CA1086907A - Method of increasing interfiber bonding among fibers of lignocellulosic material, and resultant product - Google Patents

Abstract

Abstract of the Disclosure Defiberized lignocellulosic material, such as wood, is treated with a liquid carrier containing an oxidizing agent, and the wet mat thereof is subjected to pressure, and to heat for a sufficient period of time to cause an oxidative reaction among the fibers resulting in a strong interfiber bond.
Catalysts or other reaction modifying agents may be employed.
By virtue of the enhanced interfiber bonding effect t paper sheets, such as liner board, which are usually formed of delignified cellulosic material, the fibers of which are highly refined, can be formed totally or partially of less expansive sources of material such as ground wood, semi-chemical or semi-mechanical lignocellulosic pulps without sacrifice of strength.

Description

869(1~7 -Related Application The invention hereof is related to the generic invention disclosed in applicants' copending Canadian Application No. 208,438, filed September fl, 1974, now Canadian Patent No. 1,031,925, issued May 30, 1978 but is specific to defiberized lignocellulosic material for the manufacture of paper or paper like products in which enhanced interfiber bonding is effected in contradistinction to surface to surface interface bonding of sol~d ~ood~
Back'ground'of'the'In'v'en'tion Bonding of lignocellulosic fiber materials, such as wood fiber, is widely used commercially as for example in the manufacture of paper or fiber products. In present commercial bonding procedures, bonding among the fibers is ' ' based primarily on physical forces created by the large surface of finely interlocked cellulose fibers. For increasing the bonding strength of such'product, one-may add to the pulpr before mat or sheet formation, sizing substances s'uch as starch or xesins as adhesiYes. Strength ~0 increase by such procedure is only moderate, and moreover the use thereof increases costs. Strength may also be ~ .

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;9~7 ., increased by formation (fibrillation) of lonyer and more refined fibers. This involves, however, more complicated and costly chemical pulping procedures, and results in lower yieldr of about 45% in the Kraft process, compared to 95% in mechanical pulping.
Summary 'and Ob'je'cts' of' the'In~enti'on In the invention hereof, less expensive sources of lignocellulosic fibers are rendered available for the production of paper or paper like products, which'provide physical properties comparable to more expensive fiber sources. Thus, high lignin content mechanical pu~p Cground wood), semi-mechanical or semi-chemical pulp provide sources for the production of products of incre~sed strength, such as liner board ~r other flexibIe paper, which could not normally be obtained otherwise. Such objective is achieved by increasing the interfiber bonding strength among the fibers, by~ thoroughly dispersing throughout a mat of the' fibers, an oxidizing agent which'results in foxmation of interfiber chemical linkages effected by oxidation upon application of heat.
Ground wood, whi'ch is no~ widely emp~oyed as a source for newsprint or ot~er high llgn-in content fibers, can by the invention hereof be employed for the''manufacture of much stronger flexible sheets not heretofore obtainable from ground wood, such as liner board used in the manufacture , .
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Although the invention hereof is particularly applicable to ground wood as it enables an inexpensive source of fiber to be used for paper products requiring strength properties not heretofore obtainable from ground wood, it may be employed with other sources of defiberized lignocellulosic material wherein at least some of the lignin is present such as semi-chemical and semi-mechanical pulps, which normally form weaker paper mats than fully delignified lignocellulosic material. In this connection, to obtain the oxidative bonding reaction, at least some lignin should remain in the defiberized material, or lignin like material, such as phenolics added thereto~
The chemical reactions involved in the process hereof are not fully understood, Wood is a high-polymeric substance composed of three classes of materials--carbohydrates (primarily cellulose), lignin and extractives. While cellulose is a polysaccharide built up of glucose units, lignin appears to be a polymeric phenolic material, the structure of which is still not fully understood. Not much is known about the bond between the carbohydrates and lignin, - although, generally speaking, lignin seems to function as a binder for cellulose microfibrils. The function of extractives appears to be manifold; their disease protective function is probably the most important.

