PMMA

Question 1

What are the ideal mechanical and thermal properties of a denture material?

Answer 1

• High Young’s (Elastic) Modulus – rigid (stiff)
  – ie applying a large stress produces small strain
• High Proportional Limit / Elastic Limit
  – only large stresses will cause permanent deformation
• • Dimensionally accurate and stable in use
• High Softening Temperature (Tg) (not distort during ingesting of hot fluids)
• Unaffected by Oral Fluids
• Thermal expansion similar to artifical teeth to avoid internal stresses
• High thermal conductivity (keep patient safe)
• Low density to aid retention (gravity)

Question 2

What material is used for denture bases?

Answer 2

Acrylic / Polymethylmethacrylate (PMMA)

Question 3

What is free radical addition polymerisation and what type of molecule does it involve?

Answer 3

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)

Question 4

What are the steps of acrylic polymerisation?

Answer 4

• Activation - of initiator (benzoyl peroxide/camphorquoine/ lucerin) to provide 2 free radicals
• Initiation - free radicals break C=C bond in monomer
  and transfer free radicals
• Propagation - growing polymer chain
• Termination - of polymerisation

Question 5

What are the two methods of intiator activation in PMMA?

Answer 5

heat >72 c
self cured

Question 6

What is the initiator molecule that provides the free radicals?

Answer 6

benzoyl peroxide

Question 7

What are the components of heat cured powder acrylic? What do they do?

Answer 7

• Initiator (benzyol peroxide - provides free radicals)
• PMMA Particles – pre-polymerised beads
• Plasticiser - allows quicker dissolving in monomer liquid eg dibutyl phthalate
• Pigments – to give “natural” colour
• Co-polymers - to improve mechanical properties eg ethylene glycol dimethacrylate

Question 8

What are the components of heat cured liquid acrylic? What does each component do?

Answer 8

• Methacrylate Monomer
  – dissolves PMMA particles – polymerises
• Inhibitor (Hydroquinone, 0.006%)
  – prolongs shelf life - reacts with any free radicals produced by heat, UV light
• Co-polymers
  – improve mechanical properties - particularly cross-linking of polymers

Question 9

What is the proportion of powder to liquid?

Answer 9

3/3.5 to 1

to produce a dough-like material that can be easily handled

Question 10

What does heat curing need in order to provide material with good mechanical properties?

Answer 10

must be done in a way that causes efficient polymerisation – as this will yield a high molecular weight polymer – and that means good mechanical properties, such as mechanical strength.

Question 11

What are the two heat curing cycles of the water bath? A vs B

Answer 11

Flask of ACRYLIC RESIN placed in water bath:-
(a) at 70oC for 7hrs.100oC for 2hrs. Slow Cool
(b) at 100 oC for several hours

Question 12

Why does the monomer not exceed 100 degrees in cycle A?

Answer 12

GASEOUS POROSITY is avoided (or minimised)

Question 13

Why does the monomer exceed 100 degrees in cycle B?

Answer 13

due to the exothermic polymerisation reaction it undergoes after about 2hrs– this cycle would produce material with many gaps.

Question 14

Why should cooling be done slowly?

Answer 14

If cooled too quickly this could lead to internal stresses within the acrylic resin – which would weaken it.

cooling should be done slowly to minimise internal stresses.

Question 15

What is fatigue strength?

Answer 15

where low level stresses applied repeatedly over a long time, cause fracturing

Question 16

What are potential errors that occur during curing and what do they cause?

Answer 16

Undercuring, free monomers will remain, thus leading to a lower molecular weight and poorer mechanical properties.

Overheating curing, potentially causing gaseous porosity as temperatures reached would be too high

Incorrect powder/liquid monomer ratios, leading to contraction porosity (too much) or granularity (too little).

Porosity in general, affecting strength, appearance, and hygiene. (due to overheating or curing too quickly)

Internal stresses due to different thermal expansion, coefficients of the mold material and acrylic, as well as cooling rate, shape/size, curing pressure, and curing cycle.

Question 17

How does gaseous porosity happen, what happens and how can it be avoided?

Answer 17

Reach 100 oC, it boils, producing gas bubbles close to the acrylic resin surface.

This flaw can be avoided by curing slowly. This is the only way the contribution of the heat produced by its exothermic reaction can prevent the resin’s overall temperature getting out of control.

Question 18

Where is gaseous porosity more likely to be visible in the material?

Answer 18

at the bulkier parts

Question 19

What is contraction porosity?

Answer 19

polymerisation shrinkage
the monomer, which even when mixed with the powder, will still shrink 7%

the monomer would shrink on its own (21%).

Question 20

What are the causes of contraction porosity?

Answer 20

– too much monomer
– insufficient excess material
– insufficient clamp pressure

Question 21

Where in the material does contraction porosity occur?

Answer 21

Occurs anywhere where dough is not sufficiently packed

Question 22

When would there be a thermal expansion mismatch?

between teeth and base

Answer 22

If acrylic teeth are used, then the denture base and artificial teeth will expand & contract equally – so there will be no issues.

However if porcelain teeth were used then there would be a mismatch, with acrylic expanding much more. This could lead to gaps at the interface.

Question 23

What dental material has a high thermal expansion?

Answer 23

acrylic resin
higher than composite, porcelain and enamel

Question 24

What is the thermal conductivity of acrylic resin?

Answer 24

low/ poor
patient’s may struggle in identifying hot drinks

Question 25

What is used to offset the poor mechanical properties of acrylic?

Answer 25

increase in bulk to compensate

Question 26

How is the hardness of acrylic and what does that mean?

Answer 26

The surface of acrylic resin is much softer than composite resin, - with a hardness of just 20 Vickers hardness units. This means it will experience much more abrasive wear than a composite resin.

Question 27

What is the density properties of acrylic?

Answer 27

– low (good)
– BUT the need to increase its BULK - to overcome poor mechanical properties - offsets this advantage

Question 28

At what temperature does acrylic soften and what should be avoided?

Answer 28

75 oC
– OK for ingested hot fluids
– DON’T use boiling water for cleaning

Question 29

How is the stability of acrylic?

Answer 29

Dimensionally accurate & stable in use
- Linear Contraction 0.5%
- Acceptable