PMMA Flashcards
What are the ideal properties of PMMA
Dimensionally accurate and stable in use
-must fit patient’s mouth AND be retained
High Softening Temperature (Tg)
-must not distort during ingesting of hot fluids, or during cleaning
Unaffected by Oral Fluids
-over time
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
Thermal Expansion
= Artificial tooth
-avoid internal stresses on cooling during manufacture (more on internal stresses in Metals & Alloys)
High Thermal Conductivity
-transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus
Low Density
-aid retention of upper denture
(the heavier the material, greater the gravity pull)
Colour / Translucency
-to match natural tissues
Non Toxic, Non Irritant
What is the definition of free radical addition polymerisation
“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)
What monomer undergos free redical addition polymerisation
Methacrylate monomer
ch3
|
ch2 = c
|
cooch3
What are the steps to acrylic polymerisation
Activation - of initiator to provide free radicals
Initiation - free radicals break C=C bond in monomer and transfer free radicals
Propagation - growing polymer chain
Termination - of polymerisation
How many free radicals does activation make
2
What is the name of the initiator
Benzoyl peroxide (symmetrical)
What does acrylic propagation look like
R* + M ——-> R-M*
R-M* + M --------> R-M-M*
R-M-M* + M ——–> R-M-M-M*
What are the components of the powder part of heat cured acrylic
Initiator (Benzoyl Peroxide, 0.2 - 0.5%)
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 dimet
What components does the liquid part of heat cured acrylic have
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
What are the advantages of using a powder liquid mix acrylic
to produce dough-like material that can be handled/mixed easily and customised to desired shape
reduce heat of reaction
minimise polymerisation shrinkage
When making up a heat cure acrylic from powder and liquid what proportions of each should be used
3 to 3.5 / 1 P/L by volume
(2.5 / 1 by weight)
When is the mix ready for packing into the shape
When is it dough like (not sandy - tacky)
What could too high a curing temperature lead to
Porosity within the denture due to gas produced when curing
What does a sufficiently high curing temperature ensure
An efficient polymerisation to give high molecular weight polymer
ie good mechanical properties
What are some exampe heating schedules
1) 7 hrs. to 70o C. + 2 hrs. to 100o C. + Slow Cool
2) 72o C for at least 16 hrs.
3) 20 - 20 - 20 Reverse Curing
Place flask in boiling H2O
Remove heat for 20 minutes.
Heat to 70o C. for 20 minutes.
Then heat to 100o C. for 20 minutes.
