Week 14 Polymerization Flashcards
Modern composite systems are made with:
bis-GMA
What happens when resin is exposed to light?
reorganization of the molecules
volumetric contraction of composite:
2.6% to 7.1%
Reorganization of molecules in polymerization:
gelation, contraction, or hardening
3 gel phases:
- pre-gel phase 2 gel-point 3. post-gel phase
This gel phase is viscous, flow cannot keep up with contraction:
gel-point
This gel phase is the rigid-elastic phase when contraction is obstructed stress occurs:
post-gel phase
This gel phase is the viscous-plastic phase, material is able to flow:
pre-gel phase
What results if the bond strength is lower than stress?
gap formation
What results if the bond strength is higher than stress?
deformation, shrinks toward the center of the mass
Invisible polymerization contraction:
stress
Stress due to shrinkage is approximately:
4-7 MPa
What type of stress is applied to resin?
compressive stress
What is polymerization contraction in posterior teeth associated with?
separation of the resin at the weakest margins, formation of tensional forces at the margins, fractures in the enamel, deformation of the cusps, secondary caries, postoperative sensitivity, penetration of bacteria, passage of dentinal fluid from tubules to the gap at the dentin-resin interface
Factors we need to understand to prevent the formation of stress:
Resins, preps, curing lights, polymerization techniques, restorative techniques
Each resin needs a minimum amount of ___ to obtain clinically acceptable results.
energy, from paste to hard
unit of energy is measured in __ in dentistry.
joules: power per time
1 joule =
1 milliwatt/cm^2/sec
Optimal energy to polymerize modern composite resin:
16 joules, most monomers converted to polymeric chains
40 seconds at 400 mW/cm^w =
16 (450-475 non-motile of light)
Joules =
watts X time
400 miliwatts X 40 seconds =
16 joules
800 milliwats X 20 seconds =
16 joules
1600 milliwats X 10 seconds =
16 joules
True or False? When you double the amount of energy, the curing tim could be reduced by 50%
F
What requires time to absorb energy in curing a resin?
photo-initiator, this is why the relationship between intensity and time is not proportional.
True or False? The relationship between intensity and time is proportional.
F
How do manufacturers reduce polymerization time of composite?
add proprietary initiators to the resin so they can absorb the energy faster, material absorbs energy faster
Advantages of fast polymerization resins:
rapid polymerization, place faster, deeper depth of the cure
Disadvantages of fast polymerization resins:
short working time, greater margins stress, incomplete reaction leaving a more fragile material (conversion is never 100%, more unreacted monomer?, more fragile?), greater microleakage.
Theories regarding shrinkage upon polymerization resins:
away from margins to which is bonded the weakest
True or False? Each resin needs a different amount of energy to polymerize.
T
Stress is formed when the resin:
cannot adequately contract due to the bond between the resin and the walls
What surfaces act as stress relievers during the plastic deformation of the resin (the initial stages of polymerization)?
only the free surfaces (of the tooth structure or the placed resin?)
True or False? The fewer the free surfaces, the higher the stress
F. greater
What does C-Factor stand for?
configuration factor
C =
bonded surface/ unbonded surface
The higher the C-factor,
the greater the stress generated
What classes of lesions produce the greater stress when restored?
class I and class V