Rheologically, a substantial viscosity difference was observed: the 100% NCS batter measured 132.4 mm/s, while the 100% WCS batter reached 1474.8 mm²/s (Table 2). This marked increase is attributed to the extensive, water-entrapping molecular network formed by the highly branched amylopectin in WCS [23].

The visual appearance of the final foam samples was significantly influenced by the starch blend ratio. Foams with increased proportions of WCS presented a darker yellow color, a rougher surface texture, and exhibited more pronounced structural collapse and deformation following molding [7,11]. Detailed morphological characterization was performed using Scanning Electron Microscopy (SEM) (Table 2, micrographs). A uniform cellular structure was not achieved in any formulation, with denser and smaller cells consistently observed at the sample edges, which is typical of hot-pressed starch foams where rapid edge cooling restricts localized cell expansion [11,12]. However, the central regions showed variation based on composition. The 100% NCS foam (100:0) exhibited a uniform, non-collapsed structure with large, fully expanded cells throughout the core. In stark contrast, the 100% WCS foam (0:100) presented the most severe structural collapse in the central region, characterized by smaller, more densely packed cells bounded by thicker cell walls. This structural limitation is directly linked to the high viscosity and distinct melt rheology of the WCS batter. The highly branched amylopectin structure in WCS increases the resistance to flow, which hinders the rapid escape and expansion of water vapor during the critical foaming phase, leading to internal pressure imbalances and subsequent cell wall rupture or collapse [7,23].

A clear relationship between starch composition and final bulk density was established (Table 2). The lowest density of 0.68 g/cm³ was measured for the 100% NCS foam, while the highest density of 0.88 g/cm³ was observed for the 100% WCS foam. A progressive increase in final foam density was consistently demonstrated as the proportion of WCS was elevated. These distinct structural differences confirm that cell density and wall thickness, governed by the WCS content, are the primary factors controlling the final bulk density of the foamed material. These findings are consistent with established models of starch expansion, where the higher amylose content in NCS provides the necessary melt strength to sustain large cell growth during rapid vapor expansion [7,11], whereas high-amylopectin starches (WCS) often exhibit restricted expansion and thicker cell architectures due to their highly branched molecular structure [23,24].

The flexural properties of the NCS/WCS blended foams were evaluated using the three-point bending method (Table 3). The 100% NCS foam exhibited superior flexural strength and strain compared to the WCS counterpart. A marked contrast was observed in the flexural modulus: the NCS foam registered 97 ± 5.9 MPa, while the WCS foam attained a substantially higher value of 648 ± 4.2 MPa. This increase is attributed to the WCS foam’s dense cellular structure, resulting from minimal expansion due to the highly viscous batter, which yielded an exceptionally rigid and brittle material. A slight reduction in both flexural strength and strain was observed as the WCS proportion increased, consistent with morphological analysis. Although the flexural modulus increased slightly with WCS content, this stiffening is primarily linked to the corresponding increase in bulk density, as the stiffness of cellular materials correlates directly with density [12,24].

The Izod impact strength was uniformly poor (≈0.0410 J) across all NCS/WCS blends, and results were considered unreliable due to the severe foam collapse induced by high WCS content, which violated the constant cross-section assumption of the ASTM D256 standard.

The investigation revealed a trade-off: increased WCS elevated viscosity, resulting in smaller, denser cells susceptible to collapse. This structural instability increased foam density and negatively affected mechanical performance, demonstrating decreased flexibility and increased brittleness. Consequently, the 100% Normal Corn Starch formulation was established as the optimal base material due to its superior morphological stability, low density, and greater flexibility, providing a robust foundation for subsequent optimization.

The rheological properties of the pre-foaming batters were evaluated to determine the influence of xanthan gum (XG) on processing behavior (Table 4). A base viscosity of 132.4 mm²/s for the pure corn starch batter increased progressively with XG inclusion, reaching 1908.5 mm²/s at 2.0% w/w. Notably, a non-linear rheological transition was observed at the 1.0% XG threshold, where viscosity increased sharply to 1465.3 mm²/s. This substantial jump is attributed to synergistic intermolecular hydrogen bonding between the starch amylose and the trisaccharide side chains of the XG molecules, facilitating the formation of a robust “weak gel” network [25,26].

At this 1.0% concentration, the network provides sufficient yield stress to counteract gravitational drainage and surface tension during the critical heating phase of compression molding, thereby preventing the “central collapse” observed in lower-viscosity formulations. Furthermore, this specific rheological profile is hypothesized to represent an optimal balance; while the viscosity is sufficient to stabilize nascent cell walls against coalescence, it remains below the threshold where excessive stiffness would inhibit vapor-driven expansion. Consequently, the 1.0% XG loading enables the achievement of the lowest stable bulk density (0.43 g/cm³) while ensuring structural homogeneity across the foam matrix.

