Enhanced gas and plasticizer barrier HTPB composite liner implanted with parallel orientation Fe₃O₄/RGO nanosheets by an ultrasound/magnet-coassisted method

Highlights

• •Magnetic iron oxide decorated reduced-graphene-oxide nanosheets (MRGO) is prepared.
• •Ordered MRGO/HTPB liner is fabricated via ultrasound-assisted co-precipitation.
• •The dynamic orientation process of MRGO dispersed in the HTPB matrix was observed.
• •MRGO hinders the diffusion of small molecules through the “tortuous path” effect.

Abstract

It is of great significance to prepare liners with excellent inhibition of energetic plasticizer migration and gas barrier properties. Here, we have successfully prepared magnetic iron oxide decorated reduced-graphene-oxide nanosheets (MRGO) by using ultrasound-assisted method. The obtained MRGO nanosheet-fillers were filled into hydroxyl-terminated polybutadiene (HTPB) which was exposed to a magnetic field (200 mT) to achieve ordered orientation of MRGO in the HTPB matrix (Ordered MRGO/HTPB). The laser confocal microscopy demonstrates that MRGO exhibit ordered orientation structure in HTPB matrix with good dispersion, which renders the HTPB composite liners exhibiting high gas and plasticizer barrier capability, with a reduction of 18.9 % in water vapor permeability and a decrease of 14.1 % in dibutyl phthalate (DBP) migration equilibrium concentration as compared with those of random MRGO embedded HTPB composite liners (Random MRGO/HTPB). Moreover, a theoretical model accounting for such enhanced gas/plasticizer barrier performance of HTPB due to the implantation of order aligned MRGO was established, which shows that the effective diffusion pathways of plasticizer/gas for liner penetration would be significantly enhanced when the MRGO nanosheets are oriented within the HTPB matrix. This work provides an effective and facile strategy toward the design and development of composite liners with high plasticizer/gas barrier performance for industrial applications.

1. Introduction

Solid rocket motors play an important role in the field of aerospace, as they provide reliable thrust and flexible maneuverability and are widely used in space exploration, satellite launches and other fields [1], [2], [3]. The charge composition of solid rocket motor is shell, insulation layer, liner and grain from outside to inside [4]. The liner is an adhesive transition layer between the solid propellant and the insulating layer, which plays an important protective role [5], [6]. Its main functions include providing protection, sealing and adhesion properties to ensure the stability of the solid propellant. Curing using HTPB with isocyanates is a common method of preparing propellant liners [7], [8]. This curing reaction can take place at room temperature or at elevated temperatures and results in the formation of a crosslinked structure with high strength. The prepared liners have good physical and chemical properties and have become one of the most widely used composite propellant polymer liners [9], [10], [11], [12].

However, during long-term storage, the intrusion of external gas may accelerate the aging of the liner, resulting in a reduction in its strength, and may cause undesirable conditions such as cracking, softening, and stickiness, which greatly affects its service life [13]. At the same time, small molecules such as nitroglycerin and plasticizers in the propellant may also migrate, reducing the mechanical properties of the liner. If the amount of plasticizer migration is too large, it will reduce the bond strength between the propellant and the insulation layer and may lead to interface delamination or failure. In addition, the migration of plasticizer may cause abnormal ballistic performance within the engine, thus seriously threatening the safety of solid rocket motors [14]. Thus, improving the anti-migration performance and barrier properties of liner is of great significance.

In the past decades, efforts have been made to inhibit the migration of energetic plasticizers, among which the addition of nanofillers to the liner is undoubtedly a simple and efficient method [15]. Table 1 summarizes some of the recent studies related to plasticizer migration and gas barrier properties of HTPB liner. As shown in Table 1, by adding some specific inorganic nanofillers to the liner, such as graphene oxide (GO), Carbon Black (CB), nanoclay, carbon nanotubes (CNT), etc., the density of the liner can be increased and the migration of plasticizers can be reduced [16], [17], [18], [19]. Among them, GO can play a good role as a barrier. Due to the very thin atomic thickness of graphene with high aspect ratio and high electron cloud density, it can effectively block the penetration of molecules [20], [21], [22], [23]. Moreover, GO has good compatibility with many polymers and can be well dispersed in the polymer matrix to prepare nanocomposites with excellent comprehensive properties. When GO is added to the liner, it can form a tightly clogged barrier that prevents the diffusion and migration of plasticizers.

