Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Dental Materials Exam III > Lecture 2: Elastic Impression Materials > Flashcards

Lecture 2: Elastic Impression Materials Flashcards

1
Q

Elastic impression materials, elastomers, are classified into aqueous & non-aqueous elastomers. Give two examples of aqueous elastomers and four examples of non aqueous elastomer impression materials.

A 
Aqueous Elastomers:
1. Agar-agar
2. Alginate
Non-Aqueous Elastomers:
1. Polysulfide
2. Condensation silicone
3. Addition silicone
4. Polyethers
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Generally, what type of material are non-aqueous elastomers?

A

Rubbers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What type of colloid material is Agar-agar?

A

Reversible Hydrocolloid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the active ingredient in agar-agar?

A

Agar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is K2SO4 used for in Agar-agar?

A

Potassium sulfate acts as an accelerator (die), quickening setting time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the largest component, by volume, in agar-agar impression material?

A

Water ~85.5%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the two phases of agar-agar? Under what conditions does each exist?

A
  1. Agar Sol - Occurs in Hot conditions - In order to transition to agar sol must heat to 100 degrees celcius
  2. Agar Gel - Occurs in Cold conditions - To transition to Agar gel must cool to 43 degrees celcius
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

At what temperature is it recommended agar gel be at for optimal impression taking (Tempering temperature)?

A

Tempering - 46 degrees C

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

In order to shorten the long working & setting times of Agar-agar, what special system is used?

A

Water-cooled trays. Water is flowed at 13 C through tubing w/in the impression trays to shorten the setting time.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

When removing agar-agar from the mouth what is the best method? Why?

A

‘Snap’ removal. Quick removal of the agar impression will result in quicker deformation & allow for quicker and better recovery of accurate impression

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What aqeous, elastic impression material is more accurate: Agar or Alginate?

A

Agar - Has detail reproduction down to 25 um

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

In addition to being highly accurate, agar shows very good elastic recovery; why might one NOT want to use agar? Be specific.

A

Poor Dimensional stability. Must pour models as soon as possible. Agar both loses water & absorbs water (environment dependent):

  1. Syneresis: Loss of water during storage. Causes shrinkage & loss of accuracy.
  2. Imbibition: Absorption of water. Swelling, loss of accuracy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the estimated working & setting times of agar? What is done to improve this?

A
  1. Working = 7-15 min - very long

2. Setting = 5 min - Variable dependent on water flow through coolant system in trays

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Agar-agar is neither a toxin nor an irritant, what is the major biological property, associated w/ its use, that may cause patient discomfort?

A

Thermal Shock. Abrupt changes in temperature w/in the mouth from 43 C when first introduced to 13 C when water coolant activated.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the recommended storage method & max time for set agar-agar impressions? Should this be followed?

A

100% relative humidity for no more than 30 minutes. Due to agar-agar’s very poor dimensional stability, however, cast should be poured from model as soon as possible.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are the economic considerations concerning agar-agar impression material?

A

Need for special cooling trays requires high initial investment, however, agar itself is relatively inexpensive.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Besides agar-agar, what is the other aqueous elastomer impression material talked about in class?

A

Alginate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

In what form is soluble alginate generally seen in Alginate compression material?

A

K-Alginate (~15%)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the purpose of Sodium Phosphate (Na3PO4) in Alginate?

A

Acts as a retardent. Acually, stops the reaction until ALL Na3PO4 has been used up in a reaction w/ Ca3PO4.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What are the two, sequential setting reactions for alginate?

A
  1. 2Na3PO4 + 3Ca3SO4 —> Ca3(PO4)2 + 3Na2SO4 = Retardent step
  2. Na.nAlg Sol + n/2CaSO4 —> n/2Na2SO4 + Ca.n/2Alg Gel = Setting step (Alginate Sol (powder) to Alginate Gel)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

How is Alginate set time influenced?

A

Adjusting concentration of sodium phosphate (retarder)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

How does the accuracy of alginate compare to that of agar?

A

Poorly.

  1. Detail reproduction is 75 um vs 25um
  2. Elastic recovery is only 97.3% vs 98.8% for agar
  3. Very poor dimensional stability, same as agar
  4. LOW tear strength
  5. High rate of permanent deformation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Both alginate & agar-agar are colloids, meaning they are microscopically dispersed throughout another substance. What 2 attributes, characteristic of colloids, cause poor accuracy in both alginate & agar-agar?

A
  1. Poor dimensional stability. Absorption (imbibition) & Loss (syneresis) of water, leading to change in size
  2. Low compressive tear strengths
24
Q

Why is it important to be so precise when measuring W/P ratio w/ alginate?

A

Improper W/P ratio has large effect on strength of impression material

25
Q

What are the effects of temp change on alginate setting reactions?

A

Increased Temp: Speed up

Decreased Temp: Slow down setting rxn

26
Q

What is the major biological (health) concern associated w/ alginate? How is the alginate treated to stop this?

A

Dust from alginate is a know fibrogenen & carcinogen. Don’t want to inhale. Alginate is coated w/ Glycol to make dustless alginate.

27
Q

While agar-agar is the better impression material, why is alginate often used?

A

More convenient. Used for quick, non-final impressions

28
Q

Alginate storage?

