Mechanical coating

New coating techniques in commercial use are mechanical coating (now incorporated under plating in Table 13.7) and adhesive-bonded or vapour-deposited coatings, although each of these represents less than 1% of the zinc coatings used. 

The mechanical coatings are primarily barrel processed on to parts up to about 150 mm long or 400 g mass. Adhesive bonding (with a conductive adhesive to maintain the electrochemical protection by zinc) is particularly suitable for wrapping pipes. Vapour deposition has some use in products but the newest development is application on continuous strip for car-body manufacture—the surface is smooth so that the subsequently painted surface has no unevenness. 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

The support can then be mechanically coated with a variety of liquid stationary phases. The mobile phase most commonly used in packed column GC is nitrogen with a flow rate of ca 20 ml/min. Packed column GC affords a relatively low degree of resolution compared to capillary GC typically 4000-6000 plates for a 2 m column compared to > 100 000 plates for a 25 m capillary column. The high temperature limit of packed columns is ca 280°C beyond this temperature the liquid stationary phase evaporates at a rate which creates a large background signal. However, for many routine quality control operations, they are quite adequate. 

Controlled optimal particle size and size distribution ensures superior flow properties of coprocessed excipients and reduced reliance on addition of glidants. The volumetric flow properties of SMCC were studied in comparison with those of the physical mixture of its parent excipients (42). The particle size range of the two test samples was found to be similar, but the flow of coprocessed excipient was better than that of the physical mixture. A comparison of the flow properties of Cellactose with its parent excipients was also performed (5) by measuring the angle of repose and Hausner ratio, and Cellactose was found to have better flow characteristics than lactose or a physical mixture of cellulose and lactose. The spray-dried coprocessed product had a spherical shape and even surfaces, which resulted in improved flow properties. On similar terms, mechanically coating the 2% CSD over microfine cellulose powder resulted in improving its flow properties (43). 

Mechanical coating properties good to very good very good adequate to very good... 

No. Binder composition Molar ratios Hydroxyl content, meq/100 g ED behaviour Bath stability (Td/1+0 °C) Mechanical coating properties Salt-spray rating (10 d)... 

As shown in Figure 5.48, the coatings appear to adhere well to the substrate without showing any deleterious interface porosity. Some homogenously distributed pores are visible within the coatings. However, there is no indication that lack of cohesion may affect mechanical coating integrity. The limited porosity... 

Mechanical coating suitably prepared small steel parts are tumbled in zinc dust at room temperature. Deposition from vapor is also used to give a mechanically or physically adherent coating on steel sheet. 

Nickel contents of up to about 15% are incorporated in a zinc coating, usually by electrodeposition but also by mechanical coating. (The addition of 0.06-0.11% nickel in hot dip galvanizing, to suppress excessive Zn alloy formation, does not influence the corrosion resistance Van Eijnsbergen unpublished report). 

Sherardizing and mechanical coating both involve a tumbling action within a container and are not suitable for fragile parts or very thin, unsupported parts. Flat surfaces can give problems if two parts stick together and. 

In recent years, continuous zinc ribbon anodes have been used in a variety of underground applications (Kurr, 1973 Peabody, 1976 O Connell, 1977). This type of product has broadened the applications for zinc anodes, for it provides small increments of current continuously along the entire length of a cathode. Its uses are generally considered to lie in specialty applications, where other methods of cathodic protection are either impractical or extremely costly (see later section on induced ac on pipelines). Bagnulo (1973, 1984) has developed a tape with an electrically conducting adhesive as described in the Mechanical Coatings part of Chapter 1. 

O Cone, A., Mechanical Coating, Metals Handbook, Volume 5, Surface Cleaning, Finishing, and Coating, 9th ed., ASM International, Metals Park, OH, 1982, pp. 300-302. 

Veli-Matti T. (2001) Amorphous carbon as a bio-mechanical coating-mechanical properties and biological applications. DiamondRelatMater 10 153—160. 

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