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~C~8~9~)7 , _, In oxidation of lignocellulosic materials several reaction systems may be involved at the same time. Based on the present day chemical knowledge, it can be assumed that the oxidation o~ phenolic units contained in lignin structure is either the main or at least one of the main reactions leading to self bonding of lignocellulosic materials. In this case the intermediate formation of free radicals is likely to take place, coupling under the formation of - lignin-to-lignin linkages. It cannot be excluded, however, that to some extent polysaccharide-to-polysaccharide and lignin-to-polysaccharide bonding also takes place during this oxidation.
In effecting the oxidation reaction, a mat of the defiberized material is provided in which an oxidant is thoroughly dispersed uniformly therethrough. The mat is formed into a sheet under pressure and heat for a time sufficient to effect the oxidative reaction. In this connection, the oxidizing agent may also be employed with a promoter to promote the oxidative bonding.
The invention hereof may readily be performed on a paper making machine wherein a paper mat is formed in the conventional manner. The mat is then sprayed or roller coated with the oxidant in a liquid carrier which wets the mat, and with a catalyst to promote the reaction. They may both be contained in the same carrier or applied separately to the sheet in the machine as will be discussed more fully hereinafter.
From the preceding it is seen that the invention has as its objects, among others, the provision of an improved bm:

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~ ~)86g~7 method of effecting increased interfiber bonding among fibers of defiberized lignocellulosic material by effecting an oxidative reaction among the fibers, which method is simple to perform and renders available less expensive sowrces of pulp for the manufacture of paper or paperboard sheets requiring strength, and which is economical and simple to perform. Other objects will become apparent from the following more detailed description, and accompanying drawing in which:
The single Fig. 1 is a schematic side elevational view of a conventional Fourdrinier paper making machine in which the invention hereof may be performed in various ways;
parts being broken away to shorten the view.
Prior Art Heritage U.S. Patent No. 2,125,634, dated August 2, 1938, discloses bleaching of paper pulp in a paper making machine by applying hydrogen peroxide to the wet or partially wet mat in minute concentrations in the presence of an alkali such as sodium silicate, at a point ahead OL or in advance of the dry end of the dryer, solely to bleach the sheet or pulp.
However, it has been found pursuant to this invention that hydrogen peroxide alone will not effect oxidative bonding reaction among the fibers to impart increased interfiber bonding but must be used with a catalyst such as zirconium -tetrachloride or ferrous sulfate. Moreover, the pH of the hydrogen peroxide solution should be below pH 7, and the concentration of the hydrogen peroxide in the carrier above 1% to be effective, and desirably above 5%, and may be as high an 50%.

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~8~9617 SUM~IARY OF TH~ INVENTION
In one particular aspect the present invention provides the method of increasing interfiber bonding among fibers of defiberized lignocellulosic material containing a substantial proportion of the natural lignin content which comprises dispersing substantially throughout a sheet of such material an-oxidizing agent selected from the class consisting of per compounds, nitrates and chlorates, maintaining an acid pH
where a per compound is used, and applying heat and pressure to said sheet for a time and temperature sufficient to effect said bonding by oxidative bonding reaction.
In another particular aspect the present invention provides the method of making flexible paper from wood pulp containing ground wood which comprises continuously forming a sheet from said pulp in a paper making machine, effecting penetration substantlally throughout the sheet of an oxidizing agent in a fluid carrier by wetting the sheet therewith, and roller pressing said sheet in said paper making machine at such pH
under heat and for a time sufficient to effect interfiber bonding among fibers of the mat by oxidative bonding reaction, said oxidizing agent being selected from the class consisting of per compounds, nitrates and chlorates, said fluid carrier having an acid pH where the oxidant is a per compound.
In yet further aspects the invention provides flexible paper, liner board and fiber board of improved interfiber bonding produced by appli.cant's novel methods.

