9. PMMA Flashcards by Rory Maciver

Definition of PMMA

Polymethylmethacrylate – acrylic resin

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Error sources of dentures (2)

Production

Usage

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Denture production error sources (3)

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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Denture usage error sources (4)

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

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Component of stress-strain diagram

Shows the difference between the elastic limit and the proportional limit

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Definition of elastic limit

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

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Effect of high YM

Rigid/stiff - large stress produces small strain

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Effect of high proportional and elastic limits

Only large stresses will cause permanent deformation

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Effect of high thermal expansion

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

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Definition of thermal conductivity

Transmission of thermal stimuli to mucosa

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Effect of high thermal conductivity

Avoid scalding back of throat or oesophagus

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Effect of low density

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

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Type of acrylic resin bonding

Free radical addition polymerisation of the methacrylate monomer

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Definition of free radical addition polymerisation

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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Stages in acrylic polymerisation (4)

Activation
Initiation
Propagation
Termination

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Stages in acrylic polymerisation activation (4)

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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Stages in acrylic polymerisation initiation

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

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Stages in acrylic polymerisation propagation

Growing polymer chain

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Stages in acrylic polymerisation termination

Termination of polymerisation

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Composition of heat-cured acrylic (2)

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

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Function of plasticiser in PMMA powder

Allow quicker dissolving in monomer liquid (dibutyl phthalate)

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Function of pigments in PMMA powder

To give natural colour

Function of co-polymers in PMMA powder

To improve mechanical properties (ethylene glycol dimethacrylate)

Function of methacrylate monomer in PMMA liquid

Dissolves PMMA particles (polymerises)

Function of inhibitor in PMMA liquid

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

Function of co-polymers in PMMA liquid

Improves mechanical properties, particularly cross-linking of polymers

Features of powder and liquid when mixed (5)

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)

Requirements of heat curing of PMMA

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

PMMA heating schedules (3)

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)

Factors during mould and acrylic cooling (3)

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

Effects of internal stress (4)

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

Internals stresses are related to (5)

``` Shape and size (notches) Curing pressure Curing cycle Thermal expansion Cooling rate ```

Effects of under-curing (2)

``` Free monomer (irritant) Low molecular weight (poor mechanical properties) ```

Effects of fast curing

Can potentially lead to gaseous porosity

Incorrect powder/monomer ratios (2)

Too much monomer | Too little monomer

Effect of too much monomer

Contraction porosity

Effect of too little monomer

Granularity

Effects of porosity (4)

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

Features of gaseous porosity

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

Definition of contraction porosity

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

Polymerisation shrinkage of monomer alone

21% by volume

Polymerisation shrinkage of powder and monomer mix

Causes of contraction porosity (3)

Too much monomer Insufficient excess material Insufficient clamp pressure

Ideal properties of acrylic resin (11)

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)

Actual irritation of acrylic resin

Provided no monomer is released but some patients are allergic

Actual effect of oral fluids of acrylic resin

Water absorption, virtually insoluble in fluids taken orally

Actual thermal expansion of acrylic resin

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

Actual thermal conductivity of acrylic resin

Low, which is poor

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

Low (poor) - compensated by increasing in bulk

Actual colour/translucency of acrylic resin

Good - similar to natural tissue

Actual density of acrylic resin

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

Actual softening temperature of acrylic resin

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

Actual dimensional accuracy and stability in use of acrylic resin

Linear contraction 0.5% – acceptable