These are the properties a Operator looks at, which make it easy for him/her to operate and get an Ideal Impression. The All the Properties should be fulfilled to produce an Ideal Impression material, till date such an impression material has not been discovered or invented, so we have to use different Impression materials in different situations based on the requirements.

1. Pleasant Odor (example of bad odor- Polysulphide)
2. Pleasant Taste
3. Pleasant color
4. Absence of Toxic or irritant constituent
5. Adequate shelf life for requirements of storage and distribution
6. Should be economical, in case of high cost should be having increased charecteristics
7. Should be Easy to Use without requirement of much equipments
8. Setting time should be adequate for clinical use – And such that the Practitioner can be able to alter it according to his requirement
9. Satisfactory consistency and texture
10. Should Readily wet the Oral tissues (Hydrophilic materials)
11. Should not release Gasses during setting of Cast/Die/impression material
12. Should be readily disinfected without loss of accuracy
13. Accurate in clinical use
14. Compatibility with Cast or Die materials
15. Dimensional stability over temperature and humidity changes which can be seen in clinical conditions
16. Should be stable long enough to make a cast or Die from the impression taken
17. Should have Adequate strength so it will not break, tear during removal from mouth
18. Elastic properties with freedom from permanent deformation after strain

It is called Reversible Impression material because of its property of reversing their physical state, which in reusing the impression material for multiple number of impressions.

Agar Hydrocolloid was the first successful elastic impression material to be used in dentistry.

History of Agar Impression material:

It is was first discovered by Alphous Poller of Vienna in 1925 when he was trying to develop a material that could be sterilized and applied without pressure to the exposed surface of the dura matter for perfectly recording the convulsion and the bony margins of the skull.

Which was termed as ‘Negacol’ which was termed as “Dentacol” when introduced into dentistry.

Origin Of Agar Impression material:

• An Organic Hydrophillic Colloid (Polysaccharide) extracted from certain types of Seaweed
• It is a Sulphuric ester of a linear polymer of Galactose

Available as:

• Gel in collapsible tube (Used with a Tray)
• Multiple Cylinders in a glass jar (Syringe material)
• In bulk containers (for duplication)

Composition:

• Agar: 13-17%
• Borates: 0.2-0.5%
• Potassium Sulphate: 1-2%
• Wax: 05-1%
• Thixotrophic Materials: 0.3-0.5%
• Alkylbenzoates: 0.1%
• Coloring and Flavoring agents: Traces
• Water: Balance (Around 84%)

Functions of Each ingredient:

• Agar: Basic Constituent 13-17% used for tray material and 6-8%is used for Syringe material
• Borates: Improves Strength of the gel, (Also retards the setting of plaster or stone cast when poured into the finished impression – a disadvantage)
• Potassium Sulphate: It counters the retarding effect of borates, thereby ensures proper setting of the cast or die.
• Wax: It acts a Filler. Fillers affect the strength, viscosity and rigidity of the gel. Other Examples of fillers – Zinc Oxide, Diatomaceous Earth, Silica, Clay, Rubber etc..
• Thixotrpoic materials: It acts as plasticizer. Examples are: Glycerine and Thymol. Thymol acts as bactericide also.
• Alkylbenzoates: It acts as a Preservative
• Water: It acts as a Dispersion medium
• Coloring and Flavoring agents: Used for patient comfort and acceptance.

Materials Required for Manipulation of Agar :

• Agar Hydrocolloid Conditioner
• HydrocolloidMaterial
• Water cooled rim lock trays
• Hydrocolloid Syringe Material
• Duplicating Mould
• Rubber Hose

Materials used for Agar Impression

How does Agar Hydrocolloid Conditioner work:

It has 3 sections:

• Boiling Section (or) Liquefaction section
• Storage Section
• Tempering Section

Agar Hydrocolloid Conditioner

Boiling Section (or) Liquefaction section:

• Agar is placed in Boiling water at 100 degrees Centigrade for 10 Mins
• The Solution should be Homogenous and free of Lumps
• Everytime the material is reliquefied, three minutes should be added for the heating. This is because it is more difficult to break down the agar brush heap structures  after a previous use.

Storage section:

• 65-68 Degrees Centigrade is ideal temperature for Storage of Agar in Sol condition.
• It can be stored in the sol condition till required for impression taking.

