impression materials 2 Flashcards
what are flexible materials
hydrocolloids
elastomers
hydrocolloids
heterogeneous mixture of 2 phases where the 2 phases are not readily differentiated
solution
homogenous mixture of single phase
suspension
mixture of two phases
hydrocolloids can be ..
irreversible or reversivke
example of reversible hydrocolloid
agar
agar composition and hydrocolloid properties
natural state gel heating becomes sol 1) Agar (colloid) 2) Borax (strengthen gel) 3) Potassium sulphate 4) Water (dispersion medium)
properties of agar
1) Good surface detail
2) can be used on undercuts but liable to tear on deep undercuts
3) non toxic and non irritant
4) slow setting time
5) poor tear resistance
6) adequate shelf life
7) ca be sterilised by an aqueous solution of hypochlorite
advantages/dis of agar
1) good surface edtail
2) reusable
3) easily sterilised
Disadvantages
1) Need special equipment
2) Dimensional instability
alginate type of hydrocolloid an compostion
irreversible - sodium alginate (hydrogel former) calcium sulphate dihydrate (provide calcium ions) sodium phosphate (control working time) potassium sulphate (enhance setting) fillers (consistency ) sodium silicofluoride (control ph)
how is solid alginate formed
polymer chains forma and link
- interchange of calcium and sodium
properties of alginate
1) dust free powder
2) cheap
3) limited shelf life
4) well controlled working and setting time
5) mucostatic
6) hydrophilic
7) poor surface reproduction
8) poor storage stability
- imbibition (water sorption)
- syneresis (water loss)
9) low tear strength
10) excessive permanent deformation
imbibition
shape change as water is absorbeed by solid hydrocolloids causing an increase in volume
syneresis
shape changes as loss of a liquid from a gel causing a reduction in volume
elastomer impression materials
1) Polysulphides
2) Poly ethers
3) Silicones
- condensation cured
- addition curred
polysulphide composition
Base paste - polysulphide - filler (TiO2) Activator pate - sulphur - lead dioxide - plasticiser (dibutyl phthalate)
how do polysulpides change state
liquid to solid
sulphur groups join the tw chains
condensation polumerisation
applications of polysulphides
1) Crown
2) Bridge work
3) partial denture
4) over dentures
5) implants
polyether impression material
Base paste - polyether - filler (colloidal silica) Activator paste - filler - plasticiser (dibutyl phthalate) - aromatic sulphonate ester
how is polyether set
cross linking with amine group ends
addition polymerisation
addition polymerisation sets
activation
initiaiton
propagation
termination
applications for polyether
1) crowns
2) bridge work
3) partial dentures
4) implants
5) over dentures
silicon impression composition for condensation cured
Base paste - silicone polymer - filler (colloidal silica) - organo tin compound activator paste - silicone polymer - filler (colodial silica) - tetra-ethyl silicate) Hydroxyl groups tin is the catalyst - condensation reaction with release of a a molecule eg ethanol (depends on what is joining)
silicon impression composition for addition cured
Base paste - silicone polymer - filler (colloidal silica) - silanol activator paste - silicone polymer - filler (colodial silica) - Pt catalyst
Reaction is driven by a radical by addition polymerisation
no by products
applicaitonsfor silicone
1) crown
2) bridge work
3) partial dentures
4) implants
5) over dentures
Elastomertic impression materials merits
handling
- polysulhpides and condensation cured silicone difficult to mix
Working/setting times
- polysulphides long tie
condensation cured erratic setting due to moisture contamination
polyethers good setting times
addition cured silicones can be inhibited by latex gloves
mechanical properties and order
stiffness
- PSPE>CCS>ACS
tear strength
PS»PE>CS=AS
surface detail of elastomers
polyethers best
all have good reproduction of surface detail
failures of impressions
rough or uneven surface
air bubbles
irregualr shaped voids
what can cause air bbbles
rapid set
improper miing
surface contamination e.g. moisture
poor fit of impressions
distortion
casting too big/too small
intraoral scanners ad and dis
Ad: - patient comfort - dentist auto evaluation - reduced model time - favours clinical lab communication Dis - cost investment - training - just surface registration - coating (as that laser might not get insuffiencent detail)
