Composite Solid Rocket Propellant Based on GAP Polyurethane Matrix with Different Binder Contents

Islam K. Boshra, Ahmed Elbeih, GUO Lin， Mohamed G. Zaki

(1.School of Chemistry and Environment, Beihang University, Beijing 100191，China；2.Military Technical College, Kobry Elkobbah, Cairo, Egypt)

Abstract:Different GAP-based CSRP samples with different binder contents were prepared and compared with that of conventional HTPB propellant. The crosslinker mixture of trimethylol propane (TMP) and butane diol (BD) was used in the GAP matrix beside the addition of dibutyltin dilaurate (DBTDL) to ensure cross-linking and curing completion of the prepared CSRP. The viscosity and hardness of all prepared formulations were monitored continuously during the curing process. The mechanical characteristics of cured samples were tested. The burning rate at operating pressure and specific impulse were measured, while the theoretical specific impulse (Isp) was calculated by ICT code and compared with the measured results. According to the results, DOA was found to be a suitable plasticizer for GAP when using in propellant.The mechanical properties of CSRP with 25% GAP can produce the optimum mechanical behavior, which is close to that of HTPB-based CSRP. The optimum GAP-based formulation is one candidate to replace the traditional HTPB-based CSRP with high burning rate for some applications.

Keywords:physical chemistry; glycidyl azide polymer (GAP); hydroxyl terminated polybutadiene (HTPB); cross-linker; mechanical properties

Introduction

Glycidyl azide polymer (GAP) is a remarkable energetic propellant binder which is giving enhanced performance than hydroxyl terminated polybutadiene (HTPB) the currently used propellant binder worldwide[1-2]. Thermal and ballistic performance of the HTPB propellants is suitable for application in rockets[3-4]. The structure of GAP polymer has a highly energetic azide group (C—N3group). Energetic oxidizers like ammonium dinitramide (ADN) when used with GAP can cause an enhancement of the specific impulse and also exhaust with chlorine-free[5-6]. GAP has a high heat of formation of +117.2kJ/mol and the scission of C—N3group which could generate high energy of 685kJ/mol[7-9]. The terminal hydroxyl group in GAP structure can be reacted with diisocyanate which results in a 3D network structure similar to HTPB propellant[10].

One of the most important features should the binder have is suitable physical and mechanical properties to be accepted for use in the field of propellants. HTPB based binder has the excellent elastic ability over various temperatures and it has high values of shelf life[11]. Therefore, HTPB has various applications in the energetic applications field as composite solid propellant and plastic bonded explosives[12-13]. On the other side, GAP has relative poor mechanical properties compared to HTPB and usually special additives like cross linkers and chain extenders are added to enhance the mechanical properties[14]. Using of the energetic GAP instead of HTPB is promising topic for the researchers to obtain higher ballistic performance like burning rate and specific impulse[15]. According to the reports[16-17], the crosslinking ratio and curing conditions besides the significant effect of the crosslinker have an important role in improving the cured polymer mechanical properties.

Regarding rocket propellant studies, the GAP polymeric matrix has the ability to adhere to all the solid ingredients as an oxidizer and even the metallic fuel to form a cross-linked matrix in one integral unit[18]. In the same time, the mechanical characteristics of GAP based propellant is quite poor because of low density of cross linking, curing network irregularity, and insufficient interface between binder and fillers. But it could be improved by using of GAP with high molecular weight (2000-3000g/mol) and by adding compatible crosslinker and chain extender[19-20].

GAP′s energetic properties are not the result of its oxidation products but rather due to chain splitting of the azide group, which gives nitrogen gas with a high reaction heat[21].

Compared with traditional binders, the applications of energetic binders like GAP lead to superior combustion parameters. Most of recent GAP-related research focused on synthetic optimization, structural modification and formulation development[22].

The selection of a suitable chain extender, crosslinker and curing agent is essential in order to achieve optimized stress, stress and hardness levels of mechanical properties that have an important influence on the propellant performance. To ensure that the final product has op timized processing schedules and adequate mechanical quality, the reaction between GAP and various hydroxyl additives with curing agent should be carefully studied and monitored[23-24].

In this paper, the effect of GAP based CSRP binder content was studied by preparation of several composite solid rocket propellant samples with different percentages of GAP based binder and for obtaining improved mechanical properties of these samples, suitable additives with certain values and curing ratio were used to maintain polymeric matrix of three dimensions network as concluded from reference [21]. The viscosity build-up and shore A of all the prepared formulations besides traditional HTPB based propellant were measured. The mechanical and ballistic characteristics (using 6 inch motor) of the selected GAP formulations and HTPB propellant were measured.

