CA2152345A1 - Preparation of wood laminates - Google Patents

Abstract

A method for making laminated wood products, particularly laminated veneer lumber, and especially from wood veneer of high moisture contents wherein the veneer is bonded together using a freely-flowing, finely divided spray-dried, powdered adhesive containing a phenolic resole resin.

Description

PREPARATION OF WOOD LAMINATES

FIELD OF THE INVENTION
This invention is dhe~,~d to a method for p~ h~g l~min~t~ wood products such as plywood and l-...iluled veneer lumber (LVL). The present invention is specifically directed to ple~a~ing such l~..;..A~ed wood products by consoli-l~ting a plurality of wood ven~.~ using a dry, finely powdered phenol-form~l~ehyde resin-based adhesive.
BACKGROUND OF THE INVENTION
Powdered adhesives, such as made from spray-dried urea-form~klPhyde and phenol-formAl~ehyde resins, have been used widely to produce waferboard and particleboard from such wood sources as wood wafers, wood chips, wood flakes, wood sL~vings, wood splinters, saw dust and the like.
U.S. Patents 4,098,770 and 4,424,300, for example, describe the pl~l~alion of spray-dried phenolic resole resin-based ~ow~.~ for p~il~g a&esives for making such bonded wood products.
According to U.S. 4,098,770 the resole resin is spray dried, preferably in the pl~,sellce of a non-phenolic polyl~d~ y co.llpuulld, to produce a free flowing powder which can be used directly or re~o~ t~l as a liquid by adding the desired amount of water. The dry pOW~l is said to be stable for at least six month~, and can be used in place of a liquid adhesive for m~king waÇe.l~ald and chipboard at shorter ~l~;S~hlg times and lower pressing te~ e~a~ules. Notably, when exemplified for producing plywood, the powder is first recoi~ ~ with water to produce an aqueous adhesive which then is applied to the plywood veneer. (Example 6).

-2 2l52345 U.S. 4,424,300 proposes to make a powdered resin a&esive by spray drying a liquid lui~lul. of novolak and resole phenol-formAl-lehyde prepolymers.The resin pu..~, is disclosed for use only as a waf~ 160~ binder. Use of the powdered resin for plywood m~mlfactllre is not described or suggested.
In recent years, major gains have been achieved in reAuciTI~ plywood hot press times due to the development of faster curing resins. There also have beenclaims of increased productivity of wood veneer driers ~ul~ollt;dly due to the development of more IlloL7lule tolerant aqueous-based phenol-formAl~Phyde a&esives.
At the present time, coll.lllelcial production of plywood, especially exterior grade plywood, and other lATninAt~l wood products, such as LVL, rely exclusivelyon the use of liquid a&esives (solutions and s~ ;ons). Liquid compositions COIIIA;~ th.o~mosetting phenol-formAl~ yde resole resins are the a&esives of choice for such products, particularly for exterior grade plywood products. The present invention, in collllasl, proposes to use a powdered phenolic resole resin-based a&esive in such applications. In particular, the present invention is founded on the use of a dry, freely flowing, finely divided powder adhesive based on a phenolic resole resin. This dry a&esive can be used at reduced adhesive spreads and faster press times, relative to the pl~,se.llly employed nql1eo -s liquid a&esives, to produce l~ A~ wood pl~lU;lS, especially ~ ted wood products from VellU:.., of more elevated moisture COI.t~, while yielding bond ~ lls (Illeasuled as percent wood failure) on a par with l~ d wood products made using liquid resole resin-based adhesives, even from veneers of lower llloi~lur~
COlllt:lll~,.
DESCRIPIION OF THE INVENTION
Accoldillg to the present invention, 1A~;I.AItA wood products are pl~alcd by 60lld~ ~ogelllel a plurality of wood veneers with a dry, freely flowing, finely divided particulate or powdered phenolic resole resin-based adhesive. Such l~minAtecl wood products include plywood and 1AIII;I~AIY~ veneer lumber or LVL.
Plywood is a panel-type product pl~aled by colll~lessillg, with an adhesive binder, large sheets of relatively thin wood veneer or plies. The normal 21~2345 construction has an odd number of layers with the grain of ~dj~ nt layers at an angle to one a"~ (most often 90). Inner plies are commonly referred to as the core and the outer pieces as faces, front and back. The core generally is made of wood veneer, but may be lumber or even particleboard. Total panel ll"rL .~sses are usually not less than about one-eighth inch nor more than about two inches.
At the present time, more than about 95% of all plywood produced is less than about one inch thick.
Using current con,ll,~.cial plywood m~m-f~r~lring procedures, a plurality of wood ve~ are coated with an aqueous adhesive and pre-assembled into multi-ply sheets. The pre-assembled sheets are held in open assembly at ambient ~e"~?clalu,e until a full press load is p~ ~ for the hot press cure cycle. This operation may require a half hour or more. During this open assembly time, the wood absorbs water from the ~q~leolls adhesive glue. With a highly abso,l~"l softwood, such as the spring wood portion of Southern yellow pine, rapid dehydration of the glllelinlo may occur with reslllt~nt inadequate glue flow andinadequate ~l~el,alion during the subsequent hot press cure. With a less absorptive wood veneer, like Douglas fir veneer, an aqueous glnelin~ dries more slowly and too much flow and l)el~elldlion may occur during hot p,~ssing. Eithercondition can lead to ~ub~ldard plywood.
The present invention uses a dry, freely flowing, finely divided powdered a&esive which by its nature avoids the problems of pl~ alule dry-out or adhesiveover pel~el,~alion. As described more fully h~leilldrlel, use of a powdered a&esive also allows more control over the molecular weight of the phenolic resin. Liquidadhesives, based on aqueous phenolic resole resins, have a limited shelf life due to the contimle~1 and inherent resin adv~nrP-m~nt during storage. Thus, during the p,~a,alion of such adhesives, one is limited with respect to the pcl",issible molecular weight range of the resin product. Pow~l~d resin adhesives have a broader molecular weight tolclal~ce because of their il~h~lelll stability duringstorage. Molecular weight is one of the ~llollgcr inflllrnr~s on the flow characl~ lics and thus the perf~ a~ce of the adhesive resin during hot press curing.

