Utilization of Lignin Edwin C . Jahn New York State College of Forestry, Syracuse, Ν . Y .
L
IGNIN is one of the world's greatest
Above. Plastics dies and presses at the New York State College of Forestry. Laboratory molding equipment used for the study of wood and lignin plastics is pictured. At the left are shown raw ma terial and three finished samples. Below. ing tests on wood and lignin plastics at the New York State College of Forestry.
industrial waste and stream pollution problems. In the United States the pulp industry alone must dispose of about 12,000,000 gallons of lignin solution daily. The great logging, sawmilling, and wood working industries account for over 15,000,000 tons of lignin in sawdust, shav ings, and other forms of wood waste an nually. Agriculture, with 15 to 25 per cent lignin by weight in waste straw, corn cobs, bagasse, grain hulls, and the like, has source material totaling millions of tons each year. An enormous supply of lignin taxes our ingenuity to provide methods for disposal and challenges our scientific ability to determine its nature and possible use. Pulp mill wastes and lignocellulosic wastes not chemically treated, such as sawdust and straw, are the two sources of lignin having potential use. Lignin in pulp mill wastes has been liberated from wood and is in solution in spent cooking liquors as so-called alkali lignin or as a lignin sulfonate. Alkali lignin is dis solved from wood by the alkaline processes (soda ant sulfate). It is burned in soda recovery systems at the mills, and on the basis of pulp produced (6) about 1,334,000 tons of alkali lignin were produced and burned in 1937. However, the lignin may be easily recovered from waste liquors by acidifying—for example, by carbonic acid from stack gases. Lignin sulfonates are formed from the sulfite pulping process, and exist as a lignosulfonic acid when the salt is freed of calcium. The sulfite process is the most important chemical pulping process and practically all its waste liquors (which contain nearly all the lignin of the wood) go into the sewer. About 1,000,000 tons of lignin are thereby lost annually in the United States. Time and space do not permit a com plete survey of the literature on lignin wastes. A number of uses, some minor and others involving little chemistry, include sulfite waste liquors for alcohol and yeast production, for roads, and for fertilisers; lignin for adhesives, fuels, in cements, in storage batteries, and for removal of iron from water; and sulfite lignin for leather tanning. Recently much activity has centered in lignin plastics. New developments in this field and in vanillin production will be treated here. and FoodChemistryatthe99thMeetingofof d » American ChtmiomT 8o«i«ty, Ciooinnmti, Ohio» AprU 8 to 12, 1040.
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Nature of Lignin Lignin is generally defined 8 the noncarbohydrate portion of extactive-free woody plant tissue, and thi negative definition indicates the lack of basic knowledge concerning lignin structure. Since the first work on ligna in 1838 by Payen, no one has been a be to pene trate completely its inner stucture al though many facts have been liscovered. Lignins of different plants seen to have a common kinship, yet definitcdifferences are noted, as for example in hose from straw, hardwood*, and softwoods. lignin in a given plant does not apear to be uniform, for certain of its isolaed deriva tives may be fractionated b selective solvents. The existence of lipin in the plant has even been denied bvone group of workers (16, Î7), who call it nerely a reversion product from the actin of acids on certain methylated carbhydrates. This view has many unanswered objections and is not taken seriously. A number of investigators (69, IS) give lignin a molecular weight arouid 850 or a multiple thereof. Various woiters (δ, 10, IS) agree that softwood ligna has five metboxyl groups and five hydray « groups, one or two different from thi others in their reactions, which appar to be of the normal aliphatic type. Aromatic compounds may be obtained rom wood by alkali fusion. This fact (£), as well as the isolation of aromatic eulstances by ethanolysis of lignin (8, 2S) aid of cyclohexanol derivatives by high-presure cata lytic hydrogénation (II, It) indicates possible aromatic groupings in ignin. Obscure and controversial aspects of its chemistry have undoubted^ retarded industrial development of ligiin wastes. The patent literature on utiization of waste iignin from the pulp ndustry is extensive, but the number of uggestions applied arc very few. Only n very recent years has any practice headway been made, and this is mostly in the developmental stage. Lignin Plastic Developments Alkali Lignin Phillips and Weihe (31) have described resins made by condensing aromatic amines and furfural with coricob lignin isolated by alkaline extracton. Most of them are fusible andreadilysoluble in organic solvents. A number of patents have been granted on the preparation of iignin molding powders from taste alkali pulp liquors. In some cases no organic condensing agent is employed (41). In others, phenols and aldehydes are
