PMMA

Question 1

What are the ideal properties of a denture base material?

Answer 1

Dimensionally accurate and stable in use
High softening temperature
Unaffected by oral fluids
High Young’s modulus (rigid)
High proportional elastic limit
Thermal expansion matches that of the material used for artificial teeth
High thermal conductivity
Non toxic
Non irritant
Low density

Question 2

What is Young’s modulus?

Answer 2

Measures Stress/strain
Determines elasticity of the material

Question 3

What does it mean if a material has a high Young’s modulus?

Answer 3

The material is very rigid.

Question 4

What does “elastic limit” mean?

Answer 4

The amount of stress that a material undergoes, which results in the material straining, to a point which can be reversed.

Question 5

What happens if the stress increases beyond the point of the elastic limit?

Answer 5

The material will permanently deform.

Question 6

What does “proportional limit” mean?

Answer 6

Material limit where stress and strain are linear.
The amount of stress applied to a material, is proportional tot eh amount of strain.

Question 7

By what chemical process is PMMA made?

Answer 7

Free radical addition polymerisation.

Question 8

What is the definition of free radical addition polymerisation?

Answer 8

Chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule.

Question 9

What is the monomer used to make PMMA?

Answer 9

Methacrylate monomer.

Question 10

What are the four stages of acrylic polymerisation?

Answer 10

Activation- benzoyl peroxide provide free radicals
Initiation- free radicals break C=C bone in monomer and transfer free radicals
Propagation- growing polymer chain
Termination

Question 11

What two means of activation can occur for PMMA?

Answer 11

Heat cured or self cured.

Question 12

What is the most commonly used version in the dental hospital?

Answer 12

Heat cured
- powder and liquid mixed together.

Powder- Initiator, PMMA particles. plasticiser, pigments, co-polymers.
Liquid- methacrylate monomer, inhibitor and co-polymers.

Question 13

What issues occur with using too high a temperature during heat curing?

Answer 13

Gaseous porosity.

Question 14

Why should acrylic resin be cooled slowly during the cooling process?

Answer 14

The acrylic resin is surrounded by gypsum in the flask. These materials have different thermal expansion co-efficients and will expand at different rates.
This will cause internal stresses.

Question 15

What happens to the material after it experience internal stresses?

Answer 15

Reduced strength
Reduced fatigue strength
Warping
Repair problems

Question 16

What factors may contribute to internal stresses?

Answer 16

Different thermal expansions between mould and acrylic.
Cooling the material too fast.
Shape and side
Curing pressure
Curing cycle

Question 17

What happens if the PMMA is under cured?

Answer 17

Free monomer left within the material, which can be irritant.
Low molecular weight.

Question 18

What happens if the PMMA is cured too fast?

Answer 18

Gaseous porosity.

Question 19

Why do we want to avoid porosity?

Answer 19

Reduces strength
Adversely affects appearance
Causes the surface to be rough
Absorbs saliva- poor hygiene

Question 20

How does gaseous porosity occur?

Answer 20

PMMA is cured too fast (too high a temperature for too long), the monomer starts to boil, produces gases and cause bubbles to form in the acrylic resin surface.

Question 21

What type of reaction occurs during heat curing?

Answer 21

Exothermic reaction.

Question 22

What is contraction porosity?

Answer 22

Too much monomer is present within the mixture, this causes more polymerisation contraction shrinkage, leads to porous regions.

Question 23

Name a few causes of contraction porosity.

Answer 23

Too much monomer
Insufficient pressure during clamping
Insufficient excess material

Question 24

How does granular porosity occur?

Answer 24

Too little monomer

Question 25

Why is it not a good idea to use porcelain teeth on an acrylic denture base?

Answer 25

Acrylic has a higher thermal expansion co-efficient than porcelain. When heated, there will be gaps forming between the teeth and the denture base.

Question 26

Name some disadvantages of PMMA as a denture base.

Answer 26

Low thermal conductivity
Poor mechanical properties- low EM.

Question 27

Name some advantages of PMMA as a denture base.

Answer 27

Non toxic
Non irritant
Good colour
Thermal expansion fine if you use acrylic teeth
Low density
High softening temperature (75 degrees).
Fatigue strength.

Question 28

What is the purpose of the inhibitor?

Answer 28

Hydroquinone- reacts with any free radicals produced by heat.

Question 29

What is the difference between self-cure and heat-cured PMMA?

Answer 29

Self cured is activated by promoter (tertiary amine) in liquid.

Question 30

What is the advantage of self cure acrylics?

Answer 30

Lower temperature, so less thermal contraction and better dimensional accuracy.

Question 31

What is the disadvantage of self cure acrylics?

Answer 31

Chemical activation is less efficient
- there is more unreacted monomer, which can cause irritation, reduces strength.

Water absorption gives expansion- fits the cast better initially but then falls out when water is absorbed.

Question 32

What are light activated denture resins commonly used for?

Answer 32

Master impression trays, repair of fractured dentures.

Question 33

What is a disadvantage of light-cured denture resins?

Answer 33

Light may not penetrate to appropriate depth (depth of cure).

Question 34

What alternative materials can you use if the patient has a proven allergy to acrylic?

Answer 34

Nylon- swells with water
Vinyl polymers- requires injection moulding and melts at 60 degrees
Polycarbonates- requires injection moulding