impression Materials Elastic- dr. Sumit Goel

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ppt. lecture for undergraduate dental students who r eyeing for a comprehensive knowledge regarding elastomers

Text of impression Materials Elastic- dr. Sumit Goel

DEFINITION

An

impression

is

essentially

a

negative or reverse replica of any entity. In dentistry, this replica is of the teeth and its supporting and surrounding structures.

According to the ability of the set material to be withdrawn over undercuts :Elastic impression materials Hydrocolloid Agar Alginate Rubbers Polysulfide. Poly silicones. Polyether.

Non-elastic or Rigid impression materials Impression compound ZOE

Ideal Requirements of Impression Materials

1.

Fluid enough to adapt to the oral tissues Viscous enough to be contained in the tray In mouth should transform (set) into a rubbery or rigid solid in a reasonable amount of time: setting time less than 7min

2.

3.

Set impression should not distort or tear when removed from mouth Impressions must be dimensionally stable until the cast can be poured

Impression should maintain dimensional stability after removal of cast to enable making of a second or third cast from same impression Biocompatible Material associated, processing equipment, and processing time should be cost effective

Elastomers or Rubber base Impression materials fulfills most of these

Soft and rubber-like & are known as elastomers or synthetic rubbers.

As per ADA Sp. No. 19 they are called as - non-aqueous Elastomeric dental impression materials.

Liquid polymers which are converted

into solid rubber at room temperature

Contains large molecules with weak

interaction between them, tied together at certain points and form 3D network

Basically

used for dentulous cases

by virtue of their ability of withdrawal from under cuts and their excellent surface detail reproduction.

Types:III.According to chemistry ( setting rxn)1) Polysulfides 2) Polysilicones condensation & addition 3) Polyether

2. According to Viscosity1) Light body or syringe consistency 2) Medium or regular body 3) Heavy body or tray consistency 4) Very heavy body or putty consistency

3. ADA ClassificationBased on selected elastic properties & dimensional changes 1) Type I 2) Type II 3) Type III

General Properties:Excellent reproduction of surface details

Generally hydrophobic (except poly ether)

Good elastic properties

(repeated pouring is possible)Dimensional inaccuracies are

lower but exists due to various reasons

Excellent tear strength. Extended shelf-life Generally higher cost Requires tray adhesive or

mechanical interlocking with the tray.

CHARACTERSTICS

Rheological

properties

(viscoelastic )- play major roleIntroduced

as Viscous pastes-

setting reaction- viscoelastic solids.

imp.

Mat. should be free of any

distortion dev due to strain during its removal, thus the mat. Should be visco elastic i.e. intermediate behavior between elastic solid and viscous liquid.

Supplied as: All elastomers - two paste systems (base & catalyst) Putty consistency supplied in jars

Uses:

Impression material for all applications including

Fixed partial dentures Dentures and edentulous impressions

Border molding of special trays (poly ether) Bite registration As duplicating material for refractory casts

1.

POLY SULFIDES:

Ist elastomer to be introduced mercaptan/Thiokol

Polysulfide First

dental elastomers Indications completetissue

denture removable fixed partial denture crown

and bridge

Composition:Base Paste: Liquid Inert

Polysulfide Polymer 80 to 85 %

-

fillers (Titanium dioxide, zinc sulfate ,copper carbonate or silica) 16 to 18 % Plasticizer-- dibutylpthalate (viscosity) Accelerator---

0.5% Sulfur

Reactor /catalyst/accelerator PasteLead dioxide 60 to 68 % 3% Dibutylphthalate (plasticizer) - 30 to 35 % Sulfur (accelerator) Retarder --- oleic acid or stearic acid Other substances like Magnesium & deodorants- 2 %

Tray adhesive: Butyl rubber or styrene/acrylonitrile dissolved in a volatile solvent such as chloroform or a ketone.

Chemistry and Setting ReactionsThe lead dioxide reacts with the poly sulfide polymer Oxidizing agent like lead dioxide initiates polymerization rxn through

Chain lengthening by oxidation of the terminal -SH groups and Cross-linking between pendant -SH groups.

