L10 Dental Gypsum Products Flashcards
What is a model?
Replica of several teeth and their associated soft tissues
What is a die?
Replica of a single tooth
What are the uses of dental gypsum (dental stone/plaster)?
Models - full or partial dentures
Dies - crowns and bridges
Impression material - increase K2SO4 to hamper growth of crystals to reduce amount of expansion
Moulds - denture construction
Refractory investments - cast alloys
What are the key material requirements for dental gypsum?
Dimensional accuracy - fluidity/viscosity to record fine detail
Dimensional stability - during or after setting
Mechanical properties - resist accidental fracture, abrasion resistance
Compatibility - with impression, good colour contrast to wax
What is gypsum?
Naturally occurring white powder mineral. Calcium sulphate dihydrate.
What are dental gypsum compared to gypsum?
Adaptation by dehydration to become calcium sulphate hemihydrate
Briefly describe gypsum production
Driving off part of the water of crystallisation
Open kettle to form large porous particles, like dental plaster
Pressure vessels for small dense particles, like dental stones
Adding defloculants for small very dense particles, like diestone
What is dental plaster?
Plaster of paris
What is calcination
To dessicate without melting
What are the properties of dental plaster?
Setting expansion - 0.2-0.3%
Compressive strength - 8.8MPa
Setting time - 12+-4mins (initial hardening)
What is the particle morphology of dental plaster?
Irregular large particles
Dental stone is chemically identical to dental plaster, how is it physically dissimilar?
More regular, less porous hemihydrate formed. Lower gauging volume - the amount of water needed to fill up all the gaps between the particles is lowered. Powder to liquid ratio can decrease, hence increasing mechanical properties.
What are the properties of dental stone?
Setting expansion - 0.08-0.1% (decreased)
Compressive strength - 20MPa (increased)
Setting time - 12+-4 mins
(Optimised process)
Particle morphology of dental stone
Image on pg54
How is diestone different?
Improved dental stone with improved mechanical properties.
Cl- ions act as a deflocculant to prevent agglomeration of particles and high density fine particulate is formed.
What are the properties of diestone?
Setting expansion - 0.05-0.07%
Compressive strength - 34MPa
Setting time - 12+-4mins
Basic setting reaction of dental gypsum
Hemihydrate and water to form dihydrate
Plaster slurry to solid and resultant dimensional change
Why is the gypsum reaction rapid?
Because it is very soluble in water
Describe the setting process for plaster
Hemihydrate dissolves very quickly
Dihydrate precipitation is delayed - quite unstable phase
Solubility level of dihydrate exceeded - nucleation and crystallisation
Crystals precipitate from solution - more hemihydrate dissolves until most is converted to dihydrate
Dimensional stability, how does the plaster expand on setting?
Spherulites grow from nuclei of crystallisation, which impinge on one another. Force is exerted as spherulites grow forming spaces between the crystals resulting in porosity and expansion
Why could expansion of plaster be advantageous if controlled?
Investment materials - makes up shrinkage of metal castings on cooling
Crowns, bridges and dentures - don’t fit too closely
What factors can control the setting reaction?
Water/powder ratio
Spatulation
Accelerators
Retarders
Temperature
How does the water/plaster ratio affect setting characteristics?
Dental plaster 50ml water per 100gm powder
Stone 20-35ml water per 100gm powder
How does the spatulation affect setting characteristics?
The way you mix the materials affects them. The more you mix, the faster the reaction becomes due to increasing sites of nucleation. Increasing mixing time increases setting time.
How do the accelators affect setting characteristics?
Increase rate of reaction:
Potassium sulphate encourages growth of more crystals and controls expansion
Gypsum - adding more sites of nucleation, more dihydrate
Sodium chloride (small amounts) - analogous to using gypsum, increases density and nucleation sites
How do retarders affect setting characteristics?
Slow rate of reaction:
Borax - forms calcium tetraborate on the crystals as they form, stopping them from growing as much and slowing the groups
Potassium citrate
Sodium chloride (larger amounts) - analogous to others
How does temperature affect setting characteristics?
Limited effect
Process of dissolving the hemihydrate will increase with increased temperature, but it will retard crystallisation too, having a counter effect
How does crystallisation affect setting time?
Increasing points of crystallisation by increasing powder liquid ratio, setting time increases
How does mixing time affect setting time?
Increasing mixing time breaks up dihydrate crystals and creates more nucleation sites, decreasing setting time
How does the water/powder ratio affect compressive strength?
Increasing amount of powder, compressive strength increases as water powder ratio goes up. More powder means more sites of nucleation so more crystals and higher density and reduced porosity to increase compressive strength.
Increasing amount of water does the opposite
