Dynamic Control of Moisture During Hot Pressing of Wood Composites

Cheng Piao, Todd R Shupe, and Chung I? Hse

Abgtrsct Hotpmssingfssnhportantsfepinthe~

ture of wood composites. In the conventional pns- ing system, hot press output often acts as acanstrslint to~pmdudian.Sevleredryingofthefurnish bg*, gwicles* flab, or fibers) required 'by this pm- ~ S U ~ ~ ~ t h e m a n ~ c o s t a a d g . l e a t e s ~ i F p o ~ ~ ~ o r t s d ~ ~ r g a n i ~ armpoatnds, which are a main source of air pollution b the finest products in- Wood composites madeinthem~tional~hawhfghinteKnal ~ a n d l o w d i m e n s i o n a l s t a b i t i t y d ~ . T h e dedqmmtdapressingtechndogytoimpmthe ~ i s ~ t e d b y t h e n e l e a ~ e ~ f v a p a r p r e s s u r e during hotpmssing. A perfmated caul system may en- lage the area for the moisture to escape f k n the baardrnatduringpressing,andand,makepossiMe high moiaum content (MC) ~aessing.% objectives oftbis!3tudyareto:

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I. invedgatetheMCrn~entthughdynamic CQntrnl of moisture in hot pnssing,

2. dammine the thennal conductivity, perambiz- ity, speci6c heat of the major wood compos- ites (medium density fibedmd [MDn part- icleboad, and oriented ~~ [OSBI),

3. d e V i ? l o p ~ ~ ~ f 9 r e a c h p r e s s i n g s tagedthe~t idpress isgprocess ,

4. devebpmathematicalm&forthevacud perforated platen pnsbg process to define tempature, m m and vapor pns-= p- h t s in a hot pmdng cycle, a d

5. ~ t h e v a a l u m ~ a n t h e p h y s i c a l d ine&nicalperfonnanaofthesedcom- pasites.

A p l e l i m i n a r y s b r d g o n ~ p l a t e a s ~ that the rate of core tempera- changes lwneased with inueasing flake MC and number of platen holes. Modulus ofrup~~e (MOR) was highest when the MC was at 8 percent and d e c m s d thereafter as MC in- creased forail three amounts of platen holes. Internal b o n d ~ ~ w i t h t h e i n c l . l e a s e o f p l a t e n hde number per unit area at all MC Jd except the 2 p e m e n t M C W a n d ~ w f t h t h e i n ~ o f fiake MC levels. Sbis study showed that the @rated caulhadthefuactioaofreleasingmoistureandvapor presure during hot p~pgging.

~ a r d ~ J 9 r i n g B o t ~ In the man- of dry-process wood ampas-

ites. min-coated w a d furnishes me consolidated into panels between two heated platens. The moishu'e i n a m a t i s v a g o r i z e d d ~ i n t h e ~ c a l and horizontal directions from' the mat rndaces. Since mat edges are the only areas exposed to the out- side enviroment, vapor presswe is formed in the mat and is responsible for problems such as low care densityandintemaZbond(T]B)strength,blawsandde- lamination, d resin wash-in and wash-out effects. To avoid these pblems, woad furnishes are dried to a low moisture content (MC). The drying process also d t s in volatile c#.ganic compound (VOC) emis- sions. The low MC nquhmcnt of the conventional pressing system lowers the productim of the furnish dryer as well as the entire production line. The rigid requbnent on mat MC makes the safety window of the conventional process very nanowand substantial ]assesawincurred Thesolutiontothis problem re quires the use of furnish MC that 19 higher than what iscurrmtlyusedinc~mm~plamsandanunderc standing of the mechanism of heat and mass transfer during~pressing.

The moisture in a mat of a typical dty-process w o o d ~ t ~ ~ p r l n s a r i l y c o l n e s f r o m ~ sources, i.e., the furnish, the resin applied to the fur- nish, and the mndensatfon reacton after the rcesin ams. h a typical manufacturing proce%s, laat MC fiDmthefudshmaybebetweffl2~tto6pcr- cent based an m d t y furnish weight. For stmcmd pels, wakr-bsed phenol-fddchyde (PF) resin is widely used. The solid content of PF resin is about 45 to SO percent; that is, if 1 percent mare of solid msin is added to a mat, more than 1 pacent of water is also added to the mat. The typid resin conteat of a conventional oriented strandboard (OSm manufao- twingprooess b 3 to 6 percent. That means that about 3 to 6 parent of water is added to the mat along with the resin addition. For the third so-, a small amount of extra moisture is added to the board mat by combjning two resin molecules and splitting off one watermolde. The total MC of a mat is from 7 to 12 pement, depending on the particular process.

3n the pmces of consolidation, moisture in a mat fkstEhangestovaporraderheatand~md migrates to the cooler anas of the mat am, edges, and comers along with volatile d v e s of the wood furnish and nonreactive adhesive mmwnents

(Kamh and Casey 1988a, 1988b). The fow edges and axmm am the ody amas h m which the mobtun? can escape during hot pressing. Thus, mobtm, tem- ~ , a a d v a p o r ~ ~ t 3 1 t s m f o r m e d fram face to core and from con to edges (Maloney 1977, Stickler 1959, !hcbdand 1962). The mat edges are d compared to the total surfaoe area of the b O g T d , ~ , o n l y a s m a l l a m a r n t o f 1 1 ~ ) ~ e s - capes thmugh the edges and oomm; most moistwe is released thxwgh the psnel Eaces upon opening of the hot press (Maloney 1977). Since excessive m o b tun may cause blows or delamination, the MC in a conventional process is r i g o d y controlled in a low ranseh&dty iqs .

