The dry process is the least important and is used only where cast resins are being made. Light-colored, transparent resins are obtained and the operation is carried on to the final stage (C resin). In this process the aldehyde used is solid paraformaldehyde or hexamethylenetetramine. These materials are more costly than formaldehyde solution.
Proportions of raw materials used vary widely—Baekeland suggested 7 mols of formaldehyde and 6 mols of phenol (210 parts of 100-percent formaldehyde to 564 parts of phenol), with a yield of resin equivalent to 118 percent of the phenol. Larger proportions of formaldehyde are said to increase the yield to as much as 140 percent of the phenol.
Catalysts used to aid in the condensation of the reacting bodies may be acids or bases. Certain properties of the resins may be varied by the kind and quantity of catalyst used. Large proportions of basic or acidic catalysts may affect the filler or metal inserts. Basic catalysts used include caustic soda, caustic potash, ammonia, carbonates, and alkali sulphites. Acid catalysts are usually one of the mineral acids such as hydrochloric acid or sulphuric acid.
While formaldehyde in the form of a 40-percent solution is the principal aldehyde used with the tar acids, certain other aldehydes are used in small amounts. Among these are acetaldehyde, butyraldehyde, benzaldehyde, and others. Resins from furfural and phenol are discussed as “Furfural Resins,” page [51].
Production in the United States.
The production of tar-acid resins in the United States has increased markedly in the last 10 years. Table [3] shows the production and sales of all coal-tar resins in 1927 and 1928 (when there was no further break-down available but when this classification was made up chiefly of tar-acid resins) and of tar-acid resins from 1929 to 1937. The figures given are in net resin content and do not include fillers, modifiers, or pigments. From 1929 to 1937 production increased from 26 million pounds to 80 million pounds; sales from 25 million pounds valued at 9.9 million dollars to 74 million pounds valued at 13.3 million dollars; the value per pound dropped from 39 cents to 19 cents.
In 1937 the production of tar-acid resins for molding accounted for about 40 percent of the total; those for surface coatings, about 25 percent; those for lamination, about 20 percent; and those for miscellaneous uses, about 15 percent.
Table 3. Tar-acid resins: United States production and sales 1927-37
| Year | Production (net resin content) | Sales | ||
|---|---|---|---|---|
| Quantity (net resin content) | Value | Unit value | ||
| Pounds | Pounds | |||
| 19271 | 13,452,230 | 13,084,313 | $6,094,656 | $0.47 |
| 19281 | 20,411,465 | 20,778,856 | 7,211,958 | .35 |
| 19292 | 26,235,792 | 25,129,701 | 9,869,274 | .39 |
| 19302 | 18,338,389 | 17,428,687 | 6,576,023 | .38 |
| 19312 | 22,647,000 | 21,496,000 | 6,646,000 | .31 |
| 19322 | 17,163,000 | 15,042,000 | 3,946,000 | .26 |
| 19332 | 31,697,780 | 28,002,799 | 6,564,670 | .23 |
| 19342 | 40,663,565 | 36,086,008 | 9,037,861 | .25 |
| 19352 | 52,731,728 | 46,733,378 | 8,730,438 | .19 |
| 19362 | 70,349,328 | 61,961,200 | 11,743,978 | .19 |
| 19372 | 79,844,825 | 73,545,880 | 13,300,870 | .19 |
1 All coal-tar resins.