Are hydrocolloids elastic or rigid?
Elastic
How do agar solidify?
With increasing temperature
Why do people dont use agar?
It requires expensive apparatus to work with
What is the tray we use for agar?
Water cooled rim locked tray
What is Agar used for mainly?
For removable dentures and bridges
When does agad go through deformation?
It has very poor mechanical properties and goes through deformations very easily after a few minutes of being under stress
How is the dimensional stability in agar?
It has very poor dimensional stability becuase of its water absorption and vapourizing
Is agar reversible or irreversible?
Reversible
Is alginate reversible or irreversible?
Irreversible
How does alginate changes its state?
Becuase of its chemical reactions
In what form do they provide Alginate?
Powder
How can manufacturers change alginate?
By changing the retards added during manufacturing
How can dentists change alginate?
- Changing water/paste ratio
- Changing mixing time
- Changing water temperature
How does temperature affect setting time of alginate?
Increase of temperature decreases setting time
Name the properties of Alginate?
- Pleasent smell and taste
- Flexibility
- Elastic recovery
- Lower than agar accuracy
- Low adhesion
How flexible is Alginate under strees?
About 14% flexible
If you increase the paste how will that affect flexibility?
It'll result in lesser flexibility
How much deformation will Alginate get if its highly elastic?
1.2%
How can you protect alginate against deformation?
By removing it from mouth quickly
How can you solve the adhesion problem of Alginate
By either:
- Pouring it in perforated trays
- Adding adhesive compounds
How is alginate if it filled undercuts?
Its flexible enough to be withdrawn from undercuts but if the undercuts were deep it'll break becuase it can't handle too much stress
What is the best way to store Alginate?
- Put it in a damp paper towel(slightly wet to not osorb water)and cover it with a rubber bowel
- Sealed in plastic bag
How does time affect Alginate?
Alginates become stronger if you remove it after a few minutes after it was set
What is Syneresis?
Losing water(maybe by vapourising)
What is imbibition?
Absorbing water
How long should you store Alginate for?
Maximum 1h
What are the uses for Alginate?
- For primary impressions for toothless patients
- For preliminary impressions(used for diagnoses)
- Temporary removal prosthetics
What is the advantages of Alginate?
- Ease of mix and manipulation
- No complex equipment required
- Economical
- Comes out of undercuts easily
- Has a pleasant taste and smell and is comfortable for the patient
What are the disadvantages for Alginate?
- Poor dimensional stability
- Poor strength especially if it became thin
- Distortion could happen if it moved while setting
- Can't be refined after it was set
Describe the defects of Alginate?
1-Defects:
improper mixing
prolonged mixing
w/p ratio
is too low.
2- Tearing:
inadequate bulk
moisture
contamination
premature removal from mouth
prolonged mixing
3-External bubbles:
undue gelation preventing flow
air
incorporated during mixing
4-- Irregularly shaped voids:
moisture or debris
5- Rough or chalky stone model:
inadequate cleaning of the impression
excess water left in impression
Premature removal of model.
6- Distortion:
impression not poured immediately
Movement of tray during setting.
Removal from mouth too early
Improper removal from mouth