Development of Smokeless GAP Propellants with Various Oxidizers

Transcription

1 SUMMARY Erik Unneberg, Tove K.E. Karsrud, Tom C. Johannessen and Ivar Sollien Norwegian Defence Research Establishment (FFI) Division for Protection and Materiel P.O. Box 25 NO-2027 Kjeller NORWAY A series of GAP binders and smokeless GAP propellants containing HMX, HNF and CL-20 were prepared and characterized. Controllable processing properties and satisfactory curing were obtained. Some of the propellants had fairly good mechanical properties and a glass transition temperature below 50 C. By firing small-scale motors it was found that typical burning rates at 7 MPa were 8, 15 and 19 mm/s for HMX, CL-20 and HNF propellants, respectively. The pressure exponents were about 0.8 for HMX and CL-20 propellants and 0.95 for an HNF propellant. INTRODUCTION A disadvantage with conventional AP/HTPB propellants is the production of chlorine containing smoke during burning. If oxidizers like HMX, HNF, CL-20 and ADN can be used instead of AP, smokeless ( clean ) propellants may be prepared. In addition, it can be seen from Figure 1 that some of these propellants are superior to AP propellants with respect to theoretical performance. Spec.imp. (Ns/kg) AP/HTPB 1 AP/GAP 2 3HMX/GAP4 HNF/GAP 5 6CL20/GAP7 ADN/GAP /13 75/25 75/25 75/25 75/25 75/25 Vol.spec.imp. (Ns/dm 3 ) Figure 1: Theoretical Specific (grey columns) and Volumetric (shaded columns) Impulse of various Oxidizers/Binders based on a Chamber Pressure of 6.9 MPa and 0.1 MPa at Exit [1]. The GAP binder matrix used for the calculations was GAP diol/n100 plasticized with an equal amount of TMETN. Paper presented at the RTO AVT Specialists Meeting on Advances in Rocket Performance Life and Disposal, held in Aalborg, Denmark, September 2002, and published in RTO-MP-091. RTO-MP

2 In this work we describe the preparation and characterization of propellants based on HMX, HNF, CL-20 and the energetic binder GAP. Both di- and trifunctional GAP prepolymers were used, cured with tri- and diisocyanates, respectively. Four different plasticizers were considered for use in GAP binders and GAP propellants, namely GAPA, TMETN, MEN42 and BDNPA/F. One part of the motivation for doing so, is the fact that the low-temperature properties of GAP elastomers and composites are inferior to conventional HTPB based compounds, due to the higher glass transition temperature (T g ) of GAP. Moreover, the processing properties during mixing of the propellant may be improved by use of plasticizers with lower viscosity than GAP. Finally, energetic plasticizers like those mentioned above will contribute to a better propellant performance as the ratio plasticizer/gap is increased. The type of plasticizer is important in this respect, as theoretical calculations show that the specific impulse for a GAP based compound increases in the order BDNPA/F<GAPA<TMETN<MEN42 whereas the volumetric specific impulse increases in the order BDNPA/F<GAPA<MEN42<TMETN [1]. EXPERIMENTAL Chemicals 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclo-octane (HMX), Dyno. Hexanitro-hexaaza-isowurtzitane (CL-20), Thiokol. Hydrazinium nitroformate (HNF), APP. Difunctional hydroxyl terminated glycidyl azide polymer (GAP diol), SNPE. Trifunctional hydroxyl terminated glycidyl azide polymer (GAP triol), SNPE. Desmodur N-100 (N100), i.e. biuret (trifunctional) of hexamethylene diisocyanate, Bayer. 4,4 -Methylene diisocyanate (MDI), Aldrich. Azide terminated glycidyl azide polymer (GAPA), SNPE. Trimethylolethane-trinitrate (TMETN), SNPE. Bis-(2,2-dinitropropyl)-acetal/formal (BDNPA/F), SNPE. Mixture of 58% methyl-2-nitratoethyl-nitramine and 42% ethyl-2-nitratoethyl-nitramine (MEN42), Dyno. Dibutyltindilaurate (DBTDL), Fluka, 2-nitro-diphenyl-amine (2-NDPA), lead citrate (Pbci), lead stearate (Pbst), carbon black. Mixing and Curing CL-20 and most liquid chemicals were dried before use. Batches of up to 60 g binder or propellant were mixed and automatically stirred in FFI-constructed small-scale mixers (remote controlled), whereas larger batches ( kg) were mixed in a 1-liter Planetron vertical mixer. Mixing of diol/n100 binders of up to 40 g batch size was carried out at room temperature, and larger diol/n100 binder batches were prepared at 60 C. In binders with MDI as curing agent the mixing temperature was 60 C in order to prevent crystallization of MDI (melting point 37 C). Exceptions were made for two samples (POL-14 and -15) which were mixed at 45 C. Besides sample HNFP-3, which was mixed at 45 C, all small-scale propellants (50-60 g) were prepared at room temperature. Standard mixing temperature of larger batches ( kg) was 60 C, but a small variation was done for CLP-5. Before the curing agent was added, the temperature was reduced from 60 C to 40 C in order to extend the pot life. For almost all samples, curing was performed at 60 C. There were four exceptions: POL-9, POL-14 and HNFP-2 were cured at 25 C, and POL-15 was cured at 45 C. Compositions Two types of polyurethanes were prepared: GAP diol cured with N100 and GAP triol cured with MDI. In addition, two samples were prepared with equal amounts of diol and triol in combination. These mixtures were cured by either MDI or N100. Details of the composition are presented in Table 5 and in Table 6 together with other data. In addition, GAP binders with various diol/triol ratios (from 20 to 80% triol) were mixed and cured with MDI. 6-2 RTO-MP-091

