Heat-Cure Denture Base Materials: Advantages, Disadvantages, and Uses

Polymeric Denture Base Materials

Heat-cure Denture Base Materials:

They are most widely used resins for the construction of removable or complete dentures.

Properties Of Heat-cure Denture Base Materials:

• Taste and odor:
  • Completely polymerized acrylic resin is tasteless and odorless.
  • On the other hand, poorly made dentures with a high amount of porosity can absorb food and bacteria resulting in an unpleasant taste and odor.
• Esthetics:
  • It is a clear transparent resin that can be pigmented (colored) easily to duplicate the oral tissue.
  • It is also compatible with dyed synthetic filers thus esthetics is acceptable.
• Strength:
  • These materials are typically low in strength.
  • However, they have adequate compressive (75 MPa) and tensile strength (48 to 62 MPa) for complete or partial denture applications.
• Modulus of electricity: They have sufficient stiffness (2400 MPa) for use in complete and partial dentures.
• Dimensional stability:
  • A well-processed acrylic resin denture has good dimensional stability.
  • The processing shrinkage is balanced by the expansion due to water sorption.
• Solubility: Acrylic is virtually insoluble in water and oral fluids.
• Stability to heat: Polymethyl methacrylate is chemically stable to heat up to a point. It softens at 125°C.
• Color stability: Heat-cure acrylic resins have good color stability. The color stability of self-cure resin is slightly lower.
• Biocompatibility: Completely polymerized acrylic resins are biocompatible. True allergic reactions to acrylic resins are rarely seen in oral cavity.
• Adhesion: The adhesion of acrylic to plastic denture teeth is good chemical adhesion.
• Shelf life: Acrylic resins dispensed as powder and liquid have the best shelf-life.
• Radiopacity:
  • There are instances of broken pieces of dentures being aspirated or swallowed.
  • Radiopacity is a desirable property to enable easy location of fragments.
  • Most denture base materials are radiolucent. Radiopacity is obtained by adding heavy metal salts such as bismuth or uranyl at a concentration of 10 to 15%.

Thermal properties:

• Stability to heat:
  • Polymethyl methacrylate is chemically stable to heat up to a point. It softens at 125°C.
  • However, above this temperature, i.e. between 125°C and 200°C it begins to depolymerize.
  • At 450°C, 90% of the polymer will depolymerize to monomer.

Thermal conductivity:

• They are poor conductors of heat and electricity.
• This is undesirable because patients wearing acrylic complete dentures often complain that they cannot feel.
• The temperature of food or liquids they ingest, thus reducing the pleasure.
• Replacing the palatal portion with metal is one solution because the metal is a better conductor of heat.
• Coefficient of thermal expansion: These materials have a high coefficient of thermal expansion (CTE).
• The CTE for polymethyl methacrylate resin is 81 × 106/°C.
• The addition of filers reduces CTE.
• Heat distortion temperature: This is the measure of the ability of a plastic to resist dimensional change when loaded under heat.
• It is measured by observing the temperature at which a specimen under 1.8 MPa load defects 0.25 mm.
• Heat distortion temperature for PMMA  — 71 to 91°C.
• Heat distortion temperature for vinyl resin — 54 to 77°C.
• Distortion is of concern during procedures like the repair or polishing of dentures. The temperature should be kept low to avoid distortion.

Polymerization Reaction:  Polymerization of heat-activated resin is achieved by the application of heat and pressure.

The reaction is:

Powder      +      Liquid     +    Heat    →    Polymer     +     Heat
(Polymer)      (Monomer)    ( External)        (Reaction)

Mode of Polymerization:

• Here the mode of polymerization is by application of external heat for activation of polymerization reaction which is provided by water bath or by microwave.
• Heat-activated resins are available in powder and liquid form.

Manipulation of Heat-cure Resin:

• Mix the correct proportion of polymer and monomer, i.e. polymer/monomer ratio is 3 to 3.5/1 by volume or 2.5/1 by weight.
• First, the liquid monomer is poured in a thick, clean glass jar and then the powder is added slowly to it.
• After all powder is added the mix is stirred and vibrated thoroughly so that mechanical mixed pigments and dyes are dispensed evenly throughout the material.
• On completion of stirring, a light lid must be placed on the glass jar to prevent evaporation of the monomer.
• When polymer and monomer mix is allowed to stand in a closed container it undergoes physical changes due to the interaction of monomer to polymer.

Advantages  of Heat-cure Resin:

• Aesthetically it is good.
• It consists of the high glass transition temperature.
• It is easy to fabricate.
• Its surface finish is excellent.

Disadvantages of Heat-cure Resin:

• Its flexural strength is low.
• It has short fatigue life.
• It is radiolucent.

Uses of Heat-cure Resin:

• For fabricating complete and removable partial dentures.
• For fabricating orthodontic appliances.
• For fabricating artificial teeth.
• For fabricating obturators.
• For making temporary crowns and bridges.