US2028287A - Process of making glued up plywood - Google Patents

Description

Patented Jan. 21, 1936 UNITED STATES PATENT OFFICE No Drawing. Application January 2, 1931, Serial No. 506,321

1 Claim. (01. 144-309) The present invention relates to gluing up plywood, with glues of the type involving a proteid and an alkali which constitutes-a solvent for the same. The object is to produce a fin- 5 ished plywood which will be resistant to water, by the use of glues, many of which applied and treated by the ordinarily accepted cold methods would not give water-resistant panels.

For gluing upplywood the cold process has been heretofore largely used, particularly with glues such as those made from casein or seedmeal or mixtures of the same, containing an alkaline material which will serve as a solvent for the proteids of the seedmeal, and/or forthe casein, and when water-resistance was required it has been the usual practice to employ considerable quantities of lime or other alkaline earth compound of a basic nature, such as magnesium oxide or hydroxide. The glue, preparedaccordo ing to any of the well known processes or formulas has been mixed with about two or two and a half parts of water to one part by weight of the commercial dry glue, and the usual procedure in making three ply plywood has been to 25 apply the glue to both sides of the center piece or core, then to'apply the face veneers upon the cores carrying the coating of wet glue, and the usual practice has been in many of the mills, to continue to make up the panels for about twenty 30 or twenty-five minutes, then the pile of glued panels is-placed under high pressure, say in a hydraulic press, for twelve to twenty-four hours. The pressure is then released and the panels are stacked up in a drying room for three to ten 35 days to season and dry out, prior to finishing or shipping the same.

During the period that these glued up panels are kept under pressure of 125 to 175 pounds per square inch, which usuallfis 12 to 24 hours, the

40 glue does not dry out very much, but the water present in the glue, carrying soluble matter is largely absorbed by the cores and veneers. Very often in taking down the stack of panels from the press, and handling the same in transferring 45 the same to the drying room, it will be found that there has been a certain amount of decomposition as evidenced'by the fact that a pronounced odor of ammonia will be noticed, and the wood will be warm showing some decompo- 50 sition to have taken place. Ifjythe decomposition has proceeded to a substantial extent, as indicated by heating of the panels, the absorbed alkali solution may also have caused more or less staining of the panel, which of course would 55 be objectionable in many cases. Sometimes the staining does not become entirely apparent until the plywood goes into the final heat drying operation. Staining is of course highly objectionable-when the finished veneer is to be 60 merely varnished or lacquered with a, transparent varnish. Staining may be frequently more apparent in those. cases where the face veneer is very thin, as in making imitation mahogany, oak, black walnut and the like, for which purposes it is customary to employ sur- 5 face veneers of the high priced wood, which are sometimes extremely thin, say one-thirtysecond of an inch or even one-sixty-fourth of an inch in thickness. In such cases the surface finish may be merely varnish or oil, or in many 10 cases there may be no surface finish, and of course in such cases any staining would be extremely objectionable.

There are many well known advantages in using casein glues or seedmeal glues or casein 15 and seedmeal glues or blood albumen glues, in place of ordinary animal glue, fish glue, hide glue and the like, in making up such plywood.

Hot plate gluing is of course well known, and it is alsowell known that when the panels are pressed hot there is an increased tendency for the glue to come through the face veneers and stain the faces of the panels. This is one of the reasons why cold gluing, of the character above described has been more commonly used in the prior art. Another objection to the hot gluing is that very often blisters or spots where there is no adhesion of the veneer to the core will be found in the glued up panels made by the hot pressing method. It is believed that the blisters may be largely due to the development of steam in the'glue line, the steam expanding and separating the pieces of veneer from the core, to some extent thereby obstructing adhesion of the veneer to the core. In accordance with the present invention I am able to completely avoid staining and blistering, and despite the fact that preferably I employ glues which by the cold process would not give a good degree of water resistance in the 40 glued up product, I obtain an excellent degree of water-resistance, but I may use any kind of a water-dissolved glue containing casein, seedmeal or seedmeal proteid or blood albumen, and

to some extent improve the water resisting propertiesof the final panel by my process. The glue is dissolved in the customary manner, according to the formula furnished by the manufacturer or distributor, and the glue is applied to thefcores in the usual manner, for example on the double roller machine (in making three ply plywood). But the face veneers are not applied to the coated cores at once, and the coated cores are allowed to dry to a considerable extent, for example in a warm room, or they may be run into a highly heated room to dry more rapidly. They may be allowed to dry to a point where the surface just feels slightly sticky, and is somewhat gelatinous, or the drying may be continued until the glue is dry to the feel, so that a person handling the coated cores would not notice that they are at all moist, and they would not'feel at all sticky. The degree of drying at this stage will depend to a considerable extent upon the work under treatment. If the face veneers are to be very thin then it is more essential that the drying shouldbe continued until the cores feel entirely dry to the hand, but when thicker face veneers are to be used, the cores may be dried to the same extent or to a less extent so that the glue on the surface would be in the form of a stiff jelly. Q

When the proper degree of dryness has been reached, the face veneers are laid onto the glue coated cores, and placed in a hot plate press or the like, and heat and pressure applied for a short time only, say 3 to 5 minutes. The panels are then ready to be removed from the press and may at once be trimmed, sand papered, shipped and used in the construction of theultimate objects.

It is of course understood that any number of plies can be made up, and that in making say five-ply panels the interior three plies will be considered as a core in the subsequent or final veneering operation. This interior three-ply core can be made by the process described above, or by any other suitable process.

