Properties Flashcards
What is wettability and how do you achieve it?
- In bonding, adhesion requires an intimate contact between the adhesive material and the substrate.
- To obtain a maximum contact area between a liquid and a solid surface, the substrate must show evidence of high wettability with regard to that liquid (hydrophilicity of dentine and material).
- Achieved using acid etch technique and DBA (primer acts as wetting agent); creates an opportunity for an excellent bond between composite and enamel.
What are the drawbacks of CoCr?
Low ductility - means centrifugal casting is needed which is more difficult process and difficult to produce defect free cast
Difficult to adjust once made, work hardens rapidly, therefore requires precise casting when first made
Shrinkage during casting, at 2.3%, is much higher than other materials. Requires phosphate bonded investment material.
Hardness - makes polishing time consuming and arduous
What are the differences between CoCr and type IV gold?
CoCr is less dense therefore will have more defects, however will be lighter
CoCr has a higher hardness
CoCr has lower ductility - will not deform under stress
CoCr has higher rigidity - modulus of elasticity (Young’s modulus)
CoCr has a higher shrinkage
CoCr has a lower ultimate tensile strength than type IV gold
What is the initiator in polymerisation of acrylic?
Benzoyl peroxide (symmetrical molecule of C6H5COO-OOCH5C6) -0.2-0.5% in heat cured acrylic powder
When using alginate, permanent distortion and tearing can be reduced by using large bulk of material. How much is this bulk?
5mm
Where does alginate start setting first?
Increased temperature speeds up setting reaction - sets nearest tissue first
What are the properties of alginate?
Flow, setting changes, non-toxic, non-irritant, taste, smell, setting time, ease of use = all ok
Movement causes stresses and distortion
Nearly elastic - not 100% recovery
Poor tear strength
Storage - avoid moisture (prepare cast asap)
What are the properties of impression compound?
Flow - does not give fine detail
High thermal expansion coefficient - mouth to room temp linear contraction 0.4% - use small volume and individual tray
Removal - plastic deformation if undercuts - allow to cool to minimise flow
Sterilisation - difficult (may affect properties, autoclave)
Stability, non-toxic, non-irritant, setting time, low cost (long shelf life) = all ok
What are the properties of impression paste (ZOE)?
Fine details, setting changes, storage, compatibility with stone, setting time, shelf life = all ok
Not elastic
Non-toxic, non-irritant - eugenol may cause allergic response, adheres to tissues (coat lips with petroleum jelly)
Not cheap
Define creep.
When recurrent stressing below elastic limit for period of time causes permanent deformation - sits proud of surface making it vulnerable to fracture and microleakage.
Why are spherical particles superior to lathe cut particles in amalgam?
Increase in early strength Increase in tensile strength Less mercury is required Easier to carve Less sensitive to condensation
What are the benefits of copper enriched amalgam?
higher early strength, less creep, increased corrosion resistance, increased marginal durability
What are the advantages of using elastomeric impression materials over alginate impression materials?
elastomers are less viscoelastic, therefore have better elastic recovery and less deformation on removal
higher tear strength
more accurate in recording surface detail
What is annealing and why is it used?
relieves residual stress
heat metal/alloy so greater vibrations allows migration of atoms
Describe what is meant by viscoelastic behaviour of an impression material and how you would overcome the key drawbacks this presents.
Impression materials should behave elastically, so they can undergo sufficient strain when being removed from the mouth to overcome undercuts and bulbous teeth, and then shortly return to original dimensions.
Viscoelastic materials take time to reach maximum strain needed to overcome undercuts, and when removed from the mouth recover their shape gradually but do not return to original dimensions - slight permanent deformation
Using an elastomer can limit the permanent deformation to as little as 0.5%
Removing impression tray quickly with a sharp pull reduces stain material undergoes and helps minimise permanent deformation
