L08: Impression Materials: Chemistry, Selection Flashcards
What impression materials are rigid (and not really used much anymore)
Impression compound
ZOE paste
Impression plaster
What impression materials are flexible?
Hydrocolloids (agar and alginate)
Elastomeric (polysulphide, polyether, and silicone)
What are hydrocolloids?
Polysaccharides in water
What system do hydrocolloids use?
Hydraulic system - have to combine constituents with water so they cross-link and react
What are the 2 hydrocolloids?
Agar, alginate
What are the 2 forms of hydrocolloid?
Gel from a sol
Simple cross-linking reactions
What is the chain arrangement in low and high viscosity hydrocolloids?
Low viscosity fluid - random chain arrangement (sol)
Higher viscosity fluid - more organised chain arrangement (gel)
Why is borax added to agar?
To achieve suitable viscosity
Why is potassium sulphate added to the agar?
To act as an accelerator
Why is agar not really used anymore?
Thermoplastic process - no chemical addition to the composition to cross-link it. Sol and gel made via temperature in water baths etc.
Issues with cross-infection
Why might agar still be used?
Used still because it can be reversed - reheated and redone
Describe the bonds in alginate
Gel formation provides strong cross-links, between chains of sodium alginate
Chemical cross-linking cannot be broken once formed
What material has the most preferable optimal rheology and setting reaction for taking impressions?
Sodium alginates
What are the properties of hydrocolloids?
Mucostatic
Hydrophilic
Controlled rheology
Poor dimensional stability
Flexible
Low tear resistance
Highly viscoelastic
What does mucostatic mean?
Does not displace the soft tissues
What does hydrophilic mean?
Requires water meaning no issues with repulsion of saliva (which would reduce dimensional accuracy)
What are the 2 parts of dimensional stability in alginates?
Syneresis - water loss via evaporation so will alginate will shrink
Imbibition - sucks in water since it’s hydrophilic
Why is it useful that alginate is flexible?
So it can be removed from the mouth successfully without damage to the patient mouth or impression
Why is low tear resistance bad for impressions?
For intricate impressions for crown and bridge work where there might be undercuts or areas where the impression will be thin, as soon as you pull it out, it will rip. So only good for basic impression techniques.
What does being highly viscoelastic mean for impression materials?
Never expect 100% recoil, will always be a bit of a difference
What are the components in alginate?
70% reinforcing filler
15% sodium alginate
10% calcium sulfate dihydrate