PARTIAL DENT - Dental Polymers Flashcards
what are the standard 2001 ISO requirements of an ideal denture base polymer? (9)
- bond to denture teeth
- biocompatible
- high polish
- translucent
- colour stability
- no porosity
- low residual monomer
- low solubulity
- > 65MPa of flexural strength
2 GaP of flexural modulus
what are desirable properties which aren’t part of the standard requirements? (8)
- resilient
- tough
- high impact strength
- dimensional stability
- repairable
- good thermal conductor
- radio-opaque
- cheap
define resilience
the amount of energy absorbed by a material up until the point of permanent deformation
give 3 denture materials used historically
porcelain - made clicking sound
vulcanite - high cross-linked rubber
bakelite - poor mechanical properties but was first mouldable plastic
what is PPMA?
polymethyl methacrylate
describe the chemistry of PMMA
addition polymerisation
what are the setting reactions of PMMA
addition polymerisation
- activation - heat, light or chemical
- initiation - free radical connects with a monomer
- propagation
- termination
describe the initiation phase.
- denture base resin contains benzoyl peroxide
- heat above 60 degree
- molecules yields 2 free radicals
describe the propagation phase.
- free radicals break double bond in methyl methacrylate
- multiple MMA bond together
= as a result get PMMA
give 10 advantages and 5 disadvantages of PMMA
advantages
- aesthetics
- repairable
- polishable
- high glass transition temperature
- cheap
- easy to make
- bonds to denture tooth
- light
- low water sorption
- rigid
disadvantages
- 21% shrinkage
- porosity during processing
- poor impact strength
- poor thermal conductor
- prone to fracture
what is glass transition temperature?
the temperature above which an amorphous polymer becomes soft
- the point of a glass to rubber transition
what temperature must a denture have a higher glass transition temperature?
- liquids in the mouth can be up to 70 degrees
- must be higher than 70 to prevent going soft
what is a factor affecting glass transition temperature?
molecular structure
- methyl has higher GTT than ethyl, propyl, butyl
how is the shrinkage of PMMA tackled? what does the shrinkage reduce to?
prepolymerize the base resin
shrinkage reduces to 6% and 0.5% linear shrinkage
what are the two components which need to be mixed to create the heat cured acrylic resin?
powder and a liquid
describe the powder component of acrylic resin
- what is it
- the initiator
- what is may also contain
- granules of PMMA
- initiator = benzoyl peroxide
- may contain dyes, opacifiers, plasticisers and fibres
describe the liquid component of acrylic resin.
- what is it?
- the initiator
- what else is may contain.
- monomers of MMA
- inhibitor = hydroquinone
- may contain cross linking agents
describe how the PMMA changes in sensation as it sets (4)
sandy - initial melting of beads
sticky/stringy - thickened entanglements
dough - gelation - this is when it needs to be applied into the mould
rubbery - monomer penetrates to the core of beads, plasticising them - too late, it has set
how are acrylic dentures manufactured?
- wax pattern
- flasking
- boiling
- dough moulding
- boiling - activate polymerisation
- deflasking
what are the 3 types of porosity?
gaseous
contraction
granular
describe gaseous porosity
when MMA is boiling during polymerisation
- MMA boiling point = 100.3 degree
- due to reaction being exothermic, temperature of 170 degrees can be reached
- deepest parts of the denture hardens
how do you prevent gaseous porosity?
heat MMA very slowly so it fully hardens before its boiling point it reached
describe contraction porosity
due to 6% shrinkage and because theres not enough material or pressure
how is contraction porosity prevented?
add extra PMMA dough before heating
mould is kept under high pressure to prevent bubbles growing in size
