THE GRINDING Cf HARDWOODS August 1942 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin THE GRIIJDIZTG OF HARDWOODS Studies on Swamp Tupelo, Paper Sirch, Green Ash, Sugarberry, Southern Cottonwood, Black Willow, and American Elm By E. R. SCHAZER, Senior Engineer and J. C. PEW, Associate Engineer Summary Investigations of the grinding of hardwoods have been conducted at the Forest Products Laboratory over a period of years with results that should be of interest to manufacturers of papers containing groundwood, who now obtain suitable softwood species at ccn^iierable disianceb from their mills. -\- woods , by their ready accessibility to many groundwood mills, offer distinct advantages from the standpoint of pulpv/oou transportation costs. Increased use of hardwoods would also aid in the improvement of Lixed stands of hai woods and coniferous woods, and would expand our pulpwocd resources. Swamp tupelo, if ground soon after cutting, yields a groundwood pulp that is light colored and short fibered. It lacks the strength considered essential in the pulps produced from spruce and other softwoods but, even so, can be used in appreciable quantities as filler stock in the manufacture of book, magazine, and newsprint paper. Groundwood pulp made from parser birch has similar properties. The absorbent quality of this pulp was demonstrated in the production of a highly absorbent toweling paper. The pulps made from green ash and sugarberry were stronger than those made from tupelo and birch, but not so strong as standard palp made from spruce. Satisfactory newsprint papers were made, however, in which both species constituted a considerable part of the furnish. The groundwood pulps obtained from southern Cottonwood and black willow were comparable in strength with commercial ground'-'ood. Although the color of the latter is too dark to permit its use in newsprint or other light colored papers, it should be suitable for papers and boards in which the dark color is not objectionable. Groundwood pulp made from American elm was both short fibered and dark colored, qualities which limit it? use to such products as filler stock for boards and papers of lov; color. The energy consumed in the making of groundwood pulps from hardwoods need not be so high as is generally believed necessary. For many purposes a satis- factory groundwood can be made from these woods with less energy than is ordinarily consumed in the grinding of softwoods. Some hardwoods, Boreorer, yield more pulp per cord than can be obtained from the softwoods. RIU19 Introduction Although spruce still is fairly plentiful in some sections of the United States and Canada, groundwood mills in some regions of this country have been com- pelled to transport wood from distant points. Thus, the increased cost of the wood offsets to a great extent the cheapness of the process. The spruces are the most desirable species from which to make groundwood pulp, with the true firs, western hemlock, and southern pine following in order. The idea of employing hardwoods in this process is not new, and certain of the hardwood species have "been used to a limited extent for a number of years. In fact, aspen was one of the first species used in the groundwood process. Increased use of hardwood species is worthy of consideration not only as a wartime measure to employ a cheap and abundant material resource, but also as good forestry practice resulting in lasting future benefits. For instance, in- creased utilization of hardwoods in the Southern States would be conducive to improved forest stands throughout that area. The grinding of hardwoods lias been attempted many times, but the amount of published technical information on the subject is relatively small. Thickens and McNaughton (S) studied the grinding of aspen, paper birch, and black tupelo and noted that relatively dull stone surfaces and higher energy con- sumption were required to produce pulp comparable with spruce groundwood. Running (6) reported that strength approximating only one-half to two-thirds that of spruce is obtained from aspen ground under the same conditions. Cottonwood and poplar require more power and the pulps have lower freeness for given bursting strengths than some of the coniferous species, according to experiments conducted by Wynn-Roberts ($) . Munro (£) suggests that the use of a 9- or 10-cut, high-lead burr, or even a thread burr, is better for the grinding of aspen than a 12 or lU straight or diamond burr. A moderate pres- sure and low temperature are also desirable. Benninger (l) found that beech was more easily ground than spruce. The foaming of the beech groundwood was, however, quite troublesome. He