ElastomericImpression Materials Part 1 Dr. Noland Naidoo
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 • 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 polymer • Suitable filler (titanium dioxide or lithopone) • Plasticizer ( eg. dibutyl phthalate) • Sulphur – small amount that enhances reaction ( catalyst)
Polysulphides • The catalyst paste: • Lead dioxide • Plasticizer- chlorinated parafin • Approx same amount of filler as in base paste • Oleic or stearic acid- retarders- control the rate of setting
Polysulphides • Must be used with a special tray • Reasonable tear strength and good elastic properties • Unpleasant odor and taste • Work well in the presence of moisture- moderately hydrophilic • Used commonly for crown and bridge impressions • Casts should be poured within 24hrs of impression but not immediately as to allow for full elastic recoil • Reported incidences of allergic reaction to patients sensitive to latex • Impressions 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.
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
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.
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
Comparison of the properties of elastomeric impression material McCabe and Walls, 2008