PMMA

it is a complex 3D customised object

based on patients feedback - they will either say it fits or it doesn’t fit

○ Initial impression is flawed § Material and method not 100% accurate § If these aren’t perfect then the denture cannot be perfect ○ Curing process § Several potential error sources ○ May not achieve perfect fit at first attempt § Refinements at the chairside may be required [Can make minor adjustments but the overall fit has to be close enough to the desired fit for this to work]

○ Fits patient only for short period ○ Fractures § During use § Accident ○ Uncomfortable ○ Becomes warped § The way the denture is handled ○ Surface suffers wear § Gets worn away or the surface becomes rough and becomes uncomfortable for the patient

• Unaffected by oral fluids • Replaces function of natural teeth • Is seen by other people • Dimensionally accurate and stable in use • Must fit patient’s mouth and be retained • Low density - aids retention of upper denture as the heavier the material, the greater the gravity pull • Colour / translucency matching natural tissues • Non toxic, non irritant

○ High young’s (elastic) modulus § Rigid (stiff) § Large stress produces small strain ○ High proportional limit / elastic limit - Only large stresses will cause permanent deformation

• High softening temperature (Tg) ○ Must not distort during ingesting of hot fluids or during cleaning ie won’t change with high temperatures • Thermal expansion ○ Must be the same as the artificial tooth § Avoid internal stresses on cooling during manufacture • High thermal conductivity ○ Transmission of thermal stimuli to mucosa to avoid scalding the back of throat or oesophagus

acrylic / polymethylmethacrylate (PMMA)

• Chemical union of 2 molecules either the same or different to form a larger molecule without the elimination of a smaller molecule • Involves molecules with (C=C bonds)

double bond is what we are looking to break down so it links to other molecules = so material becomes rigid, heavy molecule want cross linking happen

• Activation ○ Of initiator to provide free radicals • Initiation ○ Free radicals break C=C bond in monomer and transfer free radical • Propagation ○ Growing polymer chain • Termination ○ Of polymerisation ○ Polymerisation stage just stops

Benzoyl peroxide (NB symmetrical) C6H5COO-OOCH5C6

A) Heat > 72°C - Different heat curing cycles B) Self cured - Not in this lecture

○ Activation gives 2 free radicals R* § C6H5COO* § Breaking down benzoyl peroxide so it is split in the middle § Electrical charge

R* + M --> R-M* R-M* + M --> R-M-M* R-M-M* + M --> R-M-M-M* …. And so on Initial monomer grows and cross links with other molecules

○ Initiator § Benzoyl peroxide (0.2-0.5%) ○ PMMA Particles § Pre-polymerised beads § Speeds things up □ Introduce into the powder PMMA that has been produced before and then it is ground into little beads which then react with the liquid ○ Plasticiser § Allows quicker dissolving in monomer liquid § Eg dibutyl phthalate § Speeds up the process ○ 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 § Prevents any free radicals that might be produced inadvertently as the material is being stored somewhere § Could be triggered by heat or light § Stops it from reacting before it gets to the powder ○ 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 § Monomer on its own will shrink by 21% § Mix it with powder and it will shrink by 7% § Substantial amount of shrinkage during heat curing process

○ 3 to 3.5 / 1 § P / L by volume § Follow instructions for ratios ○ 2.5 / 1 § By weight

Sandy - tacky Dough - packing

• Metal vessel filled with mould material that is the shape of the patient’s mouth • Individual teeth placed within the mould material • Brush allows separation later in the process • Pack dough like material in - important to pack in • Allow excess material besides • Take flasks and clamp them together • Apply an appropriate pressure

• Need efficient polymerisation to give high molecular weight polymer ie good mechanical properties = Better the polymerisation then the better the material you are going to get Hence high temperature but gaseous porosity limits ○ Ideally want a high temperature ○ But if you have the wrong heat curing cycle / temperature then this will cause porosity

1) 7 hours to 70°C + 2 hours to 100°C + slow cool 2) 72°C for at least 16 hours - Simplest 3) 20-20-20 reverse curing - Place flask in boiling H2O - Remove heat for 20 minutes - Heat to 70°C for 20 minutes - Then heart to 100°C for 20 minutes Don’t be too concerned about the details of these

If temperature peaks above 100°C then gases are produced which lead to porosity which leads to a weak material Raise temp to 70°C - exothermic reaction of the polymerisation will raise it to 100°C Then it is dropped and 70°C is maintained Stay clear of exceeding 100°C within the acrylic