Dry Film Gluing in Plywood Manufacture

WDI-56-1D ry Film Gluing in Plyw ood M anufactureBy RAY SORENSEN,1 LOUISVILLE, KY.B e t t e r P l y w o o d A d h e s iv e s N e e d e dwood. It is also true that the strength of plywood varies widelyTHE limitations of recognized plywood adhesives have been with its moisture content. Only when properly dried wood isa severe handicap to the veneer and plywood industries, used can an acceptable product be obtained. It is much easierseriously retarding their development of known and to dry the individual plies before gluing, such as lumber cores,promising plywood types and products. The recognized commercialwood adhesives in the United States, until within the process, than it is to dry the jointed, glued, and veneered plywood.veneer cores, or outer veneers, by an efficient and reliable dryinglast two years, might be termed wet glues, and consist chiefly of When using fluid glues containing a high percentage of water,either animal or vegetable matter. All of these fluid glues, which we have termed wet glues, these problems will be particularlygreat. The percentage of water contained in gluesanimal, vegetable, casein, albumin, soya bean, and their combinations,have been found inadequate to meet the wonderful cannot be kept uniform nor can it be kept within certain limits.opportunities in the plywood fields immediately ahead.There is no possibility of regulating the influence of moisturePartly for this reason, and partly because of inefficient promotion,the veneer and plywood industries have been severely the plies to be glued depends greatly on the structure, dryness,on the glued plies. How deep the glue moisture penetrates intorestricted in their growth, while the metal and synthetic sheet thickness, density, and temperature of the wood, as well as onmaterial industries, with broader vision, have made big inroads the consistency and composition of the glue. It is of primeinto the well-recognized woodworking fields. It is a truism thatan inferior but well-promoted product will often be accepted inplace of an intrinsically better product that is not well presented.It is seldom possible to find new uses or new fields for plywoodproducts because of the lack of proper adhesives and the dearthof necessary industrial research. It has been well said thatplywood is no stronger than its glue line, and a glue of limitedscope is a severe handicap in every way.It is a fact that today, and for some time to come, the facilitiesfor producing plywood are far in excess of the demand, andunless new uses and new fields are found, the industry faces adrastic curtailment of volume. Consequently it is incumbenton the industry to carefully analyze and evaluate all new materialsoffered to determine to what extent the old limitations are overcome.P l y w o o d P r o g r e s s A s s u r e d b y D r y G l u e F il mThe ideal wood adhesive is one which is of a uniform quality,which can be laid or spread in an even coating, which gives aperfect glue bond, and which can be applied economically. Dryglue film stands ready to meet all of these conditions in an efficientway. The dry glue film process eliminates many complicationsthat beset the manufacturers of fine veneered work becausedry glue film is simpler to apply than wet glues; in fact, all of theuntidy and unpleasant mixing and spreading operations in wetgluing are wholly removed from the plywood factory by theuse of dry glue film. Properly manufactured glue film containsin each square foot of surface precisely the same quantity ofglue, of equal quality, of uniform composition, of exactly thesame bond strength, and of the same standard thickness.The entire industry is certain to benefit greatly by the introductionof dry glue films for the fabrication of veneers, nonporoussubstances, and insulating materials into a new and widerrange of plied-up combinations. The extent of the benefit tothe industry will depend upon the degree that this dry glue filmovercomes the known limitations of the commonly used wet glues.P r o b l e m o p W o o d M o is t u r e i n P l y w o o d G l u in gThe industry has long known that warping, winding, and twistingin plywood is greatly influenced by the moisture content of the1 Formerly Secretary, Tego Gluefilm, Inc.; present address, 131North Western Parkway, Louisville, Ky.Contributed by the Wood Industries Division and presented at theSemi-Annual Meeting, Chicago, 111., June 26 to July 1, 1933, of ThbA m e r ic a n S o c ie t y or M e c h a n ic a l E n g i n e e r s .importance in gluing to have not only even spreading, but alsostandard quality and predetermined consistency of the glue.It is known that many variations occur when glue is mixed in aplywood factory, regardless of the care exercised in its preparation.The industry has been aware of the many disadvantages ofwet glues and has tried to find or to perfect ways of evaporatingpart of the glue moisture after spreading and pressing, but withonly mediocre success. These endeavors have inevitably openedthe field to the development of the dry glue film.E a r l y D e v e l o p m e n t o f D r y G l u e F il mIn the early development of dry glue film it was discoveredthat it was impractical to produce a dry glue film having as abase the ordinary wood adhesives. The possibilities of dry gluefilms have had the attention of various individuals for a numberof years, and as a result a dry glue film has been gradually developed,improved, and finally marketed.The adhesive power to wood of phenol-formaldehyde condensationproducts has long been known, but their general useor commercial application has been prevented because of theexpense or cost.The qualities offered by phenolic resin as a wood adhesivewere quickly recognized, and attention was then turned to theapplication of the resin product, whether as a colloidal solution,dry film, or powder. Many attempts were made by industries,both in this country and in Europe. About