Energy dissipation and rate-dependent deformation behavior of STF-integrated PU foam nanocomposites

Abstract

Designing energy absorptive structures is crucial for safety and these structures withstand several loads such as compression, bending, and impact. Beside to honeycombs as a core material for sandwich structures, polymeric foams receive great attention as structural components with their outstanding effective stress transfer, viscoelastic properties, and increased surface area. Among several polymeric foams, rigid polyurethane (PU) foams are good candidates presenting high strength and lightweight characteristics and tailorability. PU is a particular polymer group that stands out with its unique chemistry, typically synthesized from polyol and isocyanate, which also has a potential to fill the vacancies between rubbers and plastics based on their mechanical thermal and viscoelastic properties. PU foams can have densities that range from 20 to 3000 kg/m3 which provide a wide range of application areas and properties. The physical and mechanical properties of PU foams are closely correlated to their morphological characteristics, which are mainly determined by several key parameters such as foam density, cell density, cell edge length, wall thickness, and thickness to length ratio (t/l). PU foams could be customized by incorporating nano and/or micro-sized reinforcing agents to obtain unique functionality and properties. Combining nanoparticle inclusion with process parameter optimization can result in improved mechanical properties and multifunctionality. Nanoparticles can act as nucleation points and lead to narrow cell edge length, increased cell wall thickness, and higher cell density, yielding advanced mechanical properties. Despite the considerable research into carbon-based nanostructures such as carbon nanotubes (CNTs) and graphene as reinforcing agents in polymeric foams to maximize their energy absorption capabilities and strength, further efforts are required to investigate the potential role of other nanomaterials, such as shear thickening fluids (STFs), which exhibit extraordinary energy absorption properties. STFs as colloid suspensions, at elevated shear rates, form hydroclusters due to particle interaction yielding a drastic viscosity increase. Thus, rapid viscosity change results in an excellent energy absorption characteristic. This study is aimed to improve the compressive and energy absorption properties of PU foams with STF integration while discussing the microstructure/mechanical property relationship. Initially, STFs were fabricated with up to 30 wt.% fumed and spherical silica content, using a mechanical stirrer at 300 rpm and horn sonicator with 30% amplitude, then investigated by a plate rheometer in order to understand the effect of particle geometry and weight fraction on the flow characteristics. The results revealed that 26 wt.% of fumed silica was the optimum suspension for integration to PU foam with the excellent thickening ratio, and viscosity values increased after critical shear rate up to 67.66 times. The 26 wt.% STFs were successfully integrated into rigid PU foams, using a mechanical stirrer prior to the foaming reaction, at 0.5, 1, and 3 wt.%, and the morphological analysis, compression, and cyclic compression tests at various strain rates up to 0.2 s-1 and 10, 40, 80% strains xxii in order to understand rate dependent properties under different deformation region such as plateau and densification, were performed. According to morphological characterizations, cell edge lengths decreased and cell wall thickness increased with increasing until 1 wt.% STF. The maximum t/l ratio, which is a favorable indicator for mechanical strength prediction, was observed with 1 wt.% STF integration into PU foam. The results showed that 1 wt.% STF presented the highest compressive strength and specific compressive strength with 33% and 10.4% increments, respectively. For the energy absorption properties, 1 wt.% STF demonstrated up to 9.4% higher loss factor and 46.9% total absorbed energy regarding cyclic compression tests strain and strain rate parameters. Dynamic mechanical analyses were also carried out, under bending loads with dual cantilever clamps, to develop the microstructure-mechanical property relationship and to examine the viscoelastic properties. Linear viscoelastic region and storage and loss moduli increased with 1 wt.% STF integration. The results were consistent with both the compression and cyclic compression tests, despite differences in the direction and frequency of the applied force. Integration of 1 wt.% STF resulted in a significant increase of approximately 50% in both storage and compression modulus, while no significant changes in loss factor were observed. By enhancing the storage modulus and broadening the linear elastic regime, STF/PU foams can effectively withstand higher levels of load and displacement without permanent deformation. Excessive STF integration, such as above 1 wt. %, caused deterioration in the foam morphology and cellular structure, resulting in lower mechanical properties and strengths, however the 3 wt.% STF integrated foam still exhibited better properties than neat foam. This study showed that the flow properties of STFs are significantly influenced by the silica surface geometry and weight fraction. Furthermore, the study demonstrated that the addition of an optimized amount of STF enhanced the compressive strength, viscoelastic properties, and energy absorption capabilities of PU foams. With the addition of STF, PU foams could be used in a wider range of application areas and their superior mechanical properties could be used to build safer and reliable structures.