Structure of Composites Flashcards
What is a composite
Two or more materials put together, with each contributing to the overall properties
Name some of the uses of composites
- Filling materials
- Luting agents
- Indirect restorations
- endodontic post and cores
- Fissure sealants
What are the advantages of Direct Filling Acrylics and give an example
- Tooth like appearance
- Insoluble in oral fluids
- Ease of manipulation
- Low cost
e.g. PMMA - polymethyl-methacrylate
What are some disadvantages of Direct Filling Acrylics
- Secondary caries due to high setting shrinkage
- Poor wear resistance
- Coefficient of thermal expansion (contraction) different to the tooth
What can be used to reduce problems with direct filling acrylic
Inclusion of quartz powder as a filler that renders the filling as a composite structure
Why is the direct filling acrylics having a different coefficient of thermal expansion than the tooth a problem
Causes stresses at the cavity margins
What is the shrinkage of methyl methacrylate monomers
about 21% volume
What is the shrinkage of PMMA/MM, powder/liquid shrinkage
about 7% volume
What is bowen’s resin made based on and how it this made
Based on Bis-GMA (a dimethacrylate) made from the reaction between Bis-phenol A and glycidyl methacrylate
When is bowen’s resin primarily used
Direct aesthetic anterior restorations.
What is a Direct composite filling material
This is a material that must be placed in the unset state and cured (set) in the oral cavity)
What are the 3 phases/components of Dental Composites
- Organic Matrix
- Inorganic Filler
- Coupling Agent
Describe the structure and function of the organic matrix (monomer phase) of dental composites
A plastic monomer/resin material that polymerises (sets, cures) to form a continuous phase, binding the filler particles (via a coupling agent)
Describe the function of the inorganic filler of dental composites
reinforcing the particles and/or fibres dispersed in the matrix
Describe the function of the coupling agent of dental composites
Bonding agent promotes adhesion between the filler and resin matrix
What are monomers and what is their role in dental composites
chemically active components of the composite that are viscous fluids (resins) that set into a rigid polymeric material via a free radical, addition polymerisation reaction
What is the organic matrix (monomer phase) of dental composites made of
- Aromatic dimethacrylate-bis-GMA and/or UDMA
- Bis-GMA = Bowens’ monomer
Why might UDMA be used as opposed to BisGMA
As BisGMA is very viscous and UDMA has a lower viscosity
What does UDMA stand for
Urethane Dimethacrylate
Name another dimethacrylate monomer that can be used as an alternative to BisGMA and UDMA
Ethoxylated Bisphenol A derivative
What is a dimethacrylate
contains 2 methacrylate groups at each end of the molecule that can both undergo polymerisation
How does the inclusion of fillers affect the viscosity of organic matrix monomers
increases viscosity
What are Diluent Monomers
Also known as viscosity controllers that help to keep the viscosity of the monomers down
Name what kind of molecule diluent monomers are and give examples
Glycol Dimethacrylates
- Triethyleneglycol dimethacrylate (TEGDMA)
- Ethylene glycol dimethacrylate (EDGMA)
- Tetrahydrofurfuryl methacrylate (THFMA)
What type of monomers are used in denture bases
Methyl Methacrylate
What is molar volume
This is the space occupied by each monomer unit
How do DMAs differ to MMAs in molar volume
MMA = low molar volume BisGMA = large molar volume
Why are DMAs preferred to MMAs in dental composites
MMA has lower molecular weight and lower molar volume and so there is more volume shrinkage of the material after polymerisation
Where in the chemical structure of methacrylates do the free radical reactions take place
between carbon=carbon double bonds, in this case at the ends of the monomer molecules
AY BAWS CAN I HABE DE NOTE PLZ
BisGMA, UDMA and diluents have 2 sets of double bonds. All of these polymerise and contribute to the overall shrinkage of the material when set.