nine years ago aGerman research and chemical manufacturing company set outto perfect the use of phenolic resin as a commercial wood adhesive,which meant bringing it to the simplest form of applicationand within a cost range for general adoption by the plywoodindustry. The final result, after seven years of research, wasthe development of a phenolic resin dry glue film. Dry gluefilms have proved to be very simple, clean and quick in theirapplication, and to eliminate many of the disadvantages unavoidablyinvolved in the wet glues of today.Because it is the first scientifically manufactured form ofadhesive which at all times has the correct quantity and qualityof glue, and sets up for the first time a scientific method of plywoodmanufacturing, it presents a distinctly new method whichpromises to revolutionize the present form of applying andusing wood adhesives. There being but one dry glue film commerciallyrecognized and used today, it may be consideredtypical of a class that is in the development stage.As mentioned before, the industry has long felt the need of37

38 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERSbetter adhesives which would permit it to seek new uses and newfields for its products. I t has been repeatedly shown that manynew outlets for plywood are necessary in order to enable theindustry to continue its sturdy progress. The newly developeddry glue film answers this need and permits the industry toproceed to enter these new markets.T y p e s a n d G r a d e s o f D r y G l u e F il mThe dry glue film, as marketed today, might well be termed asheeted synthetic or phenolic resin film because, as mentioned, it isa phenol-formaldehyde condensation product. The film is to behad in various weights, to meet a wide range of requirements.For example, what is termed a standard film which meets allordinary conditions as an adhesive for plywood, is a 60-gramfilm. This means that over a given area the film contains byweight 40 grams of resin and 20 grams of paper, which is thecarrier required in producing the film. This paper or carriermust be thin, porous, and fibrous, so as to permit a uniformimpregnation of adhesive on both sides. I t must be toughenough not to break in handling and so porous th a t under theinfluence of heat and pressure the adhesive will thoroughlypenetrate the carrier and eliminate any tendency for the gluejoint to separate. Film can be furnished with a greater or lessamount of resin, according to the requirements demanded as anadhesive and the type of materials to be laminated. The shearvalues resulting from such variations are shown in Table 1.T A B L E 1 S H E A R V A L U E O F T E G O G L U E F IL M B O N D(In p ounds p er sq i n .a s influenced b y different p ro p o rtio ns ofresin on th e sam e carrier)P ressu re: M ahogany 142, a n d B irch 284 lb per sq in.im e in P ress: 10 m inT em p era tu re : 130 CM oisture C o n te n t of V eneer: A pproxim ately 10%R esin coating M ah o gan y B irchper sq m eter, 3-ply, 6/64 in. 3-ply, x/ k in.gr D ry W et D ry W et21 154 107 261 19323 182 123 312 23726 209 145 336 22828 222 162 321 23932 253 204 325 23940 308 266 396 330T he stan d ard glue film , described as 60 gram s, is th e la s t in th e table,having 40 gram s of resin a n d 20 gram s of p ap er carrier.T A B L E 2 S H E A R V A L U E O F S IN G L E A N D O U B L E L A Y E R S O FT E G O G L U E F IL M B O N D(In p ounds p er sq uare inch)Vie 3-ply, All B irchP ressure: 284 lb p er sq in.im e in press: 10 m inT em p eratu re: 130 CM oisture C o n te n t of V eneer: 10%R esin coatingp er sq m eter, Single D ouble Increase,gr la y e r la y e r %21 248 312 2640 298 445 49In case more than 40-gram coating is required, as in gluing metalto wood, it is practicable to use two sheets of dry film glue ineach joint. While this does not double the bond strength, stillit does result in a substantial increase, as shown in Table 2.I t is frequently advisable to use double layers where the surfacesare rough, requiring more glue bulk, or where veneers varyslightly in thickness.The standard width is 51 in., but can be made up to 82 in.Rolls will average over 3900 ft in length, and a 51 in. X 3900-ftroll will weigh 225 lb crated, containing approximately 17,000sq ft.A p p l i c a t i o n o f D r y F i l m G l u e sThe assembling of veneer into plywood with dry glue filmmethods demands an entirely different technique than that whichhas been commonly employed with the usual wet glues. Thedry film glue, being a phenolic resin, is thermoplastic and thermosetting,which now permits the fabricating of plywoods withoutthe introduction of water into the carefully dried veneers, as isnecessary with the wet glues.The film glue being thermoplastic and thermo-setting, thebond is accomplished with heat and pressure, requiring a hy-C h a r t 1 I n f l u e n c e o f T e m p e r a t u r e o n G l u e F i l m J o i n t sV ie I n . , 3 - P l y B i r c h(G lued a t 10% m oisture con tent in 10 m in tim e, w ith 284 lb p er sq in.D o tte d line is te s t a fte r im m ersion. F ull line is d ry te s t before im m ersion.)draulic hot plate press. W ith the application of both heat andpressure a t the same time, as is had uniformly with the moderntype of hydraulic hot plate presses, the resin film polymerizesand becomes both insoluble in water and chemically inert.This method of producing a bond with the dry film glue mightbe termed “vulcanizing,” and has for the first time set up alaminated panel in which the fibers of the adjacent plies arebrought directly in contact with one another. In fact, theadjacent plies might be said to be welded to each other. Thusthe bond obtained has in reality converted the several originalplies into an ultimate panel of solid material, and this has beendone without the internal stress and strain which is often sotroublesome.The dry glue film process eliminates the expensive redryingmethods which have to follow gluing with wet glues. Theindustry has spent a great deal of time and money to set upproper methods for this subsequent redrying necessary to removethe glue moisture. Woods vary in their structure and density,making it impossible to accurately control the drying process,which is often the cause of distorted plywood and checked faces.Inasmuch as the dry glue film is only 0.005 in. thick, entirelynew demands must be made as to the pressure required. Thehydraulic hot plate presses must be designed and built to new