A

100% rel humidity. No more than 30 min. Same as agar-agar; pour model as soon as possible.

29
Q

What are the 2 presentations (physical forms) of non-aqueous elastomers (=rubber based impression materials)?

A
  1. Base & catalyst pastes

2. Automix systems

30
Q

What is the major use for non-aqueous elastomers?

A

Mostly used for taking final impressions for restorations

31
Q

What are the Base & Accelerator pastes that make up Polysulfide (mercaptan) impression material?

A

Base Paste: Polysulfide polymer w/ terminal & pendant -SH- groups
Accelerator Paste: Cross linking agent. Usually, Lead dioxide

32
Q

What type of setting rxn does polysulfide undergo? By-product? Exothermic or endothermic?

A

Condensation polymerization with water by-product. Exothermic rxn.

33
Q

How do the terminal & pendant groups of polysulfide play a role in the condensation polymerization setting rxn?

A

The terminal & pendant sulfhydril groups -SH- react with oxygen, resulting in the H2O byproduct.

34
Q

Too much H2O lost as a result of the condensation polymerization setting rxn will cause what? What can be done about this?

A

Shrinkage of polysulfide impression material. Therefore, polysulfide must be poured immediately.

35
Q

Why would a custom tray be recommended when using polysulfide impression material?

A

Polysulfide has a high flow rate, therefore if you use too much material it will run down the patients throat causing gagging.

36
Q

When dispensing base-catalyst non-aqueous elastomers, what is always the ratio?

A

1:1 base:catalyst ratio

37
Q

What are 5 reasons why polysulfide impression materials are not normally used.

A
  1. Flow rate is too high (Big one)
  2. Shrinkage is too great due to condensation (dehydration) rxn
  3. Short storage time
  4. Long working & setting time
  5. Bad smell & taste
38
Q

What are the base & catalyst paste/liquid for condensation silicone?

A

Base paste: Hydroxy terminated polydimethyl siloxane

Catalyst past/liquid: Crosslinking agent: Orthoalkyl silicate + Tin octane

39
Q

What is the volatile by-product of condensation silicone’s condensation polymerization, setting reaction?

A

An ALCOHOL

40
Q

What forms does condensation silicone come in?

A

Putty, heavy, medium & light bodied viscocities

41
Q

Condensation silicone has very good accuracy properties, with one exception. What is this one very important exception?

A

SHRINKAGE. Due to the loss of alcohol during the condensation polymerization reaction while condensation silicone sets, there is a lot of shrinkage (0.2-1%). Much too high.

42
Q

Setting time of condensation silicone can be controlled by the amount of accelerator used. Why might this be a problem?

A

If the correct amounts/ratios aren’t used, the accelerator may affect the final material.

43
Q

What is the best overall impression material available?

A

Addition silicone

44
Q

What are the Base & Catalyst pastes for addition silicone?

A
  1. Base: Vinyl terminated polydimethyl siloxane polymer

2. Catalyst: Chloroplatinic acid

45
Q

The setting reaction of addition silicone is what type? What is important about it?

A

Addition polymerization reaction with NO volatile by-products. This is very important b/c if no by-products are lost the impression material does not shrink

46
Q

What common substances will inhibit the setting of addition silicones?

A

Tin/sulfur compounds & powdered gloves.

47
Q

Platinum compounds are placed in modern day addition silicon materials to stop what?

A

The leakage of hydrogen gas, which decreased accuracy by causing porosity in the impression.

48
Q

What is the most important negative thing that can affect the accuracy of addition silicone impressions?

A

Distortion resulting from the poor use of adhesives during the impression.

49
Q

Addition silicones allow for multiple pours. Why?

A

B/c stable over longer periods of time…1 week

50
Q

What base & catalyst paste make up non-aqueous elastomer, Polyether?

A

Base: Polyether polymer
Catalyst: Alkyl aromatic sulfate

51
Q

What are the filler, colloidal silica, and the plasticizer, glycol ether, used for in the base paste?

A

The filler & plasticizer help to create optimal viscocity in order to allow 1:1, Base:Catalyst paste ratio.

52
Q

What is the basic mechanism by which the polyether polymerase rxn occurs? Are there any volatile by-products? What is the final product?

A

Ionic polymerization.
No volatile by-products
Polyether rubber = final product

53
Q

Polyether is not a base-catalyst paste impression material. Instead, how is polyether delivered?

A

Via the Pentamix auto mix system

54
Q

What is the major trouble w/ polyether that makes its use difficult at times?

A

Polyether is very inflexible. Can pull teeth out when set impression is being removed from the mouth. Never use for people w/ PDL problems or that have gaps b/t their teeth.

55
Q

Transmission of what viral diseases are a concern when taking impressions? Give 3 examples

A
  1. Hepatitis B
  2. Herpes simplex virus
  3. AIDS
56
Q

Name 3 disinfectants used for cleaning impressions.

A
  1. 1% sodium hypochlorite
  2. 2% glutaraldehyde solution immersion for 30 minutes
  3. Iodophors
  4. Phenolics
57
Q

What is the non-aqeous elastomer with the best combination of characteristics? What is one area that polysulfide rubbers perform much better in?

A
  1. Addition Silicone

2. Tear strength

Dental Materials Exam III (2 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions