Flashcards
Alginate and Plaster/Stone
Cards in this deck (28)
- 1What is an impression?A negative likeness of the oral environment
- 2Alginate is the material used for impressions. Where does it come from?Algenaic acid extracted from seaweed
- 3How is algenaic acid then turned into alginate?The acid is reacted to form a salt of either sodium or potassium, then mixed with water to form a hydrocolloid
- 4How does the setting reaction work?1 calcium (provided by calcium sulfate) replaces 2 Na+/K+ in the alginate, thus joining 2 alginate molecules together and making the product more rigid
- 5Can the gel become a sol again after the setting reaction?No, the sol to gel reaction in the setting reaction is irreversible, unlike something like agar jelly
- 6What are the main components of alginate?A filler like diatomaceous earth.
Na/K Phosphate to provide the monovalent cations to slow down the reaction to allow handling time.
pH indicator to show then the reaction has finished (14-7).
Water is a critical component and catalyst.
CaSO4.H20 as a source of Ca2+.
Na/ K alginate triethanolamine of course. - 7Are the exact proportions always the same in every type of alginate?No, composition is entirely dependant on the manufacturer
- 8What consequences do the different compositions have?Different setting times, different strengths and elasticities, different shelf lives, different tastes, different prices
- 9What type of material is set alginate?Viscoelastic
- 10How do you prevent permanent deformation in the impression?A 'snap removal' technique so that no deformation occurs and the correct shape can be recovered after removal
- 11What is syneresis?The evaporation of water, making alginate dry out, causing the impression to shrink
- 12How do you prevent syneresis or imbibition?Put the impression in a sealed bag at 100% relative humidity and use it asap
- 13What happens if the impression is stored in water?Imbibition - swelling of the impression
- 14Can impressions be sterilised?No, only disinfected
- 15What is the material used to make dental plaster and stone?Gypsum
- 16Give the chemical formula and name of gypsumCaSO4.2H20 -- Calcium Sulfate Dihydrate
- 17What is the structure of gypsum?Sheet form, with each sheet being joined to another by weak hydrogen bonds
- 18How is plater produced?Heat gypsum and remove water to leave calcium sulphate hemi hydrate
(CaSO4)2. H20 - 19Are dental plaster and stone chemically different?No they are chemically identical, just structurally different
- 20What is the general structure of B hemi-hydrate (plaster)irregular and porus
- 21What is the structure of a hemi-hydrate? (stone)regular prisms and dense
- 22How do you get stone to form over plaster?Higher pressure
Closed vessel as opposed to open
Chemical aids of calcium chloride - 23What are the 5 types of gypsum product?1 - dental plaster for impressions
2 - dental plaster for models
3 - dental stone for models
4 - dental stone for die
5 - stronger dental stone for die - 24Give 2 examples of additives that can change the reaction to form stone/plasterAcellerators like NaCl
Retarders like Borax - 25Give the setting reaction of calcium sulfate hemi-hydrate(CaSO4). H20 + 3H2O -- > 2(CaSO4.2H2O)
- 26Which is stronger and harder out of a and B?Stone (a)
- 27Which hemi-hydrate has a higher expansion?Plaster (B)
- 28Why isn't gypsum replaced by other more modern materials?It is so cheap that doing so wouldnt be worth it
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- Flashcards on Alginate and Plaster/Stone, created by Annabel Carnley on 03/02/2015.
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- Annabel Carnley
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