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