possible test questions Flashcards
why void-free layer?
it is important to have a void-free layer (voids are detrimental to retention and support of prosthesis)
describe the ideal surface energy and wetting abilities in dental cements?
good wettability results from low surface energy cement and high surface energy substrate
how to measure wettability, what would be good wetting?
- contact angle
- need a small contact angle for good wetting
why do bubbles form in gypsum?
if the surface tension of gypsum is higher than surface energy of impression material
what is the difference between luting and bonding
luting: no adhesive interaction, just a filler material
bonding: true adhesion between 2 materials
what is the reaction for glass ionomer cements
glass powder and PAA
why do we add tartaric acid to glass ionomers?
large molecules of polycarboxylate acid react with glass particles and dentine slowly, causing long maturation and susceptibility to moisture, so the tartaric acid shortens the setting time
benefits of tartaric acid?
- short setting time
- decrease viscosity of PAA
- actively removes Al and Ca from glass
- stabilizes matrix gelation
- improves hardness and compressive/tensile strengths
describe the mechanism of fluoride release by glass ionomers?
- rapid F release from matrix
- gradual F release from particle overtime
- if F is low, F is released from glass ionomer
- if F is high, F is recharged into glass ionomer
describe the dual bonding mechanism of glass ionomer to dentin
- chemical: displacement of phosphate/calcium ions from hydroxyapetite by carboxylate ions of polyacid and incorporation of carboxylate into hydroxyapetite structure
- mechanical: micromechanical interlocking formed by shallow hybridization of partially demineralized dentin
advantages for glass ionomer
- good fluoride release
- less shrinkage
disadvantages for glass ionomer
- susceptible to dehydration
- poor abrasion resistance
- low esthetics and color-stability
- less tensile strength than resins
- poor acid resistance
- sensitivity to moisture at placement
what is different about RMGI than GI
- has a methacrylate copolymer
- covalent link between resin matrix and polyacrylic matrix
what is the purpose of acid etching?
dissolve the smear layer and mineral matrix of dentin and enamel so that resin can penetrate into enamel prisms and collage fibrils (micro-mechanical interlocking)
what is the significance of the hybrid layer
- layer where adhesive resin infiltrates the primed demineralized dentin
- forms an interfacial micro-mechanical bond
what is the disadvantage of using smaller diluent monomers
they under greater polymerization shrinkage, which increases the risk of leakage in marginal gaps
why use a filler?
- lower polymerization shrinkage
- lower thermal expansion
- lower water sorption
- lower viscosity
- reinforcement
- radiopacity
advantages of smaller particles in fillers?
- less scattering of light
- less opacity
- better curing depth
- smooth surface
- less stains
mechanism of silanes coupling agent
- methoxy groups are hydrolyzed to silanol groups
- silanol groups bind with other silanol groups on the filler surface (siloxane bonds)
- methacrylate groups form covalent bond with resin by polymerization
example of elastic materials
- alginate
- polysulfides
- polyethers
- silicones
alginate, polyethers, and heavy-body PVS can be considered what?
- mucocompressive
- irreversible setting
compare chain growth with step growth
- chain has vinyl groups, free radicals, fast prop and MW increase, no by product
- step: difunctional monomers, no free radicals, slow prop and MW increase, by-product!
why are elastomers elastic?
they have random coils that represent maximum entropy, if they get straightened, then the entropic recoiling effect will restore random coils
why do polysac materials like alginate have poorer dimensional stability
-they only rely on 2 cross linking mechanisms: H bonding and chelation
types of elastomeric impression
- polysulfides
- condensation silicones
- polyethers
- addition silicones (PVS)
why do materials undergoing step growth have polymerization shrinkage and dimensional changes
ethanol evaporation
disadvantages of condensation PVS
- lack of dimensional stability (ethanol evap)
- short working time
- long setting time
- 1-3% permanent deformation
- hydrophobic
- pour in 1 hour
addition PVS is better because
- no ethanol or byproduct so no shrinkage/dimensional changes
- also hydrophobic, but you can add surfactant!
other advantages of addition PVS
- highly accurate
- high dimensional stability
- pour in 1 week
- multiple casts
- easy to mix
- pleasant odor
disadvantages of addition pvs
- expensive
- sulfur INHIBITS set (found in latex gloves and retraction soln)
- short working time
- hydrophobic
polyethers - also addition!!!
alkyl benzene sulfonate + imine => carbocation
advantages of polyether
- accurate and good dimensional stability
- HYDROPHILIC! so it has good wettability and tolerates moisture WITHOUT bubbles
- 3 days
disadvantages of polyether
- rigid
- HYDROPHILIC - dimensional changes when absorbs water
what factors can decrease working time and setting time
- increase in temp
- increase in humidity
- increase in viscosity
setting time (greatest to least)
condensation and polysulfide > polyether > addition (VPS)
time for pouring the impression
condensation - 1 hour
addition - 1 week
polyether - 3 days
best way to remove impression
“snap” removal - fast and least amount of plastic deformation
explain the paradox of hydrophilic vs hydrophobic materials
hydrophilic can capture better details of surfaces because it can handle residual moisture better, but has poor dimensional stability because it absorbs water, while hydrophobic materials have better dimensional stability
how do we solve the issue of hydrophobic and hydrophilic materials
add surfactant to hydrophobic materials to make the surface more hydrophilic, so it is more accurate but also has dimensional stability