Based upon my experiments, it is recommended to use proteinaceous glues containing only a relatively small amount of lime or similar alkaline earth material, and this can be omitted entirely if desired. It is also advisable to use only a relatively small amount of alkaline sodium salts, for example about the minimum which will give complete solution of the proteids of the glue base. Such glues in many cases would not, according to the cold gluing processes above described, give good water resistance, but when used in my present process they give water resistance even superior to that produced by the high lime glues.

By way of example I give the following as suitable formulas for making up glues which may be used in the present invention, but it will be understood that these examples are merely for the purpose of illustration and. not for the purpose of limiting the invention thereto: (parts are by weight).

Formula 1 casein 6 hydrated lime 5 trisodium phosphate 8 sodium fluoride Formula 2 70 casein 12 hydrated lime 7 trisodium phosphate 6 sodium fluoride Formula 3 65 casein 9 hydrated lime 5 sodium carbonate 7 sodium fluoride,

A short time before use, the dry glue is mixed with about 2 to 2.5 times its weight of water, to form a thin flowing liquid glue. No water-glass or other soluble silicate is used in the glue base or in the final dissolved glue.

No claim is made herein to the dry glue bases disclosed above or other proteinaceous glue bases, or to the liquid glues produced by ,mixing them with water or to the method of applying the cold glue to the veneer. An essential feature of my process consists in subjecting the substantially dry glue-lines on the veneers to heat and pressure, in the manner herein described.

It is my belief, based upon experiments made, that the heat softens or plasticizes the proteid compound, and constitutes a plastic cement in the hot gluing operation, when proceeding according with the present invention.

Of course in making two-ply wood, one of the veneers can be considered as the core and the other as the face. veneer: and it is not necessary to coat both sides of the corewith the liquid glue.

The amount of pressure and thetemperature to be used during the hot pressing, as well as the periods of time during which the hot pressing is continued maypbe varied between rather wide limits.

In most cases I have secured the most satis-.

factory results by pressing at subtantially above the boiling point of water, say 230 to 250 F., at pressures rather higher than those ordinarily employed in cold gluing, for example 175 to 250 pounds or more, per square inch. The time factor is of some importance, and usually periods of from 3 to 10 minutes in the press, while the heat is on, have given very satisfactory results. Of course it will be understood that there is a substantial difference between the temperature of the metal in the press and the temperature of the glue line in the plywood.

In some experiments illustrating this, I worked with a low lime casein glue, which was not water resistant after drying (by the cold gluing process, for which the same had been designed). Thus the dry glue of Formula 1 mentioned above is suitable. Particularly satisfactory results were produced in the following manner:

The non-waterproof glue, in a liquid state, (mixed up 2.25:1) was applied to the cores, and the latter then placed directly over some steam pipes, by which they were practically dried, that is to say they did not feel wet, when rubbing the hand over the same. The face and back veneer were then put onto the core and this assemblage of the face veneer, coated core and back veneer was pressed at 240 F., at 200 pounds per square inch, for 3 minutes. The three ply panel was then cut up and tested, both dry and wet. The panel showed a dry strength of 350 pounds and after soaking for two days was then tested wet and found to have a strength of somewhat over 100 pounds. Itis particularly noted that in this experiment heat has transformed a non-waterproof casein glue into a strongly water resisting plywood glue. Other experiments made with this same glue, applied to the same kind of veneer wood but pressed cold by the ordinary process, was found to fall apart upon soaking in water for an hour or so.

In other experiments with the same kind of glue the coated cores were allowed to stand in a warm room for half an hour. At the end of this time it wasfound that the coating of wet glue was jelled, but was not completely dry. It was jelled altogether too much to make even a fairly good joint by the cold gluing process. A face and back were applied to this coated core and hot pressed in accordance with the procedure of the present invention. A highly satisfactory panel was thereby produced, which showed good water resistance.

It will be understood that a particular adcan be further treated or can be shipped. The

drying before applying the face and back veneers requires only a small fractionof the amount of time necessary for drying the completed ply- 'WOOd made by the cold gluing process. Ac-

cordingly a plant of a given size can ship out the finished plywood in a small fraction'only of the amount of time now required by the cold gluing process, since it is readily possible to ship in the afternoon finished plywood made from veneers received and started upon in the moming, whereas by the cold gluing process the drying (after standing 12 to 24 hours-in the press) requires several days.

The elimination of stain on the veneers is also highly important. Not only does glue stain not occur on the outer surface of the veneer, but

I have in some instances sanded down the outer,

closing any staining of the same. It would seem that by the evaporation of the alkaline glue water before pressing, the alkali penetration which causes stain in the veneer is eliminated.

The amount of labor required in handling is also greatly reduced under the present process.

No claim is made hereinto the use of liquid glues containing water-glass, as a substantial constituent thereof.

A process of gluing together pieces of wood. more particularly for the manufacture of plywood and for veneering, comprising the use of a glue which when dry is liqueflable by heating to a low temperature, applying said glue when in liquid form to a piece of wood, immediately sub-- jecting the piece of wood to artificial and rapid drying to prevent the liquid of the glue from soaking into the wood and to attain a practically liquid-free glue film on the piece of wood, and then hot pressing the piece of wood to the piece of wood to which it is to be joined to unite the pieces together.

WILLIAM F. LEICESTER.