Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar

      where α is the polarizability, r is the distance, and I is the first ionization potential.

      The negative sign indicates the attractive interaction.

2.3.1.2 Dipole-Dipole Interaction

      Dipole–dipole interaction is between polar molecules. A polar molecule has an electric dipole moment by virtue of the existence of partial charges on its atoms. Opposite partial charges attract one another, and, if two polar molecules are oriented so that the opposite partial charges on the molecules are closer together, then there will be a net attraction between the two molecules with a potential energy V given by

      μ_i are the dipole moments

      ε is the permitivity of the medium

      T is the temperature in Kelvin

       2.3.1.3 Dipole–Induced-Dipole Interaction

      In the dipole–induced-dipole interaction, the presence of the partial charges of the polar molecule causes a polarization, or distortion, of the electron distribution of the other molecule. As a result of this distortion, the second molecule acquires regions of partial positive and negative charge, and thus it becomes polar. The partial charges so formed behave just like those of a permanently polar molecule and interact favorably with their counterparts in the polar molecule that originally induced them. Hence, the two molecules cohere with a potential energy V given by

      where μ is the dipole moment of the polar molecule, α is the polarizability of non-polar molecule, and r is the distance between them.

       2.3.1.4 Ion–Dipole Interaction

An ion-dipole force is an attractive force that results from the electrostatic attraction between an ion and a neutral molecule that has a dipole. It is most commonly found in solutions. It is especially important for solutions of ionic compounds in polar liquids. The potential energy of ion–dipole interaction is given by

      where q is the charge on the ion.

One should note that in all the above equations describing the intermolecular attractions, the denominator contains the factor r⁶. Thus, the types of intermolecular interactions described above occur only at very small distances, of the order of typical atomic bond lengths (the range of non-bonding interactions is between 0.3 and 0.5 nm). For interactions to occur, therefore, the two materials must be able to make intimate contact with each other (i.e., they must be able to approach within a nanometer). This is possible if the adhesive wets the substrate efficiently. The types of interactions and the corresponding energies are given in Table 2.1.

Table 2.1 Bond types and typical bond energies [1].

Type of interaction	Energy (kJ/mol)	Basis of attraction
Bonding
Ionic	400–4000	Cation–anion
Covalent	150–1100	Nuclei–shared electron pair
Metallic	75–1000	Cations–delocalized electrons
Non-Bonding
Ion–dipole	40–600	Ion charge–dipole charge
Hydrogen bonding	10–40	Polar bond to hydrogen–dipole charge
Dipole–dipole	5–25	Dipole charges
Ion–induced dipole	3–15	Ion charge–polarizable electrons
Dipole–induced dipole	2–10	Dipole charge–polarizable electrons
Dispersion forces	0.1–40	Interaction between polarizable electrons

2.3.1.5 Hydrogen Bonds

      This is an important intermolecular interaction specific to molecules containing an oxygen, nitrogen, or fluorine atom that is attached to a hydrogen atom. This interaction is the hydrogen bond, an interaction of the form A–H···B, where A and B are atoms of any of the three elements mentioned above and the hydrogen atom lies on a straight line between the nuclei of A and B (Figure 2.1).

       2.3.1.6 Ionic Bonds

      Salts like NaCl.

       2.3.1.7 Chemical Bonds

      The acid–base character of the substrate may influence the reactivity between adhesive and substrate. A covalent bond involves shared valence electrons (Figure 2.1).

2.4 Theories of Adhesion

      According to Schultz and Nardin (1994), the main adhesion theories are as follows:

      1 Mechanical interlocking

      2 Electronic or electrostatic theory

      3 Adsorption (thermodynamic) or wetting theory

      4 Diffusion theory

      5 Chemical (covalent) bonding theory

      6 Theory of weak boundary layers and interphases

      The adsorption hypothesis, which explains that adhesion is caused by intermolecular forces such as van der Waals forces, hydrogen bonds, and electrostatic interactions, is widely considered to be the most applicable to wood–polymer adhesion [7]. However, in a porous material like wood, penetration and mechanical interlocking must also play a significant role in the bonding process.

      Marra [5] described adhesive bond formation in wood-based panels as a dynamic process consisting of flow, transference, penetration, wetting, and solidification (cure).

The