composite resins Flashcards
classification
filler particles - conventional, microfine or hybrid
setting - light or self-cure
area of use - anterior/posterior
handling - condensable/syringeable/flowable
composition
filler particles - glass resin camphorquinone low weight dimethacrylates silane coupling agent
ideal properties
mechanical - strength, rigidity, hardness bonding to tooth/compatible with bonding systems thermal aesthetics radiopaque handling/viscosity anticariogenic polishable low setting shrinkage biocompatible
filler particles
control aesthetics
not involved in polymerisation
polymerisation shrinkage inversely proportional to filler loading
hybrid
range of filler particle sizes so can have higher %
- better mechanical properties
- less polymerisation shrinkage
- less heat of polymerisation
camphorquinone
photoinitiator
activated by blue light
produces radical molecules
initiates free radical addition polymerisation of Bis-GMA
leads to changes in resin properties
- increased molecular weight so increased viscosity and strength
can add inhibitor (hydroquinone) to prevent premature polymerisation - shelf-life (0.1%)
resin - monomers used
Bis-GMA
urethane dimethacrylates
resin - monomer characteristics
difunctional molecule
C=C bonds facilitate X-linking
undergoes free radical addition polymerisation
unreacted monomer
potential to cause an allergic reaction
low weight dimethacrylates
e.g. TEGDMA
added to adjust viscosity and reactivity
because the resin monomers are highly viscous, adding any filler would make it too stiff
why is a silane coupling agent necessary?
good bond between filler particle and resin is essential
- but normally water adheres to glass filler particles, prevents resin from bonding to the glass surface
silane coupling agent
preferentially bonds to glass and also to resin
bonds hydrophobic resins to hydrophillic glasses
hydroxyl group bonds to glass
methacrylate group bonds to resin via C=C
flowable
lower filler content
- more shrinkage
- poorer mechanical properties
effect of adding filler particles
improved mechanical properties improved aesthetics increased abrasion resistance lower thermal expansion reduced polymerisation shrinkage reduced heat of polymerisation some radiopaque
curing development
self (2 pastes) UV activation (obsolete, 1 paste) light curing (blue light, 440nm, 1 paste) direct curing indirect/post curing (in lab)
advantages of light curing
extended working time less finishing immediate finishing less waste higher filler levels - not mixing 2 pastes less porosity - not mixing 2 pastes
light cure sources
halogen or LED
LED much more efficient - light source spectra matches camphorquinone absorption much better
activation self-curing 2 pastes
benzoyl peroxide initiator
aromatic tertiary amine activator
light-curing 1 paste
camphorquinone blue light (430-490nm)
activation
free radicals break resin C=C bonds
hardness
most of blue light absorbed close to surface - where it sets most readily and becomes hard
depth of cure
depth to which it polymerises sufficiently such that its hardness is about half that of the cured surface
2mm
increment thickness
> 2mm
underpolymerised base
- ‘soggy bottom’
- poor bonding to tooth - early failure
what do manufacturers do re depth of cure?
exaggerate it
bulk fill e.g. tetric ceram
different optical absorption spectrum - UV and blue light needed to polymerise (cure) fully
lucerin initiator as well as camphorquinone
safety
exothermic reaction
divergent light beam
thermal trauma to STs
staff - ocular damage
light curing potential problems
light/material mismatch - overexposure
premature polymerisation from dental lights
optimistic DOC values
recommended setting times
polymerisation shrinkage - micro leakage - use small increments
properties
biocompatible (but unreacted monomer) quite hard but some concerns for posterior strong rigid - high YM bonding good but technique dependent hybrid best mechanical properties thermal conductivity low - good as avoids pulpal damage TEC high - poor (micro leakage) good aesthetics - translucency some radiopaque not anticariogenic on demand set and polishable polymerisation shrinkage
conventional
strong
but problems with finishing and staining
soft resins and hard particles
microfine
smoother surface - better aesthetics for longer period
reduce shrinkage stress
inferior mechanical properties
wear
resin soft relative to hard filler particles - more wear
conventional - bigger filler particles so rougher surface
factors affecting wear - material
filler material particle size distribution filler loading resin formulation coupling agent
factors affecting wear - clinical
cavity size and design tooth position occlusion placement technique cure efficiency finishing methods
bonding to tooth surface
reduce microleakage counteract polymerisation shrinkage shear bond strength minimise cavity design stress transfer to tooth and bone - good bonding = no stress concentration areas
amalgam vs composite failure rate
amalgam much lower failure rate
Filtek silorane posterior composite
not methacrylate based - lower polymerisation shrinkage
no grey/white high stress areas in stress concentration map
