Gypsum Flashcards
ILO 1.6c: have knowledge of the chemical and physical properties as well as the clinical uses of a range of dental materials
what is the purpose of a study model/cast?
- records the position, shape and dimensions of teeth
- aids visualisation/assessment of dentition
- enables manufacture of dental prosthesis (RPDs)
what are the types of gypsum?
- plaster (β-hemihydrate)
- dental stone (α-hemihydrate)
- densite (improved stone)
what are the uses of gypsum?
- cast (plaster/stone)
- die (stone/improved stone)
- mould material (stone)
- investment binder (stone)
what is the equation for the manufacture of gypsum?
manufacture conditions determine type of gypsum produced
how is plaster (β-hemihydrate) produced?
- dihydrate is heated in an open vessel with air readily available
- creates a powder with a large porous and irregular crystal structure
how is dental stone (α-hemihydrate) produced?
- dihydrate is heated in an autoclave (closed vessel)
- produces non-porous and regular shaped crystals
how is densite (improved stone) produced?
- dihydrate is heated in the presence of Ca and Mg (CaCl2, MgCl2)
- produces a more compact, less porous material, with smoother particles
what is the setting equation of gypsum?
reverse of manufacturing process
does plaster or stone require more water per 100g of powder?
- plaster requires more - 50-60ml
- stone - 20-35ml
describe the setting process of gympsum
- hemihydrate dissolves in water and dihydrate forms
- dihydrate crystals precipitate on impurities as crystals and increase in size until all hemihydrate is dissolved
- during initial set, dihydrate crystals grow until they contact and expansion starts
- during final set, crystals are large and different sizes
- gypsum is strong and hard and strength continues to develop
what is the effect of using a higher water:powder ratio than the theoretical ratio?
- excess water allows the powder and water to be mixed easily and achieve the correct consistency to produce an effective cast or die
- during setting, excess water is trapped in the powder
- on completion of setting, excess water is evaporated and leaves voids (porosity)
- evaporation may cause dissolved dihydrate crystals to cement together
what are the properties of gypsum?
- reproduction of surface detail - gypsum is porous and produces a rough surface, reducing accuracy :(
- strength - plaster-stone-densite - compressive strength develops over 24 hour period (less water=stronger) :)
- low surface hardness - easy to abrade :(
- setting time - must be convenient, not too long or short
- expansion on setting - should be low :)
- compatability with impression materials - need to wet the IM to reduce formation of voids and bubbles :/
- brittle - fractures with small amount of strain :(
how does increased powder quantity affect setting time and expansion?
- decreased setting time
- increased expansion
- more nuclei of crystallisation per unit volume so crystals come into contact sooner
how does increased spatulation affect setting time and expansion?
- decreased setting time
- increased expansion
- breaks down growing dihydrate crystals so there are more nuclei of crystallisation
- more crystals can be created and come into contact sooner
how does increased impurity quantity affect setting time and expansion?
- decreased setting time
- increased expansion
- more impurities for dihydrate crystals to precipitate on so more nuclei of crystallisation
- more crystals can be created and come into contact sooner
how does increased temperature affect setting time and expansion?
- can increase or decrease setting time depending on specific temperature range
- does not affect expansion
- water temp from 10-40 degrees reduces setting time
- water temp from 40-80 degrees increases setting time
- water temp above 80 degrees greatly increases setting time
- due to increased rate of diffusion of ions and decreased solubility of hemihydrate with increased temperature
how do chemicals affect setting time and expansion?
- can increase or decrease setting time
- decreases expansion
- borax decreases setting time as deposits calcium borate on dihydrate crystals, restricting growth
- potassium sulfate accelerates setting time as it encourages development of more crystals
what are the advantages of gypsum?
- dimensionally accurate and stable
- low expansion of stone/densite
- good colour contast
what are the disadvantages of gypsum?
- low tensile strength
- poor abrasion resistance
- very brittle
- surface detail less than elastomer impression
- poor ‘wetting’ of some impression materials