MC and its distribution are important hctom gav- e the pmperties of wood composites. Heat and moisture soften the wood furnish and facilitate plasticization so the wood can be bent without break- ing. It is reported that the &mum condition for wood deformatkin is 15 tr, 20 percent MC at 100% (Spelter 1999). The MC of a oonventional mat is nor- mally below this range and high swebgwould be ex- pected after the panels retake moistrim Rice (1960) found both bendhg properties and dhensional sta- bility p q m t i e s are impmved by inaedng the mat MC from 9 to 15 percent. Moistum may speed the heat tmnsfer from face to core (Kamke and Casey 1988b, Soemin 1974, SMckler 1959). Drying wood materi$ls is the dominate s o m of VOC emissions frwr the wood products indw (Englund and Nussbaum 2000). Englund and Nussbaum (2000) showed that emissions of tapenes and VOC per wendry weight rapidly inuwem when the sawdust MC is reduced to below 12 percwit (Granstrom 2003). Also* high mat MC may reduce closing time (Maloney 1977), board moistmy adsorption and thickness swelling (Maku and Hamada 1955), and internal strain (Kelly 1977, Sucbs?and 1962). MC a d d y influences other pm- ptdes. Excessive moisture may cause adhesive cur- ing pmhlems, longer pxxssing time, low care density, and thus low i n d bond (IB) strength (Johnson and Karnke 1994, KeUy 1977, Malmey 1977). High MC k#ds to high gas press= in the mat (Kamke and Casey 1988b). If moisture Is not deased sufficiently during pFwsiag, blows or delamination can occur: Thae are techniques to reduce or ehnimte the

negdw eflects of MC and take advantage of its posi- tive effeds. Several endeavors have been 3mplemm- tedtoachi~vethisgdOnetechniqueistodistribute higherrnoistutreinthefacethminthecaretop duce the steam-shock effect (Sosnh 1974, Strider

1959). Steam-shock is an advmce h both pressing theory and technology. Strldih (1959) showed that the heat transfer mte increases with increasing faoe furnish MC, but modulus of ruptun! (MOR), modulus of elasticity (MOE), and IB 1tnmgt.h decrease when the corn MC is at 9 mt.

Steam-injeaion pressing was designed to reduce pnssine time d frnprwe dimensional gtabiliity of wood composites. It hcludes open and c l d sys- tems. Both systems use perforated platens to heat the mat thmugh hot stepun that is ejected fmm the holes. Intheclosedsystem, thematissealedandconsali- dated under elevated temperature and pnsme (Gei- mer and Kwon 1999, Geirner et al. 1998, Shen 1973). Attheendofsteaming,pressureis~leassdandthe~ mainhg steam is e x h a d to the atmosphere. In the open system, mat edges arc exposed to the envir9n- ment, as in the conventional pressing pmss. Steam is injected~separatelyheatedphplatensandthematis consolidated by the heat fmm the pktens and the steam (Ceimer and Kwon 1999, Hse et al. 1991, John- sonand Kamke 1994). Composite panels p d with the steam-injection method have high dimensional stab- This was attributed to the combination of W e plasticization, Egrh flow, and chemical modifi- cation (Geimer and Kwon 1999). However, high MC in a steaminjected mat d t s in poor mechanical pmpdesofthemat. Thisalsoistheresultintheam ventional pressing process when the mat has high a MC (Palardy et al. 1989). The solution to the pzoblem of high mohtum pness-

ing is dependent on the release of excessive moistme i n t h e ~ e o f t h e h o t ~ p r o c e s s . Inatypical ~cycle,thesteamgressurp!ishi@estinthe middle ofa mat and diminishes toward hedges, whemthesteamisre1eased.ItiS~tedtbatone arb& ofthetwomatfaasmay beusedasavenues forvaprtoescapefromthematd hotpmsbg wenlarge the mktwxscaping m. 7 e way to im- ~thisfstohavehdesintheplatemsathat the mqmatedsteammaybe~kasedthmughtheholes &o the atmwphem while the board is being hot -. M d a l b g o n h e r t r a d s s l w s ~

Heat transfer in a medium or between media is due to tempera- differences. T h e are thrree modes af heat tram&, i.e., conduction, convection, and diation. If heat ttanski- occurs across a ma dium, then the heat is tmmferred by conduction. Convection heat t rader occurs between a surface

and a flowing fluid when they are at Wrat tan- -. Thermal radiation occurs as a surEace emit* energy in the form of dectmmagnetic waves. Allof thesa transfermodes are present during hot pnssing of wood composites.

Numerous studies have been conducted on the de- velopment of mathematical models of heat and mass transfer in hot pressing. Bolton and Humphrey (1988) conductad a review af the literatun? on heat and m o m transfer. MacLeau (1942) and Car- ruthers (1959) measured the tempemwe changes in plywood during hot pmsing. Kull calculated heat d e r in Elakeboard and b d t a onedimembnal model (1988) to predict the tempemtum changes in the mat. Since heat and moisture transfer in winpos- ites during hot pmssing cxxws in three dimensions, ollbdimensional models can d y @ct &mois- ture and temperature changes in the vedcal direc- tion. The onedimensional model cannot accurately predict moisaire and tempera- changes in a mat. Bowem (1970) evaluated the convection effects, and Cefahrt (1977) and Kayibn and Johnson (1983) in- cluded convection effects in their ontxhemional models. Humphrey and Rolton (1989) studied the heat and

mass Sansfer in pressing of partidebad. A ma& matical model was developed using a finite ~~ method to predict temperature, MC, and vapor pres- sure changes in the middle of the p-ticlebcmrd mat during pressing. Both conduction and mmeaion were included in the madeling and chmlar baards wemz used to simulate dmedhensional hot-press- ing ~ t i o z l s . The initial temp3rature in- predidedbythemoddf3tdwiththeeperimental resultr. T h e d ~ t r e n d o f ~ 1 1 p o r ~ p r e - dicted by the model agrees with the memud +, b&thepdktedvaporgressurevaluesexdtheex- peiimemtal ones often by a factor of thee.. The differ- ence was partially attributed to the long flow paths and the h o e ofpress dosun in the model. The mockl fa i i edto~c t thesecand~intemper- a~andwash~donlybrtheinitialandsub quent v a m o l l stages.

