Military smokes that efficiently obscure light across visible wavelengths and have low environmental impact and human health toxicity have not yet been developed. Hexachloroethane (HC) smokes have excellent obscuration performance with mass extinction coefficients (MEC) of > 3 m2/g across visible wavelengths, but are toxic and have limited use. A non-toxic obscuration grenade, the M106, has been produced by the Army and is currently used for visible obscurant applications. The MEC of M106 devices tested in a 190 m3 chamber at the Edgewood Chemical Biological Center (ECBC) averages 1.7 m2/g in the 400 – 700 nm regime with a grenade figure of merit (gFOM; a measure of fielded performance) of 0.67 m2/cm3. Numerical modeling predicts that titanium dioxide (TiO2) based obscurants could produce MEC that exceed 10 m2/g across the visible portion of the spectrum and gFOM of 25 m2/cm3 if TiO2 particles with a narrow size distribution were fabricated and dispersed in a non-agglomerated format.
The goal of this SERDP Exploratory Development (SEED) project was to develop a visible obscurant using low toxicity TiO2 particles that improves upon the performance of existing powder based visible obscurant grenades by a factor of 5. This goal was achieved by chemically and physically improving the dry powder fill material in a TiO2 grenade and investigating new methods of disseminating TiO2 nanoparticle to reduce agglomeration when aerosolized.
Several methods to synthesize TiO2 were developed and evaluated. One method produced stable monodisperse particles with mean diameters of 20 nm and 20% coefficient of variation (CV) and another method yielded discrete 184 nm diameter particles with an 18% CV. A variety of commercially available TiO2 powders were also acquired, processed, and functionalized with surface chemistries designed to minimize surface adhesion and to optimize dissemination. Uncoated TiO2 particles are hydrophilic and, after coating with long chain silane groups, can be rendered hydrophobic which improves their dispersibility. Techniques for coating TiO2 particles in solution and as a dry powder were successfully developed.
A wide variety of surface modified TiO2 powders were packed at precisely controlled pressures into various container geometries. The packed powders were disseminated into a custom built 2.5 m3 aerosol chamber using various dispersal methods that are optimized to generate high shear forces and minimize the agglomeration of the aerosolized materials. The high improvement due to surface functionalization was Tiona RCL-9 TiO2 (Cristal Global) coated in solution with cyclic azasilanes, lyophilized to remove the solvent without agglomerating the nanomaterials, compressing into a thin puck using a ram with 500 psi pressure, and pneumatically disseminating with a burst of 1000 psi air. Using this optimized approach, gFOMs of up to 4 m2/cm3 were obtained compared to 0.6 m2/cm3 for the currently fielded M106 grenade (a factor of 6 improvement).
The performance gains were obtained using surface coatings on commercially developed powders that were still agglomerated, and continued improvement in deagglomerating commercial powders followed by surface functionalization and effective dissemination has the potential to increase performance closer to the theoretical gFOM for TiO2 based smokes which is 25 m2/cm3. If successful, such improvements would allow for the generation of a rapid, safe, and effective new class of obscuration devices that would exceed HC based obscurant devices in performance.