Rubber Base Impression Techniques

Rubber Base Impression Techniques

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Rubber base impression materials are another group of elastomeric impression materials. They are soft and rubber-like and are known as elastomers or synthetic rubbers. Elastomeric materials contain large molecule with weak interaction between them. They are tied together at certain points to form a three-dimensional network.

Classification of Rubber Base Impression Materials:

• On the basis of their chemistry
  •  Polysulfide
  • Condensation polymerizing
  • Addition polymerizing silicones
  •  Polyether.
• On the basis of their viscosity (ISO 4823:2015)
  • Type 0—Putt consistency (very heavy)
  • Type 1—Heavy bodied consistency (tray consistency)
  • Type 2—Medium bodied consistency (regular-bodied)
  • Type 3—Light bodied (syringe consistency).
• On the basis of wettability or contact angle:
  • Hydrophilic, if their contact angle is from 800 to 1050
  • Hydrophobic, if their contact angle is from 400 to 700.
• According to ADA specification: As per the elastic property and dimensional stability of set material, ADA has classified elastomers as:
• Type –  1
• Type –  2
• Type –  3

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Polysulphide Elastomeric Impression Material:

This was the first elastomeric impression material to be introduced. It is also known as “Mercaptan or Thiokol”.

Composition of Polysulphide Elastomeric Impression Material:

• Base Paste:
  • Liquid polysulfide polymer 80 – 85%
  • Inert filer (titanium dioxide, zinc sulfate or silica) 16 – 18%
• Reactor Paste:
  • Lead dioxide 60 – 68%
  • Dibutyl phthalate 30 – 35%
  • Sulfur 3%
  • Other substances like magnesium 2% stearate and deodorants

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Setting Reaction of Polysulphide Elastomeric Impression Material:

As base and accelerator pastes are mixed, they undergo both polymerization by oxidation of terminal SH groups and the cross-linking of pendant SH groups. Reaction occur between polymer with free mercaptan, i.e.–SH groups, and the oxidizing agent lead dioxide, which lengthen and cross-link the chain through reaction of terminal and pendant –SH group. The whole reaction is exothermic which increases the temperature from 3°C to 5°C and formation of water as by-product.

Mercaptan + Lead dioxide → Polysulfied +Water

Properties of Elastomeric Impression Material:

It has an unpleasant odor and color. It stains linen and is messy to work with.

These materials are extremely viscous and sticky. Mixing is difficult.

• It has a long setting time of 12.5 minute.
• Excellent reproduction of surface detail.
• Dimensional stability: The curing shrinkage is high and continuous even after setting. It has the highest permanent deformation among the elastomers, i.e. 3 to 5%.
• It has high tear strength (4,000 g/cm).
• It has good flexibility (7%) and low hardness.
• It is hydrophobic so the mouth should be dried before making an impression.
• It can be electroplated more with silver than copper.
• Shelf-life is good, i.e. 2 years.

Advantages of Polysulphide:

• Excellent reproduction of surface detail as fie as 0.025
  mm wid
• It can be electroplated more with silver than copper
• The shelf-life is good (2 years)
• It has good flexibility (7%)
• It has high tear strength (4,000 g/cm) thus making it more resistant to tearing even when the impression is in thin section.

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Disadvantages of Polysulphide:

• Unpleasant odor and color.
• These materials are extremely viscous and sticky. Mixing is difficult.
• It has a long setting time which adds to the patient’s discomfort.
• Dimensional unstability: It has high permanent deformation. The curing shrinkage is high, and loss of byproduct (water) also causes shrinkage.
• Staining is present due to lead dioxide.

Condensation Silicone

It is also known as conventional silicone.

Composition of Condensation Silicone :

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Setting Reaction of Condensation Silicone : 

It is a condensation reaction. Polymerization occurs as a result of cross-linkage between the ortho-ethyl silicate and terminal hydroxy group of dimethylsiloxane to form a three-dimensional network. Ethyl alcohol is the by-product of the reaction which evaporates causing contraction of the set condensation silicone impression. The reaction is exothermic with a temperature rise of 1°C.

