KRAFT PULPS, PAPERS, AND LINERBOARD FROM SOUTHERN PINE THINNINGS

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1 U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS In Cooperation with the University of Wisconsin LABORATORY MADISON, WIS. U. S. FOREST SERVICE RESEARCH NOTE FPL-0182 JANUARY 1968 KRAFT PULPS, PAPERS, AND LINERBOARD FROM SOUTHERN PINE THINNINGS

2 Abstract Pulping, bleaching, and papermaking studies were conducted to compare pine thinnings with mature wood for use in the manufacture of kraft pulp and various papers and paperboard. Kraft pulps from slash pine and loblolly pine thinnings and core wood (pith to 8 or 10 year growth ring) had burst and tensile strengths equal to pulps from mature pulpwood logs, but their tearing resistance and pulp yield were lower, Pulps from the thinnings and the core wood had comparable strength characteristics. The outer wood of both pines gave pulps that were comparable in burst and tensile strengths to pulps from mature pulpwood logs, but their tearing resistance was greater. Paper and linerboard made with the pulp from the loblolly pine thinnings generally were better formed, had higher tensile and burst strengths, but had lower tearing resistance than papers made with pulp from the mature wood. The thinnings pulp gave softer and more absorbent tissue paper, smoother and more closed printing papers, and linerboard with greater compressive resistance.

3 KRAFT PULPS, PAPERS, AND LINERBOARD FROM SOUTHERN PINE THINNINGS By D. J. FAHEY, Technologist 1 and J. F. LAUNDRIE, Chemical Engineer 1 Forest Products Laboratory, 2 Forest Service U.S. Department of Agriculture ---- Introduction The availability of substantial amounts of thinnings of southern pine leads to questions by foresters and papermakers on the suitability of these thinnings for papermaking. In the 1930's at the Forest Products Laboratory (1) 3 pulps made from southern pine top wood were compared with pulps made from wood lower in the stem. It was found that in proceeding from the butt to the top of the tree, the burst and tensile strengths of the kraft pulps showed a tendency to increase and the tear to decrease with the accompanying increase in the proportion of springwood. McKee (5), a number of years later, found in kraft pulping small diameter slash pines (less than 3.5 inches in diameter) that the thinnings generally did not give as good strength as the medium-density mature wood. Others (2, 3) have investigated the relationship between anatomical properties of the wood and pulp properties, but little or no published information is available on using thinnings in the manufacture of paper. Therefore this work was undertaken to evaluate their suitability for use in papers and in linerboard. 1 Acknowledgment is made to the Alexandria Forestry Center, Southern Forest Experiment Station, Alexandria, La., and to the American Pulpwood Association, Jackson, Miss., for aid in procuring and selecting the wood used in this work. 2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3 Numbers in parentheses refer to Literature Cited at the end of this report.

4 Wood The mature slash pine logs used were plantation grown near Alexandria, La. The logs were separated into two batches. The first contained logs representative in size of the wood normally received at a typical kraft pulpmill, and the second was of logs with a minimum diameter of 10 inches. These were cut at the Laboratory to separate the 10-year-old core wood from the remaining outer wood. The slash pine thinnings were plantation grown in Shelby County, Ala. Both the loblolly pine thinnings and the mature loblolly pine were plantation grown in Tallapoosa County, Ala. The mature loblolly pine was cut at the Laboratory to separate the 8-year-old core wood from the remaining outer wood. One-inch disks were cut from the ends of the logs and used for determining age, growth rate, summerwood content, and specific gravity (table 1). The peeled logs were converted into nominal 5/8-inch chips in a commercial-sized, two-knife chipper, and the oversized and undersized materials were removed by screening. As expected, the growth rates of the slash pine thinnings and the core wood almost doubled those for either the mature slash pine pulpwood logs or the slash pine outer wood that were almost equal to each other. The growth rate of the loblolly pine thinnings was also considerably higher than that for the whole mature loblolly pine logs. The summerwood contents of the mature slash pine pulpwood and the slash pine outer wood were about the same, and surprisingly, the content was only slightly less in the slash pine thinnings. The summerwood content of the slash pine core wood was only about half that of the outer wood. In the loblolly pine thinnings, it was about two-thirds of that in the mature loblolly pine logs. The specific gravity of the mature slash pine pulpwood and the slash pine outer wood was the same, whereas that of the core wood was only slightly less. The mature loblolly pine logs had a specific gravity about 12 percent higher than that of the loblolly pine core wood and about 18 percent higher than that of the loblolly pine thinnings. With slash pine the amount of alcohol-benzene extractives in the core wood was about three times that in the outer wood, whereas in the thinnings and the mature slash pine pulpwood, it was about twice that amount. A slightly greater FPL

