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Dental Materials Y2 > Gypsum Products Plaster and Stone > Flashcards

Gypsum Products Plaster and Stone Flashcards

1
Q

What is Gypsum

A

Calcium Sulphate Dihydrate = (CaSO4)2H2O

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2
Q

What can gypsum based materials be used as

A
  • Impression materials - limited use today, used in edentulous cases (negative replica)
  • Models - poured from an impression to construct partial or full denture (positive replica)
  • Dies - positive replica of individual teeth
  • Moulds - for dentures
  • Refractory investments?*
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3
Q

What is a positive replica

A

When a mould or impression i made of the study object and then the mould is filled with a permanent substance

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4
Q

What is a negative replica

A

Inverse representation of the original specimen surface yh lol

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5
Q

How do manufacturers process gypsum

A

Convert the calcium sulphate dehydrate to Calcium sulphate hemihydrate (just one water molecule)

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6
Q

Depending on the treatment process of gypsum what forms can gypsum hemihydrate be in

A
  • Plaster - soft white powder
  • Stone - harder, yellow powder
  • Improved stone
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7
Q

What happens to gypsum powders when mixed with water on setting

A

They convert back to the dihydrate form

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8
Q

What are the differences between different gypsum hemihydrate forms

A

Chemically identical but physical properties depend on nature of dehydration process or preparation process

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9
Q

Describe the differences in physical properties between Gypsum plaster and stone/improved stone

A
  • Plaster - large, irregular, porous particles
  • Stone/Improved stone - small, regular and non-porous particles
  • Improve stone - expensive and more scratch resistant cf stone
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10
Q

What are the differences in the product formed when alpha and beta hemihydrates mix with water on setting

A

Alpha - forms stronger product due to small, regular, non-porous particles that pack closely together within the set material

Beta - forms weaker product due to large irregular and porous particles that don’t pack closely together that results in large pores in the set material

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11
Q

In what ways do alpha and beta hemihydrates differ

A

Alpha and beta hemihydrates differ in particle/crystal size and surface area

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12
Q

What are the advantageous features of using gypsum plaster

A

Softer
Cheap
Easy to use and shape

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13
Q

What are the uses of gypsum plaster

A
  • General purpose material used for mounting models into articulators
  • Flasking procedures for complete and partial denture processing
  • Basing models
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14
Q

What are the uses of gypsum stone

A

Used where strength is v important

  • Dies
  • Models of mouth - +ve replica of individual teeth, in construction of crowns, bridge and dentures
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15
Q

Describe the composition of gympsum products

A 
Hemihydrate - 75-85%
Unchanged gypsum - 5-8%
Mix of fast set soluble and slow set insoluble anhydrides - 5-8%
Impurities - ~4%
Accelerators/Retarders - ~4%
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16
Q

What do the accelerators/retarders in gypsum do

A

These increase or decrease the solubility of gypsum in water and therefore affect its setting time

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17
Q

Name some accelerators for gypsum

A

2% K2SO4 solution
CaSO4.2H2O
NaCl (<20%)

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18
Q

How does Potassium Sulfate gypsum accelerator work

A

Reacts with water/hemihydrate = Sygenite

This crystallises rapidly and encourages growth of more crystals

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19
Q

How does CaSO4.2H2O accelerate setting of gypsum

A

Provides additional nucleation sites

20
Q

How does NaCl (<20%) accelerate gypsum setting

A

Provides additional sites for crystallisation - increases reaction rate and reduces observed expansion

21
Q

Name some gypsum retarders

A

NaCl (>20%)
Borax
Potassium Citrate (gum arabic, acetates)

22
Q

How does NaCl (>20%) act as a retarder

A

Slows down reaction rate by deposition of NaCL on CaSO4.2H2O crystals and preventing growth

23
Q

AY BAWS CAN I HABE DE NOTE PLZ

A

All gypsum retarders interfere with crystal formation and affect dimensional change on setting

24
Q

Describe the setting reaction of Calcium sulphate hemihydrate with water

A

Forms calcium sulphate dihydrate (less soluble)

