US3511691A

US3511691A - Method for chemically stabilizing and protecting elongate wood strips

Info

Publication number: US3511691A
Application number: US720317A
Authority: US
Inventors: Eldon D Johnson; Randol C Crooks
Original assignee: Andersen Corp
Current assignee: Andersen Corp
Priority date: 1968-04-10
Filing date: 1968-04-10
Publication date: 1970-05-12
Anticipated expiration: 1987-05-12

Description

y 1970 E. D JOHNSON ETAL 3,511,691

METHOD FOR CHEMICALLY STABILIZING AND PROTECTING ELONGATE WOOD STRIPS Filed April 10, 1968 boob 0on0 Fig.

E LOO/V D. JOHNSON 8 RANDOL C. CROOKS INVENTORS ,1/ A 7 6 ORA/E Y United States Patent 3,511,691 METHOD FOR CHEMICALLY STABILIZING AND PROTECTING ELONGATE WOOD STRIPS Eldon D. Johnson and Randol C. Crooks, Stillwater, Minn., assignors to Andersen Corporation, Bayport, Minn., a corporation of Minnesota Filed Apr. 10, 1968, Ser. No. 720,317 Int. Cl. B44d 1/14, 1/28; B27k 2/50 US. Cl. 11772 Claims ABSTRACT OF THE DISCLOSURE Elongate strips of normally porous liquid-absorbent fibrous natural wood materials have traditionally been used as a structural material. In many structural applications, such as for siding materials for dwelling homes, window sashes and frames, doors and door jambs, and the like, the normally-porous hydrophilic wood is subjected to potentially damaging environmental conditions such as excess humidity, temperature extremes, fungus growth, and various inimical insects including termites and the like. Over the years, lumber processors, door and window manufacturers, and others in the wood working industry have invariably impregnated into wood materials a com bination of two generic classes of chemicals in order to stabilize the wood against the aforementioned kinds of potentially damaging environmental conditions. One of the two generic chemical classes is of the wood preservative class, including such specific examples as chlorinated phenols, copper sulfate, copper naphthenate and zinc oxide, the purpose of the wood preservative chemicals being to make the wood resistant to fungus growths and to termites and similar inimical insects. The second generic class of chemicals commonly utilized in wood stabilization treatments is of the water-repellent resin class, said water-repellant resins being adherently applied to the outward exposed surfaces of lumber materials so as to make the normally-porous and hydrophilic wood material resistant to humid environmental conditions.

In the prior art, the two generic components of wood stabilizers have been traditionally mixed together and in further admixture with a volatile liquid carrier, such as water or organic solvents, to provide a liquid chemical treatment bath for the wood. Following temporary immersion or similar impregnation of the chemical treat ment bath into the wood pores, the volatile liquid carrier component is allowed to evaporate resulting in an indiscriminate admixture of the Wood preservative chemical and the water-repellant resin as random chemical deposits in the Wood pores. The depth of the said randomly admixed chemical deposits into the Wood pores depends almost exclusively upon the depth to which the chemical treatment bath had been impregnated into the Wood before evaporation of the volatile liquid carrier component. Such random and indiscriminate admixture of the wood preservative chemical and the water repellant resin in the identical immersion bath or other impregnation solution, and ultimately so randomly and indiscriminately disposed within the porous wood, involves inefficient use of raw materials and inefficient and slow procice essing conditions, and the resultant chemically stabilized wood is not reliably amenable to a choice of finishing operations e.g. painting, staining, etc. For example, certain of the prior art chemicals used for stabilizing wood materials, and in particular the water-repellant resin sealer component at the wood external surface, render the wood material non-receptive to many non-pigmented wood stains, and adhesion between certain pigmented paints and the wood material is apt to be poor or unreliable.

It is accordingly a general object of the present invention to provide chemically stabilized elongate strips of wood embodying therewithin improved classes and physical relationships of wood preservative chemical and water-repellant sealer resin.

It is another general object of the present invention to provide an economical and etficient method technique for making chemically stabilized elongate wood strips embodying therewithin improved classes and physical relationships of wood preservative chemical and water-repellant resin.

It is another object of the present invention to provide chemically stabilized elongate wood strips that have special utility for window sashes and frames and for doors and door frames.

It is yet another object of the present invention to provide a resinous water-repellant sealer component for elongate wood strips that seals the external surfaces of the wood strip exceedingly well, that exhibits practically no adverse effects upon the wood preservative chemical, and that is amenable and compatible with a wide range of pigmented and non-pigmented decorative-protective external coatings for the chemically stabilized wood strip.