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Detailed Description In performing the method hereoE, a lignocellulosic mat of for example ground wood fiber is formed in the usual manner as a continuous sheet. After the sheet is formed, it is wetted with a liquid carrier containing an oxidizing agent which penetrates the sheet thoroughly and covers the surfaces of the individual fibers. The wetting may be effected in any suitable manner such as by spraying the liquid carrier containing oxidant over a surface of the sheet or by roller coating the same on such surface. Usually, as will be discussed more fully hereinafter, it is desirable to employ a catalyst which is also uniformly dispersed throughout the sheet to promote oxidation by the oxldant. Various procedures of oxidant application to the sheet may be employed, such as:
1. The lignocellulosic fiber sheet may be simply wetted with a liquid carrier containing an oxidant of the type effective without a catalyst discussed hereinafter, or with a mixture of oxidant and catalyst, followed by application of -heat and pressure. The effectiveness varles depending upon factors such as type of oxidant, temperature and time.
Hydrogen peroxide, used with a catalyst, sùch as zirconium tetrachloride, can be effectively employed in this manner of application.

2. In many instances a higher level of interfiber bonding may be obtained if the lignocellulosic sheet is first wetted with an oxidant thoroughly penetrating the sheet, presumably to increase the number of carboxyl groups in the lignocellulosic material, followed by treatment with a liquid carrier containing a transition metal catalyst. Subsequent , -6-~ ..~ .
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~8~g~7 wettiny with a liquid carrier containing hydrogen peroxide forms a Fenton reagent with the transition metal catalyst, which is a very efEective oxidizing agent for the lignocellulosic fibers. Pressing under an elevated temperature is then effected.

3. Another mode of application is first to wet the sheet with a liquid carrier contain:ing a peroxide such as a peraeid to incorporate peroxy groups into the lignocellulosie material. After sueh ineorporation, a liquid carrier eontaining a transition metal eatalyst is added to the material, followed by application of pressure at an elevated temperature to form the flexible paper sheet~

4. In some eommercial processes which are known as dry or semi-dry processes used in the production of fiberboards or hardboards, the dry or semi-dry pulp is formed as a relatively thick mat which may be 2 or 3 inches in thickness, and then compacted into a relatively thin rigid board.
Because o~ the initial thickness of such mat, it may be difficult to obtain uniform penetration or dispersion throughout the mat by spraying or roller spreading the carrier containing the desired oxidizing agent on the mat surface, To insure such uniform penetration the oxidizing agent if used along, and the catalyst if employed with the oxidant have to be thoroughly intermixed with fiber. If the catalyst does no-t react with the oxidant at ambient temperature, they may be both included in the same liquid carrier~ However some catalysts may react with the oxidant at ambient temperature, such as hydrogen peroxide and ferrous sulfate~
In such event to produce the reaction initially in the fiber, bm:

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~L~186907 the catalyst and the oxidant are applied separately in two steps. For example, the carrier and oxidant may be applied first, and then the carrier and catalyst, or vice versa Also, an oxidizing agent may be mixed with one half of the material for formation of the mat, and a transition metal catalyst thoroughly mixed with the other half, followed by mixing of the two parts together which results in uniform incorporation of oxidant and catalyst in the mat. The mat is then compacted under pressure and heat to form the desired 10 product.
From the preceding it is seen that particular procedures for performing the method hereof may vary widely.
In the manufacture of flexible paper and related products such as flexible liner board, the method hereof can be performed readily on a conventional paper making machine. It is only necessary to spray or otherwise apply to the fiber sheets in the machine a liquid carrier containing oxidant~ catalyst, or oxidant and catalyst as the case may be~ in the manner outlined above. The liquid carrier penetrates the sheet thoroughly.
20 Also, the agents might be included in the water slurry prior to dehydration of the sheet on the paper making machine~
There are numerous oxidizing agents and catalysts that may be employed as will be listed subsequently. It is only necessary, irrespective of the system of oxidant or of catalyst used, to effect the oxidative bonding reaction among the fibers of the lignocellulosic material at an elevated temperature and for a tirne sufficient to effect such interfiber bonding. The oxidative reaction is effected primarily by heat but it is desirably conducted under pressure bm:

