Principle of Adhesion

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Principle of Adhesion

• Supanee Buranadham, DDS, PhD
• Dept of Prosthetic Dentistry
• Faculty of Dentistry, PSU

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Mechanical Bonding

• Structural retention
• Gross screws, undercuts
• Microscopic irregularities crevices, pores in
  the surface of the substrate

acid etched enamel
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Definition

• Adhesion
• unlike molecules are attracted ? e.g. A-B
• Cohesion
• same kind of molecules are attracted ? e.g. B-B
• Adhesive
• material or film added to produce the adhesion ?
  B
• Adherend/substrate
• material to which adhesive is applied ? A

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Questions

• Examples of adhesion and cohesion
• Adhesion failure v.s. cohesion failure

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Adhesion ? Molecular attraction
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When two substances are brought into intimate
contact with each other, the molecules of one
substance adhere to molecules of another.
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• Why is it difficult to force two solid surfaces
  to adhere?
• Because the rough surfaces at the atomic or
  molecular scale will keep the major area of the
  surfaces apart.
• Attraction is negligible when the surface
  molecules are separated more than 0.7 nm (0.0007
  µm).

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• Wetting a surface with a material (an adhesive)
  brings the material into intimate association
  with that surface.? Why?
• Use of a fluid that flows into these
  irregularities provides contact over a greater
  part of the surface of the solid.

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Wettability

• Wettability is a measure of the affinity of a
  liquid for a solid as indicated by the spreading
  of a drop.
• ?indicates the ability of an adhesive (a liquid)
  to contact a substrate

? Spreading ? good wettability
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Contact Angle

• The angle formed by the adhesive with the
  adherend at their interface ? Contact angle
• Contact angle is used to determine how well an
  adhesive can wet the surface of an adherend.
• The tendency for the liquid to spread
  (wettability) increases as the contact angle
  decreases.

If contact anglegt90o ? that liquid cannot wet the
solid surface.
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Low contact angle
High contact angle
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• The degree of wetting depends on the relative
  surface energies of the solids and the liquids
  and on their intermolecular attraction.

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Surface Energy

• The energy at the surface of a solid is greater
  than that of its interior because the outermost
  atoms are not equally attracted in all
  directions.
• The increase in energy per unit area of surface
  is referred to as the surface energy (solid) or
  surface tension (liquid).? J/m2

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Question

• High- surface energy solids and low-surface
  tension liquids
• ? (good or bad) Wetting?
• ? (high or low) Contact angle?

Good wetting Low contact angle
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Wetting ability is influenced by

• Cleanliness of the surface
• water contamination, oxide film ? ?surface energy
  of the adherend
• Surface energy of adherends
• Materials with low surface energy
• Teflon (polytetrafluoroethylene), polymers, wax
• Materials with high surface energy
• Metals
• Comparatively low surface energies liquids can
  spread well on solids of high surface energy.

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Surface Roughness

• Increase the potential area for bonding
• But can also give rise to the entrapment of air ?
  reduce the bonding area
• Adhesives with a high viscosity are prone to
  causing entrapment of air.

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• Surface imperfections (irregularities) represent
  a potential impediment to the achievement of an
  adhesive bond.
• Cause voids at the interface

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• To improve ability of adhesive to fill in
  irregularities
• ? Contact angle between an adhesive and an
  adherend
• Ex Addition of a wetting agent
• ? Fluidity (? viscosity) of the adhesive

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Adhesion to Tooth Structure

• Fluoride products may be effective in reducing
  dental caries by providing a tooth surface that
  stays cleaner over a longer period.
• The wettability of enamel and dentine is markedly
  reduced after the topical application of an
  aqueous fluoride solution. (? enamel surface
  retains less plaque over a given period)
• Bonding of restoration material to tooth
  structure.

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Requirements for setting adhesive systems

• The adhesive must give good wetting of the
  adherend.
• The adhesive should have a suitable viscosity to
  enable it to flow readily over the surface of
  adherend.
• The morphology or surface roughness of the
  substrate.
• The setting of the adhesive should occur without
  excessive dimensional changes.
• The thickness of the adhesive must be optimal
  (thin).
• The strength of the set adhesive must be taken
  into consideration.

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Summary

• Adhesion
• Adhesion/Cohesion
• Failures
• Wetting/ wettability
• Surface energy
• Contact angle

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End of Principle of Adhesion