12. Gypsum Flashcards
Definition of gypsum
Material used to create study models/casts
Definition of study model/cast
A positive replica of the dentition produced from the impression (negative representation) of the patient’s dentition
Purpose of study model/cast (3)
Records the position and shape of teeth
Aids visualisation/assessment of dentition
Enable manufacture of dental prostheses
Uses of gypsum (4)
Cast (plaster/stone)
Die (stone/improved stone)
Mould material (stone)
Investment binder (stone)
Manufacture reaction of gypsum (2)
CaSO4.2H2O –> (CaSO4)2.H2O + H2O
Dihydrate –(heat)–> hemihydrate
Types of gypsum (3)
Plaster (B-hemihydrate)
Dental stone (A-hemihydrate)
Densite (improved stone)
Crystalline structure of gypsum determines
Properties
How is plaster heated and what does it produce
Heated in an open vessel
Produces large porous, irregular crystals
How is dental stone heated and what does it produce
Heated in an autoclave
Produces non-porous, regular crystals (requires less water)
How is density (improved stone) heated and what does it produce
Heated in the presence of calcium and magnesium chloride
Produces compact smoother particles
Setting reaction of gypsum (4)
Reverse of manufacture
(CaSO4)2.H2O + 3H2O –> 2CaSO4.2H2O
Hemihydrate –> dihydrate
Powder + water = gypsum (study cast)
Mixing ratios of gypsum - theoretical and plaster and stone (3)
Theoretical ratio - 18.6ml water and 100g powder
Plaster - 50-60ml water and 100g powder
Stone - 20-35ml water and 100g powder
Features of setting process (4)
Hemihydrate dissolves
Dihydrate forms (low solubility - supersaturated solution)
Dihydrate crystals precipitate on impurities as crystals
Impurities act as nuclei for crystallisation (hemihydrates dissolve around impurities)
More hemihydrate dissolves. This continues until all hemihydrate has dissolved
Setting process - initial set (4)
Dihydrate crystals come into contact (push apart)
Expansion starts
Properties of weak solid and will not flow
Can be carved
Setting process - final set (2)
Strong and hard enough to be worked
Strength continues to develop
Measuring initial and final setting times
Initial and final setting times are measured using Gilmore needles
Water action during setting (2)
During setting, excess water is trapped in the powder mass (excess water is required for an appropriate mix)
On completion of setting, excess water evaporates, and voids are produced (porosity
Compressive strength of gypsum
20-35MPa
Hardness of gypsum
77kg/mm2 (low)
Setting time of gypsum
At convenience of operator
Expansion of gypsum-based materials (3)
Plaster – 0.2-0.3%
Stone – 0.08-0.1%
Densite – 0.05-0.07%
Relationship of strengths of gypsum-based materials (3)
Improved stone > stone > plaster
35-40MPa > 30MPa > 12-20MPa
Stone is stronger than plaster as it requires less water for a workable mix (less porous). Improved stone is the strongest
Effect of increasing powder content on setting time and expansion (2) with explanation (2)
Setting time - decreased
Expansion - increased
Increased powder leads to more nuclei of crystallisation per unit volume
Crystals come into contact sooner causing a faster set and greater expansion
Decreased powder has the opposite effects
Effect of increasing spatulation on setting time and expansion (2) with explanation (2)
Setting time - decreased
Expansion - increased
Increased spatulation breaks down growing crystals.
Fragments act as nuclei of crystallisation
More growing crystals come into contact sooner
Effect of increasing impurites on setting time and expansion (2)
Setting time - decreased
Expansion - increased
Effect of increasing temperature on setting time and expansion (2) and explanation (2)
Setting time - decreased and increased
Expansion - no effect
Temperature has conflicting mechanisms
Rate of diffusion of ions increases as temperature increases
Solubility of hemihydrate decreases when temperature increases
Effect of chemicals on setting time and expansion (2)
Setting time - decreased or increased depending on chemical
Expansion - decreased
Features of expansion on setting (2)
Should be a small percentage
Allows for the model to be slightly too large, so crowns, bridges and dentures won’t be too tight a fit when placed in the mouth
Types of chemical additives (2)
Potassium sulphate
Borax
Features of potassium sulphate (3)
Produces syngenite (K2(CaSO4)2 . H2O)
Crystallises rapidly - encourages growth of more crystals
Decreases setting time
Features of borax (2)
Forms calcium borate – deposits on dehydrate crystals
Delays setting process (increases setting time)
Features of dental stone material surface (2)
Depends on the type of impression material – it needs to be chemically compatible
It must ‘wet’ the impression material (no resistance to flow over surface; avoid bubble/void formation)
Features of gypsum fine detail reproduction (2)
Ideally gypsum reproduces the fine detail on the impression material
Gypsum is inherently porous, resulting in a relatively rough surface (28-40um) which is acceptable - ISO guidance states a material must reproduce a 50um wide groove
Advantages of gypsum (3)
Dimensionally accurate and stable
Low expansion (<0.1%) of stone/densite
Good colour contrast
Disadvantages of gypsum (5)
Low tensile strength Poor abrasion resistance Very brittle Surface detail less than impression Poor ‘wetting’ of some impression materials
