Cellulose f rorn Hardwoods GEORGE A. RICHTER’ Brown Company, Berlin, N. H.
This is the first of a series of articles that deals with hardwood as a source of wood pulps. The hardwoods described include the birches, the maples, and beech, such as are prevalent in the northeastern states. They differ from the softwoods both physically and chemically. The unseasoned hardwoods, white birch in particular, are rich in extractables which offer an opportunity for economic recovery. Hardwoods are richer in pentosans and leaner in lignin than are spruce and fire, They are more dense and contain more cellulose per cord of stacked wood than do the conifers. This article is confined chiefly to the Characterization of the wood itself.
NTIL recently the coniferous woods have occupied an undisputed dace of preference as a raw material j o r the rianufacture of wood celluloses that are intended for the fabrication of paper and for conversion into the several cellulose derivatives. The choice of the softwoods over deciduous or hardwoods has depended upon a number of resultant advantages, some of which were enjoyed by the producer of the pulp while others have appeared important to the papermaker and the producer of cellulose esters. I n the early days of pulpmaking, the roading of logs was both uncertain and costly, and it was almost compulsory that the wood be floated to the mill. The northern forests were abundant in both spruce and fir, which require a minimum effort in cutting and which can be driven down the streams and held in the rivers a t strategic points until needed a t the mill. The beeches, birches, and maples are too heavy to float except for short distances, and if left in the water for more than a few weeks, a substantial sinkage loss is experienced. Transportation by road was then almost impossible and rail rates were prohibitive when compared to transportation by the waterways. Aside from the matter of cheap transportation, the pulping of hardwood a t the pulp mill was attended by several additional problems. Whether the pulpmaker produced an unbleached base material for newsplint or perhaps a bleached fiber for papers of higher quality, the longer fiber of the spruce and fir made it less difficult to approach the physical strength of established papers of rag origin. Of lesser importance was the greater ease with which one can avoid physical loss of fiber in the many washing steps when processing the longer fiber. And last but not least, appeared the difficulty of pulping satisfactorily the much denser birch, beech, and maple with the processes that had been developed for the softwoods. Except for a small tonnage of short-fibered poplar soda pulp which has been used for years as a filler fiber in some grades of soft papers, by far the greater part of the present wood pulp industry has been predicated upon an abundant supply of available softwood, because of the reasons given above. I n recent years, particularly in New England, the softwood forests have given evidence of substantial depletion, and the pulpmaker has been faced with a choice of curtailment of wood pulp production or of a major development t o establish ways and means of satisfactorily processing the almost limitless supply of beech, birch, and maples that have replaced the large portion of the original softwood growth. Such research and development were seriously undertaken some ten to twelve years ago. The result has been highly successful, and in the last decade the refined hardwood pulps have entered practically every field that was formerly occupied by the softwood pulps alone. In view of the fact that the softwoods and the wood pulp
U
1
of coniferous origin are more widely known than are the hardwoods and their corresponding products, it seems advisable to include with the hardwood data typical characterizations of softwood that were obtained in the same laboratory and by identical analytical procedure. Hardwoods that are indigenous to the northeastern section of our country differ greatly from the spruce and fir of the same region, not only physically but chemically. Physical differences occur both in respect to the individual fibers that make up the wood body and in the composite structure itself. I n general, the internal structure of the hardwood is much more complex. Whereas the softwoods are composed largely of one type of element that comprises upward of 80 per cent of the total volume of the wood, the hardwoods are made up of a more heterogeneous assortment of units that are present in less regular pattern than appears in the conifers. The hardwoods are further characterized by the presence of pores which sometimes occur largely on the inner side of the annular ring and in other instances are distributed throughout the growth ring. The pores are readily seen and consist of tubelike systems that the technologist terms “vessels”. The fibers in hardwood are very slender and are shorter than those in softwood. The fiber walls of the hardwood are usually thicker, and the ray cells are more numerous and variable in type than those of spruce and fir. If one accepts the school of thought that considers all cellulose as being of the same chemical composition regardless of origin, then the major differences as revealed by chemical analyses of different woods can be attributed to the secondary noncellulose substances that are associated with the cellulose itself. The pulpmaker ordinarily designates these associated substances as impurities, and they assume an important role in the characterization of woods of different species, as will be noted
Wood Density Except for the poplar which is sometimes classed with the hardwoods, the remaining prominent deciduous species of northern New England-namely, the birches, maples, and beech-are definitely more dense than are the softwoods that grow in the same region. Density may be expressed in many ways. From the viewpoint of the pulpmaker, i t is best defined as the pounds of dry wood that are contained in a solid cubic foot of undried wood as actually purchased. Even such a value does not tell the entire story, inasmuch as the purchaser ordinarily buys the wood on an unbarked cord basis, and the total cubic feet of solid wood in a stacked cord are dependent both on the assortment of diameters and the thickness of bark. An average cord stacked t o outside dimensions of 128 cubic feet will contain in the neighborhood
Present address, 158 Prospect Street, Berlin, N. H.
75
76
INDUSTRIAL AND ENGINEERING CHEMISTRY
of 90 cubic feet of solid wood. If the logs have been peeled, the solid wood value approximates 100 cubic feet. The density of wood in the case of a given species depends upon the rate of growth, the relative amounts of summerand springwood, and general environment. Differences that occur within a given species are not large enough to prohibit a general classification of the woods in question. Table I includes averages that were established from several hundred data. Wherever a direct comparison is possible, these values are consistent with those reported by Kewlin and Wilson (4). The higher value as expressed in terms of weight of a solid cubic foot of dry wood is explained by the volumetric shrinkage that takes place when the wood is dried. TABLE I. AVERAGERANGEOF WOODWEIQHTS Lb. of Dry Wood in a Solid Cu. F t . of: Red suruce (Picea rubra) Fir (Abies balsomea) White birch (Betula papyrifera) Yellow birch (Betula lutea) Rock maple (Acer saccharum) Red maple (Acer rubrun) Beech (Faous grandifdin)
Undried wood
Dried wood
21-25 20-23 31-35 33-36 36-40 29-33 35-42
22-26 21.5-24.5 33-37 35-39 38-44 31-36 38-46
The greater density of the hardwood is clearly seen. Both spruce and fir are about two thirds as dense as the birches, beeches, and maples. It does not necessarily follow that the penetration of aqueous solutions of pulping reagents is less rapid in the case of the hardwoods, but the increased density does influence the amount of reagent that can be forced into a given wood volume and hence determines the necessary increase in concentration of solute t o attain a desired percentage of delignifying agent based on the wood and contained therein. Subsequent cooking data will illustrate this point more clearly. Beech wood consistently and the rock maples usually have highest density, ranging from 40 t o as high as 48 pounds of dry wood contained in 1 cubic foot of the undried wood. Cooking data reflect this extreme density. Later figures will show that the increased density of the hardwood is translated into a corresponding increase in pulp yield over softwoods when such yields are expressed on a cordage basis.
