Adhesion thermodynamic

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

Thermodynamical adhesion. Treatment with TCI solution improves the wettability on SBR, due to the creation of surface chemistry and roughness. 

Qualitatively speaking, the term adhesion designates the cohesion between two media, whether they be identical or not. However, it is more difficult to give a quantitative definition. This concept may be understood from three complementary angles fundamental adhesion, thermodynamic adhesion and experimental adhesion. 

Thermodynamic adhesion is defined as the reversible work W. needed to create an interface... 

Moreover, A1 can also form weak interactions with COOH groups this happens preferentially to the C=0 site first, thus forming A- C-O interactions [88] but there follows the formation of (weak) interactions to the C-O site [46]. The end product of the Al-COOH reaction is AI2O3. Its formation can be considered as a redox reaction again [40, 84] because of the reduction of C=0 and C-O sites and the oxidation of Al. In this case, unspecific thermodynamic adhesion must be impHed. The exact experimental proof of all the interactions and bond formation discussed here is matter for future work. 

Figure 1 Diagrams illustrating (a) the thermodynamic cohesion" proce.ss, and (b) the thermodynamic "adhesion process. See Eqs. (2) and (3). (Redrawn from Ref. 10, with permission.)...

Thermodynamic adhesion is the term that applies to the ideal adhesion already defined in terms of reversible work needed to separate two surfaces by overcoming the molecular interactions across the interface. Chemical adhesion is a term that may be applied to adhesion involving the formation of formal chemical bonds (covalent, electrostatic, or metallic) across an interface. Mechanical adhesion refers to the situation in which actual mechanical interlocking of microscopic asperities at the interface occurs over a significant portion of the contact area. 

We have already encountered the concept of thermodynamic adhesion and its related terms such as the work of adhesion. The term is applied to a defined model system and does not take into consideration conditions before or after the formation of the interface, the presence of random flaws or defects in the system, or the bulk physical properties of the components, all of which are of primary importance in the practical application of the concept of adhesion. It is related to molecular interactions such as van der Waals, dipolar, and electrostatic forces but does not consider mechanical or chemical interactions as defined above. It is therefore not a very useful concept in terms of practical adhesion problems, but it serves as a good theoretical tool and to indicate a maximum force or work that a given interface may be expected to transmit before failure (i.e., separation) occurs. 

Because, in theory at least, file concept of ideal or thermodynamic adhesion applies equally well to liquid and solid phases, it is of interest to see how a calculation of such an ideal value compares with reality. The complete expression for the work of adhesion between two phases with each phase completely saturated by the other, denoted by A(B) and B(A) is... 

Throughout the history of adhesion science and technology, researchers have hypothesized that the practical adhesion and the thermodynamic adhesion should somehow be correlatable, but there is no unanimity on this issue. It is universally agreed that the practical adhesion cannot be equated with the thermodynamic adhesion at the most, one can expect a direct correlation between the two. This is so because (i) during the breaking of the joint irreversible processes such as Inelastic deformations take place with the consequent dissipation of energy and (ii) refers to a defect free interface whereas in real situations there are present a large number of flaws between the adhesive and the substrate. 

In the present paper, I wish to (i) review critically the literature pertaining to the relationship between the practical adhesion and thermodynamic adhesion, (ii) discuss various surface chemical criteria—Wa, interfacial free energy, penetration of the adhesive—and the conditions which optimize these criteria, (iii) test these criteria and conditions against the existing values of adhesive strengths, (iv) finally deduce the most important surface chemical criterion germane to the practical adhesion. 

Before discussing the conditions dictating optimum thermodynamic adhesion, it is in order to introduce the concept of 7q, the critical surface tension of wetting. It is obvious from equation (2) that for the determination of W, one must know the values of and Tsl (TIv can easily be measured), and these are not easily accessible because of many experimental difficulties associated with measuring solid surface free energies. However, in cases where equation (1) is valid, the difference of Tgy - Tsl can be determined by measuring 6. ... 

Table II. Values of Optimum Surface Free Energy of Liquids (Adhesives) Calculated For a Variety of Conditions Predicting Optimum Thermodynamic Adhesion for Various Pol3rmers... 

So it is obvious from the discussion presented that a variety of conditions should culminate into optimum thermodynamic adhesion. Table II lists the values of optimum 7 (as dictated by the various conditions discussed) for a variety of polymeric solids. Obviously, there is a liberal latitude in the selection of the right value of 7for an adherend in question. The next section summarizes the published experimental results on adhesive strengths, and the applicability of the various conditions discussed here will be tested. 

Finally, there is another price to be paid for using (O 4.54) The interfacial tensions )>sl ysv of the soKd have been eliminated, and hence, they cannot be deduced from wetting experiments. This obstacle was the reason for the development of so-called thermodynamic adhesion theories which introduce various proposals for a relationship between the Kquid-soKd interfacial tension ygL the surface tensions lvi Tsv- These theories are considered in Chap. 6. 

This issue and the common thermodynamic adhesion models will be discussed below. But as a first illustration, the widely forgotten approach by Antonov (1907) will be considered here. Antonov s empirical rule for low energy solids (Tlv > Tsv)... 

All these concepts suffer from the same problem as the thermodynamic adhesion model proposed by Girifalco and Good. Provided that attention is confined to polar and dispersion interactions and use of the values for and )>lv from the liquid-liquid wetting is made, one still has two unknowns in Eq. 6.38 yjy and )>sv-... 

All thermodynamic adhesion models start with the idea that wetting experiments with suitably chosen test liquids could provide the surface tension, ygy of the wetted solid. 

The thermodynamic adhesion (work of adhesion - Wa) between two polymer materials (1 and 2), in ideal contact, is given by the Dupre relation ... 

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