1 Experiment

1.1 Materials

Hydroxyl-terminated polybutadiene (HTPB R-45M obtained from Zibo Qilong Chemical Industry Co., Ltd., Shandong Province China, density: 0.9g/mL; hydroxyl value: 0.84 meq/g; molecular mass (Mn): 2800g/mol; viscosity at 25℃: 5800cps), 1,1,1-trimethylol propane (TMP) 97% (23235-61-2), 1,4-butane diol (BD) 99% (110-63-4), dibutyl tin dilaurate (DBTDL) 95% (77-58-7) , Hexa-metylene di-isocynate (HMDI), DOA, DOZ and DBP supplied by Sigma-aldrich Germany. Aluminium and Ammonium perchlorate (AP) purchased from commercial source were used. Glycidyl azide polymer (GAP) was prepared according to reference[7].

1.2 Preparation of CSRP by casting technique

Different GAP based CSRP formulations were prepared in addition to traditional propellant based on HTPB as a prepolymer. From reference[14], it was illustrated that GAP based PU compositions with curing ratio (NCO/OH =1.0-1.2) and crosslinker ratio of 5% give optimum mechanical characteristics compared with those obtained from PU compositions based on HTPB with curing ratio (NCO/OH =0.8-0.9) which used for conventional and large scale CSRP.

As it has been prepared in reference[21], traditional propellant formulation F1containing 14% HTPB binder content and the other formulations were based on GAP as an energetic prepolymer. CSRP samples F2, F3and F4have the same compositions but with different plasticizers to choose the most compatible plasticizer can be used with GAP. As result, formulations F4, F5, F6and F7have the same components (DOA as a plasticizer) but with different binder contents ranging from 14% to 30% to investigate the effect of binder content on the mechanical characteristics and performance parameters of formulations based GAP; compositions of the formulations prepared are illustrated in Table 1. For comparison, the trimodal distribution ratios was the same in all the compositions, the AP has particle sizes of 400μm (54%), (200μm) (24%) and 7-11μm (22%) in all the studied samples.

Casting technique was used to prepare the propellant samples, starting with the mixing of the various ingredients using an 8kg stainless steel mixer and vacuum casting into the pre-prepared 6-inch motors and JANAF (dumbbell-shaped) mould[21].

1.3 Characterization and analyses

1.3.1 Viscosity build up measurements

Digital Anton paar viscometer of rotational-type, Model physica MCR 101 was used to monitor all samples viscosity during the cure reaction (Viscosity build up) at temperature 55℃, the viscosity build up studies were recorded during and after casting for some time intervals to predict the pot life and maximum casting time that could be reached.

Table 1 Compositions of prepared CSRP formulations

1.3.2 Hardness measurements

Samples were taken out of curing oven daily for 5 minutes to track the sample hardening trend, using the ZWICK (model 3102) hardness tester. The apparatus measures the needle penetration as a function of curing completion through the increase of the sample hardness value by slowly immersing the device needle into the sample and reading the numerical indication shown on the device screen. When shore A value is constant for successive three days, curing is completed and samples are removed from the curing oven.

1.3.3 Determination of mechanical properties

The tensile stress-strain relationship was measured experimentally for each of the prepared samples using LLOYD universal test machine for carrying out all the tensile mechanical tests. The tensile test was carried out for at least three samples of each prepared formulation; the mean value of the obtained results was recorded. The test was conducted under normal temperature. The speed of the cross head was 50mm/min with accuracy of 0.5.

1.3.4 Determination of ballistic performance

By using 6 inch motor as shown in Figure 1, the ballistic performance characteristics of the prepared propellant formulations including burning rates, operating pressures and specific impulses were determined. This large scale motor allows to produce higher thrust which facilitates obtaining more reliable and precise specific impulse results with lower errors. Measurements will be conducted using a 16.25mm 6 inch motor nozzle to obtain certain burning rates corresponded to its operating pressures for each formulation to compare between them using the same nozzle. After curing, the tested samples were mounted into a 6kg propellant testing motor to achieve precise measurements of thrust and specific impulse with small deviations.

Fig.1 Photo of the 6 inch motor

Thermochemical calculations applied using a chemical equilibrium computer program “ICT thermodynamic code produced by Institute of Chemical Technology, version 3, Germany”[25]. ICT code was applied to obtain the theoreticalIspof both GAP and HTPB based formulations, the results were compared with that delivered from 6 inch motor test to show the relation between the theoretical and measured specific impulse.

2 Results and Discussion

2.1 Viscosity build up

Table 2 presents the viscosity build-up of the all prepared formulations. As mentioned in reference [21] propellant formulations with 25% GAP based binder F2and F3showed that both DOZ and DBP are GAP-incompatible plasticizers for using in propellants because of the separation found between the binder system and solid fillers. At the same time F4(DOA plasticizer) showed lower viscosity, more convenient mixing and casting compared to F1based on HTPB which means that DOA can be assumed as a GAP-compatible plasticizer in the propellant.

Table 2 Viscosity build up with time of CSRP formulations

As shown in Figure 2. The viscosity of F5(14% Gap binder) started with high values and by short time it reached very high values, as a result it was impossible to be casted (short pot life time), on the other side F6(20% GAP binder) shows higher viscosity than F1but still could be casted but hardly. However, in case of F7(30% GAP binder) had lower viscosity than other GAP based samples so it was clear that formulations F4and F7ensure favourable process ability characteristics for propellant slurry.