Table 1.

Recent studies related to plasticizer migration and gas barrier properties of HTPB liner.

Matrix	Fillers	Key material functionality	References
HTPB	CB	Resistant to IDP migration	[24]
HTPB	β-CD (TDI Modified)	Prevent DOP Migration	[25]
HTPB	CNT	Prevent DOP Migration	[26]
HTPB	GO	Anti-Migration Of DOS	[13]
HTPB	GO (TDI Modified)	Anti-Migration Of DOS	[11]
HTPB	GO (IPDI Modified)	Anti-Migration Of DOS	[27]
HTPB	GO (ODA Modified)	Anti-Migration Of DOS	[28]
HTPB	Nanoclay	Enhanced Gas Barrier	[29]

Isodecylpelargonate (IDP), β-cyclodextrin (β-CD), toluene diisocyanate (TDI), Dioctylphthalate (DOP), Isophorone diisocyanate (IPDI), octadecylamine (ODA).

More importantly, the barrier properties of nanocomposites depend not only on the loading of nanosheets but also on their arrangement. Previous studies have theoretically demonstrated that regularly arranged nanofillers can significantly improve the barrier properties [30], [31], [32]. Seira Morimune et al. [33] observed that when GO was aligned perpendicular to the direction of water vapor molecule diffusion, along the surface of the sample, it resulted in a substantial decrease in the water vapor diffusion coefficient. Consequently, this orientation significantly enhanced the barrier properties of the composites against water vapor transmission. In order to enhance the barrier properties of graphene in polymers, the most effective way is to have a directional distribution in the polymer matrix [34], [35], [36].

In this paper, we prepare a highly ordered liner composite with excellent barrier properties. Firstly, a simple ultrasound-assisted coprecipitation method was used to prepare magnetic nanoparticles of Fe₃O₄/reduced graphene (MRGO).This sonochemical-based method has the advantages of simplicity, directness and flexibility [37], [38], [39]. The sonochemical synthesis method utilizes the physical and chemical effects of ultrasound waves, which can generate intense vortex currents and high temperature and pressure conditions in the reaction system, thus facilitating the mixing and reaction of substances. Ultrasonic radiation excites the process of formation and collapse of tiny bubbles in the liquid, creating a region of localized high temperature and pressure that facilitates the formation of precipitates and control of particle size. Subsequently, MRGO was added to the HTPB matrix to induce ordered alignment under a low magnetic field (200 mT). Then, the morphology and microstructure of the prepared MRGO and ordered MRGO/HTPB liners were characterized. In addition, the water vapor permeability and plasticizer migration were evaluated. Finally, a theoretical model was used to further analyses the mechanism of MRGO enhanced barrier property of HTPB. This work can open up a new avenue for the development of liner materials with advanced properties for industrial applications.

2. Experimental

The chemical information and experimental details can be seen in Supporting Information (SI).

GO was prepared by a modified Hummers’ method and MRGO was prepared by a ultrasound-assisted co-precipitation method [40]. First, 0.1 g GO was dispersed in 100 mL deionized water and sonicated for 0.5 h by using an ultrasound cleaning bath (.

SCIENTZ-IID, Ningbo Scientz Biotechnology, frequency: 20 kHz, nominal power: 200 W) to obtain a uniform suspension. Next, 0.15 g FeCl₂·4H₂O and 0.41 g FeCl₃·6H₂O were dispersed in 50 mL deionized water and then added to the GO suspension. Then, ammonium hydroxide was added to the suspension dropwise until the pH reached to 11. Next, the mixture was then further reduced by hydrazine under ultrasound irradiation by a high power horn-type ultrasound sonicator (UP-250 Shanghai Qiqian Electronic Technology, frequency: 20 kHz, nominal maximum power: 1200 W). The device is directly immersed in the reaction system of the dispersion. The pulse mode of “5s on / 5 s off” is adopted to avoid excessive temperature. Finally, the final product was washed with deionized water at least three times and lyophilized for further use. For comparison, we used conventional mechanical mixing treatment instead of ultrasonic treatment to synthesize MRGO, and the conditions remained unchanged.

