DM8 - Alginates

Question 1

what are the 3 main impression materials

Answer 1

Hydrocolloids
Elastomers
Non-elastic materials

Question 2

what are the 2 types of hydrocolloid impression materials

Answer 2

Reversible agar

Irreversible alginates

Question 3

what are the 3 types of non-elastic material, elastic impression materials

Answer 3

zinc oxide eugenol
impression plaster
impression compound

Question 4

what are the 3 types of elastomers in impression materials

Answer 4

silicones
Polysulphides
polyethers

Question 5

what are the 2 types of silicones for elastic impression materials

Answer 5

condensation

addition

Question 6

why are alginates classed as irreversible hydrocolloids

Answer 6

sets by a chemical cross-linking reaction

Question 7

what is alginate made from

Answer 7

extracted from seaweed with hydroxide

NaOH forms sodium alginate

Question 8

what happens when alginate is mixed with water

Answer 8

forms colloidal suspension

sets by chemical cross linking reaction (irreversible hydrocolloid)

Question 9

what are the 6 things in alginate powder composition and what are their functions

Answer 9

◆Na/K alginate 12%
◆ diatomaceous earth (filler - body/strength) 70%
◆ CaSO4 (cross-linking agent) 12%
◆ Na3PO4 (or Na2CO3) (retarding agent) 2%
◆ Sodium silicofluoride or Sodium fluorotitinate (pH controller) 4%
◆ Magnesium oxide
(pH controller) 3%

Question 10

what is the function of diatomaceous earth in the composition of alginate powder

Answer 10

filler - body/strength 70%

Question 11

what is the function of CaSO4 in the composition of alginate powder

Answer 11

cross-linking agent 12%

Question 12

what is the function of Na3PO4 (or Na2CO3) in the composition of alginate powder

Answer 12

retarding agent 2%

Question 13

what is the function of Sodium silicofluoride or Sodium fluorotitinate in the composition of alginate powder

Answer 13

pH controller 4%

Question 14

what is the function of magnesium oxide in the composition of alginate powder

Answer 14

pH controller 3%

Question 15

where does cross linking during the setting stage occur within the sodium alginate powder

Answer 15

polymer chain

chemical bonds joining 2 polymer chains by Ca

Question 16

explain what happens for the setting reaction to occur (function of cross-linker + retarder)

Answer 16

look at ipad for notes

Question 17

why are pH controllers added to alginate mix

Answer 17

water + alginate -> pH12
setting = pH 3 –> once set pH9
◆ ↑ alkalinity + acidity give poor plaster/stone surface
◆ pH controller ↓ pH to 8 (near neutral) so its compatible with casting materials
◆ improves surface of resulting gypsum cast (mould)

Question 18

what are the ph controllers used in alginates

Answer 18

sodium silicofluoride (or fluorotitinate) 
MgO

Question 19

how is chromoclone linked to pH controllers

Answer 19

purple = water + alginate
pink = mixed well + loaded into tray
light peach = insert tray into mouth
◆ prevents unnecessary time in oral cavity

Question 20

what is the mix time for chromoclone alginate

Answer 20

90-110 secs

Question 21

what are the advantages of using alginates

Answer 21

good setting behaviour
◆ Na3PO4 (retarder) suppresses setting at first
◆ material - viscous paste while tray is in mouth
◆ once setting starts, completed real quickly -> ↓ impression time

material is cheap + reliable

Question 22

what are the 5 main disadvantages of using alginates

Answer 22

1. alginates in air loses water
2. alginate must be immersed in water/disinfecting solution
3. poor tear strength
4. ↑ viscoelastic material
5. doesn’t adhere well to tray

Question 23

how is it a disadvantage of alginate that in air it loses water?

Answer 23

◆ continual shrinkage in air (so need immediate casting, but its not practical)
◆ poor dimensional stability in air
◆ impression must be covered in damp napkin/gauze + plastic bag, sealed

Question 24

how is it a disadvantage of alginate that it needs to be immersed in water/disinfecting solution?

Answer 24

◆ initially swells, then shrinks IMBIBITION
◆ poor dimensional stability in solutions
-> so must follow disinfection protocol

Question 25

how is it a disadvantage of alginate that it is high viscoelastic material?

Answer 25

◆ snap-removal technique needed | Technique sensitive

Question 26

how is it a disadvantage of alginate that it doesn't adhere well to the tray?

Answer 26

◆ retention to tray by mechanical locking features | ◆use perforated trays or apply adhesive

Question 27

what is the disinfectant protocol for alginates

Answer 27

1. remove alginate from mouth 2. rinse in tap water -> removes blood/saliva 3. Immerse in disinfecting solution for ONLY 10MINS 4. Remove from solution + rinse in water 5. wrap in damp napkin/gauze + seal in polythene bag

Question 28

how do you properly dispense alginates + why needed

Answer 28

◆ density of diff ingredients in powder differ ◆ shaken before use, let dust settle 2mins before use ◆ silica particles in dust can be health hazard

Question 29

what do you add to get dust free alginate

Answer 29

small amount ethylene glycol

Question 30

what are the 3 types of alginates you can get

Answer 30

◆Fast ◆regular ◆slow set depending on amt of retarder and cross-linking agent

Question 31

why is agar classed as a reversible hydrocolloid

Answer 31

sets by physical process

Question 32

what states does agar exist at

Answer 32

◆ solid gel at room temp ◆ viscous liquid at 60°C ◆ cooling - gelling = physical process

Question 33

what are the 5 things in the composition of agar and their functions

Answer 33

1. Agar (colloid) 13-17% 2. Borates (strengthen gel) 3. potassium sulphates (accelerates setting of stone) 4. thixotropic materials - wax (filler) 5. water (dispersion medium)

Question 34

what is the function of agar in the composition of agar impression materials

Answer 34

colloid 13-17%

Question 35

what is the function of borates in the composition of agar impression materials

Answer 35

strengthen gel 0.2-0.5%

Question 36

what is the function of potassium sulphates in the composition of agar impression materials

Answer 36

accelerates setting of stone 1-2%

Question 37

what is the function of thixotropic materials (wax) in the composition of agar impression materials

Answer 37

filler 0.5-1%

Question 38

what is the function of water in the composition of agar impression materials

Answer 38

dispersion medium

Question 39

how do you dispense agar

Answer 39

◆ tubes - prevent water loss ◆ Tubes in water bath at 60% (liquid) -> bath at 40% before use ◆ Material squeezed out onto special metal tray in mouth ◆ Tray cooled by water (tubes attached on outer surface)

Question 40

what are the 3 advantages of using agar

Answer 40

◆ set up, easy to use ◆ Cheap material ◆ Impressions have good surface + fine detail -> due to setting behaviour

Question 41

what are the 4 disadvantages of using agar

Answer 41

◆ Impressions cast immediately - avoids shrinkage due to synerisis ◆ Absorbs water (imbibition) - can distort impression ◆ Poor tear strength but better than alginates ◆ Highly viscoelastic so technique sensitive; snap-removal technique