Elastomeric Impression 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 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.

Poor dimension stability 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

Silicones Addition 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)

Silicones Addition 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

Silicones Addition 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.

Silicones Addition 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

Silicones Addition 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