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The synthesis principle and method of polycarboxylate superplasticizer

Polycarboxylic organic material has attracted widespread attention, it is mainly used for concrete water-reducing agent, detergent additives, coating and ink pigment dispersant etc.. The surfactant has excellent washing, permeability, dispersion, emulsion, demulsification performance, especially has the advantages of good washing effect, low temperature resistance to hard water, biodegradability, good coordination capability. Therefore, a wide range of applications, the polycarboxylic acid polymer used as concrete superplasticizer has not long history, Japan is the first research and development and use of the country. In recent years, poly carboxylic acid Jianshuiji is widely accepted in the concrete industry, and by the study of admixtures concrete at home and abroad pay more and more attention and users. Investigate its reason, and the agent of naphthalene sulfonic acid and sulfonated melamine condensation compared objects by tradition, they can give concrete with high fluidity and high dispersion, the dispersion stability of the system to prevent the loss of slump in low dosage. At the same time, prices of industrial naphthalene rose, naphthalene water-reducing agent be imperative problems in application of long production cycle, environmental pollution is increasingly outstanding also makes the polycarboxylate superplasticizer. At present, the main component of Japanese common high air-entraining water-reducing agent is in transition from naphthalene sulfonate reactive polymer to polycarboxylates, Europe and the United States also followed.
The polycarboxylate superplasticizer research especially for the superplasticizer preparation principle, mechanism of action, development prospects and other aspects of reports. A review of research progress of preparation principle, mechanism of action, development prospects and other aspects to be on the reduction of the water.

1.preparation principle

Polycarboxylate superplasticizer is high molecular compound with sulfonic acid groups, carboxyl, amino and containing polyoxyethylene side chain, in water solution, by free radical copolymerization principle synthesis with comb polymer surfactant type structure.
Synthesis of polycarboxylate high performance water reducing main raw materials needed: methacrylic acid, acrylic acid, ethyl acrylate, 2-hydroxyethyl acrylate, sodium allyl sulfonate, methyl methacrylate, 2- acrylamido -2- methyl acrylate, methoxy polyoxyethylene methyl acrylate, ethoxy polyethylene glycol acrylate, allyl ether, in polymerization process can be used as initiator, benzoyl peroxide: agent, azo two isobutyl cyanide caused sulfate; chain transfer agent: 3- mercapto propionic acid, mercapto acetic acid, mercapto ethanol and isopropanol.
The synthetic method is: with a stirrer, thermometer, dropping device, and the reflux condenser tube round bottom flask, through the method of water bath heating slowly dripping polymerization of the monomer solution and initiator solution, in the selection of polymerization of the monomer, full consideration should be given to the reactivity ratio of big science. The reaction temperature can be based on the specific reaction of monomer type, generally can choose 70~95 ℃ this temperature range temperature as the reaction temperature. In one hour after dropping the monomer solution, and then in the 20min drops of initiator solution plus residual, finally the temperature 5 ℃, reaction continues 1H, cooled to 40 ℃, and discharging.