5 Table 1.--Wood and fiber properties

6 amount was found in the loblolly pine thinnings than was found in the mature logs. The core wood, however, had about twice the amount of that in the mature wood. Because the core wood from both the slash pine and the loblolly pine contained a considerably greater amount of extractives than did the thinnings of the same age, it is possible that these extractives are deposited as the trees grow beyond their juvenile stage. Procedure Kraft Pulping Small-scale pulping trials were made to determine the conditions that would produce a pulp with a kappa number of about 50 for linerboard and a pulp with a kappa number of about 30 for bleaching (tables 2 and 3). A stainless-steel digester of 0.8-cubic-foot capacity and equipped with a heat exchanger and circulating system was used. After charging the chips in a perforated basket, the digester was evacuated before the cooking liquor was drawn in. The cooked chips were partially washed inside the digester with hot water and then disintegrated in a stainless steel mixing tank with an electric-powered stirrer and hot water. The resulting pulps were again washed with hot water and then screened through a 12-cut slotted flat screen, dewatered, shredded, weighed, and sampled for moisture content and kappa number determination. The pulps were refined in the standard TAPPI beater, and handsheets made and evaluated according to TAPPI methods. Pilot-scale digestions were made using loblolly pine thinnings and mature pulpwood to produce sufficient quantities of pulp for bleaching and papermaking. These digestions were made in a tumbling, steam- jacketed digester of 13-cubic-foot capacity. At the end of cooking, the pressure was relieved to about 80 pounds per square inch, and the cooked chips were blown into a stainless steel blow tank having a false bottom perforated with 3/32-inchdiameter holes. The pulp was screened through a slotted flat screen having inch openings, passed through a 6-inch-diameter and a 3-inch-diameter centrifugal cleaner in series, and thickened to about 20 percent consistency on a vacuum decker. The primary cleaner rejects were recycled through the same system, and the accepted pulp from the rejects was mixed with the primary accepts. FPL

7 Table 2.--Kraft pulping of slash pine mature wood and thinnings and core wood and outer wood from mature trees

8 Table 3.--Kraftpulping of loblolly pine mature wood and thinnings and core wood and outer wood from mature trees

9 Bleaching The kraft pulps were bleached in four stages using an oxidative technique, which consisted of adding hypochlorite and then adding chlorine without intermediate washing in the first stage. A combination of caustic soda and hypochlorite was used in the second stage. Hypochlorite was used in the third stage and peroxide in the fourth stage. Papermaking Linerboard and offset, book-coating base, and facial tissue papers were made on the experimental paper machine to compare kraft pulp from loblolly pine thinnings with pulp from mature wood. The furnish for the experimental 42 pound linerboards contained 100 percent of each of the two unbleached pine kraft pulps refined in a pressurized disk mill to about 570-milliliters freeness (Canadian Standard). The furnishes contained 0.5 percent rosin size. The pulp furnishes for the 45-pound coating-base sheets and the 60-pound offset papers contained 40 percent of each of the two bleached experimental pine pulps and 60 percent of a commercial, bleached gum kraft pulp. To the coating-base furnish 10 percent clay was added, whereas 17 percent clay, 3 percent titanium dioxide, and 0.5 percent rosin size were added to the offset furnishes. The furnishes were refined with a jordan, lowering the freeness to about 500 milliliters. Offset papers were made with and without starch surface size. Using a horizontal size press, the starch sizing, an 8 percent mixture of a medium-viscosity ethylated cornstarch, was applied to the web when the moisture content was about 6 percent. The starch pickup on a solids basis was 3.2 percent with no difference noted between the paper with the thinnings pulp and the paper with the mature wood pulp. Experimental 12-pound facial tissue papers were made from funishes containing 80 percent of each of the two experimental bleached pine pulps with the remainder consisting of the commercial, bleached gum kraft pulp. The furnishes were refined in the jordan to about a 600-milliliter freeness. The tissues were dry creped at about 95 percent solids with 0.1 percent animal glue added continuously to the furnish for better adhesion at the creping dryer. FPL