HAULING OLD-GROWTH SPRUCE
Wood Spruce Fir White biroh
Yellow birch
Beech Rook maple
Vol. 33, No. 1
TABLE11. BARKVOLWES Diameter, Growth Bark Inches 4.15 8.72 4.15 8.47 3.88 8.27 4.23 9.34 4.23 8.89 4.11 7.77
Rings 62 112 38 65 46 68 74 32 63 125 67 114
Vol. Based on Wet Log Vol., yo 12.06 9.26 10.0 3.4 14.5
9.5 10.1 3.3 6.9 6.1 13.5 18.0
Wood Bark A few typical figures (Table 11) will illustrate the relative importance of the bark volumes of different species when the wood is procured in the rough or unbarked state. Table I1 reveals that in all cases except with the rock maple the bark volume percentage is less as the tree diameter is greater. The beech is characterized by thinnest bark. Several measurements showed that with logs ranging from 5 to 12 inches in diameter, the bark of the rock maple becomes thicker a t a rate that exceeds the increase in total diameter. This observation has importance since wood is largely purchased in the rough state and the bark represents not only a substantially valueless hauling charge, but determines the per cent wastage of each cord purchased. Barking of the logs in the woods is not practical except during the spring and early summer months. There is no marked difference in the ease with which bark is removed from each of the species in question. Although the bark is ordinarily removed from the tree before delignification of the wood, a few data are submitted as a matter of general interest. Potential utilization of the bark ingredients is receiving serious consideration. Table I11 summarizes typical test values obtained in the study of barks of several species common in northern New England. I n each case the trees were felled in late October or early November. Seasonal differences occur particularly during the spring months. The test results are strictly comparable, inasmuch as the same procedures and technique were employed throughout. U n l e s s o t h e r w i s e stated, the extractables were removed by the solvents in the order: ether, acid alcohol, hot water. This observation is important because of the presence of certain ingredients that have common solubility in more than one of the solvents employed. As noted by other investigators, the barks of all the species examined are considerably higher in lignins and are for the most part richer in total extractables than are the woods of the same tree. The pentosan values are on a lower level than is found in the corresponding woods (Table V). The high ash content of the beech bark is particularly interesting and was subsequently found to be in the same general range in the case of two or three additional samples.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1941
77
TABLE 111. WOODBARKANALYSEV IN PERCENT Tree No. 8-14 8-1-6 F-11-1
Black spruce
Fir (New Hampshire)
Beech
17.1 17.48
14.16 13.7 12.64
37.0 41.42 46.2 43.62
9.1
Hot-WaterSol. 10.46
2.3
1.7
...
5.8
12.3
5.9
i :i
1i:i
i:i
26: 3
4:o i:i
...
16.6
41.82
Acid-AlcoholSol. 12.06 4.10 12.50 6.60 12.23 13.6
8.05
16.48
37.8 41.70 36.5
EtherSol. 2.26 4.10 11.15 11.3
8.8
41.0 49.16
B-11-2 RM-1-2 RM-11-1
Rook maple
8.84 9.5 8.30
39.16
WB-11-2 WB-I14 YB-11-1 YB-11-2 B-1-2
Yellow birch
Pentosans
40.20
F-1-2
White birch
12.8 6.72 7.30 5.77
...
...
WM-1-1 10.96 WM-11-1 35.7 14.40 2:i 20:i 2.9 Entire bark uaed in all cases: bark stripped from trees less than 1 month after trees were out, and analyses made immediately. White maple
5
Lignin 45.84
METHODFOR EXTRACTABLES. Unless otherwise noted, the samples are extracted successively with ether, with acidulated denatured alcohol, and with boiling water. The alcohol extractant comprises 90 per cent of denatured alcohol ( B formula), 9.5 per cent water, and 5 per cent glacial acetic acid. The sample to be tested is air-dried. High drying tem eratures are avoided since extractables are sometimes susceptibye to heat in the presence of air, and a t such higher temperatures losses may occur. Ordinarily, 10-gram samples are used. A moisture test is made on a portion of the base material. A Soxhlet extraction apparatus is used and a suitable bath is em loyed. Temperature is controlled to maintain a vigorous boifkg of the solvent in question. Extractions are carried out for periods long enough to ensure complete removal of the soluble material. A glycerol bath at about 110' C. is used with the alcohol extraction, and in the case of the hot water extraction heat is applied by means of a hot plate which is controlled to keep the extractant at a lively boil. The extracts are filtered and evaporated in tared beakers. The residue is weighed and the percentage calculated. TABLEIV. PER CENT EXTRACTABL~S IN INNER AND OUTER BARKS Black spruce Inner bark Outer bark Fir New Hampshire inner bark 0uter.bark White birch Inner bark Outer bark
EtherSol.
Acid-Aloo- Hot-WaterSOL hol*Sol.
Total Sol.
1.60 3.01
15.00 9.12
11.35
9.76
27.85 21.70
2.88 19.42
14.74 10.26
4.88 8.56
22.50 38.24
0.56 9.91
9.16 8.35
... ...
... ...
