US2591771A - Method of gluing a plurality of wooden elements comprising curing a urea formaldehyde adhesive by means of high-frequency heating - Google Patents

Description

,part of Serial No. 100,765, filed June Patented Apr. 8, 1952 METHOD OF GLUING A PLURALITY. OF

WOODEN ELEMENT S COMPRISING CURING. A UREA FORMALDEHYDE AD- HESIVE BY QUENCY HEATING Elmer E. Bergey,

MEANS OF HIGH-FRE- Lansdale, Pa., assignor to National Casein Company, Chicago, 111., a corporation of Illinois No Drawing. Application June 9, 1950, Serial No. 167,250

3 Claims.

The object of the invention is to provide an improved hardener for use with urea-formaldehyde resins in the edge gluing of wood in particular, and other materials to a substantial degree, by means of high frequency electric currents,

now abandoned.

In recent years many new and remarkably improved gluing techniques have been developed especially for use in the woodworking industry. and of these probably the most impressively successful has been that which involves the use of high frequency energy in curing heat-sensitive glues, as ameans of both enhancing the quality of laminated wood products and in effecting substantial operating economies.

The principal methods by which high frequency energy is applied to the unification or gluing of laminated wooden bodies are known as perpendicular heating and parallel heating, and ineach case it is of the utmost. importance that proper adhesives be used, and that these shall harden or set as quickly as possible. In perpendicular heating the high frequency field is perpendicular to theglue laminations or veneers and the intervening glue joints are heated simultaneously. This method is used in the production of both flat and curved plywood, laminated wood, furniture panels and the like. is applied to the union of multiple laminations and intervening glue layerswherein the high frequency field is parallel with the glue. lines, which latter are. virtually the only part of the load that is heated even momentarily.

While it is true that both the wood and the intervening glue joints compete as conductors of the high frequency energy, the glue is comparatively much more conductive thanthe wood, and consequently conducts much more of the electric energy and is correspondingly heated to a much greater degree. Parallel heating is the more efficient method, due to the fact that the mass of the glue heated is relatively very small when compared with the bulk of. the wood being bonded, However, the glue requirements. for parallel heating are far more exacting than for perpendicular heating, as the glue must be of a heat-sensitive type, and must be just sufiithis application being a continuation-im;

lines, so that both the wood The term parallel heating. by contrast.

ciently conductive to pass the high frequency current, while not sufficiently conductive to cause an arc to be produced.

As the wet relatively highly conductive glue conducts or passes the electric current, it is caused to boil very quickly, usually within one or two seconds, and this condition continues to a diminishing degree as long as the moisture of the glue is decreasing, and in fact until it dries out completely and its conductivity becomes practically nil, or at best but slightly more conductive than the wood, which in the meantime has been but very slightly heated. The so-called curing or setting of' the glue isa combination of drying and polymerization of the molecules, depending largely upon the type of glue that is used.

Of the adhesives developed and used thus far in parallel heating, urea-formaldehyde resins have produced the best results. However, numerous problems are encountered with any glue, some being totally unadapted for use in parallel heating, as for example inherently low conductivity glues. It must therefore be a thermosetting glue of just the proper degree of conductivity to pass the electric current, and to set during the brief period during which it is subjected to the electric energy ofhigh frequency. If it is too highly conductive, arcing takesv place with resultant burning of the wood, whereas if it is too low in conductivity it will. not conduct the current sufficiently to produce the desired set, and at the same time the wood may be. heated to an undesirable degree. Likewise, the frequency of the high tension current must be. considered, for lower frequencies tend to heat the glue line more than the higher frequencies when the same power goes into the load.

The primary object, therefore, is to provide a hardener for glues or adhesives of the ureaformaldehyde type that will aid the setting of the glue during the brief period of high frequency application, and which will insure the proper degree of conductivity in the glue line.