Kayihan and Johnson (1983) developed a one- dimensional theoretical model fbr heat and moisture tmhr when pressing particleboard. The model was based on three simultaneous systems of equations de- rived by Luikov (27). Both conduction and mvection were included in the model. The model divided a half of the mat thickness into a n u m k of consecutive corn-tJ, each of which was divided into two

phases according to the solid to void d o : void space aradwodtsub&nce.Tofnchrdethebulkflowofva- porintothernade1,aItlirdngaw;lsusedasamea- suretosimulatedebulkmavlementhmhigh-u, low-pmme regions. The lateral removal of vapor was tnated as %&age" in the m d The model can b e u s e d t o p r e d i d b o u n d m b ~ , ~ ~ ~ m o i s - me, vapor pmsme, density, mpor mafsture, and to- tal pressw pnrfiles at diikmt mat depths. The tem- peratwedropginte~~ll~oftenrpem~pmfifffinthe expdments with fibe&od are consistent with the concept of "lealdnf fntrudud into the modeel. Ikd et al. (2001a, 2001b) developed a finitcelanent

m o d e l t a ~ c t t b e ~ ~ t u n ? , g a s g r e s s u r e , MC, and VDPs of wood and straw composites. Some heat conduction and convection jmmmters = ob- tained from apmhents and ovfPers wem taken fbm the litemhue. The pmdicted d t s of temperature, va- p a r y , a d MC agreed well with the -t.

Harfess et al. (1987) developed a one-dimeasional nmmical model to predict the density profile of particleboard. The model that Included heat and mass transfer simulated the physical and mechanical pnx?esses that occur in the press and mat system. The dosing pmams was modeled as a d e s of closing stages, wM& are dMnguished by input values for ram my power supply, and rate of closure. A mat was described as a series of levels, each of which oon- sisted of two or more mat l a m . Mat layers within a level were assumed and identical with respect to NC, bulk density, and specific gmvity. At the end of any simulated pmss cycle, the predicted b i t y p f U e mpmse!nts layers at difForent tempera- tumsandfll~~values.~caldaCawasusedto calculate the strain development as a of tern- pa*ture and stress without o n s i b t i o n of MC.

The&iss~te?nperaftnr:ofapolymer,TT,,is thetemperatmethatcot.respondetoa-in* when a specific volume is measured again& tempera- ture (1983). The amorphous d a l s in wood, hemi- celluloses and lignh, am glassy m@mes when the tern- is below Tg and rubbery when the tem- paatuxeisaboveTgForhydmscopicplymermatai- als such as wood, T,decmses with an in- of MC The variation of Tg Eor h e m i o e l l h and lignin with MC was desuibed by the Kwei Equation (Kelly d al. 1987). Wokott et al. (1990) used a one-dimensfoaal heat and mass transfer model in conjunction with measured temperature and gas pmssure to predict lo- caked MC. The TB of haniceMoses and lignin in wdodwastrac%edthnwglhthe~cycleandarm-

pared with the measured tempera-. The dlfFennce ~ t h e t w o t e m p e m ~ w ~ e d a s a n i a d i c a t o r o f t l a L e ~ o n a n d ~ ~ o n , w h i c S , ~ e r e usedto pdid t&e-pl~file. Thepdc?dden- s~ty~tsagreedwellwithwhatwas measuredex- c e p t w h e ! n t h e t r a c L e d T g ~ b d a w t h e ~ t e m - pemue (Wolcott et al. 1990). ?'he aumacy of the techniques is dependent on the &dent measur~

m a t of the difference between the two temperatures. For numerical simulation, e m exist to assume a c o n d n u u m a n d l o c a l ~ ~ c ~ u m , b u t errors can be minimized by meshing the simulated wl- ume dciently small. The huge praoessing capacity of modern c~mputem can reduce the errom to sd6- den@ small. The techniques of tracking Tg combined with numerical methods for MC simulation would be mom~andviableforthepn?dictionof?empkr- abre,vaporpssure,andmoisntreMaswellas density gdients.

Of cwrse, the main material in wood composites is wood. Thus, the model used to predict heat and moisture of wood can be adopted and further devel- oped to model the heat and moisturetmwfer of com- posites. Kamke and Wolmtt (1991) sixmdated the variation of temperature, vaporpressure, and MC in a pressing mat based on a mathematid model in wood drying. The model first defined the environ- mental conditions surmunding an individual flake o m the entire pressing cycle byestbating the equi- llbrium moisture content (EMC) using measured lo- cal temperature and vapor pressure. Then a one- dimensional model was developed to desuibe the heat and mass transfer inside of the individual wood flalw based on the enviromental condition and the wooddrying model. Modeling implicitly assumed that the pdcted domain was infinitely large and edge boundaries were neglected. Tempemture and EMC of a flake aad average flake MC were estixnated by the ~IEodel. The predicted tempemture variation agreed with the measurement.

Zomborl (2001) dewloped a two-dimensional model using a finite diffemnce method to @ct the variation of tanpemum, vapor pmsme, and MC of single and thme-layer OSB. Madeling was Jso devel- oped on the basis of a model on wood drying. As in most other models, a subcstantial dkrepancy existed between the predicted and expedmental values. The model qualitatively agrees with the experhentat data. This was attributed to the materids properties that wme obtained from the libmtme and not the ones on which the modd was built. Howevez; two ad-

