Synthesis Of Polycarboxylate And Its Application In Mold Gypsum

Mold gypsum was studied in this paper for overcoming its low intensity, short life andother defects. A targeted polycarboxylate with functional groups including carboxyl group,ester group, sulfonic group and side chain of polyoxyethylene polyoxypropylene ether wasdesigned and synthesized from4monomers acrylic acid (AA), methyl acrylate (MA),methacrylic sulfonate (SMAS) and allyl polyoxyethylene polyoxypropylene ether (APPE).The effects of synthesized polycarboxylate upon mold gypsum were studied by investigatingproperties such as bending strength, compressive strength and water absorption of the system.And the adsorption characteristic of polycarboxylate on the surface of mold gypsum was alsostudied. Main results were summarized as follows:1. It was found that AA and MA were easily self-polymerized while SMAS and APPE werethe opposite by investigating self-polymerization reaction and copolymerization reactionof the monomers. What’s more, the formers would promote the polymerization of thelatters. The effects of dosage of initiator, reaction temperature and reaction time onsynthesis of polycarboxylate were investigated by single factor experiments. Theoptimum process conditions were as follows: the feed moding was dropping feed for AA-MA mixture and the initiator, the temperature was80oC and the time was8h, theinitiator was ammonium persulfate. In addition, the structure and molecular weightdistribution of the products were respectively characterized by infrared spectroscopy(IR)and gel permeation chromatography(GPC). Surface tension and thermal stability ofpolycarboxylate were determined by du nouy ring method and thermal gravimetricanalysis (TGA).2. The formula for preparing polycarboxylate was preliminary determined by single factorexperiments with detecting the bending strength, compressive strength and waterabsorption of mold gypsum. On this basis, the optimal formula was obtained viaorthogonal test as nAA:nMA:nSMAS:nAPPE=6:6:3:5, WAPS=0.5%. Polycarboxylatesynthesized following this optimal formula showed good performance. Under thecondition of0.55water/gypsum and0.2%(w/w) in dosage, bending strength of gypsumspecimen reached7.14Mpa, representing a24.17%increase compared to blank gypsum,compressive strength reached20.5Mpa, representing a13.70%increase compared toblank gypsum, and water absorption reached22.76%, representing a0.50%fall comparedto blank gypsum.3. It is shown that the concentration of Na2SO4and pH of mixing water affect the propertyof polycarboxylate. The strength and water absorption of mold gypsum would decrease inthe presence of Na2SO4, and show a clear downward trend with increasing concentrationof Na2SO4in mixing water. Meanwhile, pH of mixing water was closer to neutral, theproperty of mold gypsum was better. As the standard consistency of blank gypsum is0.55the polycarboxylate can decrease the standard consistency of mold gypsum to0.45.Furthermore, the setting time of mold gypsum with polycarboxylate of0.25%(w/w) wastwice longer than that of the blank one. 4. Polycarboxylate can improve the strength of mold gypsum but can’t increase the waterabsorption. To solve this problem,6kinds of natural macromolecules with good waterabsorption such as tannin extract, chitosan, guar gum, sodium carboxymethyl cellulose,sodium alginate and gum arabic were applied to add to polycarboxylate. Adding tanninextract increased the rate of water absorption of mold gypsum of2.9%. Adding chitosanled to the improvement of water absorption and compressive strength of mold gypsum of0.5%and5.8%, respectively. However, the bending strength changed faintly. Addinggum arabic also contributed to better water absorption. Other3kinds of naturalmacromolecules can’t be benefit to the performance of mold gypsum. Duing to the poorcompatibility of polycarboxylate and natural or synthetic macromolecules, theircombination made the properties of mold gypsum worse.5. The methods of the adsorption ability of polycarboxylate were screened. it is found thatboth the weighing and UV photometric measurement resulted in positive error.Meanwhile, polycarboxylate has solubilization effect on calcium sulfate. Thus, it is notsuitable to detect the adsorption ability of polycarboxylate by the weighing and UVphotometric measurement. All-carbon analysis was finally used to determine theadsorption of polycarboxylate on the surface of mold gypsum particles. The adsorptionrates of polycarboxylate were affected by intra-particle diffusion rate and membranediffusion rate. Furthermore, the adsorption rate was in good agreement with Lagergrensimulated second order kinetic model of which Eawas21.15kJ/mol, frequency factor k0was137.28min-1. Adsorption thermodynamics was also studied, showing that adsorptionlaw of polycarboxylate macromolecules agrees well with Langmuir adsorption isothermequation. In addition, standard Gibbs free energy Δ GΘad, adsorption capacity Qemandadsorption enthalpy Δ H were calculated in the adsorption process.