LIGNIN DETERMINATION. The amount of sample depends upon the expected lignin content. With wood a 0.5-gram sample is adequate. I n the caae of a well-bleached pulp, a 10gram Sam le should be used. The volume of 72 per cent sulfuric acid is ad$sted, depending upon the size of the sample. The 0.5 gram of wood calls for 22.5 cc. of acid. The mixture of acid and sample is stirred thoroughly a t 25" C. until all ag regates are dispersed and solution is well underway. It is then afowed to stand a t 21" C. overnight. Dilution follows with distilled water; 315 cc. of distilled water are used in the case of the 0.5-gram sample, and the diluted mixture is digested on a steam bath for 2 hours. Hot water is added to maintain the original volume. A pinch of previously ignited asbestos fiber is added t o assist coagulation. The mixture is then filtered through a Gooch crucible that contains a thin mat of asbestos fiber, the crucible and mat having been previously ignited. The mixture is washed well with hot 1 per cent hydrochloric acid solution until free from sulfates. After washing, the crucible and contents are dried at 105' C. to constant weight, cooled, and weighed. The crucible is then ignited, cooled, and reweighed. The loss in weight on ignition is re orted as ash-free lignin. PENTOSAN DETERMINATION.&e method is that of the A. 0. A. C. with the following exceptions: A sample of exactly 2 grams of bone-dry material is used, and the distillation is carried out in a 500-c~.flask. A total volume of 500 cc. is distilled over; the solution of phloroglucinol is added, and the mixture is heated on the water bath at 80-85" C. for 2 hours and then allowed t o cool to room temperature. The weight of phloroglucid obtained times 0.785 is taken to be the amount of pentosans present in the 2-gram sample.
Total Sol. 24.78 21.0 30.37
Ash Content
...
2.1
26.06 21.6
i:i i:i
19:i
2:i
18.3
8:s
25.10
... ... 26.8 2i:o
4:i
...
3.1
The relatively large percentage of extractables in many of the barks suggests a more general utilization. Although the work is not complete, a further search was attempted to classify the soluble matter by means of a set of extractions in which the extractants were used with all possible sequences-i. e. : Ether + alcohol + water Ether + water + alcohol, etc. When so examined, it was found that the extractables of a white birch bark could be grouped as follows:
$ 0
sol. only in ether 0.65 sol. only in acid alcohol 1.93 sol. only in hot water 6.84
in ether or acid alcohol 1.15 9 sol. sol. in acid alcohol or hot water 8.57
It is evident that of the 19.14 total, 15.41 per cent is soluble in hot water itself. The extractables present in the outer and inner barks vary considerably with the species. A few representative tests will illustrate the degree of difference (Table IV).
Wood Analyses Other investigators (B) have reported analytical data covering a wide variety of woods. These published data are not altogether consistent. In some cases the wide variation may be attiibuted to major differences that actually occur in wood samples of a given species. Other discrepancies may be explained by differences in analytical procedure. A species of wood may vary widely not only in physical structure but also in chemical composition. Factors that determine these differences include geographic origin, season of cutting, and the peculiar environment of the individual tree. In fact, as pointed out by others, the usual analytical tests will reveal marked differences in composition of wood taken from different sections of the same tree. Unfortunately, many of the published data omit essential description which could add materially to the value of the tests so made. As is often done with other organic products that appear in nature, investigators usually focus their attention on certain recognized chemical groups present in the wood. The analyses aim to reveal quantitatively either those ingredients that one wishes to eliminate by a purification process or those which the investigator seeks to recover most advantageously. Thus he measures the so-called impurities as lignin, extractables, pentosans, and mineral matter. Inasmuch as cellulose constitutes the main interest of a pulpmaker, he also endeavors t o determine its percentage and, in more recent years, attempts to segregate modifications of cellulose that he has classified by definition. A summation of the analytical test values does not necessarily total 100 per cent of the wood, except when the investibtor calculates a last component by difference. I n some cases certain secondary components remain in an analytical residue and are reported in
INDUSTRIAL AND ENGINEERING CHEMISTRY
18
TEsrs O F TABLD v. ANALYTICAL
-
Tree
top (40 Et. or 12.2 meters)
9-1-1. middle (16 f t . or 4.9 meters)
9-1-1, butt
Oct. 10 4.6 23 26.1
Oct. 10 6.76 83 24.5
Oot. 10 8 120 26.8
9-1-1,
29.56
... ... ...
Sapwood Heartwood Pentosans, 70 ... Entire wood Sapwood ... Heartwood Ether-soluble, % 1.05 Entire wood Sanwood Hiartwood Alcohol-soluble, % 1.38 Entire wood Sapwood Heartwood Hot-water-soluble, % 0.85 Entire wood .. Sanwood .. Heartwood Total soluble, % 3.28 Entire wood Sapwood Heartwood Ash, % Entire wood ... Sapwood Heartwood
r0
a
... .. .. .. ...
,..
...
...
.... ..
1.09 1.49
...
TVB-1-1, butt
Time felled Days before testing Tree diam., in. No. tree rings Densitya Lignin, Entire wood Sapwood Heartwood Pentosans. % Entire wood Sapwood Heartwood Ether-soluble, 70 Entire wood Sapwood Heartwood Alcohol-soluble. % Entire wood ' ' " Sauwood Heartwood Hot-water-soluble, 5% Entire wood Sapwood Heartwood Total soluble, % Entire wood BaDwood Heartwood Ash % Ehtire wood Sapwood Heartwood
... 0.58 0.80
...
-
29: 66 33.38
... ...
... ...
Tree
29:is 31.38
...
... ... ... ... ... ... ... ... .
.
I
WB-1-2, top ( 2 2 ft.'or 6.7 meters)
Spruce S-1-2, butt
3-1-3, butt
Oct. 15
Oct. 15 5 22 23.0
2
33 25.6 32.56
... .
.
I
11.44
...
1.17 1.45
...
1.38 2.41
...
1.60 2.56
...
4.15 6.42
... ... ...
WB-1-2 middle,' (10ft. or 3 meters)
S-1-4. butt
... 0.77 ... ...
Oct. 15 7.5 125
...
...
30166 32,64
32.7 31.56
10:oo 10.14
9.64 9.68
... ...
...
0.82 1.39
...
0.73 0.77
...
...
.. .. .. .. .. ..
