Polyvinyl Alcohol And Sodium Hexametaphosphate Modified Thermoplastic Starch Materials Prepared With The Assistance Of Supercritical CO₂

The ease of melt-processing,moisture-resistance and tensile retention properties of sc CO₂sodium hexametaphosphate(SHMP)and polyvinyl alcohol(PVA)modified thermoplastic starch(sc CO₂TPS₁₀₀SHMP_xPVA_y)materials were explored in this investigation.These characteristics were considerably improved after modification with different contents of SHMP and PVA.The melting flow rate of optimized sc CO₂TPS₁₀₀SHMP₁₀PVA₂ was 15.3g/10min,which was 1.8/1.4 times of the corresponding TPS/TPS₁₀₀SHMP₁₀PVA₂.After conditioning at 25^oC and 50%relative humidity for 56 days,the moisture content values of optimized sc CO₂TPS₁₀₀SHMP₁₀PVA₂was only 2.7%,which was 23%/45%of the corresponding TPS/TPS₁₀₀SHMP₁₀PVA₂;The tensile strength retention values of optimized sc CO₂TPS₁₀₀SHMP₁₀PVA₂was 24.1 Mpa,which was 8.3/1.6 times of the corresponding TPS/TPS₁₀₀SHMP₁₀PVA₂.As evidenced by FTIR analyses,much more and loosely hydrogen-bonded-OH groups of starch molecules were observed in FTIR spectra of sc CO₂TPS,sc CO₂TPS₁₀₀SHMP_xand sc CO₂TPS₁₀₀SHMP₁₀PVA_y compared to TPS,TPS₁₀₀SHMP_xand TPS₁₀₀SHMP₁₀PVA_yspecimens,respectively.These loosely hydrogen-bonded-OH groups could then react to a greater extent with phosphate groups of SHMP during the modification processes of sc CO₂TPS₁₀₀SHMP_xPVA_y compared to TPS₁₀₀SHMP_xPVA_y gelatinized and modified under conventional atmospheric conditions,and also form strong hydrogen bond with the-OH groups of PVA.In addition,conditioned sc CO₂TPS,sc CO₂TPS₁₀₀SHMP_xand sc CO₂TPS₁₀₀SHMP₁₀PVA_y exhibited only Vh-type crystals with melting temperatures significantly lower than those of corresponding conditioned TPS,TPS₁₀₀SHMP_xand TPS₁₀₀SHMP₁₀PVA_y specimens.Possible reasons accounting for the considerably improved melt-processing,moisture-resistance,tensile retention and retrogradation of sc CO₂TPS,sc CO₂TPS₁₀₀SHMP_xand sc CO₂TPS₁₀₀SHMP₁₀PVA_y are proposed in this study.