Composition of heat-cured acrylics?
Powder:
- Polymer: PMMA
- Initiator: Benzoyl peroxide
- Pigments and opacifiers
- Synthetic fibres (acrylic resin)
Liquid:
- Monomer: Methyl methacrylate
- Inhibitor: Hydroquinone
- Cross-linking agent: Glycol dimethacrylate
- Plasticiser: Bibutyl phthalate
Composition of autopolymerised acrylics?
Powder:
- Polymer: PMMA
- Initiator: Benzoyl peroxide
- Pigments and opacifiers
- Synthetic fibres (acrylic resin)
Liquid:
- Monomer: Methyl methacrylate
- Inhibitor: Hydroquinone
- Cross-linking agent: Glycol dimethacrylate
- Plasticiser: Bibutyl phthalate
- Activator: Tertiary amine
Effect of P:L on mechanical properties of heat-cured resins?
Increased P:L –> More powder = more polymer = drier mix, leading to porosity = decreased mechanical properties
Decreased P:L –> More liquid = More monomer = wetter mix, mix flows too much, excess monomer uncured leads to tissue irritation
What are the advantages of heat-cured resins?
- Good colour stability
- High biocompatibility
- Low polymerisation shrinkage (but higher compared to autopolymerised)
- Dimensionally stable
- Insoluble in water (soluble in some organic solvents like chloroform)
- High MOE and proportional limit (rigid)
- Easy to polish - low abrasion resistance
- Less residual monomers compared to autopolymerised resins
What are the stages of setting reaction of heat cured resins?
- Sandy stage: No molecular interaction between powder and liquid; Mixture is grainy and coarse
- Stringy stage: Some polymer chains dispersed in liquid monomer; Mixture is stringy when touched or pulled apart
- Doughy stage: More polymer chains enter into liquid monomer forming a mixture that is smooth and dough-like; Manipulation is allowed at this stage, working time ~5min
- Rubbery stage: Mass rebounds when compressed or stretched
- Stiff stage: Excess free monomer evaporates; Mixture is very dry and resistant to mechanical deformation
What are the difference between long curing cycles and short curing cycles?
Long curing cycle: Resin is cured at a constant temperature of 74°C for 9h, without terminal boiling. This method reduces the amount of residual monomers,
Short curing cycle: Resin is cured at 74°C for ~2h, with terminal boiling at 100°C for additional 1h (this ensures adequate polymerisation of thinner portions of resin). Thereafter, resin is cooled slowly to prevent thermal shock. This method conserves less energy.
What is the setting reaction of autopolymerised resins?
The activator, tertiary amine, activates benzoyl peroxide to form free radicals and polymerisation reaction proceeds as per usual .
What are the problems with PMMA resins?
- Porosity (Gaseous, Granular and Contraction)
- Crazing
- Fracture
- Wear
What are the causes of porosity?
- Inadequate pressure/inadequate dough in mould during polymerisation, leading to generalised white spots on denture surface. This causes contraction porosity.
- Incorrect P:L leading to evaporation of monomer (usually at thinner sections of denture), this causes granular porosity
- Evaporation of monomer, vapour is entrapped within hard polymer, resulting in gaseous porosity. Often occurs in thicker sections of denture (ie. Palatal slopes of maxillary denture, lingual aspect of mandibular denture)
- Lack of homogeneity of dough, causes localised shrinkage during polymerisation and large surface voids. This causes contraction porosity.
Causes of crazing?
- Presence of mechanical stresses: For example, repeated wetting and drying of denture = alternate expansion and contraction of denture
- Cleaning of dentures with solvents (ie. Chloroform, water)
- Stresses due to variable COTE between porcelain teeth, clasps etc.
