US2537343A - Rapid gluing process - Google Patents

Description

Patented Jan. 9, 1 951 RAPID GLUING PROCESS Alexander J. Golick and Jesse J. Maas, Seattle, 1

Wasln, assignors, by mesne assignments. to Monsanto Chemical Company, St. Louis, Mo., a

corporation of Delaware No Drawing. Application January 7, 1946, Serial No. 639,693

7 Claims.

This invention relates to a rapid gluing procass for alkaline casein glues and to reagent compositions for use therewith.

Numerous attempts have been made to utilize the almost instantaneous gel-forming and insolubilizing reaction of formaldehyde with alkaline casein glues to speed up the gluing rocess in making glued structures from wood or other dry porous material. To date, these attempts have not succeeded in producing bonds of the desired full commercial strength although weak bonds can be produced in this manner with great speed. The exact cause of this unsatisfactory bond strength with formaldehyde-treated structures cannot be stated with certainty but, as a result of extensive experiments, we believe that formaldehyde per se or as such is inherently too fast a gel-former to produce satisfactory bonds between materials glued with alkaline casein glues. Apparently, if the actual formaldehyde chemical in the free state is allowed to contact the glue film before the surfaces to be glued have been properly pressed together, a gel is instantly formed which is too dense to penetrate the companion surface so that a barrier to adequate penetration is immediately created no matter how rapidly the penetration-promoting bonding pressure is applied subsequent to contact. This precludes success with a method where the glue is applied to only one surface and the'formaldehyde is applied directly to either the same surface or to the unglued surface. In both cases z'the resulting bonds are below desirable commercial strength and there is evidence of inadequate glue penetration into the member which was not originally spread with glue.

It has been proposed to avoid this difllculty by first applying glue to both surfaces and then, Just before contact, spraying or otherwise applying a dilute solution of formaldehyde to one of the glue films. This method, however, also fails to eliminate the objectionable sudden glue gellation upon initialicontact of the glue with formaldehyde and henceiiinvolves intolerance of delay in applying bonding pressure as well as the inconvenience of applying the glue to two surfaces instead of one. It has also proved very difilcult to spray or otherwise apply formaldehyde uniformly to the surface of a wet glue film and the spraying method disseminates formaldehyde fumes throughout the working space.

For mechanical reasons in most work, for instance in edge-gluing, the method of applying the glue to one of the surfaces and the reagent solution to the other is advantageous because this l 2 can be done simultaneously and uniformly by passing the strips of lumber through an apparatus which applies glue by contacting one edge of the lumber with the surface of a glue roll while, at the same time, the opposite edge is contacted with a felt roll which applies the chemical solution with excellent uniformity. However, as stated, when a solution containing free formaldehyde is used, this method has not been capable of successful operation.

A principal object of the present invention is the provision of a rapid gluing process for use with alkaline casein glues, particularly one which is effective to speed up the fabrication of glued parts made of wood or other dry porous material by mass production methods. Another object is the provision of a process that is adapted for various automatic and semi-automatic mechanized operations, such as edge-gluing strips of waste lumber in forming larger elements for packing cases, lumber cores, shelf stock and many special products where very rapid bonding of relatively small surfaces is especially desirable.

A further object is the provision of stable reagent compositions which may be employed with alkaline casein glues to make possible such processes. Still further objects include the provision of an alkaline casein gluing process which permits the application of glue to one surface and a gelling and insolubilizing composition to the other surface which, when united, produce a glue line of satisfactory physical properties; the provision of a rapid gluing process which avoids any initial contact of the glue with free formaldehyde, and the provision of stable reagent compositions which are capable of releasing a quickacting insolubilizing agent for alkaline casein glues when contacted with systems possessing a pH of 10 or above. Another object is to provide a fast gluing process for alkaline casein glues that will provide a glue bond that is highly water-resistant and yet not marred by a strongly marked alkaline glue stain. Other objects and the entire sco e of the invention will become apparent from the subsequent description.

We attain these objectives by employing, as the gelling and insolubilizing agent for alkaline casein glues, water-soluble stable materials which are characterized by being substantially incapable of causing any gelling of casein glues having a pH of 9 or less but which are capable of a progressive gelling effect upon casein glues having a pH of 10 or above.

