US2313953A - Process for resinifying lignocellulosic materials - Google Patents

Description

; density and strength.

Patented Mar. 16 1943 PROCESS FOR RESINIF'YING LIGNO- QELLULOSIC MATERIALS William Karl Loughborough, Madison, Wis, as-

signor to Claude R. Wlckard, as Secretary of Agriculture of the Unlted'States of America, and to his successors in oiilce No Drawing. Application March 17, 1941,

- Serial No. 383,798

5 Claims. (Cl.144-309) (Granted under the act or March 3, 1883, as

amended April 30, 1923; 370 o. G. 757) This application is made under the act of March 3, 1883, as amended-by the act of April 30, 1928, and the invention herein described and claimed, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes,

, without the payment to me of any royalty thereon.

My-invention relates to processes for plasticizing and resinifying'wood and other ligno cellulosic materials, and to products made from said materials.

For brevity and clarity whenever the word wood is used in this specification and the claims, it is understood that I include in its definition a ligno-cellulosic complex. 3 It is further understood that such ligno-cellulosic materials may include the natural constituents of trees and plants in varying proportions. I have in mind particularly trees and plants of all kinds.

A principal object of my invention is to produce solid compressed wood of great density and strength.

A further object of my invention is to produce molded articles from comminuted wood.

A further object of my invention is to produce plywood and laminated compressed wood of great A further object of my invention is to produce wood which can be readily formed when hot and wet.

. A further object of my invention is to .produce wood which can be readily formed when hot and A further object of my invention is to produce a material which is thermosetting, which has very low hygroscopic properties; and which has permanence of size and shape.

Other objects of my invention include the production of molded articles from solicfwood, from laminated wood, and from plywood.

My invention, in itsvarious modifications, is based upon the facts that aqueous solutions of urea and thiourea, separately or in mixture, have a marked softening eflect upon wood, and. that urea and/or thiourea can be combined with various chemicals, such asgthe aldehydes, to form f thermo-s'etting resins. l

I am aware that urea and other chemicals have been used in the chemical seasoning of wood. However, the purposes, methods employed, and results secured are all entirely different from my in my invention the entire volume of the wood should be impregnated. In chemical seasoning, the vapor-pressure characteristics of the aqueous solutions of the chemical and the anti-shrink properties of the dry chemical within the wood are of paramount importance; whereas in my invention the ability of the aqueous solution of the chemicals to soften the wood and the ability of said solution to condense and polymerize into a thermo-setting resin are most essential proper-- ties. In chemical seasoning it is desired to maintain the green size and form of the wood insofar as possible; whereas in my invention an important object is to modify the original form.

I have pointed out in a previous patent application, Serial No. 345,677, filed July 15, 1940, that urea, NHzCONI-Iz, and thiourea, NHzCSNHz, separately or in combination, have a marked softening effect upon wood at elevated temperatures, and that this effect may be applied to produce new and useful results. The discovered effects vary widely in intensity among the various ligneous and cellulosic materials of differing origins: therefore, successful application of my invention will depend in large measure upon the proper selection of materials, and successful utilization of any given material will likewise depend upon the proper selection of a suitable treating or conversion method. In general, the eifects are most intense in the case of li eous materials.

I have discovered that wood which has been treated and softened by soaking in an aqueous soltliition of urea and/or thiourea can be resinified an given superior new properties by soaking it preferably before drying, in a water solution of furfuryl alcohol (CiHaoCHzOHlor of an aldehyde, in the presence. of a catalyst.

I have also discovered that it is possible to secure a preliminary softening and a final resinification of the wood by the use of a single'aqueous treatingsolution consisting of urea and/or and ac'etaldehyde (CHsCHO). The ratio o aldehyde-to urea and/or thiourea may rangeon an atomic-weight. basis, from a. maximum of 1 to' 1 invention. In chemical seasoning it is essential that only the surface layers of the wood to be I dried be impregnated with the chemical, whereas A solution composed only of urea and/or thioto aminimum of 1 to 10. If maximum water resistance be desired this ratio should not be larger than 1 to 1.5.

urea and an aldehyde in the presence of a suitable catalyst, but without a buffer, would be unsatisfactory for the purpose intended because it would start condensing and polymerizing too soon. Therefore for most of the purposes contemplated in my invention a catalyst and a buffer must he added. Sometimes it is expedient to use an acid catalyst and sometimes an alkaline catalyst. There is an advantage in using sodium hydroxide and enough acetic acid to form sodium acetate with some free acetic acid because sodium acetate will buffer the dilute organic acid solutions at pH values between 5.5 and 6.0. To avoid using an excessive amount of hydroxide and acid an additional buffer is required. The most eflicient buffer I have discovered is borax. Some acid salts like sodium acid phosphate seem to act as a buffer as well as a catalyst. In this case the sodium acid phosphate in the presence of weak wood acids holds the pH to about 8.

