Gypsum Flashcards
What are the uses of gypsum
cast (plaster/stone)
die (stone / improved stone)
mould material (stone)
investment binder (stone)
What determines the type of gypsum
The manufacturing conditions
dihydrate –(heat)–> hemihydrate
open vessel
autoclave
CaCl2
What are the types of gypsum
Plaster ( beta-hemihydrate)
Dental stone ( alpha-hemihydrate)
Densite (improved stone)
What does manufacturing CaSO43H2O in an open vessel produce
plaster
What does manufacturing CaSO43H2O in an autoclave produce
stone
What does manufacturing CaSO43H2O with CaCl2 produce
Improved stone
What is the structure of plaster
large porous, irregular crystals
What is the structure of stone
non-porous, regular crystals, requires less water
What is the structure of densite (improved stone)
compact smoother particles
What is the gypsum setting reaction
reverse of manufacture
(CaSO4)2 .H2O + 3H2O —–> 2CaSO4 . 2H2O
What are the mixing ratios for plaster and stone
Stone - 20/35ml per 100g
Plaster 50/60ml per 100g
What is the setting process
Hemihydrate dissolves as dihydrate forms and the crystals precipitate on impurities
Hemihydrate fully disolves
Initial set - dihydrate crystals come into contact and expansion starts (properties of a weak solid)
Final set - strong and hard enough to be worked strength continues to develop
How are initial and final setting times measured
Using Gilmore needles
What role does water play in setting
During setting excess water is trapped in the powder mass
On completion of setting excess water evaporates and voids are produced
Is gypsum reliable for reproducing surface details
Ideally GYPSUM reproduces the fine detail on the impression material
BUT gypsum is inherently porous, resulting in a relatively rough surface – about 28 to 40um
What does increased spatulation do
increased spatulation breaks down growing crystals
fragments act as nuclei of crystallisation
more growing crystals -come into contact sooner
Hence
decreased setting time
increased expansion
Which is the strongest form
Densite (imrpoved stone)
What is the compressive strength of gypsum
28MPa - 38 MPa
What is the compressive strength of gypsum after 24 hrs
75MPa
What are the properties of Gypsum
Surface hardness low (not ideal - easy abrasion)
Setting time - varied
Expansion on setting should be low
Brittle
Compatable with impression materials
What does increasing and decreasing the powder in ratio do
increasing powder
-more nuclei of crystallisation per unit volume
-crystals come into contact sooner
-faster set & greater expansion
decreasing powder
-converse
What effect does temperature have on setting
Conflicting actions
-Rate of diffusion of ions increases with temp
-While solubility of the hemihydrate decreases as temp increases
What is gypsums flexural strangth
15-20 MPa
What are the advantages and disadvantages of gypsum
Advantages
-dimensionally accurate and stable
-low expansion (<0.1%) of stone/densite
-good colour contrast
Disadvantages
-low tensile strength
-poor abrasion resistance
-very brittle
-surface detail less than elastomer impression (~20um groove)
-poor “wetting” of some impression materials
What effect does potassium sulphate and borox additives have on setting
Potassium Sulphate - decreases setting time
Borox - increases setting time
