Adhesion of Polymers
Adhesives have been known to mankind for thousands of years. The earliest adhesives were based on proteins gained from boiling down bones, albumin, etc. These adhesives were often stronger than the wood they bonded together. Today’s adhesives are based on numerous chemistries. They are used to bond many different substrates. Adhesives are usually low molecular weight reactive liquids (structural adhesives) or they are (tackified) viscoelastic polymers (pressure sensitive adhesives). In the first case the adhesive is applied as a liquid to the substrate, and then chemically crosslinked (cured) and in the second case it is applied with the right amount of pressure and without any post cure. In the case of a coating/paint, both chemical and physical adhesion is equally important.
Adhesion is the tendency of dissimilar surfaces to stick to one another. It can be measured in terms of work of adhesion in J/m2. It is the energy needed to separate 1m² of joined materials. It can also be measured as peel in N/m, which is the force required to pull off a strip of material that is 1 meter wide. Thus the two measures are identical: 1 J/m2 equals 1 N/m.
Many engineers and scientist believe that surface energy is important for adhesion. However, an increase in surface energy does not necessarily improve adhesion, that is, the energy that is required to separate a (viscoelastic) polymer from a surface (often called work of adhesion) is not directly proportional to the surface tension. This can be easily shown: The typical surface energy of a polymer is in the range of 20 to 40 dyne/cm or 40 mN/m, whereas typical peel values are in the range of a 20 to 1000 N/m. Therefore, adhesion forces are at least 100 times and typically 10,000 times stronger. Thus, surface tension plays only a minor role for adhesion contrary to what is often believed. However, surface tension and wetting is still important when it comes to initiating adhesion.
So what is the root cause of adhesion? In fact, there are several types of forces that can occur when surfaces come in close contact:
The adhering polymer (adhesive) flows into the voids / pores of the substrate and thus interlocks with the micro-porosity like pieces of puzzles. If the liquid hardens as it is the case with structural adhesive, a strong bond is formed between the substrate and the adhesive.
Non-specific or Dispersive Adhesion
Dispersive adhesion is caused by rather weak non-specific intermolecular forces (Van der Waals Forces) which are present in all materials. These attractive forces originate from induced dipole - induced dipole interactions between uncharged atoms and molecules and are the main cause of cohesion in condensed matter (liquids and solids).
Electrostatic adhesion is based on the formation of an electrical double layer when two materials come in contact and exchange electrons. This creates an attractive electrostatic or Coulomb force between the two materials similar to the two plates of a capacitor.
Specific adhesion occurs when the atoms/molecules of the two adhering surfaces form specific bonds such as hydrogen bonds. These forces are responsible for the high boiling point of low molecular weight liquids with strong dipoles such as water, glycerin, methanol and so on.
Chemical adhesion is a special case of specific adhesion. The atoms/molecules of the two adhering materials form chemical bonds that can be of ionic or covalent character. This is usually the strongest form of adhesion.
Some materials may fuse at the joint by diffusion. This happens when the two polymeric materials are soluble in each other and mobile enough to interdiffuse. This phenomenon is known as interdiffusion or interpenetration. Even modest “intermingling” of the polymers of the two plastic substrates can lead to noticeable adhesion and if the polymers truly “entangle” when they cross the interface, strong adhesion will occur due to physical crosslinking (entanglement).
When separating adhering materials, two types of failure can occur. In the case of adhesive failure, the bonds between the adhesive and adherend break, i.e. no contaminations from the adhesive and adherend remain on the separated surfaces. This is the preferred failure mode for pressure sensitive adhesive tapes. In the case of cohesive failure, the fracture appears within either the adhesive or adherend. This is the preferred failure mode for structural adhesives.