lunguleasa-r32

8/11/2019 Lunguleasa-R32

1/6

139

EFECTUL TEMPERATURII ASUPRA

CONTRAGERII LEMNULUI

THE TEMPERATURE EFFECT

ON WOOD SHRINKAGE

Aurel LUNGULEASATransilvania University of Brasov, Romania

Rezumat.Lucrarea prezintcteva aspecte ale influeneitemperaturii asupra contragerii lemnului n general iasupra speciei de fag n special. Dup analizareadocumentaiei existente n acest domeniu se trece larealizarea propriilor experimentri pe aceastproblemise trag concluziile. Accentul este pus pe difereneledintre contragerile radiale i tangeniale, dar se obine irelaia de dependen pentru determinarea contrageriivolumice n funcie de contragerile radiale i tangeniale.

Abstract. The paper presents some aspects of thetemperature influence on the wood shrinkage in generaland especially on the beech specie. After analyzing theexisting documentations it goes to make ownexperiments on this problem and are drawn theconclusions. Emphasis is placed on the differencesbetween radial and tangential shrinkage but here is foundthe relationship for determining the volume shrinkagerelated to the radial and tangential ones.

Cuvinte cheie:contragerea lemnului, efectultemperaturii, uscarea lemnului

Key words:wood shrinkage, the temperature effect,wood drying

1. Aspecte introductiveCopacii pe picior conin a cantitate mare de

ap, sub forma de vapori, lichid i chimicdisociat. Coninutul de umiditate din butenivariaz cu sezonul de doborre, tipul produsuluilemnos (butean, cherestea, furnir, placaj, placi dinachii de lemn, achii, fibre etc.), diferitele pri alebuteanului lemnos (alburn sau duramen, lemntimpuriu sau trziu, cherestea tangenialsau radialetc.). Se estimeaz c 1 m3 de butean proasptdobort conine aproximativ 540 litri de ap[1].

Apa este reinut n structura celular alemnului (v. figura 1), celulcare este alctuitdingoluri celulare i membrane celulare. Apa care segsete n golurile celulare i cele intercelulare senumete ap liber, n timp ce apa din interiorulmembranei celulare (din spaiile intermicelare iinterfibrilare) se numete ap legat. Dupdoborre, dar mai ales dup transformarea ncherestea sau alte produse lemnoase, lemnul verdeva pierde umiditate de pe suprafaa expus la aer.Apa liber se pierde naintea apei legate. Aceasta

ns reduce greutatea lemnului, dar nu afecteaz

dimensiunile sale. Punctul n care apa liber s-aeliminat i a rmas numai ap legat se numetepunct de saturaie a fibrei (PSF) i are o valoaremedie procentualde 30 %. Lemnul care se usuclaun coninut de umiditate sub acest nivel va face screascnivelul contragerii, pe msurce membranacelulara lemnului va pierde umiditate.

Lemnul de fag (indiferent de tipulsemifabricatului sub care se gsete) are ocontragere sczut de 4,5 % pe direcie radial iuna mai mare pe direcie tangenial, de 9,5 %.Contragerea pe direcie longitudinal este aproapeneglijabil, n jur de 0,4 %.

1. Introductory aspectsLiving trees contain a large quantity of water,

in form of steam, liquid and chemically dissociated.The moisture content from logs will vary with theseason to felling up, type of wooden products (log,timber, veneer, plywood, chipboard, chips, fiberetc.), different parts of the wood log (sapwood orheartwood, early wood or late wood, tangential orradial timber etc). It has been estimated that 1m3offreshly felled log contains approximately 540 litersof water [1].

The water is held in the cellular structure ofthe wood (see Figure 1), which is made up of cellcavities and cell walls. The water held in the celland intercell cavities is called free water, whereasthe water inside of the cell walls (intermicelaryand interfibrilary cavities) is called bound water.After felling and especially after conversion intimber or other wooden products, green wood willstart to loose moisture from any freshly exposedsurfaces. The free water is lost before the boundwater. This reduces the weight of the wood butdoes not affect its dimensions. The point at which

the free water has been removed and the boundwater remains is called the fiber saturation point(FSP) and has a medium value of 30 %. Wood thatis dried to moisture contents below this level willexhibit some degree of shrinkage as the cell wallslose moisture.

Beech specie (regardless of the blank type thatis found) has a lower shrinkage of 4.5% on radialdirection and 9.5 % on the tangential direction. Theshrinkage on longitudinal direction is almostneglectable, namely about 0.4%.


