Adhesion

Question 1

What is adhesion?

Answer 1

The force that binds two dissimilar materials together when they are brought into intimate contact

Question 2

How do adhesion and cohesion differ?

Answer 2

Adhesion: 2 dissimilar materials
Cohesion: 2 of the same material

Question 3

Describe solid-solid adhesion

Answer 3

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

Question 4

Describe solid-liquid adhesione e.g. glass slide and water droplet

Answer 4

Intimate force throughout
Secondary bonds (van der Waals)
Low force, large contact area
Cleanliness is key!

Question 5

Describe solid-liquid-solid adhesion

Answer 5

e. g. when you glue something together
- Liquid is an adhesive, needs to bond to both materials
- Fills space between solids

Question 6

Criteria for adhesion

Answer 6

Intimate contact on molecular level
-viscosity of adhesive
-surface roughness of substrate
Formation of attractive bonds
Wettability (how well adhesive contacts substrate)

Question 7

Why is adhesion important in dentistry?

Answer 7

• Aesthetic dentistry based off adhesion
• Conserve and reinforce tooth structure
• Reduce postoperative sensitivity (hydrodynamic effect reduced)
• Reduced marginal leakage
• Ortho brackets

Question 8

Mechanisms of adhesion

Answer 8

1. Micromechanical
2. Physical
3. Chemical
4. Molecular entanglement

Question 9

Describe micromechanical adhesion

Answer 9

Interlocking of components on microscopic level

• Microscopic undercuts
• Adhesive flows into undercut (wettability is key)
• Sets

Question 10

How would micrmechanical adhesion break once set?

Answer 10

• Must fracture, cannot disengage from undercut
• Macroscopic retention is indipendent of wettability
• Cleanliness is key, otherwise won’t adhere properly

Question 11

Examples of micromechanical adhesion (metals)

Answer 11

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

Question 12

Examples of micromechanical adhesion (ceramic)

Answer 12

Heterogeneous: feldspar and leucite

• Preferentially etch leucite crystals with hydrofluoric acid
• Provides micromechanically retentive surface

Question 13

Examples of micromechanical adhesion (enamel bonding)

Answer 13

Heterogeneous: this time it is in the enamel structure

• Preferentially etch using phosphoric acid the change the surface characteristics
• Provides a micromechanically retentive surface

Question 14

Describe physical adhesion

Answer 14

• Van der Waals and hydrogen bonding

- Dipole interaction (temporary or permanent)

Question 15

Describe temporary dipole movement

Answer 15

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

Question 16

Describe permanent dipole movement

Answer 16

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)

Question 17

Properties of physical adhesion

Answer 17

• Weak, reversible, rapid
• Physical adsorption
• Precursor to chemical bonding
• Non-polar & polar substances do not interact on molecular level

Question 18

Describe chemical adhesion

Answer 18

Covalent and ionic bonding

• Sharing e-s
• Strong bond
• Chemisorption

Question 19

Describe molecuar entanglement

Answer 19

• Highly porous surface (e.g. etched), collagen left
• Add a monomer
• Polymerise
• No clear border between materials (hybrid zone)

Question 20

Describe example of molecular entanglement

Answer 20

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

Question 21

What is wettability?

Answer 21

The ability of an adhesive to contact a substrate

-Good wetting is ability to cover substrate completely