Producing Studs from Paper Birch by Saw-Dry-Rip

Transcription

1 United States Department of Agriculture Forest Service Forest Products Laboratory Research Paper FPL-RP-480 Producing Studs from Paper Birch by Saw-Dry-Rip Robert W. Erickson Harlan D. Petersen Timothy D. Larson and Robert Maeglin

2 Abstract Paper birch, the second most abundant hardwood species in the Great Lakes region, is greatly underutilized. The major obstacle to the commercial use of paper birch is warp. In this study, we compared the effect of two methods of sawing (conventional and Saw-Dry-Rip (SDR)) and two methods of drying (conventional and high temperature) on warp of paper birch studs. Effects of air and dehumidification drying were evaluated only for SDR sawing and for comparison of mean values with the other four treatments. The SDR process significantly reduced warp, particularly in 2 by 4 studs compared to 2 by 2 and 2 by 3 studs. The effect of SDR on warp was independent of drying temperature; crook values obtained with conventional temperature and dehumidification drying were equal to those obtained with high temperature drying. In general, there was a statistically significant increase in warp, especially crook, during a l-month storage period at laboratory conditions. However, dimensionally stable studs were obtained from SDR flitches kiln-dried at high temperature and maximum venting to a uniform 5 to 6 percent moisture content. SDR was most effective in reducing warp of studs from upper logs. Problems encountered included dark wood (which resulted in honeycomb and collapse) and excessive thickness shrinkage. Darkwood can be minimized by sorting logs, before processing, to eliminate darkwood. Excess thickness shrinkage can be counteracted by increasing target thickness from 1-3/4 inches to 1-7/8 inches. Our results indicate that the SDR process could be used to minimize the amount of crook in both structural and nonstructural lumber sawn from paper birch, resulting in increased recovery and greater utilization of this hardwood resource. Keywords: Paper birch, Saw-Dry-Rip, SDR, Warp November 1986 Erickson, Robert W.; Petersen, Harlan D.; Larson, Timothy D.; Maeglin, Robert. Producing studs from paper birch by Saw-Dry-Rip. Res. Pap. FPL-RP-480. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; p. A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI Laboratory publications are sent to over 1,000 libraries in the United States. The Laboratory is maintained in cooperation with the University of Wisconsin.

3 Producing Studs from Paper Birch by Saw-Dry-Rip 1 Robert W. Erickson Harlan D. Petersen Timothy D. Larson College of Forestry, University of Minnesota, St. Paul, MN Robert Maeglin, Supervisory Research Forest Products Technologist Forest Products Laboratory, Madison, WI Introduction Demand on softwood species to meet structural lumber needs in North America is increasing. However, supplies of softwood timber have not increased to meet this demand. In fact, many areas of the softwood forest in northern Minnesota have become inaccessible for commercial use because of the Boundary Waters Canoe Wilderness Act (PL95-495). This shortage could be partially alleviated by greater use of underutilized species, such as paper birch (Betula papyrifera), which is the second most abundant hardwood species in Minnesota and other parts of the Great Lakes region and Canada (Martodam 1983). In addition, the commercial use of paper birch could obviate the ever-increasing cost of transporting lumber from the southern and western lumber-producing regions to midwestern markets. Paper birch is a medium-density hardwood species with an average specific gravity of 0.48 based on green volume, which is about the same specific gravity as that of southern yellow pine (Forest Products Laboratory 1974). The major obstacle to its commercial use is warp, particularly crook. When studs are produced by conventional methods, growth stresses cause crook to occur at the headrig. Furthermore, additional crook can develop in drying because of non- uniform longitudinal shrinkage and the inherent low resistance of studs to shrinkage forces. The Saw-Dry-Rip (SDR) process can greatly reduce or eliminate crook that results from growth stresses in the log. SDR originated at the USDA Forest Service, Forest Products Laboratory (FPL) for the purpose of increasing the utilization of low- to medium-density hardwood species for structural lumber. It employs the live sawing of logs into nominal P-inch-thick flitches, drying the flitches, and then ripping them into studs. Maeglin and Boone (1983) have described in detail the principles behind SDR and its successful application to yellow-poplar. The primary objective of the research reported here was to compare the amount of warp of conventionally sawn paper birch studs with that of studs produced by the SDR method. This research was phase 2 of a three-part study: Phase 1, investigation of the drying of nominal 2-inch paper birch flitches by various methods; phase 2, comparison of the amount of warp obtained in conventionally produced and SDR-produced studs; and phase 3, comparison of the strength of studs dried at high temperature and conventional kiln temperature. Results of phase 1 have been published as an FPL research paper (Larson et al. 1986), and phase 3 research is currently underway. This paper was supported in part by Mclntire-Stennis funds and is entered as Paper No in the scientific journal series of the Agricultural Experiment Station, University of Minnesota, St. Paul.

