dental material science Flashcards
two types of curing
heat and light
why can acrylic teeth be bad
they can cause abrasion good because they can be bonded like for like
what is acrylic made. of
powder:
Beads or granules of poly-methyl-methacrylate
Initiator (benzoyl peroxide)
pigments/dyes
Optical pacifiers (TiO2/ZnO)
Plasticisers (ethyl acrylate, dibutyl phthalate)
Synthetic fibres (nylon)
liquid :
methyl methacrylate monomer Inhibitor (hydroquinone) Crosslinking agent (diethylene glycol dimethacrylate)
How is acrylic made:
polymerisation
activation - coveting from low to high molecular weight polymers .activator reacts with inhibitor in powder generating free radicals
initiation - using initiator like yielding free radicals setting off chain reaction. free radicals react with monomer to generate free radical monomers
propagation - formation of co-polymers and addition polymerisation reaction.these free radical mongers react with other to create free radical-polymer chains
termination -direct coping of free radicals.
adverse umma reactions
toxic irritants contact dermatitis ]allegic contact dermatitis oral lichennoid reactions anaphylactoid reactions contact urticaria
how to stop irritant contact dermatitis
gloves buffer creams not handling dough monomer - fully cure denture worse with cold cure resins alternative materials polycarbonate or nylon
disadvantages of pmma
- low strength and toughness
- susupeable to distortion
prone to discolouration
low thermal conductivity
radiolucent
denture cleaning problems
brush and paste - abrasion scratching and roughness and will break easily if dropped with scratches
soak- oxygenating cleaner can lead to change material properties
and water - boiling can cause colours problems denture distortion
porosity types
contraction porosity -insuffievent packaging into flask or inadequate flashing pressure
avoid- processing under pressure using dough holding technique
gaseous porosity - vaporising the monomer during processing
avoid; high processing temps
granular porosity- incorrect monomer :polymer ratio when producing dough or failing to pack the flask at dough stage
errors in packaging
pack at sandy stage
- high fluidity material that will run out of flask and cause porosity
packaging a rubbery stiff stage - material not flow so lack of denture material so fractured or moved teeth
advantages and disadvantage of PMMA
pros; excellent aesthetics /cheap/low dentistry
disadvantages; low strength and toughness/susceptible and distortion/discolouration/low thermal conductivity/radiolucent
heat cure acrylic stages
sandy/stringy/dough/rubbery/stiff
processing strains
warping, crazing,distortion
Avoid internal strain by: Slow cooling of the flask Use acrylic rather than porcelain teeth Do not overheat on polishing keep denture away from solvents