For use with molding powders, I find that buffers are not always necessary since the time required for the entire process is comparatively brief.

Five typical solution formulae are here presented:

I Urea grams 790 Formaldehyde (37% solution) do.. 714 Water ..do 485 Sodium hydroxide ....do 11 Borax ..do 79 Glacial acetic acid c.c 18

II Urea "grams" 790 Formaldehyde (37% solution) do 714 Water do 485 Sodium hydroxide do 11 Borax d 79 Glacial acetic acid c. c 18 Ammonia (19% solution) do 250 III Urea grams 790 Furfural do 1279 Water do 8000 Sodium hydroxide do 240 Borax do 800 IV Urea rams 790 Formaldehyde (37% solution) do 714 Water do 485 Sodium acid phosphate do 20 V Urea grams 790 Water do 790 Furfuryl alcohol do 1185 Phosphoric acid (85%) do 47 I have discovered and demonstrated that, to secure best results, the solution formula must be modified and adjusted to suit various species and various conditions of the process. Thus, woods like oak, containing comparatively large quantitles of acidyrequire sparing use of acid in the solution. Further, if darkening of the wood be objectionable, ammonia may not be used with oak.

The buffered urea-aldehyde solution above defined and described has outstanding advantages over other unpolymerized or partially polymerized resins. It will remain unpolymerized, at room temperatures, for weeks. It will also remain unpolymerized for days at temperatures up to 220 F. It will not completely polymerize at temperatures much below 300 F; These properties make possible the successful manipulation of the wood through the various soaking, drying, forming and compressing operations herein described and claimed.

Through proper selection of the chemicals I have been able to produce a buffered urea-aldehyde solution which permits satisfactory performance of an'operating schedule involving the following steps: (a) preliminary soaking of the wood in said solution, the soaking period, everything else being the same, is relatively short when green wood is used and much longer when dry wood is used; (b) heating and softening of the soaked wood: (0) deformation, molding, and/or compression of the heated soaked wood; and (d) final condensation and polymerizing of the chemicals in said wood and drying of the finished product.

I have also been able to secure satisfactory performance of an operating schedule involvin the following steps: (a) preliminary soaking of the wood in a buflered urea-aldehyde solution for various periods, depending upon size, species, and moisture content of-the material; (b) drying of the soaked impregnated wood; (0) deformation, molding, and/or compression of the dry impregnated wood at elevated temperature; and (d) final condensation and polymerization of the chemicals within said wood.

The preferred operating schedule, when using two separate chemical solutions, involves the following steps: (a) preliminary soaking of the wood in an aqueous solution of urea and/or thiourea; (b) heating the soaked wood; (0) deformation, molding, and/or compression of the hot soaked wood; (d) soaking said wood in an aqueous solution of an aldehyde plus a suitable catalyst; and (e) final drying of the finished product.

It is not possible, in the present state of the art, to state with complete certainty exactly what chemical reactions take place and what physical phenomena manifest themselves in the practice of my invention, but it appears now that most of the results secured support the hypothesis that the principal effective action during the preliminary soaking and heating periods is one of softening. However, it is probable, especially in the case of certain species, such as oak for instance, which contain unusually large percentages of materials which are neither cellulosic nor llgneous, that a certain amount of condensation and resinification does take place between the wood and the chemicals in the solution.

During the final resinifying steps in the various modifications of my process the urea and/or thiourea condenses with the aldehyde and polymerizes to form a urea-aldehyde resin. In my process, the various chemicals in the solutions enter into the fine microscopic structure of the wood cells, with the end result that the resins which are formed completely permeate the wood structure and do not simply form a superficial coating as in various other processes in the prior art. This complete permeationpractically eliminates swelling and shrinkage with changes in the humidity of the ambient atmosphere.

In previous experimental work I discovered that green wood may be readily impregnated with urea and/or thiourea by immersion in an aqueous solution of said chemicals. The action is princlpallyone of difiuslon and it goes forward at atmospheric temperature and pressure.

small accelerating effect.

It may be hastened by the use of elevated tem- I pressure has only a very Dry wood can also perature, but increased be impregnated in this manner. is much slower than with green wood. I'now find that wood may be impregnated with a buffered urea-aldehyde solution quite as easily-as with urea and/or thiourea alone. Other methods of impregnation may also be used in the practice ofmy invention.