4 Materials and Methods The study was designed to evaluate the effect of SDR on the reduction of warp in studs. Design The principal study design was a 2 by 2 factorial with two replications. The secondary study design was a 2 by 2 by 3 factorial for within-group factors. The principal treatments were two sawing methods (SDR live-flitch sawing and conventional stud sawing) and two drying methods (conventional and high-temperature). Half of the flitches were ripped to green stud target dimension (for conventional sawing), and the other half were dried as wide flitches and then ripped to dried stud target dimension (for SDR treatment). The within-group variables measured were the effect of storage, the position of studs in the tree (butt logs and upper logs), and stud size. Air and dehumidification drying were evaluated in addition to conventional and high-temperature drying, but only in conjunction with SDR and not with conventional sawing. Mean values of warp for air and dehumidification drying were compared with warp means for the four principal treatments, using the Bonferroni test (Miller 1966). Harvesting and Allocation of Logs Paper birch trees 8 to 12 inches in diameter at breast height (dbh) were harvested from the Superior National Forest near Grand Marais, MN, and bucked into logs 100 inches long, using an average of four to five logs per tree. Trees with excessive lean, rot, or extremely poor form, and logs with more than 3 inches of sweep per 8 feet of length were excluded. Each log was marked with its tree and log number and then end coated. Logs for the study had a minimum small-end diameter inside the bark (dib) of 4.5 inches. Two hundred and sixteen logs were used for the study. After the small end dib was measured, the logs were sorted into l-inch-diameter classes ( through in). Commencing with the 5-inch class, the logs were randomly allocated to six treatment groups; each treatment had two replications. Butt logs were allocated separately to insure an equal distribution to treatments because studs produced from such logs of other species have exhibited more crook than logs from upper tree positions (Maeglin and Boone 1983). Machining The logs were live sawn into nominal 2-inch flitches that were double end trimmed to 97 inches and then rough edged. Flitches were numbered, block piled, wrapped in plastic, and transported to the University of Minnesota for cold-room storage (@33 F) to await further processing. Flitches were ripped by a nearby hardwood lumber processor on contract. Green and dried flitches were premarked for ripping into 2 by 2, 2 by 3, and 2 by 4 studs. We attempted to achieve maximum piece yield but gave priority to the production of 2 by 4 studs. Rough dry studs, both SDR and conventionally sawn, were surfaced to a thickness of 1-7/16 inch and standard finished widths of 1-1/2, 2-1/2, and 3-1/2 inches. The less-than-standard thickness was the result of high thickness shrinkage combined with sawing variation. Drying The treatments are described in table 1. In the abbreviations used to define the treatments, the first and second letters of each abbreviation designate the type of sawing and drying, respectively; e.g., SH refers to SDR processing and high-temperature drying. Treatment 1 is subdivided into 1A and 1B because the wet-bulb water supply was lost in drying the first kiln charge. Consequently, the kiln vents were wide open rather than closed overnight, and the intended wet-bulb temperature of 190 F was not maintained. A lower and more uniform final moisture content (MC) resulted for treatment 1A compared to 1B and to the other high-temperature runs (Erickson et al. 1984). We had previously discovered in phase 1 of this study (Larson et al. 1986) that the darkwood of paper birch is impermeable and subject to severe drying degrade. Darkwood refers to tissue that is darker in color than normal heartwood. It occurs in irregular patterns near the heart of the tree, has higher MC than normal heartwood and sapwood, and appears to have its origin associated with branch stubs and possibly other tree wounds. Darkwood may be bacterially infected wetwood (Ward and Pong 1980). Some of it will develop honeycomb even at air-drying conditions. It seemed apparent that an extended equalization period would be required to reach the desired goal of a final MC range of about 8 to 12 percent. Of course, only the steam-heated kiln afforded the equalization option. Each kiln charge contained six sample boards. Four of the six boards contained varying amounts of darkwood, whereas the other two were totally whitewood. (Whitewood refers to all wood other than darkwood, and consequently a given whitewood flitch could have been all sapwood or a combination of sapwood and normal heartwood.) There were eight samples for air drying, four in each of two units. 2