standards of accuracy as to the deflection of its members. Themaintenance of proper temperatures on the entire surface of theplates is of extreme importance.R e l a t i o n o f H e a t , M o i s t u r e , P r e s s u r e , a n d T i m eWhen gluing with dry glue film there are four variables to beconsidered, namely, temperature or heat, moisture, pressure, andtime. All are closely related to one another in producing asatisfactory bond with resin adhesives, but it is best to considerthe influence of each factor separately. The strength valueof each of these variables in relation to the others has beenaccurately determined and charted. The accompanying charts1, 2, 3, 4 show definitely the strength of the bond obtainedwhen varying the factors and prove conclusively that the laminatingof plywood with dry glue film may be considered a scientificallycontrolled process. A careful study of the series ofcharts will readily show that when the same combinations ofvariables are repeated, the value or strength of the bond obtainedcomes within close limits, if the same or similar woods areused.1 Influence of Temperature or Heat. The dry glue film,being a phenolic resin and thermoplastic, requires a definiteamount of heat and a predetermined time to set up or producepolymerization.Chart 1 clearly indicates the influence of heat for a givenperiod of time, and demonstrates the greater shear values of thebond as the temperature is increased. In preparing these chartsit will be noted that certain standard time, pressure, moisture,and heat factors have been established, to show relative comparativevalues with one another. The construction of theC h a r t 2 I n f l u e n c e o f M o is t u r e C o n t e n t o f W o o d o n G l u eF il m J o in t s3/ i« I n ., 3 - P l y B ir c h(G lued a t 130 C, 284 lb p er sq in., 10 m in tim e. D o tte d line is film 14 daysold. F ull line ia film one y ear old.)WOOD INDUSTRIES WDI56-1 39C h a r t 3 I n f l u e n c e o f P r e s s u r e o n G l u e F i l m3/ie I n . , 3 - P l y B i r c h(G lued in 10 m in tim e, w ith 10% m oisture content in veneer. D o tte dlines a re testB a fte r im m ersion. F ull lines are d ry te sts before im m ersion.)plywood used is a 3-ply panel, 3/ie in - thick, all plies Vi6in.birch.The solid line indicates the strength of the bond when testingthe plywood in a dry state, having an average moisture contentof 7 to 8 per cent. The dotted line indicates the strength aftersoaking in water at room temperature for 48 hr. It will benoted that while the value obtained when testing the bond aftersoaking is somewhat lower than the dry test, this does notmean that the glue line has weakened to this extent, but ratherthat the tensile strength of the fibers of wood are lower in a wetstate. I t is also a fact that the shear value of the dry glue filmbond always increased when redried after soaking.In studying this chart it will be readily observed that the increaseof temperature in a given time materially increases thestrength of the glue bond. For instance, a panel glued at atemperature of 100 C for 10 min at a pressure of 284 lbper sq in. and with a moisture content of 10 per cent,has a shear value of 66 lb per square inch. W ith all otherfactors remaining the same and the heat increased to 140 C,the shear value increases to 396 lb per sq in. The same increaseis proportional to a lesser extent in the wet test. Increase ofheat in a given period of time always produces a better gluebond.In the manufacture of dry glue film the resin has been precuredto a definite point, and it will be observed on all the chartsthat a temperature no higher than 140 C has been shown. Testshave proved th a t no material increase in the bond is found attemperatures beyond 140 C, but any increased temperatureabove this has no detrimental effect upon the curing of the resin.The wood itself is the item that must be kept in mind whenemploying high temperatures. The gluing temperature of 140 C

40 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERSC h a r t 4I n f l u e n c e o f T im e o f P r e s s u r e o n G l u e F i l m J o i n t sV is I n - . 3 - P l y B i r c h(G lued a t 284 lb per sq in., and 10% m oisture content. F ull line is dryplyw ood te st. D o tte d line is plyw ood te s t a fte r 48 h r im m ersion. Allte m p era tu re s are given in C entigrade.)required for dry glue film does not damage the quality of thewood.This has been proved by a number of tests, especially thoseof Professor Dr. Otto Gerngross of the Technical University ofBerlin, as shown in Table 3. While the table refers only to birchand fir, these are taken as typical of the hardwoods (deciduous)and the softwoods (coniferous). I t is to be noted that bothspecies have a strength increment of more than 10 per cent dueto simultaneous heat and pressure. The heat alone on the birchdoes not alter the strength value, but heat alone on the fir willcarry the strength value to that of combined heat and pressure.This is due to the natural rosin content of the coniferous woodthat is equalized and hardened by the heat.T A B L E 3 E F F E C T O F H A T , W IT H A N D W IT H O U T P R S S U R E ,O N T E N S IL E S T R E N G T H O F S IN G L E -P L Y V E N E E RSpecim ens te ste d : \ x/% in. X 4 in. X 5/ m in. B irch a n d F ir veneerT em p era tu re : 140 CP ressures and tensile stre n g th given in lb p er sq inṪ ensile S tren gthSpecific T im e, B efore A fterSpecies pressures m in h eat h eatD eciduous (H ardw oods)irc ................................... 