Is tooth structure hydrophobic or hydrophilic?
Hydrophilic
Are most dental restorative materials hydrophobic or hydrophilic?
Hydrophobic
Define adhesion
Force that binds two dissimilar materials together by the attraction of atoms and molecules
Define Cohesion
Attraction between similar atoms/molecules within one substance
Name 3 stresses that weaken adhesive bonds
- Thermal expansion coefficient (hot drink)
- Dimensional changes
- Moisture from tooth structure hydrating active groups in the material
Was is an adherent?
Surface of a material
What binds two solid adherents together?
An adhesive interface
How does water cling to a glass surface?
Via van der waal forces
What are the 5 factors governing adhesion?
- Surface Energy/Tension
- Viscosity of the adhesive
- Wettability of substrate by adhesive
- Contact angle
- Surface roughness
Explain the atomic forces in the bulk of a solid/liquid
- Atoms have attractive forces in all directions and are in dynamic equilibrium
- The molecules inside are attracted to all
molecules around them.
Explain the atomic forces at the surface of a solid/liquid
- There is a net inward attraction to the bulk atoms
- These forces result in a Surface Energy/Tension
What is the difference between Surface energy and Surface tension?
- Surface energy (solids)
- Surface tension (liquids)
How do you measure surface tension?
Wilhelmy Plate method
Why is PTFE (polytetrafluoroethylene) difficult too wet?
Has a low surface energy
Typically do metals have high or low surface energies?
High surface energies
What can reduce surface energy/tension?
- Increasing temperature
- Presence of impurities
- Surface active agents (surfactants in soaps)
Comment on the viscosity of a good adhesive
- Should be able to spread easily but not too easily
- If too viscous then flow is too slow and cannot penetration surface crevices
Give an example of good wettability
- Wetting of metal with water
- Water flows across and spreads
Give an example of poor wettability
- Wetting of PTFE with water
- Water forms globules (does not spread)
What is wettability?
NO LECTURE DEFINITION
BUT GENERALLY….
How easy water can spread across a surface, ie droplet of not.
Name 3 factors that affect wettability
- Surface cleanliness
- Surface irregularities
- Viscosity of adhesive
What is the contact angle?
Angle between liquid and solid interface
What is the contact angle for perfect wetting adhesion? What does this mean?
- Contact angle = 0
- surface completely covered with adhesive
- Therefore maximum bond strength is achieved
How does increasing surface roughness affect adhesion?
- Improved bond strength
- More difficulty for adhesive penetration due to trapped air
