Acrylic Resins: Composition, Polymerization, and Properties

Synthetic Resins and Polymer Fundamentals

Types of Synthetic Resins

1. Acrylic
2. Vinyl (Vinyl Resins)
3. Acrovinylics
4. Dimethacrylate
5. Epoxies (Epoxy Resins)

Defining Polymers

A Polymer is a high molecular weight macromolecule composed of millions of simple, low molecular weight molecules (monomers) joined into long chains.

Polymer Classification

Polymers are classified:

• According to their structure: Linear chain, branched, and cross-linked.
• According to their chemical composition: Homopolymers, copolymers, and terpolymers.

The Polymerization Process

Polymerization is a process involving a series of chemical reactions where large quantities of simple chain molecules (monomers) join to form one large molecule (polymer). This is typically a fast phenomenon accompanied by the detachment of heat (exothermic reaction).

Acrylic Resins: Applications and Classification

Acrylic Resins are synthetic polymers with extensive applications in the fabrication of prosthetic appliances, orthodontics, orthopedics, and surgery.

Synonyms for Acrylic Resins

• Synthetic plastic
• Plastic
• Acrylic plastics
• Polymers

Classification of Acrylic Resins (By Activation Method)

• Autopolymerizable (Self-curing)
• Heat-activated (Thermopolymerizable)
• Photopolymerizable (Light-curing)

Chemical Composition of Acrylic Resins

Powder Component (Polymer)

The powder component typically contains:

• Base: Polymethylmethacrylate (PMMA)
• Initiator: Benzoyl peroxide
• Plasticizers: Dibutyl phthalate
• Opacifiers: Zinc oxide or titanium dioxide
• Synthetic Fibers: Nylon or acrylic fibers
• Colorants: Sulfides of mercury, cadmium, selenium, iron oxide, and carbon black

Liquid Component (Monomer)

The liquid component typically contains:

• Base: Methyl methacrylate
• Inhibitor: Hydroquinone (prevents premature polymerization)
• Cross-linking Agent: Glycol dimethacrylate (enhances chain cross-linking)
• Activators (Self-Curing Resins Only): Tertiary amine or sulfinic acid

Commercial Presentation

Acrylic resins are commonly presented commercially as a liquid and powder system, or sometimes as a gel.

Composition of Photoactivated Acrylic Resins

Photoactivated (light-curing) resins have a distinct composition:

• Matrix: Urethane dimethacrylate, microfine silica, and high molecular weight monomers.
• Filler: Acrylic resin globules.
• Photoinitiator: Camphorquinone amine.
• Activator: Visible light (wavelength 400–500 nm).

Polymerization Dynamics and Monomer-Polymer Interaction

Key Polymerization Activators

• Self-Curing: Tertiary amine
• Heat-Activated (Thermopolymerization): Heat
• Light-Curing (Photopolymerization): Visible light

Monomer-Polymer Proportions

Typical mixing ratios are:

• By Volume: 1 part monomer to 3 parts polymer (1:3)
• By Weight: 1 part monomer to 2 parts polymer (1:2)

Importance of the Monomer-Polymer Ratio

Maintaining the correct ratio is crucial because it:

• Ensures the mixture achieves the right consistency.
• Reduces dimensional changes (shrinkage).
• Allows control over the polymerization time.
• Influences the final compression resistance.
• Enables proper coloring and aesthetics.

Physical Stages of Monomer-Polymer Interaction

When monomer and polymer are mixed, the material passes through distinct physical stages:

1. Period 1 (Sandy Stage): A sandy mass lacking cohesion.
2. Period 2 (Stringy Stage): A stringy, fibrous, and sticky mixture.
3. Period 3 (Dough/Plastic Stage): A soft, pasty mass (the period of optimum plasticity, allowing for proper compaction and working time).
4. Period 4 (Elastic Stage): The mass becomes more cohesive, elastic, and rigid.
5. Period 5 (Hardened Stage): The final, hardened mass.