21S~g5 LVL billets normally are co~tlu~;~ed with the grain direction of all the ~elleel~ aligned parallel to one another. Though some LVL products are as thin as three-~lua~ of an inch (34 "), the predominant cc,lll,ne.~;ial constructions are one and one-half inch (1 'h ") thick and greater. Con~eque-ntly, while plywood may be pl~al~d in 3-ply to 6-ply construction~, most LVL products generally have over 10-15 plies.
LVL billets are often produced in continlloll~ lay-up operations and are cut to length after they exit from the hot press. ~lr....-~ ively, they may be constructed in predetc....il-~l Iengths. These lengths are virtually all greater than 30 feet often up to 80 feet. Inner ply veneers of LVL are often intentionally lap jointed or gap butt jointed for ~lov~d ~ and to provide steam vents to reduce blows.
Select~d veneers may also be scarf jointed to provide ~eq~te continlloll~ veneerlength for the billets being constructed. Plywood is vir~ually never produced ina continuous lay-up process and most of what is produced is laid up in 4 feet wide by 8 feet long sheets. Also, in plywood lay-up line operations, the panels are cut to length prior to the actual hot plessing operation.
As used herein, the term "veneer" refers to thin sheets of wood produced for example by skiving a thin wood layer or sheet from a log. Generally, a veneer panel or sheet will have a thirlfn~ss of about 0.1 inch (2.54 mm) to about 0.167inch (4.233 mm). Veneer is provided in 4 feet x 8 feet sheets for plywood ple~alalion, although larger or smaller ~im~n~ions obviously can be employed.
THe ~ ion of veneer used in LVL production generally can be more variable.
Replesel~l; live wood varieties which can be used to pl~ale veneers for ~la~ illg the present invention include soft woods such as Southern pine, Douglas fir, Ponderosa pine, and the like, and hard woods such as oak, walnut and birch.
Veneer for l~llli~ul~cl wood products can be l,lepal.,d by heat col--1ilio.-.n~ str~ight logs cut to length in vats cont~inin~ hot water, and then corltimlollsly p~eeling the thin veneer from the heated logs. Th~leartcl the veneer is dried to the desired moisture content.
LVL is almost exclusively made with softwood veneer; plywood, in co~ alison, often is made from softwood, mixed softwood and hardwood, and s 2152345 hardwood veneers. While plywood may be made using veneers of various thi.~L,.f,~s a"ai~ged throughout the panel, LVL tends to be pl~a,ed using a veneer of one ll~irL llf'SS.
In accor~ce with current co""--f-l~ial plywood production ~roccdul~s, wood veneers used to make wood l~ gener~lly have moisture co~ of less than about 10-15% based on the dry weight of the wood. At the present time,LVL is ~ a,cd almost exclusively from veneer dried to a total moisture content of less than about 7%. Unless-there is careful control by the plywood or LVL
mill operator, use of high ",oislu,c content veneers in conventional processes invariably leads to a large llu",~l of l~min~tion defects and an excessively large number of rejected panels beca.lse of poor l~ ;oll. The l~min~tion defects are caused by stream formation bcl~n veneer layers and a blow out of the stcam when p~e;.~ulc is released upon completion of the p~ssmg cycle. The water contributing to this steam formation origin~tes both with the wood and the adhesive. As the lc~ lu~e increases in the center of the veneers during consolidation, so does the vapor ples~ule of trapped steam. As the press is opened, the built-up vapor or steam seeks an avenue of escape and may blow the panel. Sizable o~l~lhlg and capital costs have been incurred in the prior art toassure a CO~ moisture content for wood ~eneels used in m~king wood l ..,~in~l~S so as to el;...in-t~ lion defects and reduce the number of rejected panels.
Until l, cclllly~ continl~oll~ co~ lcial producti-n of acceptable l~...i,.~t~ s from wood veneers having overall moisture contents of greater than about 5 to 10% had not been possible using convc"lional opcl~ lg procedu,~ s. With the more ,ecer,lly developed faster curing liquid resin adhesives claims have been made that good bollding can be obt~illed using higher moisture content ~,. 1l~l~.
Even as described, however, this art typically teaches that the veneer must be dried to a total ",oi~lu,~ content of less than about 15 % and usually less than about 12%. In actual practice, use of such rn~t~ri~lc and conditions on a routine basis to obtain s~ti~f~ctory bonding results has not been possible. As will be described in detail h~leartcr, the dry, freely-flowing, finely divided, spray-dried phenol--6- 21~2345 form~ld~yde resole resin-based adhesive of the present il~ ion, however, makes it possible to prepare acceptable wood l-.~.ir~les from high moisture content veneer, i.e., a plurality of wood veneer each having an overall moisture contentof greater than about 15%.
In co~ nlional, continuous lay-up LVL operations, open assembly time prior to hot press entry generally is ~le~ignffl to be less than about five ...i..~l. s.
Often no pre-p~ssh~g operation is pe,ro,..led. In single billet lay-up LVL
operations, as opposed to the continllollc lay-up, open assembly time prior to pre-pl'~ssing iS also ~lesign~A to be as short as possible, typically less than fivel..;....les. Problems frequently occur during such batch proces~ , however, thatcan stretch this time out to about 15 ~--;....tes or more. Stand time after assembly but before pre-press can range from n~gligihle to five ...;..~ s. Pre-plessillg time generally runs 1-5 ...;..~ s. When using the present invention, whether for plywood or LVL, no p.e~less operation is n.o~derl As such, the open assembly time prior to hot p~ssing is ci~nifir~ntly shortened and productivity is accoldingly mcleased.
When producing a l~...in~ted wood product, such as plywood and LVL, the most critical gll~elin~ during the hot press cure operation is the inn~ st glueline of the core. The press conditions, ten~c,~lul~ pl~,.7.7Ul~ and particularly time, must be controlled to ensure the complete cure of this glll~linP, since it wiil not be fully cured as rapidly as the other glll~linPs are. Not sul~l;s,~ly, the press time needed to obtain full cure of all the glllPlinPs, especially in-~hltlin~ the inn~rmost glueli,le, is to a large extent a function of the panel construction. For example, plywood pl~palcd using conventional liquid resole resin-based adhesivesfrom 3 plys of 1/8 inch veneer may be fully cured at a press time of about 3 ...;..~tS, while 5 ply product may take 4.5 to 6.0 .~ s. Use of a dry, freely flowing, finely divided powdered phenol-fonn~lde~yde resole resin-based adhesive, ho~ lO~S the heat ~ ,r~l during the press operation and thus allows the i~n~ .ost gll~elin.o of l~.,.;"~led wood products to achieve a fully cured condition in about a 10-25% shorter time period than now ~lacliced col~l~ ;ially with liquid a&esive resins.

