Thermoplastic starch and glutaraldehyde modified thermoplastic starch foams prepared using supercritical carbon dioxide fluid as a blowing agent

Supercritical carbon dioxide (scCO₂) processed thermoplastic starch (scCO₂ ^aTPS), cellulose nanofiber (CNF) modified scCO₂ ^aTPS (scCO₂ ^aTPS₁₀₀CNF_0.02) and glutaraldehyde (GA) modified scCO₂ ^aTPS₁₀₀CNF_0.02 (scCO₂ ^aTPS₁₀₀CNF_0.02GA_x) foams were prepared for the first time using scCO₂ as a blowing agent during their foaming processes. The expansion ratio, cell density, moisture resistance, and compressive strength (σ_c) retention properties of each foam series were considerably improved with increasing scCO₂ pressure during the foaming processes. The expansion ratios and cell densities of each scCO₂ ^aTPS₁₀₀CNF_0.02GA_x foam series were increased considerably to a maximum value, as the GA content approached an optimum value. The optimal scCO₂ ¹¹TPS₁₀₀CNF_0.02GA_1.6 foam material exhibited a high expansion ratio and cell density at approximately 50 and approximately 8 × 10⁸ cells/cm³, respectively. Compared with corresponding aged scCO₂ ^aTPS and scCO₂ ^aTPS₁₀₀CNF_0.02 foam specimens, considerably better moisture resistance and σ_c retention properties were observed for scCO₂ ^aTPS₁₀₀CNF_0.02GA_x foam specimens, when they were modified with the corresponding optimum GA content. The moisture resistance and σ_c retention for optimal prepared scCO₂ ⁷TPS₁₀₀CNF_0.02GA_0.4, scCO₂ ⁹TPS₁₀₀CNF_0.02GA_0.8 and scCO₂ ¹¹TPS₁₀₀CNF_0.02GA_1.6 foam materials improved further with increasing scCO₂ pressure. Possible reasons accounting for the highly expansion ratio, moisture resistance, and σ_c retention properties for scCO₂ ^aTPS₁₀₀CNF_0.02GA_x foams are presented.