U. S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE • FOREST PRODUCTS LABORATORY • MADISON, WIS. In Cooperation with the University of Wisconsin U. S. FOREST SERVICE RESEARCH NOTE FPL-0182 JANUARY 1968 KRAFT PULPS, PAPERS, AND LINERBOARD FROM SOUTHERN PINE THINNINGS 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. KRAFT PULPS, PAPERS, AND LINERBOARD FROM SOUTHERN PINE THINNINGS By D. J. FAHEY, Technologist1 and J. F. LAUNDRIE, Chemical Engineer1 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 gen­ erally 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 avail­ able 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. 2Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3Numbers in parentheses refer to Literature Cited at the end of this report. Wood The mature slash pine logs used were plantation grown near Alexandria, La. The logs were separated into two batches. The first contained logs representa­ tive 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 - 0182 -2­ Table 1.--Wood and fiber properties 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 dis­ integrated 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 paper- making. 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-inch­ diameter holes. The pulp was screened through a slotted flat screen having 0.012-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-0182 -3­ Table 2.--Kraft pulping of slash pine mature wood and thinnings and core wood and outer wood from mature trees Table 3.--Kraftpulping of loblolly pine mature wood and thinnings and corewood and outerwood frommature trees Bleaching The kraft pulps were bleached in four stages using an oxidative technique, which consisted of adding hypochlorite and then adding chlorine without inter­ mediate washing in the first stage. A combination of caustic soda and hypo- chlorite 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.