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Flashcards in Dental Materials Deck (293)
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121

How can ceramics, plasters and alginates effect the body?

Respiratory problems from inhalation of dust

122

How can trace anaesthetics effect the body?

NO, O2 sedation can cause neural and liver disease

123

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

124

What must there be for adhesion to occur?

Attraction between atoms in 2 surfaces

125

What 3 stresses weaken adhesion?

1. Thermal expansion coefficient
2. Dimensional changes
3. Moisture hydrating active groups in material

126

What are the 3 criteria for successful adhesion?

1. Surfaces clean and dry
2. Contaminants removed
3. In close contact

127

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

128

Describe solid-liquid adhesion

Good

Water clings to glass due to VDWs bonds (secondary), require thermal energy to remove water

129

What are the 5 factors governing adhesion?

1. Surface energy/tension
2. Wettability
3. Viscosity
4. Contact angle
5. Morphology/Surface roughness

130

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

131

What 3 things can reduce surface tension?

1. Inc. temp
2. Impurities
3. Surface-active agents/species: surfactants

132

What 2 factors make an adhesive effective?

1. Must be in close contact with substrate
2. Spread easily

133

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

134

What is wettability?

Tendency of fluid to spread on a surface

135

What's an example of good wettability?

Metal and water
Water spreads easily over the whole surface

136

What 3 factors can affect wettability?

1. Surface cleanliness
2. Surface irregularities
3. Viscosity

137

What is contact angle?

Angle between liquid and solid

138

What is contact angle dependent on?

Surface tension and surface energy

139

What does a contact angle of 0 mean?

Perfect wetting
Surface is completely covered by adhesive, max. bond strength

140

What happens as contact angle is inc.?

Air voids are introduced preventing perfect wetting
Rupture of adhesive joints

141

Explain the correlation between surface roughness and bond strength

Direct

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

142

What are the 5 types of adhesion?

1. Mechanical
2. Physical
3. Chemical
4. Molecule entanglement
5. Mixture of all

143

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

144

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

145

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

146

Describe molecular entanglement

Adhesive or component penetrates surface and absorbs into surface of substrate

147

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

148

Why is DM structure important?

Understanding structure-property relationship ensures right DM selection for specific clinical situation

149

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

150

What are the 2 classifications of chemical bonds?

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