PMMA

Question 1

State common error sources in dentures

Answer 1

1. Production
   - initial impression flawed so material + method is not 100% accurate
2. Curing process
   - refinements at chair side required
3. Usage
   - Pt may agree denture fits initially then overtime experience problems

Question 2

What denture problems can occur for the patient over time? (4)

Answer 2

1. Uncomfortable
2. Fractures
3. Becomes warped
4. Surfaces suffer wear

Question 3

State 3 ideal properties of a denture base

Answer 3

1. Dimensionally accurate + stable in use
   - Must fit in pt’s mouth and be retained
2. High softening temp (Tg)
   - Must not distort during ingestion of hot fluids/cleaning
3. Unaffected by oral fluids

Question 4

What does the elastic limit represent on the stress strain diagram?

Answer 4

The stress beyond which a material cannot reform to its original shape

PERMANENTLY DEFORMED (even after releasing the stress factor)

Question 5

Ideal mechanical properties of a denture base (2)

Answer 5

1. High YM
   - Large stress produces small strain
2. High elastic limit
   - Only large stresses will cause permanent deformation

Question 6

Define free radical polymerisation

Answer 6

The chemical union of 2 molecules, either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule

Involves molecules with C=C bonds

Question 7

What are the 4 steps in acrylic polymerisation

Answer 7

1. Activation
2. Initiation
3. Propagation
4. termination

Question 8

Describe events at ACTIVATION stage of acrylic polymerisation

Answer 8

Activation of initiator to produce 2 free radicals

Question 9

Describe events at INITIATION stage of acrylic polymerisation

Answer 9

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

Question 10

Describe events at PROPAGATION stage of acrylic polymerisation

Answer 10

Growing polymer chain

Question 11

Describe events at TERMINATION stage of acrylic polymerisation

Answer 11

Termination of polymerisation occurs

Question 12

Ideal thermal properties of denture base (2)

Answer 12

1. Thermal expansion
   - Avoids internal stresses on cooling during manufacture
2. High thermal conductivity
   - Transmission of thermal stimuli to mucosa

Question 13

Name the initiator for polymerisation

Answer 13

Benzoyl peroxide

Question 14

What makes up the heat cured acrylic POWDER (5)

Answer 14

1. Initiator
   - Benzoyl peroxide 0.2-0.5%
2. PMMA particles
   - Pre polymerised beads
3. Plasticiser
   - Allows quicker dissolving in monomer liquid
4. Pigments
   - Give natural colour
5. Co-polymers
   - To improve mechanical properties

Question 15

What makes up the heat cured acrylic LIQUID (3)

Answer 15

1. Methacrylate monomer
   - Dissolves pMMA particles
2. Inhibitor (Hydroquinone)
   - Prolongs shelf life as it reacts with any free radicals produced by heat/UV light
3. Co-polymers
   - Improve mechanical properties, particularly cross-linking of polymers

Question 16

Describe the important of the powder/liquid mix (3)

Answer 16

1. Forms a dough like material that can be handled/mixed easily and customised to a desired shape
2. Minimises polymerisation
   shrinkage
3. Reduces heat of reaction

Question 17

State the proportions for powder/liquid mix

Answer 17

3 to 3.5/1

Question 18

What can exceptionally high temperatures lead to?

Answer 18

Gaseous porosity within the acrylic weakens the material

Question 19

What are internal stresses of the denture base related to? (5)

Answer 19

1. Shape and size e.g. notches
2. Curing pressure
3. Curing cycle
4. Thermal expansion
5. Cooling rate

Question 20

What’s the effect of under curing the base?

Answer 20

• Free monomer (irritant)
• Low molecular weight
• POOR mechanical properties

Question 21

What’s the effect of fast curing the base?

Answer 21

• Creates bubbles within the material, which increase gaseous porosity and weaken the material

Question 22

What’s the effect of too much monomer?

Answer 22

Contraction porosity

Question 23

What’s the effect of too little monomer?

Answer 23

Granularity

Question 24

What does porosity affect?

Answer 24

1. Strength
2. Affects appearance
3. Rough sensation to tongue
4. Absorbs saliva - poor hygiene

Question 25

What areas of the denture is porosity more present in?

Answer 25

Bulkier parts of the denture

Question 26

Compare polymerisation shrinkage for just monomer / powder+ momomermix ? (2)

Answer 26

1. Monomer alone = 21% (by volume) | 2. Powder + Monomer mix = 7%

Question 27

State the causes of contraction porosity (3)

Answer 27

1. Too much monomer 2. Insufficient excess material 3. Insufficient clamp pressure

Question 28

Where does contraction porosity occur?

Answer 28

Anywhere where the dough is NOT sufficiently packed

Question 29

How do internal stresses lead to repair problems?

Answer 29

> Decreased strength > Decreased fatigue strength > Warping e.g. during finishing > Repair problems

Question 30

Ideal properties for patient comfort and aesthetics (3)

Answer 30

1. Low density - Aids retention of upper denture (the heavier the material, the greater the gravity pull) 2. Colour/translucency - To match natural tissues 3. Non toxic/ non irritant

Question 31

Looking at the acrylic temp changes graph, what do temperatures over 100 lead to?

Answer 31

Gaseous porosity - bubbles that weaken the material

Question 32

Purpose of efficient polymerisation?

Answer 32

Gives high molecular weight polymer | Good mechanical properties