PMMA Flashcards by niamh ritchie

what is the measure of success of a denture fit

patient feedback

- natural appearance

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3 error sources in denture production

initial impression is flawed – material and method not 100% accurate

curing process - several potential error sources
may not achieve perfect fit at first attempt

REFINEMENTS at the chairside required
- Little adjustments is feasible
initial impression is flawed – material and method not 100% accurate
curing process - several potential error sources
may not achieve perfect fit at first attempt
REFINEMENTS at the chairside required
Little adjustments is feasible

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5 error sources in denture usage

fits patient only for short period
- with time and wear fit fails

fractures

uncomfortable

becomes warped – implications
on shape

surface suffers wear

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13 ideal properties of denture base material

replaces function of natural teeth

goes into patient’s mouth

needs to be aesthetically pleasing

Dimensionally accurate and stable in use
- must fit patient’s mouth AND be retained

High Softening Temperature (Tg)
- must not distort during ingesting of hot fluids, or during cleaning

Unaffected by Oral Fluids
over time

High Young’s (Elastic) Modulus
rigid (stiff) – large stress produces small strain

High Proportional Limit / Elastic Limit
- only large stresses will cause permanent deformation

Thermal Expansion
= Artificial tooth
- Match denture base and artificial teeth
- avoid internal stresses on cooling during manufacture (more on internal stresses in Metals & Alloys)
take into account internal stresses when manufacturing

High Thermal Conductivity
transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus from hot food/drink

Low Density
- aid retention of upper denture
the heavier the material, greater the gravity pull

Colour / Translucency
to match natural tissues

Non Toxic, Non Irritant

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what is the free radical addition polyermisation reaction which occurs in PMMA

Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule

INVOLVES molecules with (C=C bonds)

Break C=C
Become a larger, longer molecule so more rigid

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what reaction occurs in PMMA on setting

free radical addition polyermisation reaction

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4 stages in acrylic polyemrsation

activation

initiation

propagation

termination

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what occurs in activation in acrylic polymerisation

of initator to provide free radicals

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what occurs in initiation of acrylic polymerisation

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

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what occurs in propagation acrylic polymerisation reaction

growing polymer chain

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what occurs in termination of acrylic polymerisation reaction

final molecule made

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what is the initiator in acrylic polymerisation

benzoyl peroxide

C6H5COO-OOCH5C6
symmetrical

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what are the 2 types of acrylic addition polymerisation activations

heat above 72 degrees C

or self cured

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what does activation of the initiation addition polymerisation reaction achieve

two free radicals C6H5COO*

electrically charged

react with another molecule/monomer
- grows and cross links with other methacrylate monomer

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monomer in PMMA

methacrylate

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2 components of heat cured accylic

powder

liquid

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5 parts of powder in heat cured acrylic

Initiator
- (Benzoyl Peroxide, 0.2 - 0.5%)

PMMA Particles

pre-polymerised beads
produced before, ground into beads, mixed into powder, reacting with liquid

Plasticiser
- allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
- improves dissolution of monomer liquid
speed up process

Pigments
- to give “natural” colour

Co-polymers

to improve mechanical properties e.g. ethylene glycol dimethacrylate
PMMA monomer cannot do on its own

what is the role of the PMMA particles/beads in the powder of PMMA?

pre-polymerised beads

- produced before, ground into beads, mixed into powder, reacting with liquid

what is the role of the plasticiser in the PMMA powder

allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
improves dissolution of monomer liquid
speed up process

what is the role of the co-polymers in the PMMA powder

improve mechanical properties e.g. ethylene glycol dimethacrylate
- PMMA monomer cannot do on its own

3 components of the liquid in PMMA

Methacrylate Monomer
- dissolves PMMA particles – polymerises

Inhibitor (Hydroquinone, 0.006%)
- prolongs shelf life
reacts with any free radicals produced by heat, UV light
- Prevent any free radicals made inadvertently when material stored somewhere

Co-polymers
- improve mechanical properties - particularly cross-linking of polymers

what is the role of the inhibitor in liquid part of PMMA?

prolongs shelf life
reacts with any free radicals produced by heat, UV light
Prevent any free radicals made inadvertently when material stored somewhere

what is the role of the co polymers in the liquid part of PMMA?

improve mechanical properties - particularly cross-linking of polymers

what is the role of the methacrylate monomer in the liquid part of PMMA?

dissolves PMMA particles – polymerises

what is the result of mixing the liquid and powder components of PMMA?

dough-like material that can be handled/mixed easily and customised to desired shape reduce heat of reaction minimise polymerisation shrinkage - monomer on its own shrink by 21% but with liquid only 7% - substantial shrinkage factored into heat curing process

2 stages of PMMA mixing

sandy - tacky | dough - packing

how to use PMMA clamp flask

Filed with investment/mould material - Shaped to pt mouth - Individual teeth placed to mould material Brush applied to palate area Dough packed into investment materials - Allow excess material at the sides Clamp together - Apply sufficient pressure

why is sufficient heat curing required for PMMA

Need efficient polymerisation to give high molecular weight polymer i.e. good mechanical properties better the polymerisation the better the material use a high temperature ideally to make efficient - but the wrong temperatures can cause porosities in the acrylic hence high temperature but gaseous porosity limits different heating schedules depending on the material used - Stay clear of 100 o C in acrylic to avoid weakness

how should PMMA be cooled

Cool slowly in flask over many hours to allow relief and internal stresses

what can internal stresses in acrylic lead to

Decreased strength Decreased fatigue Warping e.g. during finishing Repair problems

what are some causes internal stresses in acrylic

Shape and size e.g. Notches Curing pressure Curing cycle Thermal expansion Cooling rate

2 problems if PMMA undercured

Free monomer leak out of denure base - Pt irritant Low molecular weight - Poor mechanical properties

problem if PMMA cured too fast

possible gaseous porosity

if too much monomer in PMMA

see contraction porosity

if too little monomer in PMMA

see granularity of surface material

4 effects of porosity

Affects strength Affects appearance - Roughness - Plaque trap Rough sensation to tongue Absorbs saliva - poor hygiene

gaseous porosity caused by

monomer boiling occurs in bulkier parts - inspect bulky areas for subsurface imperfections

what type of reaction is the free radical addition polymerisation of acrylic

exothermic | so cure slowly to avoid gaseous poroisty

what are 3 causes contraction porosity

excess monomer causing polymerisation shrinkage insufficient excess material insufficient clamp pressure

what are the softening temperatures of acrylic like

high - OK for ingested hot fluids DON’T use boiling water for cleaning Will warp acrylic denture so no longer fits