Gypsum
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what is a study model / cast
A positive replica of dentition produced from the impression (which is a negative representation of a patient’s dentition)
what are the purposes of a study model / cast
○ Records the position, shape of teeth ○ Aids visualisation / assessment of dentition ○ Enables manufacture of dental prostheses § Eg partial dentures, crowns etc
name the uses of gypsum
• Cast ○ Plaster / stone • Die ○ Stone / improved stone • Mould material ○ Stone • Investment binder Stone
what does the manufacturing determine with gypsum
what type of gypsum is produced
what is the reaction that occurs with gypsum manufacture
CaSO4.2H2O. —[heat]—> (CaSO4)2.H2O + H2O Calcium sulphate Dihydrate –[heat]—> Calcium sulphate hemihydrate
what are the types of gypsum
• Plaster ○ Beta-hemihydrate • Dental stone ○ Alpha-hemihydrate • Densite Improved stone
what does crystalline structure determine
properties
how is plaster (beta-hemihydrate) made and what is it like
• Heated in open vessel * Large porous, irregular crystals
how is dental stone (alpha hemihydrate) made and what is it like
• Heated in an autoclave * Non-porous, regular crystals, requires less water
how is densite (improved stone) made and what is it like
• Heated in presence of Ca and Mg chloride * Compact smoother particles - Smoother then it can be packed in more readily and made more dense
what is the setting reaction
reverse of manufacture (caSO4)2.H2O + 3H2O —> 2CaSO4.2H2O hemihydrate —> dihydrate powder + water = gypsum
what are the mixing ratios
Water Powder Plaster 50-60ml 100g Stone 20-35ml 100g Theoretic18.6ml 100g
why does the theoretical mixing ratios differ from the actual mixing ratios
Practise and experience tells you that these ratios are what are needed to mix the material with water rather than the theoretic ratio Excess water - Needed for a workable mass - Affects properties
what is the setting process for gypsum
1) - hemihydrate dissolves - dihydrate forms - dihydrate solubility low - supersaturated solution - impurities present 2) • dihydrate crystals precipitate on impurities as crystals • more hemihydrate dissolved • continues until all hemihydrate dissolved (until you are just left with dihydrate crystals) 3) initial set - dihydrate crystals come into contact (ie push apart) - expansion starts - properties of weak solid and will not flow - can be carved / manipulated / shaped 4) final set - strong and hard enough to be worked (large dihydrate crystals of different sizes all packed together) - strength continues to develop
what are initial and final setting times measured using
gilmore needles
where is excess water found during setting
trapped in the powder mass
what happens to the excess water on completion of setting
excess water evaporates voids are produced (porosity) this brings a problem as any porous material is relatively weak
what is the compressive strength of gypsum
Compressive (~20-35MPa) This is around a tenth of the compressive strength of composite resin
what is the hardness like of gypsum
• Hardness - low ○ Remember hardness is not strength as it refers to just the surface ○ With gypsum you don’t want the surface to scrape parts off or be worn off as it is passed between clinic and the lab etc Want the surface to remain in tact but as it has low hardness then you need to handle it with care to protect it from chipping
what are the typical expansion values of gypsum
Plaster = 0.2-0.3% Stone = 0.08-0.1% Densite (improved stone) = 0.05-0.07% expansion should ideally be 0%
explain the strengths of gypsum
• Develops over 24 hour period • Stone stronger as it requires less water for workable mix (less porous) • Improved stone (deniste) strongest (~35MPa) but Plaster is weakest (~12MPa)
what happens to the setting time and expansion of gypsum if you increase powder, spatulation and impurities
setting time decrease expansion increases
what effect down chemicals have on the setting time and expansion
can either increase or decrease the setting time but will always decreases the expansion
how does spatulation affect the gypsum setting
• Increase spatulation breaks down growing crystals • Fragments act as nuclei of crystallisation ○ Centres around which the crystallisation takes place ○ More crystals come into contact more regularly • More growing crystals - come into contact sooner • Hence ○ Decreased setting time ○ Increase expansion Spatulation = how you mix