Synthesis Of Cyano/ester-modified Polycarboxylic Acid Superplasticizers

Polycarboxylic acid superplasticizer(PCA) is the most important concrete admixture in contemporary society. The modification of PCA has been a hot topic in the industry. In this work, according to molecular structure design, PCA had been modified by adding the functional monomer which contains cyano or ester groups to enhance its water reducing ratio and slump retention based on the binary copolymerization. The effects of modified functional groups on the properties and mechanisms of PCA had been investigated.In this paper, using acrylic acid(AA) and isoamyl alcohol polyoxyethylene ether(TPEG) as raw materials, the optimized basic PCA(PCA1) was prepared by single factor experiment in an oxidation–reduction initiator system. Based on the preparation conditions of PCA1, cyano-modified PCA(PCA2) was synthesized through AA replacing by acrylonitrile(AN) partly. Then, the structure and residual monomers of PCA2 were measured. The optimum polymerization conditions for the preparation of PCA2 are listed as follows: the reaction temperature is 45℃, n(AA + AN) : n(TPEG) = 4:1, n(oxidant) : n(reductant) = 4:1, mole ratios of AA replaced by AN is 7.0%, the dosages of reductant and mercaptoacetic acid are 0.2 wt% and 0.4wt% of total monomers, respectively, the dropping time of AA solution is 1 hour, the dropping time of reductant and mercaptoacetic acid mixed solution are 1.5 hours, and holding time are 2 hours. For PCA2, the weight-average molecular weight(Mw) is 4.46×104, Mw/Mn is 1.08, the residual rate of TPEG is less than 2%, and there is no residual AN.Based on the optimum polymerization conditions of PCA1, hydroxyethyl acrylate(HA) and dimethyl maleate(DMM) were used as ester-containing monomers. The most optimized reaction conditions of two kinds of ester-modified PCAs(PCA3 and PCA4) were found through the orthogonal experiment. The structures and residual situations of the monomers of PCA3 and PCA4 were measured. The optimum polymerization conditions for the preparation of PCA3 are listed as follows: n(HA) : n(TPEG) = 1:2,the dosage of reductant and mercaptoacetic acid are 0.1 wt% and 0.4wt% of total monomers, respectively. The optimum polymerization conditions for the preparation of PCA4 are listed as follows: n(DMM) : n(TPEG) = 1:1, the dosage of reductant and mercaptoacetic acid are both 0.4 wt% of total monomers. The Mw of PCA3 and PCA4 are 6.90×104 and 4.23×104, respectively, and the molecular weight distribution is relatively concentrated. There is no residued of HA or DMM, and residual rate of TPEG is less than 2%.According to the national standards and industry standards, the properties of the four synthesized PCAs were tested. The cement paste test results show that the three modified-PCAs have better adaptability to different cements. Meanwhile, compared to PCA1, PCA2 has advantages to improve the initial dispersion, while PCA3 and PCA4 are beneficial to enhancing the dispersion retention ability. The concrete test results show that when the dosage of admixture is 0.12%, the water reducing ratios of PCA1、PCA2、PCA3 and PCA4 are 27.78%, 32.39%, 33.76% and 34.80%, and the slump loss after 1h are-25 mm,-15 mm,-5mm and +5mm respectively. The performance tests and the pilot study of dispersing mechanism indicate that the better dispersion of the cement paste, the closer to the Newtonian fluid. Under the same dosage of admixture, the order of zeta potential absolute value is as follows: PCA1>PCA2>PCA3、PCA4, which indicate that electrostatic repulsion is not the main dispersing mechanism of the PCA. The hydrolysis of cyano-group and ester-group has significantly affects on initial dispersion and dispersion retention ability.The system research about the synthesis of cyano/ester-modified PCA in this article enriches the domestic synthesis and modification technology on high efficiency polycarboxylate superplasticizer.