ppt. lecture for undergraduate dental students who r eyeing for a comprehensive knowledge regarding elastomers
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According to the ability of the set material to be withdrawn over undercuts :Elastic impression materials Hydrocolloid Agar Alginate Rubbers Polysulfide.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
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
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
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.
(viscoelastic )- play major roleIntroduced
as Viscous pastes-
setting reaction- viscoelastic solids.
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
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
Ist elastomer to be introduced mercaptan/Thiokol
dental elastomers Indications completetissue
denture removable fixed partial denture crown
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---
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)
costto silicones and
working time High tear strength High flexibility Good detail reproductionPhillips Science of Dental Materials 1996
by-product pour within one hour single pour Custom Messy paste-paste bad
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
Also known as conventional silicone.
Available in light, medium and putty consistency
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
+ Tetraethyl orthosilicate
Stannous octate Silicone rubber
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%)
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
AdvantagesBetter Clean, Stock
elastic properties pleasant tray
working and setting time
dimensional stabilityshrinkageof ethanol
Addition silicones:Also called as polyvinyl siloxanes Better properties than condensation silicones.
and bridge denture bite registration-Improvement over condensation silicones -no by-product
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
Si-H + CH2=CH-Siactivator
Si-CH2-CH2-Si CH3 CH3
Vinyl + siloxane
No by product, but imbalance hydrogen gas air bubbles in the stone models To avoid this palladium /platinum is added.
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
in latex gloves + ferric and Al sulfate in retraction solution inhibits setting teeth with pumice req before impression.