Dental Materials Flashcards Preview

BDS 1 > Dental Materials > Flashcards

Flashcards in Dental Materials Deck (293)
Loading flashcards...

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

Not desirable

Surfaces of both rough (at atomic level) only contact at cusp tips, require adhesive to bind
Surfaces: adherent/substrate
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
2. Wettability
3. Viscosity
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
2. Impurities
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
3. Viscosity


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?

Perfect wetting
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?

1. Mechanical
2. Physical
3. Chemical
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
Small: VDWs
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


What are the 2 classifications of chemical bonds?

Primary: strong; 0.5-7 eV
Secondary: weak; <0.5 eV