ELASTOMERIC IMPRESSION MATERIALS Dr Priyadarshani Pawar Rubbery polymers

ELASTOMERIC IMPRESSION MATERIALS Dr. Priyadarshani Pawar

Ú Rubbery polymers, which are either chemically or physically cross linked. They can be easily stretched and rapidly recover their original dimensions when the applied stress is released Ú Chemically, there are four kinds of elastomers used as impression materials are: polysulfide, condensation polymerizing silicon, addition polymerizing silicon & polyether.

Polysulfide elastomer Ú main component is a multifunctional mercaptan or polysulfide polymer.

Composition(Base paste) Ingredients Weight (%) Function Polysulfide ploymer 80 -85 Main component Lithopone/titaniu 16 -18 m dioxide/zinc sulphate/copper carbonate/silica dibutyl phthalate 2 -4 Filler (to provide required strength) Plasticizer

Catalyst paste Ingredient Weight (%) Function lead dioxide 60 -68 dibutyl phthalate 30 -35 Oxidizing agent (to initiate the polymerization) Plasticizer Sulphur Accelerator 3 oleic acid/stearic 2 acid Retarder

Setting reaction Ú During setting reaction terminal hydrogen atoms of the polisulfide polymer are oxidized by the lead dioxide with a resultant increase in the degree of polymerization

Ú The reaction is slightly exothermic, with a typical increase in temperature of 3 -4 C. Ú mixes set to rubber consistency in about 10 - 20 min but polymerisation continues & the properties change for a number of hours after the material sets

Ú hot & humid conditions accelerate the setting of polysulfide impression material. Ú The reaction yields water as a by product of reaction. Loss of this molecule from the set material has a significant effect on the dimensional stability of the impression.

Ú do not adhere to impression tray- adhesives: rubber solution in acetone Ú available in three consistencies – Low (syringe or wash) – Medium (regular) – High (tray)

Condensation silicone Ú Base paste: Polydimethyl siloxane: main constituent & has reactive terminal hydroxyl groups. – Fillers: calcium carbonate or silica in concentration of 35% for low consistencies to 75% for putty like consistencies. Ú Accelerator: liquid that consists of stannous octate suspension & alkyl silicate/paste by adding a thickening agent.

Setting reaction Ú polydimethyl siloxane & accelerator are mixed leading to production of a three dimensional network with the liberation of ethyl alcohol (byproduct) & an exothermic temperature rise of about 1 C. Ú release of ethyl alcohol causes a shrinkage that is greater in the low consistency than in the putty like consistency.

Ú Available consistencies – Low – Putty Ú Catalyst of Condensation silicones can be supplied as a putty or liquid Ú do not adhere to impression tray-adhesive solutions

Addition silicone Ú Base paste: polymethyl hydrogen siloxane& other siloxane prepolymers. If catalyst paste contains platinum salt activator, then base paste must contain hybrid silicone. Fillers are also present. Ú Accelerator paste/ catalyst paste: divinyl polydimethyl siloxane & other siloxane prepolymers. Platinum salt activator may be added. Fillers are also present.

Setting reaction Ú no reaction byproducts develop as long as the direct proportions of vinyl silicone & hydride silicone are maintained & there are no impurities. Ú However, a secondary reaction between moisture & residual hydrides of the base polymer can lead to the development of hydrogen gas.

Ú Available consistencies – Extra low – Monophase – Putty (extra high)

Polyether elastomers ÚComposition: base paste: polyether polymer fillers inert oils activator paste: sulphonic acid esther in hydrocarbons fillers

Setting reaction Ú During setting reaction cross linking reaction occurs with the aziridino groups of the base and the end product is a rubber-like polymer

Ú Consistencies available – Low – Medium – High Do not adhere to impression tray-special adhesives

Mixing systems for elastomers Ú Hand mixing Ú Static Automixing Ú Dynamic Mechanical mixing

IMPRESSION TECHNIQUES Ú Simultaneous, dual viscosity technique Ú Single viscosity or monophase technique Ú Putty wash technique

Monophase

Putty wash impression

Setting properties Ú Viscosity: is the resistance of a fluid to flow Ú Polysulfides have the lowest viscosity & ranks as one of the least stiff of elastomeric impression materials Ú The most rapid increase in viscosity with time occurred with silicones & polyether materials

Shear thinning or pseudoplasticity Ú a shearing force can affect the viscosity of polyether & addition silicone impression materials

Working & setting times Ú working time is measured at room temperature where as, setting time is measured at mouth temperature Ú An increase in the temperature accelerates the curing rate of all elastomers & thus decreases both setting & working time

Working & setting times Ú Cooling is a practical method of increasing the working time Ú Working &setting time decreases with increase in viscosity. Ú Altering the base/catalyst ratio will change the curing rate. Mechanical properties can be adversely affected when marked change in base/catalyst ratio occur.

