Evaporative/diffusion adhesives

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

Solvents are selected such that some will escape relatively quickly from paint films to prevent excessive flow, while others will escape slowly to provide film leveling and adhesion. With typical alkyd coatings, the first 30% of solvent has been observed to evaporate as quickly as the neat solvents, essentially at a constant rate which is dependent on volatility. Later stage evaporation occurred several times more slowly and was rate-controlled by solvent diffusion to the surface of the paint film. The transition point between this behavior was defined as the resin solids content at which the evaporation rate due to volatility equaled that due to diffusion. Transition points have been observed to typically occur at a resin solids content of 40-50% v/v. Thus alkyd paints, normally formulated at 27-40% v/v resin solids, generally exhibit rapid initial solvent-release driven by volatility while high solids coatings (usually 65-75% v/v resin solids) dry solely by a diffusion-controlled process with negligible influence by solvent volatility (Ellis, 1983). 

A Pd film decoupler has also been constructed for amperometric detection of catecholamines. The Pd film has been thermally evaporated onto a plastic chip (without the use of the Cr or Ti adhesion underlayers). Owing to the fast diffusion of H2 on a Pd surface, gas bubbles will not form. Pd is able to absorb H2 produced at the cathode up to a Pd/H ratio of 0.6. This reduces one of the interferences to the EC signal, leading to an improvement of LOD to 0.29 pM dopamine [205,375]. With an optimal decoupler size of 500 pm, up to 6 h of operation was achieved with an electric field of 600 V/cm [375]. 

Solvents are used, however, for special applications. For example, solvents may be added to reduce viscosity and assist penetration on porous substrates. On certain polymeric substrates, solvents may be added to improve adhesion by assisting the diffusion of the adhesive molecules into the substrate. On nonporous substrates, volatile solvents must be evaporated before cure because the solvent could interfere with the degree of crosslinking, and under certain curing conditions, gaseous bubbles could form in the bond line and degrade joint strength. 

Spherical precipitates in the skin of a polymer have previously been observed after the evaporation of Cu on polyimide(PI)(liD- These two systems [Al/PET(sample(l)) and Cu/PI] present a low chemical interaction and a low adhesion, but the deposition rate was much lower for Cu/PI than for Al/PET (1 ML/mn compared with 50 nm/s). This behaviour has been interpreted by Le Goues and alfl 01 as a consequence of a poor chemical interaction between the metal and the polymer at the interface the metal being free to diffuse in the polymer to form clusters of nearly spherical shape. However, it is difficult to compare the mechanism of formation of these precipitates because of the huge evaporation rate difference this rate seems to be critical (Le Goues and al(l Oil for the formation of these precipitates. The presence of these precipitates can only be considered as a fingerprint of a poor quality interface. 

If in the case of aluminized silicone we were able to evidence a drastic difference between sputtering and evaporation, it happens not to be the case for aluminized PET (13). Our preliminary results on this latter polymer indeed show no marked differences between the two deposition processes, both giving strong chemical interaction. By contrast we have also observed that with noble metals such as Au, no chemical interaction is taking place with silicone substrate with both deposition processes. This tells us that the nature of the polymer substrate and of the metal are most important for the interfacial and adhesive properties. The fundamental parameter seems to be the reactivity of both constituents of the interface. It has been confirmed by Pireaux et al. that the carboxylic function is one of the most reactive surface entity (14) and indeed for PET, the adsorption site for the Al atoms is found to be the carboxylic function (13). During this interaction, Al is oxidized and the diffusion of O into the Al film can occur. 

This emulsion is not a liquid-liquid system, but is an aqueous dispersion of solid polymer particles. Therefore, if the Tg were above room temperature (at which the emulsion is applied), the polymer segments, lacking segmental mobility, would not diffuse readily from one particle of the emulsion into another after evaporation of the water medium in which the copolymer emulsion is prepared. The result would be a powdery film. Conversely, when the Tg is reduced below room temperature, segmental mobility in the copolymer leads to diffusion and formation of a flexible, strong adhesive film from the latex by coalescence of the emulsified particles during drying at room temperature. 

The percentage of solids in the dispersion adhesives is 35-55%, in some cases as much as 70%, the rest is water, which after the adhesive application must either be completely evaporated (ventilation time) or diffuse into the joined parts (wood, textiles, concrete, foams, leather). The film formation temperature in types containing plasticizers and flexibilizing additives is lower than in types that are free of plasticizers. Additives, so-called high boilers (ethyl glycol, ethyl glycol acetate, ethylhexanol), are used to bring the film formation temperature down. 

Evaporation of solvent residues through glueline edges, respectively, diffusion through adherends. Development of a characteristic. .adhesive layer"... 

In our study, a three-layered Al/Cu/Ti film was employed as the seeding layer for electroless Cu deposition process. These metal films were deposited using the electron-beam evaporation technique and the substrates employed were thermally oxidized <100> silicon wafers. Ti is employed as the first layer, to serve as a barrier/adhesion promotion layer since Ti adheres well to most dielectric substrates and can prevent Cu diffusion into Si02. The second layer, Cu is the best homogenous catalyst for electroless Cu deposition. The last layer, A1 is a sacrificial layer to prevent Cu oxidation before immersing into the electroless deposition solution. 

Figure 12.20 The free interface diffusion method utilizes a capillary to minimize convective mixing between the protein solution and the second layer of solution containing the crystallization agent (e.g., salt or PEG). The capillary is sealed at both ends with an appropriate adhesive or wax to prevent evaporation of the solution.

Evaporation is carried out under reduced pressure. The chamber is evacuated and back filled with the required gas at a suitable low pressure. Since evaporation is a line of sight process, the component must be rotated and turned to provide a uniform coating on complex shapes. The adhesion of the coating can be improved by heating the substrate, using quartz lamps or a diffuse electron beam. 

A special type of organic solvent-based adhesive is the so-called contact adhesive. This makes use of the fact that certain elastomeric or rubbery solids (e.g., polychloroprene) have the property of autohesion, i.e., they can stick readily to themselves, especially if compounded with resins and containing small amounts of solvents. The bonding takes place by a diffusion process, the adhesive being applied to both surfaces to be bonded. Thus substrates may be coated with a contact adhesive, the adhesives can be allowed to dry till most of the solvent has evaporated (the dry adhesive film at this point will contain... 

In thin-film metallization by evaporation or sputtering of thin metal films onto a ceramic surface (Chapter 28), it has been demonstrated that a sequence of layers of different metals is required for optimum film properties. The first layer is usually a refractory metal such as Ti, Cr, or NiCr this layer provides adhesion to the ceramic. These elements are reactive and bond through redox reactions with the substrate. The second layer acts as a diffusion barrier. The barrier material will usually be a noble metal, preferably Pt or Pd. The top layer will be the metal of choice for the particular application, for example, Au for wire-bonding applications and Ni or Ag-Pd for solderability. 

The adhesive is applied to both of the surfaces to be joined and the solvent is allowed to almost evaporate before the surfaces are joined. Joining is carried out under as high a pressure as possible and as soon as the surfaces contact, the adhesive polymers diffuse into each other and create a strong bond. It is not the duration of pressure which is important but rather the initial pressure. 

Another type of galvanization is called Dry Galvanizing because it does not involve any liquids. For this process, parts and zinc powder are tumbled in a rotating sealed drum at a specified temperature. The zinc evaporates and diffuses into the substrate to form a zinc alloy. This coating has good adhesion to paint, powder coatings, and rubber. It is used to coat small, complex-shaped metals. 

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