vances were made in this modeling# that is, the Mu- s h of dosing time and heat generated from the exo- thennfc reaction of resinpolymerizatio~~ A sqamted p r o m model was also developed based on h k e gemn&y and density and used to simulate the stme huaI changes of the mat during compression. Most of the above models began the simulation h the point a . p m c1om-e. The heat trans- fend by radiation and time difference of the upper and lower mat surfaces in aolltadq with the platens were neglected. Closing time is typically mom than 10 percent of the total pmsing time. From the moment when the mat is put in the press to the moment when the mat is pressed to target thickness, the suditce lay- ersafthenmtmwarmedandtempemtumandmois- tun~entsareformed.Enwswould&6mm the selection of boundary conditions in a model with- m t i n c l ~ t h e p r e s s c l ~ T h e d d c P e l o p e d by Zombori (2001) sixdated the enthe pressing pfo- oess including closing time, but the predicted temper- a t n m a n d t a t a l p r e s s u r e w e r e p o a i y ~ w i t h theexperhmtalmeasurementinthepmsdosing period.Amther<xnmnanstrategyofmocst~(3~~ ~ w a s t h e u s e o r o n e m o d e 1 t o ~ l a t e e l l - tire period that was modeled. The actmmly ompli- otedinteractionsamongheatan8masstransferpa- ~ t i m i n g , a n d m ~ e l ~ ~ o f t e n ~ a prohibitive computer nm time (Zombori 2001 ). To make the simukion pmadure practical, the pro- gram either sacrificed the accuracy of the prcdidtion by selecting hge meshing sizes or reduced the simu- lated dimensions or pmsing time. The heat tmnsfer rnode varies during the ~ i n g period and the made tmnsitionsamncededtobeidudedinthemd. But most of the above d* did not explicitly display the mode transition of heat transfer h the modeling. Thermal conductivity, permeability, and speci£ic

heat depend on material density, MC, and tempera- tun? (Hymphrey and Bolton 1989) as &# as furaish geometry and species. Humphrey and Bolton (1989) used correction Eactors in their model to incMe ma- terial density, MC, and temperature effects on ther- mal conductivity, pmeability, and spedfic heat of pwticleM. The parametem used to &ate the d o n k t o ~ ~ wem mostly not measured fn,m the~onwhicbthemodelwasbuilt .Si~situ- ations exist in most other simulations. The high vari- ability of particleboards and other wood c o ~ t e % makes the parametem, i.e., permeability and thennal conductMty, different from one another. Smith (1980) noted that vapor removal h m the boami edges

isafunaionofparticlegeometty.Thesteamdissidisaipa- tionofflakeboardisslowerandtbepr~sstimesare longer than those of particleboard and MDF. Kamke and Casey (1 988b) hmd that a more permeable mat

the presswe buildup and thus lead to a 1- ~pera~plateauinthecore.Thedifferew#infur- nish shapes also affects the voids in the boards (Lenth and Kamke 1996). Quantitative pdctiom depend on the accunwy of the ma& pametem on which the model is built. The way to gemdze the model to beviableforothermaterialsistoindudethe~~ characteristcs in the model.

Current Approach

M O a i m g - r r r l m When hot pressing wood composites, the k t to

moistwe vapo&ns water to vapor, but vaporization is limitedbythevapar~inthemat.Afterphtens contadthemat, heatis~rnainlybyconduc- tim (Bawen 1970, Kamke and Casey 1988b) and mois- min@clesanbothmatsurfacesisvaporized.ha mmzntiional platen system, vapor pmmm is formed inthinlayem wthetopdbottomofamatatthe be- ginnhgofapressingqde.Thus, whenpmahghigh MCfurni sh , there i s su£f i~mo~inthese layen and cxxmdk heat t rader win play a major mle in addition to conduction (Kamke and Casey 1988b). The vaporizedmoistumwouldbedrivwbyp~tothe cooleraxwsinthec~e,as sh0wninFigm-e 1 a h A to B. There would be no heat transfer in the h- direction in this period.

TheperiodhAtoBinPigumlacomspndsto A to B in Figure 1c. which shows corn temperatam varies with time in a typical pressing cycle. The in- comiogwaterfrvnnthefacetotheco~wouldin- crease the pressure in the cxm and densation is possible (Kanh and Wokott 1991). The condensa- tion of vapor on particle surfaces gives off latent heat, which increases furnish temperatun. The c u n d d water may sink into particles and increase particle MC. This scenario would p d throughout the path of the traveling vapor to the mat core until the 00n

temperature mches the vaporizing temperature. Cudemation becomes weaker and vaporization be- comes stronger as the temperature rises. The entire process is coupled with the variation in vapor pres- sure caused by the M e deformation and stress relax- ation in the core (Ihnb and Casey 1988b, Wolcott et al. 1990). Once the core d e s vaporking t e m p - ture, incoming vapor and evapomted water h m the

kbepe changjng bcfcm the platens reach Eargrt thick- ness* tht c o n d u e &), p=-MW %), and spe- dfk bat (c) would cbmge with derrra;ty 4 tern- pesatum.The~eparameterrcanbeobtainedexpexi- ~taUy.SsageIwOLlbefurtherdi~intosevlen ~ , a t E d t h e h e a t a n d m a s s ~ i n e a c h a f t h e stagewinbemodeled(Harlessetal. 1987):InSt.IL heat is mferred by both conduction and convec- tiuninthetmtiddircctionfwboahdynamicand conventiod pmxse. Them wrnrM be no heat ~ i n t h e ~ n t a l ~ o n . I n & a g e I I I , heat istransftsnedbymnducticminthevertid~n d by canvection and duct ion in the horizon&l direction. Them am moisture gmdients in both the verticd and harizontal m o m . Vapor pressun gmdients exist in the horizontal but not in the e c a l dhcti011~.InStageIV, heatistmmhedbyumduc- tion in the vertical direction d by duction and anwedion in the horizontal M a n . There are m u b t m e a n d ~ - i n t h e - a n d hor izon ta l~o l l s .

