Adhesion Flashcards
What is adhesion?
The force that binds two dissimilar materials together when they are brought into intimate contact
How do adhesion and cohesion differ?
Adhesion: 2 dissimilar materials
Cohesion: 2 of the same material
Describe solid-solid adhesion
2 solids pushed together e.g. shoe and floor
- Points of contact have high pressure (cold welding)
- Source of friction when force is sheared
- High force, but tiny contact area
Describe solid-liquid adhesione e.g. glass slide and water droplet
Intimate force throughout
Secondary bonds (van der Waals)
Low force, large contact area
Cleanliness is key!
Describe solid-liquid-solid adhesion
e. g. when you glue something together
- Liquid is an adhesive, needs to bond to both materials
- Fills space between solids
Criteria for adhesion
Intimate contact on molecular level -viscosity of adhesive -surface roughness of substrate Formation of attractive bonds Wettability (how well adhesive contacts substrate)
Why is adhesion important in dentistry?
- Aesthetic dentistry based off adhesion
- Conserve and reinforce tooth structure
- Reduce postoperative sensitivity (hydrodynamic effect reduced)
- Reduced marginal leakage
- Ortho brackets
Mechanisms of adhesion
- Micromechanical
- Physical
- Chemical
- Molecular entanglement
Describe micromechanical adhesion
Interlocking of components on microscopic level
- Microscopic undercuts
- Adhesive flows into undercut (wettability is key)
- Sets
How would micrmechanical adhesion break once set?
- Must fracture, cannot disengage from undercut
- Macroscopic retention is indipendent of wettability
- Cleanliness is key, otherwise won’t adhere properly
Examples of micromechanical adhesion (metals)
Heterogeneous: metal bonded bridge of nickel-chrome alloy
- Preferentially etch: nickel is etched with acidic ‘soup’, unfilled resin flows into holes
- Provides micromechanically retentive surface
Examples of micromechanical adhesion (ceramic)
Heterogeneous: feldspar and leucite
- Preferentially etch leucite crystals with hydrofluoric acid
- Provides micromechanically retentive surface
Examples of micromechanical adhesion (enamel bonding)
Heterogeneous: this time it is in the enamel structure
- Preferentially etch using phosphoric acid the change the surface characteristics
- Provides a micromechanically retentive surface
Describe physical adhesion
- Van der Waals and hydrogen bonding
- Dipole interaction (temporary or permanent)
Describe temporary dipole movement
e. g. monatomic gases
- electronegative repulsion pushes e-s to orbit elliptically, makes atom more positive on left
- even within 1 atom there are areas of more +ve and more -ve
Describe permanent dipole movement
e. g. water
- O has 6 e-s in outer shell, H each have 1
- O has more protons than H
- pulls e-s away from H atoms –> covalent bond
- O bit more -ve, H bit more +ve
- lots of H2Os together form permanent dipole-dipole forces (H-bonding)
Properties of physical adhesion
- Weak, reversible, rapid
- Physical adsorption
- Precursor to chemical bonding
- Non-polar & polar substances do not interact on molecular level
Describe chemical adhesion
Covalent and ionic bonding
- Sharing e-s
- Strong bond
- Chemisorption
Describe molecuar entanglement
- Highly porous surface (e.g. etched), collagen left
- Add a monomer
- Polymerise
- No clear border between materials (hybrid zone)
Describe example of molecular entanglement
e. g. dentine bonding
- Demineralise dentine
- Leaves collagen network open
- Monomer infiltrates pores (good wettability needed)
- Polymerisation solidifies resin
- Results in region of diffusion between layers
What is wettability?
The ability of an adhesive to contact a substrate
-Good wetting is ability to cover substrate completely
