prosthetic polymers and resins part II Flashcards
what are chemically activated denture base resins?
Chemical activators are used to induce denture base polymerization
No need for thermal energy. Thus can be pursued at room temperature (cold curing)
Accomplished by adding a tertiary amine such as dimethyl-para-toluidine to the monomer liquid
Once the liquid monomer is mixed with the powder, the tertiary amine triggers benzoil peroxide decomposition
Free radicals are produced and polymerization begins
How do heat activated resins compare to chemically activated resins?
Heat activated resins yield a higher degree of polymerization
Incomplete polymerization in chemically activated resins provide unreacted monomer that acts as a plasticizer that results in reduced transverse strength of the denture resin. The monomer also irritates the tissue, and biocompatibility is an issue.
Chemically activated resins exhibit less shrinkage
Heat activated resins exhibit superior color stability
Transverse strength is the pull perpendicular to the applied force.
Isotropic is when it has the same strength in all directions. If it’s not it’s anisotropic.
what are some technical considerations for chemically activated resins?
Mostly molded by compression
Same mold preparation and resin packing technique used with heat activated resins
Working time is less than heat activated resins
A lengthy initiation period (prolonged by refrigeration)
what are processing considerations for chemically activated resins?
Polymerization time varies with material chosen
Initial hardening within 30 min of flask closure
The flask should be held under pressure for at least 3 hrs
High residual monomer content (3 to 5%) as compared to heat-activated resins (0.2 to 0.5%)
what is the fluid resin technique of chemically activated resins?
Employs a pourable, chemically activated resin
Mixture of powder and liquid exhibits low viscosity
Poured via a sprue into a specially designed flask
Flask is placed in a pressurized chamber at room temperature while the mixture polymerizes
Finishing and polishing
what are advantages of the fluid resin technique?
Better adaptation to underlying soft tissues
Decreased probability of damage during deflasking
Reduced material costs
Simple flasking, deflasking, and finishing procedures
what are disadvantages of the fluid resin technique?
Noticeable shifting of prosthetic teeth during processing
Air entrapment within the denture
Poor bonding between the acrylic resin teeth and the denture base material
Technique sensitive
what is light-activated denture base resin?
Composite material made of a urethane dimethacrylate matrix, microfine silica, high molecular weight acrylic resin monomers, and acrylic resin beads filler
Activated by visible light
Camphorquinone initiator
Silica is SiO2. Silica is sand.
what are the physical properties of denture base resins with polymerization?
Polymerization shrinkage:
Up to 21% of volumetric shrinkage
Occurs with phase transformation of the soft rubbery material to a rigid glassy state at the glass transition temperature, Tg
Linear shrinkage = αT where α is the linear coefficient of thermal expansion, and T is the temperature
Glass transition temperature is different from the melting point but glass and polymers have this.
The linear coefficient of thermal expansion will be 1/degree celcius.
what are physical properties of denture base resins with porosity?
Porosity:
Can compromise the physical, aesthetic, and hygienic properties of a processed denture
Occurs due to vaporization of unreacted monomer, poor thermal conductivity of surrounding dental stone, inadequate mixing of components, or air inclusions associated with fluid resins
To minimize porosity: ensure homogeneity, and delay packing until doughlike consistency is reached
There will be stress concentration at the pores.
Initiation, propagation, fracture is how cracks go.
what are physical properties of denture base resins in terms of water absorption?
Water Absorption:
PMMA absorbs water when placed in an aqueous environment
Water molecules position themselves between polymeric chains and force them apart causing expansion and acting as a plasticizer
Affects the physical and dimensional properties
what are physcial properties of denture base resins for solubility and processing stress?
Solubility:
ANSI/ADA Specification #12: Weight loss < 0.04 mg/cm2
Processing Stresses:
Occur when natural dimensional changes are restricted such as surface stresses at the mold walls during shrinkage
Stress release results in material distortion
You don’t want the material to be too soluble and dissolve when you clean it for example.
what are physical properties of denture base resins crazing?
Crazing:
Microcracks resulting from stress release
May predispose a denture resin to fracture
May result from partial dissolution of the resin by a solvent
Oriented at right angles to the tensile forces
Originate at the surface and progress inwards through the resin
what are physical properties of denture base resins in terms of strength and creep?
Strength:
Depends on composition, processing technique, degree of polymerization, and oral environment
Should meet ANSI/ADA Specification # 12
Creep:
Viscoelastic behavior
Plastic deformation with time
Creep – primary, secondary, tertiary.
A huge chandelier hanging is an example.
what are miscellaneous resins and techniques?
Repair resins Relining resin denture bases Rebasing resin dentures Short/long term soft denture liners Denture cleansers Infection control procedures Allergic reactions
