Lecture 2: Elastic Impression Materials Flashcards by William Lowrey

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.

Aqueous Elastomers:
1. Agar-agar
2. Alginate
Non-Aqueous Elastomers:
1. Polysulfide
2. Condensation silicone
3. Addition silicone
4. Polyethers

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

Rubbers

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What type of colloid material is Agar-agar?

Reversible Hydrocolloid

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What is the active ingredient in agar-agar?

Agar

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What is K2SO4 used for in Agar-agar?

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

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

Water ~85.5%

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

Agar Sol - Occurs in Hot conditions - In order to transition to agar sol must heat to 100 degrees celcius
Agar Gel - Occurs in Cold conditions - To transition to Agar gel must cool to 43 degrees celcius

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

Tempering - 46 degrees C

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

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

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

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

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

Agar - Has detail reproduction down to 25 um

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

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

Syneresis: Loss of water during storage. Causes shrinkage & loss of accuracy.
Imbibition: Absorption of water. Swelling, loss of accuracy

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

Working = 7-15 min - very long

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

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

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

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

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.

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

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

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

Alginate

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

K-Alginate (~15%)

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

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

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What are the two, sequential setting reactions for alginate?

2Na3PO4 + 3Ca3SO4 —> Ca3(PO4)2 + 3Na2SO4 = Retardent step
Na.nAlg Sol + n/2CaSO4 —> n/2Na2SO4 + Ca.n/2Alg Gel = Setting step (Alginate Sol (powder) to Alginate Gel)

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How is Alginate set time influenced?

Adjusting concentration of sodium phosphate (retarder)

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

Poorly.

Detail reproduction is 75 um vs 25um
Elastic recovery is only 97.3% vs 98.8% for agar
Very poor dimensional stability, same as agar
LOW tear strength
High rate of permanent deformation

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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?

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

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

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

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

Increased Temp: Speed up | Decreased Temp: Slow down setting rxn

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

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

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

More convenient. Used for quick, non-final impressions

Alginate storage?

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

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

1. Base & catalyst pastes | 2. Automix systems

What is the major use for non-aqueous elastomers?

Mostly used for taking final impressions for restorations

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

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

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

Condensation polymerization with water by-product. Exothermic rxn.

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

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

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

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

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

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

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

1:1 base:catalyst ratio

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

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

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

Base paste: Hydroxy terminated polydimethyl siloxane | Catalyst past/liquid: Crosslinking agent: Orthoalkyl silicate + Tin octane

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

An ALCOHOL

What forms does condensation silicone come in?

Putty, heavy, medium & light bodied viscocities

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

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.

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

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

What is the best overall impression material available?

Addition silicone

What are the Base & Catalyst pastes for addition silicone?

1. Base: Vinyl terminated polydimethyl siloxane polymer | 2. Catalyst: Chloroplatinic acid

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

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

What common substances will inhibit the setting of addition silicones?

Tin/sulfur compounds & powdered gloves.

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

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

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

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

Addition silicones allow for multiple pours. Why?

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

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

Base: Polyether polymer Catalyst: Alkyl aromatic sulfate

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

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

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

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

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

Via the Pentamix auto mix system

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

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.

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

1. Hepatitis B 2. Herpes simplex virus 3. AIDS

Name 3 disinfectants used for cleaning impressions.

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

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

1. Addition Silicone | 2. Tear strength