# Partially-Perforated Self-Reinforced Polyurea Foams
## 1 Introduction
A promising material for foaming is polyurea, a thermoset elastomer with superior structural and mechanical properties coupled with hygothermal and chemical resistance that warranted widespread attention in industrial and scientific applications [14; 15; 16; 17; 18; 19]. The superior impact mitigation properties are driven by the underlying segmental microstructure of polyurea, which comprises of a rigid hard phase composed of aromatic moieties, dispersed in a compliant, aliphatic soft phase [20]. The ratio of these hard and soft phases is controlled by tuning the stoichiometric ratio of the isocyanate and amine constituents [21]. It is essential to differentiate between the chemical structure of the common polyurethane material, including its foam derivatives, and the polyurea discussed here. Generally, dense polyurea is well situated within the coating industry as a viable solution for a wide range of applications, e.g., water well and roof protection, given its ability to retain the majority of its mechanical properties even after exposure to ultraviolet radiation for extended periods [22; 23; 24; 25]. It was then the motivation of this research to introduce a novel polyurea foam microstructure manufactured through a simple and green manufacturing process that also self-reinforces the cell walls with polyurea microspheres (a byproduct of the process), hence, creating a new polymer/polymer foam composite [5; 6; 12]. The notion of 'composite' is motivated based on the concurrent forming of polyurea in a different form factor, i.e., the microsphere, which was shown to be stiffer than its dense counterpart [26]. The introduction of this new class of polymeric foams will promote scientific, theoretical, and experimental investigations beyond those presented within the scope of this paper. It is important to note that other researchers concurrently investigated different variations of polyurea foams, based on the same constituents, where their results corroborate and support the notion of this foam having superior impact mitigation properties [26; 27]. Polyurea foams can be further exploited in many applications, including thermal insulation, filtration, and acoustic isolation, to name a few. Previous studies confirmed that polyurea foams inherit superior mechanical properties, including viscoelastic and impact mitigation attributes, of their dense counterpart [4; 5; 6; 28; 29; 30; 31]. Different density variations were fabricated and characterized using quasi-static and impact testing. [PERSON] et al. first performed the microstructure analysis [5]. They noted that their version of polyurea foam exhibit a hierarchical microcellular structure consisting of large perforated semi-closed spherical cells surrounded by smaller closed spherical cells, where the mean diameters, calculated based on image analysis of scanning electron microscope micrographs, were found to be \(370\pm 162\)\(\upmu\)m and \(69\pm 18\)\(\upmu\)m, respectively [5]. Following the Ogden hyperfoam model, an exhaustive quasi-static characterization demonstrated the hyperelastic response of polyurea foams extending from linear elasticity to the onset of densification, and the results and the experimental data were found to be in good agreement [5]. [PERSON] and coworkers also found that the elastic and energy-absorbing properties of polyurea foams outperformed a benchmark, off-the-shelf, closed-cell foam (also with spherical microcellular structure) [5]. [PERSON] et al. then tested the same polyurea foams in response to low-velocity impacts and confirmed the predictions of [PERSON] et al. regarding the remarkable impact performance [6]. Contemporary to the investigations mentioned above, [PERSON] et al. also foamed polyurea using a vacuum environment distinctively different from the polyurea foams reported herein that were foamed and cured in ambient conditions [4; 28; 29; 30; 31].