With the above and other objects and advantages in view, which will become apparent as this description proceeds, this invention comprises certain novel chemical ingredients and gravimetric proportions thereof, together with novel processing steps to provide special physical relationships among the wood material, the wood preservative chemical component, and the water-repellant resin component, said novel and special features resulting in improved chemical stabilization and protection of elongate wood strips.

This invention contemplates as the initial processing step thereof impregnating a preservative solution into the natural pores of an elongate strip of liquid-absorbent porous fibrous natural wood material, said elongate strip including: a longitudinal axis that is substantially paralel to the wood grain, elongate surfaces surrounding said longitudinal axis and providing a cross-sectional periphery for the elongate strip, and two terminal ends positioned transversely of the said longitudinal axis and the elongate external surfaces. The preservative solution comprises a homogeneous mixture comprising a minor portion by weight wood preservative chemical and a major portion by weight a volatile liquid carrier, such as volatile organic solvent, for the wood preservative chemical. Following impregnation of the preservative solution into the porous elongate wood strip, commencing at the transverse terminal ends and extending along the wood grain and the longitudinal axis and also commencing at the elongate surfaces and extending toward the longitudinal axis, the volatile carrier component is removed, as by evaporation, whereby the wood preservative chemicalresidne deposits occupy a minor portion of those pores having been previously subjected to the preservative solution.

A relatively large class of wood preservative chemicals are suitable for the purposes of this invention including zinc oxide, chlorinated phenols such as pentachlorophenol, andvarious copper compounds such as copper sulfate, copper naphthenate and copper-S-quinolinoate.

Volatile organic solvents having vapor pressures exceeding about 0.3 p.s.i.a. are preferred over aqueous systetms as the volatile carrier component of the preservative solution, and weight ratios exceeding about 7:1 are preferred for the quantitative relationship between the volatile carrier and the preservative chemical respectively. Because certain of the conventional wood preservative chemicals become crystalline at room temperatures, so-called nonvolatile non-blooming agents are sometimes used along with the wood preservative chemical to prevent its recrystallization from the wood pores. For example, one part ethylene glycol might be added to every part pentachlorophenol used as the wood preservative chemical.

Chronologically following the steps of impregnating the wood material with the wood preservative solution and then removing the volatile carrier portion thereof to leave the solid preservative chemical residue within the wood pores, a sealing solution comprising a mixture of waterrepellent resin and a volatile carrier therefor is applied to the elongate external surfaces of the wood material. The volatile carrier portion of the sealing liquid is removed, preferably before the sealing liquid has penetrated the wood as deeply as had the previously applied preservative solution, whereby a relatively thin layer of waterrepellent resin extends inwardly into the wood material from the transverse cross-sectional periphery thereof.

For the water-repellent resin component of the sealing solution, polyurea resins are preferred because of their compatibility with the prevalent kinds of preservative chemicals, and with many varieties of pigmented and nonpigmented decorative and/ or protective coatings, and because sealing solutions containing polyurea resins are amenable to the preferential processing techniques of the present invention. Among the polyurea resins, those re sulting from the exothermic chemical reaction of polyisocyanate and amino compounds are favored, and the chemical reaction product of diisocyanate and polyamines are particularly adapted to the present invention.

Diisocyanate (hereinafter referred to as Resin A) is characterized by the structural formula:

where D is a 36 carbon hydrocarbon radical. In physical properties, the diisocyanate is a clear amber liquid having: a molecular Weight of about 600; a specific gravity of 0.924 at 77 F.; a refractive index of 1.479 at 77 F.; and an N=C=O content of about 14% by weight. Diisocyanate is further characterized in that: its vapor pressure at 77 F. is less than 0.00 1 mm. (mercury); its pour point is at less than 30 F.; and its flash point is about 560 F., partial chemical decomposition beginning at about 540 F.; and its viscosity in centipoises is about 170 cps. at 68 F.; and about 50 cps. at 122 F. Finally, diisocyanate is soluble in a wide range of polar and non-polar solvents including: ketones such as acetone and methyl isobutyl ketone; chlorinated solvents such as carbon tetrachloride; cyclic hydrocarbons such as benzene, toluene and xylene; aliphatic hydrocarbons such as heptane, hexane, and mineral spirits blends; and with higher alcohols and ether alcohols.

