Wood Treating Process

Abstract

The treatment of green or undried wood with a polyethylene adduct of a compound containing multi active-hydrogen functionality is described. Such treatment of wood improves the bondability of the wood; for example, the adhesion of veneers together to form plywood.

Description

BACKGROUND OF THE INVENTION

This invention relates to the treating of undried or green wood with a chemical material.

Broadly, chemical treating of wood is known to the art. It is generally recognized that during seasoning or drying of wood, the surface undergoes chemical change as evidenced by a decrease in surface polarity and wood hydroscopicity. This, in turn, often severely affects the bondability of the wood with conventional and economic wood adhesives. In fact, certain drying conditions will render the wood useless as a laminate member or will severely restrict the quality and end use of the composite laminate.

In the manufacture of plywood panels where thin sheets of wood (veneers) are used as the individual layers, preparation and drying of the veneers is a considerable problem to the industry. Veneers for plywood use are obtained by a technique wherein a wet log is rotary cut into a long thin sheet of wood. The wet sheet of wood is then cut into selected lengths and dried. Drying of green or wet veneer is a time consuming burden to the plywood industry because of the drying conditions customarily employed to economically obtain useful dry veneer. A major problem associated with veneer preparation is the control in degree of dryness because of variations in drying rates of the wood. Accordingly, some of the veneer invariably is underdried and some is overdried. Overdried (case hardened) veneer and underdried veneer (too high a moisture content) exhibit poor adhesive bonds when made into plywood panels. This often makes the product useless or severely restricts its end use.

Therefore, it is apparent that a treatment of wood which reduces the criticality of drying conditions, i.e., temperature and/or time would be highly desirable in the wood industry and particularly in the plywood industry.

SUMMARY OF THE INVENTION

This invention is directed to a process for treating undried or green wood, particularly veneers, with a particular class of chemical compounds.

Accordingly, typical objects of this invention are to provide (1) wood having improved surface characteristics, (2) wood having improved adhesion properties, (3) wood having improved drying characteristics, (4) an improved process for drying wood which allows shorter drying times, (5) reduction in waste wood resulting from overdrying, and (6) an improved process for the manufacture of plywood.

In accordance with this invention, wet (green) wood is surface treated with a polyoxyethylene adduct of a compound containing multi active-hydrogen functionality. The thus treated wood is dried by conventional means. In one aspect of this invention, plywood is produced by peeling veneer from a log, treating the peeled veneer with an aqueous solution of a polyoxyethylene adduct of a compound containing multi active-hydrogen functionality, drying the treated veneer, applying an adhesive to the surfaces of the dried treated veneers, laying a plurality of the adhesive coated, dried, treated veneers into a panel assembly and compressing the assembly under heat and pressure to consolidate the assembly and bond the veneers together to form the plywood panel.

The treating materials useful in the practice of this invention, generally speaking, can be classified as nonionic surface active agents. More specifically, the useful materials are obtained from the reaction product of ethylene oxide with a multi-functional active hydrogen compound. Among the useful multi-functional active hydrogen compounds are saccharides, anhydrides of saccharides, polyhydric alcohols, and polyfunctional amines, amides, fatty acids and mercaptans of aliphatic or aromatic origin. A preferred class of such compounds are the polyhydric alcohols or long chain fatty acid mono esters of these compounds. Another useful class is the formaldehyde addition products of aromatic hydroxyl compounds commonly known as resoles. These include the mono-, di- and tri-methylol phenols, cresols and resorcinols. Broadly, such useful compounds can be generally represented by the formula

wherein R is --H, --OH, --SH, --NH.sub.2 ,

or a fatty acid radical; wherein at least two of the R's are other than --H or a fatty acid radical; n is 1 to 20 ; and

is alicyclic, cyclic or heterocyclic.

