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

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the wetting of a solid substrate (S) by a liquid (adhesive) “L”. A solid–liquid interface is formed as a result according to the following equation:

      (2.2)

      If γS, γL, and γSL are the surface free energies of solid substrate, liquid (adhesive), and the interphase, then the free energy change of the process (ΔGA) can be written as

      (2.3)

      The work of adhesion WA = –ΔG can be written as

       (2.4)

      This is the thermodynamic work of adhesion or the work needed to separate unit area of the solid–liquid interface.

       (2.5)

      This is known as Young–Dupre’s equation. Thus, if the contact angle, θ, of a well-defined probe liquid against a solid is measured, the work of adhesion can be determined.

      Thus, the thermodynamic work of adhesion (W) is, by definition, the free energy change per unit area required to separate to infinity two surfaces initially in contact with a result of creating two new surfaces at the interface between two materials, for example, an adhesive and an adherend.

      It is related to the intermolecular forces that operate

      at the interface between two materials, for example, an adhesive and an adherend.

      It is related to the intermolecular forces that operate

      at the interface between two materials, for example, an adhesive and an adherend.

      It is related to the intermolecular forces that operate

      at the interface between two materials, for example,

      It is related to the intermolecular forces that operate

      at the interface between two materials, for example,

      The approach is described below:

      (1) Partitioning of surface free energies into components

      The principle of partitioning is based on the assumption that the surface free energy is determined by various interfacial interactions. These interactions in turn depend on the basic properties of the interacting liquid and that of the solid–liquid interface (SL) [23, 24].

      (2.6)

      where =

      are the dispersion, polar, hydrogen (related to hydrogen bonds), induction, and acid–base components, respectively, while o refers to all remaining interactions.

      (2) Mode of combinations of the individual energy components According to Fowkes, the dispersion component of the surface free energy is connected with the London interactions. The remaining van der Waals interactions, i.e., the Keesom and Debye ones, have been considered by Fowkes as a part of the induction interactions.

      Fowkes investigated mainly two-phase systems containing a substance (solid or liquid) in which the dispersion interactions appear only. Considering just such systems, Fowkes determined the SFE corresponding to the solid–liquid interface as follows:

      For two-phase systems comprising of a solid and liquid, in which only dispersion interactions occur, namely, between , Fowkes employed geometric mean as the mode of combination of these energy components to give the following equation:

       (2.7)

      (2.8)

      Owens and Wendt [26] significantly changed the Fowkes idea while assuming that the sum of all the components occurring on the right-hand side of Equation 2.11, namely

      except that γd can be considered as associated with the polar interaction ,

      Consequently, the following equation was obtained:

       (2.9)

      (2.10)

      van Oss, Chaudhury, and Good proposed the latest concept of partition in which surface energy is partitioned into two components [29, 30]:

      1 Long range interactions London, Keesom, and Debye called the Lifshitz–van der Waals component (γLW)

      2 The short-range interactions (acid–base), called the acid–base component (γAB) = 2(γ+ γ–)0.5 where γ+ and γ– mean the acidic and basic constituents, respectively, which are associated with the acid–base interactions.

      (2.11)

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