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
What does the calcium sulphate dihydrate (gypsum) do in the alginate setting reaction?
Provides the Ca2+
Release of Ca2+ and cross-linking determines rheology
What does sodium phosphate 2% do in the alginate setting reaction?
Regulates Ca2+ release
Controls time allowed to mix material, place in tray and place in patient mouth. It alters the calcium ion concentration.
What is formed in the alginate setting reaction?
Insoluble calcium alginate (look at pg41 for reaction)
What is the issue with polysulphides (synthetic elastomers)?
2 paste system might not be mixed correctly, porosity might be incorporated as it is blended with air, proportions (kenics mixer)
Are polysuphides hydrophilic or hydrophobic?
Very hydrophilic
What is the 2 paste system made of?
A base and catalyst
Properties of polysulphides
Moderately hydrophilic
Good tear resistance
Viscoelastic
Long setting time
Is dimensional stability of polysulphides good or bad?
Dimensional stability affected by
Water loss (condensation reaction)
Volatile components
Continued reaction (shrinkage)
Brief summary of polysulphide setting
Hydrophilic groups allow cross-linking, changing it from low viscosity to a mixture that is rigid and flexible. Water by product can affect dimensional accuracy.
What is the system used for polyethers?
2 paste system, base and catalyst
Do we use polysulphides?
More expensive, and rarely used due to unpleasant taste.
Do we use polyethers?
Yes commonly
What is the polyether setting reaction?
Addition polymerisation, no by product
What are the properties of polyethers?
Adequate mechanical properties
Hydrophilic
Relatively rigid when set (disadvantage)
Good dimensional stability (since no by product, needs dry environment otherwise will absorb water)
Why are silicones most popular choice for prosthodontic applications?
Ideal properties apart from hydrophilicity. No by product so better dimensional accuracy.
Describe condensation silicones
Not biocompatible so must be modified. 2 paste system with base and catalyst
What can impact the setting reaction of condensation silicones?
Sulfur containing products can inhibit the reaction, egg., latex gloves. Inhibits the catalyst. (But these are thus not used for impressions)
What does the by product of condensation silicones affect?
Dimensional accuracy
Describe addition silicones
2 paste system with catalyst and base
No by product formed in reaction
What are the properties of addition silicone (for impressions)?
Closest to ideal elastic properties (lowest viscoelasticity so good for high dimension accuracy)
‘Hydrophilic’
Good dimensional stability (almost no dimensional change)
Adequate tear resistance
Pseudoplastic
Condensation types exhibit dimensional change
Latex gloves may inhibit set
Why are silicones ‘hydrophilic’ and not hydrophilic?
They are hydrophobic but are adjusted
Surfactant is added to improve wettability
Describe the differences in distortion between PS, PVS, PE
PS has a larger potential distortion after removal from the mouth than PE and PVS, meaning it takes longer for it to come back to how it was before.
Which one of PE PVS PS has the smallest error margin?
PVS has a smaller margin for error. So it can be turned into a positive impression straight away.
What should you consider about PE and PS before use?
Need to think about for PE and PS: the reaction, relaxation and when you can pour the material. Want to pour as soon as you can but is probably better to leave for 10-20mins before you do.
What are the mixing options for 2 paste systems?
Mixing pads, mixing gun, mixing machine
What are Newtonian fluids?
(E.g., water) does not matter how much energy is out into stirring/manipulating the material, it’s viscosity will stay the same.
What does pseudoplastic mean?
Upon force (increased shear rate) the material will decrease viscosity (get thinner) REFER TO GRAPH PG46
What does dilatant mean?
Upon force (increased shear rate) the material will increase viscosity (get thicker) GRAPH PG46
When is pseudoplasticity an advantage?
Sheer thinning is an advantage if you are trying to squeeze a material into a small space, so silicones and polyethers are suited to this technique.
Which materials are pseudoplastic?
Polyether, PVS (addition silicones)
What materials are dilatant?
Polysulphides and condensation silicones
Which materials are mixed on a pad?
Dilatant properties in materials, so polysulphides and condensation silicones
