Principles of adhesion Flashcards
What does an adhesive stick together?
Adherents
What does adhesion depend on?
1) The surface of the substrate (smooth or rough)
2) Thickness of substrate (ideally want uniform ditstribution)
What are the 4 types of adhesion?
1) Mechanical
2) Chemical
3) Intermolecular
4) Diffusion
How does mechanical adhesion work?
It is based on the surface topography of the substrate allowing for the adhesive to intrude into the micro-voids.
Rough adhesion = stronger adhesion
Due to large interfacial area (large surface area) and micro-mechanical retention of the adhesive into the surface micro-voids
How does chemical bonding work?
Molecules of the adhesive form chemical bonds (ionic/covalent) with the molecules of the adherent.
How does diffusion bonding work?
Adhesion can occur by diffusion when the molecules of the adhesion and adherent are similar in nature.
This allows for molecule movement across the interface.
Movement of the molecules go from the adherent to the adhesive (this movement allows diffusion through the matrix forming an adhesive bond).
Example: repairing a composite (placing uncured composite on the cured composite) and DBA diffusing into the dentine tubules
What are the ideal properties of an adhesive?
1) Good wetting (low contact angle)
2) Surface energy of the adhesive < surface energy of adherent to allow spreading
3) Low viscosity of the adhesive
4) Adhesive needs to displace air to prevent formation of air pockets (air pockets prevent polymerisation)
What is the name of the layer between the substrate and the adhesive?
Boundary layer
Does the core of the adhesive tend to be more or less homogenous in comparison to boundary areas?
More homogenous
What layer controls the durability and strength of an adhesive joint?
The boundary layer
What are the 3 modes of failure?
- Structural failure : failure in one of the substrates
- Adhesive failure : failure at the adhesive interface with the substrate
- Cohesive failure : failure within the adhesive itself
Why do we etch enamel?
- To selectively dissolve the HA minerals in enamel
- To form irregular surface on enamel to facilitate penetration of the adhesive forming resin tags
- Etching removes 5 microns form surface including: absorbed pellice, amorphous calcium phosphate layer, smear layer
What are the two types of etch patterns we can have?
Which gives better bond strength?
- Perpendicular to rod direction
- Parallel to rod direction
Perpendicular rods
What are sealants and what are the 3 types?
They are used to seal fissures.
Filled
Self Cured
Light cured
Why can dentine be hard to bond to?
Hydrophilic
