Elastomeric Impression Materials

Elastic Materials: • Synthetic Elastomeres eg. Polyethers and Silicones • Natural (hydrocolloids) eg. Agar and Alginates Non elastic materials: • Plaster of Paris/Impression plaster • Impression compound • Zinc-oxide-eugenol • Impression waxes

• Solution is a homogenous mixture whereas suspensions are heterogenous • Suspensions have particles, which are microscopically seen in a dispersed medium • Colloids are heterogenous but the particles dispersed are much smaller • When the dispersion medium is water, it is known as a hydrocolloid • Colloids are basically suspensions but solute particles are smaller • Hydrocolloid impression materials used in dentistry are based on colloidal suspensions of polysaccharides

Sol-Gel State -Either as a sol or a gel state Sol • Liquid in the sol state • Polysaccharide chains are randomly distributed rendering it as a fluid with low viscosity Gel • Fibrils are aligned and acquire an ordered structure • Enclose the fluid phase giving it a gelatinous consistency • Cross linking has also occured

-Sol state is low viscosity as the polysaccharide chains are randomly distributed • Gel state is an ordered arrangement with cross linking • Fibrils are aligned and acquire an ordered structure In the gel state, elastic properties are achieved

Concentration of fibrils -Greater the concentration, the higher the viscosity Concentration of fillers -Inert powders are added to reduce flexibility

• A gel can lose water by evaporation • Gel molecules may draw close to each other squeezing the water, squeezing the water molecules to the surface and, as a result, water may appear on the surface -Phenomena called syneresis • Water uptake is called imbibition due to hydrophilicity • Often occurs in alginates if left unprotected • May be a change in dimensions • Loss of water through syneresis

Reversible • Gel formation may be induced by cooling the sol • Chains become more aligned • Elasticity of the gel is therefore enhanced In some cases, reheating of the gel may cause the bonds to readily break reverting the matrix to a sol state -These are called reversible hydrocolloids -Agar is an example of reversible hydrocolloid Irreversible: • Alginates belong to a class of polysaccharides • Alginic acid is an insolube polysaccharide • Sodium, potassium or ammonium salts are the solube alginates • Once set, it won’t transform back to the sol state • Crosslinking occurs so there is no reversibility

• Placed in the mouth in the sol state • Records sufficient detail • Then removed when it has reached the gel state • Hydrocolloid materials, especially the alginates, may display a lack of incompatibility with some makes of dental stone • Resultant model may show reduced surface hardness and possibly surface irregularities and roughness

Powder contains: 1) Alginate salt (sodium alginate) 2) Calcium salt (calcium sulphate) 3) Trisodium phosphate On mixing the powder (soluble alginate) with water, a solution is formed Powder should be shaken before use to get an even distribution of constituents -Water at RT should be used

• Material closest to the tissue sets first - Due to temperature of the tissue

-Either by perforations in the tray And/Or by adhesives

Na or K Salt of Alginic Acid (Alginate Salt eg sodium alginate): • Reactive ingredient • Forms solution with water • Becomes cross linked to form a gel CaSO4.2H20 (calcium salt/gypsum) • Source of calcium ions • Cross links alginate chains Trisodium phosphate -Controls working time Inert fillers such as diatomaceous earth -Gives body and enables ease of manipulation Reaction indicator -Gives a colour change on completetion of setting

• Alginate supplied as powders, which are then mixed with water • On mixing and spatuling, an alginate solution is formed • The sodium salt present in the powder readily dissolves whereas the calcium salt is sparingly soluble • The sodium alginate readily reacts with the calcium ions to form calcium alginate • The replacement of the monovalent ions of sodium by divalent calcium ions results in cross linking of the alginate chains to form the gel -Look at pic on slides

• The reaction occurs too quickly often during mixing or loading of the impression tray • Adding trisodium phosphate slows down the reaction • Trisodium phosphate reacts with the calciums sulphate to produce calcium phosphate, preventing the calcium sulphate reacting with sodium alginate to form a gel • Setting reaction can therefore be slowed down by appropriate amounts of tri sodium phosphate

Pros • Non toxic and non irritant • Good surface details • Ease of use and mix • Cheap and good shelf life • Setting time can be controlled with temperature of water used Cons • Poor dimensional stability (very hydrophilic so undergoes syneresis and imbibation) • Incompatibility with some dental stones • Setting time very dependent on operator handling (how long it takes to mix) • Messy to work with -Alginate powder is unstable on storage in presence of moisture or in warm temperatures

• Alginate powder is unstable on storage in presence of moisture or in warm temperatures • Poor Dimensional stability

• Modified by incorporating silicones • 2 pastes with contrasting colours to enable visual aid to thorough mixing of the two pastes • Setting is similiar to conventional alginates, they show better reproduction detail and have a better tear resistance • Still exhibit poor dimensional stability and need to be cast up soon after recording impressions in case accuracy is to be maintained

• Rubber like in nature • Consist of macromolecules held together by weak van der Waals forces of attraction • Sufficiently low inter and intra molecular forces for free energy of deformation to be mainly a function of entropy • Individual polymer chains may tend to undergo chain straightening, chain lengthening or slip between planes on application of an external stress • Thereby producing elastic deformations per unit applied stress

1) Condensation Silicones 2) Addition Silicones 3) Imine terminated polymers or poly ethers

• 0.5ml of mixed material is pressed between two flat plates using a force of 1.5N • Consistency defined as the average diameter of the resulting disc after the force has been applied and is related to its viscosity 0= Very high consistency 1= High Consistency 2= Medium consistency (medium body) 3= Low (light body) As you decrease the consistency, the test disc diameter increases

• Vinyl terminated polydimethyl siloxanes prepolymers react with silicones containing hydride groups in the presence of a platinum catalyst • Cross linking occurs via addition of hydrogen

-Look at slides

• Good dimensional stability • Good accuracy, easy of use, capable of being disinfected, available in various viscosities, lack of unpleasant odour and taste • Hydrogen evolution may occur due to reaction of platinum with moisture, scavengers such as Pd are incorporated • Pseudoplastic materials, pronounced effect of increased strain on unset material, ie, flow properties differ under different applied stresses 1) Accurate 2) Ease of use 3) Fast setting 4) Wide range of viscosities Hydrogen gas is evolved on setting which may lead to surface pitting, creating a roughened surface on the resultant model

• Sometimes difficult to remove the impression from the mouth • Too accurate in some circumstances (cast produced is not sufficiently oversized)