Лако-красочные материалы — производство

General

It cannot be overemphasized to the beginner in organic industrial work that absolute cleanliness of work is the first requirement for success, not only in analytical and research laboratories, but also in the industrial laboratory. This applies both to the apparatus and con­tainers and to the materials employed. Keeping the apparatus dean is often more difficult than it is in the analytical laboratory because the industrial work involves materials which are more strongly colored and, not infrequently, tarry. Also, non-transparent containers are usually employed and the cleanliness of these is not so easily determined. Under these conditions, special attention is given to cleaning each vessel thoroughly, immediately after emptying it. Immediate deaning is to

be recommended also because it may often be effected satisfactorily with hot water, either alone or with small quantities of acid or alkali, whereas a residue, once dried and crusted, may be much more difficult to remove, requiring powerful solvents such as sulfuric and chromic acids. In many cases, ordinary household cleaning powders are very useful in removing oily and tarry residues.

If the chemist does not dean his own apparatus, or have it cleaned under his direct supervision, he must be very careful not to leave con­tainers with residues which are highly inflammable or explosive or strongly poisonous (such as ether, alkali metals, sodium amide, dimethyl sulfate, phosgene, etc.). The neglect of this rule has resulted in many accidents, since the person cleaning the apparatus is not aware of the danger. Residues of this sort should not be emptied into the laboratory sink, but should be treated so as to make them as harmless as possible.

The work table should always be kept clean and uncluttered. It is then possible to recover substances which have been spilled and thus save an experiment which otherwise would have to be repeated from the beginning. In experiments which require prolonged strong heating, the table top should be protected from the burner by a fire-resistant, insulating cover, such as asbestos or “Transite.” Apparatus no longer in use should not be left in the working space. If space permits, it is desirable to place larger apparatus outside of the working space proper, and reserve the latter area for test tube experiments, titrations, melting point determinations, and so forth. All necessary equipment for control tests should be readily available, including clean, dry test tubes, glass rod and glass tubing, small filters and funnels, and the most common reagents and test papers.

The use of pure materials is at least as important as the cleanliness of the apparatus. In analytical and research laboratories, the use of chemically pure reagents is understandable, since, as a rule, only small quantities of materials are used and the cost is of no importance. In industrial laboratories, however, cost considerations usually prohibit the use of chemically pure materials and furthermore one is often restricted to the use of commercially available starting materials in order to duplicate plant conditions.

It is the general rule, however, in preparing new products and in working out new procedures to use the purest substances possible with­out regard to their cost and commercial availability. Later it is estab­lished whether the same results can be obtained with technical materials.

In most cases the inorganic chemicals used in industrial organic work are usable in the form supplied commercially. There are, however, important exceptions. For example, the nitric acid used in the nitration of free primary amines in sulfuric acid solution must be free from nitric oxide (see page 165). Also, the presence of sulfuric acid in technical hydrochloric acid may cause difficulties in the diazotization of amines which form difficultly soluble sulfates. In still another example, if chlorate is present in the caustic alkali used for alkali fusion, undesired oxidation or even an explosion may result. In all such instances, it is essential to use products which are free from the deleterious impurities. It should also be pointed out that certain substances deteriorate on long standing. This occurs with materials which are strongly hygro­scopic (e. g., oleum and chlorosulfonic acid), easily oxidizable (e. g., sulfite and bisulfite), or otherwise easily decomposed (e. g., hypochlorite solutions). In these cases, the compounds must be titrated before use.

In the laboratory, distilled water may be used even for technical work, in order to avoid cloudiness caused by the separation of calcium salts. Before a procedure is adapted for large scale operation, however, it must be established that tap water has no harmful effect. Water may be very injurious when present as an impurity in organic liquids, as in the chlorination of nitrobenzene (page 117), and in these cases, careful drying is necessary. With high-boiling liquids, the drying may be accomplished by distillation, discarding the forerun.

The presence of impurities in the organic starting material is gener­ally much more injurious than their presence in the inorganic chemicals. Frequently, only very small amounts of a contaminating material may produce an appreciable lowering of the yield or a marked decrease in the purity of the end product. Therefore, the organic chemical industry has gone to considerable lengths to prepare their intermediates in as pure a form as possible. Considerable progress along this line has been made in recent times, so that today many intermediates are available in almost chemically pure form. This is especially true for those substances which are purified by distillation or vacuum distillation. On the other hand, all materials which must be isolated by a salting-out process naturally and unavoidably contain some inorganic salts. This salt is harmless for the great majority of applications, but of course it must be taken into account in determining the quantities to be used. Therefore, it is necessary to determine the purity of all salt-containing starting materials. Primary amines can be titrated with nitrite solution, and compounds which undergo diazo coupling can be analyzed by titration with a diazonium compound. With other materials, other suitable methods are used (see Analytical Section).

The beginner is usually greatly impressed by the strong color ex­hibited by technical products which should be colorless when com­pletely pure. As a rule, this color has no significance for industrial use,

unless it is exceptionally strong, indicating decomposition or oxidation, and then purification may be necessary. Still more serious, however, is contamination by isomers or closely related compounds whose presence is not so easily discernible, for example, monosulfonic acid in disulfonic acids, or vice versa. Similarly, m-phenylenediamine, which contains only a few per cent of the ortho and para isomers, gives azo dyes in much lower yield and purity than the pure meta compound. The impure material is also much less stable (see pages 112 and 116). The methods used for testing the organic starting materials for purity, and for purify­ing them, are the same as those described later for end products (see page 40 ff.).