Adhesive surface tension

For example, an ink may contain a mixture of cyclohexanone with an evaporation rate of 0.2 and butoxyethanol (Butyl Cellosolve) with an evaporation rate of 0.07. Additional factors to take into account when choosing the solvents are the interaction of the solvent with the substrate, e.g., swelling of polymeric substrates enhances adhesion. Surface tension will influence print quality and dot gain with high surface tension solvents leading to smaller drops. 

The equilibrium value of the polymer surface tension decreases with increase in temperature (see Fig. 2.12), which follows from basic thermodynamic relationships. Thus, the increase of the adhesion strength for cementing at higher temperatures is caused, apart from other factors, by the improvement of wetting of the substrate by adhesive due to the decrease of the adhesive surface tension. 

Given that the metal surface tension was the same in all csises, the thermodynamic work of adhesion depends on the interphase tension and on the adhesive surface tension. In this case (see Fig. 2.22) a correlation is seen between the thermodynamic work of adhesion and the adhesion strength. This correlation was determined earlier by Lipatov and Myshko by applying the modified equation of Dupre-Young [102]. Epoxy, polyester, pol30irethane, and polyaciylate adhesives were used the type of the adhesive did not have a significant effect on the correlation dependence. 

Now let us consider the effect on the adhesion strength of surface-active substances at the boundary between the adhesive and the met d. Figure 2.22 presents the dependence of the adhesive-to-steel adhesion strength on the difference between the interphase tension of the solid adhesive-mercury system and the adhesive surface tension. Because it is impossible to measure the adhesion strength at the bound uy with mercury, it was determined at the boundary between the adhesive and steel. The interphase tension will surely differ here from that at the boundary with mercury, although this is only a quantitative difference and the qu ditative relationship will be maintained. 

Several properties of surfaces have origins that are very sensitive to the surface, but are essentially macroscopic in character. Examples include adhesion, surface tension and the contact angle. Techniques to measure these properties are not discussed in this chapter, but rather are mentioned in connection with discussions of the property involved elsewhere in this book. However, surface properties can also be probed at a microscopic level for example the forces between surfaces at microscopic separations may be probed using the surface forces apparatus (SFA). The scanning force microscope... 

Adhesive surface tension Substrate s critical siuface tension... 

Dissimilar molecules carry very powerful attractive forces referred to as adhesion. Surface tension is the result of molecular attraction in fluids, which is stronger along the outer perimeter and weaker toward the middle. Surface forces vary from those found deeper in the liquid because there are no upward forces. 

Table 14.6. Free energy of adhesion, surface tension, /pv, and contact angle, 0, with...

Friction. The coefficient of friction (COE) is a measure of the friction forces between two surfaces. It characterizes a film s frictional behavior. The COE of a surface is determined by the surface adhesivity (surface tension and crystallinity), additives (slip, pigment and antiblock agents), and surface finish. Cases in which the material s COE values require careful consideration include film passing over free-running rolls bag form-... 

From Tables 2 and 3, an epoxide adhesive (surface tension 45 dynes/cm) would not be expected to wet and effectively bond a low energy surface such as polyethylene (critical surface tension 31 dynes/cm). When the polyethylene surface was etched for increasing times in a sulfuric acid-dichromate solution, bond strengths markedly increased and the surface s contact angle with water (increasing polarity) similarly decreased (Fig. 4). ... 

Finally, Newmann and co-workers [30] (see also Ref. 31) have argued that while free energy contributions may not be strictly additive as in Eq. IV-11, there should, in principle, be an equation of state relating the work of adhesion to the separate liquid surface tensions such as... 

Water at 20°C rests on solid naphthalene with a contact angle of 90°, while a water-ethanol solution of surface tension 3S dyn/cm shows an angle of 30°. Calculate (a) the work of adhesion of water to naphthalene, (b) the criticd surface tension of naphthalene, and (c) y for naphthalene. 

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. 

The separation of two surfaces in contact is resisted by adhesive forces. As the nonnal force is decreased, the contact regions pass from conditions of compressive to tensile stress. As revealed by JKR theory, surface tension alone is sufficient to ensure that there is a finite contact area between the two at zero nonnal force. One contribution to adhesion is the work that must be done to increase surface area during separation. If the surfaces have undergone plastic defonnation, the contact area will be even greater at zero nonnal force than predicted by JKR theory. In reality, continued plastic defonnation can occur during separation and also contributes to adhesive work. 