10 Results Pulping The yields of the slash pine pulps decreased as the amount of extractives increased (table 4). The pulps from slash pine thinnings had yields only about a 1/2 percentage point lower than the pulps from the mature slash pine. The pulps from the core wood, however, had yields about 2 percentage points lower than the mature slash pine pulps, whereas the yields of the outer-wood pulps were about 1-1/2 percentage points higher than the mature slash pine pulps. Only small differences were found in the amounts of total reducing sugar and mature wood, the core wood, and the outer wood of slash pine. Although the amount of extractives in the loblolly pine thinnings was only slightly greater than that in the mature wood, the yields of the thinnings pulps were about 3 percentage points lower than those of the pulps from the mature wood. The pulp yield from the loblolly pine outer wood was about 1 percentage point lower than that for the mature wood and was about 2 percentage points lower for the core wood. An advantage to pulping the thinnings with the larger amount of extractives, however, is that higher yields of turpentine and tall oil can be expected. Strength properties of the pulps are given in tables 2 and 3. At equal freenesses, the burst and tensile strengths of the pulps from the mature slash pine and those from the thinnings, the core wood, and the outer wood were about the same. The tearing resistance of the pulp of the slash pine thinnings, however, was about 30 percent lower and the pulp of the core wood was about 20 percent lower than that of the mature slash pine. However, the tearing resistance of the pulp of outer wood was about 8 percent higher than that of the pulp from the mature slash pine. The handsheet densities of the core-wood and of the thinnings pulps were about 8 percent higher than those of either the outer-wood pulps or the pulp from the mature slash pine. However, at equal sheet densities, the tearing strengths of pulps of the core wood and of the thinnings were only about 10 percent lower than those of the outer-wood pulps and the pulps from the mature slash pine, whereas the burst and the tensile strengths of pulps from the core wood, the outer wood, and the thinnings remained about the same as for the pulps from the mature slash pine. FPL

11 Table 4.--Effect of chemical composition on kraft pulp yields from slash and loblolly pine

12 The strength properties of the unbleached loblolly pine pulps were about the same as those with the unbleached slash pine pulps. As expected, correlation was good between the properties of the various loblolly pine pulps and fiber length and fiber coarseness. The pulp fibers from the outer wood and the mature wood were longer and coarser than those from the thinnings and the core wood. The longer, thicker-walled fibers gave pulps with better tearing resistance, but they had lower burst and tensile strengths. Whereas no fibril angle measurements were available from this series of pulps, Hiller and Brown (4) have shown that the fibril angle decreases in going from the pith to the outer wood. Bleaching In bleaching on a laboratory scale to a brightness of about 85 percent, the strength of the pulps having the higher kappa numbers was maintained. There was, however, some loss in tearing resistance of the pulps at the lower range of kappa number. The pulps used in pilot-scale bleaching for papermaking were at the lower range of kappa number. These were bleached to a brightness of 88 percent; as a result, the strength loss was somewhat greater than would have occurred at a brightness of 83 to 85 percent. Despite this, the papers made from these pulps were adequate in strength properties. Papermaking The papers and the linerboard made with pulp from the loblolly pine thinnings generally had better burst and tensile strengths than corresponding papers made with the mature wood pulps (tables 5, 6, and 7). They were also better formed and possessed a smoother surface for printing, but were lower in tearing resistance. In these runs the same refining technique was used in lowering the freeness to the same level for the furnishes with the thinnings pulp and the mature wood pulp. Furthermore, only a single run of a given grade was made with each pulp. With additional trials and a different refining technique including a milder treatment of the thinnings pulp, it is reasonable to expect that a better balance between burst, tensile, and tear might be obtained. The printing qualities of the offset papers with the thinnings pulp were superior to those papers with the mature wood pulp. The surfaces of both the surface-sized and unsized papers were smoother when the thinnings pulp FPL

