DOH124 Lecture Notes - Lecture 3: Melting, Polyelectrolyte, Silica Gel
SETTING REACTIONS
For GICs --- acid-base reaction only
The setting mechanism involves the dissolution of the surface of the glass particles with
the release of calcium and aluminium ions, which then combine with the polyacrylic acid
to form calcium and aluminium polyacrylate chains. The calcium chains form first,
producing an early set, but are fragile and highly soluble in water. The aluminium chains
form thereafter; these are strong and insoluble and provide the major physical
properties of the set restoration. The essential setting mechanism is an acid—base
reaction between the poly(alkenoic acid) liquid and the glass, which leads to a
diffusion-based adhesion between the glass particles and the matrix. Calcium and
aluminium ions are released by proton attack on the surface of the glass particles; these
ions ultimately cross-link the poly-acid chains into a network that remains porous, thus
allowing free passage of both hydroxyl and fluoride ions out of and back into the
cement matrix. Three stages have been described in this setting reaction, which is
regarded as a long-term reaction continuing for at least 1 month after placement and,
probably, much longer.
Stage 1— dissolution
The surface layer of the glass particles is attacked by the polyacid to produce a
diffusion-based adhesion between the glass particles and the matrix. Approximately
20-30% of the glass is decomposed and ions (including calcium, aluminium and fluoride
ions) arc released, leading to formation of a cement sol.
Stage 2 — precipitation of salts; gelation and hardening
During this stage, calcium and aluminium ions bind to polyanions via the carboxylate
groups. The initial clinical set is achieved by cross-linking of the more readily available
calcium ions. This reaction is relatively rapid, usually forming a clinically hard surface
within 4-10 minutes from the start of mixing. Maturation occurs over the next 24 hours
as the less mobile aluminium ions become bound within the cement matrix, leading to
more rigid cross-linking between the poly(alkenoic acid) chains. Fluoride and phosphate
ions form insoluble salts and complexes. Sodium ions contribute to the formation of an
orthosilicic acid on the surface of the particles and, as the pH rises, this converts to a
silica gel which assists in binding the powder to the matrix.
Stage 3— hydration of salts
Associated with the maturation phase is a progressive hydration of the matrix salts,
leading to a sharp improvement in the physical properties.
Document Summary
The setting mechanism involves the dissolution of the surface of the glass particles with the release of calcium and aluminium ions, which then combine with the polyacrylic acid to form calcium and aluminium polyacrylate chains. The calcium chains form first, producing an early set, but are fragile and highly soluble in water. The aluminium chains form thereafter; these are strong and insoluble and provide the major physical properties of the set restoration. The essential setting mechanism is an acid base reaction between the poly(alkenoic acid) liquid and the glass, which leads to a diffusion-based adhesion between the glass particles and the matrix. Three stages have been described in this setting reaction, which is regarded as a long-term reaction continuing for at least 1 month after placement and, probably, much longer. The surface layer of the glass particles is attacked by the polyacid to produce a diffusion-based adhesion between the glass particles and the matrix.