An adhesive should possess a Hquid surface tension that is less than the critical wetting tension of the adherend s surface. 

The surface of PTFE articles is sHppery and smooth. Liquids with surface tensions below 18 mN/m(=dyn/cm) are spread completely on the PTFE surface hence, solutions of various perfluorocarbon acids in water wet the polymer (78). Treatment with alkafl metals promotes the adhesion between PTFE and other substances (79) but increases the coefficient of friction (80). 

Mechanisms of Leukocyte Adsorption. The exact mechanism of leukocyte adhesion to filter media is not yet fuUy understood. Multiple mechanisms simultaneously contribute to the adhesion of cells to biomaterials, however, physical and biological mechanisms have been distinguished. Physical mechanisms include barrier phenomenon, surface tension, and electrostatic charge biological mechanisms include cell activation and cell to cell binding. 

Surface Tension. Interfacial surface tension between fluid and filter media is considered to play a role in the adhesion of blood cells to synthetic fibers. Interfacial tension is a result of the interaction between the surface tension of the fluid and the filter media. Direct experimental evidence has shown that varying this interfacial tension influences the adhesion of blood cells to biomaterials. The viscosity of the blood product is important in the shear forces of the fluid to the attached cells viscosity of a red cell concentrate is at least 500 times that of a platelet concentrate. This has a considerable effect on the shear and flow rates through the filter. The surface stickiness plays a role in the critical shear force for detachment of adhered blood cells. 

An inversion of these arguments indicates that release agents should exhibit several of the following features (/) act as a barrier to mechanical interlocking (2) prevent interdiffusion (J) exhibit poor adsorption and lack of reaction with at least one material at the interface (4) have low surface tension, resulting in poor wettabihty, ie, negative spreading coefficient, of the release substrate by the adhesive (5) low thermodynamic work of adhesion ... 

Many of these features are interrelated. Finely divided soHds such as talc [14807-96-6] are excellent barriers to mechanical interlocking and interdiffusion. They also reduce the area of contact over which short-range intermolecular forces can interact. Because compatibiUty of different polymers is the exception rather than the rule, preformed sheets of a different polymer usually prevent interdiffusion and are an effective way of controlling adhesion, provided no new strong interfacial interactions are thereby introduced. Surface tension and thermodynamic work of adhesion are interrelated, as shown in equations 1, 2, and 3, and are a direct consequence of the intermolecular forces that also control adsorption and chemical reactivity. 

For hquid systems these surface energies expressed in mj/m are numerically equivalent to the surface tensions in mN/m(= dyn/cm). If the adhesive is phase 1 and the release coating is phase 2, then the spreading coefficient, S, of 1 on 2 is as given in equation 2. 

Nitrile latices are used ia a wide variety of appHcations, including production of dipped nitrile mbber products. In the principle use of paper saturation, adhesives and fiber bonding, small particle size and optimum surface tension is desirable to achieve rapid penetration and setup or dryiag. 

In the pendular state, shown in Figure la, particles ate held together by discrete lens-shaped rings at the points of contact or near-contact. For two uniformly sized spherical particles, the adhesive force in the pendular state for a wetting Hquid (contact angle zero degree) can be calculated (19,23) and substituted for H. in equation 1 to yield the foUowing, where y is the Hquid surface tension in N/m. 

However, the surface tension of the soHd, y, and the soHd—Hquid interfacial tension, y, caimot be measured direcdy by simple means. The work of adhesion of the soHd to the Hquid usually deterrnined by other techniques. 

The excellent chemical resistance and physical properties of PVA resins have resulted in broad industrial use. The polymer is an excellent adhesive and possesses solvent-, oil-, and grease-resistant properties matched by few other polymers. Poly(vinyl alcohol) films exhibit high tensile strength, abrasion resistance, and oxygen barrier properties which, under dry conditions, are superior to those of any other known polymer. The polymer s low surface tension provides for excellent emulsification and protective coUoid properties. 

Adhesion to Metals. For interaction between coating and substrate to occur, it is necessary for the coating to wet the substrate (107). Somewhat oversimplified, the surface tension of the coating must be lower than the surface tension of the substrate. In the case of metal substrates, clean metal surfaces have very high surface tensions and any coating wets a clean metal substrate. 

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