13 Table 5.--Comparison of propertied of book papers of two loblolly pine pulps--from mature wood and from thinnings

14 Table 6. --Comparison of properties 1 of facial tissues of two loblolly pine pulps--from mature wood and from thinnings Table 7.--Comparison of properties 1 of linerboard of two loblolly pine pulps--from mature wood and from thinnings

15 was used as shown in the photomicrographs in figure 1. These surfaces were photographed with a single lens reflex camera equipped with an 8-inch extension tube. Oblique illumination at about 15 to the horizontal plane of the paper was used. Evaluations of halftones from letterpress printing on all four offset papers, both before and after supercalendering, also showed the papers with the thinnings pulp to be significantly better than those with the mature wood pulp. In strength and other sheet properties the papers with the thinnings pulp generally had properties equal to or better than those for papers containing the mature wood pulp. The tearing strength was somewhat lower when the thinnings pulp was used, but this property was still in the range of many commercially produced offset papers. From evaluations of the properties obtained on the two coating-base papers, the thinnings pulp should yield a coated sheet with characteristics as good as or better than the mature wood pulp. The facial tissue with the thinnings pulp was softer and more absorbent than that with the mature wood pulp. Softness and absorbency are highly desirable properties in this grade of paper. In addition to the better burst and tensile strengths, the 42-pound linerboard with the thinnings pulp had higher ring crush and folding endurance than the board with the mature wood pulp (table 7). Although the tearing resistance was lower, it was still within the range of many commercially produced linerboards. Conclusions Papers and linerboard made with kraft pulp from thinnings of loblolly pine were better formed and stronger, except for tear, than they were when made with kraft pulps from mature wood. The printing papers were smoother, and the tissue paper was softer and more absorbent than that made from the pulp of mature wood. FPL

16 Figure 1.--Photomicrographs of the offset paper showing: Upper, no surface size; left, from mature wood; right, from thinnings. Lower, surface sized; left, from mature wood; right, from thinnings. M FPL-0182

17 Literature Cited 1. Bray, M. W., Martin, J. S., and Schwartz, S. L Sulfate pulping of southern yellow pines. II. Efect of growth variables on yield and pulp quality, Southern Pulp and Paper J. 2(6): Byrd, V. L., Ellwood, E. L., Hitchings, R G., and Barefoot, A.C Wood characteristics and kraft pager properties of four selected loblolly pines. II. Wood chemical constituent and their relationship to fiber morphology. Forest Prod. J. (8): Einspahr, D. W Correlations between fiber dimensions and fiber and handsheets strength properties. Tappi 47 (4): Hiller, C. H., and Brown, R. S Comparison of dimensions and fibril angles of loblolly pine tracheids formed in wet or dry growing seasons. Amer. J. Bot. 54 (4): McKee, J. C The kraft pulping of small diameterslash pines. Tappi 43 (6):202A 204A. FPL

18 PUBLICATION LISTS ISSUED BY THE FOREST PRODUCTS LABORATORY The following lists of publications deal with investigative projects of the Forest Products Laboratory or relate to special interest groups and are available upon request: Architects, Builders, Engineers, Logging, Milling, and Utilization and Retail Lumbermen of Timber Products Box and Crate Construction and Mechanical Properties and Struc Packaging Data tural Uses of Wood and Wood Products Chemistry of Wood Modified Woods, Paper-Base Drying of wood Laminates, and Reinforced Plastic Laminates Fire Performance Sandwich Construction Fungus and Insect Defects in Forest Products Thermal Properties of Wood Furniture Manufacturers, Wood Fiber Products Woodworkers, and Teachers of Woodshop Practice Wood Finishing Subjects Glue and Plywood Wood Preservation Growth, Structure, and Identification of Wood Note: Since Forest Products Laboratory publications are so varied in subject matter, no single catalog of titles is issued. Instead, a listing is made for each area of Laboratory research. Twice a year, January 31 and July 1, a list is compiled showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory s mailing roster, and it serves to keep current the various subject matter listings. Names may be added to the mailing roster upon request.