Physics and Chemistry of Light Curing Flashcards
What is a Polymerisation reaction?
Process of reacting monomer molecules together in a chemical reaction to form 3D networks or polymer chains
A monomer is a small molecule that has the potential of chemically bonding to other monomers of the same species to form a polymer
What’s Light Polymerisation?
- Using light to initiate a reaction where monomers form polymer chains (curing)
What are the advantages of Light Curing?
- Extended working time - useful if complex aesthetic restorations are being provided
- Command Set
- More consistent means of polymerisation obtained
- Amount and concentration of tertiary amine required in the material can be lowered
- Save clinic time
- Quality of the cure is improved
- Higher levels of monomeric conversion to the polymer (typically 50-70%) - unconverted can lead to leaching out and degradation of the restoration
What are the disadvantages of light curing/
- Expensive hardware required
- System compatible: excitation wavelength of chemicals and wavelength of light
- Adequate energy and correct wavelength is required; if not suboptimal restoration
What are the mechanisms of Photo-polymerisation?
- Light breaks down the photo-initiator
- Breakdown produces atoms or molecules/activated chemical which reacts with an amine
- Free radicals (highly chemically reactive) produced initiate the polymerisation reaction
- Once initiated the reaction propagates until the process goes to completion
- Chain reaction
What’s the mechanism of Cure?
- Sufficient amount of light energy/wavelength is required
- Monomers polymerise to form a rigid cross-linked polymer
- Various polymer chains linking via the methacrylate groups
- Possible to accelerate the polymerisation reaction by increasing the conc of photoinitiator
What’s the downside of adding more photoinitiator?
- Material sets quicker but propagation phase of the polymerisation process is shortened so shorter chain lengths formed and material is not as strong
What’s the setting reaction mechanism?
- Photo-initiator (a-diketone) + light specific wavelength
=
activated diketone
- Activated diketone + amine
= Free Radical (R)
- R + monomer + copolymer
= Polymer
What is the Photo-initiator?
- Most commonly a-diketone (camphorquinone)
- Yellow
- Problematic when a lighter shade of resin composite is being manufactured - influences final shade
What’s a drawback of Camphorquinone?
- Cannot be used with bleach shades
- Use others:
1. PPD
2. Lucirin TPO
3. Ivoclar Vivadent
What’s special about newer photo-initiators?
- Have higher visible light absorption rate (increased quantum efficiency)
- Improved light curing performance so a lower quantity of light is sufficient to trigger the poly. reaction
What’s Peak Absorption?
Wavelength at which maximum excitation of a photolytic chemical occurs (peak excitation wavelength)
Camphorquinone = wavelengths of 390-510nm
- Peak absorption= 470nm
PPD + Lucirin = 380-430nm
- Most light cured materials are most sensitive to light at 470nm
What’s Spectral Band?
Part of the spectrum at which the light polymerising unit may produce chemical excitation
What are the 3 types of light sources?
- Halogen
- Plasma
- LED
What’s Halogen light?
- Exhibits broad spectral range
- Intensity of light may vary at the wavelength where peak excitation of the photo-initiator
- Cooling fans are required to prevent bulb overheating
- Halogen bulb filaments change overtime = operating temp + wavelength of light change = curing efficiency is diminished and material is less well cured
What are LEDs?
- Very effective polymerisation produced
- Emits ecess of 1W of energy
- Narrow spectral band
- Consistent output of wave
- Cooling fans unnecessary even with the high powered lights
What are the new Poly Wave LED lights?
- developed to increase compatibility of peak excitation and photo-initiator
- Has more than 1 LED each operating at a different wavelength
What are the effects of incomplete curing?
Uncured or partially cured material can lead to:
- Pulpal inflammation due to uncured acidic components can cause chemical trauma
- Discolouration and marginal staining due to oral fluid diffusing into the partially set material
- Decreased wear resistance, compressive and flexure strengths
- Secondary caries and microleakage
What factors influence the rate of curing?
Manufacturer & Clinican
What are the manufacturer influences on the rate of curing?
Opacity/translucency of material:
- More opaque materials require longer curing times
- Intensity/attenuation of light is reduced
Varying diketone/amine chemistry:
- Altered concs. of diketone/amine can increase polymerisation
- Some may be more susceptible to ambient light
- EXCESS CAN LEAD TO SHORTER POLYMER CHAINS =DECREASED MECHANICAL PROPERTIES
Particle size:
- Light is attenuated slowly through fine particle bulk resins
- Leads to lower light penetration and less curing
Factors under the clinicians control?
- Reduced thickness of each increment will increase curing depth
