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Dental Materials > PMMA - Acrylic Resin > Flashcards

PMMA - Acrylic Resin Flashcards

1
Q

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

A

Initial impression is flawed

Acrylic resin curing process has several potential error sources

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2
Q

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

A

Refinements can be made at the chairside

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3
Q

For what reasons may acrylic resin fail

A

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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4
Q

What are the basic requirements of a denture base material

A

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

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5
Q

What are the ideal properties of acrylic resin

A

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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6
Q

What is the proportional limit

A

The stress that can be applied before the material behaves inelastically

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7
Q

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

A

Compressive strength
Elastic limit strength
Proportional limit stress
The elastic modulus

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8
Q

What is elastic limit strength

A

The stress beyond which deformation occurs

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9
Q

What is the elastic modulus and how is it determined

A

Stiffness

Determined by the gradient of the initial part of the curve

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10
Q

What are the ideal mechanical properties of acrylic resin

A

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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11
Q

What are the ideal thermal properties of acrylic resin

A

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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12
Q

Describe the other ideal properties of acrylic resin

A

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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13
Q

How is acrylic resin produced and what does this mean

A

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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14
Q

Describe the 4 stages of acrylic polymerisation

A

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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15
Q

What is the initiator in acrylic polymerisation

A

Benzoyl peroxide (symmetrical)

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16
Q

How may acrylic polymerisation be activated

A

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

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17
Q

Describe acrylic propagation

A

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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18
Q

What are the components of heat cured acrylic resin powder

A

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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19
Q

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

A

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

20
Q

Give an example of a co-polymer

A

Ethylene glycol dimethacrylate

21
Q

What are the components of heat cured acrylic liquid

A

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

22
Q

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

A

3-3.5/1 powder to liquid by volume

2.5/1 by weight

23
Q

How long should the powder and liquid be mixed

A

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

24
Q

Why is the powder and liquid mixed

A

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

25
Q

What should happen if acrylic heat curing is carried out ideally

A

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

26
Q

Give examples of heating schedules used for acrylic resin

A

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

27
Q

Describe 20 - 20- 20 Reverse Curing

A

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

28
Q

Why should acrylic resin heating not exceed 100*C

A

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

29
Q

What happens if acrylic is cooled too quickly

A

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

30
Q

What are the consequences of internal stress in acrylic

A

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

31
Q

What are internal stresses of acrylic related to

A 
Thermal expansion
Cooling rate
Shape and size eg - notches
Curing pressure
Curing cycle
32
Q

What happens if acrylic is under cured

A

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

33
Q

What happens if acrylic undergoes fast curing

A

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

34
Q

What happens if the incorrect powder/monomer ratio is used

A

Too much monomer can lead to contraction porosity

Too little monomer can cause granularity

35
Q

What effects does porosity have on acrylic

A

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

36
Q

What is gaseous porosity

A

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

37
Q

How can gaseous porosity be avoided

A

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

38
Q

How much polymerisation shrinkage happens to acrylic

A

Monomer alone - 21% (by volume)

Powder and monomer mix - 7%

39
Q

How can polymerisation shrinkage be minimised

A

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

40
Q

Where does polymerisation shrinkage occur

A

Anywhere where dough is not sufficiently packed

41
Q

Compare the elastic modulus of acrylic to enamel

A

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

42
Q

Compare the hardness of acrylic to composite

A

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

43
Q

How does acrylic compensate for its mechanical weaknesses

A

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

44
Q

Describe the density of acrylic

A

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

45
Q

What is the linear contraction of acrylic

A

0.5% - acceptable

Dental Materials (19 decks)

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