Properties of  Condensation Silicone:

• It has pleasant color and odor.
• Its setting time is 6 to 9 minutes while the mixing time is 45 seconds.
• It is non-toxic but direct skin contact should be avoided.
• Excellent reproduction of surface details.
• Dimensional stability is comparatively less because of high curing shrinkage (0.4 to 0.65).
• Tear strength (3,000) g/cm is lower than the polysulfides.
• It is stiffer than polysulfide.
• It is hydrophobic so field should be dried before making an impression.
• It can be plated with silver/copper.
• Shelf-life is slightly less than polysulfide.

Advantages of  Condensation Silicone:

• It does not consist of a bad odor of polysulphides.
• Basic advantages of elastomeric impression materials are present.

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Disadvantages of Condensation Silicone:

• Shelf life is limited.
• As by-products are formed after the chemical reaction, the impression cannot be kept for an extended period of time.
• Due to evaporation of ethyl alcohol dimensional stability is affected.

Addition Silicone

It has better properties than condensation silicones. It is also known as “Polyvinyl siloxane”.

Composition of Addition Silicone :

• Base:
  • Poly (methyl hydrogen siloxane)
  • Other siloxane prepolymers
  • Fillers (amorphous silica or fluorocarbon)
  • Palladium or hydrogen absorber
  • Retarders.
• Accelerator:
  • Divinyl polysiloxane
  • Other siloxane prepolymers
  • Platinum salt-catalyst
  • Fillers.

Setting Reaction of Addition Silicone :

It is an additional reaction. In this, the base polymer is terminated with vinyl groups and is cross-linked with silane. The reaction is activated by platinum salt. There are no by-products as long as, there is a balance between the vinyl siloxane and silane siloxane.  If unbalanced, hydrogen gas is produced causing air bubbles in stone models. To avoid this palladium is added to absorb hydrogen.

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Properties of  Addition Silicone:

• Its odor and color is pleasant.
• It can lead to allergic reactions.
• The reproduction of surface details is excellent.
• Setting time is 5 to 9 minutes. The mixing time is 45 seconds.
• Working time is extended by chilling the tubes.
• Dimensional stability is good. Curing shrinkage is low (0.17%) and permanent deformation is very low, i.e. 0.05 to 0.3%.
• Tear strength is good, i.e. 3,000 g/cm.
• Material is hydrophobic so care should be taken while pouring impression.
• It can be electroplated from silver or copper
• Flexibility is low. Extra spacing should be provided in impression tray. Care should be taken at time of removing stone cast from impression to avoid breakage.
• Its shelf-life ranges from 1 to 2 years.

Advantages of Addition Silicone :

• Most dimensionally stable impression material.
• These impressions can be kept for 7 days after taking impression.
• They are available in various consistency so can be used in various clinical situations.

Disadvantages of Addition Silicone :

• They are inherent and have hydrophobic nature. A rounded margin with loss of detail in impression is caused by moisture contamination of prepared tooth.
• Sulfur contamination from latex gloves inhibit setting  reaction of additional silicones.
• As residual hydrides in set material react with moisture present in the atmosphere and evolve hydrogen gas, due to this cast cannot be poured readily.
• It should be poured after 30 minutes for gypsum casts and only after 24 hours for epoxy dies.

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Polyether Rubber Impression Material

It has good mechanical properties and dimensional stability.

Composition of Polyether Rubber Impression Material:

• Base:
  • Polyether polymer 80 – 85%
  • Colloidal silica—filer
  • Glyco ether or Phthalate — Plasticizer.
• Accelerator Paste:
  • Aromatic sulfonate ester — cross-linking agent
  • Colloidal silica — Filer
  • Phthalate or glycol ether — Plasticizer.

Setting Reaction of Polyether Rubber Impression Material:

It is cured by the reaction between aziridine rings which are at the end of branched polyether molecules.The main chain is a copolymer of ethylene oxide and tetrahydrofuran. Cross-linking is brought about by the aromatic sulphonate ester via the imine end groups. The reaction is exothermic. No by-product is formed.

