Development of sodium silicate adhesives for electrical steel bonding

Abstract

Inorganic adhesives have several benefits over traditional joining methods for joining electrical steels used in magnetic cores of numerous industrial applications. As insulators with very high melting temperatures, the adhesives offer the possibility of increasing the efficiency of these machines. The aim of this project was to characterize sodium silicates as adhesives for such applications and develop methodology for their processing. The chemical and physical properties of the water-soluble sodium silicates were easily altered by changing the composition of Na₂O, SiO₄, and water, offering a spectrum of properties to investigate. Several aspects of the electrical steel provided by POSCO were also investigated, including surface chemistry and microstructure due to processing of the steel sheets. Coating efficacy was evaluated based on the adhesive's ability to wet the substrate to form a uniform coating, as well as resistance to mechanical loads, including adhesion and flexural strain. Greater degree of alkalinity in the sodium silicates resulted in improved wetting, uniformity, adhesion, and flexural strain for the range of viscosities that supported these behaviors. The microstructure of the electrical steels influenced the interaction of the adhesive with the surface, but properties still improved with higher alkalinity. Firing parameters were used to alter the mechanical properties of the silicates, as well as to determine operability limits. The best mechanical properties occurred for those coupons fired between 600°C and 800°C. The efficacy did not degrade significantly with long exposure to high temperatures, offering promise for sodium orthosilicates as appropriate adhesives for the described applications. Further study of the environmental conditions under which the adhesives will be used, as well as full characterization of the insulating properties will allow the processes developed here to be scaled up for industrial use.