Composites Flashcards
what are the 2 main components of composite resins
glass filler particles (hard) and resin (soft)
what is added to composites to give it better mechanical properties
dimethacrylates
what is added to composite to give it its light curing ability
camphorquinone - a photo initiator
what is added to composites to allow its intimate contact between glass filler particles and resin
silane coupling agent
conventional composite
large particles that take up about half of the material volume
microfine composite
tiny particles that take up a small portion of material
fine composite
small particles that take up a large portion of material
hybrid composite
mixture of tiny and large particles that take up a veyr large portion of the material
what monomer is commonly used in resins
BIS - GMA
has difunctional unit (c=c) that facilitates cross linking via free radical addition polymerisation
how does camphorquinone influence composite resins
requires blue light to be activated
once activated produces free radical molecules (electrically charges) which initiate polymerisation of monomer in resin e.g BIS-GMA
not all monomer molecules are converted
what does a greater number of filler particles result in
improved mechanical properties - increased strength, hardness, rigidity etc..
lower thermal expansion
lower polymerisation shrinkage
hardness ratio test
method of measuring depth of cure
cylinder of composite is cured
surface of column hardness determined and compared to other points. at 80% of surface value = depth of cure
depth of cure
depth at which composite resins polymerise sufficiently e.g where they are hard enough
standard depth usually = 2mm
what can happen if increments are more than 2mm in depth
under polymerised base resulting in decreased mechanical properties, poor bonding and subsequent early failure
what do bulk fill composites claim
they can have a depth of cure up to 6mm
use 2 photo initiators rather than one that have different absorption spectrums
controversy over whether claims can be replicated in mouth
is polymerisation an exothermic or endothermic reaction
exothermic meaning there is the potential for pulpal damage
hybrid layer
resin material penetrating into dentinal tubules
what dentine is most favourable for bonding
primary as open tubules allowing a good hybrid layer
tertiary dentine has an irregular structure so is harder to bond to
configuration factor
ratio of bonded to unbonded surfaces
a high configuration factor increases polymerisation contraction stress that can lead to restoration failure
polymerisation contraction shrinkage
monomers join together to form polymers during curing process (polymerisation) , as these short molecules join to form longer ones there is a decrease in volume - polymerisation contraction shrinkage
etch/bond is stronger than enamel interstitial strength so if a high configuration factor there is a risk of enamel fracture that can lead to early breakdown of restoration margins if microleakage occurs and possible secondary caries
why do dentine bonding agents contain a solvent
to dry off any moisture
dont want to over dry dentine before placement as this can cause post - operative sensitivity
pros and cons of conventional composite
strong but problems with finishing and staining as soft resin often at surface
pros and cons of microfine composite
smaller particles give good aesthetics however has lower fracture stress, rigidity and increased abrasion
pros and cons of hybrid composite
best all rounder out of composites on offer - mixture of particle sizes gives good mechanical and aesthetic properties
hardness
measure of how resistant a material is to indentation
discuss composites thermal properties
both thermal conductivity and diffusivity is low which is good
however thermal expansion coefficient is markedly higher than that of enamel or dentine so there is a risk of gaps forming
discuss biocompatility of composite resins
generally okay unless material is not fully cured e.g due to poor technique, too deep an increment, inadequate light exposure etc
risk of unpolymerised resins irritating soft tissues