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The temperature and time for obtaining the oxidative bonding reaction among the fibers will vary depending upon the oxidants and the character of the fibrous material. As usual, the lower the temperature the longer the reacting time and vice versa. The reacting temperature should not exceed the temperature at which charring of the lignocellulosic material will occur. Also, the pressure applied should not exceed that at which the lignocellulosic material is crushed.
With higher amounts of some oxidants such as hydrogen peroxide, and compatible catalysts the pressing or reacting temperature may be as low as ambient. A suitable temperature range is between 20C and 200C with a reaction time of 0.5 to 5.0 minutes at a pressure of between atmospheric and 950 psi.
As a solvent or liquid carrier for the oxidant, any liquid may be employed which does not react with the wood such as water or alcohol. The solvent readily escapes as vapor during the pressing and drying of the mat.
The amount and concentration of oxidant solution will also vary widely depending upon the chemical character of the oxidant, the type of lignocelluloslc material, and reaction conditions. In general, an amoun-t of carrier solution (which need not be a true solution but which may be a suspension) is used which will provide from 1 0 to 6.0~ of ,., _g_ bm:

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~ ~0~ )7 oxidant based on the dry weight of the lignocellulosic material but this range is not critical as even small amounts of reagent are effective. Large amounts serve no useful purpose. For any given oxidant one can readily determine the amounts and conditions of treatment which will produce optimum oxidative bonding.
As noted above, a large number of oxidants may be used for the purposes of this invention to effect the interfiber bonding of defiberized lignocellulosic material by oxidative bonding. The oxidants fall into two classes, namely, (1) oxides of chlorine and nitrogen and their derivatives and (2) per compounds.
Some of these oxidants are effective alone without catalysts while others require a catalyst in conjunction therewith to promote the oxidative bonding. Those that can be used alone without catalysts include:
Nitric acid;
Nitrates such as ammonium nitrate, magnesium nitrate, potassium nitrate and sodium nitrate;
Nitrites such as sodium nitrite;
Chlorates such as sodium chlorate and potassium chlorate;
Chlorites such as sodium chlorite;
Perchlorates such as potassium perchlorate;
Halogens such as chlorine and bromine; and Hypochlorites such as calcium hypochlorite and ammonium hypochlorite.
As will be seen, these are oxidants of the first class, namely oxides of chlorine or nitrogen and their deriva-tives.
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The following oxidants, which are of the second class (per compounds), usually require promoters or catalysts to be effec-tive comprise:
Per compounds, for example peroxides, such as barium peroxide, hydrogen peroxide and sodium peroxide;
Per acids, such as peracetic acid, persulfuric acid, and salts of per acids, such as persulfates (and other organic and other inorganic "per" compounds) such as ammonium persulfate, potassium persulfate and po-tassium perborate;
and Ozone and ozonides.
The use of chlorites and hypochlorites is effective as a peroxidizing agent in first treating the mat, followed by treatment with hydrogen peroxide oxidant and a transition metal catalyst. Also, incorporation of peracids into the sheet such as persulfuric acid followed by treatment with a transition metal catalyst is very effective.
In general, the oxidative bonding is especially effective if the pH of the carrier solution is on the acid 2- side, desirably at about the natural pH of woods used in paper manufacture, namely about 3 - 6. The pH may be adjusted by addition of~acid of alkalil as required.
Catalysts which are preferably employed can be applied in the liquid carrier mixed with the oxidant or separately. Any suitable catalysts may be employed which will advantageously speed up and enhance the bonding action of the oxidant used. They include metal salts such as ferric, ferrous, manganese, chromium, lead, copper and cobalt, or corresponding oxides which are well known promoters of many oxidative bm:

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)7 reactions. Catalysts also include various organic and inorganic reducing ayents such as hydroquinone, pyrogallol, tannins, hydrazine and bisulfites. The amount of catalyst used is relatively small compared to the amount of oxidant and usually will vary from 0.01% to 1.0~ b~ weight of the oxidant, but this range is not critical.
The following are typical examples of hand prepared samples prepared by conventional laboratory procedures demonstrating the principles of the instant invéntlon:

A mat of Western hemlock ground wood fibers about l foot square, was formed on a sieve screen of about 120 mesh from a water slurry of about 4% consistency. It was pressed between such screen and another similar sieve screen to a thickness of about 0.1 in., to partially dehydrate the resultant mat to a consistency of aboùt ~0~, and the mat while still wet was then sprayed with a water carrier containing about 15% by weight of hydrogen peroxide and about 0.75% by weight of zirconium tetrachloride; the total amount of carrier, oxidant and catalyst being about 6.5~
by weight of the dry weight of fibers. After allowing the carrier an~d its contents to penetrate the mat which took about 1 minute, the mat was promptly pressed between two 120 mesh sieve screans at a temperature of about 150C and pressure of about 700 lbs. per sq. inch ~psi) for about 2 minutes to thus form a flexible paper sheet suitable for use as liner board. The physical properties of this sheet and those of following Examples 2 and 3 are noted in subsequent Table I which also includes properties of control samples .~
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~ 869~7 which were treated in the same way as in the examples but without the oxidant.
In this example, it will be noted that the oxidants and the catalyst were both applied from the same water carrier.

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A mat of one foot squaxe was ~ormed of Western hemlock ground wood ~iber from a water slurry contai~ing about 5% by weight of the ground wood and 0.125% of sodium hypochlorite as a preoxidant thoroughly dispersed in the wood fiber. It was pressed as in Example 1 to partially dehydrate the resultant mat to a consistency of about 40~, and was then sprayed with a 2.5% water solution of ferrous sulfate catalyst in the amount of about 5~ solution to the weight of dry fibers. After the solution was allowed to penetrate the mat as in Example 1, it was sprayed with a 20%
water solution of hydrogen peroxide in the amount of about

5% of solution to the weight of dry fiber, and was then pressed between two sieve screens as in Example 1 at a temperature of about 150C and pressure of 700 psi for two minutes which resulted in a flexible paper sheet.
In this example, the impregnation with hypochlorite as a preoxidant, is followed by sequential catalyst and oxidant addition.

A mat one foot square was formed as in Example 1 from a water slurry of Western ground wood fiber. After draining and partial dehydrating by pressing between two sieve screens, the mat was sprayed with 7.5% water solution bm:
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water solution of ferrous sulfate in the amount of 10%
solution to the weight of dry fiber, and was pressed as in Examples 1 and 2 at a temperature at about 150C and pressure of 700 psi for about two minutes. This example illustrates sequential addition of oxidant and catalyst.
The physical properties of the paper sheet materials produced under conditions of Examples 1 through 3 are noted in the following Table I, which as noted above also includes the properties of control samples which were treated in the same way as in Examples 1 through 3 but without the oxidizing agents.
TABLE I

'Example Thickness I ~ensity Tensile strength Thickness _ i j swelling in. gr/ft2 dry 24 hrs. %
soaked _ _ _ ...
1 0.023 55 1987 512 39 '~ 2 0.025 54 2649 663 34 3 0.024 56 2505 495 26 L trol ~ _ 57 2037 282 51 ¦

The data set forth in the Table for each example is an average of 10 tests. From the Table, it will be noted that the thickness and density resulting from all tests are substantially the same. The dry tensile strength data of Examples 2 and 3 evidence the efficaciousness of the oxidative interfiber bonding achieved under -the conditions described in these examples.

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~8i~90~7 It is noteworthy that the tensile strengths of the sheets after they had been soaked in water for 24 hours establish the marked improvement in wet strength of Examples 1 through 3 compared to the control. Also, it will be observed that the control had a much higher percent of thickness swelling than the sheets of Examples 1 through 3, which evidences the bonding strength obtained by the method of this invention. The less the swelling, the higher the bonding strength, or decrease in hygroscopicity.