7 21523~5 Plywood generally is hot pressed at Iclll~.alul~s of 135-180C (about 275-360F), most often at ~lll~ld~ules of about 140-165C (about 285-330F), and such co~ are a~lo~ ~ for the present invention. Hot press ples~ul., is almost never greater than about 14 kgm/cm2 (about 200 PSI) and, most l~celllly, is being staged in co..---.- I~ial operations (high ples~ule to low ples~ul~) once during the cycle to m~Ximi7e panel Ih;~L..~5s. A typical staging would be an initial ples~ul~c of about 10.5-12.3 kgm/cm2 (about 150-175 PSI), reduced early in the cycle to about 7.7-9.1 kgm/cm2 (about 110-130 PSI). Again, such press pl~S~iul~,S can be used in col~eclion with the present invention.
LVL is generally hot pressed at ~~ JClalUl~,s of about 150-190C (about 300-380F), most often at tcllll~e.alules of about 155-180C (about 315-360F).Hot press ples~ulc iS generally varied throughout the duration of the ~l~SSillg cycle. P~ ules as high as about 19 kgm/cm2 (about 275 PSI) for a signifir~nt part of the cycle are often used. Press pl.,;~:jUl~;i iS controlled to m~int~in thirl~n~ss control in ope~alions that employ lapped veneer innerply lay-up plocedures. These colllllle~ial conditions remain suitable for use in connection with the present invention. Due to the thicker constluctions, press times generally are longer for LVL as cOlll~al~d to plywood. However, for the same reasons as ~ cussed above, the h~ vcd heat lla~ realized using the finely divided, dry powdered adhesive of the present invention permits a ~i~,..irir~ re~l~ction (typically 10 to 25%) in the hot press time needed to pl~luce a LVL product.
Conv~.ltiol~al adhesives for 1~ ~1 wood products col"~lise aqueous mixtures usually co~ ng from about 28 to 38% active phenolic resole resin solids and 5 to 20% additional solids of various amylaceous and fibrous fillers/eYt~n-lrrs. Powdered vegetable materials such as starch, nllt~h~ (for example, walnut shells, pecan shells, cocollul shells, ivory nut shells, horse ch~ shells, peanut shells, and the like), wood flours, barks, leaves, corn cob (Co Cob), rice hull and the like are usually mixed into the adhesive as fillers either alone or in colllbillalion with a ~lilntent such as wheat flour (Glu X), so~ghulll flour and the like. Such additives are used for reducing the a&esive cost, for preventing over pel,c~,alion of liquid a&esive into the wood veneer, for l~t~