Diisocyanate will chemically co-react with amino compounds to form a polyurea resin particularly adapted to the present invention. The preferred amino compound, coreactant is a polyamine (hereinafter referred to as Resin B) having the structural formula:

wherein R is a hydrocarbon radical having about 75 carbon atoms. In physical properties, the polyamine is a clear amber liquid having: a molecular weight of about 1100; a specific gravity of 0.927 at 77 R; an equivalent weight of about 275; and a viscosity range of about 25- 100 poises at 77 F.

The said polyamine (Resin B) will react with diisocyanate (Resin A) in the presence of a methyl ethyl ketone blocking agent (in ratio of about 7 parts polyamine by weight to three parts methyl ethyl ketone by weight) to provide an exothermic reaction product having the following structural formula (hereinafter referred to as Resin C).

When the said reaction product is homogeneously mixed with small amounts of xylene (to provide a slight tackiness to light finger touch), the reaction product will cure in about minutes at 77 F. to provide flexible and tough polyurea resin polymer particularly adapted to the present invention. Specifically, the polyurea resin resulting from the reaction of diisocyanate (Resin A) and polyamine (Resin B) provides a water-repellent resin component for the sealing solution that results in an exceedingly efiicient and functional water-repellant sealer for the external surfaces of elongate wood strips. For example, even very thin films of the polyurea resin render the wood exceedingly resistant to potentially inimical environmental conditions wherein excessive moisture is present. Moreover, sealing films of said polyurea resin bond tenaciously to the wood, apparently by a chemical combination with the wood cellulosic fibers, and also provide a tenacious bridge between the wood and pigmented paints containing polyurea resin binders. Alternatively, such sealing films of said polyurea resin, when applied to the wood in very thin layers, as by techniques described later in this patent application, do not detract from the stainability of the wood with conventional non-pigmented wood stains.

Referring now to the drawing, wherein like numbers refer to like parts in the several views, and in which:

FIG. 1 is a perspective view of a typical pre-stabilized elongate wood strip that is adaptable for subsequent chemical stabilization and protection according to the teachings of the present invention.

FIG. 2 is a sectional elevational view of a chemically stabilized and protected wood strip taken along line 22 of FIG. 1.

FIG. 3 is a detail sectional elevational view of FIG. 2 showing schematically the microscopically dentate crosssectional periphery of the fibrous wood material.

FIG. 4 is a perspective view of an apparatus that might be advantageously employed for applying a sealing solution to the elongate external surfaces of an elongate wood stri P IG. 1 shows a typical elongate strip 10 of normallyporous liquid-absorbent fibrous natural wood material that is adaptable for subsequent chemical stabilization according to the teachings of the present invention. Elongate wood strip 10 comprises natural cellulosic elongate grains extending substantially parallel to the longitudinal axis 11 of said wood strip 10, elongate external surfaces 12-15 surrounding longitudinal axis 11 and providing a cross-sectional periphery for the elongate strip 10, and two terminal ends (including forward end 16 and rearward end 17) positioned transversely of longitudinal axis 11 and also herein transversely of the external elongate surfaces 12-15. A nail 18 is shown removably embedded into strip forward end 16 along axis 11, said nail 18 being but one means for facilitating application of the chemical treatments of the present invention. The microscopically dentate cross-sectional periphery of wood strip 10, provided by the cellulosic fibers thereof, eg fibers 12A at surface 12, is shown schematically in FIG. 3.

The following three specific examples will serve to illustrate the practice of this invention. It will be understood, of course, that variations may be made without departing from the scope and spirit of the invention as specified in the appended claims.

EXAMPLE I A preservative solution was made by homogeneously blending the following ingredients:

parts by weight wood preservative chemical as crystalline pentachlorophenol 95 parts by weight volatile carrier as xylene solvent for pentachlorophenol The preservative solution is impregnated into the porous elongate wood stri 10, commencing at the several surfaces thereof including elongate surfaces 12-15 and transverse end surfaces 1617. Impregnation of the preservative solution can be accomplished by several methods such as by repeated application with a paint brush, repeated spraying, or by dipping one or more elongate wood strips into a vessel of the preservative solution wherein at least one internal dimension of the vessel exceeds the elongate length of the wood strip. Herein, the preservative solution was changed into an open top vessel having a height and solution level exceeding the elongate length of wood strip 10. Then, the wood strip was grasped by removable nail 18 and immersed for about ten minutes in the preservative solution; in so doing, preservative solution penetrates the wood strip 10 commencing at the various surfaces 1217 and the said preservative solution is absorbed into the porous volume of wood strip 10 for a given depth from elongate surfaces 12-15 to ward axis 11. Then, the so-impregnated wood strip 10 is removed from the preservative solution bath and placed in a drying chamber for example at room temperature of 77 F. for about 24 hours, whereby the xylene carrier is removed by volatilization from the wood strip. Thus, the pentachlorophenol wood preservative chemical occupies a minor portion of the porous volume commencing at surfaces 12-15 for a given depth P, as schematically indicated in FIGS. 2 and 3, and also for a much greater depth in the granular direction along axis 11 commencing at ends 16 and 17.