Inthis- theExpWt FannoftheeneqybaE mcemethod(afinite~meth0d)wiUbeused t o ~ t e t h e m r i a t i o n o f t e m p e x a a ~ i n a ~ dimensional space breachof the four stages (ie., I to nr). Each stage takes the d t s ofthepndous stage as the boundaty or initial wnditiolls. The txznpera- ttm variation lines for a mng cycle can be ob- ta inedbyco~of the l ines~tachof thefour stages-

The energy bdance method assume tbat all heat flow is into the node. The general form of the a q g y balance equation is expmsed as:

VtheE E~ = energythat00wsintothewde, E~ = e n e q y ~ t e d h t h e ~ a n d E~ =energythatismd

I n ~ o n a l s p a c e ~ e e e h n o d e h e s s i x ~ b c # i n g n o t e s ( f a r r a r r e ~ i n F i g . 2 ) . ~ e x - d.langeisinfl-dbydaand-onb tweenm, n, k,anditssixadjoiinodes,aswellasbg geneation. "ha Eqyation [Z] may be expresd as:

whem gtiwmrr f i - conduction Tate a d,

Figun2.--Conductionandcimvectiontomrhterior node f#vm its adjoinkg nodes.

S ( r h t m s r k) = convection rate benvmn nodes, - and

j X b , Ayp Ad = heat generation rate. Accuding to FOuriefs Iaw, the rat0 at which energy is a ; m s f d by conduction hpm node m - 1, rr, k to tn, n,andkmaybeexptPssed;rs:

a n d t h e ~ e c t i o n ~ x a t e f r a m n o d t m - 1 , n , k tom, n, and k is the sum of latent heat of vaporization of h water and heat produd when liquid water is ~ b y a n ~ t e ~ o f ~ . I t m a y b e e x - m a s :

when: , HL I h t heat of vaporization (J kg-I), H, = adsorptkw heat (J b-9, 4, - ~~~density(kgm-~),and

3 Vv = vaporvolume (m ). . A c c d h g to Dam$s and Fiis laws (Siau 1995), the vaporvohrme V, in Equation [S]cm be calculated by *tion 163

* K = specific permeability of wood

composites (m2)* A = cmss s e c t i d area of flow p t h (d)? t = duration of flow perlod (sec.),

L * ~0- length (m), 9 = upstream pm=u= CN m-2h PI = downmeam pressure (N m-2), n = vapor dynamic viscosity &g m - ' ~ ~ ) ,

Dm = vapor diffusivity (kg masm1 ), pw = normal density of water (kg m-3), G = spec& gravity of wood (I ,000 kg me3).

and aAa = MC difbeme (%).

where:

a =percentageofarndensador~d - (96). The heat generated by the exohrmk reaction of

gbcanbeexpressedas(Zam~2001):

& = ~ t ~ o f 8 h t e ( i t g m - ~ l . d AF = reactionindex-.

The esletgy stored in the node m, n, k may be ex- pre!Bed as:

w- p = l - m a s s a t y & t h e M

mat, c = spec& heat Q kg-'IC1), and dt = timeinaease(s8(:*).

After the calculation of the comecdon and conduo tiantransfw18tesftrrmthesixncighborlngnodes, Equation 131 can be used to calculate the temperature of this node. The temperature gradients of each step and each time period can be obtained through solving Equation 133 along with boundary and initial con&-

tiom. Since the txad-er of heat and moistme is coupled togethw, the moisture and vapor g t a d i e n t s w i l t b e o b t a i n e d s i m u l ~ i n ~ c a l ~ lation of temperature gradients.

M-=-F-= ~ o u n Q u s t a n d h e a t a n d m a s s ~ i n d e p t h ,

spe&Uydesiipedomtedplatenswi.lIbeusedto dease the vapor pmssme drning pmsshg. The hod- zontal cham& connected to the perhxations urSI1 be c d to a vacuum. Fipm? la shows the mois- twe mavement in a mat pressed wi?h such a platen wittmut a vacuum. To &ted heat W a d k the vacuum wiU not be applied before the a r e tempera- tun - 80" to 10006. When moishm is vapor- ized, a part of the vapor produced may descend in the mat, as in the canwntional process. The mn&der finds its way &mu& the holes to the end-nt, as shown in Flgun lb. Thus. "U" shapes of vapor tracb wuuk? be apeaid -out the mat thickness. Va- porprssure in regionsnearholes &be lowerthan that in the convent id pmcess. However, perha- dons haw little effect on heat transfer due! to rela- tivelysmallareasofthehales.Afterthemo~in t h e m a t c o ~ r e a c h e s ~ t o 100"C.mostwoodfur- nish hm&out the path of moistwe is plasticized. Keepingm~inthematanyfintherwouldbe d - ' r ta l to the board Tfre saturated moisture would the further irzrrease of temperatme in them(KamkeandCasey 1988b)andwashoutthe resin on furnish swfaces. The rnohtum should be rp m o v d l a s q u i d y a s ~ b l e m p x z t c t i c a l . ~ i s ~avaclsumstarts.Ifavacuumisapplidtothe holesatpointB inFigutw Icandd, vapor will be cmauted and mpor pmsure will dmp, as indicated bythedpbdlineshowninFigm Id. Also,anega- tire~wilSallowpossible=mainingfrec:water to be quickly vapized and evacuated out of the mat. Partsofthemsin thatwas "wasbdina whenthefur- nish was heated &mu@ condensation of water woddLikdybeuurashedd tothe furnish sufaceas the mohhm quicldy evaporated. The detrimental ef- fects of "wash-aut" can also be &tmed or avoided. T h i s w o u l d b e a c h i d b y ~ t h e ~ t i m e to start the vacuum. The corn tempe!mture wauld in- crease again with the quick remod of moisture and pmistm. The vaporization period, B to C in Figure Ic, will be neduced to B to C due to the evacuaton of m o h m by the vacuum. Thus, thrwgh dynamicaUy mmdng the moisture, the pressing time and "wash- out" effects wodd be reduoed. Little heat traasficr oc-

curs from B to C in the padel-to-faa direction due to~vacuum. ThevacuumwillbereducedatCand stoppedwhen thepressst~utsturelease theboar& T h e h e a t i s ~ f r P m C t o l Y b y d ~ n . A s sbowninFQtm 14 thevaporpressm(thedashed line) after evacuation will be much less than the pel's IB strength, a d thus blows ar delaminatfon would not - W e 1 su- the af hesit transfer in a typical pressing cycle. Four kinds of flaw can occur in woad (Siau 1995): 1. viscous or *, 2. Wndent? 3. -,and 4. I L W ~ ~ slip flow or Kwtdscn d i i b h .

