DML9: Elastic impression materials Flashcards Preview

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Flashcards in DML9: Elastic impression materials Deck (47)
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In elastomers how do the polymers react

The two paste-polymers cross-link via an addition or condensation reaction


What are the ideal properties of an impression material

1. Pleasant odour, taste, colour
2. Adequate shelf-life
3. Non-toxic irritants
4. Economically commensurate
5. Easy to use
6. Good setting requirements
7. Readily wets oral tissues
8. Elastic properties without permanent deformation after strain
9. Adequate strength so it doesn't break on removal
10. Dimensional stability over temp and humidity ranges
11. Compatible with cast and die materials
12. Accurate
13. Readily disinfects


When are elastomers used

- To produce accurate replica of teeth and surrounding tissues
- For construction of full/partial dentures, crowns, bridges and inlays

This is due to their strength and dimensional stability being better than alginates and agar (however, they are more expensive)


How are elastomers dispensed and when does the reaction begin

Two-paste system (the viscous pastes sets into an elastic material upon mixing) This is done via tubs of putty or tubes of the material

- the cross-linking reaction begins as soon as the two components interact via condensation/addition reaction

One-paste system begins reacting as soon as LED light unit is directed


What is the purpose of double barrel cartridges and Pentamix pouches for the dispensing of elastomers

These avoid incomplete mixing and prevent the introduction of air bubbles into the elastomer as these will be replicated in the casts and therefore affecting the products made from casts


Why does a condensation reaction of elastomer affect the dimensional stability of the set material

Because there is elimination of a small molecule which causes the shrinking of the set material

- Addition reaction does not result in this as there is no by-products so it is more dimensionally stable


What is a cross-link

Covalent bond linking one polymer to another


Outline how different paste viscosities are matched for elastomer materials

- Light-bodied wash + Putty/ High viscocity
- Heavy/medium (mono phase) + Light-bodied


Why is wash light-bodied

Because it contains the least amount of filler


Why does putty have to be hand mixed

Because it cannot be extruded through a nozzle as it contains the maximum amount of filler making it very viscous


Why can a light-bodied/wash material be used in isolation

Because it doesn't have the strength and body required as it contains little filler - this means it will tear easily when removing the tray from the mouth


Why is a light-bodied material beneficial when using a heavy-bodied base

Because it records the fine details and the heavy-bodied base can act as a support to the light material which is adhered to it


Outline the twin mix/ one stage technique for elastomer impressions

1. Syringe the wash material around the teeth
2. Load putty into the tray and insert immediately into mouth after syringing the wash material
3. Putty sets faster than the wash so make sure to leave some of both materials on the side to determine final set


Outline the double impression/ two stage technique for elastomer impressions (no-spacer method)

1. Use the putty to take a rough impression of the mouth
2. After preparing teeth syringe the wash material onto them and reinsert the putty impression over the wash


Why are relief channels needed in the double impression/ two stage technique for elastomer impressions

So that the putty can hold the wash in palce


Outline the double impression/ two stage technique for elastomer impressions (spacer method)

1. Place polythene sheet over teeth
2. Use putty to take impression over the polythene sheet
3. After set, remove the putty impression and the polythene sheet
4. Place wash material in the space provided by the putty impression and set in mouth


Why are relief channels not needed when the spacer method is used in the two stage/ double impression technique

Because the polythene sheet acts as a spacer which removes the need for relief channels


What is the dual viscosity technique for impression taking

This is where a heavy and light bodied paste is used in combination
- heavy/medium bodied material is extruded into tray
- light bodied material is syringed around teeth/ tray
- tray is seated in mouth and material sets
- light bodied material can flow into undercuts and record fine details
- heavy bodied is more rigid so acts to support the light bodied


What are the disadvantages of one and two stage techniques for elastomer impressions

One stage: both materials have to be mixed simultaneously and both of these are two-paste systems

Two stage: this takes more time to do


Give an example of a condensation silicone and the nature of these materials

Poly(dimethyl siloxane) = PDMS

These are hydrophobic materials so the mouth must be dry when taking impressions otherwise there will be many defects


Outline what is in both pastes in a condensation silicone

Poly(dimethyl siloxane)

1. Base paste
- silicon polymer with terminal OH
- inert filler

2. Catalyst paste
- tetraethocy orthosilicate (cross-linking agent)
- dibutyl tin dilaurate (catalyst)
- inert filler


What is eliminated in the setting of poly(dimethyl siloxane)



What are the advantages of Poly(dimethyl siloxane)

Condensation silicon elastomer impressions are
- stonier and have better dimensional stability
- more elastic so recover from deformation rapidly
- have adequate tear strength
- have adequate elongation at break


What are the disadvantages of Poly(dimethyl siloxane)

Condensation silicone elastomer impressions
- shrink in air
- they are hydrophobic so detergents need to be used to confer a degree of wettability
- can expand in disinfecting solution when too much detergent is incorporated
- mouth needs to be as dry as possible
- erratic setting behaviour when a liquid catalyst is used
- limited shelf-life


Why is it important that too much detergent isn't used along with condensation silicones

Because the detergent is hydrophilic so if there is too much used, the material will expand in the disinfecting solution


Give an example of an addition silicone and describe its nature

Polyvinyldimethylsiloxane = PVDMS

This is a hydrophobic material


Outline what is in both pastes of an addition silicone


1. Base paste
- silicone polymer with terminal vinyl groups
- inert filler

2. Catalyst paste
- silicone oligomer with Si-H groups (cross-linking agent)
- platinum salt catalyst (required for set)
- inert filler


What is an oligomer

Low molecular weight polymer


What are the advantages of addition silicone materials for impressions


1. Best dimensional stability
2. No by-products as set by addition cross-linking
3. Good elastic recovery from deformation


What are the disadvantages of addition silicone materials for impressions


1. When pouting, water in paster/stone reacts with unreacted Si-H groups to release hydrogen, giving a porous model (to avoid this, pour impression after 30 mins)
2. % Elongation and tear strength is less than that of condensation silicones but still adequate
3. Setting can be impaired by rubber gloves (S poisons Pt catalysts)
4. Hydrophobic material so detergents are needed for wettability