US3676169A - Fire retarding process comprising converging separate solutions of polyvinyl alcohol and alkalimetal borate - Google Patents

Abstract

AN AQUEOUS SOLUTION OF POLYVINYL ALCOHOL AND AN AQUEOUS SOLUTION AN ALKALI METAL BORATE, WHEN COMBINED, FORM A DILATANT FLUID HAVING EXCELLENT PROPERTIES AS A FIRE RETARDANT AND FIRE SUPPRESSANT.

Description

3,676,169 Patented July 11, 1972 US. Cl. 117-3 7 (Ilaims ABSTRACT OF THE DISCLOSURE An aqueous solution of polyvinyl alcohol and an aqueous solution of an alkali metal borate, when combined, form a dilatant fluid having excellent properties as a fire retardant and fire suppressant.

BACKGROUND OF THE INVENTION Forest, brush, and grass-range fires cause enormous damage each year. For example, in the decade 1948-1957, over 100 million acres of forest land were destroyed by fire. Not only is valuable timber destroyed, but frequently houses, business structures and lives are also lost. The threat of such fires is ever present in many areas of the country, particularly during the drier months of the year. Such fires will hereinafter be referred to as forest fires, although it will be understood that the burning vegetation may comprise trees, leaves, or other dead vegetation, shrubs, bushes, grass, farm crops, or any combination thereof. Likewise, buildings having a substantial amount of surrounding vegetation frequently are largely composed of combustible matter and are thereby destroyed when such vegetation catches fire.

The important common denominator of all such outdoor vegetation fires is that once ignited, the fire spreads from its source through adjacent vegetation and any other combustible matter until it is extinguished or burns up all such surrounding combustible matter. Such fires ordinarily spread by successive ignition of unburned vegetation adjacent to the burning area. This vegetation is brought to ignition temperature by heat radiated and convected from the flame front.

Generally, vegetation must be very close to or even enveloped by flame or superheated by gases in a convection column before igniting. This mechanism results in a more or less continuous advancement of such a fire. Fire spread may also be caused by spotting, that is, ignition by burning brands carried outside the fire area by wind and turbulence. Nearby buildings are very frequently ignited by burning brands falling on the roofs thereof.

One of the most common methods of controlling and suppressing a forest fire is by creating a fuel-less barrier. This may be done, for example, by digging a trench through surface vegetation down to mineral soil ahead of an approaching fire of sufficient width to prevent the fire from crossing. However, this may require a trench of considerable width, i.e., approximately the square of the height of the flames. In some instances, it is desirable to burn off an area containing noxious vegetation, e.g., poison ivy, while at the same time insuring that the fire does not spread to the surrounding vegetation. Under these circumstances, digging of a trench may not be practicable.

Since the early l950s, use has been made of chemicals to prepare a nonflammable fire-line either in preventing the spread of forest fires or in burn-off operations. An aqueous slurry or solution is sprayed in a line upon surface vegetation to provide a noncombustible line in the same manner as a trench. More commonly, in the case of forest fires, the aqueous slurry is dropped or sprayed in bulk from a low-flying airplane in a line just ahead of the advancing flame front. The fire, on reaching the slurry-coated or solution-sprayed vegetation, either stops completely or is sufliciently reduced in intensity to enable ground crews to Work safely and effectively with conventional firefighting equipment, for example, Water hoses.

Water alone, although exceedingly effective for a brief period at rendering vegetation noncombustible, is not really usable as a fire retardant since it is rapidly lost after application to the vegetation by evaporation and run-01f. Desirably, the retardant will adhere to the vegetation to which it is applied and be comparatively nonvolatile and resistant to absorption into the soil. These characteristics collectively determine the effectiveness of the retardant. Other characteristics desirable in a fire retardant include ease of aerial application and from tank trucks, low metal corrositivity and abrasivity, minimal toxicity, low cost, ease of mixing and pumping, and good storage stability. Currently used fire retardants, which generally comprise water as a major constituent, plus one or more thickening agents, have deficiencies in one or more of the above-indicated characteristics. For example, many fire-retardant formulations contain pectin or alg'in, both of which deteriorate in storage. Other retardant formulations contain high concentrations of sodium calcium borate, which is toxic to vegetation. Particularly desirable in a fire retardant is the ability to render vegetation fire resistant for a more or less prolonged period of time. This requires in general not only that the retardant have good adherence to the vegetation, but also that the retardant be resistant to wash-off by rain and evaporative water loss.

