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BDS3 Dental Materials Science > Acrylic resin > Flashcards

Acrylic resin Flashcards

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1
Q

What is the function of a denture?

A
  • 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)
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2
Q

What are the ideal properties of a denture?

A
  • 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
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3
Q

What are the Mechanical properties of denture base?

A
  • High Young’s (Elastic) Modulus
  • High Proportional Limit
  • High Transverse Strength*
  • High Fatigue Strength
  • High Impact Strength
  • High Hardness / Abrasion Resistance
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4
Q

What can cause fractures of a denture?

A
  • Acrylic resin can fracture at pivot point (middle of palate and denture) as mastication forces applied both sides
  • Being dropped on the floor
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5
Q

What is free radical addition polymerisation?

A
  • 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
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6
Q

What are the stages of Acrylic polymerisation?

A

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

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7
Q

What is the composition of the powder in heat cured acrylic?

A
  • 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
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8
Q

What is the composition of the liquid in heat cured acrylic?

A

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

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9
Q

Why is it important there if efficient polymerisation?

A
  • Need efficient polymerisation to give high molecular weight polymer
  • Gives good mechanical properties
  • Therefore use high temp but gaseous porosity limits
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10
Q

What are acrylics actual properties?

A
  • 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
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11
Q

What is the softening temp of acrylic?

A
  • 75oC
  • This is ok for ingesting hot fluids
  • Don’t use boiling water for cleaning
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12
Q

When does PMMA expand?

A
  • 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%
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13
Q

What is self-curing acrylic?

A
  • Similar composition to heat cured PMMA
  • The benzoyl peroxide is activated by tertiary amine in liquid not heat
  • e.g. Dimethyl-para-toludine
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14
Q

Why use self cure acrylics over heat cured PMMA?

A
  • Polymerisation requires no heating stage so little thermal contraction
  • Leads to better dimensional accuracy and better fitting
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15
Q

What are the properties of Self curing acrylic?

A
  • 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
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16
Q

What percentage of unreacted monomer is present in self and heat cure PMMA?

A

Self - 3-5% unreacted monomer (risk of dimensional stability)

Heat - 0.2-0.5% unreacted monomer

17
Q

Is self cure or heat cure tolerated better by patients in regard to fit and why?

A
  • 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
18
Q

Does heat or self cure have higher molecular weight?

A
  • Heat cure has higher molecular weight
  • Stronger
  • But may cause porosity (need good technician to prevent this)
19
Q

Does heat or self cure have poorer colour stability?

A
  • Self cure poorer colour stability
  • Tertiary amines susceptible to oxidation
20
Q

What is an improved form of acrylic?

A
  • 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
21
Q

What is Ultra-Hi?

A
  • 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
22
Q

What are Pour n Cure resins?

A
  • Similar to self cure
  • Smaller powder particles to produce fluid mix
  • Fluid mix pour into mould
  • Has good fitting but poor mechanical properties
23
Q

What are light activated denture resins?

A
  • 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
24
Q

Why is it useful that denture base materials are radiopaque?

A
  • If any fragments break off and there is risk of being swallowed
  • Radiograph can confirm where contents are
25
Q

How are dentures made to be radiopaque?

A
  • 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
26
Q

What alternative polymers have been explored if patient has an allergy to acrylic?

A
  • 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)

BDS3 Dental Materials Science (10 decks)

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