Rubber Base Impression Techniques

Rubber Base Impression Techniques

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

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

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.

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 :

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.

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.

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.

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.

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.

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.

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.

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.