My copending application Ser. 'No. 740,452, filed June 27, 1968, now Pat. No. 3,537,873, teaches that aqueous solutions containing from 0.5 to 5.0 weight percent water-soluble vinyl alcohol polymer, and from 0.5 to 5.0 weight percent alkali metal borate are effective fire retardants for combating fires, especially forest, brush, and range fires.

This previously filed application teaches that the application of such aqueous solution to outdoor vegetation renders such vegetation noncombustible or at least significantly reduces its combustibility. Said previously filed application disclosed and claimed the application of the above described aqueous solutions to living or dead vegetation to reduce the combustibility of said vegetation. Such aqueous solutions were indicated to be particularly advantageously applied to vegetation at or immediately ahead of a flame front.

However, my invention as described in said previously filed application did not provide a complete solution to the problem of fighting fires. It did not address itself to the problem of extinguishing or suppressing already started fires in urban areas, i.e., fires in which burning buildings or quantities of merchandise, e.g., lumber, furniture, cloth, or dried foodstuffs, or a combination thereof, are the combustible matter.

As is of course Well known, the standard method of extinguishing urban fires is to spray water, usually from hoses, onto the burning buildings, etc. Water is certainly an excellent extinguisher, but it does have certain shortcomings, the most obvious of which is that, being a free flowing liquid, it runs off vertical surfaces. To render a vertical wall sufficiently wet to be either noncombustible or at least difiiculty combustible, comparatively huge quantities of water must be applied to it. Additionally, the wetness which is imparted will tend to dissipate rapidly by evaporation and drain off. It obviously would be very useful if water could be modified in some fashion so as to stick to vertical surfaces and thereafter undergo comparatively limited loss by evaporation.

As described in my previously filed copending application, aqueous solutions containing (a) alkali metal borate and (b) a water soluble vinyl alcohol polymer, possess many desirable properties from the standpoint of fire-retardant applications, e.g., they are sticky and adhere readily to smooth or irregular surfaces. After application such solutions form a comparatively impermeable surface skin that results in significantly reduced loss of the water constituent of the solution by evaporation or drain off.

As heretofore indicated, my previously filed application Ser. No. 740,452 teaches the use of such solutions as fire retardants for vegetation.

I have now found that such solutions can also be advantageously applied as fire retardants or suppressants to buildings, furniture, stored lumber, and other nonvegetation combustibles. Although the terms fire retardant and fire suppressant have no clearly defined meaning and are frequently used interchangeably, as used in the instant application, the term fire retardant means a substance which renders flammable but non-ignited substrates either noncombustible or less combustible. A fire suppressan is a substance which serves to extinguish already burning or glowing flammable substrates by reducing or eliminating the combustibility of the unburned portions of such substrates.

In addition to vertifying that such aqueous solutions can advantageously be applied to virtually all types of combustibles, I have discovered a vastly superior procedure for applying my aqueous borate/polyvinyl alcohol solutions to all types of combustible substrates for fire retardation or fire suppressant purposes. In my previous application, Ser. No. 740,452, I described the preparation of an aqueous borate/polyvinyl alcohol dilatant solution either by separately adding borate and polyvinyl alcohol (PVA) to an agitated and preferably heated aqueous menstruum, or by combining separate aqueous PVA and aqueous borate solutions. However, in both cases, an aqueous PVA/borate solution was obtained and then applied to the combustible substrate. Since the aqueous PVA/borate solutions possessed the property of dilatancy, i.e., an increase in viscosity when subjected to shear handling of the solution and in particular, spraying or otherwise applying it to the combustible substrate, posed severe mechanical problems.

