Preparation And Mechanism Of Oxidized Starch In A Screw Extruder Under Aikali-free Conditions

In the present study, according to mechanochemistry theory, the high temperature, strongshear stress of screw extruder damaged the film, ribbon and wall of sub-granule, and madestarch crystals be dislocated. Thus reaction reagents were mixed homogeneously with starchmolecules, meanwhile chemical activity sites on starch molecules increased and shifted insidestarch granules. This work had broken some technical problems for wide application of drymethod for preparing modified starch, such as inhomogeneous mixing of starch and reactionreagents, poor sulubility of oxidized starch and so on. So preparation of oxidized starch byreactive extrusion under alkali-free conditions could be described as a new dry technology.This study also used some latest research tools to study mechanism of preparing oxidizedstarch in a screw extruder under alkali-free conditions, such as DSC, SEM, XRD, NMR,ATR-FTIR and so on. The main research contents were as follows:(1) The effects of extrusive condition on the carboxyl content (CC) and water solubilityindex (WSI) of oxidized corn starch were found out clearly. High temperature or low watercontent reduced mechanical force on starch. Increasing screw speed increased the extent ofstarch shear-induced degradation and promoted the uniformity of mixing starch with oxidantand water. However, increasing screw speed would reduce mean residence time in theextruder, the carboxyl content of oxidized starch was not promoted with the increase of screwspeed. NaClO and H2O2in the extruder had different state and boiling points, resulting indifferent optimal reaction conditions for preparing two kinds of oxidized starch.(2) Due to hydrophilicity and charge repulsion of oxidized starch, its solubility andthermal stability rose up, and its retrogradation and intrinsic viscosity decreased.(3) Oxidized starch prepared by this method could be used as materials for preparing high-strength degradable films, because it’s molecular chain stretched. Elongation at break of filmsprepared by this oxidized starch was poor (13.61%), because its molecular weight was small.(4) The mechanism of preparing oxidized starch in a screw extruder under alkali-freeconditions were found out. The high temperature, strong shear stress of screw extruder madechemical activity sites increase and shift inside starch granules, which improved distributionuniformity of functional groups in starch granules. And screw extruder also broke a lot of C- O-C, which made starch molecules generate free radical, so reaction efficiency was improved.Moreover, the strong shear stress of screw extruder could activate starch by changing D-glucopyranosyl ring configuration.(5) Different oxidants had different forms of oxidizing starch. Oxidation of ClO2is thestrongest, but oxidized starch prepared using it has the most carbonyl content, the leastcarboxyl content and poor physical and chemical properties. H2O2made starch degradeseverely. Oxidized starch prepared using H2O2had good physical and chemical properties, butpoor color. NaClO in the extrude existed in the form of small particles, and high temperatureand strong shearing did not destroy NaClO particles. Oxidized starch prepared using NaClOhas the most carboxyl content. Catalyst made oxidized starch had more carbonyl and carboxylgroups, and degraded starch molecules heavily. Meanwhile catalyst accelerated thedecomposition of hydrogen peroxide.