A sealing solution was prepared by homogeneously blending the following components:

12 /2 parts by weight diisocyanate (Resin A) 12 /2 parts by weight of a mixture comprising about 8.7 parts by weight polyamine (Resin B) and about 3.8 parts by weight methyl ethyl ketone 75 parts by weight volatile carrier as xylene.

The diisocyanate and the polyamine (in the presence of the methyl ethyl ketone blocking agent) exothermically react within the xylene solvent to form within the said sealing solution the aforedescribed intermediate (Resin C):

The Sealing solution is applied to the elongate surfaces 12-15, as by application with a paint brush, or by spraying techniques, or by dipping as was previously done with the preservative solution. Then, the sealing solution is placed under drying conditions, as at 77 F. for about 12 hours, whereby the xylene volatile carrier is removed and the Resin C intermediate cures to a tough flexible polyurea resin film 20 that adheres tenaciously to elongate surfaces 12-15, providing a water-repellant sealer for elongate surfaces 12-15. The unusually great adhesion between said film 20 and elongate wood strip 10 appears to be chemical in nature, rather than physical, i.e., there is indicated a chemical bond between the cellulosic wood fibers and the cured polyurea resin.

A homogeneous pigmented paint was prepared by ballmilling the following ingredients for about 24 hours:

12 /2 parts by weight, diisocyanate (Resin A) 12 /2 parts by weight of a mixture comprising about 8.7 parts by weight of polyamine (Resin B) and about 3.8 parts by weight methyl ethyl ketone 100 parts by weight titanium dioxide pigment 375 parts by weight xylene Thus, the vehicle of the resulting pigmented paint comprises a polyurea resin (Resin C) as was used for the sealing solution. The pigmented paint was applied upon selected elongate surfaces of wood strip 10 utilizing a spraying apparatus, although this could be done alternatively with a conventional paint brush. The liquid paint coating was allowed to dry at 77 F. for about 24 hours whereupon the xylene was fully volatilized and the paint vehicle became fully cured whereby the paint protective layer 30 adheres very tenaciously to wood strip 10, said desirable adhesion being afforded by the chemically related intermediate sealer layer 20. As indicated in FIG. 2, paint might be applied along less than all the elongate surfaces of wood strip 10, or the painting operation might be dispensed with entirely, depending upon the ultimate intended use of the chemically stabilized wood strip. When the chemically stabilized wood strip is employed for making window frames, it is commercially practicable to paint three of the elongate surfaces at the window-frame factory, and the fourth elongate surface is left to be painted in the desired color or tone after the pre-assembled window frame has been installed at the dwelling house or other building structure.

EXAMRPLE II The entire procedure of Example I was repeated except that the preservative solution used was made by homogeneously blending the following ingredients:

10 parts by weight copper naphthenate as wood perservative chemical.

parts by weight toluene as volatile carrier for copper naphthenate.

Properties of the elongate wood strip treated with the preservative solution of this Example II was in all respects substantially identical to the chemically stabilized and protected wood strip of Example I. As was alluded to in the introductory portion of this patent application, many kinds of wood preservative chemicals of the prior art are adaptable for use as the preservative solution of the present invention.

EXAMPLE III The entire procedure of Example I was repeated except that the Example I sealing solution was applied as a very thin layer (about 1 /2 mils thick) to the microscopically dentate cross-sectional periphery of wood strip 10 as schematically illustrated in FIG. 3, as by means of the vacuumatic apparatus shown in FIG. 4. Upon removal of the volatile xylene carrier component of the sealing solution, the resultant water-repellent polyurea film 20A is disposed within the interfiber crevices whereby the outward portions of the fibers 12A extend outwardly beyond the water-repellent polyurea film 20A. Thus the fibers 12A at the cross-sectional periphery remain liquid-absorbent and are readily receptive to non-pigmented wood stains. Accordingly, one or more of the elongate surfaces 12-15 may be stained rather than protected with a pigmented paint. For example, in the making of window frames, three of the four elongate surfaces of the wood strip might be protected with a pigmented paint layer e.g. 30A, whereby the fourth elongate surface, which provides that surface of the window frame located within the building interior, might be either stained and varnished or painted, de pending upon the decor of the building interior.

