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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
Describe the mechanism of the setting reaction of calcium sulfate hemihydrate with water
- 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
What is the role of unchanged gypsum dihydrate in setting mechanism
Acts as a crystallisation nuclei for growth of dihydrate crystals
27
How much water is used for 100g of different gypsum product and why do we use excess
Plaster - 50ml/100g Stone/improved stone - 20ml/100g Excess because otherwise the mix would be too viscous
28
How can increasing the proportion of water used in the setting reaction affect the reaction
Slower setting time - longer for saturation, the mix will be runny and the model will be weaker
29
How can increasing the proportion of powder used in the setting reaction affect the reaction
Difficult to mix and product will result in porosity within the set material
30
What does increasing the spatulation time (mixing) in the setting reaction do to the reaction
Reduction in setting time - break up formed crystals to form new sites for crystal growth, also increases the setting expansion
31
What does increasing the temperature in the setting reaction do to the reaction
- Little change between 0 and 50C - More than 50C = gradual retardation occurs - At 100C = no reaction
32
What methods are there to measure setting time
- Vicat needles | - Gillmore needles
33
What different Gillmore needles are there and how are they used to measure setting time
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
What is a visual sign that the material has reached its initial set
Surface of material loses its gloss or shine
35
What is the rough working and setting time for model plaster
2-3 mins working time | 5-10 mins setting time
36
What factors can increase setting expansion
- Powder/liquid ratio | - Spatulation time
37
What causes setting expansion in gypsum products
Crystals impinge on each other as they grow and push each other apart = large empty spaces between crystals = porosity = 0.6% by volume
38
What is hygroscopic expansion
When crystals grow freely in water and expand By immersing material in water while setting the setting expansion increases
39
What happens when a material is left to set in air
If set in air the surface tension of the free water in the material tends to draw the crystals together
40
Name some of the properties of gypsum products
- 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
Describe the tensile strength of different gypsum products
- Plaster tensile strength = v low | - Stone tensile strength - double plaster - crown and bridge models and dies
42
Describe the surface hardness and scratch and abrasion resistance for different gypsum products
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
What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Laboratory Plaster
``` 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
What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Stone
``` 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
What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Improved Stone - Densite
``` 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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