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Base Dimethyl siloxane polymer with terminal OH gp Copper
carbonate, silica as fillers. Catalyst Stannous octoate + alkyl
silicate as Tetra ethyl silicate Reaction silicone rubber + alcohol
exothermic 1C Condensation polymerization largest dimensional
change. Shrinkage-0.6% Consistencies: only two Lowputty very high
Limited shelf life due to oxidation of stannous octoate and ethyl
silicate is not stable in presence of tin. +
Slide 6
Condensation Silicone Base: Polydimethylsiloxanes Fillers (35%
to 75%) Accelerator: Stannous octoate Alkyl silicate By-product =
ethanol The release of ethanol is greater in the low viscosity
materials than the heavy body materials. causes more shrinkage
Slide 7
SILICONE RUBBER Manipulation and Technique Considerations a.
Limited shelf-life: Unstable in tubes. b.Requires mechanical
retention or special tray adhesives c.No syneresis or imbibition,
but continued polymerization shrinkage. d.Better dimensional
stability than RHC but more expensive e.Pouring of models: (1)More
flexible so more chance for distortion during removal (2)Wait 20-30
minutes before pour for stress relaxation to occur Has more
desirable qualities in comparison to polysulphide: Easy mix &
easy to use Better taste and odorless so much more used friendly
& much comfortable for the pat ient Shorter setting time (5-7
minutes)
Slide 8
Addition Silicone
Slide 9
III- Addition Silicone Base Polymer with silane group * Fillers
Catalyst Polymer with vinyl terminal groups *Filler *Chloroplatinic
acid
Slide 10
Reaction: Ionic Polymerization Addition type --- no by
products. Addition type --- no by products. If proper balance
between two polymers is not maintained 2 ry reaction occurs.
Evolution of the hydrogen gas bubbles on surface of impression
voids in stone model. bubbles on surface of impression voids in
stone model. Manufacturers add finely divided Platinium or
palladium to absorb Hydrogen.
Slide 11
Consistencies: low medium high light regular heavy light
regular heavy and a very high (putty). Latex or rubber gloves:
containing sulphur cause retarded or no polymerization of addition
silicones Use: Vinyl gloves.
Slide 12
Addition Silicone Polyvinylsiloxane The terminal vinyl groups
are crosslinked with hydride groups activated by a platinum salt
catalyst.
Slide 13
POLVINYLSILOXANE Manipulation and Technique Considerations a.
BEST impression material for dimensional stability(n0 byproducts):
Pouring should be delayed at least 4 hours for H2 out-gassing.
Pouring can be delayed up to 7-to-10 days (or indefinitely).
b.Stiffness makes removal difficult. c.Most material dispensed
using auto-mixing gun and mixing tips d. Expensive material
Slide 14
Polyether
Slide 15
IV- Polyethers: Base Polymer with ethylene Imine rings terminal
groups. Silica filler plasticizer: glycoether phthalate Catalyst
Cross linking agent as dichlorobenzene sulphonate Reaction:
Cationic polymerization and opening of the imine rings. Additional
polymerization exothermic 4C
Slide 16
Consistencies: *medium consistency + thinner in a separate tube
to viscosity. Recently low and high viscosity. Hydrophilic must not
be stored in H 2 O or humid atmosphere. Very short working time 2.5
minutes by thinner Stiff: difficult to remove from the patients
mouth and to separate from the cast.
Slide 17
Polyether Chemistry Aromatic sulfonate ester 2 3 1 Polyether
molecule with ethylen imine ring
Slide 18
POLYETHER RUBBER Manipulation and Technique Considerations a.
Excellent impression accuracy and dimensional stability. b.Stiff
and therefore difficult to remove without rocking. c.Break seal and
rock slightly to prevent tearing: Low tear resistance. d.Negatively
affected by H2O, saliva, and blood. (1)Since hydrophobic, moisture
increases marginal discrepancy (2)Increased water absorption occurs
if use thinning agents e.Can be dispensed from automated extruder
and mixer (ESPE PentaMix)
Slide 19
Properties Setting time Polysulphide: longest S.T 7-10 min.
Condensation silicone6-8 min. Addition silicone6-8 min.
Polyethers3-4.5 min. W.T and S.T Decreases as viscosity increases
from low to high W.T and S.T by in temperature and humidity.