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994 added to spent liquor and undergo reactions with the lignin prior to its isolation (7). In all cases acidification precipitates the lignin from the spent liquor. In a practical method for isolating and processing lignin (18), flue gas containing 16 to 17 per cent carbon dioxide is bubbled through soda black liquor until precipitation is complete and the suspension coagulated by heating. The washed and dried lignin powder is soluble in dilute alkaline solutions and in most polar organic solvents. It has a melting point of 230° to 240° C. (£9), and will react with aldehydes in the presence of phenols to form a thermosetting resin reported suitable for molded hoards (28). Lignin Sulfonates
Spent liquors from the alkali processes cause no serious problems to the pulp industry since their organic material is burned in soda recovery. In the sulfite process, however, getting rid of great volumes of waste liquor daily has been a constant and costly headache. literature on disposal and utilization is voluminous but, despite thousands of proposed schemes, only a few have a practical basis. These affect merely a minute fraction of total waste lignin. The colloidal and adhesive properties of sulfite waste liquor lignin have long been known, and it has been frequently recommended as a glue, adhesive, or cement. Its most extensive adhesive use is as a binder for secondary roads, being commonly employed for this purpose in Sweden. It is also used in a number of places in this country. The majority of recent developments have been designed to overcome the pollution problem, to recover inorganic chemicals, and to use organic matter for fuel. Probably the most extensive research has been carried out by Howard and coworkers (tO). The objectives of the Marathon-Howard process (19) are, first, to reduce stream pollution, and second, to realize commercial potentialities of the large tonnage of waste noncellulosic organic matter. The waste liquor is given a three-stage precipitation treatment with controlled amounts of lime. Calcium sulfite is the main product recovered, with basic calcium sait of lignosulfonic add next, and the primary organic product recovered for use as boiler fuel or lignin raw material. The basic calcium lignosulfonate may be converted into the free acid or into other salts. Heating the sodium salt with alkali results in the splitting out of vanillin. The ligninfissionproduct which remains is being developed for plastic products. Laminated products built up by compressing paper sheets impregnated with lignin resin are reported to have mechanical strength as well as good water and oil resistance (te). Methods of approach in utilising subite waste liquor lignin for plastics vary. The direct use of calcium lignosulfonate mixed with 60 parts of
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other lignin material for molding plastics is described in a recent patent by Howard and Sandborn (tl). Wallace (45) reports a composite resinous body made directly from sulfite vaste liquor by heating with phenol and formaldehyde. Concentrating waste liquor and precipitating the lignosulfonic acid and other organic matter by sulfuric acid is another method used for plastics molding with or without pr nol and formaldehyde (to). In Wood and Agricultural Wastes
Sawdust, straws, and other forms of forest and agricultural wastes are receiving wide attention as plastic materials. Generally these wastes are treated so the lignin content is both increased and altered chemically and physically to develop fusible and resinous properties. The various methods proposed can be classified on the basis of how the lignin component is made available for use— namely, thermal hydrolysis, acid hydrolysis, and direct chemical condensation. Thermal Hydrolysis
