Metal-matrix interface in reinforced glass ionomers.

Objective: In reinforced materials, interfacial bonding is critical for efficient transfer of stress from the matrix to the reinforcement. The goal of this study was to characterize the physical and chemical nature of this interface in three metal-reinforced glass ionomers. Two of them were commercial, ESPE-Ketac Silver (KS) and GC-Miracle Mix (MM), and the third, EX, was experimental.

Methods: The techniques of scanning electron microscopy (SEM) and infrared spectroscopy (IR) were used to accomplish the stated goal. SEM analysis utilized polished sections of set cylindrical specimens of each material prepared from their respective powder and liquid components. The glass and the liquid in EX were the same as in MM. The reinforcing agent in EX, 50% by weight, was an Ag-base spherical alloy similar in composition to that in MM. For EX, the alloy was oxidized to promote its bonding to the matrix. The specimens for IR study were prepared as follows. The metallic powders of each material were mixed with the corresponding liquid in excess, stored at 37 degrees C for 1 h, washed with warm water (60 degrees C), filtered and dried. The untreated metallic powders served as controls for IR analysis.

Results: SEM revealed a distinct halo shaped internal reaction layer surrounding each alloy particle in EX. A similar layer was not seen in MM and KS. The alloy-matrix interface was continuous and gap-free in EX. In contrast, gaps separating matrix from respective reinforcements were conspicuous features in MM and KS. The IR spectrum of the liquid treated EX alloy powder showed absorbency bands characteristic of unreacted carboxyl groups and carboxylate salts. These bands were absent in the IR spectra of all other powders--treated and untreated.

Significance: The absence of interfacial bonding in MM and KS demonstrated in this study provides a reason why these two materials, in spite of metal addition, have not proved to be any stronger or more durable than their metal-free counterparts. A means of creating interfacial bonding presented here could be useful in the design of improved reinforced glass ionomer materials.