Polymethyl methacrylate
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what are the ideal properties of PMMA
high thermal conduction - can feel temp of food high YM and EL - rigid low density - so it can be thin good aesthetics high softening temperature - doesnt distort
what are the actual properties of PMMA
poor thermal conduction, low YM and EL - have to increase density to gain mechanical properties, softening temp is okay for food but not for cleaning - boiling water will distort, good aesthetics especially if used with acrylic teeth, dimensionally stable and accurate
what is the reaction PMMA undergoes for production
free radical addition polymerisation
what is the monomer used in the reaction
methacrylate monomer
what is used in the production of PMMA
liquid - methacrylate monomer powder - benzyl peroxide, PMMA particles, plasticer, pigments
what are the stages required in production of PMMA
initiator, activation, polymerisation, termination
what happens at the initiator stage
benzyl peroxide - this produces free radicals upon heat activation to result in free radical addition polymerisation
what happens at the activation stage
heat activates the breaking of benzyl peroxide to release free radicals
what happens at the polymerisation stage
free radicals break C=C in methacrylate monomer, this transfer the free radical to the monomer, as the monomer binds to other monomers, the free radical moves along to the next monomer, allows for polymerisation
what happens if the material is over heated
cannot go above 100 degrees, if it does, results in gaseous porosity - affects strength and appearance, roughened surface can be sore on tough and plaque trap
what happens if the material is cooled too quickly
results in internal stresses, over time, material cannot withstand stress and will eventually fracture
why is it important to get the correct amount of monomer added
too much - polymerisation shrinkage not enough - reduced mechanical strength