also reported that a 50-50 mixture of beech and spruce groundwood pulps was satisfactorily used. Other foreign investi- gators (2, 2) have published results similar to those of the American ob- servers. Boiling or steaming the hardwood before grinding aids in improving strength, increasing fiber length, and lowering energy consumption per ton, but produces a brown pulp (U, S ) . Since the early work of Thickens and McNaughton, the Forest Products Laboratory has conducted various experiments in the grinding of several hardwoods. This report summarizes the results of these researches. Swamp Tupelo Table 1 contains data for the grinding of swamp tupelo ( Nyssa biflora ) (formerly swamp black gum), an abundant species in the southern United States. It ranges from Maryland to Florida and westward to eastern Texas. The samples tested were obtained from North Carolina. The wood was ground as Rl*+19 -2- received and again after treatment with sodium sulfite solutions. In all experiments on untreated wood, the pulps obtained were low in strength and average fiber length as compared to standard spruce groundwood. The pulps obtained by grinding on a dull-surfaced stone and with high pressure were more completely fibrillated and slightly stronger than those obtained with the use of sharper stone surfaces. These swamp tupelo groundwood pulps had an excellent color. The color of groundwood pulps obtained in other experiments was found, however, to be in- fluenced greatly by the conditions of storage prevailing prior to pulping of the wood. It has been pointed out in a previous publication (J) that the light-colored sapwood in the freshly cut wood became discolored in lesp than 2 months if 3tored in a warm, moist condition, such as might prevail in southern climates during the summer. The fresh wood did not discolor if dried rapidly, indicating that moisture was an essential factor in the dis- coloration. Pulping the wood soon after cutting, before discoloration can occur, would appear to be a desirable practice. The wood was chemically treated before grinding to improve the strength and increase the average fiber length of the pulp. Most pretreating processes, however, discolor the wood, and this discoloration, of course, prevents the use of the groundwood pulp in light-colored papers. Since the neutral sulfite solutions used in the semichemical process of treating chips do not discolor the wood if properly applied, the liquor used in these experiments was of that type, i.e., a mixture of sodium sulfite and sodium bicarbonate. Log sections about 28 inches long were weighted down in a treating cylinder and covered v;ith the chemical solution. The conditions of the various treatments are summarized in table 2. When the temperature of treatment was in the range of 150° to 170° C. , the wood was discolored and the brownish groundwood pulps were obtained. When alternating vacuum and pressure were used, with the temperature during the pressure period not exceeding 130° C., the pulps obtained from the treated wood were not discolored. When wood treated by the latter method was ground on a dull stone with moderate pressure, the pulps had fair strength and vrere longer fibered than the untreated wood pulps. Fapermaking experiments indicated that the tupelo groundwood pulps made from untreated wood were more useful as filler stock than to provide strength. Data for newsprint and book papers are given in table 3* 1° most of the newsprint experiments in which 70 to 75 percent of groundwood and 30 to 25 percent of sulfite pulp were used, the strengths were below standard for this grade of paper. One, however, (machine run UU7) in which 70 percent of tupelo groundwood wa3 combined with 30 percent of commercial spruce sulfite pulp, was of slightly higher weight, but otherwise equal to standard news- print. Another experiment in which 50 percent of the furnish consisted of tupelo groundwood, 20 percent of slash pine groundwood, and the remainder spruce sulfite pulp, (machine run 350) had normal strength. A run in which the tupelo groundwood content was reduced to 50 percent and the slash pine sulfite component was increased to 50 percent (machine run UU3) had higher than standard newsprint strength. Although no trials were made in which hardwood and coniferous groundwood pulps were combined with either hardwood RIU19 -3- neutral sulfite semichemical or pine a emit leached sulfate pulps, the applica- bility of swamp tupelo groundwood in such furnishes is suggested by the results of experiments, to "be discussed later, in v/hich a paper birch ground- wood of similar type was so used. Acceptable sheets of book paper were made, in which the