The success of this invention results principally from the discovery that, by employing the 3 compositions'defined above with alkaline casein glues, it is possible to produce adhesive bonds which possess high structural strength at relatively high rates of production in contrast to the usual far slower process of bond formation with unmodified casein glues or the poor structural strength resulting from direct formaldehydetreatment procedures. The general basic procedure for utilizing this discovery is as follows:

A casein glue having a pH of 10 or above is applied to one of the surfaces of the porous material to be joined. A stabilized solution of trimethylol nitromethane, diformamidomethane, or other compounds capable of releasing reactive aldehyde or other casein gel-forming material upon contact with a system having a pH of 10 or above, is applied to the opposing surface. Stabilization of the solution to insure absence of free formaldehyde is secured by adding a small amount, usually only about 0.25 per cent of the weight of the aldehyde-providing substance, of any suitable formaldehyde-fixing or binding agent, such as sodium sulfite, urea, phenol, a protein, or the like. The gelling solution may also contain a wetting agent to aid in it penetration into the treated surface and, if desired, may include a dye, such as malachite green, to identify the treated surface.

The reagent-treated surface may be allowed to dry or may be joined to the glue-coated surface while still wet. The joining of the surfaces is carried out in such a manner that no contact is permitted between the reagent-treated surface and the glue-coated surface until the exact moment of joining. Pressure is applied to the assembly as the surfaces are contacted in order to promote glue penetration and maintain them in intimate contact until chemical gellation of the glue has progressed to such a degree as to produce a bond of sufilcient strength to permit normal handling without injury, i. e., a period of time generally of the order of one minute. Finally, after release of pressure, the glued structure is allowed to stand for a sufficient time, c. g., two hours, to permit the bond to attain ample strength to permit rough handling such as machining.

A more complete understanding of this invention will be obtained from the following examples in which all parts are by weight. These examples are illustrative of various methods of using our process but should not be construed in a limiting sense.

. Example I This example illustrates the edge-gluing of strips of dry fir and hemlock to produce wide clear boards for various industrial uses.

Dry, well-matched strips of fir and hemlock vertical grain lumber, inches thick, 1.5 inches wide and 12 feet long, are passed through a glue applicator which applies, at the rate of 40 to 60 pounds of wet glue per thousand square feet of glue line surface, a commercial casein glue containing caustic soda and having an alkalinity of about pH 11.5, to one of the edges of the strips. Simultaneously, the opposite edges of the strips are passed through a felt roll applicator which applies, at the rate of 6 to 7 pounds per thousand square feet of glue line surface, a aqueous stabilized solution of trhnethylol nitromethane which contains a small amount of wetting agent to insure uniform deposit on the wood. The solution of trimethylol nitromethane was stabilized by the addition of 0.25 part of sodium sulflte for each parts of trimethylol nitromethane. As the treated strips emerge from the applicator, they are placed side by side on the bed of a press, caution being used to avoid contact of adjacent edges. In a fully mechanized operation, such undesirable preliminary contact may be avoided by the use of separator pins with a mechanical device for withdrawing them after the strips have been placed in position. In this manher, the glue-treated surface of one strip is laid adjacent to but out of contact with the agenttreated surface of the preceding strip with a slight openspace between. Twenty-four. strips are thus prepared and assembled, this requiring less than two minutes elapsed time. When the prepared strips are properly assembled, the press is closed, bringing all the strips into edge to edge contact. It is important that the press be designed to secure parallel alignment and uniform application of pressure. Air cylinders may be used for quickly applying pressure of the order of 50 to 100 pounds per square inch of edge surface. The pressure is retained for one minuteand then released and the board is removed to a pile. It is found that, at the end of one minute pressure, the bond is amply strong to stand the handling necessary for piling. The glued-up assemblies, which have a width of about 36 inches and a length of 12 feet, are allowed to stand in a pile on a truck for two hours during which period the bond attains ample strength to permit machining. Thereupon, the boards are run through a planer and trimmed and are ready for shipment. A strong and highly water-resistant bond is obtained.

Using the above procedure, three operators are able to produce five times the number of assemblies previously produced by four operators using normal adhesives and the usual clamping process.

Example II This example illustrates the edge-gluing of fir, alder and hemlock stock to form blanks for the manufacture of chair seats.

