Diffusion bonding process chain

Figure 1.13 Diffusion bonding process chain. Starting top left stacking, diffusion bonding furnace with mechanical pressure force, diffusion bonding and a cut through a microchannel system after diffusion bonding.

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... 

The kinetics and equlibria of the complexation between PAA and PEO or PVPo were studied by Morawetz s group [ 13-15], and it was shown that the complex formation consisted of an initial diffusion-controlled hydrogen-bonding process with a small activation energy and an extensive conformational transition of the two polymer chains which induces additional hydrogen bonding, thus stabilizing the complex. 

Two-Shot Injection Molding. Two-shot or oveimolding refers to a process whereby either different colors or different materials are molded into one part. In this process, the first material or color is injected, then the mold is rotated, and the second shot is injected as depicted in Fig. 1.27. An alternative method is to use a retractable core. In this case, the first material is injected, cooled to solidify, and then the core is retracted to allow injection of the second material as shown in Fig. 1.28. Bonding is accomplished through either strictly mechanical means or by adhesion between the two components through diffusion of the chains. This can result in parts with two materials combined with-... 

A second type of irreversible phenomena commonly occurs at the interface itself. They may be irreversible chemical reactions of the sort deliberately produced in certain structural adhesive joints. In addition, in certain polymeric contacts, diffusion processes may produce either the segregation of low molecular weight species at the interface or diffusion bonding by the intermixing of the polymeric chains. The former will reduce adhesion and the latter will increase it. In principle, for both processes the time dependent value of the adhesion may be used to monitor the extent of the diffusion processes" " " and hence provide an estimate of the diffusion coefficient. For these and other reasons it is often found that the adhesion is a strong function of the temperature and contact time. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

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