PMMA - Acrylic Resin Flashcards by David Jack

For what reasons may a denture not fit the patient first time

Initial impression is flawed

Acrylic resin curing process has several potential error sources

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What can be don’t if a denture doesn’t fit the patient first time

Refinements can be made at the chairside

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For what reasons may acrylic resin fail

Fracturing due to masticatory forces during use or through mishandling
Becoming warped
Surface suffering wear
Gradually becomes an uncomfortable fit
Only fits patient for a short period of time

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What are the basic requirements of a denture base material

Replaces function of natural teeth
Goes into patient’s mouth
Is seen by other people

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What are the ideal properties of acrylic resin

Dimensionally accurate and stable in use - must fit patient’s mouth and be retained
High Softening Temperature (Tg) - must not distort during ingestion of hot fluids, or during cleaning
Unaffected by oral fluids over time

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What is the proportional limit

The stress that can be applied before the material behaves inelastically

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What can be gathered from the stress/strain diagram of acrylic resin

Compressive strength
Elastic limit strength
Proportional limit stress
The elastic modulus

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What is elastic limit strength

The stress beyond which deformation occurs

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What is the elastic modulus and how is it determined

Stiffness

Determined by the gradient of the initial part of the curve

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What are the ideal mechanical properties of acrylic resin

High Young’s (elastic) modulus - stiff, don’t want it to be squeezed to readily, pressing against the palate
High proportional limit so only large stresses will cause permanent deformation

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What are the ideal thermal properties of acrylic resin

Thermal expansion equalling the artificial tooth to avoid internal stresses on cooling during manufacture
High thermal conductivity to allow transmission of thermal stimuli to mucosa to avoid scalding back of throat or oesophagus

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Describe the other ideal properties of acrylic resin

Low density to aid retention of the upper denture - the heavier the material, the greater the gravity pull
Colour and translucency to match the natural tissues
Non toxic, non irritant

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How is acrylic resin produced and what does this mean

Free radical addition polymerisation
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

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Describe the 4 stages of acrylic polymerisation

Activation - of initiator to provide free radicals
Initiation - free radicals break C=C bond in monomer and transfer free radicals
Propagation - growing polymer chain
Termination - of polymerisation

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What is the initiator in acrylic polymerisation

Benzoyl peroxide (symmetrical)

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How may acrylic polymerisation be activated

Heat >72*C
or
Self curing (a chemical setting reaction)

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Describe acrylic propagation

Electrically charged radical reacts with monomer, the charge is transferred to the monomer end of the molecule
This then reacts with another monomer molecule and the charge is transferred to the monomer end of the molecule and so on

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What are the components of heat cured acrylic resin powder

Initiator - Benzoyl Peroxide, 0.2-0.5%
PMMA particles
Plasticiser allows quicker dissolving in monomer liquid
Pigments - to give natural colour
CO-polymers - to improve mechanical properties

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What are PMMA particles and why are they used in heat cured acrylic powder

Pre-polymerised beads of PMMA
This is PMMA material that has been made before, is ground down into small particles
It’s role is to speed up the polymerisation reaction

Give an example of a co-polymer

Ethylene glycol dimethacrylate

What are the components of heat cured acrylic liquid

Methacrylate monomer - dissolves PMMA particles, polymerising them
Inhibitor (Hydroquinone, 0.006%) - prolongs shelf life - reacts with any free radicals produced by heat and UV light
Co-polymers - improves mechanical properties through cross-linking of polymers

What proportions of powder and liquid should be used to make acrylic resin

3-3.5/1 powder to liquid by volume

2.5/1 by weight

How long should the powder and liquid be mixed

Initially produces a sandy then tacky feel

Only when it has a dough-like consistency is it ready for packing into the desired denture base shape

Why is the powder and liquid mixed

To produce dough-like material that can be handled easily and customised to the desirable shape
Reduces heat of reaction
Minimises polymerisation shrinkage

What should happen if acrylic heat curing is carried out ideally

Efficient polymerisation as this will yield a high molecular weight polymer and that means good mechanical properties such as mechanical strength

Give examples of heating schedules used for acrylic resin

1 - 7hrs to 70*C + 2hrs to 100*C + slow cool 2 - 72*C for at least 16hrs 3 - 20 - 20 - 20 reverse curing

Describe 20 - 20- 20 Reverse Curing

Place flask in boiling water Remove heat for 20 minutes Heat to 70*C for 20 minutes Then heat to 100*C for 20 minutes

Why should acrylic resin heating not exceed 100*C

Due to the exothermic polymerisation reaction the cycle would produce many voids

What happens if acrylic is cooled too quickly

The acrylic and the mould material will contract at different rates when being cooled Cooling too fast can lead to internal stresses which in the acrylic resin which would weaken it

What are the consequences of internal stress in acrylic

Reduction in compressive/tensile strength Reduction in fatigue strength More likely to be warping Make minor repairs problematic

What are internal stresses of acrylic related to

``` Thermal expansion Cooling rate Shape and size eg - notches Curing pressure Curing cycle ```

What happens if acrylic is under cured

Some unreacted or free monomer will be present which may leach out over time, possibly causing irritation to oral tissues The resin will have a low molecular weight which means it will have poor mechanical properties

What happens if acrylic undergoes fast curing

There is a risk of the material exhibiting regions with gaseous porosity and that seriously compromises the denture’s strength

What happens if the incorrect powder/monomer ratio is used

Too much monomer can lead to contraction porosity | Too little monomer can cause granularity

What effects does porosity have on acrylic

Reduces strength Adversely affects appearance Results in a rough sensation when contacted by the tongue Resin may absorb saliva resulting in poor oral hygiene

What is gaseous porosity

During the heating process, if the monomer reaches 100*C, it boils producing gas that’s revealed as bubbles close to the acrylic resin surface More likely to occur in bulkier parts of the material

How can gaseous porosity be avoided

By curing slowly and preventing the resin’s overall temperature getting out of control

How much polymerisation shrinkage happens to acrylic

Monomer alone - 21% (by volume) | Powder and monomer mix - 7%

How can polymerisation shrinkage be minimised

Use as little monomer as possible Add an excess amount of the acrylic dough to the mould material Apply the right clamp pressure

Where does polymerisation shrinkage occur

Anywhere where dough is not sufficiently packed

Compare the elastic modulus of acrylic to enamel

Acrylic is 2.5GPa, much lower than composites 14, so the same stress applied to both will mean acrylic resin is undergoing almost six times more strain

Compare the hardness of acrylic to composite

The surface of acrylic is much softer than composite - 20 Vickers hardness units compared to 90 This means it will experience much more abrasive wear

How does acrylic compensate for its mechanical weaknesses

It is used in greater bulk and thickness than is actually needed

Describe the density of acrylic

Low which is good but need to increase bulk to overcome poor mechanical properties which offsets this advantage

What is the linear contraction of acrylic

0.5% - acceptable