Nitric Acid Pulping of
Whole Cotton PROPERTIES OF CELLULOSES OBTAINED J. D. FLEMING AND F. K. CAMERON University of North Carolina, Chapel Hill, N. C.
I
N T E R E S T in cellulose Whole cotton and its components have samples prepared for pulping p u b s prepared by the been pulped separately with 5 per cent were ground in a hammer mill; a 3/32-inch screen was used for action Of nitric acid On nitric acid at atmospheric pressure. There whole cotton, and a 3,50-inch various raw materials is steadily increasing. Macormac was no undue degradation Of cellulose* screen to shorten the pulping Examined for alpha- and beta-cellulose period for stems and cusps. and Cameron (18) cited data and pentosan content and for copper numBefore pulping, whole cotton on such substances as bagasse and cotton stalks; a further ber and cuprammonium viscosity, the was washed with aqueous sodium sulfide, as suggested by study Of the latter was repulps compared favorably with those from and Cameron to reported by Korzheniovskil and Raskina (11) and Shikata and commercial woods* No Practical reason move coloring agents and then appeared for separating the components extracted with a low-boiling Akagi ( 8 3 ) . Simoda and petroleum ether. The delinted Yosino (86)pulped rice straw. for the production of pulp. seeds were likewise extracted Aronovsky and Lynch (8) with petroleum ether. leaving pulped woods with alcoholic for pulping a residue of hulls and degreased seed meats. nitric acid; Aronovsky and Dryden ( I ) , Reid and Lynch (I@, The pulping was carried out in a 5-liter flask. I n it were and Whittemore, Reid, and Lynch (26) studied pulps from placed 250 grams of air-dried sample with 2500 ml. of a 5 woods and cotton linters. Routala and Sev6n (19) pulped fir. per cent solution of nitric acid. The flask was then placed Benson and Walton (4)studied the use of nitric acid for pulping on a steam bath and heated for 4 hours. During the digesDouglas fir; they reported that the acid has been used in Gertion, the temperature in the fluid mass was between 80" and many to produce 40 tons per day of high grade cellulose from 90" C. The pulp was then washed free of acid and heated on beechwood, and a t least one American producer has used it. the steam bath with 2500 ml. of a 2 per cent solution of sodium Macormac and Cameron (18) pulped whole cotton with a hydroxide. Filtered on a copper screen, the pulp was washed 2.1 per cent aqueous solution of nitric acid, after removing until the washings were neutral to litmus. It was then oil and wax from the pulverized mass of stems, cusps, and washed with 500 ml. of a 10 per cent solution of acetic acid. seed cotton. A more concentrated solution is desirable. For Washed free of this acid the pulp was bleached with 2 liters the present study a 5 per cent solution was used. The proof a 0.5 per cent solution of commercial bleaching powder. cedure of Chen and Cameron (6),who investigated the distriThe appearance of the pulps was satisfactory. A few bution of cellulose in the several components of the cotton brown spots or specks were found in a wet pulp from cusps, plant, was followed. This paper records some of the properand close inspection showed a few in a pulp from whole cotton. ties found for pulps made from whole cotton and from its Raitt (16) and Heim de Balsac and eo-workers (8) noted such several components. Because there is no lignin in seed cotspots in their work on cotton stalks. I n making p u b s from ton and not enough in stems and cusps to affect the pulp from whole cotton which had been harvested several years before, whole cotton significantly, determinations were not made. Macormac noted such spots, but they disappeared when the Nor were the physical properties of the pulps studied quantipulps were beaten. Many pulps without specks have been tatively. made from the cotton used here. However, the cotton had IMATERIALS AND PULPING PROCEDURE been in storage 18 months when our pulps were made. It is a The whole cotton was grown in 1940 and has been stored in reasonable conclusion that the specks are due to aging of the an open shed. This same cotton was used in the studies recotton. The scanty occurrence of the specks and their disappearance ported by Chen and Cameron (6) and by Powell and Cameron on beating make it improbable that they can affect any prac(14). It was harvested 25 weeks after planting and contained some immature seeds, but a fair proportion of the seeds was tical use of the pulps. It seemed desirable, nevertheless, to shown to be viable by a planting experiment. Zakoshchikov determine their origin. Raitt suggested that they come from the cuticle or outside skin of the stems. We found large and colleagues (27) reported that the lint of immature plants has the same value commercially as that from mature plants. pieces of stems pulped completely without specks. When About 400 pounds, taken a t random, were sent to the whole cusps were pulped separately, little nodules near the base of some of the cusps were found to have resisted the laboratory where the mass was quartered and several 25action of the pulping liquid. Some nodules were gathered pound samples were prepared. The content of stems was and treated separately and proved to be very resistant. 20.5 per cent; of cusps, 19.5 per cent; and of seed cotton, 60.0 per cent. Seed was delinted with sulfuric acid. All Even when broken into small fragments in the grinding of a 819
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820
sample for a regular pulping, an occasional fragment may not be completely disintegrated in the pulping and becomes a speck in the resulting relatively large mass of cellulose. PROPERTIES OF THE PULPS
