Surface wettability and adhesion

Elemental sodium, as well as other alkali metals, reacts with perfluorocarbon polymers by removing fluorine from them. This reaction has a practical application for improving surface wettability and adhesive bonding of perfluorocarbon polymers to other substrates.57... 

There are numerous techniques which provide information related to the surface energy of solids. A large array of high-vacuum, destructive and non-destructive techniques is available, and most of them yield information on the atomic and chemical composition of the surface and layers just beneath it. These are reviewed elsewhere [83,84] and are beyond the scope of the present chapter. From the standpoint of their effect on wettability and adhesion, the property of greatest importance appears to be the Lifshitz-van der Waals ( dispersion) surface energy, ys. This may be measured by the simple but elegant technique of... 

Adhesion of paints and adhesives to TPOs is especially problematical due to the aliphatic nature of the substrate material. In Europe, plasma and corona treatment is employed to render these surfaces wettable and obtain strong adhesion by adhesives and paints in automotive manufacture. In the United States, however, primers based on solvent-borne chlorinated polyolefin oligomers (CPOs) have become the treatment of choice for these substrate materials. The VOC emissions from these primers are considerable (as in all solvent-borne adhesives), but the less... 

The reason for surface modification of a polymer is in most cases a wish to improve wettability and adhesion towards more hydrophilic materials. Both adhesion and wettability are interfacial phenomena. 

Today it is claimed that the surface fluorination of polymers using F2 gas mixtures enhances a wide range of properties, e.g., low permeability to nonpolar liquids4 improved permselectivity,5-6 excellent wettability and adhesion,7 low friction coefficient (especially for elastomers),8 and chemical inertness.9 Obviously, these properties depend on the chemical composition ofthe fluorinated layer, which in turn is determined by the chemical structure ofthe base polymer, the composition of the F2 gas mixture, and the fluorination parameters. 

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adhesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

Thermoplastic materials often have a lower surface energy than do thermosetting materials. Thus, physical or chemical modification of the surface is necessary to achieve acceptable bonding. This is especially true of the crystalline thermoplastics such as polyolefins, linear polyesters, and fluoropolymers. Methods used to increase the surface energy and improve wettability and adhesion include... 

Feast WJ, Munro HS and Richards RW (Eds), "Polymer Surfaces and Interfaces II", Wiley, Chichester, 1993. Fowkes FM, In "Contact Angle, Wettability and Adhesion", Adv Chem Ser 43, Am Chem Soc 1964, p. 108. 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

The surface wettability by water drops has been clearly linked with the concentration of chemically bound nitrogen in the surface. Conversely, measurements of advancing and receding contact angles for water can provide information regarding surface composition. On the other hand, there appears to be little or no direct correlation between water wettability and adhesive strength of evaporated aluminum films. Substantial improvement in adhesion of Ai to many polymers can be achieved by 02 plasma treatment of the polymer surface before metallization. Other workers have shown this to result in chemical linkages between the metal and polymer surface moieties. 

One of the most striking differences between protein-dominated substrates e,g., skin, tissue masses, and blood) and other solid, semi-solid, or liquid surfaces is in their wettability and adhesiveness with other materials. Work on the development of surgical adhesives based upon the poly(a-cyanoacrylates) used successfully in hemostasis for massive... 

Table 9.1 Critical surface tension,yc, dispersion component, y%/, and surface tension, ySv, values of polymeric solids. (Values compiled from standard references especially from Kaelble, D.H. (1971) Physical Chemistry of Adhesion. Wi ley-lnterscience, New York, and Zisman, W.A. (1 964) in Contact Angle Wettability and Adhesion, Adv. Chem. Ser. No 43, American Chemical Society, Washington D.C.) ...

Table 9.2 Values of surface tension components of test liquids Yo/ of Fowkes y[v of Owens and Wendt and yfy, Ylv, Ylv of van Oss-Good components. (Values compiled from Fowkes, F.M., McCarthy, D.C. and Mostafa, M.A. (1980) /. Colloid. Interface Sci., 78, 200 Good, R.J. (1993) Contact Angle Wettability and Adhesion, Mittal, K.L. (ed.). VSP, Utrecht)...

The surface chemisfry of flame-freafed polypropylene closely correlates wifh fhe weffabilify of the surface. Figure 22.8 also shows fhe ESCA O/C atomic ratio of flame-freafed polypropylene as a function of fhe equivalence ratio. The amounf of surface oxidation generated by the flame follows fhe same trend as the wettability. The maximum O/C ratio of 0.18 is obtained at q) = 0.92 - 0.94. Within this range, the gas phase concentrations of surface oxidizers in fhe flame are high. This surface oxidation is fhe reason for the improved wettability and adhesion properties of flame-treated polypropylene. A detailed discussion of the flame and surface chemistry involved in surface freafing is presented in Section 22.4. 

The most important result of the plasma treatment of polymers, which are produced on an industrial scale, is the change in their wettability and adhesion characteristics. As was discussed earlier, plasma treatment can make polymers more lydrophilic as well as more hydrophobic. Both effects are widely used for practical applications. The change of wettability is usually characterized experimentally by the contact angle 9, which is formed on the solid surface along the linear solid-hquid borderline of air (see Fig. 9-25). An increase of... 

The adhesive bond strength between two surfaces, which most often involves an adhesive and a solid substrate, is a complex phenomenon. It is controlled (at least in part) by the values of surface tension and solubility parameters of the materials, and the viscosity of the adhesive. To obtain good wettability and adhesion between a polymeric substrate and an adhesive, the surface tension of the adhesive must be lower than that of the substrate. Usually, the difference between the two values must be at least 10 dynes/cm. Similarity in solubility parameters between the two phases indicates similarity of the intermolecular forces between the two phases. For good compatibility, the values of the solubility parameters must be very close. Low viscosity in the adhesive is necessary for good spreadability and wettability of the substrate. Adhesive bond strength is addressed in more detail in Section 6.3.1. 

W Duncart-Hewitt, R Nisman. Investigation of the surface free energy of pharmaceutical materials from contact angle, sedimentation and adhesion measurements. In K.L. Mittal, ed. Contact angle, wettability and adhesion. Utrecht VSP. 1993. pp 791-911. 

Dettre, R.H., Johnson, R.E.J., 1964. Contact angle hysteresis n. Contact angle measurements on rough surfaces Contact SI32. The dream of sta3ing clean lotus and biomi-metic surfaces. In Fowkes, F.M. (Ed.), Angle, Wettability, and Adhesion. American Chemical Society, Washington, DC, pp. 136—144. 

Since changes on the enviroimient can significantly affect the thermodynamics at the surface level structural reorganization may occur. Hence, reversible changes on the surface properties of these materials can be easily accomplished, thus, creating switchable materials with controlled surface wettability, charge, adhesion, and chemical functionality. 

The effect of the polyamide benzimidazole group on the surface wettability and interfacial adhesion of fiber/matrix composites of two kinds of aramid fibers from poly(p-phenylene terephthalamide) (Kevlar-49) and poly-(polyamide benzimidazole-co-p-phenylene terephthalamide) (DAFIII) have been... 

Cai RQ, Peng T, Wang FD, Ye GD, Xu JJ. Improvement of surface wettability and interfacial adhesion of poly-(/ -phenylene tereph-thalamide) by incorporation of the polyamide benzimidazole segment. Appl Surface Sci2011 257(22) 9562-7. 

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