Fig. 1 shows the process of preparing ordered MRGO/HTPB liners. Briefly, MRGO (500 mg) was dispersed in acetone (50 mL) and ultrasonically treated at 30 °C for 60 mins. Then the MRGO dispersion was mixed with 50 g HTPB at 60 °C by strong mechanical stirring to evaporate acetone. After cooling to room temperature, IPDI was introduced at a curing factor R = 1.3, and the mixture was mechanically stirred for 10 mins to obtain a uniform dispersion. Subsequently, the nanocomposite solution was subjected to ultrasound treatment for 1 h by using an ultrasound cleaning bath (SCIENTZ-IID, Ningbo Scientz Biotechnology, frequency: 20 kHz, nominal power: 200 W) and then vacuum-degassed before pouring into a 100 mm × 100 mm × 2 mm mold. To make MRGO orderly arranged in the HTPB matrix, a pair of permanent magnets was fixed along the horizontal direction to generate a parallel weak magnetic field (200 mT) (as shown in Fig. 1).The mold was placed in the magnetic field for 24 h to ensure orderly dispersion of MRGO, then ordered MRGO/HTPB samples were obtained after a 3-day curing process in a hot air oven heated to 60 °C [41], [42], [43], [44]. HTPB and randomly distributed MRGO/HTPB nanocomposites were prepared in the same way, with the difference that there was no magnetic field.

Fig. 1.

Schematic illustration for preparing ordered MRGO/HTPB liners.

3. Results and discussion

3.1. Characterization of MRGO

MRGO is prepared by using a high power horn-type ultrasound sonicator. For comparison, the effects of mixed treatment instead of ultrasonic treatment were also studied through FTIR, UV, and SEM. Fig. 2(a) and (b).are the FT-IR spectra and UV spectra of GO, samples formed under mixed treatment and ultrasonic treatment at 25 °C, respectively. The FT-IR spectrum of the sample formed under mixed treatment shows that the oxygen-containing functional groups in GO still exist. On the contrary, all peak intensities caused by oxygenation functional groups decreased after ultrasound treatment. In the UV spectra of GO, a typical peak appears at around 230 nm in the spectrum. At the same conditions, there was no peak shift at 230 nm under mixing treatment. While the absorption peak at 230 nm was significantly red-shifted to 270 nm after ultrasonic treatment. In addition, the color of the GO aqueous dispersion changed from light brown to black after ultrasonic treatment (Fig. S2), suggesting that GO was reduced. According to these results, it is found that ultrasonic treatment is an excellent technology compared with mixed treatment.

Fig. 2.

FT-IR spectra (a) and UV spectra (b), SEM images of samples formed under mixed treatment (c) and ultrasonic treatment (d), ΔA under different ultrasonic power at different time (e), TG curves of MRGO synthesized by different power (f).

We also investigated the effect of different ultrasonic powers (400 W, 600 W, 800 W) on the formation of MRGO. The absorbance is usually related to the concentration of the solute, and the rate of MRGO formation is represented by comparing the changes in absorbance at 600 nm. Fig. 2(c) shows the change of absorbance at 600 nm during the generation of MRGO under different ultrasonic powers. The results show that ΔA increases with the increase of ultrasonic power. Obviously, high power ultrasound is an important factor to accelerate the formation rate of MRGO. We also used thermogravimetric analyzer to analyze the thermal stability of MRGO synthesized by different power. As shown in the Fig. 2(d), MRGO prepared at different ultrasonic powers showed similar thermal weight loss behavior. The grain size of MRGO prepared at different powers is similar (Fig. S3), while the thermal stability of MRGO synthesized at 800 W is the best, so we use the MRGO prepared at 800 power in later experiments.