3 A bimodal particle fraction was used in the HNF propellants (HNFP) and the binders were based on POL-9, -14 and -15. The compositions are given in Table 1. A high content of DBTDL was added to HNFP-1 as this propellant was cured by the aliphatic curing agent N100 at room temperature. An excess of isocyanate was added to the HNF samples to maintain a good curing rate even if parts of HNF should react with the curing agent. No plasticizer was used. Table 1: Compositions of HNF Propellants (Batch size: g) Propellant HNFP-1 HNFP-2 HNFP-3 GAP binder system Diol/N100 Triol/MDI Triol/MDI HNF (%) DBTDL (ppm in binder) NCO/OH Bimodal or trimodal particle distributions were used in the HMX propellants (HMXP). Carbon black was added to most of the mixtures for firing purposes. The compositions are given in Table 2. Table 2: Compositions of HMX Propellants (Batch size: 500 g) Propellant HMXP-1 HMXP-2 HMXP-3 HMXP-4 HMXP-5 HMXP-6 HMX particle distribution Bimodal Bimodal Bimodal Bimodal Trimodal Trimodal GAP binder system Triol/MDI Diol/N100 Diol/N100 Diol/N100 Diol/N100 Diol/N100 Plasticizer GAPA GAPA TMETN BDNPA/F BDNPA/F BDNPA/F HMX (%) Plasticizer (%) DBTDL (ppm in binder) Carbon black No Yes Yes Yes Yes Yes Pbci (%) NCO/OH A bimodal distribution of fine and coarse CL-20 particles was used in all CL-20 propellants. The binder system was GAP diol cured with N100 (NCO/OH=1.1). A survey of the compositions is given in Table 3. Table 3: Compositions of CL-20 Propellants Propellant CLP-1 CLP-2 CLP-3 CLP-4 CLP-5 CLP-6 CLP-7 CLP-8 CLP-9 Plasticizer BDNPA/F BDNPA/F BDNPA/F BDNPA/F BDNPA/F BDNPA/F TMETN TMETN TMETN CL-20 (%) Plasticizer (%) DBTDL (1) NDPA No Yes Yes Yes Yes Yes Yes Yes Yes Carbon black No No Yes Yes Yes Yes Yes Yes Yes Pbst (%) Total solids (%) Batch size (g) (1) ppm in binder RTO-MP

4 Characterization Tap density measurements were performed on a ST5000 tap volumeter (J. Engelsmann A.G. Apparatbau) taps in 1 h were performed before the volume of the solids was measured, and the tap density was calculated. Vacuum thermal stability (VTS) tests were carried out to check if the propellant ingredients were compatible with each other. In accordance with STANAG 4147 [2], 2.5 g of each of two components were mixed and tested at the standard conditions 100 C/40 h. A pressure transducer continuously measured the pressure due to gas evolution. Some of the cured propellants were also subjected to VTS testing in order to evaluate their thermal stability. The conditions were 100 C/40 h, except for samples containing HNF where lower temperatures were used. In order to study thermal decomposition, TGA and DSC experiments were performed on a TGA 2950 (TA Instruments) and on a Perkin-Elmer DSC-7, respectively, with nitrogen as carrier gas. The curing was studied on a Physica UDS200 rheometer (plate-plate, shear rate γ=1%). FTIR spectra were recorded on a Nicolet Avatar 320 spectrometer with a DTGS KBr detector. The instrument was equipped with a ZnSe crystal plate (45 ) and a Spectra-Tech Thermal ARK temperature controller for ATR measurements. The curing was monitored in situ by analyzing the NCO ( 2270 cm -1 ) and urethane ( 1725 cm -1 ) bands quantitatively, utilizing the C-H stretching bands as internal standards. Separate standard experiments confirmed that the absorbances used for the calculations were in the linear area for the validity of Beer s law. Viscosity measurements of propellant ingredients at 20 C and 60 C were also performed on the rheometer. Hardness testing was carried out on a Bareiss BS61 Shore A instrument, whereas an MTS instrument was used for tensile testing of end-bonded samples (crosshead speed 50 mm/min). T g values were determined either by DSC (10 K/min) or on a dynamic mechanical analyzer (DMA) from TA Instruments (DMA2980, dual cantilever). On the latter instrument T g was determined by the maximum of the loss modulus at an oscillating frequency of 1 Hz, in accordance with STANAG 4540 [3]. A heating rate of 0.3 K/min was found to be sufficiently low to eliminate temperature gradients. The ballistic properties were measured by firing small-scale rocket motors with radial burning propellant grain. The propellant weight was either 40 g (2.3 mm web) or 280 g (12.5 mm web). The pressure vs. time curve was recorded during the firings and burning rate vs. pressure could then be calculated. The propellant density was determined by weighing the propellant grain in the inner steel tube of the small-scale motors. Hazard testing (BAM friction and impact, file plate) was performed by NAMMO Raufoss, whereas performance calculations were done by means of a computer program [1]. RESULTS AND DISCUSSION Characterization of Ingredients VTS testing of propellant ingredients did not reveal any incompatibilities, as less than 5 ml net gas production was detected per 5 g of most two-component combinations. Some exceptions were, however, discovered. Firstly, HNF reacted with GAP, plasticizers and curing agents even at 80 C, but were reasonably stable at 60 C. Therefore it was decided that the only way HNF could be used in a GAP propellant would be to mix and cure it at low temperature. Secondly, the curing agents also failed in the VTS test with most components at standard conditions (100 C, 40 h), but as the isocyanate groups will be consumed during the curing, they will not affect the stability of the propellant. Finally, a few 1:1 compositions including at least one of the minor additives seemed to be incompatible, but as these components were intended to be added in small amounts, they were considered to be acceptable additives in GAP propellants. 6-4 RTO-MP-091

