Biomaterials Exam - Impression Materials Flashcards
What are the requirements of ideal impression materials?
- low cost
- long shelf life
- biocompatibility
- pleasant to patient
- dimensionally stable
- good handling properties
What are the 3 main classes of impression materials? Name the 2 subtypes of each class.
- rigid: ZOE and plaster
- plastic: wax and compound
- viscoelastic: hydrocolloid and elastomers
True or false: Alginate is an irreversible hydrocolloid impression material that is water-based.
true
What is formed when alginic acid is soluble in water?
sol which resembles a solution, but is made up of colloidal particles dispersed in a liquid
What ion cross-links alginate?
calcium
What two parameters control the setting reaction of alginate?
- temperature controlled (reversible)
- chemically controlled (irreversible)
What is dimensional stability of alginate? How can you minimize it?
- Because it is a gel, it undergoes shrinkage or expansion upon loss or gain of water.
- minimize it by storing it in 100% humidity and by pouring quickly after removal from mouth
What is syneresis?
loss of water to the surroundings (part of dimensional stability)
What is imbibition?
pick up of water from the surroundings (part of dimensional stability)
What are the advantages of water-based alginate?
- economical
- easy to use
- quick setting
- fair taste
- hydrophilic
- can displace blood and saliva
- stock trays
What are the disadvantages of water-based alginate?
- limited detail reproduction
- low tear resistance
- single pour only
- quick pouring required
- low dimensional stability
Is polysiloxane impression material normally a hydrophilic or hydrophobic material?
hydrophobic
How can polysiloxane material be made hydrophilic?
- adding a surfactant
- chemical incorporation of hydrophilic moieties into the silicone backbone
Why is the hydrophobicity of polysiloxane a problem in taking impressions and transferring detail to a model?
- the oral cavity is hydrophilic so it may cause some difficulty in registering oral structure
- when pouring the stone slurry into the impression, it may cause voids since the slurry is water-based
What reaction forms the cross-links of polysiloxanes?
hydrosilation reaction
Does polysiloxanes have good or bad dimensional stability?
good
What is the role of a surfactant in polysiloxanes?
improves (along with polyethers) detail registration and detail transfer by making the polysiloxane more hydrophilic
Of these two hydrophobic compounds (polysiloxanes and polyethers), which is more hydrophilic?
polyethers
Do polyethers have good or bad dimensional stability?
good
With what is gypsum cured?
water
Through what process is gypsum dehydrated?
calcination
What is the dehydration/rehydration reaction that is responsible for the synthesis of gypsum and the curing of it?
(CaSO4/0.5H2O) + 1.5H2O –> (CaSO4/2H2O) + 3900 cal/g*mol)
- hemihydrate is converted back into a dihydrate via an exothermic reaction
The properties of gypsum are dependent on what 2 characteristics? How does this affect the gypsum?
- density and crystal structure
- if the crystals are dense, regularly-shaped, and relatively non porous, the material will not require much water and will be stronger (dental stone - alpha form)
- if the crystals are less dense, irregularly-shaped, and porous, it will require more water to mix and the material will be weaker (dental plaster - beta form)
Which type of gypsum (dental plaster or dental stone) is used to make dental models?
dental stone
How is Triad (denture base material) cured?
visible light cured (VLC)
How are provisionals cured?
chemically cured (by mixing)
Why is there a difference in composition between denture bases and provisionals?
- denture bases need to last a long time, have fillers, and dimethacrylates that cross-link
- provisionals are more temporary and need to be removed, no cross-linking; chemistry is very similar to the “cold cure” Triad (denture base)
Are mouth guards made of thermoplastic or thermoset material?
thermoplastic
low softening point, thermally processed with hot water, vacuum formed so it required a dye
What controls the properties of the thermoplastic mouth guard material?
copolymer of ethylene and acetate