The above mentioned compounds are reacted with ethylene oxide to form the adduct useful as the treating material in this invention. Accordingly, typical treating materials are tris (polyoxyethylene) sorbitan, tris (polyoxyethylene) sorbitan mono-laurate or oleate, ethylene oxide condensates of ethylene glycol glycerol, or pentaerythritol, tall oil ethylene oxide condensates, di- and tri-methylol phenol-ethylene oxide condensate and the ethylene oxide condensate of resole phenolic resins.

The treating materials useful in this invention are applied to the wood to be treated in a quantity of at least one-half ounce per 1,000 sq. ft. of surface. The maximum quantity of material applied is dictated by economics and generally not more than 1 pound per 1,000 sq. ft. of surface need be used. The treating material may be applied in any condition and manner desired which applies the material uniformly to the wood surface. A very satisfactory way of applying the material is in aqueous solution. The concentration of the aqueous solution is not critical and will depend upon the manner in which it is applied to the wood. Generally, a 1 to 10 percent aqueous solution of the material is satisfactory. The material may be applied by any conventional method such as dipping, spraying, brushing, curtain coating and the like, which will distribute the treating material uniformly over the surface of the wood.

The treating material, regardless of the manner in which it is applied, is applied according to this invention at any time prior to drying or seasoning of the wood. As hereinafter shown in the Examples, a post drying treatment of wood with the treating materials of this invention does not obtain the advantage of improved adhesion.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following Examples are given as illustrative of the invention and specifics given herein are not to be considered as limitations upon the inventive concept.

EXAMPLE 1

This Example demonstrates the preparation of a phenolic adhesive useful in the manufacture of plywood.

The phenolic resin used is an aqueous alkaline phenolic resin containing about 56 percent solids by weight, having a 70.degree. F. viscosity of about 60 on the MacMichael No. 36 wire (see U. S. Pat. No. 2,437,981 for description of this viscosity index) and prepared by condensing 1 molar proportion of phenol with about 2.15 molar proportions of formaldehyde in an aqueous medium in the presence of about 0.5 molar proportions of sodium hydroxide. The filler employed is a standard Douglas Fir bark flour. The ingredients and mixing procedure are as follows:

Material Parts __________________________________________________________________________ Water at 70.degree. F 1260 50% caustic soda 120 Filler 400 1) Digest at 200.degree. F for 30 min. 2) Cool to 180.degree. F Alkaline phenol formaldehyde resin 750 1) Mix 15 minutes at 180.degree. F 2) Cool to 100.degree.F Alkaline phenol formaldehyde resin 750 Cool to room temperature __________________________________________________________________________

EXAMPLE II

This Example demonstrates the improved bond strengths of plywood made using cores treated in accordance with this invention.

Eighteen 4 .times. 8- feet, 3 ply, 7/16 inch, Douglas Fir plywood panels are made using the adhesive of Example I with wet spreads of 55 pounds per thousand square feet of double glue line. Each of 18 core veneer pieces 27 .times. 50- inch are cut in half to provide 18 matching pairs of 13 2/3 .times. 50-inch core veneer pieces. The 18 matching pairs of core veneer pieces are divided into 3 groups of 6 matching pairs each. One of each of the matching pairs of core veneer pieces of one group is sprayed on one side with a water solution of tris-polyethylene oxide sorbitan mono oleate sufficient to apply one-half pound of said material per 1,000 sq. ft. of surface (A treatment). One of each of the matching pairs of core veneer pieces of another group is sprayed on both sides with a water solution of tris-polyethylene oxide sorbitan monooleate sufficient to apply one-half pound of said monooleate per side per 1,000 sq. ft. of surface (1 pound total-both sides) (B treatment). One of each of the matching pairs of core veneer pieces of the remaining group is sprayed on both sides with a water solution of tris-polyethylene oxide sorbitan monooleate solution sufficient to apply one-quarter pound of said monooleate per side per 1,000 sq ft. of surface (one-half pound total-both sides) (C treatment). The thus treated veneers and their matched controls are then dried in a standard veneer dryer for 26 1/2 minutes at 350.degree.F. This veneer is then combined with enough other veneer to make 18-- 4 .times. 8 feet, 3 ply, 7/16 inch plywood panels. The adhesive of Example I is applied by a conventional glue spreader to both surfaces of the core veneers in a quantity of 55 pounds per 1,000 sq. ft. of double glue line and then laid up between surface veneers to form the 3-ply assemblies. The assemblies are consolidated by pressing at 300.degree.F for 6 3/4 minutes, 2 panels per platen opening, using various assembly times for each subgroup of panels. The resulting panels are tested in standard plywood shear tests. The results are given in the following Table. ##SPC1##

