Polymerization In Dentistry Notes

Polymerization

• Polymers are long chains of large high-molecular weight macromolecules. They are formed by a chemical reaction where a large number of smaller units or monomers join to form polymer macromolecules, a process called polymerization.
• Elastomers are liquid polymers which can be converted to solid rubber at room temperature. By mixing with a suitable catalyst, they undergo polymerization and/or crosslinking (by condensation or addition) reaction to produce a firm elastic solid.

Types of polymerization reactions

In elastomers 3 types of polymerization reactions are seen.

• Addition polymerization
• Condensation polymerization
• Ring opening polymerization

Glass transition temperature in elastomers

Read And Learn More: Basic Dental Materials Notes

• The glass–liquid transition (or glass transition for short) is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state.

Glass Transition Temperature In Elastomers Types

According to Glass Transition Temperature In Elastomers chemistry

Chemically, there are four kinds of elastomers.

• Polysulfide
• Condensation polymerizing silicones
• Addition polymerizing silicones
• Polyether

Glass Transition Temperature In Elastomers Classification based on viscosity

Each type may be further divided into four viscosity classes based on consistencies determined immediately after completion of mixing.

Type 0—Putty consistency (very heavy)

Type 1—Heavy-bodied consistency (tray consistency)

Type 2—Medium-bodied consistency (regular bodied)

Type 3—Light-bodied (syringe consistency)

According to wettability or contact angle

Impression materials are also classified as

• Hydrophilic, if their contact angle is from 80 to 105°.
• Hydrophobic, if their contact angle is from 40 to 70°.

Uses Of Elastomeric Impression Materials

• In fixed partial dentures for impressions 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 edentulous custom trays.
• For bite registration.
• Silicone duplicating material is used for making refractory casts during cast partial denture construction.

Elastomeric Impression Materials Supplied As

Regardless of type all elastomeric impression materials are supplied as two component (base and catalyst) systems.

• Collapsible tubes
• Cartridges—light and regular body material are also supplied in cartridges to be used with static mixing tips and dispensers
• Putty consistency is supplied in jars

General Properties Of Elastomeric Materials

• Excellent reproduction of surface details. The low viscosity is capable of producing very fine details.
• They are generally hydrophobic (except polyether which is hydrophilic), so the oral tissues in the area of impression should be absolutely dry for better flow of the impression material. Because of their hydrophobic (water hating) nature, care must be taken while pouring stone in the impression. The poor wetting and high contact angle can result in air entrapment. Commercial surfactants sprays are available which improve wetting.
• Elastic properties of elastomers is good with near complete elastic recovery. Repeated pouring of impression is possible (though not recommended when high accuracy is critical).

• Coefficient of thermal expansion of elastomers is high. Thermal contraction occurs when impression is transferred from mouth to room temperature.
• In general dimensional changes and inaccuracies occur due to
  • Curing shrinkage.
  • Loss of by-products of reaction, e.g. condensation silicones lose alcohol and shrink. Polysulfides (hydroperoxide type) lose volatile accelerators causing contraction.
  • Polyether being hydrophilic absorbs water and loses soluble plasticizers causing change in dimension (e.g. when immersed in disinfectant).
  • Thermal contraction when transferred from mouth to room temperature.
  • Removing impression before complete setting can cause serious distortion.
  • Incomplete recovery after deformation during removal.
  • Amount of filler When filler content is increased, the polymer content is reduced and shrinkage is less. Thus, less shrinkage is seen in putty, and higher shrinkage is observed in light bodied.
  • Uniform thickness of material gives more accurate impression as the shrinkage is uniform.
  • Good adhesion of impression to the tray (using adhesives) minimizes dimensional changes as the shrinkage is directed towards the tray. In the absence of adhesion between the tray and impression, the shrinkage is directed centrally and the model prepared will be smaller in size.
  • Time of pouring Impression should be poured after elastic recovery but before dimensional changes set in.
• The tear strength of these materials are excellent, thus making it more resistant to tearing even when the impression is in thin sections.
• Electroplating Elastomers can be copper and/or silver plated.
• Radiopacity Radiopacity of impression materials is important for radiographic identification of excess material which may be accidentally swallowed, aspirated or left in gingival tissues. Presently, only the polysulfide materials exhibit significant radiopacity due to their lead dioxide content.
• Retention to tray Elastomeric materials do not adhere well to the impression tray. They may be retained by
  • Mechanically by using perforated trays (only in case of putty).
  • Tray adhesives These are tacky liquids that are applied with a brush. Each elastomer type has a specific adhesive which is not interchangeable.
• The shelf life is about two years. The silicones have a slightly lower shelf life. Storage under cool conditions increases shelf life.
• Color They come in a variety of colors.