DM- CH 8 Impression Materials

Impression materials

A. Used to make replicas

• Impression is a negative reproduction

• Model, Cast, Die are positive reproductions

B. Impression material systems used for:

• Study Model

• Cast

• Die

Impression Material Systems:

Water/Powder

Heated/melted materials

*Paste/Paste

*Used with impression trays

Impression Materials costs

• Can be inexpensive - very expensive

• Depends on type of impression taken

Impression Trays

• Used to carry impression material to mouth, support impression and improve accuracy

Types of trays

• Impression

• Stock

• Custom

• Special-use

Custom trays

Made on study models, 2 steps:

• Make study model from alginate imp.

• Then make custom tray from that model

Special-use trays

Triple Tray

• Used to obtain replica of biting surfaces of teeth

Chemical reaction or physical change

thermoset and thermoplastic

Thermoset

Material sets by a chemical reaction

- Strong cross-link bond, strong covalent bonds bwetween polymer chains

Classification of impression materials

1. Chemical reaction or physical change

2. Use:

a). Elastic/inelastic

b). Accuracy

c). Flow and detail reproduction

3. Types of impression materials

Thermoplastic

Material sets by a physical change when it cools

- Weak intermolecular forces between polymer chains, no cross-links between chains (softens when heated)

Elastic

Used for dentulous patients, flows well

Inelastic

Used for edentulous pts. Sets hard/rigid

Accuracy

Different materials used for more or less accurate

impressions

Flow and detail reproduction

Viscosity of material; is it thick or thin

Inelastic impression material

1. Plaster

2. Wax and Impression Compound

3. Zinc Oxide-Eugenol (ZOE)

Aqueous elastomeric

1. Alginate: Irreversible Hydrocolloid

2. Agar: Reversible Hydrocolloid

Nonaqueous elastomeric

1. Polysulfide

2. Condensation Silicone

3. Polyethers

3. Addition Silicone

Plaster

Chemically set, edentulous pt. for dentures, seldomly used

Wax and Impression Compound

• Wax: Physically set, edentulous pts., inexpensive, can also be used to extend trays

• Impression Compound: Physically set, edentulous pts., it's wax with fillers, cools at temperature of the mouth

Zinc oxide-eugenol (ZOE)

Chemically set, limited use as imp. of edentulous ridges, hard/brittle, bad taste

b) Aqueous elastomeric

c) Nonaqueous elastomeric

Aqueous elastomeric material

Has water

• Water = Dehydration of material when exposed to air, leads to deformation

Alginate (Irreversible Hydrocolloid)

Chemically set, used for preliminary denture imp., custom trays, study models

Agar (Reversible Hydrocolloid)

Physically set, used for partial dentures, inlays, crowns, bridges

Nonaqeuous elastomeric material

Has no water

All are chemically set, used for full/partial dentures, inlay, crown, bridge, when very detailed impressions are needed

No water = No dehydration and deformation

• Polysulfide

• Condensation Silicone

• Polyethers

• Addition Silicone

Sol

fluid state

Reversible

Will return to sol state when heated

Irreversible

Not able to return to sol state

Alginate (Irreversible Hydrocolloid) Properties

• Aqueous (need water), elastic, chemically set, mix powder/water

• Easy to use, not accurate, not for detailed replicas

Alginate (Irreversible Hydrocolloid) Composition

• Powder has potassium alginate (seaweed), silica (provides body)

Alginate (Irreversible Hydrocolloid) Setting Reaction

• Reactor: Calcium sulfate

• Retarder: Sodium potassium

• Carboxylate groups work with calcium ions to form the gel

Alginate (Irreversible Hydrocolloid) Use and Handling

• Advantages: easy to pour, wets tooth surface and absorb oral fluids to decrease air bubbles

• Disadvantages: Distorts & shrinks when exposed to air

Dimensional change

expansion and shrinking

Taking an impression with Alginate:

A. Fluff container (wear mask)

• Silica can cause lung issues

B. Mix powder & water

• Cold = sets slower, warm = sets faster

C. Spatulate with pressure to remove air bubbles

D. Load tray

E. Take impression

F. Removal; quick snap motion to decrease deformation

G. Sets in 3-4 min (Reg) 1-2 min (fast set)

H. Disinfect, wrap, pour with gypsum

Agar (Reversible Hydrocolloid) Form of Material

Semisolid material in tubes and sticks

• Mostly water

• Used as a growth medium in biology

• Components include mold inhibitors and sulfate

compound; this improves hardness of gypsum when

poured

Agar (Reversible Hydrocolloid) Equipment

• Requires equipment to heat, cool and store agar material

• Specialty trays

Agar (Reversible Hydrocolloid) Preparation of material

• Boil to soften, store at 150 degrees until using, temper at 110 degrees when ready to use

Agar (Reversible Hydrocolloid) Hysteresis

• The characteristic of melting and gelling at different temperatures

• Melts at very high temperature, gels at cool temperature

Agar (Reversible Hydrocolloid) Advantages/Disadvantages

• Works well in wet environment, but very expensive for equipment

Agar (Reversible Hydrocolloid) Popularity

• Not popular due to expense and process

Nonaqueous Elastomeric (Rubber) Impression Materials

AKA rubber-based materials, elastomers, elastomeric

• Used for more detailed impressions for indirect restorative processes

Nonaqueous Elastomeric (Rubber) Impression Materials is set by

polymerization reaction

Polymerization reaction

Chemical process where many small molecules (monomers) combine to form larger molecules (polymers), leaves no biproducts and does not evaporate

Nonaqueous Elastomeric (Rubber) Impression Materials are

• More stable and accurate than hydrocolloid materials

• More expensive than hydrocolloid materials

ex: Polysulfide, Polyether, Condensation Silicone, Addition Silicone

Polysulfide Impression Materials Advantages

• Elastic, chemically set, accurate and stable

Polysulfide Impression Materials Disadvantages

• Smell and taste bad

• Need custom trays

• Long working time, up to 10 minutes

Mixing polysulfide

• 2 pastes; white base and brown accelerator

• Mixed for 30-90 seconds until homogenous

Use of polysulfide materials

• Used for final impressions of dentures, partials, crowns, bridges, inlays

• Much more accurate than alginate

Condensation Silicones

• Not used, difficult to pour

• Make many air bubbles

• Have an alcohol byproduct

Polyethers

1960's development

• Shorter working time than polysulfide material

• Stiff and accurate

• Easy to pour with gypsum

• Use disposable trays

• Bad taste

Addition Silicones

Most popular, most accurate and stable, used for crowns, bridges, etc

Addition Silicones Polymerization Reaction

• No evaporation or byproducts

Addition Silicones Viscosities and mixing

• Various available, automix gun can be used

Addition Silicones Use of putty

1. Purpose

• Fills majority of the tray, high viscosity, low viscosity material covers it

2. mixing, working, and setting times

• Manually knead 2 materials together with fingers

Addition Silicones Additives to the material

1. Surfactants: Increase wetting and decrease bubbles

2. Hydrogen absorbers: Prevent hydrogen bubbles when pouring up with gypsum