Mechanical Retention and Waterproof Properties of Bacterial Cellulose-Reinforced Thermoplastic Starch Biocomposites Modified with Sodium Hexametaphosphate

The waterproof and strength retention properties of bacterial cellulose (BC)-reinforced thermoplastic starch (TPS) resins were successfully improved by reacting with sodium hexametaphosphate (SHMP). After modification with SHMP, the tensile strength (σ_f) and impact strength (I_s) values of initial and conditioned BC-reinforced TPS, modified with varying amounts of SHMP(TPS₁₀₀BC_0.02SHMP_x), and their blends with poly(lactic acid)((TPS₁₀₀BC_0.02SHMP_x)₇₅PLA₂₅) specimens improved significantly and reached a maximal value as SHMP content approached 10 parts per hundred parts of TPS resin (phr), while their moisture content and elongation at break (ɛ_f) was reduced to a minimal value as SHMP contents approached 10 phr. The σ_f, I_s and ɛ_f retention values of a (TPS₁₀₀BC_0.02SHMP₁₀)₇₅PLA₂₅ specimen conditioned for 56 days are 52%, 50% and 3 times its initial σ_f, I_s and ɛ_f values, respectively, which are 32.5 times, 8.9 times and 40% of those of a corresponding conditioned TPS₁₀₀BC_0.02 specimen, respectively. As evidenced by FTIR analyses of TPS₁₀₀BC_0.02SHMP_x specimens, hydroxyl groups of TPS₁₀₀BC_0.02 resins were successfully reacted with the phosphate groups of SHMP molecules. New melting endotherms and diffraction peaks of V_H-type crystals were found on DSC thermograms and WAXD patterns of TPS or TPS₁₀₀BC_0.02 specimens conditioned for 7 days, while no new melting endotherm or diffraction peak was found for TPS₁₀₀BC_0.02SHMP_x and/or (TPS₁₀₀BC_0.02SHMP_x)₇₅PLA₂₅ specimens conditioned for less than 14 and 28 days, respectively.