Morphological characterization via SEM revealed that XG incorporation significantly altered the cellular architecture of the starch foam. Distinct morphological zones were identified: a dense, small-celled perimeter “skin” layer formed by rapid heat transfer at the mold interface, and a more expansive central core. To ensure a consistent assessment of the expansion process, all quantitative descriptions regarding cell size and density (Table 4) refer to the overall bulk structure of the specimens, encompassing both the stabilized core and the integral skin layer formed during processing. This skin-core morphology is characteristic of compression-molded starch systems, where thermal lag prevents uniform vapor expansion near the cooled mold surfaces [12,24].

The 1.0% XG concentration was identified as a critical morphological inflection point. At this threshold, the pore structure transitioned from large, irregular voids to a refined, uniform closed-cell network. A detailed analysis of the 1.0% XG core revealed a quantifiable thinning of the cell walls relative to the control. This thinning of the cell walls, occurring concurrently with a reduction in mean cell size, serves as evidence of XG’s role in interfacial stabilization. By increasing the viscosity of the starch batter and forming a protective hydrocolloid layer at the nascent air-starch interface, XG effectively inhibits cell coalescence [19,26]. This mechanism stabilizes the interface during the rapid vapor-driven expansion phase, facilitating the development of a more delicate yet mechanically stable cellular matrix. These findings align with previous studies demonstrating that xanthan gum’s “weak gel” rheological behavior provides the necessary melt strength to withstand the internal pressures of expanding steam without rupturing the thin cell walls [27,28].

The influence of XG on foam density was quantified, showing a pronounced inverse relationship (Table 4). The density of the pure foam (0.68 g/cm³) was systematically reduced with increasing XG concentration, culminating in the lowest density of 0.24 g/cm³ at 2.0%. This reduction is directly substantiated by the morphological stabilization, confirming XG as an effective agent for producing lightweight, low-density corn starch foams.

The flexural properties of the XG-modified foams were characterized via three-point bending tests according to ASTM D5943-96. As summarized in Table 5, the unreinforced corn starch foam displayed the maximum flexural strength (3.66 ± 0.36 MPa) and flexural modulus (96.65 ± 5.91 MPa). The incorporation of XG across the investigated range (0.5%–2.0% w/w) induced a marginal reduction in flexural strength and a progressive attenuation of the flexural modulus, indicating a transition toward a more compliant material.

Notably, the 1.0% w/w XG concentration was identified as the optimal threshold for industrial cushioning applications. At this loading, the material achieves a critical balance between significant lightweighting—associated with the stabilized, thin-walled cellular morphology—and sufficient structural integrity. While the decline in flexural modulus signifies a decrease in intrinsic stiffness, this modification is conducive to enhanced energy-absorption performance. The increased flexibility at the 1.0% level facilitates a broader area under the stress-strain curve during deformation, indicating superior toughness and energy dissipation capacity compared to the more brittle control foam. This trade-off between peak mechanical strength and viscoelastic energy absorption positions the 1.0% XG formulation as the most viable candidate for protective packaging, where the prevention of impact-force transmission to heavy or delicate goods is the primary performance objective.

The cohesiveness and springiness, critical performance indicators for cushioning materials [29], were evaluated through TPA. Foams without XG and with a low concentration (0.5%) exhibited severe structural collapse post-molding, precluding TPA testing. However, sufficient structural integrity was achieved at XG concentrations from 1.0% to 2.0%, making these samples suitable for TPA evaluation. As presented in Fig. 1 and quantified in Table 6, a clear dependency on XG concentration was established: both the compressive strength and the cohesiveness of the foam were found to increase with rising XG content. This result suggests that the enhanced molecular network provided by the XG effectively reinforces the cell walls, leading to a more robust and unified structure. The incorporation of XG results in a marked improvement in foam stability and formability, attributed to its gel-forming ability which establishes a thermo-reversible hydrocolloid system [26]. This system stabilizes the starch matrix, mitigating water loss and increasing material toughness. Despite these structural benefits, the influence of XG concentration on most measured mechanical properties was generally not statistically significant. While marginal decreases in flexural strength and modulus were observed with increasing XG, no statistically significant differences in compressive strength, springiness, or cohesiveness were detected across the tested XG concentrations by TPA.

The cellular structure of the neat control foam (1% xanthan gum only) was characterized by small, uniformly distributed cells. Upon fiber inclusion, a noticeable alteration was observed (Table 7). At the 2% loading, the central cells appeared smaller and more densely packed than the control, attributed to the physical obstruction of cell expansion by the fibers. Conversely, increasing the fiber loading to 4% and 6% resulted in larger foam cells. This enlargement is hypothesized to be due to a mechanical obstruction effect where the rigid fibers impede the expansion of nascent cell walls, leading to the rupture and coalescence of adjacent cells [16]. This ultimately yields the formation of larger, irregularly shaped cells. Across all samples, the fibers were generally well-dispersed, though localized clumping was occasionally present, and the overall fiber density increased proportionally with the elevated content. Consistent with prior research [12], the peripheral cells remained small and dense across all fiber loadings due to rapid heat transfer at the mold surface.