... ... ... ... .. .. ..
... ...
.. .. .. ... .... ..
White Birch WB-1-2, butt
22.48
...
2i:i2 24,46
22:06 23.60
23.58 23.74 23.42
22,80
... ... ...
2i:96 23.42
.. .. ..
3.04 2.30 3.46
3.52
...
1.71
2.13
... ...
4.30
...
1.81
...
...
... ... .. .. ... ...
...
... ...
...
... 9.63
.. .. ..
...
...
28.4 29.8
...
9.1 10.5
...
0.72 0.87
...
...
0.31 0.64 WB-1-4, butt
...
2.97 3.89
...
2.19 2.13
...
2.56 3.09
...
...
...
... ...
...
1.75 4.30
...
...
...
TVB-11-4, butt
...
22.2 20.8
...
...
... ... ...
...
... ...
...
...
...
... ... ...
.. .. ..
...
1:20 1.63 ..t
4.21 8.30
...
0.20 0.58
...
...
... ...
... ,..
...
. I .
0.33 0.53 ---White WM-1-1
July 10 6.5 26 35.2
22:86 24.36
20.8 22 2
20: 30 16.78
16.6 15.6
.
I
.
... ...
... ... ... ..*
3:i4 2.69
I . .
0.97 0.70
...
3.40 2.37
,..
...
...
... ...
MapleWM-11-1
Oot. 15 8 56 33.0
0.51 0.31
... ... ... .
... ...
...
0.16 0.21
...
--Yellow YB-11-1
Oot.
16 8 51 38.0
...
...
4.93
...
024 0 73.
BirchYB-11-2
July
lo
7 101 38.2
...
ii:is
20.8 19.8
21.76
...
0 44 0.94
...
...
0 26 0 37
...
4 21
23.0 26.4
...
...
0'34 2.09
23.0 26.36
... ...
5.08 4.86
I
...
i'78 2.38 2.13 2.32
.
...
0.30 0.42
, , .
...
...
0.23 0.29
...
,,,
...
0.26 0.33
...
...
7.13 7.44
0.25 0.93
...
2.24 1.30
...
4.35 9.28
23:i6 24.54
...
.
10.6 15.4
... ... ...
... ...
I
16:h 15.94
...
...
...
.
22.4 22.3
3.30 3.70
...
... ... ... ,. . ...
1i:is
I . .
0.19 0.38
2.04 5.10
...
23.4 26.6
...
..... .. ..
7.24 8.91
24126 23.54
...
0.26 0.38
... ...
7.72 9.11
23.2 26.3
2.90 3.00
1.65 3.8
20.0 28.1
...
... ... ...
22144 23.32
0.93 0.74
...
...
July 10 7.6 97 34.3
...
0.95 1.18
0.98 1.57
2.64 2.06
Oct. 15 9 47 38.0
19.54
1.79 3.11
I
...
July 10 7.5 145 44.6
...
... ... .
Oct. 15 8 84
7.03 7.60
2.76 3.45
.
RM-1-2
28.9 31.5
9.7 10.6
23: 54 24.10
21.92
1.84 3.40
MapleRM-11-1
R-11-2
la
28.7 30.7
Oct. 10 5.7 42 36.0 23.42
-Rock
--Beech-B-11-1
Feb;
~
25.10 24.88 25.32
...
...
4.34 5.32
WB-1-3, butt
F-11-2
10 74 22.6
... .. .. ..
Oct. 10 4.6 31 35.4
...
Fir-
July 10 6 33 22.8
...
...
F-1-2
July 10 8.6 83 25.2
2.62 3.39
... ...
7 -
butt
...
Oct. 10 9 44 35.6
...
-
5-14,
1.11 1.06
...
... ... ...
V.4RIOUS W O O D SPECIES
...
... ... ...
...
...
Vol. 33, No. 1
... .
I
.
... ...
... ... .... ..
...
0:ia
1.18
...
2.14 6.52
...
1.39 1.41 4:i3 9.10
...
0.19 0.68
Expressed as pounds of bone-dry wood contained in 1 cu. ft. of undried wood.
terms of the main ingredient of the residue. It is well known, for instance, that cellulose as isolated analytically contains some of the furfural-producing groups which appear as well in a total pentosan determination. A lignin residue will sometimes show the presence of a substance that develops furfural upon further distillation. Some of the extractables, if present when the lignin is isolated, will appear in part in the lignin residue. The empirical method for determining alpha-cellulose, almost universally adopted by cellulose laboratories, does not correct for other than cellulose ingredients that resist the solvent action of the mercerizing caustic. In other words, a comprehensive and accurate analysis of wood material awaits a better procedure than is available a t present. Test values cited below are largely
confined to lignin, extractables, total pentosans, and ash. Total cellulose and holocellulose determinations were not considered pertinent to the investigation, which had for its main purpose the evaluation of the several species of wood as raw materials in the production of refined wood pulps. Except as specifically indicated, the values reported have been determined by the usual practice of applying the test to the unextracted dry wood. A number of examples are cited to indicate the degree to which the pentosan and lignin determinations are affected when the base wood is first subjected t u the solvent action of the extractants. Regardless of the indefinite nature of test values as ordinarily established, one recognizes certain obvious relations that exist. To illustrate, most of the northern hardwoods
lanuary, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
when contrasted to the northern spruce and fir are characterized by lower lignin and much higher contents of pentosans, and this relation exists irrespective of differences that are found from tree to tree and in different portions of a tree. Likewise, one finds a much higher percentage of pentosans present in the hardwood cellulose as isolated by the accepted methods. To the author's knowledge, no convincing work has been published t o show quantitative relation of hexosans and pentosans as differentiated in the hardwoods and the softwoods, but one might expect that the hardwoods if so analyzed would show a lower pentosan-free cellulose than would the softwoods. Table V comprises an assortment of analytical values. I n some instances complete data were not obtained; in other instances the tree was sampled at the butt, a t the middle, and at the top, and some sections were further tested both as sapwood and heartwood. The data in Table V include only those analytical tests that were made shortly after the tree was felled. As shown later, marked changes take place, particularly in the extractables, when wood is aged naturally or artificially. Results appearing in Table V may be generally summarized as follows : LIGNIN. Whereas the lignin in spruce and fir approximates 28 to 30 per cent, the hardwoods have materially less, ranging for the most part from 20 to 24 per cent. The heartwood is substantially richer in lignin than is the sapwood, and this appears more marked in the case of the birches. To demonstrate further, both white birch and yellow birch were explored more fully by zoning plaques of the logs from the peripheral sapwood to the innermost heartwood. Results appear i n Table VI. PENTOSANSTable V shows that the hardwoods are much richer in pentosans than are spruce and fir. The beech and the birches contain from 20 t o 22 per cent and the maples from 15 to 16 per cent, whereas the softwoods containapproximately 10 per cent. There seems to be no consistent variation in pentosans as found in the sapwood and in the heartwood of a given tree, regardless