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DMS :( > PMMA (2 lectures) > Flashcards

PMMA (2 lectures) Flashcards

1
Q

free radical addition polymerisation

A

“chemical union of two molecules same/different to form a larger molecule without the elimination of a smaller molecule”
C=C bonds
methacrylate monomer

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2
Q

stages of free radical addition polymerisation

A

activation
initiation
propagation
termination

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3
Q

activation

A

of initiator to provide free radicals

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4
Q

initiation

A

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

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5
Q

initiator

A

benzoyl peroxide

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6
Q

activation methods

A

heat >72 degrees

self-cured

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7
Q

why is heat curing efficient?

A

produces a high molecular weight polymer with good mechanical properties

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8
Q

heating schedules

A

7hrs to 70 degrees and 2hrs to 100 degrees and slow cool
72 degrees for at least 16hrs
20-20-20 reverse curing

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9
Q

why should you cool v slowly?

A

mould material and acrylic have different thermal expansion coefficients
internal stresses

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10
Q

under-curing

A

free monomer - irritant

low molecular weight - poor mechanical properties

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11
Q

heat cured acrylic powder

A 
initiator - benzoyl peroxide 0.2-0.5%
PMMA particles
plasticiser
pigments
co-polymers
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12
Q

heat-cured acrylic liquid

A

methacrylate monomer
inhibitor (hydroquinone 0.006%)
copolymers

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13
Q

inhibitor in heat cured acrylic liquid

A

hydroquinone 0.006%

reacts with any free radicals produced by heat, UV light

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14
Q

copolymers in heat cured acrylic liquid

A

improve mechanical properties

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15
Q

consistency of acrylic and why?

A

‘dough-like’
reduce heat of reaction
minimise polymerisation shrinkage

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16
Q

effects of porosities

A

affects strength and appearance
rough sensation to tongue
absorb saliva - poor hygiene

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17
Q

types of porosities

A

gaseous
contraction
granular

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18
Q

gaseous porosity

A

monomer boiling - 100 degrees

bulkier parts

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19
Q

contraction porosity

A 
polymerisation shrinkage - monomer alone 21%, powder and monomer mix 7%
causes
 - too much monomer
 - insufficient excess material
 - insufficient clamp pressure
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20
Q

ideal properties

A 
dimensionally stable and accurate in use
high softening temp
unaffected by oral fluids
mechanical properties
 - high YM
 - high proportional/EL
 - high transverse, fatigue, impact strength
 - high hardness/abrasion resistance
thermal properties
 - thermal expansion = artificial tooth
 - high thermal conductivity
low density
colour/translucency
non-toxic/non-irritant
radiopaque
easy to manufacture
easy to repair
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21
Q

actual properties

A

non-toxic
non-irritant - provided no monomer released, but some pts allergic
unaffected by oral fluids
thermal expansion ok if acrylic teeth used, significantly higher than porcelain
low thermal conductivity - pt may scald throat
poor mechanical properties - increase in bulk to compensate
good aesthetics
low density - but have to increase in bulk to overcome poor mechanical properties
softening temp 75 degrees
- ok for ingested hot fluids
- don’t use boiling water for cleaning
quite dimensionally accurate and stable in use
- linear contraction 0.5% - acceptable
fatigue/impact strength - fairly resistant but can be a cause of failure
high hardness/abrasion resistance - retains good polish, some wear over time

22
Q

transverse strength - 3 point loading - flexural

A

how well does upper denture cope with stresses that cause deflection?
palate (fixed)
stress (masticatory)

23
Q

impact strength

A 
may break upon impact
or microcracks (surface cracks) may form - invisible but over time act as weak points in denture
24
Q

propagation

A

of growing polymer chain

25
Q

termination

A

of polymerisation

26
Q

heat cured acrylic powder

A 
initiator - benzoyl peroxide 0.2-0.5%
PMMA particles - pre-polymerised beads
plasticiser
pigments - natural colour
co-polymers
27
Q

heat cured acrylic powder - plasticiser

A

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

28
Q

heat cured acrylic powder - co-polymers

A

to improve mechanical properties e.g. ethylene glycol dimethacrylate

29
Q

heat cured acrylic liquid

A

methacrylate monomer - dissolves PMMA particles - polymerises
inhibitor - hydroquinone 0.006%
co-polymers