T h e l a m i n a r f l o w ~ w k t h e ~ o l d s ' s ~ k i s b e b w a b a u t 2 , 0 0 0 , a t w h i c h p o i m a n ~ ~ causesasmooth,streamiinedflowwithrtohrrbu- ~ I n ~ ~ t i o n ~ f ~ c o ~ t i d p r e s s i n g process, a laminar flow was assumed and Daxy's law w a s u s e d t o d d b e t h e m a s s ~ f e P . W h e n t h e ~ l d s ' s number exceeds the critical dues, lami- nar flow begins to break down d eddies and d h u - bancesocc~r.TheThe~~lmdesthbe~~lditi(~is~- lentk.Whmadcalvanuunisappliedtothe ~ t ~ ~ ~ p d e s s i n g s b q p l a i r a n d v a p o r an?eraa;latedfr-om~piky-pon?reghesandtufbu- ~ f b w u p a t i a n s a r e n e e d e d t o ~ b e t h e m a s s and heat t r a d k Under the tddent am&ions, an exponential rehionship exists between the &ow rate andpnessurediffkmn~wthertbantheliwar& tionshipdes&bedbyDaFcy'skw,asshownin~ tkmC61. Thegovemingequationforturbulentflowis given below (Kuiko 1%6, Siau 1995):

k = pamabWty of wood mmposites (rd), AP = &?a=sm-(Pa), Stawnstant,

q = v-hxmsity of vapor (lg m-1s-1). r 2 dkncterofaca.piby(m),and

P = d-wdvapor(kgm4). The first term on the right of the Equation is the flow & in the qdhy-pornus regions formed between parti~andthedtermontherightisthe~8pil- Euy flow rate inside particles.

The perforations in the platens will be separated by an equal distance. Xu modeling, a grid will be v k d l y supesimpoeed over the board domain, Each hole will be located at the center of one element area in the grid. A finite difference method will be used to sitnu- late the variation of temperatuxel vapor p m , and MC in the pressing mat Since the evacuation is un- steady state mass transfer, the time diffmmtial wlll be smallmallto have an acemate simulatioa The turbulent lIlodelingwillstartfmmtheisothcrmalstageandwill model each element volume to save amputmpro- cesshg time. The solution of one dement will be the lwnmby and initial cxditions of surrPunding ele- ments. Heattranskin the vacuum period is assumed to be c#mdudan.

The exprbmtal mrbibles of this study included thme levels of platen hole number (PHN) (no holes is PHNI; low nmhberof hdes is PHN2; and Mgh num- ber of holes is PHN3) and five levels of £lake MC (2%- 8%, 12%, 17%, and 20%). For each PHN 1 4 , the holes were udomdy distributed thxwghout the aIu- minum platens, which wcm 0.32 cm (118 h) in thick- ness. FigPrt 3 ilhstrates one design of the platens.

Southern pine (EM sp.) flakes wem obtained fnom a k d OSB company in centd Louisiana The flakesu~nftutherhammemdedjatosmaIlerBaka usingarnotormiUer(lndustrhlPhsman~ by Briggs & Strattcm Gorp.. 4 133412) at 8 per- cent MC. The @cles wee screened on a 6-cm mesh screen, and particles on the sueen were then uni- fomdy divided into five gmups. Eve mobtam treat- m e were randody applied to the five groups of particles with each gmup d v i n g one treatment. Theg~iupwlthatargetedMCof2peroentwas o~ed.Theotherfourgnxlpsofpart ic les~

* . condmanod acconhg to the taqeted MC of the group in a conditioning *bec The PF resin used

u s e d i t t t h e m m r ~ o f ~ ( a ) a O Q U I p h and (b) &&n ofa particular hole a the cad

was obtained f n m Borden Chemical Company The solid content of the resin was 51 percent. P d wem made with a miin c o n k of 4.5 per-

cent. Resin was applied to the particles in a drum blender by an ait.atonMng nozzle. The targetsd den- sity of the boards was 0.65 @an3. he target thickness ofthebaardswas 12.7~1nthemiddleofthefonn- h g operation, t b m d couple wlr;#s were buried in the geometric center of the randomly formed mats to measure the temperature chmgm of the mat core during hat After hmhg, one of the treat- ment platens (no holes, low PHN, high PHN)was mi- do& selected and put on the top of the formed mat as theupperplaten.'Rw,boardswenmadeforcach combination of MC and PHN variables. For each ~ , t h e t e m p e r a ~ o f t h e c o m w e n e ~ i m - mediately after the pressure reached the maxh~urn and kept neconbg until the opening of the press.

The baards were tested for MOR, MOE, and inter- nal bnd (XB) stmngth, water absorption OVA), a d thickness swelling (TS) according to the ASTM D 1037-98.

