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Dental materials > impression materials: elastomeric dental impressions > Flashcards

impression materials: elastomeric dental impressions Flashcards

1
Q

non aqeous elastomers are a compound of what?

A

rubber molecules

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2
Q

what are the three sub groups on aqueous elastomers?

A

polysulfides, silicones,polyethers

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3
Q

what are the two types of reactions that occur from silicon

A

condensation reaction and an addition reaction

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4
Q

elastomeric impression materials offer potential solution to the two main problems associated with hydrocolloids, what are they?

A
  1. poor tear resistance

2. dimensional stability

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5
Q

elastomeric impression materials can be stretched and recover their original dimensions (to a limit)
T OR F

A

true

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6
Q

what are the uses for elastomeric impressions?

A
  1. fixed and removable prothetics

2. anay situation where high accuracy is required

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7
Q

what are the mixing options for elastomeric materials?

A
  • 2 pastes on mixing pad
  • 2 pastes in mixing gun
  • 2 pastes in mixing machine
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8
Q

what is the sructure of elastomeric imp?

A

polymeres with large macrmomolecules with a folded or coiled structure with a small number of primary bonds

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9
Q

how does setting occur?

A

by chain lengthening polymerization and chemical cross-linking by either condensation or addition reactions

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10
Q

what is vicscoelasticity?

A

the ability of a material to strain instantaneously

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11
Q

Manipulation: compression set

A

measure of the ability of a material that was compressed to return to its original shape

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12
Q

manipulation: compression set-what does it depend on?

A

-time and strain

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13
Q

what does strain depend on?

A
  • the shape and tilt of the tooth

- the thickness of material between the tooth and the tray-thicker sections reduces strain

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14
Q

how to reduce compression set?

A
  1. use recomended mixing time
  2. maximize tray tooth distance
  3. material must be completely set before removal
  4. SNAP removal
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15
Q

general formulation for elastic impression materials

A
  1. flexible matrix : polyemere, crosslinking agent, curing agent (catalyst or initiator), modifiers:accelerators, retarders, plasticizers, flavouring agents, colorants
  2. filler (dispersed phase)
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16
Q

polyemere and filled polymere: shrikage

A

polymer-high shrinkage

-filled polymer-low shrinkage

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17
Q

what do fillers do?

A

change the viscosity of polymers

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18
Q

fluid and dough like material: fillers

A

-fuild materials use lots of fillers, dough like materials use very little fillers

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19
Q

why do we use fillers in elastomeric impressions

A

-because the matrix is a polymer and when polymers react (set) it contracts (shrinks) if you add filler to the material it reduces contraction

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20
Q

what is the cause of elastomeric impression materials shrinking?

A

polymerization and materials with reaction by products underfo additional contraction

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21
Q

which type of elastomeric impression materials have the largest dimensional change during setting?
-what type of reaction do they have?

A
  1. polysulfides
  2. condensation silicone’s
    - condensation reaction-produces byproduct
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22
Q

which materials have smaller dimensional change during setting?

A
  1. polyethers
  2. addition silicones (smallest chnage)
    - addition reactions produce no biproducts
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23
Q

elastomeric impressions: management of shrinkage-what type of technique is it?

A

-2 step technique- load tray with heavy bodied IM and cover prep with light bodied IM from syringe

24
Q

during setting shrinkage, distortion of impression is towards the tray.

A

therefore the cast will be oversized

25
Q

elastomeric materials: management of distortion during tray removal

A

-use snap removal it uses less stress and strain

26
Q

why do you need to use both a fluid and dough like consistency type of material

A

because fluid material provides a more detailed copy but also causes polymerization shrinkage therefore use the dough like material with lots of fillers so it reduces shrinkage. you cant use only dough like material though because you would get no detail

27
Q

types of none aqueous elastomers

A
  1. polysulphide rubber
  2. polyether rubber
  3. silicone rubber : condensation-cured and addition cure silicone: polysilloxane, rubber (vinyl)
28
Q

Polysulphide rubber

A

-first elastomeric rubber to be developed for dentistry

29
Q

contraindications of polysulphide rubber

A
  1. brown
  2. stains clothes
  3. produces stong odor
  4. not well accepted by patients
30
Q

indications of polysulphide rubber

A
  1. long tooth preparations
  2. most hydrophilic of non-aqueous elastomers
  3. best tear stength and elasticity
31
Q

polysulphide rubber

-type of reaction

A

condensation reaction: bad because you get biproducts thereofre the product evaporates and the product shrinks more

32
Q

Polysulphide rubbber (rubber base,mercaptan rubber or thiokol rubber)

  1. what is the equation and what does it create
  2. what is the filler used to reduce shrinkage
A
  1. polysulphide polymere—–>crosslinked polysulphide polymere + water—->water is created and stays inside the polymere
  2. filler= TiO2, zinc sulfate
33
Q

polysulphide rubber:what does it work best with?