0 45 7892 7864B irc h ................................... , , 142 15 8788 8674C oniferous (Softw oods)i r ....................................... 0 45 5631 6897F ir ....................................... , , 142 20 6541 6826A dapted from article in “ S p e rrh o ltz,” 1930 (S-382) b y Prof. O tto G erngross,of B erlin, G erm any.When gluing with dry film glue it is always safer to employthe higher temperatures, but seldom over 140 C. This usuallydepends on the type of material to be laminated.2 Influence of Moisture Content in Wood. Methods ofgluing with dry glue film differ from those of ordinary wetgluing because the natural moisture in the wood is not increasedby glue water. Experiments have shown that, regardless ofthe type of glue used, warping and wavy surfaces in lumbercore plywood are not only caused by irregular thickness andresistance against compression by the core, but that they alsooccur if the moisture content of the core exceeds 5 to 6 per cent.This limit of moisture content, however, does not apply to crossbandingand outer veneers.I t will be noted from chart 2 that the shear value or bondstrength depends to a great extent on the moisture content ofthe veneers. A careful study of the chart shows the bond thatcan be obtained when using a dry glue film that is several monthsold, which is the average age of the film when used. This isindicated by the solid line, while the dotted line represents thebond obtained when using a freshly manufactured film aboutten to fourteen days old.A certain amount of moisture is necessary to plasticize theresin, and this moisture may come either from a freshly madeglue film or from the veneer. Consequently it is important toregulate the moisture of the veneer within safe operating limits.The chart indicates that freshly made glue film must be usedwhere veneers are substantially less than 4 per cent moisturecontent, but that even better joint strength is obtainable fromaged glue film when the veneer is between 4 and 12 per centm.c., which is the normal condition found in veneer as storedin the average warehouses. It is obvious that this eliminatesthe ordinary redrying required prior to wet gluing.Experience has proved that the most useful medium for themoisture necessary in plasticizing dry glue film is the crossbandingin 5-ply construction. This is so situated in the assemblingoperation as to supply the required moisture to the dryglue film adjacent to both the core and face. It has been pointedout that excessive moisture in a lumber core is dangerous, andit is apparent that surplus moisture in face veneer may resultin open joints or hair lines and checks.It might be mentioned that in gluing very fragile face veneers,such as highly figured crotches and burls, tests have shown amoisture content of about 6 per cent to be the most satisfactory.3 Influence of Specific Pressure. The pressure applied onthe surface of the plywood is the specific gluing pressure, or, asit is frequently called, the platen pressure in pounds per squareinch and should not be confused with the higher pump pressurethat is required. The application of pressure with dry glue filmis no different than when gluing with wet glues, except that wetgluing usually requires 75 to 150 lb specific pressure, while drygluing ranges from 150 to 250 lb and in some instances up to300 lb per sq in. The amount of pressure exerted on a paneldepends on the density of the wood and the construction of theplywood.It is also indicated on chart 3 that the strength of the glueline increases with additional pressure. The maximum pressuredepends upon the compression loss in plywood thickness thatcan be allowed. Some woods and synthetic boards will crushunder 50 lb, while denser woods and non-porous synthetic productswill permit greater pressure. Consequently the adhesionon the softer products will not be as great as on the hardermaterials. The average compression percentages on the commonerwoods are given in Table 4.4 Influence of Time of Pressure. The time required to completethe bond, which is often referred to as gluing time, isgenerally understood to be the time that elapses after obtainingthe correct specific pressure and until the reopening of the press.It is also essential that the temperature of the plates is securedbefore closing the press. The gluing time depends chiefly uponthe time necessary for the heat to penetrate from the steam-

44 TRANSACTIONS OF T H E AM ERICAN SOCIETY OF MECHANICAL EN G IN EERSretained, finishing costs are lowered, and a rich, deep tone ofartistic character is produced.The bond effected with dry glue film resin is not only waterresistant,but is actually insoluble in water and chemically inert.Extensive tests have not only shown the glue line absolutelywaterproof, but on redrying after immersion the bond alwaysshows increased strength. Not only is the bond chemically inertbut it is resistant and retardant to the attack of mold, fungi,and bacteria. Tests show that termites and wood vermin willnot attack plywood laminated with a phenolic resin film whenlayers are Vs in. or thinner.Chart 5 shows the life of various wood adhesives when subjectedto the attack of mold and fungi.2 I t should be mentionedthat the time shown in weeks for full decomposition ofthe glues was partly absorbed by the building up of the moldculture. This time is shortened when subjecting the specimensto a culture of mold and fungi. For instance, in place of requiringapproximately four weeks to decompose a casein glueline when applied in the cold method, the time is reduced toapproximately ten days when subjected to a culture. I t willalso be observed that the hot plate press method of gluing withwret glues not only increases their water resistance but alsoimproves their resistance to the mold attack. This shows thatall of the wet adhesives are subject to full decomposition, whilethe bond made with the film resin is 100 per cent resistant to2 Since preparation of this article Naval Aircraft SpecificationsNo. 