Assessment of High Energy / High Elongation Propellant

Transcription

1 Jim Dodds, Gaynor Kavanagh, John Theobald, David Tod QinetiQ Future Systems Technology Division Fort Halstead, Sevenoaks Kent, TN14 7BP UNITED KINGDOM ABSTRACT QinetiQ has a continuing interest in all propellant materials of potential service use and routinely conducts screening tests on candidate materials. Chemical stability and mechanical integrity are of particular concern where a material is being considered for rocket motor applications. A range of high energy / high elongation propellant materials, with a polyethylene glycol binder are being assessed as part of a general materials evaluation programme. Test materials have been artificially aged under a variety of environmental conditions and compared against natural ageing. A number of analytical techniques are used to follow the ageing process. Of particular concern is the depletion of stabiliser content on ageing, which is followed using an HPLC method. With this particular class of propellant, using polyethylene glycol as binder, the procedure used to extract the stabilisers has proved to be a critical process. The optimisation of the extraction procedure is presented together with some of the data obtained during the ageing trials. 1.0 INTRODUCTION QinetiQ, FST, Life Assessment group has a continuing interest in the development and characterisation of potential materials for use as rocket propellants. Since safety is of paramount importance, tests to confirm chemical stability and mechanical integrity are carried out at an early stage of the screening process. Materials are artificially aged at elevated temperature and in some cases at varying levels of relative humidity, followed by testing of both chemical and mechanical properties. As this process is being carried out at an early stage, where supplies of material are limited, the testing regime is designed to maximise the data obtained while limiting the quantity of sample consumed. A limited small-scale hazard assessment is often included in the programme. One series of materials that we have been studying is a range of high energy / high elongation propellants based on polyethylene glycol (PEG) as binder. Samples of different composition have been artificially aged at various temperatures and relative humidity for chemical and mechanical testing. For each test condition, two rectangular test pieces 10 x 6 x 40mm were cut or milled from the bulk material. Changes in bulk mechanical properties were followed using dynamic mechanical analysis (DMA) using one of the rectangular pieces. The second test piece was used to evaluate chemical changes using a range of chromatographic and spectroscopic techniques. Stabiliser depletion was monitored using solvent extraction, followed by reverse phase HPLC with uv detection. Breakdown of the binder cross-linking was followed by NMR analysis of free PEG, from a solvent extract. 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 2.0 MATERIALS A range of high energy / high elongation propellants were prepared using a binder system based on polyethylene glycol incorporating a nitrate ester as plasticiser. The composition of these materials is given in table 1. Table 1: Test Materials Propellant AP (%) Aluminium (%) Nitramine (%) Binder (%) A B C D EXPERIMENTAL Samples of each propellant formulation were prepared as rectangular blocks of size 10 x 6 x 40 mm. This geometry was chosen as it is convenient for the measurement of mechanical properties using DMA and can also be readily prepared for chemical analysis. Replicates samples were then aged over saturated salt solution in sealed desicators placed within temperature controlled ovens. In this way, both temperature and humidity could be controlled during the accelerated ageing process. At an early stage in the evaluation process, small-scale hazard tests were carried out for each of the formulations under test. Tests were conducted in accordance with the Sensitiveness Collaboration Committee Manual of Tests. 3.1 Mechanical Testing Assessment of the bulk mechanical properties of the materials before and after ageing was carried out by DMA using a Rheometrics RDA2 rheometer, with the torsion rectangular sample of previously noted dimensions. When only small samples are available, as in this case, DMA is a particularly useful technique for evaluating ageing trends in the bulk mechanical properties of energetic materials. The test provides a fingerprint of the mechanical response of the material over a wide temperature range. A rotary motor applies a sinusoidal strain at one end of the sample and the transmitted torque is measured at the opposite end using a torque transducer. Results of DMA testing provide the shear modulus G, the loss modulus G and the loss angle Tan (δ) as a function of the test temperature. The shear modulus is a measure of the amount of elastic energy stored within the sample, whereas the loss modulus is a measure of the amount of energy lost due to various dissipating processes. The loss angle is a function of the two moduli. The sample is mounted into the rheometer, without any further preparation and is tested in sinusoidal deformation to 0.1% maximum strain, at a rate of 1 radian/second, typically over the temperature range of -110 to +100 C. A typical set of DMA results, for one of the test materials, is shown in figure 1. QinetiQ has over several years, built an extensive data-base of DMA results on energetic materials from munition life assessment and surveillance programmes. 4-2 RTO-MP-091

