Classification of 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
Hydrocolloids definition
- 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
States in which a hydrocolloid can exist in
Properties of these states
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
Difference between sol and gel states in term of structure
-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
Strength and toughness of hydrocolloid gel depends on:
Concentration of fibrils
-Greater the concentration, the higher the viscosity
Concentration of fillers
-Inert powders are added to reduce flexibility
How can a hydrocolloid gel lose water
Name of phenomenon
Clinical significance
- 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
Types of hydrocolloids
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
How are hydrocolloids used as impression materials
Issues with this
- 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 in alginate constituents
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
Where does the alginate set first
- Material closest to the tissue sets first
- Due to temperature of the tissue
How is retention of the alginate onto the tray achieved
-Either by perforations in the tray
And/Or by adhesives
Composition of alginate and function of each ingredient
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
Chemistry of alginates
- 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
What can be added to slow down the reaction and chemsitry of this
- 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
Properties of alginate
Pros and Cons
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
Storage of alginates
- Alginate powder is unstable on storage in presence of moisture or in warm temperatures
- Poor Dimensional stability
Modified alginates
- 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
Elastomeres definition
- 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
Types of Elastomeric Impression Materials
1) Condensation Silicones
2) Addition Silicones
3) Imine terminated polymers or poly ethers
What is the ISO standard for consistency
- 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
Addition Cured Silicones definition
- 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
Chemical Reaction of addition cured silicones
-Look at slides
Properties of addition cured silicones
- 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
Issues with addition cured silicones
- Sometimes difficult to remove the impression from the mouth
- Too accurate in some circumstances (cast produced is not sufficiently oversized)
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Biocompatibility of materials for use in the body21
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Failure of Materials54
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Polymers in Dentistry33
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Ceramics59
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Polyalkenoate Cements27
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Composites39
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Metals in Dentistry56
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Adhesion to Enamel and Dentine40
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Amalgam39
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Impression Materials36
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Elastomeric Impression Materials36
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Acrylic Materials22
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Cements27