-8- 21~;2345 a ullifu~ liquid adhesive visco~ily to f~ilhqte the spreading of the adhesive onthe veneer surface, and for p~ the formation of interstices and cracks accol~dl,ying the shrinkage and aging of the cured resin after adhesion. Fillersoften are used in an amount of from about 8 to 14% based on the weight of the phenolic resin. I~ ganic eyten~lers such as ~qttqrll~ite clay also can be uæd.
Although not needed for many of the same reasons, such additives can also be used in co~-lu~ n with the preænt invention primqrily to reduce adhesive cost.
Depellding on the wood species, prior art aqueous adhesive application rates often ranged bclwwn about 160 and 270 grams per square meter (0.033 to 0.055 pound per square foot) of single glue line. This corresponds to an application level based only on resin solids of typically bclWccll about 45 to 103 gMms per square meter (0.009-0.021 pound per square foot) of single glue line.
Another advantage of the present invention is that l~ ~l wood products of a collll~aldble ~llcl~lll can be pr~d,~,d using a glue spread, based on resin solids, of as much as 15 to 45% lower than when using a liquid a&esive. Glue spreads, based on resin solids, uæful in the present invention generally will vary bclwwn30 and 90 gMms per square meter.
Aqueous phenol-fonnq~ hyde resins useful for plc~al ng the solid dry, frwly-flowing, finely divided particulate or powdel.,d resole-based adhesives ofthe present invention con~ .J~ thermosettin~ co~ e ~ n products produced by l~acling at least an equal molar amount of formqk1ehyde with phenol. P~cf~ ,d phenol-fonnql-lf-hyde con~f-n~-qtion products are ~l~al~,d using a molar Mtio ofphenol to aldehyde in the range of about 1:1.5 to 1:3 with a phenol to formql-lf-~lyde molar ratio of about 1:2 usually being plcfellcd. Suitable aqueous resole resins are plcpa~d by l~,aclillg phenol and formql~ehyde at these molar Mtios in an aqueous reaction ",P1i..", in the p,e3el~ce of an qlkqlin~ catalyst. In the broad ~la~;lice of the present invention, ~dald reaction conditions, procedures and r~artqnt~ widely used for pl~alillg aqueous phenol-form-q-l-lehyde resole resins can be used.
Sodium hydroxide generally is the vlkqlinf- catalyst of choice in the present invention. A list of other potential alkqlin~ catalysts in~ des, inter alia, other aLkali metal hydluxides such as pot~sillm hydroxide, aLkali metal ca~l~tes such as sodium and pot~csillm call)olld~s7 and ~lk~linl~ earth metal oxides and hydroxides such as barium hydroxide, c~lrillm hydroxide and r~lc;-lm oxide.
A variety of te~hniq~les are known in the art for reacting phenol and fonn~ hyde in the ~ se,lce of the alk~lin~- catalyst. Typically, the resin is reacted in stages with Se~alt; partial additions of either one or both of the l~c~ and the ~lk~lin~ catalyst. For example, one common plocedul~ is to react the phenol with a portion of the fonn~ldehyde, in the pl~se.l~e of a portion of the ~lk~linP catalyst. After a brief, initial exothermic reaction, additionalamounts of ~lk~lin~ catalyst and fonn~ hyde are added to the reacting llli~Lule and the reaction is continued with careful control of the reaction tell.pela~ule.
Once all of the re~ct~nt~ and catalyst have been added, the reaction is allowed to proceed to a suitable end point, which may be dele ...in~d by measuring the refractive index of the l~,ac~ g mixture or by medsuling the viscosity of the reacting lllL~lule or by some combination thereûf as recognized by those skilled in the art. Once the selectçd end point is achieved, the reaction lllLl~lule is ready for pl~d~ a dry, freely-flowing, finely divided powdered adhesive mixture by spray drying.
The amount of ~lk~lin~ catalyst added during resin ~r~aldtion to produce a resole resin solution is often s~ur~ to produce an ~lk~linity content, in the range of about 1% to about 15%, usually about 3% to about 9%, and most often about 4% to about 8% based on the weight of the resole resin solution. As used herein ~lk~linhy content means the content of ~lk~lin~ catalyst in the resin solution ~"~lessed as a weight percent of the solution on the basis of the equivalent (mole) weight of sodium hydroxide. For eA~le, in the case where the aqueous resin solution collt~i ls about 6.4 % by weight potassium hydroxide, the ~lk~linity content of the resin solution would be about 4.6% based on the equivalent (mole)weight of sodium hydro~ide since potassium hydroxide has a greater molecular weight than sodium hydl~ide. Normally, from about 50 mol % of sodium hydroxide up to about 100 mol % of sodium hydroxide based on the mols of phenol used in pl~lmg the resole resin will provide a desired level of ~lk~linity.

