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Dental Materials > Chapter 12- gypsum > Flashcards

Chapter 12- gypsum Flashcards

1
Q

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

A

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

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2
Q

what other industries is gypsum used in?

A

plaster in drywall

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3
Q

in dentistry, what is gypsum used for?

A

fabrication of models and in mounting these models on articulators

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4
Q

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

A

gypsum dental investment

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5
Q

what is a gypsum dental investment used for?

A

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

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6
Q

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

A

get distortion and poor fit of the restoration

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7
Q

Compare beta hemihydrate and alpha hemihydrate gypsum powders

A

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

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8
Q

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

A

False, it has been replaced by polymer based materials

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9
Q

______ 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?

A

Dental stone

stronger

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10
Q

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

A

less

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11
Q

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

A

requires less water…makes it stronger

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12
Q

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

A

dental stone

dental plaster

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13
Q

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?

A

*calcining* calcium sulfate dihydrate to form calcium sulfate hemihydrate

YES, HEAT IS ADDED

the starting material for gypsum casts/plasters

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14
Q

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

A

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)

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15
Q

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

A

colloidal theory

hydration theory

dissolution-precipitation theory

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16
Q

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

what happens in the sol (solution) state?

A

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

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17
Q

Describe hydration theory for plaster setting.

A

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

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18
Q

Describe disollution precipiation theory for the setting rxn for plasters

A

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

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19
Q

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

A

dissolution precipitation theory

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20
Q

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

What happens once the solution is saturated?

A

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

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21
Q

what happens as the dihydrate precipitates out?

A

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

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22
Q

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?

A

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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23
Q

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

setting time?

strength?

setting expansion?

A

settine time increases

strength decreases

setting expansion decreases

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24
Q

Define mixing time.

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

A

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

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25
Q

what is working time?

how is it measured?

Generally, how long of a working period do you need with gypsum?

A

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

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26
Q

what happens to compressive strength during setting/hardening

A

it increases in a sigmoidal manner

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27
Q

What is Setting time?

How do u measure it?

A

Setting Time is the time from the beginning of the mixing until the material hardens

measured by a hardness test (indentation test)

28
Q

name 3 ways we can control setting time

A
  1. Increasing or decreasing solubility of the hemihydrate…increasing it means supersaturation of the Calcium sulphate increases and crystals are formed faster
  2. The 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.
29
Q

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

A

change the W:P ratio or change mixing time

30
Q

The more water used for mixing, the _____ nuclei there are per unit volume

Consequently, the setting time is ____

A

fewer nuclei per unit volume

setting time is longer

31
Q

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

A

setting time decreases

32
Q

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

A

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

33
Q

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

A

FALSE! GYPSUM BEHAVES WEIRD

increasing temp to 0-50 degrees causes no change in rxn rate
and temperatures >50 degrees actually DECREASE RXN SPEED

34
Q

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

What happens at 100 degrees?

A

Reaction 2 reverses and gyspum crystals become hemihydrate again

once the rxn reaches 100 deg no rxn takes place

35
Q

to ensure a long setting time, what two things can a clinician do?

will using cold water help?

A

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

36
Q

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 ___%

A

expansion of mass

0.06%- 0.5%

37
Q

Describe how setting expansion changes over the course of a reaction

A

initial shrinkage from the start
shrinkage peaks at about 20 minutes

shrinkage decreases for a bit then plateaus

38
Q

what causes expansion in the setting rxn?

what does this expansion do to the texture of the material?

A

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

39
Q

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 _____

A

after the initial set

fluid at this stage

40
Q

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

A

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

41
Q

is setting expansion advantageous or a disadvantage’?

A

can be both depending on what the clinician wants

42
Q

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

why?**

A

lower, longer

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

43
Q

what is the best way to reduce setting expansion?

hint: the manufacturer has to do it

A

manufacturer adds: PBS

potassium sulfate,

borax

sodium chloride

44
Q

The strength of gypsum products is generally expressed in terms of ________ strength

Does the compressive strength increase or decrease as the plaster hardens?

A

compressive strength

increases

45
Q

T/F bound water content of the gypsum affects strength

A

False, it is free water content that affects strength

46
Q

what are the 2 ways we measure gysum strength?

A
  • wet strength (also known as green strength) –> think of a wet green swamp
  • dry strength
47
Q

what is wet strength?

what is dry strength?

A

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

48
Q

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

A

FALSE…DRY STRENGTH IS 2X higher than wet strength

49
Q

does drying increase or decrease compressive strength?

when does drying stop having an effect on compresive strength?

A

increases it

after 24 hours drying has no effect

50
Q

What effect does increasing W/P ratio (ie increasing water) have on:

porosity

dry strength…explain how it effects the strength too

A

incd porosity
decd dry strength

increased porosity means less crystals which means less strength

51
Q

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

How would low W:P ratio effect tensile strength?

A

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

52
Q

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

A

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

53
Q

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

A

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

54
Q 
Type 1 (impression plaster):
what was it made of?

what was it used for

is it used anymore and why?

A

plaster of paris

impression for a full denture

no, replaced by less rigid materials, such as hydrocolloids and elastomers

55
Q

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?

A

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

56
Q

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?

A

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)

57
Q

Why cant we use type 3 gypsum for stone dies?

note: Stone dies are reproductions of prepared teeth, on which prostheses are constructed

A

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

58
Q

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

A

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

59
Q

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

A

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

60
Q

Type IV (Dental Stone, High Strength)

What are they used for? why are they used for this reason?

A

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

61
Q

Again, what are the key propertis of a good die stone?

What type of hemi-hydrate should be use for a die stone?

A

“The die they used at the casino was a SHAM”

strength

hardness

abrasion resistance,

minimum setting expansion

Alpha- hemihydrate (remember the strong alpha male)

62
Q

why is a hard surface necessary for a die stone?

What things can be added to gypsum to increase its abrasion resistance?

A

wax must be carved off the die stone with sharp instruments so it must be abrasion resistant

addition of silver plating, hardening solutions

63
Q

What does type V stone have over type IV stone?

Explain this advantage

A

higher compressive strength

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

64
Q

Does Type V have high or low expansion setting?

why is this useful?

A

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

65
Q

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

A

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

Dental Materials (15 decks)

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