Chapter 12
A Recovery Process for Spent PolyurethaneBased Propellants Feasibility Studies 1
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Frank J. Y. Shiu , Iris C. Y. Yang , T. F. Yen , and Donald D. Tzeng
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1Environmental and Civil Engineering Department, University of Southern California, Los Angeles, CA 90089-2531 United Technologies Chemical Systems, San Jose, CA 95161-9028 The valuable inorganic components—energetics and fuel—of solid propellants are successfully separated and recovered by using swelling and ultrasound. In this study, the inert polyurethane-based propellant is first comminuted in various solvents as swelling media, and an appropriate solvent is then selected. Subsequently an ultrasound induced cavitational reaction for interfacial treatment is performed with the use of a mild oxidant. Through the process, the polymer binders are partially degraded and the propellant network is destroyed, enabling inorganic and metallic constituents to be released and separated.
To meet military need, solid composite propellants have been extensively developed and manufactured in the past. Following the end of the cold war, these large quantities of propellants from the demilitarization of missiles brought about the urgency of safe propellant disposal. While noted for their high energy release arid explosion, the spent propellants become hazardous wastes and need to be disposed with caution. In compliance to the Resources Conservation and Recovery Act (RCRA) and EPA regulations, the conventional and primary disposal method of spent propellant—open-pit burning—is no longer legal. These stringent environmental regulations and uprising pollution concerns have prompted the study of the recovery process for spent propellants. To recover the useful inorganic components from the spent propellant, the most common methods of breaking the polyurethane backbone macro-structure under vigorous conditions, such as thermal and hydrolytic cleavage, are not applicable. Currently, several techniques are actively being pursued by different groups. These include: aqueous maceration and extraction of propellant by Thiokol, cryogenic washout by General Atomic, and ammonia extraction at supercritical or near-supercritical conditions by Hercules. In addition to these, alternative methods leading to a mild recovery process are being sought. 0097-6156/95/0609-0139$12.00/0 © 1995 American Chemical Society
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Approach for The Feasibility Studies At the outset of the research, we considered our approaches to be a matrix modification. Previously we have successfully separated naturally-occurring composite systems, such as oil shale or coal, into meaningful fractions (1-2). The fragmentation of crosslinked systems, such as asphalt, through solvent swelling has been achieved in several studies (3-5). As shown by Young's modulus and other mechanical properties, the physical integrity of the composites has been drastically changed due to embrittlement, so that the composite integrity is destroyed. For a number of years, ultrasound irradiation has been utilized as a method of tar sand separation to aid the recovery of bitumens in our research (6-7). The ultrasound method has been used for heavy oil upgrading to crack the high molecular weight asphaltene into low molecular weight oil (8-9), as well as for the dechlorination of trihalocarbons in water (10). The results are very promising and may lead to some novel petroleum recovery techniques (11). Therefore, the approach which included solvent swelling and ultrasonic degradation is included in this study. Later, a feasible process to recover the useful components from the spent propellant is also outlined. Quantitative as well as qualitative determinations were performed in order to follow the changes in polyurethane after the treatment processes. These measurements included a) mass balance analysis by dry weight measurements of each separated fraction, and confirmation of the targeted substance by chemical analysis; and b) molecular structure determination by UV/visible spectroscopy for qualitative detection of the degraded products. Propellant Composition Polyurethane-based propellants have become more important since the mid1950's due to the need for higher ballistic performance and the precise mechanical requirements of large rocket motors. The composition of the solid propellants is a 3-dimensional cross-linked matrix with oxidizer, metallic fuel, and other solid components uniformly dispersed throughout. Regardless of types, all polyurethane propellants contain binders that can be cured through the reactions of polyols with isocyanates, with or without a plasticizer. These substances belong to the formal class of polyurethane (PU) elastomers, especially those that recently contain hydroxyl-terminated polybutadiene (HTPB) (12-14). The HTPBbased polyurethane elastomers are superior to the common polyester- and polyether-based elastomers because they possess properties of lower water permeability, better stability in a moist atmosphere, higher electrical insulation, and lower glass-transition temperature (Tg) (15-17). The solid components of the propellants range between 70 to 90%, and their higher limit depends on the processability. The energetic component is the most important and accounts for the largest percentage in the composition. The preferred energetic compounds used in current solid propellants are nitrate or perchlorates, such as AP(ammonia perchlorate), RDX(trinitrohexahydrotriazine),
