Gypsum Flashcards
What is a study model/case a positive replica of?
A patient dentition
What is a study cast produced from?
- From the IMPRESSION (negative representation of the patients dentition)
What are the purposes of a study model? (3 points)
- Records the position and shape of teeth
- Aids visualisation/assessment of dentition
- Enables manufacture of dental prostheses e.g. partial dentures
How is gypsum manufactured?
Calcium sulphate dihydrate is converted into calcium sulphate hemihydrate by using a heating proces and water is released
What is the chemical name of the gypsum used by technicians?
Calcium sulphate hemihydrate
What are the 3 different types of gypsum?
- Plaster (beta-hemihydrate)
- Dental stone (alpha-hemihydrate)
- Densite (improved stone)
(3 different types depending on how the heating process is carried out)
What is the heat process used to produce plaster?
- Heated in an open vessel
What is the structure of plaster?
- Large porous
- Irregular crystals
What is the heating process used to produce dental stone?
Heated in an autoclave
What is the structure of dental stone?
- non-porous, regular crystals, requires less water
What is the heating process sued to produce densite?
Heated in presence of Ca & Mg chloride
What is the structure of densite?
- Compact, smoother particles
What does the different crystalline properties of the different forms of gypsum determine?
Determines the properties of the material
What is the setting reaction of gypsum?
- Reverse of manufacture
- (CaSO4)2.H2O + 3H2O -> 2CaSO4.2H2O
How much water should be used when mixing with 100g of plaster?
50-60ml
How much water should be used when mixing with 100g of stone?
20-35ml
What is the first stage in the setting process of gypsum? (4 points)
- Hemihydrate dissolves
- Dihydrate forms
- Dihydrate solubility low - supersaturated solution
- Impurities present
What is involved in the second stage of the setting process of gypsum? (3 points)
- Dihydrate crystals precipitate on impurities as crystals
- More hemihydrate dissolves
- Continues until all hemihydrate is dissolved
What is involved in the third stage of the setting reaction of gypsum and what is this stage called? (5 points)
- Initial set
- Dihydrate crystals come into contact i.e. push apart
- Expansion starts
- Properties of weak solid and will not flow
- Can be carved
What is involved in the 4th stage of the setting reaction of gypsum and what is this stage called? (3 points)
- Final set
- Strong and hard enough to be worked
- Strength continues to develop
What are initial and final setting times of gypsum measured using?
- Measured using Gilmore needles
Suring setting of gypsum where is excess water trapped?
Trapped in the powder mass
On completion of setting of gypsum excess water evaporates. What does this cause?
- Voids are produced (porosity)
What is the compressive strength of gypsum?
(~20-35MPa) - 1/10 of composite
What is the hardness gypsum like?
Low
- Refers to the surface of gypsum
Why do you want a good setting time of gypsum?
For convenience
What would be the ideal expansion for gypsum?
0
What is the expansion for plaster?
0.2-0.3%
What is the expansion for stone?
0.08-0.1%
What is the expansion for densite?
0.05-0.07%
Which type of gypsum has the best expansion properties?
- Densite
How long does it take for the strength of gypsum to develop?
- Develops over a 24hr period
Which type of gypsum is the strongest?
Improved stone (densite)
Why is stone stronger than plaster?
It requires less water for a workable mix so is less porous
What does increasing the amount of powder in the mix do to the setting time and expansion of the material?
- Decreases setting time
- Increases expansion
What does increasing the spatulation of the mix do to the setting time and expansion of the material?
- Decreases setting time
- Increases expansion
What does increasing the amount of impurities in the mix do to the setting time and expansion of the material?
- Decreases setting time
- Increases expansion
What does increasing temperature of the mix do to the setting time of the material?
Can increase or decrease
What does increasing the amount of chemical (borax, NaCl) in the mix do to the setting time and expansion of the material?
- Can increase or decrease setting time
- Decreases the expansion
What does increased spatulation do to the crystals in gypsum?
- Breaks down the growing crystals
What do the fragments of crystals caused by increased spatulation of the material act as?
- Nuclei of crystallisation
more growing crystals = come into contact sooner
Increase spatulation of the material causes more growing crystals which will come into contact sooner. How does this affect setting time and expansion?
- Setting time decreased
- Expansion increased
What happens to the mixture when you increase the amount of powder in it and what effect does this have on setting time and expansion? (3 points)
- More nuclei of crystallisation per unit volume
- Crystals come into contact sooner
- Faster set & greater expansion
- It is the converse when decreasing the powder
Does the rate of diffusion of ions increase or decrease when increasing the temperature of the material?
Increases
Does the solubility of hemihydrate increase or decrease when increasing the temperature of the material?
Decreases
What does the addition of potassium sulphate do to the material?
- Produces syngenite
- Crystallises rapidly - encourages growth of more crystals
- DECREASES the setting time
What does the addition of borax do to the material?
- Forms calcium borate - deposits on dihydrate crystals
- Delays setting process (increases setting time)
Dental stone model surface detail depends on the type of impression material used. What are the requirements of this? (2 points)
- Needs to be chemically compatible
- Must ‘wet’ the impression material (i.e. no resistance to flow over surface; avoid bubbl e formation)
Ideally gypsum should reproduce the fine details on the impression material BUT gypsum is inherently porous, resulting in a relatively rough surface. How rough is the surface and what is the standard requirement for this?
- About 28-40um
- Standard is 50um wide so gypsum is slightly better than the standard
What are the 3 main advantages of gypsum?
- Dimensionally accurate and stable
- Low expansion (<0.1%) of stone/densite
- Good colour contrast
What are the 5 main disadvantages of gypsum?
- Low tensile strength
- Poor abrasion resistance
- Very brittle
- Surface detail less than elastomer impression
- Poor ‘wetting’ of some impression materials
