DML1: Structure of composites Flashcards
What is a compostie
two or more materials put together with each contributing to the overall properties (thus if one was missing the material would fail)
Why are silicate cements not used any more
- they are soluble in oral fluids and so the properties are weakened over time
- it means that the material is swallowed
- it also discolours
Why were silicate cements originally used
- they used to be the only tooth coloured restorative
- they released fluoride which reminineralises
What are the advantages of direct filling acrylics such as polymethyl methacrylate (PMMA)
- tooth like appearance
- insoluble in oral fluids
- easy to manipulate
- low cost
What are the disadvantages of direct filling acrylics such as polymethyl methacrylate (PMMA)
- There is high setting shrinkage and so secondary caries occurs at the margins
- Poor wear resistance (especially in posterior cavities)
- The coefficient of thermal expansion is different to the tooth and so there are stresses at cavity margins
How can the disadvantageous properties of PMMA direct filling acrylics be reduced
Inclusion of quartz power as a filler - this makes the filling a composite
What are resin based direct composite filling materials such as Bowen’s resin made from
Bis-GMA (dimethacrylate made from Bis-phenol A and glycidyl methacrylate)
The filling material consists of three phases
Why are resin based direct composite filling materials used
They are tooth coloured and so a choice for direct aesthetic anterior restorations
What are the three phases in dental composites
- Organic matrix = plastic monomer/resin viscous fluid material that polymerises to form a continuous phase, binding the filler particles via a coupling agent (this is hydrophobic)
- Inorganic filler = reinforcing particles/fibres dispersed in the matrix which enhances the materials mechanical properties (this is hydrophilic)
- Coupling agent = bonding agent promotes adhesion between the filler and resin matrix
How does the organic matrix set
It cure-polymerises into a rigid polymeric material via a free radical, addition polymerisation reaction
What does the organic matrix in the monomer phase consist of
- Aromatic dimethacrylate-Bis-GMA
- Bis-GMA
As this is not pure, it will not polymerise to 100% and so the two components will leach into the patients mouth
What are the constituents of Bis-GMA
- Bisphenol A
2. Glycidyl methacrylate
Why is UDMA (urethame dimethacrylate) used instead of Bis-GMA
Because it is a monomer with a lower viscosity because it doensn’t have aromatic rings
Why are diluent monomers added to the organic matrix
Because both Bis-GMA and UDMA become unworkable due to their viscosity which is further increased by the addition of fillers
Diluent monomers act as viscosity controllers
Give an example of a diluent monomer
Glycol dimerthacrylates
Why are dimethacrylates used instead of methyl methacrylates
because methyl methacrylate has a lower molar volume and molar weight than dimethacrylates like bis-GMA have larger monomers and this reduces the linear shrinkage that will occur (this is because the C=C double bonds will be broken)
What additives does the organic matrix contain and why
- hydroquinone
- DHPT activator/initiator system for room temperature cure
- camphorquinone activator/initiator system for light cure
- fillers to reduce polymerisation shrinkage
- pigments to give correct shade
- UV stabilisers
- optical brighteners to give flouresence
What are the benefits of including a filler phase within composites
- reduces setting shrinkage as less resin is required
- reduces coefficient of thermal expansion (this has to be close to that of the tooth tissue)
- Enhances modulus, compressive strength and hardness
- Provide radiopacity
- Control of aesthetics
Mechanical properties of composite improve with higher amounts of filler
Why must the refractive index of filler and resin match
To avoid scattering of light otherwise the full depth of cure will not be achieved when light curing the resin
What do smaller filler particles result in
A smoother finish of the restoration and they can be polished
Why are macrofillers no longer used
Because they give a poor finish with a dull appearance as the filler particles protrude through the resin surface
What are the positives of macrofillers
They have a high maximum filler loading
What is the issue with microfillers
A very large surface area of filler is in contact with the resin and so there is difficulty obtaining a high filler loading - this gives worse mechanical properties even though it is more aesthetic
How is adequate microfiller loading achieved
2 STAGE PROCESS (has greater cost implications)
- A microfilled composite is polymerised and ground to particles 10-40um in size
- These particles are then used as a filler
Thus these are composite containing composite filler particles
