Study On The Retrogradation Behavior Of Thermoplastic Cassava Starch Based On Sisal Fiber

Sisal fiber is similar to starch in structure,both of they are renewable natural polymers with biodegradability.Sisal fiber has light weight,large aspect ratio,high strength ratio and modulus ratio,and has obvious advantages in enhancing the mechanical properties of starch.The retrogradation of starch will affect the performance and structure of starch.In this paper,cassava starch is used as the raw material,glycerin is the plasticizer,and sisal fiber is the reinforcement.After the sisal fiber was treated with alkali?Na OH?and silane coupling agent?KH550?,the thermoplastic cassava starch?TPS?/sisal fiber?SF?composite material was prepared using a torque rheometer.The thermal behavior of the composite material retrogradation process was studied by the Differential Scanning Calorimeter?DSC?,the Thermal Gravimetric Analyzer?TG?,the Thermal Gravimetric?Mass Spectrometer analyzer?TG-MS?.The retrogradation structure of the composite material was studied by the Fourier Transform Infrared Spectrometer?FTIR?,the Polarized Light Microscope?PLM?,the Scanning Electron Microscope?SEM?,the X-ray Powder diffractometer?XRD?,the Nuclear Magnetic Resonance?NMR?.The performance of the composite material after retrogradation was studied by the Universal testing machine,the photoelectric haze meter,the Contact angle measuring instrument.The retrogradation behavior of thermoplastic cassava starch/sisal fiber composites under constant strain was studied by the Differential Scanning Calorimetry?DSC?,the Polarized Light Microscope?PLM?,the Contact angle measuring instrument,the Dynamic thermomechanical analysis?DMA?.The results are as follows:When the amount of SF is increased,the plasticizing torque of TPS/SF composite is increased and the plasticizing time is shortened;when the amount of SF is 5 phr and 10 phr,Na OH treated SF is more conducive to starch plasticization than KH550 machining.As the retrogradation time increases,the melting enthalpy of the TPS/SF composite increases and the degree of retrogradation increases;The amount of SF increases,the melting enthalpy and the degree of retrogradation of the composite also increase;After the treatment of SF with KH550,the degree of retrogradation of TPS/SF composite is larger;the retrogradation degree?DR?of TPS and TPS/SF composites increases with the increase of retrogradation time,the retrogradation index n is 2.53 and 2.49,and the crystal growth mode is two-dimensional nucleation growth.With the increase of the retrogradation time,at first the thermal stability of TPS/SF composites increases and then decreases.The addition of SF enhances the thermal stability of TPS.After Na OH treatment of SF,the thermal stability of the composites is better.After adding SF,the content of OH⁺,H₂O,HC=CH⁺and CO₂produced by thermal degradation of TPS/SF composite is reduced.The hydrogen bonding effect of the TPS/SF composite material increases with the retrogradation time and the amount of SF.The treatment of SF with KH550 will enhance the hydrogen bonding effect of the composite material.The increase in the retrogradation time and the increase in the amount of SF can increase the number of spherulites in TPS/SF composites,and more spherulites in TPS/SF composites with KH550 treated SF.The more the amount of SF is,the easier it is to agglomerate.Under the same amount,the SF is more evenly dispersed in the starch matrix after being treated with KH550.With the increase of the amount of SF in the TPS/SF composite material,the crystal structure changes.The tensile strength of TPS/SF composite material increases with the increase of SF content.After the SF is treated with KH550,the tensile strength of the composite material is improved.With the increase of the regeneration time,the light transmittance of the TPS/SF composite material decreases.The more the amount of SF,the lower the light transmittance of the composite material;after KH550 treatment of SF,the light transmittance of the TPS/SF composite material is slightly Has improved.With the increase of regeneration time,the contact angle of TPS/SF composites decreased first and then increased.The addition of SF increased the hydrophilicity of TPS/SF composites;the treatment of SF with KH550 increased the hydrophobicity of TPS/SF composites.With the increase of sisal fibers,the tensile strength of TPS/SF composites increases and the strain decreases;after SF treatment with KH550,the tensile strength of TPS/SF composites is higher and should be changed less.After applying constant strain,the melting enthalpy of TPS and TPS/SF composites increases,and the degree of regeneration increases.After applying constant strain,the TPS and TPS/SF composites have increased hydrophilicity.Adding SF can increase the glass transition temperature of TPS/SF composites;after applying constant strain between TPS and TPS/SF,the cassava starch Tg in the material disappears,and the thermoplastic cassava starch Tg decreases.