5 Table l--treatment description and drying conditions Treatment Equalizing Treatment description Drying conditions conditions a,b SH wbu SDR/high temp drying/wet bulb DB temp of 240 F. Kiln vents wide DB temp of 200 F and WB temp of uncontrolled. open for 14 h followed by 24 h at 180 F for 27 h. WB temp of 190 F. SH wbc SDR/high temp drying/wet bulb DB temp of 240 F and WB temp of DB temp of 200 F and WB temp of controlled. 190 F for 26 h. 180 F for 141 h. SC SDR/conventional temp drying. Dried by FPL schedule T8C3. Average DB temp of 180 F and WB temp of drying time of 397 h for each of the 160 F. Approximately 24 h for each two runs. of the two runs. SD SDR/dehumidification drying. 1 HP unit. Average drying time of None. 460 h for each of the two runs. SA SDR/air drying. Northern Minnesota air-drying yard None. from May through November. Stickered units were then covered with plastic and stored outdoors at Kaufert Laboratory during December and January. CH Conventional sawing/high temp DB temp of 240 F and WB temp of DB temp of 200 F and WB temp of drying. 190 F for 24 h. 180 F. About 44 h for each of the two runs. cc Conventional sawing/conventional Dried by FPL schedule T8C3. Average DB temp of 180 F and WB temp of temp drying. drying time of 380 h. 160 F for about 90 h for each run. a With the exception of treatment SH wbu, equalizing began when the driest sample board reached 7 to 8 percent MC and continued until the wettest reached a MC of about 12 percent. In treatment SH wbu, the equalizing commenced when the wettest kiln sample was about 10 percent and continued until the average of all six sample boards was approximately 6 percent. There was a top load restraint of 60 lb/ft 2 for all treatments except SA. b Total kiln residence was equal to the hours given under Drying Conditions plus the hours given under Equalizing Conditions; DB = dry bulb, WB = wet bulb, temp = temperature. Measurement of Moisture Content of Dried Flitches and Studs A resistance moisture meter was used to take readings on about one-third of the flitches and conventionally sawn studs soon after drying and again after machining and storage. Readings were taken about 18 inches from each end and at midlength. When a band of darkwood was present, two readings were taken at each location: one within the darkwood and another in the whitewood adjacent to the darkwood. The insulated needles were driven to a depth of 3/8 inch. After MC measurement the lumber was stickered 4 feet on center, using 3/4-inchthick stickers, and stored indoors. After the lumber had been stored for approximately 1 month, warp was remeasured on all studs. MC was redetermined on the same studs that were tested prior to storage. Collection of Warp Data Crook, bow, and twist were measured to the nearest 1/32 inch with a calibrated metal wedge. The initial measurement was completed within 2 or 3 days after planing, the second after storage. For crook measurement, the stud was placed on a perfectly flat table with the concave narrow edge down. The wedge was inserted to refusal at the point of greatest crook, which was normally at midlength, and the crook was read to the nearest 1/32 inch. Bow was measured similarly, with the wide face of the stud down. For the 2 by 2 s, bow was measured perpendicular to a 1-1/2-inch face and crook perpendicular to a 1-7/16-inch face. For twist, three of the four corners of the stud were made to contact the table top, and the wedge was inserted under the raised corner. 3

