Elastomeric impression materials (elastomers) Flashcards

1
Q

What is an elastomer?

A

A polymer with elastic (rubber-like) properties

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2
Q

Elastomers can be natural or synthetic - which are only used in dentistry?

A

Synthetic

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3
Q

In what form do elastomeric impression materials come? Explain

A

Liquid polymers (pre-polymers (short chains thin grow via polymerisation or cross linking)

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4
Q

Elastomers often have repeating units of Si-O or C-O

Why is this better than C=C?

A

More flexible

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5
Q

Why is filler added to liquid polymers?

A

Make a paste

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6
Q

What property of the liquid polymer makes the elastomer?

A

Crosslinking

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7
Q

Why were elastomers developed in terms of addressing problems with alienates?

A

Alginates not accurate enough for crown and bridge work
Large dimensional changes
poor tear resistance - problems in thin sections

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8
Q

Name 2 useful elastomers?

A

Silicones

Polyether

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9
Q

By what reaction are condensation silicone elastomers formed?

A

Condensation reaction

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10
Q

Describe the condensation reaction to form silicones?

a) reactors
b) catalyst
c) setting reaction
d) initial and final characteristic
e) by-product

A

a) silicone prepolymers and silica to from paste
b) tin
c) chain extension and cross linking
d) fluid to elastic
e) alcohol

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11
Q

Significance of alcohol by product in silicone setting reaction?

A

Significant dimensional changes over time - pour cast as soon as possible

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12
Q

Why are addition silicones used more than condensation silicones?

A

Less dimensional changes

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13
Q

What are the 3 paste systems that addition silicones come in?

A

Hand mixed
Cartridge - expensive, more waste
Mechanical mixers - even more expensive and more waste

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14
Q

Advantages of addition silicones?

A

Addition reaction = no-by product = no dimensional changes

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15
Q

Why wait a certain amount of time before pouring the impression of addition silicones?

A

hydrogen may be evolved - form pits

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16
Q

How is the viscosity of addition silicones controlled?

A

Amount of filler

Makes ultra-light, light, regular, heavy and putty

17
Q

Why is it necessary to have different viscosities of silicones?

A

Use a mix of different viscosities to get high level of accuracy e.g. place putty into impression tray and syringe low viscosity onto teeth

18
Q

Why not use light silicone at all times?

A

Light body = low viscosity = finest detail but highest shrinkage (less filler)

19
Q

Heavy vs light body silicone?

A

Heavy = less shrinkage on setting, less thermal contraction, more stable, easier to handle, no special tray (acts like tray for light body)

20
Q

Physical properties of addition silicones?

A

Good dimensional stability
elastic when set
tear resistance

21
Q

Problems with addition silicones?

A

Hydrophobic - don’t interact well with moist tissues = impression faults
Need a dry field (surface treatment)

22
Q

Polymerisation of polyether?

A

Addition polymerisation
No by product
Chain extension and cross linking to make it elastic

23
Q

Polyether properties

a) setting characteristics (WT and ST)
b) properties when set
c) properties before setting
d) dimensional stability

A

a) enough WT, short ST
b) rigid - difficult to remove from mouth - can remove mobile teeth
c) fluid and pseudoplastic (good detail), hydrophilic
d) good when dry but can absorb water and swell - model made quickly so not a problem

24
Q

What can elastomers be used for?

A

undercuts e.g. crowns, bridges

25
Q

Addition silicones vs polyethers uses?

A

Addition = very popular and material of choice but hydrophobicity is a problem - if you are careful e.g. dry field - this is not a problem
Polyethers - can be used for implants as hydrophilic