PMMA

Dimensionally accurate and stable in use​ -must fit patient’s mouth AND be retained ​ ​ High Softening Temperature (Tg)​ -must not distort during ingesting of hot fluids, or during cleaning​ ​ Unaffected by Oral Fluids​ -over time High Young’s (Elastic) Modulus​ -rigid (stiff) ​ -ie applying a large stress produces small strain​ ​ High Proportional Limit / Elastic Limit​ -only large stresses will cause permanent deformation Thermal Expansion ​ = Artificial tooth​ -avoid internal stresses on cooling during manufacture (more on internal stresses in Metals & Alloys)​ ​ High Thermal Conductivity​ -transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus Low Density ​ -aid retention of upper denture ​ (the heavier the material, greater the gravity pull)​ ​ Colour / Translucency​ -to match natural tissues​ ​ Non Toxic, Non Irritant

“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)

Methacrylate monomer ch3 | ch2 = c | cooch3

Activation - of initiator to provide free radicals​ Initiation - free radicals break C=C bond in monomer and transfer free radicals​ Propagation - growing polymer chain​ Termination - of polymerisation

2

Benzoyl peroxide (symmetrical)

R* + M ——-> R-M*​ R-M* + M --------> R-M-M*​ R-M-M* + M ——–> R-M-M-M*

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 dimet

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

to produce dough-like material that can be handled/mixed easily and customised to desired shape​ reduce heat of reaction​ minimise polymerisation shrinkage

3 to 3.5 / 1 P/L by volume (2.5 / 1 by weight)

When is it dough like (not sandy - tacky)

Porosity within the denture due to gas produced when curing

An efficient polymerisation to give high molecular weight polymer​ ie good mechanical properties​

1) 7 hrs. to 70o C. + 2 hrs. to 100o C. + Slow Cool​ ​ 2) 72o C for at least 16 hrs.​ ​ 3) 20 - 20 - 20 Reverse Curing​ Place flask in boiling H2O ​ Remove heat for 20 minutes. ​ Heat to 70o C. for 20 minutes. ​ Then heat to 100o C. for 20 minutes.​

20 - 20 - 20 Reverse Curing​ -Place flask in boiling H2O ​ -Remove heat for 20 minutes. ​ -Heat to 70o C. for 20 minutes. ​ -Then heat to 100o C. for 20 minutes.

2 hr mark

Thermal Expansion​ Cooling Rate ​ Shape and Size eg. Notches​ Curing Pressure​ Curing Cycle

slowly to allow relief and keep internal stresses as low as possible

when undercured

gaseous porosity (possibly)

Free monomers present (irritant)​ Low molecular weight​ - poor mechanical properties​

Affects strength​ ​ Affects appearance​ ​ Rough sensation to tongue​ ​ Absorbs saliva - poor hygiene

Too much monomer -​ Contraction Porosity​ Too little monomer - ​ Granularity