The very thin layer application of sealing solution, necessary for alternate paintability or stainability of an elongate surface of the wood strip, might be accomplished with the vacuumatic applicator device 40 shown in FIG. 4. Apparatus 40 comprises an upright rectangular receptacle 41 for the sealing solution 25, said receptacle including a forward wall 42 and a rearward W11 43. Receptacle 41 has a pair of horizontally-aligned transverse openings (not shown), and an annular adapter plate 44 is removably attached over each said trnsverse opening. Each adapter plate 44 has an outwardly-extending rectangular tapered portion 44A that dimensionally substantially equals the cross-sectional dimension of the wood strip 10, whereby the said wood strip 10 may be slidably moved through the two horizontally-aligned tapered portions 44A. An annular gasket (not shown) might be used to line tapered portion 44A to enhance tight slidable engagement with wood strip 10.

The upper panel 45 of apparatus 40 has a tubular collar opening 46 and is provided with a removable air-tight cover 47 (shown removed of collar 46) whereby sealing solution 25 might be charged into the receptacle 41. The sealing solution level 25 extends above adapters 44 i.e. above the pathway of wood strip 10, but below receptacle upper panel 45. Tubular suction line 48 communicates with the interior of receptacle 40 above upper panel 45 and a positive action blower unit 49 is disposed along suction line 48 whereby sub-atmospheric conditions can be induced within receptacle 41.

In operation, the centrifugal blower speed is regulated so that the absolute pressure within receptacle 41 is about p.s.i. Consecutive wood strips are fed through the aligned openings 44A. The positive forces in suction created by blower 49 not only prevents the sealing solution 25 from flowing out of receptacle 41 through the two openings 44A, but also draws away all sealing solution not actually adhering to the elongate surfaces 12-15 of wood strip 10, exiting through opening 44A of receptacle rearward wall 43. Moreover, excess sealing liquid is stripped from the elongate surfaces 12-15 by a positive wiping action of the resilient gasket lining the respective tapered openings 44A.

What is claimed is as follows:

1. The method of chemically stabilizing an elongate strip of liquid-absorbent porous fibrous natural wood material having a cross-sectional periphery surrounding the longitudinal axis of the elongate wood strip, the fibrous nature of said natural wood material providing a microscopically dentate cross-sectional periphery therefor, said method comprising the following steps in order:

(A) impregnating a preservative solution into the porous volume of said elongate strip of liquid-absorbent porous fibrous natural wood material commencing at the microscopically dentate cross-sectional periphery thereof and extending into the fibrous =wood material for a given depth, said preservative solution comprising a homogeneous mixture comprising one part by weight wood preservative chemical and at least seven parts by weight of a volatile liquid carrier for the wood preservative chemical;

(B) removing the volatile carrier portion of the preservative solution whereby the wood preservative chemical residue deposit occupies a minor portion of the porous volume of the natural wood material commencing at the microscopically dentate crosssectional periphery of the fibrous wood material and extending inwardly therefrom for said given depth; and

(C) applying a sealing solution comprising a waterrepellaut polyurea resin having the chemical formula where D represents a 36 carbon hydrocarbon, n signifies that the formula may be repeated any number of times, and R represents a hydrocarbon radical having about 75 carbon atoms and a non-aqueous volatile carrier therefor to the microscopically dentate cross-sectional periphery of said fibrous wood material and then removing the volatile carrier to deposit the water-repellant polyurea sealing resin within the microscopic inter-fibers crevices of the said cross-sectional periphery, the outward portions of said microscopic fibers being relatively free of the polyurea resin deposits whereby said fibers outward portions remain relatively more liquid-absorbent than do the microscopic inter-fibers crevices of the said cross-sectional periphery whereby the chemically stabilized elongate wood strip is alternatively amenable to staining and painting operation.

2. The method of claim 1 wherein the water-repellant polyurea resin is a chemical reaction product of polyisocyanate and an amino bearing compound.

3. The method of claim 2 wherein the polyurea resin is a chemical reaction product of diisocyanate and a polyamine whereby the resultant polyurea resin deposits chamically bond to the cellulosic wood fibers.

