Adhesion critical surface tension

The low energy surface of some polymers can create problems with adhesion. Critical surface tension values for some common materials are given in the following table. Substrates such as the polyolefins, polypropylene and polyethylene are extremely difficult surfaces on which to achieve good adhesion. Techniques such as flame treatment have been developed, which oxidise the surface of the componenL but these still tend to be used in conjunction with the standard chlorinated polyolefin primer in a belt and braces approach to adhesion. The most reliable form of polyolefin pretreatment is plasma surface treatment. 

The interesting implication of Eq. XII-24 is that for a given solid, the work of adhesion goes through a maximum as 7b(a) is varied [69]. For the low-energy surfaces Zisman and co-workers studied, )3 is about 0.04, and Wmax is approximately equal to the critical surface tension yc itself the liquid for this optimum adhesion has a fairly high contact angle. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Critical surface tension (Yc) of the oiled CDC Ni-plated sheets apparently is determined by the polar group concentration of oil and metallic elements (Cr and Ni). Yc increases with increasing the polar group concentration of oil and, therefore, the improvement of adhesion due to the higher polarity of oil was also supported by thermodynamical aspect (wettability). 

Zisman discovered that there is a critical surface tension characteristic of low-energy solids, such as plastics and waxes. Liquids ihat have a lower surface tension than the solid will spread on that solid, while liquids with a higher surface tension will not spread. Examples of critical surface tension values for plastic solids in dynes per cm are "Teflon/ 18 polyethylene, 31 polyethylene terephthalate, 43 and nylon, 42-46. As one indication of the way this information can be used in practical applications, one can consider the bonding of nylon to polyethylene. If nylon were applied as a melt to polyethylene, it would not wet the lower-energy polyethylene surface and adhesion would be poor. However, molten polyethylene would spread readily over solid nylon to provide a strong bond. 

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. 

As explained under the adsorption theory of adhesion,3 an adhesive must first wet the substrate and come into intimate contact with it. (A brief description of the adsorption theory of adhesion is presented in the section below.) The result of good wetting is simply that there is greater contact area between adherend and adhesive over which the forces of adhesion (e.g., van der Waals type of forces) may act. For good wetting, the surface free energy (surface tension yLV) of the liquid adhesive must be less than that (critical surface tension yc) of the solid adherend, or... 

Table 3.3 provides surface tensions yLV for common adhesive liquids and critical surface tension yc for various solids. A brief discussion of how these properties are measured follows. 

TABLE 3.3 Surface Tension of Several Liquids Including Epoxy Adhesive Formulations (Top) and Critical Surface Tension of Various Substrate Materials (Bottom)... 

FIGURE 3.4 Contact angle of an uncured epoxy adhesive on four substrates of varying, critical surface tension. Note that as the critical surface tension of the substrates decreases, the contact angle increases, indicating less wetting of the surface by the epoxy adhesive.4... 

The critical surface tension value for most inorganic solids is in the hundreds or thousands of dynes per centimeter. For polymers and organic liquids, it is at least an order of magnitude lower. Critical surface tension is an important concept that leads to a better understanding of wetting and adhesion. 

To obtain a usable adhesive bond with polyolefins, the surface must be treated. A number of surface preparation methods, including flame, chemical, plasma, and primer treatments, are in use. Figure 16.4 illustrates the epoxy adhesive strength improvements that can be made by using various prebond surface treatments to change the critical surface tension of polyethylene. 

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),... 

Another important example of the diflFerential adhesiveness which protein-dominated surfaces can display, is in the development of artificial skin especially for wound dressings and for temporary covers of extensive burns. C. W. Hall and co-workers 17) showed that relative tissue adhesion to mechanically identical velour fabrics constructed of various materials follows the order predicted by the critical surface tensions of construction material. 

Mohandas and co-workers (18), confirming previous findings of Weiss and Blumenson (19), have also shown that cells in an environment free of adsorbable proteins (which rapidly modify the surface properties of polymeric or inorganic substrates) will exhibit a similar direct relationship between their adhesion and the critical surface tension of the surface they contacted. DiflFerential adhesion of red blood cells was measured by determining the fraction of cells retained on a surface after the application of well-calibrated shear stresses (IS). In protein-free experiments, the red cells (themselves dominated in adhesive interactions by their protein membranes) had greatest adhesion to glass, intermediate adhesion to polyethylene and siliconized glass, and least adhesion to Teflon. 

Biofilm development is affected by both physical and chemical factors. The abundance and condition of bacteria in the water column plays a major role in initial rate of settlement on a surface 181]. Surface factors such as wettability [82] and critical surface tension [83], surface hydrophobicity [84], fluid dynamic forces [85], shear stress [86], electrolyte concentration [87] and metabolic inhibitors [88] can all affect microbial attachment, adhesion or growth. The low surface energy of a gorgonian octocoral has been implicated as a passive fouling resistance mechanism used in conjunction with other antifouling defences [82]. 

It has generally been observed that urethane systems, which are cured on the surface of some low energy materials, are also free from adhering to them, i.e. they are self-releasing. Materials which show this characteristic relative to urethanes in a very effective way include at least three different types of plastics polytetrafluoroethylene, polyethylene, and polypropylene. These materials all have a defined critical surface tension sc less than about 30 dynes/cm. Assuming that this value is near, the liquid vapor surface tension lv value of an effective IMR urethane systems, then the work of adhesion as given by Equation 4 is as follows ... 

Figure 4. Polyethylene adhesion of machine made papers at different critical surface tension levels. (Reproduced, with permission, from Ref. 4. Copyright 1966,...

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.) ...

Many interesting correlations have been established between the critical surface tension of materials (or other approximations of surface free energy) and protein adsorption, cell adhesion, and thrombus formation (41-48). Unfortunately, very few studies in which a biological response has been related to a specific surface chemistry exist. One study in which such a relationship was established, demonstrated the power of the contact angle method in analyzing surface structure related to blood compatibility (40). The blood compatibility of Stellite alloy heart valves was not due to the alloy itself, but to the closely packed methyl group structure associated with a tallow polishing compound used to finish the valve. Very recently, the power... 

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