5 According to the values in Table 4 the viscosity of GAP triol was lower than for diol, which indicates that propellants based on triol might have favorable processing properties. Triol also produced less gas in the VTS test than diol did, and this may be advantageous with respect to stability. Based on the VTS results in Table 4, BDNPA/F should be the most thermally stable plasticizer, whereas MEN42 should be the least stable one. However, reactions with other ingredients in the propellants are not considered here. Due to the viscosity measurements presented in Table 4, the plasticizers should contribute to better processing properties in the sequence GAPA<BDNPA/F<TMETN<MEN42. Table 4: Viscosity and VTS Values of GAP and Plasticizers Compound GAP diol GAP triol GAPA MEN42 BDNPA/F TMETN Viscosity at 20 C (Pa s) Viscosity at 60 C (Pa s) VTS, 100 C/40 h (ml/g) GAP Binders Mixing and Curing The diol/n100 sample POL-9 cured successfully at room temperature due to a high concentration of DBTDL. This result was important, as such a polymer matrix could be suitable for preparing propellants containing temperature sensitive oxidizers. The other diol/n100 samples cured nicely at 60 C. This was confirmed by rheological monitoring of the curing in addition to FTIR investigation of curing samples. By visual inspection, the time needed to complete the curing varied from 1 to 6 days. Other things being equal, it was noticed that presence of plasticizer seemed to increase the necessary time of curing. Nevertheless, sample POL-1 (without plasticizer) and POL-2 (with GAPA) cured at about the same rate, but the DBTDL content of the latter binder was one order of magnitude higher than in POL-1 due to earlier experience with this plasticizer [4]. Instead of DBTDL, 1% Pbst was added to sample POL-8 during the mixing process. This was done to examine the possible catalyzing effect this potential burning rate modifier had on the curing rate. It was found that the curing rate was about the same as for an analog mix but with 50 ppm DBTDL instead of Pbst. This observation must be taken into account when pot life of propellant mixes containing Pbst shall be controlled. In general, the curing rates of the triol/mdi binders were high due to the fact that MDI is an aromatic diisocyanate. It was found that no or only a small amount of curing catalyst was needed, as most of the binders based on GAP triol and MDI underwent satisfactory curing within 3 days at 60 C. The only problem in this series occurred for POL-14, as this sample cured too fast to cast proper samples. In spite of the fact that curing was carried out at room temperature, the DBTDL content in this sample was obviously too high, and must be reduced before this binder can be used in propellants. Hardness, T g and VTS Shore A and T g values of diol/n100 binders are presented in Table 5. The plasticizer lowered the hardness as well as the T g of the samples. In the series POL-1, -2, -3 and -5 it was seen that the sample plasticized with MEN42 (POL-3) was softest (Shore A=0), whereas the corresponding GAPA and BDNPA/F samples exhibited values of 25 and 21, respectively. From a T g of 43 C for the unplasticized sample (POL-1), the value was lowered to 50 C by BDNPA/F addition and to 52 C by using GAPA. By using TMETN (POL-4) an even lower T g was observed, but the lowest T g (below 60 C) was measured for the sample with 30% MEN42 (POL-3). The effect MEN42 had on T g was much less pronounced for sample POL-6 that contained only 15% MEN42. The fact that this sample had a slightly higher NCO/OH ratio RTO-MP

6 than POL-3 could only account for a minor part of the T g difference. The low Shore A value of MEN42 samples, the high gas production of MEN42 in the VTS test (Table 4) and the higher VTS value of POL-6 (containing MEN42) compared to POL-7 (Table 5) were the reasons why this plasticizer was not used in propellants in this work. In addition, upon aging POL-6 at 60 C, its Shore A value decreased from 12 to 5 in 60 days. After 90 days the value was zero, showing clearly that the sample was not very resistant to aging. Table 5: Compositions and Properties of GAP Diol/N100 Binders Binder POL-1 POL-2 POL-3 POL-4 POL-5 POL-6 POL-7 POL-8 POL- 9 (1) Plasticizer type None GAPA MEN42 TMETN BDNPA/F MEN42 BDNPA/F BDNPA/F None Plasticizer (%) DBTDL (ppm) (2) 500 NCO/OH Shore A n.m n.m T g ( C) 43 (3) 52 (3) 64 (3) 56 (4) 50 (3) 46 (4) 47 (4) n.m. n.m. VTS (ml/g) n.m. n.m. n.m. n.m. n.m n.m. n.m. Batch size (g) n.m. = not measured. (1) Cured at 25 C. (2) 1.0% Pbst added. (3) Determined by DSC. (4) Determined by DMA. Compared to POL-5, sample POL-7 had a slightly higher T g. One explanation for this may be that the NCO/OH ratio of POL-7 was somewhat higher than for POL-5 (Table 5). However, POL-5 contained a few percent more BDNPA/F than POL-7, and this variation may rather account for the T g difference. This assumption was supported by the observation that changing NCO/OH from 1.0 (sample POL-10) to 1.2 (POL-15) did not have any significant effect on T g, but led to an increase of the Shore A value. The triol/mdi binders had higher T g values than the diol/n100 samples (Table 6). By comparing POL-1 with POL-10, a T g difference of 34 degrees is seen. The hardness of POL-10 was also higher than that of POL-1. These results show that the polymer network is denser in the case of triol/mdi. As for the diol/n100 binder, addition of plasticizers reduced T g for triol/mdi in the same sequence BDNPA/F<GAPA<MEN42 for increasing effect. On the other side, addition of plasticizers lowered T g more for triol/mdi than for diol/n100 compositions. It may be assumed that the hard segments in pure triol/mdi are more capable than diol/n100 to establish physical crosslinking between the polymer chains. The plasticizer has the ability to loosen up this network structure of weak bondings, and will thus be most effective in such systems. Table 6: Compositions and Properties of GAP triol and GAP Diol/Triol Binders Binder POL-10 POL-11 POL-12 POL-13 POL-14 (1) POL-15 (2) POL-16 POL-17 GAP type Triol Triol Triol Triol Triol Triol Diol/Triol (1:1) Diol/Triol (1:1) Curing agent MDI MDI MDI MDI MDI MDI MDI N100 Plasticizer type None GAPA MEN42 BDNPA/F None None None None Plasticizer (%) DBTDL (ppm) NCO/OH Shore A n.m T g ( C) 10 (3) 37 (3) 49 (3) 32 (3) n.m. 9 (4) 29 (4) 34 (4) Batch size (g) n.m. = not measured. (1) Cured at 25 C. (2) Cured at 45 C. (3) Determined by DSC. (4) Determined by DMA. 6-6 RTO-MP-091

