# Development and Characterization of Tailored Polyurethane Foams for Shock Absorption
## 1 Introduction  
Human protection systems such as bullet vests, frames, and helmet or nonlethal projectile weapons are increasingly used [1]. Protection materials must be resistant to fire and shock to reduce accidental injuries caused by explosives usage. In addition to their advantage and usefulness (costs, infrastructure, ease of handling, superior performance), polymers can offer the design of safer and cheaper protection systems with suitable fire resistance and shock resistance. Polymer foams, thanks to their cellular structure made up of open or closed cells, are promising candidates with strong shock absorption, excellent acoustic and thermal isolation, and the ability to achieve intumescence [2]. The optimization of properties requires considering the relationship between microstructure and properties. The mechanical response of such materials is based on the nature and the intrinsic characteristics of polymer cell walls such as thickness, distribution, and cell type, which are the basic units for the foam architecture [3]. The mechanical properties can also be tuned by mixing high and low densities [4]. Polyurethane foams can be made of various chemicals and are very tunable. The alveolar polyurethanes are produced by combining poly(isocyanate) with polyols in ablowing agent [5,6]. Flexible, rigid or semirigid foams can be designed according to the composition and chemistry of the reagents in use. [PERSON] examined the NCO/OH ratio effects between 1.00 and 1.06 [7]. The fatigue resistance was enhanced and the final polyurethane molar weight was increased, along with strength, variability, and better-controlled alternating flexible segments (SS) and rigid segments (SR) [8]. A high ratio increases the risk of cracking. Thus, a balance between mechanical strength and usage must be established. The number of urethane and urea bonds is usually increased when the ratio of NCO/OH increases, thus raising the possibility of hydrogen bonding. This effect increases the amount of the rigid segments, leading to an increase of elastic module E, an increase of the crystallinity degree to which rigid segments crystallize, and a decrease in the elongation of the polymer [7]. With a ratio of NCO/OH less than 1, mechanical properties (modulus, tension, and stress at break) are poor because of the low molar mass of the shaped PU chains [9]. The improvement in the total NCO/OH ratio a small amount above 1 gives the possibility to compensate for the lack of isocyanate feature by side reactions and therefore provide a higher modulus by optimizing the chain's elongation and growing the intensity of the urethane bonding device [10]. Recently, one of the main facets of change in the processing of polyurethane products is the exclusion of hazardous substances from the production method and their substitution with environmentally safe ones [11]. Concerns about materials derived from petroleum resources are common owing to their environmental and healthcare implications. That is why scientists were keen to develop polyurethane foam of vegetable origin. [PERSON] et al. studied the biosynthetic pathways for synthesizing cyclic carbonates and non-isocyanate polyurethane (NIPUs) [12]. [PERSON] et al. prepared Mannich-based polyol for hard foams [13] and the promotion of a homogeneous dispersion of fillers within the polymer matrix [14]. [PERSON] et al. used synergistic interactions to fabricate transparent and mechanically robust nanohybrids based on starch, non-isocyanate polyurethane, and cellulose nanocrystals (CNCs) for the development of sustainable, high-performance materials [15]. Since [PERSON] suggested the manufacture of silica aerogels in the 1930s [16], great attention has been paid to new advancements in fillers with alumina (Al\({}_{2}\)O\({}_{3}\)) [17,18] and multilayered fillers mainly from clays [19,20]. Their unique form impacts polyurethane foam, leading to extremely exceptional durability and stability.