BDS2 > PMMA > Flashcards

PMMA Flashcards

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1
Q

How is denture success measured?

A

by patient feedback

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2
Q

When can error sources for dentures arise?

A

during production or usage

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3
Q

What are some error sources that can arise during the production of a denture?

A
  • flawed impression
  • curing process (several potential error sources)
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4
Q

Why are denture refinements required at chairside?

A

dentures don’t often fit the patient first time

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5
Q

What are error sources that may arise during usage of a denture?

A

fractures, uncomfortable, becomes warped, surface wear, fits patient only for a short time

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6
Q

Why is it important for acrylic / PMMA to exhibit ideal properties?

A

PMMA must replace function of natural teeth, goes in patient’s mouth, is seen by others

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7
Q

What are the ideal properties of a denture material?

A

dimensionally accurate and stable in use, high softening temperature (Tg), unaffected by oral fluids, high Young’s / elastic modulus, high proportional limit / elastic limit, thermal expansion equal to teeth, high thermal conductivity, low density, colour/translucency, non-toxic, non-irritant

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8
Q

Why must PMMA be dimensionally accurate and stable in use?

A

must fit patient’s mouth and be retained

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9
Q

Why must denture materials have a high softening temperature (Tg)?

A

so it does not distort during ingestion of hot fluids or during cleansing

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10
Q

How is Young’s (elastic) modulus calculated?

A

stress / strain (gradient of line)

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11
Q

What happens if a material’s elastic limit is exceeded?

A

material is permanently deformed

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12
Q

Why is it important for PMMA to have a high Young’s (Elastic) Modulus?

A

to be rigid (stiff) so that applying a large stress produces a small strain

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13
Q

Why is it important for PMMA to have a high proportional / elastic limit?

A

so only large stresses will cause permanent deformation

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14
Q

Why is thermal expansion an important property in PMMA?

A

to avoid internal stresses on cooling during manufacture

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15
Q

Why should denture materials ideally have a high thermal conductivity?

A

to transmit thermal stimuli to mucosa - preventing scalding of pharynx or oesophagus

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16
Q

What is a disadvantage of PMMA?

A

poor thermal conductivity therefore need to warn patient’s to not eat/drink too hot

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17
Q

Why is it advantageous for a denture material to have a low density?

A

aids retention of upper denture (the heavier the material, the greater the gravitational pull)

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18
Q

What material is used for the construction of dentures?

A

acrylic / polymethylmethacrylate (PMMA)

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19
Q

How long has acrylic / PMMA been around for?

A

since 1937

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20
Q

What is the name of the reaction that acrylic resin undergoes?

A

free radical addition polymerisation

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21
Q

What is the definition of free radical addition polymerisation?

A

chemical union of two molecules either the same or different to form a larger molecule without the elimination of water

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22
Q

What bond do molecules that participate in a free radical addition polymerisation have?

A

C=C bonds

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23
Q

What is the monomer of PMMA that undergoes free radical addition polymerisation?

A

methacrylate monomer CH2C(CH3)COOCH3

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24
Q

What are the steps in acrylic polymerisation?

