Andrew R. Demko

In situ synthesis of polyurethane–TiO2 nanocomposite and performance in solid propellants

Achieving a homogeneous nanoparticle dispersion is a frequent challenge in the preparation of polymer–matrix nanocomposites. In this paper, we report a polyurethane–TiO2 composite with improved nanoparticle dispersion by the in situ synthesis of TiO2 nanoparticles in a solution containing the pre-polymer hydroxyl-terminated polybutadiene (HTPB). The coordination chemistry of titanium with the functional groups of HTPB was studied by 1H NMR spectroscopy, and protective methods were developed to prevent undesired side reactions. The resulting synthesis approach produced a uniform dispersion of mesoporous anatase TiO2 nanoparticles with minimal agglomeration in HTPB, which could then be cast and cured into bulk polyurethane composites. The performance of the polyurethane–TiO2 composite as a solid propellant binder was investigated. The in situ TiO2 nanoparticles demonstrated substantially higher activity as combustion catalysts compared to TiO2 nanoparticles incorporated into polyurethane by conventional powder or solution mixing methods.

Modeling Composite Solid Propellant with Catalytic Additives

This paper presents current work on improving the accuracy of solid composite propellant modeling to include catalytic additives and using it to develop a better understanding of the working mechanism of burning rate enhancement. To this end, isolating the physical mechanism where the catalyst impacts the composite propellant burning rate was studied. To accomplish this goal, a parametric study was conducted by systematically varying different aspects of the burning that are covered by the simple model. The results from the model were compared to experimental data collected from propellants formulated in the authors’ laboratory both with and without catalytic nanoparticle additives, namely TiO2. Some variations of the TiO2 additive tested included powder that was produced by spray-drying with and without heat treating as well as titania that was premixed into the binder before making the ammonium perchlorate (AP)/binder mixture. The propellants are compared to two ammonium perchlorate and hydroxyl-terminated polybutadiene (HTPB) baselines with 80% and 85% solids loading. Advancements on the Beckstead-Derr-Price (BDP) model to incorporate catalytic additives were accurately demonstrated using the technique outlined in this paper. This study determined that the catalyst primarily impacted the condensed phase by increasing the reaction rate of the condensed-phase AP; this conclusion was based on the fact that only changes in the condensed-phase AP reaction rate produced the pressure dependence and absolute magnitude of the increased burning rates due to the TiO2-based additive that were seen in the data. In contrast, an unrealistically high increase in the pressure dependence was found if the binder kinetics were modified to match the observed burning rates, and changes in the primary flame kinetics only varied the slope of the burning rate curve; these results are not supported by the experimental data. Furthermore, an empirical constant (Ωc) was found to model the effect of the additives on the AP reaction rate in the form of a burning-ratemagnitude modifier. Typically, an increase of around 50 to 60% in the reaction rate was observed for the use of nano-titania in an 85% AP propellant.

Facile nanoparticle dispersion detection in energetic composites by rare earth doped in metal oxide nanostructures

The segregation and agglomeration of nanoparticles dispersed in polymer matrices play important roles in nanocomposite performance. A method of rapid and simultaneous visualization of macroscopic and submicron particle dispersion properties is presented, based on nanoparticle luminescence induced by europium doping. The luminescence intensity of polymer composites containing Eu-doped TiO2 nanoparticle catalysts varied with the nanoparticle agglomerate size between 90 nm and 10 μm, and with concentration variations from segregation. These variations were detected by photoluminescence spectroscopy and visible-light photography, making this a facile characterization method for bulk composites without affecting the nanoparticle performance.

Aging Effects of Composite AP/HTPB Propellants Containing Nano-Sized Additives

The aging behavior of a composite propellant is a key indicator in the service life of a practical rocket motor. Advancements in the development of nano-particle additives and their burning rate enhancing characteristics give rise to examining the long-term impacts on the service life of the motor. Recent research at Texas A&M has focused on tailoring ballistic characteristics through the addition of nano-additives specifically synthesized for composite propellants. This paper focuses on an AP/HTPB propellant with a dry-powder nano-titania additive to influence ballistic characteristics and fluorescing Lumidots to assess morphological changes associated with accelerated aging. Sample sets were aged at 85 o C in ambient humidity conditions for six and twelve days, simulating four and eight years of aging, respectively. Data were correlated to a set of naturally aged samples. Acceleratedaged monomodal AP formulations with 80% solids loading and nano-additive demonstrated an approximate 7% decrease in burning rate, with accompanying morophological changes. Accelerated-aged bimodal AP forumlations with 85% solids loading and nano-additive demonstrated an approximate 12% increase in burning rates, with accompanying morphological changes.