For this purpose it has been found that a combination of an ammonium salt of a strong acid, such as ammonium chloride, ammonium sulphate, ammonium nitrate, ammonium bromate, ammonium fluoride, etc., in combination with an alkaline earth phosphate, such as tricalcium phosphate, tribarium phosphate, tristrontium phosphate, trimagnesium phosphate, trizinc phosphate, etc., or one of the said alkaline earth salts of phosphoric acid, produces the desired degree of conductivity and speed of set required together with all of the other chemical and physical characteristics desirable in a liquid ureaformaldehyde resin adhesive. It should also be understood that for practical purposes it is possible to use any one of the said groups of ammonium salts in combination with any one of the said alkaline earth phosphates, or the reaction products of said alkaline earth salts with phosphoric acid.

The resin used is a water-soluble condensation product of urea and formaldehyde, that it is essential that it shall not have been previously dried and remixed with water. Any resinforming urea may be used, c. g., urea or thiourea. Any aldehyde may be used, e. g., formaldehyde, acetaldehyde or furfural. The resin most widely used by the industry is a condensation product of urea (carbamide) and formaldehyde.

The urea (1 mol) is reacted under proper conditions with between 1.7 and 2.5 mols of 37% formaldehyde, and the resin concentrated by vacuum distillation to about 60 to 70% solids. The remainder is water and some free formaldehyde. The ureaaldehyde resin may be modified with other aldehyde reactive substances to form resin, e. g., resorcinol, aniline, xylenol, and the like.

Any commercial high frequency generator may be employed, though they vary quite widely in electrode voltages and frequency, the latter ranging between one and fifty megacycles, wherefore the proportion of the hardener to the quantity of glue used depends largely upon the particular generator used. Some generators that employ higher voltages across the load tend to are with high proportions of the composite hardener, while with the characteristically lower voltages of other generators higher proportions of the improved hardener produce a definite advantage by concentrating more power in the glue lines.

Formula I .An example of a glue that is best adapted for use with generators of the lowest has given best results with most generators of intermediate power (i. e., in the neighborhood of 6 to 15 kw.)

100 parts liquid water-soluble urea-formaldehyde condensation product.

2 parts of an ammonium salt selected from the group herein before listed.

4 parts of an alkaline earth phosphate selected from the group hereinbefore listed, or a reaction product of one or the same with phosphoric acid.

10 parts filler (omitted if desired).

Formula IV.An example of a glue that is best adapted for use with generators of somewhat higher than average power:

100 parts liquid water-soluble urea-formaldehyde condensation product.

1 part of an ammonium salt selected from the group hereinbefore listed.

1 part of an alkaline earth phosphate selected from the group hereinbefore listed, or a reaction product of one of the same with phosphoric acid.

' 10 parts filler (omitted if desired).

Formula V.An example of a glue that is best practical power (i. e., in the neighborhood of c Formula II .An example of a glue that is best adapted for use with generators of somewhat lower than average power:

urea-formalde- 100 parts liquid water-soluble urea-formaldehyde condensation product.

5 parts of an ammonium salt selected from the group herein before listed.

3 parts of an alkaline earth phosphate selected from the group hereinbefore listed, or a reaction product of one of the same with phosphoric acid. 10 parts of filler (omitted if desired).

Formula III.An example representing approximately proportions of a glue that thus far practical power (i. e., 25 to 40 kw.)

parts liquid water-soluble urea-formaldehyde condensation product.

part of an ammonium salt selected from the group hereinbefore listed.

5 part of an alkaline earth phosphate selected from the group hereinbefore listed, or a reaction product of one of the same with phosphoric acid.

10 parts filler (omitted if desired).

The curing properties of a particular glue formula of this nature will be found to vary with the power of the generator, roughly between kw. and 40 kw., above which latter figure it is not practical to go, while the average usually vary between 6 and 15 kw. However, the curing depends upon a number of variables such as frequency and voltage as well as the capacity of the generator, while the power required is obviously increased by an increase in the number, thickness, longitudinal measurement between electrodes and the area of the glue lines. Commercial generators in use today employ from 1200 to 4000 volts at the electrodes, while the frequencies most often employed vary from 5 to 7 megacycles, although as high as 25 megacycles have been used.

