*  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.