EXAMPLE III

This Example illustrates the preparation of ready-to-use phenolic adhesives for use in the manufacture of plywood in accordance with this invention.

Molar Material Parts by Weight Proportion __________________________________________________________________________ Water 28.2 -- Furafil 9.9 -- 50% Aqueous Sodium Hydroxide 3.66 0.20 100% Phenol 21.6 1.0 50% Aqueous Formaldehyde 24.3 1.75 50% Aqueous Sodium Hydroxide 3.94 0.30 __________________________________________________________________________

The water and furafil are charged to a reactor equipped with means for agitation, cooling and reflux and then the charge is stirred to thoroughly wet the powdered extender. The first sodium hydroxide is charged to the reactor and the total charge is then thoroughly mixed. Next the phenol and formaldehyde are charged to the reactor slowly while maintaining the total charge mixture at 110.degree.- 115.degree.F. with cooling and/or refluxing. Next the temperature of the reaction mixture is slowly raised to 150.degree.F., over a period of about 20 minutes, and maintained thereat for one hour. The reaction temperature is then raised to atmospheric reflux and after about a 25 minute reflux period the reaction mixture is rapidly cooled by vacuum reflux to 180.degree.F. (and maintained thereat until a 70.degree.F. viscosity of about 60 on the MacMichael 26s scale is reached.) The second sodium hydroxide is charged and the temperature of the reaction mixture is reduced to 170.degree.F. and maintained thereat until a 70.degree.F. viscosity of about 2,350 centipoises is reached. The reaction mixture is then cooled. The resulting adhesive contains about 45.5 percent total solids, has a 180.degree.F. gel time of about 7,990 seconds and a useful storage life of about 3 weeks at 70.degree.F.

This Example demonstrates that the treating of wood in accordance with this invention prior to drying is most effective in obtaining excellent bond strengths.

Eight 5 .times. 10 inch, 3 ply, one-half inch, Idaho White Pine plywood panels are made using the adhesive of Example III with wet spreads of 50 pounds per thousand square feet of double glue line. The veneers for a set of four panels are dipped in a 2 percent water solution of polyethylene oxide sorbitan monooleate and then dried in an air circulating oven at 390.degree.F. for 30 minutes. The veneers for the other set of four panels are dried under said conditions followed by dipping into said treat-ing solution and drying under mild conditions (30 minutes at 200.degree.F). Each veneer is thus treated with about one-half pound of said monooleate per 1,000 sq. ft. of veneer surface. The veneers are assembled and each panel assembly is pressed at 300.degree.F. and 200 psi for 5 minutes. The panels are tested with the following results:

Wood Failure (%)

Assembly Knife Boil Time Pretreat- Posttreat- Pretreat- Posttreat- (minutes) ment ment ment ment __________________________________________________________________________ 3 10- 9 3- 1 95 30 10 10- 10 8- 7 100 65 30 10- 10 6- 8 100 50 40 10- 8 2- 1 92 45 __________________________________________________________________________

EXAMPLE V

This Example demonstrates the increased drying rate of wood treated in accordance with this invention.