Taperatun changes of the board core with press- ing time forthe five MCs and thrree PHNlevels are pre- sented in Hgnm 4. Each temperature-time line in

Figme 4 can be divided into three parts. The first pat% is the Ztntar iwmw part of the temperatme--time Unes. The water in the bo9t.d core began vaporhhg at the end of this period, which lasted 1 to 2 rnirmtts af- ter press dosum, dqxading on mat MC. Vaporizing t e m ~ ~ ) o f P H N l worethehig3lestatthr: five flake MC levels and all pattr than those of other P H N l e v e l s . T h e V B o f ~ ~ w i t h P H N 3 (i.e., the highest hole density) were the lowest and were almost egual to 100% for the five rnoistum lev- els. Since the vapor pmssure is proportional to tem- p e r a m the vapor pressure inside the mat farPHN3 level was similar to that of ambient conditiotls,

One h & t of a high MC is the @ck heat transfkr f r o m b o a v d ~ t o ~ r e , o r t h e " ~ o c k " e f F e c t (29). To analyve the effects of MC on heat tamdkr f m m b o a r d f k c e t o w r e , h e a r ~ o n ~ wore conducted for the data that repmsented the lin- ear part of the tempmituretimc lines in Figum 4. The slopes ("CIsec.) of the mpssion lines m summa- rizedin~b2.Asshownin~~2,theholesonthe platens slowed the heat transfer from panel face to coreatafhkeMCbelow 13.7~torndhadUttleef- feet after this MC point. However, the theheat transfer rates i n d with the inuease of mat MC for dl

UC: 2.1% HC: 6.79b Mi2 13.m MC: 17.0% MC: 19.7%

1 r

M MC (96) 2.0 8.0 13.0 17.0 20.0 Actual MC (%) - - 2.1 8.7 13.7 17.0 19.7

PHNl"" 2.08 (0.066) 2.89 (0.105) 2.77 (0.038) 327 (0.066) 411 (0.430) PHNZ 2.08 (0.032) 2.85 (0.133) 2.48 (0.095) 3.15 (0.501) 4.19 (0.522)

PHN3 - 2.05 --- (0.054) 2.66 (0.275) - 2.54 (0.142) 3.70 (0.145) 453 (0.2971 ' O ~ f a p u e s k e s ~ t h b ~ ~ ~ f ~ n p r r s e n t s p a * D w b a Z e - ~ ~ ~ l - m r ~ ~ ~ ~ a - h b a k l l l m t k r ~ ~ n d ~ 3 - ~ b o l s ~ b c r ~ ~

platen number levels. The amage temperatwe Mi mtes were 3.M0, 2.9S0, a d 3.109: per second for PHN1, PHN2, and PHN3, mspective137; for difkmnt MC l d , the average rates wen 2.07*? 2.809 2.6E 3.37*, and 4.31% per d for 2.1, 8.7, 13.7, 17.0, and 19.7 m n t flake MC, reqedvel~

The second part of the temperahue-time lines was frwn the time when vaporization began to the dme when core ternperam i n d again. Moisture in the mat was vaporized in this period. This period lastedhmsecondsfor2.1 pe~mtMCtornorr?thanZ minutes for the 17 percent MC for both PHN2 and PHN3. It is noted that the second part never came to anendintheprwsingcycleforPHN1 ktheflakeMC of 17pemmtand 19.7pe~ent. Thetimeusedinthis period i n d with kcmasing MC and decnasad w i t h i n d g P N N . The rest of the temperatme-time line is the third

put. For the last two Bake moisture I d , the third part never came to the PHN1 (17% and 19.7%) be cause the press was opened before the ternperam bmxsed. The highest temperature mached at the end of gmssing cycles derreased with increasing flake MCs.

After the boards were pressed? no blow or de- hination was found with furnish NIC up to 17 per-

cent even though total mat MC was about 22 parent for the fifth group furnish. Although boards pn?ssed with PNNl did not fail, the bonding between flakes was weak. Boa& with 19.7 percent moisture level were delaminated except for one board of F'HN3.

A - values and standard deviations of the phy- sical and mechanical prop&= of tested flakelmad specimens are s d z e d in W e 3. Most boards at 19.7 pement ffake MC were delaminated, and the data for these boards was not available. The VDPs of Merent MC and PHN Iwls are shown in Figum 5.

Moisture bad significant effects on flexutd proper- ties. Figure 6 shows that both MOR and MOE m u k d the highest level when the flake MC was about 9 percent. It is evident that the f l d pmper- ties of boards at 2.1 m t ftake MC w e -or to those of b o d at other MC levels except MOR'at the 19.7 percent level. Both MOR and MOE demwd with an increase in MC after the 9 percent £lake MC. Both MOR a d MOE of PHN3 were lower tban the other two PHN levels. However, PHNl and PHN2 lost more men& than PHN3 as the MC i n d from 8.7 to 17.0 percent. fBvahresofboardsatd@erentPHNandMCleveIs

are shown in Table 3. As a high MC deep ened the VDP cvves for platens without holes; while

spedfic -# t F f W MOR MOE IB TSb W A ~ NTS"

- PHN3 - ~ L T n PHNl

Pion. S b . and H.w 4 37

FiguFt6.-~sctsOfPH~ and MC on (a) MOR and (b) MOE of southern pine fZakeboard

there was Iittle in the VDP h r PHN3 In the Eaur MC levels. XB stmngth of the boards inawmd with the number of holes on the platens To compare the dlikence in IB values among three PWNs at each MC level, pirwise cornprisons were made and the resultingp-values of t-tests arelisted in Table 4. At 2.1 pemznt MC level, PHN had no effects on the bonding pl.opertiesof~board.Sincesomemois~wasre- leased from the board mat once it was vaporized dur-

ing hot pnessing, the "wash-in" and "wash-out* effects were d u d at high MC levels. When the flake MC was 13.7 percent, IB strength increased by 12.9 percent and 30.8 percent as the PHN increased from 1 to 2 and from 1 to 3, nspedvely. The same per- centages of increase were found when the £lake MC was17pemnt. TS and W A after 48-hour soaking and non-

rec~vcrablle TS after 2 1 &hour soakfng aru! jmsatd

B o P r Q i n t h i s g r o u p b k w ~ ~ a n ~ k Values a m significant at the 5 pe~cent &&cant led.

