impression materials 2

Question 1

what are flexible materials

Answer 1

hydrocolloids

elastomers

Question 2

hydrocolloids

Answer 2

heterogeneous mixture of 2 phases where the 2 phases are not readily differentiated

Question 3

solution

Answer 3

homogenous mixture of single phase

Question 4

suspension

Answer 4

mixture of two phases

Question 5

hydrocolloids can be ..

Answer 5

irreversible or reversivke

Question 6

example of reversible hydrocolloid

Answer 6

agar

Question 7

agar composition and hydrocolloid properties

Answer 7

natural state gel
heating becomes sol
1)	Agar (colloid)
2)	Borax (strengthen gel)
3)	Potassium sulphate
4)	Water (dispersion medium)

Question 8

properties of agar

Answer 8

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

Question 9

advantages/dis of agar

Answer 9

1) good surface edtail
2) reusable
3) easily sterilised
Disadvantages
1) Need special equipment
2) Dimensional instability

Question 10

alginate type of hydrocolloid an compostion

Answer 10

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)

Question 11

how is solid alginate formed

Answer 11

polymer chains forma and link

- interchange of calcium and sodium

Question 12

properties of alginate

Answer 12

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

Question 13

imbibition

Answer 13

shape change as water is absorbeed by solid hydrocolloids causing an increase in volume

Question 14

syneresis

Answer 14

shape changes as loss of a liquid from a gel causing a reduction in volume

Question 15

elastomer impression materials

Answer 15

1) Polysulphides
2) Poly ethers
3) Silicones
- condensation cured
- addition curred

Question 16

polysulphide composition

Answer 16

Base paste
-	polysulphide
-	filler (TiO2)
Activator pate
-	sulphur
-	lead dioxide
-	plasticiser (dibutyl phthalate)

Question 17

how do polysulpides change state

Answer 17

liquid to solid
sulphur groups join the tw chains
condensation polumerisation

Question 18

applications of polysulphides

Answer 18

1) Crown
2) Bridge work
3) partial denture
4) over dentures
5) implants

Question 19

polyether impression material

Answer 19

Base paste
-	polyether
-	filler (colloidal silica)
Activator paste
-	filler
-	plasticiser (dibutyl phthalate)
-	aromatic sulphonate ester

Question 20

how is polyether set

Answer 20

cross linking with amine group ends

addition polymerisation

Question 21

addition polymerisation sets

Answer 21

activation
initiaiton
propagation
termination

Question 22

applications for polyether

Answer 22

1) crowns
2) bridge work
3) partial dentures
4) implants
5) over dentures

Question 23

silicon impression composition for condensation cured

Answer 23

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)

Question 24

silicon impression composition for addition cured

Answer 24

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

Question 25

applicaitonsfor silicone

Answer 25

1) crown 2) bridge work 3) partial dentures 4) implants 5) over dentures

Question 26

Elastomertic impression materials merits

Answer 26

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

Question 27

mechanical properties and order

Answer 27

stiffness - PSPE>CCS>ACS tear strength PS>>PE>CS=AS

Question 28

surface detail of elastomers

Answer 28

polyethers best | all have good reproduction of surface detail

Question 29

failures of impressions

Answer 29

rough or uneven surface air bubbles irregualr shaped voids

Question 30

what can cause air bbbles

Answer 30

rapid set improper miing surface contamination e.g. moisture

Question 31

poor fit of impressions

Answer 31

distortion | casting too big/too small

Question 32

intraoral scanners ad and dis

Answer 32

``` 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) ```