Impression materials( hydrocolloid alginate,elastomeric impression materials) Flashcards
what is the function of an impression material
to accurately record the dimensions of oral tissues and their special relationships
is an impression a negative or positive reproduction
negative reproduction
give an eg of a + reproducion
model/cast-if large areas of the teeth are involved)
die-when a single or multiple tooth preps arerecorded-
what are the 2 classifications of impressions
flexible or rigid at the time of removal
when can a rigid material be used
it is limited to tooth prepared for a full crown or an edentulous arch
name one of the most widely used aqueous dental impression materials
alginate
name 5 advantages of alginates
- ease of mixing and manipulation
- low cost
- flexibilityof set impression
- high accuracy
- min needed equipment
what are the 2 disadvantages of alginates
- low tear strength
2. they do not transfer much detail to gypsum dies like agar or elastomeric impressions do.
what is the time of setting in the rxn controlled by
the amount of sodium phosphate present in the alginate powder.
what is the sodium phosphate called
a retarder because it delays the setting time
why are these materials referred to as irreversible
because once the paste sets into a gel, the process cannot be reversed.
which ingredient of the alginate powder serves as a retarder
sodium phosphate
which ingredient controls flexibility of impression
silicate powder
what is the relationship between the percentage of permanent deformation and consistency of mix
when thinner mixes are used permanent deformations increases
alginate impressions are less likely to tear during removal from the mouth when removed slowly or rapidly
rapidly
what are the best conditions to store alginate in
high humidity
What happens tot the set alginate impression if stored in air
water will evaporate and it will shrink
what happens to a set alginate impression is placed in water
it will absorb it and expand
what is the spatulation time of an alginate
45 sec
how can we avoid the patient to gag while taking a max impression
the posterior of the tray must be seated first followed by the anterior which avoids a large portion of the alginate to flow into the palatal area
define elastomeric impression materials
they are flexible crosslinked polymers when set
what is the main reason for the domination of elastomeric impression materials in the market
1.their greater accuracy
2.dimensional stability with time
3.and ability to record detail
as compared with the hydrocolloid materials
name 4 elastomeric impression materials
- polysulfides
- condensation silicones
- polyethers
- addition silicones
what is also known as vinyl polysiloxanes (VPS)
addiition silicone impression materials
addition silicone impression materials were developed as an alternative to which two materials
- polysulfides
2. condensation silicones
what is the catalyst in addition silicone impressions
chloroplatinic acid
the addition reaction occurs between which two groups
vinyl and hydrogen groups
what is the final product in addition silicone impression reactions
rubber silicone
why is addition silicone impressions not subjected to dimensional changes with time
because no volatile byproducts (water or ethanol) is formedin this reaction
what product is added to the product to permit the immediate pouring of gypsum/epoxy dies
a hydrogen absorber, palladium, because when H is released gradually from the set impression it produces bubbles in gypsum dies.
what are 2 noteworthy improvements of the additin silicones over condensation silicones
- less dimensional change
2. higher elastic recovery
why should latex rubber gloves not be worn during the manipulation of addition silicone impressions
because components in the latex rubber will retard or prevent the setting by poisoning the platinum catalyst
what are 2 disadvantages of polyether impression materials
- poor taste
2. limited dimensional stability over time.
how are polyether impression materials supplied
base and catalyst system
base: polyether
catalyst: sulfonic ester
which impression material is not commonly used in the dental office but rather as an accurate duplicating material in the dental labratory
condensation silicone impression material
what byproduct is produced that contributes to the rather high shrinkage durng the first 24 hours after settin of condensation silicone impressions
ethyl alcahol
what impression material is also known as rubber bas
polysulfide impression materials
how are polysulfide impression materials supplied
as two pastes, 2 2 tubes each conatining a base and an accelerator respectively
how many types are available for polysulfides and how are they classified
3, according to viscosity, low, medium and hiigh viscosity
what type of polysulfide is used for denture impressions
low-viscosity polysulfides
what are dentures
are prosthetic devices constructed to replace missing teeth, and are supported by the surrounding soft and hard tissues of the oral cavity.
what doe the base-catalyst systems in polysulfides contain respectively
BASE: 80 % low-molecular weight organic molecule whith reactive mercaptan ( -SH ) groups
20% reinforcing agents such as silica
CATALYST : contains compounds which causes the mercaptan groups tp react to from polysulfide elastomer
what is the catalyst of polysulfides carries in
an inert oil ( dibutyl )
what is the most common catalyst used in polysulfides
lead dioxide which causes the paste to be dark brown or grey
when copper hydoxide catalyst is mixed with the white base paste of polysulfides what is the resulting color
blue-green
