Understanding various adhesion mechanisms is important to understanding why good adhesion is achieved with certain materials but not with others. Adhesion between two polymer substrates can occur by a variety of mechanisms. In decreasing order of typical adhesion strength, the various adhesion mechanisms are:
- Chain entanglement
- Covalent bonding
- Hydrogen bonding
- Dipole-dipole non-hydrogen bonding
- Van der Waals forces
Covalent bonds are formed by the reaction of two chemical groups and typically result in the highest adhesion. Covalent bonds typically provide excellent adhesion since they require significant energy to break. In anhydride-modified polyolefins (AMP), covalent bonds can result from the reaction that occurs between the functional group in the tie-layer and the bonding substrate. Anhydrides will react with groups that contain labile hydrogen, such as hydroxyls (OH) and amines (NH 2), but otherwise do not typically form a covalent bond. For example, the anhydride group in the AMP will react with the hydroxyl group in the EVOH to form an ester linkage, but AMP will not react with the side-chain aromatic group on polystyrene.
Besides covalent bonding, the other kinds of chemical bonding are dipole-dipole interactions and Van der Waals forces. Dipole-dipole interactions occur when the positive end of one polar molecule is attracted to the negative end of another polar group. Conversely, two identical atoms bonded together are non-polar as with polyethylene. The C-H bond has very limited polarity which does not result in dipolar bonding. An especially strong kind of dipole-dipole attraction is hydrogen bonding. The positively charged hydrogen is attracted to, but does not react with, a negatively charged functional group. To have a sufficient positive charge for hydrogen bonding, the hydrogen atom must be connected to an electronegative atom such as nitrogen or oxygen. Acids (COOH), amines (NH 2 ), amides (NHCO) and hydroxyls (OH) groups can therefore have hydrogen bonding, while a methyl group is essentially non-polar and is not capable of hydrogen bonding. The strength of the hydroxyl hydrogen bond (H...O-H) is about one tenth that of the hydroxyl (O-H) covalent bond. An even weaker interaction, but still important factor, is Van der Waals forces. These forces result from the small dipoles that exist in all molecules and require that the polymer chains be in very close proximity to have an effect.
While chemical interactions are important for bonding, resins such as low-density polyethylene (LDPE) and EVA typically have excellent adhesion to each other although no covalent bonding or dipole-dipole interaction occurs. These materials adhere well due to chain entanglement whereby polymer chains diffuse into the other material and become entangled. If the materials are semi-crystalline, co-crystallization can occur which further improves adhesion compared to chain entanglement in the amorphous state. In order for chain entanglement to occur, the materials must be in the melt state, be compatible and have sufficient contact time. The ability of the materials to diffuse is related to the wetability of the materials. If the resins have similar surface energies and are compatible, the resins are considered to have good wetability. The table below shows examples of polymer substrates and the adhesion mechanisms that are present.