L4- Composite structures Flashcards
what was the earliest direct tooth coloured restorative material? (1950-1960s)
silicates
how is silicate formed?
Reaction product of an acid base reaction (giving a salt
plus water) between – Aluminosilicate glass and Phosphoric acid
what are properties of silicate?
- Releases Fluoride
- Moisture sensitive
- Erosion prone
- Brittle
what restorative material was used between 1960 -1980s?
acrylics
what was acrylics derived from?
products used to fabricate dentures
how is acrylics presented?
powder and mixer in which it is mixed to a precise ratio for correct properties
why is working time limited?
chemical set
what is included in the powder of acrylics?
– Pmma beads of no greater than 50 μm
– Chemical Initiator (commonly a peroxide)
– Pigment
what is included in the liquid of acrylics?
– Mma monomer
– Chemical activator (tertiary Amine)
what type of reaction is the setting reaction in acrylics?
free radical addition polymerisation
what are the stages of free radical addition polymerisation of methyl methacrylate?
- activation
- initiation
- propagation
- termination
what occurs in activation?
free radical breaks double bond and joins together
what occurs in propagation?
linking one by one like building blocks
what is termination?
when chain attaches to impurity / free radical or further growing chain
why is this free radical reaction important for the clinician?
• Material contracts on polymerisation • Reaction is exothermic • Tertiary amine is not all used up – Poor colour stability (clear – brown) • Must use correct P:L
what are acrylics compared to silicates?
acrylics are less : -prone to erosion -soluble -acidic -brittle Exhibit lower thermal diffusivity
Describe the properties of acrylics.
- Match tooth shade (but not in long term if have amine)
- Are exothermic when setting
- Irritant to pulp due to residual mma
- Interact with Eugenol
- Contract (6% by volume)
- Have low hardness
- Poor abrasion resistance
what was introduced to acrylics in 1951 ?
Introduced inert filler particles to reduce shrinkage
– Though successful in this regard they weakened the material
what is in the 1963 composite ?
- BIS GMA resin matrix
- Coupling Agent
- Filler particles
what is a resin composite?
A combination of two chemically different materials with a distinct interface separating the components and having properties which could not be achieved by any of the components acting alone
how are resin composites retained?
acrylics free radical addition polymerisation
Describe early products in resin composites.
two pastes were mixed together :
– Limited working time
– Incorporation of porosity
Describe later products in resin composites.
light curing technology was developed to give command curing
what is included In resin matrix?
– Generally based upon methacrylate or dimethacrylate monomers. • E.g. BIS GMA, Urethane dimethacrylate – Include comonomers such as TEGMA – Inhibitors
what does comonomer such as TEGMA control?
controls viscosity :
- facilitates manufacturers filler addition
- clinical handling
why are inhibitors in the resin composites?
to stop early setting accidentally
In terms of resin matrix, discuss viscosity of matrix polymer.
• Reduces mobility of unreacted monomers
• A consequence that in any composite of this type there
are unreacted monomers offering scope for chairside
repair.
• C=C take part in free radical polymerisation
• In new products, based on Siloranes, ring opening
occurs (not free radical), producing expansion that offsets shrinkage. Still however an overall contraction.
what 3 fillers can you get?
- quartz
- silica
- glasses
what is the coupling agent in resin composites?
δ – methacryloxypropyltrimethoxysilane
what are the functions of methacryloxypropyltrimethoxysilane?
- Methacrylate charcteristics
* Silane interacts and bonds with glass
How does the coupling agent spread?
good wetting
What happens to surface hardness as %filler content by volume increases?
increases
what happens to % volume setting reaction as %filler content by volume increases?
decreases
what happens to coefficient of thermal expansion as %filler content by volume increases?
decreases
what are the two ways to activate resin composites?
- chemical
- light
How can you chemically activate of resin composites?
- Powder Liquid
- Paste Liquid
- Encapsulated
How can you activate resin composites by light?
• Early ones U V (health risks)
• Today Camphorquinone
– Protect eyes
– Will set under operating light to a certain extent
what does Camphorquinone do?
Yieldsthenecessary free radicals to start the polymerisation
At what wavelength does camphorquione become excited?
460-480nm
visible blue light
what is camphorquinone blue light most readily generated by?
quartz halogen lights
what do all types of resin composites contain?
- resin
- filler
- coupling agent
what is resin susceptible to upon polymerisation?
shrinkage
how can resin be modified?
-methacrylate/acrylate
-OR a chemical that upon setting expands due to a ring
opening mechanism eg Oxirane
what effect does expansion of resin have?
-This expansion in resin volumes offsets to a degree the
polymerisation shrinkage
» Still however a net shrinkage
what filler properties control resin composite properties?
- type
- concentration
- particle size
- particle size distribution
what role does the coupling agent have?
The coupling agent transfers the stresses generated under loading from the rigid and brittle filler to the more flexible and ductile polymer matrix
what is the filler regarded as?
shock absorber
how is resin composites classified?
- method of activation (chemical/light)
- filler particle size and distribution
- handling characteristics
- intended clinical application
what are the 4 types of resin composites?
- conventional
- microfilled
- hybrid (conventional + microfilled)
- nanocomposites
What is conventional resin composite?
- 1-50 micrometers
- 60-80% by weight
What is microfilled resin composite?
- 0.01-0.1 micrometers
- 30-60% by weight
What is hybrid resin composite?
- blend of conventional and microfilled
- 83-90% by weight
What is nanocomposite resin composite?
uses particle less than 1 micrometer diameter
what are the different handling characteristics?
• Packable – Highly viscous – Presents packaging challenges to manufacturers • Flowable – More fluid – Less filler
what are different types for clinical application (ISO 4049)?
– Type 1 – restoration of cavities involving occlusal surfaces
– Type 2 – All other polymer based filling and restorative
materials
what does resin composite need to be on the market?
conform with ISO 4049
are resin composites biocompatible?
• Less scrutiny than amalgam
– Cytotoxicity of components
– Oestrogenicity of commonly used resin precurrsors
– Tumour producing aerosols of fine glass particles
what happens to viscosity of resin composite when leaving the tube?
Viscosity increases on leaving tube - filler particles become bunched up in the mix as its pushed out
Describe the setting of resin.
- exothermic set
- contract not towards light source
what are the thermal properties of resin?
– Mismatch tooth for thermal
expansion
• Risks marginal percolation
– Thermal diffusivity matches dentine
what is the difference in working time between chemically and light activated?
- Limited working time if chemically activated
* Extended working time if light activated
how does the resin set?
gradual set from outer surface within
When are physical properties reduced?
with porosity
what strength is considered more important?
Tensile and flexural now considered more important than compressive
what is tribology?
changes with function and loss of resin rich outer layer
why is thermal properties important to filler content?
As filler increases coefficient of thermal expansion
decreases
why is thermal properties important mechanically?
• Dependent upon
– Filler content and type
– Efficiency of coupling
– Degree of porosity
what happens to strength with increased porosity?
decrease
what happens to strength with decreased coupling?
decrease (by 30%)
what happens to fatigue with increased porosity?
decrease
what happens to fatigue with decreased coupling?
decrease (by 30%)
Describe the staining properties.
– Initially good appearance but may stain
• Microfine best at maintaining – In general:
» Resin matrix can take up fluids eg wine
» Picture Frame stain if marginal deterioration eg wine/smoking.
Does cavity shape influence overall success of restoration?
Yes
what is the C factor?
ratio of number of nodes surfaces to number of un-bonded surfaces
why does C factor matter?
composite shrinks
what should clinical application be based upon?
– Awareness of shortcomings
– Understanding of properties