The XRD characteristic diffraction peaks of Fe₃O₄ NPs exist in the obtained MRGO products can be assigned to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) planes, which match well with the cubic spinel crystal structure of Fe₃O₄ (JCPDS No.: 19–0629) (Fig. S4a). The hysteresis loops show that Fe₃O₄ and MRGO exhibit the same superparamagnetic behavior without remanence and coercivity, but the magnetic saturation value of MRGO is significantly lower than that of pure Fe₃O₄ (Fig. S4b). This decrease can be attributed to the nano-size and content of Fe₃O₄ NPs and the presence of graphene sheets [45], [46]. In addition, MRGO can be easily separated under a magnet. The TEM image show that Fe₃O₄ particles are uniformly dispersed on the surface of layered RGO (Fig. S4c). Few of them aggregate to form larger particles. This is because the strong micro-turbulence and micro-jet generated by ultrasonic cavitation can significantly reduce the secondary aggregation of particles. Ultrasound treatment can improve the dispersion of particles and effectively control their size, which has been widely applied in various fields.

3.2. Structure of MRGO/HTPB liners

Confocal laser scanning microscopy was utilized to observe the distribution of MRGO in HTPB. Fig. 3 (a) and (b) demonstrate that MRGO is dispersed randomly within the HTPB matrix, with some agglomerated MRGO nanofillers present. To improve the distribution of MRGO and achieve a specific orientation in the HTPB matrix, a magnetic field of 200mT was applied during the curing stage of the composites. The influence of the static magnetic field resulted in the formation of beam-like structures composed of MRGO (Fig. 3c and d), indicating its orderly dispersion in the HTPB matrix. SEM was also employed to observe the ordered distribution of MRGO in HTPB. Fig. 3 (e) reveals that the MRGO/HTPB composite did not exhibit the characteristic arrangement of MRGO in the polymer matrix. However, after molding in a 200mT magnetic field, it became evident that MRGO was parallelly aligned along the direction of the magnetic field (Fig. 3f). The SEM images further confirmed the significant orientation of MRGO in the polymer matrix. This magnetic field-induced orientation of MRGO nanofillers in the HTPB matrix may contribute to the improvement of the barrier performance of HTPB. The alignment of MRGO in the HTPB matrix is a critical factor that significantly enhances the barrier properties of solid rocket motor liners. The ordered arrangement of MRGO nanosheets in the HTPB matrix may create a complex network structure known as the “tortuous path”. This unique architecture hinders the diffusion of gas molecules and plasticizers, thereby improving the overall gas barrier performance of the liner.

Fig. 3.

Optical micrographs of random MRGO/HTPB (a), (b) and ordered MRGO/HTPB (c), (d). SEM images of random MRGO/HTPB (e) and ordered MRGO/HTPB (f).

We also observed the dynamic orientation process of MRGO in HTPB matrix under the influence of a parallel magnetic field. When a parallel magnetic field is applied, the flaky nanoparticles develop a magnetically induced dipole with an N-pole at one end and an S-pole at the other end. The interaction between the external magnetic field and the magnetic induction dipole causes the nanoparticles to align along the long axis in parallel to the magnetic field direction. Fig. 4 illustrates the rapid movement of MRGO upon the application of the magnetic field, and after approximately 30 s, a beam structure becomes visible. After 120 s, it becomes evident that MRGO has formed a distinct bundle structure and is neatly arranged within the HTPB matrix.

Fig. 4.

Optical micrographs of MRGO in the HTPB exposured to the magnetic field: 0 s (a), 10 s (b), 30 s (c), 60 s (d), 90 s (e) and 120 s (f).

