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Dental Material Science > PMMA > Flashcards

PMMA Flashcards

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1
Q

State common error sources in dentures

A
  1. Production
    - initial impression flawed so material + method is not 100% accurate
  2. Curing process
    - refinements at chair side required
  3. Usage
    - Pt may agree denture fits initially then overtime experience problems
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2
Q

What denture problems can occur for the patient over time? (4)

A
  1. Uncomfortable
  2. Fractures
  3. Becomes warped
  4. Surfaces suffer wear
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3
Q

State 3 ideal properties of a denture base

A
  1. Dimensionally accurate + stable in use
    - Must fit in pt’s mouth and be retained
  2. High softening temp (Tg)
    - Must not distort during ingestion of hot fluids/cleaning
  3. Unaffected by oral fluids
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4
Q

What does the elastic limit represent on the stress strain diagram?

A

The stress beyond which a material cannot reform to its original shape

PERMANENTLY DEFORMED (even after releasing the stress factor)

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5
Q

Ideal mechanical properties of a denture base (2)

A
  1. High YM
    - Large stress produces small strain
  2. High elastic limit
    - Only large stresses will cause permanent deformation
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6
Q

Define free radical polymerisation

A

The chemical union of 2 molecules, either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule

Involves molecules with C=C bonds

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7
Q

What are the 4 steps in acrylic polymerisation

A
  1. Activation
  2. Initiation
  3. Propagation
  4. termination
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8
Q

Describe events at ACTIVATION stage of acrylic polymerisation

A

Activation of initiator to produce 2 free radicals

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9
Q

Describe events at INITIATION stage of acrylic polymerisation

A

Free radicals break the C=C bond in monomer and transfer free radicals

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10
Q

Describe events at PROPAGATION stage of acrylic polymerisation

A

Growing polymer chain

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11
Q

Describe events at TERMINATION stage of acrylic polymerisation

A

Termination of polymerisation occurs

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12
Q

Ideal thermal properties of denture base (2)

A
  1. Thermal expansion
    - Avoids internal stresses on cooling during manufacture
  2. High thermal conductivity
    - Transmission of thermal stimuli to mucosa
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13
Q

Name the initiator for polymerisation

A

Benzoyl peroxide

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14
Q

What makes up the heat cured acrylic POWDER (5)

A
  1. Initiator
    - Benzoyl peroxide 0.2-0.5%
  2. PMMA particles
    - Pre polymerised beads
  3. Plasticiser
    - Allows quicker dissolving in monomer liquid
  4. Pigments
    - Give natural colour
  5. Co-polymers
    - To improve mechanical properties
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15
Q

What makes up the heat cured acrylic LIQUID (3)

A
  1. Methacrylate monomer
    - Dissolves pMMA particles
  2. Inhibitor (Hydroquinone)
    - Prolongs shelf life as it reacts with any free radicals produced by heat/UV light
  3. Co-polymers
    - Improve mechanical properties, particularly cross-linking of polymers
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16
Q

Describe the important of the powder/liquid mix (3)

A
  1. Forms a dough like material that can be handled/mixed easily and customised to a desired shape
  2. Minimises polymerisation
    shrinkage
  3. Reduces heat of reaction
17
Q

State the proportions for powder/liquid mix

A

3 to 3.5/1

18
Q

What can exceptionally high temperatures lead to?

A

Gaseous porosity within the acrylic weakens the material

19
Q

What are internal stresses of the denture base related to? (5)

A
  1. Shape and size e.g. notches
  2. Curing pressure
  3. Curing cycle
  4. Thermal expansion
  5. Cooling rate
20
Q

What’s the effect of under curing the base?

A
  • Free monomer (irritant)
  • Low molecular weight
  • POOR mechanical properties
21
Q

What’s the effect of fast curing the base?

A
  • Creates bubbles within the material, which increase gaseous porosity and weaken the material
22
Q

What’s the effect of too much monomer?

A

Contraction porosity

23
Q

What’s the effect of too little monomer?

A

Granularity

24
Q

What does porosity affect?

A
  1. Strength
  2. Affects appearance
  3. Rough sensation to tongue
  4. Absorbs saliva - poor hygiene
25
Q

What areas of the denture is porosity more present in?

A

Bulkier parts of the denture

26
Q

Compare polymerisation shrinkage for just monomer / powder+ momomermix ? (2)

A
  1. Monomer alone = 21% (by volume)

2. Powder + Monomer mix = 7%

27
Q

State the causes of contraction porosity (3)

A
  1. Too much monomer
  2. Insufficient excess material
  3. Insufficient clamp pressure
28
Q

Where does contraction porosity occur?

A

Anywhere where the dough is NOT sufficiently packed

29
Q

How do internal stresses lead to repair problems?

A

> Decreased strength
Decreased fatigue strength
Warping e.g. during finishing
Repair problems

30
Q

Ideal properties for patient comfort and aesthetics (3)

A
  1. Low density
    - Aids retention of upper denture (the heavier the material, the greater the gravity pull)
  2. Colour/translucency
    - To match natural tissues
  3. Non toxic/ non irritant
31
Q

Looking at the acrylic temp changes graph, what do temperatures over 100 lead to?

A

Gaseous porosity - bubbles that weaken the material

32
Q

Purpose of efficient polymerisation?

A

Gives high molecular weight polymer

Good mechanical properties

Dental Material Science (7 decks)

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