7 It was observed that in a 1:1 diol/triol mixture, the Shore A value was higher for a sample cured with N100 (POL-17) than with MDI (POL-16). This may be explained by the fact that N100 is a trifunctional curing agent, and may therefore contribute to a higher density of chemically crosslinked polymer chains. T g was, however, slightly lower for POL-17 than for POL-16. Therefore, N100 is a better curing agent than MDI with respect to T g even though MDI is only difunctional. In Figure 2 the hardness and T g are plotted as a function of the triol content in diol/triol polymers. It can be seen how the Shore A value and T g increase with the content of triol. Shore A T g ( o C) Triol (% of total GAP) Figure 2: Shore A and T g Values (determined by DSC) of GAP Diol/Triol Mixtures cured by MDI. HNF Propellants HNF Content A maximum tap density of 1.13 g/cm 3 was obtained for the bimodal HMX particle distribution that was used in the HNF/GAP propellants. Based on this value, a theoretical maximum for the HNF load in a GAP binder matrix would be 68.5%. However, not more than 50% HNF was used in the mixes, because 1) the crystals were needle shaped, 2) mixing was performed at fairly low temperature, 3) possible side reactions involving HNF should be limited and 4) HNF crystals are sensitive to friction and could cause hazardous reactions at high viscosity in the mix. Mixing and Curing The HNF propellant based on GAPdiol/N100 (HNFP-1) was mixed without any viscosity problems and it cured nicely at 25 C. No color change appeared during curing, and the propellant seemed to be homogeneous with no loose HNF particles (Figure 3). FTIR measurements showed that 50% of the curing agent was consumed at about 6 h curing time, whereas rheological monitoring of the curing showed that the shear storage modulus (G ) become larger than the loss modulus (G ) after 8 h. A complete curing was achieved after 3 days. After that time no change in the shear moduli was observed. Figure 3: Picture of Propellant HNFP-1. Longest side is 5 cm. RTO-MP

8 On the other hand, HNF propellants based on the GAP triol/mdi binder system did not cure well. These samples underwent color change from yellow to light brown after a few hours curing, and both of them, especially HNFP-2, were inhomogeneous and soft after 3 days. Further curing did not improve the quality of the samples. The main reason for this observation may be that the aromatic diisocyanate MDI reacts fast with HNF compared to the desired curing reaction GAP+MDI. FTIR showed that isocyanate disappeared during the curing, but polyurethane was not formed in predicted amounts. Another problem by using MDI as a curing agent in HNF propellants is its high melting point (37 C), and this is unfavorable for curing at room temperature, which may be important to suppress undesired HNF reactions. As a consequence of these findings, only HNFP-1 was subjected to further characterization. Characterization of HNF Propellant Some properties of HNFP-1 are listed in Table 7. The measured density of 1.51 g/cm 3 was very close to the theoretical value (1.52 g/cm 3 ) and supported the visual observation that curing had went on nicely. The Shore A values in Table 7 represent the upper side of the sample. On the bottom side the Shore A value was 64, indicating that some degree of sedimentation had occurred. This was expected due to the fairly low HNF content in the sample. Sample Density (g/cm 3 ) Shore A T g ( C) (1) Table 7: Properties of HNFP-1 and HNF Crystals Decomposition temp. ( C) (2) VTS (ml/g) (3) Impact (J) Friction (N) Burning rate at 7 MPa (mm/s) Pressure exponent (4) HNFP > HNF-E HNF-S (1) Determined by DMA. (2) At maximum of DTG peak, 1 K/min. (3) At 80 C, 40 h. (4) Pressure range 1-19 MPa. I sp Ns/kg TGA with slow heating rate (1 K/min) showed that HNFP-1 underwent a 44% weight loss between 110 and 180 C with a maximum in the DTG peak at 126 C. This weight loss may be assigned to HNF decomposition in the propellant. Between 180 and 250 C a weight loss of 23% with a DTG maximum at 225 C was registered. It is suggested that the main part of this weight loss was due to decomposition of the azide groups in GAP. Above 250 C HNFP-1 continued to lose weight, but no clear DTG peak was observed. It should be noted that higher heating rates (e.g. 10 K/min) resulted in a nearly 100% weight loss in a narrow temperature region around 138 C. For comparison, TGA data of HNF crystals (1 K/min) are included in Table 7. It was obvious that the thermal decomposition of the HNF propellant started at a lower temperature than what was observed for pure HNF. The vacuum thermal stability of HNFP-1 was also lower than pure HNF crystals (Table 7). Whereas HNF after 40 h at 80 C had produced 3-4 ml gas per g sample, the propellant produced more than 20 ml/g after 9 h at this temperature. At 60 C this level was reached after 3 days (Figure 4), but the second derivatives of these curves were negative, which indicates that the decomposition was autocatalytic. FTIR (ATR) spectra of the sample were recorded before and after it had been subjected to the VTS test. If GAP should have decomposed during the test, the most probable reaction would have been release of dinitrogen from the azide groups. However, the relative intensity of the azide band was not reduced after the VTS test. In fact, no significant difference in the spectra before and after could be seen (solid particles like HNF are hardly detected in the spectra of these kinds of ATR measurements). The HNF decomposition must therefore be the primary cause for the gas production in the VTS test. 6-8 RTO-MP-091