3.3. Barrier properties of MRGO/HTPB liners

The gas barrier performance of the HTPB liner was evaluated through the utilization of the pressure difference method [25], with the resulting water vapor permeability being graphically depicted in Fig. 5. The water vapor permeability of pure HTPB liner and random MRGO/HTPB liner are 1.258 g/cm²·day and 0.997 g/cm²·day, respectively. The incorporation of random MRGO into the HTPB liner demonstrated a significant reduction in water vapor permeability when compared to pure HTPB. This reduction can be attributed to the substantial presence of randomly distributed MRGO, which effectively impedes the transmission of water vapor molecules within the HTPB matrix. In this case, the random MRGO/HTPB liner exhibited a noteworthy decrease in water vapor permeability by 20.7 % in comparison to pure HTPB. However, there are still certain imperfections, such as gaps, observed between the randomly distributed MRGO layers and HTPB molecular chains, which allow water vapor molecules to easily pass through. Upon the introduction of a parallel magnetic field, the MRGO layers aligned parallelly and uniformly dispersed within the HTPB matrix, thereby enhancing the barrier effect. Consequently, the least amount of water vapor permeated through the material after magnetic field orientation. The water vapor permeability of ordered MRGO/HTPB liner is 0.809 g/cm²·day, in comparison to pure HTPB and random MRGO/HTPB liner, the ordered MRGO/HTPB liner exhibited a reduction in water vapor permeability by 35.7 % and 18.9 %, respectively, significantly improving the barrier performance. This improvement in barrier performance can be attributed to the ordered arrangement of MRGO filler layers within the HTPB matrix, which acts as a robust physical barrier, impeding the diffusion of water vapor through a “tortuous path” mechanism. The findings of this study effectively demonstrate that the establishment of ordered MRGO through the application of a magnetic field can significantly enhance the gas barrier properties of HTPB.

Fig. 5.

Water vapor permeability of the different HTPB liners.

The joint specimen method was employed to investigate the interfacial migration of DBP. This method entails the bonding of propellant and liner to simulate the actual conditions within an engine charge. HPLC is used to measure the migration of DBP content, so as to determine the degree of migration [14]. Calibration process and results of HPLC external standard method are shown in Fig. S5. We can study the migration of DBP by measuring and comparing the migration equilibrium concentration and diffusion coefficient of various samples. Fig. 6(a) shows the migration concentration curve of DBP in the HTPB liner at 25 °C. Given the significant concentration disparity of DBP between the propellant and HTPB liner, DBP is expected to migrate from the propellant to the liner. Initially, due to the large concentration difference, the migration speed of DBP is relatively fast. However, as the migration process continues, this speed starts to slow down, eventually reaching a state of concentration equilibrium. In the pure HTPB liner, the equilibrium concentration of DBP migration is 18.33 %. When randomly dispersed MRGO is added, the equilibrium concentration continues to decrease. This is mainly because the layered structure of MRGO hinders the diffusion of DBP, making it more difficult for DBP to migrate from the propellant to the liner. As a result, the measured equilibrium concentration in the random MRGO/HTPB liner is 15.26 %, which is 3.01 % lower than that of pure HTPB.

Fig. 6.

Time-dependent DBP concentration in HTPB liner at 25 °C (a), diffusion coefficients of the different HTPB liners (b).

After the introduction of a parallel magnetic field, the equilibrium migration concentration of ordered MRGO/HTPB liner is 13.11 %, which decreases 14.1 % compared to the random MRGO/HTPB liner. This indicates that the addition of ordered MRGO can further suppress the migration and diffusion of DBP. This phenomenon is due to the formation of a dense and directionally aligned barrier by the ordered arrangement of MRGO layers within the HTPB matrix, making it more difficult for DBP to migrate from the propellant to the liner. On the contrary, in the randomly arranged MRGO/HTPB liner, although the presence of MRGO can already inhibit the migration of DBP, DBP may still migrate through these voids or discontinuous regions due to the presence of more voids and discontinuous regions between the MRGO sheets. Therefore, the ordered MRGO/HTPB liner exhibits the best migration resistance, and the migration resistance is 28.5 % higher than that of HTPB. This result suggests that we can effectively improve the migration resistance of HTPB liner by adjusting the arrangement of MRGO.

