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BDS2: Dental Material Science > PMMA 1 (Polymethylmethacrylate) > Flashcards

PMMA 1 (Polymethylmethacrylate) Flashcards

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1
Q

What are some ideal properties of denture materials?

A
  1. Dimensionally accurate and stable in use
    - must fit patients mouth and be retained
  2. High softening temperature (Tg)
    - must not distort during ingesting of hot fluids, or during cleaning
  3. Unaffected by oral fluids
    - over time
  4. Low density
    - aid retention of upper denture (the heavier the material, greater the gravity pull)
  5. Colour/translucency
    - to match natural tissues
  6. Non toxic, non irritant
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2
Q

What is the elastic limit in relation to stress strain diagrams

A

Stress beyond which deformation of the material occurs

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3
Q

What is compressive strength in relation to stress strain diagrams?

A

Where the curve ends

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4
Q

What is the proportional limit in relation to stress/strain diagrams?

A

Maximum stress where stress and strain are linearly related

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5
Q

How is elastic modulus calculated?

A

Stress divided by strain (in linear/elastic region)

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6
Q

What mechanical properties are ideal for denture base materials?

A
  1. High Young’s (elastic) modulus
    - rigid (stiff)
    - applying a large stress produces small strain
  2. High proportional limit/elastic limit
    - only large stresses will cause permanent deformation
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7
Q

What thermal properties are ideal for denture base materials?

A
  1. Thermal expansion
    = artificial tooth
    - avoid internal stresses on cooling during manufacture (more internal stresses in metals and alloys)
  2. High thermal conductivity
    - transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus
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8
Q

What is free radical addition polymerisation?

A

Chemical union of two 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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9
Q

What are the 4 stages of acrylic polymerisation?

A
  1. Activation - of initiator to provide free radicals
  2. Initiation - free radicals break C=C bond in monomer and transfer free radicals
  3. Propagation - growing polymer chain
  4. Termination - of polymerisation
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10
Q

What is the initiator molecule in polymerisation of acrylic?

A

Benzoyl peroxide:

C6H5COO-OOCH5C6

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11
Q

What temperature is the initiator (benzoyl peroxide) activated at?

How is it cured?

A

> 72 degrees Celsius

Self cured

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12
Q

What components make up heat cured acrylic resin powder?

A
  1. Initiator (benzoyl peroxide, 0.2-0.5%)
  2. PMMA Particles - pre-polymerised beads
  3. Plasticiser - allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
  4. Pigments - to give “natural” colour
  5. Co-polymers - to improve mechanical properties e.g. ethylene glycol dimethacrylate
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13
Q

What components make up heat cured acrylic liquid?

A
  1. Methacrylate monomer
    - dissolves PMMA particles - polymerises
  2. Inhibitor (hydroquinone, 0.006%)
    - prolongs shelf life - 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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14
Q

Why is heat cured acrylic powder and liquid mixed together?

A
  1. Produce dough like material that can be handled / mixed easily and customised to desired shape
    - reduce heat of reaction
    - minimise polymerisation shrinkage
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15
Q

What are the proportions of powder and liquid that are mixed together?

A
  • powder to liquid ratio = 3 to 3.5 - 1 (by volume)

- 2.5-1 by weight

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16
Q

What are 3 heating schedules used for acrylic?

A
  1. 7hrs. To 70 degrees C + 2hrs to 100 degrees C + slow cool
  2. 72 degrees C for at least 16 hrs
  3. 20-20-20 reverse curing
    - place flask in boiling water
    - remove heat for 20 mins
    - heat to 70 degrees C for 20 mins
    - then heat to 100 degrees C for 20 mins
17
Q

Why is it best to avoid heating cycles that rise over 100 degrees C?

A

Gaseous porosity will result in voids occurring in the cured material

18
Q

Why is it better to allow acrylic to cool slowly?

A

Mould material and acrylic have different thermal expansion coefficients, so will contract at different rates, if cooled too quickly; this can result in internal stresses within the acrylic

19
Q

What problems do internal stresses in acrylics cause?

A
  1. Decreased compressive strength
  2. Decreased fatigue strength (low level stresses applied repeatedly over a long time, causing fractures)
  3. Warping e.g. during finishing
  4. Repair problems
20
Q

What factors are related to internal stresses?

A
  1. Thermal expansion
  2. Cooling rate
  3. Shape and size e.g. notches
  4. Curing pressure
  5. Curing cycle
21
Q

What are problems caused by under curing and fast curing?

A
  1. Under cured:
    - free monomer (irritant)
    - low molecular weight = poor mechanical properties
  2. Fast curing:
    - possibly gaseous porosity
22
Q

What are problems caused by porosity in dental materials?

A
  1. Affects strength
  2. Affects appearance
  3. Rough sensation to tongue
  4. Absorbs saliva - poor hygiene
23
Q

Briefly describe gaseous porosity

A
  1. Monomer boiling (100 degrees C)
  2. Occurs in bulkier parts
  3. Be careful (exothermic reaction)
    - hence cure slowly
24
Q

Where does contraction porosity most often occur?

A

Anywhere where dough is not sufficiently packed

25
Q

How much polymerisation shrinkage (%) occurs in monomer alone vs powder + monomer mix?

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

2. Powder + monomer mix = 7%

26
Q

What causes contraction porosity?

A
  1. Too much monomer
  2. Insufficient excess material
  3. Insufficient clamp pressure
27
Q

What problems are caused by incorrect powder to monomer ratios?

A
  1. Too much monomer
    - contraction porosity
  2. Too little monomer
    - granularity
28
Q

When can acrylic cause irritation in the mouth?

A

If monomer is left unreacted it can be released and cause irritation

29
Q

Briefly describe some general properties of acrylic resin

A
  1. Non toxic - yes
  2. Non irritant
    - provides no monomer is released
    - some patients are allergic
  3. Unaffected by Oral fluids
    - water absorption, virtually insoluble in fluids taken orally
  4. Colour/translucency = good
30
Q

What thermal properties are of concern in acrylic resin?

A
  1. Thermal expansion = artificial tooth
    - Ok if acrylic teeth are used, significantly higher than porcelain teeth
    - acrylic resin:- 80ppm/degrees C
  2. High thermal conductivity
    - low (poor)
31
Q

List the following mechanical properties of acrylic resin (elastic modulus, tensile strength, hardness)

A
  1. Elastic modulus = 2.5GPa
  2. Tensile strength = 85MPa
  3. Hardness = 20VHN
32
Q

How would you describe the density of acrylic resin?

A
  1. Density low (good)

- but need to increase in bulk to overcome poor mechanical properties offsets this advantage

33
Q

What is the softening temperature of acrylic?

A

75 degrees C

  • ok for ingested hot fluids
  • don’t use boiling water for cleaning
34
Q

Is acrylic dimensionally accurate and stable in use?

A

Yes

- linear contraction 0.5% = acceptable

35
Q

Summarise the acceptability of PMMA properties

A

Most properties (mechanical and thermal) are satisfactory, but dentures

  • may not fit the patient first time
  • may not last long

Alternative materials being sought

BDS2: Dental Material Science (8 decks)

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