Idaho White Pine veneer is dipped in a 2 percent water solution of polyethylene oxide sorbitan monooleate and is thus treated with about one-half pound of said monooleate per 1,000 square feet of glue line surface. This veneer and an untreated Idaho White Pine veneer are dried in an air circulating oven at 230.degree.F. The drying times required to reduce the moisture content of these veneers to selected percentages of moisture are as follows:

Drying time (minutes) Moisture Content % Treated untreated __________________________________________________________________________ 30 7 8 20 10 12 10 15 18 5 21 25 __________________________________________________________________________

EXAMPLE VI

This example demonstrates the effectiveness of this invention on improving the bondability of severely overdried Douglas Fir veneer.

A series of Douglas fir 3/16 inch 3-ply plywood panels are prepared using old growth heart veneer treated with 1 pound per 1,000 square feet of surface with tris-polyethylene oxide sorbitan monooleate. The veneers are dried 45 minutes at 390.degree. F. The thus treated dried veneers and control veneers are laid up into assemblies after applying the adhesive of Example I in a quantity of 55 pounds per thousand square feet of double glue line.

The assemblies are pressed at 300.degree.F. The resulting panels are subjected to the standard plywood shear tests and the percent wood failure obtained. The results are listed in the following table together with the pressing conditions of assembly time and press time for each of the control and treated panels.

TABLE A

Assembly Press Boil Wood Failure (%) Panel time(min.) time(min.) Control Treated __________________________________________________________________________ 1 30 7 45 97 2 30 6 1/2 80 100 3 30 6 5 90 4 30 5 1/2 -- -- 5 30 5 34 100 6 30 4 1/2 0 90 7 15 7 74 100 8 15 6 1/2 30 100 9 15 6 10 100 10 15 5 1/2 7 100 11 15 5 0 100 12 15 4 1/2 0 90 __________________________________________________________________________

EXAMPLE VII

This example demonstrates the adhesion benefit obtained with this invention for a wide spectrum of phenolic resin adhesives.

Several panels of mild overdried Idaho White Pine veneers are laid up into 3-ply assemblies using various types of adhesives. The assemblies are consolidated under a temperature of 300.degree.F. The characteristic variables of the veneers and adhesives are listed in the following Table.

TABLE B

Assembly Press Boil Wood Failure Adhesive time(min) time(min) Control Treated __________________________________________________________________________ High mol wt. Phenolic Resin- 3 5 30 77 Furafil filled adhesive 30 5 93 100 40 5 30 100 Low mol wt. Phenolic Resin- 3 5 Delam 17 Bark filled adhesive 30 5 Delam 90 40 5 30 100 Medium mol wt. Phenolic Resin- 3 5 53 97 Furafil filled adhesive 30 5 100 100 40 5 93 100 __________________________________________________________________________

EXAMPLE VIII

This example demonstrates the effectiveness of this invention with regard to improving adhesion of Southern Pine Veneer at various levels of dryness.

Several 5 .times. 10 inch, 3 ply, one-half Southern Pine plywood panels are prepared using the adhesive of EXAMPLE III at a spread level of about 55 pounds per 1,000 sq. ft. of double glue line surface. The panels are consolidated at a temperature of 300.degree.F. and 200 psi. Characteristic variables of each panel and the resulting adhesive strength test data are given in the following Table:

Wood Failure (%) Dry tr- Boil Assembly Press con- ea- C'- T'- Panel Dryness time(min) time(min) trol ted trol td __________________________________________________________________________ 1 Normal.sup.1 10 6 40 94 70 92 2 " 10 5 20 80 30 75 3 " 3 7 25 98 40 100 4 " 20 7 96 98 98 96 5 " 30 7 90 90 86 96 6 " 40 7 78 100 36 96 7 Slightly.sup.2 10 6 100 100 98 98 overdried 8 " 10 5 85 98 80 98 9 " 3 7 34 100 60 98 10 " 20 7 10 64 18 58 11 " 30 7 80 98 40 88 12 " 40 7 74 92 74 96 13 moderately.sup.3 10 6 20 98 0 100 overdried 14 " 10 5 0 0 0 0 15 " 3 7 28 88 0 50 16 " 20 7 0 92 0 80 17 " 30 7 58 60 0 50 18 " 40 7 56 58 0 86 __________________________________________________________________________

EXAMPLE IX

This example demonstrates the effectiveness of this invention with regard to improving adhesion of overdried wood.