The equilibrium concentration represents the limit of plasticizer diffusion and migration, while diffusion coefficient describes the rate of diffusion of molecules from the high concentration region to the low concentration region, reflecting the speed of DBP migration [47], [48]. A higher diffusion coefficient indicates an increased migration rate of DBP molecules, leading to accelerated diffusion and enhanced migration. According to Fick’s second law, the diffusion coefficient was obtained by processing the migration data [44]. The linear fitting curves are shown in Fig. S6. The experimental results of the diffusion coefficients for the three liners (HTPB, random MRGO/HTPB, ordered MRGO/HTPB) show a decreasing trend (Fig. 6b), which were 1.47E-04 m²s⁻¹, 1.22E-04 m²s⁻¹ and 1.02E-04 m²s⁻¹, respectively. Among them, pure HTPB has the highest diffusion coefficient, indicating that DBP migrates most rapidly in pure HTPB. In contrast, the ordered arrangement of MRGO layers forms a more effective barrier in the ordered MRGO/HTPB liner, resulting in the longest diffusion path for DBP and thus the slowest diffusion rate and the smallest diffusion coefficient. The diffusion coefficient results validate the previous conclusion that the ordered MRGO/HTPB liner exhibits the best migration resistance. By inducing the ordered dispersion of MRGO in the HTPB matrix through a magnetic field, the migration of DBP can be effectively suppressed. Compared with previous reports (Table S1), ordered MRGO/HTPB liner has a relatively low diffusion coefficients, which could be used to inhibit the migration of DBP.

3.4. Contact angles

Fig. 7 present the contact angle test results for HTPB liners, which reflect the affinity between the HTPB liners surface and liquids, further indicating the improved barrier performance with the introduction of MRGO. From Fig. 7(a), it can be observed that the contact angle between the filler-free HTPB liners and deionized water (DI) is 81.6°. After the addition of randomly dispersed MRGO, the contact angle for the random MRGO/HTPB liner increases to 94.8°. Furthermore, with the application of a parallel magnetic field, the contact angle for the parallel ordered MRGO/HTPB liners reaches 101.6°. A significant increase in the contact angle between the HTPB liner and DI is evident with the inclusion of layered MRGO, and an even greater increase is observed after adding parallel magnetic field induces MRGO ordering, confirming the ordered dispersion of MRGO in HTPB is useful to enhance the barrier properties. In Fig. 7(b), it can be seen that the contact angles between HTPB, random MRGO/HTPB, and ordered MRGO/HTPB liners with DOP gradually increase, indicating a reduced affinity between them. The contact angle data for DOP reaffirms that the introduction of a parallel magnetic field to achieve parallel ordered dispersion of MRGO in HTPB is beneficial for enhancing the barrier performance of the HTPB liner, effectively suppressing DBP migration and diffusion. These findings are of substantial importance for industrial applications, especially in the manufacturing of solid rocket motors. The improved resistance to gas and plasticizer migration can contribute to better longevity and reliability of solid rocket motors.

Fig. 7.

Contact angles of DI and DBP.

3.5. Analysis of the barrier mechanism

By analysing the above barrier property test results, the theoretical model of diffusion and migration of small molecules in random and oriented dispersed MRGO/HTPB liner can be obtained to further analyse the mechanism of MRGO enhanced barrier property of HTPB. Fig. 8 shows the model diagrams of the diffusion mechanism of small molecules in pure HTPB, random and ordered MRGO /HTPB liners. In pure HTPB, small molecules diffuse from the high concentration region to the low concentration region due to the concentration difference, and the branching path of their diffusion is denoted by P_vk. The path taken by small molecules in penetrating the HTPB matrix can be calculated by using Eq (1) [49]:

$$l_0=\sum _{k=1}^n{P}_{\mathit{vk}}$$ (1)

Fig. 8.

Diffusion mechanism model diagram of small molecules in HTPB liners.

When the flake MRGO is added to HTPB, small molecules cannot penetrate the flake of MRGO. As a result, the diffusion path inside HTPB becomes tortuous. Small molecules first diffuse inside the HTPB through the branching path P_vk, and when they encounter the flake MRGO, they need to diffuse through the branching path P_sk that surrounds the MRGO. The total diffusion path length of small molecules through HTPB was l, includes the branching path P_vk in the HTPB matrix and the branching path P_sk surrounding the lamellar MRGO. The incorporation of the flake MRGO significantly lengthens the effective diffusion path length of small molecules inside HTPB, and the total diffusion path length can be calculated using Eq. (2):

$$l=\sum _{k=1}^n(P_{\mathit{vk}}+P_{\mathit{sk}})$$ (2)