Slide 20
Double mix technique-single impression Syringe or light bodied
material is injected in and around preparation. Heavy body is
mixed, inserted in tray and placed over syringed material. Two
materials bond, set at about the same time.
Slide 21
Relining or Putty wash techniques- Double Impression 1 st heavy
body is mixed and impression is made before preparation in a stock
tray (set alone). Impression is removed from patients mouth. Heavy
body act as special tray. Light body is mixed, placed over heavy
body, reinserted in patients mouth to record fine details.
Impression is placed twice in patients mouth.
Slide 22
POLYSULFIDE Vs SILICONE Vs POLYETHER
Slide 23
Dimensional Changes on Setting Addition silicone: smallest
change -0.15% Polyethers-0.2% Polysulphides-0.4% Condensation
silicone: largest change -0.6%
Slide 24
Elastic Recovery from Deformation: Addition silicone: shows the
best recovery from deformation Condensation silicone. Polyethers
Polysulfies shows greatest deformation i.e., poorest recovery.
Visco elastic Mechanical properties are time dependent, higher rate
of deformation higher tear strength (sudden pull).
Slide 25
Detail Reproduction: All reproduce 0.02mm wide line except the
very high viscosity 0.075mm line. Compatible with gypsum.
Slide 26
Strain in compression Measure of flexibility. Low consistency
more flexible than high consistency. Polyethers highest stiffness
Addition silicone Condensation silicone Polysulfides lowest Lowest
strain (flexibility) Highest strain
Slide 27
Shore A Hardness Highest: polyethers. Lowest: polysulfides
Hardness and flexibility (strain) affect the force necessary for
removal of impression from mouth.
Slide 28
Tear strength: Tear strength: withstand tearing in thin
interproximal areas. Much higher than hydrocolloids 350-700 g/cm.
Elastomers: Elastomers: 2000-4000g/cm. Polysulfides: Polysulfides:
shows the highest tear strength. But due to high permanent
deformation inaccurate impression
Slide 29
1- Automatic mixing systems (mixing gun).
Slide 30
Advantages Uniform dispensing of catalyst and base. Fewer
bubbles. Less waste than in hand mixed.
Slide 31
Slide 32
2- Hydrophilic Addition Silicone: Hydrophobic materials has
poor wettability: high contact angle: casts with voids (water
repellent). In hydrophilic: surfactants (wetting agents) added
contact angle, improve wettability, simplify pouring of gypsum
models.
Slide 33
Disadvantage: cannot be electroplated as metallizing powder
does not adhere to surface of hydrophilic silicone. 3-Addition
Silicone Containing Hydrogen Absorbers So that dies can be poured
as soon as desired without bubbles.
Slide 34
4- Single viscosity (Monophase) addition silicone with
sufficient shear thinning qualities So they can be used as both
syringe and tray material. Decrease in viscosity at high shear
stress as during syringing or mixing, high viscosity at low stress
as when inserted in the tray.
Slide 35
Disinfection of Elastomeric Impression Materials Done by:
Immersion Spraying Solutions used : Chlorine: harmful to skin,eye,
unpleaseant odour, bleach clothes Aldehyde solutions 2%:
suffocating odour irritating to skin Iodine (iodophors 1%)
Phenols
Slide 36
Disinfection of Elastomeric Impression Materials Polysulphide :
can be disinfected by immersion up to 30 mins Polyether :should be
sprayed < 10min not immersed due to swelling Cond. And Addition
silicone are more dimensionally stable even if immersed for long
periods
Slide 37
Slide 38
Impression During setting shrinkage, distortion of impression
is toward tray. Dies tend to be oversize and so the casting is
oversize. ELASTOMERIC IMPRESSIONS Management of Shrinkage
Impression Impression Tray Use 2-step techniques: (a) Light-Heavy
Body, OR (b) Wash-Putty Load tray with heavy-bodied IM Cover prep
with light-bodied IM from syringe
Slide 39
Impression Impression Tray IMPRESSION MATERIALS Key Properties
Accuracy a.Accuracy = ability to replicate the intraoral surface
details. Dimensional Stability b.Dimensional Stability = ability to
retain its absolute dimensional size over time. Tear Resistance
c.Tear Resistance = ability to resist tearing in thin sections
(such as through the feather-edged material within the gingival
sulcus.