A

activation, initiation, propagation, termination

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25
What is the initiator in acrylic polymerisation?
benzoyl peroxide (C6H5COO-OOCH5C6)
26
What is notable about the structure of benzoyl peroxide?
symmetrical (produces two free radicals C6H5COO*)
27
What happens during the activation stage of acrylic polymerisation?
benzoyl peroxide (C6H5COO-OOCH5C6) is heated > 72 degrees and self cured to produce 2 free radicals (R* = C6H5COO*)
28
How is benzoyl peroxide activated?
heated > 72 degrees and self cured
29
What happens during initiation?
free radicals (C6H5COO*) break C=C bond in monomer (methacrylate, CH2C(CH3)COOCH3) and transfer free radicals
30
Give a general equation to represent initiation
R* + M -> R-M*
31
What happens during propagation?
polymer chain grows (therefore increasing in strength)
32
Give 2 generalised equations to represent propagation in a free radical addition polymerisation reaction?
R-M* + M -> R-M-M* R-M-M* + M -> R-M-M-M* ... and so on
33
What are the 2 components of heat cured acrylic?
powder and liquid
34
What are the contents of the powder component of heat cured acrylic?
initiator, PMMA particles, plasticiser, pigments, co-polymers
35
What is the initiator in the powder component of heat cured acrylic?
benzoyl peroxide, 0.2-0.5%
36
What are the PMMA particles in the powder component of heat cured acrylic?
pre-polymerised beads
37
What is the role of the plasticiser in the powder component of heat cured acrylic?
allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
38
What is the role of the pigments in the powder of heat cured acrylic?
gives "natural" colour
39
What is the role of the co-polymers in the powder of heat cured acrylic?
improve mechanical properties e.g. ethylene glycol dimethacrylate
40
What are the contents of the liquid component of heat cured acrylic?
methacrylate monomer, inhibitor (hydroquinone 0.006% of liquid), co-polymers
41
What is the role of the methacrylate monomer in the liquid component of heat cured acrylic?
dissolved PMMA particles and polymerises
42
What is the role of the inhibitor in the liquid component of heat cured acrylic?
hydroquinone prolongs shelf life by reacting with any free radicals produced by heat / UV light to prevent polymerisation before powder is even mixed with liquid
43
What is the role of the co-polymers in the liquid component of heat cured acrylic?
improve mechanical properties - particularly cross-linking of polymers
44
Why is heat cured acrylic made up of a powder and liquid component (3 reasons)?
to produce a dough-like material that can be handled/mixed easily and customised to desired shape, reduce heat of reaction, reduce polymerisation shrinkage
45
What are the proportions of the powder / liquid mix?
3-3.5 to 1 powder to liquid by volume (or 2.5 to 1 by weight)
46
What are the mixing stages called?
sandy (tacky) and dough (packing)
47
How is acrylic resin moulded into shape?
as much dough-like acrylic resin as possible is packed into metal flask containing the mould, 2 halves of the flask are brought together and screws are tightened
48
How is acrylic cured?
heat
49
Why is efficient acrylic polymerisation required?
to give high molecular weight polymer (ie good mechanical properties) - therefore high temp required
50
What factor limits the temperature at which acrylics heat curing takes place?
gaseous porosity (gases produced if temp too high, resulting in a porous material - bad)
51
What are the 3 possible heating schedules for heat curing acrylics?
1. 7hrs in 70 degrees (water bath) then raise to 100 degrees for 2 hours, slow cool 2. 72 degrees for at least 16 hrs 3. 20-20-20 reverse curing (place flask in boiling H2O, remove heat for 20 min, heat to 70 degrees for 20 min, heat to 100 degrees for 20 min)
52
Why does the temperature of the acrylic resin increase further at the 2 hour mark of each curing method?
polymerisation exotherm (exothermic reaction)
53
Why is it more favourable to heat the acrylic resin at 70 degrees for 7hrs before raising to 100 degrees for 2 hours?
temperature will not exceed 100 degrees (including polymerisation exotherm at 2 hrs)
54
Why is it important to allow the acrylic to cool slowly?
the mould material (in flask) and acrylic have different thermal expansion coefficients which can cause internal stresses. Therefore, cool slowly to minimise
55
What are the consequences of internal stresses?
decrease strength, decrease fatigue strength, warping (e.g. stresses during finishing), repair problems
56
What is fatigue?
low level stresses applied repeatedly over a long time, causing fracturing
57
What factors can contribute to internal stresses?
thermal expansion, cooling rate, shape and size (e.g. notches), curing pressure, curing cycle
58
What are the potential problems associated with curing?
under curing, fast curing
59
Why is under curing not favourable?
free monomer can leak out (irritant) and low molecular weight (poor mechanical properties)
60
What is the issue with fast curing?
possibly gaseous porosity
61
What may result if there is too much monomer (incorrect powder/monomer ratios)?
contraction porosity
62
What happens if there is too little monomer (incorrect powder / monomer ratios)?
granularity
63
Why is porosity unfavourable?
affects strength, affects appearance, rough sensation to tongue (wears more easily), absorbs saliva (poor hygiene)
64
Why should the curing cycle of a denture avoid reaching >100 degrees?
monomer boils at 100 degrees leading to gaseous porosity
65
Where does gaseous porosity occur?
bulkier parts of the denture
66
Why is it important to cure an acrylic denture slowly?
to avoid a sudden temperature increase due to an exothermic reaction
67
What is contraction porosity?
polymerisation shrinkage of the materials
68
By what percentage does the volume of the monomer decrease?
21% decrease of monomer volume
69
What percentage does the volume of the powder and monomer mix decrease by?
7%
70
Where does contraction porosity occur?
anywhere the dough is not sufficiently packed
71
What are the causes of contraction porosity?
too much monomer, insufficient excess material, insufficient clamp pressure
72
What are the properties of acrylic?
non-toxic, non-irritant, unaffected by oral fluids, good colour/translucency, thermal expansion is ok, low thermal conductivity, poor mechanical properties, low density, sufficiently high softening temperature, dimensionally accurate and stable in use
73
In what scenarios may acrylic be an irritant?
if monomer is released due to under curing, or allergic patient
74
What is the thermal expansion of acrylic resin?
80 ppm/degrees C (enamel 11.4, composite resin 15-40)
75
What are the disadvantages of acrylic?
low thermal conductivity (warn patients not to drink too hot fluids), poor mechanical properties (Young's modulus and proportional limit / elastic limit)
76
How are the poor mechanical properties of acrylic compensated for?
by increasing the acrylic in bulk (increases resistance to fracture)
77
How does the elastic modulus (GPa) of acrylic resin compare to composite resin?
acrylic 2.5 GPa, composite 14 GPa
78
How does the tensile strength (MPa) of acrylic resin compare to composite resin?
acrylic 85 MPa, composite 50 MPa
79
How does the hardness (VHN) of acrylic resin compare to composite resin?
acrylic resin 20 VHN, composite resin 90 VHN
80
How does the thermal expansion (ppm/C) of acrylic resin compare to composite resin?
acrylic resin 80 ppm/C, composite resin 32 ppm/C
81
What is the disadvantage of having to increase the bulk of acrylic to overcome poor mechanical properties?
offsets the advantage of the low density of acrylic (becomes heavier)
82
What is the softening temperature of acrylic?
75 degrees C (fine for ingested fluids but DO NOT use boiling water for cleaning)
83
What is the percentage of linear contraction that acrylic undergoes?
0.5% (acceptable for dimensionally stable and accurate)
84
What do you need to warn patients who get an acrylic denture about?
not to drink too hot fluids to avoid burning throat, not to use boiling water for cleaning
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