Impression materials Flashcards
What is the purpose of using impression materials and gypsum materials?
fabricate diagnostic cast
What is the purpose of impression materials?
makes NEGATIVE mold of dental structures
What is the synonymous term for irreversible hydrocolloid?
alginate for impression
What are the characteristics of ideal impression materials?
low cost, long self life, bio-compatible, stable, good handling, pleasant to patient
What are two rigid impression materials?
zinc oxide eugenol and plaster
What are two plastic impression materials?
wax and compound
What are two elastic impression materials?
hydrocolloids and elastomers
What are the limitations of alginate?
not used for crown or bridge impression because of the lack of accuracy
define sol
resembles a solution but is made of colloidal particles dispersed in liquid
What is the product of alginic acid in water?
sol
How can sol transform into gel?
through control of temperature and chemical
Define gel
entangles network of solid colloidal particles in which liquid is trapped in the interstices and held by capillary forces
When transforming from sol to gel, which control is reversible
temperature is reversible
chemically is irreversible
What is the purpose of a reactant in impression material?
allows for cross linking
What is the purpose of a retarder in impression material?
it controls the reaction
What is the purpose of a reinforced filler in impression material?
adds body and strength
What is the purpose of potassium sulfate or alkani zinc fluorides in impression material?
provides good surfaces on gypsum dies
What is the affect of increasing the powder and decreasing the water in alginate?
Lowering the W/P ratio causes an increase in strength, increase tear resistance, decreases working/setting time
How does lowering the temperature affect alignate?
increasing working/setting time
What is mixing time a measure of?
how long it takes to form uniform blend
What is working time a measure of?
how long from the beginning of the reaction until the setting begins
What is setting time a measure of?
how long from the beginning of the mix until the setting reaction is over
Is syneresis the gain or loss of eater to the surrounding?
loss
Is imbibition the gain or loss of water into the surrounding?
gain
What are the advantages of alginate?
economical, easy to use, quick setting, quick setting, fair tasting, hydrophilic, displace saliva/blood, stick trays
What are the disadvantages of alginate?
limited detail reproduction, low tear resistance, single pour only, quick pouring required, low dimensional stability
What are three well known hydrophobic impression materials?
polysulfides
polysiloxanes
polyethers
Doe hydrophobic materials have a high or low contact angle?
high angle because the water beads up
How are polysiloxanes made less hydrophobic?
use of a surfactant or incoperating moieties into the silicone backbone
How is polysiloxanes controled?
by molecular weight of the oligomers
What is the major advantage and disadvantage of hydrophobic impression materials?
advantage = dimensionally stable disadvantage = bubbles because of body fluids
Can polysiloxane be re-poured?
yes
Why should the cast wait to be poured from a polysiloxane
all of the hydrogen gas needs to be freed from the mold
What are the major disadvantages of polysiloxane?
cannot vary setting time, expessive, can produce bubbles in the cast
Is polyether more or less hydrophobic than polysiloxane?
less hydrophobic = better w/ water
What is the disadvantages of polyether?
extreemely stiff, low shear strength, bad taste, can cause contact dermatitis
Are the different types of gypsum chemically different?
No, only the crystal structure is different
What form (1-5) of gypsum is most commonly used?
type 3
What is gypsum composed of?
calcium sulfate and two molecules of water
What is the process of the removal of water to form the hemihydrate?
calcination
Is the gypsum reaction reversible ?
Yes
What does the reverse reaction entail?
it is exothermic
When the solid is mixed with water, it refers back to a dihydrate (powder)
Can the physical properties of gypsum be manipulated?
yes, through the solubility and growth of dihydrate crystals
What is the difference between type 1 and 2 dental plaster?
type one is the mounting stone while type two is the laboratory plaster used as 2nd pour
Why is dental plaster weaker than dental stones?
contained pores, thus needs more water for mixing, and the irregular shapes of particles prevents uniform fitting
What form (alpha or beta) is the hemihydrate form of gypsum?
alpha
Why is dental stone used for making casts and molds?
high crushing strength and abrasion resistance
How is the strength of plaster, stone, and high-strength stone differ?
based on shape and form of the hemihydrate crystals
What is the characteristics do dental stone and high-strength dental stone crystals possess?
more dense and regular in shape
What is the greatest disadvantage of gypsum products?
poor resistance to abrasion
What are some gypsum hardeners?
colloidal silica or synthetic resin
Are manipulation and setting times directly or indirectly related to the W/P ratio?
directly
Are strength and setting time directly or indirectly proportional to W/P ratio?
indirectly
How is the higher setting expansion in dental stone masked?
by the additives used in the fomulation
What is characteristic of TRIAD?
custom tray material, visible light cured, no mixing, and no methyl methacrylate monomers
What is the use of dimethacrylate in TRIAD?
acts as a softener but after curing allows for a stronger cross linked material
What is characteristic of trim and jet materials?
provisional, similar chemistry to TRIAD, COLD cured (via mixing)
Why is trim and jet used in temporaries rather than composite?
much simpler, cheaper, and can be easily manipulated
Does an ethyl group add or decreased rigidity?
decrease,
and more ductile
What is characteristic of mouthguard materials
can absorb high impact, high toughness, low rigidity, thermoplastic
What is a thermoplastic material?
pliable at certain temperatures
What is characteristic of a thermoplastic polymer>
low softening point, processed in hot water
What does a vacuum formed thermoplastic polymer require?
die
