US3547687A - Fire-proofing of lignocellulosic material in two stages - Google Patents

Description

United States Patent 3,547,687 FIRE-PROOFING 0F LIGNOCELLULOSIC MATERIAL IN TWO STAGES Menachem Lewin, Jerusalem, Israel, assignor to The State ff lsfael, Ministry of Trade and Industry, Jerusalem,

srae No Drawing. Continuation-impart of application Ser. No. 551,526, May 20, 1966. This application Jan. 9, 1969, Ser. No. 790,165

Int. Cl. C09k 3/28; 344d 1/44; B27k 3/52 US. Cl. 117-138 9 Claims ABSTRACT OF THE DISCLOSURE Fire-proofing of lignocellulosic material by bromination. The bromination is effected by a two-stage procedure in the first of which the substratum is impregnated with at least one bromide or bromate or both, and in the second stage some further reactants are added so that the substratum is in contact with at least one bromide and a least one bromate, and, when necessary, the physical conditions, such as temperature or pressure, are so changed that a reaction delivering free bromine and consequently the bromination of the lignin is started. Due to the two-stage procedure the reactants penetrate into the depth of the substratum and the bound bromine is distributed throughout the brominated material.

This application is a continuation-in-part of co-pending application Ser. No. 551,526, filed May 20, 1966, now abandoned.

The present invention relates to the flame-proofing of lignocellulosic pulps and articles with an object of improving their resistance to flame initiation and flame propagation. The expression lignocellulosic pulp will be understood as meaning a lignin containing cellulosic pulp suitable for the manufacture of paper, paper board, insulation board, hardboard and related products. The expression lignocellulosic articles will be understood to mean lignin containing cellulosic bodies or structures such as wooden beams, veneers or rods; paper sheets, rolls of newsprint; straw board or chip board; yarns, woven or knitted fabrics, ropes or belts made of lignocellulosic fibres such as for example jute, kenaf, sisal, hemp or the like.

Wood is composed of three major ingredients, namely cellulose, hemicellulose and lignin. The first two are long chain carbohydrates 'whilst the lignin is a compound containing condensed phenyl-prop-ane units. Most other plant materials contain these three major ingredients although the relative amounts vary widely. Although in some cases, for example in the case of cotton, a cellulosic material manufactured from the plant is substantially lignin-free, there are many other products in which at least some lignin is retained. This applies in particular to groundwood which serves as the main ingredient in the manufacture of newsprint. Such groundwood flour may contain up to about 27% of lignin depending on the species of wood from which it has been prepared and also depending on the bleaching process employed. Semichemical pulps serving for the production of cardboard and of corrugated sheets for the manufacture of cardboard boxes contain varying amounts of lignin. Jute and sisal fibres contain substantial amounts, so do also other bast and leaf fibres.

The lignin and hemicellulose form the so-called middle lamella which is found between the fibrous cells in wood and many other plant tissues; lignin also occurs in lignocellulosic fibres such as those of jute, hemp and sisal. The middle lamella surrounds the fibrous cells, which themselves are composed mostly of cellulose, and ce- 3,547,687 Patented Dec. 15, 1970 ments the fibres together, thereby serving to maintain the structure of the fibre bundle.

From Israel Pat. No. 15481 and corresponding patents in other countries, e.g. US. Pat. 3,150,919, it is known to flameproof lignocellulosic pulp or articles by halogenation. The halogenation treatment affects the lignin component of the lignocellulosic pulp or article which becomes halogenated, and. it is this halogenated lignin which has an improved degree of fire resistance and imparts to the entire pulp or article a high degree of flame resistance, stable against prolonged leaching in water. According to this method, different concentration levels of the halogenating agent may be used to conform 'with extent and intensity of the fire exposure that the particular lignocellulosic article will experience and the amount of fire resistance desired.

According to Israel Pat. 15481 a lignocellulosic article or pulp is halogenated in what amounts to basically a continuous one-step operation. By one method according to said patent the material is brominated with elementary bromine or with an aqueous solution of elementary bromine or an aqueous solution. capable of producing elementary bromine in situ, for example, a bromidebromate solution that is gradually acidulated or a bromide solution into which elementary chlorine is gradually introduced whereby elementary blromine is produced in situ.