In considering the foregoing, it is to be understood that the liquid urea-formaldehyde condensation product resin should be in its initially prepared state of physical condition, and not previously dried and subsequently mixed with water, since a resin of this nature has been definitely found to give markedly better results than when the resin has been dried, or permitted to dry, and then at some future time mixed with water.

While the chemistry of the reaction that takes place in the combined urea-formaldehyde resin and hardener may not be fully understood at this time, it is believed that there occurs a double decomposition type of reaction, probably beginning with the addition of the hardener to the liquid resin, followed by further reaction upon their being simultaneously subjected to the effects of the high frequency current. In any case, a chemical reaction definitely takes place and is completed upon the setting of the glue as hereinbefore described.

An additional. advantage of the improved hardener described herein resides in the fact, that in combination with urea-formaldehyde resins it prevents the formation or development of. too greata degree of acidity in the glue, orin other words it functions as an acidity control, since too high a degree of acidity is definitely known to be detrimental to the durability of the bond. The alkaline earth phosphates assist by absorbing the excess acid.

Practically innumerable tests and experiments have been carried out in an effort to discover or develop the most efficient proportions of both hardener per se, and of the combined liquid adhesive and dry hardener, and it has been found that the above formulas may be varied considerably, depending largely upon the electrical characteristics of the high frequency current and the makeup or composition of the base adhesive, but that under the conditions set forth the specified proportions seem to produce the best results. Obviously many modifications and variations may be made without departing from the spirit and scope of the invention.

During the research work that preceded the perfection of the hereinbefore described formulas, and the determination of the approximate range of alternatives among the many available ammonium salts of strong acids .and alkaline earth phosphates, a great many experiments were made and tests run, of which the following are illustrative. Experiment I shows that certain ammonium salts used as catalysts in urea formaldehyde resin adhesives do not act or function independently, but depend for their activity upon the presence of the formaldehyde in the resin, and also that the action with respect to ammonium chloride differs from the action with ammonium silicofiuoride.

EXPERIMENT I [No formalin (HCHO) in Nos. 1 and 2.]

'6. EXPERIMENT II Water Parts 0.65 Powdered urea resin g 100'. Ammonium silicofluoride heated to 73 F g 2.

Gelling at 18 minutes.

Firm gel at 20 minutes.

Water parts 0.65 Powdered urea resin g 100. Ammonium chloride heated to 73 F g 2.

Good flow at 32 minutes (pH 4.80). Usable flow at 128 minutes (pH 4.20). Very heavy at 155 minutes.

Beyond use at 178,minutes.

Working life, 2% to 3 hours.

Resin used was powdered urea-formaldehyde resin without catalyst.

Having thus described my invention, what I claim and desire to protect by Letters Patent of the United States is:

1. The method of edge-gluing a plurality of wooden elements which consists in providing a pair of elements with an intervening layer of adhesive consisting of an aqueous urea-formaldehyde resin in its original liquid state and containing about 60 to 70% urea resin solids, and a hardener composed of an ammonium salt of a strong acid selected from the group which consists of ammonium bromide, ammonium chloride, ammonium fluoride, ammonium nitrate and ammonium sulphate, and an alkaline earth phosphate, selected from the group which consists of tribarium phosphate, tricalcium phosphate, trimagnesium phosphate, tristrontium phosphate and trizinc phosphate, clamping said elements together tightly, and then passing a high frequency electric current through and in the plane of said adhesive to cure the same.

