Organic and inorganic compounds have been used as a modifier for enhancing bonding force and strength of sodium silicate sand at room and high temperatures. Sodium silicate is generally used as an inorganic binder for producing molds and cores which are hardened by CO2 or ester.
The additive amount of sodium silicate should be decreased by raising its binding capacity to improve the strength and collapsibility. Therefore, it is necessary to refine colloidal particles and change the structure of sodium silicate by modifying it.
Many studies on modification of sodium silicate have been concentrated on application of organic compounds with functional group.
O Kwang Hi, an institute head at the Faculty of Materials Science and Technology, has modified the size of colloidal particles of sodium silicate by sodium phosphate. Then, he has measured and compared the sizes of colloidal particles of modified and unmodified sodium silicate by using SEM. After that, he has observed the change of crystal of modified and unmodified sodium silicate by XRD.
For modification, he set the pressure at 0.4~0.5MPa, time at four to five hours and additive amount of modifier at 4%.
The main conclusions of his study are as follows:
Firstly, the heating temperature of sodium silicate core modified by sodium phosphate is 760~1 400℃ for cast iron and steel castings with wall thickness of 8 to 60mm, and after being heated at the temperature of 800~1 200℃ the retained compressive strength and the knockout energy of the modified sodium silicate sand are about 1/3 and about 1/2 lower than those of unmodified one.
Secondly, when the modified sodium silicate is heated and cooled, collapsibility of the mold is improved due to the difference in the shrinkage stresses between silicate glass and phosphate glass.
Thirdly, when sodium phosphate is added to sodium silicate as a modifier, Na2O-SiO2 binary system is changed into Na2O-SiO2-P2O5 ternary system, and net colloidal structure is formed to enhance the binding strength as a result of interaction between phosphate tetrahedroid and silicate tetrahedroid.
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