2/6

RECENT, Vol. 12, nr. 2(32), July, 2011

140

Figura 1. Localizarea apei n lemn: 1-membrancelular; 2-golurile dintre microfibrele i micelele celulozice (aplegat); 3-lumenul celulei (apliber); 4-goluri intercelulare (apliber)

Figure 1. Location of water in wood: 1-cell wall; 2-cavities between cellulose micro fibrils and micelles (bound water);3-cell lumen (free water); 4-intercellular cavities (free water)

Cnd lemnul pierde ap, echilibrul existent n

membrana celular (dat de forele de atracie irespingere din interiorul membranei celulare) serupe. Mai nti se rupe legtura apei (dipolii OH-)cu dipolii H+ai lemnului i acest lucru va conducela aproprierea micelelor ntre ele i micorareamembranei celulare. Acesta reprezint de faptcontragerea lemnului [2]. Toate modificriledimensionale au loc numai n domeniul apei legate,adicsub punctul de saturaie al fibrei (sub 30 %).

Contragerea lemnului apare cnd apa legatdinlemn scade, mai ales cnd lemnul se usuc. Prininfluena temperaturii de uscare, mrimea

tensiunilor interne i calitatea lemnului obinut dupuscare vor depinde semnificativ, aa c estenecesar analiza influenei temperaturii asupracontragerii lemnului masiv. Contragerea lemnuluise exprim prin coeficieni liniari, pe direcieradial, tangenial i longitudinal i coeficienivolumici. Toi acetia pot fi pariali sau totali, aacum se observn figura 2, pentru cei volumici.

When wood loose water, the equilibrium of cell

wall (given by the rejection and attraction forcesinside of cell wall) is broken, firstly the waterlinking (OH- dipoles) with the H+ dipoles areruptured, and will conduct at the coming near of themicelles and decreasing of cell wall. This is theactually wood shrinkage [2]. All modifications ofwood sizes take place only for bound water range,respectively bellow the fibre saturation point(approximately 30 % of moisture content).

Wood shrinkage appears when the bound waterin wood decreases, especially when the wood dries.By the dependence of drying temperature the size of

internal stresses and quality of wood obtained afterdrying depend significantly, so it is necessary tostudy the influence of temperature on the shrinkageof solid wood. Wood shrinkage is expressed bylinear coefficients, on radial, tangential andlongitudinal direction and volume coefficients. Allof these coefficients can be totally or partially, as itcan see in figure 2 for volume ones.

Figura 2. Coeficienii volumici totali (stnga) i pariali (dreapta)Figure 2. Total (left) and partial (right) volume coefficients of shrinkage

Stevens [3] a investigat n 1963 efectultemperaturii asupra contragerii i a gsit c, ngeneral, contragerea tinde s creasc uor cucreterea temperaturii. Cu creterea vitezei de

uscare contragerea cherestelei va descrete. Toateacestea se ntmplnumai sub 30 % umiditate, care

Stevens [3] had investigated in 1963 the effectof temperature on shrinkage and found that, ingeneral, the shrinkage tends to increase slightly withincreasing of temperature. With increasing of

drying speed the timber shrinkage will decrease. Allthese are happened only under 30% moisture


3/6


141

este sub punctul de saturaie al fibrei. Oricum, sub25 % coninut de umiditate, valoarea contragerii nupare sfie afectatde temperatursau de viteza deuscare a lemnului, conform aceluiai autor.

n 1970 Sitova [4] afirmcatunci cnd lemnul

se usucncet, contragerea lemnului va fi mai maredect la uscarea rapid, elementul de cretere avitezei fiind temperatura de uscare. Explicaia sebazeazpe creterea vitezei procesului de uscare iapariia deformaiilor plastice reziduale pe seciuneatransversal datorit temperaturii nalte. Pentru aelucida acest lucru, experimentrile au fost fcutepe scnduri de brad, pin i fag la temperaturi de 60,70, 80, 90, 100 i 110 C. Pentru a nltura crparealemnului uscat sau apariia altor defecte ale uscriila temperaturi nalte, s-au efectuat cteva ntreruperi

n timpul testrii, prin decuplarea termostatului

electric cteva ore, avnd intenia clarde a eliminatensiunile interne din lemn i de a obine relaxareamaterialului. Uscarea lemnului a pornit de la oumiditate iniialde 70 % (dar contragerea a plecatde la punctul de saturaie al fibrei, de 30 %) pnlao umiditate final de 7 %. Rezultatele au fostsintetizate pentru cele trei specii examinate n figura3. Se observ o curb cu descretere constantpentru fag i o tendin general de descreterepentru rinoase, dar cu o uoar cretere pentrubrad pnla 90 C i apoi o descretere la 110 C.

content, that is until fibre saturation point FSP.However below 25% moisture content the shrinkagevalue does not seem to be affected by temperatureor by speed of wood drying, says the same author.

