PMMA

Question 1

What are the ideal properties of PMMA?

Answer 1

Dimensionally accurate and stable
High softening temp
Unaffected by oral fluids
Low density
High thermal conductivity
Thermal expansion equal to artificial tooth
Radiopaque
Non toxic, non irritant
Acceptable colour and translucency
Inexpensive
Easy to repair

Question 2

What are the ideal mechanical properties of PMMA?

Answer 2

High Youngs Modulus (elasticity)
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistant

Question 3

Describe transverse strength in an acrylic denture?

Answer 3

3 point loading
Has to cope with forces applies from palate and teeth on both sides which could potentially cause fracture at pivot point

Question 4

What is free radical addition polymerisation?

Answer 4

Chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
Involves C=C bonds

Question 5

What are the stages of acrylic polymerisation?

Answer 5

Activation
Initiation
Propagation
Termination

Question 6

What is the initiator in heat cured acrylic?

Answer 6

Benzoyl Peroxide 0.2-0.5%

Question 7

What are the other components of heat cured acrylic powder?

Answer 7

PMMA particles
Plasticiser
Pigments
Co-polymers

Question 8

What are the components of heat cured acrylic liquid?

Answer 8

Methacrylate monomer
Inhibitor - hydroquinone 0.006%
Co-polymers - play crucial role in enhancing the mechanical properties

Question 9

Describe how heat cured acrylic is made?

Answer 9

Vessel contains mould
Artificial teeth placed ready for acrylic resin in dough like form to be packed into place
Acrylic needs to be cured to form a strong solid denture base
Two halves of vessel are clamped together ready to be subjected to the heating cycle required to cause polymerisation

Question 10

What is needed to ensure acrylic heat curing is successful?

Answer 10

Need efficient polymerisation to give high molecular weight polymer with good mechanical properties

Question 11

Why is acrylic used in bulk?

Answer 11

To compensate for the poor mechanical properties

Question 12

What are the actual properties of acrylic/

Answer 12

Reasonable impact/fatigue strength
High hardness/abrasion resistance
Thermal expansion = artificial tooth
High thermal conductivity
Low density
High softening temp
Dimensionally accurate and stable in use

Question 13

Describe the dimensional accuracy of heat cured acrylic

Answer 13

0.5% linear contraction during manufacture
0.4% expansion during usage
Just about makes up for the contraction during heat curing process

Question 14

How does self curing acrylic differ in polymerisation?

Answer 14

Same as heat cured except benzoyl peroxide is activated by tertiary amine in liquid - not the heat

Question 15

Why are self cured acrylics used?

Answer 15

Lower temperature needed during polymerisation so less thermal contraction and better dimensional accuracy

Question 16

What are the disadvantages of self cure acrylic activation?

Answer 16

Less efficient than heat cured as it yields less
Hence lower molecular weight and poorer mechanical properties and softening temp
Hence more unreacted monomer which softens the denture base reducing transverse strength and is a potential tissue irritant

Question 17

Compare the unreacted monomer in self cure and heat cure acrylic

Answer 17

Chemical cure (self cure) - 3-5% unreacted monomer
Heat cure - 0.2-0.5% unreacted monomer

Heat curing process is more effective

Question 18

Describe the dimensional accuracy of self cure acrylic and which acrylic is better tolerated

Answer 18

Fits original cast better than heat cured but then water absorption gives expansion
Hence self cure is over sized, heat cured is under sized but better tolerated

Question 19

How does heat cure acrylic compare overall to self cure?

Answer 19

HC - higher molecular weight so stronger but curing process may cause porosity
SC - higher monomer levels which is an irritant, fits cast better but water absorption makes it oversized, poorer colour stability from oxidation of tertiary amines

Question 20

What are the different variations of acrylic?

Answer 20

Acrylic resin
Radiopaque polymers
Alternative polymers

Question 21

What are the different types of acrylic resin?

Answer 21

Ultra-Hi
Pour n Cure resins
Light activated denture resins

Question 22

Describe Ultra-Hi

Answer 22

A high impact heat cures acrylic resin
Has exceptional flexural strength and superior fracture toughness (ductility)

Question 23

Give examples of alternative polymers

Answer 23

Nylons
Vinyl polymers
Polycarbonates