Acrylic resin

• 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)

• 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

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

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

• 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

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

• 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

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

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

• 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

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

• 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%

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

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

• 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

Self - 3-5% unreacted monomer (risk of dimensional stability) Heat - 0.2-0.5% unreacted monomer

• 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

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

• Self cure poorer colour stability • Tertiary amines susceptible to oxidation

• 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

• 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

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

• 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

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