It is recognized that certain properties of the finished products, such as hygroscopicity, fire-resistanoe, decay-resistance, resistance to insect but the process v treating molding,

attack, and weather-resistance, may be modified by the addition of various well-known chemi-. eels-principally water-soluble chemicals-to the aqueous treatingv solutions. My invention contemplates the use of such chemicals added to the buffered urea-aldehyde solution.

If wood be impregnated with buffered urea-aldehyde solution and then heated it becomes very soft and flexible, so that it may be easily bent, twisted, molded, compressed, or otherwise formed or deformed.

Subsequent drying and finalheating to a temperature higher than that of the preliminary aldehyde solution; dry said wood at room tem- I I perature; place said wood into a suitable mold and apply pressure in asuitable press; heat said wood under 300 F.,' release the'pressure after polymerization has been completed.

My invention also contemplates a method of treating, laminating, compressing and/or moldheating remove excess moisture and cause final polymerization of the resin-forming chemicals.

Likewise, wood impregnated in accordance with my invention andthen dried and heated becomes quite soft andfiexible, so that it may be formed or deformed, but to a lesser degree than when hot and wet.

The bufiered urea-aldehyde has an advantage over other thermo-setting resin-forming chemical combinations in that it remains thermoplastic for a long time in the range of temperatures normally .employed in kiln drying; and it does not become thoroughly cured until exposed to a temperature of about 300 F.

Final heating to a temperature higher than that of the preliminary heating causes finaLpolymerization to take place.

I have discovered that wood which has been impregnated with a buffered urea-aldehyde solution possesses such a high degree of adhesiveness after proper polymerization that, under tion of any adhesive.

suitable conditions, sheets can be laminated and comminuted material molded without the addi- My invention contemplates a method of treating, bending or otherwise forming, and resinifying wood which consists essentially of the following steps: impregnate the wood with a buffered urea-aldehyde solution; heat said wood to a temperature of about 212 F. to 220 F.; bend or otherwise form said wood; dry said wood; heatsaid wood to a temperature of about 300 F.

My invention also contemplates a method of treating and resinifying woodwhich consists essentially of the following steps: impregnate said wood with a buffered urea-aldehyde solution; dry said wood; heat said wood to a temperature of about 300 F.

My invention also contemplates a method of treating, bending or otherwise forming, and

resinifying wood which consists essentially of the following steps: impregnate the wood with a buffered urea-aldehyde solution; dry said wood;

heat said wood to av temperature of about 212 vF. to 220 F.; bend or otherwise form saidwood;

heat said wood to a temperature of about 300 F.

My invention also contemplates a method of treating, densifying, and resinifying wood which consists. essentially ofthe following steps: im-

- stack of sheets in a suitable press and resiniiying sheets of wood which consists enough pressure to said stack to sired degree of compression and/or molding; and simultaneously heat said stack under pressure to a temperature of about 300 F.; release the pressure after polymerization has been completed.

Specific illustrations of how my present invention may be practiced follow:

. To produce a boat rib. Soak a stick of green white oak heartwood,. one'inch square and 30 inches long, for a; period of 10 days, at room tem perature; in a buffered urea-aldehyde solution prepared as follows:

Mix together and heat to 150 F. Water grams 485 Borax o 79 Urea do 790 Sodium hydroxide do 11 Glacial acetic acid c.c. 11

0001 to room temperature. When cool add 714 grams of 37 percent formaldehyde solution.

After the soaking period, heat the stick to a temperature of from 212 be done conveniently by immersing said stick in a bath of boiling urea solution. Maintain said stick at this temperature for half an hour.

Remove said stick from the bath, immediately bend said stick over a previously prepared form, and clamp said stick in place on the form. In performing this operation, ordinary steam bending may be followed.

Allow'the bent rib to dry on the form. .Heat said bent rib, while still on the form, to a temperature of about 300 F. and maintain said temg perature for an hour. This completes the process and said bent and resinifledrib may be removed from-the form and immediately used as soon as it is cool enough to handle.

. To produce a light boat rih soak a stick of green white oak heartwoqd inch by inch in cross section and 30 inches long, for a period of- 5 days,

urea-aldehyde solution. After the soaking period, dry said stick at a temperature of about F. Then heat said stick to a temperature of about 212 F. Then bend said stick,,while hot, over'a previously prepared form and clamp said stick in place on the form. In performing this opera-- tion, practices customary in ordinary steam bend-'- pressure after poly contemplates a method of pressure to a temperature of about I impregnate secure the de F. to 220 F. 'This may practices customary in at room temperature, in abuifered' ing may be followed.

Then heat the bent rib, while still on the form, to a temperature of about 300 F. and maintain said temperature for 40 minutes. This completes'the process and said bent and resinified rib may be removed from the form and immediately used as soon as it is cool enough to handle.