of variety. Freshly cut white birch contains a high EXTRACTABLES. percentage (2 to 4 per cent) of material that responds to ether extraction. The recovered refiidue is light yellow, fluid, and oily. It has a n odor that resembles some vegetable oils that have undergone slight rancidity. It is about two thirds saponifiable. Unseasoned yellow birch contains less of the ether-soluble material (0.7 t o 1.5 per cent), whereas the maples and the beech have relatively little. The ether-solubles in freshly cut spruce and fir such as are native t o New England range from 0.7 to 1.5 per cent. All of the hardwoods that were tested, as well as the spruce and fir, contain appreciable amounts of a brown waxy material that resists the solvent action of ether but dissolves in acid alcohol, and also an additional percentage of an amorphous substance resembling a hard gum, which resists the solvent action of both ether and acid alcohol but will dissolve in hot water. Of the hardwoods, the birches contain by far the largest total percentage of extractables as obtained by the successive treatments with ether, acid alcohol, and hot water, I n practically all cases the heartwood is richer in material that can be extracted by the reagents in question. An
79
examination of pulps that are produced from heartwood and sapwood, respectively, shows similar relations in regard to the resinous residues that remain with the pulp.
TABLE VI.
LIGNINCONTENT OF BIRCHES % Lignin
Zone Section A Heartwood, 1-in. (2.5-om.) radius B
Annulus from 2-in. i. d. t o 4-in.
(5-10 om.) (10-15 om.) D Outside annulus &in. (15-om.) i. d. to periphery Additional test of innermost heartwood
C Annulus from 4-in. i. d. t o 6-in.
0.d. 0. d.
White blroha 35.14 23.08 21.26 19.54 37.44
Yellow birch6 29.14 22 .OO 21.82 21.40 29.84
Diameter 7.3 inches (18.5 om.). 29 growth rings. 7 inches (17.8 om.); '101 growth rings.
b Diameter:
Further Investigation of Extractable5 A comprehensive analysis of the extractables that occur in the base wood should ultimately determine its value if recovered. Such methods of separation, if applied to the pulps that survive a chemical digestion of the wood, should also show which of the original substances are readily removed during the pulping procedure and indicate what type of posttreatment will be most effective in an ultimate purification of the fiber. A few preliminary attempts were made to characterize the ether-soluble material, inasmuch as i t is the chief offender in causing disturbance in the later processing of the wood cellulose. Approximately 30-gram portions of ether-soluble material were collected from unseasoned spruce and unseasoned white birch, and from two sulfite pulps that had been prepared from hardwood and softwood, respectively. Each of the extracts was submitted to an analytical procedure as recommended by Hibbert and Phillips ( 3 ) . The remlts appear in Table VII. Although more workmust follow before the relations can be fully accepted, and although the pulps were not produced from the identical logs that had been used for the wood extractions, there is nevertheless a strong indication that: ( a ) I n the case of the hardwood the cooking process was very effective in removing the resin acids, mod-
SPRUCE AND WHITE BIRCH
INDUSTRIAL AND ENGINEERING CHEMISTRY
80
Spruce
Fir
White Birch
erately effective in removing the fats, and relatively ineffective in the elimination of the fatty acids and the unsaponifiable matter. In checking these conclusions one must remember that the pulp yield is about 45 per cent. (b) With the softwood the resin acids underwent a substantial but not so great a reduction as in the case of the hardwood. The fats were somewhat more readily removed than those of the hardwood, whereas the fatty acids and the unsaponifiable matter follow approximately the same lesser degree of removal as takes place with the hardwood. These findings deserve further attention. Similar work should be done with the ether-soluble material that is present in the corresponding pulps after they have undergone successive purification steps. Results of such analyses may suggest better methods for the removal of the ether-soluble material than are now generally available.
Analysis of Extracted Wood Unless otherwise stated, the lignin and pentosan figures were obtained on the original wood and were calculated on a bone-dry basis. The corresponding values on the extracted wood will be higher or lower, depending upon whether the extractables are richer or leaner in components that will behave like lignin and like pentosans in the analytical procedure than is the wood itself. A few trials were made to throw some light on this question. A white birch wood was chosen because of its relatively high content of extractables. Adjacent portions of the wood were prepared as a n unextracted sample, one that had been subjected to extraction by ether and acid alcohol, and one that had been extracted with ether, acid alcohol, and hot water. The results are as follows: %
Unextracted Extd. with ether and acid alcohol Extd. with ether, acid alcohol, and hot water
Lignin 22.4 19.8 20.4
Vol. 33, No. 1
Yellow Birch
time exposure also causes reduction in the extractables, particularly those that are soluble in ether and in acid alcohol. This reduction in the oleoresins is accompanied by a corresponding reduction of the extractables in the pulp produced. Substantial elimination of resins, the fatty acid compounds, and the waxes is important both for the papermaker and for the producers of cellulose esters. A high resin content in the pulp fouls the papermaking wires and causes a deposit of pitchy material which loosens periodically, with a subsequent formation of resin spots in the paper. The resinous material is also highly objectionable in the manufacture of waterleaf papers such as blottings, saturating stock, parchment base, and the like, where high and uniform absorption properties are needed. I n the papermaking field the alcohol-soluble waxes and the hot-water-soluble gums are harmful mostly from the standpoint of brightness of sheet and color stability,
TABLE VII.