It has now been observed that when the bromination with such a bromide-bromate solution is conducted as a continuous one-step operation, the distribution of the bound bromine through the cross-section of the treated pulp or article is uneven, the degree of bromination of the lignin decreasing towards the .interior of the wood or the pulp. There is thus obtained a concentration gradient of bound :bromine. This gradient may in some cases be very steep and amount to the formation of a relatively thin skin with a relatively high content of brominated lignin and a core with a low content of brominated lignin, if any. This observation is explained by the fact that a relatively large proportion of the lignin near the surface of the article or pulp will react and consume the greater part of the available bromine made available by the gradual acidulation of the bromide-bromate mixture and only very little bromine will penetrate to the inner part of the article or pulp.

This poses a difiicult problem in the fireproofing of lignocellulosic material by bromination directed to substantially uniform distribution of the brominated lignin in a bulky lignocellulosic product, whether in pulp form or in solid wood form such as in boards, timber, veneer or the like. Because of the above mentioned concentration gradient of the brominated lignin one is faced with two possibilities, namely either to brominate only a small outer layer and leave the core of the wood or pulp with a low halogen content inefiicient for flameproofing, or alternatively to conduct the bromination intensively so that the core of the wood or pulp is brominated to the desired degree which means that the outer portions thereof will be excessively brominated.

It is the object of the present invention to provide an improved process for the bromination of lignocellulosic articles and pulp with the object of imparting to them an improved degree of fire resistance, by which process a better and more even distribution of the brominated lignin throughout the cross-section of the article or pulp is obtained.

It is a further object of this invention to brominate the lignocellulose articles utilizing a bromination composition that is substantially inactive and innocuous until reacted or activated in such a manner that a substantial part of its bromine content is liberated and thereby made available for bromination.

The invention consists in a process for the bromination of lignocellulose containing materials with the object of improving the resistance of said material to flame initiation, flame propagation and biological decay, wherein said material is first impregnated with a reactant or reaction mixture capable of participating in a lignin bromination reaction (impregnation stage), the impregnated material is then developed under conditions which cause the bromination of the lignin, (development stage), characterized in that at the development stage there are present at least one bromide selected from the group consisting of hydrobromic acid, ammonium bromide, alkali metal bromides and alkali earth metal bromides, and at least one bromate selected from the group consisting of bromic acid, ammonium bromate, alkali metal bromates and alkali earth metal bromates.

By conducting the bromination of the lignocellulosic material in two stages in the manner specified a better and more even distribution of the brominated lignin is ensured. This so because during the impregnation stage the reaction mixture penetrates into the inner interstices of the treated material and thus becomes available throughout the material. Consequently, during the development stage, bromination of the lignin occurs substantially throughout the material.

It is known that by impregnating lignocellulosic material with a bromide, the material is only partly flame proofed. However, such a salt is leachable, whereby the flameproofing efi'ect is lost. Such a disadvantage does not occur in lignocellulosic material flame proofed in accordance with the invention since the bromine is chemically bound and can therefore not be leached out.

In addition to flameproofing the lignocellulosic material, the treatment according to the invention also imparts to the material a considerable resistance against biological decay caused by fungi. This is a further great advantage of the invention.

Preferably, any hydrobromic acid and/or other acid remaining on the substratum after the development should be neutralized before the brominated lignocellulosic material is processed any further.

In accordance with one embodiment of the invention the reactant applied during the impregnation stage compries at least one bromide and the development is brought about by contacting the impregnated substratum with a bromate. An acid may be added at either stage in order to be available during the development for the formation of bromine.

In accordance with another embodiment of the invention, said reactant applied during the impregnation stage comprises at least one bromate and during the development stage the impregnated substratum is contacted with a bromide, e.g. an aqueous bromide solution. Again, an acid may be added at either stage in order to be available during the development for bromine l formation.

In yet another embodiment of the invention the reaction mixture used during the impregnation stage con tains both at least one bromide and at least one bromate, the composition of the reaction mixture being such that no appreciable conversion of bromide into bromine occurs, and an acid is used for the development. For instance, it is possible to impregnate with an aqueous bromide-bromate solution and to develop with sulfuric acid, hydrochloric acid, perchloric acid or any other suitable acid, thereby to release the formation of free bromine from the bromide-bromate mixture. Thus, here again the bromination of the lignin will occur only in the development stage in an essentially evenly distributed manner.