The density of the fiber-reinforced composite foams was quantified (Table 7). The control foam (1% xanthan gum) exhibited a density of 0.43 ± 0.04 g/cm³. A clear trend of decreasing density was observed as the fiber loading was increased. The lowest density of 0.36 ± 0.04 g/cm³ was achieved at the 6% fiber loading. This reduction in bulk density is primarily attributed to the increased porosity resulting from the morphological changes induced by the fibers. The fiber-induced cell wall rupture and subsequent coalescence led to the formation of larger voids, increasing the material’s overall porosity and confirming the fibers’ role in reducing density and shrinkage during cooling [16].

The flexural properties of the corn husk fiber-reinforced foams were evaluated (ASTM D5943-96). Contrary to expected polymeric composite behavior [30], the flexural strength, strain, and modulus displayed no statistically significant change with increasing fiber content (Table 8). This was attributed to a competing effect: the reinforcing contribution of the fibers was counteracted by a concurrent negative influence on the foam matrix. Specifically, increased fiber loading correlated with an increase in foam cell size and a measurable decrease in composite density. This cell enlargement reduced the intrinsic strength of the foam structure, effectively negating the positive mechanical effects of the fibers.

The impact absorption capacity was quantified using an Izod impact tester. A clear and positive correlation was established between corn husk fiber content and resulting impact strength (Table 8). The unreinforced base foam exhibited an average impact strength of 1.27 ± 0.21 kJ/m². This value was systematically enhanced with increased fiber loading, culminating in the highest average impact strength of 1.87 ± 0.50 kJ/m² achieved by the 6% fiber formulation. This improvement confirms the fibers’ effectiveness in augmenting the foam’s ability to absorb and dissipate impact energy, a trend consistent with observations in other lignocellulosic-reinforced starch systems [13,16].

This increase is primarily attributed to the good interfacial adhesion facilitated by the TEOS-treated fibers, which enables efficient stress transfer from the starch matrix to the high-strength fibers [15,21]. The silane-coupling agent effectively bridges the hydrophilic starch and the fiber surface, creating a more cohesive network that promotes energy dissipation through mechanisms such as fiber pull-out and crack bridging during sudden loading. Fiber reinforcement is thus affirmed as a highly effective strategy for improving the impact resistance of these bio-based foams, bridging the performance gap between sustainable alternatives and synthetic petroleum-based cushioning materials.

TPA was performed to comparatively assess the internal bonding (cohesiveness) and elasticity (springiness) of the developed composite foams, as high values in these parameters are crucial for preventing fragmentation and enabling repeated impact absorption in cushioning materials [31]. The TPA results for the control foam (1% xanthan gum) showed a compressive strength of 0.12 ± 0.10 N/mm², springiness of 0.61 ± 0.08 (approximately 60.6% recovery), and cohesiveness of 0.33 ± 0.02. Upon the initial incorporation of corn husk fibers, a marginal enhancement was observed. Crucially, a clear trend of compressive strength improvement was established with increasing fiber loading, culminating in a significantly elevated strength of 0.56 ± 0.17 N/mm² at the highest (6%) loading (Fig. 2a). Concurrently, cohesiveness also increased notably, suggesting that the fibers effectively reduced the material’s propensity for fracturing. However, the springiness (elastic recovery) lacked statistically significant improvement across the increased fiber loading range. This outcome indicates that while the fibers successfully enhance the foam’s strength and internal bonding, they do not substantially contribute to its elasticity or capacity for deformation recovery.

The optimized 6% fiber composite foam was subjected to a comparative Texture Profile Analysis (TPA) against commercial polyethylene (PE) and polystyrene (PS) foams (Fig. 2b). An exceptionally high compressive strength of 0.56 N/mm² was recorded for the bio-based composite, exceeding PE foam by approximately 3.8 times and PS foam by 5.7 times. This high rigidity, however, raises concerns regarding potential impact force transmission rather than absorption, which is undesirable for fragile goods. Conversely, the springiness (elastic recovery of ≈52%) was substantially lower than that of PE (≈94.6%) and PS (≈86.4%) foams, indicating a significantly reduced capacity for recovery after single deformation and limiting suitability for multi-cycle impact applications. Furthermore, the cohesiveness of the bio-based foam (0.47) was considerably lower than commercial alternatives (PE: 0.79, PS: 0.92), suggesting a greater propensity for fragmentation under impact stress. While the composite demonstrated superior load-bearing capacity, its practical applicability requires further enhancement of elasticity and internal bonding properties before it can serve as a direct replacement for conventional petrochemical cushioning foams.