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Flashcards in 10-Midterm Deck (39)
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31

1)Sodium Phosphate
2) Filler Particles

1)-slows down reaction of Alginate Impression
-it reacts with Ca, delaying the setting reaction
2) strengthens the set gel (zn oxide)

32

Valuable Properties of Alginate Impression

-good elasticity---removed easily
-good elastic memory
-easy to use
-sets quickly
-good accuracy
-inexpensive


---restorations, ortho, peds, oral surgery

33

Invaluable Alginate Impression

-good but not great accuracy, cant reproduce fine details, not recommended for fix restorations
-elastic memory doesnt remain stable over time
-water makes up dispersion phase that evaporates over time---shrinks and distorts impression
-water will form exudate(ooze) on to surface of alginate impression: syneresis
-if soaked in H20 it will absorb it and distort anyway via imbibition

34

Dental Casts

-electroplated casts
-epoxy resin
-digital

35

Gypsum Casts over the other Casts

-easy 
-compatible w/ physical impression materials
-inexpensive

36

Gypsum

-grinding, heating, and pressure
-drives off portion of H20 found in gypsum converting it from dihydrate to hemihydratecalcining
---summary---gypsum= heating dihydrate

37

1- a- hemihydrate
2-b-hemihydrate

1-die stone or improved stone---calcining process results in gypsum particles that are small, smooth, and dense (microstone)
2-dental plaster---calcining process results in gypsum particles that are large, porous, and irregular (mounting stone)


---both are chemically the same, but the makeup of these particles= diff dental casts

38

Making Dental Casts

-Added H20 causes dissolution of CaSO4 hemihydrate creating a saturated solution of CaSO4
-no longer calcining
-will eventually be followed be precipitation of CaSO4 dehydrate crystals w/ heat generation (exo) equivalent to heat used to original calcining process of gypsum
-crystalization= expansion 
-continues until no more CaSO4 dehydrate can precip= cast
-but H20 leaves voids and that reduces the strength of cast

39

1-a hemihydrate

2-b hemihydrate

1-require less H20 as part of setting so the cast=harder and more resistant to fracture/abrasion
2-require more H2O as part of setting so casts= softer and more easily abraded/fractured

---altering amt of H20 added to powder, alters expansion of gypsum(not good idea)