PMMA and investment materials Flashcards
name 5 ideal properties of a denture base
high conductivity
high softening temperature
low density
non toxic/ irritant
unaffected by oral fluids
easy to repair
low thermal expansion
free radical addition polymeristation
Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule
(via C=C bond)
contents of heat cured acrylic powder
initiator
PMMA particles
plasticiser - allows quicker dissolving in liquid
pigments - give good colour
co - polymers - improve mechanical properties
contents of heat cured acrylic liquid
methacrylate monomer
inhibitor - extends shelf life
co polymers - improve mechanical properties
what are the 4 stages of acrylic polymerisation
- activation - of initiator to provide free radicals
- initiation - free radicals break C=C bonds in monomer
- propagation - growing polymer chain
- Termination - of polymerisation
what initiator is used in both heat cured and self cured acrylic
benzoyl peroxide
how is the initiator activated in self cure vs heat cure acrylic
heat cure - activated by heat
self cure - activated by a promoter (tertiary amine) present in the liquid
compare heat and self cure acrylic
- chemical activation
self cure has less efficient chemical activation meaning less efficient polymerisation resulting in lower molecular weight and decreased mechanical properties
compare heat and self cure acrylic
- unreacted monomer
self cure sees more unreacted monomer therefore an increased chance of irritation, a softer denture base and decreased transverse strength
compare heat and self cure acrylic
- polymerisation contraction
self cure acrylic sees no contraction during polymerisation so sees a better initial fit. However, expands due to water absorption in mouth
(heat cure acrylic has this expansion compensated for by polymerisation contraction)
what other denture base materials are available if a patient had an acrylic allergy
nylons (absorb water)
vinyl polymers (soften at low temperatures)
what is an investment material
material that is suitable for forming a mold into which a molten metal or alloy can be cast to make a indirect restoration
why do investment materials need to be able to expand
to compensate for the cooling shrinkage of alloys
why do investment materials need to be porous
to allow trapped gasses to escape
what is a sprue in relation to investment materials
a channel allowing the molten metal or alloy access to the mould of the indirect restoration
process of creating and using an investment material (lost wax technique)
- create wax pattern e.g of crown
- investment material poured around wax pattern and allowed to set to create a mold
- wax is then eliminated via boiling water or burning in oven
- molten alloy is then forced into mold cavity now vacated by wax via sprues
what are the two categories of components that make up an investment material powder
binder - forms coherent solid mass and determines what type of investment material it will be
refractory - withstands high temperatures and gives expansion
binder component of investment materials
forms coherent solid mass and determines what type of investment material, there are 3 available:
gypsum , phosphate or silica
refractory component of investment materials
withstand high temperatures and allow expansion
silica
green strength
strength at room temperature
what is investment material powder mixed with
water
shrinkage of gold and cobalt chrome when cooled
gold - 1.4%
CoCr - 2.3%
why can gypsum bonded investment material not be used for casting cobalt chrome
it is only capable of expanding 1.4% versus CoCr’s shrinkage of 2.3% - not compatible
can be used for gold
properties of gypsum bonded investment material
- expansion
- particle size
- ease of use
- porosity
- strength
- 1.4% expansion , not suitable for CoCr but can be used for gold
- fine particles which produce a good smooth surface
- easy to use
- good porosity
- adequate strength if produced correctly
