Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar. Читать онлайн. Newlib. NEWLIB.NET

Автор: R. N. Kumar
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119605669
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liquid, e.g., diiodomethane.

Liquid Density (kg/m3) Viscosity (mPas) Surface free energy (mJm–2) (LW–AB approach)
γlv
Water 1000 1.00 21.8 25.5 25.5 51.0 72.8
Formamide 799 1.02 39 2.28 39.6 19 58
Ethylene glycol 1109 19.9 29 1.92 47 19 48
Diethylene glycol 1130 26.8 44.7
Diodomethane 3325 2.8 50.8 0 0 0 50.8
1-Bromonaphthalene 1483 44.4 0 0 0 44.4

      The acid–base interaction and its relevance to adhesives and adhesive bonding have been reviewed in detail by Chehimi et al. [32].

      For spreading, another parameter, the spreading coefficient γSV – γSL – γLV appears to be important in classifying liquids that have a tendency to form good films on a given substrate. In general, the larger and more positive value of the spreading coefficient (S), the more energy is gained by intercalating a liquid film between a solid and air. Thus,

      S > 0, spontaneous spreading

      S > 0, not spontaneous spreading

      Though the condition S > 0 is necessary for a liquid to spread spontaneously on a solid, it is insufficient to describe the final state of the film.

Figure shows the Zisman’s plot (https://www.researchgate.net/publication/279532584_Analysis_of_the_Results_of_Surface_Free_Energy_Measurement_of_Ti6Al4V_by_Different_Methods/figures?lo=1), which is the well-known concept of critical surface tension, γc, where in the method adopted by Zisman, the contact angles θ for a series of organic homologous liquids were measured on a solid and a plot of cos θ vs. surface tension of the liquids gave a straight line.

      The above equation is true for smooth, contamination-free surface. However, the real solid surface is not smooth and the roughness of the surface has a profound effect on the wetting and adhesion. The innumerable small hills, valleys, and crevices on the solid surface entrap and occlude air or vapor within them. Even if θ = 0, it is not possible under real conditions to ensure that an intimate contact between the adhesive and adherend is established. Surface roughness plays therefore an important role in the wettability of a solid surface.

      The impact of roughness on the contact angle is given by the Wenzel equation (Equation 2.12)

       (2.12)

      The advancing and receding contact angles can throw light on the magnitude of roughness of the wood surface. The difference between advancing and receding contact angles is the contact angle hysteresis. The magnitude of contact angle hysteresis is dependent on roughness, topography, morphology, and chemical homogeneity of the solid surface [31]. Good [34] suggested that the advancing contact angle represents hydrophobic areas on the surface, while the receding contact angle characterizes hydrophilic areas.

      The weak boundary layer theory explains the loss of adhesion as a failure in an intermediate molecular layer between adhesive and adherent [35]. This molecular layer consists of low-molecular-weight impurities of various origins including water. This theory has never been verified for wood, but it is known that low-molecular-weight extractives