Study On The Synthesis, Properties And Adsorption-Dispersing Behavior Of Polycarboxylate Superplasticizers

Used as a super dispersant, superplasticizers are key materials for preparation of stable suspensions. Comb-structure polycarboxylate grafted copolymer containing polyethylene oxide is a new generation and novel type superplasticizer, which becomes the most important admixture for high performance concrete, and is also used for gypsum dispersing applications. Cement and gypsum are both hydrated-hardened materials. Thus, during the process of dispersing of these powders into water, they will be hydrated by water. The suspensions are a complex dispersion system with multi-phases and multi components. Molecular structure characters are the most important factors which determine the dispersing ability and stability of suspensions. It is also necessary to investigate other impact factors such as particle characters, temperature, pH value, concentration of electrolytes, as well as rheological properties to know much better of suspensions characters. This dissertation studied on synthesis, properties and characterization of polycarboxylate superplasticizer, investigated the interaction between copolymer and powders, and the correlations between molecular structures and properties, and discussed adsorption behavior of copolymers on solid-liquid interface, arid examined stability and rheology of suspensions.Polyester type comb-structure polycarboxylate grafted copolymers with good dispersing ability and retention have been synthesized via molecular design means, and by using MAA unsaturated acid and MPEGMA macromonomers. The correlations between molecular structure characters and dispersing properties, set time of concrete, strength and cement hydration heat were investigated. Copolymer structures were characterized by IR, HNMR, GPC and other means.Polyether type comb-structure polycarboxylate grafted copolymers with good dispersing ability and retention have been synthesized by using unsaturated acid monomers and APEG macromonomers. Polymerization rate of APEG was examined. The correlations between molecular structure characters and properties were also investigated.Adsorption behavior of copolymers on cement particle surface was studied. Adsorption of copolymer on cement particle is an exthothermal reaction. Adsorption amount decreases with increasing of temperature. Adsorption heat is calculated to be 17.4 kJ/mol, by Clausius-Clapeyron equation. IR analysis indicates that surface complexes and chemical adsorption are formed by carboxylate functional groups-COO- of copolymers with Ca2+ on particle surface. Kinetics and thermodynamics of adsorption behavior are greatly depending on molecular structures. It is also indicated that adsorption configurations of copolymers on cement particle are wrap, loop or horizontal. Adsorption configurations depend on molecular structure and suspension colloidal conditions.Polycarboxylate copolymers also have good dispersing ability to ultra-fine powders. Cement paste suspension with polycarboxylate copolymers is accordance to Bingham rheological model. But it also shows time-depending properties. The modification of rheology of cement paste by ultra-fine powders and polycarboxylates are also investigated.Studies on gypsum suspensions show that dispersing ability depends on molecular structure of polycarboxylate copolymers. The dispersion mechanism of polycarboxylate copolymers to gypsum mainly attributes to electrostatic repulsion than steric hindrance. Surface complexes are formed by carboxylate functional groups-COO- of copolymers with Ca2+ on particle surface. The binding strength of different copolymers on gypsum particle surface has following sequence:maleic copolymer> acrylic copolymer> methacrylic copolymer. Adsorption of polycarboxylate copolymer on gypsum particle is also an exthothermal reaction. Adsorption heat is calculated to be 7.7 kJ/mol, by Clausius-Clapeyron equation. Adsorption isothermal of polycarboxylate copolymer on gypsum belongs to Langmuir type. Polycarboxylate copolymer can significantly improve microstructure and pore structures of hydrated gypsum. Polycarboxylate copolymer will decrease porosity but increase pore size and distributions. The decrease of porosity attributes to high solid of gypsum paste caused by high efficiency of water reduction of polycarboxylate copolymer, while increase of pore size and distributions attributes to air entrainment of copolymer as a kind of surfactant.