Exothermic

25
Q

Describe the mechanism of the setting reaction of calcium sulfate hemihydrate with water

A
  • Hemihydrate slowly forms dihydrate
  • Some CaSO4 hemihydrate dissolve in water due to low solubility and reacts to form dihydrate
  • Dihydrate’s lower solubility in water = unstable supersaturated solution
  • So CaSO4.2H2O precipitates forming stable crystals
  • As crystals form, more hemihydrate dissolves in water and process continues until setting
26
Q

What is the role of unchanged gypsum dihydrate in setting mechanism

A

Acts as a crystallisation nuclei for growth of dihydrate crystals

27
Q

How much water is used for 100g of different gypsum product and why do we use excess

A

Plaster - 50ml/100g
Stone/improved stone - 20ml/100g

Excess because otherwise the mix would be too viscous

28
Q

How can increasing the proportion of water used in the setting reaction affect the reaction

A

Slower setting time - longer for saturation, the mix will be runny and the model will be weaker

29
Q

How can increasing the proportion of powder used in the setting reaction affect the reaction

A

Difficult to mix and product will result in porosity within the set material

30
Q

What does increasing the spatulation time (mixing) in the setting reaction do to the reaction

A

Reduction in setting time - break up formed crystals to form new sites for crystal growth, also increases the setting expansion

31
Q

What does increasing the temperature in the setting reaction do to the reaction

A
  • Little change between 0 and 50C
  • More than 50C = gradual retardation occurs
  • At 100C = no reaction
32
Q

What methods are there to measure setting time

A
  • Vicat needles

- Gillmore needles

33
Q

What different Gillmore needles are there and how are they used to measure setting time

A

1/4 lb needle measures initial setting time, mould cannot be moulded but can be carved, needle won’t mark material

1 lb needle measure final setting time = when material can be removed from the impression without distortion or fracture

34
Q

What is a visual sign that the material has reached its initial set

A

Surface of material loses its gloss or shine

35
Q

What is the rough working and setting time for model plaster

A

2-3 mins working time

5-10 mins setting time

36
Q

What factors can increase setting expansion

A
  • Powder/liquid ratio

- Spatulation time

37
Q

What causes setting expansion in gypsum products

A

Crystals impinge on each other as they grow and push each other apart = large empty spaces between crystals = porosity = 0.6% by volume

38
Q

What is hygroscopic expansion

A

When crystals grow freely in water and expand

By immersing material in water while setting the setting expansion increases

39
Q

What happens when a material is left to set in air

A

If set in air the surface tension of the free water in the material tends to draw the crystals together

40
Q

Name some of the properties of gypsum products

A
  • Once set - little or no dimensional change
  • Storage - excellent
  • Compressive strength affected by: too much water (inferior strength), too little water (thick mix, porosity, incomplete reaction)
  • Dry strength = 2x that of wet strength
41
Q

Describe the tensile strength of different gypsum products

A
  • Plaster tensile strength = v low

- Stone tensile strength - double plaster - crown and bridge models and dies

42
Q

Describe the surface hardness and scratch and abrasion resistance for different gypsum products

A

Surface hardness:
- plaster = very low
- improves for stone and even better for improved stone
Scratch and abrasion resistance:
- Plaster - low resistance
- Stone + improved stone - better resistance

43
Q

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Laboratory Plaster

A 
P/W ratio = 100g/45-50ml
Initial setting time - 5-10 mins
Linear expansion - 0.2-0.4%
Compressive strength 2hrs - 10MPa
Compressive strength 24hrs - 24MPa
44
Q

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Stone

A 
P/W ratio = 100g/25-40ml
Initial setting time - 7-15 mins
Linear expansion = 0.08-0.1%
Compressive strength 2hrs - 30MPa
Compressive strength 24hrs - 70MPa
45
Q

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Improved Stone - Densite

A 
P/W ratio = 100g/20-40ml
Initial setting time - 5-12 mins
Linear expansion = 0.05-0.07%
Compressive strength 2hrs - 35MPa
Compressive strength 24hrs - 80MPa

Dental Materials Y2 (13 decks)

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