PMMA

• 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 • • Dimensionally accurate and stable in use • High Softening Temperature (Tg) (not distort during ingesting of hot fluids) • Unaffected by Oral Fluids • Thermal expansion similar to artifical teeth to avoid internal stresses • High thermal conductivity (keep patient safe) • Low density to aid retention (gravity)

Acrylic / Polymethylmethacrylate (PMMA)

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 (benzoyl peroxide/camphorquoine/ lucerin) to provide 2 free radicals • Initiation - free radicals break C=C bond in monomer and transfer free radicals • Propagation - growing polymer chain • Termination - of polymerisation

heat >72 c self cured

benzoyl peroxide

• Initiator (benzyol peroxide - provides free radicals) • 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

3/3.5 to 1 to produce a dough-like material that can be easily handled

must be done in a way that causes efficient polymerisation – as this will yield a high molecular weight polymer – and that means good mechanical properties, such as mechanical strength.

Flask of ACRYLIC RESIN placed in water bath:- (a) at 70oC for 7hrs.100oC for 2hrs. Slow Cool (b) at 100 oC for several hours

GASEOUS POROSITY is avoided (or minimised)

due to the exothermic polymerisation reaction it undergoes after about 2hrs– this cycle would produce material with many gaps.

If cooled too quickly this could lead to internal stresses within the acrylic resin – which would weaken it. cooling should be done slowly to minimise internal stresses.

where low level stresses applied repeatedly over a long time, cause fracturing

Undercuring, free monomers will remain, thus leading to a lower molecular weight and poorer mechanical properties. Overheating curing, potentially causing gaseous porosity as temperatures reached would be too high Incorrect powder/liquid monomer ratios, leading to contraction porosity (too much) or granularity (too little). Porosity in general, affecting strength, appearance, and hygiene. (due to overheating or curing too quickly) Internal stresses due to different thermal expansion, coefficients of the mold material and acrylic, as well as cooling rate, shape/size, curing pressure, and curing cycle.

Reach 100 oC, it boils, producing gas bubbles close to the acrylic resin surface. This flaw can be avoided by curing slowly. This is the only way the contribution of the heat produced by its exothermic reaction can prevent the resin’s overall temperature getting out of control.

at the bulkier parts

polymerisation shrinkage the monomer, which even when mixed with the powder, will still shrink 7% the monomer would shrink on its own (21%).

– too much monomer – insufficient excess material – insufficient clamp pressure

Occurs anywhere where dough is not sufficiently packed

If acrylic teeth are used, then the denture base and artificial teeth will expand & contract equally – so there will be no issues. However if porcelain teeth were used then there would be a mismatch, with acrylic expanding much more. This could lead to gaps at the interface.

acrylic resin higher than composite, porcelain and enamel

low/ poor patient’s may struggle in identifying hot drinks