APPROXIMATE ANALYSIS OF ETHERSOLWLES
Base substance Unseasoned from: spruce
Unbleached softwood sulfite Duln Thin oil, soft grease
- -
Unseasoned white birch Thin oil
Physical consist- Heavy oil, ency of ether hard fat sol. 0.832 0.85 0.2 0.68 10.6 29.0 fatty acidsa 22.6 17.63 24.64 fatsb 20.3 unsaponifi19.77 33.3 ablec %;ct in analy8.38 13.0 a WeiFhed as ethyl and methyl eaters. b Originally glycerides end esters. but weighed 0 Sterols, resenes, polymerized terpenes.
...
Unbleached hardwood sulfite nulo Soft grease
. .
0.52 31.05 6.14 24.90
0.889 0.16 3.14 11.8 29.00
30.00
44.70
7.39
11.20
as acids.
%
Pentosans 22.8 23.8 24.6
The results indicate that the extractables in white birch are richer in lignin-simulating material and leaner in substances that form furfural on acid distillation than the resinfree wood itself. This finding deserves further attention and should be extended to include other wood species.
Changes in Extractable8 on Seasoning Although green wood can be pulped by chemical digestion immediately after it has been cut, the pulpmaker finds in the case of many species that it is advantageous to season the wood before it is further processed. Seasoning evens out many of the great differences in moisture content which, in turn, determines the uniformity of penetration of cooking liquor into the chips that are charged into the digester. Long-
A highly resinous wood pulp is not suitable for the processes that involve xanthation, nitration, or acetylation, and is equally unsatisfactory for use as a base in the more recent plastics industry. Here also the alcohol- and hot-watersoluble material is objectionable in so far as it affects color stability. The loss in the extractables by aging is not satisfactorily explained. There is evidence that it may be partly ascribed to volatilization and to splitting of the glycerides with subsequent leaching of the soluble products. Other changes in chemical composition may take place by polymerization and oxidation. Many data substantiate the fact that the ether-soluble component of wood decreases on aging and that the reduction can be hastened by elevation of temperature. The rapidity of change that can take place in a pulpwood pile is probably influenced by the degree of surface exposure,
January, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
81
PUOTOQRAP€iS
OF
VARIOCS
WOOD SPECIES
Red Maple
Rock Maple
the ease with which air circulates through the mass, the effect of the sun, as well as the humidity and temperature. It is also to be expected that the particular wood species and the chemical composition of the extractables play an important role in the aging step. Inasmuch as birch is considerably richer in extractables than the other hardwoods, most of the present investigation was carried out with that species. A first experiment was made to obtain some idea of the rate of change in the percentage of extractables when white birch is artificially aged. The particular tree was cut in January. The butt log showed thirty-eight growth rings and was 7.25 inches in diameter. This log was held in its original form with the bark on for 45 days. It was then converted in part into shavings which were immediately tested. Extraction showed 1.96 per cent ether-soluble and 3.05 per cent acidalcohol-soluble. A portion of the shavings was heated in a current of air at 40" C. for 48 hours and retested. The ethersoluble had dropped to 0.98 per cent, and the acid-alcoholsoluble was 3.14 per cent. A second sample of the shavings was heated a t 100" C. for 5 hours. The extractables were then 0.82 and 3.06 per cent, respectively. These first results gave evidence that the ether-soluble can be greatly reduced by artificial aging and that the alcohol-soluble waxes are not greatly affected. DATAON WHITEBIRCHCUTIN APRIL TABL VIII. ~ % Ether-
% ' AoidAlcohol-Sol.
2.26 2.10 1.11 0.90 1.15
1.70
0.83
1.17
Sol.
Wood 40 days old: Sapwood Heartwood Peeled log held 10 mo. Shavings from peeled log heated 48 hr. at 40" C. Unpeeled log held 10 mo. Shavin 8 from unpeeled log heated 48 hr. at 40"
6.
1.23 1.26 1.16
A second set of experiments was carried out with two white birch logs cut from a given tree in April. A corresponding log was held for 40 days and extracted. One of the original logs was held in unbarked form, the other as a peeled log for 10 months, a t the end of which time they were again extracted and found to contain substantially less of the ether-soluble and of the acid-alcohol-soluble materials. When the seasoned logs were subjected to an additional artificial aging, the ethersoluble was further reduced but slightly, and the alcoholsoluble not a t all (Table VIII). It appears that the ether-soluble content of white birch can be reduced to about half of its original value by seasoning for a total of 10 months, part of which is the summer period. The natural aging also reduces the acid-alcohol-solubles
Beech
similarly. Very little is to be gained by subjecting the seasoned wood to a high-temperature artificial aging, even though that treatment is favored by the increased surface of the shavings. It is significant that it is difficult to reduce the ether-soluble material much below 1 per cent, either by natural aging or by application of heat. This indicates the difference in behavior of the several components of the extract. As additional evidence it is noted that when an ethersoluble residue from a birch pulp was heated as a thin film a t 105' C. for a 48-hour period, about 30 per cent of the original weight could be redissolved in ether. The remaining insoluble matter was converted from a yellow, oily mass to a hard, dark brown, glassy consistency. An analysis of the ether-soluble material obtained from green and from seasoned birch would help in establishing which of the original components are inert to the chemical changes that occur during the aging of the wood. The interesting behavior of the alcohol-soluble component in resisting the high-temperature aging appears significant. We conclude that the change occurring with long-time seasoning a t normal temperatures cannot be duplicated by the high-temperature air treatment. This also deserves further attention. Another practical test will throw further light on the changes of the oleoresins that occur in the birch when seasoned. I n this set of experiments both white and yellow birch logs were cut in October. Portions of the trunk were extracted within a 15-day period after the tree had been felled. Unbarked logs of each variety were stored on a roof exposed to the wind and sun for 2 years, and the wood was again extracted with ether. I n this case, however, the seasoned logs were sampled a t three points, and the composite heartwood and sapwood from each section were extracted. The points of sampling were: z, the end of the log; y, 1 foot (30.5 cm.) from the end; and z, the midpoint or 2 feet (61 cm.) from each end. The following table shows changes that occurred and confirms the idea that surface exposure is important. % Ether Solubles White birch Yellow birch Ureen wood Sapwood Heartwood Unpeeled wood, seasoned 2 yr. 5
Y t
3.04 2.30 3.46
0.90 0.44 0.94
0.88 1.24 1.34
0.45 0.74 0.71
The results give additional evidence that there is a minimum ether-soluble content of about 0.9 per cent that can be realized by seasoning white birch. It is probable that the minimum can be reached more quickly when the logs are of smaller diameter and fully exposed to the elements. The results also suggest t h a t 2-foot (61-cm.) logs will undergo the changes more rapidly than will the usual '&foot (122-cm.) logs.