Subjecting lignocellulosic material to high temperatures and pressures in the presence of water results essentially in a partial hydrolysis, owing in part to formic and acetic acids developed from the woody material. According to Boehm (8) hemicelluloees are largely hydrolyzed and removed, and the lignin altered to develop plastic properties. In this process (8, ttf tT) wood chips are treated for a few seconds with steam at pressures up to 1200 pounds per square inch, or above the critical temperature for lignocellulose. Sudden release of pressure results in a disintegrated mass which, when dried to about 4 per cent moisture, may be ground and molded or formed into sheets. These, piled together and hot pressed, form a homogeneous board. Hard black products obtained may be machined, and the mechanical strength and dielectric properties are said to be very good. A series of Russian papers (/, £4) describe the preparation and properties of a molding material by heating sawdust with 0.5 to 1.5 parts of water at 7 to 10 atmospheres9 pressure for 2 or 3 hours. The dried ligneous powder may be molded at 400 to 1000 kg. per sq. cm. at 200° to 250° C. for 5 to 6 minutes. In making the powder it was found tliat lignin increased and cellulose decreased only slightly with increased pressure and temperature (82), and final distribution of particle sizes depended more on initial material than on pressure (80). In molding, addition of a mixture of phenol and glucose was found to increase the flowability and permit molding of figured articles (4). A d d Hydrolysis
If wood or agricultural wastes are hydrolyzed completely or in part by mineral acids, the ligneous residue has plastic properties when subjected to heat and pressure. Chemists have developed a
Vol. 18. No. 22 plastic in this way (37, 39). Sawdust is digested with dilute sulfuric add for 30 minutes at 135 pounds per square inch, the ratio of lignin to cellulose determining properties of thefinishedmolded product Higher lignin content decreases strength but increases water resistance. Add concentration and cooking conditions, therefore, are determined by the nature of the product desired. After washing and drying, the material may be molded at 190° C. with no plasticixer except water, but best results are obtained by adding aniline and furfural, which lower the required molding temperature to 150° C. and improve the properties (40). Various modifications of the add hydrolysis procedure have been described, especially in the patent literature. A patented continuous process employs high pressures and temperatures for 2 to 5 minutes (55). The product is mixed with suitable plasticizers and pressed at 2000 to 7500 pounds per square inch at 140° to 200° C. (84), or it may be incorporated into phenolic molding powders. In Germany wood is commercially saccharified by mineral acids to produce glucose, and by-product lignin is briquetted for fuel. It is claimed (2) that it can be molded for building purposes—flooring and wall material—without use of a binder. Generally, however, free lignin molded without condensing agents or plasticizers gives a brittle product of low mechanical strength. Lignin Condensation Products
Lignin in situ in sawdust, etc., will re--t and condense with a number of chemicals to give moldable products. A process (88) has been developed whereby sawdust or agricultural wastes are digested at 160 pounds per square inch for 3 hours with water and aniline. An aniline-lignin complex results and some hydrolysis of the carbohydrates takes place. When furfural is added as a plasticiser the material forms a satisfactory molding composition having somewhat higher strength and water resistance than plastics prepared by the acid hydrolysis method (40). Various patents have been granted for processes in which lignocellulosic substances react with phenols or aldehydes to produce molding materials. Properties vary widely, but in general the mechanical strength is not high. General Properties of Lignin Plastics
Owing to great variation in properties, it is not possible to give general figures which apply to all lignine prepared for plastic molding. However, products from wood and agricultural lignocellulose wastes by the methods outlined require molding conditions somewhat as follows: pressures of 1500 to 4000 pounds per square inch and temperatures of 150° to 250° C. (mostly 150° to 175° C.) for periods of 3 to 10 minutes. Higher temperatures aie usually necessary if no plaetidsing or re-
Left. At the New York State College of Forestry, stainless steel digesters, equipped with outside circulation and heat exchangers are used for research in the acid pulping of wood and autoclaving of wood wastes for plastics of lignin.
Right. A 65-cubic-foot capacity semicommercial pulp digester. New York State College of For* estry, is constructed of stainless steel. Two stainless steel liquor tanks are in the foreground. Large amounts of sulfite waste lignin are obtained for study by use of the equipment.
