US2656296A - Process of manufacturing fibrous sheet covered plywood - Google Patents

Description

D. H. GRANsAA-RD PROCESS OF MANUFACTURING FIBROUS SHEET COVERED PLYWOOD Filed June 5, 1951 oct. 2o, 1953 3 snets-sheet 1 Patented Oct. 20, 1953 PROCESS F MANUFACTURING FIBROUS SHEET COVERED PLYWOOD Donald H. Grangaard, Appleton, Wis., assignor to Paper Patents Company, Neenah, Wis., a corporation of Wisconsin Application June 5, 1951, Serial No. 229,951

The present invention relates generally to the manufacture of plywood and similar sheet materials, and has for its principal object the improvement of the surface characteristics of these materials, especially plywood made from Douglas fir.

, As will hereinafter appear, this object is accomplished by the provision of a masking or cover sheet of novel type, which is adapted to be attached to the sheet material with which it is used so as to constitute the exposed surface thereof.

Very large amounts of plywood are manufactured and used each year in the United States for buildings and for other purposes, and a large portion of this plywood is made from Douglas fir. This wood' has the advantages of being strong and relatively inexpensive, while at the same time being comparatively easy to fabricate. Douglas fir plywood has, however, certain very important disadvantages. Particularly, it is difiicult to obtain logs which are completely free from knots or other blemishes, and as a result, the available surface veneers frequently contain relatively large knot holes, pitch pockets or other defects. Moreover, even when it is possible to Obtain a substantially perfect surface ply, Douglas fir is subject to checking and to very severe grain-raising, both of which render the obtaining of a smooth, grain-free finish on the plywood surface almost impossible.

The phenomenon of grain-raising is caused by the variation in density in the wood due to differing rates of growth in the spring and summer growth periods. The differing densities in the wood result in differing rates of expansion or contraction with change in moisture content, and the grain becomes clearly visible on the face of the sheet or panel. This is especially objectionable in cabinets, doors, etc., which are to be subsequently painted, or finished, with a glossy paint or enamel.

Checking is evidenced by relatively short, hair line cracks, the cracks developing most rapidly when the face veneer is dried out, a condition which commonly occurs in the interior of houses, etc.,when the relative humid-ity is low. In aggravated cases, the wood may actually split. It is not possible to completely prevent or overcome checking or splits by painting, enameling, or

pear as cracks in the nish.

The defects resulting from knots, pitch pockets, and the like are corrected by the use of plug patches, the most commonly used types being 4 Claims. (Cl. 154-132) known as cookie patches and boat patches. In the use of plug patches, a hole is cut or punched from the wood sheet or veneer of sufiicient size to include the entire defective area,l

and an insert of similar shape, which has been cut from a perfect sheet, is inserted in the cutout. These inserts are, however, clearly visible in the completed plywood sheet, and since at least the outline will show through paint or other finishes, they have the effect of lowering the quality of the sheet. To this extent, the defects resulting from the use of plug patches are most serious, from an economic viewpoint, Vbecause a plywood sheet which contains any visible defects on either surface must be sold as a lower grade sheet than one which is completely free from defects.

In the practice of the present invention, a sheet of resin containing paper is applied to 1 the exposed surface of the plywood. This resinbearing sheet has certain very definite requirements and characteristics, as will hereinafter appear, and when applied to the plywood surface, lit

effectively masks all of the ordinary surface de-v fects, including raised grain, ordinary checking and small splits, and both cookie and boat type plug patches. The principal object of the in.- Vention is thus accomplished and a smooth yfinish, free from grain or other imperfections is obtained on the plywood or other sheet material with which the masking sheet is used.

The present application is a continuation-inpart of my prior abandoned applications Serial No. 779,889, and Serial No. 116,810, which were filed on October 15, 1947, and September 20, 1949, respectively, and which were assigned to the assignee of the present invention. In those applications I have disclosed certain particularly satisfactory masking sheetsA which are likewise capable of accomplishing the above stated principal object of the invention. More particularly, the earlier led application, Serial No. 779,889, is principally directed to masking sheets containing resins of the phenol-aldehyde type, while the second application, Serial No. 116,810, is concerned primarily with masking sheets containing 'other nishing methods, and these defects apresins of the urea-aldehyde type.

This application combines the disclosures of these two prior applications, and includes, 'in addition, certain extensions of, and additions to, the disclosures madeV these said applications, particularly with regard to the addition of the resin to the sheet. Hence, a further object of the present invention is to provide improved methods o f manufacturing resin-containing ply- `wood masking sheets, in accordance with the 'general principle disclosed in my said applications Serial No. 779,889 and Serial No. 116,810. A still further, and even more specific object of the invention is to produce a masking sheet of this type which contains one or morenf the beater addition resins', alone o r in` combination With one or more other resins and which, at the same time, has very high wet abrasion and scuff resisting surfaces.

In Figure 1 of the drawings; there s'hematic illustration of a method of manufacturing the improved cover or masking sheet of the invention;

Figure 2 shows, in schematic illustration, another method of manufacturing the improved masking sheet of the invention;

Figure 3 is an enlarged, fragxnentaryperspe tive View, showing the completed masking sheet having. e glee-lirrrler-erhneeive @replied .the-rete; ....Eisurefr free eieererrmetwiew .Off .e ply-W09@ mess and illustrates .the meenerirr rb ierrmesk? irrasheets. .irr-eeeerderreewithiihe.irrirfrriierl'rlei' besepplied .te the,..s1;1 eeee Oiihe. .Plrweed panels.; .Figure 5. is .a .seetreeel new illustrating.. e. Per: of e. elifweee .Darrel-with.- eerer .Qrmeslrirle sheets. `in accorda nce .vvith the invention, app1ieslf..te. beth .serfeees .heref;. Y