9 100 Pressure (kpa) Time (h) Figure 4: VTS Test of HNFP-1. Left curve: 80 C. Right curve: 60 C. In these experiments a pressure of 100 kpa corresponds to a gas volume of approximately 20 ml per g sample. Hazard data (friction and impact) are presented in Table 7. HNF is a very sensitive material, but the values of the HNF/GAP propellant were definitively better. Still, much concern must be taken regarding the sensitivity of HNF propellants. Another unfavorable property of HNFP-1 was the high pressure exponent (close to 1). A suitable burning rate modifier will be needed for future applications in rocket motors. A specific impulse of 2254 Ns/kg has been calculated for HMXP-1 (Table 7). The value might seem low, but it must be remembered that the HNF content was only 50%. HMX Propellants HMX Content A bimodal HMX particle distribution with a maximum in the tap density of 1.24 g/cm 3 was selected for propellants HMXP-1, -2, -3 and -4. A maximum theoretical HMX content in a GAP/N100 binder with 30% BDNPA/F was calculated to be 73%. Due to this result, the propellants HMXP-1, -2, -3 and -4 contained 70% solids (Table 2). Tap density measurements of trimodal HMX compositions have revealed that up to 85% HMX may theoretically be introduced in a similar binder [5]. HMXP-5 and -6 were therefore prepared with a higher solids load (75%) including a trimodal HMX distribution (Table 2). Mixing and Curing All HMX propellants had satisfactory pot lives, and it could thus be concluded that the amount of curing catalyst was within reasonable limits. Suitable curing time was found to be between 3 and 6 days for all these propellants. The viscosities of the mixtures were sufficiently low during mixing and casting, but in the last part of the mixing process the viscosities of HMXP-1, -2 and -4 were somewhat higher than the others were. The lower viscosity in HMXP-3 at this stage may be due to the fact that BDNPA/F and GAPA are more viscous than TMETN (plasticizer in HMXP-3), see Table 4. Compared to HMXP-5 and -6, the solids load in HMXP-1, -2 and -4 were closer to the theoretical maximum value, and a higher viscosity should therefore be expected in the latter compositions. RTO-MP

10 Structure build-up during the first hours of curing was followed by rheology, an example is shown in Figure 5. When the storage modulus (G ) became larger than the loss modulus (G ) the sample went from being mainly viscous to become predominantly elastic. For some samples the depletion of isocyanate and increase of urethane linkages was monitored by FTIR during curing. The half-lives of the isocyanate are given in Table 8. This time is very dependent on the amount of curing catalyst and of the viscosity in the mixture G' G, G'' (Pa) and η* (Pa s) G'' η Curing time (h) Figure 5: Rheometer Curves of HMXP-3 during Curing. Table 8: Properties of HMX Propellants Propellant HMXP-1 HMXP-2 HMXP-3 HMXP-4 HMXP-5 HMXP-6 Plasticizer GAPA GAPA TMETN BDNPA/F BDNPA/F BDNPA/F Half-life NCO (h) n.m. n.m n.m. 4.3 Curing quality Some cracks Cracks, swelling Good Good Good Good Shore A T g ( C), (1) n.m. n.m n.m. Density (g/cm 3 ) n.m Theor. density (g/cm 3 ) Tensile stress (MPa) (2) 0.44/0.23 n.m. n.m. 0.37/0.23 n.m. n.m. Tensile elongation (%) (2) 3.4/6.5 n.m. n.m. 5.1/6.7 n.m. n.m. Young s modulus (MPa) 15 n.m. n.m. 7.5 n.m. n.m. VTS (ml/g) Calculated I sp (Ns/kg) n.m. n.m n.m. = not measured/determined. (1) Measured by DMA. (2) At max stress/at rupture, 21 C. Physical and Mechanical Properties The density of the propellants reflected the curing quality, as a sample that did not cure satisfactory (HMXP-2) had a significantly lower density than the theoretical value (Table 8). The TMETN sample (HMXP-3) had a high density, and this correlated well with the fact that TMETN has a higher density than the other plasticizers. The densities of HMXP-5 and -6 were also high in this test series. The higher amount of solids may account for that. The propellant based on the GAP triol/mdi binder system had the highest Shore A value in the test series, even though this sample did not cure perfectly (Table 8). An explanation for this may be that 6-10 RTO-MP-091

11 the polyurethane produced from this system has a higher crosslinking density (physical and chemical). As expected, it was also observed that the samples with 75% solids (HMXP-5 and -6) exhibited higher Shore A values than similar samples with 70% solids. A slightly higher NCO/OH ratio in the binder matrix of HMXP-5 and -6 may also contribute to the higher Shore A values. From Table 8 it is seen that T g of HMXP-4 was 47 C. The effect of changing plasticizer from BDNPA/F to TMETN was clearly demonstrated as T g of HMXP-3 was 6 degrees lower. An increase of the solids from 70 to 75% did not lead to any significant increase in T g as HMXP-5 had a value of 46 C. HMXP-1 and -4 were tensile tested at 21 C, but their mechanical properties derived from these experiments were quite poor. Elongations just below 7% were observed for both propellants, whereas the maximum stress was measured to 0.44 MPa (HMXP-1) and 0.37 MPa (HMXP-4). For practical use of these propellants, the mechanical properties would have to be improved. Addition of a suitable bonding agent, changing the NCO/OH ratio or changing the particle distribution may be tried. Thermal Stability The propellants underwent only small gas release when subjected to VTS test at 100 C for 40 h (Table 8). The lowest VTS values (<0.2 ml/g) were recorded for the samples plasticized with BDNPA/F, whereas HMXP-3 (TMETN as plasticizer) exhibited the highest value in the test series. This value was, however, as low as 0.59 ml/g, and the thermal stability of all HMX propellants was therefore considered as good. A thermogram (TGA) of HMXP-3 is shown in Figure 6. The sample was stable during heating to 120 C (10 K/min) in nitrogen atmosphere. From that temperature and up to 210 C a weight loss of 10.7% was detected, and may be assigned to decomposition of the plasticizer. Separate experiments have revealed that TMETN decomposes in this temperature region, and the weight loss seen in Figure 6 corresponds well with the TMETN content in HMXP-3 (Table 2). Figure 6 also shows that a major weight loss occurred around 228 C. Decomposition of GAP and HMX should account for this observation. The final 3.4% of the sample remained in the TGA crucible when further heating to 280 C was carried out. This residue was probably a carbon rich material that would have to be oxidized to leave with the carrier gas % (1.224mg) C 80 4 Weight (%) % (9.820mg) 2 Deriv. Weight (%/ C) 0 20 Residue: 3.403% (0.3893mg) Temperature ( C) Universal V2.5H TA Instruments Figure 6: TGA Measurement of HMXP-3. Heating Rate: 10 K/min. RTO-MP