acting chemical is u ed. The molded products are dark to lustrous black, have a density of 1.35 to 1.45, and machinabUity is usually fairly good. The compressive strength varies from 21,000 to 25,000 pounds per square inch, theflexuralfrom 6000 to 8000 pounds per square inch, and moisture absorption from 1 to 3 per cent Thesefiguresare average values for a few products made from sawdust by different methods. The technique of preparing and molding various types of lignin powders is undoubtedly only partially perfected. Many severe difficulties have been encountered, such as blistering and failure to flow and completely fill the mold, which result in imperfect articles, poor in mechani al
strength and other properties. These have been reduced, and it is reasonable to expect further improvement The impact strength is on the whole fairly low and a tendency to absorb small amounts of moisture is a disadvantage for some uses (in electrical work). Certain lignin products, especially the lignocelluloee types from wood and agricultural wastes, appear best suited for panels and flat boards. Some of the pulp waste lignins, however, apparently are satisfactory for use as adhesive resins and, when mixed with synthetic resin molding powders, for the more rapid molding of various articles. Use as a reacting diluent in certain synthetic resins, thereby reducing costs for more ex-
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pensive chemicals, appears feasible. Two great advantages favor the use of lignin— availability of the raw material and low cost of processing—and should stimulate further efforts to improve the prépara· tion and properties of lignin plastics. Other Uses of Lignin Production of vanillin from sulfite waste liquor is the most interesting development in the use of lignin as a source of organic chemicals. Only sulfonated lignins yield vanillin. On boiling with alkali, calcium lignosulfonate treated with hot caustic alkali yields 6 to 8.5 per cent (based on free lignosulfonic acid lignin) (45, 44)Industrial processes have been developed by two American and one Canadian pulp
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Semicommercial pulp and paper laboratory, New York State College of Forestry. Facilities of this building are devoted to instruction and research in pulp and paper manufacture and related problems including the utilization of waste lignin. companies. In the HibbertrTomlinson (15) And Hatch (14) processes, the sulfite waste liquor is subjected to hot caustic soda treatment and vanillin extracted after acidification. In the Marathon-Howard process (35, 36') the isolated calcium lignosulfonate is treated and the vanillin removed from an alkaline system. Two of these processes are in operation. A small amount of lignin from sulfite waste liquors is used for tanning leather. In Scandinavia particularly the carbohydrates are fermented for alcohol production and yeast growing. Other interesting uses include lignin as a depolarizer in the negative plates of storage batteries and as an agent for removal of iron from water. Many valuable commercial organic compounds have been obtained from lignin by vacuum distillation, hydrogénation, alkali fusion, and nitric acid oxidation These include hydrocarbons, large numbers of phenols and other aromatic compounds, cyclohexanols, and oxalic acid.
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Conclusion
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In spite of extensive studies on the nature and utilization of lignin, only very small amounts find industrial use. The different types of lignin products for plastics result in materials with varyimr properties, and what has been produced apparently represents an incomplete stage in development of lignin plastics. This fact, as well as present success with commercial production of vanillin and laboratory isolation of many important chemicals, indicates a probable increase in the importance of lignin to chemical indusfnes.
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Literature Cited (1) Barkalai, O. £.. Novoeti Tekhniki, 1938, No. 10,39; C. Α., 33,1409 (1939). (2) Bergiae. F., Koch, F., and Faorber, E., U. 8. Patent 1,890,491 (Dec. 13, 1932). (3) Boehm, R. M., Modern Plastics, 17, 65 (1939). (4) Bofdanov, A. M., Trudy Nauch.IeeUdoeaUL Lab. i Op. StanUii
