.

Ii

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(ISSN 0735-6161)

Volume 33 ..

JOURNAL OF THE

SOCIETY OF

W(XJDSCIENCE AND TECHNOLOGY

;i~i~~~Jt

CONTRIBUTION FACfOR OF WOOD PROPERTIES OF nlREE POPLARCLONES TO STRENGTH OF LAMINATED VENEER LUMBER

Fucheng BaoProfeISCK

Research IDItitute of Wood bxIusttyatinese Academy of Forestry

Beijing 1(XX)91, PRC

Feng FuProfealOr

Reacan:b IDstihite of Wood IIMIuSb'yChinese Academy of Foratty

Beojing 1(XX)91, PRC

Elvin T. Choongt

and

Chung-yun HsePrincipal Wood Scientist

SoudIem ~ S~USDA, Forest ServicePiJIevil1e. LA 71360

(Received (ktober 1998)

ABSTRACI'

The tenn "Contribution Fac1or" (c.) was introduced in this p8per to indicate die contributionratio of solid wood properties to laminated veneer lumber (LVL) strength. Three poplar (Populusap.) clones were studied, and the results showed that poplar with good solid wood pr()perties baahigh Contribution Factor. The average Contribution Factor of Poplar 69 (PopullU delt0id4,r CV. 1-69/SS), Poplar 72 (P. euramericana cv. 1-72/S8), and Poplar 63 (P. deltoide,r cv.I-63-S1) was 76.2%,68.6%, and 66.1%, ~pectively. The avenge Contribution FICtor of dte ~ clonea for shearstrength, modulus of elaaticity (MOE), and impact fOUgbDesS wu 8IJIXOximaldy 80'11, which ishigher than that for modulus of rupture (MOR), compreaaive strength, and hardness. The averageContribution Factor of die six properties tested was highest in Poplar 69 (76.2%) and dte lowest inPoplar 63 (66.1%), indicating that dte Contribution FICtor is positively affected by soUd woodproperties. Densification also significantly affects L VL MOE in Poplar 72, as compared to that ofPoplar 69. Poplar 63, bowever, sbowed highest improve~t in MOR strength from solid wood toL VL and also highest specific L VL MOR. even though it baa dte lowest solid wood MOR &mODIthe ~ clones.

Keywords: Cootributioo factor, PopulIU Ip., solid wood, I__rnioated v~ Iumia. wood~,strenath, plantation.

t Mcmba' of SWST

w..w ~ ,., ScI 33(3), _I. w. 34S-3520 _I by die Scx:iMy of Wood ~ .. Ted8qy

ProfessorSchool of Forestry, Wlldlife, and Fisheries

Louisiana State Univenity Agricultural Center88tO11 Rouge, LA 70803

346 WOOD AND FIBER ~CB, JULY D)I. V. 33(3)

all veneers glued parallel to the longitudinalaxis of the billet; therefore, L VL was chosenas the end-use product, whose properties areusually superior to those of solid wood.

INI'RODucnON

Plantation forests are important for the sus-tainable development of forestry. China hasmore than 33 million hectares of planted for-ests to increase its fiber resource and improveits ecosystem, and is now a world leader inafforestation programs (Wang 1995). Efficientuse of this resource, however, requires com-prehensive understanding of the relationshipbetween wood properties and processing ofplantation-grown wood for end-use products.There is voluminous information in the liter-ature on the growth-quality relations of Poplar(Populus sp.) wood, which Uu (1988) hascompiled, but only a few studies have beenconducted addressing the effects of silvicul-tural treatments and/or juvenile wood on prop-erties of wood composites (Pugel et al.1989a.b; Wasniewski 1989; Kretschmann etat. 1993). Recent research has focused on thesilvicultural effects of specific products, i.e.,laminated veneer lumber (Shupe et aI. 1997a),plywood (Shupe et at. 1997b; Baa and Jiang1998), and fiberboard (Shupe et at. 1998).These investigations, however, were per-formed mainly on the southern pines (Pinusspp.), and did not address the effects of plan-tation-grown genetically engineered specieson the performance of specific end-use prod-ucts. With regard to poplar plantation wood,preliminary studies have been done to deter-mine the relationships between wood proper-ties and end-uses (pu and Bao 1999; Bao etat. 2000); however, these studies did not iden-tify the "Contribution Factor" of propertiesother then the inherent solid wood strength.

