resin based composites Flashcards
for aesthetic restorative materials what are the advantages of silicates, PMMA, and Bis-GMA?
History:
Silicates: released fluoride but severely eroded
PMMA: aesthetic, insoluble, good plasticity, and cheap. However, they exhibit poor wear resistance and shrink drastically upon curing, leading to microleakage. Also, develops stresses with thermocycling. As a remedy, quartz powder added, but their lack of bonding to the PMMA lead to leakage, staining, and poor wear resistance.
Bis-GMA: a dimethacrylate resin. Advantage: an organic silane coupling agent was added to bond filler particles to the matrix (Matrix-Filler-Coupling agent)
You have to have a material to assist in bonding the fiber to the matrix. In this way, the fibers will carry the load rather than the matrix. This is a coupling agent. Bis-GMA has estrogenic effects, because of this it was removed from baby bottles.
what are the uses and applications of resin based composites?
Direct aesthetic anterior restorations Restoration of posterior occlusal areas Restoration of high stress bearing sites Sealant for pits and fissures Bonding agent for ceramic veneers Cement for fixed prosthodontics
what is the composition of dental composites?
Composition: Resin Matrix Filler Coupling Agent Activator / Initiator Pigments Ultraviolet absorbers Inhibitors
for dental composites what is the resin matrix?
A blend of aromatic / aliphatic dimethacrylate monomers such as bis-GMA
Form highly cross-linked polymer structures
Polymerization shrinkage –> unrelieved stresses at the tissue / composite interface –> restoration margin gap –> secondary caries
High MW monomers reduce curing shrinkage and improve the mechanical properties; however, they exhibit high viscosity which can be reduced by diluent monomers
Polymerization shrinkage, wear resistance and manipulation properties place strict limits on optimizing the performance of composite materials
Sulfur can be used to cross link polymer structures.
Diluent monomer – diluting the monomer with other monomer to dilute it.
what are the fillers like in composites?
Milled quartz, and silica are examples. Their addition:
Reinforces the matrix
Reduces polymerization shrinkage
Reduces thermal shrinkage / expansion
Improves workability
Reduces water sorption, softening, and staining
Increases radio-opacity
how are fillers produced and what are microfillers?
Produced by grinding or milling quartz or glasses
Range in size between 0.1 to 100 μm
Microfillers refer to submicron silica particles of colloidal size that are produced by a pyrolytic or precipitation process
To get to 20 nm you would precipitate the product. If you could control the precipitation rate, you could control the size of the particle.
Pyrolytic is heating it up to a high temperature and then making smaller particles.
how are composites classified?
Composites are classified according to average particle size, particle size distribution, major filler component, volume level, index of refraction, radiopacity, and hardness of the filler
what are coupling agents for composites?
Essential for bonding filler particles to resin matrix
Allows flexible polymeric matrix to transfer stresses to the higher modulus filler particles
Organosilanes used most commonly
what is the activator-initiator system?
Free radical activation of the polymerization process
Chemical activation
External energy activation (light, heat, or microwave)
what are inhibitors for?
Added to minimize or prevent spontaneous or accidental polymerization
Strong reactivity with free radicals
Faster reactivity with the free radical than the reactivity of free radical with the monomer
Also prevent oxidation and rancidity
Rancidity – takes off the bad smell and old tastingness of it.
what are optical modifiers for?
Added to yield a more natural appearance
Pigments made of metal oxide particles
Adjusted translucency and opacity
what is curing of resin-based composites by chemical activation?
Cold curing / self curing
Initiated by mixing two pastes just prior to usage
Pores form during mixing. They weaken the structure and the oxygen inhibits polymerization during curing
Operator has no control over working time; therefore, insertion and contouring must be completed quickly
what is curing of resin-based composites by light activation?
Resins do not require mixing
Uses a photosensitive initiator and an activating light source
Avoids porosity
Allows operator to complete insertion and contouring before curing
Takes 40 sec to cure a 2mm thick layer. In contrast, chemically activated resins take several minutes
Not sensitive to oxygen inhibition
what are the drawbacks of light activation?
Must be placed incrementally if bulk exceeds 2 to 3 mm due to limited depth of light penetration
Thus, may take more time in some cases, like in Class II cavity preparations
More expensive (added cost of light curing unit)
Relatively poor accessibility in certain posterior and interproximal locations
Sensitive to room illumination
what types of curing lamps are there? (4)
Light-Emitting Diodes (LED) Lamps: Emit radiation only in the blue part of the visible spectrum between 440 and 480 nm, and do not require a filter.
Quartz-Tungsten-Halogen Lamps: Irradiate both UV and white light, and use filters to remove heat and all wavelengths except those in the violet-blue range (~400 to 500 nm).
Plasma Arc Curing (PAC) Lamps: Use ionized Xenon gas to produce plasma, and use filters to allow only blue light (~400 to 500 nm) to be emitted.
Argon Laser Lamps: Have the highest intensity, and emit a single wavelength (~490 nm)
