Polymerization Techniques in Dentistry

Injection Molding Technique

• Vent allows for attachment of an external injector
• Soft resin is contained in the injector and forced into the mold space
• No trial closure is necessary
• Ensures proper mold filling

Colorgard

• Separating medium to prevent union between investing medium and denture base halves
• Two layers are applied while the mold space is still warm

Monomer-Polymer Reaction

• Monomer's function is to produce a plastic mass that can be packed into the mold
• Prevents resin adhesion to the mold surface during flask closure
• Pressure should be applied slowly for even dough distribution in the mold space
• Excess overflow is called flash
• Insufficient dough results in no flash

5 Stages of Polymerization

• Damp Sand Stage: Polymer gradually settles in monomer
• Sticky Stage: Monomer attacks, creating stringiness and adhesiveness
• Dough Stage: Mass becomes more saturated with polymer, creating a smooth, non-tacky texture, ready for mold packing
• Rubbery Stage:
• Stiff Stage:

Denture Base Resins

• Types:
  • Heat-Cured Resin
  • Chemically Cured Resin
  • Light-Cured Resin
• Dispensing: Powder (polymer) and liquid (monomer)
  • Monomer (methyl methacrylate):
    • Clear, transparent liquid at room temperature
    • Highly volatile
    • Light sensitive
    • Boiling point at 100.8°C

Components of Powder (Polymer)

• Inorganic pigments:
  • Mercuric sulfide (red)
  • Cadmium sulfide (yellow)
  • Ferric oxide (brown)
  • Added to match denture base color to soft tissues
• Multifunctional Methacrylates and Acrylate Resins:
  • Bowen’s resin (or BIS-GMA) bisphenol glycidyl methacrylate
  • TEGDMA, UDMA, PENTA-P
• Dyed Synthetic Fiber: Esthetics, simulates blood vessels
• Dibutyl Phthalate: Plasticizer
• Talc or gelatin: Prevents bead/pearl cohesion during storage
• Inorganic materials: Zirconium, silicone, or alumina
  • Controls thermal expansion coefficient
  • Increases resin stiffness

Components of Liquid (Monomer)

• Methyl methacrylate: Monomers, highly volatile
• Hydroquinone: Organic inhibitor (less than 0.1%), prolongs storage, UV-sensitive
• Glycol Dimethacrylate: Cross-linking agent, resists surface cracks
• Organic Amine: Organic accelerator, decomposes peroxide at room temperature

Steps of Denture Base Construction (Double-Pressing Technique)

• Impression making
• Master/working cast construction
• Flask construction
• Packing
• Polymerization
• Finishing and polishing

Hydrocolloid Impression Materials

• Definition: Solids suspended in liquids, termed sols
• Sol to Gel Formation: Temperature change
• Hydrophilic (liquid loving): Term for hydrocolloid impression materials
• Types:
  • Reversible Hydrocolloids (SOL-GEL-SOL)
  • Irreversible Hydrocolloids (SOL-GEL)
• Gelation Temperature: Temperature at which sol changes to gel

Reversible Hydrocolloids

• Composition:
  • Agar: 12-15% (tray material)
    • 6-8% (syringe material)
    • Extracted from seaweed
  • Water: 80-85% (principal ingredient)
  • Borax: 0.2%
    • Strength improver
    • Increases viscosity
    • Acts as a retarder
  • Potassium sulfate: 1-2%
    • Ensures proper setting of gypsum models and dies
  • Fillers: Diatomaceous earth, clay, silica, wax, rubber
    • Control strength, viscosity, and rigidity
  • Coloring and flavoring materials
  • Plasticizer: Glycerin
  • Antimicrobial agents: Thymol (bactericide)
  • Preservatives: 0.1% Benzoates
• Advantages:
  • Impression accuracy
  • Easy missing and manipulating
  • Minimal equipment required
  • Flexibility of the set impression
  • Low cost
  • Comfort for the patient
• Disadvantages:
  • Poor shelf life
  • Storage of agar impressions should be avoided, limited to 1 hour in 100% relative humidity
  • Syneresis: Dehydration
  • Imbibition: Storage in water causes swelling
• Classification: Thermoset, elastic, Preliminary/Final Impression Material

Irreversible Hydrocolloids (Alginate)

• Composition:
  • Sodium or Potassium alginate: Main ingredient, soluble salt of alginic acid, forms sol in water
  • Calcium sulfate: Reactor (dihydrate or hemihydrate)
    • Dihydrate: Common
    • Hemihydrate: Increases shelf life and dimensional stability
  • Trisodium phosphate: Retarder
  • Diatomaceous Earth or Silicate Powder (Fillers):
    • Increase strength and stiffness
    • Produce a smooth texture
    • Ensure a firm, non-tacky gel surface
    • Disperse alginate powder particles in water
  • Potassium titanium fluoride: Accelerates stone setting, plaster hardener effect
  • Organic glycol: Makes alginate "dust-free"
  • Quaternary ammonium compounds: Self-disinfection
  • Flavoring and coloring materials
• Advantages:
  • Easy to use
  • Economical
  • Accurate reproduction detail
• Disadvantages:
  • Setting reaction is irreversible
  • Dimensional instability due to syneresis and imbibition
• Shelf Life:
  • Storage temperature
  • Moisture contamination from air
• Storage: Cool, dry environment in tightly sealed containers

Manipulation of Alginate

• Powder/Water Ratio: 15-18 grams powder: 40-50 CC water
• Mixing: Water first, then powder, horizontal figure-8 motion, 45 seconds to 1 minute
• Result: Smooth, creamy mixture
• Tray Selection: Proper size before mixing
• Tray Loading: Posterior portion first, pushed toward anterior
• Gelation Time: 2-3 minutes, single, firm, and sudden withdrawal
• Factors Affecting Gelation Time:
  • Water/Powder ratio: More water = increased gelation, more powder = decreased gelation
  • Mixing time: Shorter = increased gelation, longer = decreased gelation
  • Temperature: Low = increased gelation, high = decreased gelation

Explore the various techniques involved in the polymerization process, including injection molding and monomer-polymer reactions. This quiz covers the critical stages of polymerization and their significance in creating dental prosthetics. Test your understanding of the methods used for effective mold filling and resin application.