Acrylic resin Flashcards
What is the function of a denture?
- Replaces function of natural teeth
- Goes into patient’s mouth
- Is seen by other people
- Has to give value for money (either NHS or patient)
What are the ideal properties of a denture?
- Dimensionally accurate and stable in use
- High softening temp (Tg)
- Unaffected by oral fluids
- Thermal expansion
- Low density
- High thermal conductivity
- Radiopaque (in case denture fractures)
- Non toxic, non irritant
- Colour/translucency
- Easy and inexpensive to manufacture
- Easy to repair
What are the Mechanical properties of denture base?
- High Young’s (Elastic) Modulus
- High Proportional Limit
- High Transverse Strength*
- High Fatigue Strength
- High Impact Strength
- High Hardness / Abrasion Resistance
What can cause fractures of a denture?
- Acrylic resin can fracture at pivot point (middle of palate and denture) as mastication forces applied both sides
- Being dropped on the floor
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 molecules with C=C bonds
What are the stages of Acrylic polymerisation?
Activation - Of initiator to provide free radicals
Initiation - Free radicals break C=C bond in monomer and transfer free radical
Propagation - Growing polymer chains
Termination - of polymerisation
What is the composition of the powder in heat cured acrylic?
- Initiator (Benzoyl Peroxide, 0.2 - 0.5%)
- 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
What is the composition of the liquid in heat cured acrylic?
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
Why is it important there if efficient polymerisation?
- Need efficient polymerisation to give high molecular weight polymer
- Gives good mechanical properties
- Therefore use high temp but gaseous porosity limits
What are acrylics actual properties?
- Non toxic
- Non irritant if it is fully polymerised
- Unaffected by oral fluids as it is insoluble in them
- Mechanical properties are poor so increase in bulk to compensate
- Fatigue Strength / Impact strength is “fairly resistant” but can be cause of failure
- High Hardness / Abrasion Resistance so retains good polish, some wear over time
- Thermal Expansion = Artificial tooth
OK if acrylic teeth used, significantly higher than porcelain teeth - Low thermal conductivity which is poor
- Low density
- Ok dimensionally stable and accurate
What is the softening temp of acrylic?
- 75oC
- This is ok for ingesting hot fluids
- Don’t use boiling water for cleaning
When does PMMA expand?
- Expands through water absorption during usage about 0.4%
- This just makes up for contraction that took place during the heat-curing stage 0.5%
What is self-curing acrylic?
- Similar composition to heat cured PMMA
- The benzoyl peroxide is activated by tertiary amine in liquid not heat
- e.g. Dimethyl-para-toludine
Why use self cure acrylics over heat cured PMMA?
- Polymerisation requires no heating stage so little thermal contraction
- Leads to better dimensional accuracy and better fitting
What are the properties of Self curing acrylic?
- The chemical activation is less efficient (less efficient polymerisation)
- So has lower molecular weight and poorer mechanical properties
- More unreacted monomer
- This acts as plasticiser, softening denture base, reducing transverse strength
- Has potential tissue irritant which compromises biocompatibility