To resinify a bearing block.Soak a board of green white heartwood 12 inches square and one inch thick, for a period of 10 days, at room temperature, in an aqueous solution prepared as above specified.

Remove said board from said solution and allow said board to dry.

After said board has dried to a moisture content of about 10 percent, heat said board to a temperature of about 300 F. and maintain said temperature for an hour. This completes the process and the resinified board may be cooled and used immediately.

To resinijy and compress a die block.--Soak a board of green white oak heartwood 12 inches square and 2 inches thick, for a period of 20 days, at room temperature, in an aqueous solution as above specified.

Remove said board from said solution and heat said board to a temperature of from 212 F. to 220 F. by immersing said board in a bath of boiling urea solution. Maintain said board at said temperature for an hour. 7

Remove said board from said urea solution, place said board in a suitable press and apply suificient pressure to reduce the thickness of said board to 1% inches.

Dry said board under pressure.

When dry, raise the temperature of said board to about 300 F. and maintain said temperature for one hour.

Release the pressure and remove said board.

To mold an ash tray.Soak a suitable quantity of fine green oak sawdust for a period of 15 minutes, at room temperature, in an aqueous solution prepared as above specified.

Remove said sawdust from said solution and dry said sawdust at room temperature.

When dry fill the ash tray mold with said sawdust, place said filled mold in a hot plate press and apply a pressure of 1000 pounds per square inch of mold area to said mold.

Raise the temperature of said mold to 300 F. and maintain said pressure and said temperature for a period of 30 minutes.

Release said pressure, open said mold and remove said molded ash tray.

To make a compressed plywood plate.- Soak 13 pieces of -inch green white oak veneer 12 inches square for a period of hours, at room temperature, in an aqueous solution prepared as above specified.

Remove said veneer from said solution and dry at room temperature.

Arrange said dry veneer in a stack, with the grain of adjacent plies at right angles.

Place said stack in a hot plate press and apply a pressure of 2000 pounds per square inch of surface area of said stack.

Raise the temperature of said stack to 300 F. and maintain said temperature for a period of one hour.

Open said press and remove said plate.

Having thus described my invention, what I claim for Letters Patent is:

l. A process for producing a wood product which is initially highly plastic, and which finally becomes resinified, comprising the following steps: soaking wood in an aqueous formaldehyde urea solution which contains an excess of urea, on an atomic weight basis, the mixture containing suitable buflers which maintain its pH at about 8; removing said wood from said solution; heating said wood to a temperature of substantially 212 F.; altering the shape of said heated wood; drying said wood; heating said wood to a temperature of substantially 300 F.; and maintaining said temperature until resiniflcation is complete.

2. A process for producing resinified wood comprising the following steps: soaking wood in an aqueous formaldehyde urea solution containing an excess of urea, on an atomic weight basis, the mixture containing suitable buifers which maintain its pH at about 8; removing said wood from said solution; drying said wood; heating said wood to a temperature of substantially 300 F.; and maintaining said temperature until resinification is complete.

3. A process for producing densified resiniiled wood comprising the following steps: soaking wood in an aqueous formaldehyde urea solution containing an excess of urea, on an atomic weight basis, the mixture containing suitable buffers which maintain its pH at about 8; removing said wood from said solution; heating said wood to a temperature of substantially 212 F.; compressing said heated wood; drying said wood under pressure; heating said wood under pressure to a temperature of substantially 300 F.; maintaining said temperature and said pressure until resinification is complete; and releasing said pressure.

4. A process for producing laminated resinifled wood comprising the following steps: soaking sheets of wood in an aqueous formaldehyde urea solution containing an excess of urea, on an atomic weight basis, the mixture containing suit able buffers which maintain its pH at about 8; removing said sheets from said solution; drying said sheets; assembling said sheets into a stack; pressing said stack with a pressure of about 400 pounds per square inch of stack area, simultaneously increasing the temperature oif said stack to substantially 300 F. and maintaining said temperature and said pressure until resiniilcation is complete; and releasing said pressure.

5. A process for producing laminated resinifled densiiied wood comprising the following steps: soaking sheets of wood in an aqueous formaldehyde urea solution containing an excess of urea, on an atomic weight basis, the mixture containing suitable buffers which maintain it pH at about 8; removing said sheets from said solution; drying said sheets; assembling said sheets into a stack; pressing said stack at a pressure suificient to produce the desired densification, simultaneously increasing the temperature of said stack to substantially 300 F. and maintaining said temperature and said pressure until resinification is complete; and releasing said pressure.

WILLIAM KARL LOUGHBOROUGH.