A rigid hard board suitable as a building board panel was produced in the following manner. Western hemlock ground wood fibers were sprayed with a 1.25% water solution of sodium hypochlorite followed by spraying with a 1.25% -water solution of ferrous sulfate both in the amount of about 100~ solution to the weight of dry fibers. After thorough mixing, a mat was formed from a water slurry containing about 5~i by weight of treated fibers. After draining and partial dehydration by pressing the sheet between t~o sieve screens as in the previous examples, the sheet was sprayed with a 20~ water solution of hydrogen peroxide in the amount of 10 to dry weight of ibers. After such treatment, the sheet was pressed between two sieve screens at a temperature of 150C and pres~ure of about 850 psi for five minutes to produce hardboard of 0.117 in thickness and 1.055 speciEic gravity. Table II, below, depicts the physical data obtained by an average oE ten tests on samples produced by Example ~, compared to a control which was not treated with oxidizing agents, also an average oE 10 tests.

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TABLE II

Example Thickness Specific Tensile strength Thickness gravity P' ,i swelling _in. dry ~ éd %

4 0.117 1.055 4322 1424 26,6 Control 0.123 1,034 4103 667 52.2 This example is one wherein hard board is produced from a relatively thick ma-t which is compacted to a relatively thin rigid board. One part of ground wood fiber particles was sprayed with a 1.25% water solution of sodium hypochlorite as a preoxidizing agent followed by spraying with a 1.25% water solution of ferrous sulfate both in the amount of about 10% by weight of the fiber on a dry basis. The other part was sprayed with a 20% water solution of hydrogen peroxide also in the amount of 10% by weight of the dry weight of fibers, The thoroughly wet sprayed parts were then thoroughly mixed together; and a sheet of about a thickness of about 2 inches was formed and then pressed between sieve screens of about 120 mesh to dehydrate the mat to a water consistency of about 40%. The mat was conveyed on the screens into a press in the usual manner, and the mat was compressed to a thickness of about 1/8 inch under a temperature of about 150C and pressure of about 850 psi for about 2 minutes which resulted in a rigid hard board suitable for building purposes.
Thickness of the board was 0.120 in.; specific gravity 1.071; dry tensile strength 4,416 psi; tensile strength after 24 hrs. soa~ing in water 1,519 psi~ and thickness swelling 24.4%.

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86~7 As was noted above, the method hereof is particularly adapted for performance in a paper making machine, Referring to Fig. 1, a conventional type of Fourdrinier machine is schematically illustrated. It comprises headbox 2 from which a slurry of defiberized material, such as ground wood, is discharged onto a Fourdrinier wire or table 3 on which the mat is initially formed. From wire 3, the wet web of paper is continuously discharged into press section 4 through which it is continuously conveyed through press rolls 6, and wherein the moisture content is reduced by mechanical pressure effected by the ral:ls. The thus partially dehydrated sheet is continuously conveyed through dryer section 7 which removes remaining moisture from the sheet by means of heat and vapor -transfer; the dryer section comprising a large number of heated drying rolls 8. From the dxyer section, the now substantially dehydrated sheet passes through calender stack 9 comprising a series of smooth surfaced, heated calender rolls 11 which control the thickness of the sheet, its smoothness and other characteristics. The calendered sheet is then wound into a roll 12.
As previously related, the oxidant or oxidant and catalyst may be applied to the defibered lignocellulosic material in various ways rendering the method hereof very versatile. For example, with reference to paper ma]sing machine application, if only an oxidant or oxidant and catalyst i5 applied, the liquid carrier containing the oxidant or mixture of oxidant and catalyst may be suitably added at positions indicated at A, B or C in the machine, which results in penetration of the oxidant, or catalyst and oxidant, into the bm:

