DML1: Miscellaneous properties of Composites Flashcards
What are the advantages of light cured composites
- single component systems
- no mixing so minimal porosity
- less decolouration
- virtually command set (when light source is directed)
- rapid polymerisation
- thin inhibited layer
What are the issues with light cured composites
- dont leave on bench because direct sunglight can initiate the free radical substitution
- it is light sensitive during application so reduces working time
- retinal damage can occur
- there is a limited depth of cure (2mm) so large cavities have to have composites built up incrementally
Why is a high degree of conversion desirable
Because for optimum mechanical properties the majority of C=C bonds need to be converted otherwise the restoration will fracture
What is the degree of conversion for light cured composites
65-80%
What is the degree of conversion for self cured composites
60-75%
What is the degree of conversion for heat cured composites
> 90%
How does one light cure a composite
- Tip of light source must be close to restoration surface
- Light tip must not be contaminated (check for remnants)
- Cure for manufacturers recommended time
- Make sure there is no fanning and that curing spots overlap
- Each time the light is used check the required wavelength is output as this deteriorates with time
What can the depth of cure be affected by and why
Type of composite because when light hits, some of it is reflected scattered and absorbed; absorption is dependant on the composite shade so the darker the shade, the greater time required to cure
What are the issues using a UV light for curing
This uses benzoin methyl ether
- causes damage to cornea
- soft tissue burns
- limited depth of cure
What do all light curing composites contain
- alpha-di ketone (photoinitiator e.g. camphorquinone)
OR
- Amine activator (e.g. DHPT)
These are a source of free radicals which attack the double bonds within the matrix
What are the issues with quartz-tungsten halogen light curing units
they are bulky and so aren’t good for paediatrics and the bulb life decreases which means effective cooling is important; as the bulb deteriorates, the depth of cure may not be complete
What are the benefits of using a LED for light curation
- less lateral heat production
- long lasting light source
- requires low wattage
- typically cordless
- has a narrow emission spectrum as it is created for camphorquinone, this means it is energy efficient
- newer versions have an increased range of wavelength so other photoinitiators can be used
What are the issues with LEDs
- high cost
- low intensity
- increased exposure time
How do plasma-arc light curing units work
Uses xenon gas-ionisation; high intensity white light is filtered which decreases heat and allows emission of blue light
- this has a 1-3 second cure time which is rapid
What are the issues with plasma-arc light curing units
The rapid conversion of resin means there is high shrinkage stress and it is also very expensive to get bulbs for
What are the positives and negatives of using an argon laser light curing unit
- high energy
- high intensity
- emit light at a single wavelength
- very expensive and warning signs are required as it is a laser
What are the three strengths of using composites
- excellent aesthetic results
- less tooth tissue is removed
- command set when light cured
What are the disadvantages of using composites
- Lining materials are limited
- Setting is inhibited with eugenol based materials
- Doesn’t adhere intrinsically to enamel and dentine because the composite is hydrophobic and teeth are hydrophilic
- Incremental placement and light-cure are required so takes 3 times longer to place the restoration
- Caries tends to progress more rapidly at margins due to shrinkage on polymerisation
- Sticks to instruments so makes marginal adaptation harder
- The glass filler adsorbs water onto its surface leading to hydrolytic breakdown of bond between filler and resin
What happens when water is adsorbed into composite fillings
- unreacted monomer and highly soluble fractions in the composite are released
- this leaves pores which the water will fill within the cured resin
- this affects the wear resistance and colour staining
Why can composites have oestrogenic effects
- this is because uncured resin leaches out
- a component of the monomer Bis-GMA (Bisphenol A) leaches out
- this can cause developmental disruption in foetuses and babies
What is oxygen inhibition in regards to composites and what are the benefits of this
Where the air and resin interface results in a sticky resin surface; this is because oxygen inhibits the cure of the resin - this is good because it means the incremental placements are well bonded
What are the issues with oxygen inhibition in regards to composites
The final incremental surface will be sticky and this is an issue because food will get caught
How can the issues surrounding oxygen inhibition be resolved
- a thinner sticky layer results when using a light cure
- cure through a clear matrix strip so that oxygen isn’t competing with the surface
- overfill and polish down to remove the sticky layer
- apply bonding agent
How can a bonding agent be used to get rid of the sticky final layer resulting from oxygen inhibition
The bonding agent is placed over the composite and this contains C=C bonds which means oxygen will compete for this and yet again form another sticky layer
However the final composite increment is protected and so when the bonding layer comes away, the final layer stays
