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.
What is reverse curing
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.
When is the polymerisation exotherm reached in curing methods
2 hr mark
What can cause internal stresses
Thermal Expansion
Cooling Rate
Shape and Size eg. Notches
Curing Pressure
Curing Cycle
How should acrylic be cooled
slowly to allow relief and keep internal stresses as low as possible
When might acrylic contain free monomers
when undercured
What can fast curing cause
gaseous porosity (possibly)
What would undercuring of a denture lead to
Free monomers present (irritant)
Low molecular weight
- poor mechanical properties
What affects with porosity have on a denture
Affects strength
Affects appearance
Rough sensation to tongue
Absorbs saliva - poor hygiene
What are the affects of having the incorrect poweder/monomer ratios
Too much monomer -
Contraction Porosity
Too little monomer -
Granularity
What type of reaction is curing of acrylic
Free radical addition polymerisation
Exothermic
Why can boiling water not be used to clean dentures
High Softening Temperature - 75 oC
What is the polymerisation shrinkage of the monomer
21%
When mixed what is the polymerisation shrinkage of acrylic
7%
What causes contraction porosity
too much monomer
insufficient excess material
insufficient clamp pressure
Where will contraction porosity occur
Anywhere where dough is not sufficiently packed
What are the actual properties of acrylic
Non Toxic, - yes;
Non Irritant
- provided no monomer released
- but, some patients allergic
Unaffected by Oral Fluids
water absorption, virtually insoluble in fluids taken orally
Colour / Translucency - good
Thermal Expansion = Artificial tooth
OK if acrylic teeth used, significantly higher than porcelain teeth
Acrylic resin:- 80 ppm/°C
(enamel: 11.4ppm/°C; composite resin: 15 to 40 ppm/°C)
High Thermal Conductivity
low – poor
Low Density
low (good)
-BUT the need to increase its BULK - to overcome poor mechanical properties - offsets this advantage
High Softening Temperature - 75 oC
-OK for ingested hot fluids
-DON’T use boiling water for cleaning
Dimensionally accurate & stable in use - OK
- Linear Contraction 0.5% - Accepta