In 1970 Sitova [4] says that when the wood is

dried slowly the wood shrinkage will be more thanquick-drying, the element of speed increasing beingthe drying temperature. The explanation for that isplaced on speeding up the drying process and theappearance of residual plastic deformation on thecross section of wood due to high temperatures. Toelucidate that, the experiments were done onwooden planks of fir, pine and beech attemperatures of 60, 70, 80, 90, 100 and 110 C. Toavoid cracking of dry wood or other defectsresulting from high-temperature drying, there weredone some interruptions during the tests, performed

by decoupling the thermostat few hours, which areintended to defuse tensions and obtain relaxation ofthe material. Wood drying was determined from aninitial moisture content of 70% (but the shrinkagestarted from the fiber saturation point of 30%) to afinal moisture content of 7%. The results aresummarized for the three examined species in Figure3. A steadily decreasing curve is observed for beechspecie and the general tendency to decrease forresinous wood but a slight growth of the fir specie upto 90 C and then decreases to 110 C.

Figura 3. Influena temperaturii asupra contragerii lemnului (dupSitova): 1-brad; 2-pin; 3-fagFigure 3. Influence of temperature on wood shrinkage (after Sitova): 1-fir, 2-pine, 3-beech

Filipovici n 1965 [3] a stabilit c contragerealemnului depinde de temperatura de uscare. Spreexemplu, piesele de fag uscate la temperatura de25 C au nregistrat o contragere tangenial totalde 11 %, iar dacsunt uscate la 75 C au avut 15,5 %.n consecin, valoarea contragerii crete cu cretereatemperaturii, ceea ce sugereaz c proprietileplastice ale fagului cresc cu creterea temperaturii.Acelai autor a reliefat de asemenea influenadimensiunilor epruvetelor, anume c piesele mai

groase se usucmai greu, cu deformaii plastice maimari, i se contrag mai puin dect cele subiri.

Filipovici in 1965 [3] stated that, the woodshrinkage depends on drying temperature. Forexample, pieces of dried beech at a temperature of25% recorded a total tangential shrinkage of 11%and if are dried to 75% of 15.5%. So, size ofwooden shrinkage increases with the temperatureincreasing, which suggests that plastic properties ofthe beech wood increases with temperature. Thesame author also highlights the influence of samplesizes, saying that the thicker pieces have dried more

difficult, with larger plastic deformation and theywill shrinks less than thinner materials.


4/6


142

2. Epruvete i metodologia de lucruPentru a studia influena temperaturii asupra

contragerii lemnului masiv s-au folosit epruveteprismatice din fag, debitate din aceeai scndur, pea se anihila influena condiiilor de cretere.

Epruvetele au fost rindeluite pe toate feele, au fostsortate cele fr defecte i s aib toate feeleradiale sau tangeniale. Forma i dimensiunileepruvetelor se vd n figura 4.

2. Samples and working methodologyIn order to study the influence of temperature

on shrinkage of solid wood, there were usedprismatic beech specimens, cut from the same boardto annihilate the influence of growth conditions.

The samples were planned on their faces, weresorted on defects and to have all faces perfect radialand tangential. The shape and sizes of the sampleswere shown in Figure 4.

Figura 4. Dimensiunile epruvetelor lemnoase: R - radial; Tg - tangenial;L - longitudinal

Figure 4. Dimensions of wooden samples: R - radial; Tg - tangential;L - longitudinal

n timpul experimentrilor au fost analizaicoeficienii totali ai contragerii liniare a lemnului,pe direcie tangenial i radial. Msurareadimensiunilor pe seciunea transversal s-a realizatcu un ubler, avnd o precizie de msurare de 0,02mm. Pentru o mai bun sistematizare a valorilor,epruvetele lemnoase au fost grupate n mai multecategorii, anume pentru msurtori tangeniale iradiale, n grupe de 10 buci. naintea msurrii,

epruvetele lemnoase au fost puse n ap, 2 cm subnivelul apei, i au fost inute pn cnd acestea auatins nivelul punctului de saturaie al fibrei PSF,respectiv pn cnd s-a trecut de la starea apeilegate la cea a apei libere din lemn.

Aceastcondiie s-a bazat pe urmtoarea relaiede calcul:

During the experiments it was taken intoaccount the total coefficients of linear shrinkage, ontangential and radial direction. Measuring thedimensions on the cross section was performed witha caliper, with measurement accuracy of 0.02 mm.For a better systematization of all values thespecimens were grouped into many categories,respectively for combined tangential and radialmeasurements in groups of 10 pieces.