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Where mild preoxidation o~ the sheet is desirable, a small amount of the preoxidizing agent, such as sodium hypochlorite, may be added in the slurry in the headbox, or at position A. The carrier containing the transition metal catalyst may be added midway in the dryer section indicated at position B, and the carrier containing hydrogen peroxide oxidant at position C just ahead of calendar stack or rolls Where the sheet is to be treated with a peracid or peroxide, it may be added at position D~ just before the press section; and the carrier contain.ing a transition metal catalyst at position B or C Both surfaces or only one surface of the sheet may be wetted. Also, a catalyst solution may be applied to one surface and the oxidant solution to the other surface of the sheet as long as they are thoroughly intermixed in the mat.
From the preceding, it is seen that the procedure comprises a two step process, namely (a) treatment of the defibered lignocellulosic material with oxidant or oxidant and catalyst before pressing, namely before bringing the fiber surfaces into sufficient contact, and (b) effecting the bond formation reaction by temperature increase, and desirably under pressure.

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Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of increasing interfiber bonding among fibers of defiberized lignocellulosic material containing a substantial proportion of the natural lignin content which comprises dispersing substantially throughout a sheet of such material an oxidizing agent selected from the class consisting of per compounds, nitrates and chlorates, maintaining an acid pH where a per compound is used, and applying heat and pressure to said sheet for a time and temperature sufficient to effect said bonding by oxidative bonding reaction.

2. The method of Claim 1 wherein the oxidizing agent is applied to said sheet in a liquid carrier which penetrates the sheet.

3. The method of Claim 1 wherein a catalyst for said oxidizing agent is also dispersed in said sheet to promote said oxidative reaction.

4. The method of Claim 1 wherein said defiberized material is partially delignified wood fiber containing at least some lignin.

5. The method of Claim 1 wherein the defiberized material is ground wood, semi-chemical or semi-mechanical pulp all containing lignin.

6. The method of Claim 2 wherein said oxidative reaction of said defiberized lignocellulosic material is effected in a paper making machine wherein said sheet is formed and which has calender rolls adjacent its end from which a flexible paper sheet is continuously discharged and said sheet is wetted in said machine by said liquid carrier containing said oxidant at a location ahead of said calender rolls.

7. The method of Claim 6 wherein said sheet is liner board.

8. The method of Claim 1 wherein said oxidative reaction of said lignocellulosic material is effected in a press to form fiberboard.

9. The method of Claim 1 wherein said sheet is first wetted with an oxidant to preoxidize the fibers, and the sheet is subsequently wetted with a catalyst and with an oxidant in a liquid carrier.

10. The method of Claim 9 wherein said last mentioned oxidant is hydrogen peroxide.

11. The method of Claim 3 wherein the oxidant is hydrogen peroxide, and the catalyst is a transition metal salt.

12. The method of Claim 2 wherein said liquid carrier containing said oxidizing agent is sprayed or roller spread onto a surface of the sheet.

13. The method of Claim 1 wherein the oxidant is a nitrate.

14. The method of Claim 13 wherein the nitrate is sodium nitrate.

15. The method of Claim 1 wherein the oxidant is a chlorate.

16. The method of Claim 15 wherein the chlorate is sodium chlorate.

17. The method of making flexible paper from wood pulp containing ground wood which comprises continuously forming a sheet from said pulp in a paper making machine, effecting penetration substantially throughout the sheet of an oxidizing agent in a fluid carrier by wetting the sheet therewith, and roller pressing said sheet in said paper making machine at such pH under heat and for a time sufficient to effect interfiber bonding among fibers of the mat by oxidative bonding reaction, said oxidizing agent being selected from the class consisting of per compounds, nitrates and chlorates, said fluid carrier having an acid pH where the oxidant is a per compound.

18. The method of Claim 17 wherein said carrier liquid contains a catalyst to promote said oxidative bonding reaction.

19. Flexible paper obtained by the method of Claim 6.

20. Liner board obtained by the method of Claim 7.

21. Fiberboard obtained by the method of Claim 8.