39P13a, April 1, 1933, provide:E-2c. The adhesive used shall be resistant to decompositionfrom mold, fungi, bacteria, and shall show a minimum average shearvalue of 250 lb per square inch when subjected to a culture of theabove for a period of ten days.these attacks. Other tests show no deterioration to the bondafter being exposed to the attack of mold and fungi for two years.The old theory that plywood must be made with a balancedconstruction has been disproved. It is now possible to produce2-ply panels with lumber core and face veneer with substantialeconomy and showing no distortion. Many new forms ofconstruction not heretofore possible can be developed whengluing, without introducing moisture into the veneer or lumber.After gluing, plywoods can be placed in hot water or steamedwithout injury to the bond. This permits use of flat plywoodconstruction in the forming of bent and curved products, and theresulting work is very satisfactory.The dry glue film bond will not react, thus eliminating acorrosive attack.I t also has the quality of extreme flexibility. There is nocrystallizing and cracking, and the bond cannot be destroyedby distortion.The dry film is completely dissolved under pressure and heatand for the first time has effected a bond which is stronger thanthe contracting and expanding power of wood, when subjectedto excessive moisture. This is illustrated in Fig. 8, showing a3-ply flooring test, using 6/ 16-in. oak face and Vie-in. chestnutcore and back. The outer surface expanded in a one year’simmersion test some 5 per cent, while the glue side of the oakwas held to approximately original dimension by the dry gluefilm bond.Dry glue film is very adaptable for the gluing of crotch andhighly figured veneers in either a 5-ply construction or a 2-plyreinforced face construction. I t insures these veneers againstchecking because the adhesive bond is stronger than the expansivepower of the wood. For the same reason it also eliminates

the so-called hair-lining troubles which are so objectionable toall plywood and furniture manufacturers.Another interesting feature is that dry glue film has made itpossible to use the low-priced Western coniferous woods for corematerials, and veneer over them with a thin face (Vs6 in.) veneerwithout showing the effect of the underlying grain in the coarserwoods. Absence of moisture in the gluing is the chief factor inmaking this possible.There are many other advantages known and in the processof development, should space permit their discussion.R e c e n t A d a p t a t io n s o p P l y w o o d t o N e w P r o d u c t sThe plywood industry has been eager to enter broader avenuesof trade, but up to the present time the handicap of the standardwet glues has seriously retarded such expansion. The widerange of opportunities for these newer types of plywood arebriefly suggested.1 Construction Trades. To the building trade, dry gluefilm products offer a new material heretofore limited. Theincreased use of Ferroclad materials to meet fireproof requirementshas demanded an adhesive which will not corrode thematerials and will not crystallize and decompose under rapidchange of temperature and humidity. The adaptation of ply-T A B L E 5 E F F E C T O N PLY W O O D S T R E N G T H O F C O N T IN U E DH E A T IN K IL N S(A fter rem oval from h o t press)Specific pressure in press: 285 lb p er sq in.No pressure in kilnsShear te sts: et or d ry , in lb p er sq in.W et te sts after soaking in w ater a t room te m p era tu re for 48 h rem peratures: D eg C, auto m atically m ain tain edT im e: H r and m inPressing (before kilning)K ilning (after rem oval from press)PressTim e, tem p., Shear ShearH — M . C d ry w etl/i in., 3-ply, All B irch Plyw ood0— 3 130 il Nilil NilN il Nil0— 4 130 1101161430— 5 130 160165187K ilnT im e, tem p ..H .— M . CfWOOD INDUSTRIES WDI-56-1 45S heard ry0— 30 140 253253176S hearw et253264275227 264770— 30 140 264 34177264 34199286 301123 84 271 3281120— 30 140 286 286118286 275143281 264171 124 284 2750— 6 130 206 1760— 30 140211220 294184242 319242 184330 319220 181 264 3100— 7 130 242 206O— 30 140 253 268253 209308 297259 209330 297251 208 297 2873/i6 in., 3-ply, All B irch Plyw ood0—10 110 187220940— 30 140 429 462140506 462264 Nil519 308224 78 485 410Sam e Same Sam e Sam e 2— 0 90 358358352356266279232292Same Same Sam e Sam e 17— 0 90 341 365484 407462 462s/i6 in., 3-ply, All B irch Plyw ood • 429 4110—10 110 187220940— 10° 130 534 341140484 319264 N il418 341224 78 478 334° R e-pressed and re-h eated b y retu rnin g to press, as in case of u nderplasticizedplyw ood, w here kilns n o t available.wood to the building trade demands th a t the bond shall withstandall conditions of exposure, is not subject to vermin, termites,mold and fungi attack, is fully waterproof, and resistantto rapid change in temperature, both wet and dry. A few suggesteduses are as follows:Plywood flooringFlush and panel doorsVeneer faced synthetic fiber boardsPrefabricated unit housesConcrete formsPaneling with built-in electrical heating unitsWall paneling.2 Automotive Industry. The automobile industry is alwayssearching for material of the greatest strength with the leastweight. While plywood has been considered by designers inthe past, the lack of waterproofness and resistance to decompositionby atmospheric elements, such as wet and dry decay, hasretarded its adoption by the industry. The dry resin film hasentirely overcome these difficulties and the designers are eagerto specify plywood for the following items:Floors, sides, and roofs of refrigerated and ordinary trucksRoofs, floors, and interiors of busesRoofs of closed passenger carsRunning boards and insulated floor boardsBattery separators and containers.The plywood roof not only eliminates the deterioration nowso common but supplies an unusual degree of rigidity.The plywood battery separator and container offer a new andrepetitive m arket to the plywood industry.3 Ship and Boat Building. Due to the advent of the dryresin glue film the entire ship and boat building field is now openfor plywood utilization. Waterproof plywood made possibleF i g . 