SUMMARY OF THE INVENTION It is an object of this invention to provide a superior process for fire retardation and fire suppression. It is a further object of this invention to provide an improved technique for applying substantial quantities of water to a combustible substrate in such a form that said water will be resistant to run-off and evaporative loss.

Further objects and advantages will become apparent from the description of the invention which follows in greater detail.

It has now been found in accordance with the instant invention that if separate aqueous solutions of alkali metal borate and PVA, respectively, are brought together in the form of converging streams of the respective solutions, a viscous dilatant solution is instantly formed which solution possesses excellent fire retardant and fire suppressant properties. The great advantage of my invention lies in the fact that the mechanical problems entailed in handling a dilatant solution are completely avoided, since such solution is formed exterior to all mechanical dispensing equipment and only immediately prior to the solutions application to, or indeed even at the point of first contact with the surface of the combustible substrate being treated. In the practice of my invention, the separate aqueous solutions of alkali metal borate and PVA would be separately stored, ordinarily in tanks or similar containers, although for certain large scale operations, other type containers can also be utilized.

When it is desired to utilize the dilatant composition for treating vegetation or other flammable substrates to render the same noncombustible, converging streams of the two solutions are sprayed at the substrate. The two streams, on converging, essentially instantly form the viscous dilatant solution having the aforementioned fire suppressant and fire retardant properties. The convergence of the two streams can be effected at any point from the point at which the two streams exit from the dispensing apparatus up to the point at which the streams impinge on the substrate since, as above indicated, the dilatant solution forms almost instantaneously. Preferably, the streams will be converged to form the dilatant solution as soon as possible after leaving the dispensing apparatus. This is most readily achieved by utilizing two immediately adjacent nozzles which are so shaped and positioned that the streams issuing from the respective nozzles converge essentially instantly after such emergence. Alternatively, the two nozzles can be arranged in concentric fashion with an outer nozzle essentially coaxially concentrically disposed around an inner nozzle with the exit apertures of both nozzles likewise being essentially concentric and coterminous. The exact fashion in which convergence of the streams is achieved is not critical since it is only necessary that convergence take place at some point between or even on the combustible substrate, as heretofore indicated. The advantage of utilizing a dilatant solution which is formed only immediately prior to its application to the substrate being treated is that a dilatant solution is not handled by mechanical equipment, that is, the convergence of the two streams is exterior to the dispensing mechanism.

The dilatant solution of the instant invention contain from about 0.5 to 6.0 weight percent PVA and 0.5 to 6.0 weight percent alkali metal borate. Since such dilatant solution is obtained by the combination of two separate solutions as flowing streams, possibly at differing flow rates, the operable concentration of PVA and borate in the respective separate solutions prior to combination cannot be given with exactitude. Assuming equal volumes of the two solutions are being combined, the concentrations of the PVA and borate in the respective separate solutions could vary from 1.0 to 12.0 weight percent, which on combination would provide the final desired concentration of 0.5 to 6.0 weight percent. It is, of course, not necessary that the concentration of PVA and borate in the combined stream be identical since dilatant solutions are obtained as long as both components are each present in the combined stream in a concentration ranging from 0.5 to 6.0 weight percent. The preferred concentration of alkali metal borate and vinyl alcohol polymer in the fire retardant dilatant solutions of this invention will range from about 2:0 to 6.0% by weight of each.

The term alkali metal borate. as used in the instant specification and in the appended claims, embraces not only the alkali metal salts of the common boric acids,

i.e., tetraboric acid, H B O meta-boric acid, HBO and orthoboric acid, H BO but also the other boric acids Such as H2B204, HZBGO'H), H2B12019, 116E409, and H B O With the exception of the metaand orthoborate salts, su'ch alkali metal borates have the general formula: M O'mB O wherein M denotes an alkali metal and m can range from 1 to 4. Hydrates of the above enumerated borate salts are also suitable.

