Light-activated polymerization Flashcards
List 5 polymerization activation mechanisms
Chemical Heat Light Microwave Combination
What are the polym activ. mechs that we use intraorally?
Chemical, Light & combination of both
- Heat also plays a role in that the resin rxns are exothermic
Where does the light portion of the initiation process come from? How does this impact us clinically?
- the visible light portion of the electromagnetic spectrum (btwn 380 and 780 nm)
- if resin is left exposed on a table, it’s being exposed to visible light that contains the wave lengths required to initiate the polymerization. Thus, can impact your working time.
Within the visible electromagnetic radiation, three factors impact photoinitiation
i) Wavelength – color (hue) (i.e. at the blue end NOT red end)
ii) Power – amount of radiation produced and directed to the resin (mW)
iii) Intensity (Power density)
= power/unit of surface area
=mW/sq. mm
One factor that hugely impacts the degree of polymerization (the free radicle initiated polymerization rxns)
Energy density
Energy density (J/cm2)
= Intensity (power density) X Time (sec)
(W/sq. sm)
Factors impacting the ability of radiations to reach a given point
- lamp output
- exposure time
- distance from light source (ex. opacity of the material impacts intensity experienced beyond the surf.; type of filler etc)
- intervening materials (ex. enamel)
- curing depth
How does the type of filler impact the ability of radiation to reach a given point?
- how much of the filler, the size of the filler impact how much light is being scattered instead of being transmitted through the resin
Time req’d for Visible Light curing is affected by (5)
i) Lamp - Quartz-Tungsten Halogen, plasma arc, laser, LED
ii) Distance - inc. dist, incr time
iii) Resin thickness - inc thick, inc time
iv) Shade - darker shade, inc time
v) Illumination thru tooth structure inc time
What conclusions have ben drawn regarding the use of PAC (plasma arc) lights for resin polymerization?
i) High output PAC (1800 mW/cm2) lights with 6 second cure will polymerize CQ initiated resins
ii) Behaves like rapid halogen polymerization regardless of the power source BUT, it takes a pretty high output in order to get that accomplished within 6 seconds
One application where PAC lights (lower energy densities n shorter times) have the most traction is in_________. List 2 advantages.
- orthodontics
i) rapid polymerization of light activated resin when bonding brackets;
ii) no worrying about full polymerization & max bonding strengths coz they will eventually come off
How do LEDs work?
- they have light emitting diodes in them that rcv energy; as they receive that energy they are throwing off photons that are being transmitted through fiberoptics within the light wand
2 main Advantages of LEDs over QTH lights
i) the LED light produced tends to be a much more collimated light. QTH lights on the other hand, when QTH produces energy it scatters pretty rapidly as it comes out of the end of the wand
ii) LEDs do not generate the same amt of heat as QTH lights; hence less pulpal damage. (Note: LEDs do generate heat output as well)
Blue LED energy range and peak
440-495nm (470 peak of absoptn of CQ)
Properties of photoinitiators:
- Identify the 3 primary photoinitiators
- found in single component materials (one paste)
- most common one is CQ (yellow)
- others are PPD & Lucirin TPO
Mechanism of CQ
when exposed to light energy, it rxts w/aliphatic amines to form free radicles that initiate the polymerization rxn
Diff btwn CQ and other photoinitiators like PPD (phenyl-prpanedione) and Lucirin TPO
CQ is yellow whereas the others are mo common in lighter shade composites
For each of the common known curing lights identify the photoinitiators which they are able to activate in their emission wavelength
- QTH - broadest wavelenth hence all three (CQ, PPD and Lucirin TPO)
- Lasers - more focused in theri output hence limited to CQ and PPD
- Plasma - CQ & PPD
- LEDs (w/multiple diodes) - CQ, PPD and Lucirin TPO
Xstics/advantages of light-activated systems
Need no mixing - No air incorporation Very long working times Rapid polymerization on demand 40 second exposure for 2 mm thickness of composite
Main disadvantage to incorporating air in resin i.e how are the mechanical props affected? (2)
- the air-holes become stress concentration points that will weaken the material
- the oxygen in the incorporated air inhibits free radicle polymerization- hence very long working times
Mechanisms of photoinitiation/ polymerization(3)
- Polymerization begins closest to the light
- Polymerization shrinkage tends to pull the unset material away from the unbonded walls of the preparation
- Photoinitiation does not occur beneath opaque materials
(hence chemical or dual activated systems are still needed)
3 approaches used to combat polymerization stress
i) Low elastic modulus liner
ii) Incremental placement of the composite
iii) Reduce the polymerization rate
Can a reduced polymerization rate result in less stress but maintain the degree of conversion and associated volumetric contraction?
With rapid polymerization, increased stress is developed within the composite. Explain the hypothesis
Increased interfacial stress –> more marginal defects i.e larger gaps
If you elect to use high energy halogens, (i.e. high energy, short curing times) what are 2 recommendations following this?
- Resealing restoration margins
- Using low modulus liners on inaccessible areas
Degree o conversion
- the bottom shld be at least 80% cured relative to the top of the 2mm increment because the clinical data seems to support that as being adequate
Bottom knoop hardness/Top knoop hardness >/= 0.80
