Adhesive bond diffusion theory

The diffusion theory states that interpenetration and entanglement of polymer chains are additionally responsible for bioadhesion. The intimate contact of the two substrates is essential for diffusion to occur, that is, the driving force for the interdiffusion is the concentration gradient across the interface. The penetration of polymer chains into the mucus network, and vice versa, is dependent on concentration gradients and diffusion coefficients. It is believed that for an effective adhesion bond the interpenetration of the polymer chain should be in the range of 0.2-0.5 pm. It is possible to estimate the penetration depth (/) by Eq. (5),... 

A number of adhesion theories have been proposed to identify the formation of adhesive forces. The contributed adhesion mechanisms are (1) chemical bonding such as chemisorption theory (2) physical interaction such as polarization, electrostatic, and diffusion theory (3) thermodynamical interpretation such as adsorption theory and (4) mechanical interlocking. No single theory exists to explain the entire property of adhesion oti various substrates and adhesives. However, those theories may provide a guideline to understand the principle of the adhesion as the following details (Fig. 2). 

It should be pointed out that, interestingly enough, in the diffusion theory of adhesion (Voyutskii and Vakula, 1969), polymer molecular diffusion is regarded as the mechanism behind the creation of an adhesive bond, and the strength of the bond is attributed to intermolecular forces. 

Historically, mechanical interlocking, electrostatic, diffusion, and adsorp-tion/surface reaction theories have been postulated to describe mechanisms of adhesion. More recently, other theories have been put forward for adhesive bonding mechanism (Table 1.1). It is often difficult to fully ascribe adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding within a given adhesive system. The extent of the role of each mechanism could vary for different adhesive bonding systems. An understanding of these theories will be helpful to those who plan to work with adhesives. 

This theory suggests that adhesion is developed through the interdiffusion of molecules in between the adhesive and the adherend. The diffusion theory is primarily applicable when both the adhesive and the adherend are polymers with relatively long-chain molecules capable of movement. The nature of materials and bonding conditions will influence whether and to what extent diffusion takes place. The diffuse interfacial (interphase) layer typically has a thickness in the range of 10-1,000 A (1-100 nm). Solvent cementing or heat welding of thermoplastics is considered to be due to diffusion of molecules. ... 

Thus, when investigating the nature and mechanism of adhesion between an adhesive, coating or polymer matrix and the substrate, it is important to consider the possibility of primary bond formation in addition to the interactions that may occur as a result of Dispersion forces and Poiar forces. In addition to the Adsorption theory of adhesion, adhesion interactions can sometimes be described by the Diffusion theory of adhesion, Electrostatic theory of adhesion, or Mechanical theory of adhesion. Recent work has addressed the formation of primary bonding at the interface as a feature that is desirable from a durability point of view and a phenomenon that one should aim to design into an interface. The concept of engineering the interface in such a way is relatively new, but as adhesives become more widely used in evermore demanding applications, and the performance XPS and ToF-SIMS systems continues to increase, it is anticipated that such investigations can only become more popular. 

The theories of adhesion that have been described in this chapter pertain to the adhesion of any two substrates. There is, however, one further theory which should be discussed that pertains in particular to the adhesive bonding of polymers to polymers. This is the diffusion theory of adhesion. 

The theory of adhesion by diffusion was espoused by various Russian scientists, the most prominent of whom was Voyutskii, who wrote a monograph on the subject. The theory was given a quantitative theoretical basis by the work of Vasenin, who derived the relationship for bond strength in autohesion shown in Eq. (108). 

The diffusion of the interfacial system is similar to the dissolving process of substances. They are all hybrid processes. The diffusion effect leads to the Interface becoming fuzzy and even disappearing (e.g., solid dissolved in liquid). The diffusion process correlates with molecular weight, flexibility, temperature, solvent, plasticizer, and other factors of the molecular chain. The interface diffusion between the polymer matrix and the filler could improve bonding performance. However, the diffusion theory cannot explain the adhesion phenomenon between the polymer matrix and inorganic reinforced material without interfacial diffusion. 

The interdiffusion of the polymer molecules of adhesive and substrate is dependent upon various parameters, such as pressure, time, temperature, molecule size, and, of course, the reciprocal solubility, as shown by the correlation between the compatibility of the polymers and the quality of the bond [21]. Examples of bonds to which the diffusion theory is applicable include the bonding of PVGU adherents to PVC in solvents containing tetrahydrofuran and so-called contact bonding, where the diffusion... 

The probability of a major role of the polyester group in reaction with dip chemicals during cure is quite low. The polyurethane-polyester bond is pictured primarily as a physical interaction rather than a chemical one. Iyengar" reported that the polyurethane from methylene-few(4-phenyl isocyanate) and the epoxide have a cohesive energy density of 10.5. Polyethylene terephthalate has a cohesive energy density of 10.3. The compatibility between these two networks would be quite high and the probability exists for adhesion by the diffusion theory of Voyutskii. ... 

We will not explain here why a given adhesive should stick to a given material, because this is explained in the chapter Theory of adhesion in Volume 2 in the light of wetting, surface energy, adsorption, work of adhesion, electrostatic, diffusion, covalent bonds and van der Waals forces, and it is also discussed in all the chapters dealing with the various chemical families of adhesives. 

Some adhesives will diffuse into the surface. The polymer chains of the adhesive interact locally with the polymer chains of the substrate and the two fuse together. Solvent-based adhesives are probably the best example of this adhesion theory. Cyclohexanone and similar solvents are widely used in the medical industry for bonding tubular polyfvinyl chloride) (and other plastic) medical devices. The cyclohexanone is usually applied to the male part by dipping the tube into a container. The solvent evaporates quite quickly but there is sufficient time to assemble the components and as the solvent evaporates, it bonds the two parts together to give a very effective bond. 

---