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Dynamic Control of Moisture During Hot Pressing of Wood Composites

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Dynamic Control of Moisture During Hot Pressing of Wood Composites Dynamic Control of Moisture During Hot Pressing of Wood Composites Cheng Piao, Todd R Shupe, and Chung I? Hse Abgtrsct Hotpmssingfssnhportantsfepinthe~ I. invedgatetheMCrn~entthughdynamic ture of wood composites. In the conventional pns- CQntrnl of moisture in hot pnssing, ing system, hot press output often acts as acanstrslint 2. dammine the thennal conductivity, perambiz- to~pmdudian.Sevleredryingofthefurnish ity, speci6c heat of themajor wood compos- bg*,gwicles* flab,or fibers) required 'by this pm- ites (medium density fibedmd [MDn part- ~SU~~~theman~costicleboad, and oriented ~~[OSBI), aadg.leates~iFpo~~~ortsd~~rgani~3. deVi?lop~~~f9reachpressing armpoatnds, which are a main source of air pollution stagedthe~tidpressisgprocess, bthe finest products in- Wood composites madeinthem~tional~hawhfghinteKnal 4. devebpmathematicalm&forthevacud ~andlowdimensionalstabitityd~.The perforated platen pnsbg process to define dedqmmtdapressingtechndogytoimpmthe tempature, mmand vapor pns-= p- ~is~tedbythenelea~e~fvaparpressure htsin a hot pmdng cycle, ad during hotpmssing. A perfmated caul system may en- 5. ~thevaalum~anthephysicald lage the area for the moisture to escape fkn the ine&nicalperfonnanaofthesedcom- baardrnatduringpressing,andand,makepossiMe pasites. high moiaum content (MC)~aessing.% objectives Apleliminarysbrdgon~plateas~ oftbis!3tudyareto: 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 saapc: was at 8 percent and decmsdthereafter as MC in- ~~,Sdtodd~Natlaat~, ~statcItsfverJfty~~Rdulps,LA creased forail three amounts of platen holes. Internal HmK bond~~withtheincl.leaseofplaten PaJt~ResearchScienrid,USDAFonraSaulce,Poaprt per at MC Produds--w hde number unit area all Jd except the 2 ffa: pementMCWand~wfththein~of ~wood~UglAFanartSaoioe.SaabanRb fiake MC levels. Sbis study showed that the @rated seuch Station, Phdk,LA Ihiepapa(No. 06~)isplbllehsd~thtrgpm#ldtbeDi- caulhadthefuactioaofreleasingmoistureandvapor rectorof the lmManaAgiculW lkpe&m@statiosL presure during hot p~pgging. (Kamh and Casey 1988a, 1988b). The fow edges and axmm am the ody amas hmwhich the mobtun? ~ard~J9ringBot~can escape during hot pressing. Thus, mobtm, tem- In the man- of dry-process wood ampas- ~,aadvapor~~t31tsmformed ites. min-coated wad furnishes me consolidated fram face to core and from con to edges (Maloney into panels between two heated platens. The moishu'e 1977, Stickler 1959, !hcbdand 1962). The mat edges inamatisvagorizedd~inthe~cal are dcompared to the total surfaoe area of the and horizontal directions from' the mat rndaces. bOgTd,~,onlyasmallamarntof11~)~es- Since mat edges are the only areas exposed to the out- capes thmugh the edges and oomm; most moistwe side enviroment, vapor presswe is formed in the is released thxwgh the psnel Eaces upon opening of mat and is responsible for problems such as low care the hot press (Maloney 1977). Since excessive mob densityandintemaZbond(T]B)strength,blawsandde- tun may cause blows or delamination, the MC in a lamination, d resin wash-in and wash-out effects. conventional process is rigodycontrolled in a low To avoid these pblems, woad furnishes are dried to ranseh&dtyiqs. a low moisture content (MC). The drying process also MC and its distribution are important hctom gav- dtsin volatile c#.ganic compound (VOC) emis- ethe pmperties of wood composites. Heat and sions. The low MC nquhmcnt of the conventional moisture soften the wood furnish and facilitate pressing system lowers the productim of the furnish plasticization so the wood can be bent without break- dryer as well as the entire production line. The rigid ing. It is reported that the &mum condition for requbnent on mat MC makes the safety window of wood deformatkin is 15 tr, 20 percent MC at 100% the conventional process very nanowand substantial (Spelter 1999). The MC of a oonventional mat is nor- ]assesawincurred Thesolutiontothis problem re mally below this range and high swebgwould be ex- quires the use of furnish MC that 19 higher than what pected after the panels retake moistrim Rice (1960) iscurrmtlyusedinc~mm~plamsandanunderc found both bendhg properties and dhensional sta- standing of the mechanism of heat and mass transfer bility pqmtiesare impmved by inaedng the mat during~pressing. MC from 9 to 15 percent. Moistum may speed the heat The moisture in a mat of a typical dty-process tmnsfer from face to core (Kamke and Casey 1988b, wood~t~~prlnsarilycolnesfrom~Soemin 1974, SMckler 1959). Drying wood materi$ls sources, i.e., the furnish, the resin applied to the fur- is the dominate somof VOC emissions frwr the nish, and the mndensatfon reacton after the rcesin wood products