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BDS2 - DMS > PMMA > Flashcards

PMMA Flashcards

1
Q

Why is manufacturing a denture challenging?

A

As it is complex, customized 3D object

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2
Q

Often dentures don’t fit a patient first time. What problems in the production process could cause this? (2 points)

A
  • Initial impression is flawed (material and method not 100% accurate)
  • Curing process - several potential error sources
  • May not achieve perfect fit at fist attempt but REFINEMENTS at chairside can be made
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3
Q

Often dentures don’t fit a patient first time. What problems in the usage of the denture could cause this? (5 points)

A
  • Fits patient only for a short period
  • Fractures
  • Uncomfortable
  • Becomes warped
  • Surface suffers wear
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4
Q

What are the basic requirements for a denture material (PMMA)? (6 points)

A
  • Replaces function of natural teeth
  • Goes into patient’s mouth
  • Is seen by other people
  • Dimensionally accurate and stable in use (must fit patient’s mouth and be retained)
  • High softening temperature (must not distort during ingestion of hot fluids, or during cleaning)
  • Unaffected by oral fluids (over time)
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5
Q

What does the linear slope of a stress/strain diagram indicate?

A
  • The rigidity of a material (young’s modulus)
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6
Q

What is the elastic limit of a material?

A
  • Represents the stress beyond which the material will become deformed
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7
Q

What do you want the young’s modulus to be for denture material?

A
  • High Young’s modulus

- Rigid (Stiff) - large stress produces small strain

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8
Q

What do you want the proportional/elastic limit of a denture material to be?

A
  • High limit

- Only large stresses will cause permanent deformation

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9
Q

What do you want the thermal expansion of a denture material to be?

A

Want it to match an artificial tooth

  • Avoid internal stresses on cooling during manufacture
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10
Q

What do you want the thermal conductivity of a denture material to be?

A
  • High (can conduct heat readily)

- Transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus

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11
Q

What do you want the density of a denture material to be?

A
  • Low

- Air retention of upper denture (the heavier the material, greater the gravity pull)

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12
Q

What do you want the colour/translucency of a denture material to be?

A
  • To match the natural tissues
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13
Q

Do you want denture materials to be non-toxic and non-irritant?

A
  • Yes
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14
Q

PMMA is produced by a free radical addition polymerisation. What is this?

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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15
Q

What is the ‘activation’ stage of acrylic polymerisation?

A
  • Activation of initiator to provide free radicals
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16
Q

What is the ‘initiation’ stage of acrylic polymerisation?Term

A
  • Free radicals break C=C bond in monomer and transfer free radical
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17
Q

What is the ‘propagation’ stage of acrylic polymerisation?

A

Growing polymer chain

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18
Q

What is the ‘Ternimation’ stage of acrylic polymerisation?

A

End of polymerisation

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19
Q

What is the initiator in the polymerisation reaction of PMMA?

A
  • Benzoyl peroxide (it is symmetrical)
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20
Q

What is the polymerisation reaction of PMMA activated by?

A

Can be heat at >72 degrees

or can be self cure

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21
Q

What is the free radical that is produced in the polymerisation reaction of PMMA?

A

C6H5COO*

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22
Q

What is included in the powder of heat cured acrylic? (5 points)

A
  • Initiator
  • PMMA particles
  • Plasticiser
  • Pigments
  • Co-polymers
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23
Q

What is the initiator in heat cured acrylic?

A

Benzoyl Peroxide, 0.2-0.5%

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24
Q

What are PMMA particles already in heat cured acrylic?

A

To speed up the reaction

25
Q

What does the plasticiser in heat cured acrylic do?

A

Allows quicker dissolving in monomer liquid e.g. dibutyl phthalate

26
Q

What do the pigments in heat cured acrylic do?

A
  • Give the heat cured acrylic a natural colour
27
Q

What does the co-polymer in heat cured acrylic do?

A
  • Included to improve mechanical properties e.g. ethylene glycol dimethacrylate (because PLL methacrylate monomer will not do this by itself)
28
Q

What does the methacrylate monomer in the liquid of heat cured acrylic do?

A
  • Dissolves PMMA particles - polymerises
29
Q

What does the inhibitor (Hydroquinone, 0.006%) in the liquid of heat cured acrylic do?

A
  • Prolongs shelf life - reacts with any free radicals produced by heat, UV light
30
Q

What do the co-polymers in the liquid of heat cured acrylic do?

