PMMA (acrylic resin) Flashcards
what usage errors can cause a denture to fail?
- fractures
- uncomfortable
- becomes warped
- surface suffers wear
- fits patient only for short period
what are the ideal properties of PMMA?
- dimensionally accurate and stable in use
- high softening temperature
- unaffected by oral fluids
what does stress/strain measure?
determines a materials stiffness
what determines the stress/strain of a material?
Young’s (elastic) Modulus
what are the ideal MECHANICAL properties of PMMA?
- high Young’s (elastic) modulus
* high proportional limit/elastic limit
what is meant by a high Young’s (elastic) modulus?
- material is rigid (stiff)
* applying a large stress produces a small strain !
what is meant by a high proportional limit/elastic limit?
• only large stresses will cause permanent deformation
what should the thermal expansion of PMMA match & why?
the artificial teeth to avoid internal stresses on cooling during manufacture
why is it important that a denture has a low density?
to aid retention of upper denture
how long has PMMA been around?
since about 1937
what type of setting reaction does PMMA undergo?
a free radical addition polymerisation
what is meant by a free radical addition polymerisation reaction?
chemical union of two molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecules
what type of bonding is seen in a free radical addition polymerisation reaction?
involves molecules with carbon to carbon double bonds (C=C)
how many stages are their in the acrylic polymerisation process?
4 stages
what are the stages in the polymerisation of acrylic?
- activation
- initiation
- propagation
- termination
what is the activation stage of polymerisation?
activation of initiator to form free radicals
what is meant by the initiation stage of polymerisation?
free radicals break the C=C bond in monomer and transfer free radicals
what is meant by the propagation stage of polymerisation?
growing polymer chain
what is the initiator in the acrylic polymerisation reaction?
benzoyl peroxide
how is the polymerisation of PMMA activated?
(a) a heat > 72°C
(b) self cured
what does activation give?
two free radicals
what is the heat cured acrylic mixture made up of?
- powder
* liquid
what components make up the powder aspect of heat cured acrylic?
- initiator (benzoyl peroxide)
- PMMA particles
- plasticiser
- pigments
- co-polymers
what is the function of plasticiser?
allows quicker dissolving in monomer liquid
what is the function of co-polymers?
to improve mechanical properties & enhance performance
what components make up the liquid aspect of heat cured acrylic?
- methacrylate monomer
- inhibitor
- co-polymers
what is the function of methacrylate monomer?
dissolves the PMMA particles
what is the function of the inhibitor?
• prolongs shelf life (by reacting with any free radicals produced by heat/UV light)
why is the powder/liquid mixture used?
- to produce dough-like material that can be handled/mixed easily and customised to desired shape
- reduce heat of reaction
- minimise polymerisation shrinkage
what proportions should the powder to liquid mixture be?
- 3 to 3.5 / 1 (P/L) by volume
* 2.5 / 1 (P/L) by weight
what occurs once the dough like acrylic has been packed into mould?
it is CLAMPED & ready for heat curing
what are the heating schedules of PMMA?
(1) 7 hrs. to 70°C. + 2 hrs. to 100°C. + slow cool
(2) 72°C for at least 16 hours
(3) 20-20-20 reverse curing
do the mould material and the acrylic have the same thermal expansion coefficients?
NO
what may occur if the mould & mixture is cooled too quickly?
internal stresses
what are the consequences of internal stresses?
- lower compressive and tensile strength
- lower fatigue strength
- warping
- repair problems
what can occur if PMMA is under cured?
- free monomer (irritant to oral tissues)
* low molecular weight (poor mechanical)
what can occur if PMMA is cured too fast?
possible gaseous porosity (compromises denture strength)
why is porosity undesirable?
- affects strength
- affects appearance
- rough sensation to tongue
- absorbs saliva (poor hygiene)
