bonding agent, with terminal groups substituted by cyano, ester and hydroxyl functional groups, were used. Branched polymers, containing many modifiable terminal active sites [
23,
24], are suitable as bonding agents for the chemical modification of terminal groups into different polar groups in order to evaluate the effects of the type and number of polar groups on the interfacial interaction performance of the bonding agent/CL-20. This study aimed to demonstrate the affinity of different polar groups to CL-20, thus providing a basis for the design and selection of the structure of the bonding agent. The static contact angle, FTIR and XPS were utilized as test methods to demonstrate the interfacial interaction between the branched bonding agents and CL-20, and compared with that of boron trifluoride triethanolamine complex (TEA·BF
3).
2. Materials and Methods
2.1. Materials
CL-20 (γ-polymorph, 20 µm, white crystal) was obtained from Liaoning Qingyang Chemical Industry Co., Ltd., China and dried at 60 °C before use. The branched polyether bonding agents (cyano-terminated branched polyether (CBPEs) and branched polyether (BPEs)) were synthesized in our laboratory. The functionality of the groups and the molecular weight for branched polyether bonding agents are shown in
Table 1. TEA·BF
3 was supplied by Xi’an New Chemical Material Company, China. Ethanol (analytical grade) was obtained from Beijing Chemical Plant, China. Diiodomethane (analytical grade) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Formamide (analytical grade) was purchased from Tianjing Fuchen Chemical Reagent Co., Ltd., China. Ethylene glycol was purchased from Beijing Tongguang Fine Chemicals Company, China.
2.2. Preparation of CL-20 Coated with Bonding Agents
CL-20 (0.002 mol) was mixed with the bonding agent (0.001 mol) and dissolved in ethanol (10 mL). After being continually stirred for one hour at room temperature, the mixture was filtered, and the precipitate was washed with ethanol several times to remove the surface bonding agent. After this, the CL-20 samples coated with different bonding agents were kept in a dry oven at 60 °C for two days.
2.3. Characterization
Scanning electron microscopy (SEM) measurements were performed with a thermal field emission scanning electron microscope instrument (JSM-7610F, JEOL, Tokyo, Japan) at an operating voltage of 5 KV. An ultrathin conductive coating was deposited before it was analyzed.
The contact angle was measured using an OCA contact angle analyzer (Datephysics Co, Stuttgart, Germany). Diiodomethane, formamide, and ethylene glycol were selected as the test fluids. The drop volume was 1 µL and the drop flow was at medium speed.
Fourier transform infrared spectra were recorded (Nicolet FTIR-8700, Thermo, Waltham, Massachusetts, USA) with a wavenumber resolution of 4 cm−1 and a single average of 64 scans at room temperature. KBr pellets of the samples were used.
X-ray photoelectron spectroscopy of pure CL-20 and CL-20 coated with bonding agents were recorded with an ESCALAB 250Xi electron spectrometer (Thermo Fisher Scientific Co, Waltham, Massachusetts, USA). Monochromatic Al-Kα irradiation at 72 W (12 kV at 6 mA) was used for the target (1486.6 eV) and under a vacuum of less than 10−6 MPa. The beam spot size was 50 µm. Survey scans were recorded with a 1 eV step and 150 eV pass energy and the high-resolution regions were recorded with a 0.1 eV step and 20 eV pass energy. Before data analysis, the high-resolution measurements were charge corrected using the C 1s signal at 284.8 eV. The XPS-PEAK software was used to process the data, including curve smoothing, deconvolution, background subtraction, normalization and curve fitting.
4. Conclusions
In this work, the interfacial interaction performance between the bonding agents (CBPEs, BPEs and TEA·BF3) and CL-20 particles was studied. The SEM images show that CL-20 particles were coated with bonding agents. The results of the interfacial tension measurement indicated that the CBPEs and BPEs showed a higher ability to wet CL-20 than TEA·BF3. The affinities of the polar groups to CL-20 were in the order–CN > -OH > -COO. Furthermore, the change in the number of cyano groups had the greatest influence on the reduction of the values of the contact angle, which makes it easier to improve the wettability of the bonding agents. According to the FTIR and XPS results, the interaction mainly originated from the interaction between the -NO2 and the polar groups in the bonding agents, and the interaction between the -CN and -NO2 was stronger than the interaction between the –COO and -NO2. The shift of the characteristic peak positions of -NO2 is attributed to hydrogen bonding, electronic effects and induction effects. The results of the interfacial characteristics suggest that the bonding agent CBPE-10,10 shows the strongest interfacial bonding interaction with CL-20, as the adhesion degree reaches 41.04%.