Elastometric Impression Materials

Question 1

What are the 2 main classes of impression materials?

Answer 1

1. Elastic materials

Synthetic elastomers like silicones and polyethers

Hydrocolloids like agar and aliginates

1. Non elastic materials

Impression plaster like impression compound, zinc oxide pastes and impression waxes.

Question 2

Why are colloids terms as heterogeneous?

Answer 2

Because they contain particles dispersed in a medium

Question 3

When are colloids termed hydrocolloids?

Answer 3

When water is the dispersion medium.

Question 4

What are hydrocolloid impression materials used in dentistry based on?

Answer 4

They are based on colloidal suspensions of polysaccharides.

Question 5

What are the 2 states dental hydrocolloids exist in and give properties of each state.

Answer 5

1. Sol state
   polysaccharide chains are randomly distributed.
   As a result it is rendered as a fluid with low viscosity.
2. Gel state

Can transition from a sol to a gel state.

Forms a gelatin-like, viscous consistency.

Fibrils align into an ordered structure, trapping the fluid phase.

This structure gives it viscosity and elasticity.

Question 6

What does the strength or toughness of a hydrocolloid gel depend on?

Answer 6

Concentration of fibrils: Higher fibril concentration increases viscosity.

Concentration of fillers: Inert powders are added to reduce flexibility.

Question 7

What are the terms for water loss and water uptake in hydrocolloid gels?

Answer 7

Syneresis = water loss
Imbibtion = water uptake

Question 8

How are hydrocolloid gels formed from a sol and what does this enhance as a result?

Answer 8

cooling the sol
more chains align
increasing elasticity

Question 9

How can gel hydrocolloids go back to a sol state and what are these hydrocolloids called?

Answer 9

reheating
break bonds
returns to sol state

reversible hydrocolloid

Question 10

Give an example of a reversible hydrocolloid.

Answer 10

Agar is an example of a reversible hydrocolloid.

Question 11

From what material on earth does alginate originate from?

Answer 11

Alginates are derived from seaweed.

Question 12

What soluble and insoluble component is alginate composed of?

Answer 12

Alginate consists of soluble salts (sodium, potassium, or ammonium) and insoluble polysaccharide (alginic acid).

Question 13

Before the hydrocolloid is placed into the mouth, what state is it in, then once removed, what state does it reach?

Answer 13

Before the hydrocolloid is placed into the mouth, what state is it in, then once removed, what state does it reach?

Question 14

What is a potential problem with alginates displaying a lack of compatibility with some makes of dental stones?

Answer 14

Certain salts in alginate can react with the stone

weakening the cast

leading to reduced surface hardness and irregularities

This can be addressed by using alternative impression materials.

Question 15

What 5 materials is alginate made of and their functions?

Answer 15

Na or K salt of alginic acid: Forms a sol with water.

CaSO4.2H20 (gypsum): Cross-links to form a gel, provides Ca²⁺ ions for cross-linking.

Sodium phosphate (Na₃PO₄): Controls working time.

Inert filler (diatomaceous earth): Adds body and enables easy removal.

Color indicator: Shows when the reaction is complete.

Question 16

What chemical process happens once water is mixed with alginate?

Answer 16

mix aliginate and water sol is formed

sodium salt readily dissolves - sodium alginate

the sodium alginate is displaced by calcium ions to form calcium alginate. results in cross linking of alginate chains to form the gel.

Question 17

When can bubbles form when mixing alginate with water and what is recommended to avoid this?

Answer 17

Bubbles can form if alginate is beaten during mixing, compromising the impression accuracy.

To avoid this, use a vacuum mixer or spatulation to mix the alginate and water.

This minimizes air entrapment, ensuring a smooth, homogeneous mixture.

Question 18

What 3 things does alginate powder contain?

Answer 18

Alginate salt (e.g., sodium alginate).

Calcium salt (e.g., calcium sulfate).

Trisodium phosphate.

Question 19

How can the setting reaction between alginate and water be made to slow down and how does this work?

Answer 19

can be slow down by Adding trisodium phosphate to the powder.

trisodium phosphate reacts with calcium ions preventing the calcium ions reacting the sodium alginate.

setting reaction is slowed down

Question 20

Before mixing aliginate and water what should be done to the powder and why?

Answer 20

1. Shaken
2. To get an even distribution of constituents

Question 21

What is the recommended temperature for water to be mixed with alginate?

Answer 21

Room temperature

Question 22

Why do some users prefer using cold water with alginate?

Answer 22

Longer setting time so more time to mix and put on tray.

Question 23

How does alginate stay in the impression tray(2 ways)?

Answer 23

1. Perforations in the tray
2. Adhesives.

Question 24

Give 5 positives of alginate.

Answer 24

biocompatible
good surface detail
easy to use
cheap
setting time can be controlled

Question 25

Give 5 negatives of alginate.

Answer 25

poor dimensional stabilty incompatible with some dental stones poor tear resistance setting time is dependant operator messy

Question 26

What do modified alginates contain?

Answer 26

Incorporated silicones.

Question 27

Give 2 benefits of using modified alginates.

Answer 27

1. Better reproduction detail 2. Better tear resistance

Question 28

What do modified alginates still lack?