8 S h o w i n g T h a t t h e W i d t h w i s e S w e l l i n g o f 'V ih- I n .,3 - P l y O a k P l y w o o d A f t e r 1 Y e a r ’s S u b m e r s io n I s H e l d b y t h eD r y G l u e F il m B o n d A g a i n s t t h e C o r e , b u t A s s u m e s N o r m a lE x p a n s io n a t t h e O u t e r S u r f a c e sthe fabrication of canoes, racing hulls and outboard motor boatsinsuring greater strength and reduced weight. The new waterproofplywood insures indefinite durability under continuousimmersion, and the bond is not affected by oil, acid, or saltwater. Some of the watercraft applications are:CanoesOutboard motor boats and racing hullsShip bulkheads and partitionsMarine wall panelingShip furniture.4 Aircraft. Innumerable tests in commercial and aircraftlaboratories, both here and abroad, have conclusively establishedthat the bond in phenolic resin plywood is far superior in strengthand endurance to any type of plywood heretofore used.The waterproofness of the bond, the absolute resistance toatmospheric elements, the tests that have proved the shear

46 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERSvalue of the bond to be the highest ever recorded, and the continuousincrement of strength or frequent absorption of moistureall strengthen the hold of plywood in aircraft industry.Plywood of unusual strength as thin as 3-ply, ‘/as in-> is nowproduced commercially for aircraft fabrication. This type ofmaterial now available to the industry permits the manufactureof aircraft at a reduced cost, with less weight, with increasedstrength, with longer life regardless of atmospheric conditions,and with a higher factor of safety. This will lead to greaterdemands for plywood and will mean the ultimate increase of theshear value of the glue bond in the present government andcommercial specifications.In addition to the major applications by industry, there areseveral outstanding combinations of dry glue film plywoods withsynthetic sheet products which enter too many fields to permitsuch industrial classification.5 Asbestos Clad Plywood. In the development of asbestosslate, manufactured under various trade names, phenolic resinplywood has again found many new applications.Asbestos clad plywood, or plywood faced with asbestos slate,glued with dry glue film, has introduced many new possibilitiesfor plywood for interior and exterior purposes. The slate canbe had in various thicknesses from 1/ 1o in. up, and can be nailedor riveted in a very satisfactory manner. As the slate is resistantto acids and alkalies, is non-porous, is incombustible, and has smooth surface for paint or enamel, it may be used for either aface or a cross band. If used as a cross band, it can be facedwith any type or thickness of fine veneers. With dry glue filmunusually thin veneers can be applied to the slate without dangerof staining the work. Tests show that the dry film bond isstronger than the slate. A few of its many adaptions might belisted as follows:Wall panelingTelephone booths(Interior or exterior)Laundry equipmentHospital interiorsAir-conditioning unitsFireproof partitionsElectrical equipmentGreenhouse equipmentElevator cabsLavatories and shower rooms Railroad freight carsLaboratory equipmentIce cream cabinetsKneading boardsRefrigerators.Steaming roomsRubber Vulcanizing Directly on Plywood. The vulcanizing ofrubber directly on plywood is a recent development which opensanother field for the veneer and plywood industries. The rubberresearch laboratories have been eagerly searching for a suitablewood product on which rubber may be vulcanized. It wasfound that neither solid wood nor plywood glued with wetadhesives would answer the requirements of the vulcanizingprocess.Successful vulcanizing of rubber to wood demands that thewood shall not have more than 2 per cent moisture content, andthat it must withstand a heat from 350 to 380 F, under a specificpressure of 250 lb for a time of 20 to 30 min. It is immediatelyrealized that neither solid wood nor wet glued plywood canmeet such exacting demands. A dry glue film bond does notdeteriorate under this heat, and since it permits the manufactureof plywood with not more than 2 per cent moisturecontent under 250 lb specific pressure, the new plywood answersfully all the requirements for vulcanizing rubber to wood. Someof the many uses for this type of material are as follows:Desk topsRunning boards (auto)Rubber covered refrigerators Toilet seatsTable topsChair and stool seatsCounter topsCasketsFloor , boards (auto)Flooring and tile.Other Plywood Products. Dry glue film plywood applicationsfor other new products, discussion of which space does not permit,might be enumerated as follows:Agricultural equipmentPlywood milk racksBrush backsArchery bowsAthletic equipmentGolf shaftsArtificial blackboardsFret molding plywoodWood-metal desk topsTennis racket framesFlameproof plywoodHockey sticks2- and 3-ply shipping containers Curved and bent plywood.Foundry pattern stockThe phenolic resin bond of fret moldings from y»-in. andVei-in., 3-ply plywood has no injurious effect on the cutter headsor punches and dies. It is surprising to note the volume thatcan be made without renewing the cutting edges. The characterof the phenolic resin glue line does not dull saws, knives, andcutter heads.CONCLUSIONThe development of dry glue film has introduced a numberof distinct and definite advantages which the wet glues havenot offered. The result is that new possibilities and applicationsfor plywood are constantly being discovered and developed,and the horizon of the plywood industry is thereby greatlybroadened.N o t e : Various U. S. patents are applied for and pending onphenolio resin plywood, and prospective users should investigatecarefully.BIBLIOGRAPHY“Glues Used in Airplane Parts,” Report No. 66. Forest ProductsLaboratory, Madison, Wisconsin, 1920.