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Dental Materials Y2 > Structure of Composites > Flashcards

Structure of Composites Flashcards

1
Q

What is a composite

A

Two or more materials put together, with each contributing to the overall properties

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2
Q

Name some of the uses of composites

A
  • Filling materials
  • Luting agents
  • Indirect restorations
  • endodontic post and cores
  • Fissure sealants
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3
Q

What are the advantages of Direct Filling Acrylics and give an example

A
  • Tooth like appearance
  • Insoluble in oral fluids
  • Ease of manipulation
  • Low cost

e.g. PMMA - polymethyl-methacrylate

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4
Q

What are some disadvantages of Direct Filling Acrylics

A
  • Secondary caries due to high setting shrinkage
  • Poor wear resistance
  • Coefficient of thermal expansion (contraction) different to the tooth
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5
Q

What can be used to reduce problems with direct filling acrylic

A

Inclusion of quartz powder as a filler that renders the filling as a composite structure

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6
Q

Why is the direct filling acrylics having a different coefficient of thermal expansion than the tooth a problem

A

Causes stresses at the cavity margins

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7
Q

What is the shrinkage of methyl methacrylate monomers

A

about 21% volume

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8
Q

What is the shrinkage of PMMA/MM, powder/liquid shrinkage

A

about 7% volume

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9
Q

What is bowen’s resin made based on and how it this made

A

Based on Bis-GMA (a dimethacrylate) made from the reaction between Bis-phenol A and glycidyl methacrylate

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10
Q

When is bowen’s resin primarily used

A

Direct aesthetic anterior restorations.

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11
Q

What is a Direct composite filling material

A

This is a material that must be placed in the unset state and cured (set) in the oral cavity)

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12
Q

What are the 3 phases/components of Dental Composites

A
  • Organic Matrix
  • Inorganic Filler
  • Coupling Agent
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13
Q

Describe the structure and function of the organic matrix (monomer phase) of dental composites

A

A plastic monomer/resin material that polymerises (sets, cures) to form a continuous phase, binding the filler particles (via a coupling agent)

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14
Q

Describe the function of the inorganic filler of dental composites

A

reinforcing the particles and/or fibres dispersed in the matrix

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15
Q

Describe the function of the coupling agent of dental composites

A

Bonding agent promotes adhesion between the filler and resin matrix

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16
Q

What are monomers and what is their role in dental composites

A

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

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17
Q

What is the organic matrix (monomer phase) of dental composites made of

A
  • Aromatic dimethacrylate-bis-GMA and/or UDMA

- Bis-GMA = Bowens’ monomer

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18
Q

Why might UDMA be used as opposed to BisGMA

A

As BisGMA is very viscous and UDMA has a lower viscosity

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19
Q

What does UDMA stand for

A

Urethane Dimethacrylate

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20
Q

Name another dimethacrylate monomer that can be used as an alternative to BisGMA and UDMA

A

Ethoxylated Bisphenol A derivative

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21
Q

What is a dimethacrylate

A

contains 2 methacrylate groups at each end of the molecule that can both undergo polymerisation

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22
Q

How does the inclusion of fillers affect the viscosity of organic matrix monomers

A

increases viscosity

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23
Q

What are Diluent Monomers

A

Also known as viscosity controllers that help to keep the viscosity of the monomers down

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24
Q

Name what kind of molecule diluent monomers are and give examples

A

Glycol Dimethacrylates

  • Triethyleneglycol dimethacrylate (TEGDMA)
  • Ethylene glycol dimethacrylate (EDGMA)
  • Tetrahydrofurfuryl methacrylate (THFMA)
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25
Q

What type of monomers are used in denture bases

A

Methyl Methacrylate

26
Q

What is molar volume

A

This is the space occupied by each monomer unit

27
Q

How do DMAs differ to MMAs in molar volume

A 
MMA = low molar volume
BisGMA = large molar volume
28
Q

Why are DMAs preferred to MMAs in dental composites

A

MMA has lower molecular weight and lower molar volume and so there is more volume shrinkage of the material after polymerisation

29
Q

Where in the chemical structure of methacrylates do the free radical reactions take place

A

between carbon=carbon double bonds, in this case at the ends of the monomer molecules

30
Q

AY BAWS CAN I HABE DE NOTE PLZ

A

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.

