DML1: Structure of composites Flashcards Preview

BDS2; Dental materials > DML1: Structure of composites > Flashcards

Flashcards in DML1: Structure of composites Deck (33)
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1
Q

What is a compostie

A

two or more materials put together with each contributing to the overall properties (thus if one was missing the material would fail)

2
Q

Why are silicate cements not used any more

A
  • they are soluble in oral fluids and so the properties are weakened over time
  • it means that the material is swallowed
  • it also discolours
3
Q

Why were silicate cements originally used

A
  • they used to be the only tooth coloured restorative

- they released fluoride which reminineralises

4
Q

What are the advantages of direct filling acrylics such as polymethyl methacrylate (PMMA)

A
  • tooth like appearance
  • insoluble in oral fluids
  • easy to manipulate
  • low cost
5
Q

What are the disadvantages of direct filling acrylics such as polymethyl methacrylate (PMMA)

A
  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
6
Q

How can the disadvantageous properties of PMMA direct filling acrylics be reduced

A

Inclusion of quartz power as a filler - this makes the filling a composite

7
Q

What are resin based direct composite filling materials such as Bowen’s resin made from

A

Bis-GMA (dimethacrylate made from Bis-phenol A and glycidyl methacrylate)

The filling material consists of three phases

8
Q

Why are resin based direct composite filling materials used

A

They are tooth coloured and so a choice for direct aesthetic anterior restorations

9
Q

What are the three phases in dental composites

A
  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
10
Q

How does the organic matrix set

A

It cure-polymerises into a rigid polymeric material via a free radical, addition polymerisation reaction

11
Q

What does the organic matrix in the monomer phase consist of

A
  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

12
Q

What are the constituents of Bis-GMA

A
  1. Bisphenol A

2. Glycidyl methacrylate

13
Q

Why is UDMA (urethame dimethacrylate) used instead of Bis-GMA

A

Because it is a monomer with a lower viscosity because it doensn’t have aromatic rings

14
Q

Why are diluent monomers added to the organic matrix

A

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

15
Q

Give an example of a diluent monomer

A

Glycol dimerthacrylates

16
Q

Why are dimethacrylates used instead of methyl methacrylates

A

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)

17
Q

What additives does the organic matrix contain and why

A
  • 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
18
Q

What are the benefits of including a filler phase within composites

A
  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

19
Q

Why must the refractive index of filler and resin match

A

To avoid scattering of light otherwise the full depth of cure will not be achieved when light curing the resin

20
Q

What do smaller filler particles result in

A

A smoother finish of the restoration and they can be polished

21
Q

Why are macrofillers no longer used

A

Because they give a poor finish with a dull appearance as the filler particles protrude through the resin surface

22
Q

What are the positives of macrofillers

A

They have a high maximum filler loading

23
Q

What is the issue with microfillers

A

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

24
Q

How is adequate microfiller loading achieved

A

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

25
Q

What are the benefits of hybrid fillers

A

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

26
Q

What are the diadvantages of hybrid fillers

A

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

27
Q

What are the benefits of nanofilled composites

A

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

28
Q

What are microfilled composites used for and why

A

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

29
Q

Why are silane coupling agents needed

A

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

30
Q

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

A

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

31
Q

What happens if bonding is unsuccessful between the filler and resin via the silane coupling agent

A
  1. Unstable bonds cause fracture and disintegration of material
  2. If no bond is made there will be stress transfer due to phase separation
  3. Crack initiation sites will occur which will cause fatigue failure of composite
32
Q

What are the three initiator systems for polymerisation and curing of composites? Outline their strengths and weaknesses

A
  1. Heat cured - impractical for direct filling materials and cannot be used within patients mouth; it is used for indirect composites (inlays/onlays)
  2. Room temperature - dispensed as two pastes; on mixing air bubbles are incorporated and this makes mixing difficult due to viscosity of pastes and porosity will occur
  3. Light cured - react in blue light to form free radicals which initiate addition polymerisation
33
Q

What are the two pastes used in room temperature cured composites

A
  • benzoyl peroxide

- dihydroxyethyl-p-toulidine (DHPT) = tertiary amine