A

Improve mechanical propertied - particularly cross-linking polymers

31
Q

What is PMMA made of a powder/liquid mix? (3 points)

A
  • To produce dough-like material that can be handled/mixed easily and customised to desired shape
  • Reduce heat of reaction
  • Minimise polymerisation shrinkage
32
Q

What are the proportions of the powder/liquid mix of the PMMA?

A
  • 3 to 3.5/1 (P/L) by volume

- 2.5/1 (P/L) by weight

33
Q

What are the mixing stages of the powder/liquid mix of PMMA?

A

Sandy - tacky

Dough - packing

34
Q

Why do you need efficient polymerisation of PMMA?

A
  • To give a high molecular weight polymer i.e. good mechanical properties
  • Hence high temperature but gaseous porosity limits
35
Q

What are 3 examples of heating schedules of PMMA?

A

1) 7hrs. to 7C + 2hrs. to 100C + slow cool
2) 72C for at least 16hrs (easiest one)
3) 20-20-20 Reverse curing
- Place flask in boiling H2O
- Remove heat for 20 mins
- Heat to 70C for 20 mins
- Then heat to 100*C for 20 mins

36
Q

When cooling the acrylic material, the mould and the acrylic have different thermal expansion coefficients which could cause internal stresses. How can this be avoided?

A
  • Cool slowly - allow relief

- Cool gradually over many hours to reduce stresses

37
Q

What are the disadvantages of having internal stresses? (4 points)

A
  • Decreased strength
  • Decreased fatigue strength (reduced ability to withstand low level stresses over a long time)
  • Warping e.g. during finishing
  • Repair problems e.g. if needs repair over time - if internal stresses, that’s when it really shows - will lead to fracture of the denture
38
Q

What are internal stresses related to (what can cause them)? (5 points)

A
  • Shape and size e.g. notches
  • Curing pressure
  • Curing cycle
  • Thermal expansion
  • Cooling rate
39
Q

What happens if the PMMA is under-cured?

A
  • Free monomer is an irritant

- Low molecular weight - poor mechanical properties

40
Q

What happens if the PMMA is cured to quickly?

A
  • Possible gaseous porosity
41
Q

What happens if there is too much monomer in the PMMA?

A

Causes contraction porosity

42
Q

What happens if there is too little monomer in the PMMA?

A
  • Causes granularity of the material
43
Q

What does porosity affect? (4 points)

A
  • Affects strength
  • Affects appearance
  • Rough sensation to tongue
  • Absorbs saliva - poor hygiene
44
Q

Where does gaseous porosity occur?

A
  • In bulkier parts
45
Q

How is gaseous porosity caused?

A
  • Monomer boiling (100*C)
46
Q

How much polymerisation shrinkage will there be of the monomer on its own?

A

21% by volume

47
Q

How much polymerisation shrinkage will there be of the powder + monomer mix?

A

7%

48
Q

What are possible cases of contraction porosity? (3 points)

A
  • Too much monomer
  • Insufficient excess material
  • Insufficient clamp pressure

(occurs anywhere where dough is not sufficiently packed)

49
Q

Is acrylic non-toxic?

A

Yes

50
Q

Is acrylic non-irritant?

A
  • Yes, provided there is no monomer release

- But, some patients are allergic

51
Q

Is acrylic affected by oral fluids?

A
  • No

- Water absorption, virtually insoluble in fluids taken orally

52
Q

Is the thermal expansion of acrylic okay with artificial teeth?

A
  • Yes, OK if acrylic teeth are used, significantly higher than porcelain teeth
53
Q

Does acrylic have a high thermal conductivity?

A
  • No, which is poor (ideally look for a high one)
54
Q

Are the mechanical properties of acrylic good?

A
  • No, they are poor

- Increase in bulk to compensate

55
Q

Is the colour/translucency of acrylic appropriate?

A
  • Yes, it is good (natural looking)
56
Q

Is the density of acrylic good?

A
  • Yes, it is low which is good

- BUT need increase in bulk to overcome the poor mechanical propertied offsets this advantage

57
Q

Acrylic has a high softening temperature (75*C) is this good?

A
  • OK for ingested hot fluids

- DON’T use boiling water for cleaning

58
Q

Is acrylic dimensionally accurate & stable in use?

A
  • Pretty much

- Linear contraction 0.5% - acceptable

BDS2 - DMS (14 decks)

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