30
Q

heat cured acrylic liquid - inhibitor

A

hydroquinone 0.006%

prolongs shelf life - reacts with any free radicals produced by heat, UV light

31
Q

heat cured acrylic liquid - co-polymers

A

improve mechanical properties - particularly cross-linking of polymers

32
Q

heat cured acrylic technique

A

vessel - mould material in inverse shape of denture base required
denture base material will be placed in the recess of the RH clamp
acrylic dough
clamp flask, cure to form a strong solid denture base

33
Q

acrylics heat curing pros and cons

A

need efficient polymerisation to give high molecular weight polymer i.e. good mechanical properties
therefore high temp
BUT gaseous porosity limits
always some unreacted monomer, over time can go into pts mouth - irritant

34
Q

heat cured contraction/expansion

A

manufacture - 0.5% linear contraction

usage - 0.4% expansion

35
Q

self-curing acrylic composition

A

as heat cured except benzoyl peroxide is activated by promoter e.g. dimethyl-paratoluidine (tertiary amine) in liquid

36
Q

advantage of self-cured

A

lower temp - less thermal contraction so better dimensional accuracy

37
Q

disadvantages of self-cured

A

chemical activation = less efficient
poorer mechanical properties
Tg lower
more unreacted monomer
- acts as plasticiser, softening denture base, reducing transverse strength
- potential tissue irritant, compromising its biocompatibility

38
Q

comparing properties - unreacted monomer

A

chemical cure - 3-5%
- unreacted monomer - risk of dimensional instability
heat cure 0.2-0.5%

39
Q

comparing properties - dimensional accuracy

A

self-cured fits original cast better than heat cured
BUT water absorption gives expansion
- SC oversized
- HC undersized - better tolerated

40
Q

SC higher monomer levels - irritant

A

inform pt of risk - instruct them to notify you ASAP if any signs

41
Q

colour stability

A

SC poorer - tertiary amines susceptible to oxidation

42
Q

attempts to strengthen

A 
high impact resistant materials
 - incorporate rubber toughening agent (butodienstyrene) - stop crack propagation - long term fatigue problems
incorporate fibres
 - carbon, UHMPE, glass
 - difficult processing - ongoing
43
Q

heat cure denture base product - Ultra-Hi

A

a high impact heat cure acrylic resin

  • flexural strength
  • superior fracture toughness (ductility) - a slight bending aspect which keeps the material from being brittle and subject to cracking/breaking
44
Q

pour n cure resins

A

similar to self-cure
smaller powder particles
fluid mix pour into mould
good fitting but poor mechanical properties

45
Q

light activated denture resins

A

urethane dimethacrylate matrix plus acrylic copolymers
microfine silica fillers - small amounts to control rheology
photo initiator systems
adapted to cast - no heat curing cycle
cured in light chamber - but limited depth of cure
used mostly as customised impression tray material and for repair of fractured dentures

46
Q

radiopaque polymers

A 
metal inserts - weaken, poor aesthetics
inorganic salts e.g. barium sulphate
 - low conc - not radiopaque
 - high conc - weak base
comonomers containing heavy metals e.g. barium sulphate - poor mechanical properties
halogen containing comonomers or additives e.g. tribromophenylmethacrylate
 - may act as plasticiser
 - expensive
 - ? promising
47
Q

alternative polymers

A 
PROVEN allergy to acrylic?
try
 - nylons
 - vinyl polymers
 - polycarbonates
48
Q

nylons

A

water absorption - swelling - softening

49
Q

polycarbonates

A

injection moulded
- Tg 150 degrees
internal stresses - distortion in use
good impact strength

50
Q

vinyl polymers

A

e.g. polyvinyl acetate, polyvinylchloride, styrene
injection moulding
Tg = 60 degrees - softening in use

51
Q

which type is most commonly used and why?

A

heat cured - alternatives all have deficiencies

DMS :( (18 decks)

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