Answer 28

1. Dimensional stability As a result still need to be cast up soon after recording impressions to maintain accuracy.

Question 29

What in nature are elastomers like?

Answer 29

Elastomers are like rubber.

Question 30

What do elastomers consist of and what holds the molecules together?

Answer 30

lastomers consist of macromolecules. They are held together by weak van der Waals (VdW) forces. Their low intermolecular forces make deformation primarily dependent on entropy.

Question 31

How do elastomers produce elastic deformations per unit applied stress?

Answer 31

Elastomers deform under stress and return to their original shape when the stress is removed. Polymer chains undergo straightening, lengthening, or slipping between planes, producing elastic deformations. This makes them ideal for impression materials as they capture and reproduce fine details accurately.

Question 32

What are the 2 categories of elastomeric impression materials?

Answer 32

Silicones (addition and condensation silicones). Imines or polyethers.

Question 33

How are addition cured silicones formed?

Answer 33

1. Vinyl terminated polydimethyl siloxanes prepolymers. 2. React with silicones that contain hydride groups. 3. In presence of Pt catalyst

Question 34

How does cross linking occur in addition cured silicones?

Answer 34

Via addition of hydrogen onto the double bond.

Question 35

How can hydrogen evolution occur with addition-cured silicones and what can this result in?

Answer 35

Hydrogen evolution occurs due to the reaction of the platinum catalyst with moisture leading to surface pitting and a roughened surface on the model.

Question 36

How can hydrogen evolution with addition-cured silicones be prevented?

Answer 36

By incorporating scavengers such as palladium (Pd).

Question 37

Why are addition-cured silicones pseudoplastic, and why is this beneficial?

Answer 37

Pseudoplasticity means that external force causes a dip in viscosity, which allows the material to flow better, resulting in a more accurate impression.

Question 38

Give 6 positives of silicones.

Answer 38

1. Good dimensional stability 2. Good accuracy 3. Ease of use 4. Capable of being disinfected 5. Available in various viscosities 6. Lack of unpleasant odour and taste

Question 39

Give 6 positives of addition cured silicones.

Answer 39

1. Accurate 2. Ease of use 3. Fast setting 4. Wide range of viscosities 5. Adequate tear strength 6. Dimensionally stable

Question 40

Give 2 negatives of addition cured silicones.

Answer 40

difficult to remove from the mouth

Question 41

Give 2 uses of addition cured silicones.

Answer 41

1. Crowns 2. Bridges

Question 42

What can poison the platnium catalyst used in addition cured silicones?

Answer 42

Rubber glove contact

Question 43

What are the 2 components and their respective functions of condensation-cured silicones?

Answer 43

Base paste: Hydroxy-terminated polydimethyl siloxane (forms rubber) and silica filler (modifies viscosity and physical properties). Liquid component: Alkyl silicate and tin compounds (act as a catalyst).

Question 44

What 2 components in condensation-cured silicones react together, how long does this reaction take to set, what is this reaction called, and what is lost in this reaction?

Answer 44

Hydroxy-terminated polydimethyl siloxane and alkyl silicates react. The reaction takes 7 minutes to set. This is a condensation reaction. 2 molecules of ethyl alcohol are lost during the reaction.

Question 45

Give 8 positives of condensation-cured silicones.

Answer 45

Rapid elastic properties with near ideal recovery. Adequate dimensional stability. 300% tear strength. High viscosity putty pastes available. Non-toxic. Accurate. Easy to use. Can be used on severe undercuts.

Question 46

Give 5 negatives of condensation-cured silicones.

Answer 46

Short working time. Hydrophobic, causing blow holes (requires drying). Alcohol loss leads to shrinkage (requires immediate casting). Contains heavy metal catalyst. Liquid component may cause irritation.

Question 47

What chemistry are polyethers based on?

Answer 47

Polyethers are based on Imine chemistry. Setting occurs via a cationic ring opening involving the imine ring, with no by-products.

Question 48

What are the 2 pastes polyethers consist of?

Answer 48

Base paste: Polyether and filler. Catalyst paste: Sulphonic acid ester (enhances polymerization and crosslinking) and inert oils.

Question 49

When the 2 pastes of polyethers are mixed, what is formed and how long is the setting time?

Answer 49

Regular bodied Light + heavy bodied (diluent)

Question 50

What speeds up the setting reaction of polyethers, and what is important that we must do as a result?

Answer 50

Moisture speeds up the setting reaction. It is important to keep the tray dry.

Question 51

Give 10 positives of polyethers.

Answer 51

Easy to use Quick setting Adequate strength High hardness Good shelf life (up to 2 years) Good elastic recovery Non-toxic Low setting contraction Excellent surface detail Good dimensional stability

Question 52

Give 5 negatives of polyethers.

Answer 52

High hardness can cause stress on removal, possibly leading to rupture. Hydrophilic, so needs to be cast up quickly. May cause allergic reaction due to sulphonic acid ester. Poor tear strength. Rapid setting time (short working time). - amount of catalyst controls setting Stiff set material (can be hard to remove from mouth).

Question 53

Give 5 uses of polyethers.

Answer 53

crowns bridges denture implants

Question 54

What trays, what should be applied on the trays for polyethers and how many steps does the technique require?

Answer 54

special/stock tray adhesive cast wothin 24 hours