3 oC 32wks bq Control ba 20%RH bb 30%RH bc 40%RH G' (b Q ) [Pa] Temp [ C] Figure 1: DMA Data for Propellant B. 3.2 Binder Degradation An understanding of the degradation of the binder system is important to support the assessment of the bulk mechanical properties of the material by DMA. In the case of this particular range of materials based of polyethylene glycol, nuclear magnetic resonance spectroscopy (NMR) was carried out on solvent extracts of propellants before and after ageing. The soluble fraction of each sample was extracted using acetone-d6 and 1 H NMR spectra of the extract recorded on a Brucker MSL-300 multinuclear spectrometer at a frequency of MHz. 3.3 Stabiliser Determination Stabiliser levels were determined by solvent extraction of the propellant samples followed by high performance liquid chromatography (HPLC). For much of the work, a laboratory standard procedure involving shaking a weighed quantity of the chopped propellant (2mm cubes) with dichloromethane, overnight, was followed. This initial extraction step is followed by evaporation of the solvent to dryness, then transfer of the extracted stabilisers to a clean flask using aliquots of diethyl ether, eliminating much of the co-extracted nitramine. The ether extract is then evaporated to dryness, the residue dissolved in an acetonitrile solution of internal standard, followed by HPLC. The HPLC analysis is carried out using reverse phase on a Lichrosorb PR-18 5µ column maintained at 30 C, with an eluent consisting of water and acetonitrile 60:40 and UV detection. Flow programming is used to optimise the chromatography and a typical chromatogram of a stabiliser standard solution is given in figure 2. RTO-MP

4 Figure 2: HPLC Chromatogram of Stabiliser Standard Solution. 3.4 Stabiliser Extraction Procedure The relatively small sample weights used in this screening approach can produce higher variability in results than a method optimised for quality control. In general this is not a problem as valid trend data can be obtained. However, with this particular range of propellant materials, a larger variability than normal was found. Experiments were carried out on propellant B to try to develop an analytical procedure with improved reproducibility. The standard procedure outlined in section 3.3 contains a number of steps where stabiliser can be lost. Several experimental procedures were tested, which reduced the number of steps. In particular, it was felt that with the improvements in chromatography data handling software packages, the ether extraction step to eliminate nitramine, was probably not necessary. Three alternative extraction procedures were tested: a) The chopped propellant was extracted with dichloromethane using a Soxhlet extractor, followed by addition of internal standard solution and evaporation to dryness. The residue was then re-dissolved in acetonitrile, filtered and separated using the standard HPLC conditions. 4-4 RTO-MP-091

5 b) The chopped propellant was extracted by shaking with acetone overnight, followed by addition of an acetone solution of internal standard, filtration and analysis by the standard reverse phase HPLC method. c) The chopped propellant was extracted by shaking overnight with a solution of internal standard in dichloromethane, followed by evaporation to dryness. The residue was dissolved in acetonitrile, filtered and analysed by the standard reverse phase HPLC method. In order to develop as robust a method as possible, two different batches of propellant B both unaged and aged at 70 C for 3 weeks were tested. Replicate analyses were carried out using each test procedure, together with the standard method for comparison. 4.0 RESULTS The results of the small scale hazard testing are given in table 2. It is evident that propellant C is less sensitive than the others. This would be expected, as this formulation is more like an explosive than a propellant. Table 2: Small Scale Hazard Data Test / Material A B C D Factor of Insensitiveness Factor of Friction Temperature of Ignition (/ C) Binder Degradation Proton NMR results for propellant B before and after ageing in a humid environment, are shown in figure 3. The peak at 3.72 ppm is characteristic of free PEG and is relatively small for the control sample, but much larger for the aged sample. This and other data clearly show an increase in free PEG with ageing, where some of the cured binder is reverting to the uncured state. Figure 4 shows the relative levels of free PEG for all of the test propellants, after ageing at 70 C at a relative humidity level of either 20% or 40%. The degree of binder reversion is higher at 40% RH for all of the test propellants, but varies significantly between the different materials. Since the percentage of binder is constant for all the formulations, additional factors besides the relative humidity level must be influencing the reaction. RTO-MP

6 PEG 3.72 ppm 3.72 ppm Figure 3: NMR Spectra. 1.2 Relative amount PEG A B C D 40% RH 20% RH Test Material Figure 4: NMR Results. 4-6 RTO-MP-091

7 4.2 Stabiliser Determination Some typical results of stabiliser loss on ageing under various conditions of temperature and humidity are shown in figures 5 and 6 for one of the test propellants (B). These two examples at 50 C and 40 C with 20%, 30% and 40% relative humidity have been chosen to illustrate the complex behaviour observed with this particular range of propellants. PNMA DEPLETION AT 50C % PNMA % RH 30% RH 40% RH AGEING TIME IN WEEKS Figure 5: PNMA Depletion at 50 C. PNMA DEPLETION AT 40C % PNMA % RH 30% RH 40% RH AGEING TIME IN WEEKS Figure 6: PNMA Depletion at 40 C. RTO-MP