-Usually about 0.60 mol to about 0.80 mol of sodium hydroxide per mol of phenol lS used.
Preferably, hydro~be~ e (phenol) is the phenol l~c~ll of choice for making the aqueous resole resin, although s.lb~ ~ phenols such as cresol and higher îul~lionAl phenols such as l~;solchlol or bisphenol-A, can be used.
Form~l~lehyde is the pleÇ~ ,d aldehyde co..~ Generally, the forrnql-lehyde is supplied as an aqueous solution known in the art as "formAlin". Formalin is an aqueous solution that generally contains from about 37% to about 50% by weight formAl~lehyde. Other forms of forrnAI~lehyde such as palafo....A1(3e-hyde also can be used. Other aldehydes, which may be used in combination with formql-l~hyde, include aliphatic aldehydes such as Ac~tAl~hyde and propionaldehyde; aromatic aldehydes such as benzylaldehyde and furfural and other aldehydes such as aldol,glyoxal and crotonaldehyde.
One advantage of the present invention is that the molecular weight of a resole resin suitable for pre~dling the dry, freely-flowing, finely divided particulate or powdered resole resin-based adhesive by spray drying in accor~ ce with the present hlvel~ion can be higher than for a liguid resole resin that is to be stored for subsequent use in form-llAtin~ an aqueous adhesive. This is because the spray-dried, heat hardenable resin solids are more stable than the precursor aqueous th~rrnosetting resin, which generally has a limited shelf life due to the inherent molecular weight alvAI-~mr-~.l that occurs on storage of such aqueous resins.
Thus, when used in comle~;lion with the present invention, the aqueous resole resin prior to spray drying can normally be advanced to a higher molecular weight end point than is collv~n~ional for aqueous resole resins p~ ~ for plywood and LVL
adhesive applications. The desired molecular weight of the spreay-dried resin solids will be inflllenr~ to a large extent by the veneer moisture content. At higher veneer moisture contents, a resin with a higher molecular weight and lower flow ch~a~t~ lics often will be favored.
For pl~à~ion of 1~ ;llAI~ wood products according to the present invention, aqueous resole resins, prior to spray drying, may exhibit a number average molecular weight in the range of about 2,000 to 10,000 and a weight 11- 21~2345 average molecular weight of about 20,000 to 50,000. The aqueous-based resin's poly~ ;'y often is ~lweell about 7 and 14. For purpose of the present invention, the weight and l,wll~l average molecular weights of the phenol-form~ld~Pllyde resole resin can be de~ .n~d by gel pe ...~lion cln.)llld~ography(GPC) using tetrahydlorw~ (THF) solvent, pol~ yl~,~e standards and procedures well known to those skilled in the art.
The process of making the spray dried pl~;w~or, ~q~leoll~ resole resin of the present invention is directly adaptable to eqllirmP-nt conventionally used for making phenol-form~ Phyde resole resins. As noted above, the reaction is con-l~lcted in aqueous solution. Normally, the reaction is con-1ucte~ so that the ltim~te resole resin has a non-volatile material (NVM) content of at least about35 % by weight based on the weight of the resole resin solution. NVM contents of up to about 55-60% are possible, ~lthou~h the NVM content often does not exceed about 50% in order to avoid solutions that have such high Vi~CQSitiPS that they present problems in mixing, ~wll~mg and spray drying.
In using the resole resin for p~alillg a dry, freely-flowing, finely divided particulate or powdered, spray-dried resole resin-based adhesive useful for bol~ding a l~...in~d wood product such as plywood or LVL, it is common to blend the aqueous resin with additional ill~;l~iell~ before spray drying. In addition to the resin so1uti-~n itself, which co,.~ ,s the major COIlll~Olle,lll of the aqueous adhesive llli~lule (generally from about 75 to 100% by weight of the a&esive solids), it is common to include the various fillers, i.e. inert materials added to increase the weight of the adhesive ll~iAlW._, adhesive çxtpn~lprs~ additional caustic and other known adjuva-l~ as in-1ic~tP~l to a certain hereinabove. Generally, a suitable ~qu~Polls adhesive before spray drying will contain from about 25 to 50%
resin solids, from about 3 to 7% fillers, from about 3 to 7-% extenders and fromabout 1 to 10% a~l~lhion~l ~lk~linP catalysts. As noted above, suitable fillers and extenders include starch, wood, flour, nut shell flour, bark products or ~rir,lllblr~l recidue,s, clays, and corn husks. Starch and clays generally are used as fillers, often in amounts of 1% to 10% based on the weight of the resin solids. On a water-free basis the adhesive generally contains about 75 to 100% resin solids.

