Notes on pros exam Flashcards
What are the types of porosity?
Internal/gaseous porosity
Contraction porosity
Granular porosity
Inhomogeneity of dough -> Localised shrinkage -> Large internal and superficial voids
What causes internal/gaseous porosity?
Monomer entrapment (Especially if the denture base is thick)
What causes contaction porosity?
Lack of dough in the mould and inadequate pressure during polymerisation
What causes granular porosity?
Insufficient monomer due to:
Incorrect ratio, evaporation
What are the problems with PMMA?
Porosity: Leads to poor aesthetics, difficulty cleaning, weakened structure, and warping
Fracture: Due to fatigue
Crazing: Incomplete crack due to relaxation of surface stresses -> Tensile stress
Wear: Generally related to teeth, rather than denture base
Others that include possible cytotoxicity from leaching chemicals, brittleness/less mechanical strength, relatively low modulus of elasticity, high coefficient of thermal expansion, faster deterioration
What causes crazing in PMMA dentures?
Expansion and contraction on repeated wetting and drying
Organic solvent: Exacerbated by drinkers
Stresses due to coefficient of thermal expansion
How are polymers classified?
According to origin: Natural vs synthetic
According to physical state: Elastomer, hard amorphous, hard partially crystalline, and fibrous
According to thermal behaviour: Thermoset and thermoplastic
According to spatial structure: linear, branched, cross-linked
According to polymerization reaction: Addition vs condensation polymerization
Why is crosslinking of polymers useful?
Increase strength, and Tg
Decreased water sorption and ductility
Elastic deformation rather than plastic.
Better resistance to organic solvents
Why are copolymers used?
They can provide the material with better physical properties
How do plasticizers affect materials?
Liquids can penetrate between the chains of polymer driving the molecules further apart and decrease forces between the moleucles thus:
They soften the material
Increase flexibility
Decrease glass transition temperature
Decreased strength and hardness
Increased brittleness
What are the acrylic-type denture resins?
methyl methacrylate
poly (methyl methacrylate)
Light cured resin (triad system)
What are the types of denture resins according to activator system?
Heat-cured denture resins
Autopolymerised denture resins
Light-cured denture resins
What are the types of addition polymerisation?
Free-radical polymerisation (Denture resins)
Ring-opening polymerisation (polyethers)
Ionic polymerisation (Addition polymerised silicones)
How does free-radical polymerisation work?
Head activates an initiator (such as benzoyl peroxide) which releases free radicals that initiate polymerisation
What are the stages of free-radical polymerization?
Relies on unsaturated molecules filling up with free radicals; 3 stages to this reaction:
Initiation (initiator is activated to form free radicals and start to transfer their energy to monomer molecules)
Propagation (Rapid addition of monomers to the free radicals)
Termination (Inhibition by material that reacts with the free radicals instead of the monomers)