Name some of the additives that can be found in the Organic Matrix
- Activator/Initiator system for room temp cure e.g. DHPT
- Activator/initiator system for light cure e.g. DHPT
- Fillers that reduce polymerisation shrinkage
- Pigments e.g. iron oxide
- UV stabilisers
- Optical brighteners - fluorescence
- Hydroquinone - polymerisation inhibitor, increases half life of monomer
What are the advantages of incorporating the Filler Phase of dental composites
- Reduce setting shrinkage of the organic resin phase (less resin required)
- Reduce coefficient of thermal expansion (80ppm/C –> 10ppm/C)
- Enhance modulus, strength (compressive) and hardness
- May provide radio-opacity e.g. quartz, fused silica, radio-opaque glass
- Control of aesthetics e.g. translucency, fluorescence and colour
AY BAWS CAN I HABE DE NOTE PLZ
Mechanical properties of composite improve with high amounts of filler incorporated (>70%)
Why must the refractive index of filler and resin match closely
To avoid scattering of light otherwise the full depth of cure is not achieved on light curing the resin
What are the refractive indices of BisGMA, TEGDMA and most glasses
BisGMA = 1.55
TEGDMA = 1.46
Most glasses = 1.5
What must a filler be treated with
Silane
Why are multiple fillers used rather than just one
One filler is not enough to attain most of the above goals
What determines the amount of filler that can be incorporated in the resin
Average particle size and particle size distribution
What kind of filler results in a smooth finish
smaller filler particles
Name the 3 types of filler
Conventional (Macrofillers)
Hybrid/Small Particle Hybrid
Microfine
Name examples for each type of filler
Conventional - Quartz, Borosilicate glass
Hybrid - Lithium/Barium aluminium silicate
Microfine - Strontium glass, colloidal silica
What process is used to allow the use of microfillers
- Microfilled composite is polymerised and ground to particles
- These particles used as fillers in further composite formulations
- Allows high loading of pre-polymerised particles
What are the advantages of Hybrid composites and high filler loading
- Favourable physical, mechanical and optical properties
- Improved wear resistance
- Surface morphology superior to conventional but inferior to microfilled resins
- Radiopacity possible
What are some disadvantages of hybrid fillers
- Increased surface roughness with time, resin wears away
What is the typical inorganic filler loading vol. %
- 30-70%
What is the maximal theoretical packing density
74%
What are microfilled composites used for and why
Anterior restorations due to lower filler loading and hence mechanical properties are compromised
What are hybrid composites used for and why
Posterior restorations due to higher filler loading
Describe the hydrophilicity of the organic resin and the filler
Organic resin - Hydrophobic
Filler - Hydrophilic
What is silane used for in dental composites
Silane is a coupling agent that is used to bond the filler to the resin
What is silane referred to as
gamma MPTS
What part of silane structure bonds to resin on polymerising
the double bond in the methacrylate group (free radical polymerisation)
What part of the silane structure binds to the filler particle
The hydrophilic OCH3 group reacts with the surface OH bonds on the filler (condensation)
What is the method for silane application called and what happens in it
The Silane treatment of the filler is when the agent is coated around the filler particles
What effect does acid treatment have on silane and what reactions follow the treatment
Hydroxyl groups replace the methoxxy groups and these OH groups then react with the hydroxyl groups on the glass filler via condensation
What happens if the bond is unstable between the filler and resin
The composite may/will fracture, subsequently leading to disintegration of the material
What happens if no bonds forms between the filler and resin
Stress transfer between the filler (glass) and resin is insufficient, resulting in creep, fracture and water of the composite material. Crack initiation sites occur which will grow in time resulting in fatigue failure of the composite
What kinds of composites are heat cured composites ideal for
Indirect, inlays/onlays
Not direct
How do room temperature cured composites work
- Dispensed as 2 pastes, benzoyl peroxide and DHPT
- On mixing, air bubbles are incorporated due to difficulty in mixing as a result of the viscosity of the pastes, which in turn affect the mechanical properties
How are most direct filling materials cured
Light
Describe the reaction when blue light is directed onto the paste
Polymerising reaction occurs and the tertiary amine (eg DHPT) reacts with the light initiator (camphorquinone), to form free radicals that then start the addition polymerisation reaction
