Flashcards in Dental Materials Deck (293)
How can ceramics, plasters and alginates effect the body?
Respiratory problems from inhalation of dust
How can trace anaesthetics effect the body?
NO, O2 sedation can cause neural and liver disease
Define adhesion and cohesion
Adhesion: force that binds 2 dissimilar materials together by attraction between atoms and molecules
Cohesion: attraction between atoms/molecules within 1 substance
What must there be for adhesion to occur?
Attraction between atoms in 2 surfaces
What 3 stresses weaken adhesion?
1. Thermal expansion coefficient
2. Dimensional changes
3. Moisture hydrating active groups in material
What are the 3 criteria for successful adhesion?
1. Surfaces clean and dry
2. Contaminants removed
3. In close contact
Describe the adhesion between 2 solids
Surfaces of both rough (at atomic level) only contact at cusp tips, require adhesive to bind
Where they meet: interface
Describe solid-liquid adhesion
Water clings to glass due to VDWs bonds (secondary), require thermal energy to remove water
What are the 5 factors governing adhesion?
1. Surface energy/tension
4. Contact angle
5. Morphology/Surface roughness
Compare the surface energy/tension of bulk and surface molecules
Bulk: each molecule attracted in all directions to molecules around it, dynamic equilibrium
Surface: molecules only attracted down and to other surface molecules, thus have higher surface energy
Molecules try to maintain min. surface area so more molecules have lower energy state
What 3 things can reduce surface tension?
1. Inc. temp
3. Surface-active agents/species: surfactants
What 2 factors make an adhesive effective?
1. Must be in close contact with substrate
2. Spread easily
What 3 factors affect spreading?
1. Wettability: resistance to viscosity
2. If too viscous flow too slow so can't penetrate crevices before set
3. Misleading if adhesive is solvent w/ additives
What is wettability?
Tendency of fluid to spread on a surface
What's an example of good wettability?
Metal and water
Water spreads easily over the whole surface
What 3 factors can affect wettability?
1. Surface cleanliness
2. Surface irregularities
What is contact angle?
Angle between liquid and solid
What is contact angle dependent on?
Surface tension and surface energy
What does a contact angle of 0 mean?
Surface is completely covered by adhesive, max. bond strength
What happens as contact angle is inc.?
Air voids are introduced preventing perfect wetting
Rupture of adhesive joints
Explain the correlation between surface roughness and bond strength
Roughness inc. bonding area but creates areas difficult for adhesive to penetrate
Need adhesive of high surface tension to penetrate crevices but not too high as must be able to wet substrate
What are the 5 types of adhesion?
4. Molecule entanglement
5. Mixture of all
Describe mechanical adhesion
Retention by mechanical interlocking of components or penetration of 1 phase into surface of other
Attraction between substrate and adhesive not necessary but is weak, unable to withstand debonding
Describe physical adhesion
Dipole-dipole attractions between polar molecules
Weak adsorption: not suitable for long adhesion
Rapid bonding, reversible
Thermal energy breaks bond
Describe chemical adhesion
Bonding at molecule/atomic level
Dissociation of molecule after adsorbs to surface, constituents then bond again separately via ionic/covalent forces
Strong adhesive bonds and attraction
Describe molecular entanglement
Adhesive or component penetrates surface and absorbs into surface of substrate
What is molecule entanglement enhanced by?
1. Good wetting
2. Absorbing component long chain or forms long chain within penetrate layer
3. Entanglement between adhesive and substrate
4. V high bond strength
5. Adhesives must be strongly chemically attracted to surface
Why is DM structure important?
Understanding structure-property relationship ensures right DM selection for specific clinical situation
Compare protons, neutrons and electrons
P: +ve charge, 1 amu, relatively large, inside nucleus
N: 0 charge, 1 amu, relatively large, inside nucleus
E: -ve charge, 0 amu, relatively small, outside nucleus