82
INDUSTRIAL AND ENGINEERING CHEMISTRY
Recovery of Extractables The large amounts of extractables in the birches particularly invite attention to the possibility of combining a simultaneous removal and recovery of that material from the substantially green wood. The fresh birch oleoresins contain substances which are saponifiable and which, as soaps, possess detergent TABLEIx. EXTR.4CTIONS
OF
Vol. 33, No. 1
with water beyond a 90 per cent concentration. The residue recovered from an alcohol extract is about one third soluble in ether. A single set of values will illustrate the effect of alcohol extraction of white birch chips on the extractables that are associated with the white birch pulp produced therefrom.
WHITE BIRCHSAWDUST
AFTER
DIGESTION WITH VARIOUS LIQUORS 0 36%
Original Sawdust
0.6% NaOH 79.0 0.33 0.70 90.5
Water 95.4 Yield % 2:98 2.63 Ethd-sol., % 4.10 0.60 Alcohol-sol. % ,Original etder-sol. removed % 15.5 Original acid alcohol-sol. 're86.1 86 moved, % Mixture made volumetrically. a Denatured B formula.
..
gk: 79.3 0.53 1.18 85
1% NazCOz 84.3 1.26 1.18 61
76
75
properties. There is also the possibility of separation of the ether-soluble material into its several components and the selection of some of those fractions Tyhich are particularly susceptible to polymerization to yield relatively hard inert products whose properties have been little explored. Recovery of extractables from the birch may take several forms, among which are the following: 1. A steam distillation of the wood chips in much the same manner as practiced with highly resinous wood, and a subsequent pulping of the steam-distilled chips. 2. A large-scale aaulication of solvent extraction urior to the pulping stel;. 3. Digestion of wood chips in weak alkaline solutions under conditions that will not interfere with a second step of delignification. Subsequent recovery of the extractables from the alkaline extract offers several possibilities. 4. Direct digestion of the fresh birch chips in an alkaline ulping liquor and recovery of soaps from the so-called black gquor that results.
__
In all cases it would prove highly advantageous to employ green wood in order to realize greatest recovery of oleoresins, and to segregate the wood species that contain the highest amounts of the extract that is sought. The recovery process suggested under item 4 is predicated upon an alkaline cooking process, typified by the soda or kraft type. When a sulfite process is used, other methods must be adopted. A few experiments were made to explore the possibilities of a recovery step such as indicated under items 2 and 3. A green, white birch sawdust was used as base material. It was subjected to a variety of treatments as indicated in Table IX. In each case the suspension of sawdust in liquor was digested a t the boiling point and with an attached condenser to avoid loss of vapors so far as possible. At the end of the 4-hour period the mass was filtered to remove sawdust residue, which was carefully washed and submitted for test. Yields were recorded and, except with the alcohol digestions, show a substantially higher loss than can be accounted for by the removal of the extractables. .4 parallel investigation gave evidence that a large part of this shrinkage can be attributed to the removal of some of the hemicellulose and lesser amounts of the substances that respond to the lignin test. Removal of the acid-alcohol-soluble material is readily accomplished. Elimination of the ether-soluble constituents by water alone or by weak sodium carbonate solution is inappreciable. The sodium sulfite solution serves remarkably well as does the dilute sodium hydroxide. A combination of these latter reagents gives excellent results. The alcohol is interesting in that by itself it is very effective, whereas it loses that property in a large degree when diluted
17
NaOH i1% Iia290~
...
79.2 0.17 0.62 95
Sa&OOa 89.1 0.44 87
...
90%
0.30%
Na2S
86.3 0.73
...
79
fw;keF
B alcohola
92.0 0.15 0.33 95.5
96.2 2.20 0.38 30
...
88
60%
B alcohola
91
Chips were prepared from a single unseasoned white birch log. The chips measured l/z inch (1.27 cm.) long with the grain and were '/8 inch (3.18 mm.) thick. One portion was cooked by a standard bisulfite procedure that prescribed an 8-hour digestion, a cooking acid that tested 5 per cent free sulfur dioxide and 1 per cent combined sulfur dioxide, and a maximum temperature of 140" C. (The combined sulfur dioxide is that part which can be considered as combined to the base as a neutral salt. The free sulfur dioxide includes the portion that demands alkali to convert from a bisulfite to a neutral sulfite as well as that which is present as sulfurous acid.) A second portion of the original chips was first extracted for several hours with denatured B alcohol a t 100" C. in a closed vessel. The alcohol took on a brown shade, and the chips were slightly darker after the treatment. The extracted wood was drained free of excess liquor but was not washed to remove residual alcohol. The recovered alcohol contained 0.42 per cent solids on evaporation. From this figure one can compute that a total of 2 per cent extractables based on dry wood had been removed in the drained liquor. The alcohol-extracted chips were then pulped by the same procedure that was used for the unextracted wood, and the pulps were washed in identical manner. The results are assembled in Table X. TABLE X. RESULTS OF ALCOHOL EXTRACTIOS OF WHITEBIRCH CHIPS (Original white birch ether-soluble material, 1.74% : acid-alcohol-soluble, 3.63%) Pulp from: Unextd. chips Extd. chips W t . dry wood in chips, grams 680 680 W t . pulp recovered, grama 318 374 46.8 55.1 Yield, 9% Screenings Trace Trace 1.39 1.46 Lignin, ?& Result8 of beating for 35 min.a streneth 103 148 Tear 147 138 1.26 3.11 Ether-sol., % Acid-alcohol-sol., %, 0.31 0.31 a The procedure and units used were described in B previoue article ( 6 ) 7 -
I
The pulp obtained from the alcohol-extracted chips is characterized by an appreciably lower content of ether-soluble material. The two pulp products show no difference in the amount of that extractable which resists the solvent action of ether but does dissolve in alcohol. One concludes that those waxes are readily removed during the high-temperature acid digestion. The residual alcohol that remains in the wood chips after the preliminary extraction is largely responsible for the larger yield of somewhat less thoroughly cooked pulp.
January, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
An attempt was made to strike a balance of the ethersolubles in the unextracted chips, the I drained alcoholic extract, and the resulting pulp. The total residue recovered from the alcohol extract amounted to 12.8 grams, of which 3.4 grams were soluble in ether. From the data in Table X one can compute the ether-solubles in the original wood to be 11.83 grams and in the resulting pulp from the alcoholextracted chips to be 4.71 grams. Hence, 11.83 - (3.4 4.71) = 3.72 grams of the ether-soluble material must have left the system by way of the relief gas from the digester and with the waste liquors from which the pulp was freed. I n other words, of the original ether-soluble material present in the wood, about 29 per cent was removed with the alcohol extract, about 31 per cent was lost in the relief gases and the waste liquor, and about 40 per cent remained with the pulp. On the other hand, the pulp from the unextracted chips contained 9.89 grams of ether-solubles, showing in this case a loss of (11.83 - 9.89) = 1.94 grams, or about 16 per cent of the original, in the relief and in the waste liquors. It is probable that the alcoholic residues which were left in the extracted chips in the parallel cook experiment were largely responsible for the greater eiimination of resins in the waste
+
a3.
liquors. It is probable that materially greater percentages of the original ether-soluble matter can be recovered in the alcohol if the chips are smaller and if more favorable conditions of extraction are observed. Other low-cost solvents such as carbon tetrachloride and trichloroethylene should be studied.
Acknowledgment The author wishes to extend full credit to D. H . McMurtrie, M. W. Hayes, C. W. Thing, E. W. Lovering, and others who took active part in the laboratory work.
Literature Cited (1) Assoc. Official Agr. Chem., Methods of Analysis, pp. 96-7 (1920). (2) Hawley and Wise, “Chemistry of Wood”, p. 17B, A. C. S., Monograph, New York, Chemical Catalog Co., 1927; Dore, J. IND. ENG.CHEM.,12, 476, 984 (1920); Ritter and Fleck. I b i d . , 15, 1055 (1923), 18, 608 (1926); Miiller, “Pflanzenfaser”, p. 163 (1876); Konig and Becker, 2. angew. Chem., 32, 155 (1919); Schwalbe and Becker, Ibid., 32, 229 (1919); Barnes, Chem. & M e t . Eng., 28, 504 (1923); Aschan and Rantalainen, BrennstoffChem , 4 , 101 (1923). (3) Hibbert and Phillips, Can. J. Research, 4 , 1-34 (1931). (4) Newlin and Wilson, Dept. Agr., Bull. 556 (1917). ( 5 ) Richter, G. A., IND.ENG.CHEM.,23, 266 (1931).
Distribution of Pectic Acid in Cotton Fibers R. F. NICKERSON AND C. B. LEAPEl, Mellon Institute, Pittsburgh, Penna. The investigation reported in this paper was undertaken as an inquiry into the function of pectic matter in raw cotton. By direct and indirect analyses it is shown that the pectic material occurs, as does the natural wax, almost exclusively on the outside fiber surfaces where i t can have but little influence on fiber structure and properties.
-HE
presence of pectates in cotton was noted first 1 Schuick (19) wbo isolated a pectic acid from kier liquor. Knecht and Hall (14) affirmed that caustic
soda removed pectic substances from cotton. Clifford and Fargher ( 7 ) detected methanol and acetone in the volatile products of raw cotton and concluded that a pectic material was the precursor. Harris and Thompson (12) made a careful study of the pectic acid from a cotton; they obtained a yield of 0.68 per cent pectic acid which had an equivalent weight of 201 and a n of +225.4 and which released 21.8 per cent of carbon dioxide upon decarboxylation; they concluded that the material occurred in raw fibers as the calcium and magnesium salts of pectic acid. Their figure for the equivalent weight is in good agreement with the corresponding value for fruit pectates (16). Whistler, Martin, and Harris (23) estimated 1.15 per cent of pectic acid in a cotton by converting the differential carbon 1 Present address, Westinghouse Research Laboratories, East Pittsburgh, Penna.
dioxide yield of a raw and a purified sample with a factor of 4.8. The latter value was obtained from isolated cotton pectic acid and corresponds to 20.8 per cent yield of carbon dioxide upon decarboxylation. A conversion factor of 4.5 for carbon dioxide to pectic acid is indicated by the data of Harris and Thompson. Anderson and Kerr (1) reported that the pectic substances of cotton were surface constituents, as a number of earlier investigators (4,9,24) had implied. Several different lines of evidence lead independently to the conclusion that native cellulose contains an amorphous substance or gel in addition to the crystalloid elements (3,I S , 80). Farr (11) made the suggestion that this substance is a pectin or, a t least, a polyuronide which, to a considerable extent is responsible for the high viscosity of cotton dispersed in cuprammonium hydroxide reagent. This hypothesis is objectionable because pectins do not dissolve in cuprammonium hydroxide ( 2 ) ; in fact, the insolubility in this reagent has been used as the basis for the separation of cellulose from pectin preparations (16). While the literature indicates that pectic substances are constituents of raw cotton, it does not show clearly their distribution in the fiber. The present work was undertaken to determine whether or not a polyuronide occurs in appreciable quantities in the internal fiber structure.
Experimental Procedure A fairly good correlation is known to exist between the average fiber length and the fineness of cotton. Fineness,, which depends upon the average thickness of fiber walls and is measured in terms of fiber weight per centimeter, increases with the staple length of the cotton; in general, the longer