“Modem Glues and Glue Testing,” C. H. Teesdale, Periodical PublishingCo., Grand Rapids, Michigan, 1922.“A Dry Glue Method of Laying Veneers," J. R. Truax, Veneers,Indianapolis, Indiana, vol. 24, no. 10, pp. 30-31, October, 1930.“Gluing Wood in Aircraft Manufacture,” J. R. Truax, TechnicalBulletin No. 205, U. S. Department of Agriculture, 1930.“Hydraulic Presses for Plywood,” Harry G. Francis, A.S.M.E.Trans., WDI-53-21, September-December, 1931.“Development of Wood Adhesives and Gluing Technique,” J. R.Truax, A.S.M.E. Trans., WDI-54-4, February, 1932.“The Dry Powder Glue Process,” Veneers, pp. 13-14, Indianapolis,Indiana, March, 1932.“Laminating With Phenolio Resins,” E. H. Merritt, presentedat Seventh National Meeting of the A.S.M.E. Wood Industries Division,Jamestown, N. Y., November, 1932.FOREIGN“The Approach to Superior Plywood,” L. M. Cohn-Wegner.Werkstattstechnik., vol. 22, no. 11 (June, 1928), Berlin.“The Development of Plywood Manufacture” and what the practisingengineer must know about it. L. M. Cohn-Wegner. Zeitschriftfiir Wirtschaftliche Technik, November, 1929.“Veneers and Plywood,” 2 volumes. Knight, Wulpi, and L. M.Cohn-Wegner. The first volume a translation into German of theAmerican publication and the second volume a complete descriptionof the plywood industry in Germany. M. Krayn, Berlin, 1930.“Wood Working Glues and Their Testing,” with special referenceto gluing wood with adhesive films. Otto Gemgross, Zeitschrift farAngewandte Chemie, vol. 44, p. 231, 1931, Berlin.“Plywood, Its Production, Use and Properties.” A. Mora, Timberand Plywood, 131-133 Middlesex St., London, 1932.DiscussionC h a r l e s B. N o r r i s .8 This is a valuable description of theuse of phenolic sheet glue, particularly the technical data includedin tables and charts. As Mr. E. H. Merritt has pointed>Mechanical Engineer, Haskelite Mfg. Corp., Grand Rapids,Mioh. Mem. A.S.M.E.

WOOD INDUSTRIES WDI-56-1 47out,4 the sheet glue process is only one of several processes ofgluing made possible by the development of phenolic resins.Mr. M erritt favors a dry powder process. My company hasdeveloped a process of gluing with which the sheet glue could beused or, with the addition of some of Mr. M erritt’s equipment, thedry powder could be used. However, we prefer using thephenolic resin in the form of a colloidal suspension in water.In the process we have developed, the glue is spread upon theveneers by means of a glue roll of the usual design. Special rollsand scraper bars are necessary, and the condition of the air surroundingthe rolls has' to be controlled. The veneer passes disoth a t each panel has two heating elements adjacent to it, one oneach side. The stack is moved into the press, pressure is applied,and the heating elements are energized. The pressing operationtakes about 20 min. Fig. 11 shows one end of the press and thepiling mechanism for placing the heating elements upon the stock.There is a similar unpiling device at the other end of the press.Fig. 12 shows the press-control apparatus, including an automaticvalve for controlling the pressure and an automatic devicefor controlling the temperature. The apparatus shown makespanels 8 ft long and 6 ft wide. I t has an output of 3000 sq ft ofVr-in. panel per hour. The full production possibilities of thisFia. 9Fio. 10Fia. 11rectly from the spreader into a simple surface drier which we havedesigned. High-velocity air is passed over the surfaces of theveneer in such a way th a t even thin veneers (V64 in.) are not disturbed.Fig. 9 shows the glue spreader and the entrance end ofthe drier. Fig. 10 shows the exit end of the drier. After theveneers leave the drier they are ready for the gluing process.The gluing process is similar to the usual cold gluing process inthat a stack of panels is formed and moved into the press. Thinelectric heating elements are piled alternately with the panels,4 “ Laminating With Phenolio Resins,” Trans. A.S.M .E., paperno. WDI-55-3 (June 30,1933).F ig . 12method of manufacturing panels are shown by plans of a largermachine, which we have completed. This larger machine makespanels 10 ft square at a rate of 22,000 sq ft per hr.The author describes the ideal wood adhesive as “one which isof a uniform quality, which can be laid or spread in an even coating,which gives a perfect glue bond, and which can be appliedeconomically.” There are also other requirements which itshould meet. There is almost an infinite number of possiblechemical combinations of phenol and formaldehyde. In makinga phenol formaldehyde glue it is possible to end up with any oneof a great number of these compounds by slight changes in the

48 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERStemperature-time relations of the process. It is very unlikelythat the manufacturer of the glue will ever make two batches ofglue that are chemically identical. Luckily the user of the glueis not interested in its chemical nature but in its physical properties.Uniform gluing results cannot be expected unless thephysical properties of the glue are uniform. The user should bein a position to test these properties himself. The form in whichhe receives the dry glue film makes this almost impossible. Wehave developed simple physical tests which can be applied to thecolloid glue to tell whether the glue is suitable for our purpose.Regarding the uniformity 6l glue spread, in using a liquid gluewe have at our command all the experience gained by the printingindustry in spreading inks uniformly. We find that we can obtainvery uniform spreads indeed. We also found that the dryglue film made by the author’s company is not uniform in thickness.Fifty measurements taken around the edge of a piece 18in. long and 5 in. wide show an average thickness of 0.00229 in., amaximum thickness of 0.0029 