PMMA Flashcards
What are the ideal properties of PMMA?
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
What are the ideal mechanical properties of PMMA?
High Youngs Modulus (elasticity)
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistant
Describe transverse strength in an acrylic denture?
3 point loading
Has to cope with forces applies from palate and teeth on both sides which could potentially cause fracture at pivot point
What is free radical addition polymerisation?
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
What are the stages of acrylic polymerisation?
Activation
Initiation
Propagation
Termination
What is the initiator in heat cured acrylic?
Benzoyl Peroxide 0.2-0.5%
What are the other components of heat cured acrylic powder?
PMMA particles
Plasticiser
Pigments
Co-polymers
What are the components of heat cured acrylic liquid?
Methacrylate monomer
Inhibitor - hydroquinone 0.006%
Co-polymers - play crucial role in enhancing the mechanical properties
Describe how heat cured acrylic is made?
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
What is needed to ensure acrylic heat curing is successful?
Need efficient polymerisation to give high molecular weight polymer with good mechanical properties
Why is acrylic used in bulk?
To compensate for the poor mechanical properties
What are the actual properties of acrylic/
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
Describe the dimensional accuracy of heat cured acrylic
0.5% linear contraction during manufacture
0.4% expansion during usage
Just about makes up for the contraction during heat curing process
How does self curing acrylic differ in polymerisation?
Same as heat cured except benzoyl peroxide is activated by tertiary amine in liquid - not the heat
Why are self cured acrylics used?
Lower temperature needed during polymerisation so less thermal contraction and better dimensional accuracy
What are the disadvantages of self cure acrylic activation?
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
Compare the unreacted monomer in self cure and heat cure acrylic
Chemical cure (self cure) - 3-5% unreacted monomer
Heat cure - 0.2-0.5% unreacted monomer
Heat curing process is more effective
Describe the dimensional accuracy of self cure acrylic and which acrylic is better tolerated
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
How does heat cure acrylic compare overall to self cure?
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
What are the different variations of acrylic?
Acrylic resin
Radiopaque polymers
Alternative polymers
What are the different types of acrylic resin?
Ultra-Hi
Pour n Cure resins
Light activated denture resins
Describe Ultra-Hi
A high impact heat cures acrylic resin
Has exceptional flexural strength and superior fracture toughness (ductility)
Give examples of alternative polymers
Nylons
Vinyl polymers
Polycarbonates