Tempering Section:

• 46 degrees Centigrade for about 2 minutes with the material loaded in the tray.
• This is done to reduce the temperature so that it can be tolerated be the sensitive oral tissue
• It also makes the material viscous suitable for impression taking

 How to Use Water Cooled Rim Lock Impression Tray:

• Rim Lock Impression Trays have a feature of Water Circulation through the Tray.
• The Rim Lock is a Beeding on the inside edge of the tray which helps to retain the material (as Agar does not adhere to the tray)
• It has an Inlet and Outlet for connecting the water tubes
• The Tray should allow a space of 3mm occlusally and laterally and extend distally to cover all teeth

Water Cooled Rim Lock Impression Tray - Agar

How to take an Agar Impression:

Steps to Take the Impression:

1. Remove the Tray containing the Agar from the Tempering Section of the Hydrocolloid Conditioner.
2. The Outer surface of the Agar sol is scraped off
3. The water hoses are connected and the tray is positioned in the mouth by the dentist
4. Water is circulated at 18 to 21 degrees Centigrade through the tray until gelation occurs.
5. Rapid Cooling: (e.g ice cold water) is not recommended as it can induce distortion
6. To guide the tray into position, 3 stops of compound are prepared on non-involved teeth
7. A Post dam is constructed with compound to prevent distal flow of the impression material
8. In a deep palate case, compound is placed on the palatal aspect of the tray in order to provide a uniform thickness of the Hydrocolloid
9. The mandibular tray is prepared by placing compound on the distal aspect to limit the impression material
10. Black Tray compound is used as it is not affected in the tempering bath

Steps to Remove the Impression:

1. Wait for the Agar to turn into a Gel
2. Break the Peripheral seal around the Impression
3. Remove the Impression rapidly from the mouth with a Single stroke or snap (This is done to avoid distortion of the impression)
4. The Impression is Rinsed thoroughly with water ans the excess water is removed by shaking the impression

How to Store Agar Impression:

1. The Cast should be poured immediately
2. Storage of Agar Impression material should be avoided at any cost, as there is no storage medium available
3. Storage in Air results in 100% relative humidity which results in Shrinkage as a result of continued formation of the agar network agglomeration
4. If Storage is unavoidable, it should be limited to one hour in 100% relative humidity

How to Pour and Separate the Cast:

1. Use Gypsum products for pouring the Cast
2. Mix the Gypsum product and Pour it into the impression
3. When the Gypsum products have set, the agar impression must be removed promptly since the impression will Dehydrate and become Stiff and difficult to remove
4. Be careful while removing the Cast as the weaker portions may fracture
5. Prolonged contact will result in a rougher surface on the model, so  make sure the cast is removed immediately after the setting time of the gypsum product is finished

Uses Of Agar:

1. Used as a Cast Duplicating material (during fabrication of the cast metal removable partial denture)
2. For Full mouth impressions without deep undercuts
3. Used for Crown and and Bridge impressions before Elastomers were Discovered
4. Used as Tissue Conditioners

Materials Used To Disinfect Agar Hydrocolloid Impression:

1. Iodophor
2. Bleach
3. Glutaraldehyde

Advantages Of Agar Hydrocolloid Impression Material:

1. Helps in preparing Dies Accurately
2. It has Good Elastic Properties and Reproduces most undercut areas correctly
3. It has Good Recovery from Distortion
4. It is not Hydrophobic, hence gives good model surface
5. It is Palatable ans well tolerated by the patient
6. It is cheap when compared to synthetic elastic materials
7. The Main Advantage is: Agar can be Reused when used as a Duplicating Material

Note: Reuse is not recommended when used as an impression material

Disadvantages Of Agar Hydrocolloid Impression Material

1. Does not Flow as good as the newly developed materials
2. It cannot be Electroplated
3. Due to the heat it may be painful to the patient during Insertion or Gelation
4. Tears relatively easily
5. Greater Gingival retraction is required for providing adequate thickness of the material
6. Only one model can be poured
7. Expensive Equipment is required
8. A Plaster Hardener should be used to get a soft surface on Gypsum Cast
9. Contamination is very prevalent with this material due to Reuse and the lack of Sterilization technique

• It was the first type of silicone impression material.
• Also known as conventional silicones.
• Setting occurs in room temp , so called as RTV silicones.

Composition:

Base paste

1. Polydimethyl siloxane (hydroxy terminated)
2. Colloidal silica or micro sized metal oxide filler
3. putty viscosity – 60-70%
4. medium viscosity – 35-75 %
5. low viscosity – 5-15%
6. color pigments

accelerator paste

1. alkyl silicate (ortho ethyl silicate)
2. stannous octate
3. inert filler.

Setting reaction:

Dimethyl        orthoethyl                                                    silicone            ethyl

+                    —^{STANNOUS OCTATE—–}>                           +

Siloxane          silicate                                                         rubber           alcohol

Properties:

1. Setting time – 8-9 min.
2. Mixing time – 45 sec.
3. Excellent detail reproduction
4. Dimensional stability – high curing shrinkage.
5. Permanent deformation – 1-3 %
6. Ttear strength – 3000gm/cm
7. Sitffer & harder than polysulfide material.
8. Hydrophobic
9. Can be electroplated with silver & copper.
10. Shelf life – 2 yrs.

Advantages:

1. Adequate working & setting timer.
2. Pleasant odor & no staining.
3. Adequate tear strength
4. Better elastic properties on removal.
5. Less distortion on removal.

Disadvantages:

1. Adequate accuracy if poured immediately.
2. Poor dimensional stability.
3. Potential for significant distortion.
4. Putty-wash method is technique sensitive.
5. Slightly more expensive.