In still another embodiment of the invention the reaction mixture used during the impregnation stage comprises a bromide-bromate mixture, the impregnation conditions being so selected that only a part of the elementary bromine needed for the treatment, if at all, is

liberated during the impregnation and during the development stage the physical conditions are so changed as to accelerate the reaction of the reactants with each other to liberate elementary bromine. The change of physical conditions which, in accordance with this embodiment, are capable of starting the development, are a raise of temperature, the application of pressure or any combination thereof.

Thus, in accordance with this embodiment the reaction mixture used for the impregnation will include a bromide, a bromate and a compound capable to release an acid and thereby to lower the pH of the reaction mixture. The acid releasing compound may be a Lewis acid such as, for example, ammonium chloride or zinc chloride. If a lignocellulosic article or pulp is impregnated with such a mixture at ambient temperature, the impregnation is not accompanied by any discernible chemical reaction, and the mixture penetrates as it is into the material. When thereafter the material is heated, possibly by steaming, the pH of the reaction mixture drops to about 2-3 whereupon the bromide and bromate start reacting with each other and the lignin is brominated. When the bromination is completed the pH is normally found to be between 3.5 to 6.0 which is as a rule satisfactory and safe for solid wood products.

Also in accordance with the last-mentioned embodiment it is possible to incorporate a proton acid in the reaction mixture used for the impregnation stage. In this case the pH of the reaction mixture applied during the impregnation stage is substantially that required for the development, the composition of the reaction mixture being such that under the physical conditions applied during the impregnation stage the lignin bromination reaction is slow as compared to the rate of penetration of the reaction mixture into the lignocellulosic substratum. When proceeding in this manner it is possible in some cases merely to leave the reaction mixture without change of physical conditions thereby to allow the bromination to proceed. In other cases it may, however, be preferable to somewhat change the physical conditions, e.g. by heating and/ or by the application of pressure in order to accelerate in this manner the halogenation reaction.

If desired, the even distribution of the reactants employed in the impregnation stage throughout the lignocellulose structure or pulp may be improved by conducting the impregnation under reduced pressure and conducting the development under atmospheric or superatmospheric pressure.

Further, when a second reactant or reaction mixture is applied during the development stage, it is possible, if desired, to put the impregnated lignocellulosic material resulting from the impregnation stage under reduced pressure which reduced pressure is then released by the introduction of said second reactant or reaction mixture.

The proportion of bromine used for the bromination of the lignocellulosic material depends on various factors such as, for example, the nature of the material to be fiameproofed and the required degree of flame resistance. For a given degree of bromination, the time required to efiect bromination will in general be less where the lignocellulose to be treated is finely comminute. This applies, for example, in the case of ground wood or wood pulp and in other cases where a large specific surface is available, such as for example in the case of yarns and rope, as contrasted with chips for the manufacture of chipboard or with wooden veneers or structural wood.

It is known that a bromide ion in the form of bromide salts, can be oxidized to form bromine. However, upon brominating an organic material by a replacement reaction, half of a mole of bromine reappears as bromide ion, namely as hydrogen bromide. In situ reoxidation of the newly formed bromide ion, whenever it is formed, is a key to a high bromine utilization and the more so, if the oxidizing agent also contains bromine, as is the case with bromate, which then becomes available for bromination as well, thus avoiding the formation of undesirable residues from the reduction of the oxidizing agent. A typical oxidation reaction system of the kind mentioned, is represented by the following equations:

when R is a lignin residue. Equation 1 requires 1 mole of bromate for moles of bromide to convert all of the available bromine to elementary bromine for bromination. As evident from Equation 2 only of the bromine is utilized in the bromination reaction. Addition of Equation 1 to Equation 2 yields the following equation:

where R is again a lignin residue. It will be immediately noted that the molar relationship of lBrO for every 2Br" required in the presence of a brominatable lignin material RH, for bromination by a replacement reaction, is greater than the stiochiometric ratio of Bro for every 5Br shown in Equation 1. It will further be noted that the oxidation of Br by BIO3 requires acidification to take place, whatever the ratio.

