Acrylic Resins in Dentistry: Polymerization, Properties, and Clinical Applications
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Acrylic Resins: Polymerization Stages and Properties
Polymerization Stages
Plastic Stage (Pastoso Period)
The Plastic Stage (Pastoso Period) is the interval between Period 1 and Period 3 of the monomer-polymer physical interaction.
Working Time (D-Time)
The Working Time (D-Time) is the time elapsed between the end of Period 2 and the beginning of Period 4. This is the duration during which the material maintains a plastic consistency suitable for manipulation.
Factors Modifying Working Time
Key factors that modify the working time include:
- Temperature
- Polymer particle size
- Polymer molecular weight
Monomer-Polymer Interaction: Chemical Periodicity
The chemical periodicity of polymerization involves the following steps:
- Activation: Formation of free radicals (e.g., decomposition of initiator: ROOR → 2R•).
- Initiation: The radical unites with a monomer molecule (R• + M → R-M•).
- Propagation: The radical chain grows through the union of multiple monomer units.
- Termination: The polymerization chain is completed, usually by coupling or disproportionation of two growing chains.
Properties Comparison
Properties of Self-Curing Resins
- High strength
- Low color stability
- Increased residual monomer content
- Increased toxicity (due to higher residual monomer)
Properties of Heat-Cured (Thermoset) Resins
- High strength
- Better color stability
- Lower residual monomer formation
- Lower toxicity
- Low solubility
- Good dimensional stability
Clinical Applications
Uses of Self-Polymerizable Resins
- Temporary crowns
- Acrylic patterns
- Impression trays
- Prosthetic base repair
- Veneer repairs
- Splinting devices
Uses of Heat-Cured Resins
- Artificial teeth
- Denture bases for complete and removable partial dentures
- Combined crowns
- Maxillofacial orthopedic and orthodontic devices
- Palatal plates
- Habit eradicating plates
- Splints
- Impression trays
- Definitive prostheses
Specialized Acrylic Resin Types
Modified Acrylic Resins
Modified acrylic resins include:
- Hydrophilic acrylic resins
- Photoactivated acrylic resins for prosthetic bases
- High-impact acrylic resins (e.g., those incorporating butadiene rubber)
- Acrylic resins for fluid mixture prostheses
Hydrophilic Acrylic Resins
Manipulation Technique
Manipulation requires specific ratios and tools:
- Technique: Use of glass or glass-metal spatulas.
- Powder Ratio: Larger amount.
- Liquid Ratio: Smaller amount.
Advantages
Hydrophilic resins offer several advantages:
- They absorb some masticatory energy, ensuring that shock is not fully transmitted through the prosthesis into the underlying soft tissues.
- Increased prosthesis retention due to greater adherence to the oral cavity.
Chemical Composition
Typically composed of Butadiene-Styrene combined with Polymethyl Methacrylate (PMMA).
Fluid Acrylic Resins
Fluid Acrylic Resins are self-curing resins characterized by smaller powder particles. When mixed with the monomer, the compound is highly fluid, making it easy to pour into a mold.