Dental Materials Science and Biomaterials
What is examined
Properties, handling and failure modes of the materials used daily - gypsum, impression materials, polymers, ceramics, alloys and cements - and the ISO standards that define them.
Gypsum
- Type I impression plaster, II plaster, III stone, IV die stone, V high-expansion die stone.
- Water-to-powder ratio controls everything: extra water gives a weaker, more porous, less expanding set. Weigh it; do not guess.
- Setting expansion is roughly 0.2–0.3% for plaster, 0.08–0.2% for stone, and lowest for die stone. Vacuum mixing reduces porosity and raises strength.
Impressions
- Alginate is an irreversible hydrocolloid: it imbibes water and undergoes syneresis, so pour within 10–15 minutes or store sealed and damp.
- Addition silicone is dimensionally stable for days, hydrophobic, and inhibited by sulfur from latex gloves.
- Polyether is hydrophilic and stiff - useful in wet fields, difficult to withdraw from undercuts.
- Disinfect exactly as the manufacturer states; prolonged immersion distorts alginate.
Polymers, ceramics, metals
- PMMA polymerises with about 6% volumetric shrinkage; the curing cycle (for example 74 °C for 9 hours or 100 °C for 20 minutes) controls porosity and residual monomer.
- Glass ceramics (leucite-reinforced, lithium disilicate) etch with hydrofluoric acid and bond with silane. Zirconia does not etch usefully - abrade and use a 10-MDP primer.
- Cobalt-chrome is hard and stiff; titanium is reactive at casting temperature and is usually milled or printed instead.
Worked example
A die stone cast feels chalky and margins crumble. The cause is over-dilution: too much water dropped compressive strength and changed expansion. Weighing powder and water and vacuum mixing corrects it.
Traps
- Assuming material that feels hard has finished setting - stone keeps expanding afterwards.
- Soaking an alginate impression in disinfectant for far longer than specified.
- Mixing brands of addition silicone without checking compatibility.