Polymeric Denture Base Materials

Polymeric Denture Base Materials

Stages Occur during the Mixing of Polymer and Monomer:

After mixing, the material goes through various physical states. No polymerization reaction takes place during these stages. Plastic dough is formed by a partial solution of the
polymer in the monomer.

Physical Stages

• Stage I: Wet Sand Stage:
  • Polymer gradually settles into the monomer forming a fluid, incoherent mass.
  • Mix exhibits a sandy consistency during this stage.
• Stage II: Sticky/Stringy Stage:
  • Monomer attacks the polymer by penetrating into the polymer.
  • The mass is sticky and stringy (cobweb-like) when touched or pulled apart.
• Stage III: Dough or Gel Stage:
  • As the monomer diffuses into the polymer, it becomes smooth and dough-like.
  • It does not adhere to the walls of the jar.
  • It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer.
  • The mass is plastic and homogenous and can be packed into the mold at this stage.
• Stage IV:
  • Rubbery Stage Monomer disappears by further penetration into the polymer for evaporation.
  • The mass is rubber-like, non-plastic, and cannot be moulded.
• Stage V: Stif Stage Formal mass is totally unworkable and should be discarded.

Compression Molding Technique

The compression molding technique is also known as the pressure packing technique. Heat-activated denture bases are commonly processed by this technique.

Steps in Compression Molding Technique:

1. Preparation of the waxed denture pattern: The structure to be created is fist constricted in wax; the wax portion will be replaced with acrylic.
2. Preparation of split mold: The waxed denture is invested in a dental flask with dental stone or plaster by three-pour technique. After the stone or plaster sets, it is dewaxed by placing it in a hot water bath. Removal of the wax leaves an empty space or creates mold. Mold is flashed further with hot water and mild detergent to remove any wax residue and is cleaned.
3. Application of separating medium: A tin foil substrate, i.e. cold mold seal is applied both on the investment and the cast. Generally, two coats in succession should be applied.
4. Mixing of powder and liquid: Polymer-monomer proportion 3:1 by volume or 2:1 by weight. The measured liquid is poured into a clean, dry mixing jar. The powder is slowly added allowing each powder particle to become wetted by monomer. The mixture is then stirred and allowed to stand in a closed container. Once the material reaches dough stage it is kneaded by a wet cellophane sheet.
5. Packing: The powder liquid mixture should be packed into the mold space at the dough consistency and the flask is reassembled.
6. Trial closure: Mold should be overfilled by the acrylic which should be removed by trial closures till no flesh is evident. A sharp scalpel is preferred to cut the flesh.
7. Now flask is transferred to a hydraulic bench press which maintains the flask under compression for half an hour for bench curing during which the monomer completely dissolves the polymer. For curing to get complete, flask should be placed in a controlled water bath.