- 12- 215234~j Resin catalysts for accel~tillg the curing (hardcning) Mte of the resole resin, such as resGl.;i.lol-formql-l~hyde resins and phenol-lesolcillol-formql~lehyde resins, also can be added. Such i~l~iellt~ may be added to the aqueous adhesive before SpMy drying or they can be added as SpMy dried powders th~ lves by dry blending subsequent to the SpMy drying of the aqueous phenolic resole resin.These resins are known catalysts for ~ P the cure Mte of phenol-form~klellyde resole resins. The ~ al~lion and use of these resins in the context of formlllqting adhesives for making wood l~"~ es is well-known to those skilledin this art.
The a~eo~ls resol resin-based adhesive then is fed to a SpMy drier for producing the dry, freely-flowing, finely divided particulate or powdered adhesive used in accordal~ce with the present invention. As used herein, the term "spray dryer" refers to the te~h~ lly sophi~ti~-q~t~oA process of at~ g (in the form offinely divided droplets) a solution or slurry into a hot gas stream under controlled lcl~ela~ul~ conditions and under s~ecirlc gas/liquid cont-qctin~ conditions to effect evaporation of water from the ~o...i~ droplets and production of a dry freely flowing, finely divided particulate product.
Spray drying, as used herein, is typically carried out with p~S~ulc nozzles or centrifugal qlc..~i,.,.~ opeldLillg at speeds of up to 10,000 to 16,000 RPM or more. At these speeds, one millilitçr of liquid feed can be converted to over 100 million fine droplets. Despite this high velocity gell~ration of droplets, the spray dryer is ~e~ignPA so that the droplets do not contact the spray dryer wall underproper opelaling procedures. This effect is achieved by a precise balance of o.--i,~r velocity, air flow, spray dryer tiim~n~ions of height and ~ el~ and inlet and outlet means to produce a cyclonic flow of air in the chqmhçr. This highly speciqli7~A art is clearly to be dislhl~,uished from the random and pl`O--Ii.~ QUS S~lajillg of liquid droplets such as one might do with a garden hose.
In spray drying, the liquid adhesive feed is converted to a fine spray; the water in the feed is evaporated by means of a stream of hot gas, usually air; and the dry, powdered product is sepalàlcd from the stream of hot air. Moisture evaporation is controlled by controlling of the inlet and outlet Iclll~elalul~c of the 13 Zi5~4~