Although a Br":BrO ratio of 2:1 can theoretically give 100% bromine utilization, in practice it has to be taken into consideration that natural lignocellulosic materials can also be oxidized by bromine. While it has been found that the cellulose contained in the these materials is virtually not oxidized under the acidic conditions of the bromination, it is known that a slight oxidation of the lignin does occurs. Furthermore, the natural lignocellulosic materials contain varying amounts of oxidizable components, such as reins, called in the trade extraneous materials. The oxidation reactions proceed according to the following equation:

(4 Br +2RH+2H O=2R0H+2H++2Brand in the case of the bromate-bromide system: 5 BrOy-l-5Br-+6H++3RH=3ROH+6BF+6H+ R in both the above equations being again a lignin residue. Thus bromate is being consumed in the oxidation reactions. Consequently, when carrying out brominations of lignocellulosic materials, it is preferred to use a ratio of BITZBI'O3T smaller than 2:1 or in other words, the proportion of Ero in the mixture has to be increased.

Consequently, in accordance with a preferred embodiment of the invention the amounts of bromide and bromate are so proportioned that during the development the bromide-bromate molar ratio is within the range of from 2:1 to 1:2. A bromide-boromate molar ratio found to be useful in the examples of this specification is 1:1, which is equivalent in the case of sodium ions to 40 weight parts of sodium bromide and weight parts of sodium bromate, but this ratio may be moderately lowered or raised in dependence on the nature of the lignocellulose which determines the bromate consumption in consequence of Z 1 side reactions.

The molar ratio of BI /BlO of 1:1 is shown in equation 6 as follows:

where R is again a lignin residue.

It is thus a further object of this invention to provide a salt mixture capable of producing a bromination reaction in the two-stage bromination process according to the invention and comprising a bromide and a bromate in a molar ratio of bromidezbromate within the range of from 2:1 to 1:2. Preferred mixtures are these in which the molar BrmBrO; ratio is from 1:1 to 1: 1.5.

The above compositions may be provided in dry form or in the form of solutions.

A further basic feature of this invention is concerned with the pH of the brominated lignocellulosic article. If the bromination is carried out in two stages according to the above description, the pH of the Water extract of the wood is low, i.e. in the range of 2.0-2.5 depending on the extent of the bromination. This low pH may be harmful to the wood and may cause degradation and weakening upon storage. The brominated article can be neutralized after the bromination with lime or with borates or any other suitable alkaline agent. This constitutes an additional step. It is, however, in many cases, particularly in the case of not comminuted lignocellulosic articles, not desirable to introduce an additional step which prolongs considerably the bromination procedure. The present invention provides a new way to effect the neutralization after the bromination.

According to the above Equations 3 and 6 in order to effect the bromination in the desirable manner, it is necessary to use one equivalent of a proton acid for each equivalent of bromine, irrespective of whether it originates from the bromide or from the bromate. In accordance with one embodiment of the present invention an amount of proton acid smaller than the equivalent amount needed according to Equations 3 and 6 is used, within the range of 3070% of the equivalent amount. To provide the balance acidity a so-called latent acid, i.e. an acid capable of bringing about the formation of protons during the development, is added, e.g. aluminium sulfate. During the second development stage of the bromination the proton acid is exhausted first, whereupon the bromination is continued with the aid of the acidity developed by the latent acid. When the bromination is completed no free acid is present in the substratum, and the pH of the water extract of the brominated article is found to be in the range of 3.56.0, depending on the amount of the latent acid salt. The latent acid thus acts as a buffer built-in in the bromination mixture. This action is characteristic of the development stage in which the main bromination and buffering or neutralization is effected. The latent acid may be added together with the acid in the first, impregnation stage along with the bromide-bromate mixture.

The invention is illustrated by the following examples without being limited thereto.

