DML1: Structure of composites Flashcards Preview

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Flashcards in DML1: Structure of composites Deck (33)
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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)

1. There is high setting shrinkage and so secondary caries occurs at the margins

2. Poor wear resistance (especially in posterior cavities)

3. 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

1. 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)

2. Inorganic filler = reinforcing particles/fibres dispersed in the matrix which enhances the materials mechanical properties (this is hydrophilic)

3. 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

1. Aromatic dimethacrylate-Bis-GMA
2. 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

1. 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

1. reduces setting shrinkage as less resin is required
2. reduces coefficient of thermal expansion (this has to be close to that of the tooth tissue)
3. Enhances modulus, compressive strength and hardness
4. Provide radiopacity
5. 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)

1. A microfilled composite is polymerised and ground to particles 10-40um in size

2. These particles are then used as a filler

Thus these are composite containing composite filler particles


What are the benefits of hybrid fillers

The packing density is increased while the size of the filler has been reduced
- improved wear resistance
- radiopacity

This is why they are used for posterior restorations because they are stronger


What are the diadvantages of hybrid fillers

They have an increased surface roughness with time as the resin wears away and the filler particles protrude through the matrix


What are the benefits of nanofilled composites

They have the strength of a hybrid and polishing finish of a microfilled material


What are microfilled composites used for and why

For anterior restorations due to lower filler loading and this decreased mechanical properties


Why are silane coupling agents needed

Because there is phase separation due to hydrophobic resin and hydrophilic filler and these need to be bonded to eachother for the composite to have the desired mechanical properties - coupling agents thus give improved wear resistence


How does the silane coupling agent react to bond to the filler and resin

Filler: hydrophilic end of coupling agent reacts with hydroxy groups of filler via condensation reactions

Resin: bonds form to the organic matrix via free radical addition polymerisation