Either a single borate salt or any mixture thereof can be used. Ordinarily, no advantage accrues from the use of a mixture of borates. Likewise, although alkali metals other than sodium and potassium are perfectly usable, they are not preferred for economic reasons.

The preferred borate is borax, i.e., sodium tetraborate decahydrate.

The term water-soluble vinyl alcohol polymer as used herein and in the appended claims embraces vinyl alcohol polymers having up to 50% of the hydroxyl groups theeof replaced by methoxy, ethoxy, acetyl, propionyl, or butyryl radicals, i.e., partially etherified or esteri-fied polyvinyl alcohol. Said methoxy and ethoxy ether radicals can be unsubstituted or substituted with hydroxyl or carboxyl groups. Said acetyl, propionyl and butyryl radicals can likewise be unsubstituted or can be halogenor hydroxyl-substituted. Preferably, no more than about 20% of the polyvinyl alcohol hydroxyl groups will be replaced by any of the aforementioned ether or ester radicals. The term water-soluble means that the polymer is soluble to the extent of at least 5.0 weight percent in water at room temperature, although heating the water to a higher temperature of up to 95 C. may be necessary in some instances to initially dissolve the polymer.

The vinyl alcohol polymers utilized in the practice of the instant invention can, therefore, be represented by the structural formula -CH2CH L at wherein x can range from about 1200 to about 5000, preferably 1600 to 3000, and wherein R represents hydrogen-, methyl-: ethyl-, acetyl-, propionyl-, butyryl-, hydroxylor carboxyl-substituted methyl or ethyl, or halogenor hydroxyl-substituted acetyl, propional or butyryl, and wherein at least 50% of said R groups are hydrogen. The Water-soluble vinyl alcohol polymers of the instant invention can have molecular weights ranging from about 50,000 to about 450,000, and preferably from about 70,000 to 200,000. As above indicated, preferably at least 80% of said R groups will be hydrogen.

Vinyl alcohol polymers are conventionally obtained by polymerizing esters of vinyl alcohol followed, where appropriate, by saponification of the ester groups. To prepare the polymers of the instant invention, wherein up to about 50% of the R groups are acyl, one polymerizes the corresponding vinyl ester to afford the polyvinyl ester having all R groups acyl and then partially saponify said polyvinyl ester and thereby remove 50% or more of the acyl groups. For example, to prepare polyvinyl alcohol containing 20% acetyl groups, polyvinyl acetate would be -80% saponified and the remaining 20% acetyl groups be left unsaponified. To prepare the methoxy and ethoxy ether derivatives of polyvinyl alcohol, a fully saponified material, i.e., polyvinyl alcohol having 98+% hydroxy groups, as would be obtained by total saponification of a polyvinyl ester, is etheri-fied up to the desired degree, that is, up to about a maximum of 50% of the hydroxyl groups can be etherified using conventional etherification agents such as diazomethane, dimethyl sulfate or diethyl sulfate.

The separate solutions of PVA and borate are readily prepared by simply adding PVA or borate to the correct amount of water to provide the desired concentration. Dissolution is facilitated if the addition is accompanied by agitation and heating of the water.

The pumping or other solution transfer equipment utilized to spray the converging streams is a matter of choice and any of the conventional equipment which is currently available for spraying essentially noncorrosive, aqueous solutions of wide viscosity range can be utilized.

The rate of application of the dilatant solution will obviously vary with the substrate being treated. Ordi narily, from about /2 to 10 gallons per square feet of substrate is a suitable application rate. Where the substrate is vegetation, obviously, the thicker the vegetation, the greater the amount of solution which must be applied to reduce the combustibility of said vegetation.

A particular advantage which accrues from treating combustible substrates with the adherent dilatant compositions of my invention is that if some portions of the substrate surface are missed, heat caused by nearby fire will reduce the viscosity of the adhering dilatant solution and cause it to flow on to adjacent hitherto untreated portions of the substrate.

