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Dental Material Science > Elastomeric Impression Materials > Flashcards

Elastomeric Impression Materials Flashcards

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1
Q

Classification of impression materials

A

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
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2
Q

Hydrocolloids definition

A
  • 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
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3
Q

States in which a hydrocolloid can exist in

Properties of these states

A

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
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4
Q

Difference between sol and gel states in term of structure

A

-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

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5
Q

Strength and toughness of hydrocolloid gel depends on:

A

Concentration of fibrils
-Greater the concentration, the higher the viscosity

Concentration of fillers
-Inert powders are added to reduce flexibility

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6
Q

How can a hydrocolloid gel lose water
Name of phenomenon
Clinical significance

A
  • 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
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7
Q

Types of hydrocolloids

A

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
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8
Q

How are hydrocolloids used as impression materials

Issues with this

A
  • 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
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9
Q

Powder in alginate constituents

A

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

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10
Q

Where does the alginate set first

A
  • Material closest to the tissue sets first

- Due to temperature of the tissue

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11
Q

How is retention of the alginate onto the tray achieved

A

-Either by perforations in the tray

And/Or by adhesives

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12
Q

Composition of alginate and function of each ingredient

A

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

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13
Q

Chemistry of alginates

A
  • 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

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14
Q

What can be added to slow down the reaction and chemsitry of this

A
  • 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
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15
Q

Properties of alginate

Pros and Cons

A

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

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16
Q

Storage of alginates

A
  • Alginate powder is unstable on storage in presence of moisture or in warm temperatures
  • Poor Dimensional stability
17
Q

Modified alginates

A
  • 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
18
Q

Elastomeres definition

A
  • 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
19
Q

Types of Elastomeric Impression Materials

A

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

20
Q

What is the ISO standard for consistency

A
  • 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

21
Q

Addition Cured Silicones definition

A
  • 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
22
Q

Chemical Reaction of addition cured silicones

A

-Look at slides

23
Q

Properties of addition cured silicones

A
  • 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

24
Q

Issues with addition cured silicones

A
  • Sometimes difficult to remove the impression from the mouth
  • Too accurate in some circumstances (cast produced is not sufficiently oversized)
25
Q

Composition and function of condensation silicones

A

Base Plate:
-Hydroxy terminated poly(dimethyl siloxane) and silica as filler to form pastes
Function:
-Undergoes cross-linking to form a rubber
-Silica filler gives body and modifies viscosity and influences the physical properties

Liquid Component:
-Alkyl silicate eg. tetra ethyl or triethyl silicate
-Tin compounds as catalyst
Silicates act as a crosslinking agent

26
Q

How does crosslinking occur in condensation cured polymers

A

-Cross linking occurs via reaction of polydimethylsiloxane with alkyl silicates

  • On mixing the two pastes react, cross linking occurs and setting takes about 7 minutes
  • Condensation reaction
  • 2 molecules of ethyl alcohol are lost during the course of reaction
  • By losing molecules, the accuracy decreases, leading to poorer dimensional stability compared to addition cured polymers
27
Q

Setting rxn of a condensation cured polymer

A

-Check the slides

28
Q

Properties of condensation cured silicones

A
  • Short working time
  • Attain elastic properties rapidly and near ideal elastic recovery
  • Hydrophobic so cause blow holes to occur (need to dry area where you are going to take an impression
  • Adequate dimensional stability
  • Adequate tear strength
  • High viscosity putty pastes are available
  • Loss of alcohol gives measurable shrinkage usually needs immediate casting
  • Non toxic but do contain heavy metal catalyst
29
Q

Advantages and disadvantages of condesnation cured polymers

A

+:

  • Good accuracy
  • Ease of use
  • Used on severe undercuts

-:
Liquid component of paste/liquid system may cause irritation

  • Silicones have a shorter setting time and elasticity develops early on
  • Hydrophobic and wettability is a problem
  • Advisable to dry the area of the mouth in order to take an accurate impression
30
Q

How the properties of silicones change from light to heavy body

A

-With increasing consistency (light to heavy bodies silicones):

  • Decreasing setting contraction
  • Decreasing thermal contraction
  • Increasing dimensional stability
  • Increasing viscosity
31
Q

Indications for condensation cured silicones

A

-Crown, bridge and occasionally for partial dentures

32
Q

Polyethers components

A
  • Based on imine chemistry
  • Supplied in 2 pastes

Base Paste:

  • Polyether
  • Filler

Catalyst Paste

  • Sulphinic acid ester (enhances further polymerisation and crosslinking)
  • Inert Oils
  • When mixing, the polymer and sulphonic acid ester react to form a stiff polyether rubber
  • Setting time occurs in about 6 minutes
  • Usually comes in one viscosity- regular bodied, but can also come as light and heavy bodies (diluent)
  • Moisture speeds up the reaction
33
Q

Chemistry of polyethers and eg

A
  • Impregum
  • Sed via a cationic ring opening involving the imine ring: no by products
  • Easy to use, quick setting, adequate strength and high hardness
  • The high hardness can cause sufficient stress on removal leading to possible rupture
  • Hydrophilic
34
Q

Properties of Polyethers

A
  • Hydrophilic
  • Good shelf life
  • Good elastic recovery
  • Non toxic
  • Low setting contraction
  • Low tear strength
  • Excellent surface detail
  • Good dimensional stability
  • May cause an allergic reaction due to sulphonic acid ester
  • Poor tear strength
  • Rapid setting time (Short working time)
  • Stiff set materials (sometimes hard to remove from the mouth)
35
Q

Polyethers mixing and indications of use

A
  • Crown and bridge work, partial dentures, implants and over dentures
  • Mixed in a 1:1 ratio until homogenous colour, the amount of catalyst used can be used to control the setting time
  • Used in special or stock trays with an adhesive
  • One or two stage technique can be used
  • Although dimensionally stable, the impression should be cast within 24 hours
36
Q

Comparison of Impression Materials (Addition v condensation v Polyethers)

A
  • Both available in 4 viscosities
  • Both have adequate tear resistance
  • Both have very good elasticity
  • Addition has better accuracy with stock trays
  • Addition has very good dimensional stability, condensation models should be poured shortly after taking impression

Polyethers:

  • Available in a single viscosity (Regular)
  • Adequate tear resistance
  • Adequate elasticity
  • Very good accuracy and dimesnional stability
  • But their hardness means their getting used less

Dental Material Science (13 decks)

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