Processing Technology With Supercritical Carbon Dioxide Assistant And Foaming Properties Of Glutaraldehyde Modified Thermoplastic Starch Foams

Supercritical carbon dioxide(scCO₂)processed thermoplastic starch(scCO₂^aTPS),cellulose nanofiber(CNF)modified scCO₂^aTPS(scCO₂^aTPS₁₀₀CNF_x)and glutaraldehyde(GA)modified scCO₂^aTPS₁₀₀CNF_0.02(scCO₂^aTPS₁₀₀CNF_xGA_y)foams were prepared for the first time using supercritical carbon dioxide as a blowing agent during their foaming processes.The expansion ratios of scCO₂^aTPS were increased significantly with increasing supercritical carbon dioxide pressure.After addition of 0.02 PHS proper amounts of cellulose nanofiber,scCO₂^aTPS₁₀₀CNF_0.02 foams displayed a considerable increase in expansion ratios compared to scCO₂^aTPS foams prepared at the same supercritical carbon dioxide pressure.By futher modification with proper amounts of glutaraldehyde,scCO₂^aTPS₁₀₀CNF_0.02GA_y foams displayed a considerable increase in expansion ratios compared to scCO₂^aTPS₁₀₀CNF_0.02foams.The expansion ratio,cell density,moisture-resistance and compressive strength retention properties of scCO₂^aTPS,scCO₂^aTPS₁₀₀CNF_0.02 and scCO₂^aTPS₁₀₀CNF_0.02GA_y foam series were considerably improved with increasing supercritical carbon dioxide pressure during the foaming processes.The expansion ratios and cell densities of each scCO₂^aTPS₁₀₀CNF_0.02GA_yfoam series were increased considerably to a maximum value,as the glutaraldehyde content approached an optimum value.The optimal scCO₂¹¹TPS₁₀₀CNF_0.02GA_1.6 foam material exhibited a high expansion ratio and cell density at?50 and?8×10⁸ cells/cm³,respectively.Compared to corresponding aged scCO₂^aTPS and scCO₂^aTPS₁₀₀CNF_0.02 foam specimens,considerably better moisture resistance and compressive strength retention properties were observed for scCO₂^aTPS₁₀₀CNF_0.02GA_y foam specimens,when they were modified with the corresponding optimum glutaraldehyde content.The moisture resistance and compressive strengthretentionforoptimalpreparedscCO₂⁷TPS₁₀₀CNF_0.02GA_0.4,scCO₂⁹TPS₁₀₀CNF_0.02GA_0.8 and scCO₂¹¹TPS₁₀₀CNF_0.02GA_1.6 foam materials improved further with increasing supercritical carbon dioxide pressure.In fact,the increase in expansion ratios and cell densities for each scCO₂^aTPS,scCO₂^aTPS₁₀₀CNF_0.02and scCO₂^aTPS₁₀₀CNF_0.02GA_yfoam series with increasing supercritical carbon dioxide pressure is most likely attributed to the presence of increased soluble supercritical carbon dioxide during the foaming processes.This generates more nuclei,leading to higher expansion ratios and structures with increased cell density as supercritical carbon dioxide pressure increases.As evidenced by Fourier transform infrared and melt flow rate analysis,chemical(e.g.crosslinking)reaction of starch hydroxyl groups with the aldehyde functionality of glutaraldehyde occurred during the modification processes of scCO2^aTPS100CNF0.02GAy.Presumably,crosslinking reaction between glutaraldehyde and starch molecules can increase the melt strength and viscosity of scCO2^aTPS100CNF0.02GAy foams,and hence,considerably enhance the nucleation of cell structures,cell densities and expansion ratios during their scCO₂-assisted foaming processes.In addition,gelatinization,complete disruption of molecular organization and/or modification within amylose or even amylopectin chains of the starch granules,became much easier with the aid of supercritical carbon dioxide in the processing of scCO₂^aTPS₁₀₀CNF_0.02 and scCO₂^aTPS₁₀₀CNF_0.02GA_y.These would imply that the dissociated amylopectin chains would have the possibility and time to entangle and form hydrogen bonds,not only with themselves and the plasticizer(i.e.glycerol),but also with amylose molecules during the gelatinization process.Presumably,these stronger but dissociated amylopectin intermolecular interactions present in scCO₂^aTPS₁₀₀CNF_0.02 and scCO₂^aTPS₁₀₀CNF_0.02GA_y would improve their moisture resistance,compressive strength retention properties and inhibit their recrystallization and/or retrogradation during their conditioning processes.