.1.. rieure elan vrevmf e. eie.rr1...mber rise@ in.. determining ..eertein ...ebereeteristes 0f the masking sheets eff the irri/entier;

f Figure? ,is en.. enlarged. iraginentary.. seerienal new... .illustratirrehemanner .in which the die memberef Fieur.e8..i.sa erephehQwi.. e....verieiir1. in the per cent of extractable resin With ur ing .time ron-three .representative .types ef.. .meskineehets made in accordanoe v vith thepresent invention from,e. .'relativelyr s.1.evf....am,edir1rrry.er1d e feet curing, .resir.1..,..-...."1,`he..e'reeh also., .illustrates .e .resi procedureyvhich will hereinafter be described ir;sreater...rietei 1... .,ln...the.merruieeture ...er .the 'meshing street .Qf thepresent invention... .resin is uniformly distrib.- uted .threuehoutthesheet and thsresinis. then advanced to. a. high. .degreeef eure. ,the ed weer? desreenof .eure being.. V011e ior.. the imperia?? .iee- .tures ,or lthe..inventier1 ...The resrnrnerhe ubi.-

iormlynistributed.threusheut e besesheetlefier that sheethas beenformedaonirr .the alternative., theresrr may .be introduced.. .inte elle .Dreduet-by mixng..a.resin..or ...resins .with .the 9.1.1.1r,... ,The masking sheet...ey.also.:bemenufeetrlrerlby dis.- ibutng...the.resin. or-.resns irl the sheet by e eombnatonof. .these,methods....m.. M .when in@ -resin is. to be. .irrtrodueedinie erre,- iormed. sheet, ...n..bese sheet. 0fy .rather moderately.. absorbent .paper,.-,mey be -peseed through.resiny apply-ins .Ineens .seas te eieet .errirorm .distribution of .resin throughout thaesheet.- The. .resinfleeeurrlg.` sheet. .may..,the.n. be.. mei/eel threuehdryirreend..eurir1e..erer1s tei. eerrlplisb eeritrelleci, @urina-.0r the.. resin- Felewirre.. the curing. eneratieaiheresin.,bearing.,eheetlrleS/.Pe

.conducted threuehaeeolmazene, after, .whieh .er1 adhesive coatingmay. be applied to Qnesurfae thereetier. the purpese-frrerldrle glue; line- As a final s tep, .the glue ,..if used, 'should be dried. .tee nereteekr.. eeneitierlf If` ainpr.part.lof,..the.resirris.t9 be. .impregnated inithe. masking .sheet as errineident,tethemermf .facture .of .the .leasey sheet,..e beatervedeliten. .type resin..,whiehhesrelatively v.reuiirl eurngnehereeteristies, isfintrosiueed into .the peper furnish preferably at the beater, the resin being in either liquid or solid form. When this is done, the beater thoroughly disperses and uniformly distributes the resin throughout the pulp ber. The furnish is then pumped to a conventional type paper-making machine, such as a Fourdrinier machne, and formedintoga sheet. A glue vline iriaybefpplidltothe sheet atthe end of the paper making operation and, if the g1ue line is used, it is dried suiiciently to produce a non-tacky condition as a nal step in the formatioilbfwthewfrs'liig sheet of the invention.

'Rgardless` of the particular method employed i0 ierreet-rheintnqgnuon of resin into the sheet, itis portanti-hat the resin shall be uniformly distributed throughout the sheet, and that it shall be curedlto ith e e,xtent to be hereinafter defined. When this isudne, the sheets resulting from any ofhtheseunethods Will have excellent masking characteristics. In this;v connection, it should be noted that. .the preferredinethods of manufacturing sinfbeariigfhiaskng sheetsin `ac rdancez'wthuthe invention d'o not use press efoi the purpselef df 'ing theresin eure. aridi @i .bhe .resmfeureisi .eereeeedion hier. 'degree 'e er to lany use of thev masking sheet. The latter chai? acteristiclothe.-,invention is of signiancej'r thereafsonthatit isco'ntrary'to the commercial practicel of preparing ,resinbearing ,laminates and, indeed, has been carefully avoidedheretofore. The dravving's Villustrate tivo general typ s of apparatus for manufacturingthe masking vsheet negentien. reuse. eiserne., ressens wherebythe resrl...`lrrillfe l'ation may be...in'tro` duced into'a preformed sheetand ,Figure '2,'.illu`s iretirisapparet's whereri .resin may AJee infiel .duced intola tjf .paper .during @H manu'- factre of Lthat sheet. AIfrth'e. particular. apparatus illustrated v.in.,figure-. l, a base 'sheet is;` con- .tainedpn .1 supmyron, illustrated 'at 1 and uns `sheet lis guided tl1roug-h;` a pan 8 .which contains' a solutionfof impregnatinaresin, by rreans. of fguidenrollsjandill). Ifo assurethat the proper amount fofAresfn'nvvill be contained in the sheet, anditoeffect even. resin i; jip 'gnation and. diseieeeee. .e .perr'geffs'eleeze .51.1 end fillers arranged toengagejthebas'e 'she' tasIit emerges frrr-rnthe. batl i.o f resin.. .Je 'Squee'ie'rolls I I erre... l?. ere desireels'z eldl eerily sbrierea'iri order that the amounnonresin .retention in .une vererben. .eerr...lee. eeerrtelv. 'erirf'Hed. .After leavingthe s'iueeael rollsal I2, theshfeet is c nveyed through. an oven I3. by4 means of a v.Belt e9r1veyer;.l.5....Theme- 'lineledes threaten@ rate...heating2 sections: |1 If'and` |9, which. are adapted.. tube maintained 'fat different inter'de'-, renderer. een... ree@ .temeereferes in. order 'ther the. .drying ...elle euries 'er the resin. rrlpreeeied erreeee'err Jee ,eee'ereelr eenlrelled. ...Trie '0i/'er1 .!3..e1ee.ir1,elrrdes ...e-.remeber @Oelregseeten f vvhich may be provided with fans .orfoolin'g coils, er..beth...rererfeeting eee1ir1e..ef..rhe sheet in geert .inierve1eitime- Thesrevisien er e @e01- .irre.z...eer1e ....immedietely... renewing ...the ...heeiei .eerirrerenes .else eisisin ...s eeurirreeeeurere eers..- trel .ef the .eurir1g-91eeration- ...It will .be underv .SOQQL that.. .other means .and procedures.. can, 4be .employed .for .ineerperating .ther resimime .the sheetnerrdr effeetirieihe. curing Qf. the .seme inthe.epparatusillustrated in Figurel; aelue line adhesive is appliedto one f acef the cured. resin-.bearing sheet; .immeeietelyerterit leaves .thepvenherrleerrsfer eeeemplishirg'this .mehr esererrr! reneerrteinine the. adhesive irr liquid form, a dip roll 22 and a pair of squeeze rolls .23 and 24. After passing through the adf- ,i The glue line adhesive can be applied as a completely separate operation, or it may be )omitted entirely. In some instances, it will be .found more desirable to apply the adhesive for joining the masking sheet to the surface of the .plywood or other product with which the sheet is used at the time that the sheet is to be attached and the adhesive may then be applied to .the Wood rather than to the masking sheet, or a separate sheet or film of the adhesive may be lused in accordance with the known practices of the art.

The base sheet used in apparatus of the type shown in Figure 1 preferablyy comprises a 100% .kraft sheet.` However, satisfactory sheets can be manufactured from pulp furnishes of other types, for example, sheets manufactured from sulphite pulp, ,semi-chemical pulp, groundwoodsulphite mixtures, etc., such sheets giving good results in most instances. The `particular pulp or pulp mixture used will depend to some extent vupon the handleability conditions required in the `final `sheet and upon the degree of scuff and .abrasion resistance required in the end product.