Polyether + Sulfonic ester→ Crosslinked rubber

Properties of Polyether Rubber Impression Material

• Its taste and odor are pleasant.
• It can lead to skin reactions due to presence of sulfonic ester.
• Thorough mixing is done before making an impression.
• Its setting time is 6 to 8 minutes. Mixing is done quickly, i.e. in 30 seconds. Heat decreases the setting time.
• Dimensional stability is good. Curing shrinkage is low, i.e. 0.24%.
• Permanent deformation is low, i.e. 0.8 to 1.6%. Polyether absorbs water and can change dimension, so prolonged contact with water and humid climate is not recommended.

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The material is stiff and it is difficult to remove it from undercuts so additional spacing is required.

• Tear strength is good, i.e. 3000 g/cm.
• Since the material is hydrophilic, moisture in the impression field is not critical. It has the best compatibility with stone among
  all elastomers.
• Its shelf-life is excellent, i.e. more than 2 years.
• It can be electroplated with silver or copper.

Uses of Rubber Base Impressions Materials:

• In fixed partial dentures for an impression of prepared teeth.
• Impressions of dentulous mouths for removable partial dentures.
• Impressions of edentulous mouths for complete dentures
• Polyether is used for border molding of special trays.
• For bite registration.
• Silicone duplicating material is used for making refractory casts during cast partial denture condensation.

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Manipulation of Rubber Base Impression Materials:

Following are the methods of manipulating rubber base or elastomeric impression materials, i.e.

• Hand mixing
• Static auto-mixing system
• Dynamic mechanical system

Hand Mixing:

• High-viscosity material, i.e. putt is usually supplied in small tubs with different colored scoops for identification and dispensing.
• Equal quantities of both the pastes are taken. Mixing is done by hand by kneading the material together to obtain a uniform mix without any streaks.
• Manufacturers also supply base and catalyst in separate tubes with a much smaller diameter opening for a catalyst to enable dispensing equal lengths of base and catalyst, though the volume is different.
• For polysulfides and additional silicone mixing of low-viscosity material is accomplished by taking equal lengths of base and catalyst on a paper pad and manipulating by stiff bladed spatula in a circular motion for 45 seconds till the mix is free from streaks.

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For condensation silicone the quantity of reactor paste needed is very little

• So the base and reactor pastes are extruded on a mixing pad with unequal lengths of base and catalyst on a paper pad and manipulated by stif bladed spatula in a circular motion for 45 seconds till the mix is free from streaks.
• In polyether rubber base impression material the quantity of reactor paste needed is very little.
• So the base and reactor pastes are extruded on mixing pad with unequal lengths without touching each other.
• Required amount of thinner when supplied is added to the base and accelerator depending on the viscosity needed.
• Reactor paste is incorporated into base paste. Mixing is done by using tapered stiff bladed metal or plastic spatula. A streak-free mix is obtained in 45 seconds.

Static Automixing System:

These days many of the materials are now dispensed by an extruder gun with a mixing tip.

• Base and catalyst are stored or supplied in separate cylinders of the plastic cartridge.
• Extruder gun contains two plungers on which these cylinders are placed.
• Now the base and catalyst are forced through the mixing tip.
• It consists of a stationary plastic internal spiral that brings together the correct volume of both base and catalyst, and dispenses it premixed.
• Apply the mixed material directly over the prepared teeth and/or impression tray.
• The advantages of the mixing tip include dispensing of the correct ratio of base and catalyst, and uniform mixing without incorporation of air bubbles.’
• Addition and condensation silicones, and polyethers are available in these systems.

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Dynamic Mechanical System:

It is also known as automated mixing system.

• It consists of a separate mixing machine with cartridges for the base and catalyst, and a plastic mixing tip.
• Base and catalyst are supplied in collapsible plastic bags.
• After housing of the base and catalyst in the cartridge, the mixing tip is placed in the front of the machine.
• On pressing the button on the top, the machine automatically mixes the base and catalyst by the movement of the plungers pushing against the collapsible plastic bags.
• Here in this system the internal spiral is motor drive that rotates. A uniform, thorough mixing can be accomplished even for higher viscosity materials.
• In this system benefit is of uniform mixing and the speed of mixing and the disadvantage is the equipment is expensive.
• Only addition silicones and polyethers are available in these systems.