The invention can be more fully understood by reference to the following examples. All parts are parts by weight unless otherwise expressly noted.

Example 1 175 grams of 98+% saponified polyvinyl alcohol (molecular weight about 100,000) was added with stirring to 4200 cc. of water heated to 85 C. forming a homogeneous solution. There was added with agitation 175 grams of borax to 4200 cc. of water heated to 85 C. forming a second homogenous solution. The borate and polyvinyl alcohol solutions were both cooled to about 40 C. and separately charged to two 2 /2 gallon fire extinguishers which were then pressurized with nitrogen to psi. The outlet nozzles of the two fire extinguishers were fitted together so that when the PVA and borax solutions emerged from the respective nozzles, the two solutions merged to form a single unitary, substantially homogenoous stream. Rates of flow were adjusted by valves so that equal rates of delivery were obtained.

A flat, rectangular area of bare ground 10' x 20' was covered with a 6-inch deep layer of dead and dried vegetation consisting of leaves, twigs, and branches up to 1 inch in diameter. A 2-foot wide band of this vegetation running between approximately the mid-points of the two opposite long sides of the rectangle, i.e., a 20lsquare foot area of vegetation, was uniformly sprayed by means of the fire extinguishers with about one-fourth of the solution present in each tank. The merged streams emitted from each tank formed a dilatant solution almost instantaneously so that the solution actually deposited on the vegetation was a dilatant solution.

After a wait of 2 hours, an upwind corner of the vegetation was ignited. Wind velocity was approximately 5 knots. All brush contiguous to the point of ignition was eventually consumed by fire up to the treated band. The treated band did not burn nor did the untreated vegetation on the other side of the band from the fire ignite. Two hours after the ignited portion had completely burned, the treated band could still not be ignited with burning newspaper.

Example 2 Simulated natural brush consisting of douglas fir pine needles was used to test a number of other solutions for fire retardance. The test consisted of covering a 2' x 5' rectangular steel plate with a 2 inch thick layer of pine needles. A 4 inch wide band of needles completely across the pine needles at about the mid-point of the 5' sides of the needle layer was sprayed with a variety of dilatant solutions in the amounts shown in the table below. In each case, the dilatant solution was formed by charging and pressuring with nitrogen two extinguishers, one with PVA solution (Solution 1) and the other With alkali metal borate solution (Solution II), and dispensing both said solutions simultaneously in converging streams of equal volume which formed the dilatant solution essentially instantaneously prior to the deposition of the latter on the needles. After standing for 2 hours, a corner of the needle bed was ignited. In all cases, the area of needles contiguous to the point of ignition up to the treated band was completely burned up. In all cases, the treated band did not ignite and the fire did not cross the treated band to the area of needles on the other side thereof.

The following solutions were used. In all cases, sufficient water was used to yield 100 grams of each solution.

Weight percent Approx. grams of each solution Polyvinyl alcohol 1 Borate 2 in (I and II) dein Solution I Solution II posited on band A 1. A 1. 0 75 B 1. 0 B 1. 0 85 C 2. O C 2. 0 85 D 4. 0 D 4. 0 90 E 5. 0 E 6. 0 90 A 2.0 B 2.0 100 A 2. 0 A 2. 0 100 B 2.0 A 2.0 100 B 2.0 C 2.0 100 B 2. 0 B 2. 0 100 C 2. 0 A 2. 0 100 D 2.0 A 2.0 100 E 2. 0 A 2.0 100 A 3.0 A 4.0 100 A 3. 0 A 3 5. (l 100 A 4. 0 A 3 6. 0 100 17 A 12. 0 A 3 12.0 100 1 See the following table:

Molecular weight Polyvinyl alcohol:

A=98+% polyvinyl alcohol (98+% saponified polyvinyl acetate) 85, 000

Referring to the tables, it is thus apparent that in Run 1, for example, a total of 150 cc. of dilatant solution formed by the combination of 75 cc. each of PVA and borate solution, while in Runs 6-17, 200 rc. of dilatant solution was deposited.