Investigations have shown that the base sheet should have a basis weight when dry within the range of from about lt-o 210 pounds per standard ream, 500 sheets 24 by 36 inches (bone dry basis), with a density Within the range of from about .55 .to .80 gram per cubic centimeter and that the optimum sheet has a dry basis weight within the range of from about 125 to 135 pounds per standard ream, at densities Within the range of from .65 to .'75 gram per cubic centimeter. If

the density is too low, the resulting masking sheet will lack abrasion or scuff resistance, and if the density is too high, it will become unduly Vdiillcult to accomplish the particular kind of resin impregnation that is required in the attain- .ing of the objects of the invention. Generally stated, the base sheet should be a relatively `dense sheet which, at the same time, is absorbent. 'Ihe heavier basis weight sheets tend to give lbetter masking, but sheets having basis weights ,materially greater than the optimum range specified are uneconomical.

. The caliper or thickness of the base sheet may vary between rather wide limits, but the caliper lis desirably in the range of from about 10 to 13.5

thousandths of an inch. The pH of the base sheet is not particularly important, although it will usually be found most convenient to operate with a base sheet which has a pH which is some- ,:whatnon the acid side, i. e. a pH in the range of from about 4.0 to 4.5. The tensile strength of the `sheet is also not of too great importance, but it should be sufficiently great so that the sheet may vbe drawn through the impregnating apparatus IWithout tearing or breaking.

In one particularly satisfactory embodiment :of

Ythe invention the base sheet constituted a 100 per vcent kraft sheet, having an unimpregnated basis 'weight of approximately 125 pounds per standard ream of500sheets 24 by 36v inches (bone dry basis). The sheet had an average caliper or thickness Within the range of about 12.5 to 13.5

one-thousandths of an inch and a density within f the range `of from .56 to .65 gram per cubic centimeter. The raverage net wet tensile strength. of the sheet was about 10 pounds per inch of width. The resin impregnating material used in the manufacture of this embodiment of the invention consisted of an aqueous solution of a phenol formaldehyde resin having high penetrability and low viscosity at the concentrations used for impregnating the sheet, and the actual impregnating solution had a solids content of approximately 25 per cent and a viscosity of `about 50 centipoises at the impregnating temperature, which was within the range of from about to F. Elevated temperatures of this order are desirable during the impregnating operation in order to improve .the penetration of the resin into the base sheet and to facilitate the subsequent drying operati'on. Y. l

After passage through the squeeze rolls III and I2, the impregnated sheet had a wet weight of approximately 365 pounds per standarddream. This resulted in a resin content in the nal dry sheet, as it came from the oven I3, of approximately 55 pounds per standard ream, or a resin content of about 30.5 per cent, based on the finished total weight yof the dry sheet. i In the oven I3, the initial zone I'I was maintained at a temperature of about 325 F. Vflf'he temperature of the next succeeding zone I8 was approximately 350 F. and the temperature in the nal heating zone I9 was maintained at approximately 375 F. The entire oven I3, excluding the cooling zone 20, had a length of approximately 100 feet, andthe three heating zones were of equal length. The temperature in the cooling zone was maintained substantially equal to room temperature by the use of circulating fans. During the entire operation, the sheet was moved at a uniform speed within the range -of from about 25 to 30 feet per minute. Under these conditions, the resin was dried and cured within the oven structure I3 to the highly advanced state which is required in carrying out the invention. A portion of a completed sheet is illustrated at 29 in Figure 3. It will be understood that the impregnating resin is distributed throughout the base sheet and that the glue line resin comprises a thin surface coating. f Y

In another highly satisfactory embodiment of the invention urea-aldehyde resins were employed. Urea-aldehyde resins are either white or substantially colorless when cured,l and when these resins are used, masking sheets which are light in color may be produced. Such masking sheets contrast with the relatively dark sheets which result from the use of Phenol-aldehyde resin in the masking sheets. In order to obtain the maximum advantage from the use of ureaaldehyde resins, base sheets which are light in color should be employed, as for example, sheets which are made from unbleached sulphite pulp, bleached sulphite pulp, semi-chemical pulp, groundwood pulp, alpha sulphite pulp, soda pulp, and bleached kraft pulp, or mixtures thereof. course, sheets made from other pulps may be employed if the color of the sheets is not objectionable.

A masking sheet containing a urea-formaldehyde resin was produced by taking a selected base sheet made from all sulphite furnish (unbleached balsam sulphite) having a basis weight, when dried, of approximately 119 pounds per standard ream of 500 sheets, 24 by 36 inches (bone dry basis). The sheet possessed sufficient wet strength to enable it to be drawnthrough the impregnating bath and thedrying oven, and

, zer'rresinv content, However, such-flisht f st '"f lmarmfafcture andflandlealiilty of the 'sheet "ae fnottoo'fimportant; the resin cement' een be rirv`creased to'fas 'highas-B per'fcent. The us'pf sheetsfhalving'a resin content `onacry sheet Al'niajsis of 1less Vthanvl'O tper cent or greater than'ffpr cntfis fnotfgenerally recommended.

Thefamo'unt `of `'uncured' resin 'applied" t'o-the base ;sheet 'during the impregnating operation should p e suh, -When--nreaformaldehyde resins are employed, to produce laconcentrationV 'of eure@ resin in the impregnated andl ldried sheet @11.1 the. range. of imm. 5 it@ 535 .--per cent A,by .Wght preferred resin content 'the caseAl or phenolic Iresin impregnation, is-within trie range of-froin about 2'5 to SOfper cent by Wigi'ihro'n abne dry basis.

The d'ensityof the i'she'diinasking sheet as nico es jfrein "or tbfs "bwww used, is'a particularly ugem index brine exenicef the sheet. Generally stated, the maskingsheet at this time, should have a density within the range of from about .60 to .95 gram per cubic centimeter, and sheets within this range Will have satisfactory Wet and dry abrasion resistance. In

`this connection, it is important to note that this density range is attained Without the application of pressure during the curing operation.

It will be noted that the carrying out of the impregnation processes on apparatus of the type shown in Figure 1 causes the resin reaction to be completed and the resin to be formed within the cellulosic web of theV base sheet." This is believed to be of some importance in accomplishing the objects of the invention, and especially increasing wet abrasion, by the procedures hereinafter set forth.

The `drawings also illustrate apparatus for preparing a masking sheet by the addition of all or part ofthe resin-impregnation to the pulp prior to formation of the sheet, this apparatus `being illustrated in Figure 2.' 'I'he incorporation of all or part of the resin into the pulp has several distinct advantages. In the first place, such ja ymethod is inherently economical because the resin is incorporated into the sheet Without the necessity of an additional step and the need for impregnating apparatus is eliminated. In the second place, the resin can be uniformly distributed in the pulp through the inherent operation of the beaters, which are normally employed in preparing the pulp for the papermaking operation, independently of the characteristics of the resulting sheet.

