final exam questions Flashcards
Two accelerators for gypsum?
- Sodium sulfate (low conc)
- Potassium sulfate and sodium chloride (low conc)
how does tri-sodium phosphate act as a retarder
tri-sodium phosphate reacts first with calcium sulphate. then sodium alginate reacts with calcium sulphate
should impression materials be plastic or elastic? why?
elastic; required to be stretched to a fairly large degree without suffering any permanent deformation (as in during removal of impression – put under compressive and tensile stress); plastic materials lack ability to record any undercut
why do u need adhesive on the tray
detached is grossly distorted and will lead to ill-fitting resto
Two retarders for gypsum?
- borax
- sodium chloride (high conc)
- sodium sulfate (high conc)
- glue
- agar
- coagulated blood
What provides the driving force for the setting reaction of gypsum?
water molecules in the dihydrate are found in alternating double layers
-dehydration process requires a lot of energy because the dihydrate is more stable at room temperature
What is the base, reactor, and retarder for alginate?
Base – Na alginate
Reactor – calcium sulphate
Retarder – tri-sodium phosphate
what is the activator for cold-cure PMMA
dimethyl-p-toluidine
criteria for good impression
- uniform, homogenous mix
- tray adhesive applied
- rigid impression tray
- no void or pulls on margins
- detailed margin
- no burn through of tray
- strong bond between light and heavy
shrinkage of polymeric material is caused by
- VDW volume reduction (due to single bonds)
2. free volume reduction (less rotation and distance)
4 reasons why we use custom trays
- Record most out of reach anatomical landmarks
- Minimize impression material distortion
- Prevent tissue distortion (can use low viscosity materials)
- Reduced cost
what can affect the mechanical/physical properties of a polymer
molecular weight and degree of polymerization
effect of oxygen on poly
- decrease rnx velocity
- decrease deg of poly
syneresis
loss of water from alginate
-excess water in contact with gypsum
name all components of alginate
- na/k alginate
- calcium sulphate
- tri-sodium phosphate
- mineral/silicon fillers
- potassium fluoride
- pigment particles
- organic glycol
imbibition
alginate absorbs water from gypsum
why might you have lack of detail on impression
- low temp (prolonged setting time; increase viscosity)
- too much pooled liquid (impression material doesnt flow)
What stage(s) does expansion occur during gypsum setting?
Plastic and friable stages
what type of impression material is alginate
irreversible aqueous hydrocolloid
Is alginate a hydrocolloid? what does it mean?
yes. hydrocolloids are uniform distribution of individual particles of one substance in water
list all properties of PMMA
- colourless
- any shade
- radiolucent
- chemically stable
- knoop hardness 15 to 18
- Tg=125 deg c
- absorb water
- low thermal conductivity
components of powder PMMA
- pre-polymerized beads of PMMA
- initiator: benzoyl peroxide
- plasticizer: dibutyl phthalate
- inorganic particles (hardness)
- organic pigments and dyes
list all physical properties of MMA
- transparent
- dissolves PMMA
- tg -45 deg C
- excellent organic solvent
- not chemically stable
3 practical ways to decrease setting time of gypsum
- Add slurry water to water/powder mixture
- Use mixing bowl with some gypsum particles
- Mix water and powder faster and more vigorously
What is gypsum?
Calcium sulphate dihydrate
what can cause premature termination of poly and how
impurity in monomer; react with activated initiator OR activated growing chain
What is the effect of low water/powder ratio?
- Quick setting
- High expansion
- More strength
- More surface hardness
- Less porosity
properties of stone
Uniform, dense, small particles Rods and prism-like crystals Pack tightly Little water Organized crystal growth Strong structure (20% porosity)
properties of plaster
Irregular (size/shape), porous particles Needle-like crystals with capillary pores Loosely packed Needs lots of water for mixing Water absorbed into pores bw particles Weak structure (35% porosity)
Define the dissolution-precipitation theory.
