Silicone Elastomeric Impression materials

CONDENSATION SILICONES

  • It was the first type of silicone impression material.
  • Also known as conventional silicones.
  • Setting occurs in room temp , so called as RTV silicones.

Composition:

Base paste

  1. Polydimethyl siloxane (hydroxy terminated)
  2. Colloidal silica or micro sized metal oxide filler
  3. putty viscosity – 60-70%
  4. medium viscosity – 35-75 %
  5. low viscosity – 5-15%
  6. color pigments

accelerator paste

  1. alkyl silicate (ortho ethyl silicate)
  2. stannous octate
  3. inert filler.

Setting reaction:

Dimethyl        orthoethyl                                                    silicone            ethyl

+                    —STANNOUS OCTATE—–>                           +

Siloxane          silicate                                                         rubber           alcohol

Properties:

  1. Setting time – 8-9 min.
  2. Mixing time – 45 sec.
  3. Excellent detail reproduction
  4. Dimensional stability – high curing shrinkage.
  5. Permanent deformation – 1-3 %
  6. Ttear strength – 3000gm/cm
  7. Sitffer & harder than polysulfide material.
  8. Hydrophobic
  9. Can be electroplated with silver & copper.
  10. Shelf life – 2 yrs.

Advantages:

  1. Adequate working & setting timer.
  2. Pleasant odor & no staining.
  3. Adequate tear strength
  4. Better elastic properties on removal.
  5. Less distortion on removal.

Disadvantages:

  1. Adequate accuracy if poured immediately.
  2. Poor dimensional stability.
  3. Potential for significant distortion.
  4. Putty-wash method is technique sensitive.
  5. Slightly more expensive.

ADITION SILICONES

  • Introduced after condensation silicones
  • Have better properties than condensation silicones.
  • Also known as poly vinyl siloxane.

Composition:

Base paste

  1. Poly(methyl hydrogen siloxane)
  2. Other siloxane prepolymers
  3. Fillers

putty viscosity – 60-70 %
medium viscosity – 35-75 %
low viscosity – 5-15%

accelerator paste

  1. Divnyl polysiloxane
  2. Inert oils & fillers
  3. Platinum salt
  4. Palladium
  5. Retarders
  6. Fillers.

Setting reaction:

Poly vinyl siloxane + silane siloxane     —–Pt ,Salt —–>   silicone rubber

Properties:

  1. Setting time: – 5.9 min.
  2. Mixing time  – 45 sec
  3. Excellent surface detail reproduction.
  4. Highest dimensional stability amongst elastomers
  5. Lowest curing shrinkage – 0.17 %
  6. Good tear strength – 3000gm/cm
  7. Extremely hydrophobic
  8. Low flexibility
  9. Electroplated with copper & silver.
  10. Shelf life – 1-2 yrs.

Advantages:

  1. Short setting time
  2. Adequate tear strength
  3. Extremely high accuracy
  4. Minimal distortion on removal
  5. Dimensionally stable  even after 1 week
  6. If hydrophilic, good compatibility with gypsum.

Disadvantages:

  1. Hydrogen gas may cause dimensional change.
  2. Hydrophobic & hence requires a very dry field.
  3. Expensive.