Exothermic reaction - 3 to 40 C rise in temp. leaving water as by product. It is accelerated by heat and moisture

PbO2 + S HS - R- SH HS - R-S-S-R-SH + H2O Poly sulfide +Water

Mercaptan + Lead dioxide

T-butyl hydro peroxide alternative to PbO2

Polysulfide Reaction--SH O = Pb = O H S S H O = Pb = O HS---------------------SH H O S-= Pb = O -S-S---------------S-S-

S + 3PbO + H O 2 S

mercaptan + lead dioxide

polysulfide rubber + lead oxide + water

OBrien Dental Materials & their

Properties: Unpleasant odor and colour - stains linen & messy to work with Extremely viscous and sticky - mixing is difficult Mixing time is 45 seconds Long setting time of 12.5 (at 370C) - Patient discomfort Excellent reproduction of surface detail

Dimensional stability: - Curing shrinkage is high 0.45%. It has the highest permanent deformation (3 to 5%) among the elastomers It is hydrophobic - so the mouth should be dried thoroughly before making an impression The shelf life is good (2 years)

AdvantagesLower

costto silicones and

compared

polyethersLong

working time High tear strength High flexibility Good detail reproductionPhillips Science of Dental Materials 1996

Disadvantages Poor

dimensional stability

water

by-product pour within one hour single pour Custom Messy paste-paste bad

traysmix

odor may stain clothing Long

setting timePhillips Science of Dental Materials 1996

Silicone Rubber Impression Material:Developed to over come some of the disadvantages of poly sulfide

TypesBased on the type of polymerization reaction

1) Condensation silicones 2) Addition silicones

CONDENSATION SILICONE:

Also known as conventional silicone.

Available in light, medium and putty consistency

Indications

complete crown

dentures

and bridge

COMPOSITION:Base pasteAccelerator paste

Polydimethyl siloxane Tetraethyl (hydroxyl-terminated) Orthosilicate (cross linking agent) Colloidal silica or Stannous octoate microsized metal (catalyst) oxide (filler) 35-75% Color pigments

SETTING REACTION: CH3 OC2H5

OH-Si-OH + C2H5O-Si- OC2H5 Sn octoate CH3 OC2H5 Silicone + CH3CH2OH

Cross-linking between terminal group of the silicon polymers and alkyl silicate to form 3D network

Dimethyl siloxane

+ Tetraethyl orthosilicate

Stannous octate Silicone rubber

+ ethyl

alcohol

Ethyl alcohol byproduct-evaporate -shrinkage &dimensional instability

Tray adhesive Polydimethyl siloxane & Ethyl silicate

Properties: Pleasant odor and color. Mixing time of 45sec & setting time of 8-9mins. Excellent reproduction of surface details and highly elastic. Lesser dimensional stability - high curing shrinkage (0.4 - 0.6%)

permanent

deformation due to shrinkage caused by the evaporation of ethyl alcohol is also high (1-3%). - needs a dry field.. inert. with all gypsum

Hydrophobic Biologically Compatible

products.

AdvantagesBetter Clean, Stock

elastic properties pleasant tray

putty-wash

Good

working and setting time

DisadvantagesPoor

dimensional stabilityshrinkageof ethanol

high

polymerization evaporation

pour

immediately30 minutes

within

Hydrophobicpoor

wettability

Addition silicones:Also called as polyvinyl siloxanes Better properties than condensation silicones.

Indicationscrown

and bridge denture bite registration-Improvement over condensation silicones -no by-product

Composition:

Base: Poly (methyl hydrogen siloxane) Other siloxane prepolymers Fillers

Accelerator:Divinyl poly siloxane Other siloxane pre polymers Platinum salt: Catalyst (chloroplatinic acid) Palladium (Hydrogen absorber) Retarders Fillers

SETTING REACTION:CH3 CH3Pt salt

CH3

CH3

Si-H + CH2=CH-Siactivator

Si-CH2-CH2-Si CH3 CH3

CH3

CH3

Vinyl + siloxane

Silane siloxane

Silicone rubber

No by product, but imbalance hydrogen gas air bubbles in the stone models To avoid this palladium /platinum is added.

CLINICAL SIGNIFICANCE:

Sulfur compounds retard the setting of silicones One source of sulfur contamination is from latex gloves Vinyl gloves should be used

Properties: Pleasant odor and color Excellent reproduction of surface details Mixing time of 45 secs ,setting time of 5-9 mins. Best dimensional stability - low curing shrinkage (0.17 %)

-

lowest permanent deformation (0.05 0.3 %) Stone pouring delayed by 1-2 hours Extremely hydrophobic, some manufacturers add a surfactant (detergent) to make it more hydrophilic Good shelf life of 1-2 yrs Good tear strength (3000gm / cm2)

Advantages Highly pour

accurate High dimensional stabilityup to one week Stock

or custom trays Multiple casts Easy to mix Pleasant odor

DisadvantagesExpensive Sulfur

in latex gloves + ferric and Al sulfate in retraction solution inhibits setting teeth with pumice req before impression.

Polishing