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False, it has been replaced by polymer based materials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
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
What is Setting time?
How do u measure it?
Setting Time is the time from the beginning of the mixing until the material hardens
measured by a hardness test (indentation test)
name 3 ways we can control setting time
- Increasing or decreasing solubility of the hemihydrate…increasing it means supersaturation of the Calcium sulphate increases and crystals are formed faster
- The number of nuclei of crystallization can be increased or decreased:more nuclei of crystallization means faster crystal formation, which means faster hardening
- The setting time can be accelerated by increasing rate of crystal growth. The setting time can be decreased by retarding the rate of crystal growth.
In conclusion, what two things then can the dentists do to change setting time?
change the W:P ratio or change mixing time
The more water used for mixing, the _____ nuclei there are per unit volume
Consequently, the setting time is ____
fewer nuclei per unit volume
setting time is longer
What happens to setting time if you mix the plaster and water very fast and rigorously?
setting time decreases
Describe what happens on the molecular level for why fast mixing causes a shorter setting time?
Crystals are broken up by the mixing spatula and are distributed throughout the mixture, resulting in the formation of more nuclei of crystallization
Thus, the setting time is decreased
True or false, Adding warm water instead of cold water speeds up the setting reaction
FALSE! GYPSUM BEHAVES WEIRD
increasing temp to 0-50 degrees causes no change in rxn rate
and temperatures >50 degrees actually DECREASE RXN SPEED
what happens if the temp of the setting rxn occurs between 50-100 degrees?
What happens at 100 degrees?
Reaction 2 reverses and gyspum crystals become hemihydrate again
once the rxn reaches 100 deg no rxn takes place
to ensure a long setting time, what two things can a clinician do?
will using cold water help?
should purchase a regular setting product and/or use minimum mixing times and slower rates of mixing
NO, COLD WATER WILL NOT SLOW IT DOWN
during gypsum conversion from hemihydrate to a dihydrate, a _____ of mass occurs
Depending on the composition of the gypsum product, this observed linear expansion maybe as low as __% or as high as ___%
expansion of mass
0.06%- 0.5%
Describe how setting expansion changes over the course of a reaction
initial shrinkage from the start
shrinkage peaks at about 20 minutes
shrinkage decreases for a bit then plateaus
what causes expansion in the setting rxn?
what does this expansion do to the texture of the material?
from the growing crystals hitting each other and pushing out
it creates pores in the material
As far as the technician or dentist is concerned, only the setting expansion that occurs ______ the initial set is of interest
Any initial contraction/expansion that occurs before the initial set does not affect the accuracy, because the mix is _____
after the initial set
fluid at this stage
can the initial shrinkage in a setting rxn be measured? why or why not
no, the initial shrinkage cannot be measured because it occurs while the reaction is still in a fluid state
is setting expansion advantageous or a disadvantage’?
can be both depending on what the clinician wants
A _____ W/P ratio and a _____ mixing time i**ncreases the setting expansion.
why?**
lower, longer
Each of these factors increases the nuclei density, so the growing crystals impinge upon each other sooner
what is the best way to reduce setting expansion?
hint: the manufacturer has to do it
manufacturer adds: PBS
potassium sulfate,
borax
sodium chloride
The strength of gypsum products is generally expressed in terms of ________ strength
Does the compressive strength increase or decrease as the plaster hardens?
compressive strength
increases
T/F bound water content of the gypsum affects strength
False, it is free water content that affects strength
what are the 2 ways we measure gysum strength?
- wet strength (also known as green strength) –> think of a wet green swamp
- dry strength
what is wet strength?
what is dry strength?
wet strength- the strength obtained when excess water is left in the hemihydrate
dry strength- When the excess water in the specimen has been driven off by drying
T/F? the wet strength is usually 2 or more times greater than the dry strength
FALSE…DRY STRENGTH IS 2X higher than wet strength
does drying increase or decrease compressive strength?
when does drying stop having an effect on compresive strength?
increases it
after 24 hours drying has no effect
What effect does increasing W/P ratio (ie increasing water) have on:
porosity
dry strength…explain how it effects the strength too
incd porosity
decd dry strength
increased porosity means less crystals which means less strength
What would increasing W:P ratio do to tensile strength?
How would low W:P ratio effect tensile strength?
A high W/P ratio results in tensile strengths as high as 25% of the corresponding compressive strength
A low W/P ratio results in tensile strengths less than 10% of the corresponding compressive strength
should u ever mix for longer than 1 minute? why?
NO, mixing longer than 1 minute breaks up crystals and weakens the material
Briefly Describe the 5 Types of gypsum (ie type I, type II…)
Type I: Impression Plaster
Type II: Model Plaster
Type III: Dental Stone
Type IV: Dental Stone, High Strength
Type V: Dental Stone, High Strength, High Expansion
Type 1 (impression plaster): what was it made of?
what was it used for
is it used anymore and why?
plaster of paris
impression for a full denture
no, replaced by less rigid materials, such as hydrocolloids and elastomers
Type 2 plaster (model plaster)
when do we use it?
what color is it?
what is it like in terms of compressive and tensile strength?
mounting casts on articulators. also used to fill a flask in denture construction when setting expansion is not critical and the strength is adequate.
white
weak compressive and tensile strength
Type 3 (Dental Stone)
when was it discovered and what was it called?
What is it used for?
What was the major disadvange of it?
Describe its compressive strength?
1930: called α-gypsum
making casts for full dentures
too hard so it made the indirect pattern impossible
strength of 20.7 MPa (3000 psi), but it does not exceed 34.5 MPa (5000 psi)
Why cant we use type 3 gypsum for stone dies?
note: Stone dies are reproductions of prepared teeth, on which prostheses are constructed
they are too weak…cannot withstand stresses during try in, carving of wax, etc.
Can setting expansion be allowed to occur when using type III gypsum to make a cast?
setting expansion is only allowed to occur if the case is just reproducing soft tissue. If teeth are being reproduced then it cannot be allowed
Is type III or type 4 preferred for casts and why?
Type III is generally preferred because the strength is usually good enough AND it is way easier to remove after processing
Type IV (Dental Stone, High Strength)
What are they used for? why are they used for this reason?
used to make a die material stone
Have the principle properties of what makes a good die stone: s
strength, hardness, abrasion resistance, and a minimum setting expansion
Again, what are the key propertis of a good die stone?
What type of hemi-hydrate should be use for a die stone?
“The die they used at the casino was a SHAM”
strength
hardness
abrasion resistance,
minimum setting expansion
Alpha- hemihydrate (remember the strong alpha male)
why is a hard surface necessary for a die stone?
What things can be added to gypsum to increase its abrasion resistance?
wax must be carved off the die stone with sharp instruments so it must be abrasion resistant
addition of silver plating, hardening solutions
What does type V stone have over type IV stone?
Explain this advantage
higher compressive strength
makes it possible to lower the W/P ratio more than u can with Type IV
Does Type V have high or low expansion setting?
why is this useful?
high expansion setting (iincreased from a maximum of 0.10% to 0.30%)
new alloy metals shrink a lot –>the expansion of type V gypsum makes up for this shrinkage
Why cant you use type V gypsum to make a cast?
the high setting expansion in type V causes fits that are way too tight