Before the size measuring, the samples wereplaced in water, 2 cm below the water level andhave been kept until they reached the fibersaturation point FSP, until when it moved their starefrom the area of bound water to free water.

This condition was based on the followingrelationship for calculus:

( )Ummu + 10 [g] (1)

unde:mu- este masa epruvetelor umede, exprimate n g;m0- masa epruvetelor absolute uscate, n g;U- coninutul de umiditate, exprimat n g/g.

Spre exemplu, dacmasa epruvetei uscate estede 25 g i dup introducerea n ap se doretedepirea punctului de saturaie al fibrei PSF de30 %, masa trebuie sdepeasc32,5 g. Pentru aasigura validitatea condiiei de mai sus pentru totgrupul de epruvete, condiia se stabilete pentrufiecare epruveti se reine cea mai micvaloare aepruvetei cele mai defavorizate [5, 6, 7]. Sefolosete relaia urmtoare, iar masele pentru toateepruvetele trebuie s respecte aceast valoare dereferin:

where:muis mass of wet wooden specimen, in g;m0- absolutely dry mass of wooden specimen, in g;U- moisture content, in g/g.

For example, if the dry specimen mass is 25gand after introducing in water it wants to exceed thefiber saturation point FSP of 30%, the specimenmass must exceed the value of 32.5g. To ensure thevalidity of above condition for the entire group ofwooden samples, the condition was established foreach specimen and it retained the lowest value ofthe specimen most deprived [5, 6, 7]. Nextrelationship is used and the subsequent weightingsof all specimens had to respect this last benchmarkvalue:


5/6


143

Ma> max{m1, m2, , m10} (2)

Duptrecerea tuturor epruvetelor peste punctulde saturaie, acestea sunt scoase din ap, sunttamponate pe o hrtie sugativ pentru nlturarea

excesului de apde pe fee, apoi li se msoarceledou dimensiuni ale seciunii transversale i secntresc pentru determinarea masei. Mai departe,epruvetele au fost plasate in etuva de laboratorpentru uscarea la 0 %. Temperatura a fost reglatla120 C i 60 C, pentru a vedea dac existdiferene ale contragerii funcie de temperatur.

After passage of samples over the fibersaturation point, they were removed from the water,were buffered on blotting paper to remove excess

water on their faces, then they were measured thetwo dimensions of cross section and then weighedall specimen for obtaining their mass. Further,samples were placed in laboratory oven to dry down0%. Drying temperatures were set at 120 C and60 C to see if there are differences of woodshrinkage depending of drying temperature.

3. Calculul i exprimarea rezultatelorRezultatele msurtorilor i a calcului au fost

introduse n tabele, valorile cumulative fiindprezentate n tabelul 1. Relaiile pentru

determinarea coeficientului de contragere radial(Sr), tangenial (St) i volumic (Sv) a lemnuluimasiv au fost urmtoarele [8]:

3. The calculus and expression of resultsThe results of measurements and calculations

were introduced inside of tables, cumulative tablebeing introduced in table 1. Relations for the

determination of radial shrinkage coefficient (Sr)tangential (St) and the volume one (Sv) were the nextones [8]:

[%]100

[%]100

max

minmax

max

minmax

=

=

t

ttS

r

rrS

t

r

100tr

rtvSS

SSS

+=

(3)

unde (figura 5):rmaxi tmaxsunt dimensiunile radiale i tangeniale

maxime;rmini tminsunt dimensiunile radiale i tangeniale

minime.

where (figure 5):rmax and tmax are radial and tangential maximal

dimensions;rmin and tmin are radial and tangential minimal

dimensions.

Figura 5. Modelul contragerii lemnului masiv: lmax, rmax, tmax- dimensiuni longitudinale, radiale i tangeniale maxime;lmin, rmin, tmin- dimensiuni longitudinale, radiale i tangeniale minime

Figure 5. Sketch for solid wood shrinkage: lmax, rmax, tmax- longitudinal, radial and tangential maximal dimensions;lmin, rmin, tmin- longitudinal, radial and tangential minimal dimensions

Au fost efectuate i alte experimentri cu altetemperaturi intermediare, respectiv 80 i 100 C,dar rezultatele obinute au avut valori neglijabile, cudiferene sub 3 %.

There were made other testing withintermediate temperatures of 80 and 100 Celsiusdegrees, but the results have negligible values, withdifferences under 3%.