12 Ballistic Properties The results from motor firings (radial burning grain) are given in Table 9 and Figure 7. The pressure exponent was quite high for all samples, but it was slightly lower for the TMETN propellant (HMXP-3) than for the similar BDNPA/F propellant (HMXP-4). Addition of a potential burning rate modifier (2% Pbci and carbon black) gave a reduced pressure exponent in the low-pressure regime (2-6 MPa), but did not have any effect on it at higher pressure (7-16 MPa). A similar effect has been described by Kubota et al. [6]. The burning rate (at 7 MPa) was higher for HMXP-5 and -6 than for the propellants with 70% content of bimodal HMX particles, but still below 10 mm/s. Table 9: Ballistic Properties of HMX Propellants Propellant HMXP-3 HMXP-4 HMXP-5 HMXP-6 (2 firings) Pressure (MPa) Burning rate at 7 MPa (mm/s) (1) 9.0 Pressure exponent (1) Extrapolated value HMXP-6 HMXP-5 Burning rate (mm/s) HMXP-3 HMXP Pressure (MPa) Figure 7: Burning Rates for HMX Propellants (Radial Burning Grain). CL-20 Propellants CL-20 Content A maximum tap density of 1.17 g/cm 3 was found for bimodal CL-20 fractions. This implied that this fraction might constitute up to 67% of a CL-20 propellant. As a consequence of that, the CL-20 propellants in this work were prepared with up to 65% solids (Table 3). Mixing and Curing The curing quality was good for all CL-20/GAP propellants. Some of the rheology and spectroscopy results from the curing are given in Table 10. It was seen that as in binder systems without solids, the presence of Pbst increased the curing rate, leading to a pot life that would be too short for practical use. The results show, however, that reducing the DBTDL content, or even exclude DBTDL completely can lower the curing rate. Another possibility would be to add the curing agent at 40 C and cast it at that 6-12 RTO-MP-091

13 temperature, as it was done with CLP-5. The viscosity of this mixture was satisfactory low throughout mixing and casting. Physical and Mechanical Properties The densities of the CL-20 propellants in this work were close to 1.7 g/cm 3 and in good agreement with the theoretical values (Table 10), and confirmed that a curing without gas bubbles had taken place. The Shore A values measured at the upper side of the propellants were around 60. At the bottom side the values were slightly higher (up to 10 units) for most of the samples. The TMETN propellants seemed to be a little softer than the BDNPA/F propellants, but the differences were small. Table 10: Properties of CL-20 Propellants Propellant CLP-1 CLP-2 CLP-3 CLP-4 CLP-5 CLP-6 CLP-7 CLP-8 CLP-9 Solids (%) Half-life NCO (h) (1) Curing time when G =G (h) (1) Shore A T g ( C), measured by DMA n.m n.m Density (g/cm 3 ) 1.63 n.m Theoretical density (g/cm 3 ) Decomposition temperature ( C) (2) n.m n.m. n.m. n.m. VTS (ml/g) Friction (N) 110 n.m. n.m. n.m. 110 n.m. n.m. n.m. n.m. Impact (J) 1.5 n.m. n.m. n.m. 1.5 n.m. n.m. n.m. n.m. Calculated I sp (Ns/kg) n.m. n.m. n.m n.m. n.m. n.m. n.m n.m. = not measured/determined. (1) Measured at 40 C. (2) At maximum of DTG peak, 10 K/min. The T g value of the CL-20 propellants (Table 10) did not seem to be dependent on the solids load (60 or 65%). However, the type of plasticizer had an influence on T g, as propellants with TMETN exhibited values 4 degrees lower than propellants with BDNPA/F. Even though the former type of propellants had T g values below -50 C it will be a future challenge to lower this value further. One way of doing this is to increase the plasticizer content or use even more effective plasticizers, but GAP binders will anyway have great problems matching T g of HTPB propellants (typically 80 C). Tensile properties of the CL-20 propellants with 65% solids load are listed in Table 11. At room temperature the CL-20 propellants plasticized with BDNPA/F had roughly twice as high stress and elongation values than their HMX analogs with 70% solids (Table 8). Even better elongation was experienced for the propellant plasticized with TMETN (CLP-9), but the tensile stress and Young s modulus were somewhat lower than for the other propellants entered in Table 11. Another observation quoted in Table 11 was that tensile stress and elongation were markedly better at 30 C than at room temperature. This trend was not dependent on the plasticizer used, and it was found that also at this temperature CLP-9 had lower stress values than the other CL-20 propellants. The elongation of CLP-9 seemed to be better than the other propellants, the relative differences were smaller than those at room temperature were. RTO-MP

