Flashcards in L4- Composite structures Deck (76)
what was the earliest direct tooth coloured restorative material? (1950-1960s)
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
what restorative material was used between 1960 -1980s?
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?
what is included in the powder of acrylics?
– Pmma beads of no greater than 50 μm
– Chemical Initiator (commonly a peroxide)
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?
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
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
what does comonomer such as TEGMA control?
controls viscosity :
-facilitates manufacturers filler addition
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
• 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?
what is the coupling agent in resin composites?
δ – methacryloxypropyltrimethoxysilane