In the particular apparatus shown in Figure 2, a beater, which is employed in the usual manner, is illustrated generally at 3|. During opthroughout the paper furnish. After the pulp and resin are thoroughly mixed,

the contents of the beater are delivered to a pa- 'permaking machine which maybe the Fourdrinier 'type as shown at 33. The furnish is transferred from the beater 3| to `the papermaking machine lvia the usual stock chest 35, and stock pump 31,v which, may be of the-centrifugal type, as -ilillustrated. From the stock pump 31, the pulpV passes through a Vdistributing iiow box 39, ma-

chine screens 4|', and flow box 43 in accordance with known practices. The masking sheet formed onl the Fourdrinier wire 44 is conveyed through 4thefusual press rolls 45 into `a drier section 41.

Afterpassing through the dryer section, -the sheet may be passed through calenders 49, and

' finally it is wound upon a take-up reel 5|.

lIf a glue line is to be applied to one face of the resin-bearing sheet, the line maybe applied after the sheet leaves the calenders 49. l A suitable `means` for accomplishing the rapplication of Atheglue line is shown in the illustrated apparatus, and includes a pan 52 for containing the adhesive in liquid form, a dip roll 53 and a pair i of squeeze rolls 55 and 51. After passing through the adhesive'applying meansthe sheet is conducted through a drying chamber 59,which is operative to dry the adhesivev on the sheet merely to a non-tacky condition. .After application Y of resin the glue line, the sheet may be taken up on the reel 5|.

.As in the case' of the impregnated sheet resulting from the immersion process, the glue line adhesive can be applied as a completely separate operation, or it may be omitted entirely. When it is omitted, the adhesive for joining the masking sheet to the surface of the plywood, or other product with which the sheet is used, may be applied to the surface of the wood or a separate sheet or film of adhesive may be `used intermediate the masking sheet and wood in accordance with Well known practices. Generally, however, better masking characteristics are obtained when the glue line adhesive is applied to the sheet and dried to a non-tacky condition.

In the manufacture of resin-containing maskingsheets in accordance with the invention, by processes'whereby all or part of the resin impregnation is by beater addition procedures, the sheet may be made by the use of almost .any of the known types of pulp. For example, I have found that unbleached kraft pulps, unbleached sulphite pulps, groundwood sulphite pulp mixtures, and semi-chemical pulps provide highly satisfactory results. While the results obtained are not dependent upon the kind of pulp, they are directly related to the pulp freeness. For obtaining most satisfactory abrasion resistance in the resulting masking sheet, lower pulpV freenesses are desirable. However, if the pulp freeness is too low, the handleability of the resulting sheet may be impaired and the sheet will be more diilicult to manufacture. Most satisfactory masking sheets are obtained when thefreeness of the pulp lies in the range of from about 300 to 400 when measured by the Canadian freeness tester. 1

I vhave found that particularly good results can be obtained through theuse of a pulp comprising a mixture of chemical pulp such as unbleached balsam sulphite and groundwood. In general, pulps of the desired freeness are obtained when the ratio of chemical pulp to ground- Wood is in' therange of from about 50 to 75 parts of chemical pulp to from'about 50 to 25 parts groundwood. f

When comminuted insoluble resins are employed as beater addition resins, such as a water insoluble phenolic resin, further treatment of the pulp, after resin addition is not required. However, when water soluble, alkaline soluble resins, or resin emulsions are used, it will generally be necessary to acidify or otherwise treat4 the pulp 'in order to deposit the resin upon the fibers comprising the pulp. If the pulp is not acidied, the =resin may not bind satisfactorily to the pulp and, consequently, will drain from the pulp in the paper making operations'.` Accordingly, after a soluble resin or resin emulsion is added to the beater, the Vusual Apractice will be to reduce the pH of the pulp to avalue from between aboutV 4 and about 7 through the addition of an acidifying agent such as alum,-sulfuric acid, mixtures of alum and sulphuric acidI mixtures of acetic and phosphoric acid, etc.

The addition of the acidifying agent causes vthe resin to comev out `of solution and deposit upon the cellulose fibers, In the case of i some beater addition resins, this deposit constitutes a precipitation but, on the other hand, when other beater addition resins are employed, acidification results in the breaking of an emulsion or colloidal suspension causing the resin to coagulate upon the vbers.l

Generally speaking, thelower the pH, the greater the percent of resin retention .by the iib'ers. However, lower pH values interferewith the pulp processingand paperrnaking equipment. llorrbesti results, thepulp shouldnbeacidied to a pH iny the range of:4;5 to 5;5.

Best" masking `qualitiesl result when the' basis Weight ofv the sheet made! by the use" of `a vresin containing'pulp is relatively high,: inthe range `oli'rom. about 150i to =300fpounds per 500.- ream of sheets-.241W 36 inches (bone dry basis). The thiokness'of the'vsheetshould' be in thefrangeof -fromfabout 141 to .28. mils. However; in' some instances, :proper equipment. may notl be available for-obtaining -sheetsY of suoli high basis weight necessitatingl the manufacture of sheets 'having much. lower :basis weights. 'If such lightweight sheets: are'. to bei satisfactorily used for' masking purposes, multiple llayups willl ordinarily be used, ine..several?ofthe` sheets will be superposed upon one another, andi the individual sheets maybe 'heid togetherby' means of a lightglue lineal;-

to one side of. the'sheet or by means"v ot a separate-'attachment sheet.

The resin content of` themasking sheet'im- Ypregnatedby the beater additionV processmay "vary between rather broad limits, depending upon the properties 'desired in the' resulting sheet. Generally, i thel resinl content of the sheet is. directly*proportionalito the. amount of"ahrasion resistentie, kwai-,erA resistance, etc. However, if thev` resin content of the sheet-is too high, hanelleability of the sheet is lowered andthe mask- `ing?sheet'fibecornes'tooexpensive. It has been 'found thatoptimum overall results are obtained 'when thermal sheet' contains between about 25 'percent amd'aboutao percentgresin, onxa bone dry basis, as in the case of resin impregnated `fslreets prepared bythe immersion process. However, a' masking sheet satisfactoryV formany purisattainable 'with resin contents as low vas Aabout 1'0 per cent. The resin content should not bei overabout 35 per cent.

llt will. be understood, of course,` that. substanftiallyfgreaterfamounts off resinv -rwill'have to be fadded 'to' 'the beater than is contained' 'in the resulting sheet to compensate for inherentresin solubility, 'physical 'lossesy etc. Investigations have shown that the 'losses may equal the amount fof resin 'in the masking isheetfand, accordingly, twice as 'ni-uch res-in should be added tothe beater islpreserrt iin the vfinished, sheet.

