| The preparation of dual-modified maize starch was studied by two kinds of reaction sequences respectively, etherificated with 1,2-epoxybutane and crosslinked with epichlorohydrin. The physicochemical properties, the granule structure, as well as the application of the dual-modified maize starch in the mortar were researched systematically. In the following, detailed processes and results of the study were presented.The preparation of hydroxybutyl starch by water-phase method was studied by single factor tests and orthogonal tests. From the perspective of increasing hydroxybutyl molar substitution, the optimal conditions for preparing hydroxybutyl starch were obtained by orthogonal tests as follows: 1,2-epoxybutane was 12% based on the mass of starch, reaction temperature was 50°C, the reaction time was 20 h, starch concentration was 40%, sodium sulfate was 11% mass of starch, sodium hydroxide was 1% mass of starch. Under these conditions, the molar substitution of hydroxybutyl starch was 0.0923.The preparation of hydroxybutyl starch with high degree of substitution by alcohol-phase method was studied by single factor tests and orthogonal tests. From the perspective of increasing hydroxybutyl molar substitution, the optimal conditions for preparing hydroxybutyl starch were obtained by orthogonal tests as follows: 1,2-epoxybutane was 150% based on the mass of starch, reaction temperaturewas 70°C, the reaction time was 12 h, ethanol was 250% mass of starch, sodium hydroxide was 3% mass of starch. Under these conditions, the molar substitution of hydroxybutyl starch was 0.4156. The droxybutyl with different molar substitution were investigated respectively. Hydroxybutyl starch showed new absorption peaks at 1370 cm-1, which indicated that hydroxybutyl groups were attached to the native starch. A type X-ray spectra was assigned to modified starch, which confirmed the reaction mainly occurred in the amorphous region. The surface transformation changes of starch granules before and after the hydroxybutylation were observed. Brabender showed that pasting temperature and set back values of the modified starches decreased significantly, whereas peak viscosity increased in comparison with the native starch, with the increase of the degree of substitution, the changes were more significant. When the molar substitution degree reached a certain extent, the hydroxybutyl starch was soluble in the cold water. Also with the increase of the degree of substitution, the transparency and freeze-thaw stability were substantially improved.Using the hydroxybutyl starch of highest degree of substitution as raw materials, the cross-linked process of hydroxybutyl starch was studied. From the perspective of increasing degree of cross-linking, the optimal conditions for preparing hydroxybutyl cross-linked were obtained by orthogonal tests as follows: reaction temperature was 60 °C, reaction time was 4.5 h, p H value was 11, epichlorohydrin was 0.06% the mass of starch, the degree of cross-linking was 1.92 m L. The relative crystallinity decreased weakly after cross-linked, however, The crystalline type was not changed. SEM indicated that the reaction only occurred in the amorphous region. Physical and chemical characteristics of hydroxybutyl cross-linked starch were discussed, hot and cold stability increased significantly. hydroxybutyl cross-linked starch had good properties of viscidity, resistance to freezing, rheology, salt tolerance and acid resistance.The effect of replacement ratio of hydroxybutyl cross-linked starch(HBCLS) on the property of fresh mortar was studied,the result showed that HBCLS could increase the viscosity of cement paste significantly under certain dosage although its viscosity was lower. Moreover, as mortar additives, HBCLS was not as effective as hydroxypropyl methylcellulose(HPMC) to improve the water retention, but can significantly increase the viscosity fluidity, apparent density, retarding and adhesion strength of mortar, and the function was better than hydroxypropyl starch(HPS). |
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