- dissolution of hemihydrate molecules
- recrystallization of dihydrate particles, resulting in interlocking of crystals, to form the set solid gypsum
how can we control the speed of phase transformation from sol to gel
add tri-sodium phosphate as a retarder
What are two properties that can control setting time of gypsum?
- Solubility of hemihydrate (higher solubility = shorter time)
- Number of nuclei of crystallization (more nuclei = shorter time)
purpose of preliminary impression vs final impression
prelim: record dimensions of edentulous arch
final: record fine details of edentulous arch
auto-mix or spatula mixed and why?
auto-mixed:
- faster
- more convenient
- less technique sensitive
- no voids
- no risk of cross cont
- ideal phys/mech properties
How is beta-calcium sulphate hemihydrates formed?
For plaster: dry calcination, without pressure, heated to 120 deg C in an open vessel
clinical acceptable range of alginate
1-3%
Explain the 4 steps/mechanisms to dissolution precip theory
- hemihydrate powder mixed in water to form a workable suspension. Particles are only moderately dissolved in water
- continuous dissolution forms a super-saturated hemihydrate solution
- super-saturated solution forms dihydrate crystals by gradual precipitation
- solution is “not-saturated” so more hemihydrate particles are dissolved (cycle continues)
6 criteria for custom trays.
- Close adaptation
- No impingement
- Adequate space
- Rigid, thick, and fracture resistant
- Handles don’t interfere with movements of oral structures
- Smooth surfaces and flanges, no sharp corners/edges
Name a retarder for gypsum and how it works?
Agar and alginate can retard crystallization by being adsorbed on calcium sulfate nucleating sites and interfere with crystallization on the surface of gypsum model. Gives it a chalky view and powder texture
How is alpha-calcium sulphate hemihydrates formed?
For dental stone: wet calcination, under pressure (117 Kpa for 5-7 hrs), heated to 125 deg C; For die stone: without pressure, boiled to 100 deg C in 30% CaCl2 solution
what if you compress more than 1/3 original thickness of VPS or polyether
- elastic recovery is not 100%
- plastic deformation
- distance to wall is twice the depth of undercut
why do we use both light body and heavy body
- light: very accurate; low dimensional stability
- heavy: support light body; maintain dimensional stability
2 ways of keeping alginate fresh
- store in 100% relative humidity inside a plastic bag
- wrap in damp (not dripping) paper towel
properties of die stone
Large, uniform, dense particles Short and thick crystals Tightly packed Little water Organized and extensive crystal growth Strong structure (10% porosity)
components of liquid PMMA
- monomer: MMA
- inhibitor: hydroquinone
- cross linking agent: glycol dimethacrylate
- activator (cold-cure): dimethyl-p-tonuidine
clinical properties of alginate
acceptable surface detail; faster reaction with warm water; enough elasticity for moderate undercuts; time-dependent dimensional stability; non-toxic; can be manipulated
what is the reaction transformation for alginate soln to gel
Na alginate +CaSO4 =>(water) Ca alginate + Na2SO4
practical way of alternating setting time of alginate
higher temp of water (shorter setting time)
What is the starting spot for dihydrate crystals?
Nucleation centers in the suspension
What can act as nucleation centers for dihydrate crystals.
- Impurities, such as residual gypsum particles
- particles of gypsum
- strained areas on undissolved hemihydrate particles
PMMA reaction for heat cure liquid
PMMA (activator: heat)+BP=active BP
active BP+MMA=active MMA=PMMA
how long can you leave alginate on bench
30 min
PMMA reaction for cold cure liquid
PMMA (activator: dimethyl-p-toluidine) + benzoyl peroxide = activated benzoyl peroxide
activated BP +MMA = activated MMA = PMMA
function of inhibitors in MMA
- inhibit polymerization to increase storage stability
- retard poly to adjust working time
what does adding barium salts do to PMMA
- reduce strength of denture base
- reduce translucency and vital appearance of denture
- more expensive
