Alginates Flashcards
What are impression materials used for?
Used to produce a detailed replica of the teeth and oral tissues
What are the 3 types of elastic impression materials?
- Hydrocolloids
- Elastomers
- Non-elastic materials
Give an example of a reversible and irreversible hydrocolloid
- Reversible = Agar
- Irreversible = Alginate
What are the 3 types of elastomers?
- Silicones
- Polysulphides
- Polyethers
Name 3 non-elastic impression materials
- ZOE
- Impression plaster
- Impression compound
What makes alginates irreversible hydrocolloids?
The reactions sets by a chemical cross-linking reaction
What are alginates extracted from?
Extracted from seaweed with a hydroxide
Ie with NaOH forms sodium alginate
What happens when alginate is mixed with water?
When alginate is mixed with water, it forms a colloidal suspension in it, and then it sets by a chemical cross-linking reaction
What is a typical alginate powder composition? (6)
Include percentage composition
- Na/K alginate (12%)
- Diatomaceous earth (70%)
- CaSO4 (12%)
- Na3PO4 (2%)
- Sodium silicofluoride or Sodium fluorotitinate (4%)
- Magnesium oxide (3%)
What is the role of Diatomaceous earth in alginate powder?
Acts as a filler to strengthen alginate
What is the role of CaSO4 in alginate powder?
Cross-linking agent
What is the role of Na3PO4 in alginate powder?
Retarding agent
What is the role of Sodium silicofluoride, Sodium fluorotitinate and Magnesium oxide in alginate powder?
pH controller
In sodium alginate polymer where does cross linking occur?
At COONa
Define cross link
Chemical bonds joining two polymer chains together
What joins 2 polymer chains of alginates?
What is displaced?
- Ca
- Na in COONa is displaced
What is the function of the retarder in alginate powder?
Without retarder alginate gels immediately, so a retarder gives working time
How much cross-linking occurs when phosphate ions are present in the powder?
The cross-linking reaction is suppressed while the phosphate ions are being used up
So no cross-linking
Why are pH controllers added to alginate powders?
- When setting pH of alginate initially decreases to below pH 3.5 and rises to pH 9 on setting
- When set gel is too acidic or alkaline it will give a poor plaster/stone surface
- So pH controllers are added to decrease the pH to near neutral (~8)
- In order to make it compatible with casting materials
What feature does Chromoclone have that makes the alginate easier to use?
Powder colour changes at different pH’s
With Chromoclone what colour appears when water is added to alginate?
Purple
What colour change of chromoclone needs to occur in order to indicate the alginate is ready to be loaded in a tray?
Purple to Pink
Which colour change indicates that chromoclone in a tray can be inserted into the mouth?
Pink to light peach
What are some advantages of alginates? (2)
- Very good setting behaviour
- Cheap and reliable
What are some disadvantages of alginates? (5)
- Once alginates are set they lose water in air
- Immersing the alginate in disinfectant causes initial swelling then shrinking causes poor dimensional stability
- Poor tear strength
- Does not adhere well to tray
- Highly viscoelastic
Alginates losing water air once set means what for general properties? (2)
How do you manage this?
- Continual shrinkage in air
- Poor dimensional stability
- Impressions must be covered with a damp napkin and placed in a sealed plastic bag
Explain step by step the disinfection protocol for alginates (5)
- Remove set alginate/tray from the mouth
- Rinse in tap water to remove blood/saliva
- Immerse in disinfecting solution for only 10 minutes
- Remove from disinfecting solution and rinse in tap water
- Wrap in a damp napkin and seal in a polythene bag (100% relative humidity)
What is a disinfectant solution used in alginates?
Sodium hypochlorite
Why do you need to shake the alginate tub before using?
- Density of the various ingredients of an alginate powder differ
- So there is a tendency for settling out in the tin/tub
Why are dust free alginate powders used today?
What particle in particular is the cause of this?
Silica particles in the dust are a possible health hazard
How do you produce dust free alginate?
Ball-mill (mix/grind) alginate powder with a small amount of ethylene glycol
What are the different types of alginate bases? (4)
- Sodium alginate
- Potassium alginate
- Sodium and Potassium alginate
- Triethanolamine alginate
What were polyvinyl alcohol sachets?
- Pre-weighed alginate powders in sachets
- Sachet made from polyvinyl alcohol, a water soluble plastic
- Sachets would dissolve in water
- No longer used as expensive
By what process is agar set?
Set by a Physical Process
What state is agar in at room temperature?
What state is agar in at 60°?
What occurs to agar when cooled form 60°?
- A solid (gel) at room temperature
- A viscous liquid at 60°C
- On cooling, it reverts to the gel
What is the composition of agar impression material? (4)
- Agar (13-17%)
- Borates (0.2-0.5%)
- Potassium sulphates (1-2%)
- Thixotropic materials (0.5-1%)
What is the function of agar in agar impression materials?
Colloid
What is the function of borates in agar impression materials?
Strengthens gel
What is the function of potassium sulphates in agar impression materials?
Accelerates setting of stone
What is the function of Thixotropic materials in agar impression materials?
Filler
Why are agar dispensed in tubes?
Prevent water loss
What is agar squeezed into?
Special metal tray
What occurs before placing agar into its metal tray?
- Tubes are put into a water bath at 60°C
- Transferred to a bath at 40°C before use as 60°C is too hot for patient
How is agar cooled?
Cooled by water circulated via cooling tubes attached on outer surface of tray
What are some advantages of agar? (3)
- Once the equipments’ set up it is easy to use
- The material is cheap
- Impressions have good surface detail
What are some disadvantages of agar? (5)
- Impressions must be cast up immediately to prevent shrinkage
- Agar absorbs water during disinfection
- Poor compatibility with model materials (ie stone)
- Poor tear strength
- Highly viscoelastic