31
Q

Name some of the additives that can be found in the Organic Matrix

A
  • 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
32
Q

What are the advantages of incorporating the Filler Phase of dental composites

A
  • 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
33
Q

AY BAWS CAN I HABE DE NOTE PLZ

A

Mechanical properties of composite improve with high amounts of filler incorporated (>70%)

34
Q

Why must the refractive index of filler and resin match closely

A

To avoid scattering of light otherwise the full depth of cure is not achieved on light curing the resin

35
Q

What are the refractive indices of BisGMA, TEGDMA and most glasses

A

BisGMA = 1.55
TEGDMA = 1.46
Most glasses = 1.5

36
Q

What must a filler be treated with

A

Silane

37
Q

Why are multiple fillers used rather than just one

A

One filler is not enough to attain most of the above goals

38
Q

What determines the amount of filler that can be incorporated in the resin

A

Average particle size and particle size distribution

39
Q

What kind of filler results in a smooth finish

A

smaller filler particles

40
Q

Name the 3 types of filler

A

Conventional (Macrofillers)
Hybrid/Small Particle Hybrid
Microfine

41
Q

Name examples for each type of filler

A

Conventional - Quartz, Borosilicate glass
Hybrid - Lithium/Barium aluminium silicate
Microfine - Strontium glass, colloidal silica

42
Q

What process is used to allow the use of microfillers

A
  • Microfilled composite is polymerised and ground to particles
  • These particles used as fillers in further composite formulations
  • Allows high loading of pre-polymerised particles
43
Q

What are the advantages of Hybrid composites and high filler loading

A
  • Favourable physical, mechanical and optical properties
  • Improved wear resistance
  • Surface morphology superior to conventional but inferior to microfilled resins
  • Radiopacity possible
44
Q

What are some disadvantages of hybrid fillers

A
  • Increased surface roughness with time, resin wears away
45
Q

What is the typical inorganic filler loading vol. %

A
  • 30-70%
46
Q

What is the maximal theoretical packing density

A

74%

47
Q

What are microfilled composites used for and why

A

Anterior restorations due to lower filler loading and hence mechanical properties are compromised

48
Q

What are hybrid composites used for and why

A

Posterior restorations due to higher filler loading

49
Q

Describe the hydrophilicity of the organic resin and the filler

A

Organic resin - Hydrophobic

Filler - Hydrophilic

50
Q

What is silane used for in dental composites

A

Silane is a coupling agent that is used to bond the filler to the resin

51
Q

What is silane referred to as

A

gamma MPTS

52
Q

What part of silane structure bonds to resin on polymerising

A

the double bond in the methacrylate group (free radical polymerisation)

53
Q

What part of the silane structure binds to the filler particle

A

The hydrophilic OCH3 group reacts with the surface OH bonds on the filler (condensation)

54
Q

What is the method for silane application called and what happens in it

A

The Silane treatment of the filler is when the agent is coated around the filler particles

55
Q

What effect does acid treatment have on silane and what reactions follow the treatment

A

Hydroxyl groups replace the methoxxy groups and these OH groups then react with the hydroxyl groups on the glass filler via condensation

56
Q

What happens if the bond is unstable between the filler and resin

A

The composite may/will fracture, subsequently leading to disintegration of the material

57
Q

What happens if no bonds forms between the filler and resin

A

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

58
Q

What kinds of composites are heat cured composites ideal for

A

Indirect, inlays/onlays

Not direct

59
Q

How do room temperature cured composites work

A
  • 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
60
Q

How are most direct filling materials cured

A

Light

61
Q

Describe the reaction when blue light is directed onto the paste

A

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

Dental Materials Y2 (13 decks)

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