Pull-off adhesion

Fig. 2. Maps (2x2 pm) of. sample topography (height variation) and pull-off adhesive force for a Langmuir-Blodgett film on mica consisting of a I I mixture of palmitic (Cl6) and lignoceric (C24) fatty acids [46].

Fig. 23.4 Time dependence of (a) load, (b) displacement, and (c) temperature for a typical pull-off adhesion test.

Figure 5.3 Effect of water on the specific electrical resistance and the adhesion strength (steel 3-steel 3) of the adhesive based on macrophenylmethane diisocyanate (a), and on macrodiisocyanates made of POPG-1000 and diphenylmethadiisocyanate (b), POPG-2000 and TDI (c), and POPG-1000 and TDI (d) containing 0.2% catalyst and 10% filler (b, A c d), and without filler (b, B) (1) adhesive 0.25 mm thick applied to steel foil (2) electrode in the bulk adhesive (3) electrode on the substrate-adhesive boundary (4) pull-off adhesion strength.

NMR and Si-NMR spectra were obtained in CDCI3 with chemical shifts (6) referenced to tetramethylsilane. Elemental silicon was analyzed by Galbraith Laboratories. Instron Universal Tester (Model 1000) was used to measure the tensile properties of the cured coatings. Pull-off adhesion tester (Elcometer, Model 106) was used to evaluate the adhesion of the coatings to aluminum substrates. 

Friction commonly is described with two coupled terms, interfacial (chemical) forces and ploughing (mechanical loss) forces. It is important to consider both when interpreting results to assign a mechanism of fnction. Figure 7 shows no pull-off (adhesive) dependence on temperature except when the temperature is sufficiently into the rubbery regime. Above Tg the contact zone will increase due to a decrease in polymer modulus. In the case of PMMA this increase in contact zone did not... 

In a recent case study (see Svendsen et al, 2007 and also Problem 6.1), in collaboration with a paint company, the adhesion of six different epoxies-silicon systems has been studied. These paints are used in marine coating systems. Some epoxies showed adhesion problems in practice while others did not. The purpose of the study was to understand the origin of these problems and whether adhesion could be described/ correlated to surface characteristics, e.g. surface tensions. An extensive experimental study has been carried out including both surface analysis (contact angle measurements on the six epoxies, surface tension of silicon at various temperatures, atomic force microscopy (AFM) studies of the epoxies), as well as measurements of bulk properties (pull-off adhesion tests and modulus of elasticity). Theoretical analysis included both estimation of Zisman s critical surface tensions and surface characterization using the van Oss-Good theory. 

Tape test A series of cross-hatched cuts are applied to the coating, a pressure-sensitive tape is applied and pulled off. Adhesion is measured by the percent of material that is not pulled off by the tape. Human elements are present in this test because of variations in tape application pressure and the rate and angle of pull. ASTM D 3359... 

X-Cut Adhesion by Wet Tape ASTM D 3359 and FED-STD-141 Tensile (Pull-Off) Adhesion ASTM D 4541... 

Epilation is required for permanent hair removal. The most effective epilation process is electrolysis or a similar procedure. Epilation can also be achieved by pulling the fibers out of the skin. Eor this purpose, wax mixtures (rosia and beeswax) are blended with Hpids, for example, oleyl oleate, which melt at a suitable temperature (about 50—55°C). The mixture is appHed to the site (a cloth tape may be melted iato the mass) and after cooling is rapidly pulled off the skin. A similar process can be carried out with a tape impregnated with an aggressive adhesive. 

When the surfaces are in contact due to the action of the attractive interfacial forces, a finite tensile load is required to separate the bodies from adhesive contact. This tensile load is called the pull-off force (P ). According to the JKR theory, the pull-off force is related to the thermodynamic work of adhesion (W) and the radius of curvature (/ ). 

