Gypsum Products Flashcards
(41 cards)
Gypsum Classifications:
type I= impression plaster
type II= model plaster
Type III= dental stone
type IV= die stone or high strength stone
type V= die stone with high expansion
important that chemically all forms are identical just differ in shape and porosity
Gypsum is made up of:
- dehydrate form of calcium sulfate (CaSo4 x 2H2O)
Gypsum equation:
- through heating, dehydrate form loses water of crystallization and is converted to hemihydrate
- CaSo4 x 2 H2O + heat –> CaSo4 x 1/2H2O + 1.5 H2O
- done via pressure
Setting Reaction of Gypsum:
- opposite of heating
- hemihydrate (plaster, stone, destine) is mixed with water, reaction converts hemihydrate to dehydrate
- CaSo4 x 1/2H2O + 1.5 H2O –> CaSo4 x 2 H2O + heat
- exothermic reaction
How to mix hemihydrate:
- use EXCESS water
- this is so the excess water wets the powder so that particles can react
- plaster= requires MORE water (50ml/100g)
- die stone= requires LEAST water (24ml)
How is the excess water lost?
- evaporation
- but make sure you still measure it out or it will take longer to set and cause porosities in the material
What is working time?
- Time during which material can be mixed/poured into impression
- detected by loss of gloss (excess water has been absorbed)
- 8-16 minutes from start of mix
What is final setting time?
- time when you can separate the cast from impression without distortion/fracture
- 45-60 minutes
What is compressive strength after one hour?
- measure of wet strength
- excess water not completely lost
What is dry strength?
- when cast is completely dry
What makes die stone the strongest?
- mixed with least amount of water, densest, shrinks/expands less than others
The more water you have to put into material….
the weaker
The greater the surface hardness….
the greater the resistance to abrasion
Expansion ranks:
plaster (expands most)< dental stone < die stone (expands least)
What is hygroscopic expansion?
if gypsum material immersed in water during setting, the setting expansion will increase
What does increasing the rate of the reaction do? And what is added to do this?
- reduces the setting time
done via: - accelerators (chemicals that increase the rate)
- include: potassium sulfate, calcium sulfate dihydrate crystals (slurry water)
- increasing temperate to body temperature (37 degrees)
What does decreasing the rate of reaction do? And what is added to do this?
- prolongs the setting time
done via: - retarders
- include: blood, saliva, alginate, borax
- increasing above body temperature (> 37 degrees)
boiling eater will stop reaction
What does high relative humidity do to products?
- can initiate setting reaction
- can prevent normal setting
- reduce strength and hardness
now, store powders in airtight containers/pre-measured packets
What will an increased water/powder ratio do?
- thinner mix, longer setting time, weaker, less expansion
What will a decreased water/powder ratio do?
- thicker, shorter setting time, stronger, increased expansion
The longer the mixing time, the —- the setting time and —- the expansion
the shorter the setting time and greater the expansion.
How long do you mix gypsum products?
1 minute with spatulate
- vibrate bowl for a few seconds to remove air bubbles
Desirable properties of gypsum involved investment:
- adequate expansion
- easy to handle
- produce smooth surface/fine detail
- stable at high temps
- porous
- high heat strength
Components of investment:
- refractory filler (expand under heat):
form of silicon dioxide specifically Cristobalite (60-65%) - binder: calcium sulfate hemihydrate (30-35%)
- modifiers: 5%