Fourteen inch pieces of random width 1% inch fir, alder and hemlock are brush-treated on one edge with a stabilized 20% solution of diformamide-methane containing a small amount of wetting agent and the water-soluble dye, malachite green. This solution was stabilized with about 0.25% of sodium sulfite as in Example I. The treated pieces are allowed to dry, well in advance of the gluing operation. The untreated edges, which are easily recognized by the absence of the dye, are then passed over a glue roll where they are coated with the glue of Example I. These materials are applied at a rate similar to that described in Example I. The assemblies are then laid up on press beds which are wide enough to produce an eighteen inch blank as required for an individual chair seat. Assemblies may be pressed as fast as the glue is applied to the stock required to make one blank. In placing the assemblies on the press beds, care should be used to avoid contact between adjacent edges until ready to apply pressure. The assembly time for each blank will be of the order of thirty seconds. Pressing time, as in the previous example, should be about one minute. The final bond is strong and water-resistant.

Using this procedure, two operators can operate two presses, including handling the stock, applying the glue, and piling up the blanks. The color or the dye disappears from the alkaline glue line.

Example III This example illustrates the use of our process with a very Simple improvised air-operated clamping press greatly speeding up the manufacture of blanks for wood louvre ventilating units for prefabricated house construction.

Stock consisting of smoothly-surfaced reclaimed waste pieces of dry pine lumber, 1.5

' inches thick, '7 inches wide and 14 inches long, is employed, two such units being edge-glued together to form one blank. The gluing operation, which may be performed by two unskilled operators, consists of the application of the casein glue as in Example I to one member by one operator by means of a roll applicator and the application as in Example I of the catalyst (consisting of a stabilized solution of trimethylol nitromethane) to the adjacent unit with a brush by the second operator who also assembles each unit in the press and operates the valve to apply and release pressure. The solution used is the same as described in Example I. These units can be built up as rapidly as the two operators can prepare them which is generally at the rate of four units per minute. The pressure is retained for approximately 15 seconds, after which time the unit is removed from the press and piled on a load for the required conditioning before flnal machining.

This process makes possible the use of a very simple but highly efficient production unit and eliminates the use of the more cumbersome, expensive and slower-operated clamp carriers.

The short pressure time of approximately 15 seconds in this example is practical because the units are small and light and the area secured by the adhesive is proportionately large. Under these conditions, it is not necessary to wait for one minute, as in Example I, in order to secure an initial bond sufficiently strong to permit handling. The final bond is strong, uniform and water-resistant.

The foregoing examples illustrate the operation of our process with the usual readily available highly alkaline casein glues. Such glues react with some species of wood to produce dark glue lines which are occasionally objectionable in exposed locations. Such high stain character in casein glues is, of course, dependent on the presence of excess of strong alkalies such as lime and caustic soda. Since the release of effective amounts of insolubilizing agent from our reagent solutions is dependent on a critical pH level in the casein glue and not upon the total available alkali, it is apparent that, where such glue stains are objectionable, they can be minimized by known methods of special glue formulaability, cost, ease of shipping and handling and freedom from objectionable characteristics. We have found that trimethylol nitromethane and diformamidomethane are examples of chemicals which, when stabilized, meet the requirements particularly well. Either of these compounds can readily be prepared by known methods by reacting nitromethane or formamidc with formaldehyde. Aldehyde reaction pro ducts of nitroparafflns (particularly the mono-nitroparafflns) and amides (particularly the mono-amides, e. 3.. the methylol derivatives of nitroparafiins and the methylol or ethylol derivatives of amides) may also be employed. Dimethylol urea, for example, is a chemical of this type, although we find it too slow in its gelling characteristics for general use. Other compounds capable of decomposing in strongly alkaline media to yield casein gelforming products may be utilized. As noted, it is important in making up or using these reagents in our process to maintain them entirely free of uncombined aldehyde. These aldehyde addition compounds are all somewhat unstable and, even in neutral solution, may contain or release enough free aldehyde to interfere with the operation of our process but we find they can be readily stabilized by including with them a small content of some inexpensive aldehydebinding substance, for example, sodium sulflte. We find an effective amount with sodium sulflte is about 0.25% based on the'weight of the aldehyde-providing material. The presence of the stabilizer binds and renders inactive any content of free formaldehyde present in the reagent solution or developed therein during storage. The stabilizer, however, does not prevent the ultimate release of active aldehyde by alkaline hydrolysis tion to eliminate the presence of staining agents to the extent which is feasible and still provide a casein dispersion having a pH of 10 or above. It is thus possible to obtain the benefits of our process by special formulation without producing glue lines of marked discoloration. However, in