For the ordered MRGO/HTPB liner, the branching path P_sk has a longer diffusion path around the MRGO layer. Osmotic diffusion of small molecule plasticizers in the HTPB liners increases the small molecule content of the HTPB, where the one-dimensional Langmuirian formula (3) applies [50]:

$$\omega (t)={\omega }_{\infty }-(\frac{v}{u+v})\exp [-ut]-(\frac{u}{u+v})8\frac{{\omega }_{\infty }}{{\pi }^2}\exp [-{(\frac{\pi }{l})}^{2}Dt]$$ (3)

where u is the free diffusion index and v is the hindered diffusion index, both values are normal numbers. The value of v/(u + v) in the formula can be calculated from the results in the weight gain curve in Fig. S7.

For pure HTPB, the total length of the diffusion path for small molecules penetrating the HTPB liner can be calculated from the Langmuirian prediction curves is l₀. The total lengths of the diffusion paths for small molecules in the random and ordered MRGO/HTPB liners are 1.32 l₀ and 1.83 l₀, respectively. The longest diffusion paths for small molecules in the ordered MRGO/HTPB liners are due to the fact that the diffusion branching paths of small molecules around the MRGO are extended when the MRGO sheets are arranged in a parallel manner. This is because when the MRGO lamellae are arranged in a parallel and orderly manner, the orderly arranged MRGO extends the diffusion branching paths of small molecules around the MRGO (P_bk), which ultimately lengthens the effective diffusion paths and slows down the migration rate of small molecules, which further enhances the barrier performance. The experimental test results and the analysis of the diffusion mechanism demonstrate that the oriented dispersion of MRGO in the HTPB matrix is beneficial to enhance its barrier properties.

4. Conclusion

In this study, we synthesized magnetic MRGO nanofillers employing an ultrasound-assisted co-precipitation technique. For enhancing the barrier characteristics of HTPB, we incorporated MRGO into the HTPB matrix and achieved orderly distribution by using a magnetic field. The findings reveal that MRGO arranged in an orderly manner boosts the barrier properties of HTPB more effectively than randomly dispersed. Compared with the random MRGO/HTPB liners, the water vapor permeability and the migration equilibrium concentration of DBP of the ordered MRGO/HTPB liners reduced by 18.9 % and 14.1 %, respectively. This improvement is primarily due to the robust physical barrier created by the orderly distributed MRGO flake fillers within the HTPB matrix, which obstructs the diffusion of water vapor and DBP molecules via the “tortuous path” effect. The exceptional barrier properties of these ordered MRGO/HTPB composites make them promising candidates for advanced liners applications.

However, there are certain limitations to this approach. Firstly, the synthesis and dispersion of MRGO within the HTPB matrix require precise control and optimization to ensure consistent results. The process is also dependent on the use of a magnetic field, which may not be feasible in all production environments or scales. Additionally, while our study has shown improvements in barrier properties under controlled conditions, further testing is needed to evaluate the performance of these liners under real-world conditions, such as varying temperatures and pressures, long-term storage, and actual flight conditions.

Future work should focus on addressing these limitations. More extensive investigations into the synthesis and dispersion techniques for MRGO could optimize this process for larger scale production. Moreover, comprehensive durability and performance tests under more challenging conditions will provide a better understanding of the long-term stability and effectiveness of these enhanced liners. Lastly, exploring other innovative filler materials or additional methods to improve liner properties could open new avenues for research and development in this field. These efforts would pave the way for the practical implementation of this technology, bringing us one step closer to safer and more reliable solid rocket motors.

CRediT authorship contribution statement

Zhehong Lu: Conceptualization, Data curation, Investigation, Methodology, Validation, Writing – original draft. Qiang Zhou: Conceptualization, Funding acquisition, Supervision, Validation, Writing – review & editing. Yulong Zhang: Conceptualization, Funding acquisition, Supervision, Validation, Writing – review & editing. Abdullah Atya: Investigation, Methodology. Tengyue Zhang: Investigation, Methodology. Guangpu Zhang: Investigation. Yanan Zhang: Resources. Guigao Liu: Supervision, Writing – review & editing. Wei Jiang: Investigation. Yubing Hu: Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Data availability

Data will be made available on request.