Investigation On Melt Flow Rate And Water-resistance Properties Of Thermoplastic Starch Biocomposites Modified With Sorbic Acid

In recent years,biodegradable polymers have attracted intensively attention due to their environmental friendliness.It is expected that in the next few decades,biodegradable polymers will gradually replace some of the traditional petroleum-based materials.In particular,starch-based composites have been widely used.However,traditional thermoplastic starch（TPS）are strongly inclined to absorb moisture,recrystallization and/or retrogradation during long-term storage,that yields very poor strength retention properties of the traditional TPS materials.However,the moisture absorption,retrogradation and strength retention properties of TPS materials during long-term storage were not systematicaly reported in the literatures or patents.After being modified by glutaraldehyde（GA）,the modified TPS materials exhibit good water-resistance and strength retention properties,however,their melt flow rates（MFR）are very small that makes them very difficult for melting-processing in the industries.In order to improve the melt processibility and strength retention properties of GA modified TPS materials,PLA were melt-blended with GA modified TPS hydrolyzed with sorbic acid.The main contents and results are summarized as follows:（1）Investigation on the properties of sorbic acid hydrolyzed thermoplastic starchThe tapioca starch was used as the raw material,water and glycerin as the plasticizer.The thermoplastic starch materials with water-resistance were prepared by crosslinking the tapioca starch with a common dialdehyde cross-linker（glutaraldehyde）.Then,the sorbic acid（SA）is used to hydrolyze the tangled thermoplastic starch molecular chain segment under the external mechanical force to make it smaller and shorter,so as to improve its melt processibility.The structure,morphology and mechanical properties of the sample were characterized by melt flow rate,Fourier transform infrared,differential scanning calorimetry,wide angle X-ray diffraction,water content,tensile strength,impact strength and scanning electron microscopy.The results have showed that the melt flow rate increases with the increasing of sorbic acid content and acid hydrolysis time.The optimum acid hydrolysis time was determined to be 1.5 hours,differential scanning calorimetry and wide angle X-ray diffraction analysis also showed that the hydrolyzed thermoplastic starch of SA acid induced the recrystallization of starch during long-term storage.The recrystallization melting point of the sample was significantly decreased with the increase of SA content,and the intensity of the recrystallization of the corresponding V_H type starch increases obviously with the increase of SA content.After the tensile and impact tests were carried out on the samples,it was found that the tensile strength and impact strength gradually decreased with the increase of SA content and the placing time,and the elongation at break was improved.To summarize the melt index and mechanical properties,the optimum SA content is 0.5 PHS（part per hundred parts of starch）.（2）Investigation on properties of acid hydrolyzed thermoplastic starch/polylactic acid compositesIn this chapter,thermoplastic starch-based composites of melt processibility and water-resistance were prepared by 15,20,25 and 30 wt%of water-resistant biodegradable material PLA were blended in the thermoplastic starch with an SA content of 0.5 PHS and acid hydrolysis for 1.5 hours(SAMWRTPS₁₀₀^0.5/1.5GA_0.5),respectively.The structure,morphology and mechanical properties of the sample were characterized by Fourier transform infrared,differential scanning calorimetry,wide angle X-ray diffraction and so on.The results showed that SAMWRTPS₁₀₀^0.5/1.5GA_0.5 sample and PLA was simply blended and without chemical bond changes,differential scanning calorimetry and wide angle X-ray diffraction analysis showed that the recrystallization melting point of PLA was higher than that of the unmixed SAMWRTPS₁₀₀^0.5/1.5GA_0.5 sample and the corresponding peak intensity of the recrystallization of the V_H type starch was obviously inhibited.The mechanical properties showed that the tensile strength and impact strength of acid hydrolyzed thermoplastic starch/polylactic acid are higher than that of unmixed SAMWRTPS₁₀₀^0.5/1.5GA_0.5 samples,and the elongation at break decreases,where the content of PLA is 25 wt%,the retention and impact strength of the sample is best after the initial and 20℃/50%RH conditions are placed at different times.