DMT Flashcards
definition of working time
from start of mix till can’t be used
measure at room temp
definition of setting time
from start of mix to setting
measured in mouth
what is a pseudoplastic material
viscosity decreases with pressure
what is elastic and plastic strain
elastic = recovered plastic = permanent - occurs when yield stress applied
definition of hardness
resistance to indentation under load
definition of strength
maximum stress that can be withstood
what are added during polymerisation to reduce shrinkage
fillers
advantages of alloys compared to pure metal
harder
stronger
melting range
cheaper
strength properties of amalgam
develops over 24hrs
high compressive strength
low tensile/flexural strength
weak in this sections, unsupported edges
cavity design for amalgam
undercut for mechanical retention rounded internal angles to reduce stress keys/grooves for retention remove unsupported enamel flat floor
thermal disadvantages of amalgam
high thermal diffusivity - use liner to insualte
high coefficient of thermal expansion - more expansion/contraction than tooth = microleakage or fracture
what must be removed before bonding to dentine
remove smear layer
2 types of elastic impression material
alginate, silicone
3 types of non-elastic impression material
Zinc oxide
plaster
compound
2 uses of impression wax
correct airblows in impressions
produce mucocompressive impression of lower free ed saddles
properties of alginate
mucostatic
flexible - undercuts
low strength - tears easily
poor dimensional stability
properties of silicone
viscoelastic- flexbile - undercute
good tear resistance
good dimensional stability
dry field needed
properties of acrylic
weak + flexible
low fatigue resistance - crack propagation
low impact strength - notch sensitive
low abrasion resistance - scratches
slowly absorbs water - colonising organisms - needs cleaning
modified by adding elastomers
why do tissue conditioners cause increased risk of candida
rough + irregular surface
advantages of ZOE as a temp restoration
analgesic if pulp inflamed
how to decrease chance of porcelain fracturing in PFMC
porcelain and metal thermal expansion matched
how does porcelain bond to metal
mechanical retention
chemical bonding to oxide layer
how is crack prorogation risk decreased in PFMC
bonding inner surface of porcelain to metal
outer surface of porcelain being under tension
