elastomeric impression materials part 1 dr. noland naidoo
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ElastomericImpression Materials
Part 1
Dr. Noland Naidoo
Classification
Impression Materials
Non-elastic
Plaster
Compound
Impression wax
ZnO- Eugenol
Elastic
Aqueous Hydrocolloids
Agar (Reversible)
Alginate (Irreversible)
Non-aqueous Elastomer
Polysulfide
Silicones
Condensation
Addition
Polyether
Introduction
Rubberlike elastic impression materials
Currently referred to as elastomers or elastomeric impression materials.
Elastomeric material consists of large molecules or polymers that are joined by a small amount of cross-linking
Cross linking form a three-dimensional network
The amount of cross-linking determines the stiffness and elastic behaviour of the material
Requirements
Accuracy- must reproduce the entire surface upon which the prosthesis to be made will fit
Elasticity- low elastic modulus and high elastic range
Dimensional stability- the way in which accuracy varies with time after recording the impression
Ease of handling
Good tear strength
Non-toxic and non-irritating
Compatible with die and model materials
Classification
Impression Materials
Non-elastic
Plaster
Compound
Impression wax
ZnO- Eugenol
Elastic
Aqueous Hydrocolloids
Agar (Reversible)
Alginate (Irreversible)
Non-aqueous Elastomer
Polysulfide
Silicones
Condensation
Addition
Polyether
Classification
The ADA considers three types of elastomeric material and each type can be further divided into four viscosity classes:
light body
medium body
heavy body
putty
Polysulphides
Supplied in the form of two pastes- a base and a catalyst
Examples: Permlastic (SDS/Kerr), Coe-Flex, Omniflex (GC America)
The base paste consists of:Polysulphide polymerSuitable filler (titanium dioxide or lithopone)Plasticizer ( eg. dibutyl phthalate)Sulphur – small amount that enhances reaction ( catalyst)
Polysulphides
The catalyst paste:Lead dioxidePlasticizer- chlorinated parafinApprox same amount of filler as in base pasteOleic or stearic acid- retarders- control the rate of setting
Polysulphides
Must be used with a special trayReasonable tear strength and good elastic propertiesUnpleasant odor and tasteWork well in the presence of moisture- moderately hydrophilicUsed commonly for crown and bridge impressionsCasts should be poured within 24hrs of impression but not immediately as to allow for full elastic recoilReported incidences of allergic reaction to patients sensitive to latexImpressions made from polysulfide materials should be poured within 30 minutes following removal from the mouth, because in the first hour they undergo 50% of their 24-hour shrinkage.
Polysulphides
Advantages Disadvantages
Good tear strength Poor dimension stability
Good elasticity Offensive odor
Low cost Stains clothing
Long setting time
Polysulphides
Silicones Condensation silicone impression material
Supplied in a base paste and low viscosity liquid
Examples: Speedex (Coltene/Whaledent), Accoe (GC America)
McCabe and Walls, 2008
Silicones Condensation silicone impression material
• Setting characteristics more favourable than polysulphide• Setting time shorter and elasticity developed earlier• Very hydrophobic- repelled by water or saliva therefore
necessary to dry areas of the mouth for an acurate impression
• Adequate tear resistance for most purposes• Models must be poured as soon as possible after
impression recordings• The increased use of addition silicones has lead to a
gradual decline in the use of addition silicone
Silicones Condensation silicone impression material
Advantages Disadvantages
Moderate tear strength Poor dimensional stability
Good working time Poor wetability
Good setting time
SiliconesAddition silicone impression material
Also supplied as two paste
The hydroxyl group from the condensation silicone is replaced by vinyl group
Examples: Extrude, Take 1 (SDS/Kerr), President (Coltene/Whaledent), Express, Imprint II (3M ESPE), Examix, Exaflex (GC America), Reprosil, Aquasil, Hydrosil (Dentsply/Caulk)
SiliconesAddition silicone impression material
• Properties are similar to condensation silicones
• They have near ideal elasticity with adequate tear resistance and setting characteristics
• The use of the light body and putty enables accurate impressions to be recorded
• Dimensional stability is greater than condensation silicones
• Also inherently hydrophobic therefore a dry field is required during impression recording
• Although surface active agents have been added to newer materials to make it less hydrophobic
SiliconesAddition silicone impression material
• Handling characteristics similar to condensation silicone
• Routinely used for crown and bridge impressions
• Preferred to condensation silicones due to its dimensional stability.
SiliconesAddition silicone impression material
Advantages Disadvantages
Excellent dimensional stability Hydrogen gas release
Good tear strength Inhibition of setting by sulphur containing materials
Good working and setting time Expensive
Excellent wettability Stiff
Automix system
SiliconesAddition silicone impression material
Polyethers
Also supplied as two pastes and automix
Examples: Impregum F, Permadyne (3M ESPE), Polyjel
(Dentsply/Caulk)
McCabe and Walls, 2008
Polyethers
• Tear resistance and elasticity approaching that of silicones
• Relatively rigid when set• It is dimensionally stable under relatively low humidity
conditions• The set material is hydrophilic and absorbs moisture
under high humidity conditions• Can record accurate impression even when perfect
moisture control is not achieved
Polyethers
• The major disadvantage is its rigidity during its set state- but this is also an advantage , e.g. when taking impressions for implants
Advantages Disadvantages
Excellent wettabillity Poor tear strength
Good dimensional stability Short working and setting time
Stiff
Difficult to disinfect
Expensive
Patient sensitivity
Polyethers
Comparison of the properties of elastomeric
impression material
McCabe and Walls, 2008
End of Part 1
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