Adhesion: Types and Properties

• Adhesion involves a physicochemical process at the point of contact between two materials, creating an attractive force.
• Adhesion relies on the adhesive physically contacting the substrate by wetting the surface.
• Covalent or ionic chemical bonds are not essential for adhesion.
• An effective adhesive joins two substances (adherends) without bond failure.
• The most crucial adhesive property is its ability to make close contact with a surface for strong bonding.

Types of Adhesion

• Chemical

• Mechanical

• Electrostatic

• Diffusive

• Dispersive

• A thin adhesive layer is preferable for uniformly distributing stresses and loads applied to the substrate.

Mechanical Adhesion

• Rough or porous surfaces enhance adhesion due to a larger interfacial area.
• Micromechanical retention of the adhesive occurs within surface micro-voids.
• An example of mechanical adhesion is acid etching of enamel.

Chemical Adhesion

• Adhesive molecules are able to form chemical bonds (ionic or covalent) with the adherend's molecules.
• Free radical reaction of vinyl groups shows chemical adhesion.

Diffusive Adhesion

• Molecules move across the interface for adhesion if the adhesive and adherend molecules are similar.
• Composite repairs and dentine bonding agents are examples of diffusive adhesion.

Ideal Properties of Adhesives

• Should wet the adherend surface with a low contact angle and appropriate surface energy.

• The adhesive's surface energy must be lower than the substrate's for proper wetting

• Should flow easily over the substrate with low initial viscosity.

• Ability to displace air from the interface minimizes stress concentration and avoids inhibiting polymerization.

• The physical and chemical properties of the adhesive can vary at non-contact points, while the core is typically homogenous.

• The durability and strength of an adhesive joint is thought to be controlled by the boundary layer composition.

• The boundary layer composition is responsible for stress transfer between adherends.

• The boundary layer composition is frequently a site of environmental attack and subsequent failure.

Modes of Failure in Adhesion

• Structural failure: Failure within the material

• Adhesive failure: Failure at the adhesive interface

• Cohesive failure: Failure within the adhesive itself

• Shear and tensile tests are used in dentistry to test adhesion due to tooth structure substrate limitations.

• Enamel etching creates an irregular surface, enabling micromechanical retention as the adhesive penetrates.

• Acid etching of enamel selectively dissolves hydroxyapatite, allowing adhesive penetration and forming resin tags.

• Standard etchant: 30-50% phosphoric acid greatly increases surface area, with etched enamel patterns classified as:

  • Type 1: Preferential dissolution of the prism core
  • Type 2: Preferential dissolution of the periphery of the prisms
  • Type 3: a mix of Type 1 & 2

• Etching time:

  • 15 seconds on cut enamel
  • 30-60 seconds on uncut enamel

• Etching removes approximately 5 microns of the surface, including:

  • Adsorbed pellicle
  • Amorphous CaP layer
  • Smear layer (if present on cut surfaces)

• Local fracture usually goes through the interrod and parallel to the rod head.

• Fracture parallel to rods results in a 40% reduction in bond strength

Types of Bonding Agents

• One type involves phosphoric acid etching and rinsing before applying acidic monomers and resin in solvent.

• The other type (self-etch) uses an acidic monomer primer to dissolve the surface HA, followed by resin application in solvent.

• Sealants: A cost-effective caries preventive measure.

Types of Sealants

• Filled, unfilled

• Self-cured

• Light cured

• There is no difference in efficacy with type

• Sealants typically use phosphoric acid etching and self-etching, which is less effective in clinical studies.

• Dentine is hydrophilic, composites are hydrophobic.

• Dentine is vital, with organic and inorganic components, and is covered by a smear layer.

Dentine Bonding Agent Components

• Etchant, primer, and sealant/bonding agent OR

• Weak or strong self-etching primer combined with a bond

• Adhesive monomers enhance dentine bonding

4-META Monomer

• Reacts in the presence of moisture to form two carboxyl groups

• Responsible for adhesion

• Requires a special catalyst for polymerization

• Bonds to metal (base) and enamel/dentine

• MDP is a key component in many dentine and metal primers

• Adhesive monomers are effective at concentrations between 2-10% by weight of total monomers.

• Smear layer creation: Any mechanical manipulation of dentine creates a smear layer and is acid labile, and can be easily washed off with acid.

• Dentinal Tubules near EDJ have higher bond strengths and tubules near the pulp have lower bond strengths.

• The ability of primers and resins to infiltrate demineralized dentine leads to the formation of a hybrid layer at the dentine-resin interface.