PMMA (2 lectures) Flashcards
free radical addition polymerisation
“chemical union of two molecules same/different to form a larger molecule without the elimination of a smaller molecule”
C=C bonds
methacrylate monomer
stages of free radical addition polymerisation
activation
initiation
propagation
termination
activation
of initiator to provide free radicals
initiation
free radicals break C=C bond in monomer and transfer free radical
initiator
benzoyl peroxide
activation methods
heat >72 degrees
self-cured
why is heat curing efficient?
produces a high molecular weight polymer with good mechanical properties
heating schedules
7hrs to 70 degrees and 2hrs to 100 degrees and slow cool
72 degrees for at least 16hrs
20-20-20 reverse curing
why should you cool v slowly?
mould material and acrylic have different thermal expansion coefficients
internal stresses
under-curing
free monomer - irritant
low molecular weight - poor mechanical properties
heat cured acrylic powder
initiator - benzoyl peroxide 0.2-0.5% PMMA particles plasticiser pigments co-polymers
heat-cured acrylic liquid
methacrylate monomer
inhibitor (hydroquinone 0.006%)
copolymers
inhibitor in heat cured acrylic liquid
hydroquinone 0.006%
reacts with any free radicals produced by heat, UV light
copolymers in heat cured acrylic liquid
improve mechanical properties
consistency of acrylic and why?
‘dough-like’
reduce heat of reaction
minimise polymerisation shrinkage
effects of porosities
affects strength and appearance
rough sensation to tongue
absorb saliva - poor hygiene
types of porosities
gaseous
contraction
granular
gaseous porosity
monomer boiling - 100 degrees
bulkier parts
contraction porosity
polymerisation shrinkage - monomer alone 21%, powder and monomer mix 7% causes - too much monomer - insufficient excess material - insufficient clamp pressure
ideal properties
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
actual properties
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
transverse strength - 3 point loading - flexural
how well does upper denture cope with stresses that cause deflection?
palate (fixed)
stress (masticatory)
impact strength
may break upon impact or microcracks (surface cracks) may form - invisible but over time act as weak points in denture
propagation
of growing polymer chain
termination
of polymerisation
heat cured acrylic powder
initiator - benzoyl peroxide 0.2-0.5% PMMA particles - pre-polymerised beads plasticiser pigments - natural colour co-polymers
heat cured acrylic powder - plasticiser
allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
heat cured acrylic powder - co-polymers
to improve mechanical properties e.g. ethylene glycol dimethacrylate
heat cured acrylic liquid
methacrylate monomer - dissolves PMMA particles - polymerises
inhibitor - hydroquinone 0.006%
co-polymers
heat cured acrylic liquid - inhibitor
hydroquinone 0.006%
prolongs shelf life - reacts with any free radicals produced by heat, UV light
heat cured acrylic liquid - co-polymers
improve mechanical properties - particularly cross-linking of polymers
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
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
heat cured contraction/expansion
manufacture - 0.5% linear contraction
usage - 0.4% expansion
self-curing acrylic composition
as heat cured except benzoyl peroxide is activated by promoter e.g. dimethyl-paratoluidine (tertiary amine) in liquid
advantage of self-cured
lower temp - less thermal contraction so better dimensional accuracy
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
comparing properties - unreacted monomer
chemical cure - 3-5%
- unreacted monomer - risk of dimensional instability
heat cure 0.2-0.5%
comparing properties - dimensional accuracy
self-cured fits original cast better than heat cured
BUT water absorption gives expansion
- SC oversized
- HC undersized - better tolerated
SC higher monomer levels - irritant
inform pt of risk - instruct them to notify you ASAP if any signs
colour stability
SC poorer - tertiary amines susceptible to oxidation
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
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
pour n cure resins
similar to self-cure
smaller powder particles
fluid mix pour into mould
good fitting but poor mechanical properties
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
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
alternative polymers
PROVEN allergy to acrylic? try - nylons - vinyl polymers - polycarbonates
nylons
water absorption - swelling - softening
polycarbonates
injection moulded
- Tg 150 degrees
internal stresses - distortion in use
good impact strength
vinyl polymers
e.g. polyvinyl acetate, polyvinylchloride, styrene
injection moulding
Tg = 60 degrees - softening in use
which type is most commonly used and why?
heat cured - alternatives all have deficiencies
