Structure of Composites Flashcards
Define Composite
two or more materials put together, with each contributing to the overall properties
Disadvantages of Silicate Cements - 3
- Cannot be used for permanent tooth erosion in a few years
- Soluble in oral fluids
- Discolouration
Advantages of PMMA - 4
- Tooth like appearance
- Insoluble in oral fluids
- Easy to manipulate
- Low Cost
Disadvantages of PMMA - 4
- High setting shrinkage (causes secondary caries)
- Poor wear resistance
- Different coefficient of thermal expansion to tooth
- Problems with reduced inclusion of quartz powder as filler
What type of composite is currently used?
Resin Based Direct Composite filling material
What are the 3 Phases of Dental Composites
- Organic Matrix
- Inorganic Filler
- Bonding Agent
Why is Bis-GMA used instead of MMA in composite fillings?
MMA has a lower molecular weight -> more volume shrinkage of the material after polymerisation
Where does the Free Radical Substitution take place?
In the C=C double bond in the methacrylates.
What are the other additives in the Organic Matrix?
- Hydroquinone
- DHPT ( Activator for RTP Cure and LIGHT CURE)
- Camphorquinone (LC)
- Filler
- Iron oxide (pigment)
- UV stabilisers
- Optical Brighteners
How do direct composite fillings work?
Placed in the unset state and then is cured (set) in the oral cavity
What is the role of the organic matrix in a dental composite?
It is the plastic resin that polymerises to form a continuous phase
What is the role of the coupling agent in a dental composite?
To promote adhesion between filler and matrix resin
What monomer does the organic matrix consist of?
Bis-GMA
What is Bis-GMA prepared from?
Bisphenol A
Glycidyl Methacrylate
Viscosity of Bis-GMA
700,000cps
What is the role of hydroquinone in the organic matrix?
Acts as inhibitor to prevent the setting composite in the lab
Advantages of incorporating fillers 5
- Reduced setting shrinkage
- Reduced coefficient of thermal expansion
- Enhanced strength and compressive hardness
- Provides radio-opacity
- Control of aesthetics
Why must the refractive index of both the filler and resin be the same?
To avoid the scattering of light, otherwise full depth light cure is not achieved
Why are many fillers used in composites?
1 filler is insufficient to attain most of the desired outcomes
What are the 3 different types of fillers?
- Conventional (micro and macro)
- Hybrid
- Microfine
What did the first composites use as macro fillers?
Used ground quartz
What material is used as a micro filler?
Collodial silica
What does a macro filler do?
Increases viscosity of monomers so method of incorporation is further improved
Advantages of Hybrid fillers? 4
- Favourable physic, mechanical and optical properties
- Improved wear resistance
- Surface morphology superior to conventional but inferior to microfilmed resins
- Radiopactiy possible
Disadvantage of Hybrid Fillers
increased surface roughness with time
Resin wears away
Usually how much inorganic filler is loaded into a composite?
30-70%
Why are fillers hydrophobic?
Surface layer of OH groups
What coupling agent is used to bond the filler to the resin?
Silane coupling agent
How does a silane coupling agent work?
The hydrophilic end of the coupling agent (OCH3) reacts with surface hydroxyl groups (OH) of the filler - via condensation reaction
How are room temp. composites dispensed?
2 Paste containing:
- Benzoyl peroxide
- DHPT
How are light cured composites dispensed?
1 paste