normal component of soda pulp was substituted in whole or in part with tupelo groundwood (machine runs 339 » ^li ancL HH2). These papers possessed good color, finish, and strength. One of the papers in this group (run 3^0) was, except for its low bursting strength, typical of rotogravure paper in its properties. The papers having treated tupelo groundwood as part of their furnish were rough of surface and weak. Further study of pulps of this type is necessary to learn more about their papermaking characteristics. Paper Birch Paper birch ( Betula papyrifera ) is an important northern hardwood. It ranges from Labrador to Hudson Bay, southward to Long Island and northern Pennsyl- vania, and westward through Ontario, Michigan, and northern Wisconsin to western Minnesota and eastern Manitoba. The wood used in the experiments re- ported here was obtained from northern Wisconsin and Michigan. The grinding data are given in table U. The pulps produced were, in general, like those from the swamp tupelo; short fibered and low in strength, but, in spite of their short-f ibered character- istics, quite free. Increasing the pressure of the wood on the stone surface from 20 pounds per square inch to 30 pounds and then to Uo pounds, successive- ly, reduced both the strength and the unit energy consumption. Freeness was increased, but there was little change in fiber length. Contrary to experience with softwoods, raising the pit temperature (without change in consistency) produced a negligible effect. Although the birch wood, especially when green, is quite white, the pulps were inclined to have a more or less pronounced orange tint. The coloring matter appeared to be water-soluble, but showed a tendency to become adsorbed on the pulp to some extent when standing in suspension. A sample of the pulp washed immediately after discharge from the grinder was decidedly improved in color. The properties of the birch groundwood described above did not bar its use in several types of paper. Table 5 shows data for newsprint paper. When combined with birch neutral sulfite semichemical pulp and a coniferous ground- wood pulp, birch groundwood may be used in proportions up to 30 percent of the total furnish with satisfactory results. The 50 percent of birch semi- chemical pulp used with this amount of birch groundwood brought the total hardwood content of the sheet to SO percent. Comparing the properties of the experimental papers with the averages for commercial newsprint papers, it is noted that nearly all of the experimental papers are equal to or better than the commercial standard in most properties, RIH19 -U- Certain grades of toweling paper contain from Uo to 50 percent of groundwood pulp, usually made from spruce and balsam, the remainder of the furnish "being spruce sulfite pulp. The short fiber, softness, and apparent absorbent qualities of birch groundwood suggested its possible use in this grade of paper. The principal data obtained in several experiments in the making of toweling paper are given in table 6. The birch groundwood was substituted for part or all of the spruce groundwood in amounts varying from 15 to Vj percent of the total furnish. The birch groundwood lowered the strength slightly, but this was overcome to some extent by a little beating or jordan- ing. The drainage from the wire was slower and the wet strength of the web before passing the creping doctor was lowered. Considerable picking occurred on the dryers. In commercial operation, these machine-operating character- istics will need to be controlled by appropriate count ermeasures. The water- absorption rate of the toweling paper was greatly increased by the addition of the birch pulp. One of the best papers, meeting most of the strength re- quirements of the standard toweling paper, contained 25 percent of birch groundwood (machine run 1707). This paper had more than 3 times the absorbency of the standard. Green Ash, Sugarberry, and Southern Cottonwood Green ash, sugarberry, and Cottonwood were received and tested at the same time. For this reason, it is convenient to consider them as a group. Green ash ( Fraxinus, Pennsylvania lanceolata ) is widely spread throughout southern Canada and the United States as far west as the eastern ranges of the Rocky Mountains. Sugarberry ( Celtis laevigata ), also commonly known as hackberry, is a bottomland hardwood growing in the southern United States, being quite plentiful In the Mississippi valley. Southern cottonwood ( Fopulus deltoides virgir.iana ) occurs In river valleys throughout the eastern, southern, and central United States. The woods used in this study were received from the delta region of Arkansas. The