Addition silicones (polyvinyl siloxane)

Question 1

base paste

Answer 1

• polyethyl hydrogen siloxane
• other siloxane prepolymers
• fillers to give bulk and viscosity

Question 2

accelerator paste

Answer 2

• divinyl polysiloxane
• inert oils and fillers(forms bulk of paste)
• platinum salt (chloroplatinic acid)
• palladium or hydrogen absorber
• retarders
• fillers

Question 3

setting rxn

Answer 3

• addition polymerization
• vinyl groups of base paste react w/ silane groups of accelerator paste and cross-linking occurs
• if in improper proportion, H gas may evolve during setting rxn
• Palladium is added to absorb H to prevent dimensional change
• time: 5-9 min
• mix: 45 sec

Question 4

other characteristics

Answer 4

• excellent surface detail reproduction
• high dimensional stability among elastomers
• lowest curing shrinkage-0.17%
• good tear strength 3000 gm/cm
• extremely hydrophobic
• low flexibility
• electroplated w/ copper and silver
• shelf life 1-2 years

Question 5

advantages

Answer 5

• short setting time
• adequate tear strength
• extremely high accuracy
• minimal distortion on removal
• dimensionally stable even after 1 week
• if hydrophilic, good w/ gypsum

Question 6

disadvantages

Answer 6

• may cause dimensional change
• hydrophobic
• expensive