The additionk resinsemployed'zpreerably lconfstitutel one yor 'more fof-fthe relatively rapid curing Such characteristics are. important beca-use the resins should' advance Sto highdegree oicuref during :manufacture Lof the sheet. 'In "ftl'is'fconnection, iteshould be noted that hereto- *ffore it has -b'een considered 4undesirable to permit `tl'ie"'res"in sheet material for laminating f-purposes, fto advance in cure and such a result was avoided. natersoluble resins, or .resins soluble in- 'alkaline solutions Jare '-preerred, `'and .there :are various gsuch phenol 'formald'ehyde resins avail- --afble `4cornrnereiiatlly. 'Water dispersableresins lin emulsion or dry powdered form mayalso be used.

*It-will be understood `thaturea formaldehyde and melamine resins 'having satisfactory fc'hartaeter'isticsfm'ay 'also :be used. In -the'latt'er coninecti'on, investigations 'have shown that fitis very lillciilt 't'o obtain sheets containing more ithan ellooi't 56 -res`in 4f'tbon'e 'dry ibasis) when Ypresently availablebeater addition .resins of fthe urea and fmlaininetype are employed. '.nccordinglyfitfmay be necessary to incorporate one or more-'addil2 tional.- resinsy hr suchsheets Lbrother impregnationprocedures,` as for example Vbythe procedures hereinbeforei-described, toobtain a sumcient total resin; content to provide a` satisfactory masking sheet.

:When resins are' added totheV beater in Comminutedf. solids form. they tend. to form 4gliibulfs in theresulting masking sheet and, these globules are apparent in thermaskng sheet, thereby decreasing the rna-rketabilityI ofthe sheet.. Therefore, the-resin. is preferably added'vto thenulp in liquidv'form and for this reason. water or alkaline soluble.- or emulsion forming resins are employed.

'In. the manufactureV of a 'satisfactory embodiment of .thenin-venton by the beater addition resin-impregnation procedure, a beater mixture consisting of 1170; pounds' of unbleached V11emlockfsulphite pulpand 630 poundsof unbleched sprucergroundwood was prepared. To this was .addedan alkaline solution of.- a kpher-iol-aldehyde :resin` lof .the beater addition type.: the solntimri comprisingabout Y- 46% resin solids. .Sufeen-t solntionwas added-to provide900 poundszo resin. The resin solutionv was allowed to mix with, the pulp in the beater for approximatelyll) minutes and anaqueous solution-.containing .25 per cent alum was-.added slowly tothe mixtureof pulp and resin'until thepI-I .of the beater @entente was iieduoed. to between 4.5 to 5.0.. Thestok had .a freenessofzeto 352` (C-anadiani-reeness tester) afterlbeating, and Wasi delivered directly. to `a papennaking machine. Dependingnpon various operating speeds and machineondions, the

resulting masking sheet had a basis` weight between 104- and 148 pounds per standard ream and a resin content of between 13 and ..277 yper cent. .The -densityof the sheet varied between 6.3.v and .72y igram percubic centimeter .and thefthickness .or vcaliper. `ofthe sheet varied between 19,3 and 27.3 one-.thousandthsf-of an As before noted, the resin is `cured to a highly advanced state prior .to taking up. of. .the sheet on the ,reel I The masking sheet prepared by the.. use .of addition resins. thus includes .a resin impregnation havingfa. .state .oiurc Substantially .thesameasthose .resulting when the :Resin impregnation is incorporated into .the Sheet by .other processes. ,.Moreover, as .before noted,

.sheets .made by any of/-theprocessesare orten times ,d'icult .to dilerentiate.

As has 'been indicated` above, thefmasking .sheet yof .the .invention ,may .be .preparedby .a lcombination of thefaboveprooesses. Such a combination procedureis desirable whenit is advantageous to .incorporatetwo types .o f resins in `the masking sheet, as...flor..example.z-a phenol-.aldehyde resin and .a i-urea-laldehyde resin. may benone by .add-ing .a .beater .addition resinto-the pulp and ,theniurther impregnating thesheet made `from the v,nullo with `anotl'ier Lyne 0f. resin by .an immersion .or other process. When this p rp- .cedure isemployed, the'freeness. of the pulp .should lbe somewhaltlhigher than before indicated to assure adeguate and uniform impregna- .tion .of the second resin. yHowever, the -freeness should not 'beyso great as to interfere with the abrasion and -scu'ff resisting properties 'of vthe sheet. *It lappears that Athe proper freeness 'range `for the usual Ymultiple type impregnation should be'fromabou't 400 to 500, when measured `by the 'Canadian 'Sfreeness tester. l'It =wl1.=be :apparent lthat'two for v:more types of; resins 'may also be incorporated int-o ,fa .ibase zsheet 'by carrying :out theai'rmmersion impregnation procedure `Suc- ,abrasion resistance.

cessive steps. It is desirable to advance vthe cure of the resin to a high degree before carrying out a second impregnation process but, under some circumstances, two or more types of resins may be applied to a base sheet in seriatim prior to curing.

. As fully described in my aforementioned copending application Serial No. 116,810, where urea resins are employed in the treatment of the masking sheet, very remarkable improvement in .the wet-abrasion resistance of the masking sheet material can be obtained by the introduction vinto the sheet, preferably before the impregnation of that sheet with resin, of small percentages 4of specific water soluble, inorganic acid salts of aluminum and ammonium. Specifically, these compounds are ammonium chloride, ammonium phosphate, ammonium sulfate, ammonium thiodesired increase in wet-abrasion resistance is to 1 be obtained. It isalso important that at least a part of the catalytic material, and preferably all of it beadded to the base sheet prior to the resin-impregnating operation, this being particularly true when using aluminum sulfate f which is the best of the 4several catalytic materials described. W'hen a catalytic material oi the type described above is used, the total amount -ofsuch material present in the sheet during the resin-curing operation should be within the range of from .4 to 1.5 per cent of the dry weight of the resin in the base sheet. The optimum amount of aluminum sulfate is within the range of` from .4 to .8 per cent. A lesser amount of the catalytic material, or a greateramount, will .substantially decrease the resulting benecial effect and will result in a sheet of much less Part of the catalytic material may be introduced into the base sheet as a component of the impregnating resin, although the resultant sheets areusually somewhat inferior in wet-.abrasion `resistance to the sheets obtained when all of the catalytic material is added to the base sheet prior to impregnation. Also it is possible to add all of the catalytic material to the impregnating resin, although this practice results in a much lessy wet-abrasion resistance sheet than' is the case when at least part of the catalytic material is added to the base sheet. Y

. The catalytic materials disclosed are all water soluble materialsl and may be conveniently introduced into the basesheet by passing the sheet through a water solution of the catalytic material. Generally, whenA using base sheets of the 'type described, the concentration of the catalytic salt in the water solution should be about half `the concentration required in the base sheet.