most work, the usual highly alkaline glues are during our process. In the case of these aldehyde derivatives, it is not theoretically necessary that these compounds be formaldehyde derivatives but the analogous compounds of higher aliphatic aldehydes are more expensive, less reactive and hence not found to be economic;

Since the variety of possible compounds of this nature which can decompose to release gel-forming material is extensive, a convenient method of determining relative suitability of such a reagent for our process is as follows:

An alkaline dispersion of casein is made up to glue viscosity atpH 9 and the material to be tested is added thereto in the required amount. Effective reagents should not produce any gelling action on such a solution within a period of about 15 minutes but, if the solution is adjusted to about pH 11, then satisfactory reagents will produce marked gelling action within 5 or 10 seconds on admixture with the alkali. However, it has been found that some compounds, which do not cause gelling at pH 9 but produce rapid gellation at pH 11, will still produce a slow gellation at a pH as low as 10. For most purposes,

rapid gel formation, once the surfaces to be.

joined have been united under pressure, is desirable. Hence, it is generally preferable, except when a low stain glue line is required, to use a glue that is at least as alkaline as pH 11 and a reagent that, due either to its nature or concentration, produces rapid gellation at that pH. For the purpos of obtaining a wide range of control, glues having a pH as low as 10 with a reagent which produces gellation at that pH may be used. All 'pH values noted herein are values after correction for the sodium ion.

Upon contact with a strongly alkaline casein glue, the gel-forming agents of this invention advantageous. As

release substances which gel the glue but this occurs only as the result of a decomposition reaction and hence is not instantaneous as when formaldehyde chemical "as such" is directly contacted with the glue. It is surprising that this slight retardation of the reaction has such a desirable effect. However, this retarded action appears to be responsible for the success of our process by making possible adequate penetration of the adhesive into both members to be united. By the use of these reagents and process, instead of a bond being formed between the surfaces by a relatively gradual process of dehydration, as in normal gluing with casein glues, or a bond being formed of poor strength, as in gluing with casein glue and formaldehyde as such, this invention makes possible a controllably rapid but not instantaneous chemical gellation of the glue which quickly develops bond strength adequate to permit early release of pressure and handling of the parts without injury to the final bond. This initial pressure and chemical gellation stage is then succeeded by a second stage during which the bond spontaneously reaches final full commercial strength without pressure and largely by the usual progressive dehydration. With most soft woods, the final bond is stronger than the wood itself and, due to insolubilization of the protein, has higher water-resistance than that developed by other casein glues.

Thus, while avoiding the difiiculties associated with the direct formaldehyde gellation of casein glue, our process produces bonds with great speed since it is not necessary to wait for slow dehydration. As a result, the joining pressure can be, applied entirely in a simple press without the use of presssure or retaining clamps, and the pressure can be released very soon after the parts are contacted. Furthermore, the speed of the process can be controlled as required to meet various specific gluing requirements. In general, the pressure can be released after an interval which does not exceed an extreme duration of about minutes and, in most cases, after a pressing period of only 1 minute. In some instances, the prsssing period may be reduced as low as 10 seconds. These new features permit numerous useful applications.

Through the use of a stabilized non-volatile hardener, materials to be thus glued can be treated with the hardener at any stage during manufacture. These insolubilizing chemicals are effective regardless of whether dried or still wet from recent application. This is often very indicated, small amounts also of any desirable wetting agent may be incorporated in the reagent solution to insure uniform disposition on the surface. Where the solution is to be applied in advance of gluing, a dye may be included to identify the treated surface, as has been indicated in Example 11 above. By selecting a dye such as malachite green, which is destroyed by alkali, no color is left in the final glue line.

