Лако-красочные материалы — производство

Wood assumes first place among the natural materials. It is used in making vats for dye manufacture, for stirring equipment, for frame­work, and particularly for the construction of factory buildings. In re­cent years, a trend has started toward the use of reinforced concrete for buildings in place of wood, and it remains to be seen how they will stand up. Wood is surprisingly resistant to all chemicals since it is at­tacked only on the surface, and the degraded surface layer then acts as a protective film for the interior.

American pitchpine, Scotch pine, and larch are used predominantly. Birch cannot be used because of its tendency to split, but oak, which is costly but very resistant, is often used. Other woods, except for ash, are not considered because they are too expensive.

Vats up to 20,000 liters in size are used, with stirrers made of ash and attached to the driving assembly by means of a chuck. Such large vats are rarely placed on a framework, as shown in Figure 56, but usually rest directly on the floor. A vat which is to be evacuated must be braced internally with beams, and one to withstand 2 to 3 atmospheres of pressure must be held together with strong iron rods (so-called tie rods). A very small air tube is always installed next to the syphon tube so that any precipitated dye can be stirred up with compressed air. If this precautionary measure is not taken, a large part of the dye may be left behind on the bottom of the vat. All iron hoops should be painted with red lead; frequently, the whole vat is painted. If the liquid in the vat is to be heated to boiling, the vat should be covered, for the sake of

convenience and safety, and a suitable flue for steam is necessary. Fig­ure 56, page 376, shows this arrangement clearly. Vapor stacks are provided with an air or steam tube so that a strong draft can be pro­duced.

Wood is often used for filter press frames; larch or oak, instead of pitchpine, which is rich in tar, is used for presses which are to be used with alcoholic liquids. Also, small spigots made of pear wood are used on filter presses.

Leather is used for transmission belts, for pump leathers on hydrau­lic presses, and for other minor purposes.

Rubber is the outstanding artificial organic material used for con­struction purposes. It is used in many forms, such as tubing, coatings on centrifuges (hard rubber), scoops, and hard rubber stopcocks. Rub­ber coatings on centrifuges have proven very satisfactory, but are not used widely in the dye industry except for centrifuging gallic acid. With this compound, however, copper centrifuge baskets can be used, or iron baskets lead coated by the Schoop spraying process. Chlorinated rub­ber, and synthetic chlororubbers, such as neoprene, are used as acid — resistant coatings for supply lines, vessels, etc.

Synthetic resins, such as Bakelite, Pollapas, Cibanite, and similar materials, are used only for special purposes, since their mechanical strength is generally quite low. Hard rubber is a better material, and can be used also for concentrated hydrochloric acid lines.

Filter cloths are made from organic materials including cotton, jute, hemp, and wool. Press cloths are made from cotton, rarely wool. Strongly acid precipitates are pressed out in camel hair cloths. At one time, cloths made from Chinese hair were available on the market and were superior to all others for durability. As filter cloths, but not press cloths, the so-called nitro filters are often used. (Nitro filters have only moderate mechanical strength.) These are prepared by nitrating special cotton filters. Since cotton shrinks on nitration, the warp and weft of the material must be equally strong. The dry cotton material is stretched lightly on an aluminum frame and suspended for 1 hour in 85 to 88 per cent nitric acid at 15-20°C., and then for 20 minutes in sulfuric acid (66° Вё), after which it is washed thoroughly. These filters withstand 60 per cent sulfuric acid at 100°, but they are immediately destroyed by acid solutions of iron salts. Polyvinyl chloride filters have recently been introduced. They are characterized by exceptional resistance and strength, and are very stable to acids. Class filter cloths are also used, they too being stable to acids.