hot air used for drying. Due to the thermos~U~ nature of the product, it is pl.,f~ d that the outlet ~lu~lalule be regulated within the range from about 70to 90C. Variables such as droplet size, air flow rate, air ~elll~.alur~ and hl-~irlity also affect the rate of drying and nltim~te moisture content of the powdered product. The particle size di~lflbulion of the spray-dried adhesive powder, which varies with the degree of ~to..~ ion of the liquid adhesive and amount of solids in the liquid feed, should pl~ r~lably be from about 20 lllicrolls and up, with no particles larger than about 60 microns. A ~h~illg disc-type :ItO~ " iS preferably used for spray drying the liquid a&esive. Before storing the particulate adhesive, it is desirable to cool the material quickly to room t~lllpc,~lure. This can be accon~lished by cont~ tin~ the powdeled adhesive withchilled air.
The freely-flowing, finely divided resole resin-based adhesive powder of the present invention is to be di~lhl~,uislled from dry, pulverized phenolic con-l~n~tion product ler~ lellced in U.S. 2,150,697. When used in c~ clion with dried heat-hardenable resole resins, pul~,~,.~dlion pl~lules likely cannot provide the degree of subdivision without undesired resin ad~ nt realized via spray drying and çsse~ l to the suc~es~rul practice of the present invention.
Moreover, the use of such pulverized material even for the pr~aldlion of plywood, with a small number of veneer plies, is far afield from the ~l~yalalionof l~ ,d veneer lumber with 10 or more plies using a spray-dried a&esive.
T ~ ed wood products can be pr~ from the spray dried phenol-form~klehyde resole resin-based adhesive by applying a thin, subst~nti~lly unirollll layer of the freely flowing particulate dry adhesive to the mating snrf~ces of aplurality of wood vellc~l~. The powdered adhesive can be applied using any one of a wide variety of solids h~n(llin~ means for laying down a UlliÇollll layer of the powdered a&esive material onto the mating surface of each veneer. Such e~ ip...f-.~l would include devices that operate on gas entr~inm~-nt such as pl.~.. ~lic spray guns or curtain-type coaters with spaced nozzles or orifices and devices that operate with me~h~ni~l/gravity type pow~r feed such as a sifting - 14- 21a2~5 -means. Leveling means may be inrl~lded to ensure that a subst~nti~lly u~ ifo layer of adhesive is applied to each veneer mating surface.
De~n~ on wllc~l plywood or LVL is being p,~aled the veneer sheets with the applied layer of adhesive are assembled into a l~ e structure so that the pcwdelcd adhesive is sandwiched bclweell mating surfaces of the wood v~ e,~. The 15,...i.~:~te then is consolidated at a t~ ~lalul~, and p,~,s~u.c and for a time s~rri~;p~ to cure the resole resin, as desc~ ed above in co~-~-Pclion with plywood and LVL m~mlf~cl..~;..g proccdu es.
A particularly ~u~lisillg feature of the present invention is that a plurality of wood veneers each with total moisLulc content of 15% and above can be s~lccç~sfully bonded into a l~ l wood product using an a&esive application level of 30 to 90 grams of resole resin solids per square meter of single glue line.
The following examples are pleselllcd to illustrate and explain the invention. Unless othelvvise il~ ted, all .eç~ ,llccs to parts and ~r~enlages are based on weight and all tell.pclalu.es are c~lessed in degrees Celsius.
Example 1 An aqueous resole resin suitable for spray drying to produce a freely-flowing, finely divided pow~led adhesive can be pl~a.ed as follows. About 25 parts phenol is added to a suitable reaction vessel. Then about 13 parts of a 50%
aqueous forrn~l~ellyde solution, 26 parts additional water, 0.3 parts cresylic acid, a minor amount of a defoall-iug agent, 0.4 parts pearl starch and about 5.7 parts of 50% solution of sodium hydroxide (caustic) are added. The reaction n~lule is allowed to exothem to about 80C and the reaction ~lixlule is held at that lem~lalul~ for about 10 to 20 ~ e~. Additional form~l~el-yde is then slowly added over about a 3/4 to 1 hour period while m~int~ining the batch at a lt;lu~lalule of about 80C to increase the form~l~1el-yde to phenol (plus cresylic acid) mole ratio to about 2.2:1. The batch is then heated quickly to about 95-100C and held at that te...pe-alu.e until the reaction Il~L~lule exhibits a Gardner-Holt (G-H) viscosity of about A-B. The batch is ll~ ,arler cooled to about 80C
and held until a G-H viscosity of about R-S is reached. About 5 . 7 parts additional caustic is added and the batch is reacted to a G-H viscosity of about U-V. Once 21S2:345 the target visco~ily is reached, the batch is cooled and can be stored for a short time before spray drying. The resin solution typically co~i~s about 43 % solids,has an alkalinity content of 5.7 and a Brookfield visco~ily of about 1200 cps.
Example 2 LVL billets (15 ply - 1.5" thick) were produced by applying a powdered phenolic-formql-lehyde resole resin ple~al~d by spray drying an aqueous resole resin to the veneer in an arnount of about 43 grams per square meter of single glueline. The billets were hot pressed using typical LVL press conditions to cure the resin. Wood failure rates, using All~e.ican Plywood ~Csociqtion (APA) test procedures for LVL, were 93 to 98%.
Example 3 Douglas Fir veneer, one-tenth inch thick, was used to ~l~are 1'h inch-15 ply panels with the ~lim~oncions 12 inches by 12 inches. GP 5478 phenolic resoleresin, a spray dried aqueous resole waferboard resin, commercially available from Georgia-Pacific Resins, Inc., was applied to the veneer using a salt shaker-typeapplicator. Four panels were fabricated at a resin application level of. about 43 grarns per square meter (4 gm per ft2) and four panels were fabricated at a resin application level of about 64 grams per square meter (6 gm per ft2). Another four panels were made by applying the resin at 43 grams per square meter (4 gm per ft2) followed by a water spray on the glue line. Finally, a final set of four panels were ~ d by applying GP 52C59 resin spray dried aqueous resole waferboard resin, collllll~cilly available from Georgia-Pacific Resins, Inc., at an applir-q-tion level of about 43 grams per square meter (4 gm per ft2). The following table ~.. z. i~;s other pr~es.~ p parameters. Based on a visual eX~ on of the bond quality, the majority of the panels had a good wood failure in the core.

