Polymerization (Week 14) Flashcards
Most modern resins are made with what monomer molecule?
Bis-GMA
What happens to the resin when exposed to light
polymerization ( monomers form polymeric chains)
During polymerization the resin composite (contracts/expands)
contracts
By what percentage does the volume of the resin contract to
2.6-7.1%
The process of reorganization of the molecules (or polymerization is also called (3 names)
gelation, contraction and hardening
What is the pre-gel phase
viscous-plastic phase, material is able to flow
What is the gel- point phase
viscous, flow cannot keep up with contraction
What is the post-gel phase
rigid elastic phase when contraction is obstructed stress occurs all around where the composite is bonded
What happens when the bond strength to the wall is lower than the stress generated
gap formation
What happens with the bond strength is higher than the stress
deformation
During deformation (bond strength>stress) the resin shrinks (contracts) to…
the center of the restoration
Contraction leads to __
stress
Stress (units of Megapascal) is about
4-7 MPa
The resin undergoes __ stress during the polymerization process
compressive
The adhesive undergoes __ stress during the polymerization process
tensile
Does the tooth undergo stress during the polymerization process
yes
Polymerization contraction in posterior teeth is associated with what clinical problems?
- 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 sensitity
- Penetration of bacteria
- Passage of dentinal fluid from tubules to the gap at the dentin-resin interface
What technique do you use to prevent the fracturing of enamel in posterior teeth
do not restore two cusps together (use the oblique method)
What factors are important in prevention of formation of stress?
resins
preparations
curing lights
polymerization techniques
What is the optimal energy needed to polymerize resin?
16 joules
the unit joule represents what relationship…
the amount of power per time
1 joule= 1…
milliwatt/cm^2/sec
How long do you have to polymerize resin if you have a 400 milliwatt light?
400 x 40 sec = 16 J
(T/F): When you double the amount of energy, the curing time can be reduced by 50%
F
Why is it that the intensity of the light and the time polymerizing are not proportional to one another
because the photo-initiators require time to absorb energy
How can you reduce polymerization time of resin
add more photo initiators to absorb the energy faster
What are the advantages of rapid polymerizing resins?
- Placed faster
- Deeper depth of cure
What are the disadvantages of fast polymerizing resins
- Short working time
- **Greater marginal stress
- Incomplete reaction leaving a more fragile material? –may end up with more unreacted monomer
- Greater micro leakage?
A (Fast/slow) polymerizing resin should be used for the first increment and why?
-Slow best we want this layer to mimize marginal stress
There are 2 theories as to where the resins contract upon polymerization, what are the two theories
- Shrinks towards the center (debonds from the walls to the center of its mass)
- Away from margins were it is bonded the weakest
(T/F) Each resin requires the same amount of energy to polymerize
F- different
(T/F) There are both slow and fast polymerizing resins
t
When does stress form
when the resin cannot adequately contract due to the bond between the resin and the walls
During the initial stages of polymerization, what are the only surfaces that can act as stress relievers during plastic deformation of the resin
Free surfaces- not coded to anything
The (more/less) free surfaces a restoration has the greater the stress
less
C-factors stands for__ and is the relationship between__
Configuration factor…. Ration of bonded to unbounded surface
Class I preps and deep boxy Class V preps have a C factor value of __
5
What is the C fact value for a class II restoration (MO or DO)
2
What is the C factor value of a shallow class III
2
What is the Cfactor of a deep Class III (facial and lingual exposed
1
What is the Cfactor value of a veneer or shallow class V
0.2 or (1/5)
What is the Cofactor value of a class IV
0.5 or (1/2)
What are the 4 types of curing lights
- Halogen
- Plasma arc
- Argon laser
- LED
What is the value range of intensity (mW/cm^2) of a halogen light
600-800
What are LED lights popular now
last a long time because they do not degrade the filter like halogen lights do
Low intensity is within the range of…
50-400 mW/cm^2
Medium intensity is within the range of…
400-800 mW/cm^2
High intensity is within the range of…
1000 + mW/cm^2
What are the two theories that aim to reduce stress
- Regulate the amount of energy given to the resin
- Control the amount of resin polymerized at a given time
Describe how we can regulate the amount of energy given to the resin
-Soft (low intensity to start with)
Describe how we can control the amount of resin being polymerized
When you add composite in bulk it will add more stress than in thin layers (indirect)
What are the two rules to minimize stress
- Keep the 1st layer thin
- Never connect opposing cusps (enamel) with single layer –> use oblique increments
(T/F): After application of the adhesive, you can add a thick layer of flowable composite to reduce stress
F- THIN layer of flow