8 For test propellant B, the depletion of pnma stabiliser at 40 C is virtually independent of the level of relative humidity over the range studied. By contrast, at 50 C there is a strong dependence between the stabiliser depletion and level of relative humidity. As might be expected, samples aged at 40% RH show a more rapid loss of stabiliser than those samples aged at either 20% or 30%. However, samples aged at 20% RH show a more rapid loss of stabiliser than those aged at 30%, which is counter intuitive. Work is in progress to investigate this apparent anomaly. Figures 5 and 6 also illustrate the variability experienced with this particular range of propellants, which is greater than normally observed for other systems. It is for this reason that more detailed tests were conducted to try to improve the analytical procedure. 4.3 Extraction Procedure The results obtained for each of the four test propellant B samples (batch 1 and batch 2, aged and unaged) using the different analytical procedures are given in tables 3 6. Table 3: pnma Stabiliser Content, Batch 1 unaged Method Mean Standard dev. RSD Standard Soxhlet Acetone DCM (undried) DCM (dried) Table 4: pnma Stabiliser Content, Batch 1 aged at 70 C for 3 weeks Method Mean Standard dev. RSD Standard Soxhlet Acetone DCM (undried) DCM (dried) RTO-MP-091

9 Table 5: pnma Stabiliser Content, Batch 2 unaged Method Mean Standard dev. RSD Standard Soxhlet Acetone DCM (undried) DCM (dried) Table 6: pnma Stabiliser Content, Batch 2 aged at 70 C for 3 weeks Method Mean Standard dev. RSD Standard Soxhlet Acetone DCM (undried) DCM (dried) The results clearly show that in some cases, poor reproducibility is obtained using the standard method. The acetone extraction procedure was the simplest method evaluated, since it did not contain an evaporation step, but it did not always give good reproducibility. The method using Soxhlet extraction generally gave good reproducibility figures, but it is more complex and since it involves boiling solvent, there is some risk of affecting the stabiliser levels. Of the three alternative methods tested, the dichloromethane total solution procedure gave the most consistently low value for the relative standard deviation. Compared to the standard method, it is less complex and requires less consumable materials. In each of the tables 3 6 there are two entries for the dichloromethane total solution method marked DCM (undried) and DCM (dried). This corresponds to whether the propellant sample was dried over silica gel before being cut into cubes and testing. Since these particular propellant samples also contain 2-Nitrodiphenylamine (2NDPA) as a secondary stabiliser, the HPLC data from the dichloromethane total solution method was processed to calculate the 2NDPA levels. The results for both stabilisers are given in table 7. RTO-MP

10 Table 7: pnma & 2NDPA Stabiliser Content, Dichloromethane Solution Method Batch Treatment Dried pnma Mean 2NDPA Mean 1 Unaged No Unaged Yes Aged No Aged Yes Unaged No Unaged Yes Aged No Aged Yes For all four samples tested (batch 1 and batch 2, unaged and aged), drying the sample before analysis reduces the apparent level of pnma stabiliser. Conversely, drying the sample before analysis increases the apparent level of the 2NDPA stabiliser. The reason for this difference is believed to be due to the relative polarity of the two stabilisers and the strength of their adsorption onto active sites within the propellant. Further work will be needed to prove this theory. 5.0 CONCLUSIONS The programme of work described is typical of the approach taken in QinetiQ to study the ageing process in energetic materials. The particular propellant studied, using a cured PEG binder system, has shown some interesting effects with humidity. NMR has shown that there is a reversion of the binder system to the uncured state influenced by the level of relative humidity. Also a strong relationship has been demonstrated between the loss of stabiliser during accelerated ageing and the level of relative humidity, although the relationship does not appear to be straightforward. The procedures using small samples normally give poorer precision than would be expected for quality control purposes, but are acceptable for assessing ageing trends. With this particular range of propellants based on PEG as binder, the standard extraction procedure for HPLC analysis of stabilisers gave unacceptable precision. An improved extraction procedure has been devised, which gives superior results. Using this new procedure, differences between the results of samples with and without drying have been observed. Work is continuing to further understand the effect of humidity and sample conditioning on this class of propellant RTO-MP-091

11 6.0 ACKNOWLEDGEMENTS The authors wish to acknowledge the support of Lisa Cribb and Rosalind Dale, of QinetiQ, who conducted much of the experimental work presented. 7.0 REFERENCES [1] A.V. Cunliffe et al, Proceedings of the Predictive Technology Symposium, p37, (1991). Life Prediction of Composite Propellant Motors. [2] A.V. Cunliffe et al, NDIA Symposium on Energetic Materials (1998), A Screening Approach to Propellant Aging Assessment. QinetiQ Ltd RTO-MP

12 SYMPOSIA DISCUSSION PAPER NO: 4 Discusser's Name: Bruno D Andrea Question: Did you test samples aged at high humidity and re-dried? Author's Name: Jim Dodds Author s Response: Some samples were dried before testing after ageing. However, the softening of the material has been identified as being due to irreversible chain scission. This cannot be recovered by drying. Discusser's Name: Luigi DeLuca Question: Based on your experience, could you say how ballistic properties are related in general to mechanical properties? Author's Name: Jim Dodds Author s Response: The loss of mechanical properties could eventually lead to a catastrophic failure of the motor at ignition RTO-MP-091