4. The method of claim 3 wherein the sealing step i followed by the coating of a pigmented liquid paint upon the cross-sectional periphery of the elongate fibrous wood material, said pigmented paint comprising as the vehicle component thereof a polyurea resin provided by the chemical reaction product of diisocyanate and a polyamine.

5. The method of claim 1 wherein the sealing solution comprises one part polyurea resin and less than about twelve parts volatile non-aqueous carrier, said sealing solution being applied at subatmospheric pressures to the elongate surfaces of the fibrous wood material, and wherein the wood preservative chemical extends deeper into the wood from the cross-sectional periphery than does the polyurea sealing resin deposits.

6. The method of claim 5 wherein the preservative solution comprises one part pentachlorophenol wood preservative chemical and about twenty parts by weight xylene carrier therefor.

7. The method of chemically stabilizing an elongate strip of liquid-absorbent porous natural wood material having a cross-sectional periphery surrounding the lougitudinal axis of the elongate wood strip, said method comprising the following steps in order:

(A) impregnating a preservative solution into the perous volume of said elongate strip of liquid-absorbent porous natural wood material commencing at the cross-sectional periphery thereof and extending inwardly therefrom toward the longitudinal axis for a given depth, said preservative solution comprising a homogeneous mixture comprising one part by weight wood preservative chemical and at least seven parts by weight of a volatile liquid vehicle for the wood preservative chemical;

(B) removing the volatile vehicle portion of the preservative solution whereby the wood preservative chemical residue deposit occupies a minor portion of the porous volume of the natural wood material commencing at the cross-sectional periphery of the natural wood material and extending inwardly therefrom toward the longitudinal axis for said given depth;

(C) applying a sealing solution comprising a waterrepellant polyurea resin and a non-aqueous volatile carrier therefor to the cross-sectional periphery of said natural wood material and then removing the volatile carrier to deposit the water-repellent polyurea sealing resin at the cross-sectional periphery of the natural wood material; and

(D) coating upon the polyurea sealing resin a liquid pigmented paint comprising as the vehicle component thereof a water-repellant polyurea resin and drying.

8. The method of claim 7 wherein the water-repellent polyurea resins of the sealing solution and pigmented paint are a chemical reaction product of diisocyanate and a polyamine whereby the resultant polyurea sealing resin deposits of the sealing solution chemically bond to the wood and the vehicle component of the pigmented paint becomes in uni-phase relationship with the sealing resin to provide a tenacious bond between the wood and the pigmented paint.

9. The method of claim 8 wherein the preservatives solution comprises one part pentachlorophenol wood preservative chemical and about twenty parts by weight xylene carrier therefor; wherein a plurality of elongate strips of liquid-absorbent porous natural wood material are immersed into a bath of said preservative solution; wherein the sealing solution comprises one part polyurea resin and less than about twelve parts volatile non-aqueous carrier; and wherein the wood preservative chemical extends deeper into the wood from the cross-sectional periphery thereof than does the polyurea sealing resin.

10. The method of chemically stabilizing an elongate strip of liquid-absorbent porous fibrous wood material having a cross-sectional periphery surrounding the longitudinal axis of the elongate wood strip, the fibrous nature of said natural wood material providing a microscopically dentate cross-sectional periphery therefor, said method comprising the following steps in order:

(A) impregnating a preservative solution into the porous volume of said elongate strip of liquid-absorbent porous fibrous natural wood material commencing at the cross-sectional periphery thereof and extending into the fibrous wood material, said preservation solution comprising a homogeneous mixture comprising one part by Weight wood preservation chemical and at least seven parts by weight of a volatile liquid carrier for the wood preservative chemical;

(B) removing the volatile carrier portion of the preservative solution whereby the wood preservative chemical residue deposit occupies a minor portion of the porous volume of the natural wood material commencing at the cross-sectional periphery of the fibrous wood material and extending inwardly therefrom; and

0 (NHgNHDNH-KI JNHR)n where D represents a 36 carbon hydrocarbon, n signifies that the formula may be repeated any number of times, and R represents a hydrocarbon radical having about 75 carbon atoms.

References Cited UNITED STATES PATENTS 2,382,374 8/1945 Wood 117147 X 2,820,020 1/ 1958 Franko-Filipasic 26077.5 X 2,852,494 9/1958 Lehmann 26077.5 X 3,006,898 10/1961 Walter 260858 X 3,097,192 7/1963 Schilit 260858 X 3,105,773 10/1963 Frank 117148 X 3,248,424 4/1966 Mullen 260-775 X WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. 117116, 148