In all cases, it is essential that the insolubilizing agznt shall not supply any appreciable amount of gelling action instantaneously upon contact with the adhesive but only after a chemical reaction with the glue. While this reaction in the case of satisfactory materials appears to commence very soon after contact between the reagent and the alkaline glue, it is probably progressive, although it is impossible to measure it by any general analytical method. However,

it appears that the resulting occurrence of gellation is sufficiently retarded so that the formalation of the peneration barrier of initially strongly gelled adhesive, which results from initial contact with formaldehyde per se or as such," does not occur in our process.

With regard to control of the process to meet varied conditions: it will be readily understood that the specific kind of rea ent and itsconcentration permits considerable variation of the rate of bond formation. For instance, it has been found that the action of diformamidomethane is somewhat slower than that of trimethylol nitromethane and, of course, lowering the adhesive-reagent ratio tends to retard action. In addition, time of application affords a control since, if the reagent had been permitted to dry on the surface, it must re-dissolve before it can be decomposed by the alkali in the glue, whereas a reagent which is wet at the time of contact may start the process of decomposition immediately.

This invention makes possible high speed, low cost production of a great variety of strong, uniformly-bonded, glued products from lumber or similar materials. The process facilitates salvage of low-grade material because it dispenses with .clamp holders and yet produces strong, uniform products at a rate which permits utilizing small pieces of waste wood.

Since other methods of practicing the present invention will be readily apparent to those skilled in the art, it is understood that the foregoing description is illustrative of the preferred method of practicing our inventon and is not to be construed as limiting its application except as set forth in the appended claims.

We claim:

1. As a new article of manufacture, a structure of wood glued together with an alkaline casein glue having a pH of at least 10 which has been insolubilized by a member of a group consisting of a reaction product of an aldehyde and a nitroparaflinand a reaction product of an aldehyde and an amide and characterized by bein incapable of gelling a casein solution having a pH of 9 and lower when associated therewith, but capable of progressively decomposing to release an aldehyde gelling agent for casein when associated with a casein solution having a pH of at least 10.

2. As a new article of manufacture, a structure comprising sections of wood glued together with an alkaline casein glue having a pH of at least 10 which has been insolubilized by the reaction of trimethylol nitromethane with said glue.

3. As a new article of manufacture, a structure comprising sections of wood glued together with an alkal ne casein glue having a pH of at least 10 which has been insolubilized by the reaction of diformamido-methane with said glue.

4. The process of gluing with a casein glue having a pH of at least 10 which comprises the steps of applying sad glue to one of the surfaces to be joined, applying to the other surface an aqueous solution free of uncombined aldehyde comprising a member of a group consisting of a water soluble reaction product of an aldehyde and a nitroparaffm and a reaction product of an aldehyde and an amide, contacting the treated surfaces to be joined under pressure, progressively decompos'ng, said aldehyde reaction product by intermingling therewith said casein glue having a pH of at least 10 thereby gelling the casein glue, and maintaining the treated surfaces in contact under pressure until a preliminary bond of a water soluble reaction product of formal-' dehyde and nitroparafiin and a reaction product of formaldehyde and an am de, contacting the treated surfaces to be joined under pressure, progressively decomposing said formaldehyde reaction product by intermingling therewith said casein glue having apH of at least 10 thereby gelling the casein glue, and maintain'ng the treated surfaces in contact under pressure until a preliminary bond has been formed by the gelling action of the released formaldehyde.

ALEXANDER J. GOLICK. JESSE J. MAAS.

REFERENCES CITED The following references are of record in the 'file of this patent:

UNITED STATES PATENTS Number Name Date 1.170,021 Tischer Feb. 1, 1916 1,895,433 Bradshaw Jan. 24, 1933 1,967,688 Rozema July 24, 1934 2,018,733 Meyercord et al Oct. 29, 1935 2,101,574 Dangelmajer Dec. 7, 1937 2,126,560 Kyrides Aug. 9, 1938 2,141,313 Osgood et al. Dec. 27, 1938 2,156,642 Slagh May 2, 1939 2,238,928 Cahn Apr. 22, 1941 2,250,255 Cox July 22, 1941 2 261,788 Wyler Nov. 4, 1941 2,279,096 Sparre Apr. 7, 1942 OTHER REFERENCES Walker: Formaldehyde, 1944, pages 44, 256-258.