Aftery passing the sheet through the solution of vcatalytic material, the sheet is desirably, but

not necessarily, dried prior to the impregnation with resin.

The degree'of curing which is accomplished when the resin-bearing sheet moves through the heated zones of the oven I3, or when themasking sheet is formed from resin containing pulp, is of the greatest importance in accomplishing the objects rof the invention. In Aorder that the completed sheetshall be capable of masking the y v `various defects discussed above, the resin conbe impractical or impossible to use.

14 tained in the sheet must be cured to such adegree that it will exhibit essentially non-flow characteristics when subjected to the pressures and temperatures normally employed in the manufacf ture of plywood, i. e. to 300 pounds .per square inch and from about 260 to 325 F. The cure must also be carried up to the point where the sheet is non-laminating at pressure in excess or 1000 pounds per square inch at a temperature in excess of 250 F. Unless curing is advanced to this extent, raised grain, patch outlines and splits will show through the cover or masking sheet, and the principal object of the invention, which is the production of a smooth, blemish freesurf face, will not be accomplished. However, the cure is desirably not carried to the point where the sheet would be non-laminating at pressures over 2000 pounds per square-inch and temperatures over 350 F. If the cure is carried very much beyond this limit condition, the nal sheet may be too brittle to handle in roll form and may As used herein, the term` non-laminating refers to the inability of the resin containing sheets to vadhere to one another. Thus, if the resin containing sheets are placed in face to face relation and subjected to pressure and heat, as specied above., and do not adhere, the sheet is said to be nonlaminating under those conditions.

The desired procedure of the invention requires that the cure be carried to the point where4 masking will be obtained, but not materially beyond that point. The determination of when the` cure has proceeded to this optimum value, in the case of urea-aldehyde and melamine resins, can be readily accomplished by a mechanical testing procedure which will be described hereinafter in complete detail. When the resins employed are of the phenolaldehyde type, the state of `cure may be ascertained by either this mechanical test or by a chemical test which will also be set lforth hereinafter.

Cover or masking sheets manufactured as described above, or by equivalent processes, can be conveniently applied to the .surfaces of plywood sheets in a conventional plywood hot press. Such a press is illustrated schematically in Figure 4, andas shown in that iigure, includes a plurality of relatively movable, flatsurfaced platens 55 disposed between top and bottom members 51 and 59, which also have flat platen surfaces thereon. Means are provided Afor heating the various platen surfaces, and when the press is in use,

4the plywood or other material'is pressed between sure. As previously noted, pressures of the order of from 150 to 30G pounds per square inch with platen temperatures of the order of 260'to 325 E are ordinarily employed in plywood hot presses; The masking sheet can be applied to a single veneer, or it can be applied to a complete plywood assembly lof multiple cross grain veneer, and in either case, the masking sheet may be applied to either one or both surfaces. In the case of a plywood assembly, the cover or masking sheet is most conveniently joined tothe other elements of the assembly during the manufacture of the plywood itself, but it can be attached to a completed sheet of plywood as a separate operation.

' Regardless of the particular combination which is being effected, itis `desirable to introducethe masking sheet into the press in such manner that it will be in contact with a caul'plate fi'i's evaporated under controlled conditions, and

`the residue remaining constitutes the extractable portion. Then by simple mathematics, the percentage of extractable resin can be calculated.

Similarly, extended tests and investigations have shown that satisfactory masking will result l provided that the maximum quantity of phenoliclike resin material which is extractable from the cured masking sheet, as described, does not exceed a percentage figure which is equal to four plus i r the change in cure, expressed as a whole number sheet and not on the resin content alone.) Stated inanother way, a resin sheet impregnated with a phenolic resin in the manner described will be vsatisfactory for masking purposes if, upon additional curing of the sheet for one minute at the temperatures and pressures employed in the manufacture of the commercial plywood products, it contains four or less percent of extractable resin.

-In determining the percentage change values,

it is most convenient to plot a curve of the percent extractable resin against the time of cure, and illustrative curves of this type are-illustrated in Figure 8. Data for curves of this type is obtained by selecting a series of samples ofthe base sheet containing the resin which is to be evaluated, and curing the samples to various degrees by placing the sample sheets in an oven for various intervals of time, the oven being maintained at the temperature vat which the sheet isnormally cured. Following the curing operation, the per cent of extractable resin contained in each of the samples is determined according to the ASTM procedure described above. A plot or graph may then be obtained, illustrating the variation in the per cent of extractable resin against` curing time as illustrated in Figure 8.

To determine the minimum permissible degree of cure `from one of these curves, a line is drawn i parallel to the abscissa at the ordinate value equal to-4 per cent extractable resin. Then, by extrapolation as shown by the dotted lines in Figure 8, the amount of cure obtainable in the minute preceding the arrival `at 4 per cent extractable resin can be accurately determined. This value, addedto the empirically determined constant 4, as described above, will give the maximum percentage of extractable resin which can be tolerated in the masking sheet. It will be found that theV permissible maximum amount of extractable resin will vary, depending upon the particular resin or resin mixtures used for impregnation. For example, a fast curing resin or resin mixture,` as illustrated in the curve numbered I in Figure 8, will give good masking with a maximum extractable resin percentage of 12.4 per cent, whereas a slow' curing resin or resin mixture, as illustrated in the curve numbered III, requires that the cure be continued to a point where the maximum extractable resin is no greater than 4.9 per cent. If the samples are cured at the same temperature as that used in the curing oven for the masking sheet, the proper curing time can be determined directly therefrom. Otherwise tests,v may be required.

When the masking sheet is made from resin containing pulp, it has been found more desirable to employ a mixture of 5 parts hydrochloric acid and parts acetone for extraction purposes in the above chemical test. However, if such va mixture is used for extraction purposes then the standard A. S. T. M. extraction procedure described above ean not be used because of the difference in vapor pressures of the components of the solution. The extraction procedure used in such instances then is as follows. A weighed sample of the material is suspended in a hot mixture consisting of 95 parts acetone and 5 parts concentrated hydrochloric acid and occasionally stirred. After about l0 minutes the sample is removed from the acetone hydrochloric acid mixture and suspended in a hot, fresh solution of acetone hydrochloric acid comprising the same proportions as in the first extraction step. After a second ten minute interval with occasional stirring, the extraction step is repeated a third time. The sample is then removed from the mixture, air dried to remove excess acetone, and dried in C. oven for 10` minutes. The weight of the dried sample is then determined. From this Weight together with the initial weight of the sample the per cent of acetone extractable material can readily be determined by simple calculation.

