Elastic Impression Materials Pt. 3 Flashcards
What is a colloid
A heterogeneous mixture of two phases where are the two phases are not readily differentiated
What are the types of hydrocolloids impression material
- Reversible agar
- irreversible alginate
What are the intermolecular bonds between molecules in gel formation during cooling in reversible hydrocolloids
- Van Der walls forces
- hydrogen bonds
How are the molecules in irreversible hydrochlorides bonded
Cross-links between polysaccharide chains
What is Agar
It is a complex polysaccharide (galactose sulphate)
What is the composition of reversible hydrocolid Agar 
- 15% colloidal suspension of Agar water
- Borax
- Potassium sulphate
- Glycerine
- Alkylbenzonate
- Thymol
- Fillers and pigment
What is the function of borax in Agar
It gives body to the gel and it retards setting
What is the function of potassium sulphate in Agar
To counteract the effects of borax alternative is dipping the impression in solution of Accelerator
What is the structure of the molecules in Agar
Helical structure consisting of hydrogen bonding
Classify agar 
- Type one - high consistency
- Type two - medium consistency
- type three - low consistency
What is the property that allows agar to be removed from undercuts
Viscoelasticity
The amount of permanent deformation exhibited by viscoelastic impression material agar depends on what
- Severity of undercuts
- the time for the materials under stress during removal of the impression
Which areas are Difficult to record using Agar 
Inter-proximal and sub gingival areas
What are the properties of Agar
- Low Viscosity
- can record undercuts
- flexible
- poor mechanical properties
- tears at low stress
- Poor dimensional stability
What is syneresis
Squeezing of water from between polysaccharide chains
What is imbibition 
In the presence of access water agar gel may absorb water
What does syneresis do to Agar
Cause shrinkage And effects accuracy
What does imbibition due to water Gel agar 
Separation of polysaccharide chains and swelling of impression 
What are the uses of Agar
- Impression of partial denture, crown and bridge
- used as a duplicating material to duplicate casts
What are the Advantages of Agar 
- Material in contact with Tray sets first
- Uptake of moisture
- Reproduce finer details
- Used for model duplication
- Flexible
- reusable
What are the disadvantages of Agar
- Dimensionally unstable
- impression cannot be electroplated
- low tear strength
- specialized equipment is required
- unacceptable to reuse
Why is it unacceptable to reuse Agar
- Cross infection control
- Alteration of materials physical properties
- incorporation of small chips of dental stone 
What is the composition of alginates
- Sodium or potassium salt of alginic acid
- gypsum
- NA3 PO4
- inert filler
- reaction indicator
What affect does warm water have on alginate manipulation
Warm water reduces working time and setting time by accelerating the rate of cross-linking
Which part of Alginate sets first in the tray 
The material adjacent to the oral tissues sets more rapidly than the cooler tray wall
What are the properties of alginate
- Viscosity
- accuracy
- viscoelasticity
Describe the decontamination of alginate impression
Aqueous solution of sodium hypochlorate or aldehyde (formaldehyde or glutaraldehyde) for 10 minutes
What are the applications of alginate
- Impression for complete and partial dentures and orthodontic appliances
- Study models
- duplicating material
What are the advantages of alginate
- Low cost
- easy to mix and manipulate
- gives good surface details even in the presence of saliva
- flexibility
- minimum requirement of equipment
What are the disadvantages of alginate
- Cannot be electroplated
- Poor tear strength
- cannot be corrected
- not recommended where high level of accuracy is needed
What is the purpose of alkylbenzonate, Thymol and glycerine in agar 
Preservative
bacteriocidal
plasticizer
What is the function of fillers in Agar
Controls strength, viscosity and rigidity