ALPRA-,BHTA-AND GAMMA-CELLULOSE. Alpha-cellulose is defined as that part of the total pulp which is not dispersed when the pulp is digested in a 17.5 per cent solution of sodium hydroxide; beta-cellulose is defined as the precipitate when an excess of acetic acid is added to the dispersion. The unregenerated material in the dispersion is estimated by difference as gamma-cellulose. The alpha-cellulose of all these pulps compares favorably with those usually reported for commercial bleached wood pulps. This is true for the figures for whole cotton, although the latter is not quite so high as might be expected, assuming the lint to be practically pure cellulose. In a pulp from another sample of this same crop, Chen and Cameron (6) found 97.34 per cent alpha-cellulose; Macormac and Cameron (If?) found 86.7 to 94 per cent in five pulps from the crops they investigated. The beta-cellulose content of the pulps from stems, cusps, and seeds are in accord with the 1.2 and 2.5 per cent for whole cotton, as reported by Macormac and Cameron. The figure for whole cotton in Table I is comparable with that commonly reported for commercial pulps but must be regarded as high and an indication of degradation in the preparation of this particular pulp. OF CELLULOSE FROXI WHOLE TABLEI. CHARACT~RISTICS COTCOX PULPED WITH NITRIC ACID
Source Alpha-cellulose, Bets-cellulose, Pentosans Z ’ Copper ndmber Cuprsmmonium viscosity, centipoises Ash, %
2
*
Stems
Cusps
Seeds
Whole Cotton
82.10 1.09 8.78 1.89
82. 2.89
9.30 2.07
83.00 3.24 14.61 2.03
84.11 10.58 4.44 1.17
14.3 0.93
17.4 0.23
21.2 0.18
7.42 0.26
i5
COPPERNuhfBER. The standard method of the Technical Association of the Pulp and Paper Industry (2%)is the Braidy modification (6) of the original Schwalbe method (10)and was studied critically by Clibbens and Geake (7). With it, we obtained good duplicate determinations. The values obtained for stems, cusps, and seeds are somewhat low as compared with those for commercial wood pulps, while the value for whole cotton is that characteristic of highly purified wood cellulose. Two possible expIanations of these low copper numbers have been published. Korzheniovskil and Raskina (11) suggest that the cellulose fibers are protected from attack by nitric acid by a buffer action of impurities in the crude plant material. Simoda (24) boiled purified cellulose with dilute solutions of nitric acid and found that in solutions of 10 per cent or less little oxycellulose was formed. He did find some hydrocellulose, but it had little effect on copper number. PENTOSANS. Although considered impurities in a pulp, the presence of pentosans is sometimes desirable or even necessary for certain kinds of paper. They are thought to interfere with esterification and must be removed from pulps so destined. The method employed for their determination is that of the Association of Official Agricultural Chemists (S), based on the fact that pentosans yield furfural, whereas cellulose yields glucose. The high value found for the seed is probably due to content of hulls. Ivanova and Kurennova (9) reported 15 per cent furfural from hulls and predict much higher results from delinted hulls. The low value for whole cotton is due to
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the high content of lint, which itself contains less than 1 per cent of pentosans. The pentosan content found for whole cotton is about that in a bleached coniferous wood pulp; that for stems and cusps, for a bleached hard wood or aspen Pulp (18). VISCOSITYOF DISPERSIONS IN CUPRAMMONIUM SOLUTIONS. Although Rich (17),Richter (18), and others reported results by methods requiring less than 30 minutes, the standard method of the Technical Association of the Pulp and Paper Industry requires 20 hours, special equipment, and rigid control. Essentially, the standard method was used to obtain the results here recorded, following the specifications of Mease ( I S ) and Scott ( d l ) , and satisfactory duplicates were obtained without difficulty. Korzheniovskil (10) found that purified cellulose, after treatment with nitric acid, gave higher viscosities when dispersed in cuprammonium solutions. Moreover, high temperatures in pulping tend to lower this viscosity. The moderate temperature and short pulping period when nitric acid is the reagent would be expected to be followed by relatively high viscosities. ASH AND COLOR. The ash contents of the pulps are as low as generally found for commercial wood pulps; but since they were due largely to adhering soil, they could have been materially lowered by mechanical manipulation in the preparation of the pulps. These pulps were not “purified”. In all cases the color was a good clear white, and bleaching was easily accomplished. SUMMARY
Cotton stalks and cusps are essentially wood and, it is to be expected, would react as do woods generally towards pulping agents. The degreased seeds, a mixture of meat residue and hulls, might be suspected of showing a different response. The present study confirms previous work in this laboratory and shows that all the components of the cotton plant, treated either separately or as a whole, are pulped readily by a 5 per cent solution of nitric acid, at atmospheric pressure and a moderate temperature, within a short time interval, and without undue degradation of cellulose. It is not suggested as having general application, since Chen and Cameron (6) showed that some woods were pulped with difficulty and with a decomposition of nitric acid which implied a considerable chemical change in the rellulose content. The pulp obtained from whole cotton compares favorably with the pulps from tree woods, commonly used as raw materials. The composition and properties of the components, including seeds (see meats and hulls), have been found to be fairly close, and the datu, obtained suggest that each can be retained with advantage in whole cotton which is to be processed for its cellulose content. It has been found that the dark colored specks occasionally observed in pulps made from relatively old cotton plants are undigested or partially pulped fragments of a concretion sometimes (but not always) to be found a t the baee of a cusp. They have been observed only as a few widely scattered spots in any one pulp. LITERATURE CITED
Aronovsky, 9. I., and Dryden, E. C., Paper I n d . Paper World, 22. 253 (1940). Aronovsky, S . I., and Lynch, D. F. J., IND. ENG.CHmhc., 38, 790 (1938). Assoc. of Official Agr. Chem., Methods of Analysis, 4th ed., p. 344 (1935). Benson, H . K., and Walton, A. T., Paper Trade J . , 111, No. 10, 27 (1940) ; Tech. Assoc. Papers, 24,552 (1940). Braidy, H., Rev. g6n. mat. color., 25, 35 (1921). Chen, W. W., and Cameron, F. K., IND.ENG.CHBM.,34, 224 (1942). Clibbens, A. C., and Geake, Arthur, J . Textile Inst., 15, 27T (1924).
