Elastomers - Copy

Elastomeric Impression Materials

PolysulfideCondensation SiliconesAddition SiliconePolyether

What are we using?

Polymers that can be converted to Elastomers at room temperature by being mixed with a suitable catalyst

Compared To Hydrocolloids, Elastomers Offer:

Stability in air after settingExcellent detail reproductionHigher tear strengthAbility to prepare dies other than that of

gypsum types

Polysulfide Rubber Impression Materials

Come as:

– Base– CatalystMixing time: 45 seconds

Polysulfides are presented in three consistencies:

Light body

Regular body

Heavy body

Some brand names

• Permlastic (Kerr)• Coe-flex (GC-America)• Omniflex (GC-America)• Neo-plex (Heraeus Kulzer)

Base is Composed of:

• 80% low m.wt. polymer with reactive mercaptan groups –(--SH)

• 20% reinforcing agents as TiO2, ZnSO4 or SiO2

Catalyst

• Catalyst (accelerator) causes oxidation of mercaptan groups resulting in chain growth and cross-linking.

• Most common: Lead dioxide (brown).• Copper hydroxide (green) is more pleasant

but less accurate

Setting Reaction of Polysulfides

Mercaptan + Lead Dioxide →Polysulfide + Water

increase in m. wtincrease in m. wt

elasticityelasticity

Polymerization: Exothermic ReactionAccelerated by Heat & Moisture

Dimensional Changes in Polysulfides

• Polysulfides shrink 0.25% in the first 24h• Models should be poured promptly (within 1

h)

Tray adhesive may affect unset polysulfide

The solvent from the adhesive layer should be allowed to evaporate prior to loading of the impression trays

Polysulfide becomes stronger with time

If tearing of polysulfide impression occurs, second impression should be left in the mouth longer to obtain a higher tear strength

Remember Please!

Allow impression to sit for 20 – 30 minutes before casting to allow for ELASTIC RECOVERY

Properties of POLYSULFIDES

• Toxicity: Safe despite presence of PbO2

• Working time: 4 – 5 min (↓ with ↑ temp)• Final setting time: 8 – 12 min from start• Polymerization shrinkage: 0.25 – 0.40 % / 24h• Flow of set material: 0.3 – 0.9 % in 15 min

Properties of POLYSULFIDES

• Permanent deformation: slightly higher than hydrocolloids

• Flexibility: ↓ with ↑ consistency• Tear strength: 22 lbs/inch2 (8 times higher

than alginate)• Detail reproduction: Excellent

Disadvantages of Polysulfides

• Poor color & offensive smell (sulfur)

• Staining of linen and uniforms (Lead)

• Exhausting mixing • Long setting time• Shrinkage on setting• Relatively high permanent

deformation (visco-elastic)

Silicone Rubber Impression Materials

• Condensation Type• Addition Type

Condensation Silicones are presented as:

• Activator Paste (or liquid)• Base Paste or Putty

Base Paste Composition

• Dimethyl Siloxane: moderately low m.wt polymer with siloxane chain backbone (-Si-O-Si-) and reactive –OH gps

• Reinforcing agents (SiO2) to give proper consistency and strength.

Accelerator Composition

• Tin organic ester (Tin Octoate)• Alkyl Silicate (Ortho-ethyl silicate)

Setting is by condensation & and alcohol bi-product is released

Setting Reaction

Dimethyl Siloxane + Ortho-ethyl Silicate + Tin Octoate → Silicone Rubber + Ethyl Alcohol

Properties of Condensation Silicones

Toxicity: Safe, Catalyst may cause skin allergyWorking time: Shorter than polysulfideSetting time: 6- 10 minutesViscosity: less than polysulfide (easier to mix)

Properties of Condensation Silicones

Polymerization shrinkage: more than polysulfide (0.5 %)– ↓ with ↑ filler content– Occurs mostly in 1st h

Permanent deformation: Less than Polysulfides (higher cross-linking)

Properties of Condensation Silicones

Flexibility: ↓ with ↑ viscosityFlow after setting: Less than polysulfideDetail Reproduction: ExcellentTear Strength: Lower than polysulfide but higher

than alginate

ADDITION SILICONES“Poly Vinyl Siloxane”

• Presented as:- Light body- Regular body- Heavy body- Putty

Light & regular bodies can be supplies in a dual cartridge (auto mixing system)

Composition

• Base Paste (or putty) composition;

1. Hydrogen-containing siloxane2. Reinforcing filler

• Reactor Paste (or putty) composition1.Vinyl-terminal siloxane2. Chlorplatinic acid catalyst 3. Reinforcing filler

Setting Reaction

• Hydrogen-containing siloxane + vinyl-terminal siloxane + Chlorplatinic acid → polyvinyl siloxane

If hydroxyl groups are present, a side reaction occurs resulting in the release of hydrogen that produces bubbles in models prepared within the 1st hour

Inclusion of a hydrogen absorber such as Palladium in the impression material eliminates hydrogen evolution

The issue of the gloves !

• Putties should not be mixed when latex rubber gloves are worn.

• Wash hands thoroughly prior to hand mixing • Components in the rubber (NOT THE

POWDER) will retard setting by poisoning the platinum catalyst

Properties of Addition Silicones

• Dimensional changes in 1st 24 h: -0.1 %• Permanent deformation on removal: 0.2 %

(lowest of all impression Materials)• % flow: lowest of all impression materials• Hydrophobic (Some types have surfactants

added to paste)• Putty displaces wash

Properties of Addition Silicones

• Superior dimensional accuracy• Working time: 5 – 8 minutes• Flexibility: LOWER than all other materials

except Polyether• Tear strength: Moderate

Polyether Impression Material

Supplied as:

• Base, Catalyst and thinner (Regular body)

Or• Light and heavy

bodies and catalyst

Properties of Polyether Impression Materials

• Working time: 2 minutes (the shortest)• Setting time: 3- 5 minutes• Flow: ↑ with ↑ shear stress (Thixotropic –

pseudo plastic ?) • Permanent deformation: less than polysulfide

but more than addition silicone• Modulus of elasticity: much higher than other

impression materials except for addition silicone

Properties of Polyether Impression Materials

• Greatest tendency for elastic behavior• Flexibility: low (3 %) compared to polysulfide

(7%) and condensation silicone (5%)• Tear strength: lower than others. 4 mm

thickness is required rather than 2 mm• Dimensional changes: Lower than others

except addition silicone. Swells if stored in water

• Catalyst can cause skin irritation