Adhesion at Interfaces

Adhesives and glues were known to antiquity. Early materials were based on proteins from boiled-down fish, bones, albumin, and so on, making gelatinous water solutions. Many animal glues form bonds stronger than the wood used in furniture construction. However, most such materials were restricted to indoor uses, for the polymers rapidly dissolved in rainwater. Later, rubber dissolved in solvents formed the so-called rubber cement, all stiU in service. 

Adhesives may be used to bond two different structures together to form a composite, as in polymer-polymer welding. The problem of adhesion also extends to the bonding of an engineering polymer directly to a nonpolymer substrate. 

Mechanical adhesion is defined as when the adhesive flows around the substrate surface roughness. An interlocking action takes place, like the pieces of a puzzle. 

Specific adhesion is defined as the case where secondary bonds, such as hydrogen bonds, are formed between the adhesive and the substrate. 

A special case of specific adhesion is when there are primary chemical bonds present. For example, graft or block copolymers may bond the different phases of a multicomponent material together see Chapter 13. Direct bonding to substrates is also encouraged in many systems. For example, maleic anhydride comonomers are used to bond to metallic surfaces. 

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Many of the important results for diffusion and adhesion at interfaces discussed in this chapter are summarized in Table 3. 

In 1989 I again retired from full time employment into a new position of Scientist Emeritus created by Dow Corning. In this position I still keep contact with Dow Corning, have an office, a laboratory, a secretary, a telephone and mailing address and am free to write, travel, lecture and consult with outside companies. As long as my health allows, I hope to continue active participation in the study of adhesion at interfaces. 

The capillary forces have been recognized as intermolecular forces which are created by cohesion and adhesion at interfaces. From the evaluation of the thermomechanical treatment we could clearly make out that these forces depend on the free Helmholz energy functions of the solid phase and the density gradient of the liquid. 

In this chapter, we concentrate on the role of the intermolecular interactions at interfaces based on the surface dynamics, the surface free energy at a given temperature and the surface energy (or enthalpy) of a solid. Contact angle and gas chromatographic techniques are respectively chosen for the studies of wettability and adsorption phenomena because of their simplicity and clearness. The degree of adhesion at interfaces between unlike solid substances is also discussed from a viewpoint of intermolecular interactions. 

In comparing the acido-basicity of the untreated and electrochemically (or anodically) treated fibers in Table 11, it is observed that the amine-treatments increase significantly the basicity of the fibers (Samples II and III), while commercial oxidation increases the acidity (IV), as expected. This behavior is probably explained in terms of the presence of any physisorbed species (such as, amine and oxygen functional groups for the fiber surfaces amined and oxidized, respectively). This can improve the degree of adhesion at interfaces of the composites for fiber surfaces incorporated with epoxy matrix for playing an important role in forming the fiber-matrix bond. 

Since we will focus below primarily on adhesion at interfaces between two immiscible polymers, it is appropriate to describe briefly what is known about such interfaces. The Gibbs free energy of mixing (per segment) of any two homopolymers A and B is given approximately by the Flory-Huggins expression ... 

WITH the easy-processing properties in the liquid crystalline phase, main-chain TLCPs have been widely used as high-strength fiber, fiber reinforcement, in situ reinforcement additive, and injection molded articles, etc, [ 1 -4]. The successful applications are quite dependent on the adhesion at interface of the liquid crystalline polymer and the conventional engineering resin, which is indeed affected by surface tension and/or interfacial tension between the two phases [1-2],... 

Grunze M, Unertl W N, Ganara]an S and French J 1988 Chemistry of adhesion at the polyimide-metal interface Mater. Res. Soc. Symp. Proc. 108 189... 

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

Deruelle, M., Leger, L. and Tirrell, M., Adhesion at the solid-elastomer interface influence of the interfacial chains. Macromolecules, 28(22), 7419-7428 (1995). 

When considering relea.se mechanisms, the physical and chemical heterogeneity of the adhesive/release interface cannot be ignored. At its most basic level, roughness of the release and PSA surface, the stiffness of the PSA and the method in which the PSA and release surface are brought together define the contact area of the interface. The area of contact between the PSA and release material defines not only the area over which chemical interactions are possible, but al.so potential mechanical obstacles to release. In practice, a differential liner for a transfer adhesive can be made to depend in part on the substrate roughness for the differences in release properties [21],... 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

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. 

Weak boundary layer. WBL theory proposes that a cohesively weak region is present at the adhesive-substrate interface, which leads to poor adhesion. This layer can prevent the formation of adhesive bonds, or the adhesive can preferentially form bonds with the boundary layer rather that the surface it was intended for. Typically, the locus of failure is interfacial or in close proximity to the silicone-substrate interface. One of the most common causes of a WBL being formed is the presence of contaminants on the surface of the substrate. The formation of a WBL can also result from migration of additives from the bulk of the substrate, to the silicone-substrate interface. Alternatively, molecular... 

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

There are many applications in which it is necessary to put a plastic coating on to paper or metal sheets and the extruder provides an ideal way of doing this. Normally a thin film of plastic is extruded from a slit die and is immediately brought into contact with the medium to be coated. The composite is then passed between rollers to ensure proper adhesion at the interface and to control the thickness of the coating (see Fig. 4.26). 

At temperatures in excess of 750°C the addition of water vapour accelerates the rate of growth of FeO (Fig. 7.10) by producing large pores in the FeO" . At much higher temperatures (1 200°C) Sheasby et have found that addition of steam to Oj-Nj, Oj-HjO-Nj and HjO-Nj, in a simulated reheating atmosphere furnace, caused increases in scale growth due to improved adhesion at the scale/metal interface. They concluded that water vapour enhances scale creep as previously reported by Tuck et... 

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