In order to obtain increased wet strength in the base sheet used in the manufacture of masking sheets in accordance with the invention, it will sometimes be found desirable to add to that sheet a wet-strength resin, or other material, in accordance with known practices in the papermaking art. Some of these resins are of the beater addition type, although others are'added to the sheet by impregnating operations. Insofar as the production of the masking sheets of the present invention are concerned, the presence or absence of these wet-strength producing additives in the percentages normally employed (1/2 to 3% of the dry Weight of the cellulose),

has little eifect. Also, it makes no difference Whether the additive is of a resinous or other nature. Similarly, the base sheet may contain pigments or printing, if decorative surfacings are to be produced.

The cover or masking sheets described in the foregoing make possible the attainment of a very smooth, easily finished surface on plywoodjand similar materials. The surface vprovided may be either light or dark in color; Vit may include decorative effects; and it will receive paint and enamel finishes particularly Well. Of even greater importance, from a commercial standpoint, the masking sheets of the invention 4effectively mask plug patches, grain raising and checking, and can be manufactured and applied to` the plywood or other sheet materials withv which-they are used at relatively low cost.k

Various of the features ofthe invention are set forth in the following claims.

v I claim: Y p

1. The improvement in the process of manufacturing brous sheet covered plywood the face of which plywood adjacent the fibrous sheet contains surface defects, which comprises the steps of impregnatingi a paper sheet having a basis weight, when dry, within the range of from to210 pounds for 500 sheets 24 by 36 inches, and

a density within the range of from .55 to .80 gram per cubic centimeter, with a thermosetting Yaldehyde resin to provide a resin content in said sheet, atfer drying, within the range of from about 5 to- 35% of the weight of the dry impregtaining sheetand i9 thatyvhena action of y the .,cnredsheetisgpressed jtiyeriaf essentie-basking-plate-andaiiat, L etali'i'c. die-biete. @bel-1 1 .approximately- .-inones (inches sanare, said @die plate .having o-ver eIg-reafter ,portion .of its .surface ,concentric 4.anfiiiehiiiiNvidia spaced apa-ruby coneertric Vgrooves .f of :aninch in `.v fiith, with fafoce of Vabout 12,300 pounds, uniformly I:disbtedlbveilthe plate, lthe sheet being ,heated r gLthe pressing operation ,to a itemperature jofrabout 300 WhileL-Inaintained.under pressure L f iba'jerio'd gof fsixfto pig-htgninutes, said sheet ree of' visiblelnrarls corresponding -,to .the die hat r1 he sidgthereof-ODDOSite to ithe side iii 4101i @et :Wihilie di? Serf-ac e5r1acins at least .y -Of S9491 @Bred Sheet over the exposed .-face of im? Qlkir 1V I 'lis O a Plywood assem- "Qli ieee semaine serio-oe defects, when essably is elated in a plywood press;- pro- -/ieine' .a e

n-eeef-r lly subi ure g Sheet and the adjacent veneer; 'ses 'tesserati-ain said press,

t. anee O f .from about 150 i 3.99 rieur- 151s Seilere and a temperature Within the range of from about 260 to 3259 is? .dieci the ifi-ning of the plies of the oi-ywood e Y ali-,fi the attachment of the cured resin- QOP Iig sheet i9 the suriaee Veneer solely by the ve action of said eine line. 2V he Jment the process of manuf p fatigues tier-ees sheet covered plywood the face -o-fwhicl PIYWQQQ. agissent the brous sheet contains" "surface defeetsz which comprises the steps g a raser sheet havinga basis dry-, Within the range of .from about ,fer- 500. sheets `2 4 by 36 inches, hin.- the.- renee, oi from .55 to. .80 'Mieter-, with thermosettine d resi- 1.1 @0 QiQ-Vide a resincontent in said 4, after, dryirig, Within the, range 0i`v from i? 5.` Q- 3@ Gf; the Weight Gfx' the dry impregd. Sheet-: drying the resineeentaining sheet. imireanaied: .Sheetha 'ne basis. weight dry Within the range ot-.frorn about :11u

vvhen @5..- grem- Per- Gabin centimeter-: heating said. dry,

lie mf-@Qntaiengf s .lieet to, cure. the resin conta `edthereintotsuch anv exten that when. a .101,105 fhfcllredisheetis pressed-'between a1. f1 metallie backing.) plats and.- a fia-t, meta-ino die plate,

cppjtagt i die-LSurface-si: piaeing. ati least:

allied-1sheetiover the exposed iface- 1Miersfoi1afplyyvoodfassembly I surface; defects.- when that'- .-,afplywoodpresse providingedadhesivefintermedi-atestile-resin-cone the adjacent -veneer ;-and -naily' subjectingisaid .assembly,-. in-said-fpress, topres-isurawithinthe range sof -fromaabout --150-to-300-' pounds peisquare ninch and a temperature Withinthegrange of--from about 26o-'to 325 Fito eiect the joiningor -theplies of the plywood. assembly @dille ait'chment-b1ihee1ired resin.- contamme-- asin .e adhesive intermediate the 9 both approximately 8- inches: by.- 81 L Lsheetto-.thesurfaceve eersolely by the adhesive @action ofsaid glue= line.

.-3. ,The ,improvement :in `the .--process of -manufaeturing -broussheet covered plywood `the --fa-ce .of-Which plywood adjacent-the brous-sheet contains f surface l defects `which comprises .-placing at Kleast one ply sof -a `masking -sheet -having -high ,Wet-and abrasion-resistanceover the-eXposed--face Lolf-,one of the outer veneers -of a plywood-assembly ,which face -contains surface defects when that :assembly is placed in a plywood ipress, said fmasking :sheet iconsisting of -a -cellulose fiber base sheet having -an -unimpreg-nated basis weight when-drywithin the range -of -from about 110 to 21d-pounds :for 500 sheets :24 by 36 inches andan uni-mpregnated densi-ty within the range of from about .55 to l gram per Acubic -centi-meter, and containing from about 5 4to 35%-, based on the finished total Weigh-t of the dry, resinecontaining sheet, fof a thermosetting aldehyde resin, "said resin being :cured to such an extent that when a section of 'a cured sheet is pressed between a at, metalli-c, backing plate and a dat, metalli-Tc, die plate, both approximately 8 inches by 8 inches square, said die plate having over the greater portion of itsl surface concentric lands 5/{34 of an inch in width, spaced apart by concentric grooves si; of an inch in width, with a force of about 12,800 pounds, uniformly distributed over the plate; the sheet being" heated during the pressing operation to a temperature of about 300 F. and being maintained under pressure for a period of six'to eight minutes, said sheet is freeV f visible marks corresponding to the die patternon th side thereof opposite tol ther side in cntaict with the die surface, providing a g-le lin'eadhesive intermediate thecured resin-*containing masking sheet and the adjacent- Veneer; and nally sub-f jecting said assembly, in. said press, to` pressure- Within the range'of fr'ornf about 150' to300 pounds per'V squareinch-x and' a' temperature withirl the" range of from' about 260 to 325" F. to' eifect the` joining of the plies of the plywood assemblyr and the attachment of. the'v curedr resin-'containing masking; sheet: tot the surface' veneensblely' by thefadhesive action'- ofrsaidfglueline.

