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DM > Impression Materials > Flashcards

Impression Materials Flashcards

1
Q

why is the gypsum water ratio important?

A

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

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2
Q

define viscoelasticity

A

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

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3
Q

how to reduce compression set?

A
  • 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
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4
Q

what is compression set?

A

permanent deformation of a material when a force that was applied to it was removed

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5
Q

why is working time important?

A
  • 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
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6
Q

what are the desirable properties of an impression material?

A
  • 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
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7
Q

study page 6

A

ye

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8
Q

how are impression materials categorized on viscosity?

A
  • mucostatic –> impression plaster, agar, light body elastomer
  • mucocompressive - impression compound
  • pseudoplastic – addition silicones
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9
Q

what are the advantages of impression compound

A
  • non toxic and non irritant
  • hardens in a good time
  • compatible with gypsum
  • no separating medium required
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10
Q

what are the disdv of impression compound?

A
  • cannot record fine detail
  • distorts over undercuts
  • shrinkage upon cooling
  • dimensional change on storage
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11
Q

what are the properties of kemco impression compound?

A
  • 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??
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12
Q

what are the advantages of kemco impression material?

A
  • can be reused several times
  • innaccurate portions cna be remade without having to make entire impression
  • acuracy can be improved by flaming surface material
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13
Q

what ar ethe disadvantages of kemco impression compound?

A
  • 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
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14
Q

when would one use ZOE?

A
  • 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
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15
Q

what factors control setting time of ZOE?

A
  • 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
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16
Q

what are the properties of ZOE?

A
  • good consistency and flow
  • good detail repro
  • rigid and having good strength
  • good dimensional stability
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17
Q

what are the advantages of ZOE?

A
  • 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
18
Q

what are the disadv of ZOE?

A
  • requires special tray for impression making
  • sticky in nature and adheres to tissues
  • burning sensation of eugenol –> tissue irritation
  • very inelastic
19
Q

what are non-eugonal pastes for?

A
  • alternatives to ZOE for pts who are sensitive to the burning and tissue irritation cause by ZOE
20
Q

what are the uses of agar hydrocolloid?

A
  • as a tissue conditioner
  • full mouth impression without deep undercuts
  • cast dupes
21
Q

what are syneresis and imbibition of agar hydrocolloids and why is it significant?

A
  • 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

22
Q

what is the use of alginate?

A
  • orthodontic and study models
  • prelim impressions
  • cast: to fabricate mouth cards
  • bleaching and fluroide trays
23
Q

what are the properites of alginate?

A
  • 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
24
Q

what composes elastomeric impression materials?

A
  1. flexible matrix - (continuous phase)
    • polymer
    • x-linking agent
    • curing agent
    • modifiers
      • accelerators
      • retarders
      • plasticizing
      • flavoruing agents
      • colourants
  2. fillers (dispersed phases)
25
Q

polysulfides and condesnation silicones have the LARGEST DIMENSIONAL CHANGES during setting

A

ye

26
Q

what are the types of non-aq elastomers?

A
  • polysulphide rubber
  • polyether
  • silicone rubber
27
Q

what are the adv of polysulphide rubber

A
  • most hydrophilic of non-aq impression materials

- best tear strength and elasticity

28
Q

what are the disadv of of polysulphide rubber

A
  • brown (non esthetic)
  • stains clothes
  • produces a strong odor
  • non-well acepted by most pts
  • long tooth prep
29
Q

what are the advantages of polyether rubber?

A
  • stable for long periods of time if stored dry

- excellent impression accuracy and dimensional stability

30
Q

what are the properties of polyether rubber?

A
  • 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
31
Q

polyether has low tear resistance

A

ye

32
Q

what is silicone rubber made of?

A

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

33
Q

which impression materials require tray adhesives?

A
  • condensation silicone rubber
34
Q

what are the disadv of silicone rubber?

A
  • polymerization shrinkage
  • limited shelf life (silica fillers tend to settle out of material over time)
  • poor wetting characteristics
  • so syneresis or imbibition
35
Q

what are the adv of silicone condensation reubber?

A
  • no odor
  • sets faster
  • does not stain clothing
36
Q

how is silicone rubber/VPS made?

A

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

37
Q

what is the polymer, x-linking agent, modifers, and fillwers for addition-cured silicone rubber?

A

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
38
Q

for ADDITION-CURED SILICONE, impressions do not need to be poured immediately

A

ye

39
Q

study page 29

A

ye

40
Q

What does compression set depend on?

A

Strain and time;

Strain depends on shape and tilt of tooth; thickness of mayerial between tooth and tray (thicker = reduced strain)

DM (18 decks)

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