6 Results and Discussion The results showed that SDR improved stud quality in paper birch. However, some adjustment may be necessary in drying schedules and thickness of flitches. Warp Data With the exception of SA, all SDR treatments resulted in considerably lower crook and less twist than treatments using conventional sawing (table 2). The values for average bow were approximately the same for SDR and conventionally sawn studs. The poor performance of SAtreated studs is believed to have been caused by the high MC of the flitches before ripping; the average MC was 16.1 percent, with a range of 11 to 23 percent. Crook increased in all studs during storage, but the increase was least for the kiln-dried SDR treated studs. Of the SDR treatments, crook-increased most in SAtreated studs. Again, this is believed to have been caused by high MC at the time of ripping and subsequent drying of the machined studs during storage. Average bow for initial and after storage measurements followed the same pattern as crook for all six treatments. With the exception of SH-treated studs in which twist decreased, there was generally a slight increase in average twist during storage. CC, CH, SC, and SD studs were similar in their initial average MC (9.6, 9.3, 9.9, and 8.9 pct, respectively). After storage, however, crook of CC and CH studs was double that of SC and SD studs. This illustrates that drying in the flitch form restrains and/or removes longitudinal growth stresses. The conventionally sawn studs, which warped at sawing, did not receive the balanced stress reduction of SDR. Consequently, the warp remained to interact with longitudinal shrinkage while the pieces further dried during storage. The effect of SDR can best be seen in a comparison of the results of SC and CC treatments. The two treatments, which used conventional-temperature drying, resulted in similar final average MC (9.9 and 9.6 pct, respectively). The total drying times were also similar, with 397 hours for SC and 380 hours for CC (table 1). Yet, the average crook of SC studs both before and after storage was about one-half that of CC studs (table 2). Because final MC, drying schedule, and drying time of SC and CC were equal, the crook reduction for SC was apparently directly and totally attributable to the SDR process, Both SC and SD, which were dried at comparatively low kiln temperatures, had crook values essentially equal to those for the more successful high-temperature SDR treatment, SH wbu. This suggests that SDR can significantly reduce crook in the production of 2 by 4 s from paper birch independent of kiln-drying temperature. Consequently, the SA treatment may have produced warp results equivalent to those from SH wbu if drying had produced comparably low and uniform levels of MC in the flitches. Figure 1 compares the percent of 2 by 4 s rejected because of crook, by treatment and time of measurement. Rejection is based on warp limits for STUD grade lumber set by the National Grading Rule for softwood structural lumber (Northern Hardwood and Pine Manufacturers Association 1978, U.S. Department of Commerce 1970). The best treatment was SH wbu, with no rejects at the initial measurement and 4 percent rejects after storage. About 20 percent of the conventionally sawn studs were rejected at the initial measurement and about 22 percent after storage. With the exception of SA, the rejection rates for SDR-treated studs were far lower than for conventionally processed studs. Table 2 Warp and moisture content of paper birch 2 by 4 studs after initial drying and after storage of 30 or more days Average Range of Average crook Average bow Average twist moisture content moisture content Number (1/32 inch) (1/32 inch) (1/32 inch) (percent) (percent) Treat- of ment 2 by 4 s Initial Remeasure Initial Remeasure Initial Remeasure Initial Remeasure Initial Remeasure SC ( ) b ( ) b < 7-7 ( ) b SH wbu ( ) b ( ) b < 7-8 < SH wbc 39 CC CH SD c SA c a Moisture content values determined using a resistance moisture meter. b Meter readings were below the minimum dial reading of 7 percent. c Sawing methods were not compared for these drying trials; all studs were processed by SDR. 4