– Polysulfides have the longest working & setting times followed by silicones & polyethers. – Polyethers show a clearly defined working time with a sharp transition into the setting phase. This behaviour is often called snap test.

Requirements by ANSI/ADA specification no. 19 Viscosity Low Max. mixing Max. time (min) working time (min) 1 2 Medium 1 2 High 1 2 Very high 1 2

Dimensional change – Polymerization shrinkage – Loss of byproduct (water or ethyl alcohol) – Thermal contraction from oral to room temperature – Imbibitions when exposed to water, disinfectant or a high humidity environment over a period of time – Incomplete recovery of deformation because of viscoelastic behaviour

Ú In ANSI/ADA specification number 19 for elastomeric impression materials, a disk of impression material is placed on a talc covered glass plate. At the end of 24 hours, the contraction should not exceed o. 5 -1. 0% Ú The change is greater in magnitude for the polysulfide & condensation silicone.

Ú One property that has a negative effect on the polyether impression is the absorption of water or fluids & the simultaneous leaching of water soluble plasticizer. Thus the stored impression must be kept in a dry, cool environment to maintain its accuracy.

Mechanical properties Ú Elastic recovery – The relative amount of permanent deformation following strain in compression increases in the following order: addition silicone, condensation silicone, polyether & polysulfide. – Addition silicones are the most ideally elastic. Distortion on removal from undercuts is virtually nonexistent because these materials exhibit the lowest permanent distortion after strain in compression

Ú Despite the possibility of a large dimensional change occurring when a polysulfide impression is removed from the mouth, there is no advantage to “bench cure” the material.

Ú Minimum elastic recovery required for elastomeric material is 96. 5% & minimum strain in compression required is 0. 8% & maximum is 20%.

Strain in compression Ú The stiffness of elastomers increase in order of: polysulfide, condensation silicone, addition silicone & polyether.

Flow Ú It is measured on a cylindrical specimen 1 hour old & the %age flow is determined 15 minutes after a load of 1 N is applied. Ú Silicones & polyether have the lowest values of flow & polysulfides have the highest.

Hardness Ú The shore A hardness increases from low to high consistency. Ú The low, medium & high viscosity addition silicones & polysulfides don’t change hardness significantly with time, whereas hardness of condensation silicone, addition silicone putties & polyethers does increase with time.

Ú Low flexibility & high hardness can be compensated for clinically by producing more space for the impression material between the tray & the teeth. Ú This can be accomplished with additional block-out for custom trays or by selecting a larger tray when using disposable trays.

Tear strength Ú It indicates the ability of a material to withstand tearing in thin interproximal areas & margins of periodontally involved teeth Ú The ranking of tear strength from the lowest to highest of all impression materials generally is as follows: hydrocolloids (agar & alginate), silicones, polyether & then polysulfide

Ú In addition to chemical composition of the material, tear strength is also influenced by the consistency & manner of removal of the material

Creep compliance Initial creep compliance illustrates that polysulfide is the most flexible & polyether is the least flexible

Ú The flatness or parallelism of the curves with respect to the time axis indicates low permanent deformation during the removal of an impression material; polyether & addition silicones have the best elastic recovery followed by condensation silicones & then polysulfides

Detail reproduction Ú Expect for the very high viscosity products, all should reproduce a V-shaped groove & a 0. 02 mm wide line in elastomeric. Ú The rubber impression materials are capable of reproducing detail more acuurately than can be transferred to the stone or die cast

Addition silicone

Polyether elastomer

WETTABILITY & HYDROPHYLIZATION OF ELASTOMERs Ú Wettability may be measured by advancing contact angle of water on the surface of the set impression material

Ú All the elastomeric impression materials possess advancing & receeding contact angle greater than 45 degrees. Ú There are however, differences in wetting among & with in types of elastomers. Ú Traditional addition silicone is not as wettable as polyether.

