Dipping process

Another familiar commercial method is the immersion or hot-dipping process. The article to be coated is immersed in a molten metal bath. Usually httie else is done to change the properties of the coating, which adheres to the surface upon removal of the article from the bath. For a successful coating, an alloying action must take place between the components to some extent. Zinc and tin coatings are appHed to sheet steel by hot-dipping. 

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

Dipping solution Carefully mix equal volumes of solutions I and II immediately before the dipping process. 

Note The full fluorescence intensity usually only develops about 30 min after the dipping process it then remains stable for several days if the chromatograms are stored in the dark (1, 5]. Fluorescein sodium can be employed in the reagent in place of 2, 7 -dichlorofluorescein [5]. The detection limits lie in the lower nanogram to picogram range [1, 5]. 

The dipping processes are of three basic types (1) continuous lengths of strip, wire or tube pass through the molten coating metal (2) semimanufactured products such as cut tube lengths or sheets are dipped into the molten metal in batches in specially designed and automatically operated plants and (3) fabricated products are dipped into the molten metal. 

Mechanical assemblies may be made rigid by hot-dip processing since all crevices should be filled with the coating metal, and this will also provide a seal where required. 

It is essential that the effect of heating of the basis metal during the hot-dipping process be borne in mind at the design stage, especially with aluminising when temperatures of about 700°C are reached and also with... 

Hot-dipped tin coatings are difficult to apply outside the thickness range of 8-38 jtm and hot-dipped zinc coatings do not normally greatly exceed 50 m in thickness. Hollow sections and excessively large articles may be impractical to handle by the hot-dipping process and very thin sections may be subject to much distortion. 

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

Detection and resnlt The chromatogram was freed from mobile phase for 5 min in a stream of cold air, immersed twice in the dipping solution for 2 s and then dried for 5 min in a stream of cold air. In order to stabilize and enhance the fluorescence intensity it was then immersed twice for 2 s in a solution of Triton X-100 — chloroform (1+4), with the chromatogram being kept in the dark between and after these dipping processes. 

In situ quantitation The photometric measurement in reflectance was carried out at X = 525 nm (Fig. IB). In order to ensure that the zone coloration had stabilized, scanning was not commenced iintil ca. 30 min after the dipping process. The detection limit for sugars was of the order of 25 ng substance per chromatogram zone. 

Anodization generally results in the formation of films with limited thickness, uncertain composition, defects, and small crystallite size. Thus, the barrier nature of the n-type semiconducting CdS film obtained in the previous manner makes it too thin to form the basis of Cu2S/CdS or CdTe/CdS solar cells by the normal dipping process. Heterojunction cells of low efficiency have, however, been made by anodization followed by vacuum deposition of the added layer (CU2S). 

Smoking. The effects of smoking on the formation of N-nitros-amines in bacon has been investigated recently by Bharucha et al. ( ). They reported that unsmoked bacon samples generally tended to contain more N-nitrosamines, presumably because of their higher nitrite content at the time of frying. Sink and Hsu (55) showed a lowering of residual nitrite in a liquid smoke dip process for frankfurters when the pH also was lowered. The effects of smoke seem to be a combination of pH decrease and direct C-nitrosation of phenolic compounds to lower the residual nitrite in the product (56). This is an area which requires further study since certain C-nitrosophenols have been shown to catalytically transnitrosate amines in model systems (57). 

The Colton machine illustrated in Fig. 1 is a fully automatic implementation of the dipping process. The steps are as follows ... 

In the manufacture of fast cord the first process is to coat paper or textile yarns with a blackpowder/nitrocellulose dope by a dipping process. These yarns are thoroughly dried and a number of them passed through an extruder and given a thick covering of plastic incendiary composition. The cord thus produced is covered with a protective layer of plastic, usually polyethylene. The overall diameter of the cord is about 2-5 mm. 

A true solution consists of molecules or ions of a solid substance uniformly distributed throughout a liquid. A rubber solution is colloidal, i.e., intermediate between a true solution and a coarse suspension. Rubber solutions are used as adhesives and in the manufacture of rubber products by the dipping process. 

The dipping process, as the name implies, involves the immersion of a former into either a latex, or a rubber solution, and then its slow withdrawal to leave a uniform deposit on the former. Subsequent processes involve, amongst others, drying the deposit, and curing it, if this is required. 

Figure 1 Experimental set-up for vertical dipping processes of monolayers on a quartz-crystal microbalance (QCM) substrate.

Figure 2 Frequency changes of the QCM substrate in air during 4 cycles of vertical dipping processes of cadmium octadecanoate LB films (surface pressure 20 mN m->, 20 °C, dipping speed 100 mm min- )-...

When the LB film-deposited QCM was lowered and raised through the air-water interface at 20 °C, the frequency of the QCM was observed to increase (the mass decrease) compared with that before soaking into the water phase. This is due to the detachment of LB films from the QCM substrate during dipping processes. Figure 17 shows the typical frequency increase (mass decrease) of the QCM deposited with cadmium octadecanoate LB films (10 layers on each side of the QCM, 1130 5 ng), when the QCM substrate was lowered and raised repeatedly through the interface at a rate of 100 mm min. Any increase in surface pressure wasn t observed during the experiments. The frequency of the QCM was... 

An ideal inhibitor is one which burns through at the same time that the powder web is exhausted so-that no residue remains in the catapult. A dipping process was... 

The use of porous formers in the dipping process, or porous molds prepared from plaster of Paris or unglazed porcelain with a surface pore size smaller than the majority of mbber particles, has been widely adopted in the latex industry. With the porous porcelain formers, the mbber particles are filtered on the surface of the formers. The mbber latex coagulates because of its high concentration to form a film of increasing thickness as more water is absorbed into the ceramic. Its rate of increase diminishes sharply beyond an optimum period of time, however, depending on the various characteristics of the ceramic. 

Dipping. A process applied to treatment of ammunition and its component metal parts in an effort to protect prevent corrosion of the surfaces. Various chromate dippings (such as Cronak, Irridite, Yellow Black Anodize have been used especially when plating is first applied. The usual requirement of a dipping process is that the 24-hour salt spray test must be met Ref Ohart (1946), 14... 

Dipping of Cellulose in Tanks used in DuPont s Mechanical Dipper Process of Manufacture of Nitrocellulose is briefly described in Vol 2 of Encycl, p C102-R, under Industrial Manufacture of NC . Schematic arrangement of DuPont s Process is shown in Vol 3, p C539 (See also 1 Direct Dipping Process or Nobel s Method of Manufacture of Cordite )... 

Steel sheets are aluminized by a hot-dip process similar to galvanizing. The principal applications for such a product are furnaces and ovens, automobile mufflers, and other equipment requiring heat and corrosion resistance. When a sheet which has been coated with aluminum by a hot-dip process is exposed to a temperature over l,000°F (538aC). the aluminum forms an iron-aluminum alloy which is heat- and corrosion-resistant. 

---