7 Statistical Analysis Results of the Bonferroni test (Miller 1966) for differences in the means showed that the kiln-dried SDR-treated studs had significantly less crook than conventionally sawn or SDR air-dried material (a = 0.05) both initially and after storage. Additional statistical comparisons were restricted to the studs dried by conventional and high temperatures. These were the only drying methods which were used in conjunction with both sawing methods. Figure l-percent of 2 by 4 s rejected because of crook, by treatment and time of measurement. (ML ) The percent of rejects for all studs (2 by 4, 2 by 3, 2 by 2), by treatment and time of measurement, once again shows the superiority of the kiln-dried SDR studs to the conventionally sawn studs (table 3). All three forms of warp, i.e. crook, bow, and twist, were represented in the percent of rejects. For further clarity, table 4 shows the number of studs rejected by warp type. At the initial measurement, a total of 72 studs were rejected for the 6 treatments; 2 studs were rejected because of bow, 4 because of twist, and the remainder because of crook. After storage, the total number rejected was 115: 4 because of bow, 13 because of twist, and the remainder because of crook. It is apparent that crook was the major form of warp and that the kiln-dried SDR treatments were quite effective in reducing crook. This is especially true for the SH wbu treatment, which dried the material to a uniform 5 to 6 percent MC. As a consequence, only one SH wbu treated stud was rejected after storage because of warp, and that for twist. The other drying methods could probably produce equally good results if drying times were sufficiently extended to produce comparable uniformity of MC. This certainly suggests that the current practice of drying studs to a maximum MC of 19 percent is grossly inadequate because of the potential for drying and warp during storage. The failure to make STUD grade because of warping can mean significant dollar losses. These data show the ability of SDR to greatly increase the recovery of STUD grade material. The warp reduction demonstrated for paper birch may be important not only for the production of studs but also for non-structural use such as furniture, paneling, etc. In the analysis of variance, the effect of sawing type on crook was highly significant; there was no significant effect on bow or twist (table 5). This was expected, for SDR is designed to minimize crook, whereas bow and twist are affected by uniform sawing, proper stacking, and stress balance in flitches (Maeglin and Boone 1983). Drying method did not have a significant effect on the three forms of warp or on the sawing/drying interaction. The effect of stud size was significant for all three forms of warp (a = 0.05) (table 6). This is attributable to the greater amount of crook and bow in 2 by 2 s and the larger values of twist in 2 by 4 s. A significant interaction of size with sawing method existed only for crook (a = 0.05), probably due to greater crook in 2 by 2 s. Increases in warp during storage were significant at the 5 percent level for all three types of warp. Crook and twist are most subject to increase during storage with additional drying. As discussed earlier, the increase in crook during storage of conventionally sawn 2 by 4 s was about twice that of SDR-processed studs. A significant interaction was observed between crook and tree position of logs after storage. The average increase in crook for studs from butt logs was 0.9/32 inch. For the studs from upper logs the increase was 0.4/32 inch. Therefore, the difference in the increase in crook for studs sawn from butt and upper logs can probably be attributed to the interaction of log position and storage. Table 3 summarizes the rejects by treatment and log position. All stud sizes were combined for the purpose of these comparisons. The percentage of rejected conventionally sawn studs was approximately four times higher than that of kiln-dried SDR-treated studs. Of further interest is the origin of the rejected studs. Thirty-two percent (13 of 41) of the rejected conventionally sawn studs came from butt logs; 75 percent (9 of 12) of the rejected kiln-dried SDR studs came from butt logs. These data illustrate the ability of SDR to almost totally eliminate rejection caused by warp of studs produced from the upper logs. The greater tendency for warp to occur in studs from butt logs agrees with data reported by Maeglin and Boone (1983). 5

8 Table 3 Rejection rates of treated studs according to log position in the tree a Treat- Number Number of Percent Number Number Percent Number Number of Percent ment of studs, rejects, rejects, of studs, of rejects, of rejects, rejects, all sizes b all sizes b all sizes b butt logs rejects, butt logs upper logs upper logs upper logs butt logs INITIAL MEASUREMENT SH c SC CH cc SD SA MEASUREMENT AFTER STORAGE SH c SC CH CC SD SA a Based on STUD grade warp requirements (Northern Hardwood and Pine Manufacturers Association 1978). b Includes 2 by 2, 2 by 3, and 2 by 4 studs. c Combined data for SH wbu and SH wbc. Table 4 Number of studs rejected, by warp type, before and after storage a Number Crook Bow Treatment of studs Initial Remeasure b Initial Remeasure Twist Total (all warp) Initial Remeasure Initial Remeasure a All sizes (2 by 2, 2 by 3, 2 by 4) combined. b Measurements were retaken after 30 or more days of storage. c Sawing methods were not compared for these drying trials. All studs were processed by SDR. 6

