denture base materials

• replaces function of natural teeth • appropriate aesthetics • dimensionally accurate and stable in use • high softening temperature • unaffected by oral fluids • no thermal expansion • low density • high thermal conductivity • radiopaque • non-toxic, non-irritant • accepted colour/translucency • easy and inexpensive to manufacture • easy to repair

• high Young’s modulus • high proportional limit • high transverse strength • high fatigue strength • high impact strength • high hardness/abrasion resistance

• flexural • 3 point loading • denture needs to be able to cope with impact

• the palatal - forces applied on either side, potentially causing fracture at the pivot point

• chemical union of 2 molecules either the same or different to form a larger molecule WITHOUT the elimination of smaller molecules • involves molecules with C=C bonds • acrylic resin undergoes this

• activation • initiation • propagation • termination

• of initiator to provide free radicals

• free radicals break C=C bond in monomer and transfer free radical

• growing polymer chain

• end of polymerisation

• initiator -> benzoyl peroxide 0.2-0.5% • PMMA particles -> pre-polymerised beads • plasticiser ->allows quicker dissolving in monomer liquid e.g. dibutyl phthalate • pigments -> to give ‘natural’ colour • co-polymers -> to improve mechanical properties e.g. ethylene glycol dimethacrylate

• methacrylate monomer • inhibitor (hydroquinone 0.006%) • co-polymers

• dissolves PMMA particles - polymerises

• prolongs shelf life - reacts with any free radicals produced by heat, UV light

• improv mechanical properties - particularly cross-linking of polymers

• contains mould material • artificial teeth are placed ready for acrylic resin • acrylic resin in dough-like form is inserted into the mould - takes up patients dentition • 2 halves of the vessel are clamped together ready to be subjected to the heating cycle required to cause polymerisation

• efficient polymerisation - gives high molecules wight polymer which gives good mechanical properties

• lots of cross linking of MMA molecules - essential

• high temperature but gaseous porosity

• non-toxic • non-irritant • unaffected by oral fluids • poor mechanical properties • fairly good fatigue strength/impact strength • high hardness and abrasion resistance - retains good polish but can wear over time • thermal expansion = artificial tooth if acrylic used, if porcelain then acrylic much higher • low thermal conductivity • low density = good as it is lightweight and comfortable for patient • softening temperature = 75 degrees, good for oral fluids but not for cleaning in boiling water • dimensionally accurate and stable in use = contraction 0.5%

• 0.5% contraction from manufacturing - 0.4% expansion from usage = through water absorption, almost makes up for contraction

• similar to heat cured - except benzoyl peroxide is activated by a promoter in the liquid

• tertiary amine - dimethyl-para-toluidine

• lower temperature • which means less thermal contraction • hence better dimensional accuracy