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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
termination
of polymerisation
26
heat cured acrylic powder
``` initiator - benzoyl peroxide 0.2-0.5% PMMA particles - pre-polymerised beads plasticiser pigments - natural colour co-polymers ```
27
heat cured acrylic powder - plasticiser
allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
28
heat cured acrylic powder - co-polymers
to improve mechanical properties e.g. ethylene glycol dimethacrylate
29
heat cured acrylic liquid
methacrylate monomer - dissolves PMMA particles - polymerises inhibitor - hydroquinone 0.006% co-polymers
30
heat cured acrylic liquid - inhibitor
hydroquinone 0.006% | prolongs shelf life - reacts with any free radicals produced by heat, UV light
31
heat cured acrylic liquid - co-polymers
improve mechanical properties - particularly cross-linking of polymers
32
heat cured acrylic technique
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
acrylics heat curing pros and cons
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
heat cured contraction/expansion
manufacture - 0.5% linear contraction | usage - 0.4% expansion
35
self-curing acrylic composition
as heat cured except benzoyl peroxide is activated by promoter e.g. dimethyl-paratoluidine (tertiary amine) in liquid
36
advantage of self-cured
lower temp - less thermal contraction so better dimensional accuracy
37
disadvantages of self-cured
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
comparing properties - unreacted monomer
chemical cure - 3-5% - unreacted monomer - risk of dimensional instability heat cure 0.2-0.5%
39
comparing properties - dimensional accuracy
self-cured fits original cast better than heat cured BUT water absorption gives expansion - SC oversized - HC undersized - better tolerated
40
SC higher monomer levels - irritant
inform pt of risk - instruct them to notify you ASAP if any signs
41
colour stability
SC poorer - tertiary amines susceptible to oxidation
42
attempts to strengthen
``` 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
heat cure denture base product - Ultra-Hi
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
pour n cure resins
similar to self-cure smaller powder particles fluid mix pour into mould good fitting but poor mechanical properties
45
light activated denture resins
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
radiopaque polymers
``` 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
alternative polymers
``` PROVEN allergy to acrylic? try - nylons - vinyl polymers - polycarbonates ```
48
nylons
water absorption - swelling - softening
49
polycarbonates
injection moulded - Tg 150 degrees internal stresses - distortion in use good impact strength
50
vinyl polymers
e.g. polyvinyl acetate, polyvinylchloride, styrene injection moulding Tg = 60 degrees - softening in use
51
which type is most commonly used and why?
heat cured - alternatives all have deficiencies

DMS :( (18 decks)

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