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DMT > Non-elastic Materials > Flashcards

Non-elastic Materials Flashcards

1
Q

General features of non-elastic material

A

Too rigid/stiff to be removed past undercuts or elastically deforms when removed past undercuts

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2
Q

Examples of non-elastic impression materials

A

Impression plaster
Impression compound
Impression pastes

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3
Q

What does hemihydrate mean?

A

Some water left in the material

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4
Q

What type of setting reaction occurs for impression plaster?

A

Crystallisation

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5
Q

Disadvantage of crystallisation setting reaction?

A

Expansion

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6
Q

K2SO4 added to impression plaster - what effect does this have?

A

Reduces expansion (anti-expansion agent) but accelerates setting too much

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7
Q

What kind of agent is added to impression plaster to counteract the negative effects of K2SO4?

A

Retarder (borax) to slow down reaction

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8
Q

100g of powder added to 50-60ml of water to make impression plaster - this creates a material with low viscosity - why is the necessary?

A

Mucostatic - won’t displace the patients soft tissues

BUT need special tray to transport the material as it is fluid

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9
Q

Disadvantages of impression plaster when set - how does this effect its use?

A

Very rigid and brittle - can’t move past undercuts - can only be used on edentulous patients

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10
Q

What is the purpose of a separating agent on impression plaster?

A

Allows separation of impression material from model

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11
Q

Why do patients find impression plaster uncomfortable?

A

Drying sensation on setting

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12
Q

What 2 materials are combined to make impression compounds?

A

Waxes and resins

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13
Q

How can characteristics fo impression compounds be controlled?

A

Wax to resin ratio

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14
Q

Function of plasticiser in impression compound?

A

Improves flow of material as waxes and resins are brittle

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15
Q

Function of filler in impression compound?

A

Control viscosity, rigidity and thermal contraction

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16
Q

Function of pigment in impression compound?

A

To trace where it is

17
Q

Thermoplastic definition

A

Hard when cool, soft when warm, harden when cooling

18
Q

IS impression compound thermoplastic or thermosetting?

A

Thermoplastic

19
Q

What are 2 types fo impression compound?

A

I/II

20
Q

Difference between Type I and II impression compound?

A 
I
lower temperature fusing 
High flow value just above mouth temp
sheet form for preliminary impression (edentulous)
Stick for to add marginal addition 
II
High temperature fusing
low flow value or just above mouth temp
rigid at mouth temp - use as tray material
21
Q

Impression compounds - issues

a) viscosity
b) thermal characteristics
c) setting reaction
d) applications

A

a) high viscosity - mucocompressive (compresses the soft tissues), poor detail reproduction
b) contraction - from softening to removal from mouth
stress relief can occur after removal
c) no setting reaction - final material is rigid and non-elastic
d) edentulous areas only (can’t move past undercuts)
tray extensions

22
Q

How is impression compound softened?

A

Water bath at 55-60 degrees

23
Q

Characteristics of impression material when too cool or too hot?

A

Too cool - won’t flow sufficiently

Too hot - sticky and painful for patient

24
Q

Characteristics of impression compounds if left in water bath too short or too long?

A

Too short - insufficient softening

Too long - leaching of plasticiser - won’t flow

25
Q

Softening technique of sheet vs stick impression compound

A 
Sheet = water bath
Stick = bunsen
26
Q

Typical pastes in impression pastes?

A

Zinc oxide and eugenol

27
Q

Setting reaction of impression pastes

A

Chelation reaction - ionic reaction so sets will in ionic environment

28
Q

How to accelerate setting reaction of impression pastes. Why is this good?

A

Aqueous environment - longer WT (shorter ST)

29
Q

Describe viscosity of impression pastes (zinc oxide eugenol)

A

low viscosity - mucostatic - won’t displace soft tissues

Pseudoplastic - viscosoty reduces under stress

30
Q

Why are impression pastes used in thin sections?

A

Such low viscosity difficult to produce thick films

31
Q

Describe dimensional stability and accuracy if impression pastes?

A

Good accuracy and only slight dimensional change on setting

But as it is a thin section the overall change is very minimal

32
Q

Can set impression pastes be used to record undercuts?

A

No - rigid and non-elastic when set

33
Q

Disadvantage of zinc oxide eugenol?

A

Eugenol can be an irritant

34
Q

Describe the 2 stage impression technique for edentulous arches?

A

1) primary impression
- impression compound in tray (mucocompressive so will displace soft tissues - allows record of full depth of sulcus)
This is sent to lab to special tray
2) major impression
Use impression paste in special tray - extend with compound when needed
records detail and maintains sulcus impression

DMT (11 decks)

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