6/6


144

Tabelul 1. Valorile contragerilor radiale, tangeniale i volumiceTable 1. Values of radial, tangential and volume shrinkage

Radial shrinkage, % Tangential shrinkage, % Volume shrinkage, %No. 60 C 120 C 60 C 120 C 60 C 120 C1 5.2 5.4 10.1 9.7 14.77 14.57

2 5.6 5.3 9.8 9.6 14.85 14.593 5.4 5.3 9.6 9.8 14.45 15.044 5.3 5.2 10.2 10.2 14.95 14.865 5.5 5.3 10.0 10.1 15.55 14.59

Mean 5.4 5.3 9.9 9.8 14.91 14.72

4. Concluzii finaleLucrarea abordeaz o chestiune de finee a

influenei asupra contragerii lemnului, respectivinfluena temperaturii asupra contragerilor liniare ivolumice. Rezultatele proprii au artat c influenatemperaturii asupra contragerii lemnului este uordescresctoare. Aceast influena uoar este uordovedit de rezultatele contoversate existente nliteratura de specialitate.

Lucrarea s-a concentrat pe contragerile liniareradiale i tangeniale, considernd c acestea voravea diferene mari n funcie de temperatur, darrezultatele obinute nu au confirmat acesta ipotez.

S-a calculat de asemenea contragerea volumic,dar rezultatele au fost tot nesemnificative. S-a dovedit

n acest fel ctemperatura nu este un factor importantal valorii contragerii lemnului, dar viteza de uscare a

lemnului, dimensiunile cherestelei, specia lemnoasigradientul de uscare pot fi importani. Un factor foarteimportant este viteza de uscare, care prin tensiunilecreate, n special prin apariia unor alte defecte lasuprafa, cum ar fi colapsul i cementare superficial.Aceste defecte grave nu vor lsa lemnul sse contragliber, cauznd n acest fel alte defecte, precumcrpturile i fisurile.

4. Final conclusionsThe paper addresses a sensitive issue of

influence on wood shrinkage, respectively theinfluence of drying temperature on linear andvolume shrinkage. The own results showed that theinfluence of temperature on shrinkage is slightlydecreasing. This slightly influence is easily provedby the controversial results existing in literature.

The paper has focused on linear tangential andradial shrinkage, believing that they will have bigdifferences related to temperature, but the result didnot confirm this hypothesis.

It has been also calculated the volumeshrinkage, but results still remained insignificant. Itturns out that temperature is not an important factorof wood shrinkage value, but wood drying speed,

size of timber, wooden specie and rate of drying canbe important. A very important factor is the dryingrate that by the tensions created especially by theappearance of such defects on the surface ascollapse and surface cementation. These worsedefects will not let wood to be free subjected atshrinkage, causing a lot of others defects as cracksand fissures.

References1. Coma, Gh., Brenci, L.M.:Modern technologies for laminated parquet. Proceeding of Conference Modern technologies, quality

and restructuring, 2003, p. 41-44, Technical University of Moldova, ISBN 9975-9748-0-5, Chiinu, Moldova2. Kadlec, J.: Biological Wastes from Forestry Industry. Proceeding of Conference Tends of wood working, forest and

environmental technology, 2006, Technical University in Zvolen, Slovakia, ISBN 80-228-1650-7, p. 101-105,3. Filipovici, J.: Wood study. Vol. I, II. Ed. Didactici Pedagogic, 1965, Bucharest, p. 44 (in Romanian)4. Boieriu, C., Lica, D., Mihailescu, T.: Technology of furniture. Transylvania University Press, 2008, Brasov, ISBN 978-973-598-

120-4 (in Romanian)5. Moura, S., Abella, D., Anjos, O.: Evaluation of wood basic density as an indirect measurement of the volume of wood raw

material. Proceeding of International Conference Wood science and engineering in the third millennium, June, 2007, Brasov,ISSN 1843-2689, Transylvania University Press, p. 72-78

6. Cmpean, M.: Study concerning the hysteresis of sorption and desorption for wood of various species and dimensions ,Proceedings of Conference Wood science and engineering in the third millennium, 2009, Transylvania University Press,Brasov, p. 52-57, ISSN 1843-2689

7. Kollmann, F., Cote. Jr.: Principles of Wood Science and Technology- Solid Wood(vol I),Springer, Berlin, New York, 19688. Bekhta, P., Potapova, O., Sedliacik, J., Hijiroglu, S.: Technology of low-temperature veneer gluing in the manufacture of

plywood. Proceedings of Conference Wood science and engineering in the third millennium, 2009, Transylvania UniversityPress, Brasov, p. 270-275, ISSN 1843-2689

Lucrare primitn Mai 2011 Received in May 2011

lunguleasa-r32

Documents