(5) (6)
(7) (8) (9)
(11) (12) (13) (14) (16) (16)
(19) (20)
(21) (22) (23) (24)
(25) (26) (27)
Barkalaitu, 1937, 110-35; C. Α., 33, 0472 (1939). Braune. F., and Hibbert. H., J. Am. Chem. Soc., 55, 4720 (1933). Census of Manufactures: 1937, Pulp, Part I, p. 556. U. 8. Dept. of Commerce. Bureau of Census, Washing ton, D. C . 1938. Conner, L. C , U. S. Patent 1,614,025 (Jan. 11, 1927). Cramer, A. B., Hunter, N. J., and Hibbcrt, H., J. Am. Chem. Soc., 61, 500 (1939). Fucha, W., and Horn. O., Ber.t 62B, 1691 (1929). Harris, £. E., J. Am. Chem. Soc., 58, 894 (1936). Harris, E. E., and Adkine, H., Paper Trade J., 107, No. 20, 38-40 (1938). Harris, E. E., D'lanni, J., and Adkin*, H.. J. Am. Chem. Soc.. 60, 1467 (1938). Harris, E. E., Sherrard, E. C , and Mitchell, R. L., Ibid., 56, 889 Π934). Hatch, R. 8., U. S. Patent 2,099.014 (Nov. 16. 1937). Hibbert, H., and To inline© η. G. H., 2nd, U. 8. Patent 2,069,185 (Jan. 26, 1937). Hilpert, R. 8.. and Hellwage, H., Ber., 68B, 380 (1935); CeUuloêtchemie, 17, 25 (1936). Hilpert, R. 8., and Littmann, E., Ber., 68B, 16 (1935). Hochwalt, C. Α., and Reboulet, H. J., British Patent 460,939 (Feb. 2, 1937); Reboulet, H. J., Canadian Patent 384,151 (Sept. 19, 1939). Howard, Q. C , Chem. A. Met. Eng.t 46, 618 (1939). Howard, G. C , U. S. Patente 1,551,882 (Sept. 1, 1925); 1,699,845 (Jan. 22, 1929); 1,848.292 (Mar. 8, 1932); 1.856.558 (May 3. 1932). Howard. G. C . and Sand born, L. T.. U. 8. Patent 2,077.884 (April 20, 1937). Howard. G. C , and Sandborn. L. T.. U. 8. Patent 2,080.077 (May 11, 1937). Hunter, M. J., Cramer, A. B., and Hib bert. H., J. Am. Chem. Soc.t 61, 516 (1939). Iv. B. T., and Iv. O. B.. J. Applied Chem. {U. S. S. R.), 9. 322 (in Gr.man. 334) (1936); C. Α.. 30, 6086 α 936). Liach, P. W. E., U. 8. Patent 1,977,728 (Oct. 23, 1934). Marathon Paper Mills Co., Chemical Products Division, Bulletin (Aug. 7 and 14, 1939). Mason, W. H., Boehm, R. M., and
Vol. 18, No. 22 Koonce. W. E., U. 8. Patent 2,080.078 (May 11. 1937). (28) Moad Corp.. British Patent 484,248 (May 3, 1938). (29) Mead Corp., private communication, New York State College of Forestry (Jan., 1940). (30) Natradse, A. G., Trudy Nauch.lêdedovatd. Lab. % Op. Stanltii Barkalaitu, 1937, 61-79; C. Α., 33, 6472 (1939). (31) Phillips, M.. and Weihe H. D.. Ind. Bng. Chem., 23, 286 (1931). (32) Plungyanskaya, M. N., and Gan, M. G., Trudy Nauch.-Iuledovatd. Lab. i Op. SlanUii Barkalaitu, 1937, 33-47; C. Α., 33. 6471 (1939). (33) Olson. E. T.. Katsen. R., and Plow, R. H.. U. 8. Patent 2.156,159 (April 25. 1939). (34) Olson, E. T.. and Plow. R. H.. U. 8. Paten, 2.156.160 (April 25, 1039). (35) Sandborn, L. T., U. 8. Patent 2,104,701 (Jan. 4.1938). (36) Sandborn, L. T., 8alversen, J. R., and Howard, G. C , U. 8. Patent 2,057,117 (Oct. 13. 1936). (37) Sherrard, E. C . BegUnger. E.. U. 8. Patent 1.932.255 (Oct. 24. 1933). (38) Sherrard. E. C . BegUnger. E.. and Hohf. J. P.. U. 8. Patent 2.130,783 (Sept. 20. 1938). (39) Sherrard. E. C . BegUnger, E., Hohf, J. P., and Bateman, E., U. S. Patent 2,153,316 (April 4, 1939). (%0) Sherrard, E. C , BegUnger, E., and Hohf, J. P.. "Wood Plastics as De veloped at the Forest Products Laboratoiy and Their Future Im portance*'. Mimeograph R1209 (June, 1939). (41) Smyser. F. H.. U. S. Patent 1.792.254 (Feb. 10. 1931). (42) TomUnson. G. H.. 2nd, and Hibbert, II., J. Am. Chem. Soc., 58,340 (1936). (43) Ibid., 58, 345 (1936). (44) Ibid., 58,348(1936). (45) Wallace, F. J.. U. S. Patent 2,159,411 (May 23. 1939).
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Billion Dollar Breathless Moment THHE ' 'billion dollar breathless moment" A is the descriptive term coined by the American Petroleum Institute to name the day of the year when the Nation's gasoline tax bill passes the billion-dollar mark. Last year this "billion dollar breathless moment" feU on December 15 and motor ists, truckers, service station operators, and others interested in highway trans portation paused a moment at 11 o'clock in the forenoon to mark the event. It is interesting to note that this moment ar rived 30 days earlier in 1940 than in 1930 and fell on Friday, November 15.