2. The method of edge-gluing a plurality of wooden elements which consists in providing a pair of elements with an intervening layer of adhesive consisting of approximately 100 parts of an aqueous urea-formaldehyde resin in its original liquid state and containing about to 70% urea resin solids, and a hardener composed of between one-half and eight parts of an ammonium salt of a strong acid selected from the group which consists of ammonium bromide, ammonium chloride, ammonium fluoride, ammonium nitrate and ammonium sulphate, and between one-half and Water cc 30 Ammonium silicofluoride g 2 pH at 5 minutes 3.72 pH at 70 minutes 3.72 pH at 135 minutes 3.72 No. 2

Water cc 30 Ammonium chloride g 2 pH at 5 minutes 3.50 pH at '70 minutes 3.50 pH at 135 minutes 3.50 No. 3

Water cc 30 Ammonium silicofluoride g 2 Formaldehyde (37%) cc 2 pH at 5 minutes 3.68 pH at 70 minutes (added additional 5 cc.

formaldehyde, 37%) 2.70 pH at 60 minutes 1185 No. 4

Water cc 30 Ammonium chloride g 2 Formaldehyde (37%) cc 2 pH at 5 minutes 3.25 pH at 70 minutes (added additional 5 cc.

formaldehyde, 37 2.00 pH at 60 minutes -1 1.30

eight parts of an alkaline earth phosphate selected from the group which consists of tribarium phosphate, tricalcium phosphate, trimagnesi-um phosphate, tristrontium phosphate and trizinc phosphate, clamping said elements together tightly, and then passing a high frequency electric current of between /2 kw. and 40 kw. at a frequency of between 5 and 25 megacycles through and in the plane of said adhesive to cure the same.

3. The method of edge-gluing a plurality of wooden elements which consists in providing a pair of element with an intervening layer of adhesive consisting of approximately parts of an aqueous urea-formaldehyde resin in its original liquid state and containing about 60 to 70% urea resin solids and a hardener composed of 7 from /2 to eight parts ammonium chloride and UNITED STATES PATENTS from to eight parts of tricalcium phosphate Number Name Date clamping said elements together tightly, and 2019 834 viernng N0 5 1935 then passing a. high frequency electric current 2236184 Menger 1941 through and in the plane of said adhesive to cure 5 2312210 Bearing 1943 the same 2,372,929 Blessing Apr. 3, 1945 ELMER BERGEY- 2,399,980 Bradshaw May 7, 1946 2,413,624 Harris Dec. 31, 1946 REFERENCES CITED 2,453,135 Bilhuber Nov. 9, 1948- The following references are of record in the 10 2,513,338 simons 8 1950 file of this patent:

Claims (1)

1. THE METHOD OF EDGE-GLUTING A PLURALITY OF WOODEN ELEMENTS WHICH CONSISTS IN PROVIDING A PAIR OF ELEMENTS WITH AN INTERVENING LAYER OF ADHESIVE CONSISTING OF AN AQUEOUS UREA-FORMALDEHYDE RESIN IN ITS ORIGINAL LIQUID STATE AND CONTAINING ABOUT 60 TO 70% UREA RESIN SOLIDS, AND A HARDENER COMPOSED OF AN AMMONIUM SALT OF A STRONG ACID SELECTED FROM THE GROUP WHICH CONSISTS OF AMMONIUM BROMIDE, AMMONIUM CHLORIDE, AMMONIUM FLUORIDEM AMMONIUM NITRATE AND AMMONIUM SULPHATE, AND AN ALKALINE EARTH PHOSPHATE, SELECTED FROM THE GROUP WHICH CONSISTS OF TRIBARIUM PHOSPHATE, TRICALCIUM PHOSPHATE, TRIMAGNESIUM PHOSPHATE, TRISTRONTIUM PHOSPHATE AND TRIZINC PHOSPHATE, CLAMPING SAID ELEMENTS TOGETHER TIGHTLY, AND THEN PASSING A HIGH FREQUENCY ELECTRIC CURRENT THROUGH AND IN THE PLANE OF SAID ADHESIVE TO CURE THE SAME.