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BDS2 CDS DMS > 9. PMMA > Flashcards

9. PMMA Flashcards

1
Q

Definition of PMMA

A

Polymethylmethacrylate – acrylic resin

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2
Q

Error sources of dentures (2)

A

Production

Usage

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3
Q

Denture production error sources (3)

A

Initial impression is flawed – material and method not 100% accurate
Curing process – several potential error sources
May not achieve perfect fit at first attempt, so refinements at the chairside may be required

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4
Q

Denture usage error sources (4)

A

Fits patient only for short period
Fractures and warping
Discomfort
Surface suffers wear

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5
Q

Component of stress-strain diagram

A

Shows the difference between the elastic limit and the proportional limit

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6
Q

Definition of elastic limit

A

The limit at which the material will return to its original shape if distorted

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7
Q

Effect of high YM

A

Rigid/stiff - large stress produces small strain

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8
Q

Effect of high proportional and elastic limits

A

Only large stresses will cause permanent deformation

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9
Q

Effect of high thermal expansion

A

If cooled incorrectly, susceptible to internal stresses that can lead to fracture

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10
Q

Definition of thermal conductivity

A

Transmission of thermal stimuli to mucosa

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11
Q

Effect of high thermal conductivity

A

Avoid scalding back of throat or oesophagus

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12
Q

Effect of low density

A

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

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13
Q

Type of acrylic resin bonding

A

Free radical addition polymerisation of the methacrylate monomer

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14
Q

Definition of free radical addition polymerisation

A

The chemical union of two molecules, either the same or different, to form a larger molecule without the elimination of a smaller molecule
PMMA free radical addition polymerisation involves molecules with C=C bonds

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15
Q

Stages in acrylic polymerisation (4)

A

Activation
Initiation
Propagation
Termination

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16
Q

Stages in acrylic polymerisation activation (4)

A

Activation of initiator to provide free radicals
The initiator is usually benzoyl peroxide (C6H5COO-OHCH5C6 – symmetrical)
Acrylic resin can be heat activated (>72C) or self-cured
Activation gives two free radicals: R* and C6H5COO*

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17
Q

Stages in acrylic polymerisation initiation

A

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

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18
Q

Stages in acrylic polymerisation propagation

A

Growing polymer chain

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19
Q

Stages in acrylic polymerisation termination

A

Termination of polymerisation

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20
Q

Composition of heat-cured acrylic (2)

A

Powder (initiator, RMMA particles, plasticiser, pigments, co-polymers)
Liquid (methacrylate monomer, inhibitor, co-polymers)

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21
Q

Function of plasticiser in PMMA powder

A

Allow quicker dissolving in monomer liquid (dibutyl phthalate)

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22
Q

Function of pigments in PMMA powder

A

To give natural colour

23
Q

Function of co-polymers in PMMA powder

A

To improve mechanical properties (ethylene glycol dimethacrylate)

24
Q

Function of methacrylate monomer in PMMA liquid

A

Dissolves PMMA particles (polymerises)

25
Q

Function of inhibitor in PMMA liquid

A

Prolongs shelf-life – reacts with any free radicals produced by heat/UV light (0.006% hydroquinone)

26
Q

Function of co-polymers in PMMA liquid

A

Improves mechanical properties, particularly cross-linking of polymers

27
Q

Features of powder and liquid when mixed (5)

A

Produces a dough-like material that can be handled/mixed easily and customised to desired shape
Reduces heat of reaction
Minimises polymerisation shrinkage
Is mixed 3-3.5:1 (P/L) by volume (2.5:1 by weight)
Mixing stages: sandy (tacky) and dough (packing)

28
Q

Requirements of heat curing of PMMA

A

Requires efficient polymerisation to give a high polymer molecular weight (good mechanical property)

29
Q

PMMA heating schedules (3)

A

7hrs to 70C and 2hrs to 100C and slow cool
72C for 16hrs
20 - 20 - 20 reverse curing (place flask in boiling water, remove heat for 20mins, heat to 70C for 20mins, heat to 100C for 20mins)

30
Q

Factors during mould and acrylic cooling (3)

A

Mould material and acrylic have different thermal expansion coefficients
Can lead to internal stresses
Slow cooling helps to relieve this

31
Q

Effects of internal stress (4)

A

Decreased strength
Decreased fatigue strength
Warping (during finishing)
Repair problems

32
Q

Internals stresses are related to (5)

A 
Shape and size (notches)
Curing pressure
Curing cycle
Thermal expansion
Cooling rate
33
Q

Effects of under-curing (2)

A 
Free monomer (irritant)
Low molecular weight (poor mechanical properties)
34
Q

Effects of fast curing

A

Can potentially lead to gaseous porosity

35
Q

Incorrect powder/monomer ratios (2)

A

Too much monomer

Too little monomer

36
Q

Effect of too much monomer

A

Contraction porosity

37
Q

Effect of too little monomer

A

Granularity

38
Q

Effects of porosity (4)

A

Affects strength
Affects appearance
Rough sensation to tongue
Absorbs saliva – poor hygiene

39
Q

Features of gaseous porosity

A

Occurs when curing temp. exceeds 100C
When this happens, monomers ‘boil’ and cause gaseous bubbles in the acrylic
Usually occurs in bulkier parts

40
Q

Definition of contraction porosity

A

Voids due to polymerisation shrinkage where the acrylic ‘dough’ is not sufficiently packed

41
Q

Polymerisation shrinkage of monomer alone

A

21% by volume

42
Q

Polymerisation shrinkage of powder and monomer mix

A

7%

43
Q

Causes of contraction porosity (3)

A

Too much monomer
Insufficient excess material
Insufficient clamp pressure

44
Q

Ideal properties of acrylic resin (11)

A

Replace function of natural teeth
Non-toxic and non-irritant
Unaffected by oral fluids
Thermal expansion (artificial tooth)
High thermal conductivity
High mechanical properties (YM, proportional limit and elastic limit)
Colour/translucency
Low density
High softening temperature (must not distort during ingestion of hot fluids or during cleaning)
Dimensionally accurate and stable in use (must fit patient’s mouth and be retained)
Fits well in patient’s mouth
Is seen by other people (who should be unaware it is a prosthesis)

45
Q

Actual irritation of acrylic resin

A

Provided no monomer is released but some patients are allergic

46
Q

Actual effect of oral fluids of acrylic resin

A

Water absorption, virtually insoluble in fluids taken orally

47
Q

Actual thermal expansion of acrylic resin

A

Adequate if acrylic teeth are used; significantly higher than porcelain teeth

48
Q

Actual thermal conductivity of acrylic resin

A

Low, which is poor

49
Q

Actual mechanical properties (YM, proportional limit and elastic limit) of acrylic resin

A

Low (poor) - compensated by increasing in bulk

50
Q

Actual colour/translucency of acrylic resin

A

Good - similar to natural tissue

51
Q

Actual density of acrylic resin

A

Low (good),but the need to increase in bulk to overcome poor mechanical properties offsets this advantage

52
Q

Actual softening temperature of acrylic resin

A

75C - fine for ingested hot fluids, do not clean using boiling water

53
Q

Actual dimensional accuracy and stability in use of acrylic resin

A

Linear contraction 0.5% – acceptable

BDS2 CDS DMS (15 decks)

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