Chapter 12- gypsum Flashcards
what mineral makes up the majority of gypsum? what is this minerals chemical formula
Dental gypsum is nearly pure calcium sulfate dihydrate (CaSO4•2H2O)
what other industries is gypsum used in?
plaster in drywall
in dentistry, what is gypsum used for?
fabrication of models and in mounting these models on articulators
When plaster is mixed with fillers, such as silica, it is known as a ______
gypsum dental investment
what is a gypsum dental investment used for?
used to make the mold in which molten metal (e.g. gold) is cast to make a restoration
what happens if you add too much water to gypsum dental investment when mixing?
get distortion and poor fit of the restoration
Compare beta hemihydrate and alpha hemihydrate gypsum powders
Beta- spongy and irregular particles Alpha- crystals are prismatic and more regular in shape
Prof said the way to remember this is beta males are soft and wimpy while alpha males are stronger
T/F? Gypsum plaster has been used as an impression material in the past, and continues to be the best impression material to use.
False, it has been replaced by polymer based materials
______ is another variety of plaster that is poured into the impression to form a positive model master cast.
Is dental stone stronger or weaker than dental plaster?
Dental stone
stronger
the ______ the water:powder ratio, the stronger the final set
less
does alpha hemihydrate require more or less water than beta hemihydrate? what implication does this cause?
requires less water…makes it stronger
alpha hemihydrate is better known as ______ beta hemihydrate is better known as _______
dental stone
dental plaster
what is the first reaction involved in the production of gypsum (done in the manufacturer’s factory)
IS HEAT ADDED?
what does this rxn create for dentists?
*calcining* calcium sulfate dihydrate to form calcium sulfate hemihydrate
YES, HEAT IS ADDED
the starting material for gypsum casts/plasters
what is the second reaction that occurs which ACTUALLY ENDS UP PRODUCING GYPSUM? is heat released or added to reaction 2?
the REVERSE OF RXN 1… Reaction of calcium sulfate hemihydrate powder with water to produce gypsum
Heat is released by this rxn (this is the rxn that occurs in a dental clinic)
what are the 3 different theories about how setting reactions occur in gypsum?
colloidal theory
hydration theory
dissolution-precipitation theory
what does the colloidal theory say about adding water to plaster?
what happens in the sol (solution) state?
when mixed with water, plaster enters into the colloidal state through a sol-gel mechanism
solution state- hemihydrate particles absorb the water and become a gel
Describe hydration theory for plaster setting.
Rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material
Describe disollution precipiation theory for the setting rxn for plasters
is based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid
What is the most widely accepted theory for how plaster setting reactions occur?
dissolution precipitation theory
According to dissolution precipitation theory, what happens to the hemihydate when water is added?
What happens once the solution is saturated?
When the hemihyrdate is mixed with water, a suspension is formed that is fluid and workable. The hemihydrate eventually dissolves and forms a saturated solution
Dihydrate precipates out after saturation
what happens as the dihydrate precipitates out?
As dihydrate precipitates, the solution is no longer saturated with the hemihydrate, so it continues to dissolve
As the reaction continues, interlocking crystals will form until no further dihydrate precipitates out of solution
what happens to the compressive strength of gypusm as the water:plaster ratio increases? decreased?
what gypusm has the weakest compresssive strength?
what gypsum has the strongest compressive strength?
if W:P increases= compressive strength decreases exponentially
if W:P decreases- compressive strength increases exponentially
type1= weakest
type V= strongest
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If the W:P ratio increases (AKA you add more water), what happens to:
setting time?
strength?
setting expansion?
settine time increases
strength decreases
setting expansion decreases
Define mixing time.
How long does it take a machine to mix versus a spatula?
the time from the addition of the powder to the water until the mixing is completed.
machine= 20-30 seconds
stirring with a spatula= at least 1 minute
what is working time?
how is it measured?
Generally, how long of a working period do you need with gypsum?
the time you have to use a workable mix (a workable mix is one that has a uniform consitency)
measured from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose
3 minutes
what happens to compressive strength during setting/hardening
it increases in a sigmoidal manner
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