This study was undertaken to investigate thecontribution ratio of solid wood (SWD) prop-erties to laminated veneer lumber (LVL)strength. The objective was to determine therelationship between SWD and L VL proper-ties as observed in poplar clones and to pro-vide a scientific basis for better utilization ofpoplar plantation resources. Among the vari-ous types of wood composites, L VL has morestructural similarities to natural wood becauseit is a wood-matrix laminated assembly with

MATERIALS AND MEmODS

Three clones of a poplar plantation, i.e., Po-pulus deltoides cv. 1-63/51 (poplar 63), P. del-toides cv. 1-69/55 (poplar 69), and P. eura-mericana cv. 1-72/58 (poplar 72), were select-ed. Two ll-year-old trees were obtained foreach of the three clones from a plantation for-est having a stand density of 270 trees per hagrowing in a sandy soil site and located at11 r E longitude and 38° N latitude near theYangtze River at Ann Qing, Anhui Province.Logs 3.5 m in length with base diameter from24 to 26 cm were cut from each tree. To min-imize variability, test samples for detennina-tions of wood and L VL properties were ob-tained from the same log. Each log was cross-cut into two parts-the lower part (1.3 m) wasused to manufacture into L VL, and the upperpart (2.1 m) was used to detennine solid woodproperties. The L VL in this study were madefrom 2.61-mm-thick veneers.

L VL manufacture

The veneers used in the manufacture ofLVL were peeled with a laboratory peeler. Af-ter peeling, the veneers were clipped to 50-6O-cm size and dried to a moisture content be-low 6% in a "roller" dryer. L VL was preparedat the Research Institute of Wood IndustryLaboratory in Beijing, China, using a 600- x6OO-mm plywood press. The plies consisted ofthirteen layers of 2.61-mm-thick veneer. Theresin spread was 300 g/m2 (liquid basis) ofunextended phenol-formaldehyde adhesive(52.7% solid content, 700 MPa viscosity,10-12 pH). The press temperature, pressure,time, and spacer were 1 ~C, 1.5 MPa, 20min, and 25 mm thick, respectively.

Specimen preparation and testingThe various SWD and LVL properties test-

ed are shown in Table I. In loading direction,

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WOOD AND FIBER SCIENCE. JULY 2001. V. 33(3)348

higher values than those of SWD of the samepoplar clone. In impact toughness, the L VLwas better than that of SWD wood in the ra-dial direction, but not in the tangential direc-tion. It is further noticed that the impact tough-ness is greater in the tangential than the radialdirection in SWD, but it is the reverse in LVL.Radial toughness was higher than tangentialtoughness due mainly to the L VL structure ofmultiple gluelines. When L VL was impactedradially, the gluelines were in series with oneanother; thus only the wood veneer in the con-tacted L VL surface absorbed or buffered theimpact energy. On the other hand, when L VLwas impacted tangentially, the alternate ve-neers and adhesive films were in parallel withone another, therefore, the adhesive restrictedthe absorbing action of contiguous veneers toreduce the impact toughness.

A comparison of the Contribution Factorsof SWD strength to those of L VL strength indifferent clones is shown in Table 3. Amongindividual properties, the average Cf of thethree clones for shear strength, MOE, and im-pact toughness were near 80%, whereas thoseof the other strength properties were muchlower (i.e., 72.9% for MOR, 54.9% for com-pressive strength, and 61.2% for hardness).The differences in c, in individual clones aresummarized as follows:

1. In shear strength and MOR. the c, in bothtangential and radial directions were high-est in Poplar 69, and lowest in Poplar 63.

2. In compressive strength, the Cr showed thesame trends as in shear strength, and theaverage c, in the longitudinal direction wasalmost 30% higher than in the tangentialdirection.

3. In MOE, the c, of Poplar 69 were highest,but those of Poplar 72 were the lowest.

4. In impact toughness, the C,s were calculat-ed only in the radial direction but not in thetangential direction because the tangentialtoughness of solid wood was greater thanthat of L VL. When L VL was impacted inthe radial direction, the Cr was highest inPoplar 63 and least in Poplar 72.

349BcIo - aL-CONTRIBUTION FACTOR OF WOOD PROPER11ES TO STREN<mi OF LVL

5. In hardness, Poplar 72 had the highest c"and Poplar 63 the least.

The Contribution Factors in individualproperties varied greatly among the clones.Even though a large portion of the L VLstrength originated from solid wood proper-ties, the differences in Cf that existed amongthe various mechanical properties cannot beexplained entirely by the effect of wood prop-erties alone. An improvement in L VL strengthis also due to the contributory effect of severalvariables, such as adhesive and manufacturetechnique. Consequently, the c, were furtherevaluated to identify the most probable causeof the L VL performance. Since the bendingstrength is considered most critical to L VL'sstructural performance, MOE and MOR werechosen for the analysis. The relationships be-tween c, and average MOE for the threeclones are shown in Fig. I, and the relation-ships between Cf and average MOR are shownin Fig. 2.

