Glass Ionomer Cements Flashcards
Describe GIC reactions
- Acid base reaction
- Release of fluorine ion make it excellent at preventing secondary caries.
Describe base component of GIC
Base component: FAS (fluoro-alumino-silicate).
- Grind down material and heat to make glass:
1) Silicon dioxide (30%) SiO2
2) Aluminium oxide (20%) Al2O3
3) Calcium fluoride (33%) CaF2 - Addition of SrF2 makes more radio-opaque.
Describe acid component of GIC
Water-soluble polymer:
- Polymer with -COOH group is used.
- Most commonly: acrylic acid with itoconic acid or maleic acid.
1) pure polyacrylic acid starts to gel over time (storage problem)
2) Other acids are added (itaconic and maleic) to prevent gelling. - Tartaric acid allows delayed setting after missing, but fast setting once viscosity increases.
Describe GIC setting event
Two ions release:
- Calcium ion - initial setting; no hydrolysis or covalent bonds
- Aluminium ions - delayed setting: hydrolysis or covalent bonds.
Final set is unreacted glass surrounded by silica gel and salts.
Calcium released first.
Fluorine and aluminium released second.
Describe steps of GIC setting
- Acid attacks glass surface and ions released (Decomposition stage)
- Silica gel forms and calcium ion binds with polyacid chains to form salt (Precipitation stage)
- Aluminium leaches out and binds with polymer to form salt (aluminium chemical binding with polyacrylic molecules is stronger) - (Maturation stage)
Describe events after calcium and aluminium leach out
Since they have positive charge, they attach to the COO-.
- Calcium is divalent, and can hold onto the negative charge of the COO- group of two polyacrylic molecules, and move them closer together and establish chemical bonding between two molecules:
1) This squeezes other molecules of water out: No hydrolysis of covalent bonds.
2) This makes the material becomes more viscous and eventually solidifies. - Aluminium is trivalent, and can hold onto three polyacrylic molecules.
1) This increases the chemical bonding between polyacrylic molecules.
2) This causes the cement to be stronger.
Describe tartaric acid (chelating agent)
- Tartaric acid forms a complex with calcium that is initially released from glass.
- Allows time for aluminium to bind with polyacrylic acid.
- Result of tartaric acid inclusion:
1) Initial viscosity of material is lower for longer: working time of material increased.
2) Once aluminium starts binding, the stronger chemical bonds cause viscosity to increase rapidly.
Describe GIC bonding to tooth:
- Bonds to enamel and dentine -> adhesives forces are stronger than cohesive forces of GIC
- Bonds via ionic (via calcium ions in hydroxyapatite) and hydrogen bonding: chemical bonding.
- Etching of surface is not required (micromechanical retention is not needed for bonding).
Describe GIC hydration
- Water needed for acid-base reaction
- Shrinkage is small if kept hydrated
- Too much water leaches out ions during setting (need isolation)
Describe GIC uses:
- Luting cement
- Lining material
- Fissure sealant
- Restorative material
What is resin-modified GIC?
Add resin to GIC to increase strength
Define ‘working time’
Time from start of mixing to when material can’t flow under pressure.
Define ‘setting time’
Time period during which matrix formation has reached a point which it can’t change shape.
Describe Non-GI cement: calcium hydroxide cement
Low strength, partly soluble in pulp, and poor bonding to dentine.
Describe non-GI cement: zinc phosphate cement
Metal crowns (very strong), porcelain fused to metal.
