DMS- PMMA

Question 1

What is PMMA?

Answer 1

Polymethylmethacrolate. Which is the acrylic used for denture bases.

Question 2

What are the ideal properties for a denture material?

Answer 2

Heat

• High thermal conductivity
• Similar thermal expansion to teeth.
• High softening temperature- don’t want it to distort when cleaning or ingesting hot fluids.

Mechanical

• High young’s modulus (rigidity)
• High elastic limit
• Dimensionally accurate and stable in use- To stay and be retained in the mouth
• Unaffected by oral fluids.

Question 3

What are the two components PMMA is made up of?

Answer 3

Powder and liquid.

Question 4

What is found in the powder of the PMMA?

Answer 4

Pigment (for aesthetic colour)

PMMA particles (so curing is quicker)

Initiator Benzoyl peroxide.

Plasticiser- allows quicker disolving in the monomer liquid.

Co-polymer to improve the mechanical properties.

Question 5

What is found in the liquid of PMMA?

Answer 5

Methacrylate monomer- Dissolves PMMA Particles.

Co-polymer- improves mechanical properties (especially cross linking)

Inhibitor - Hydroquinone- prolongs shelf life and prevents liquid reacting before it mixes with the powder.

Question 6

Why do we mix the powder and the liquid in PMMA?

Answer 6

• To gain a dough that can be handled, mixed easily and customised to the desired shape.
• To reduce the heat of the reaction.
• To minimise polymerisation shrinkage.

Question 7

Describe how pMMA cures?

Answer 7

By Free radical addition polymerisation of methomethylacrolate monomers.

Activation- activation of the (initiator) benzozyl peroxide to produce free radicals.

Initiation- free radicals break up the methomethylacrolate C-C bonds

Polymerisation- growing of the polymer chain

Termination- of polymerisation.

Question 8

What happens if the heat is too high when heat curing PMMA?

Answer 8

At over 100*C Gas is produced. This causes porosities which reduce the strength of the material.

Question 9

What effect does the difference in thermal expansion co efficents between the mould material and the acrylic have?

Answer 9

This causes Internal stresses;

Decreased strength

Decreased fatigue strength (endurance)

Material is more prone to warping.

Question 10

How do we reduce the internal stresses of PMMA?

Answer 10

We cool the flask down gradually.

Question 11

What happens if the acrylic is not cured properly?

Answer 11

• The uncured monomer will leech out of the material causing irritation
• There is a low molecular weight- causing poor mechanical properties.

Question 12

How is the PMMA affected if there is incorrect powder to monomer ratio?

Answer 12

Too much monomer- contraction porosity

Not enough monomer- causes granularity (roughened surface)

Question 13

Why pick PMMA?

Answer 13

• Will retain in the mouth
• Is not affected by oral fluids (not water soluble)
• Has a simular thermal expansion to the acrylic teeth
• non toxic
• Non-irritant if properly cured.
• Good colour.
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Question 14

Why not pick PMMA?

Answer 14

• Low young’s modulus (rigidity)
• Low elastic limit (resists less stress before it breaks)
• Low thermal conductivity
• Lower softening temperature (this is fine for oral fluid but you cannot wash the denture using hot water)

Question 15

Compare gaseous porosity and contraction porosity.

Answer 15

Gaseous porosity- if you do not heat the material gradually- gas is produced. This gas causes porosities.

Contraction porosities- If you do not put the material under pressure- the material will expand to fill the space and then contract. The contraction causes air bubbles i.e porosities.

Under pressure it doesn’t have the space to expand- unless the material is underfilled.