Structural Characteristics And Adsorption-Dispersion Performance Of Series Of Naphthalene Sulfonate Superplasticizers

Naphthalene sulfonate formaldehyde condensate (FDN) is a kind of anionic surfactant which has the hydrophobic group composed of naphthalene chain linked by methylene and hydrophilic group composed of sulfonate group. Since its synthetic technique is simple and dispersion performance is excellent, FDN is used widely for dispersion agent in concrete, coal-water slurry, and dye, etc. FDN is synthesized from naphthalene by four steps:firstly sulfonated by H2SO4, then hydrolysised by H2O, then condensated by HCHO and neutralized by NaOH. Recently, the increase of price of industrial naphthalene extracted from oil and coal makes the cost of FDN increasing. So, it is very important significance and necessary for FDN to improve its performance by physical or chemical modification methods or to reduce costs of raw material.Considering the molecular weight distribution of FDN are extremely nonhomogeneous, FDN was separated into four fractions with the following cut-off molecular weight ranges: less than 2500 (FDN-1),2500-10000 (FDN-2),10000-30000 (FDN-3) and more than 30000 (FDN-4), using a hollow fiber membrane ultrafiltration.The molecular weight and contents of sulfonic group with FDN fractions was investigated by Mass spectrum and element analysis method. The influence of FDN fractions with different molecular weight on the performances of cement paste were investigated. In addition, adsorption behaviors of FDN on the surface of cement particles including Zeta potential, adsorption amount, thickness of adsorption layer was tested. At the same time, the factors affecting the adsorption behavior, for example, molecular weight, Na2SO4, and urea, was investigated by self-assemble method. At last, the ideal structure of FDN and methylnaphthalene sulfonate formaldehyde condensate (MNSF) was simulated by Chembio3D software, and the optimum synthesis conditions for preparing MNSF using the industrial methylnaphthalene as raw materials were determined.Structural characteristics of FDN with different molecular weight show that the contents of sulfonic group are nearly the same; the main differences of FDN fractions are the molecular and their distributions. The effects of FDN fractions on the performance of cement paste show that FDN with higher molecular have better workability. For FDN-4 which cut-off molecular weight is more than 30000, when the mass dosage of FDN was 0.5%, the water-reducing ratio of mortar was up to 16%, increased by 4% than that of original FDN. At the same time, the compressive strength of responded mortar rised by 30% compared with blank sample after 7 days, accompanying with flexural strength increased by 23%. FDN fractions with higher molecular weight have some retarding effect comparing with the lower, and can help the formation of uniform and densely thin rod-shaped ettringite during cement hydration early by SEM.Adsorption isotherm of different FDN fractions on the surface of cement particle shows that adsorption behaviour of FDN with high molecular follows Langmuir model; For FDN-4 which cut-off molecular weight is more than 30000, its saturated adsorption amount is up to about 7.5 mg/g and the thickness of adsorption layer is about 2nm. The adsorption feature for FDN without ultrafiltration displays the S type which means multi-layer layers adsorption; its thickness of adsorption layer is about 9.53nm. However, FDN with low molecular displays adsorption in the hole of cement particle mainly; it don't show the saturated adsorption for FDN-1 which cut-off molecular weight is less than 2500 when the equilibrium concentration of solution is up to 140 mg/L.Aadsorption behavir of FDN on the plate glass shows that the more greater of the molecular weight of FDN, the much of its adsorption on the plate glass if adding Na2SO4 or not in the FDN solutions. Na2SO4 makes its adsorption amount of low molecular weigh FDN decrease by electrostatic shielding effect, but make its adsorption amount of high molecular weigh FDN increase by effecting their conformation in solution. Urea makes FDN adsorption amount decrease by destroying hydrogen bonding.It shows by ChemBio3D software simulation that the structure of FDN is not the regular linear structure like some references inferring. It means there are some polymers existing sheet structure. Also, the structure of MNSF is forecasted in this article, and the temperature of methylnaphthalene sulfonated is lower than that of naphthalene,experiments of synthesizing MNSF with better dispersing ability using industrial methylnaphthalene as raw material shows that the optimum reaction conditions were described as below:the mole ratio of material is n(methylnaphthalene): n(H2SO4): n(H2O of hydrolysis): n(HCHO):n(H2O of condensation)=1:1.25:1.25-1.5:0.92:4.6; sulfonation temperature was 163-165℃; sulfonation time was 3h, hydrolysis temperature was 110-120℃; hydrolysis time was 15-30 min; condensation temperature was 110℃; condensation time was 4 h; the acidity value before and after hydrolysis was controlled near 30%. When the dosage of MNSF was 0.5%, the water-reducing ratio reached to 16% and 4% more than that of FDN, the flexural and compression strength of added MNSF cement mortar approached that of FDN.