Contact angle and adhesion

Adsorption and desorption of liquid molecules at the spreading or receding liquid are accompanied by the dissipation of energy and are thus one source of hysteresis [254,255], At this point we would like to point out the similarity between contact angle and adhesion experiments. Adhesion is dominated by the solid-solid attraction, while contact angles reflect the solid-liquid attraction. 

Table 3 gives similar results for the reactions with CAC and TAA. This time CAC reacts directly with the enolic -OH. Also shown are the water contact angles. Clearly there is no correlation between contact angle and adhesion. Broadly speaking the contact angle hysteresis is constant... 

It is most impressive to find how theoretical knowledge has led to some fascinating developments in the technology. The purpose of this handbook is also to further this development. The molecular description of liquid surfaces has been obtained from surface tension and adsorption studies. The emulsion (microemulsion) formation and stability are described by the interfacial film structures. The surfaces of solids are characterized by contact angle and adsorption studies. The ultimate in interfaces is an extensive description of chemical physics of colloid systems and interfaces. Contact angle and adhesion is described at a very fundamental level. The thermodynamics of... 

Figure 10.44. Change of receding contact angle and adhesion to ink of LDPE film containing 2,000 ppm emcamide vs. corona treatment time. [Data from Chen, B.-L., TappiJ., 81-8,185-89, 1998.]...

D. Platikanov and M. Nedyalkov, Contact Angles and Line Tension at Microscopic Three Phase Contacts, in Microscopic Aspects of Adhesion and Lubrication, J. M. Georges, ed., Elsevier, Amsterdam, 1982. 

The effect of the chemical makeup of the adhesive/adherend system on contact angle and wetting is manifest through the influence of such chemistry on the surface free energies of the adhesive-air (or other fluid medium), adherend-air... 

Majumder and Bhowmick [381] have investigated the influence of the concentration of TMPTA on the surface properties of EPDM mbber, modified in the presence of EB. The surface energy of the TMPTA-modified EPDM mbber has been observed to increase as compared to the unmodified one. Table 31.4 displays the contact angles and the work of adhesion of water, for the control and the modified EPDM surfaces. 

The decrease in the contact angle and corresponding increase in the work adhesion of the modified surfaces are due to the generation of polar carbonyl (C=0) and ether (C=0=C) groups on the surfaces formed through interaction of macroradicals on EPDM backbone with atmospheric oxygen. The results have been confirmed from IR (Figure 31.4) and X-ray photoelectron spectroscopic (XPS) observation (Table 31.5). 

This relation is known as the Young-Dupr6 equation. It shows that the energy of adhesion can be determined by measuring the contact angle and the surface... 

These comments should not be interpreted to mean that measures of wettability are useless at predicting adhesion. They do seem clearly to indicate that contact angles and critical surface tensions reported for wood are not necessarily thermodynamic quantities or well-defined material parameters. Because most contact angles are dynamic values, they should be interpreted with caution and considered as relative measures of adhesion, for which the absolute scale is yet unknown. Further, we need to keep in mind that although wetting is necessary for adhesion, it may not be the limiting factor in many real situations. 

In this chapter, we concentrate on the role of the intermolecular interactions at interfaces based on the surface dynamics, the surface free energy at a given temperature and the surface energy (or enthalpy) of a solid. Contact angle and gas chromatographic techniques are respectively chosen for the studies of wettability and adsorption phenomena because of their simplicity and clearness. The degree of adhesion at interfaces between unlike solid substances is also discussed from a viewpoint of intermolecular interactions. 

As far as the available evidence goes, it appears that the cleaner the surface, the smaller is the hysteresis of the contact angle. It seems possible that the large advancing contact angle (small adhesion between liquid and solid) is due to the presence of some film which prevents the liquid adhering closely to the solid after contact with the liquid, this film is wholly or... 

Our own investigations have concerned (a) liquid spreading on solids and the laws relating the equilibrium contact angle and the critical surface tension of wetting to solid and liquid constitution (26, 27, 28, 53, 54,62), (b) liquid/liquid displacement from solid surfaces (1,5), (c) the properties of adsorbed monolayers on solids and their relation to the monolayer retraction method (28, 54, 62), (d) the surface electrostatic potentials of adsorbed organic monolayers on metals (9, 10, 11, 58, 59), (e) the effects of surface constitution on adhesion and abhesion (60),... 

Polyimide surface modification by a wet chemical process is described. Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) and poly(bisphenyl dianhydride-para-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution. These modified surfaces are further treated with aqueous HC1 solution to protonate the ionic molecules. Modified surfaces are identified with X-ray photoelectron spectroscopy (XPS), external reflectance infrared (ER IR) spectroscopy, gravimetric analysis, contact angle and thickness measurement. Initial reaction with KOH transforms the polyimide surface to a potassium polyamate surface. The reaction of the polyamate surface with HC1 yields a polyamic acid surface. Upon curing the modified surface, the starting polyimide surface is produced. The depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the KOH reaction temperature and the reaction time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. Relationship between surface structure and adhesion is discussed. 

An attempt is made to evaluate the accuracy of data obtained by these techniques, to determine the main causes of scattering and the optimal conditions for contact angle and work of adhesion measurements. 

Sessile drop experiments have been used extensively to derive quantities characterising spreading and penetration phenomena, such as the work of adhesion and work of immersion, given by equations (1.45) and (1.54), using a single sessile drop experiment to measure both the contact angle and the liquid surface energy. 

When the optimal conditions are satisfied, i.e., a well-controlled atmosphere, a pure, homogeneous and smooth solid surface and an accurate measurement system, the Young contact angle and the liquid surface energy can be derived with an accuracy of 3 deg and 2% respectively allowing the work of adhesion (Wa = [Pg.144]

Table 6.5. Contact angle and work of adhesion of Cu on different oxides at 1423K (Naidich 1981). Values of the gap energy Eg of oxides come from Duffy (1990).

Consider a non-reactive system consisting of a binary liquid alloy A-B and an oxide substrate such as AI1O3 at constant temperature. A simple statistical thermodynamic model has been developed (Li et al. 1989) to predict the contact angle and the work of adhesion isotherms, 0(XB) and Wa(XB), from the known values of contact angles... 

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