Fig. 2 The viscosity build up with time of propellant formulations

2.2 Hardness

Due to the incompatibility of both plasticizers with GAP, the curing period of F2and F3which contained DOZ and DBP as plasticizer was 14 and 13 days respectively (plasticizer migrated on the propellant surface). F5couldn′t be casted due to its high viscosity, and no hardening or mechanical measurements existed. It was noted that F4, F6and F7(with DOA plasticizer) had a reduced curing period from 12 to 6-7 at 55℃ compared to the HTPB formulation F1as shown in Figure 3. The binder content has a clear effect on the hardness value as recorded in Table 3 which consequently effects on formulation tensile stress which will be discussed later.

Table 3 Hardening measurements of prepared CSRP formulations

Fig. 3 Hardening measurements (Shore A) with days variation curves

2.3 Mechanical properties of the casted CSRP formualtions

Although poor mechanical properties of GAP prepolymer, all GAP based formulations have higher tensile stress than F1(HTPB based) because of the presence of crosslinker and higher curing ratio. F6has the highest tensile stress because it contains the highest solid content but F1has higher strain because of self-crosslinking of HTPB and low curing ratio as shown in Table 4.

Table 4 The mechanical properties of casted CSRP at 25℃

Fig. 4 Effect of binder content on the tensile stress of casted CSRP

Strain values of GAP based samples were effected by binder content which caused F4to have the highest strain but for F7(30% GAP binder), it has the lowest strain although its high binder content but maybe that the presence of high content of bubbles in this amount of binder system has an effect on the result. So the optimum strain gained for GAP based formulations was of formulation F4which was hard to be obtained due to the presence of crosslinker and high curing ratio as illustrated in Figures 4, 5 and 6.

Fig. 5 Effect of binder content on the strain of casted CSRP

Fig. 6 Tensile stress-strain relationship of CSRP formulations

2.4 Ballistic performance

Operating pressure, burning rate and both measured and calculated from thermochemical calculations (ICT code) specific impulse results are listed together in Table 5. The error in the measurements of the burning rate of each sample was ± 0.1mm/s as maximum. Figure 7 shows a comparison between the burning rate and the pressure of all samples tested. From the figure, it is clear that the burning rate of GAP based CSRP with binder content of 25% and 30 % have almost similar burning rate which are higher than the burning rate of HTPB based CSRP. The effect of inserting the energetic binder GAP in propellant compositions were significant in compensating the lower ratios of AP oxidizer in samples F4and F7compared to F1which based on HTPB binder with AP content of 69%. In addition, the pressure of CSRP based on GAP content of 30% is the highest of all the studied samples with the same tested nozzle even that the solid content of this sample is lower than the CSRP based on HTPB. These results show the advantage of using GAP instead of HTPB in propellant formulations.

Table 5 Ballistic performance results of different propellant formulations

Fig. 7 Burning rate-pressure relationship of CSRP formulations

On the other side, Figure 8 shows a comparison between the measure specific impulse and the calculated specific impulse of all the studied samples versus the binder content of each. Samples F4and F7based on GAP binder have higher specific impulse than CSRP based on HTPB. The energetic characteristics of GAP binder caused an increase in the specific impulse of the CSRP even that the content of the metal fuel was decreased in these samples. By comparing the theoretical and practical specific impulse values, it was clear that they were close to each other but the theoretical results of specific impulse have higher valued with a deviation of about 7%.

Fig. 8 Effect of binder content on the theoretical and measured specific impulse of propellant formulations

3 Conclusions

(1)Both viscosity build-up and hardness results show that DOZ and DBP are GAP-incompatible plasticizers for using in propellant due to the separation between the binder system and solid fillers occurred. Formulation F5containing 14% binder could not be casted under normal conditioning due to the short pot life while formulations containing 20% and 25% GAP based binder with DOA as a plasticizer show favourable process-ability characteristics for propellant slurry.

(2)GAP based formulations had a reduced curing time compared with HTPB based formulations from 12 days to 6-7 days at 55℃. Tensile stress of GAP samples were successfully enhanced and gave higher values than HTPB sample F1while the strain results of GAP samples effected by the binder content, which lead to obtain optimum mechanical properties for formulation contains 25% GAP based binder F4with higher stress and acceptable strain compared with traditional HTPB propellant sample F1, this result was hard to be obtained without the addition of crosslinker because of GAP poor mechanical characteristics.

(3)GAP based samples with binder content higher than 25% showed greater operating pressure, burning rate, and specific impulse compared to HTPB based sample under the same condition, although the lower solid content of GAP samples. Comparison between measured specific impulse (by 6 inch motor) and the calculated one (by ICT code) was performed and showed a small deviation of 7%. After all, it′s clear that in the presence of crosslinker, GAP based propellant of 25% binder with DOA as a plasticizer shows a promising performance for the idea of replacing the conventional prepolymer HTPB.