In an appropriately designed experiment, it is possible to measure the pull-off force (Ps), contact radius (a versus P, ao and aj, and the separation profile outside the contact zone (D versus j ). From these measurements, it is possible to determine the thermodynamic work of adhesion between two surfaces, if the contacting bodies are perfectly elastic. 

Israelachvili and coworkers [64,69], Tirrell and coworkers [61-63,70], and other researchers employed the SFA to measure molecular level adhesion and deformation of self-assembled monolayers and polymers. The pull-off force (FJ, and the contact radius (a versus P) are measured. The contact radius, the local radius of curvature, and the distance between the surfaces are measured using the optical interferometer in the SFA. The primary advantage of using the SFA is its ability to study the interfacial adhesion between thin films of relatively high... 

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

Fig. 13. Measurement of surface energies of PS and PMMA. It can be seen that there was a finite adhesion hysteresis. At a given load, the contact radius during loading was less than the contact radius during unloading. From the unloading data, we get yi>s = 45 1 mJ/nr, and yi),viMA = 53 1 mj/m . These number are in good agreement with the values of surface energies determined from the pull-off force measured using the SFA.

Mangipudi et al. [63,88] reported some initial measurements of adhesion strength between semicrystalline PE surfaces. These measurements were done using the SFA as a function of contact time. Interestingly, these data (see Fig. 22) show that the normalized pull-off energy, a measure of intrinsic adhesion strength is increased with time of contact. They suggested the amorphous domains in PE could interdiffuse across the interface and thereby increase the adhesion of the interface. Falsafi et al. [37] also used the JKR technique to study the effect of composition on the adhesion of elastomeric acrylic pressure-sensitive adhesives. The model PSA they used was a crosslinked network of random copolymers of acrylates and acrylic acid, with an acrylic acid content between 2 and 10%. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

We have recently been exploring this technique to evaluate the adhesive and mechanical properties of compliant polymers in the form of a nanoscale JKR test. The force and stiffness data from a force-displacement curve can be plotted simultaneously (Fig. 13). For these contacts, the stiffness response appears to follow the true contact stiffness, and the curve was fit (see [70]) to a JKR model. Both the surface energy and modulus can be determined from the curve. Using JKR analyses, the maximum pull off force, surface energy and tip radius are... 

Specification for electroplated coatings of 65/35 tin/nickel alloy Method for the evaluation of results of accelerated corrosion tests on metallic coatings Methods of test for paints Cross-cut test Pull-off test for adhesion Resistance to artificial weathering (enclosed carbon arc) and Addendum No. 1 Resistance to continuous salt spray Notes for guidance on the conduct of natural weathering test... 

Several models have been proposed to predict adhesion force—the maximum force required to pull off the surfaces. Among these, the JKR theory is one receiving the greatest attention [2], which says that for an elastic spherical body in contact with a semi-infinite plane, the adhesion force can be estimated by... 

The magnitude of the pull-off force depends on the natnre of the tip-sample interaction during contact. Adhesion depends on the deformation of the tip and the sample, because attractive forces are proportional to the contact area. Quantifying the work of adhesion is difficult. The measured magnitude of A7 is strongly dependent on environment, surface roughness, the rate of pull-off, and inelastic deformation surrounding the contact. 

An important consideration for the direct physical measurement of adhesion via pull-off measurements is the influence of the precise direction of the applied force. In AFM the cantilever does not usually lie parallel to the surface, due to the risk that another part of the cantilever chip or chip holder will make contact with the surface before the tip. Another problem relates to the fact that the spot size in the optical beam deflection method is usually larger than the width of the lever. This can result in an interference effect between the reflection from the sample and the reflection from the cantilever. This is reduced if the cantilever and sample are not parallel. Most commercial AFM systems use an angle in the range of 10°-15° between the sample and the cantilever. Depending on this angle and the extent to which the cantilever is bent away from its equilibrium position, there can be a significant fraction of unintentional lateral forces applied to the contact. 

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