| Four macromolecules (PC-1, PC-2, PC-3, PC-4) with different hydrophilic groups were synthesized respectively employing allyl polyoxyethylene polyoxypropylene ether (APEP), methacrylic sulfonate (SMS), N-hydroxymethyl acrylamide (N-MAM), methyl acrylate (MA), acrylic acid (AA) as monomers. Aiming to overcome the disadvantage of low strength and easy disrepair of gypsum mold, the effect of addition of the macromolecules above, polyethylene glycol, polyvinyl alcohol, polyacrylamide, vinyl chloride-vinyl acetate copolymer on the properties of a or β-calcium sulfate hemihydrates (α-H or β-H) was studied in this thesis. Subsequently, the structure-activity relationships were also studied. Finally, the mechanism of macromolecules during the hydration of β-H was preliminarily investigated. The conclusions were summarized as follows:1. Under the precondition that gypsum can be casted, with the increase of ratio of water to gypsum, the mechanical strength of α-H or β-H decreased, while the corresponding water absorption increased. It is difficult to have a common workable range of water-hemihydrate weight ratio for α-H, β-H. The suitable water-hemihydrate weight ratio is benefit to produce gypsum mold. the best range of water-hemihydrate weight ratio for β-H was0.675-0.725. The optimal water-hemihydrate weight ratio for a-H was0.55. The best mass mixing proportion of α-H to β-H was obtained (α-H:β-H=6:4) by studying the performance of mixing α-H,β-H with different proportions under different ratio of water to gypsum. With the increases of mixing ratio of a-H at low water-hemihydrate weight ratio, the water absorption increased, Except the situation that the water-hemihydrate weight ratio was0.7, water absorption changed slightly.2. The double bond retention rate was employed to characterize the progress of polymerization reaction. Accordingly, the optimal reaction conditions were screened as follows:ammonium persulfate was served as initiator, reaction temperature was at80℃, reaction time was8h and the aging time was1h as the temperature increased to90℃. Moreover, based on single factor experiment of monomers composition, the orthogonal test was designed, and finally the optimal formula was obtained as nAA:nMA:nSMS: nAPEP=0:10:15:8, WAPS=0.6%.3. Based on orthogonal test, four kinds of sulfonate macromolecules (PC-1, PC-2, PC-3PC-4) with different structures were synthesized by adjusting the ratio of main group in molecular structure. The effect of macromolecules on the performance of gypsum was investigated. The results show that SO32-groups can be used as a coagulant, which has a competitive adsorption with-COO-groups, and the macromolecules (PC-1, PC-2, PC-3) with-COO-group in molecular structure have a certain retarding effect, reducing the mechanical strength of gypsum. However, PC-4without-COO-group can increase the strength of gypsum mold under the condition that the water absorption of gypsum mold was constant. Compared with the blank sample, the bending and compressive strength of gypsum mold are both improved by9%, and water absorption is decreased by0.5%at the optimumal dosage (0.1%). 4. The effect of macromolecules with different structure on the performance of-H or β-H was investigated. The result shows that the strong hydrophilic macromolecules (polyethylene glycol, polyvinyl alcohol) not only improve the mechanical strength of gypsum mold, but also maintain or enhance the water absorption of gypsum mold. However, the weak hydrophilic macromolecules (polyacrylamde, vinyl chloride-vinyl acetate copolymer) reduce the strength of gypsum as they maintain or enhance the water absorption of gypsum mold. It can be explained that there is a relationship between the solubility of macromolecules and the viscosity of macromolecules solution.5. The combination of PC-4and PC-3did not have a synergistic effect. The increase of complex ratio of PC-3resulted in the gradually reduced strength; The combination of PC-4and PEG is better than thesinple PC-4or PEG, the optimum complex ratio is1:1. Moreover, simple PVA is better than the compound of PC-4and PVA, which indicate that PC-4undermined the role of PVA in a certain extent.6. Macromolecules (PC-3) can alleviate the hydration of β-H, however, the macromolecules (PC-4, PEG, PVA) have little effect on the hydration process. Compared with the blank sample, the macromolecules (PC-4, PEG, PVA) improved the mechanical strength of the gypsum mold with content in the range of less than0.2%to some extent. The flexural strength and compressive strength of gypsum mold reduced by17%and35%respectively with high dosages (0.3%) of PC-3. Scanning electron microscopy showed that the crystal morphology of gypsum with PC-3is regular columnar at low dosage (0.02%), while it becomes irregular flaky granular at high dosages (0.2%), and the crystal morphology of gypsum with PEG or PC-4is also regular columnar, which agrees well with the results of strength experiments. In addition, experimental results of XRD show that PC-3delay the hydration of β-H. |
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