General Materials

39 General Materials

Impression Materials:

Material	Process	Mechanism	Curing Time	Shelf Life / Storage
Plaster of Paris	Hydration/Setting	Chemical reaction: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (i.e. gypsom)	10-20 minutes	Long
Dental Stone	Hydration/Setting	Same chemical reaction, denser crystal formation	15-25 minutes	Long
Silicone	Vulcanization/Condensation Cures with air moisture	Crosslinking of polymer chains	15-25 minutes	Limited shelf life, sensitive to humidity
Alginate	Gelation	Ionic crosslinking with calcium ions	2-5 minutes	Limited shelf life, sensitive to humidity

P&O plaster
- The curing process for plaster of Paris is hydration (add water).
- The hydration reaction forms crystalline structures and is exothermic, meaning it releases heat as it cures.
- The curing process can be controlled by adding salt or higher temperature water
Dental stone
- The curing process for dental stone is hydration (add water).
  - Dental stone requires less water content (around 24-30ml per 100g) to set up vs. plaster of Paris (around 45-50ml per 100g) and is much more dense.
- Dental stone produces much smoother, more detailed surfaces
Silicone
- The curing process for most silicones used in P&O is room temperature vulcanization (RTV) But, not all silicones use RTV – some require heat curing (high temperature vulcanization or HTV)
- Because RTV silicone cures through a condensation reaction, it’s shelf life is limited: 1 year shelf life, if in freezer. Additionally, the positive model must be very dry when molding silicone over the model because water affects curing process.
- The curing process is affected by humidity.
Alginate
- Alginate doesn’t actually “cure” in the traditional sense like plaster or silicone. Instead, it undergoes a process called gelation or gel formation
- Add water to the powder alginate.
- The curing process can be controlled by adjusting calcium ion concentration or higher temperature water.