EXAMPLE 1 Samples of wood having the dimensions of one inch square by 6 inches long are placed in a pressure vessel and the vessel is evacuated to a pressure in the range of 0.1 to 500 mm. of mercury and this pressure is held for pcriods of at least 1 minute. To improve the removal of the moisture content of the wood, the vacuum may be released and reapplied at intervals for several times. The vacuum is then released by the introduction of the respective solutions of treatment reagents listed in column 1 of Table I given hereinafter. Owing to the reduced pressure the treating liquor is drawn into the inner interstices of the wood. To further intensify the impregnation, pressure is applied after completion of the first stage reagent to a level consistent with the rating of the pressure vessels. Applied pressures of 02-20 kg/cm. are found to be useful. After completion of the impregnation the pressure is released, the excess liquor is drawn off and, if desired, the wood is subjected to a drying operation, e.g. by several cycles of evacuation and vacuum release.

In the second stage, the previously treated specimens are again put under vacuum in the same manner as in the first stage and any of the various chemical agents listed in the second column of Table I is introduced into the evacuated vessel. Pressure is applied to further diffuse the second stage liquor into the inner interstices of the wood. When the impregnation is completed the pressure is released and the samples are dried and are ready for testing. Where the oxidation produces an acidic byproduct, an ad ditional evacuation and release with an alkali for neutralization is recommended. The neutralization can be done with borax, boric acid or mixture thereof, sodium or ammonium phosphate, sodium carbonate, calcium hydroxide etc.

The treated samples are inserted in flames and are found to have increased resistance to burning.

7 The reagents used in each stage in accordance with the present example and the results obtained are indicated in the following Table I:

The same treatments apply to lignocellulosic pulp. The vacuum and pressure treatments may in case of a pulp not be necessary. The final pulp is washed and neutralized with lime slurry or other alternative material additions and then is formed into sheets or boards by pressure pressing. These specimens are also found to be flame resistant in the test of insertion into the flame.

For example, groundwood pulp containing after treatment in accordance with the invention from 7 to 18% by weight of bound bromine Will produce a paper which does not burn when ignited with a match.

Control specimens are prepared by releasing the first applied vacuum with a solution capable of causing bromination in one stage and containing the required amount of brominating agent. These specimens are found to have increased flame resistance as well. However, when one millimetre layers are cut from the specimens beginning at the outside and proceeding inwardly it is found that the distribution of bromine is much higher at the surface and quite low at a position several millimetres in from the surface. Where the flameproofing has been accomplished by the method of this invention the distribution of bromine in the specimen is more uniform across an equivalent cross-section.

EXAMPLE 2 3 mm. thick veneers of Ponderosa Pine weighing each 140 g. were heated to dryness at 105110 C. Thereafter the veneers were impregnated at 20 C. for 30 seconds with an aqueous solution containing 141.4 g./l. of NaBrO 96 g./l. of NaBr and g./l. of Ultravon JU (trade name for wetting agent produced by Ciba Ltd.). The veneers absorbed 60% by weight of the solution calculated on the dry weight of the wood.

After impregnation the veneers Were dried at 105 C. for 30 minutes and then dipped in a 2 N H 80 solution for one minute. Thereafter the soaked veneers were immersed in a 5% aqueous solution of sodium borate. Subsequently, the veneers were dried and tested for fire resistance. The test according to British Standard 47653 showed a rating of Class 1 spread of flame.

mm. thick plywood panels were produced using as the two external plies veneers treated as described above and as the inner ply a 4 mm. thick untreated veneer. The plywood panel was produced in the conventional way by gluing with adhesive and hot pressing in a plywood press.

The resulting panels had the same flame-spread rating as the individual panels.

EXAMPLE 3 3 kg. of Okoumchips were sprayed with 2.4 kg. of an aqueous solution containing 114 grams of ammonium bromide, 168 grams of sodium bromate, 70 grams of anhydrous aluminium sulfate (Al (SO and grams of sodium chloride. The solution had a pH of 3.6. After sufficient soaking the chips were steamed for 30 minutes in a closed steaming vessel at 100 C. and then dried up to 23% moisture content at the same temperature. The pH of the water extract of the chips was 4.4. With this the fire-proofing treatment according to the invention was completed.

8 For producing a finished board the chips were then sprayed with 480 ml. of a commercial urea-formaldehyde solution containing 50% by weight of solids. A mat was then formed and pressed in a press at C. for 8 minutes, at a pressure of 19 kgs./cm. The particle board obtained in this manner had a specific gravity of 0.65 g./cm. The bending strength was kg./cm. which was similar to the bending strength of a particle board prepared in exactly the same way from the same chips without the fireproofing pretreatment.