9 Problems Due to Darkwood Darkwood for all kiln-dried treatments had a higher MC than the whitewood. The range in final average MC for many of the treatments was quite large, which illustrates the difficulty of removing moisture from darkwood and the effect of darkwood moisture on total drying time and final uniformity of MC. Honeycomb occurred in all treatments. It was most severe in conventional and high-temperature drying, least severe in air drying, and essentially restricted to the darkwood. Darkwood also tended to collapse and surface check, commencing in the early stages of drying. Selection of logs to reduce the presence of darkwood is recommended. It may even be beneficial to dry flitches with darkwood separately under a different schedule. Thickness Shrinkage of Flitches The flitches for this study were sawed to 1-3/4-inch thickness. Shrinkage was too great to dress the dried lumber to 1-1/2-inch thickness, so the final product was dressed to 1-7/16 inch to achieve fully dressed studs. Thickness target size should therefore be increased to 1-7/8 inch to compensate for shrinkage. Table 5 Analysis of variance for crook, bow, and twist in relationship to sawing and drying method Mean Source df F square CROOK Sawing (S) a Drying (D) S X D Error BOW Sawing Drying S X D Error TWIST Sawing Drying S X D Error a Significant at the 1 percent level. Table 6 Analysis of variance for crook, bow, and twist. The relationship of stud size on sawing and drying Mean Source df F square CROOK Stud Size (Sz) a Sz X Sawing (S) a Sz X Drying (D) Sz X D X S BOW Stud Size a Sz X S Sz X D Sz X D X S TWIST Stud Size a Sz x S Sz X D Sz X D X S a Significant at the 5 percent level. 7

10 Literature Cited 1. The SDR process significantly reduced crook in kilndried studs. Neither SDR nor conventional sawing significantly affected bow and twist. SDR was most effective in reducing warp of studs from upper logs; only 1.3 percent of studs from upper logs were rejected after treatment. 2. SDR air-dried studs were not significantly different in warp from conventionally sawn studs regardless of drying temperature. This was apparently due to insufficient drying. We expect that SDR would have reduced the warp of air-dried flitches if a lower MC had been achieved. SDR is most effective in reducing warp when the final MC is low and uniform. 3. The effect of SDR on warp was independent of kilndrying temperature. Crook values for SDR with either conventional-temperature or dehumidification drying were essentially equal to those obtained for SDR with high temperature drying. 4. After approximately 1 month of stickered storage, the percent of rejected studs from all but one of the treatments increased significantly, mostly because of increased crook. (Most of the increased warp was attributed to postdrying of darkwood.) SDR treatments had the fewest rejects. Dimensionally stable studs were obtained from SDR flitches kiln-dried at high temperature and maximum venting to a uniform 5 to 6 percent MC. 5. Darkwood caused degrade because of warp and honeycomb. Therefore, selection of logs with clear, whitewood ends is recommended in order to reduce the amount of darkwood. 6. High-temperature drying of paper birch flitches caused thickness shrinkage as high as 12 percent. This should be taken into consideration when setting green target sizes for paper birch. Perhaps 1-7/8-inch thickness rather than 1-3/4-inch thickness should be used. Erickson, Robert W.; Petersen, Harlan D.; Larson, Timothy D. Obtaining uniform final moisture content in the high temperature drying of paper birch flitches. Forest Products Journal. 34(2): 27-32; Forest Products Laboratory. Wood handbook: Wood as an engineering material. Agric. Handb. 72, revised. Washington, DC: U.S. Department of Agriculture, Forest Service: Larson, Timothy D.; Erickson, Robert W.; Boone, R. Sidney. Comparison of drying methods for paper birch SDR flitches and studs. Res. Pap. FPL 465. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; p. Maeglin, Robert R.; Boone, R. Sidney. Manufacture of quality yellow-poplar studs using the saw-dry-rip (S-D-R) concept. Forest Products Journal. 33(3): 11-18; Martodam, Dave. Evaluation of Minnesota white birch for potential use in a system 6 manufacturing processwhite birch resource analysis. Minnesota Department of Natural Resources; Miller, R. G. Simultaneous statistical inference. New York: McGraw Hill Book Co p. Northern Hardwood and Pine Manufacturers Association. Standard grading rules. Chicago, IL: Northern Hardwood and Pine Manufacturers Association 1978; 125 p. Rasmussen, Edmund F. Dry kiln operator s manual. Agric. Handb Washington, DC: U.S. Department of Agriculture; p. U.S. Department of Commerce. American softwood lumber standard PS Washington, DC: U.S. Government Printing Office; p. Ward, James; Pong, W. V. Wetwood in trees: A timber resource problem. Gen. Tech. Rep. PNW-112. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station p. 8