Flashcards in impression materials: elastomeric dental impressions Deck (56):
non aqeous elastomers are a compound of what?
what are the three sub groups on aqueous elastomers?
what are the two types of reactions that occur from silicon
condensation reaction and an addition reaction
elastomeric impression materials offer potential solution to the two main problems associated with hydrocolloids, what are they?
1. poor tear resistance
2. dimensional stability
elastomeric impression materials can be stretched and recover their original dimensions (to a limit)
T OR F
what are the uses for elastomeric impressions?
1. fixed and removable prothetics
2.anay situation where high accuracy is required
what are the mixing options for elastomeric materials?
-2 pastes on mixing pad
-2 pastes in mixing gun
-2 pastes in mixing machine
what is the sructure of elastomeric imp?
polymeres with large macrmomolecules with a folded or coiled structure with a small number of primary bonds
how does setting occur?
by chain lengthening polymerization and chemical cross-linking by either condensation or addition reactions
what is vicscoelasticity?
the ability of a material to strain instantaneously
Manipulation: compression set
measure of the ability of a material that was compressed to return to its original shape
manipulation: compression set-what does it depend on?
-time and strain
what does strain depend on?
-the shape and tilt of the tooth
-the thickness of material between the tooth and the tray-thicker sections reduces strain
how to reduce compression set?
1. use recomended mixing time
2.maximize tray tooth distance
3. material must be completely set before removal
general formulation for elastic impression materials
1. flexible matrix : polyemere, crosslinking agent, curing agent (catalyst or initiator), modifiers:accelerators, retarders, plasticizers, flavouring agents, colorants
2.filler (dispersed phase)
polyemere and filled polymere: shrikage
-filled polymer-low shrinkage
what do fillers do?
change the viscosity of polymers
fluid and dough like material: fillers
-fuild materials use lots of fillers, dough like materials use very little fillers
why do we use fillers in elastomeric impressions
-because the matrix is a polymer and when polymers react (set) it contracts (shrinks) if you add filler to the material it reduces contraction
what is the cause of elastomeric impression materials shrinking?
polymerization and materials with reaction by products underfo additional contraction
which type of elastomeric impression materials have the largest dimensional change during setting?
-what type of reaction do they have?
-condensation reaction-produces byproduct
which materials have smaller dimensional change during setting?
2. addition silicones (smallest chnage)
-addition reactions produce no biproducts
elastomeric impressions: management of shrinkage-what type of technique is it?
-2 step technique- load tray with heavy bodied IM and cover prep with light bodied IM from syringe
during setting shrinkage, distortion of impression is towards the tray.
therefore the cast will be oversized
elastomeric materials: management of distortion during tray removal
-use snap removal it uses less stress and strain
why do you need to use both a fluid and dough like consistency type of material
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
types of none aqueous elastomers
1. polysulphide rubber
2. polyether rubber
3. silicone rubber : condensation-cured and addition cure silicone: polysilloxane, rubber (vinyl)
-first elastomeric rubber to be developed for dentistry
contraindications of polysulphide rubber
3.produces stong odor
4. not well accepted by patients
indications of polysulphide rubber
1.long tooth preparations
2.most hydrophilic of non-aqueous elastomers
3. best tear stength and elasticity
-type of reaction
condensation reaction: bad because you get biproducts thereofre the product evaporates and the product shrinks more
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
1.polysulphide polymere----->crosslinked polysulphide polymere + water---->water is created and stays inside the polymere
2.filler= TiO2, zinc sulfate
polysulphide rubber:what does it work best with?
custom acrylic impression tray with adhesive
-mix with a stiff spatula
-wait 20-30 minutes after tray removal (stress relaxation)
polyether rubber: cons
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
polyether rubber: pros
-very stable-lasts a long time
polyether rubber : reaction
-good because it produces no byproducts--addition reaction
polyether rubber : filler
polyether rubber-setting reaction
exothermic-affected by temperature
polyether rubber: Manipulation and technique considerations
-good impression accuracy and dimensional stability
-very stiff and difficult to remove
what is added to reduce polyether stiffneness?
two kinds of silicone elastomer impression materials
1. polymerization by condensation reaction-biproducts produced
2. vinyl types, polymerization by ADDITION reaction- no bi products
commercially available silicone rubber
-very toxic because of the dibutyl tin dilaurate catalyst
silicone rubber condensation cured-silicone
formation of biproduct
-base paste= dimethyl sioxane polymere
-catalyst paste: tin octoate-cataylst
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?
3.organo tin compounds
silicone rubber condensation-cured silicone: pro and con
see slide 8
silicone rubber addition cured silicone: what is it called?
Vinyl polysiloxane (VPS)
silicone rubber addition-cured silicone: does it generate a bi-product?
-the catalyst is consumed as it provides radicals-may generate gas as a by-product
silicone rubber addition-cured silicone: how do you eliminate the hydrogen gas?
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
silicone rubber addition-cured silicone-why is the impression not subject to distortion?
reaction is complete at the time of impression removal
silicone rubber addition-cured silicone:how long do you have to wait for elastic recovery?
does the impression need to be poured imeddiatlye ie)pouring gypusm?
side effects of the silicone rubber addition-cured silicone reaction
-decomposition of crosslinking agent (Hydrogen gas)
-newer materials (palladium or platinum) catch the hydrogen before it can create hydrogen bubbles
properties and clinical significance of elastomeric materials
2.working and setting time
8.compatibility with die material
9. patient/opperator appeal
properties and clinical significance of elastomeric materials
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
elastomeric materials: 4 major sources of dimensional inaccuracy are:
1. compression set'
2.evaporation of constituents
4.water absorption -polythers only