Chapter 12- gypsum

Question 1

what mineral makes up the majority of gypsum? what is this minerals chemical formula

Answer 1

Dental gypsum is nearly pure calcium sulfate dihydrate (CaSO4•2H2O)

Question 2

what other industries is gypsum used in?

Answer 2

plaster in drywall

Question 3

in dentistry, what is gypsum used for?

Answer 3

fabrication of models and in mounting these models on articulators

Question 4

When plaster is mixed with fillers, such as silica, it is known as a ______

Answer 4

gypsum dental investment

Question 5

what is a gypsum dental investment used for?

Answer 5

used to make the mold in which molten metal (e.g. gold) is cast to make a restoration

Question 6

what happens if you add too much water to gypsum dental investment when mixing?

Answer 6

get distortion and poor fit of the restoration

Question 7

Compare beta hemihydrate and alpha hemihydrate gypsum powders

Answer 7

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

Question 8

T/F? Gypsum plaster has been used as an impression material in the past, and continues to be the best impression material to use.

Answer 8

False, it has been replaced by polymer based materials

Question 9

______ 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?

Answer 9

Dental stone

stronger

Question 10

the ______ the water:powder ratio, the stronger the final set

Answer 10

less

Question 11

does alpha hemihydrate require more or less water than beta hemihydrate? what implication does this cause?

Answer 11

requires less water…makes it stronger

Question 12

alpha hemihydrate is better known as ______ beta hemihydrate is better known as _______

Answer 12

dental stone

dental plaster

Question 13

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?

Answer 13

*calcining* calcium sulfate dihydrate to form calcium sulfate hemihydrate

YES, HEAT IS ADDED

the starting material for gypsum casts/plasters

Question 14

what is the second reaction that occurs which ACTUALLY ENDS UP PRODUCING GYPSUM? is heat released or added to reaction 2?

Answer 14

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)

Question 15

what are the 3 different theories about how setting reactions occur in gypsum?

Answer 15

colloidal theory

hydration theory

dissolution-precipitation theory

Question 16

what does the colloidal theory say about adding water to plaster?

what happens in the sol (solution) state?

Answer 16

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

Question 17

Describe hydration theory for plaster setting.

Answer 17

Rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material

Question 18

Describe disollution precipiation theory for the setting rxn for plasters

Answer 18

is based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid

Question 19

What is the most widely accepted theory for how plaster setting reactions occur?

Answer 19

dissolution precipitation theory

Question 20

According to dissolution precipitation theory, what happens to the hemihydate when water is added?

What happens once the solution is saturated?

Answer 20

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

Question 21

what happens as the dihydrate precipitates out?

Answer 21

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

Question 22

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?

Answer 22

if W:P increases= compressive strength decreases exponentially
if W:P decreases- compressive strength increases exponentially

type1= weakest

type V= strongest

Question 23

If the W:P ratio increases (AKA you add more water), what happens to:

setting time?

strength?

setting expansion?

Answer 23

settine time increases

strength decreases

setting expansion decreases

Question 24

Define mixing time.

How long does it take a machine to mix versus a spatula?

Answer 24

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

Question 25

what is working time? how is it measured? Generally, how long of a working period do you need with gypsum?

Answer 25

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

Question 26

what happens to compressive strength during setting/hardening

Answer 26

it increases in a sigmoidal manner

Question 27

What is Setting time? How do u measure it?

Answer 27

Setting Time is the time from the beginning of the mixing until the material hardens measured by a hardness test (indentation test)

Question 28

name 3 ways we can control **setting time**

Answer 28

1. **Increasing or decreasing solubility of the hemihydrate**...increasing it means supersaturation of the Calcium sulphate increases and crystals are formed faster 2. T**he number of nuclei of crystallization can be increased or decreased:**more nuclei of crystallization means faster crystal formation, which means faster hardening 3. **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.**

Question 29

In conclusion, what two things then can the dentists do to change setting time?

Answer 29

change the W:P ratio or change mixing time

Question 30

The more water used for mixing, the _____ nuclei there are per unit volume Consequently, the setting time is \_\_\_\_

Answer 30

fewer nuclei per unit volume setting time is longer

Question 31

What happens to setting time if you mix the plaster and water very fast and rigorously?

Answer 31

setting time decreases

Question 32

Describe what happens on the molecular level for why fast mixing causes a shorter setting time?

Answer 32

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

Question 33

True or false, Adding warm water instead of cold water speeds up the setting reaction

Answer 33

FALSE! GYPSUM BEHAVES WEIRD increasing temp to 0-50 degrees causes no change in rxn rate and temperatures \>50 degrees actually DECREASE RXN SPEED

Question 34

what happens if the temp of the setting rxn occurs between 50-100 degrees? What happens at 100 degrees?

