Intro to Dental Midterm PDF

Intro to Dental quiz 1 Calcium sulfate hemihydrate CaSO4 ½ h2O powder Calcium sulfate dihydrate CaSO4 2H2O is hardened stone difference is water molecule anhydrous /// anhydrate is stone heated with no water ( weak and crumbly) Plaster = Beta- hemihydrate Stone and die stone...

Intro to Dental quiz 1 Calcium sulfate hemihydrate CaSO4 ½ h2O powder Calcium sulfate dihydrate CaSO4 2H2O is hardened stone difference is water molecule anhydrous /// anhydrate is stone heated with no water ( weak and crumbly) Plaster = Beta- hemihydrate Stone and die stone is Alpha-hemihydrate Die stone is boiled in a 30% calcium solution Water released as steam is water crystallization Products Process of heating gypsum to drive off water is called Calcining Plaster- large irregular, porous crystals stone - smaller denser and regular sized crystals Mixing ratios Plaster = 50-55 ml water stone =32-40 ml Die stone = 22-24 ml Amount of water needed for chemical reaction is the same for all (18.6 ml) w/p ratio 0.5, 0.3, 0.24 Setting reaction Caso4 ½ h2o + 1 ½ h2o = caso4 2h2o += heat Amount of heat released = amount of heat while calcining 25 degrees Expansion through seeds called Nuclei of crystallization Seeds grow “arms” as they set like a snowflake 8-16 mins for stone Controlling setting time Temp w/p ratio Amount of mixing Chemical additives Less water= thicker mix,faster set, more expansion, harder final set Polysulfide- elastomeric Excellent in areas with undercuts Used for dentures Working time 5 mins Setting time 10 mins plus Smells bad High resistance tear(good for undercuts) most flexible elastomer, long shelf life Polyether Two paste system Low molecular weight polyether + catalyst Most hydrophilic used when moisture control is difficult Swells in high humidity Used for crowns bridges partial dentures and implants  Working time 2:30 Setting 5 min Adequate setting time monophase impression Rigid when set (bad for undercuts) absorbs water Silicone Was made as an upgrade from polyether Addition reaction-pvs ○ Two paste system ○ Mixed equal parts ○ No byproduct, no dimensional change ○ 2:15 working ○ Setting 5 ○ Hydrophobic, platinum catalyst reacts with powdered gloves,gic and rubber dam ○ Temp sensitive Condensation reaction ○ Base and catalyst ○ Ethyl is alcohol byproduct ○ Evaporation causes impression to shrink ○ Used for crowns bridges partial dentures and implants ○ Working times 2:30 ○ Setting 9 ○ Hydrophobic Rigid impression materials Impression compound (type 1) ○ Used for crown impressions, or with edentulous arches with little undercuts Tray compound (type 2) ○ Prepare custom trays ○ Natural resins make up the bulk rendering the materials thermoplastic ○ Waxes and stearic acid helps with thermoplastic properties and act as plasticizers Terms imbibition - when liquid is absorbed from a colloid causing increase in volume Syneresis - contraction from a colloid, separation of liquid > causes shrinkage Hysteresis- melting temp of a gel(semi-solid) differs from melting a sol(liquid) Gel- semi solid state of hydrocolloid Sol - liquid state of hydrocolloid Dispersed phase- discontinuous phase that is distributed throughout the dispersion medium Continuous phase-suspension of a colloid. Disperse phased is suspended Dispersed phase + continuous phase= colloid Dispersed phase + continuous phase (water) = hydrocolloid Dimensional stability- when a material maintains its original dimensions when subjected to changes in temp and humidity. Aka resistance to distortion in various storage conditions