14 Table 11: Tensile Properties of CL-20 Propellants Propellant CLP-4 CLP-5 CLP-6 CLP-9 Temperature ( C) Tensile stress (MPa) (1) 1.7/ / / / / /0.48 Tensile elongation (%) (1) 21/34 9.3/ /26 9.8/ /37 17/23 Young s modulus (MPa) (1) At max stress/at rupture. Thermal Stability and Hazard Testing Preliminary results indicated that 2-NDPA might be a suitable stabilizer for CL-20/GAP propellants, as a mechanical blend of CLP-1 and 5% 2-NDPA produced 20% less gas than CLP-1 in the VTS test. The stabilizing effect of 2-NDPA was confirmed by the fact that the VTS value of CLP-2 (with 2-NDPA) was only half the value of CLP-1 (without 2-NDPA), see Table 10. It is expected that the effect of the stabilizer will decrease with increasing time and temperature, as there will be less and less available addition positions at the aromatic rings of 2-NDPA. In order to check the stability after storage, two pieces of CLP-2 were subjected to VTS testing. One of the samples was stored for 445 days at room temperature and produced a modest amount of 1.7 ml gas per g. The other sample was values stored for 445 days at 60 C end yielded a VTS value of 2.2 ml/g. Even though some increase was observed (from 1.5 ml/g, see Table 10) the value was lower than that of a fresh sample without stabilizer (CLP-1), showing that the stabilizer still had an effect. It was a general trend that samples containing Pbst had higher VTS values than similar samples without Pbst. Furthermore, it can be seen from Table 10 that TMETN propellants were not as thermally stable as BDNPA/F propellants. The VTS values were, however, acceptable for both types of propellants. An important observation was that in contrast to the HNF propellant (Figure 4), no catalytic decomposition was observed in the VTS tests of the CL-20 propellants. The amount of produced gas increase almost linearly with time. An example is shown in Figure Pressure (k Pa) Time (h) Figure 8: VTS Test of CLP-6 (100 C) RTO-MP-091

15 All CL-20 propellants containing BDNPA/F and 2-NDPA were subjected to TGA experiments with a heating rate of 10 K/min. A one-step weight loss of more than 95% occurred for all samples at approximately 210 C (Table 10). At 1 and 0.1 K/min, however, the decomposition of about 75% of the sample seemed to proceed in three steps (Figure 9). It may be suggested that the second and third steps are due to CL-20 and binder decomposition, or vice versa. 10 K/min was obviously a too high heating rate to observe this separation, probably due to temperature gradients in the sample. This assumption was confirmed by the observation that the decomposition temperatures were higher at 1 K/min than at 0.1 K/min K/min 1 K/min 0.1 K/min 80 Weight (%) Temperature ( C) Universal V3.4C TA Figure 9: TGA of CLP-6 at various Heating Rates. Propellant CLP-6 was also subjected to DSC analysis. One endothermic and two exothermic peaks could be observed. The former peak was quite small, and may indicate the phase transition ε γ in CL-20 [7]. The exothermic peaks may represent the same decompositions as the two largest mass losses in the TGA experiments at low heating rates. Hazard properties of two CL-20 propellants with BDNPA/F are presented in Table 10. These propellants contained 60 and 65% CL-20, respectively, but they gave the same friction (110 N) and impact (1.5 J) values. This means that these kinds of propellants were sensitive to impact, but their friction sensitivity was acceptable. The values showed that the sensitivity was lower than for the HNF propellant (Table 7). The sensitivity could not be directly compared with that of the HMX propellants prepared in this work, as friction and impact sensitivity was not measured for them. However, data from other HMX/GAP samples with BDNPA/F with 80% solids were J (impact) and N (friction) [5]. This means that the CL-20 propellant is expected to be somewhat more sensitive than HMX propellants. RTO-MP

16 Ballistic Properties and Calculated Performance TGA experiments showed that the presence of Pbst reduced the decomposition temperature of CL-20 (Figure 10) and this potential burning rate modifier was therefore tried out in some of the CL-20 propellants. Results from firing experiments are presented in Table 12. Typical firing curves are shown in Figure 11 (pressure vs. time) and Figure 12 (burning rate vs. pressure). The burning rate at 7 MPa was close to 15 mm/s for all samples in this test series. The pressure exponents were in general high, but some variations were observed between the various samples. Firing of thin-web radial burning motors (propellant mass approximately 40 g) indicated that Pbst reduced the pressure exponent, as it was 0.94 for a sample without Pbst (CLP-1) and 0.77 for a similar sample with Pbst (CLP-3). In the same way, the pressure exponent measured for CLP-7 (no Pbst) was 0.82, whereas the value of its analog with Pbst (CLP-8) was The pressure regions for these firings were, however, quite narrow, and to check the validity of these results, firings of larger motors (280 g propellant) were carried out. By comparing the firing results of CLP-4 (no Pbst) with CLP-6 (Pbst) it could be concluded that presence of Pbst had no significant effect on the ballistic properties of the CL-20 propellants, at least this was true in the pressure regions for these firing experiments (above 5 MPa). Table 12: Ballistic Properties of CL-20 Propellants Propellant CLP-1 CLP-3 CLP-4 CLP-5 CLP-6 CLP-7 CLP-8 Plasticizer BDNPA/F BDNPA/F BDNPA/F BDNPA/F BDNPA/F TMETN TMETN Pbst No Yes No Yes Yes No Yes Web thickness (mm) Pressure (MPa) (1) (1) (1) Burning rate at 7 MPa (mm/s) Pressure exponent (1) Extrapolated value. 235 Decomposition temperature ( C) Lead stearate (%) Figure 10: Decomposition of CL-20/Pbst Mixtures RTO-MP-091

17 Figure 11: Firing Curve of CLP-4 (280 g Propellant). Burning rate (mm/s) CLP-5 CLP Pressure (MPa) Figure 12: Burning Rates of two CL-20 Propellants. Although the pressure interval for the thin-web experiments was small, they showed that CL-20 propellants containing TMETN might have lower pressure exponents than propellants employing BDNPA/F as plasticizer. This was in accordance with the similar effect on HMX propellants (see Table 9). Besides searching for suitable burning rate modifiers, the choice of plasticizer may therefore be carefully considered in the endeavor to reduce the pressure exponent for future CL-20/GAP applications. The specific impulse of a propellant with 65% CL-20 (CLP-4) was calculated to 2333 Ns/kg, see Table 10. This propellant contained 30% BDNPA/F in the binder matrix. When TMETN was used as plasticizer (CLP-9), the impulse was increased to 2384 Ns/kg, see Table 10. RTO-MP