indw (Englund and Nussbaum ams.h a typical manufacturing proce%s, laat MC 2000). Englund and Nussbaum (2000) showed that fiDmthefudshmaybebetweffl2~tto6pcr- emissions of tapenes and VOC per wendry weight cent based an mdtyfurnish weight. For stmcmd rapidly inuwem when the sawdust MC is reduced to pels, wakr-bsed phenol-fddchyde (PF) resin below 12 percwit (Granstrom 2003). Also* high mat is widely used. The solid content of PF resin is about MC may reduce closing time (Maloney 1977), board 45 to SO percent; that is, if 1 percent mare of solid moistmy adsorption and thickness swelling (Maku msin is added to a mat, more than 1 pacent of water and Hamada 1955), and internal strain (Kelly 1977, is also added to the mat. The typid resin conteat of a Sucbs?and1962). MC addyinfluences other pm- conventional oriented strandboard (OSm manufao- ptdes. Excessive moisture may cause adhesive cur- twingprooess b 3 to 6 percent. That means that about ing pmhlems, longer pxxssing time, low care density, 3 to 6 parent of water is added to the mat along with and thus low indbond (IB) strength (Johnson the resin addition. For the third so-, a small and Karnke 1994, KeUy 1977, Malmey 1977). High amount ofextra moisture is added to the board mat MC k#ds to high gas press= in the mat (Kamke and by combjning two resin molecules and splitting off Casey 1988b). If moisture Is not deased sufficiently one watermolde. The total MC of a mat is from 7 to during pFwsiag, blows or delamination can occur: 12 pement, depending on the particular process. Thae are techniques to reduce or ehnimte the 3n the pmces of consolidation, moisture in a mat negdw eflects of MC and take advantage of its posi- fkstEhangestovaporraderheatand~md tive effeds. Several endeavors have been 3mplemm- migrates to the cooler anas of the mat am,edges, tedtoachi~vethisgdOnetechniqueistodistribute and comers along with volatile dvesof the higherrnoistutreinthefacethminthecaretop wood furnish and nonreactive adhesive mmwnents duce the steam-shock effect (Sosnh 1974, Strider 1959). Steam-shock is an advmce h both pressing and a flowing fluid when they are at Wrat tan- theory and technology. Strldih (1959) showed that -. Thermal radiation occurs as a surEace the heat transfer mte increases with increasing faoe emit* energy in the form of dectmmagnetic furnish MC, but modulus of ruptun! (MOR), modulus waves. Allof thesa transfermodes are present during of elasticity (MOE), and IB 1tnmgt.h decrease when hot pnssing of wood composites. the corn MC is at 9 mt. Numerous studies have been conducted on the de- Steam-injeaion pressing was designed to reduce velopment of mathematical models of heat and mass pnssine time d frnprwe dimensional gtabiliity of transfer in hot pressing. Bolton and Humphrey wood composites. It hcludes open and cldsys- (1988) conductad a review af the literatun? on heat tems. Both systems use perforated platens to heat the and momtransfer. MacLeau (1942) and Car- mat thmugh hot stepun that is ejected fmm the holes. ruthers (1959) measured the tempemwe changes in Intheclosedsystem, thematissealedandconsali- plywood during hot pmsing. Kull calculated heat dated under elevated temperature and pnsme (Gei- derin Elakeboard and bdt a onedimembnal mer and Kwon 1999, Geirner et al. 1998, Shen 1973). model (1988) to predict the tempemtum changes in Attheendofsteaming,pressureis~leassdandthe~ the mat. Since heat and moisture transfer in winpos- mainhg steam is exhadto the atmosphere. In the ites during hot pmssing cxxws in three dimensions, open system, mat edges arc exposed to the envir9n- ollbdimensional models can dy@ct &mois- ment, as in the conventional pressing pmss. Steam is ture and temperature changes in the vedcal direc- injected~separatelyheatedphplatensandthematis tion. The onedimensional model cannot accurately consolidated by the heat fmm the pktens and the predict moisaire and tempera- changes in a mat. steam (Ceimer and Kwon 1999, Hse et al. 1991, John- Bowem (1970) evaluated the convection effects, and sonand Kamke 1994). Composite panels pdwith Cefahrt (1977) and Kayibn and Johnson (1983) in- the steam-injection method have high dimensional cluded convection effects in their ontxhemional stab- This was attributed to the combination of models. Weplasticization, Egrh flow, and chemical modifi- Humphrey and Rolton (1989) studied the heat and cation (Geimer and Kwon 1999). However, high MC mass Sansfer in pressing of partidebad. A ma& in a steaminjected mat dtsin poor mechanical matical model was developed using a finite ~~ pmpdesofthemat. Thisalsoistheresultintheam method to predict temperature, MC,and vapor pres- ventional pressing process when the mat has high a sure changes in the middle of the p-ticlebcmrd mat MC (Palardy et al. 1989). during pressing. Both conduction and mmeaion The solution
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