Resin Based Materials Flashcards
Why are poly methyl methacrylate granules used in the power to form heat cure acrylic resins
Lower polymerisation shrinkage from 20% (pure monomer) to 5-8%
What is present in polymer powder of heat cure acrylic resin
Poly methyl methacrylate granules
Benzoyl peroxide (initiator)
Pigments and opacifiers
Synthetic fibres
What is the nature of the liquid component in heat cure acrylic resins
Highly flammable and volatile
What is the inhibitor present in heat cure acrylic resin
Hydroquinone
What is function of plasticiser in heat cure acrylic resin liquid
Make polymer softer
Make polymer more resilient as plasticiser helps to absorb shock by breaking some existing bonds, making material less brittle
What happens if too little liquid is used in mixture toform heat cure acrylic rein
Too little monomer, lessened mechanical properties
What happens if too little powder is used in mixture to form heat cure acrylic resin
Too much residual monomer, cause tissue irritation
What are some good properties of heat cure acrylic resin
Good aesthetics, good colour stability
In expensive
Easy to process
Low thermal conductivity to insulate tissues from thermal sensation
High biocompatibility as few patients allergic, if problem is due to residual monomer causing irritation
Good dimensional stability during function
Virtually insoluble in water (though soluble in organic solvents)
What are the mechanical properties of heat cure acrylic resin
Moderate mechanical properties
Low impact strength, will fracture when dropped, but adequate to resist fracture in normal use
Rigid, low proportional limit and high elastic modulus
What does knoops test measure. What is its value and what does it indicate in heat cure acrylic resin
Measure of hardness of material.
20KHN in acrylic resin ie low hardness. Easy to polish but low abrasion resistance and is susceptible toa ttrition
DIsadvantages of heat cure acrylic resin
Susceptible to distortion during processing
High CLT
No anti microbial properties and can get colonised by C. Albicans
What may cause denture whitening
Soaking it in hot water
Contact with organic solvents (alcohol degrade polymer network, loss of colour pigments)
What is curing temperature of acrylic resin to be maintained at
74 degrees celsius
Exothermic reaction will add heat to raise material to POLYMERISATION temperature
What are the stages of polymerisation of acrylic resin
- Sandy
Fluid incoherent mass - Stringy
Monomer penetrate polymer, cannot be manipulated - Dough
>5min working time, undissolved polymer suspended in matrix of monomer and dissolved polymer. Plastic and homogenous - Rubbery
Monomer disappears by further penetration or evaporation. cannot be molded - Stiff
What is the purpose of flasking acrylic resin
Remove excess material
Advantages of long curing cycle acrylic resin
Increased polymerisation time —> less residual monomers
Less distortion and warpage as more heating time for molecules to find ideal position
Fewer bubbles and pores
Describe slow cure cycle
Heat flask at 74 degrees celsius for 90 min then increase temperature to boiling for 1 hour
Should you place acrylic dough in hot water to cure it
No. Place in cold water and increase temperature slowly. Rapid initial heating will produce internal stresses and warpage after deflasking, cracking and crazing
What causes internal porosities in acrylic resin
Vaporisation of unreacted monomer (low molecular weight polymer, temperature surpass boiling temperature)
What causes external porosities in acrylic resin
Lack of homogenity of dough
Lack of adequate pressure
What causes cracking and crazing of acrylic resin
Increased mechanical stress eg during flasking
Incorporation of water
Rapid cooling
How is benzoyl peroxide activated in autopolymerised acrylic resins
Tertiary amine in liquid activates it
Degree of conversion of autopolymerised acrylic resins
At least 5% still monomers
Properties of autopolymerised acrylic resin
Less colour stability
Less processing shrinkage since lower peak temperature
Greater porosity
Initial strength not as high but similar over time
What is contraction porosity
Inadequate pressure during polymerisation
Disadvantage of using polymethyl methacrylate to make dentures
Susceptible to denture wear due to low hardness
What is granular porosity
Localised porosities
Too much powder, unwet resin that does not completely mix. Tends to happen at thinner parts of denture as volatilisation takes place more readily there
Creates stress in denture making it prone to fracture
Can fix using autopolymerised resin