ADITION SILICONES

• Introduced after condensation silicones
• Have better properties than condensation silicones.
• Also known as poly vinyl siloxane.

Composition:

Base paste

1. Poly(methyl hydrogen siloxane)
2. Other siloxane prepolymers
3. Fillers

putty viscosity – 60-70 %
medium viscosity – 35-75 %
low viscosity – 5-15%

accelerator paste

1. Divnyl polysiloxane
2. Inert oils & fillers
3. Platinum salt
4. Palladium
5. Retarders
6. Fillers.

Setting reaction:

Poly vinyl siloxane + silane siloxane     —–Pt ,Salt —–>   silicone rubber

Properties:

1. Setting time: – 5.9 min.
2. Mixing time  – 45 sec
3. Excellent surface detail reproduction.
4. Highest dimensional stability amongst elastomers
5. Lowest curing shrinkage – 0.17 %
6. Good tear strength – 3000gm/cm
7. Extremely hydrophobic
8. Low flexibility
9. Electroplated with copper & silver.
10. Shelf life – 1-2 yrs.

Advantages:

1. Short setting time
2. Adequate tear strength
3. Extremely high accuracy
4. Minimal distortion on removal
5. Dimensionally stable  even after 1 week
6. If hydrophilic, good compatibility with gypsum.

Disadvantages:

1. Hydrogen gas may cause dimensional change.
2. Hydrophobic & hence requires a very dry field.
3. Expensive.

First elastomeric impression material to be introduced, also known as mercaptan / thymol, it is a type of Rubber based impression material

Supplied as

• Base paste (white)
• Accelerator (brown/grey)

Available in 3 viscosities:

• Light bodied
• Medium bodied
• Heavy bodied

Composition:

Base paste:

• Liquid polysulfide polymer                                                  80-85%
• Inert fillers (titanium oxide, zinc sulfate, silica)         16-18%

Reactor Paste:

• Lead dioxide                                                                            60-69%
• Dibutyl phthalate                                                                  30-35%
• Sulfur                                                                                          3%
• Other substances (like magnesium stearate)             2%
• and deodorants

Setting reaction:

HS – R – SH  ———-^PbO₂+ ^S—->    HS – R- S – S – R – SH + H₂O

Properties:

1) Setting time – 5-8 min.

• colder climate increases setting time.
• Drop of water accelerates the reaction.

2) Excellent detail reproduction.

3) Dimensional stability

• curing shrinkage – 0.45% due to continued reaction.
• Shrinkage due to loss of byproduct.

4) Permanent deformation – 3-5%

• highest amongst elastomers.

5) High tear strength – 7000gm/cm²

6) Good flexibility of 7%, allows elastic recovery of the material.

7) Hydrophobic- tissue should be thoroughly dried before making the impression.

8) Can be electroplated with copper.

Advantages:

1. long working time.
2. proven accuracy
3. high tear resistance.
4. inexpensive to use
5. less hydrophobic
6. longer shelf life.

Disadvantages:

1. must be poured with dental stone immediately.
2. potential for significant distortion.
3. odor is offensive.
4. messy & stains clothes.
5. second pour is less accurate.

Classification of Elastomeric Impression materials:
Till now the elastomeric impression materials are divided into 3 types based on the material used

1. Polyether
2. Polysulfide
3. Silicone

• Addition
• Condensation

POLYETHER IMPRESION MATERIAL

• Introduced in Germany in late 1960′s
• 1^st Elastomers to be developed as impression material.

Composition:

Base Paste:

1. imine terminated polymer(polyether)
2. colloidal silica
3. glycol ether or phthalate

Accelerator Paste:

1. alkyl aromatic sulfonate
2. colloidal silica
3. plasticizers ( glycoether )

Setting Reaction:

polyether + sulfonic ester –> cross linked rubber.

Properties:

1. setting time:  8.3 min.
2. mixing time – 30 sec.
3. dimensional stability is good.
4. decreased flexibility – 3%
5. hydrophilic
6. activator paste can produce allergy if handled frequently.
7. tear strength – 3000gm/cm
8. shelf life > 2 yrs.

Advantages:

1. short working & setting time
2. proven accuracy
3. adequate tear strength
4. hydrophilic
5. long shelf life
6. less distortion on removal
7. good dimensional stability.

Disadvantages:

1. stiffness requires blocking of undercuts
2. slightly more expensive
3. multiple casts can’t be poured due to stiffness of the material.

Manipulation of Elastomers:

Manipulation of two paste systems

• equal length of base & activator paste are taken.
• Paper pad with graduations is provided by manufacturer.
• Pastes are mixed till no streaks are present.
• Tray coated with tray adhesive. Uniform two-layer coat is applied & allowed to dry.
• The mixed material can either be taken in a syringe or loaded directly in the tray.

Manipulation of Putty Material:

• Putty dispenser is supplied with scoops for dispensing.
• Activator may be a liquid or paste.
• Putty material is taken in a scoop & the activator is added. Usually the activator is of different color.
• It is kneaded till no streaks are present.
• Putty material is used with stock trays.