18 CONCLUSIONS Several GAP binders and smokeless GAP propellants have been prepared and characterized. The main findings were: Curing of GAP diol with N100 could be performed at room temperature. This polymer was used as a binder for an HNF propellant that were successfully mixed and cured at this temperature. GAP triol underwent good curing by MDI, but T g of this polymer system was in general higher than similar GAP diol/n100 compositions. The processing properties and curing quality of the samples could be controlled by the mixing parameters (compositions and conditions). The plasticizer MEN42 was most effective in reducing T g. T g of a GAP diol/n100 composition with 30% MEN42 was 64 C compared to 43 C without plasticizer. The other plasticizers employed in this work did also reduce T g. Their effect increased in the order BDNPA/F<GAPA<TMETN. MEN42 was not used as a plasticizer in propellants prepared in this work due to a high gas evolution in the VTS test and a low Shore A value of the MEN42 binder. The tensile properties were studied mostly for CL-20 propellants. At room temperature, about 10% elongation was observed for samples containing BDNPA/F, whereas a similar propellant with TMETN as plasticizer had twice as large value. At 30 C the elongation of both types of CL-20 propellants was better than at room temperature. It was suggested that the properties may possibly be improved by adding bonding agents, reducing the NCO/OH ratio or employing other particle distributions. The HNF propellant was the least thermally stable one, and it was also more sensitive to friction and impact than the HMX and CL-20 propellants. The HMX propellants seemed to be least sensitive. Small-scale radial burning motors were fired. At 7MPa, typical burning rates were 19 mm/s (HNF propellant), 8 mm/s (HMX propellants) and 15 mm/s (CL-20 propellants). The pressure exponents were fairly high, but some experiments indicated a lower value in TMETN propellants than in propellants with BDNPA/F as plasticizer. The highest specific impulse calculated for propellants prepared in this work was 2384 Ns/kg. This propellant contained 65% CL-20 and was plasticized with TMETN. By employing more favorable particle distributions, the CL-20 content can be increased, leading to an increased performance. The specific impulse can be further increased by increasing the ratio TMETN/GAP. REFERENCES [1] Gordon and McBride, NASA SP-273, Computer Program for Calculations of Complex Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks and Chapman- Jouguet Detonations, (1971). [2] NATO STANAG 4147, Chemical Compatibility of Ammunition Components with Explosives. (Test 1: Procedure B: The Vacuum Stability Test, Transducer Method, Method No 1). [3] NATO STANAG Explosives. Procedures for Dynamic Mechanical Analysis (DMA) and Determination of Glass Transition Temperature. [4] E. Unneberg and T.C. Johannessen, Synthesis of Polymer Matrices for Composite Rocket Propellants, Internal FFI Report, FFI/NOTAT-2000/00448, (2000) RTO-MP-091

19 [5] T.K.E. Karsrud and T.C. Johannessen, Explosive Formulations of Energetic Binders and Plasticizers with NTO and HMX, Internal FFI Report, FFI/NOTAT-97/04589, (1997). [6] N. Kubota and T. Sonobe, Burning Rate Catalysis of Azide/Nitramine Propellants, Twenty-Third Symposium on Combustion, The Combustion Institute, , (1990). [7] M.F. Foltz, C.L. Coon, F. Garcia and A.L. Nichols III, Thermal Stability of the Polymorphs of Hexanitrohexaazaisowurtzitane, Part II, Propellants, Explosives, Pyrotechnics, 19, , (1994). ACKNOWLEDGEMENT We performed part of this work within the EUCLID Research and Technology Program Clean Rocket Propellants. RTO-MP

20 SYMPOSIA DISCUSSION PAPER NO: 6 Discusser s Name: D. Bohn Question: Does FFI plan to continue the formulation work with HNF? Will stabilized HNF and burning rate modifiers be used in future studies? Author s Name: Eric Unneberg Author s Response: FFI has no concrete plans to continue working with HNF. Currently, FFI is strongly involved in other projects. On the other hand, FFI has obtained many good results and made good progress during with our HNF work. As a result, FFI might perform more work with HNF. This work should consider specific new (and better) HNF crystals so that we could increase the solids loading in the propellant. As the thermal stability of HNF propellants is low, and the pressure exponent is high, it would of course be necessary to look for possible stabilizers and burning rate modifiers. Discusser s Name: Klaus Menke Question: (1) What were the particle sizes of HNF, HMX, and CL-20 you have used in our study? (2) What are the hazards connected to processing and handling of HNF and CL-20? Author s Name: Eric Unneberg Author s Response: (1) A bimodal HNF particle distribution was used. The length of the longest crystals (HNF-E7) was approximately 1mm whereas the HNF-S18 crystals were on order of magnitude smaller. For the CL-20 propellants, a bimodal distribution of particles was used. For the HMX propellants, a bimodal distributions of particles was used in HMX-1, -2, -3, and -4, whereas trimodal distributions were used in HMXP-5 and -6. (2) The hazards of HNF are mainly connected with the friction sensitivity of HNF crystals. But as long as mix viscosity is low, the hazards should be acceptable. We used quite low HNF content in HNFP-1 (50%) because1) we prepared the propellant at room temperature and 2) the crystals were needle shaped. If HNF crystals with better morphology can be used, the solids content may be significantly increased without increasing the viscosity too much. The hazard problem with CL-20 is the impact sensitivity. But so far we have only had good experience preparing propellants based on this material. Better impact results than those published in our paper (1.5 Joules) can be obtained by changing the particle distribution RTO-MP-091