4., The improvement.- iny the" process off manur facturing fibrous sheetl covered: plywood the face of: which plywoodv adjacent` the-l bi'ous sheet? contains surface: defectsk which' comprises pla'cl" ing ati leastone:k plyY ofV a' masking;- sheet'- Having' high Wet" andi abrasion resistaiieeover the ex"- posed i face of. one' ofi thef oterfveneersof a" ply" press;` said rmaskingesh'eetf-consistingf of a cellu'' losef ber(basersheetrwhiehfis-impreghated with arthermosetting.: aldehyde resin; said ibasel sheet-f having.r an.- unimpregnatedbasisl' weight; when dry,- within fthefrange of from@ about 110y to '210 pounds for 500-sheets--24 by.- 36 inches,-l and anunimpregnated density Within-the range of from about .55 to .BOfgram-sperautrici-centimeter; said'-l impregnated sheetcontaining from@ about 5 to-Y basedv on -thef-nishedtotal-iweight of--theA dryimpregnated sheet,-.- of theimpregnatingy resin, said.` impregnatedsheethaving a -basis Weight when'drywithin the-rangeof from about .60` .toA .95.,gramiperacubiocentimeter, and-said resinbeingocured.,to=such-an extent that When-- a section ofi-*the ,curedsheet --ispressed-between a flat, metallic, backing vplateand a flat, metallic,. die plate; bothiapproximately.,V S-inches byf8 -inches square, .said -die plate" having Vover the greater. portion -of its sur-face concentric 2T. lands 5A, of an inch in width, spaced apart by concentric grooves g3g of an inch in width, with a force of about 12,800 pounds uniformly distributed over the plate, the sheet being heated during the pressing operation to a temperature of about 300 F. and being maintained under pressure for a period of six to eight minutes, said sheet is free of visible marks corresponding to the die pattern on the side thereof opposite to the side in contact with the die surface, providing a glue line adhesive intermediate the cured resin-containing masking sheet and the adjacent veneer; and nally subjecting said assembly, in said press, to pressure within the range of from about 150 to 300 pounds per square inch and a temperature within the range of from about 260 to 325 F. to effect the joining of the plies of the plywood assembly and the attach- 22 ment of the cured resin-containing masking sheet to the surface veneer solely by the adhesive action of said glue line.

DONALD H. GRANGAARD.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date Re. 23,347 Redfern Mar. 20, 1951 1,857,690 Mellanoff May 10, 1932 2,031,260 Gams Feb. 18, 1936 2,325,302 Britt July 27, 1943 2,343,740 Birmingham Mar. 7, 1944 2,383,283 Auxier Aug. 21, 1945 2,442,422 Loetscher June 1, 1948 2,497,712 Auchter Feb. 14, 1950 2,565,251 Malmstrom Aug. 21, 1951

Claims (1)

1. THE IMPROVEMENT IN THE PROCESS OF MANUFACTURING FIBROUS SHEET COVERED PLYWOOD THE FACE OF WHICH PLYWOOD ADJACENT THE FIBROUS SHEET CONTAINS SURFACE DEFECTS, WHICH COMPRISES THE STEPS OF IMPREGNATING A PAPER SHEET HAVING A BASIS WEIGHT, WHEN DRY, WITHIN THE RANGE OF FROM 110 TO 210 POUNDS FOR 500 SHEETS 24 AND 36 INCHES, AND A DENSITY WITHIN THE RANGE OF FROM .55 TO .80 GRAM PER CUBIC CENTIMETER, WITH A THERMOSETTING ALDEHYDE RESIN TO PROVIDE A RESIN CONTENT IN SAID SHEET, AFTER DRYING, WITHIN THE RANGE OF FROM ABOUT 5 TO 35% OF THE WEIGHT OF THE DRY IMPREGNATED SHEET; DRYING THE RESIN-CONTAINING SHEET; HEATING SAID DRY, RESIN-CONTAINING SHEET TO CURE THE RESIN CONTAINED THEREIN TO SUCH AN EXTENT THAT WHEN A SECTION OF THE CURED SHEET IS PRESSED BETWEEN A FLAT, METALLIC, BACKING PLATE AND A FLAT, METALLIC, DIE PLATE, BOTH APPROXIMATELY 8 INCHES BY 8 INCHES SQUARE, SAID DIE PLATE HAVING OVER THE GREATER PORTION OF ITS SURFACE CONCENTRIC LANDS 5/64 OF AN INCH IN WIDTH, SPACED APART BY CONCENTRIC GROOVES 3/32 OF AN INCH IN WIDTH, WITH A FORCE OF ABOUT 12,800 POUNDS, UNIFORMLY DISTRIBUTED OVER THE PLATE, THE SHEET BEING HEATED DURING THE PRESSING OPERATION TO A TEMPERATURE OF ABOUT 300* F. WHILE MAINTAINED UNDER PRESSURE FOR A PERIOD OF SIX TO EIGHT MINUTES, SAID SHEET IS FREE OF VISIBLE MARKS CORRESPONDING TO THE DIE PATTERN ON THE SIDE THEREOF OPPOSITE TO THE SIDE IN CONTACT WITH THE DIE SURFACE; PLACING AT LEAST ONE PLY OF SAID CURED SHEET OVER THE EXPOSED FACE OF ONE OF THE OUTER VENEERS OF A PLYWOOD ASSEMBLY WHICH FACE CONTAINS SURFACE DEFECTS, WHEN THAT ASSEMBLY IS PLACED IN A PLYWOOD PRESS; PROVIDING A GLUE LINE ADHESIVE INTERMEDIATE THE RESIN-CONTAINING SHEET AND THE ADJACENT VENEER; AND FINALLY SUBJECTING SAID ASSEMBLY, IN SAID PRESS, TO PRESSURE WITHIN THE RANGE OF FROM ABOUT 150 TO 300 POUNDS PER SQUARE INCH AND A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 260 TO 325* F. TO EFFECT THE JOINING OF THE PLIES OF THE PLYWOOD ASSEMBLY AND THE ATTACHMENT OF THE CURED RESINCONTAINING SHEET TO THE SURFACE VENEER SOLELY BY THE ADHESIVE ACTION OF SAID GLUE LINE.

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