Table 1.
LVL Panel Properties Resin Panel #Wood Resin Time to Max.
Failure % used 220F temp.
Top Cntr (gms/ft2) (,,,;,,.,l~) F

GP 5478 429 99 100 4.3 16.4 240 4 gms/ft2 430 92 92 4.4 15.7 246 431 100 95 4.5 16.1 242 432 100 100 3.7 17.0 234 GP 5478 433 52 13 6.1 15.4 249 6 gms/ft2 434 80 11 6.4 15.1 251 435 80 58 6.2 15.1 239 436 94 74 6.5 15.0 252 GP 5478 453 93 100 4.7 14.7 249 4 gms/~t2 454 98 100 4.1 16.0 237 with water 455 98 92 4.2 14.9 249 spray 456 90 99 4.3 14.3 250 GP 52C59 449 96 100 4.9 15.1 255 4 gms/ft2 450 100 99 3.9 16.7 249 451 92 98 4.4 14.4 257 452 88 98 4.7 14.8 244 -17- 21~2345 Example 4 An aqueous resole resin suitable for spray drying to produce a freely-flowing, finely divided powdered adhesive can be pl~cd as follows. Phenol (about 29 parts), formql~ehyde (about 10 parts of a 50% aqueous solution) and about 23 parts water are added to a suitable reaction vessel. Then about 6.3 parts of a 50% aqueous sodium hy~uAide solution (caustic) is added and the reaction iAlUlC is heated, at least in part by the reaction exothern, to a tclll~elalulc of about 85C. Additional forrnq-lde~yde is added in an amount sufficient to bring the fonnql~e~lyde to phenol mole ratio to about 2.2:1 and the batch is reacted to a G-H viscosity of about B. The batch then is cooled to a tclll~lalul~ of about 80C, reactcd to a G-H viscosity of EF, cooled quickly to a tcl~ alulc of about 65C and reacted to a G-H viscosily of about J. A small amount of tall oil fattyacid (TOFA) and ammonium hydroxide (1 part of a 28% aqueous solution) are added during the cooling step. As produced, the resin solution has a solids content of about 45%, an qlk~linhy content of about 3.5% and a Brookfleld viscosity of about 275 cps. The resin is suitable for spraying to a freely-flowing, finely divided powder.
While certain specific embo~ x of the invention have been described with partir, ll-q-rity herein, it will be recognized that various mo~ifir-q-tion.c thereof will occur to those skilled in the art and it is to be lmflerstood that such molifi~tions and variations are to be inr1llded witbin the preview of this application and the spirit and scope of the appended claims.

Claims (6)

We claim:

1. A method for producing a wood laminate from a plurality of wood veneers comprising (a) applying a freely-flowing, finely divided particulate dryadhesive to at least one mating surface of each of said plurality of wood veneers, said adhesive having been prepared by spray drying an aqueous adhesive mixture containing a phenol-formaldehyde resole resin, said resin having been prepared as an aqueous resole resin by reacting at least an equal molar amount of formaldehyde with phenol in the presence of an alkaline catalyst, said freely-flowing, finely divided particulate dry adhesive being applied to said mating surfaces in an amount to provide 30 to 90 grams of resole resin solids per square meter of single glue line, (b) assembling said plurality of wood veneers into a laminate so that said particulate dry adhesive is sandwiched between mating surfaces of said wood veneers and (c) consolidating said laminate at a temperature, pressure and for a time sufficient to cure the resole resin.

2. The method of claim 1 wherein each of said wood veneers have a total moisture content of above about 15%.

3. The method of claim 2 wherein said alkaline catalyst is present in said aqueous resole resin in an amount sufficient to provide a resole alkalinitycontent of between 1 and 15%

4. The method of claim 1 wherein said freely-flowing, finely divided particulate dry adhesive has a particle size distribution with no particle larger than about 60 microns.

5. The method of claim 1 wherein said wood laminate is laminate veneer lumber.

6. The method of claim 5 wherein said laminate veneer lumber has at least 10 plies of veneer.