Preparation,structure And Properties Of Thermoplastic Cassava Starch /LDPE/SiO₂ Composites

Starch based films have many advantages such as biodegradability,green environment protection and wide source,and have a good prospect in the application of starch based films.But because of its easy to absorb water,brittleness,and poor tensile strength,it limits their application and development in film packaging and other industries.In this paper,cassava starch,LDPE as raw materials and glycerin as plasticizers,three kinds of nano silica(SiO₂)with different surface properties were selected as reinforcing materials,respectively,hydrophilic TSP-H10,hydrophobic TSP-L12 and hydrophobic TSP-L15.Torque rheometer was used the thermoplastic cassava starch(TPS)/LDPE/SiO₂composite was prepared by melting method by plate vulcanizer.The rheological properties of the composite were studied by torque rheometer and capillary rheometer;The thermal properties of composite were studied by DSC and TG;The mechanical properties of composites were studied by universal testing machine;The stability of composites was studied by using photoelectric Haze Meter and contact angle meter;The structure of TPS/LDPE/SiO₂composite was studied by means of PLM,SEM and XRD.The results are as follows:The rheological properties of TPS/LDPE/SiO₂composites were characterized by plasticizing test and torque rheometer capillary test.The results show that the processing properties of TPS/LDPE/SiO₂composites are improved by adding LDPE and SiO₂,and the plasticizing torque of the composites increases and the equilibrium torque decreases;Shear rate of composites?As the viscosity increases,the shear stress decreases?With the shear rate increasing?It increases with the increase of population;The addition of hydrophilic TSP-H10has the best plasticizing property,and the shear stress of the composite is lower?maximum.The minimum non Newtonian index of TPS/LDPE/SiO₂composites with 5 phr of hydrophilic TSP-H10 and 5 phr of hydrophobic TSP-L12 is 0.03,and the composites show pseudoplastic fluid properties.The smaller the non Newtonian index is,the stronger the non Newtonian property is,that is,the stronger the pseudoplasticity is,and the more obvious the viscosity changes with the change of shear rate.The mechanical properties of TPS/LDPE and TPS/LDPE/SiO₂composites were tested and analyzed by universal testing machine.The results show that with the increase of LDPE content from 0 phr to 30 phr,the stress of TPS/LDPE Composites increases from 1.18 MPa to 2.85 MPa,and the strain decreases from 89%to 8.2%;The tensile strength increased from 1.2 MPa to 3.26MPa.The mechanical properties of the composites were further improved after adding SiO₂.The mechanical properties of the composites with 7 phr of hydrophobic TSP-L12 were the best,and the tensile strength was 5.01 MPa,which was 3.83 MPa higher than that of TPS.The results of creep behavior experiment show that the composites have no fracture,and the creep behavior experiment can be carried out smoothly.After adding LDPE,the flexibility and elongation of TPS increase,and SiO₂can improve the creep resistance of TPS/LDPE Composites.The results show that with the increase of LDPE content,the melting enthalpy of TPS/LDPE composites increases,and the crystallinity of the composites increases,and the crystallinity of the composites increases,reaching 32.24%at 30 phr.The melting peak,crystallization peak temperature and crystallinity of TPS/LDPE/SiO₂composite were higher than those of hydrophilic SiO₂;The thermal degradation temperature of TPS/LDPE/SiO₂composite with hydrophobic SiO₂was higher than that of the addition of hydrophilic SiO₂.The thermal performance of the composite was further analyzed by Kissinger thermal degradation kinetics.The results showed that the activation energy of thermal degradation of the composite was improved,among which the composite with hydrophilic SiO₂was lower than that of hydrophobic SiO₂,so adding hydrophobic SiO₂can greatly improve the thermal stability of TPS/LDPE/SiO₂composite.The stability of TPS/LDPE Composites and TPS/LDPE/SiO₂composites were characterized by contact angle meter,photoelectric Haze Meter,water absorption analysis and natural degradation test.The results show that the contact angle of the composite increases with the addition of LDPE and SiO₂,and the composite changes from hydrophilic material to hydrophobic material.The water absorption of the composite decreases with the increase of LDPE and SiO₂;Secondly,the probability of forming hydrogen bond with hydroxyl group of starch molecular chain is reduced,the degree of crystallization of starch is improved,the density is improved,and the hydrophilicity is reduced.The more SiO₂content,the lower the transmittance of the composite;The results show that LDPE and SiO₂can improve the natural degradation rate of the composites,and can meet the needs of various stages of crop growth.When the natural degradation time is 21 days,the degradation rate of 5 parts of hydrophilic TSP-H10,5 parts of hydrophobic TSP-L12 and 5 parts of hydrophobic TSP-L15 is 60.22%,62.38%and 68.64%,respectively.The degradation rates of the three kinds of SiO₂with the same content and different content have little change.SEM results showed that with the increase of SiO₂content,the compatibility of the composites became worse,and agglomeration would occur,which made the dispersibility of the composites worse.When the amount of hydrophilic TSP-H10 is 2 phr,the dispersibility of the composite is good and it is easy to be wrapped in the starch matrix.PLM results show that with the increase of LDPE content,the spherulites decrease,and the spherulites size increases with the increase of LDPE content.However,with the increase of SiO₂content,the surface luminosity of the films decreased,and the number of spherulites decreased,which made the uniformity of the films worse and the transparency of the composite films decreased.XRD results show that the addition of LDPE changes the diffraction peak of TPS/LDPE Composites,and the crystal form of TPS/LDPE Composites changes from A-type+V-type to B-type+V-type;The addition of Nano-SiO₂can enhance the diffraction peak of TPS/LDPE/SiO₂composites,but it has no obvious change on the overall crystal structure of TPS/LDPE/SiO₂composites,and the crystal type is still B+V.FTIR analysis showed that c-o-si bond was formed in the composite under the action of SiO2,which inhibited the intermolecular force of starch,reduced the number of hydroxyl groups,and strengthened the molecular force of the composite.The results show that the hydrophobic properties of the composite are better than that of the composite with hydrophilic SiO2 because the amount of hydroxyl groups between starch is less and the intermolecular force is larger.In this paper,combined with the regional advantages of Guangxi,making full use of local resources,using cassava starch,low-density polyethylene and nano silica to prepare degradable film,suitable for all stages of plant growth,to provide a theoretical basis for the application of cassava starch film.