grinding data are given in table 7i which includes for comparison purposes data on paper birch and southern pine and average values for commercial news- print groundwood pulps. By lowering the grinding pressure on the sugarberry from 30 t0 20 pounds per square inch of wood on the stone surface the pulp strength was raised a little but a large increase in energy consumption per ton was attained. Vhen the pressure was rained to Uo pounds, on the other hand, very little lowering of strength was produced and both a marked increase in production and a low- ering of energy consumption were achieved. The response with ash was similar to that with sugarberry, except that the increase in pressure reduced the unit energy consumption only slightly. Comparing the groundwood pulps produced at 30 pounds per square inch, it is apparent that the unit energy consumptions for these three hardwoods are in the range of commercial practice for newsprint groundwood, with that from sugarberry being somewhat high. The fibers of the sugarberry and ash were much shorter (as indicated by the screen analysis) than average fibers of RIU19 -5- the commercial groundwoods and slightly shorter than those of "birch pulp. The cottonwood was low in the coarsest mesh fraction (2U-mesh), but otherwise approached commercial pulps in screfen analysis. The sugarberry and ash had only fair "bursting and tensile strengths and poor tearing strength. The cottonwood equaled the commercial average in bursting and tensile strength, "but was somewhat lower in tearing strength. The sugarberry, ash, and cottons- wood were all superior in Gtrength to the birch groundwood although none was as strong as the southern pine groundwood. All of them were comparable with spruce groundwood in color. These hardwood groundwood pulps were used in newsprint paperraaking experiments with southern pine groundwood pulp and semibleached southern pine sulfate pulp. The data are given in table 8. On the "basis of machine runs 1770. and 1771. it was found that a furnish containing 20 percent of cottonwood groundwood, 60 percent of pine groundwood, and -20 percent of pine sulfate, lightly jordaned, gave a paper about equal to the average commercial newsprint. The whiteness, at 65 percent, was very good. When the cottonwood groundwood content was increased to *+0 percent (machine run 177^) and the same total groundwood content was maintained, the tensile strength was lowered about one- fifth, but there was practically no change in bursting strength. The tearing strength was higher than the standard average. The relative effect of the three hardwood species may be noted by comparing machine runs 177 2 . 1773. and 1775. The paper of run I77 2 i containing cotton- wood groundwood, was about equal in qiality to that of run 1773. which con- tained an equal amount of ash groundwood, while the paper from run 1775. containing sugarberry groundwood, was poorer. Machine run 1777 w & s made with a furnish consisting of equal amounts of the three hardwood groundwoods, 39 percent in all. The rest of the furnish was pine groundwood and semibleached sulfate in about the proportions used in the three runs just discussed. With the exception of tearing resistance, the test values for all strength proper- ties are less than the average for commercial newsprint. In these newsprint experiments it appeared that at least 20 percent of semibleached sulfate pulp was required to bring the bursting and tensile strengths up to values com- parable with commercial newsprint. This amount of sulfate pulp produced a tearing strength higher than the standard. Although none of the hardwood- containing papers were quite as strong as the average for pine groundwood and sulfate alone, they are considered satisfactory for use as newsprint paper. Summarizing these experiments, it is indicated that groundwood pulp of acceptable quality can be made from cottonwood, green ash, and sugarberry. In the making of newsprint paper, these pulps can be combined, in quantities as high as ho percent, with southern pine groundwood and semibleached sulfate pulps* to produce a satisfactory furnish. Black Willow and American Elm » Black willow ( Salix nigra ) occurs in bottomlands throughout the northeastern, eastern, central, and southern United States except in Florida and the south- eastern parts of South Carolina, Georgia, and Alabama. American elm ( Ulmus americana) ranges from southern Canada south to c erttral Florida and west R1U19 -6- through the Central States to the foothills of the Rocky Mountains. The samples tested at the Laboratory were received from the Arkansas delta region, as were the throe hardwoods discussed in the