A

custom acrylic impression tray with adhesive

  • mix with a stiff spatula
  • wait 20-30 minutes after tray removal (stress relaxation)
34
Q

polyether rubber: cons

A
  1. reaction is affected by heat:exothermic
  2. short working time
  3. stiff, may break stone single teeth when separated from the impression
  4. stable for long periods only if stored dry or else it absorbs moisture and undergos dimensional chnage
  5. allergic hypersensivity
35
Q

polyether rubber: pros

A

-very stable-lasts a long time

36
Q

polyether rubber : reaction

A

-good because it produces no byproducts–addition reaction

37
Q

polyether rubber : filler

A

silica

38
Q

polyether rubber-setting reaction

A

exothermic-affected by temperature

39
Q

polyether rubber: Manipulation and technique considerations

A
  • good impression accuracy and dimensional stability

- very stiff and difficult to remove

40
Q

what is added to reduce polyether stiffneness?

A

plasticizer

41
Q

silicone elastomer:
two kinds of silicone elastomer impression materials
1.original
2.recent

A
  1. polymerization by condensation reaction-biproducts produced
  2. vinyl types, polymerization by ADDITION reaction- no bi products
42
Q

commercially available silicone rubber

A

-very toxic because of the dibutyl tin dilaurate catalyst

43
Q

silicone rubber condensation cured-silicone

A 
formation of biproduct 
-base paste= dimethyl sioxane polymere
filler=silica
-catalyst paste: tin octoate-cataylst
-filler=silica
44
Q

silicone rubber condensation-cured silicone:

  1. what is the polymere?
  2. what is the cross linking agent?
  3. what is the catalyst?
  4. what are the modifiers?
  5. what are the fillers?
A
  1. polydimethyl siloxane
  2. ehtyl silicate
  3. organo tin compounds
  4. colorants, flavorants
  5. silica
45
Q

silicone rubber condensation-cured silicone: pro and con

A

see slide 8

-pros:

46
Q

silicone rubber addition cured silicone: what is it called?

A

Vinyl polysiloxane (VPS)

47
Q

silicone rubber addition-cured silicone: does it generate a bi-product?

A

-the catalyst is consumed as it provides radicals-may generate gas as a by-product

48
Q

silicone rubber addition-cured silicone: how do you eliminate the hydrogen gas?

A

the new materials produced contain scavenger that eliminate the hydrogen gas productions—therefore no biproduct is formed as long as the ingredients are at the correct proportion

49
Q

silicone rubber addition-cured silicone-why is the impression not subject to distortion?

A

reaction is complete at the time of impression removal

50
Q

silicone rubber addition-cured silicone:how long do you have to wait for elastic recovery?

A

20-30 mins

51
Q

does the impression need to be poured imeddiatlye ie)pouring gypusm?

A

no

52
Q

side effects of the silicone rubber addition-cured silicone reaction

A
  • decomposition of crosslinking agent (Hydrogen gas)

- newer materials (palladium or platinum) catch the hydrogen before it can create hydrogen bubbles

53
Q

properties and clinical significance of elastomeric materials

A
  1. viscosity
  2. working and setting time
  3. overall accuracy
  4. dimensional stability
  5. tear strength
  6. rigidity
  7. biocompatility
  8. compatibility with die material
  9. patient/opperator appeal
  10. shelf life
  11. manipulation
54
Q

properties and clinical significance of elastomeric materials

  1. working time
  2. overall accuracy
  3. dimensional stability
A
  1. all types must have at least two minute working times , setting time in mouth is about two-3 minutes longer than working time
  2. excellent accuracy -better than alginate
  3. in general very good accuracy and stability
55
Q

elastomeric materials: 4 major sources of dimensional inaccuracy are:

A
  1. compression set’
  2. evaporation of constituents
  3. polymerization
  4. water absorption -polythers only
56
Q

compression set:typical values for: addition silicone, polyether and condensation silicone, polysulfide

A
  1. addition silicone: less than 0.5 %
  2. polyether and condensation silicone: 1-2%
  3. polysulfide: 3-6%

Dental materials (2 decks)

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