9. PMMA Flashcards
Definition of PMMA
Polymethylmethacrylate – acrylic resin
Error sources of dentures (2)
Production
Usage
Denture production error sources (3)
Initial impression is flawed – material and method not 100% accurate
Curing process – several potential error sources
May not achieve perfect fit at first attempt, so refinements at the chairside may be required
Denture usage error sources (4)
Fits patient only for short period
Fractures and warping
Discomfort
Surface suffers wear
Component of stress-strain diagram
Shows the difference between the elastic limit and the proportional limit
Definition of elastic limit
The limit at which the material will return to its original shape if distorted
Effect of high YM
Rigid/stiff - large stress produces small strain
Effect of high proportional and elastic limits
Only large stresses will cause permanent deformation
Effect of high thermal expansion
If cooled incorrectly, susceptible to internal stresses that can lead to fracture
Definition of thermal conductivity
Transmission of thermal stimuli to mucosa
Effect of high thermal conductivity
Avoid scalding back of throat or oesophagus
Effect of low density
Aid retention of upper denture (the heavier the material, the greater the gravity pull)
Type of acrylic resin bonding
Free radical addition polymerisation of the methacrylate monomer
Definition of free radical addition polymerisation
The chemical union of two molecules, either the same or different, to form a larger molecule without the elimination of a smaller molecule
PMMA free radical addition polymerisation involves molecules with C=C bonds
Stages in acrylic polymerisation (4)
Activation
Initiation
Propagation
Termination
Stages in acrylic polymerisation activation (4)
Activation of initiator to provide free radicals
The initiator is usually benzoyl peroxide (C6H5COO-OHCH5C6 – symmetrical)
Acrylic resin can be heat activated (>72C) or self-cured
Activation gives two free radicals: R* and C6H5COO*
Stages in acrylic polymerisation initiation
Free radicals break C=C bond in monomer and transfer free radical
Stages in acrylic polymerisation propagation
Growing polymer chain
Stages in acrylic polymerisation termination
Termination of polymerisation
Composition of heat-cured acrylic (2)
Powder (initiator, RMMA particles, plasticiser, pigments, co-polymers)
Liquid (methacrylate monomer, inhibitor, co-polymers)
Function of plasticiser in PMMA powder
Allow quicker dissolving in monomer liquid (dibutyl phthalate)
Function of pigments in PMMA powder
To give natural colour
Function of co-polymers in PMMA powder
To improve mechanical properties (ethylene glycol dimethacrylate)
Function of methacrylate monomer in PMMA liquid
Dissolves PMMA particles (polymerises)
Function of inhibitor in PMMA liquid
Prolongs shelf-life – reacts with any free radicals produced by heat/UV light (0.006% hydroquinone)
Function of co-polymers in PMMA liquid
Improves mechanical properties, particularly cross-linking of polymers
Features of powder and liquid when mixed (5)
Produces a dough-like material that can be handled/mixed easily and customised to desired shape
Reduces heat of reaction
Minimises polymerisation shrinkage
Is mixed 3-3.5:1 (P/L) by volume (2.5:1 by weight)
Mixing stages: sandy (tacky) and dough (packing)
Requirements of heat curing of PMMA
Requires efficient polymerisation to give a high polymer molecular weight (good mechanical property)
PMMA heating schedules (3)
7hrs to 70C and 2hrs to 100C and slow cool
72C for 16hrs
20 - 20 - 20 reverse curing (place flask in boiling water, remove heat for 20mins, heat to 70C for 20mins, heat to 100C for 20mins)
Factors during mould and acrylic cooling (3)
Mould material and acrylic have different thermal expansion coefficients
Can lead to internal stresses
Slow cooling helps to relieve this
Effects of internal stress (4)
Decreased strength
Decreased fatigue strength
Warping (during finishing)
Repair problems
Internals stresses are related to (5)
Shape and size (notches) Curing pressure Curing cycle Thermal expansion Cooling rate
Effects of under-curing (2)
Free monomer (irritant) Low molecular weight (poor mechanical properties)
Effects of fast curing
Can potentially lead to gaseous porosity
Incorrect powder/monomer ratios (2)
Too much monomer
Too little monomer
Effect of too much monomer
Contraction porosity
Effect of too little monomer
Granularity
Effects of porosity (4)
Affects strength
Affects appearance
Rough sensation to tongue
Absorbs saliva – poor hygiene
Features of gaseous porosity
Occurs when curing temp. exceeds 100C
When this happens, monomers ‘boil’ and cause gaseous bubbles in the acrylic
Usually occurs in bulkier parts
Definition of contraction porosity
Voids due to polymerisation shrinkage where the acrylic ‘dough’ is not sufficiently packed
Polymerisation shrinkage of monomer alone
21% by volume
Polymerisation shrinkage of powder and monomer mix
7%
Causes of contraction porosity (3)
Too much monomer
Insufficient excess material
Insufficient clamp pressure
Ideal properties of acrylic resin (11)
Replace function of natural teeth
Non-toxic and non-irritant
Unaffected by oral fluids
Thermal expansion (artificial tooth)
High thermal conductivity
High mechanical properties (YM, proportional limit and elastic limit)
Colour/translucency
Low density
High softening temperature (must not distort during ingestion of hot fluids or during cleaning)
Dimensionally accurate and stable in use (must fit patient’s mouth and be retained)
Fits well in patient’s mouth
Is seen by other people (who should be unaware it is a prosthesis)
Actual irritation of acrylic resin
Provided no monomer is released but some patients are allergic
Actual effect of oral fluids of acrylic resin
Water absorption, virtually insoluble in fluids taken orally
Actual thermal expansion of acrylic resin
Adequate if acrylic teeth are used; significantly higher than porcelain teeth
Actual thermal conductivity of acrylic resin
Low, which is poor
Actual mechanical properties (YM, proportional limit and elastic limit) of acrylic resin
Low (poor) - compensated by increasing in bulk
Actual colour/translucency of acrylic resin
Good - similar to natural tissue
Actual density of acrylic resin
Low (good),but the need to increase in bulk to overcome poor mechanical properties offsets this advantage
Actual softening temperature of acrylic resin
75C - fine for ingested hot fluids, do not clean using boiling water
Actual dimensional accuracy and stability in use of acrylic resin
Linear contraction 0.5% – acceptable
