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