Polycarboxylate superplasticizer is a new type of water reducing agent, has many advantages, but its mechanism is not fully understood, the following are some points:
(1) polycarboxylic acid polymer on cement obvious retarding effect, mainly due to the carboxyl acts as retarder components, R-COO and Ca2+ ions to form a complex, reduce the Ca2+ ion concentration in solution, delay Ca (OH) 2 crystals to form, reduce the formation of C-H-S gel, retard the hydration of cement.
(2) the carboxyl (-COOH), hydroxyl (-OH), amine (-NH2), polyoxyethylene alkyl (-O-R) n and the group of polar water affinity mainly through adsorption, dispersion, wetting, lubrication, surface activity, dispersion and fluidity of cement particles, and by reducing the frictional resistance of cement particles, reduce the cement particles and water interfacial free energy to increase the workability of fresh concrete. At the same time, poly carboxylic acids adsorbed on the surface of the cement particles, carboxylate ions to cement particles with negative charge, so that the electrostatic repulsion and dispersion of cement particles between the cement particles, leading to inhibition of aggregation tendency of cement paste (DLVO theory), increasing the contact area of cement particles and water, make cement sufficient water of. In the process of diffusion of cement particles, releasing condensation free water lock around, improve the workability, reduce the mixing water.
(3) polycarboxylate molecular chain of the steric hindrance effect (i.e., steric repulsion). Poly carboxylic acid molecular adsorption was "comb" on cement particle surface, adsorption layer formed on the surface of gel material, polymer molecular adsorption layer near the intersection between the polymer chains, the physical space hindrance, preventing condensation of cement particles, which is a carboxylic acid water reducing agent has an important reason than dispersion other system stronger.
(4) polycarboxylate superplasticizer dispersive mechanism can keep from cement slurry mixing the relationship between time and Zeta potential to understand. In general, the use of naphthalene and melamine superplasticizer concrete after 60min slump loss was significantly higher than that of polymer concrete superplasticizer carboxylic acid. This is mainly the cement particle adsorption model with different forces between particles, cement polymer adsorption layer is a three-dimensional electrostatic repulsion, Zeta potential of small changes.
The study found that the mechanism of dispersion of cement, DLVO only a theoretical explanation for the ionic repulsion between the often differ considerably from the experimental results. Uchikawa and Tanaka et al. Experimental results show that, the steric effect can successfully explain the polycarboxylic type superplasticizer on the mechanism of the dispersion of cement, namely polymer adsorption on cement particle surface, its extension into the solution of branched generated steric particles can't each other near, so that the dispersion of cement particles and stability. At present, the mechanism of widely accepted. Kiho****a et al, in the study of similar molecular weight branched chain length for different polymer on cement isothermal adsorption pointed out, with long branched polymers have low potential and high steric repulsive, so after adsorption on cement dispersing performance is very good, but the particle dispersion stability is not good. The author thinks, branched chain is too long may lead to mutual winding has dispersed surface branched between particles, but cause of particle coagulation.


3.The advantages and disadvantages of Polycarboxylic Water
3.1 of polycarboxylate superplasticizer advantages
With naphthalene, aliphatic, sulfonated melamine superplasticizer compared advantages such as, polycarboxylate superplasticizer mainly has following several points:
(1) slump retention is good, 90min slump without loss or loss is smaller;
(2) in the same liquidity situation, on the setting time of cement smaller effect, can well solve the problems of water, air, retarding, bleeding;
(3) polycarboxylate superplasticizer can adjust the molecular structure, high water reducing agent, preparation with special performance and use such as: low temperature and high early strength type, zero slump loss, shrinkage resistance etc..
(4) the use of poly carboxylic acid water reducing agent, the more slag or fly ash replacing cement, thereby reducing the costs;
(5) the synthesis of the molecular chain wide raw material sources, monomer are: acrylic, methyl acrylic acid, maleic acid, (methyl) ethyl acrylate, 2-hydroxyethyl acrylate (methyl), sodium allyl sulfonate, methyl methacrylate;
(6) on the molecular structure parameters of freedom, additive manufacturing technology can be controlled, high performance potential;
(7) polymerization approach diversification, such as copolymerization, grafting, block etc.. The synthesis process is relatively simple, because it does not use formaldehyde, naphthalene and other harmful substances, will not cause pollution to the environment.

3.2 of polycarboxylate superplasticizer.
Polycarboxylate superplasticizer has some disadvantages in the use of the process, mainly in the following points:
(1) stability is poor product performance. To some extent, this defect is due to too much, China's cement admixture complex, preparation technology immaturity, poly carboxylic acid.
(2) in the complex process of elimination, air-entraining agent, strong selective agent. By trial experiment and experience can be found, different manufacturers, different brands of polycarboxylate superplasticizer must choose to appropriate air-entraining agent and defoaming agent through a lot of experiments. This phenomenon is mainly due to the synthesis of polycarboxylate superplasticizer, the selective polymerization of monomers, different manufacturers may monomer type and synthesis process of polymerization using different, which makes the final synthesis of poly carboxylic acid water reducing aspects in molecular weight, molecular weight distribution and chain structure will have greater differences, so the air will itself is very different.
(3) in the configuration of high strength and high performance concrete, self-compacting concrete process, there is a concrete viscosity too much, too high pump pressure problems. This is because the current domestic market more than 95% of the polycarboxylic acid salt products, polycarboxylic acid belongs to the first generation of methyl acrylic superplasticizer, defect of the structure is the basic reason of viscous too big it appears in the preparation of high strength concrete.