PHNZ 0.13 (0.02) 023 (0.06) 0.31 (0.02) 0.28 (0.03)

PHN3 0.17 (0.02) 027 (0.04) 0.26 (0.03) 0.33 (0.03)

val~esinpaFmtbeses9nJtandardarrre PHN npnsen~~platen hdenumbcc PHNl-m~es: PIINZ- low hole m r m b e r ~ d ) ; P H N ~ - high hole nrmba ( b b 1 d ) .

in Table 3. Flake MC had signi6cant e&cb on TS, WA, and non-nxmverable TS (NTS). Both 'IS and NTS imseased as MC increased from 8.7 to 13.7 pemxnt. FHN had no efkcts on TS d WA, but hid significant effiects on NTS. NlSs were significantly reduced when the boards were pressed wit31 perkated plat- ens mable 3).

The Linear dationship between WA and TS was fMlnn by fahmann (1978). A s e mktionsbip be- tween WA and TS was f d in this study. The slope duesofthisfinear&-mF&~+~~tedinS & 5 . S l o p e v a l u e s i u c m ~ e d a s f l a k e M C i n ~ from 2.1 to 13.7 mt. Thisindicates that boards ~ a t a ~ f l a k e M C s v v d l d n w r r e a f t e p ~ unitpexenhgeofwaterthantheboardspressedata hfiake MC. PHN had no eflktontheslapevalues.

The pr$iminary msu.lts show that the -ted ~ h a d t h e f u n c t i o n a f ~ 1 1 1 0 i s t u r e d ~ ~ p o r fiomthematdurh.lghotpnssing.h*aurrat +tar design. boards can be p m s d without Mows up to 17 pemxmt flake MC. The perforated caul

changed the heat and mass pmpertiesesof boardmatmuinghot~Fkxuralandbonding proprtiEsofllakdmd~fiegativelycodted w i t h ~ f h k e M C w h e n t h e f l a k e M C w a s greater than 8.7 pacent. IB strength of boolrds pressed at modemte to high flake MC was signjfi- d y improved with increasing; PHN Iev&. Dimen- sional stability in thickness was negatively correlated with flake MC and impmved with incmasing PHN lev- els.Ourcumntandfirtundinclude..

1. establish a heat and mass traask database and finite di&rence modas that quantitatively sim- W a pressing cycle of particleboard, OSB, and MDF for both conventional and dynamic pressing%-=#

2. ascertain the gas permeability of pqrticle bods, OSB,andMDFasafkmdbydensity and MC,

3. detedqe the t h d co-vity of partide- b u d and OSB as affected by d-, MC, tern- - and furnish sizes, and

4. dedopaldynamicpressingsystemtooptimk the p m p r t i e s of wood composites.

mrslmm Amekan Society for Rsting and Mat- (ASTM). 1998.

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Bolton, AJ., PK Humpany, ,and PX. Kavwms. 1- Tha h o r ~ o f m y - ~ w a o d - ~ ~ t e b . P a I t m.Pnaictedvepor~andtemperatlnr?variadon witla time, armpared with experimental data for hhatory txlardn 43(4b199-206.

Bdton, AJ., RE. Humphrey, and PX. lbwmms. 1989b. The trot~ofdrp-~woOd-~~~lgpoeites.PartW. ~ v a r i a t i o n o f m a t ~ e o n t e n t w i t h t i m e .

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i n s p m c b s e s . . ' I ; e c h n i c a l r e p o r t , F ~ ~ C a r p . 18 PP-

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40 6 Recent Dtmkpmm in Wood Companites

Recent Developments in Wood Composites

Edited by ToddE Shupe

~ z t e ~ c p s o r ;

Louisian~ Fopwf Mum DeveIspment Cenrer; Schoor of lbmvable N a t u m l ~ ~

LouisiQna Stilte univm- Center; Batan Rouge, Louisianq USA

Forest Produds Socfety 2801 M a r s h a Court

Madfson, WI 53705-229s phone: 608-23 1-1 361 Fa: 60%-231-2152

www.forestpmLorg

Table of Contents

Assessment of Screw Holding and Other Property Variations of Canadian Furniture-Grade Particleboards

by E m m d K Sackey, K d e E. Semple, Je Jan Park, and Gregoty D. Smith . 1

Properties Survey d FurnittursGrade Partic1elxmrck MS and M2 Grade Comparison and a Practical In-Situ Test for Internal Bond Strength

by Kate E. W p k , E m d Sacw, H e h n Park., a d Gegory D. Smith. ... . 9

f l d Properties, ][ntemd Bond Strengthf and Dimensional Stability of Medium Density Fiberboad Panels Made From Hybrid Poplar Clones

by Jun fi &if S.X Zhang, W d Riedl, and G i k B w t t e . ........ 7

Dynamic Contd of Moisture During Hot Ressing of Wood C o m t e s ........................ byChengPiaoIToddF.Shupe,MdCluurgY.61se 27 3

C o m p d o n Behavior of Resinless Oriented Strandbard Mat: Effect of Internal Mat Errvironrnent and Void Volume Fraction

by Jong N. Lee, L a p T. Watsotz, and Frederick A Kbmke ... 4 1

Properties of Bio-Based Medium Density ~~ ..................... bySa@Lee, T d F . S I r u p e , d h y . Hse.. 0 89J

a Manufamng Particleboard from Under-Utilized Spedes in Oklahoma

................................................. by Sdim H i z i ~ ~ g l u . .59

Development of Binderless Particleboard h m Kenaf Core Using Steam-Injection Pressing by f ianying Xu, Ragil Wyourini Gwn- Han, ED. Wmg, Yuzo Okudawa,

............................................... and Shuichi Kawai 65

Optimizing Natural FibReinfod Polymer Composites Through Experimental Design by Rupert Wmmer, Andm Ganz, Cunter W d a d R u d o f f k & r . . . . . 73

Characterizing Wet-Proass Hardbr>ard Man- by the Use of Eixpaimental Design by Thom~s H u t t q Rupert Wiynmer, a d Rudolfksler . . . . . . . 8 1

Heterogeneous Nucleation of a Semiaystdline Polymer on Fiber SurFaces bySangyeob~,ToddEShu~e,LesI ieH.Gnw,m,dChungKHse .... 6