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Flashcards in DML8: Alginates Deck (30)
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What is the use of impression materials

- Used to produce detailed replica of teeth and oral tissues
- Used to make a model from the impression in stone/paster
- The model is then used for pull/partial dentures, crowns, bridges and inlays or as study casts


What are the different types of elastic impression materials

1. Hydrocolloids
- reversible agars
- irreversible alginates

2. Elastomers
- polysulphides
- polyethers
- silicones; condensation and addition

3. Non-elastic materials
- impression plaster
- impression compound


Why are alginates irreversible impression materials

Because they are set by chemical cross-linking and so this irreversible hydrocolloid structure cannot be reverted into its alginate powder form after set


What is alginate

It is a natural extract from seaweed with Na/K hydroxide solution to give sodium/potassium alginate


Define hydrocolloid

A colloidal suspension in water


What does alginate contain and what is the role of each component

Sodium/potassium alginate
Diatomaceous earth = filler (reduces tear)
CaSO4 = cross-linking agent
Na3PO4 (Na2CO3) = retarding agent
Sodium silicofluoride/sodium fluorotitinate = pH controller
MgO = pH controller

All these components within the alginate powder are mixed with water


How is alginate cross-linked

Chemical bonds form which join two polymer chains together via Ca2+ ions which replace Na+


What is the importance of retarders in alginate

If these weren't present then the alginate would immediately form a gel in water so there would be no working time; the retarder allows time to mix the material into a paste and remove agglomerates as well as allowing time for placement in the mouth where the final set will occur


How do retarders work in alginates

The cross linking agent CaSO4 is spraining soluble so will slowly ionise, the retarder Na3PO4 is very soluble so will ionise quickly; the Ca2+ ions of the cross-linker will react with PO4^2- to form insoluble Ca3(PO4)2

After all of the PO4^2- ions are used up, the Ca2+ ions will then react with the alginate to set the material


What is the importance of pH controllers in alginates

When setting the pH decreases to pH 3.5 initially and then rises to pH 9 on setting - alkalinity/ acidity of the set gel will give a poor plaster/stone surface

Therefore pH controllers are added to decrease the pH to near neutral so that the alginate impression is compatible with casting materials


What is chromoclone

This is an alginate which has indicators incorporated into the powder to give colour changes at various pH's

1. Water added; PURPLE
2. Mixed thoroughly; PINK
3. Loaded in tray and inserted; LIGHT PEACH


What is the benefit of chromoclone

It prevents unnecessary time in the oral cavity and the time taken from mix to set time = 90 - 110 seconds


What are the advantages of alginates

1. Good setting behaviour; Na3PO4 surpasses setting, and when setting begins it is very quick

2. Cheap and reliable material


What are the disadvantages of alginates

1. Alginate in air loses water resulting in shrinkage
2. This means immediate casting is needed (not practical)
3. Alginate is immersed in water/disinfecting solution which causes swelling and then shrinkage so there is poor dimensional stability in solutions
4. Poor tear strength (remnants left in mouth)
5. Highly viscoelastic material
6. Doesn't adhere well to tray and retention is achieved by perforations


How can alginate impressions be preserved in-between taking the impression and making the cast

By covering with a damp napkin/gauze and placing in a sealed plastic bag (only sufficient for a few hours)


What are the issues surrounding alginate being a highly viscoelastic material

- snap removal techniques needed
- permanent deformation occurs <1.5% (can be minimised if undercuts aren't deep)


Outline the disinfection protocol for alginate impressions

1. Remove set alginate/tray from mouth
2. Rinse in tap water to remove blood/saliva
3. Immerse in disinfecting solution (<10mins)
4. Remove from solution and rinse in tap water
5. Wrap in damp napkin/gauze and seal in polythene bag


How are alginates dispensed

Make sure to shake box as the dense particles will settle, tap lid before opening to prevent cloud of dust


Why is a cloud of dust from alginates dangerous

It contains silica particles which can trigger asthma


How are dust free alginates attained

Ball-mill alginate powder with a small amount of ethylene glycol


What does the amount of sodium phosphate in alginate determine

The higher the amount, the slower the set and there is more retarder present (this is used in hotter countries)

The lower the amount, the faster the set (cold countries)


Why is agar a reversible impression material

Because it has a reversible hydrocolloidal structure which is set by a physical process


What are the properties of agar

- solid gel at room temperature
- viscous liquid at 60 degrees
- on cooling it reverts to the gel
- gelling is a physical process


What are the components in agar impressionants and what are their roles

1. Agar = colloid
2. Borates = strengthen gel (filler is not comparable with stone because it slows the set)
3. Potassium sulphates = counteract borate so accelerates setting of time
4. Thixotropic materials (wax) = filler
5. Water = dispersion medium


How is agar dispensed

- in tubes to prevent water loss (placed in water bath 60)
- transferred to water bath 40 before use
- material squeezed onto metal tray seated in mouth
- cooled by water and circulated by tubes attached on outside of the tray


What are the advantages of agar

1. Cheap and easy once equipment is set up
2. Impressions have good surface detail due to setting behaviour


What are the disadvantages of agar

1. Bulky metal tray cannot be used for paediatrics
2. Impressions must be cast immediately to prevent shrinkage due to loss of water
3. Absorption of water occurs which causes distortion
4. Agar has poor tear strength (better than alginates)
5. Highly viscoelastic material (snap-removal technique and permanent deformation <1%)


Why do agars have good surface details

Because the material near the tray surface sets first as water circulates here, this gives more time for the viscous paste to flow into the undercuts and so there is increased time before the inner part of the material properly sets



Shrinkage in air due to loss of water



Absorption of water causing distortion