BDS2 - DMS > PMMA > Flashcards

PMMA Flashcards

(58 cards)

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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?

Study These Flashcards
A

To speed up the reaction

25
What does the plasticiser in heat cured acrylic do?
Allows quicker dissolving in monomer liquid e.g. dibutyl phthalate 
26
What do the pigments in heat cured acrylic do?
- Give the heat cured acrylic a natural colour 
27
What does the co-polymer in heat cured acrylic do?
- Included to improve mechanical properties e.g. ethylene glycol dimethacrylate (because PLL methacrylate monomer will not do this by itself)  
28
What does the methacrylate monomer in the liquid of heat cured acrylic do?
- Dissolves PMMA particles - polymerises 
29
What does the inhibitor (Hydroquinone, 0.006%) in the liquid of heat cured acrylic do?
- Prolongs shelf life - reacts with any free radicals produced by heat, UV light 
30
What do the co-polymers in the liquid of heat cured acrylic do?
Improve mechanical propertied - particularly cross-linking polymers 
31
What is PMMA made of a powder/liquid mix? (3 points)
- To produce dough-like material that can be handled/mixed easily and customised to desired shape - Reduce heat of reaction - Minimise polymerisation shrinkage
32
What are the proportions of the powder/liquid mix of the PMMA?
- 3 to 3.5/1 (P/L) by volume | - 2.5/1 (P/L) by weight
33
What are the mixing stages of the powder/liquid mix of PMMA?
Sandy - tacky Dough - packing
34
Why do you need efficient polymerisation of PMMA?
- To give a high molecular weight polymer i.e. good mechanical properties - Hence high temperature but gaseous porosity limits
35
What are 3 examples of heating schedules of PMMA?
1) 7hrs. to 7*C + 2hrs. to 100*C + slow cool 2) 72*C for at least 16hrs (easiest one) 3) 20-20-20 Reverse curing - Place flask in boiling H2O - Remove heat for 20 mins - Heat to 70*C for 20 mins - Then heat to 100*C for 20 mins
36
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?
- Cool slowly - allow relief | - Cool gradually over many hours to reduce stresses
37
What are the disadvantages of having internal stresses? (4 points)
- 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
What are internal stresses related to (what can cause them)? (5 points)
- Shape and size e.g. notches - Curing pressure - Curing cycle - Thermal expansion - Cooling rate
39
What happens if the PMMA is under-cured?
- Free monomer is an irritant | - Low molecular weight - poor mechanical properties
40
What happens if the PMMA is cured to quickly?
- Possible gaseous porosity 
41
What happens if there is too much monomer in the PMMA?
Causes contraction porosity 
42
What happens if there is too little monomer in the PMMA?
- Causes granularity of the material 
43
What does porosity affect? (4 points)
- Affects strength - Affects appearance - Rough sensation to tongue - Absorbs saliva - poor hygiene
44
Where does gaseous porosity occur?
- In bulkier parts 
45
How is gaseous porosity caused?
- Monomer boiling (100*C)
46
How much polymerisation shrinkage will there be of the monomer on its own? 
21% by volume 
47
How much polymerisation shrinkage will there be of the powder + monomer mix?
7%
48
What are possible cases of contraction porosity? (3 points)
- Too much monomer - Insufficient excess material - Insufficient clamp pressure (occurs anywhere where dough is not sufficiently packed)
49
Is acrylic non-toxic?
Yes 
50
Is acrylic non-irritant?
- Yes, provided there is no monomer release | - But, some patients are allergic
51
Is acrylic affected by oral fluids?
- No | - Water absorption, virtually insoluble in fluids taken orally
52
Is the thermal expansion of acrylic okay with artificial teeth?
- Yes, OK if acrylic teeth are used, significantly higher than porcelain teeth 
53
Does acrylic have a high thermal conductivity?
- No, which is poor (ideally look for a high one) 
54
Are the mechanical properties of acrylic good?
- No, they are poor | - Increase in bulk to compensate
55
Is the colour/translucency of acrylic appropriate?
- Yes, it is good (natural looking)
56
Is the density of acrylic good?
- Yes, it is low which is good | - BUT need increase in bulk to overcome the poor mechanical propertied offsets this advantage
57
Acrylic has a high softening temperature (75*C) is this good?
- OK for ingested hot fluids | - DON'T use boiling water for cleaning
58
Is acrylic dimensionally accurate & stable in use?
- Pretty much | - Linear contraction 0.5% - acceptable
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