Several 5 .times. 10 inch, 3 ply, one-half inch Douglas Fir plywood panels are prepared using the adhesive of Example I at a spread level of 50 pounds per 1,000 sq. ft. of double glue line surface. The veneers are dried at 390.degree.F. for 25 minutes and have a moisture content of 0- 2 percent. Characteristic variables of each panel and the resulting adhesive strength data are given in the following table:

Assembly Press Boil Wood Failure (%) Panel time(min) time(min) Control Treated __________________________________________________________________________ 1 20 15 65 100 2 30 15 85 98 3 40 15 93 100 4 20 30 13 93 5 30 30 68 93 6 40 30 60 98 7 20 45 60 98 8 30 45 63 95 __________________________________________________________________________

As seen from the above examples, the treating of wood in accordance with this invention prior to drying exhibits greatly improved adhesive bond strengths as demonstrated by glue line wood failure compared to untreated wood.

Any type and cut of wood can be treated in accordance with this invention, for example, Douglas Fir, Southern Pine, Idaho White Pine, Hemlock, Spruce, etc.

Many advantages are obtained in wood processing and utility through the chemical treatment of this invention. The treating of the wood offers many advantages, such as (1) improved bondability of the wood, even if the wood is overdried, (2) reduces criticality of assembly time in preparing plywood, (3) allows lower spreads of adhesives for satisfactory bonds, (4) reduces press time in bonding wood, (5) reduces criticality of adhesive spread uniformity, (6) increases dryer capacity by allowing shorter drying cycles, (7) improves moisture content uniformity in drying wood by reducing wet spots, and (8) improves the coatability of wood.

Any useful plywood adhesive such as phenolic resin base, urea resin base, melamine resin base, resorcinol or phenol resorcinol base and protein base adhesives (blood, soy bean, casein), can be used with wood veneer treated and dried in accordance with this invention.

Various modifications and ramifications of the inventive concept disclosed herein can be made without departing from the scope thereof.

Claims

What is claimed is:

1. The process of treating wood which consists of uniformly applying as an aqueous solution to the surface of green wood from 0.5 to 16 ounces per 1,000 surface square feet of an ethylene oxide adduct of a compound containing multi active-hydrogen functionality and then drying the wood.

2. The process of claim 1 wherein said compound is aliphatic, aromatic, cyclic or heterocyclic and the multi active-hydrogen functionality is OH, SH, NH.sub.2,

or combinations thereof.

3. The process of claim 1 wherein the said ethylene oxide adduct of a multi functional active-hydrogen compound is tris (polyoxyethylene) sorbitan monooleate.

4. In a process of manufacturing plywood wherein the veneers are prepared by rotary cutting a log into a long thin sheet of wood and cutting the sheet of wood into selected lengths, drying the veneers, applying adhesive to the veneers, preparing a plywood assembly of the veneers and consolidating the assembly under heat and pressure, the improvement which consists of applying as an aqueous solution to the wood veneer prior to said drying from 0.5 to 16 ounces per 1,000 surface square feet of an ethylene oxide adduct of a compound containing multi active-hydrogen functionality.

5. The process of claim 4, wherein said compound is aliphatic, aromatic, cyclic or heterocyclic and the multi active-hydrogen functionality is OH, SH, NH.sub.2,

or combinations thereof.

6. The process of claim 5, wherein the said ethylene oxide adduct of a multi functional-active hydrogen compound is tris (polyoxyethylene) sorbitan monooleate.