Preparation Of Modifiers And Their Effects On The Properties Of Sodium Silicate Sand

Sodium silicate bonding in foundry sands can reduce industrial pollution and achieve green casting,which is in line with current concept of green development in China.However,there are some problems in the process of using sodium silicate sand,such as large amounts of added sodium silicate and poor collapsibility of molds and cores,so it is very significant to improve the comprehensive mechanical properties of sodium silicate sand by preparing the modifiers.In this paper,we adopted the method of CO₂ curing sodium silicate sand in the casting process.Three kinds of inorganic powders,Al₂O₃,ZrO₂ and Si₃N₄,were selected as the main components of the modified powders.And their influences on the properties of the sodium silicate sand were investigated by the comparative experiments,to determine the composition ratio,the amount of additives and inorganic powder particle size.In addition,methyl hexadecanoate,methyl stearate,dioctyl phthalate,erucic acid amide,eicosanol and cyclohexanone were selected as the main organic modifier,and their effects on the properties of sodium silicate sand was studied via two different orthogonal experiments.The modifiers were characterized by XRD,SEM and EDS and their principles of modification will be proposed.When the content of sodium silicate was 6wt.% of sand,the content of inorganic modifier was 1.5wt.% of sand and the composition ratio of Al₂O₃,ZrO₂ and Si₃N₄ is1.5: 1: 0.67,and their particles size is 1⁵0μm,it has a better modification effect.The results show that the initial strength,retained strength at 24 hours and the residual strength after 800℃ of the samples were 1.26 MPa,2.64 MPa and 6.67 MPa,respectively,which was 174% and 38% higher than that of the samples with only6 wt.% sodium silicate,but decreased by 42% to the residual strength after 800℃.While adding modifier of Al₂O₃,ZrO₂ and Si₃N₄,the addition of methyl hexadecanoate,methyl stearate and dioctyl phthalic（6wt.%,2wt.%,4wt.% of sodium silicate,respectively）,made the proportion of sodium silicate decreased to 3wt.% of sand,and greatly improving the performance of sand.The initial strength,retainedstrength at 24 hours and the residual strength after 800℃ of the prepared sodium silicate sand samples were 0.65 MPa,2.77 MPa and 2.83 MPa,respectively,and the gas evolution of the samples was 13.7ml/g.With the optimized powder modifier of Al₂O₃,Zr O2 and Si₃N₄ with sodium silicate（3wt.% of sand）,organic ester,erucic acid amide and cyclohexanone（10wt.%,1wt.% and 2wt.% of sodium silicate）,the initial strength,retained strength at 24 hours and the residual strength after 800℃ of the samples were 0.72 MPa,2.79 MPa and1.62 MPa,respectively,and the gas evolution of samples was 11.7 ml/g.The addition of the optimized modifier can ensure the decrease of the residual strength of the sodium silicate sand molds with great initial strength and retained strength at 24 hours and improve the collapsibility of samples with the low amount of gas evolution to meet the requirement of production.The results of XRD,SEM and EDS showed that owing to the dispersing modifier in the sodium silicate binder,the initial strength and retained strength at 24 hours of the sand mold are improved.In the process of heating at 800℃ for 30 min then cooling down in furnace,inorganic powders due to the small coefficient of linear expansion would play a role in the formation of cracks in the sand surface and the bonded bridge.Organics fully volatile,which resulted a large number of pores generated in the bonded bridge of sand and a significant reduction of the residual strength of sand mold.The composition of the sodium silicate sand modifier developed in this paper are as follows: the composition ratio of Al₂O₃,ZrO₂ and Si₃N₄ is 1.5: 1: 0.67（1.5wt.% of the sand）,and proportion of the organic methyl hexadecanoate,methyl stearate,dioctyl phthalate,erucic acid amide,cyclohexanone were 5wt.%,1.67 wt.%,3.33 wt.%,1wt.% and 2wt.% of sodium silicate,respectively.