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Recovery Process for Spent Polyurethane-Based Propellants 141
and HMX(tetranitrotetraazacyclooctane). Aluminum powder is the most frequently used fuel to boost specific impulse and may be found in quantities up to 25%. These materials are trapped in the cross-linked network without chemical bonds. The inert portions of coloring matter, and other plasticizers such as dioctyl adipate (DOA), are also added in some instances (18). Due to their highly explosive nature, inert samples were initially used instead of the original propellant in this research. In the inert propellant the energetic, AP or H M X , is excluded and replaced with a combination of salts having a similar density, the composition of which is given below: TABLE I. Inert Propellant Composition Ingredient Sodium chloride Aluminum, MD-10 Ammonium sulfate R-45 (HTPB) Isophorone diisocyanate (IPDI) HX-752 (aromatic difunctional aziridine) Protec 2705 Blue Dye
Percentage (WAV) 35.1 31.3 18.7 14.4 0.3 0.1 0.1 100.0
The HTPB/IPDI based polyurethane matrix forms the main structure of the propellant, and aziridine is a bonding agent which brings inorganic salts into the organic environment. In the formulation, the organic, inorganic, and ammonium powder are 14.9%, 53.8%, and 31.3% respectively. To eliminate the interference of the fillings in the inert propellant during the investigation, a oneshot polyurethane elastomer was synthesized with 3% plasticizer and a HTPB to IPDI ratio equal to 9:1, and used for the experiments. Solvent Swelling The chemical linkages for a linear chain of homopolymers are rather easy to break and form into a uniform polymer solution just by stirring, and bond breakage can often be detected by viscosity reduction. For the three-dimensional cross-linked network, however, this type of bond breakage is often ineffective. The introduction of an organic solvent to a cross-linked system can cause the network to expand. In many chemical comminutions, this expansion can exceed the internal force holding the network or crystallite system together, thus causing the dissolution process. The dilation of a brittle solid can be expressed by
where S is strain energy per unit volume, v is the Poisson's ratio, G is shear modulus, and V is volume. The fracture of a brittle solid can be expressed as
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where F is fracture energy per unit volume, o is tensile stress, and E is bulk modulus. Comminution will occur when S > F. The swollen network can be broken in order to cause the linkage or the bonds to rupture. Based on statistical mechanical principles, the internal energy of the substrate should be matched by the internal energy of the invading solvent. Interpreting this in another way, the selection of the solvent must be based on the concept of Hildebrand solubility parameters (19). In many cases of matrix modification methods, we have experienced a total dissolution by the solvent introduced. The swelling measurement plays a critical role in the study of polymers because it can reveal the polymer structure, density, and other significant information (20). In order to find the suitable solvent for the polyurethane, swelling experiments were conducted with several kinds of solvent. A good solvent can extend the polymer bonding and loosen up the entire structure, which will make the polymer network easier to destroy. The swelling experiment was performed to select proper solvents for the sonication process. The samples were cut into 4x4x4 mm cubes and submerged in various solvents for one week. For the most effective solvents, detailed measurements were further conducted by recording the change in size daily. Swelling of a network will always exert strain on the whole system. Doubtlessly the binder-filler interaction will be decreased until the filler particles can be rejected. Our experiments will show the evidence of filler separation, which is the major objective of this study. We noticed in our initial experimental work that the compatibility of solubility parameters of the polymer network and those of the solvents should be matched. For solvent systems, this match cannot be made with dispersion effects alone; the best match is with the total cohesion parameter, i.e.,
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