The inflammability test according to the British Standard 47653 showed on the samples of this example a rating of Class 1 spread of flame.

EXAMPLE 4 3 kgs. of pine wood slabs having a specific gravity of 0.4 g./cm. and dimensions of 15 x 30 x 3 cm. were placed in an autoclave of 10.5 liters capacity. The autoclave was evacuated to a pressure of 0.1 atmospheres and thereafter filled with a solution containing 240 grams of NaBr, 340 grams of NaBrO grams of H 80 120 grams of Al (SO and 45 grams of Na Cr O under a pressure of 8 atmospheres. After 10 minutes of pressure application the excess liquor was rapidly drawn off. This completed the first, impregnation stage during which a part of the reaction only took place. The second, development stage comprised introduction of steam at a temperature of 115 C. into the autoclave. The steam was held for 30 minutes. Thereafter the steam was released and the autoclave was filled with a solution of 2 g./l. of Na S O for 15 minutes. The excess thiosulfate solution was then drawn off and the wood slabs taken out and dried.

Samples of the treated wood were tested for their inflammability by the British Standard 476-53 showing a Class I spread of flame. The pH of the water extract of the wood was 4.0.

Samples of the treated wood were tested for resistance to fungi according to the ASTM standards: D-1413-56T and D2017-62T using as test fungus a sub-culture of Poria monticola Murr. While the unbrominated control samples were completely covered with fungus after one week, the brominated samples remained completely unaffected after six months.

EXAMPLE 5 3 kgs. of pine wood slabs similar to those of Example 4 were treated in exactly the same way as in Example 4 but in the present case no steam was introduced into the autoclave during the second stage. Instead of steaming the wood slabs were left in the autoclave with the liquor for 3.5 hours and then further processed as in Example 4. The testing of the treated samples gave results similar as in Example 4.

I claim:

1. A process for improving the resistance of lignocellulosic containing material to flame initiation, flame propagation and biological decay by substantially uniformly brominating the material throughout, said process including a first impregnation stage consisting of the step of impregnating said material into the depths of the substratum thereof with a liquid solution composed of a water carrier containing at least one first impregnating compound selected from the group consisting of hydrobromic acid, ammonium bromide, alkali metal bromides, alkali earth metal bromides, bromic acid, and at least one second impregnating compound selected from the group consisting of ammonium bromate, alkali metal bromates and alkali earth metal bromates, the molar ratio between said first and second impregnating compounds being within the range of from 2:1 to 1:2, followed by the step of withdrawing the water carrier containing excess impregnating compounds, and a subsequent development stage consisting of the step of activating the impregnating compounds within the depths of the substratum of said material left by the impregnating stage to cause a reaction of said bromide with said bromate to form free bromine in situ which substantially uniformly brominates the said material throughout.

2. A process according to claim 1, wherein the molar ratio between said first and second impregnating compounds is within the range of from 1:1 to 1:15.

3. A process according to claim 1, wherein any acid remaining on the brominated lignocellulosic material after the development stage is neutralized.

4. A process according to claim 1, wherein the development stage comprises treating the lignocellulosic material with an acid.

5. A process according to claim 1, wherein said liquid solution contains a mixture of a proton acid and a latent acid substance capable of causing proton formation, in such relative proportions that at the termination of the development stage a water extract of the brominated material has a pH of at least 3.5.

6. A process according to claim 5, wherein the latent acid substance is aluminum sulfate.

7. A process according to claim 1, wherein during the development stage the lignocellulosic material is subjected to superatmospheric pressure.

8. A process according to claim 1, wherein during the development stage the lignocellulosic material is subjected to steaming.

9. A process according to claim 1, wherein the impregnation stage is carried out under subatmospheric pressure.

References Cited UNITED STATES PATENTS 2,355,410 8/1944 Bergel 260694X 2,861,901 11/1958 Reeves et a1 117136 3,150,919 9/1964 Lewin 117-138X 3,297,675 1/1967 Fuhrman et a1. 260694X H. J. GWINNELL, Assistant Examiner US. Cl. X.R.