Impression Materials Flashcards
why is the gypsum water ratio important?
depending on the type of gypsum, if too much water is added, compressive strength can be extensively diminished, especially for type IV and V
should have 0.5 W/P for plaster, 0.3 for stone, and 0.22 for die stone
define viscoelasticity
the ability of a material to strain instanaeously like an elastic solid during rapid stretching or to resist shear flow and to strain lineraly over time when a stress is appllied slowly
how to reduce compression set?
- use recommended mixing time
- maximize tray-tooth distance
- material miust be completely set before removal
- snap removal
- provide recovery time (8-30 min) before pouring
what is compression set?
permanent deformation of a material when a force that was applied to it was removed
why is working time important?
- if you go past the working time, the material will be partially set –> therefore it will not compress under the object, the elastic recovery will be too much and the negative will be smaller than the actual object
what are the desirable properties of an impression material?
- have a pleasant taste, odor, and esthetic colour
- not contain any toxic or irritating ingredients
- be economical
- have adequate shelf life for storage and distribution
- be easy to use with minimal equipment
- dimensionalstability
- have adequate strength so that it will not break or tear while removing from the mouth
- possess elastic properties and lesser plastic deformation
- be compatible with die and cast materials
- must be hydrophilic
study page 6
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how are impression materials categorized on viscosity?
- mucostatic –> impression plaster, agar, light body elastomer
- mucocompressive - impression compound
- pseudoplastic – addition silicones
what are the advantages of impression compound
- non toxic and non irritant
- hardens in a good time
- compatible with gypsum
- no separating medium required
what are the disdv of impression compound?
- cannot record fine detail
- distorts over undercuts
- shrinkage upon cooling
- dimensional change on storage
what are the properties of kemco impression compound?
- glass transition temp at 39 degrees centigrade
- fusin temp = 43/5 degrees centigrade
- poor conductors of heat
- coeff of linear expansion is HIGH
- good dimensional stability
- good dimension stability????
- fine repro of detail??
what are the advantages of kemco impression material?
- can be reused several times
- innaccurate portions cna be remade without having to make entire impression
- acuracy can be improved by flaming surface material
what ar ethe disadvantages of kemco impression compound?
- distortion due to poor dimensional stability
- compress soft tissue while making impression
- difficult to remove if there are undercuts
- difficult to record details due to high viscosity
when would one use ZOE?
- cementing and as an insulating medium
- temp filling
- root canal filling material
- bite reg paste
- impression material for edentulous pts
- temp relining material for dentures
what factors control setting time of ZOE?
- particle size of the ZOE powder
- longer mixing time = decrease setting time
- high atmospheric temp and himidity accelerates setting
- setting can be delayed via a cooling mixing slab, spatula, or adding small amount of retarder or oils or waxes
what are the properties of ZOE?
- good consistency and flow
- good detail repro
- rigid and having good strength
- good dimensional stability
what are the advantages of ZOE?
- enough working time to complete border molding
- can be checked in mouth repeatedly without deforming
- registers accurate surface details
- dimensionally stable
- does not stick to cast material–> no separating material
- minor defects can be corrected
what are the disadv of ZOE?
- requires special tray for impression making
- sticky in nature and adheres to tissues
- burning sensation of eugenol –> tissue irritation
- very inelastic
what are non-eugonal pastes for?
- alternatives to ZOE for pts who are sensitive to the burning and tissue irritation cause by ZOE
what are the uses of agar hydrocolloid?
- as a tissue conditioner
- full mouth impression without deep undercuts
- cast dupes
what are syneresis and imbibition of agar hydrocolloids and why is it significant?
- syn = loss of water
- imbib = gain of water when immerse in water
important because can cause dimensional instabilities; to avoid, impressions should be poured immediately
what is the use of alginate?
- orthodontic and study models
- prelim impressions
- cast: to fabricate mouth cards
- bleaching and fluroide trays
what are the properites of alginate?
- alginate has pleasant taste and smell
- highly flexible
- good elasticity and elastic recoveru
- fine repro of detail
- compressive strength
- tear strength
- dimensional unstable
- poorly adheres to tray
- can be toxic bc of silica powder being inhaled
what composes elastomeric impression materials?
- flexible matrix - (continuous phase)
- polymer
- x-linking agent
- curing agent
- modifiers
- accelerators
- retarders
- plasticizing
- flavoruing agents
- colourants
- fillers (dispersed phases)
polysulfides and condesnation silicones have the LARGEST DIMENSIONAL CHANGES during setting
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what are the types of non-aq elastomers?
- polysulphide rubber
- polyether
- silicone rubber
what are the adv of polysulphide rubber
- most hydrophilic of non-aq impression materials
- best tear strength and elasticity
what are the disadv of of polysulphide rubber
- brown (non esthetic)
- stains clothes
- produces a strong odor
- non-well acepted by most pts
- long tooth prep
what are the advantages of polyether rubber?
- stable for long periods of time if stored dry
- excellent impression accuracy and dimensional stability
what are the properties of polyether rubber?
- filled with silica for stability
- rxn affected by heat (is exothermic)
- stiff– may break stone or teeth when separated from from impression
- allergenic hypersensitivity
- negatively affected by saliva, water, and or blood
- important to wait 20-30 minutes after stress relaxation occurs
polyether has low tear resistance
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what is silicone rubber made of?
polydimethyl siloxane in x-linked by alkyl orthosilicate molecules; silica is the PRIMARY filler, and you have ethanol as a by product
rxn: siloxane + ethyl silicate –> silicone rubber +ethyl alcohol
which impression materials require tray adhesives?
- condensation silicone rubber
what are the disadv of silicone rubber?
- polymerization shrinkage
- limited shelf life (silica fillers tend to settle out of material over time)
- poor wetting characteristics
- so syneresis or imbibition
what are the adv of silicone condensation reubber?
- no odor
- sets faster
- does not stain clothing
how is silicone rubber/VPS made?
polymerized by free rads from chloroplatinic acid –> catalyst is consumed to generate free rads, and H2 is created as a by-product
filled with silica
what is the polymer, x-linking agent, modifers, and fillwers for addition-cured silicone rubber?
polymer: double-bond-functional silicone polymer
x-linking agent = chlrotoplatinic acid
modifiers = colorants, flavorants, plasticizers
fillers = silica
- during the first 3-4 hours small gas bubbles may edscape from the surface
- can collect at the bondaries and create bubbles
for ADDITION-CURED SILICONE, impressions do not need to be poured immediately
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study page 29
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What does compression set depend on?
Strain and time;
Strain depends on shape and tilt of tooth; thickness of mayerial between tooth and tray (thicker = reduced strain)
