Synthesis and characterization of partially silane-terminated polyurethanes reinforced with acid-treated halloysite nanotubes for transparent armour systems

Abstract

& oren e. petel in the present work, nanocomposites based on the partially silane-terminated polyurethanes reinforced with sulfuric acid-treated halloysite nanotubes were synthesized and evaluated as a potential candidate for transparent blast resistant configurations. The polyurethane must present high tensile ductility at high strain rates to be able to contain fragments and increase the survivability of the system. Gas-gun spall experiments were conducted to measure the dynamic tensile strength (spall strength) and fracture toughness of the nanocomposite and neat polyurethane. the nanocomposite presented a 35% higher spall strength and 21% higher fracture toughness compared to the neat polyurethane while maintaining transparency. the recovered samples following the spall tests were analysed via scanning electron microscope fractographies. the nanocomposite and neat polyurethane samples were chemically characterized via fourier transform infrared spectroscopy and melting behaviour via differential scanning calorimetry. The improved properties can be attributed, in large part, to the presence of more rigid spherulitic structures, and a rougher fracture surface constituting of several micro-cracks within the nanocomposite. Transparent armour systems must protect against blast and ballistic threats while maintaining structural integrity and optical transparency. Generally, transparent armour systems are composed of laminated glasses sheets bonded together by thin adhesive interlayers of polyvinyl butyral, polyurethane, and/or ethylene-vinyl acetate films, normally combined with polycarbonate as a backing layer. To achieve ballistic protection requirements; the glass layers are generally much thicker than the polymeric layers, which leads to thick and heavy armour solutions 1,2. Material improvements that allow weight reductions among transparent armour systems are of great interest for personal and vehicular applications. Polymers are extensively used in armour applications. Stretched polymers fibers (e.g., Aramids) are widely used in the ballistic fabrics integrated into soft armour and spall liner applications 3. Transparent polymers have historically seen broad use in transparent armour applications, and while they continue to be used as costeffective solutions in some visor and ballistic eyewear applications, their primary role in more robust armour solutions has been relegated to interlayer or backing support for transparent ceramics 1,4. Figure 1a shows an illustrative representation of a typical transparent armour structure. Polyurethanes (PU) are commonly adopted as interlayers in ceramic laminated systems due to their high-tensile ductility and adhesive properties, which provides the containment of armour fragments and increases the spall resistance of the ceramic layers 5. Figure 1b presents a relative transparency comparison between a 9.5-mm-thick polycarbonate plate with and without a halloysite/polyurethane nanocomposite adhesive backing layer. The adhesive layer consisted of a 1.5-mm-thick layer of s...

Microstructural evidence of the toughening mechanisms of polyurethane reinforced with halloysite nanotubes under high strain-rate tensile loading

“…PU (approximately 32 wt.%), and that the bonding of the HNTs to the hard segments has been shown to promote the micro-phase separation of the nanocomposite 37 , we expect the occurrence of spherulites with a higher concentration of crystallized hard segments in the nanocomposite. Indeed, DSC results from our previous study 27 showed that the melting point of the hard phase crystallites was approximately 15 °C higher in the nanocomposite compared to that of the neat PU.…”

“…The fracture toughnesses (K C ) were determined from spall tests that had similar values of shock stresses (0.96 GPa for the neat PU and 0.99 GPa for the nanocomposite). The fracture toughnesses were found to be 3.41 and 4.13 MPa m 1/2 for the neat PU and nanocomposite, respectively 27 .…”

“…In (d) , we show the neat PU again at a different magnification, with a spherulite and macro-fibril highlighted. (a) is reproduced from 27 under license CC BY 2.0. …”

“…Thus, the adhesion between the PU and nanotubes is promoted. The detailed synthesis and post-curing process can be found in our previous work 27 .…”

“…In our previous work 27 , we have described the synthesis process and characterization of HNT-PU nanocomposites based on PU prepolymer partially end-capped with a secondary aminoalkoxysilane, and an acid-treated HNT content of 0.8 wt.%. The partial prepolymer termination was performed in order to improve the interfacial compatibility between HNT and PU, and the selected weight fraction of HNT was low enough to maintain the transparency of the polymer as a thin film.…”