Hydrophilizer addition silicone

Ú diffusion controlled transfer of surfactant molecules from the polyvinyl siloxane into the aqueous phase, thereby altering the surface tension of surrounding liquid. Ú By observing water droplets on impression surfaces, it has been shown that hydrophilized addition silicone & polyether are wetted the best, & condensation silicones & traditional addition silicones the least

Ability of polyether & addition silicone to reproduce detail under wet & dry condition

DISINFECTION Material Method Recommended disinfectant Polysulfides & silicone Immersion Polyether Immerse with caution (<10 min) Gluteraldehyde, chlorine compounds, iodophores, phenolics Chlorine compounds & iodophores

BIOCOMPATIBILITY Ú Cell cytotoxicity test for different materials reveals that polysulfides result in the lowest cell death count & the set polyethers produce highest cell cytotxicity scores. Ú problem occurs when a segment of impression material is lodged in patient’s gingival sulcus. It may cause gingival inflammation & can be misdiagnosed.

SHELF LIFE Ú Properly compounded impression material doesn’t deteriorate appreciably in the tube or container when it is stored in a dry, cool environment. Ú The tubes should always be kept tightly closed when they are not in use.

COMMON FAILURES Ú Rough/uneven surface of impression – Incomplete polymerization caused by premature removal from mouth – Improper ratio/mixing of components – Presence of oil/other organic material on teeth – Contamination with latex gloves for addition silicones – Too rapid polymerization from high humidity or temperature – Excessive high accelerator/base ratio with condensation silicone

Ú Bubbles – Too rapid polymerization, preventing flow – Air incorporated during mixing – Irregularly shaped voids • Moisture or debris on surface of teeth

Rough/chalky stone casts Ú Inadequate cleaning of impression Ú Excess water left on the surface of impression Ú Excess wetting agent on the impression Ú Premature removal of cast/improper manipulation of stone Ú Failure to delay pour of addition silicone at least 20 min

Distortion Ú Resin tray not aged sufficiently; still polymerization going on Ú Lack of adhesion of rubber to impression tray caused by not applying enough coats of adhesive/filling the tray with material too soon after adhesive application or using wrong adhesive Ú Lack of mechanical retention for those materials where adhesive is ineffective Ú Development of elastic properties in material before the tray is seated

Ú Excessive bulk of material Ú Insufficient relief for the reline material (if such technique is used) Ú Continue pressure against impression material that has developed elastic properties Ú Movement of tray during polymerization Ú Premature removal from the mouth Ú Improper removal from the mouth Ú Delayed pouring of the polysulfide or condensation silicone impression

Polysulfides : advantages Ú Long working time Ú High tear resistance Ú Margins easily seen Ú Modest cost

Disadvantages Ú Requires custom tray Ú Stretching leads to distortion Ú Stains clothing / Obnoxious odour Ú Pour within 1 hour

Condensation silicone Ú Advantages – Putty for custom tray – Clean & pleasant – Good working time – Easily seen margins

Disadvantages Ú High polymerization shrinkage Ú Volatile byproduct Ú Hydrophobic Ú Pour immediately

Addition silicone: advantages Ú Putty for custom tray Ú Automix dispense Ú Clean & pleasant Ú Easily seen margins Ú Ideally elastic Ú Pour repeatedly Ú Stable: delay pour

Disadvantages Ú Hydrophobic Ú No flow if sulcus is moist Ú Low tear strength Ú Putty displaces wash Ú Putty too stiff Ú Difficult to pour a cast

Polyether : advantages Ú Fast setting / Clean Ú Automix dispense Ú Least hydrophobic Ú Easily seen margins Ú Good stability Ú Delay pour Ú Shelf life: 2 yrs

Disadvantages Ú Stiff / high modulus of elasticity Ú Bitter taste Ú Needs to block undercuts Ú Absorbs water Ú Leaches components Ú High cost

“ No impression material fulfills all the requirements” the selection of the material best suited for a particular clinical situation and technique rests with the dentist.

References Ú Phillip’s – 11 th edition Ú Craig’s – restorative dental materials – 12 th edition Ú Craig’s – dental materials – 11 th edition Ú Textbook of complete denture – heartwell – 5 th edition