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Flashcards in Dental Materials Deck (293)
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151

What is a bond?

Interaction between atoms

152

What 3 bonds are primary bonds?

1. Covalent
2. Ionic
3. Metallic

153

Describe covalent bonds

Strongest: 4-6 eV
Atoms share electrons to achieve noble gas configuration
Least reactive bond
Electron orbital overlap forms molecular orbital w/ shared electrons

Directional and rigid

154

Describe ionic bonds

Strong electrostatic (>1.5) interactions: 3-4 eV
Results in formation of ions: cations and anions
Non-directional as ions interact w/ any ions in vicinity

155

Describe metallic bonds

Non-directional, relatively weak 0.5-2 eV

Overlapped orbitals of metallic atoms forms electron gas
Electron has and metallic cations: electrostatic integration

156

What bonds are secondary bonds?

VDWs
H bonds

157

Describe secondary bonds

No electron sharing
Charge induced dipole interactions

158

Define phase

Physically and chemically homogenous part of system/material that has clear boundaries

159

Define phase transition

Change from one phase to another
e.g. solid -> gas sublimation

160

What is a phase diagram?

Presentation of stability and phase transitions for a system/material of phases in equilibrium at range of temps.

161

Why are phase diagrams useful?

Identify phase against the composition at a given temp

162

What is the eutectic point?

Lowest melting point of system: lower than constituents and may other mixture of them

163

Describe crystalline solids

Consist of crystals
Form crystalline lattice: ordered (symmetric and periodic) and specific arrangements of atoms

164

Describe non-crystalline solids

Disordered solids, contain no crystals
Inorganic, rapidly quenched: glass
Organic: polymers
Phase transition specific for non-crystalline solids

165

Describe the phase transitions in non-crystalline solids

Glass transition: change in viscosity/rigidity from brittle to rubbery w/ inc. temp

Crystallisation (devitrification): conversion to crystalline solids if heated up to specific temp

166

What is polymerisation?

Synthesis of polymers from monomers

167

What are the 2 types of polymerisation?

1. Condensation
2. Addition

168

Describe condensation polymerisation

Reaction between 2 molecules producing larger molecule and water/small molecule byproduct
Byproduct has to be removed during polymerisation

169

Describe addition polymerisation

Reaction between 2 molecules giving larger molecule with no byproducts

Monomers added 1 by 1 per chain to active site on growing chain

170

What is the most common type of addition polymerisation?

Free radical polymerisation

171

What is the Thiokol reaction?

Condensation polymerisation that forms polysulphide polymers
Used in base paste of polysulphide impression materials

172

How is 2-polyglycolic acid synthesised and what is its function?

Condensation polymerisation

Synthetic, absorbable, suture material
Biodegrades back to acid in 60-90days

173

What is a free radical?

Molecules w/ an unpaired electron

174

How is methyl methacylate synthesised and how is it used?

Addition polymerisation

Used as denture base

175

How are free radicals formed?

When initiation decomposes into 2 highly reactive free radicals in presence of monomers

176

What is the common initiator?

Benzoyl peroxide

177

What are the 4 stages of free radical polymerisation?

1. Activation: formation of FRs from initiators
2. Initiation: FR attacks monomer units
3. Propagation: chain growth, monomer units add together
4. Termination: 2 growing chains meet, FRa combine forming stable covalent bond

178

What are the 3 ways in which FRs are formed in dentistry?

1. Heat
2. Room temp
3. Light

179

Describe heat formation of FRs

Usually powder/liquid, 2-paste or 1-paste

When mixed and heated, BP splits into FRs
FRs attack double bonds in monomers

180

Describe how FRs are formed at room temp

Usually powder/liquid or 2-paste

Tertiary amine activator (dimethyl-p-toludine) added to monomer (MMA) reacts w/ BP in powder to form FRs
FRs attack double bonds