Acrylic Materials Flashcards
(15 cards)
ideal properties
dimensionally stable/accurate
high softening temp
unaffected by oral fluids
high YM (elastic)
rigid (stiff)
high proportional/elastic limit [only large stress = perm deformation]
similar thermal expansion to artificial teeth
low density
high thermal conductivity
non-toxic
good aesthetics
components of heat cured acrylic
powder =
initiator - benzoyl peroxide
PMMA particles
plasticiser - fast dissolve
pigments
co-polymers
liquid =
MMA monomer
inhibitor - hydroquinone [prolongs shelf life]
co-polymers
setting reaction of PMMA
free radical addition polymerisation
activation; of initiator benzoyl peroxide, provides free radicals
initiation; free radicals break c=c in monomer, transfer free radical
propagation; growing polymer chain
termination
what heat is needed for activation of setting reaction
> =72 C*
ratio of powder to liquid
2.5P:1L weight
3.5P:1L volume
stages of setting of PMMA
sandy, tacky, dough, packing
what activates setting reaction in self-cure PMMA
tertiary amine
activates benzoyl peroxide
NOT HEAT
why is self-cure PMMA used
BUT what is disadv
low temp = low thermal contraction and higher dimensional accuracy
BUT
chemical activation less efficient
lower molecular weight = lower mechanical prop
disadvantages of self-cure PMMA vs heat-cure
more unreacted monomer - irritant, reduced transverse strength
oversized result due to water absorption in mouth
reduced colour stability
name faults
contraction porosity
granularity
gaseous porosity
crazing
undercuring
describe contraction porosity
too much monomer
describe granularity
too little monomer
describe gaseous porosity
cured/heated unequally
usually in bulkier parts
fast curing
describe consequence of undercuring
free monomer = irritant
low molecular weight = reduced mechanical properties
describe crazing
internal stresses due to fast cooling rate