foregoing section. The data are given in table 7. The black willow groundwood, like the cottonwood, was low in the 2U-nesh screen fraction, but was about equal to the commercial groundwoods in strength. The elm groundwood was more like the sugarberry in fiber length and strength. Under comparable grinding conditions, the energy consumption for the elm was higher than that for the willow, which was normal for groundwood of newsprint grade. The dark color of these pulps was their principal defect, and it was for this reason that papermaking tests were not made with them. It is entirely possible that willow and elm groundwood pulps can be used in papers and boards in which color is not so important, as, for instance, in insulating board or container board filler, but no experiments were made to ascertain this. • | General Considerations It is generally believed that the production of groundwood pulp from hardwoods requires a high expenditure of energy. This is often true when it is sought to produce pulp comparable to spruce groundwood in strength and fiber length. In fact, it is doubtful whether these properties can be obtained with such dense, short-f ibered hardwoods as tupelo, birch, beech, maple, sugarberry, ^and ash. There arc uses for groundwood pulp, however, in which the strength/length normally obtained from spruce are not necessarily required. The data in tables 1, U, and 7. give evidence- that pulps satisfactory for certain purposes can be made from this class of hardwoods with average or below average energy consumption. Hardwoods of lower density such as cottonwood, aspen, willow, etc., yield relatively free groundwoods with average strength and normal energy consumption. The fiber length of these groundwoods is, as a rule, somewhat less than that of spruce, arid the color of some of them limits their field of usefulness. Because of the short fiber of some hardwood groundwood pulps, considerable difficulty is experienced in attempting to form laps on the wet machine in the customary manner. It might be better, in such instances, to take the pulp off the machine in loose, crumbled form and store it in bales instead of laps. Better still, perhaps, would be the use of a slush system entirely when grinding hardwoods. Close attention must also be paid to the prevention of white water fiber losses when operating with hardwood groundwood pulps, by providing a properly closed system and adequate save-alls. Some mill operators have abandoned the grinding of hardwoods, for the reason that low yields were obtained. A modification of the wet machine operation and white water system might have been a solution to these difficulties. Some of the hardwoods have fairly high densities in contrast to the softwoods usually used for groundwood manufacture. The resultant higher weight of wood per cord, and the consequent higher weight yield of pulp are factors to be considered in their favor. For instance, the hardwoods studied, with the RIU19 _7_ exception of Cottonwood and willow, ranged from about 2^00 to 2700 pounds of moisture-free wood per standard cord. The cottonwood and willow were compar- able with spruce in weight per cord, namely around 2000 pounds. In these days of uncertain transportation, it might be advantageous to procure hardwoods, even at a higher price per cord, in stands close to the mill than to purchase softwoods at a lower price, but requiring a longer haul. Literature Cited (1) Benninger, Frits. Groundwood from beech. Wochbl. Papier fabr. 72:118-120 (19^+1). (2) Brecht, W. ; Schrotter, IL : Suttinger, R. A comparison of various woods in mechanical pulp manufacture. Papier fabr. (Tech. Wiss Teil) ^6 U13-U20 (1938). (3) Briefkasten. Groundwood from aspen. Wochbl. Papier fabr. 68 (33): 622; (35): 655 1 (36):681; (Aug. lU, 28, Sept. U, 1937) . (1+) Kin, Myron; Libby, C. E. The manufacture of brown mechanical pulps from hardwoods. I yellow birch. Paper Ind. and Paper World 22 (7): 675 (Oct. 19^); (9): 918-923 (Dec. 19U0); (l0):10U3-10Ug (Jan. 19^1). (5) Munro, W. A. Poplar wood as a substitute in the manufacture of groundwood. Paper Trade Jour. j7 (20):6o (Nov. 15, 1923) . (6) Running, K. D. Hardwood utilization by the paper industry. Pulp and Paper Mag. Can. ^2 (2) loU-106 (Convention issue I9H1). (7) Schafer, B. R. ; Pew, J. C. ; Pillow, M. Y. Discoloration of swamp black gum pulpwood in storage. Tech. Assoc. Papers 22:U05-8 (1939). (8) Thickens, J* H* and McUaughton, G* C. Groundwood pulp. U. 3. DepU Agric* Bull, 3*+3 (April 26, 1916). (9) Wynn-Roberts, R. 1. Grinding characteristics of various woods. Paper Trafle Jour. 10*+ (6) 46-1+8 (Feb. 11, 1937). 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