Answer 34

Reaction 2 reverses and gyspum crystals become hemihydrate again once the rxn reaches 100 deg no rxn takes place

Question 35

to ensure a long setting time, what two things can a clinician do? will using cold water help?

Answer 35

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

Question 36

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 \_\_\_%

Answer 36

expansion of mass 0.06%- 0.5%

Question 37

Describe how setting expansion changes over the course of a reaction

Answer 37

initial shrinkage from the start shrinkage peaks at about 20 minutes shrinkage decreases for a bit then plateaus

Question 38

what causes expansion in the setting rxn? what does this expansion do to the texture of the material?

Answer 38

from the growing crystals hitting each other and pushing out it creates pores in the material

Question 39

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 \_\_\_\_\_

Answer 39

after the initial set fluid at this stage

Question 40

can the initial shrinkage in a setting rxn be measured? why or why not

Answer 40

no, the initial shrinkage cannot be measured because it occurs while the reaction is still in a fluid state

Question 41

is setting expansion advantageous or a disadvantage'?

Answer 41

can be both depending on what the clinician wants

Question 42

A _____ W/P ratio and a _____ mixing time i**ncreases the setting expansion. why?**

Answer 42

lower, longer Each of these factors increases the nuclei density, so the growing crystals impinge upon each other sooner

Question 43

what is the best way to **reduce setting expansion?** hint: the manufacturer has to do it

Answer 43

manufacturer adds: **PBS** potassium sulfate, borax sodium chloride

Question 44

The strength of gypsum products is generally expressed in terms of ________ strength Does the compressive strength increase or decrease as the plaster hardens?

Answer 44

compressive strength increases

Question 45

T/F bound water content of the gypsum affects strength

Answer 45

False, it is **free** water content that affects strength

Question 46

what are the 2 ways we measure gysum strength?

Answer 46

- wet strength (also known as green strength) --\> think of a wet green swamp - dry strength

Question 47

what is wet strength? what is dry strength?

Answer 47

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

Question 48

T/F? the wet strength is usually 2 or more times greater than the dry strength

Answer 48

FALSE...DRY STRENGTH IS 2X higher than wet strength

Question 49

does drying increase or decrease compressive strength? when does drying stop having an effect on compresive strength?

Answer 49

increases it after 24 hours drying has no effect

Question 50

What effect does increasing W/P ratio (ie increasing water) have on: porosity dry strength...explain how it effects the strength too

Answer 50

incd porosity decd dry strength increased porosity means less crystals which means less strength

Question 51

What would increasing W:P ratio do to tensile strength? How would low W:P ratio effect tensile strength?

Answer 51

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

Question 52

should u ever mix for longer than 1 minute? why?

Answer 52

NO, mixing longer than 1 minute breaks up crystals and weakens the material

Question 53

Briefly Describe the 5 Types of gypsum (ie type I, type II...)

Answer 53

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

Question 54

``` Type 1 (impression plaster): what was it made of? ``` what was it used for is it used anymore and why?

Answer 54

plaster of paris impression for a full denture no, replaced by less rigid materials, such as hydrocolloids and elastomers

Question 55

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?

Answer 55

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

Question 56

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?

Answer 56

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)

Question 57

Why cant we use type 3 gypsum for stone dies? note: Stone dies are reproductions of prepared teeth, on which prostheses are constructed

Answer 57

they are too weak...cannot withstand stresses during try in, carving of wax, etc.

Question 58

Can setting expansion be allowed to occur when using type III gypsum to make a cast?

Answer 58

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

Question 59

Is type III or type 4 preferred for casts and why?

Answer 59

Type III is generally preferred because the strength is usually good enough AND it is way easier to remove after processing

Question 60

Type IV (Dental Stone, High Strength) What are they used for? why are they used for this reason?

Answer 60

used to make a die material stone ## Footnote Have the principle properties of what makes a good die stone: s strength, hardness, abrasion resistance, and a minimum setting expansion

Question 61

Again, what are the key propertis of a good die stone? What type of hemi-hydrate should be use for a die stone?

Answer 61

"The die they used at the casino was a SHAM" strength hardness abrasion resistance, minimum setting expansion Alpha- hemihydrate (remember the strong alpha male)

Question 62

why is a hard surface necessary for a die stone? What things can be added to gypsum to increase its abrasion resistance?

Answer 62

wax must be carved off the die stone with sharp instruments so it must be abrasion resistant addition of silver plating, hardening solutions

Question 63

What does type V stone have over type IV stone? Explain this advantage

Answer 63

higher compressive strength makes it possible to lower the W/P ratio more than u can with Type IV

Question 64

Does Type V have high or low expansion setting? why is this useful?

Answer 64

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

Question 65

Why cant you use type V gypsum to make a cast?

Answer 65

the high setting expansion in type V causes fits that are way too tight