Composite Resins Flashcards
When were composite resins patented?
1960s
Although greatly improve performance in more recent times
Name a challenge about composite resins
Technique sensitive
Properties of ideal direct restorative material
Safe for dental team and patient Aesthetic Durable, chemically stable, tasteless Poor conductor of heat Minimal tooth prep Easy to use Cost effective
Properties of typical composite resin
Safe if handled correctly (& used in correct location)
Aesthetic
Durable, chemically stable, tasteless
Porr conductor of heat (good insulators)
Minimal tooth prep (do not need undercut due to adhesive bonding strategies)
Technique sensitive (not easy to use)
Not cheap but highly effective (durable)
What is a composite material?
Where two or more materials (metals, ceramics & glasses, polymers) are combined in such a way to produce improved properties
E.g. fibreglass is a tough glass-fibre reinforced polymer used in wide range of applications
Composite ‘rules’
In a composite, component materials are largely unchanged by the presence of others (unlike a soln)
There must be some form of interfacial bonding between components (unlike a mixture)
How do composites work?
Different components bring diff. properties to final material
E.g. fibreglass: thin glass fibre filler is strong but brittle, & polymer matrix is tough but flexible.
Additional toughness may be imparted by crack deflection. Fillers also change other properties e.g. light scattering.
Composition of dental composite resins
Matrix component typically mixture of methacrylate resins (most commonly bis glycidyl methacrylate or bisGMA, but others as well)
Filler typically a silica (SiO2) powder
Modern dental composites usually contain a photoinitiator (most commonly camphorquinone)
-Polymer formed from monomers, filler and photoiniatiator (and some pigment), fillers become trapped in polymer matrix
How is bisGMA formed?
By reaction of glycidyl methacrylate with bis-phenol A
Rafael Bowen
Inventor of dental composite
-Reality more complex: key patents also held by ICI Ltd
Setting
May set by chemical cure or light activation (polymerisation)
Chemical cure: 2 pastes mixed (an activator with free-radical initiator)
-this approach obsolete in restorative materials
Light activated systems contain a photoinitiator; setting initiated by exposure to appropriate lamp
-camphorquinone most common, activated at 470nm- hence visible blue light used
Dental composite resins and water
Hydrophobic so need bonding system to adhere to hydrophilic tooth tissue
-no intrinsic ability to bond to tooth
Cons of composites
Hydrophobic will not bond directly to hydrophilic dentine or enamel
Polymerisation shrinkage
What has changed since 1960s in evolution of composites?
Way we cure the resins (initiation of polymerisation)
Size of filler particles
Bonding to tooth tissue
What size fillers do we now use?
Nanofillers: 0.01 - 0.01p μm
We have moved down size over time
Advantages of smaller filler sizes
> resistance to wear & potentially greater strength
ability to polish
translucency
Combined with changes in filler volume, potential to decrease polymerisation shrinkage
Coupling agents
Improve adhesion of resin to filler surfaces to > strength and toughness
-chemically coat filler particle surfaces
What are used to coat fillers?
Silanes have been used for > 50 years
Disadvantages of silanes
Age during storage
Lose potency
Sensitive to water: water absorbed into composite restorations results in hydrolysis of silane bond and eventual loss of properties
Common silane agents
Vinyl triethoxysilane
Gamma or 3-methacryloxypropyltrimethoxysilane
Overcoming polymerisation shrinkage
It is inevitable
Risk of bulk placement:
-inadequate cure at depth
-weak or incomplete interface with tooth tissue = microleakage and secondary caries
Composites should be placed and cured in layers to reduce shrinkage and stresses, gives good marginal integrity
Layering techniques
2mm is considered reasonable thickness of individual composite layer
Oxygen inhibition is an additional complcication
Three phases in composite resin system
Organic phase (resin matrix) Dispersed phase (inorganic filler) Interfacial phase (coupling agent)
Organic phase (resin matrix)
Monomer, initiator, inhibitors, pigments
Forms polymer backbone to provide tensile strength
Dispersed phase (inorganic filler)
Glass, quartz, colloidal silica
Improves mechanical properties (wear and compressive strength) and decreases shrinkage
Interfacial phase (coupling agent)
Provides adhesive bond between organic and dispersed inorganic phases
Resin monomers in composites
The main resin monomer in all composites is bisphenol A-glycidyl methacrylate (bisGMA)
Other monomers are added, for example to reduce viscosity and improve mixing:
-triethylene glycol dimethacrylate (TEGDMA)
-urethane dimethacrylate (UDMA)
It may be that new resin monomers will be incorporated in future innovations
How do composites set?
Free-radical polymerisation
-leads to polymerisation shrinkage
Photo initiators
e.g. camphorquinone
Diff manufacturers today utilise range of photoinitiators in their products
Intended to aid efficiency of photoinitiation of polymerisation, and should match output of light curing unit
When did the first composites enter dental market?
1960s
- early versions were macrofilled composites
- high wear rate, polymerisation shrinkage, technique sensitivity (bad)
Development of microfilled composites
1970s
- increased wear resistance, more highly polishable surface
- technique sensitive
Introduction of hybrid composites
By mixing of filler particles of various sizes
-improved mechanical properties, good polishability
Introduction of nanofilled composites
Early 2000s
- with dispersed nanoscale filler
- simplified but effective bonding systems reduced sensitivity and impvoed clinical success
Claimed advantages of nanofilled over conventional composites
Potentially superior fracture toughness, high strength, and excellent wear resistance
High polishability and low polymerisation shrinkage
> continuity between nanoscopic tooth structure and nonfillers to provide more stable and ‘natural’ interface
> surface to volume ratio allow high filler loading to give workable consistencies
Nanosized fillers are able to scatter or absorb visible light which increase translucency
