Flashcards in Composite Properties Deck (46)
what is the fracture stress of composite
It is strong
what is the Young's modulus of composite
it is rigid
what does a large posterior cavity require from composite
composite that is high strength, high rigidity and high abrasion resistance as it needs to survive its function and withstand biting forces
what do deciduous teeth require from composite
a composite that is strong in thin sections and it’s wear is the same of that of the tooth (i.e not amalgam) as well as other important properties such as bonding, microleakage etc
describe conventional composite
strong but problems with finishing and staining due to soft resin and hard particles. The softer the resin then the softer the resin on the surface and the more likely it is to uptake stain from food and drink etc.
describe microfine composite
smaller particles, smoother surface for better aesthetics for longer period but inferior mechanical properties (elastic limit and young’s modulus)
describe hybrid composite
originally compromise between conventional and microfine. Most modern composites are hybrids - improved filler loading (higher percentage of filler) and coupling agents have led to improvement in mechanical properties
what is hardness
refers to the materials surface – we want to know whether a material is resistant to scratching, whether it is able to resist indentation.
what does abrasion result in
removal of surface layers when two surfaces make frictional contact as these are the forces that happen when we bite and grind surfaces against the tooth/restoration
what is the hardness test
test uses an indentor made of stainless steel that has a specific shape at the end. It weighs around 100g and we leave it for a period of time, 30 seconds usually and this allows an indentation to appear.
If the material is hard then the indentation should be relatively small.
what is abrasion
when the tooth grinds/slides along the opposing tooth surface (or the restorative material at its surface.
what does the tooth surface being abraded result in
a loss of material surface layers and a roughened surface.
what does the surface roughness of composite affect
o Plaque retention
o Sensation when in contact with the tongue
what happens to composite in wear
As the surface is removed, the resin is removed, and the glass filler particles are exposed and it is only when they are exposed do we lose them.
how does the size of the filler particle affect surface roughness
The size of the filler particles on the surface will determine the degree of surface roughness.
what are the material factors affecting wear
o Filler material
o Particle size distribution
o Filler loading
o Resin formulation
o Coupling agent
what are the clinical factors affecting wear
o Cavity size and design
o Tooth position
o Placement technique
o Cure efficiency
o Finishing methods
what are the bonding properties of composite
Generally okay but some mismatches have been reported
how does composite bond to enamel
acid etch technique
what is acid etch technique
uses 30% phosphoric acid, put on enamel for 20 seconds. It takes the original enamel surface and leave ‘microtags’ within the enamel structure which you can fill with an unfilled resin
how does composite bond to dentine
through dentine/universal bonding systems
what is the typical bond strength of composite to enamel/dentine
why is bonding crucial
a good bond will reduce micro leakage and counteract polymerization's stress
what happens in polymerization shrinkage
material is trying to pull away from tooth surface it is bonded to and it tries to pull away with sufficient force and break away from tooth surface then this can create gaps
what does composite bonding to tooth mean for cavity design
no need for retention undercut.
why is bonding crucial for stress transfer
Good bonding means that when a force is applied the forces acting on the base of the composite are spread uniformly and evenly and we have a good bond then that the restoration is likely to survive a long time. If you have a poor bond then there is lots of gaps between the restoration and the tooth tissue and the stress that you put will not be spread evenly and there will be greater stress on certain regions and if there is greater stress then the material is more vulnerable to fracture.
why are hybrid composites better than microfilled composites
o elastic limit is almost double that of microfilled composite
o hybrid is almost 3 x as hard making it more resistant to abrasion
how does amalgam compare in terms of elastic modulus
double the elastic modulus meaning it is twice as rigid meaning it is twice as able to resist any movement when you subject it to stress
what are the thermal properties of composite
low thermal conductivity (good) but has a high thermal expansion coefficient which is poor