Gypsum Products Plaster and Stone Flashcards
What is Gypsum
Calcium Sulphate Dihydrate = (CaSO4)2H2O
What can gypsum based materials be used as
- 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?*
What is a positive replica
When a mould or impression i made of the study object and then the mould is filled with a permanent substance
What is a negative replica
Inverse representation of the original specimen surface yh lol
How do manufacturers process gypsum
Convert the calcium sulphate dehydrate to Calcium sulphate hemihydrate (just one water molecule)
Depending on the treatment process of gypsum what forms can gypsum hemihydrate be in
- Plaster - soft white powder
- Stone - harder, yellow powder
- Improved stone
What happens to gypsum powders when mixed with water on setting
They convert back to the dihydrate form
What are the differences between different gypsum hemihydrate forms
Chemically identical but physical properties depend on nature of dehydration process or preparation process
Describe the differences in physical properties between Gypsum plaster and stone/improved stone
- Plaster - large, irregular, porous particles
- Stone/Improved stone - small, regular and non-porous particles
- Improve stone - expensive and more scratch resistant cf stone
What are the differences in the product formed when alpha and beta hemihydrates mix with water on setting
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
In what ways do alpha and beta hemihydrates differ
Alpha and beta hemihydrates differ in particle/crystal size and surface area
What are the advantageous features of using gypsum plaster
Softer
Cheap
Easy to use and shape
What are the uses of gypsum plaster
- General purpose material used for mounting models into articulators
- Flasking procedures for complete and partial denture processing
- Basing models
What are the uses of gypsum stone
Used where strength is v important
- Dies
- Models of mouth - +ve replica of individual teeth, in construction of crowns, bridge and dentures
Describe the composition of gympsum products
Hemihydrate - 75-85% Unchanged gypsum - 5-8% Mix of fast set soluble and slow set insoluble anhydrides - 5-8% Impurities - ~4% Accelerators/Retarders - ~4%
What do the accelerators/retarders in gypsum do
These increase or decrease the solubility of gypsum in water and therefore affect its setting time
Name some accelerators for gypsum
2% K2SO4 solution
CaSO4.2H2O
NaCl (<20%)
How does Potassium Sulfate gypsum accelerator work
Reacts with water/hemihydrate = Sygenite
This crystallises rapidly and encourages growth of more crystals
How does CaSO4.2H2O accelerate setting of gypsum
Provides additional nucleation sites
How does NaCl (<20%) accelerate gypsum setting
Provides additional sites for crystallisation - increases reaction rate and reduces observed expansion
Name some gypsum retarders
NaCl (>20%)
Borax
Potassium Citrate (gum arabic, acetates)
How does NaCl (>20%) act as a retarder
Slows down reaction rate by deposition of NaCL on CaSO4.2H2O crystals and preventing growth
AY BAWS CAN I HABE DE NOTE PLZ
All gypsum retarders interfere with crystal formation and affect dimensional change on setting
Describe the setting reaction of Calcium sulphate hemihydrate with water
Forms calcium sulphate dihydrate (less soluble)
Exothermic
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
What is the role of unchanged gypsum dihydrate in setting mechanism
Acts as a crystallisation nuclei for growth of dihydrate crystals
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
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
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
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
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
What methods are there to measure setting time
- Vicat needles
- Gillmore needles
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
What is a visual sign that the material has reached its initial set
Surface of material loses its gloss or shine
What is the rough working and setting time for model plaster
2-3 mins working time
5-10 mins setting time
What factors can increase setting expansion
- Powder/liquid ratio
- Spatulation time
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
What is hygroscopic expansion
When crystals grow freely in water and expand
By immersing material in water while setting the setting expansion increases
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
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
Describe the tensile strength of different gypsum products
- Plaster tensile strength = v low
- Stone tensile strength - double plaster - crown and bridge models and dies
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
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
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
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