Example 3 A special nozzle was fabricated consisting of two coaxially concentric coterminous metal tubes. The inner tube was connected by flexible pressure tubing to a first fire extinguisher tank. The outer concentric tube was connected in similar fashion to a second fire extinguisher tank. Tank I was charged with gallons of 4 weight percent aqueous solution of 100% saponified PVA of 75,000 molecular weight. Tank II was charged with 5 gallons of 4 weight percent borax. Both tanks were pressurized with CO gas and approximately one half the contents of each tank simultaneously sprayed through the special nozzle at equal rates over the entire surface of a x 10 vertical wall of /2" unpainted white pine boards. A dilatant solution was formed as soon as the two solutions emerged from the special nozzle. This solution adhered and formed a viscous coating over the wall surface. Brush was piled at the base of the Wall and ignited minutes after spraying the wall surface. The brush burned completely up, but the wall did not ignite. It was noted that dilatant solution from the upper portions of the wall ran down and replenished any lost from the lower wall surface through the action of the burning brush.

An identical wall was sprayed over its entire surface with 5 gallons of water. After 20 minutes, a similar amount of brush to that placed at the base of the first wall was placed next to this wall and ignited. The second wall caught fire from the brush and eventually burned completely up.

Example 4 Separate 5 gallon tanks were charged with solutions of borax and PVA as in Example 3. However, no special nozzle was utilized and one half the contents of each tank was sprayed through separate nozzles over the entire surface of a test wall of the same type as in Example 3. The two solutions were, however, both sprayed at the same spot on the wall at any one time. When the two solu tions simultaneously struck the same spot on the wall, a dilatant solution was formed, although some of each of the solutions did not combine to form dilatant solution and therefore ran off. After 20 minutes, ignition of brush piled at the base resulted in much charring of the wall although it did not actually ignite.

Various modifications will be apparent to one skilled in the art, and it is not intended that this invention be limited to the details in the specific examples presented by way of illustration. Accordingly, the scope of the invention is limited only by the appended claims.

20 I claim:

1. -'In a process for reducing the combustibility of a flammable substrate comprising applying to the surface of said substrate an aqueous dilatant solution, the improvement comprising forming said solution containing from about 0.5 to 6.0 weight percent alkali metal borate and from about 0.5 to 6.0 weight percent water soluble polyvinyl alcohol by emitting from a dispensing mechanism a first stream of aqueous polyvinyl alcohol solution and a second stream of aqueous alkali metal borate solution and converging said first and second streams, said convergence being achieved exterior to said dispensing mechanism for said streams.

2. A process in accordance with claim 1 wherein said dispensing mechanism comprises adjacent nozzles so disposed that a first stream of aqueous polyvinyl alcohol solution emerging from one of said nozzles converges with a second stream of aqueous alkali metal borate solution emerging from a second of said nozzles, said convergence 4 being between said point of emergence and the surface of said substrate.

3. A process in accordance with claim 1 wherein said dispensing mechanism comprises two substantially co axially concentric coterminous nozzles so disposed that a first stream of aqueous polyvinyl alcohol solution emerging from one of said nozzles converges with a second stream of aqueous alkali metal borate solution emerging from the second of said nozzles, said convergence being substantially instantaneously following said emergence.

4. A process in accordance with claim 1 wherein said alkali metal borate is sodium tetraborate decahydrate.

5. A process in accordance with claim 1 wherein said vinyl alcohol polymer contains at least about 80% unsubstituted hydroxyl groups.

6. A process in accordance with claim 1 wherein said 55 substrate comprises vegetation.

7. A process in accordance with claim 1 wherein said solution contains from about 1.0 to 6.0 weight percent alkali metal borate and from about 1.0 to 6.0 weight percent water soluble polyvinyl alcohol.

ROBERT F. BURNETT, Primary Examiner D. J. FRITSCH, Assistant Examiner US. Cl. X.R.