Quasi-Static Mechanical Response of Density-Graded Polyurea Elastomeric Foams

Abstract

Density gradation of foam structures has been investigated and found to be a practical approach to improve the mechanical efficacy of protective padding in several applications based on nature-based evidence of effectiveness. This research aims to disclose a discrete gradation approach without adhesives by relying on the properties of the frothed foam slurry to bond and penetrate through previously cured foam sheets naturally. As confirmed by electron microscopy observations, bilayer-and trilayergraded elastomeric polyurea foam sheets were fabricated, resulting in seamless interfaces. The mechanical performance of seamless, graded foam samples was compared with monolayer, mono-density benchmark foam, considered the industry standard for impact mitigation. All foam samples were submitted to compressive loading at a quasi-static rate, reporting key performance indicators (KPIs) such as specific energy absorption, efficiency, and ideality. Polyurea foams, irrespective of gradation and interface type, outperformed benchmark foam in several KPIs despite the drastic difference in the effective or average density. The average compressive stress−strain curves were fitted into empirical constitutive models to reveal critical insights into the elastic, plateau, and densification behaviors of the tested foam configuration. The novelty of these outcomes includes (1) a fabrication approach to adhesive-free density-graded foam structures, (2) implementation of a diverse set of KPIs to assess the mechanical efficacy of foams, and (3) elucidation of the superiority of polyurea foam-based lightweight protective paddings. Future research will focus on assessing the dynamic performance of these graded foam structures under impact loading conditions at a wide range of velocities.

Full‐Field Analyses of Density‐Graded Elastomeric Foams Under Quasistatic and Impact Loadings

“…Two assembly strategies (Figure 1a) were used to produce densitygraded polyurea foams using seamless and adhered interfaces. [6] Seamless interfaces were fabricated by pouring subsequent layers of foam slurry directly onto the exposed surface of a previously cured layer while it remained in the mold. As a result, polyurea foams inherit the naturally strong adhesion properties of the uncured frothed slurry, seamlessly joining the two layers.…”

“…Polyurea foams have recently emerged as a prime example of elastomeric foams for impact mitigation applications, including shoe insoles, [4] helmet liners, [5] and protective pads for lateral falls. [6] The motivation for foaming polyurea was the extensive body of research on the bulk counterpart, elucidating the multiscale mechanical behavior using experimental and computational approaches. [7][8][9][10][11][12] Aromatic polyurea, synthesized based on the stochiometric mixture of oligomeric diamine and modified methylene diisocyanate, exhibits excellent impact-resistant and shock-tolerant responses in harsh environments despite its low acoustic impedance due to its chemical, moisture, and ultraviolet radiation resiliency.…”

“…[26][27][28] Several recent experimental and optimization studies reported the mechanical behavior of density-graded polyurea foams based on the potential provided by this gradation strategy. [4,6,29] Uddin et al studied the mechanical behavior of densitygraded polyurea foams by assembling bilayer structures of %1 mm-thick adhesives. [29] Positively graded foam stacks with a stiff thermoset interlayer enhanced strength and energy dissipation when subjected to various rates of uniaxial loading, while mechanically compliant interlayers only improved energy absorption efficiency.…”

“…Consequently, Smeets et al constructed bilayer-and trilayer-graded polyurea foams, leveraging two interfacing strategies, that is, using the natural adhesiveness of uncured frothed foam slurry or thin adherent. [6] Smeets and collaborators found that monolayer foams with homogenous density outperformed the graded counterparts under quasistatic and low-velocity impact. Nonetheless, they hypothesized that density gradation is more useful in high-speed loading, leading to a companion investigation using a shock tube to launch projectiles at %25 m s À1 .…”

Self‐foaming polyurea foams reinforced by nano SiO2 via a non‐isocyanate route

“…For instance, the group of Youssef synthesized PUrFs with different densities, by mixing modified methylene diphenyl diisocyanate compound (Isonate 143 L MDI; Dow Chemical) with oligomeric diamine (Versalink P1000; Air Products). [9][10][11][12][13][14][15][16] They studied further the mechanical response of those density-graded polyurea elastomeric foams. Gupta and coworkers used the same method to prepare PUrFs and explored their energy absorption ability for applications as lattice filling materials or helmet liner materials, etc.…”