A comparison of the strength propertiesamong the poplar clones shows three types of

relationship:Case I Lower SWD strength and smaller c,

value with lower LVL strength (e.g.,

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Densification effect on L VL MOE was sig-nificant in Poplar 72, as compared to Poplar69. The most surprising results on MOR, how-ever, were in Poplar 6~. With the lowest SWDspecific MOR. Poplar 63 yielded the largestimprovement in L VL MOR and resulted in thehighest specific L VL MOR.

summarized in Table 5. An analysis of vari-ance indicated that the differences in specificMOE of solid wood between Poplar 69 andPoplar 72 were not significant. However, theLVL's specific MOE (24.81) of Poplar 69 wassignificantly lower than that of SWD woodspecific MOE (27.83), indicating that the den-sity of L VL from Poplar 69 had the least effecton LVL MOE. This phenomenon supportsCase 3, i.e., the higher compaction ratio ofPoplar 72 resulted in higher improvement ofL VL strength. The analysis of variance alsoshowed that the specific LVL MOR (153.07)of Poplar 69 was significantly lower than thespecific SWD MOR (177.17), but the differ-ences in specific MOR between L VL and solidwood of Poplar 72 were not significant, indi-cating again that the density effects on L VLMOR were less in Poplar 69 than Poplar 72.The most surprising results on MOR. however.were in Poplar 63. Even though the SWD spe-cific MOR had the lowest value (132.13 inTable 5), Poplar 63 yielded the largest im-provement in LVL MOR (70.22% in Table 4),and resulted in the highest specific L VL MOR(169.27). The reason for the high specific LVLMOR of Poplar 63 is not clear in this study;nevertheless, the potential of improving L VLproperty with poplar clone needs further in-vestigation.

REFERENCES

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-, -, AND S. Lw. 2(XX). A IIKKIeling ~to the relabonship between pIant8Iion poplar propertieaand qualities of Verl«r and plywood. Scientia SilvaeSinicae 36(1):91-96.

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plywood. JAS (JapaIIeIe Agricultural Stalldald), Tokyo,Japan.

CONO-USIONS

The tenD "Conbibution Factor" can beused as an index to evaluate the conbibutionof solid wood strength to L VL strength. It isdirectly related to die inherent properties ofwood. A genetically good IK>plar tree had highconbibution factors, and vice versa. The Con-bibution Factors in shear strength, modulus ofelasticity, and impact toughness were near80%, and higher than those in modulus of rup-ture, compressive strength, and hardness. Theaverage Conbibution Factors of die six prop-erties tested were highest in Poplar 69, andlowest on Poplar 63, indicating that the Con-bibution Factor is positively affected by solidwood nrooerties.

-, 1991 S~ laminated lumber. lAS (11p8DC5eAgricultural Standard). Tokyo. lapan.

~. D. E.. R. C. Mcxx>Y. R. f: PBu.aIN. B. A.BENDnEN. 1. M. CAIm.u. R. H. McAU$1B. AND D. W.SHARP. 1993. Effect of various proportions of juvenilewood OIl 1amiDIIed ~ lumber. Res. Pap. PPL -RP-521. USDA Forest Service, Forest PrOO. Lab.. Madison,WI. 31 pp.

NAT.:.cAL T8::HNQJE M~ BUREAU ~ PRC. 1991.Test medMJdI f« wood physical aIKI D.:ban~l ~ertiea. GB 1927-1943-91 (a.i~ National Standard).Beijina PRC.

PtxEL. A. D.. E. W. PaD AND C. Y. H.v.. 19898. C0m-posites from southern pine juvenile wood. Part I. Panelf8bricatiCHI and initial ~uiXo--. ~ PrOO.l. ~I):29-33.

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SHUPE, T. f:. C. Y. UsE, L. H. GROOM. AND E. T. CHOONO.I997L Effect of silviculblral ~ and ~ layupOIl ICMM mecbanicaI p-~-- of loblolly pine LVL.Foreat PrOO. 1. 47(9):63-69.

352 WOOD AND FIBBR SCIENCE, .JULY D»I. V. 33(3)

IIf-.1IuiIII IX8:tices in die United S USDA Fc.atSa-vK:e AgiX:. HaD(fxxIk 640. Wasbingtm. oc:. p. 30.

WASNlEWSu, J. L. 1989. Evaluation of juvenile wood IIKiits effects on Douglas-fir stnx:tura1 composite panels.pages 159-173 in 1: M. Maloney. ed. Pnx:. 23m WISb-ingtoo State Univ. Intern. Particl~omposite Ma-terials Symposium, Pullman, WA.

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