in., and a minimum thickness of0.0020 in. This is a variation of 30 per cent above the averageand 12 per cent below.A further requirement of an ideal wood adhesive is that itshould make as good a glue joint at the center of a thick panel as itdoes near the face. If a thick panel is placed between the platesof a hot plate press, the wood near to the plates is heated muchmore rapidly than the wood near the center of the panel. Thereforea glue line near the face of the panel is subjected to conditionsdifferent from those of a glue line near the center of thepanel. The glue must be such that a good glue joint is obtainedunder both these conditions. The author does not give any datashowing the properties of dry glue film joints made at differentrates of heating.Another requirement of an ideal wood adhesive is that it shouldgive good results over quite a range of moisture content of thewood. A plywood panel should be glued up at a moisture contentwhich is the mean moisture content the panel will obtain inuse. For example, if a genera] purpose panel is to be made, itmay be used indoors or outdoors. Indoors in winter its moisturecontent will probably drop to 4 or 5 per cent. Outdoors itmay reach a value of 20 per cent. The panel should be made,therefore, at a moisture content of about 12 per cent in order thatthe shear stresses developed in the panel at the low and highmoisture contents will not be great enough to cause failure of thewood. Chart 2 of the paper shows that the dry glue film issuitable for this purpose. In our process the water is dried outof the spread colloid so rapidly by the surface drier that the moisturecontent of the wood is not materially changed.So far as we know there are only three reasons for the introductionof phenolic resin glues into the plywood industry: thegreater strength of the glue joint, the greater water resistance ofthe glue joint, and the immunity of the glue to attack by fungiand bacteria. All other advantages claimed for the resin gluesare obtainable with the older forms of glues. The authorstresses the advantages of gluing with “dry” glues rather than“wet” glues, saying that the amount of water contained in gluescannot be kept uniform and that there is no possibility of regulatingthe influence of moisture upon the glued plies. We believethat these statements are an exaggeration of the facts.We have been controlling these factors in the use of blood-albuminglues for a number of years.The warning that very excellent hot plate presses are necessaryfor making plywood is timely. However, we do not agree withthe reason given regarding the extreme thinness of the dry gluefilm.An elementary discussion of the theory involved will showthe reasons for the need of very excellent presses when any kind ofglue is used. We can assume that the modulus of elasticity ofwood is 80,000. If we assume also the variation in the press of0.003 in. that the author allows, we find that the variation inpressure upon a plywood panel */4 in. thick is 960 lb per sq in.That is, under normal gluing pressures certain parts of the plywoodreceive no pressure whatever. If it were not for the factthat wood becomes somewhat plastic when it is moist and hot,the steam platen method of gluing plywood could not be successful.It is a precarious method at best. The process we havedeveloped has the advantage of the cold process method withregard to pressures. Under the same condition of 0.003 in.variation in the press, the variation in pressure is only 3 b persq in., figured by the method outlined.The impression is gained from the paper that it is not practicalto glue thick panels with the dry glue film if some of the glue linesare near the center of the panel. This is a real difficulty that isvery often encountered with the use of phenolic resins. However,this difficulty can be overcome, and in fact is being overcomein our factory.We are in perfect accord with the author that the phenolicresins make remarkable glues and that the dry glue film is a verygood glue indeed. We think, however, that the use of the colloidovercomes some of the difficulties encountered in the use of theother forms of the resin and that our gluing method overcomesthe difficulties inherent in the hot plate process.A. J. N o r t o n .5 There are now three commercially developedmethods of using phenolic resin glue lines. In addition to thefilm glue discussed by the author, there is the dry-powder methodof spreading resins which was discussed by Mr. Merritt in theOctober meeting at Jamestown, and the method of spreadingthe synthetic resin in colloidal form as was brought out by Mr.Norris, of the Haskelite Company.The colloidal method of spreading has proved satisfactory inlarge-scale production and lends itself to commercial operationvery satisfactorily. The spreading equipment is slightly differentfrom that already used, and hot plate presses, steam orelectric, are the same as are used in any hot press gluing operation.The colloid lends itself to more widely diversified typesof application, due to the ease of spreading low or high amountsof resin, and while it was not brought out at the meeting, wehave since shown that the colloid works successfully over alumber core under usual operating conditions. It was broughtout at the Chicago meeting that the use of the colloid on sheetlumber might develop a panel core which would compete activelywith the lumber core in construction work.Dry resin and colloid are manufactured by General Plastics,Inc., and these products have sold under the trade name ofDurez.With the three methods of using phenolic resin glue line, andwith the merits of such a glue line definitely established, thereseems to be no doubt of a distinct and revolutionary changetaking place in the lumber field.1 General Plastics, Inc., North Tonawanda, N. Y.N o t e : Statements and opinions advanced in papers are to beunderstood as individual expressions of their authors, and not thoseof the Society.