Intermediate-modulus adhesives

Property Adhesive A (high modulus) Adhesive B (high modulus) Adhesive C (intermediate modulus) Adhesive D (low modulus) ... 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

Formulated primers. The use of formulated primers in which a silane (or silanes) is blended with a film former, a solvent, and perhaps a hydrolysis catalyst, has been described by Plueddemann [16] and in the Dow Corning trade literature [17]. Primers of this type which contain a film former should not be confused with the previously mentioned silane pretreatment primers, as they deposit a film of measureable thickness and strength. Such formulated primers are required to wet the substrate, be readily wet by the adhesive or coating and should provide a layer of intermediate modulus between substrate and resin system. As with the pretreatment primers a formulated primer can be tailored to both the substrate and the resin. Ageing of formulated primers may be a problem both in the storage container and on the substrate before subsequent coating. 

Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

In effect, this theory postulates a chemical reaction between promoter, polymer, and mineral substrate as in the chemical bond theory but also suggests that the presence of a region of intermediate modulus between polymer and substrate which transfers stress from the high modulus surface to the relatively low modulus polymer. Adhesive technology has long recognized this principle in specially formulated primers for use when bonding rubbery polymers to metals. 

At intermediate fi equendes, monodisperse polymers exhibit a well-defined "plateau region" where G = constant Gn (Figure 2). For a given macromolecular species, the value of the plateau modulus is a characteristic feature that does not depend on molecular weight. The only way to lower the plateau modulus is to add small compatible molecules, either of the same species or not this is, for example, what is done for Hot-Melt Adhesives (HMAs) when adding a "tackifying resin" which softens the polymer and improves the "tack". 

In another evaluation, silver-glass, polyimide, and silicone adhesives were analyzed for residual stress. The silver-glass adhesive showed the greatest deflection the silicone, the lowest and the polyimide was intermediate. This order is explained by the high modulus of the silver-glass and the low modulus of the silicone relative to the polyimide. ... 

Improved bonding at the interface can be achieved by the deposition of pyrolytic carbon directly onto the carbon fiber [58] and poor adhesion to P-100 fiber and other high modulus fibers can be improved by an intermediate coat of amorphous carbon [59]. 

Recently, Sharpe noted a fundamental difference between two definitions interface and interphase. He asked the question what plays the major role in determining ultimate properties of an adhesion joint and its behavior Is it interface Evidently, it is not, because a two-dimensional array of atoms or molecules, such as an interface, is impossible to measure, whereas interphase has a sufficiently large assembly of atoms or molecules to have a modulus, strength, etc. According to Sharpe, interphase is a region intermediate to two (usually solid) phases in contact, the composition and/or structure and/or properties of which may be variable across the region and which also may differ from the composition and/or structure and/or properties of either of the two contacting phases. ... 

From the above derivation, it is important to note that the effect of constraint increases the stiffness of the component by the factor (1 +a l2h ). As seen in Fig. 15, for large thin blocks of rubber, the effective modulus approaches that of the bulk modulus, B, for compression when a/h is greater than about 10. Thornton et al. analyzed the interfacial stresses for the intermediate case when some amount of friction or adhesion occurs between the rubber disk and the rigid blocks [8]. A new variable, ii, defines the static coefficient of friction for which a critical radius exists at which the shear stresses cannot overcome the coefficient... 

One classification scheme for carbon fibers is by tensile modulus on this basis, the four classes are standard, intermediate, high, and ultrahigh moduli. Fiber diameters normally range between 4 and 10 p-m both continuous and chopped forms are available. In addition, carbon fibers are normally coated with a protective epoxy size that also improves adhesion with the polymer matrix. 

Historically, the modulus problem has been dealt with on a purely empirical basis and it is only recently that advances in mathematical modelling have given an insight into what is really required — the modification of the elastic/plastic behaviour of adhesives (shown schematically in Fig. 7.5 where the two extreme curves bracket the possibilities generated by the manipulation of the intermediate example). 

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