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
What percentage of unreacted monomer is present in self and heat cure PMMA?
Self - 3-5% unreacted monomer (risk of dimensional stability)
Heat - 0.2-0.5% unreacted monomer
Is self cure or heat cure tolerated better by patients in regard to fit and why?
- Heat cure better tolerated
- Self cure undergoes less thermal contraction during polymerisation
- Initially gives better fit but water absorption in mouth gives expansion
- Self cure ends up over sized and will fall out
- Heat cure is undersized but tolerated better
Does heat or self cure have higher molecular weight?
- Heat cure has higher molecular weight
- Stronger
- But may cause porosity (need good technician to prevent this)
Does heat or self cure have poorer colour stability?
- Self cure poorer colour stability
- Tertiary amines susceptible to oxidation
What is an improved form of acrylic?
- Attempts to strengthen acrylic resin denture and have poor strength and 10% fracture within 3 years
- One type incorporated rubber toughening agent to stop cracks forming in denture bases when experiencing impact
- Another incorporated fibres to strengthen but these materials difficult to manufacture
What is Ultra-Hi?
- High impact heat cure acrylic resin
- Used in GDH
- Has exceptional flexural strength
- Has superior fracture toughness (ductility)
- Has slight bending aspect that keeps material from being brittle and subject to cracking/breaking
What are Pour n Cure resins?
- Similar to self cure
- Smaller powder particles to produce fluid mix
- Fluid mix pour into mould
- Has good fitting but poor mechanical properties
What are light activated denture resins?
- Developed using urethane dimethacrylate matrix plus acrylic copolymers
- Use microfine silica fillers to control rheology (flow of material during manufacture)
- Use photoinitiators
- Adapted to cast and is light cured
- Used in mostly customised impression tray material and for repair of fractured dentures
Why is it useful that denture base materials are radiopaque?
- If any fragments break off and there is risk of being swallowed
- Radiograph can confirm where contents are
How are dentures made to be radiopaque?
- Use metal inserts but these weaken the base and give poor aesthetics
- Barium sulphate has been added but a low conc mean denture not radiopaque and a high conc gives a weak base
- Comonomers containing heavy metals like barium sulphate give poor mechanical properties
What alternative polymers have been explored if patient has an allergy to acrylic?
- Nylons (absorbs water and causes denture to swell and soften)
- Vinyl polymers (soften at temp of 60oC and require injection moulding which is expensive)
- Polycarbonates (use injection moulding process but internal stresses during use cause distortion and are a poor fit)
