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Flashcards in Dental Materials Deck (293)
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181

Describe light formation of FRs

Usually 1-paste

Tertiary amine activator (dihydroxymethyl-p-toluidine) and light initiator (camphorquinone) added to monomer and react in presence of visible light source to form FRs

182

How does free radical formation being exothermic lead to porosity?

Boiling causes monomer to bubble creating airspaces which then set as material polymerises

183

Why can a molecule weight not be given for a polymer?

All polymers are not the same length

184

What 2 methods can be used to give a polymer a MWt?

1. Number average MWt (Mn)
2. Weight average MWt (Mw)

185

Define number average MWt

Total weight of sample divided by number of molecules in sample

186

Define weight average MWt

Sum of the number of molecules multiplied by their weight average

187

What are the 3 states polymers exist in? Give examples

1. Rubbers: silicone rubber
2. Hard resins: poly(methyl methacrylate)
3. Fibres: polyethylene

Rubber and hard resins: long chains exist in randomly coiled configuration and thermal motion
Fibres: long chains stiff and straightened

188

How are polymer chains held?

Weak 2ndary bonds: VDWs
Entanglement of chains

189

Describe the effect of stress on rubbers

Easy deformation due to low intermolecular forces
Reversible due to 1% crosslinks within polymer
Elastic modulus in MPa

190

Describe the effect of stress on hard resins

Difficult to deform due to high intermolecular forces
Elastic modulus in GPa

191

What are crosslinks?

Covalent bonds in polymers joining one polymer chain to another
Prevent polymers flowing irreversibly

192

Compare 1% and 30% crosslinks

1%: elastic
30%: rigid material

193

Describe the effect of crosslinks on glassy polymers

Improves some properties such as impact strength (relevant for dentures)
Too many will make them brittle

194

Define thermosetting resin

Initially rubbers w/ low intermolecular forces, due to 30% crosslinks end up as rigid polymers
Vulcanite

195

Compare thermoplastics and thermosetting plastics

Thermoplastics: once heated and formed into shape can be reheated and reshaped but red. quality of plastic

Thermosetting: once hearted and shaped can't be reheated and reshaped, difficult to recycle

196

Define glass transition temp
Explain with an example

Temp at which rigid material becomes soft and rubbery

Intermolecular forces are temp dependent
PMMA is glassy, rigid w/ high intermolecular forces
As temp inc., intermolecular forces dec. until material is soft and rubbery

197

Describe the Tg fro natural rubber

Elastomer w/ low intermolecular forces
As temp dec, forces inc. until reach Tg temp at which point material becomes glassy rigid

198

What is the significance of Tg to dentistry?

Denture bases require Tg for exceeding temps likely to be exposed to
Polishing be technician, cleaning by patient

199

What is a plasticiser?

Simple, organic liquid
Dibutyl phthalate

200

What are plasticisers used for?

Added to monomer before polymerisation to red. Tg of material i.e. make naturally rigid, glassy material soft and rubbery at RT

201

How do plasticisers function?

Depress Tg by lubrication
Flow between chains of polymer red. high intermolecular forces
Rigid material becomes soft and rubbery w/ low intermolecular forces

202

What are the 2 disadvantages to plasticisers?

1. Material becomes hard again w/ time
2. Phthalates are carcinogens and leach out into patients mouth

203

What are crystalline polymers and what are their uses in dentistry?

Polymers w/ high degree of order formed by folding and stacking of polymer chains: long, stiff, straightened chains

1. Polypropylene: Instrument trays; withstand high pressure, temp
2. Polyethylene: reinforce acrylic dentures

204

What is a homopolymer?

Polymer formed when polymerised alone

205

What are the 3 stereoisomers that polymers can obtain?

1. Atactic: random arrangement of head - tail configurations along chain
2. Isotactic: all substituents on same side of chain
3. Syndiotactic: alternating head-tail linkages

206

What is a copolymer?

Product of polymerisation involving multiple types of monomer

207

What are the 3 types of copolymer?

1. Random: random arrangement of monomers along chain
2. Block: blocks of monomer groupings of 1 type along chain
3. Graft: main chain 1 monomer w/ branches of other monomers

208

Why impact can water uptake have on materials?

1. Red. strength
2. Extract potentially toxic materials
3. Can induce microorganism formation

209

What are 3 useful water soluble materials used in dentistry?

1. Alginates: impression
2. Poly(acrylic acid): cements, polymer is solid but dissolves readily and ionises, anion is polymeric
3. Hydroxyethyl methacrylate: resin modified GIC, can be used as hydrogel

210

Define ceramic

Inorganic and non-metallic compound formed between metallic and non-metallic elements
Usually oxides (MgO) can be nitrides, carbides, borides (SiC)
Crystalline or non-crystalline
Combination of compounds