Light-activated polymerization

Question 1

List 5 polymerization activation mechanisms

Answer 1

Chemical
Heat
Light
Microwave
Combination

Question 2

What are the polym activ. mechs that we use intraorally?

Answer 2

Chemical, Light & combination of both

- Heat also plays a role in that the resin rxns are exothermic

Question 3

Where does the light portion of the initiation process come from? How does this impact us clinically?

Answer 3

• 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.

Question 4

Within the visible electromagnetic radiation, three factors impact photoinitiation

Answer 4

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

Question 5

One factor that hugely impacts the degree of polymerization (the free radicle initiated polymerization rxns)

Answer 5

Energy density

Question 6

Energy density (J/cm2)

Answer 6

= Intensity (power density) X Time (sec)

(W/sq. sm)

Question 7

Factors impacting the ability of radiations to reach a given point

Answer 7

• 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

Question 8

How does the type of filler impact the ability of radiation to reach a given point?

Answer 8

• how much of the filler, the size of the filler impact how much light is being scattered instead of being transmitted through the resin

Question 9

Time req’d for Visible Light curing is affected by (5)

Answer 9

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

Question 10

What conclusions have ben drawn regarding the use of PAC (plasma arc) lights for resin polymerization?

Answer 10

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

Question 11

One application where PAC lights (lower energy densities n shorter times) have the most traction is in_________. List 2 advantages.

Answer 11

• 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

Question 12

How do LEDs work?

Answer 12

• 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

Question 13

2 main Advantages of LEDs over QTH lights

Answer 13

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)

Question 14

Blue LED energy range and peak

Answer 14

440-495nm (470 peak of absoptn of CQ)

Question 15

Properties of photoinitiators:

- Identify the 3 primary photoinitiators

Answer 15

• found in single component materials (one paste)
• most common one is CQ (yellow)
• others are PPD & Lucirin TPO

Question 16

Mechanism of CQ

Answer 16

when exposed to light energy, it rxts w/aliphatic amines to form free radicles that initiate the polymerization rxn

Question 17

Diff btwn CQ and other photoinitiators like PPD (phenyl-prpanedione) and Lucirin TPO

Answer 17

CQ is yellow whereas the others are mo common in lighter shade composites

Question 18

For each of the common known curing lights identify the photoinitiators which they are able to activate in their emission wavelength

Answer 18

1. QTH - broadest wavelenth hence all three (CQ, PPD and Lucirin TPO)
2. Lasers - more focused in theri output hence limited to CQ and PPD
3. Plasma - CQ & PPD
4. LEDs (w/multiple diodes) - CQ, PPD and Lucirin TPO

Question 19

Xstics/advantages of light-activated systems

Answer 19

Need no mixing
  - No air incorporation
Very long working times
Rapid polymerization on demand
40 second exposure for 2 mm thickness of composite

Question 20

Main disadvantage to incorporating air in resin i.e how are the mechanical props affected? (2)

Answer 20

• 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

Question 21

Mechanisms of photoinitiation/ polymerization(3)

Answer 21

• 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)

Question 22

3 approaches used to combat polymerization stress

Answer 22

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?

Question 23

With rapid polymerization, increased stress is developed within the composite. Explain the hypothesis

Answer 23

Increased interfacial stress –> more marginal defects i.e larger gaps

Question 24

If you elect to use high energy halogens, (i.e. high energy, short curing times) what are 2 recommendations following this?

Answer 24

• Resealing restoration margins

- Using low modulus liners on inaccessible areas

Question 25

Degree o conversion

Answer 25

• 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