Electro-adhesion

The science of adhesion, or "adhesiology", started to take shape in the last decade. With the cooperation of surface scientists, polymer chemists, and rheologists, we gradually started to understand the mechanisms of adhesion. Interestingly, adhesion is a universal phenomenon which can take place even in the absence of an adhesive. New terminologies have been introduced to describe various aspects of adhesion, e.g. bio-adhesion, electro-adhesion, particle adhesion, and photo-adhesion. 

Electro-adhesion was established by Krupp and Schnabel as the cause of light-modulated adhesion. Further evidence for electroadhesion is given in a new book by Derjaguin. The importance of electrostatic adhesion in electrophotography or xerography will be further discussed at the end of this book. For example, the attachment of toner particles on the surface of a carrier is determined predominantly by the electrostatic attraction between the two materials. This attraction is also called triboelectricity on the surfaces. Besides electrostatic attraction. Van der Waal forces can influence the toner particle adhesion depending upon the size of the toner. 

Figure 3. (a) Robot with electro-adhesive flaps (Prahlad, et al, 2008), adhesion stick and (b) gecko robot (Kim, etal, 2008 Unver, etai, 2006)... 

The selection of a particular deposition process depends on the material to be deposited and its availabiUty rate of deposition limitations imposed by the substrate, eg, maximum deposition temperature adhesion of deposit to substrate throwing power apparatus required cost and ecological considerations. Criteria for CVD, electro deposition, and thermal spraying are given in Table 2 (13). 

Electroforrning is the production or reproduction of articles by electro deposition upon a mandrel or mold that is subsequendy separated from the deposit. The separated electro deposit becomes the manufactured article. Of all the metals, copper and nickel are most widely used in electroforming. Mandrels are of two types permanent or expendable. Permanent mandrels are treated in a variety of ways to passivate the surface so that the deposit has very Httie or no adhesion to the mandrel, and separation is easily accompHshed without damaging the mandrel. Expendable mandrels are used where the shape of the electroform would prohibit removal of the mandrel without damage. Low melting alloys, metals that can be chemically dissolved without attack on the electroform, plastics that can be dissolved in solvents, ate typical examples. 

Fig. 5-8 Total adhesion loss of a 500-/xm-thick coating of EP (liquid lacquer), 0.2 M NaCI, galvanostatic = -1.5 /tA nrr, 5 years at 25"C. Left coating with a pin pore loss of adhesion due to cathodic disbonding. Right pore-free coating loss of adhesion due to electro-osmotic transport of H O. In both cases the loose coating was removed at the end of the experiment.

This review will highlight the interrelationships between basic photopolymer science and practical applications of this technology. Each application of photopolymer technology can be described in terms of three primary descriptors the mode of exposure, the mechanism of the photopolymer reaction employed and the visualization method used. Using this foundation, the widely diverse applications of photopolymer technology to electronic materials, printing materials, optical and electro-optical materials, the fabrication of devices and polymeric materials, adhesives and coating materials will be discussed. 

These differences in film morphology were also reflected as differences in film formation conditions, film adhesion, and in electrochemical properties. The pyrazoline beads readily formed films from solvents such as benzene. For the phenoxy TTF system, however, only CH2Cl2 was effective in forming films. In general, the TTF cross-linked polymers were found to be less adherent to the metallized substrates than the pyrazoline cross-linked polymers. Electro-chemically, it was found that the pyrazoline films showed complete activity after one potential sweep. The TTF polymer films, on the other hand, required from 5 to 20 cycles to reach full electrochemical activity as evidenced by a constant voltammogram with cycling. Furthermore, it was observed that the TTF polymer films were much less electroactive than the pyrazoline materials as shown by optical densities and total coulombs passed which were several times less for the TTF systems. 

In the crosslinked state, epoxy resins are highly resistant to chemicals, temperature, and solvents and are also endowed with good electrical properties. They are therefore employed, for example, as casting resins in electro- and electronic industry as well as resistant lacquers and coatings. Moreover, they possess excellent adhesive power for many plastics, wood, and metals ( reaction adhesives two-component adhesives ). 

With polymeric materials it is essential first to provide on the surface a conductive film as a basis for electro-deposition there must be good adhesion of this film to the substrate, and the quality of the finished work will depend on the method used to apply it and the standard attained. 

Some other elution modes have been described. They are induced by various factors — cyclical field, secondary chemical equilibria, adhesion chromatography, asymmetrical electro-osmotic flow for a review, see Ref. 2. However, the number of their implementations is rather limited, and for this reason, these modes are not discussed here. 

Natural and synthetic iron oxides not possessing pigment properties are used as raw materials in the production of hard and soft ferrites (see Section 5.5.5.2, Electro- and Magneto-Ceramics) for radio, television and telephone technology, for adhesive magnets, for rotors in dynamos, for low-loss magnetic layers, for DC-motors, for transformer cores, for electronic calculators and high frequency furnaces. This amounts to an annual worldwide production of more than 300 10 t/a. 

Electro engineering, the chemical industry, and motor vehicle production are the growth branches, the construction and furniture industries are showing a downwards productiOTi index curve. Adhesive manufacturers will therefore concentrate development efforts on the first-mentioned areas. 

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